Hydrogen bonds in polymer blends

Hydrogen bonds in polymer blends

Yong He, Bo Zhu, Yoshio Inoue*

Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259-B-55 Nagatsuta,

Midori-Ku, Yokohama 226-8501, Japan

Received 8 April 2004; revised 1 July 2004; accepted 21 July 2004

Available online 2 October 2004

Abstract

The hydrogen bond in polymer blends is an interesting and important subject of research as its presence usually enhances the

miscibility of the blend. Here, the factors affecting the formation and stability of hydrogen bonds, the effects of hydrogen bonds

on the miscibility and the properties of blends, and methods to incorporate inter-associated hydrogen bonds into immiscible

blends are reviewed, based on work over the last twenty or so years.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Hydrogen bonds; Polymer blends; Miscibility; Glass transition temperature; Melting points

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1022

2. Hydrogen bonds and characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024

2.1. Hydrogen bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024

2.2. Characterization of hydrogen bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024

3. Influence factors of hydrogen bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025

3.1. Acidity of the proton donor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1026

3.2. Basicity of the proton acceptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

3.3. Bulky side group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

3.4. Spacer length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028

3.5. Tacticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1030

3.6. Rigidity of polymer chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031

3.7. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031

Prog. Polym. Sci. 29 (2004) 1021–1051

www.elsevier.com/locate/ppolysci

0079-6700/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progpolymsci.2004.07.002

* Corresponding author. Tel.: C81 45 924 5794; fax: C81 45 924 5827.

E-mail address: [email protected] (Y. Inoue).

http://www.elsevier.com/locate/ppolysci

Y. He et al. / Prog. Polym. Sci. 29 (2004) 1021–10511022

4. Hydrogen bond and miscibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032

4.1. Basic principles of polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032

4.2. Free energy of mixing for hydrogen-bonded polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033

4.3. Hydrogen bonds and miscibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033

4.3.1. Blends of poly(4-vinylphenol) (PVPh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1034

4.3.2. Blends of poly(vinyl alcohol) (PVAL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1037

4.3.3. Blends of poly(acrylic acid) (PAA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1037

5. Incorporation of inter-associated hydrogen bonds: a strategy for compatibilization of polymer blends . . . . . . . . . 1038

5.1. Incorporation of terminated functional group as proton acceptor or donor . . . . . . . . . . . . . . . . . . . . . . . . . 1038

5.2. Inclusion of hydrogen-bonding monomer into the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1038

5.3. Addition of third polymer containing hydrogen-bonding moieties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039

5.4. Introduction of an inert diluent moiety to the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040

6. Effects of hydrogen bonds on the properties of polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040

6.1. Glass transition temperature (Tg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040

6.2. Melting temperature (Tm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1042

6.3. Crystallization behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1042

6.4. Surface enrichment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1042

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044

1. Introduction

The production of polymer materials has grown

rapidly in the past 50 years. The versatility of

plastics, not exceeded by any other class of

materials, guarantees that polymers will continue

to be very important in the future. However, at

present a distinct change is taking place in polymer

research and development. In the pioneering days

of plastics, new polymer properties were deter-

mined by the choice of suitable new monomers.

Today the commercialization of polymers from new

monomers is restricted to a few specialities. On the

other hand, the number of new polymer blends and

alloys based on known polymers is increasing very

rapidly. The market for polymer blend based

materials has increased continuously during the

past two decades.

Polymer blending is a convenient route for the

development of new polymeric materials, able to yield

materials with property profiles superior to those of the

individual components. This method is usually cheaper

and less time-consuming for the creation of polymeric

materials with new properties than the development of

new monomers and/or new polymerization routes.

An additional advantage of polymer blends is that

the properties of the materials can be tailored by

combining component polymers and changing the

blend composition.

Three different types of blends can be distin-

guished: completely miscible blends, partially mis-

cible blends (for the definition of partially miscible

blends, see the third paragraph of Section 4.1) and

fully immiscible blends. As the gain in mixing

entropy is negligible due to the high molecular weight

of polymer and the mixing is endothermic in the

majority of cases, miscible polymer blends are the

exception rather than the rule. Indeed, only a few

miscible blends have been identified. Most polymer

mixtures form immiscible blends, in which the

interphase is sharp. Since adhesion between the two

polymer components is poor in such blends, they are

usually useless unless than can be compatibilized.

On the other hand, it has becomes clear through

recent work that a large class of polymers form

miscible blends with appropriate partners through the

formation of inter-associated hydrogen bonds. Con-

sequently, it is now a well-known strategy to enhance

Nomenclature

DDA dodecanedioic acid

DMBAMA 2,3-dimethyl butadiene-stat-n-alkyl

methacrylate copolymer

DMBVPh poly(2,3-dimethylbutadiene-stat-4-

vinyl phenol)

DMTA dynamic mechanic thermal analysis

DSC differential scanning calorimetry

EVA ethylene/vinyl acetate copolymers

EVAL ethylene/vinyl alcohol copolymer

FTIR Fourier transform infrared

spectroscopy

NMR nuclear magnetic resonance

spectroscopy

P(S-b-EOx) poly(styrene-b-2-ethyl-2-oxazoline)

P(VA-co-VAL) poly(vinyl acetate-co-vinyl alco-

hol) copolymers

P2VPy poly(2-vinylpyridine)

PAA poly(acrylic acid)

PAMA poly(n-alkyl methacrylate)

PAMP poly(N-acryloyl-N 0-

methylpiperazine)

PAPP poly(N-acryloyl-N 0-

phenylpiperazine)

PAS poly(p-acetoxystyrene)

PBA poly(butyl acrylate)

PBAAA poly(butyl acrylate-co-acrylic acid)

PBMA poly(n-butyl methacrylate)

PCHMA poly(cyclohexyl methacrylate)

PCL poly(3-caprolactone)

PDI poly(dialkyl itaconate)

PDLLA poly(D,L-lactide)

PDMA poly(N,N-dimethyl acrylamide)

PEAA poly(ethylene-co-acrylic acid)

PEMA poly(ethyl methacrylate)

PEO poly(ethylene oxide)

PEOx poly(2-ethyl-2-oxazoline)

PHB poly(3-hydroxybutyrate)

PHV poly(3-hydroxyvalerate)

PLLA poly(L-lactide)

PMMA poly(methyl methacrylate)

PMMAA poly(methyl methacrylate-co-acrylic

acid)

PMOCMA poly(methoxycarbonylmethyl

methacrylate)

PMVAc poly(N-methyl-N-vinylacetamide)

PMVT poly(4-methyl-5-vinylthiazole)

POM polyoxymethylene

PP polypropylene

PPO poly(2,6-dimethyl-1,4-phenylene

oxide)

PS polystyrene

PSAA poly(styrene-co-acrylic acid)

PSSA poly(styrenesulfonic acid)

PVAL poly(vinyl alcohol)

PVAc poly(vinyl acetate)

PVDF poly(vinylidene fluoride)

PVME poly(vinyl methyl ether)

PVP poly(N-vinyl-2-pyrrolidone)

PVPA poly(vinylphosphonic acid)

PVPh poly(4-vinyl phenol)

PVPy poly(4-vinylpyridine)

SBA suberic acid

SCA succinic acid

STVPh styrene/vinyl phenol copolymer

SVBDEP styrene/4-vinylbenzenephosphonic

acid di-ethyl ester copolymer

VBDEP 4-vinylbenzenephosphonic acid di-

ethyl ester

VPh 4-vinyl phenol

XPS X-ray photoelectron spectroscopy

aPHB atactic poly(3-hydroxybutyrate)

aPMMA atactic poly(methyl methacrylate)

iPHB isotactic poly(3-hydroxybutyrate)

iPMMA isotactic poly(methyl methacrylate)

sPHB syndiotactic poly(3-

hydroxybutyrate)

sPMMA syndiotactic poly(methyl

methacrylate)

Y. He et al. / Prog. Polym. Sci. 29 (2004) 1021–1051 1023

the compatibility of the immiscible blends

by the incorporation of inter-associated hydrogen

bonds.

A lot of our work over the preceding decade has

involved polyester blends with inter-associated

hydrogen bonds. The formation of inter-associated

hydrogen bonds not only promotes the miscibility,

or at least the compatibility, of the polyester blends

but also effectively modifies the properties of the

polyesters.


In 1995, Coleman and Painter published an

excellent review on hydrogen-bonded polymer blends

[1]. This review mainly dealt with the association

models and the related theories. Since then, there has

been significant progress in the field of hydrogen-

bonded polymer blends through the work of many

research groups, with several hundred papers pub-

lished since 1995. This paper will review the progress

of hydrogen-bonded polymer blends made in the last

twenty years.

2. Hydrogen bonds and characterization

2.1. Hydrogen bonds

In general, the hydrogen bond is a directed,

attractive interaction between electron-deficient

hydrogen and a region of high electron density [2–4].

Most frequently, a hydrogen bond is of the X–H/Y

type, where X and Y are electronegative elements and

Y possesses one or more lone electron pairs. In most

cases, X and Y are F, O, and N atoms. The hydrogen

bonds are generally much weaker than covalent bonds

or other polar bonds, but much stronger than the van

der Waals interaction. The characteristic features of

the X–H/Y hydrogen bond are as follows: (i) the

X–H covalent bond stretches in correlation with the

strength of the hydrogen bond; (ii) a small amount of

electron density (0.01–0.03 e) is transferred from the

proton-acceptor (Y) to the proton-donor molecule

(X–H); (iii) the infrared absorbance band, which

corresponds to the X–H stretch, shifts to lower

frequency (red shift), broadens and increases in its

intensity upon formation of the hydrogen bond. The

value of the red shift and the strength of the hydrogen

bond are well correlated [4,5].

The hydrogen bond is the research subject of many

scientists from different branches of science. A

general review and historical survey, dating back to

the beginning of 20th century, has been published by

Jeffrey as a monograph: An Introduction to Hydrogen

Bonding [2]. The importance of the hydrogen bond is

obvious. It is responsible for the structure and

properties of water, an essential compound for life.

It is a fundamental component of the structure and

function of biomolecules, such as polysaccharides,

proteins and nucleic acids [6].

For polymer scientists, the hydrogen bond in

polymer blends is also an important issue. The presence

of inter-associated hydrogen bonds between the

components in a blend can promote compatibility and

also miscibility and has significant effects on the

properties of the blends. In fact, now-a-days the

introduction of hydrogen bonds is a routine and

effective strategy to achieve the compatibility and to

modify the properties of blends [7–11].

2.2. Characterization of hydrogen bonds

A variety of experimental methods, such as

infrared spectroscopy (IR), Raman spectroscopy,

gas–phase microwave rotational spectroscopy, neu-

tron inelastic scattering, nuclear magnetic resonance

spectroscopy (NMR), deuteron nuclear quadrupole

coupling, neutron diffraction, X-ray diffraction, and

so on, are employed to characterize the hydrogen

bonds in systems of low-molecular weight com-

pounds [2]. Among these methods, IR and NMR

spectroscopy are the most effective and widely

used methods to investigate the hydrogen bonds in

polymer systems.

Although the changes of energies, bond lengths

and electron densities with the formation of hydrogen

bonds are actually quite small in most cases, about

two or more orders of magnitude smaller than typical

chemical changes, IR spectroscopy is very sensitive to

the formation of hydrogen bond. In common, the

formation of hydrogen bond X–H/Y results in: (i) a

‘significant’ charge transfer from the proton acceptor

(Y) to the proton donor (X–H); (ii) weakening of the

X–H bond, which is accompanied by bond elongation

and a decrease of the X–H stretch vibration frequency

compared to the noninteracting species. This shift to

lower frequencies is called a red shift and can be

easily detected in liquid and solid phases by IR

spectroscopy. The red shift of the X–H stretch

vibration, which varies between several tens or

hundreds of wavenumbers, provide unambiguous

information about the formation of hydrogen bond.

Furthermore, in species Y, the bond that is adjacent to

the hydrogen bond, is also weakened somewhat with

the formation of the hydrogen bond. In most cases, a

red shift of the vibration of this bond occurs and can

be observed by IR spectroscopy though this red shift

may be much smaller than that of the X–H stretch

Fig. 1. The 13C chemical shift of the carbonyl (C) and the carbon of

–CH2–O– (B) resonances of PCL component in PCL/TDP (4,4 0-

thiodiphenol) blends with different TDP contents. The chemical

shifts were determined from the high-resolution 13C DD/MAS

(dipolar decoupling/magic angle spinning) NMR spectra of the

blends [14]. Reprinted from Macromol Chem Phys 2001;202:1035–

43, Copyright (2001), with permission of Wiley-VCH.


vibration, which presents further evidence of the

formation of the hydrogen bond.

The weakening and elongation of the X–H bond

with the formation of a hydrogen bond leads to

changes of the local chemical and electronic environ-

ment for protons and other nuclei involved in the

bond. Thus, variations of chemical shift and line shape

observed for the hydrogen-bonded nuclei by NMR

spectroscopy provide evidence of the formation of

hydrogen bond. The resonance peak of carbon nuclei

involved in the hydrogen bond tends to shift down-

field in the high-resolution solid-state 13C NMR

spectrum. In most cases, the resonances of carbonyl

carbons show downfield shifts of several ppm

[12–15]. An example for this downshift is shown in

Fig. 1, for the 13C chemical shift of the carbonyl and

–CH2–O– (B) carbon resonances of blends of poly

Scheme 1. Chemical structures

(3-caprolactone) (PCL) with 4,4 0-thiodiphenol (TDP)

for different TDP contents [14]. A downfield shift of

about 2 and 0.8 ppm is observed for the carbonyl and

–CH2–O– carbon resonances, respectively, on the

formation of inter-associated hydrogen bonds

between PCL and TDP. Besides the NMR chemical

shift and line shape, the nuclear magnetic relaxation

times, which are related to dynamic properties of the

sample, can provide further information about the

hydrogen bond.

3. Influence factors of hydrogen bonds

Hydrogen bonding has been reported for a variety

of polymer blends. The most common proton-

donating polymers are poly(4-vinyl phenol) (PVPh)

[16–21], poly(vinyl alcohol) (PVAL) [22–27], poly

(acrylic acid) (PAA) [28–31] (see Scheme 1 for the

chemical structure), their copolymers, analogs, and

so on. Proton-accepting polymers include polyesters

[32–37], polyacrylates [38–42] and polyether [27,

43–50] (Scheme 2). Depending on the combinations

of the proton donors and acceptors, the hydrogen

bonds are strong or weak, with hydrogen bond

energies ranging from 4 to 170 kJ/mol. In general,

hydrogen bonds with strength of 60–170 kJ/mol are

called as strong hydrogen bonds, 15–60 kJ/mol as

moderate hydrogen bonds, and 4–15 kJ/mol as weak

hydrogen bonds [2].

Beside the bond energy, equilibrium constants

provide another parameter to evaluate the extent of

hydrogen bonds formation. Following the Painter–

Coleman association model [1,51,52], the formations

of self-associated hydrogen bonds between B seg-

ments and inter-associated hydrogen bonds between

A and B segments can be expressed as:

of PVPh, PVA and PAA.

Fig. 2. The experimental fraction of hydrogen bonded carbonyl

group within various PCL blend systems at 75 8C (above the

melting temperature of PCL): (&) phenolic/PCL, (C) PVPh/PCL,

(:) phenoxy/PCL. The lines are predicted from the Painter–

Coleman association model (PCAM) [32]. Reprinted from Macro-

molecules 2003;36:6653–61, Copyright (2003), with permission of

American Chemical Society.

Scheme 2. Chemical structures of polyester, polyacrylate and polyether.


Bn CB1#KB

BnC1 (1)

Bn CA1#KA

BnA1 (2)

where Bn denotes the n-mers of B units. Accordingly,

the equilibrium constants for self-association and

inter-association, KB and KA, can be defined by

Eq. (3), using the volume fraction F:

KB Z nFBnC1=ðFBnFB1ðn C1ÞÞ (3a)

KA Z nrFBnA=ðFBnFA1ðn CrÞÞ (3b)

where FA1 and FB1 represent the volume fraction of

A and B segments, respectively, that are not hydrogen-

bonded, and r is the ratio of the molar volume of the

A and B segments. The inter-association equilibrium

constant KA, often given for a particular temperature,

e.g. 25 8C, provide a measure of the tendency of the

inter-associated hydrogen-bonding interaction. They

may be utilized to evaluate the effects of structure or

other factors on hydrogen bond formation.

As the hydrogen bond is a directional attractive

interaction between electron-deficient hydrogen and a

region of high electron density, the strength of the

hydrogen bond is directly related to all the elements

affecting the acidity of the proton donor, the basicity of

the proton accepter, and the accessibility of the donor

and acceptor. Among these, the chemical and stereo

structures of the donors and acceptors are essential in

determining the strength of the hydrogen bonds, with

environment conditions, such as the temperature and

pressure, also important to the properties of the

hydrogen bond.

3.1. Acidity of the proton donor

Recent reports of Kuo and Chang have addressed

the role of the chemical structure of proton donating

polymers on the strength of hydrogen bonds in binary

blends with poly(3-caprolactone) (PCL) [32,53].

With differential scanning calorimetry (DSC) and

Fourier transform infrared spectroscopy (FTIR), they

investigated the hydrogen bonds formed between the

carbonyl group of PCL and the hydroxyl group of the

phenolic formaldehyde–phenol copolymer, PVPh,

and the phenoxy bisphenol A–epichlorohydrin copo-

lymer). From Fig. 2, it is clear that the fraction of

hydrogen bond formation with PCL occurs in the

order phenolic/PCLOPVPh/PCLOphenoxy/PCL

blend [32]. Furthermore, the inter-association equili-

brium constants and relative ratio of KA/KB calculated

from the Painter–Coleman association model are in

the same order. Thus, the author concluded that the

strength of hydrogen bonds decreased in the order of

phenolic/PCLOPVPh/PCLOphenoxy/PCL, which is

the order of the acidity of the proton donors.

Goh et al. have studied the nature of interpolymer

interactions of poly(4-methyl-5-vinylthiazole) (PMVT)

with PVPh, PAA and poly(vinylphosphonic acid)


(PVPA), as well as those between poly(N-acryloyl-N 0-

phenylpiperazine) (PAPP) and PVPh, poly(styrenesul-

fonic acid) (PSSA), PVPA, and PAA, using FTIR and

X-ray photoelectron spectroscopy (XPS) [54,55]. As the

acidities of PSSA and PVPA are quite strong, each of

them formed complexes with either PMVT and PAPP or

both, where the ionic hydrogen bonds were detected by

XPS. However, PVPh and PAA only formed moderate

hydrogen bonds with PMVT and PAPP, and miscible

blends; no precipitate (complex) was obtained on

mixing them in solution. It was also pointed out that

the interactions of PAPP with acidic polymers are less

intense as compared to those of poly(N-acryloyl-N 0-

methylpiperazine) (PAMP) because of the steric

shielding effect of the phenyl groups, and the lower

basicity of the tertiary amine nitrogen [55].

Li and Goh used FTIR to study the specific

interactions between three aliphatic dicarboxylic acids:

succinic acid (SCA), suberic acid (SBA) and dodecane-

dioic acid (DDA), and two proton-accepting polymers,

poly(2-vinylpyridine) (P2VPy) and poly(N-vinyl-2-pyr-

rolidone) (PVP) [56]. They found that P2VPy interacts

with the acids through hydrogen-bonding as well as ionic

interaction, while PVP interacts with the acids only

through hydrogen-bonding. The intensity of interaction

decreases in the order of SCAOSBAODDA, which is in

the reverse order of the spacer length between two

carboxylic groups of the acid.

Kondo et al. have extensively studied the nature of

inter-associated hydrogen bonds among cellulose,

PVAL, and poly(ethylene oxide) PEO [27,57–59].

In particular, it was demonstrated by FTIR that the

inter-associated hydrogen bonds in cellulose/PVAL

blends were formed mainly between the glucose ring

ether oxygen and hydroxyl groups (OH) in PVAL

while they are also formed between secondary OH at

either the C-2 or C-3 position of the glucose ring and

the OH of the PVAL component. However, inter-

associated hydrogen bonding are formed only

between the primary OH at the C-6 position in

cellulose and the skeletal oxygen of PEO in cellulose/

PEO blends [57]. As to the PVAL/PEO blend system,

the authors found that the two polymers appeared to

be immiscible on the basis of results obtained from

optical microscopic observations, small-angle light

scattering, DSC and FTIR [27]. The immiscibility of

PVAL and PEO was regarded as evidence for the lack

of hydrogen bonds between the polymer pairs.

In contrast, the miscibility was improved, and inter-

associated hydrogen bonds were formed, in PEO

blends with hydroxypropylated PVAL derivatives,

where primary OH groups are included. Considering

that PVAL only has secondary OH, while the

derivatives have primary OH, the authors concluded

that only primary OH could engage in inter-associated

hydrogen bonding with the skeletal oxygen of PEO.

3.2. Basicity of the proton acceptor

The impact of the chemical structure of proton-

accepting polymers on the strength of hydrogen bonds

has been investigated by several groups. Goh et al.

studied binary blends of phenoxy with three isomeric

tertiary amide polymers: poly(N-methyl-N-vinylaceta-

mide) (PMVAc), poly(N,N-dimethyl acrylamide)

(PDMA) and poly(2-ethyl-2-oxazoline) (PEOx) [60].

Phenoxy forms interpolymer complexes with PMVAc

and PDMA from tetrahydrofuran solutions over the

entire feed composition range. However, complex

formation does not occur between phenoxy and PEOx

from THF solutions, indicating a weaker intermolecular

association in the phenoxy/PEOx blends than those in

the phenoxy/PMVAc and phenoxy/PDMA blends.

When using N,N-dimethylformamide as the solvent,

only miscible blends are formed between phenoxy and

all three tertiary amide polymers. FTIR studies provide

evidence of inter-associated hydrogen bonding between

the hydroxyl groups in phenoxy and the carbonyl

groups in tertiary amide polymers. The IR frequency

shifts for the phenoxy hydroxyl groups hydrogen-

bonded to the three tertiary amide polymers decrease in

the order of PMVAcOPDMAOPEOx [60].

3.3. Bulky side group

Coleman and Painter have studied the hydrogen

bonds in blends of poly(2,3-dimethylbutadiene-stat-4-

vinyl phenol) (24 wt% 4-vinyl phenol) (DMBVPh)

with a series of poly(n-alkyl methacrylate)s (PAMAs)

bearing side chains with different length (n-alkylZmethyl, ethyl, n-butyl, n-hexyl, n-octyl, n-decyl,

n-lauryl, n-hexadecyl) [61]. They utilized the inter-

association equilibrium constant KA at 25 8C as a

measure of the accessibility of the carbonyl groups for

inter-associated hydrogen bonding, where KA was

obtained from the FTIR spectra of the blends.

Table 1

Dimensionless inter-association equilibrium constants for PAMA and DMBVPh [61]

Polymer Molar vol

(cm3/mol)

KA

(100 8C)

KA (25 8C)

(hAZK4.1 kcal/mol)KSTD

A

(VBZ100 cm3/mol; TZ25 8C)

R

(cm3/mol)

PMMA 84.9 2.57 10.4 54.6 0

PEMA 101 2.58 10.4 54.7 16.5

PBMA 134 2.17 8.78 46.1 49.5

PHMA 167 1.93 7.82 41.1 82.5

POMA 200 1.91 7.72 40.5 116

PDMA 233 1.79 7.25 38.1 149

PLMA 266 1.70 6.88 36.1 182

PHDMA 332 1.56 6.3 33.1 248

PODMA 365 1.30 5.26 27.6 281

EMMA[47]a 218 3.65 14.8 77.4 133

EMMA[38] 277 3.98 16.1 84.5 192

EMMA[33] 324 4.09 16.6 86.9 239

Reprinted from Macromolecules 1996;29:6820–31, Copyright (1996), with permission of American Chemical Society.a The weight percent of methyl methacrylate in the copolymer.


A decrease in KA of the carbonyl group of PAMA,

indicating a decrease in the extent of inter-associated

hydrogen bonding, was observed for blends of

DMBVPh/PAMA as the length of the side chain

increased (Table 1) [61]. The authors interpreted these

results as evidence that the bulky side groups of the

acrylates inhibit the formation of hydrogen bonds in

the blends by limiting the ability of the two chains to

arrange and orient themselves correctly to form stable

inter-associated hydrogen bonds [61].

The effect of the side group in poly(dialkyl

itaconate)s (PDIs) and poly(methoxy-carbonylmethyl

methacrylate) (PMOCMA) on the hydrogen bonds in

their binary blends with PVPh has been examined by

Goh et al. with DSC and FTIR [62]. Even though the

existence of hydrogen-bonding interactions in all the

binary blends of PDIs with PVPh has been confirmed

by FTIR, it was found that the carbonyl groups of

PDIs become less accessible to OH groups of PVPh

with increasing alkyl chain length. As a result, binary

blends of PVPh/PDI change from miscible to partially

miscible with increase of the alkyl chain length. Due

to the bulky side group of PMOCMA, the hydrogen

bonds between it and PVPh are too weak to promote

complete miscibility of PVPh/PMOCMA blends.

The effect of chain connectivity and steric crowding

on the extent of hydrogen bonding in polymer

solutions has also been studied by Coleman et al.

[63]. They examined concentrated solutions of

PAMAs (n-alkylZmethyl, ethyl, n-butyl, n-hexyl,

n-octyl, n-decyl, n-lauryl, n-hexadecyl) in 4-ethyl

phenol. The results showed an effect of steric

crowding on the extent of hydrogen bonding between

PAMAs and 4-ethyl phenol that is similar to that in

blends of PAMA/DMBVPh, with a decrease in KA

with increasing length of the alkyl groups of PAMAs.

3.4. Spacer length

The preceding section addresses the effects of

different kinds of side groups, not located in the main

chain. To some extent, larger side groups tend to

suppress the formation of hydrogen bonds. This

section deals with spacers located in the main chain,

between functional groups along the chain. Generally,

longer spacers favor the formation of strong hydrogen

bond, as shown in the following paragraphs.

Coleman and Painter have studied the hydrogen

bonds in blends of DMBVPh (vinyl phenol content:

24 wt%) with a series of ethylene/vinyl acetate

copolymers (EVA, 70, 45, 25, 18 wt% vinyl acetate)

[61]. The standard inter-association equilibrium con-

stant for the DMBVPh/EVA blends, and therefore the

accessibility of the acetate group to form inter-

associated hydrogen bonds, increases as the spacing

between the vinyl acetate groups in the ethylene/vinyl

acetate copolymer increases, until it reaches a plateau

value for a copolymer composition with about 20 wt%

vinyl acetate [61]. Subsequently, Coleman and Painter

studied the hydrogen bonds in blends of DMBVPh


and EVA as a function of spacer length between

specific interaction sites in DMBVPh and EVA, where

both the 4-vinyl phenol (VPh) content in DMBVPh and

the VA content in EVA vary from 100 to 9 wt% [64].

The IR spectra shown in Fig. 3 were recorded at 100 8C

in the carbonyl stretching region (1675–1775 cmK1)

for DMBVPh[9] blends (DMBVPh[9] content:

90 wt%) with a series of ethylene/vinyl acetate

copolymers [64]. Two IR bands can be observed: the

band at about 1738 cmK1 is assigned to ‘free’ (i.e. non-

hydrogen-bonded) carbonyl groups, while that at

1710 cmK1 is attributed to hydrogen-bonded carbonyl

groups. With decrease of VA content in the EVA

copolymer, the relative intensity of the band corre-

sponding to the hydrogen-bonded carbonyl groups

increases significantly (Fig. 3). The calculated stan-

dard inter-association equilibrium constant increases

Fig. 3. Left: Infrared spectra of DMBVPh[9] blends (DMBVPh[9]

content: 90 wt%) with PVAc and a series of ethylene/vinyl acetate

copolymers, EVA[70], EVA[45], EVA[25], EVA[18], EVA[14],

and EVA[9] recorded at 100 8C in the carbonyl stretching region

(1675–1775 cmK1). The numbers in the bracket indicate the content

of the second comonomer unit in weight percent. Right:

Corresponding synthesized spectra for single-phase blends calcu-

lated from the Painter–Coleman association model. The numbers in

the bracket of the sample code indicate the content of the second

comonomer unit in weight percent [64]. Reprinted from Macro-

molecules 1997;30:3671–7, Copyright (1997), with permission of


with increasing spacer length between the VPh units in

DMBPVPh, as well as the length between VA units in

EVA. The authors suggested that as the average

distance between vinyl acetate units increases with

decreasing vinyl acetate content in the copolymer, the

rotational motion of the acetate groups became

independent of each other. This independence allows

more acetate groups to orient themselves properly to

form inter-associated hydrogen bonds. This process, of

course, starts to decrease when the number of acetate

groups in the copolymer becomes so low as to limit the

possible number of inter-associated hydrogen bonds

that can be created [61,64].

Coleman et al. further studied the blends of a series

of 2,3-dimethyl butadiene (DMB)-stat-n-alkyl metha-

crylate (AMA) copolymers (DMBAMAs) containing

AMA segments of increasing side chain length

(i.e., methyl, ethyl, n-propyl, n-butyl, n-decyl, and

n-hexadecyl) with DMBVPh (containing 24 wt% VPh

unit) [65]. The DMB segment in the DMBAMA

copolymers acts an ‘inert’ (non-hydrogen bonding)

diluent, spacing the AMA segments apart to a point

where the AMA segments do not interfere with one

another in forming hydrogen bonds with VPh segments

of the DMBVPh copolymer. This ‘spacing’ out of the

AMA carbonyl groups limits the effect of steric

crowding on functional group accessibility. As the

spacing between the carbonyl groups along the

polymer chain increases, the magnitude of the standard

inter-association equilibrium constant KA increases

until a plateau is reached, with no further increase in KA

for further separation of the carbonyl groups.

Coleman and Painter also examined concentrated

solutions of EVA (70, 45, 40 wt% vinyl acetate) in

4-ethyl phenol. Similar to the EVA/DMBVPh blend

system, an increase in the accessibility of the carbonyl

group to form inter-associated hydrogen bonds with

the hydroxyl group of 4-ethyl phenol was observed

with increased spacing between acetate groups in the

EVA copolymers [63]. In common with the trends

observed for miscible DMBVPh blends with EVA and

DMBAMA copolymers [61,65], the results of stan-

dard inter-association equilibrium constant KA

increased from 110, 153, 163 to 164 dimensionless

units as the VA content decreases from 100, 70,

45 to 40 wt%, respectively, demonstrating

that the ‘spacing’ effect also occurs in concentrated

4-ethyl phenol solutions.

Fig. 4. Carbonyl-stretching bands, measured at 30 8C, of aPHB,

sPHB, iPHB and their blends with catechin. In the blend, the molar

ratio of catechin to 3-hydroxybutyrate unit is 0.25 [69]. Reprinted

from Green Chem 2003;5:580–6, Copyright (2003), with per-

mission of The Royal Society of Chemistry.


Radmard and Dadmun varied the composition of

styrene/VPh copolymers (STVPh) to adjust the space

length between hydroxyl groups in an examination of the

impact of the distance between hydroxyl groups along the

STVPh chain on the formation of inter-associated

hydrogen bonds between STVPh and polyethers [66].

Utilizing the difference between the stretching frequency

of the hydroxyl band in the pure styrenic polymer and that

in the blend as a measure of the extent of inter-associated

hydrogen-bonding, they revealed that the propensity for

the hydroxyl groups to form inter-associated hydrogen

bonds with the polyethers improves as the distance

between the hydroxyl groups of the STVPh copolymer

increases, consistent with the results of Coleman and

Painter [61,64,65]. The authors attributed the results to a

decreasing of the extent of self-associated hydrogen-

bonding within the STVPh, in addition to an increase in the

extentof rotational freedomof thevarioushydroxylgroups

with respect to each otherwith increasingdistancebetween

hydroxyl groups of STVPh [66].

3.5. Tacticity

An FTIR study has been performed by Iriondo and

co-workers on blends of PVPh and poly(3-hydroxy-

butyrate) (PHB) with different tacticity to check the

effect of tacticity on the strength of hydrogen bonds

[67,68]. Using the association model of Painter

and Coleman, the inter-association constant KA has

been determined, as well as the related enthalpy of

hydrogen bond formation. The absolute value of both

the hydrogen bond enthalpy and KA obtained for PVPh/

isotactic PHB (iPHB) are lower than those for PVPh/

atactic PHB (aPHB), indicating the hydrogen bonds

between PVPh and iPHB are weaker than those

between PVPh and aPHB [67].

Inoue et al. have studied the hydrogen bonds

between catechin and PHB as a function of the tacticity

of PHB [69]. FTIR measurements at 30 8C demon-

strated that strong inter-associated hydrogen bonds are

formed in aPHB/catechin and syndiotactic PHB

(sPHB)/catechin blends, while no evidence was

found to confirm the existence of inter-associated

hydrogen bonds in iPHB/catechin blends (Fig. 4) [69].

As the crystallinity of PHB is significantly affected by

the tacticity, and the crystalline phase restrains

the formation of hydrogen bonds, they also carried

out FTIR measurements on the melt at 190 8C to

separate the effect of tacticity from that of crystallinity.

In this case, strong inter-associated hydrogen bonds

were detected in iPHB/catechin blends as well as in

aPHB/catechin and sPHB/catechin blends (Fig. 5)

[69]. Furthermore, they revealed that in the melt,

higher tacticity of PHB benefits access of the OH group

of catechin to the carbonyl group of PHB, and thus

the formation of hydrogen bonds, based on a

quantitative analysis of the FTIR spectra. In the case

of poly(lactide)/catechin blends, the effect of the

tacticity of poly(lactide) on the inter-associated

hydrogen bonds between the two component is not

obvious [70].

Fig. 5. Comparison of carbonyl-stretching spectra of aPHB, sPHB

and iPHB with those of their blends with catechin at 190 8C. In the

blend, the molar ratio of catechin to 3-hydroxybutyrate unit is 0.25

[69]. Reprinted from Green Chem 2003;5:580–6, Copyright (2003),

with permission of The Royal Society of Chemistry.

Fig. 6. The FTIR spectra of the PCL/TDP 60/40 blend in the

hydroxyl vibration region (a) and the carbonyl vibration region (b)

recorded at different temperatures. From top to bottom, the

temperatures are 26, 35, 45, 65, 90, 120 and 160 8C [14]. Reprinted

from Macromol Chem Phys 2001;202:1035–43, Copyright (2001),

with permission of Wiley-VCH.


3.6. Rigidity of polymer chains

Radmard and Dadmun have addressed the

influence of the rigidity of the aromatic polyether

on the formation of inter-associated hydrogen

bonds between the polyether and STVPh [66].

They revealed that the extent of inter-associated

hydrogen bonding increases when the polyether

becomes more flexible if all other parameters are

equal.

3.7. Temperature

In general, the inter-association equilibrium con-

stant KA and the number of the hydrogen bonds for a

given system decrease with increasing temperature

because the enthalpy of hydrogen bond formation is

usually negative. FTIR spectra as a function of

temperatures are summarized in Fig. 6 for a PCL/TDP

Fig. 7. Fractions of hydrogen-bonded CaO groups in percentage

versus temperature: (%) 50/50 and (C) 80/20 wt% PVPh/PMMA

blend [72]. Reprinted from Macromolecules 1997;30:286–92,

Copyright (1997), with permission of American Chemical Society.


60/40 blend in the hydroxyl and the carbonyl vibration

regions [14]. The absorbance of the hydroxyl becomes

weak and shifts to high wavenumber with increasing

temperature. In the carbonyl vibration region, the

intensity of the peak corresponding to the hydrogen-

bonded carbonyl decreases with increasing tempera-

ture. These facts indicate that the inter-associated

hydrogen bonds become weak and the number of

hydrogen bond decreases with the increasing

temperature.

As was expected, the inter-association equilibrium

constants in blends of DMBVPh/EVA [61] and PVPh/

poly(vinyl acetate) (PVAc) [64], and all the fraction of

hydrogen-bonded pyridine rings in both phenolic/

P2VPy and phenolic/PVPy (poly(4-vinylpyridine)

drop with the increasing temperature. However, in a

multicomponent hydrogen-bonding system containing

competing acceptor groups with disparate enthalpies of

hydrogen bond formation, the hydrogen-bonded frac-

tion of one acceptor group may increase with increas-

ing temperature. Coleman et al. found that the fraction

of hydrogen-bonded carbonyl groups increased with

temperature in BMAVPh/poly(styrene-co-2-vinyl

pyridine) blends, in which the hydroxyl group form

hydrogen bonds with both the carbonyl groups and

pyridine groups, with very different strengths for the

resulting hydrogen bonds [71].

The number of hydrogen bonds formed with

semicrystalline polymer may increase with increasing

temperature as the crystalline phase progressively

melts. In the case of iPHB/catechin, the hydrogen

bonding between iPHB and catechin could not be

detected by FTIR at room temperature. However,

strong hydrogen bonds were confirmed at 190 8C,

which is above the melting point of iPHB (Fig. 5) [69].

In PVPh/PMMA blends, increased temperature

and annealing significantly enhance the formation of

hydrogen bonds [72]. In these blends, the extent of

hydrogen-bonding of the CO groups increases after

heating the casting film to 170 8C, and this tendency

does not stop during subsequent slow cooling from

170 to 40 8C. Overall, more hydrogen-bonded CO

groups are formed after the thermal cycle. The

percentage of CO groups bonded with the OH groups

versus temperature is shown in Fig. 7 [72]. A notable

increase of the fraction of hydrogen-bonded CO group

can be observed after heating to temperatures above

the Tg of PMMA. This result may suggest that

the increasing of the temperature above the Tg

enhances the motion of the side chain and backbone

chain and thus favors the formation of hydrogen

bonds.

4. Hydrogen bond and miscibility

4.1. Basic principles of polymer blends

Polymer blends are either homogeneous or hetero-

geneous. In homogeneous blends, the final properties

are often an arithmetic average of the properties of the

blend components. In heterogeneous blends, the proper-

ties of all blend components are present. A deficiency in

the properties of one component can be camouflaged to

a certain extent by strengths of the others.

Homogeneous miscibility in polymer blends

requires a negative free energy of mixing, that is

DGmix!0. According to Flory–Huggins equation:

DGmix=RT ZF1 lnðF1Þ=N1 CF2 lnðF2Þ=N2

Cc12F1F2 (4)

where DGmix is the change of free energy on mixing

two polymers, R is the gas constant, T is the

temperature, F1 and F2 are the volume fractions and


N1 and N2 are the segment numbers of the two blend

components, respectively, and c12 is the Flory

interaction parameter. When two high molecular

weight polymers are blended, the gain in entropy,

F1 ln(F1)/N1CF2 ln(F2)/N2, is quite small, and the

free energy of mixing, DGmix, can be negative only

if the heat of mixing is near zero or negative,

c12!w0.002.

Basically, three different types of blends can be

distinguished. In completely miscible blends, for

which c12!w0.002, homogeneity is observed at

least on a nanometer scale, if not on the molecular

level. This type of blend exhibits only one glass

transition temperature (Tg), between the Tgs of the

blend components, and in a close relation to the blend

composition. In partially miscible blends, a part of one

component is dissolved in the other. As a result, two

phases are observed, one phase rich in one component

and the other phase rich in the other component. Each

phase exhibits a Tg, which is between the Tgs of the

pure components. In this case, the interphase is wide

and interfacial adhesion is good. This type of blend,

which exhibits satisfactory properties, is often

referred to as compatible. By far most blends are

fully immiscible. They have a coarse phase mor-

phology, with a sharp interphase, and the adhesion

between the phases, each exhibiting the Tg of the pure

component, is poor.

Theoretically, it has become clear that the

miscibility of polymer blends is mainly determined

by the chemical structure, composition and molecular

weight of each component. In some cases, the

preparation condition of the blends is also a decisive

factors. Experimentally, various techniques have been

used to characterize the miscibility of polymer blends,

such as optical transparency, FTIR, electron

microscopy, DSC, dynamic mechanic thermal analy-

sis (DMTA), dielectric relaxation and high-resolution

solid state 13C NMR spectroscopy [73,74]. Tradition-

ally, DSC has been used to determine the phase

diagram of polymer blends, a single glass transition

temperature denoting a single-phase blend. DMTA

and dielectric relaxation spectroscopy have the

advantage over calorimetric techniques in that they

are more sensitive to smaller domain size, and

therefore can reveal finer details [75–77]. Among

several spectroscopic techniques, high-resolution

solid state NMR has proven to be a powerful method

to characterize the structure of polymer blends.

Detailed information about the miscibility, inter-

associated interaction and morphology of polymer

blends can be obtained by examining NMR para-

meters, such as chemical shift, line shapes and

relaxation times [78–80].

4.2. Free energy of mixing for hydrogen-bonded

polymer blends

For hydrogen-bonded polymer blends, the extent

of inter-associated hydrogen bonds is an important

parameter in determining the miscibility of the blend.

Although strong specific interactions can produce a

favorable enthalpy of mixing in the blend, it should be

noted that such interactions tend to further minimize

the entropy of the system due to limitations of the

chain mobility and its rotational freedom as the

polymer chains are forced to assume nonrandom

configurations. Thus, the effect of specific interactions

on the free energy of mixing two polymers is complex

and must be thoroughly understood to utilize them to

induce miscibility in the blends.

Painter and Coleman have proposed to add a term

of DGH/RT to the classic Flory–Huggins equations to

account for the free energy of hydrogen bonding

formation [1,52,81,82]:

DGmix=RT Z F1 lnðF1Þ=N1 CF2 lnðF2Þ=N2

Cc12F1F2 CDGH=RT (5)

where the first two terms are the combinatorial entropy.

The third term c12F1F2 is an unfavorable contribution

to the free energy of mixing from so-called ‘physical’

force, while the fourth term DGH/RT is a favorable

contribution derived from hydrogen bonding. With the

‘physical’ force deduced from solubility parameters,

DGH may be determined from the equilibrium

constants and enthalpies of hydrogen bond formation.

As the combinatorial entropy is very small, the free

energy of mixing, and thus the miscibility, are

dominated by the balance of the ‘physical’ force and

enthalpy of hydrogen bond formation.

4.3. Hydrogen bonds and miscibility

The formation of hydrogen bonds usually induces

the miscibility of the blends, as has been shown for


many hydrogen-bonded blends. Almost all blends

found to be either partially or fully miscible with

the presence inter-associated hydrogen bonds; other-

wise, miscibility is the exception rather than the rule.

In the following, the results on several kinds of

important hydrogen-bonded polymer blends are

summarized to address the relationship between

hydrogen bonds and miscibility.

4.3.1. Blends of poly(4-vinylphenol) (PVPh)

A variety of PVPh blends with side-chain polyesters,

main-chain polyesters, polyethers, and so on, have been

investigated for the last twenty years. Indeed, blends

of VPh (co)polymers are the most widely studied

hydrogen-bonded blends. A short survey of PVPh

blends was published by Landry et al. in 1994 [83]. The

following mainly summarizes work on blends of PVPh

and VPh copolymer since 1994.

The miscibility of blends of PVPh with poly(methyl

methacrylate) (PMMA) has been evaluated by several

groups. It has found that miscibility depends on the

tacticity of PMMA and the preparation condition.

Zhang et al. examined the hydrogen bonds and

miscibility of PVPh/PMMA blends using solid state13C NMR [84]. No obvious downfield shifts of 13C

resonances were observed for the carbonyl carbon of

PMMA or the phenolic hydroxyl carbon of PVPh in

PVPh/PMMA blends, suggesting that no strong hydro-

gen bond is formed between PVPh and PMMA. The

results of proton relaxation indicated poor miscibility of

PVPh/PMMA blends, with a domain size O20–30 nm.

The miscibility of PVPh/PMMA blends has also

been investigated by Dong et al. using FTIR and FT-

Raman spectroscopy [72]. Although both techniques

provided evidence for the formation of hydrogen bonds

between the hydroxyl group of PVPh and the ester

group of PMMA, the blends showed only a limited

degree of hydrogen-bonding interactions, and were

partially miscible on initial observation. However, they

gradually approached complete miscibility with pro-

longed heating. Landry and Teegarden addressed the

effect of casting solvent on the miscibility of blends of

PVPh with atactic PMMA (aPMMA) [85]. They

revealed that THF induced phase separation in these

blends, while 2-butanone promoted the miscibility of

the blends. Hsu studied the miscibility of PVPh/PMMA

blends as a function of the PMMA tacticity [17].

Isotactic, atactic, and syndiotactic PMMAs (iPMMA,

aPMMA, and sPMMA, respectively) were mixed with

PVPh in THF (or 2-butanone) to obtain blend solutions.

When the blends were cast from THF solution,

calorimetry data indicated that iPMMA was miscible

with PVPh, while partial miscibility or immiscibility

was found with either aPMMA or sPMMA. When

2-butanone was used as the solvent, both iPMMA and

aPMMA were miscible with PVPh, but sPMMA/PVPh

blends remain partially miscible, as indicated by the

observation of two Tgs for most compositions.

The effect of the PMMA tacticity on the miscibility

changes on replacing PVPh by STVPh copolymer. The

miscibility of blends of STVPh/sPMMA and

STVPh/iPMMA was studied by Jong et al. with 13C

solid-state NMR, complemented with cloud point and

DSC measurements [86]. On the time scale of the

proton spin–lattice relaxation, or a dimensional scale

of about 25 nm, sPMMA appears to be miscible with

STVPh (VPh monomer units content: 5%) in all blend

compositions studied, e.g. 30, 50, and 70% STVPh. By

contrast, iPMMA was miscible at only one compo-

sition, e.g. 70% STVPh. The glass transition tempera-

ture data of these blends were consistent with the NMR

observations. On increasing the VPh content in the

copolymer to 15 mol%, the tacticity of PMMA was no

longer an important factor in the miscibility of the

blends [87], e.g. iPMMA, aPMMA, and sPMMA

blends prepared from their 2-butanone solution were

found to be miscible, as all the blend films were

transparent, with a single glass transition temperature

observed for each composition.

Coleman and co-workers reported that PVPh was

miscible with poly(ethyl methacrylate), poly(n-propyl

methacrylate) and poly(n-butyl methacrylate) over the

entire blend composition range at temperatures

between ambient and 200 8C [88]. However, PVPh

was immiscible with poly(n-hexyl methacrylate) and

poly(cyclohexyl methacrylate) as the bulky groups

inhibited the formation of strong inter-associated

hydrogen bonds [89,90].

A number of poly(methacrylate) derivatives have

been reported to be miscible with PVPh, including

poly(2-dimethylamino ethyl methacrylate) [91],

poly(N-methyl-3-piperidinemethyl methacrylate)

[92], poly(methyl thiomethyl methacrylate) [93],

poly(2-ethoxyethyl methacrylate) [94], poly(N-

methyl-4-piperidinyl methacrylate) [95].

Fig. 8. Theoretical miscibility windows at room temperature calculated from the Painter–Coleman association model for STVPh blends with

PEMA, PBMA, PHMA, POMA, PDMA, and PLMA [61]. Reprinted from Macromolecules 1996;29:6820–31, Copyright (1996), with

permission of American Chemical Society.


Based on experimental results and theoretical

analysis, Coleman and Painter constructed a miscibility

map of poly(alkyl methacrylate) (PAMA) blends

with VPh copolymers poly(2,6-dimethyl-4-vinyl

phenol) [96,97], poly(2,6-diisopropyl-4-vinyl phenol)

[96,97], STVPh [51,61,65] and DMBVPh [51,61,65].

The accessibility of the carbonyl groups of PAMA to

the hydroxyl groups of VPh copolymer decreased

with increasing length of the side chain in PAMA, due

to steric shielding. As a result, the blend changed from

miscibility to an immiscibility with increasing length

of the side chain in PAMA (Fig. 8) [61].

Using the modified equations for the free energy of

mixing of hydrogen bonded polymer blends (Eq. (5)),

Coleman, and Painter predicted the miscibility

windows of EVA blends with DMBVPh and

STVPh, finding the experimentally observed misci-

bility maps (Figs. 9 and 10) [64,98]. EVA copolymers

Fig. 9. Theoretical miscibility map at 100 8C calculated from the

Painter–Coleman association model for DMBVPh/EVA blends

using a constant KSTDA value of 58.0. The areas encompassed by the

small black dots denote predicted two-phase regions. Experimen-

tally determined single- and two-phase blends are denoted by the

unshaded and black-filled large circles, respectively [64]. Reprinted

from Macromolecules 1997;30:3671–7, Copyright (1997), with

permission of American Chemical Society.

Fig. 10. Theoretical miscibility maps calculated from the Painter–

Coleman association model. Top: STVPh/EVA blends at 25 8C.

Bottom: DMBVPh/EVA blends at 100 8C. The areas encompassed

by the small black dots denote predicted two phase regions.

Experimentally determined single and two phase blends are denoted

by the unshaded and black filled large circles, respectively [98].

Reprinted from Macromol Chem Phys 1998;199:1307–14, Copy-

right (1998), with permission of Wiley-VCH.


with low ethylene unit content are miscible with

DMBVPh and STVPh, while those with high ethylene

unit content are immiscible with the VPh copolymers,

due to the decreasing number of the hydrogen bonds

(Figs. 9 and 10).

A number of main-chain polyesters are reported to be

miscible with PVPh and many VPh copolymers. The

DSC traces of pure aPHB, PVPh, and their blends with

PVPh content of 20, 30, 40, 50, and 60 wt% are shown

in Fig. 11 [35]. It is clear that a single composition-

dependent Tg is observed for the blend, indicating that

aPHB is miscible with PVPh. Other main-chain

polyester, such as sPHB [35], iPHB [67,68,99],

poly(3-hydroxyvalerate) (PHV) [100], PCL [11,33,53,

101,102], poly(ethylene succinate) [103], poly(ethylene

adipate) [103], poly(butylene adipate) [103], poly(pro-

pylene carbonate) [104], are also miscible with PVPh

and many VPh copolymers [35]. By contrast, phase

separation was observed over a wide composition

range for blends of PVPh/poly(L-lactide) (PLLA) [34]

and PVPh/poly(D,L-lactide) (PDLLA) [105]. Only

weak hydrogen-bonding interaction exists between

the carbonyl groups of PLLA (PDLLA) and the

hydroxyl groups of PVPh as evidenced in the FTIR

spectra. As a result, the blends are only partially

miscible.

Polyethers like PEO [19,50,106–112] and poly

(vinyl methyl ether) (PVME) [46,49,110,113,114] are

reported to be miscible with PVPh or VPh copolymers

over the whole composition range, due to the

hydrogen bonds between VPh unit and polyethers.

Beside polyesters and polyethers, PVP [115,116],

poly(N-acryloylthiomorpholine) [117], PEOx [118],

PAPP [55], PAMP [119], poly(1-vinylimidazole)

Fig. 11. First-run DSC traces of pure aPHB, PVPh, and their blends

with PVPh content of 20 (80/20), 30 (70/30), 40 (60/40), 50 (50/50),

and 60 wt% (40/60) [35]. Reprinted from Macromolecules

2001;34:8166–72, Copyright (2001), with permission of American

Chemical Society.


[120] form miscible blends with PVPh (or VPh

copolymers) over the entire composition range as a

result of the formation of strong inter-associated

hydrogen bonds between them and VPh unit.

4.3.2. Blends of poly(vinyl alcohol) (PVAL)

Strong hydrogen bonds between PVAL and

phenolic [121], PVP [24,26,122–124], poly(aspartic

acid) sodium [125], PAA [22], hydroxypropyl lignin

[126], poly(N,N-dimethylacrylamide) [127] and copo-

lyamide [128] have been detected by FTIR and other

techniques. As a result, these PVAL binary blends are

miscible over the whole composition range. The

miscibility of methyl cellulose/PVAL [129], poly(N-

methylglycine)/PVAL [130], b-chitin/PVAL [131],

polyanilines/PVAL [25] and PVAc/PVAL [132]

blends depends on composition, but PLLA [133],

kraft lignin [134] and PVPy [135] are reported to be

immiscible with PVAL. Katime et al. have reported

that blends of PVAL and PEOx with relatively high

molecular weight (MnZ50000) prepared by casting

from water at 120 8C are opaque, and show immis-

cibility in DSC analysis [136]. However, Aoi et al.

reported that PVAL is miscible with a low molecular

weight PEOx (MnZ10300) [137]. They suggested

that the molecular weight of PEOx affects the

miscibility.

The incorporation of a second commoner unit,

such as vinyl acetate and ethylene, into the main chain

of PVAL limits the formation of self-association

hydrogen bonds, and thus enhances the miscibility

with many polymers. Copolymers (poly(vinyl acetate-

co-vinyl alcohol) and poly(ethylene-co-vinyl alcohol)

(EVAL)) with a certain composition are reported to

be miscible with PVPy [135,138–141] and PEOx

[136,142], even though PVAL itself is immiscible or

only partially miscible with them.

4.3.3. Blends of poly(acrylic acid) (PAA)

PAA forms miscible blends with PEO [143,144],

PAPP [55], poly(1-vinylimidazole) [120], or PMVT

[54], due to the inter-associated hydrogen bonds

between the blend pairs. There are a number of papers

addressing hydrogen bonds and miscibility of blends

of acrylic acid containing copolymers. Miscible

blends are reported for poly(methyl methacrylate-

co-22 mol%-acrylic acid) (PMMAA)/poly(butyl

methacrylate-co-26 mol%-4-vinylpyridine) blends

[145], poly(styrene-co-8 wt%-acrylic acid) (PSAA)/

PMMA [146], poly(ethylene-co-20 mol%-acrylic

acid) (PEAA)/PVP [147], poly(ethylene acrylate-co-

14 wt%-acrylic acid)/poly(bisphenol A dicyanate)

[148], PSAA (with acrylic acid content O20%)/

polyamides (nylon 6, 11, and 12) [149], PEAA (with

acrylic acid content of 20 wt%)/PEOx [150]. Partially

miscible blends have been reported for crystallizable

PSAA (acrylic acid content: 28 mol%)/polyurethane

[151], and immiscible blends are seen for PMMAA/

PVC [145], PEAA (acrylic acid content: 14 mol%)/

poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [152].

The addition of diblock copolymer of poly(styrene-b-

2-ethyl-2-oxazoline) (P(S-b-EOx), 2-ethyl-2- oxazo-

line content: 51 wt%) can enhance the compatibiliza-

tion between PPO and PEAA, as the polystyrene (PS)

and PEOx blocks are miscible with PPO and PAA,

respectively [152].

The miscibility of blends of PAA copolymer is a

function of the content of acrylic acid in the

copolymer. For example, PSAA are miscible with

polyamides (nylon 6, 11, and 12) if the acrylic acid

content in PSAA is higher than 20%; at a content of

14%, partial miscibility is observed with each


polyamide, and with a content of 8%, phase separation

is observed with the crystalline polyamides [149].

The effect of the copolymer sequence distribution

on the miscibility of PEO/PSAA blend has been

investigated by molecular simulations [153]. It is

observed that the sequence distribution of the PSAA

significantly affects the degree of miscibility [153].

5. Incorporation of inter-associated hydrogen

bonds: a strategy for compatibilization

of polymer blends

The formation of hydrogen bonds usually induce

the miscibility of the blends as DGH/RT!0, and is

always a significant contribution to the free energy of

mixing. In application, the incorporation of hydrogen

bonds into immiscible polymer blends has become a

routine strategy to achieve compatibilization of

otherwise immiscible blends. There are four methods

to incorporate the inter-associated hydrogen bonds:

(i) incorporation of terminated functional group as

proton acceptor or donor; (ii) inclusion of hydrogen-

bonding monomer into the main chains of the

components by copolymerization; (iii) addition of a

third polymer containing hydrogen-bonding moieties

into the immiscible blends; and (iv) introduction of an

inert diluent to the main chain to reduce any strong

self-association, and thereby enhance inter-associ-

ation hydrogen bond formation. These are addressed

in turn in the following subsections.

5.1. Incorporation of terminated functional group

as proton acceptor or donor

Massa et al. evaluated the phase behavior of blends

of acetoxyphenyl- and hydroxyphenyl-terminated

hyperbranched all-aromatic polyesters with linear

polymers, such as polycarbonate, polyesters, and

polyamides [154]. The acetoxy-terminated hyper-

branched polyester showed miscibility with few linear

polymers. However, the miscibility of hydroxy-

terminated hyperbranched polyester blend was iden-

tical to that of PVPh blends. This suggests that the

incorporation of inter-associated hydrogen bonds by

replacing the terminated acetoxy group with the

hydroxy group induces miscibility in the blend [154].

5.2. Inclusion of hydrogen-bonding monomer

into the main chain

PVPy is incompatible with PS [155]. Miscibility

between PVPy and styrene-rich copolymers is

achieved by introducing the VPh unit into the PS

main chain, which is able to complex with PVPy

through hydrogen bonding [155]. PVPy is miscible

with STVPh copolymer containing 50 mol% VPh

over the entire range of blend composition. However,

if the styrene copolymer contains only 20–30 mol%

VPh, compatibility is only achieved for the blends

which are rich in PVPy. Similarly, miscibility

between PS and PVPh, which are incompatible over

the entire range of composition, is only achieved if PS

is modified by incorporating large amounts of

4-vinylpyridine [155], e.g. miscibility is achieved

for PVPh and styrene-4-vinylpyridine copolymer

containing 50 mol% 4-vinylpyridine, but a similar

copolymer containing only 20 mol% 4-vinylpyridine

does not form a miscible blend with PVPh. These

results confirm that miscibility can only be achieved

through cumulative interactions of added functional-

ities for such dissimilar polymers.

Similarly, poly(2,2,3,3,3-pentafluoropropyl metha-

crylate) is immiscible with PVPh, while their blends

change from heterogeneous to homogeneous on

incorporation of a sufficient amount of 4-vinylpyr-

idine into the poly(2,2,3,3,3-pentafluoropropyl metha-

crylate) [156]. As demonstrated by FTIR and XPS,

inter-associated hydrogen bonds are the driving force

leading to miscibility of the blends.

Compatibility can also be achieved between

PVPh and PS by introducing 4-vinylbenzenepho-

sphonic acid di-ethyl ester unit (VBDEP) into the PS

chains, to permit hydrogen bond formation with VPh

[157]. Styrene/VBDEP copolymer (SVBDEP) con-

taining 4.3 mol% VBDEP units was immiscible with

PVPh, but with increasing VBDEP content in the

copolymer to 7.5 mol%, a single Tg was observed

when the content of SVBDEP was higher than

60 wt%. With a VBDEP content of 13.3 mol% in

SVBDEP, homogenous phase behavior was observed

for SVBDEP/PVPh blends over the entire compo-

sition range as a result of the formation of a lot of

hydrogen bonds [157].

Similar to PVPy, PVP was immiscible with PS

[116,158]. However, the blends become miscible with


11 mol% or more VPh in the PS chains, and the

resultant formation of hydrogen bonds between the

hydroxyl in STVPh and the carbonyl in PVP [116,158].

Similarly, whereas PS is not miscible with PEMA or

PMMA, polystyrene modified to contain a certain

amount of vinyl phenol unit to form STVPh can

become miscible with both PEMA and PMMA [159].

Poly(alkyl isocyanate)s are immiscible with

STVPh (VPh content: 9 mol%), while miscible blends

were suggested when the polyisocyanates are side-

chain-functionalized with ether, ester, and ketone

groups [160]: IR spectrometry demonstrates the

presence of hydrogen bonding interaction between

the phenol hydroxyl groups and the polar side-chain

functional groups.

5.3. Addition of third polymer containing

hydrogen-bonding moieties

Although PMMA and PEMA are only slightly

different in structure, they are known to be immiscible

[159]. STVPh with VPh content of 5 mol% was added

to an immiscible PMMA/PEMA blend to improve the

compatibility, as STVPh is miscible with both PMMA

and PEMA. As expected, the ternary blends composed

of PEMA, PMMA, and STVPh show a wide

miscibility window [159]. Similarly, introduction of

STVPh into immiscible blends of PS/PEOx, PS/

PMMA and PS/PBMA caused a notable reduction of

the phase domain size with the formation of hydrogen

bonds (Fig. 12) [41].

Poly(styrene-graft-ethylene oxide) was used to

enhance the compatibility of PS blends with poly

Fig. 12. Optical micrographs of 50/50 PS/PEOx (MWZ5!104/5!104) b

copolymer [41]. Reprinted from Macromolecules 1997;30:7119–26, Copy

(butyl acrylate) (PBA) and poly(butyl acrylate-co-

acrylic acid) (PBAAA) [161]. Although no significant

effect of the graft copolymer on the domain size were

found with PS/PBA blends, the addition of 3 wt%

graft copolymer reduced the domain size of the

PBAAA phase by a factor of 10 in a PS/PBAAA

blend. After functionalizing PBA with acrylic acid,

the average size of the polyacrylate domains was also

reduced considerably by the graft copolymer [161].

These results suggest that the compatibilization effect

of the graft copolymer comes from the formation of

hydrogen bonds between the PEO side chains in the

graft copolymer, and the acrylic acid segments in the

PBAAA phase.

Xu et al. examined the compatibilization effect of

diblock copolymer P(S-b-EOx), on immiscible blends

of PPO and PEAA [152]. The hydrogen bonds

between PEAA and P(S-b-EOx) were detected and

the diblock copolymer found to be an effective

compatibilizer for PPO/EAA blends, with the diblock

copolymer mainly located at the interface between the

PPO and PEAA phases. The interfacial tension

reduced and the blends exhibit a more regular and

finer dispersion on addition of a small amount of

P(S-b-EOx). Furthermore, the tensile strength and

elongation at break increased with the addition of the

diblock copolymer for the PPO-rich blends.

Frechet et al. compared the compatibilization

effect of graft and block copolymers with that of

random copolymer containing hydrogen-bonding

moieties [162]. The presence of block and graft

copolymers based on PS and PVPh at the PS/PVPy

lends: (a) without STVPh; (b) with 5% copolymer; (c) with 10%

right (1997), with permission of American Chemical Society.


interface can significantly enhance the interfacial

fracture toughness. The PVPh segment length was

surprisingly effective at anchoring into the PVPy phase,

even at low degrees of polymerization, suggesting that

hydrogen bonds contribute significantly to the pull-out

resistance. Although the block and graft architectures

afford strengthening of the PS/PVPy interface, their

performance appears to be inferior to that of the simple

random copolymer structures.

As the ethylene segments in EVAL is partially

miscible with polypropylene (PP), whereas the vinyl

alcohol segments can form the hydrogen bonds with

polyoxymethylene (POM), EVAL acts as a surfactant

to reduce the interfacial tension and to increase the

interfacial adhesion between PP and POM [163].

Addition of a small quantity of EVAL into the PP/

POM blend results in a better compatibilized blend, in

terms of finer phase domains and better mechanical

properties [163].

5.4. Introduction of an inert diluent moiety

to the main chain

Several blends are reported to be immiscible due to

the strong self-associated hydrogen bonds of (at least)

one component, though it would seem at first glance

that inter-associated hydrogen bonds would easily

form between the components. In this case, it is

possible to render miscible blends by incorporation of

an inert diluent moiety into the main chain, to reduce

the self-associated hydrogen bonds and thereby

enhance inter-associated hydrogen bonds.

PVPh/PCHMA blends show two phase behavior

because strong self-associated hydrogen bonds are

formed between the units of PVPh. However,

miscibility was observed for the blends when styrene

was copolymerized with VPh at high styrene content

[90]. Another example is the blend of PVPh/poly(p-

acetoxystyrene) (PAS) [8]. PVPh is immiscible with

PAS as indicated by the existence of two glass-

transition temperatures over the entire composition

range by DSC. However, incorporation of an inert

diluent moiety such as styrene into the PVPh chain

renders the modified polymer miscible with PAS.

These results were explained by the reduction of the

strong self-associated PVPh and the increase of the

inter-associated species between PVPh and PAS

segments with the incorporation of styrene on

the PVPh chain. Similar results have been observed

for PVPh/4,4 0-bis(6-hydroxy hexyloxy) biphenyl-

TDI polyurethane [7,9]. In such cases, miscibility

can be achieved by slight structural modification of

the amorphous polymer to eliminate the self-

associated hydrogen bonds.

6. Effects of hydrogen bonds on the properties

of polymer blends

As discussed in the following, hydrogen bonding

can have a significant effect on the thermal properties,

crystallization behavior, mechanical properties, and

so on of polymer blends.

6.1. Glass transition temperature (Tg)

As emphasized above, hydrogen-bonded polymer

blends are often miscible, with a single glass

transition temperature as a function of composition

for the blend. Over the years, numerous equations

have been proposed to express the Tg-composition

dependence in miscible polymer blends including

expressions due to Fox [164], Gordon–Taylor [165],

Couchman–Karasz [166–169], Kwei [170] and

Braun–Kovacs [171] (see Table 2 for the equations).

In general, it is observed the preceding expression

do not provide a satisfactory estimate of Tg for

hydrogen-bonded blends. The difference between

experimental and predicted Tg values is sometimes

considered as a measure of the strength of interactions

between the blend components [170,172–177].

Due to the effects of strong interactions between

the components, Tg observed for a hydrogen-bonded

blend is usually higher than that predicted by a linear

additivity rule, e.g. the discrepancy for blends of

PVPh with PAMP is shown in Fig. 13 [119].

The behavior is similar for PVPh blends with

PMMA [17,87,90], poly(2-dimethylamino ethyl

methacrylate) [91], poly(N-methyl-3-piperidine-

methyl methacrylate) [92].

PVPy blends with poly(hydroxyethyl methacry-

late) [178], with polybenzoxazine [179], and with

STVPh [116] show a single glass transition which

is higher value than the glass transitions calculated

on the basis of additive behavior. Such a

positive deviation of Tg is also observed for

Table 2

The composition dependence of the glass transition temperature (Tg) of miscible blends

Equationsa Parameters

Additivity rule TgZw1Tg1Cw2Tg2

Fox [164] 1

Tg

Zw1

Tg1

Cw2

Tg2

Gordon–Taylor [165]Tg Z

w1Tg1 Ckw2Tg2

w1 Ckw2

k: adjustable parameter

Couchman [166–169]ln Tg Z

w1 ln Tg1 Ckw2 ln Tg2

w1 Ckw2

k: adjustable parameter

Kwei [170]Tg Z

w1Tg1 Ckw2Tg2

w1 Ckw2

Cqw1w2

k: adjustable parameter, q:

interaction parameter term

Braun–Kovacs [171]Tg ZTg1 C

f2fg2 Cgf1f2

f1Da1

g: interaction parameter term

a Tgi, wi, fi are the weight fraction, glass transition temperature in pure state, volume fraction of component i, respectively. Da: difference

between the volume expansion coefficients in the glassy and liquid state. fg: the free-volume fraction of one component at its Tg.


poly(styrene-co-4-vinylbenzoic acid)/poly(ethyl

methacrylate-co-4-vinylpyridine) blends [180], poly

(styrene-co-cinnamic acid)/poly(ethyl methacrylate-

co-2-dimethylaminoethyl methacrylate) [181], and

so on. In cases for which the components form a

complex through hydrogen bonds, the mobility of

polymer can be greatly restrained, and Tg of the

system is not only above the weight averaged Tg, but

also higher than Tg of either component, as reported

for the systems of poly(N,N-dimethylacrylamide)/

phenol-formaldehyde resins [182], PAA/PEOx [183,

Fig. 13. Tg-composition curve of PAMP/PVPh blends (C) and

PAMP /PVPh complexes (:) [119]. Reprinted from Macromol-

ecules 1999;32:1967–71, Copyright (1999), with permission of


184], PVP/novolac resin [185] and PVPh/poly(ethy-

leneimine) [186].

A negative deviation from linearity of the blend’s Tg

is sometimes attributed to weak specific interactions

[173]. The Tg of blends of PHB/P(VA-co-VAL9)

(poly(vinyl acetate-co-vinyl alcohol) copolymers with

vinyl alcohol content of 9 mol%) blends shown Fig. 14

demonstrates a negative deviation of the observed Tg

Fig. 14. The glass transition behavior of PHB/P(VA-co-VAL9)

blends: (C) experimental points, (– –) the Fox equation, (—) the

Gordon–Taylor equation. P(VA-co-VAL9) is the abbreviation of

poly(vinyl acetate-co-vinyl alcohol) copolymers with vinyl alcohol

content of 9 mol% [187]. Reprinted from Macromolecules 1998;

31:6898–907, Copyright (1998), with permission of American

Chemical Society.


from the predicted value [187]. The Tg of PEO blends

with epoxy [188], PVP [189–191], lignin [48]

and poly(methyl vinyl ether-co-maleic acid) [192]

and blends of PVAL/PDMA [127] are all reported to

show negative deviation.

In a few cases, a sigmoid Tg-composition behavior

is observed for hydrogen-bonded blends. For example,

Tg of the phenoxy blends with PVPy [13], PMMA

[193] and PVP [194], and blends of PEOx/poly

(2-hydroxyethyl methacrylate) [195], PEOx/poly

(2-hydroxypropyl methacrylate) [195] and PCL/cate-

chin [196] all show both positive and negative

deviation, depending on the blend composition.

6.2. Melting temperature (Tm)

The depression of the melting point of a crystalline

polymer component in a blend provides important

information about its miscibility and its associated

polymer–polymer interaction parameter c12. The

temperature reduction is caused by thermodynamic

depression arising from a reduction in the chemical

potential due to the presence of the polymeric solvent.

When two polymers are miscible in the melt, the

chemical potential of the crystallizable polymer is

decreased due to the addition of the second component.

This leads to a reduction in the equilibrium melting

temperature with increasing amorphous polymer

content, especially in blends containing specific

interactions between the components. An immiscible

blend will typically not show depression of the

equilibrium melting point. Using the equation of

Nishi and Wang, which is based on the Flory–Huggins

theory [197], the melting point depression is given by

the following equation [197,198]:

1=T0mb K1=T0

m ZKRV2f21c12=ðDH0

f V1Þ (6)

where T0m and T0

mb are the equilibrium melting points

of the crystallizable polymers in the bulk and the

blend, respectively. Subscripts 1 and 2 denote the

amorphous and crystalline polymers, respectively; f

is the volume fraction; V is the molar volume of the

repeat units; and H0f is the heat of fusion per mole of

crystalline units.

Coleman and Painter modified the Nishi–Wang

equation to account for hydrogen bonding by simply

including a contribution DGH to the partial molar free

energy [199]:

1=T0mb K1=T0

m ZKRV2ðf21c12 CDGHÞ=ðDH0

f V1Þ (7)

In general, the melting point depression in hydrogen-

bonded blends is somewhat larger than that in blends

devoid of hydrogen bonds, due to significant effect of

DGH. Obvious melting point depression is observed

for almost all hydrogen-bonded blends if the blend

contains a semicrystalline component [200–202]. For

example, a melting point depression of 15 8C has been

observed for the PHB component in a PHB/PVPh

blend with PVPh a content of 30 wt% [203].

However, the melting point depression of PHB was

found to be only 1–2 8C in a miscible blend with

30 wt% PVAc.

6.3. Crystallization behavior

The formation of inter-associated hydrogen bonds

between the components usually induces miscibility

in a blend. At the same time, the crystallization

of the crystalline component is greatly suppressed

[204–210]. In some blends, the hydrogen bonds are so

strong that the crystallization of the crystalline

component is totally prohibited when its content is

low. For example, PHV was completely prevented

from crystallization in its blends with PVPh for PHV

content lower than 50 wt% [100]. In PHB/PVPh

blends, PHB loses its crystallizability in the presence

of 40 wt% PVPh [68,203]. Also, PCL, a semicrystal-

line polymer in the pure state, exists as a fully

amorphous elastomer in PCL/dihydric phenol blends

with dihydric phenol content higher than 40 wt%

(Fig. 15) [211,212].

Hydrogen bonds can also affect the crystalline

polymorph form of crystalline components. For

example, in poly(vinylidene fluoride) (PVDF)/PVP

blends with low PVP content, the specific interaction

between segmental PVDF and PVP transforms the

crystalline state of PVDF from a to g phase [213].

6.4. Surface enrichment

Many important properties of polymer materials are

determined by the composition and structure of its

surface. In a multicomponent polymer system, the

surface composition can differ greatly from that in

Fig. 15. Relationship between the crystallinity of the PCL

component and the TDP content in the blends [212]. Reprinted

from J Polym Sci Part B: Polym Phys 2000;38:1848–59, Copyright

(2000), with permission of John Wiley and Sons, Inc.


the bulk, as the components of lower surface

energy tend to enrich in the surface to minimize the

free energy of the system. A lowered equilibrium

surface energy, resulting from the placement of the

lower-surface-energy component at the surface, is

achieved at the cost of maintaining a gradient between

the surface and bulk compositions. Surface segregation

in polymer blends thus reflects a balance of surface

forces and bulk mixing thermodynamics. The surface

composition of polymer blends has been found to

depend strongly on the surface energy of the

components, as well as the magnitude of the interaction

between the two polymer components that determines

their miscibility [214–216].

In hydrogen-bonded blends, specific interactions

play an important role in the surface composition and

structure. Cowie et al. compared the surface enrich-

ment in PVME/PS blends, for which no hydrogen

bonds were involved, with that for hydrogen-bonded

blends of the PVME/PS copolymer [217,218]. They

found that the surface enrichment with PVME in the

hydrogen-bonded blends is markedly less than that

observed in PVME/PS blends, and that the extent of

this effect depends on the strength of the hydrogen

bond involved. Pearce et al. have investigated the

surface enrichment in polymer blends involving

hydrogen bonding, and demonstrated that hydrogen

bond interactions reduce surface enrichment [219].

Jiang et al. characterized the surface structure of

poly(styrene-co-p-hexafluoro-hydroxyisopropyl-a-

methylstyrene)/PVPy blends spanning the immisci-

bility–miscibility–complexation transition [220]. By

increasing the hydroxyl content in the copolymer and

thus increasing the density of hydrogen bonds in the

blends, the system changed from immiscible to

miscible blend and to complex. An excess of the

copolymer was found at the surface of the immiscible

blends. In miscible blends, an increase in the PVPy

concentration was observed at the surface and the

surface excess of the copolymer decreased dramati-

cally. In complexes, the surface and bulk compo-

sitions were very similar.

7. Conclusions

On the whole, this paper summarizes the works on

hydrogen-bonded polymer blends investigated in the

last twenty years. From these work, it is clear that the

main factors that influence hydrogen bond in polymer

blends are the acidity of the proton donor, the basicity

of the proton acceptor, bulky side groups, spacer

length, stereo structure, the rigidity of polymer chain,

and so on. The strength of hydrogen bonds increases

with the acidity of the proton donor and the basicity of

the proton acceptor. Bulky side group hinder hydrogen

bond formation, while a long spacer tends to favor their

formation. In addition, good flexibility of the polymer

chain benefits the formation of hydrogen bonds.

Theoretical analysis reveals that the formation of

strong inter-associated hydrogen bonds between the

components enhances the miscibility of the polymer

blends, and experimental work has also accumulated to

support this conclusion. Based on these studies, the

incorporation of hydrogen bond has become a routine

strategy for the compatibilization of polymer blends.

There are four methods to incorporate the inter-

associated hydrogen bonds into the immiscible blends:

(i) incorporation of terminal function group as proton

acceptor or donor; (ii) introduction of hydrogen-

bonding monomer into the main chains of the blend

components by copolymerization; (iii) inclusion of

copolymers containing hydrogen-bonding moieties

into the immiscible blends; and (iv) introduction of

an inert diluent in the main chain. These methods are

effective to enhance the compatibility and to improve

the properties of the immiscible blends.


The presence of hydrogen bonds significantly

affects the properties of polymer blends. In general, a

positive or negative deviation from linearity of the

blend’s Tg is observed. For hydrogen-bonded blends

with a crystalline component, crystallization is usually

suppressed and the melting point is lowered due to

the specific interaction. Other properties, such as

surface and mechanical properties, are also signifi-

cantly modified by the formation of hydrogen bonds.

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Hydrogen bonds in polymer blends