Miscibility and carbon dioxide transport properties of poly(3-hydroxybutyrate) (iPHB) and its blends with different copolymers of styrene and vinyl phenol

Miscibility and carbon dioxide transport properties of

poly(3-hydroxybutyrate) (iPHB) and its blends with different copolymers

of styrene and vinyl phenol

A. Gonzalez*,1, M. Iriarte, P.J. Iriondo2, J.J. Iruin

Polymer Science and Technology Department and Institute for Polymer Materials (POLYMAT), University of the Basque Country, P.O. Box 1072,

20080 San Sebastian, Spain

Received 18 August 2003; received in revised form 23 March 2004; accepted 30 March 2004

Abstract

This work summarizes the miscibility and transport properties of different polymer blends obtained by mixing a bacterial, isotactic poly(3-

hydroxybutyrate) (iPHB) with copolymers of styrene and vinyl phenol (Sty-co-VPh copolymers). Given that iPHB and pure commodity

poly(styrene) (PS) form immiscible blends, PS has been modified by copolymerizing it with vinyl phenol (VPh) units, in an attempt to

promote blend miscibility. VPh units have appropriate functional groups that interact with iPHB ester moieties. The potential miscibility was

investigated by differential scanning calorimetry (DSC) measuring the glass transition temperatures of blends of different compositions. As

an additional test, the interaction parameter between the two components, using the iPHB melting point depression caused by the second

component, was also measured. Copolymers containing less than 90% styrene showed miscibility with iPHB.

Given the remarkable barrier properties of iPHB to gases and vapours, the study has been completed by measuring transport properties of

carbon dioxide through different iPHB/Sty-co-VPh copolymer blends, using gravimetric sorptions in a Cahn electrobalance. A clear

difference was observed between the behaviour of rubbery blends and those that exhibit a glassy behaviour at the selected experimental

temperature (303 K).

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Poly(3-hydroxybutyrate); Miscible blends; Transport properties

1. Introduction

High molecular weight poly(hydroxyalkanoates) (PHA)

are synthesized and stored in the cell cytoplasm (as water-

insoluble inclusions) by several microorganisms [1]. They

are naturally biodegradable polyesters. Isotactic poly(3-

hydroxybutyrate) (iPHB) is the most important member of

this family. It contains repeating units of R-3HB (3-

hydroxybutyric acid) and serves as an intracellular reserve

for carbon and energy in bacteria. It is a very crystalline

polymer (55–80% crystallinity) with a melting temperature

around 449 K and a glass transition temperature close to

277 K.

In response to an increased awareness of global

environmental problems, iPHB has gained serious attention

as a potential substitute for non-biodegradable polymers.

iPHB has also attracted attention because of its high barrier

properties to gases and vapours. On the other hand,

mechanical properties like the Young’s modulus and the

tensile strength of iPHB are close to those of isotactic

poly(propylene) (iPP), although the extension to break is

clearly lower than that of iPP. This biodegradable PHA has

other poor physical properties like stiffness and brittleness.

An important work has been carried out in order to know the

reasons of these inadequate properties [2–5]. It is now

recognised that its high crystallinity degree is behind of this

behaviour. Moreover, its low thermal stability near the

melting point causes problems during the processing of the

material.

Many attempts have been made in order to improve the

stability and mechanical properties of iPHB. For instance,

with the help of genetic engineering techniques, HB can be

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.03.105

Polymer 45 (2004) 4139–4147

www.elsevier.com/locate/polymer

1 On leave from the Chemical Engineering Faculty. Universidad de

Oriente, Santiago de Cuba (Cuba).2 Present address: Tuboplast Hispania. 01510 Minano, Alava (Spain).

* Corresponding author. Tel.: þ34-943-018192; fax: þ34-943-212236.

E-mail address: [email protected] (A. Gonzalez).

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

copolymerized with various hydroxyalkanoate monomers

and other monomers as a way to regulate and to improve

various of the physical properties above mentioned. These

new iPHB copolymers can be potential candidates for

further commercial exploitation.

As another way to improve and regulate the physical

properties of iPHB, polymer blends with different second

components have been investigated. Given that physical

properties may be tailored changing the composition of a

miscible polymer blend, miscibilities and phase behaviour

of different iPHB-based blends have been evaluated

extensively. For instance, iPHB miscible blends have been

obtained with poly(ethylene oxide) (PEO) [6–9], poly(epi-

chlorohydrin) (PECH) [10], atactic poly(3-hydroxybuty-

rate) (aPHB) [11–14], poly(acrylonitrile-co-vinylidene

chloride) (AN-co-VCD) [15–17], with a copolymer of

epichlorohydrin and ethylene oxide (ECH-co-EO) [18,19],

poly(vinyl alcohol) (PVOH) [20,21], poly(vinyl phenol)

(PVPh) [22,23] and other second components.

On the other hand, mixtures of iPHB with poly(b-

propiolactone) [24,25], poly(ethylene adipate) [24], poly

(butylene adipate) [26], poly(1-caprolactone) [26] and

polystyrene [27] are all immiscible.

The aim of this paper is to obtain iPHB miscible blends

by modifying polystyrene with the inclusion of vinyl phenol

units. The possibility of increasing the miscibility between

the resulting copolymer and iPHB seems reasonable, given

the existence of hydrogen bonds between vinyl phenol and

HB units, as it has been previously reported [23]. It can be

supposed that this type of specific polymer–polymer

interactions will mainly govern the phase behaviour of

these iPHB/Sty-co-VPh blends.

Taking into account these premises, the miscibility of

different copolymers of styrene and vinylphenol with iPHB

has been determined by thermal analysis (glass transitions

and melting point depression). Due to the potential use of

these biodegradable polymers in packaging applications,

specially derived from the adequate barrier properties of

pure iPHB, the transport properties of an atmospheric gas

(CO2) through the above-mentioned blends have been also

measured.

2. Experimental

2.1. Materials

Bacterial iPHB was obtained from Biomer, Germany. Its

melting temperature TM was near 450 K and the glass

transition temperature ðTgÞ was in the vicinity of 275 K.

The average molecular weights were �Mn ¼ 220; 000 �

g=mol and �Mw ¼ 374; 000 g=mol (referred to polystyrene

calibration standards), as determined by SEC/GPC at 303 K

using chloroform as solvent. However, it is necessary to

report that the Biomer iPHB is not completely soluble in

chloroform, a different behaviour to that exhibited by other

iPHBs, like the one commercialized by Aldrich. Such

differences could arise from the different natural origins of

the samples. Because of the partial solubility of the Biomer

sample, it was necessary to filter the samples before

introducing them in the SEC chromatograph. Consequently,

the results here reported must be taken with caution.

Different copolymers of styrene and vinylphenol were

synthesized by solution free radical copolymerization of

appropriate mixtures of styrene and acetoxystyrene. The

preparation of these copolymers has been reported [28] in

the literature, and requires the use of a protective group. It is

necessary to choose carefully the protective group in order

to ensure that the deprotection can be achieved quantitat-

ively and without occurrence of undesirable reactions.

Many authors have demonstrated that the acetyl protection

group can be removed under mild conditions using

hydrazine hydrate. Details of the synthesis and the

characterization of the copolymers have been reported

elsewhere [28,29].

Copolymers with different molar compositions of VPh

were characterized by 13C NMR (Varian 200 spectrometer).

The results are shown in Table 1 together with the

corresponding glass transition temperatures measured by

DSC and molecular weight averages determined by SEC/

GPC. All the copolymers are in the glassy state at room

temperature and their real compositions are similar to those

of the original (nominal) feed in the copolymerization. This

is due to the value of the product of the copolymerization

reactivity ratios of the two comonomers, close to the unity.

2.2. Blend preparation

iPHB and copolymers with different compositions in

styrene monomer (50, 60, 70, 80 and 90%) were premixed

in the desired concentrations (50, 60, 70, 80, 90 and

100% w/w iPHB), by a solution/precipitation method, using

epichlorohydrine at reflux as solvent and cold hexane as

precipitant. They were repeatedly washed with n-hexane in

order to eliminate epichlorohydrin. After being kept under a

UV lamp for 24 h, the samples were stored for 4 days under

vacuum and 373 K in order to remove the precipitant. The

process was followed by a compression moulding process in

a Graseby Specac hot press at 458 K for 2 min under a

pressure of 2 T/m2. After this process, the mould was

transferred to the cooling system attached to the press in

order to allow iPHB crystallization. The blend films were

stored for other 4 days under vacuum and 373 K for removal

of the residual traces of precipitant. Finally, they were

stored under vacuum and room temperature until thermal or

sorption experiments were performed.

3. Methods

Thermal analysis was performed in a Perkin–Elmer

DSC-2 apparatus, with a TADS Data Station. Melting point

A. Gonzalez et al. / Polymer 45 (2004) 4139–41474140

temperatures and crystallinities of the films were deter-

mined during a first scan from 250 to 473 K at 20 K/min.

After 1 min at this temperature, the samples were quenched

down to 250 K at 320 K/min. A second scan, in identical

conditions to the first one, was performed in order to

determine the glass transition temperature.

Transport properties of carbon dioxide through the

different investigated films were measured by a sorption

gravimetric method based on a Cahn D-200 electrobalance

enclosed in a thermostated chamber at 303 K. After hanging

up a polymer film into the balance sorption chamber, it was

evacuated overnight. Carbon dioxide was then admitted in

the balance at different pressures below atmospheric

pressure, and the weight change of the sample was recorded.

The sorption curves were corrected by subtracting a blank

run, obtained under the same conditions but without sample.

They were also corrected in terms of the buoyancy effect.

Densities of the samples used in sorption experiments

were measured at 296 K in a density gradient column

containing aqueous solutions of sodium bromide. They were

estimated with an accuracy of ^0.0007 g/cm3.

The required thickness of the different films ð‘Þ was

measured by a Duo Check gauge with an accuracy of 1 mm.

4. Results and discussion

4.1. Thermal analysis

A thermal characterization of the different polymer

blends was undertaken as a macroscopic evaluation of the

miscibility between the blend components. Their glass

transition temperatures and melting points were determined

by DSC. The values obtained for iPHB/Sty-co-VPh (90:10)

mixtures at different compositions of iPHB in the blend are

resumed in Table 2. Crystallinities are referred to the iPHB

content of the samples in order to quantify the effect of the

presence of the amorphous component (the copolymer) in

the iPHB crystallinity.

As it can be seen, only one Tg was detected for each

blend composition. Although this is generally associated to

a miscible behaviour of the blends, these Tg values are very

similar to that of pure iPHB. It seems that, even at

concentrations such as 50/50, the second Tg value,

corresponding to the copolymer rich phase, has not been

detected. The overlapping of this transition with the melting

peak of iPHB and the low Dcp value associated to the

copolymer glass transition are presumably in the origin of

the difficulties in detecting it. An additional proof of the

blend immiscibility is that no iPHB melting point

depression was observed. Moreover, iPHB crystallized

practically in the same extent with independence of the

copolymer content. In Fig. 1 thermograms corresponding to

different compositions of blends of iPHB with the 90:10

Sty-co-VPh copolymer are shown. The final conclusion is

that the inclusion of only 10% of VPh is not enough to

provide the desired miscibility with iPHB.

The situation was completely different when the relative

amount of VPh in the copolymer was progressively

increased. In all cases a single glass transition temperature

was observed, intermediate between those of the two

components of each blend (see Tables 3–6). Simul-

taneously, melting point diminishes with increasing the

copolymer content in the blend. The third parameter

evidencing miscibility is the crystallinity degree that

becomes progressively smaller (from 66.8 to 32.5%,

referred to iPHB mass), with the copolymer content. So,

in these blends iPHB does not crystallizes in a similar extent

to that in the pure state (the opposite behaviour to that of the

occurring in the immiscible mixtures with the 90:10

copolymer). It can be finally concluded that copolymers of

styrene and vinyl phenol starting from 20% of vinyl phenol

are miscible with iPHB.

The melting point depression has been usually employed

in characterizing the miscibility of semicrystalline/amor-

phous polymer blends. From this depression it is possible to

determine the polymer–polymer Flory–Huggins inter-

action parameter ðx12Þ and the related interaction energy

density ðBÞ:

The interaction parameter was calculated by applying the

Table 1

Sty-co-VPh copolymer characterization by DSC, NMR and SEC/GPC

Nominal mole % Sty-co-VPh Tg (K) (DSC) Real styrene mol% (NMR) �Mn (SEC/GPC) �Mw (SEC/GPC) IP

50:50 417 50.4 17,300 36,100 2.1

60:40 411 59.0 14,400 46,800 3.2

70:30 399 70.2 39,600 74,400 1.9

80:30 392 80.1 31,000 60,600 1.9

90:10 383 89.4 37,500 70,900 1.9

100:0 373 — — — —

Table 2

Thermal characterization of iPHB and its blends with a Sty-co-VPh 90:10

copolymer

% iPHB in the blend Crystallinity degree

(% referred to iPHB)

Tg

(K)

TM

(K)

100 63.7 277 446

90 63.6 278 446

70 64.8 278 443

50 60.0 278 445

A. Gonzalez et al. / Polymer 45 (2004) 4139–4147 4141

Nishi–Wang treatment [30], according to the following

expression:

1

TM

¼1

T0M

2R

DHu

V2

V1

� �x12f

21

� �ð1Þ

where TM and T0M are the melting temperatures of the

crystallizable polymer in the blend and in the pure state,

respectively. DHu is the enthalpy of fusion of the 100% pure

crystalline material per mole of repeating units (that can be

known by extrapolating data of semicrystalline samples), V1

and V2 are the molar volumes of the repeating units of the

amorphous and the semicrystalline polymers and f1 is the

volume fraction of the uncrystallizable component.

From and adequate plot of the data (1=TM vs. f21) the

interaction parameter between both components of the

blend can be calculated. Fig. 2 shows the well-known

Nishi–Wang plot corresponding to a 70:30 copolymer.

Similar plots were obtained with the rest of the copolymers

and are not presented by simplicity, although can be

obtained from the data of Tables 3–6. From the slope of the

different plots, the x12 parameters were calculated for each

copolymer with iPHB. In this analysis the following values

were employed: V2 ¼ 68:03 cm3=mol [31], T0M ¼ 443 K and

DHu ¼ 12:50 kJ=mol [32].

As it can be seen in Table 7 all values are negative,

including that corresponding to the 90:10 copolymer (an

immiscible system), although the value for this system is

close to zero with a standard deviation of the same order

than the reported value. For the rest of the mixtures we

cannot conclude a clear dependence with the copolymer

composition. Although the inclusion of more VPh units in

the copolymer could be considered as a way to increase the

strength of the interactions in the mixtures, styrene units

between vinylphenol groups can be necessary in order to

favour the mobility and accessibility of the interacting units.

These two effects could compete and be in the origin of the

erratic behaviour of the interaction parameter.

The interaction energy density, B is a different form to

Fig. 1. Thermograms corresponding to different blends of iPHB with a 90:10 Sty-co-VPh copolymer.

Table 3

Thermal characterization of iPHB and its blends with a copolymer Sty-co-

VPh 80:20



Tg

(K)

TM

(K)

100 66.8 277 446

90 67.4 284 438

80 63.5 293 435

70 55.9 305 431

60 32.5 315 429

Table 4


VPh 70:30



Tg

(K)

TM

(K)

100 66.8 277 446

90 69.1 284 439

80 63.2 297 434

70 55.6 307 427

60 54.2 319 423


express the Flory–Huggins interaction parameter,

B ¼RTx12

Vr

ð2Þ

eliminating the use of a reference volume Vr implicit in the

definition of the interaction parameter. This reference

volume is usually defined in terms of the molar volume of

the amorphous component of the mixture, V1. B values are

given in J/cm3, allowing an easier comparison of inter-

actions between different functional groups. The molar

volumes of the different copolymers have been calculated

supposing that the volume of the copolymer unit can be

calculated in an additive way. We have used the following

relationships for the specific volumes of the comonomers:

vPS ¼ 1=ð1:0865 2 0:000619 TÞ

ln vPVPh ¼ 20:2757 þ 1:73 £ 1025T3=2

with T in K [29].

An average value of B ¼ 232 (^8) J/cm3 was finally

obtained as representative of the four miscible iPHB/copo-

lymer pairs. The value is lower than that reported by

Martuscelli et al. [22] for iPHB/PVPh system ðB ¼ 250 �

J=cm3Þ: This could be a normal behaviour if the miscibility is

supposed to be only generated by the number of specific

interactions occurring between the iPHB carbonyl and the

hydroxyls of the VPh unit. However, as previously

mentioned, the situation can be something more compli-

cated. For instance, recent literature [33] contains data of a

very similar system, constituted by blends of poly(1-

caprolactone) (PCL) and copolymers of styrene and

vinylphenol. In this case, there is an optimum vinyl phenol

content in the copolymer to have the most favourable trend

to form miscible blend with PCL. This behaviour has been

interpreted in terms of the Painter and Coleman Association

Model [34].

4.2. Carbon dioxide sorption measurements

The sorption kinetics recorded in the Cahn electroba-

lance for pure iPHB and its blends with the copolymers

(Sty-co-VPh, from 90 to 50% Sty) are very similar to that

shown in Fig. 3. The weight gained by the sample ðMtÞ tends

to an asymptotic value, M1, the final equilibrium weight of

gas dissolved in the polymer film. This value can be used to

calculate the concentration of the gas (measured in standard

conditions) per volume unit of the polymer, C; using the

following equation:

C ¼22414M1rF

MgpmF

ð3Þ

where mF (mg) is the mass of the film exposed to the

penetrant, rF (g/cm3) is the density of the polymer film and

Mgp the molecular weight of the permeant (44 g/mol in the

CO2 case).

In order to proceed with the adequate data treatment

explained below, densities and thicknesses of the samples

are required. The thicknesses of the films were between 35

and 45 mm, except in the case of the pure copolymers,

where films of 200–300 mm were used due to the problems

in preparing the films. The densities varied from 1.22 to

1.05 g/cm3, approximately, when the amount of copolymer

in the blends increased.

Table 5


VPh 60:40



Tg

(K)

TM

(K)

100 66.8 277 446

90 71.1 285 436

80 66.2 297 434

70 62.9 308 433

60 43.5 318 428

Table 6


VPh 50:50



Tg

(K)

TM

(K)

100 66.8 277 446

90 72.7 289 432

80 62.1 299 433

70 57.3 310 427

60 42.7 322 422

Fig. 2. Inverse melting temperature vs. square of the amorphous phase

fraction in blends of iPHB with a 70:30 Sty-co-VPh copolymer.

Table 7

Interaction parameters in iPHB / Sty-co-VPh blends as a function of the

copolymer composition

Sty-co-VPh (mol%) x12

90:10 20.1 ^ 0.1

80:20 20.7 ^ 0.1

70:30 21.3 ^ 0.2

60:40 20.7 ^ 0.2

50:50 21.0 ^ 0.3


Sorption isotherms are obtained by plotting values of C

against the applied pressure p. In Fig. 4 isotherms obtained

for iPHB pure, and for its blends with two different

copolymers are shown. We have selected only three systems

in order to show the different behaviour when the system is

at the glassy or the elastomeric state.

The equilibrium concentration ðCÞ of the gas dissolved in

a polymer is a pressure function. At every pressure, a

solubility coefficient ðSÞ is usually defined as:

S ¼C

pð4Þ

In some cases, S is a constant over an extended range of

pressures. This is the well-known Henry’s law behaviour

that is followed, for example, by the pure iPHB samples,

mainly due to the rubbery character of its amorphous phase.

In these cases the solubility coefficient and the Henry’s law

constant are identical.

The other two isotherms correspond to two different

compositions of blends of iPHB with the 50:50 copolymer.

When the composition is 90:10 (B) the blend is rubbery at

303 K and the isotherm looks practically linear (Henry’s

law behaviour). However, when the composition is 70:30

(A), the blend is in the glassy state and the isotherm deviates

slightly from linearity. This non-linearity implies that the

solubility coefficient is pressure dependent, a common

occurrence in glassy polymers. This dependence is usually

explained in terms of the so-called dual mode sorption

model, where a Langmuir sorption term is added to the

Henry’s sorption one.

In this paper, our main objective was to study changes in

the transport properties of iPHB in the vicinity of the

atmospheric pressure when it is mixed with different Sty-co-

VPh copolymers. In order to compare the transport proper-

ties between different blends we have used a constant

pressure (0.5 bars) and we have calculated the solubility

coefficient as the ratio between C and p (see Eq. (4)).

Because of the differences in the physical state of the

samples (rubbery or glassy) S is not really a constant

coefficient like the one obtained from a Henry’s law

behaviour, but it allows to compare different blends.

Moreover, at these low pressures, the deviation from the

Henry’s law is not very important.

Data of solubility coefficients corresponding to all

investigated miscible systems at this intermediate pressure

are summarized in Table 8. Each column corresponds to

Fig. 3. CO2 sorption curve for a 90:10 blend of iPHB with an 70:30

copolymer of Sty-co-VPh, at 303 K and 0.5 bar.

Fig. 4. Sorption isotherms of different blends at 303 K. A: 70.30 blend (glassy) of iPHB and Sty-co-VPh 50:50 copolymer. B: 90.10 blend (elastomeric) of

iPHB and Sty-co-VPh 50.50 copolymer. C: Pure iPHB.


solubility coefficients of mixtures of iPHB with a copolymer

of a given composition.

The addition of a 10% of the different copolymers to

iPHB provokes a decrease in the solubility when the

copolymer is rich in VPh unities (50 and 60%), solubility

that progressively increases when the amount of styrene

units increases. For this first composition in the cases of

copolymers having 70 and 80% styrene and for the rest of

the compositions, solubility is always higher that that of the

pure iPHB, probably due to the fact that the copolymer

addition decreases the crystallinity of the samples and the

solubility takes place basically at the amorphous phase.

In general, there is an inflexion point at the third row of

the table. It must be taken into account that the first two rows

of the table correspond to rubbery samples whereas the

other three correspond to glassy mixtures. In these glassy

compositions, their sorption behaviour should be better

explained in terms of the dual sorption model.

Diffusion coefficients ðDÞ for these systems can be

obtained from an adequate use of the following equation:

Mt

M1

¼ 1 28

p2

X1n¼0

1

ð2n þ 1Þ2exp

2Dð2n þ 1Þ2p2

‘2t

!ð5Þ

where the amount of the diffusant, Mt, taken up by the

polymer sheet in a time, t; is related [35,36] to M1,

previously defined, ‘, the film thickness and D; the diffusion

coefficient (in cm2/s). This equation can be obtained by

solving the differential equation corresponding to the

second Fick’s law after considering the same concentration

at both faces of the film (instantaneously established) and

without taking into account the effects of diffusion at the

edges of the film. Eq. (5) converges rapidly for long times

and it can be rewritten approximately (taking into account

only the first term of the summatory) by:

ln 1 2Mt

M1

� �¼ ln

8

p2

� �2

Dp2

‘2

!t ð6Þ

The diffusion coefficient D can be calculated from the

slope of an adequate plot of the sorption data. The values

obtained for the different blends with the different

investigated copolymers are shown in Table 9.

Due to the markedly larger values of the diffusion

coefficients of the different Sty-co-VPh copolymers, the

diffusion coefficients of the different blends tend to increase

when the copolymer content increases. In all columns, D

decreases in going from the second row to the third one.

Once again, the change from rubbery samples to glassy

samples could be in the origin of such change. Although we

have more amorphous material in the blend in the third and

fourth rows than in the previous one, the mobility of the

glassy chains decreases and D becomes smaller.

In some cases, the trends followed by a specific row or

column seem to be certainly erratic, probably due to the

small values of the weight gain during the sorption

experiments, which introduce important errors in the

application of the long-time approximation to calculate D.

Apart from changes between rubbery and glassy regimes,

other different factors must be taken into account. The

different evolution of the crystallinity with the composition

of the blends could affect the evolution of D. On the other

hand, increasing VPh content in the copolymer should

increase the interactions between the components of the

blend. These interactions could affect D in different ways.

They could increase the packing between polymer chains

obstructing diffusion of the penetrant. Or they could reduce

the crystallinity of the blend, improving the diffusive

process.

From a technological point of view, the use of the

permeability coefficient ðPÞ is much more used to

characterize the transport properties of a polymer film,

given that it is a measure of the barrier protection offered by

the polymer. In the case of atmospheric gases, this

permeability coefficient can be written as the product of

the diffusion coefficient D and the solubility coefficient S :

P ¼ DS ð7Þ

Since diffusion coefficient may be concentration depen-

dent and Henry’s law may not apply, the permeability of a

polymer is, in general, not a fundamental property.

In obtaining Eq. (7), it is necessary to assume that the

equilibrium at the polymer gas interfaces has been reached

and that the Fick’s first law is obeyed. It is also necessary to

assume both the steady state and a downstream pressure and

a gas concentration substantially lower than the upstream

pressure and gas concentration.

In a similar way to the last mentioned properties (D and

S), there is a clear frontier between rubbery and glassy states

(see Table 10). Taking into account this change, the

Table 8

Solubility coefficients (in cm3 STP/cm3 cm Hg) for iPHB/Sty-co-VPh

miscible mixtures at 303 K and 0.5 bar. S (iPHB) ¼ 0.0114

cm3STP/cm3 cm Hg

% iPHB (w/w) S (50:50) S (60:40) S (70:30) S (80:20)

90 0.0077 0.0081 0.0126 0.0252

80 0.0428 0.0428 0.0429 0.0380

70 0.0152 0.0308 0.0209 0.0350

60 0.0289 0.0221 0.0499 0.0426

0 0.0415 0.0491 0.0287 0.0240

Table 9

Diffusion coefficients (in cm2/s) for different miscible blends of iPHB with

Sty-co-VPh copolymers

% iPHB

(w/w)

D £ 109

(50:50)

D £ 109

(60:40)

D £ 109

(70:30)

D £ 109

(80:20)

90 1.4 1.8 2.1 1.6

80 1.7 1.6 1.6 2.1

70 1.0 1.4 1.1 1.1

60 1.8 2.2 3.6 5.3

0 14.5 11.2 18.7 24.4

T ¼ 303 K and p ¼ 0:5 bar. D (iPHB) ¼ 1.2 £ 1029 cm2/s.


observed trend in P with increasing copolymer amount in

the blend tends to reach the value corresponding to pure

copolymer in each case. Looking at the change with the

styrene amount in each copolymer, the behaviour is not so

clear, probably as a consequence of the erratic behaviour on

D and on S. In all cases the value obtained for different

transport parameters are in the same order to those

corresponding to barrier materials frequently used in the

market.

5. Conclusions

Blends prepared by mixing iPHB and different copoly-

mers of styrene and vinylphenol are miscible starting from

20% of styrene in the copolymer. These blends show a

single glass transition intermediate between those of the two

components of the blend. The inclusion of VPh units only in

10% in the copolymer is not enough to provoke the

miscibility between iPHB and the modified polystyrene.

Using the melting point depressions observed in the studied

miscible systems the interaction parameter has been

calculated. It does not show a clear dependence with the

VPh content in the copolymer, giving and average value of

20.9. The corresponding interaction energy density has

been 232 J/cm3. These values are in good agreement with

data recently reported [22] for a similar system.

The solubility, diffusivity and permeability of carbon

dioxide have been determined for iPHB blends containing

up to 60% STY-co-VPh copolymers and varying the styrene

copolymer composition from 80 to 50%. The addition of the

different copolymers provokes irregular effects both on S

and D, because there are many effects that can overlap each

other. For instance, the incorporation of the copolymer

increases the amorphous content of the system, but also

increases the glass transition temperature of the system, and

we are comparing glassy and rubbery systems. In general,

the permeability of the blend always increases with

increasing the copolymer content. The behaviour with the

styrene content of the copolymer is more erratic and it

seems that several effects have to be taken into account.

Although P increases with the addition of different

copolymers, the behaviour is very similar in comparing

different copolymers. In all cases the P values are adequate

for their use as packaging material.

According to the results, it can be concluded that it is not

necessary to increase VPh content in the copolymer more

than 20 or 30% in order to obtain blends with transport

properties not very far from those of pure iPHB. In fact, the

transport properties do not change too much from one

copolymer to other one. The melting point depression is also

similar in using the different copolymers, the most

important factor in changing it being the blend composition

and not the copolymer composition.

Acknowledgements

Financial support of the University of the Basque

Country (project 203.215-13519/2001) and MCYT (project

MAT98/0530) is gratefully acknowledged. A.G. thanks the

AECI for a PhD grant.

References

[1] Sudesh K, Abe H, Doi Y. Prog Polym Sci 2000;25:1503.

[2] Barham PJ, Keller AJ. Polym Sci B: Polym Phys Ed 1986;24:69.

[3] Martinez-Salazar J, Sanchez-Cuesta M, Barham PJ, Keller A. J Mater

Sci Lett 1989;8:490.

[4] Barham PJ, Barker P, Organ SJ. FEMS Microbiol Rev 1992;103:289.

[5] De Koning GJM, Lemstra PJ. Polymer 1993;34:4089.

[6] Avella M, Martuscelli E. Polymer 1988;29:1731.

[7] Avella M, Greco P, Martuscelli E. Polymer 1991;32:1647.

[8] Avella M, Martuscelli E, Raimo M. Polymer 1993;34:3234.

[9] Kumagai Y, Doi Y. Polym Degrad Stab 1992;35:87.

[10] Miguel O. PhD Thesis. San Sebastian, Spain: University of the Basque

Country; 1999.

[11] Kumagai Y. Makromol Chem Rapid Commun 1992;13:179.

[12] Pearce R, Jescucason J, Orts W, Marchessault RH, Bloembergen S.

Polymer 1992;33:4647.

[13] Pearce R, Marchessault RH. Polymer 1994;35:3990.

[14] Abe H, Doi Y, Satkowski NN, Noda I. Macromolecules 1994;27:50.

[15] Finelli L, Sarti B, Scandola M. J Mater Sci Pure Appl Chem 1997;

A34:13.

[16] Jeong Chang L, Nakajima K, Ikehara T, Nishi T. J Polym Sci, Part B:

Polym Phys 1997;35:2645.

[17] Gonzalez A, Iriarte M, Iriondo PJ, Iruin JJ. Polymer 2002;43:6205.

[18] Zhang LL, Goh SH, Lee SY, Hee GR. Polymer 2000;41:1429.

[19] Gonzalez A, Iriarte M, Iriondo PJ, Iruin JJ. Polymer 2003;44:7701.

[20] Azuma Y, Yoshie N, Sakurai M, Inoue Y. Polymer 1992;33:4763.

[21] Yoshie N, Azuma Y, Sakurai M, Inoue Y. J Appl Polym Sci 1995;56:

17.

[22] Xing P, Dong L, An Y, Feng Z, Avella M, Martuscelli E.

Macromolecules 1997;30:2726.

[23] Iriondo PJ, Iruin JJ, Fernandez-Berridi MJ. Polymer 1995;36:3235.


[25] Cao A, Asakawa N, Yoshie N, Inoue Y. Polym J 1998;30:743.


[27] Felisberti MI. de Lucca Freitas LL. Polymer Preprints 1990;31:1441.

[28] Arshady R, Kenner GW. J Polym Sci, Part A: Polym Chem 1974;12:

2017.

[29] Gonzalez A. PhD Thesis. San Sebastian, Spain: University of the

Basque Country; 2002.

[30] Nishi T, Wang TT. Macromolecules 1975;8:909.

Table 10

Permeability coefficients for different miscible blends of iPHB with Sty-co-

VPh copolymers, at 303 K and 0.5 bar. P (iPHB) ¼ 0.13 Barrer

%iPHB w/w P

(Barrer)

(cop 50:50)

P

(Barrer)

(cop 60:40)

P

(Barrer)

(cop 70:30)

P

(Barrer)

(cop 80:20)

90 0.11 0.14 0.27 0.40

80 0.72 0.70 0.70 0.81

70 0.16 0.44 0.23 0.39

60 0.49 1.49 1.80 2.27

0 6.62 5.50 5.37 5.86


[31] Abraham PJ, Seller A, Otum EL, Holmes PA. J Mater Sci 1984;19:

2781.

[32] Sanchez-Cuesta M, Martinez SJ, Barker PA, Abraham PJ. J Mater Sci

1992;23:5335.

[33] Kuo SW, Chang FCh. Polymer 2001;42:9843.

[34] Coleman MM, Painter PC, Graf JF. Specific interactions and the

miscibility of polymer blends. Lancaster, PA: Technomic Publishing

Inc; 1990.

[35] Crank J, Park GS. Diffusion in polymers. London: Academic Press;

1968.

[36] Crank J. The Mathematics of diffusion, 2nd ed. Oxford: Clarendon

Press; 1975.


Documents

Miscibility and carbon dioxide transport properties of poly(3-hydroxybutyrate) (iPHB) and its blends with different copolymers of styrene and vinyl phenol