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Colloids and Surfaces B: Biointerfaces 103 (2013) 544– 549

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces B: Biointerfaces

jou rna l h om epa g e: www.elsev ier .com/ locate /co lsur fb

ynergistic behavior of poly(aspartic acid) and Pluronic F127 in aqueous solutions studied by viscometry and dynamic light scattering

oredana E. Nitaa,b,∗, Aurica Chiriaca, Maria Berceaa,b, Bernhard A. Wolfb

“Petru Poni” Institute of Macromolecular Chemistry, 41-A Grigore Ghica Voda Alley, 700487 Iasi, RomaniaInstitut fur Physikalische Chemie der Johannes Gutenberg-Universitat Mainz and Materialwissenschaftliches Forschungszentrum der Universitat Mainz, Welder-Weg 13, D-55099ainz, Germany

r t i c l e i n f o

rticle history:eceived 12 September 2012eceived in revised form 26 October 2012ccepted 31 October 2012vailable online 9 November 2012

eywords:

a b s t r a c t

Pluronic F127/poly(aspartic acid) mixtures were investigated in dilute solutions by viscometry anddynamic light scattering. The two polymers were chosen due to well known applications in biomedicalfield, taking into account the final purpose (the use of the complex structure as drug delivery systems).The central item was to identify the possibility of complexation between the poly(carboxylic acid) and anon-ionic polymer and to investigate the conditions of the interpolymer complex formation. The abilityof Pluronic F127 to form micelle is well known. Poly(aspartic acid), as a polycarboxylic acid with resem-

nterpolymeric complexelf assemblingluronic F127oly(aspartic acid)

blance with polyacrylic acid, can act as dispersant, antiscalant, superabsorber, being also able to formmicelles. Due to its functional groups, COOH and NH2, poly(aspartic acid) can make physical and/orchemical bonds with other macromolecular compounds.

The intrinsic viscosity and the dynamic light scattering data obtained for PLU/PAS mixtures at 25 ◦Chave shown that interpolymer complex formation occurs around 1/1 wt. ratio between the two polymers.

. Introduction

Compared to the ever increasing variety of synthetic polymertructures reported, or even in relation to the number of polymersommercially available, the group of water-soluble copolymers iselatively small. If we include the common water-soluble biopoly-ers and their synthetic derivatives, this combined group, although

till not numerous in comparison to all synthetic polymers avail-ble, has a large number of industrial, environmental, household,nd medical applications, and it is of significant commercial impor-ance [1].

The solubility of a polymer in water is determined by the bal-nce between the interactions of the hydrophilic and hydrophobicolymer segments with themselves and with the solvent. Simi-

arly, the behavior of surfactants in aqueous solution is governed by

he subtle balance of hydrophilic, hydrophobic, and ionic interac-ions. As a result, aqueous solutions containing polymer/surfactant

ixtures display an apparently infinitely varied and indeed

∗ Corresponding author at: “Petru Poni” Institute of Macromolecular Chemistry,1-A Grigore Ghica Voda Alley, 700487 Iasi, Romania. Tel.: +40 232 217454;ax: +40 232 211299.

E-mail addresses: lnazarie@yahoo.co.uk (L.E. Nita), achiriac1@yahoo.comA. Chiriac), bercea maria@ymail.com (M. Bercea), bernhard.wolf@uni-mainz.deB.A. Wolf).

927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfb.2012.10.054

© 2012 Elsevier B.V. All rights reserved.

sometimes bewildering pattern of properties, due to the manyvariations in molecular structures available to the investigatoror formulator [1]. One important property of surfactants is theformation of colloidal-sized clusters from solutions, known asmicelles, which have particular significance in pharmacy becauseof their ability to increase the solubility of sparingly soluble drugin water [2]. Polymer micelles of amphiphilic copolymers havebeen also extensively used for drug carrier systems to enhancedrug solubility, stability, and biopharmaceutical properties, thatis, the permeability across membranes and permanence in bloodcirculation. Core segregation from aqueous milieu, which is thedirect driving force for micellization, is made by association ofthe hydrophobic moiety of the amphiphilic copolymer because oftheir cohesive association, including hydrophobic interaction, elec-trostatic interaction and hydrogen bonding. The hydrophilic shellsurrounding the inner core plays a role in stabilizing the poly-meric micelle and avoids uptake by the reticuloendothelial system[3]. The formation of polymeric micelles by the self-association ofdiblock copolymers consisting of hydrophilic and hydrophobic seg-ments in aqueous medium is currently a topic of great interestthe attractive applications in drug delivery carrier technology. Inthe biomedical materials field, polymeric micelles must possess

several specific properties to be of use. These include biocom-patibility, biodegradability, target specificity, and stability in thebody. Polypeptides consisting of R-amino acids are syntheticallyintriguing as polymeric micelles due to their biodegradability and

es B: B

vcbcaacapcwAt

isPiocmmdabrhaPft(

tcitt

baohftakie

2

2

sppa8sfasc

L.E. Nita et al. / Colloids and Surfac

ariation in functionality that results from the differences in sidehain groups [4]. Between them, the poly (aspartic acid) (PAS),elonging to the family of synthetic polypeptides, is a typical bio-ompatible, biodegradable water-soluble polymer with dispersingctivity. Derivatives of poly(aspartic acid) are also biodegradablend biocompatible, which are the main prerequisites for pharma-eutical applications. Their multifunctional character can afford

variety of tailor made modifications following simple chemicalrocedures. These properties make PAS and its derivatives a goodandidate for obtaining micelle drug delivery systems. In deionizedater, the carboxylic acid groups of PAS are partially ionized [4–7].lso, due to the ionizable groups, the PAS chains show polyelec-

rolyte properties.The present study is focused on investigation of the interactions

n a biodegradable polypeptide polyelectrolyte (PAS) – polymerurfactant system. As macromolecular surfactant, it was chosenluronic F127 (PLU), which is a triblock copolymer with the chem-cal formula PEO99–b–PPO69–b–PEO99, where PEO is polyethylenexide and PPO is polypropylene oxide. PLU, at high enough con-entration, forms close-packed medium consisting of sphericalicelles (18 nm in diameter) in aqueous solution. The nanoscaleicelles completely fill space with a locally ordered lattice. The

ifference in the water affinity of PEO and PPO depends on temper-ture. The difference is not significant at lower temperatures, andoth subunits are generally hydrophilic and soluble in water. Atoom temperature, however, PPO subunits become preferentiallyydrophobic and more insoluble than PEO subunits. As the temper-ture increases, the PPO block’s hydrophobicity increases, so thatPO subunits attract each other but repel water, being screenedrom water by the PEO subunits. PPO blocks overlap, surrounded byhe hydrophilic ends in order to form micelles at room temperaturehigher than 17 ◦C) [8].

PAS and PLU were chosen, taking into account the final purpose,he use of the complex structure in the biomedical field (espe-ially as drug delivery systems) due to well known applicationsn biomedical field. By combining the pH sensitivity of PAS withhe thermal sensitivity of PLU we follow obtaining a new dual pH-hermosensitive micellar system.

The current study is intended to investigate the interactionsetween the PAS and PLU in dilute solution, the most favor-ble ratio between these two polymer copartners as well as theptimum conditions for the interpolymer complex formation. Itas to be realized that the dilute solutions are important step

or understanding of the concentrated systems, since a concen-rated polyelectrolyte/surfactant aqueous mixture can be regardeds systems where polymer/macromolecular surfactant complexesnown to exist in dilute solution, interact with each other. Becauset is a part from a more widespread study, we discuss here only theffects of polymer composition at a constant temperature of 25 ◦C.

. Experimental

.1. Materials

Poly(aspartic acid) was synthesized through a reaction in twoteps. Firstly, poly(succinimide) (PSI) the PAS precursor it was pre-ared by thermal polycondensation in dodecane (Fluka Chemikarovenience) of l-aspartic acid (Fluka BioChemika provenience),t 180 ◦C, for 6 h, with o-phosphoric acid (analytical reagent of5%, Chemical Co. provenience) as catalyst. Second step was con-tituted by in situ hydrolysis in alkaline medium of PSI at −5 ◦C

or 1 h. After the hydrolytic reaction, the pH of the solution wasdjusted to be neutral by adding HCl solution. Then, methanol,aturated with NaCl, was poured into the breaker, and the pre-ipitate was recovered by filtration and then dried at 40 ◦C under

iointerfaces 103 (2013) 544– 549 545

vacuum conditions. The prepared polymer has the molecularweight (from GPC) of about 15 kg/mol and the polydispersity indexof 1.32. Pluronic F127 (known as poloxamer 407 having the molec-ular weight of 12.6 kg/mol) is a triblock linear copolymer consistingof polyoxyethylene (POE) units (70%) and polyoxypropylene (PPO)blocks (30%) (PEO99–b–PPO69–b–PEO99) (purchased from Fluka).This copolymer was used as received.

2.2. Mixture preparation

Stock aqueous solutions of PLU and PAS of the same concentra-tion were prepared and kept at rest for 24 h. The PLU/PAS mixturesin aqueous solutions were prepared by direct mixing of solutionsof both homopolymers for 60 min. In order to have different ratiosbetween the polymeric compounds, solutions of different weightfractions of PAS (denoted as w*) in the binary PLU/PAS mixtureswere prepared. Deionized water was used for the solutions prepa-ration as well as for the characterization steps.

2.3. Viscosity measurements

Viscometric measurements were performed at 25 ± 0.1 ◦C usingUbbelohde capillary viscometers for dilution sequences of type 0awith a capillary diameter of 0.53 mm, in combination with AVS 310(Schott, Mainz, Germany). The viscometric measurements werecarried out in deionized water. All samples were free of dust bymeans of filters with a pore diameter of 0.45 �m (Spartan 30/B,Schleicher&Schuell, Dassel, Germany). Only freshly prepared sam-ples were used. Because of the fact that the intrinsic viscosityobtained by means of the new approach stems from the initial slopeof ln�rel versus c (i.e., from data for c « [�]−1), possible influences ofshear rate at high concentrations remain inconsequential. Solutionswith different polymer concentrations were prepared by sequen-tial dilution of initial stock solutions, directly inside the viscometer.Also, the solutions were kept about 15 min prior the measurementsfor the temperature equilibrium.

Steady shear measurements were carried out at 25 ◦C with aBohlin CVO rheometer equipped with a Peltier device for tempera-ture control. The measurements were performed by using parallelplate geometry, the upper plate having a radius of 30 mm (gap of500 �m). 2 mL solution was poured on the lower plate for eachdetermination.

2.4. Particle size analysis

These measurements were performed by a dynamic light scat-tering technique (Zetasizer model Nano ZS, Malvern Instruments,UK) with a red laser wavelength of 633 nm (He/Ne). The systemuses a non-invasive back scatter (NIBS) technology (which reducesthe multiple scattering effects), wherein the optics is not in contactwith the sample, back scattered light being detected. On the wholemeasuring range from 0.6 nm to 6 �m the system applies the Miemethod. Dynamic light scattering (DLS) measurements yield the Z-average of the aggregate’s apparent hydrodynamic diameter (DH),according to the following equation:

DH = kT

3��D(1)

where DH is the hydrodynamic diameter, k is the Boltzmann con-stant, T is the temperature, � is the viscosity and D is the diffusioncoefficient. The hydrodynamic diameter, often expressed by sym-

bol Z or z-average, is calculated from signal intensity. In our study,the dynamic light scattering was carried out to investigate theformation of PAS/PLU interactions in 1 g/dL solutions of differentweight fractions of PAS in the polymer mixture (w*).

5 es B: Biointerfaces 103 (2013) 544– 549

2

Zmif

�

wrr

Atms

3

wtdwo

3c

coea

Ioiti

pefite

l

l

�ipafi

e[d

0.0 0.5 1. 0 1.50.0

0.1

0.2

0.3

0.4

PLU

lnre

l

c / g dL-1

PAS 0.3 3

0.2 5

w* = 0. 50

Fig. 1. Relative viscosities of dilute aqueous solutions of PAS and of PLU as a function

able as shown in Fig. 2. The errors for B (Fig. 3) are larger only forw* = 0.25, but taking into account these large errors the trend ofB = f(w*) dependence is not affected.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

dL

g-1

dL

g-1

[η]/

[η]•

46 L.E. Nita et al. / Colloids and Surfac

.5. Zeta potential

Zeta Potential values, �, were also determined on the Malvernetasizer ZS (Malvern Instruments, UK) based on electrophoreticobility (�), the equation of Henry and Smoluchowski approx-

mation for f(Ka) = 1.5, of the unloaded clay particles, under theollowing equation:

= ��

ε(2)

ith � is the viscosity, electrophoretic mobility noted � and ε whichepresents the dielectric constant of the medium and f(Ka) whichepresent Henry’s function.

The pH was adjusted at 7.4 by means of HCl or NaOH, with theutotitrator Malvern MPT2 device. The temperature was main-

ained constant at 25 ◦C (±0.1 ◦C) with a Peltier device. Eacheasurement was made 3 times and the average values were con-

idered.

. Results and discussion

This chapter is subdivided into three sections. In the first part,e describe and discuss the viscometric behavior of aqueous solu-

ions of different PLU/PAS compositions. In the second part, theynamic light scattering data are presented and in the third parte corroborated the experimental results in order to explain the

bserved behaviors.

.1. The dependence of viscometric parameters on polymeromposition

The traditional methods for the determination of intrinsic vis-osities, like the Huggins extrapolation formulated in Eq. (3), cannly be successfully applied to uncharged polymers and to poly-lectrolytes under the condition that the solvent contains sufficientmounts of salt.

� − �s

�sc= [�] + kH [�] c + . . . (3)

n the above relation � is the viscosity of the polymer solutionf composition c (mass of polymer/volume of solution), [�] is thentrinsic viscosity and �s the viscosity of the pure solvent; kH ishe so called Huggins coefficient quantifying the hydrodynamicnteraction between the solute.

Starting from the realistic assumption that viscosities of diluteolyelectrolyte solutions constitute a function of state, a math-matical expression describing the measured viscosities as aunction of composition can be used in order to obtain character-stic, system specific parameters [9,10]. Two relations which fulfillhis condition within experimental error can be applied to poly-lectrolytes as well as to uncharged macromolecules:

n �rel = c [�] + Bc2 [�] [�]•

1 + Bc [�](4)

and

n �rel = A(1 − e−([�]−[�]±)c/A) + [�]±C (5)

rel = �/�s is the relative viscosity, B and A are hydrodynamicnteraction parameters, [�]• and [�]± represent two adjustablearameters which are used to model the observation that ln �rels a function of c becomes linear within experimental error at suf-ciently high concentrations.

The description of polyelectrolyte solutions requires one param-ter more than equations of the Huggins type. In addition to the�] of prime interest, these relations contain the respective hydro-ynamic interaction parameters B and A (corresponding to kH) plus

of polymer concentration plus some data for joint solution of PAS and PLU, wherethe weight ratios of the polymers are indicated in the graph. The lines are calculatedaccording to Eq. (4).

the parameters [�]• and [�]± which are only required for the solu-tions of charged macromolecules and have the same dimension as[�]; for uncharged polymers they become zero. Eqs. (4) and (5) areequivalent, Eq. (4) is usually preferred because of its simplicity.

The primary data obtained from capillary viscometry for dilutesolutions of binary PAS/water and PLU/water as well as of ternaryPAS/PLU/water systems are presented in Fig. 1.

The initial slopes of the dependences of ln �rel as a function ofpolymer concentration (according to Eq. (4)) yield the intrinsic vis-cosities and the parameters B and [�]• determine the curvature ofthis function. One observes that the experimental data obtainedfor PLU and for PLU/PAS mixtures up to w* = 0.5 gives well distin-guished curves, whereas the further addition of PAS has a smallerinfluence on the viscosity of the system. The experimental dataobtained for w* > 0.5 are closed to those obtained for PAS in aqueoussolutions. For a better clarity, these curves are not given in Fig. 1.

The parameters obtained from the evaluation of the depen-dences shown in Fig. 1 are presented in Figs. 2 and 3. The accuracyof the intrinsic viscosities and [�]• values is in all cases reason-

PLU w* PAS

Fig. 2. Intrinsic viscosities of the neat polymers and of PLU/PAS mixtures in water asa function of composition of polymer mixture determined according to Eq. (4). Thesolid line is adjusted to a Boltzmann relation excluding the data point for w* = 0.5.

L.E. Nita et al. / Colloids and Surfaces B: Biointerfaces 103 (2013) 544– 549 547

0.0 0.2 0.4 0.6 0.8 1.00

5

10

15

20B

PLU w* PAS

Fp

aimebstsbmta(

cmt

3

brcatb

FaT

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

w*

DH

(n

m)

PLU PAS

The polyelectrolyte/surfactant complex formation depends onthe structure of the involved co-partners, in particular on thehydrophobicity and the molecular weight of the polymer, and the

ig. 3. Viscometric interaction parameter, B, as a function of composition of theolymer mixture.

The data given in Fig. 2 show that the interaction between PASnd PLU is favorable, the intrinsic viscosities are larger than antic-pated; an exception to the normal pattern is observed for the

ixture with w* = 0.5 for which the intrinsic viscosity is (within thexperimental errors) as predicted by the additive rule. This coulde an experimental coincidence. On the other hand, the viscometrictudy reveals the existence of the cumulative effects due to addi-ive and/or compensative contributions of each macromolecularpecies in solution. One can suppose that for the same amount ofoth polymers in the solution, the size and shape of PAS coils areodified by the macromolecular surfactant (PLU) which screens

he polyelectrolyte chains and diminishes the electrostatic inter-ctions, and, as a consequence the viscosity reaches a lower valuesome segments of PAS disappear in PLU coils).

The values of the parameter B are very high for the systemontaining PAS, suggesting strong interaction between macro-olecules due to the presence of COOH groups. An exception to

his trend is observed, again, for the mixture with w* = 0.5.

.2. Comparison capillary versus shear viscosity

In shear flow conditions, all systems present a Newtonianehavior, the viscosity remains constant for shear rates in theange 0.01 –50 s−1. A comparison of the capillary and shear vis-

osity of different PLU/PAS compositions measured in all cases at

constant polymer concentration of 1 g/dL is given in Fig. 4. Forernary PLU/PAS/water solutions, the viscosity values determinedy both methods are closed. The difference between the shear

w*1

1.1

1.2

1.3

1.4

1.5

0 0.2 0.4 0.6 0.8 1

shear viscosity

capillary viscosity

η(m

Pa.s

)

PLU PAS

ig. 4. Comparison of the viscosities measured for a total polymer content of 1 g/dLt different polymer compositions in shear conditions and capillary measurements.he solid line is to guide the eyes.

Fig. 5. Plot of coil dimensions, DH, as a function of PAS weight fraction (w*) in thepolymer mixture.

and capillary viscosities, which appears for binary polymer/solventsystems, can be included in the range of the experimentalerrors in Fig. 4.

Owing to the affinity between PAS and PLU, the addition of PLU isaccompanied by a conformational transition of the macromolecularchains, which leads to the micelle structure formation. These tran-sitions are at the same time accompanied by the reduction of thevolume, phenomena sustained by the dimension of colloidal parti-cles as shown by DLS determinations. The viscosity is also, relatedon the surfactant organization mode onto the polymer chains, bythe interactions between the polymeric acid and nonionic surfac-tant, respectively binding of the surfactant as a whole molecule,and the degree of hydrogen bonding in the hydrophilic moiety.However, the change of the reduced viscosity is more a hydrogenbond-sensitive phenomenon then directly related to the binding ofthe surfactant as a whole molecule [11].

3.3. Dynamic light scattering study

-26

-21

-16

-11

-6

-1 0 0.2 0.4 0.6 0.8 1

w *

Zeta

po

ten

tial (m

V)

PLU PAS

Fig. 6. Plot of zeta potential as a function of PAS fraction (w*) in the polymer mixture.

548 L.E. Nita et al. / Colloids and Surfaces B: Biointerfaces 103 (2013) 544– 549

echan

cmor

mf

wapdamhsaa

Scheme 1. The proposed self-assembling m

harge and shape of the surfactant. For linear polymers, a generalodel has emerged, often referred to as the “necklace” or “beads-

n-a-string” model, in which one or more small surfactant micelleseside within the random coil of the polymer [1].

In Fig. 5, average of the hydrodynamic diameter (DH) deter-ined for different polymer compositions as function of the weight

raction of PAS in the polymer mixture is plotted.DLS experiments showed a good reproducibility of the data

ith the deviation up to 5% of the average diameter. Also, forll PLU/PAS mixtures in solution we observed a high polydis-ersity (>0.4). In the case of binary systems, the measured sizeistribution of PLU in water shows spans from 1 to 26 nm, withn average hydrodynamic diameter around 10 nm, implying thatost of PLU molecules still exist as unimers in the solution. The

ydrodynamic diameter of PAS is about 280 nm. The dynamic lightcattering is also carried to prove the formation of PAS/PLU inter-ctions. DLS characterization of the PLU/PAS complex in solutionfter passing though 0.45 �m filter indicates reasonably narrow

ism between PAS and PLU around w* = 0.5.

size distributions. The hydrodynamic diameter increases with theincreasing of PAS content determined for the PLU/PAS mixtures, anexception is constituted by the domain with PAS content aroundw* = 0.5, when a strongly decrease of the hydrodynamic diameterwas registered.

3.4. Self-assembling of PAS in the presence of PLU in aqueoussolution

In order to discuss the possibility of the self-assembling of PAS inthe presence of PLU or about the intervened intermolecular inter-actions, we must take into account that both polymers are watersoluble and present dispersant properties. The self-assembling pro-cess between PAS and PLU takes place due to particular favorable

interactions between them. The molecules of PAS, with certaininterdependence between the repulsive forces among the simi-lar functional groups and the steric hindrances producing rigidity,interact with PLU molecules, which are more flexible.

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oscvbcwbpoic

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4

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[

[[

L.E. Nita et al. / Colloids and Surfac

From the investigation of the binary systems (PLU/water andAS/water) we observed that, despite the fact that the molecu-ar weights are closely, the intrinsic viscosities and hydrodynamiciameter are very different owing to their different chemical com-osition. We assume that the specific intramolecular interactions ofAS chains determine the increase of the dimension of the macro-olecular coils in water, comparative with PLU coils in water. PAS

s a synthetic polypeptide in which the amino acid units joinedogether by peptide bonds carry a carboxyl group. Interactionsetween these negatively charged molecules are repulsive, and inhysiological salt solutions PAS chains assume a random coil con-guration. The persistence length of PAS is about 2 nm [11,12]. In

stiff backbone such as PAS, backbone bending for hydrophobicomain formation is so difficult that secondary aggregate formationy intermolecular interactions is favored over primary aggre-ate formation by intramolecular interaction [13]. Micellizationccurs in dilute solutions of block copolymers in selected solventsbove the critical micellar concentration, at a given tempera-ure. At higher concentrations, above a critical gel concentration,he micelles can order into a lattice [14]. Low concentrations ofLU in aqueous solutions form monomolecular micelles. Higheroncentrations result in multimolecular aggregates or gels [15]onsisting of a hydrophobic polyoxypropylene central core withheir hydrophilic polyoxyethylene chains facing the external

edium.From the investigation of PLU/PAS/water ternary systems, it was

bserved that for compositions w* < 0.3 the PLU chains form clas-ic micelles in water with hydrophobic core (PPO) and hydrophilichains at surface (PEO). For w* > 0.3, the micelles split and thenanish and the PEO blocks can interact with PAS by hydrogenonds. We can conclude upon the evolution of the intrinsic vis-osity and hydrodynamic diameter as a function of w*, that for* value near 0.5, the maximum of intermolecular interactionsetween the two polymeric partners occurs. At the same time, zetaotential becomes zero (Fig. 6), indicating the absence of chargesn the surface; practically the functional groups are involvedn bonds inside the globule (micelle), the shape of the PAS/PLUomplex.

PAS chains interact with the PEO blocks through the functionalroups ( COOH, NH2 from PAS and O from PEO) and form thecore” of a “micellar” structure, whereas the PPO blocks form theshell” at w* = 0.5. At 25 ◦C and at concentration of Pluronic F127elow 1 g/dL, the hydrophobic interactions increase and becomeredominant [16]. In these conditions, the rupture of hydrogenonds of PPO leads to an increase of hydrophobicity, while the PEOhains remain hydrophilic [17]. By adding PAS into the PLU solu-ion (the polymer concentration remains always constant, 1 g/dL),he PLU content decreases bellow critical micelle concentration ands the micelles disappear, new interactions with PAS are formed.round w* = 0.5, we suppose that there are enough hydrophiliclocks of PEO to interact with PAS chains and to form the “core”,hile the “shell” is assured by PPO blocks. This packaging between

AS and PEO blocks is constrained by PPO ones (see Scheme 1). Theormation of the hydrophobic shell can explain the decrease of thentrinsic viscosity (Fig. 2) and the hydrodynamic diameter (Fig. 5).

. Conclusion

The central item of the present paper was to investi-ate the behavior during the interpolimer complex formationetween poly(aspartic acid) (PAS) and a macromolecular surfac-

ant (Pluronic F127 – PLU) in dilute aqueous solutions. Despite theact that PAS and PLU have similar molecular weights, the intrin-ic viscosities and hydrodynamic diameters in aqueous solutionsre very different owing to their different chemical composition

[

[[

iointerfaces 103 (2013) 544– 549 549

as well as physical interactions. Due to the PAS superabsorbentparticular characteristics, we assume that specific interactions ofthe chains with the solvent molecules cause the increase of themacromolecular coils dimension in water comparative of PLU, asnon-ionizable molecules less disposed for hydrogen bonds withwater. Also, the helical conformation of polyaspartic acid, spe-cific for polyaminoacids, is more available for hosting the solventmolecules of water than that of the PLU stick.

The viscometric measurements of ternary systems for whichone polymer is in excess, reflect the favorable interactions betweenPAS and PLU, the intrinsic viscosities being larger than expected.An exception from the normal pattern was registered for thew* = 0.5 when the intrinsic viscosity was (within the experimentalerrors) as predicted by the additive rule. It is assumed, that forthe same amount of PAS and PLU in solution, the size and shapeof PAS coils are modified because the intramolecular interactionsare interchanged with the intermolecular interactions betweendifferent chains, the interpolymer complex formation is favoredand the complex adopts a collapsed conformation. The segmentsof PAS are hidden by the PLU coils, the surfactant determines thescreening the polyelectrolyte chains and diminishes the electro-static interactions and as a consequence the intrinsic viscosityassumes lower values. These changes in chain conformation areaccompanied by the reduction of the colloidal particle diameteras evidenced by DLS experiments. At the same time, for w* = 0.5the zeta potential becomes zero, indicating the absence of chargeson the colloidal particle surface as well as a core/shell shape forthe colloidal particles. The functional groups of PAS are involvedin hydrogen bonds with PLU inside the globule (micelle) of thePAS/PLU interpolymer complex. Considering the complex formedjust by 1 molecule of PAS and 1 molecule of PLU in the limit of infi-nite dilution, this would lead (because of the similar molar massesof the two polymers) to the conclusion that the interpolymercomplex is formed around w* = 0.5.

Acknowledgements

This work was supported by European Social Fund – ′′CristoforI. Simionescu′′ Postdoctoral Fellowship Programme (IDPOS-DRU/89/1.5/S/55216), Sectoral Operational Programme HumanResources Development 2007–2013 and by a grant of the RomanianNational Authority for Scientific Research, CNCS-UEFISCDI, projectnumber PN-II-ID-PCE-2011-3-0199 (contract 300/2011).

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