Miscibility Behavior of Polyacrylamides Poly(Ethylene Glycol) Blends: Flory Huggins Interaction Parameter Determined by Thermal Analysis

doi:10.4236/jmp.2013.47A2007

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Journal of Modern Physics, 2013, 4, 45-51

http://dx.doi.org/10.4236/jmp.2013.47A2007 Published Online July 2013 (http://www.scirp.org/journal/jmp)

Miscibility Behavior of Polyacrylamides Poly(Ethylene

Glycol) Blends: Flory Huggins Interaction Parameter

Determined by Thermal Analysis

Maria Elisa S. R. Silva1*, Valdir Mano2, Raquel R. J. Pacheco1, Roberto F. S. Freitas1

1Department of Chemical Engineering, Federal University of Minas Gerais, Belo Horizonte, Brazil

2Department de Natural Sciences, Federal University of São João del Rei, São João del Rei, Minas Gerais, Brazil

Email: *[email protected]

Received May 1, 2013; revised June 5, 2013; accepted July 3, 2013

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Blends of polyacrylamide—PAM, poly(N-isopropylacrylamide)—PNIPAAm, poly(N-tert-butylacrylamide)—PTBAA,

poly(N,N-dimethylacrylamide)—PDMAA and poly(N,N-diethylacrylamide)—PDEAA with poly(ethylene glycol)—

PEG were prepared by casting in methanol and water at concentrations of 20 wt%, 40 wt%, 60 wt%, and 80 wt% in

PEG. The miscibility of the components was studied by Differential Scanning Calorimetry—DSC. All blend systems

are characterized by a single glass transition temperature (Tg), close to the Tg of the amorphous component. The Hoff-

man Weeks method was used to determine equilibrium melting temperature (Tm) data. The determination of the melt

point depression of the blends allowed the calculation of Flory-Huggins interaction parameter (χ12) of the two polymers

in the melt, by using the Nishi Wang equation. The interaction parameters, calculated for all the blends, are slightly ne-

gative and close to zero, suggesting a partial miscibility between the components.

Keywords: Polyacrylamides; Poly(Ethylene Glycol); Polymer Blends; Flory-Huggins Interaction Parameter;

Differential Scanning Calorimetry

1. Introduction

Most of the proposed applications of polyacrylamides

rely primarily on their behavior in aqueous solution [1-4].

On the other hand, this polymer and some of its N-alkyl

substituted derivatives show great potential in the prepa-

ration of miscible polymer blends due to their ability to

interact through hydrogen bonding. Polyacrylamide—

PAM, and some of its N-alkyl substituted derivatives,

such as poly(N-isopropylacrylamide)—PNIPAAm,

poly(N-tert-butylacrylamide)—PTBAA, poly(N,N-dime-

thylacrylamide)—PDMAA, and poly(N,N-diethylacryla-

mide)—PDEAA, are amorphous and water-soluble poly-

mers with great industrial and agricultural interest [5-7].

In previous work, Silva et al. [8] showed that their ther-

mal behavior is strongly dependent on the alkyl bonded

to the amide group. Polyacrylamides form gels, or poly-

mer networks that are able to expand in compatible sol-

vent, like water, exhibiting a great potential for concen-

tration of macromolecules from solutions, water purifica-

tion, enzyme immobilization, drug delivery, sensors and

for several biomedical uses [1,9-13].

Poly(ethyleneglycol), PEG, is a semi-crystalline poly-

mer that shows applications as chromatographic support

and polymeric electrolyte [14-18]. In the last ten years,

interest has been attracted to systems in which at least

one of the components is crystallizable. In this case, a melt

point depression of the crystalline phase relative to its

melting point (Tm) in a non-interacting medium pro-

vides additional evidence of miscibility, since the kinet-

ics and morphological effects over Tm are eliminated [19].

Measurement of the melting temperature (Tm) depression

for blends allowed the determination of the Flory-Hug-

gins interaction parameter (χ12) of polymers in the melt

state, by using the Nishi-Wang equation [20,21]. The

Hoffman-Weeks method is used to determine the equi-

librium data [22].

There have been many studies on miscible blends con-

taining poly(ethylene glycol) and poly(ethylene oxide).

For example, blends of PEO/poly(epichlorohydrin) and

PEO/poly(epichlorohydrin-co-ethylene oxide) were stud-

ied by Silva et al. [19]. For these systems, the polymer-

polymer interaction parameters are all negatives and ex-

*Corresponding author.

M. E. S. R. SILVA ET AL.

hibit dependence on the blend composition, decreasing

for blends rich in PEO, suggesting that the polymer pairs

are thermodynamically miscible.

Cortazar et al. [23] and Cimmino et al. [24] studied

blends of PEO/poly(methyl methacrylate) with several

molecular weights, finding χ12 values in the range of

−0.131 to −0.290. Blends of PEO (MW = 2.0 × 103) and

poly(p-hydroxibenzoic acid)—PHB, (MW = 2.79 × 103)

are miscible and exhibit χ12 values of −0.21 to −2.00,

depending on the concentration of the crystalline com-

ponent [25].

The miscibility of PVC/PEO blends was studied using

viscosimetry, thermal analysis and microscopy by Neiro

et al. [26]. χ12 values obtained from the melting point

depression are dependent on the molar mass of PVC,

varying from −0.102 to −0.028, indicating that the pair is

miscible in the melt.

In this paper, we report the study on the miscibility of

blends of PAM, PNIPAAm, PTBAA, PDEAA e PDMAA

with PEG, using differential scanning calorimetry (DSC).

The Flory-Huggins interaction parameters (χ12) were de-

termined by the melting point depression method.

2. Experimental

2.1. Materials

PAM, PNIPAAm, PTBAA, PDMAA and PDEAA were

synthesized via free radical mechanism, under nitrogren

atmosphere, according to the method described by Frei-

tas and Cussler [1].

Monomers (Aldrich and Polyscience) and the initiators,

ammonium persulfate, sodium metabisulfite (Reagen),

and 2,2’-azobisisobutyronitrile (Polyscience), analytical

grade, were used as received.

PEG (Aldrich), 2000 g·mol−1, was used as received,

without additional purification.

The weight-average molecular weight (MW) of amor-

phous homopolymers was determined by light scattering

using a Brookhaven Instruments equipment.

2.2. Blends Preparation

Binary blends of varying compositions were prepared by

casting from water and methanol solutions. To ensure

complete removal of the solvent, the blends were kept

under vacuum at 41˚C for ten days.

2.3. Blends Characterization and Determination

of Crystallization Temperatures by DSC

The DSC curves for homopolymers and blends were ob-

tained on a DSC-50 Shimadzu module. Samples weights

were maintained in the range of 10 mg. All experiments

were performed under helium flow of 70 mL·min−1. The

samples were heated from ambient to 210˚C at a rate of

20˚C·min−1, held at this temperature for 10 min to elimi-

nate thermal history. After cooling the samples to −20˚C

at a rate of 20˚C·min−1, they were heated again to 210˚C

at 10˚C·min−1.

The glass transition temperature and the melting en-

thalpies (Hm) values were taken from the second heat-

ing scan. The cooling scan was used to select the crystal-

lization temperatures.

2.4. Isothermal Crystallization

Isothermal crystallization was performed on a DSC-50

Shimadzu module based on the Hoffman-Weeks method

[22]. Samples weights of 10 mg were heated from 25˚C

to 200˚C at a rate of 20˚C·min−1, held at this temperature

for 5 min, rapidly cooled (cooling rate~ 40˚C·min−1) to

the desired crystallization temperature (Tc), maintained at

this temperature for 2 min, and then cooled to 10˚C at a

rate of 20˚C·min−1. After that, the samples were heated to

200˚C, at a rate of 20˚C·min−1, for the measurement of

the Tm. This procedure was repeated for different crystal-

lization temperatures.

2.5. Scanning Electron Microscopy (SEM)

SEM micrographs of the blends were obtained using a

JEOL JSM-5410 microscope and the samples were frac-

tured in nitrogen and covered by sputtering with a Au/Pd

alloy.

2.6. Density Measurements

Density measurements of each homopolymer were car-

ried out using a picnometer in a non-solvent (heptane and

cyclohexane).

3. Results and Discussion

The weight-average molecular weight of PAM and its

N-alkyl substituted derivatives are, as seen in Table 1, at

the same order of magnitude.

Density values of PAM and its N-alkyl substituted de-

rivatives are showed in Table 2.

It’s shown in Figure 1 DSC curves of blends with 20

Table 1. Weight-average molecular weight of PAM and its

N-alkyl substituted derivatives

Homopolymer PAMPNIPA Am PTBAA PDMAAPDEAA

MW × 10−5/g·mol−12.531.20 2.07 2.74 1.15

Table 2. Density values of homopolymers.

Polymer PAMPNIPAAmPTBAA PDMAA PDEAAPEG

Density/

g·mL−1 1.050.97 0.89 0.89 0.89 1.31

M. E. S. R. SILVA ET AL. 47

wt%, 40 wt%, 60 wt% and 80 wt% PEG. Tg values are

listed in Table 3. A single Tg is observed for all blends

and it does not change appreciably with the composition,

showing values around Tg of the amorphous polymer

(PAM and its N-alkyl substituted derivatives), suggesting

that the amorphous phase of the blend is rich in this

component. The blends also show a single endothermic

peak due to the melting of crystalline phase (PEG).

Figure 2 shows scanning electron micrographs of

PAM, PEG and PAM/PEG blends in various composi-

tions. It is observed that the crystal sizes are dependent

on the amorphous polymer concentration. As the amor-

phous polymer concentration increases, PEG crystalline

phase size decreases (Table 4). It is known that the ki-

netics of crystallization of the crystalline component in a

blend is affected by the presence of the amorphous po-

lymer [27,28]. Both, the overall kinetic rate and the

spherulites growth rate decrease significantly. Changes

on crystallization behavior are due to low mobility of

PEG related with Tg of the blends and with the possibility

of favorable interactions between PEG and the amor-

phous polymer.

Figure 1. DSC curves for PAM/PEG.

Table 3. Tg values of PAM/PEG and their blends.

PEG/

%.ww−1

PMA/

PEG

PNIPAAm/

PEG

PTBAA/

PEG

PDMAA/

PEG

PDEAA/

PEG

0 188 142 128 111 91

20 178 133 84 98 88

40 174 126 84 90 85

60 166 126 87 83 83

80 163 126 83 79 83

100 −52* −52* −52* −52* −52*

*Silva et al. [8].

(a) (b)

(e) (f)

Figure 2. SEM micrographs; (a) pure PAM; (b) 20% PEG;

Table 4. PEG crystals average diameters (dm) of PAM/PEG

blends.

PAM/% m·m−80

12040 60

dm/m 2424 20 14

Determination of Flory-Huggins Parameter

In blends in which at least one component is crystalliz-

able, thint depressio of the crystalline pase

releltinoint pides adnal evce

of mis. The mg pof a poer is affected

not only by the thermodynamic factors but also by the

hick-

, the

by the extrapola-

tio

e melt po

ative to its m

rov

ideng pditio

cibilityeltinoint lym

morphological aspects such as crystalline lamellar t

ness. As described by the Flory-Huggins theory

equilibrium melting point should be used to separate

morphological effect from thermodynamic effect in dis-

cussing the melt point depression [29].

Morphological effects are associated with changes in

crystal perfection or geometry and with different thermal

history of the samples. The contribution of such mor-

phological effects can usually be removed by construct-

ing a Hoffman-Weeks [22] plot by using melting data for

PEG and for blends isothermally crystallized at different

temperatures (Tc); Tme is determinated

n of the experimental curve of Tm versus Tc to theo-

M. E. S. R. SILVA ET AL.

-Wang, by Equation (1) [20,21].

retical curve, Tm = Tc.

Thermodynamic considerations predict that chemical

potential of a crystallizable polymer will decrease caused

by the addition of the miscible diluent [28]. The expres-

sion to describe the dependence of the melting point de-

pression due only to thermodynamic effects on the blends

composition is given, according to the Flory-Huggins

theory modified by Nishi

1121

2122 1

11 11

THVmmm











 











(1)

Tme e 0

T are the equilibrium melting temperature of

PEG in blends and pure PEG, respectively. Subscripts 1 e

2 are referred to the crystallizable and non-crystallizable

polymers, Vu is the molar volume of the repeating unit

Hu is the heat of fusion of the perfectly crystallizable

polymer per mole of the repeat unit, m is the degree

polymerization;



are the volume fractions, R is the u

ves c

general, Tme values showed a slight decreasing tendency

relative to me value as the concentration of the amor-

phous component increases.

Table 6 shows



1 and



2 values used to calculate χ12. R

and H values were 8.31 J·K−1·mol−1 and 7.90 K·J·mol−1,

respectively.

In Figure 4 and in Table 7, χ12 values are represented

as a function of PEG composition.

For all systems, χ12 values are dependent on the blend

composition. Avella [25] and Yoo [30] attribute the de-

pendence of χ12 on the blend composition to morpho-

logical and kinetics factors such as recrystallization, phase

segregation, etc. Painter et al. [31] proposed that this ef-

fect results from the strong interactions between the com-

ponents as hydrogen bonding observed in poly(vinyl

Table 5. Equilibrium melting temperatures of PEG in the

blends.

ni-

Tme/˚C

PEG/

% m·m−1

PAM/

PEG

PIPAA/

PEG

PTBAA/

PEG

PDMAA/

PEG

PDEAA/

PEG

20 54 52 53 56 52

40 56 56 55 53 55

60 56 56 56 55 56

80 56 56 56 56 56

rsal gaonstant, and χ12 is the polymer-polymer in-

teraction parameter.

Figure 3 shows the Hoffman-Weeks plots for iso-

thermally crystallized blends.

Table 5 summarizes the equilibrium melting tempera-

tures of PEG in the blends (Tme). For pure PEG, equilib-

rium melting point



T was determined as 57˚C. In

(a) (b) (c)

(d) (e)

Figure 3. Hoffman-Weeks plots of isothermally crystallized blends. PEG (wt%): () 100; () 80; () 60; () 40 e () 20%

m·m−1; (a) PAM/PEG; (b) PNIPAAm/PEG; (c) PTBAA/PEG; (d) PDMAA/PEG; (e) PDEAA/PEG.

M. E. S. R. SILVA ET AL. 49

Table 6.



1 and



2 values.

PAM/PEG PIPAA/PEG PTBAA/PEG PDMAA/PEG PDEAA/PEG

PEG/% m·m−1



20 0.84 0.16 0.85 0.15 0.85 0.15 0.85 0.15 0.86 0.14

40 0.66 0.34 0.67 0.33 0.69 0.31 0.69 0.31 0.70 0.30

60 0.47 0.53 0.48 0.53 0.49 0.51 0.49 0.51 0.51 0.49

80 0.25 0.75 0.25 0.75 0.27 0.73 0.27 0.73 0.28 0.72

12 versus bleomposition: ())

NIPAAm/PEG, () PTBAA/PEG, () PDMAA/PEG e ()

PDEAA/PEG.

Table 7. χ12 values as a function of PEG concentration.

χ12

Figure 4. χnds c PAM/PEG, (

PEG/

% m·m−1

PAM/

PEG

PIPAA/

PEG

PTBAA/

PEG

PDMAA/

PEG

PDEAA/

PEG

20 −0.032 −0.15 −0.15 7.2 × 10−4−0.18

40 0.012 −0.015 −0.081 −0.16 −0.13

60 −0.024 −0.072 −0.043 −0.11 −0.070

80 −0.20 −0.33 −0.074 0.060 2.0 × 13

0−

phenol) and poli(ethylene-co-acrylic acid) blends. Lee et

al. [32] studying, by DSC, blends of polystyrene and po-

0. e

molamass of the homopolym Linares and Acosta

[33ied /Pdl

fo alues in the o3

and of −0.153 to 2.201−3ectivdi,

according to thors, that these syste

mis

Bs ofinhoNywe-

ied by Koulouri and Kallitsis [34], through dynamic-me-

chanl analMC Na-

on to determine Flory-Huggins parameters. The results

for blends can be found in Zhang

blends of poly(butylene

(PBSU) and poly(vinyl phenol) (PVPh) by

Qiu et al. [36]. The existence of single composition de-

p transitre and tva-

lu of χ12 lculated using tshi-Wang equon,

cate thBSU/h blends are thenamly

le inmelt.

éfice . [29rmine degf interaction

een tmpof acarb-polyne

blend through the Flory-Huggins interaction parameter.

resuled tlend highnteno-

lycarbonate tend to present χ closer to the critical val-

nents was observed in all blends. Melting

ly utadiene, found χ12 values in the range of 0.0040 to

0102, depending on the blend composition and on th

r ers.

] stud

und χ12 v

PVDAVA an

PVDF/P

MMA b

4 02

ends and

5−

e ranf −0. to 2. × 10

× 10, respely, in

ms are partially

cating

he aut

cible.

lend poly(vyl alcol) and lon 6 re stud

icaysis (DA), DS and theishi-Wng equa

showed a single Tg and the χ12 value was –0.05, suggest-

ing miscibility. Others χ12 values and their analysis as

ues, suggesting greater compatibility in these composi-

tions.

High molecular weight samples of the novel biode-

gradable polyester poly(ethylene sebacate) (PESeb) with

poly(4-vinyl phenol) (PVPh) blends were studied, with

respect to miscibility, by Papageorgiou et al. [37]. A sin-

gle glass transition temperature intermediate to those of

pure compo

criterion of miscibility

et al. [28], Marcos et al. [35], and Cimmino et al. [24].

Miscibility was investigated in

succinate)

endent glassion temperatuhe negative

es , cahe Niati

indi

miscib

at P

the

PVPrmody ical

Or et al] deteed three o

betwhe conents o polyonatestyre

The ts showhat bs wither cot of p

int depression was observed with increasing content of

the amorphous polymer. The equilibrium melting points

were estimated using Hoffmann-Weeks extrapolation. The

interaction parameter was calculated and it was found to

be χ12 = −1.3. Blends miscibility was attributed to the

formation of hydrogen bonds between the hydroxyl groups

of PVPh and carbonyl groups of PESeb.

In the present work, χ12 values are dependent on th

end composition and are near zero or slightly negatives

(Table 7), suggesting partial miscibility between PEG

and the amorphous polymers in the melting, maybe as a

result of interactions through hydrogen bonds. Addition-

ally, Tg behavior and SEM micrographs corroborate this

result.

4. Conclusion

Binary blends of polyacrylamide and some of its N-alkyl

substituted derivatives with PEG, obtained by co-disso-

lution method, are partially miscible, as demonstrated by

the smooth lowering of Tg values relative to the Tg value

of the pure amorphous component. The polymer-polymer

M. E. S. R. SILVA ET AL.

interaction parameters obtained from thermodynamic

melt-

g temperature depression analysis are slightly negative

and close to zero, suggesting partial miscibility among

the components. Furthermore, for all blends studied, there

is an endothermic peak corresponding to the melting of

the PEG crystalline phase. For all blends studied, SEM

micrographs indicate the presence of PEG crystalline

phase.

5. Acknowledgements

The authors thank the Brazilian Agencies CNPq, CAPES

and FAPEMIG for financial support.

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