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
Received May 1, 2013; revised June 5, 2013; accepted July 3, 2013
Copyright © 2013 Maria Elisa S. R. Silva et al. This is an open access article distributed under the Creative Commons Attribution
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.
C
opyright © 2013 SciRes. JMP

M. E. S. R. SILVA ET AL. 46
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)
(c) (d)
(e) (f)
Figure 2. SEM micrographs; (a) pure PAM; (b) 20% PEG;
(c) 40% PEG; (d) 60% PEG; (e) 80% PEG; (f) pure 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
n
rov
h
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. 48
-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
2
22
1121
0
2122 1
ln
11 11
m
u
uu
m
ee
RV
THVmmm T






 





(1)
Tme e 0
me
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
0
T
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.
,
of
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

0
me
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

1

2

1

2

1

2

1

2

1

2
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, (
P
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-
b
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
po
e
bl
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
g
PVDF/P
16
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
ti
showed a single Tg and the χ12 value was –0.05, suggest-
ing miscibility. Others χ12 values and their analysis as
12
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. 50
interaction parameters obtained from thermodynamic
in
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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