Elashmawi et al, Physica B 403 (2008) 3547– 3552

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Spectroscopic and thermal studies of PS/PVAc blends

I.S. Elashmawi a,Ã, N.A. Hakeem a, E.M. Abdelrazek b

a Spectroscopy Department, Physics Division, National Research Centre, Giza, Dokki, Cairo, Egypt b Physics Department, Faculty of Science, Mansoura University, 35516, Mansoura, Egypt

a r t i c l e i n f o

Article history:

Received 20 February 2008

Received in revised form

22 April 2008

Accepted 22 May 2008

Keywords:

Polymer blends

Miscibility

FT-IR

X-ray diffraction

UV-visible

DSC

Optical energy gap

a b s t r a c t

Polystyrene and polyvinyl acetate (PS/PVAc) films were blended with different contents using casting

method. The effect of PS content on PVAc blends was investigated by Fourier transform infrared (FT-IR),

X-ray diffraction (XRD), Ultra violet and visible studies (UV/VIS), differential scanning calorimetry (DSC)

and thermogravimetric analysis (TGA). Significant changes in FT-IR, XRD and DSC analysis are observed

which reveals an interactions between the two polymers and PS/PVAc blends had good or certain

miscibility. XRD scans show some changes in the intensity and the height of the amorphous halos with

increased PS. UV/VIS analysis revealed that the optical band gap decreases with increasing content of PS

from 5 to 4.11 eV. A single glass transition temperature for each blend was observed, this DSC results

supported that the miscibility existed in the blend. The apparent activation energy (E) of the blends was

evaluated using TGA analysis. The value of E was increased with the increase of PS content.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Polymer blends have attracted the attention of materials

researchers, because the structural and some physical properties

of polymers can be modified to a specific requirement by blending

two or more polymers [1–4]. Blending involves physical mixing of

polymers and allows one to create a new material having some of

the desired properties of each component.

There has been considerable interest in the study of polymer

blends because of their importance in academic and technical

aspects. Particularly, much attention has been paid to miscibility

and phase behavior in polymer blends [5–7]. Many techniques can

be used to study the miscibility and phase behavior of polymer

blends.

In order to investigate the miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been

frequently used for the determination of the glass transition

temperature (T g) [8]. A miscible polymer blend would exhibit a

single transition between T g of the two components. With

increasing immiscibility there is a broadening of the transition,

whereas an incompatible system would be marked by separate

transitions of the polymer components in the blends [9].

The miscibility of the binary blends was investigated earlier.The miscibility of PVAc with PCL (polycaprolactone) was studied

using DSC and a single transition temperature was observed for

the complete range [10]. The miscibility of PVAC and PVC was also

investigated by DSC and their miscibility depends on the solvent

used during solution blending [11].

FT-IR spectroscopy is a rapidly expanding area in polymer

miscibility determination and has provided much information

over the years on molecular vibrations. Recent interest in

vibrational spectroscopy has been focused on instrumentation,

method development and vibrational analysis. The rapid devel-

opment of FT-IR has revolutionized the applications of infrared

spectroscopy and over recent years has been used to elucidate the

interactions present in blends and from this information the blend

miscibility has been deduced [12]. In this work, the objective is toinvestigate the miscibility of PS/PVAc blends and to examine

phase behavior and spectroscopic studies with various composi-

tion ratios of these blends. PS was chosen as one of the blend

components in view of its lack of crystallinity and its miscibility

with PVAc.

2. Experimental

2.1. Samples preparation

Polystyrene (PS) from BDH chemicals Ltd. (Poole, England)

and polyvinyl acetate (PVAc) from Aldrich chemical co. Ltd.

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Physica B

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Ã Corresponding author. Tel.: +20 502536150.

E-mail addresses: [email protected], [email protected]

com (I.S. Elashmawi).

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(Gillingham, England) were used as received. Pure acetone was

used as a common solvent for the two polymers at 50 1C. After

complete dissolution, the blend was prepared by casting onto a

Petri dish glass, then left to evaporate the solvent slowly. The

resulting PS/PVAc films were then dried in a vacuum oven at 50 1C

for three days to ensure the removal of the solvent traces. Blends

of PS/PVAc were prepared in different weight concentration

(0/100, 25/75, 50/50, 75/25 and 100/0) using a casting technique.The thickness of the films was in the range of 100–130 mm.

2.2. Measurements

FT-IR measurements were carried out using the single beam

FT-IR (FT-IR-430, Jasco, Japan). FT-IR spectra of the samples were

obtained in the spectral range of 4000–400 cmÀ1. X–ray diffrac-

tion scans were obtained using DIANO corporation-USA equipped

using Cu-Ka

radiation (l ¼ 1.540 A, the tube operated at 30 kV, the

Bragg angle (2y) in the range of 51–351. Ultraviolet–visible

(UVVIS) absorption spectra were measured in the wavelength

region of 210–600 nm using spectrophotometer (V-570 UV/VIS/

NIR, Jasco, Japan).

The differential scanning calorimetry of the prepared films wascarried out using an equipment type (Shimadzu DSC–50) from

room temperature to 350 1C with a heating rate of 10 1C/min. A

Perkin-Elmer (US, Norwalk, CT) TGA-7 was used for the thermo-

gravimetric analysis of the samples. A small amount (5–10 mg) of

the sample was taken for the analysis and the samples were

heated from 50 to 6001C at a rate of 10 1CminÀ1 in nitrogen

atmosphere.

3. Results and discussion

3.1. FT-IR spectroscopic analysis

If two polymers are completely incompatible, each individual

polymer does not recognize, in infrared spectral terms, the

existence of the other in the blend. On the other hand, if the

polymers are compatible, there should be considerable differences

between the infrared spectrum of the blend and the spectra of the

pure components. These differences would be derived from

chemical interactions resulting in band shifts and broadening.

Fig. 1 depicts the FT-IR absorption spectra in the range

4000–400 cmÀ1 of PS/PVAc blends with different blend ratios.

For pure PVAc, the vibrational bands observed at 2927 and

2854 cmÀ1 are ascribed to O–CH3 (ester group) asymmetric

stretching and symmetric stretching vibrations, respectively. The

intense band at around 1736 cmÀ1 represents the CQO stretching

band of an unconjugated ester. At 1373 cmÀ1, a prominent band is

evident, here the CH3 (CQO) group strongly absorbs acetateesters; these acetate esters show a corresponding weak band at

629cmÀ1. The strong band at 1243cmÀ1 and the band at

1122 cmÀ1 are ascribed to C–O–C symmetric stretching and C–O

stretching vibrations, respectively. Also the peak at 947 cmÀ1 is

ascribed to CH bending vibrations and there is a significant band

at 606 cmÀ1 which is assumed to be linked to CH3 (CQO) group

[13–15].

It is known that, PS consists of alternating methylene and

methane groups. However, each repeat unit in PS contains a

pendant benzene ring. The spectrum of pure PS is shown in Fig. 1.

The main PS characterizing bands are observed. The methylene

(CH2) asymmetric and symmetric stretching bands are observed

at 2924 and 2852 cmÀ1. There is a group of aromatic C–H stretches

around 3026 cmÀ

1 and benzene ring modes are found at 1600 and1491 cmÀ1. The out-of-plane C–H bending mode of the aromatic

ring is shown at 756 cmÀ1 and the ring-bending vibrational band

appears at 698 cmÀ1 [16]. These last two bands confirm that PS

contains a monosubstituted benzene ring.

For all blends, a triple split band appearance is identified at

3465, 3535 and 3637 cmÀ1 as shown in Fig. 2. This is the C–H

stretching band, typical C–H stretching being usually present as a

distinctively large band above 3000 cmÀ1. Some bands are

disappeared in the blends and the intensity of some bands was

changed. All results data suggest that homogeneous polymer

composites are formed over all the blend compositions.

3.2. X-ray diffraction analysis

The measurement of XRD diffraction scans of the polymerblends is also used as a criterion to determine its miscibility. If

two polymers have low compatibility then each polymer would

have its own crystal region in the blend films and the X-ray scans

of the samples would be expressed as simply mixed scans of the

two polymers with the same ratio as those for blending [17].

XRD diffraction scans of PS blends with different PVAc contents

are depicted in Fig. 3. All the blend compositions show a broad

and diffuse peak, which indicates the amorphous nature of the

blends.

Fig. 3(a) shows two broad peaks at angles 2y$15.071 and

$22.71, which reveal the amorphous nature of the PVAc polymer.

The observed scans of pure PS (as shown in Fig. 3e) exhibits an

amorphous character of two halos of approximately equal

intensities centered at 2y ¼ 111 and 23.51. The first halo may beattributed to the size of the side group which corresponds to an

ARTICLE IN PRESS

Fig. 1. FT-IR absorption spectra of PS/PVAc blends.

I.S. Elashmawi et al. / Physica B 403 (2008) 3547–35523548



approximately hexagonal ordering of the molecular chains. The

second amorphous halo corresponds to Van der Waals distances

[18]. Changing in the intensity and the height of the two halos

with increase of PS content was observed. This indicates that the

miscibility existed in the blends. The outcome, deduced from XRD,

is consistent with that of FT-IR and DSC.

3.3. Optical spectra analysis

Fig. 4 shows the optical absorption measurements carried out

in the spectral range (210–600 nm) for the present system. The

observed spectrum of pure PVAc has a small absorption band

(shoulder) at about 263 nm. This band is attributed to the

carbonyl group in polymeric macromolecule; the intensity of theabsorption peak slightly increases with increasing the concentra-

tion of PS. This may be due to the increase of the number of

carbonyl groups of the PVAc macromolecules. On the other hand,

the observed spectra have a sharp edge about 250 and 290 nm.

The position of the edge is slightly shifted towards higher

wavelength side suggests the miscibility of the blend between

the two polymers.

The nature of the optical transition involved in the blends can

be determined on the basis of the dependence of absorption

coefficient (a) on photon energy (hn). The total absorption could

be due to the optical transition which is fitted to the relation [19]:

ahn ¼ BðhnÀ E gÞn (1)

where E g is the optical energy gap between the bottom of theconduction band and the top of the valance band at the same

value of wavenumber and B is a constant related to the properties

of the conduction and valance bands. On the basis of Eq. (1) and

the value of n ¼ 12 for allowed direct transitions, E g can bedetermined.

ARTICLE IN PRESS

Fig. 2. FT-IR absorption spectra of PS/PVAc blends in the range of 2750–4000 cmÀ1.

Fig. 3. X-ray diffraction scans for pure PVAc, pure PS and their blends.

Fig. 4. UV–visible absorption spectra of PS/PVAc blends.

I.S. Elashmawi et al. / Physica B 403 (2008) 3547–3552 3549



The plot of (ahn)1/2 versus the photon energy hn shows a linear

behavior which are presented in Fig. 5. Each linear portion

indicates an optical energy gap E g. The values of E g are listed in

Table 1. The data in this table show that the band gap decreases

with increasing the content of PS from 5 to 4.11 eV. The existence

and variation of optical energy gap E g may be explained by

invoking the occurrence of local cross linking within the

amorphous phase of PVAc, in such a way as to increase thedegree of ordering in these parts. It is noticed that the curves are

characterized by the presence of an exponentially decay tail at low

energy [20]. These results indicate the presence of a well defined

p-p* transition associated with the formation of conjugated

electronic structure [21].

3.4. Differential scanning calorimetry analysis

DSC is one of the most convenient method to determine the

miscibility and thermal properties of the polymer blends.

Measurement of the glass transition temperature, T g, (from the

DSC thermograms) of the polymer blends is used to determine itsmiscibility [22]. A miscible polymer blend possesses a homo-

geneous amorphous phase and hence will exhibit a single glass

transition temperature (T g) between the T gs. of the two polymers.

The DSC curves of PS/PVAc blends are shown in Fig. 6. The

thermograms show that the T g values for pure PS and pure PVAc

are about 95 and 381

C [23], respectively. The curves of the othersamples also show one T g in the temperature range from 38 to

ARTICLE IN PRESS

Fig. 5. The dependence of (ahn)1/2 verses the photon energy (hn) of PS/PVAc blends.

Table 1

The values of optical energy gap (E g) and the activation energy (E ) for PS/PVAc

blends

PS: PVAc F g (eV) E (J/mol)

0:100 5.00 69.03

25:75 4.81 82.37

50:50 4.76 89.45

75:25 4.21 99.58

100:0 4.11 129.2

Fig. 6. DSC thermograms of PS/PVAc blends.




95 1C and T g value increases with increasing the PS content. This

results supports that PS/PVAc blends have miscibility in the

amorphous state and this is due to the interaction occurs between

PS and PVAc.There are several classical equations that correlate the glass

transition temperature of a miscible blend system with its

composition [24–26]. These equations are expressed by using a

modified Fox [24] equation

1

T gðblendÞ¼

w1

T g1þ

w2

T g2(2)

where w1 and w2 are the weight fractions and T g1, T g2 are the

respective glass transition temperatures of the homopolymers.

The glass transition temperature estimated using Fox equation

and T g from the DSC curves (experimental) for the studied blends

as a function of PVAc content are shown in Fig. 7.

In addition, when the PS content is increased, the melting

point of the blends decreased slightly due to favorable interac-tions between the two polymers and the miscibility take place

between PS and PVAc. No endothermic peaks appear in the scans,

indicating that crystalline regions do not exist in our samples. This

results data were correlated to X-ray analysis.

3.5. Thermogravimetric analysis

TGA is widely used to investigate the thermal decomposition of

polymers and to determine the kinetic parameters such as

activation energy and order of reaction. These parameters can

be used to give a better understanding of the thermal stability of

polymer blends.

Fig. 8 shows TGA thermograms of weight loss as a function of

temperature for pure PS, pure PVAc and their blends with aheating rate of 5 1C/min in the temperature range from 50 to

600 1C. It is clear that, the initial weight loss for all the samples

occurs at 70–90 1C due to the moisture evaporation and it is stable

up to 120 1C, above which the solvent evaporated. The major

weight losses are observed in the range of 270–360 1C for all the

samples. This may be correspondent to the structural decomposi-

tion of the polymer blends.

Table 2 summarizes the percentage weight loss at different

decomposition temperatures of PS/PVAc polymer blends taken

from the TGA thermograms. It can be seen that (from the

thermograms) pure PS and PVAc polymers lose about 5% of

weight at 256 and 250 1C, respectively. This behavior indicates

that the initial decomposition reaction for PS begins at a slightly

shifted towards lower temperature than PVAc. By increasing theheating temperature from 270 to 360 1C, the loss in weight of the

PVAc is less than the weight loss of the PS. Thus, it may be

concluded that the addition PS polymer is more stable against

thermal decomposition than the pure PVAc.

3.6. Determine the activation energy

The methods used to calculating kinetic parameters from TGA

data are classified into two groups: integral and differential

methods. The most suitable method has, however, not yet been

clarified. For both methods, the basic equation for the fraction of

conversion, a, for a weight loss system, is given by:

a ¼wo Àw

wo Àwf

(3)

where w, wo and wf are the acual, initial and final weight of the

samples, respectively.

The activation energy for the thermal decomposition of the

present samples depends on the residual mass that can be

calculated using integral equation of Coats and Redfern [27]:

log1À ð1 À aÞ1Àn

T 2

" #¼ log

R

DE 1À

2RT

E

À 0:434

E

RT(4)

where, T is the absolute temperature in Kelvin, E is the activation

energy in J /mol, R is the universal gas constant (8.3136 J/mol K)

and n is the order of reaction.

For n ¼ 1, Eq. (4) reduces to:

log À logð1À aÞT 2

¼ log R

DE 1À 2RT

E

À 0

:

434 E RT

(5)

ARTICLE IN PRESS

Fig. 7. Glass transition temperature, T g (K ) as a function of weight fraction of PS.

Fig. 8. TGA thermograms of weight loss as a function of temperature for PS/PVAc

blends.

Table 2

Weight loss (%) at different decomposition temperatures for different polymer

blends of various ratios of PS/PVAC

Polymer blends PS/PVAc Temperature (1C) at different weight loss

10% 20% 30% 40% 50% 60% 70%

100/0 278 305 320 330 340 352 360

25/75 270 282 290 295 300 305 320

50/50 273 287 295 307 314 328 340

75/25 278 278 286 295 307 310 330

0/100 270 282 290 296 303 308 316

I.S. Elashmawi et al. / Physica B 403 (2008) 3547–3552 3551



From this method by plotting the dependence of Àlog

[Àlog(1Àa)/T 2] versus 1000/T for each sample, we obtain straight

lines as shown in Fig. 9. The apparent activation energies are

calculated from the slopes of the lines using the expression:

E ¼ 2:303RÂ slope (6)

The values of the apparent activation energy, E , of the blends are

listed in Table 1. From this table, it clears that, the values of the

activation energy increases with the increase of PS content.

4. Conclusions

In this study, we prepared PS/PVAc blends by casting method.

The results data suggest that homogeneous polymer composites

are formed over all the blend compositions. Various types of the

bands for the two polymers and its blends were assigned in FT-IR

spectra. XRD scans show a broad and diffuse peak, which indicates

the amorphous nature of the blends. A single glass transition

temperature for each blend was observed, its value increased

upon increasing the content of PS, this finding suggests that this

blend system is only miscible. From UV/VIS data, the position of

the edge was slightly shifted towards higher wavelength side

suggests the miscibility of the blend. The optical gap decreases

with increasing the content of PS from 5 to 4.11 eV. It is clear that

from TGA studies, the initial weight loss occurs at 70–90 1C due to

the moisture evaporation and it was stable up to 1201C, the major

weight losses were observed in the range of 270–360 1C due to the

structural decomposition of the polymers. The values of the

apparent activation energy, E , increases with increase PS content

from 69.01 to 129.2 J/mol.

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ARTICLE IN PRESS

Fig. 9. Àlog [Àlog(1Àa)/T 2] against reciprocal absolute temperature of PS/PVAc

blends.


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Elashmawi et al, Physica B 403 (2008) 3547– 3552