Intermolecular interactions in blends of poly(vinyl alcohol) with poly(acrylic acid)—1. FTIR and DSC studies

Eur. Polym. J. Vol. 28, No. 11, pp. 1365-1371, 1992 Printed in Great Britain. All rights reserved

0014-3057/92 $5.00+0.00 Copyright © 1992 Pergamon Press Ltd

INTERMOLECULAR INTERACTIONS IN BLENDS OF POLY(VINYL ALCOHOL) WITH POLY(ACRYLIC ACID)--1. FTIR AND DSC STUDIES L. DANILIUC,C. DE KESELand C. DAVID ) Universit6 Libre de Bruxelles,Facult6 des Sciences,Campus Plaine,CP 206/I, Boulevard du Triomphe, 1050 Brussels, Belgium

(Received 16 March 1992) Abstract--The i.r. spectra and the melting behaviour of blends of poly(vinyl alcohol) (PVAI) and poly(acrylic acid) (PAA) have been studied as a function of blend composition. Replacement of H-bonding interactions in pure PAA and PVAI by intermolecular interactions has been proved by analysis of the OH and C----K)stretching region in the i.r. spectra. The efficiencyof these interrnolecular interactions results in a rapid decrease of the crystallinity of PVAI when the PAA content increases and its complete supression when the PAA content equals or exceeds 50 wt%. A very high negative value has been obtained for the interaction parameter by measurement of the melting point depression.

INTRODUCTION It is well known that the miscibility of polymers is promoted by intermolecular interactions such as ion-ion, ion-dipole, dipole-dipole, donor-acceptor and hydrogen-bonding interactions. They bring about not only homogeneous mixing but often unique physical properties. In this respect, H-bonding between a proton donor and a proton acceptor has been used to achieve miscibility in many cases [I-6]. The thermodynamic models have to be adapted to include specific interactions. Indeed, the Flory-Huggins model in its first version is only valid for Van der Waals or dispersion interactions. The interaction parameter X is then composition independent. Dependence of X on composition appears in the generalized Flory-Huggins model if Van der Waals and specific interactions are both involved. For systems involving H-bonding, Coleman et al. [7, 8] have recently developed a simple associated model. The free energy of mixing AG M can be expressed by

AG u

ckl

~

AGa

RT = ~ l n ~ l + _ , l n q ~ 2 + ~ b t ~ 2 Z 1 2 + - - ~

(1)

where the first two terms correspond to the entropy of mixing, the third to the Van der Waals interactions of the first Flory-Huggins model and the fourth to intermolecular H-bonding. This fourth term has been expressed as a function of the equilibrium constants for H-bonding and applied with success to the prediction of phase diagrams [7, 8]. If one or both components of the blend are crystallizable, specific interactions can induce miscibility in the amorphous phase and bring a lowering of the melting point of the crystalline phases. This effect opens a wide area of interesting morphologies combining partial miscibility induced by spinodal or *To whom all correspondence should be addressed.

binodal phase separation and crystallization of one or both polymers. Various hydrogen-bonded polymer blends have been studied by i.r. spectroscopy. They include polyurethane/poly(ethylene oxide-co-propylene oxide) [1], poly(ethylene-co-methacrylic acid)/poly(ethylene oxide-co-propylene oxide) [2], poly(ethylene-comethacrylic acid)/poly(2-vinylpyridine) [3], poly(vinyl phenol)/poly(vinylmethyl ketone) [4], poly(¢-caprolactone)/poly(2-hydroxypropyl ether of bisphenol-A) [5], poly(vinylpyrolidone)/poly(vinyl alcohol) [6]. These systems have two common characteristics: ----only one component of the blend is self-associated, --the functional groups involved in H-bonding interactions are different in each component. This feature simplifies the interpretation of the i.r. spectra and allows an easier assignment to the frequency shifts resulting from H-bonding interactions since the characteristic bands of donor and acceptor polymers do not overlap. The present paper concerns blends of poly(vinyl alcohol) (PVA1) and poly(acrylic acid (PAA). Both components are self-associated and contain OH groups which absorb in neighbouring frequency ranges. Each component can thus play the role of donor and acceptor. Furthemore, one of them, PVA1, is crystallizable. Infrared spectroscopy, DSC and SEM have been used to investigate intermolecular interactions. Special attention has been paid to the frequency shift of the OH and C----"Ogroups, and to the lowering of the melting temperature. These blends offer interesting possibilites. Both components are hydrophilic water-soluble polymers which can be easily transformed into insoluble swellable networks. Indeed, intermolecular ester groups are formed by a moderate thermal treatment. Such aliphatic ester linkages are susceptible to microbiological degradation in the environment with subsequent solubilization of both components. PVAI has

1365

L. DA~ILIuc et al.

1366

often been reported to be biodegradable [9] while PAA is less documented. The biodegradation of these polymers is currently being investigated in our laboratory.

PVAI and PAA solutions were then mixed at room temperature in the desired proportions. Weight % of PAA ranged from 10 to 90. After stirring for two days, films were cast from solutions of PAA/PVA1 mixtures on polystyrene plates. They were dried in air for I day at room temperature and then at 60 ° for 2 days. PAA and PVA1 homopolymer films were obtained in the same way. For FTIR measurements, thin films of homopolymers and mixtures were cast on AgCI windows at room temperature. After most of the solvent had evaporated, the films were transferred to a vacuum oven at 60° for 30hr to remove residual solvent. All the blend compositions given in the tables, texts and figures are expressed in weight percent PAA unless otherwise stated.

EXPERIMENTAL PROCEDURES

Materials Molecular weight (MW), glass transition temperature (Ts) and melting temperature (Tm) of the polymers used are listed below. PAA was purchased from Aldrich Chemical and PVAI was received from Hoechst. PVAI was obtained by hydrolysis of poly(vinyl acetate); the degree of hydrolysis is 98 tool%. Polymer Poly(acrylic acid) (PAA) Poly(vinylaleohol)(PVAI)

MW 250,000 48,000

T~(°C) 108.7 82.9

Apparatus and techniques

Tm(°C) -219.6

Different scanning calorimetry (DSC) measurements were performed on ca 10 mg samples with a Perkin-Elmer DSC7, in a N 2 atmosphere with heating rate or 20°/min. The instrument was calibrated with an Indium standard. The thermal properties of the blends and homopolymers were analysed in one scan. The heating scan was carried out from 20 to 250 °. The calorimetric melting temperature (7"=) and the heat of fusion (AHf) of each sample were determined from the maximum and the area of the melting peak, respectively.

Sample preparation Individual solutions of the homopolymers in water were prepared (10 wt% for DSC and 0.9 wt% for FTIR measurements). Care was taken to completely dissolve PVA1 in water by stirring at 90° for 30 min. Polymer solutions were further stirred continuously at room temperature for 24 hr.

Table 1. Absorption maxima of the OH stretching frequencyof H-bonded alcohols and acids Iree secondary alcohol e.lo

alco~ single H-bona ( Intramokcular )1o amomhous PV~Js~'~e H-bo~ alcohol single H- bond ( Intemtolecular)1o Experimental 'Vmx of PVAI OH

I

A

1,

3600

s 3500

3400

3300

akx)hol poiymedc H-bortds 1o [ crystallinePVAI multiple H-bondse

t 3200

, 3100

3000

Experimental 'Vml x of PAA

free OH acidI°

cyclic Oimer1°

I B 3600

3500

3400

I

I

I

3300

3200

3100

3000

~ a c l d OH bondedto alcohol OH10

~

I 360O

I 3500

.--- alooholOH b~-,~,~lto acid C=Oe

fI 3400

I 3300

.¢. A, Alcohol OH stretchingfrequency. B, Acid OH stretching frequency. C, Alcohol-acid interactions: OH stretching frequency.

3200

I

I

3100

3000

Intermolecular interactions in blends of PVAI and PAA Infrared spectra were recorded on a FTIR Bruker IFS-45 spectrophotometer. At least 64 scane were signal averaged at a resolution of 2 em- ~. SEM measurements were performed with a Jeol JSM-840 scanning electron microscope. Samples were prepared by freeze fracture in liquid N2 of films cast from solution and covered with a thin layer of gold.

1367

3250 //

3392

t

"~\

\

RESULTS AND DISCUSSION

I. Infrared spectroscopy Infrared spectroscopy has been used in many cases to identify interactions in polymer blends. Three absorption bands have been considered in the present work, viz. the OH stretching near 3500-3000cm -l, the C-----O stretching near 1700 cm -~ and the C---O stretching at 1141 cm -~. Since several comprehensive studies have been concerned with the changes induced by blending on the C-----Ostretching, we shall pay more attention to the OH stretching which has seldom been studied. The modifications of the C----O absorption will be qualitatively analysed and shown to support the interpretation resulting from analysis of the 3500-3000 cm -~ region. 1.1. Hydroxyl stretching. Two parameters can be used to identify interaction, viz. the position of the maximum and the shape of the absorption band. A summary of the types of H-bonding involving the OH groups of PVA1, PAA and model acids and alcohols

l

4000

3500

3000

2500

2000

cm-1 Fig. 2. Infrared spectrum of a 50/50 blend of PVA1 and PAA ( ) and of the superposition of the same amounts of the components ( - - - ) .

are given in Table 1, with the corresponding absorption ranges given in the literature. The OH stretching region of the pure polymers will be first considered. The spectrum of self-associated PVA1 is given in Fig. 1. It consists of a rather broad band (Vmax=3344cm-~). Pure PVA1 is partly crystalline but contributions of the amorphous and crystalline regions are not resolved. Therefore, we propose that this unique broad band results from the superposition of two types of interactions, viz. multiple polymeric H-bonds characteristic of the crystalline phase (viaX= 3260 cm -~) and single Hbonds associated with the amorphous phase (Vm==3500cm - l ) [6]. Free OH groups are not detectable. The absorption corresponding to the OH stretching in PAA is a very broad band with Vr~x at 3126cm -1, extending up to 3700cm -~ in the high frequency range and showing secondary absorption at 2674cm -t (Fig. 1). Aliphatic acids usually form very stable intermolecular cyclic dimers of the type [10, 1 l]:

//o-- .--o\

/

--C\o__.__

i 4000

3500

3000

2500

2000

cm-1

Fig. 1. OH stretching of PAA, PVAI and their blends (relative weight). EPJ 28/II--D

absorbing near 3000cm -1. Since the absorption maximum of PAA is displaced to higher frequency, it can be assumed that a broad range of cyclic dimer interactions but also non-cyclic single and multiple H-bonds are present, displacing the maximum to higher frequency and responsible for the width of the band. The spectra of the blends will now be discussed. Figure 2 gives the OH absorption of a 50% PAA blend (Vm~=3392cm -1) and the spectrum of the superposition of equal weights of the components (Vma, = 3250cm-1). This figure clearly shows that blending strongly modifies the OH stretching of PAA. The same conclusion is obtained whatever the composition. The modifications of the OH stretching introduced by blending will be analysed. The i.r. spectra of

L. DANILIUCet al.

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Table 2. OH stretchingfrequencyVm~,meltingpoint Tin,meltingpoint depressionATm,heat of fusion AHr, calculatedcrystallinityratio for PAA/PVAIblends PAA/PVAI Vmx~(OH) AHf Crystallinity wt.% (cm-I) Tm(°C) ATm(°C) J/g PVAI ratio*(%) 0/100 3344 219.6 0 72.1 47 10/90 3336 211.8 7.8 53.5 39 20/80 3338 207.7 11.9 42.2 35 30/70 3347 199.1 20.5 29.1 27 40/60 3368 181.2 38.4 21.2 23 50/50 3392 167.6 52 I0.0 13 60/40 3343 . . . . 62/38 (equi) 3248 . . . . 70/30 3141 . . . . 80/20 3110 . . . . 90/10 3109 . . . . 100/0 3126 . . . . *Correspondingto PVAI fraction. various blends are given in Fig. 1. Table 2 and Fig. 3 give the position of the maximum as a function of composition. The spectrum is dominated by the absorption of PVAI between 0 and 50% PAA and by the absorption of PAA between 70 and 100% PAA. Both components progressively shift and broaden when the composition tends to the equimolar ratio (62% PAA) where the broadest absorption is observed. This effect gives strongly overlapping unresolvable absorptions between 50 and 70% PAA. Comprehensive analysis of the shift and broadening of the individual bands as a function of composition is thus not possible. Semi-quantitative interpretation of the general features of the spectra can nevertheless be obtained using the information given in the literature and summarized in Table 1 for the various types of hydrogen-bonding. Those involving pure alcohols and acids have been used in the preceeding paragraph to interpret the spectra of PVAI and PAA. For the analysis of the spectra of the blends, we shall use as reference the characteristic contributions of the spectra of the pure polymers (Table 1), amorphous PVAI, crystalline PVAI and cyclic dimeric structures of acids. The rule giving the following sequence for the displacement of Vm~ as a function of H-bonding interactions will also be applied [10, 11]: Vm~free > Vm~single H-bonding > Vma~multiple

H-bonding

According to this, the following changes of interaction justify the observed shifts of the absorption maxima as a function of the composition of the blends. 3400

'e

J

\\\

Q L

3300

II

,

~

>E 3200

3100 0

I 20

I 40

I 60

80

100

PAA weight (%) Fig. 3. Vm~(OH stretching) as a function of composition for PAA/PVAI blends.

Between 0 and 20% PAA, the single H-bonds of the amorphous PVA1 phase (Vm~= 3500cm -~) are progressively replaced by intermolecular bonds with acid functions (3440-3300 cm -1). As PAA increases up to 50%, intermolecular interactions between PVAI and PAA progressively inhibit phase separation and crystallization of PVA1. Less than 10% of the PVAI phase still crystallizes in blends containing 50% PVAI as demonstrated by the decay of the absorption situation at 1141 cm -~ (Section 1.3) and of the heat of fusion of PVA1 (Section 2). As a consequence, the 3260cm -~ contribution of crystalline PVA1 is also replaced by bonds characteristic of alcohol-acid interactions (3440-3300 cm-~). These complex modifications result in a broadening of the PVAI band and a shift of Vm~ to lower frequency between 0 and 20% PAA and to higher frequency between 20 and 50% PAA (Figs 1 and 3; Table 1). In the PAA rich side of blends composition, similar complex changes occur. The weak self-associated structures of PAA absorbing in the highest frequency range are first replaced by intermolecular interactions with PVA absorbing at 3440-3300 cm -1. The more stable associated structures including cyclic dimers are then replaced by H-bonding with PVAI when PAA falls below 80%. These changes give the broadening and displacements of the spectra reported in Figs 1 and 3 and Table 2. Between 70 and 50% PAA, in the neighbourhood of the equimolar ratio (62% PAA), the spectrum of the blend is very broad. 1.2 Carbonyl stretching. The bands corresponding to the C----O stretching of PAA will now be considered. The spectra of pure PAA and of blends are given in Fig. 4. In pure PAA, two maxima are observed at 1710 and 1736cm -~. They respectively correspond to the intermolecular cyclic dimers and to free C----O (not involved in H-bonding). As PVA1 content increases in the blend, spectral changes are observed. The maximum at 1710 cm -l shifts to higher frequency revealing that the intermolecular cyclic dimers of PAA are progressively replaced by interactions with the alcohol. Simultaneously, the relative importance of free C----O increases and the bands at 1710 and 1736cm -~ lose resolution indicating the formation of new intermolecular interactions between PVAI and PAA. These spectral modifications support the interpretation of the complex OH band given in the preceeding paragraph. 1.3. Crystallinity related C = O stretching. The peak situation at l l 4 1 c m -~ is related to the

Intermolecular interactions in blends of PVA1 and PAA

1369

EQUI

I 1775

1850

.........

--+ 1700

1625

1550

era-1

Fig. 4. C=O stretching band of PAA, 80/20, equimolar and 30/70 blends. crystallinity of PVA1 [6]. Its position does not change with the composition of the blend but its intensity decreases with increasing PAA content (Fig. 5) and becomes negligible for 40 wt% of PAA. The strong interactions prevailing in binary blends of PVA1 and PAA inhibit crystallization of PVA1.

2. Melting point depression An expression for the melting point depression of the crystalline component of two polymers compatible in the liquid phase was established many years ago with the condition that the chemical potentials of the crystalline component in the crystalline and liquid phases should be identical at the melting point Tm of the mixture [12-16] 1

1

RV2u

Tm

T°

AH2u VI.

× L[In ( 1~2- l+' ~kX2 ( l x 2 Xl J

--~2)+ XI2(1 -- ~2) 2] (2)

where the subscript I refers to the amorphous component and the subscript 2 to the crystallizable component. Vu is the molar volume of the repeating unit; x is the number of segments per molecule; AH2u is the enthalpy of fusion of 100% crystalline polymer per mole of repeating unit; T°mis the melting point of the pure component; ~ is the volume fraction, X~2is the polymer interaction parameter which is independent of composition in the first version of the Flory-Huggins thermodynamic treatment of polymer mixtures. When both x, and x: are very large, equation (2) reduces to [16]: 1 1 RV2u - Z12(1 - ~b2)2. (3) Tm T ° AH2u Vlu Equation (3) is often written in the form ATm= Tin_0 Tm = - T ° ( ~ ) B c ~

(4)

where B, the interaction energy density, is related to XJ2 by:

8

RT (ZI:'~ =

"iv,.)

B is obtained by plotting ATm vs ~b~.

(5)

Specific interactions in hydrogen-bonded polymer blends have been investigated by Coleman et al. by developing the fourth term of equation (1) as a function of the volume fraction of the associated species and the equilibrium constants for association measured by i.r. spectroscopy [1-3, 7, 8]. This treatment has been presently limited to polymer pairs where only one component is the donor in H-bonding interactions. It has recently been extended to melting point depression in the same systems [17]. Owing to the lack of theoretical treatment for melting point depression in the present system where both components are H-donors, the present DSC results will be interpreted using equation (3) to evaluate the parameter considered as the sum of the dispersion, dipolar and H-bonding enthalpic interactions. The experimental melting points and AHf corresponding to PVAI, PAA and their blends are given in Table 2 and Fig. 6. These data show an important depression of the melting point of PVA1 when PAA increases. A simultaneous decrease of the heat of fusion of PVA1 (in J/g PVA1) indicates that the crystallinity of PVA1 decreases with increasing PAA and is completely suppressed when PAA exceeds 50 wt%, in qualitative agreement with the i.r. data given in Fig. 5. The melting point results are plotted according to equation (4) in Fig. 7. A small intercept (3.3 °) is observed. It can be assigned to residual entropic effects. The slope of the line calculated by linear regression gives for B a value of - 19.9 cal/cm 3 (using Vlu = 61.0 cm3/mol and T,, ° = 219.6°). Values of 1.6 kcal/mol and 32.7 cm3/mol have been reported in the literature for AH2u and V2o corresponding to the 100% crystalline PVA1 [6]. This gives for l a value of - 1.24. This high negative value is characteristic of very strong intermolecular interactions and can be compared with other values for H-bonded blends (Table 3). Various authors have given experimental evidence that the value of the X parameter depends on composition when specific interactions are responsible for compatibility. Such a composition dependence of X has however not been detected here although it has been demonstrated in the previous section that the nature of intermolecular interactions depends on composition. It must be remembered that the

1370

L. DANILIUC et al. 6050 -

~" 40 o

bE 30 20

10 0

I 0.1

I 0.2

I 0.3

2 (I)1

Fig. 7. Melting point depression of PAA/PVA1 blends as a function of volume fraction of PAA. results reported in this work. Phase separation could also not be detected by SEM. 4. The structure o f the blend

1250

1 2 0 0 1150

1100

1050

1000

950

Wavenumbers (cm-1)

Fig. 5. Absorption at 1141 cm -~ as a function of blend composition. composition dependent Z parameter is usually displayed using the Hoffman-Weeks extrapolation to measure the equilibrium melting points [21]. Such an accurate experimental method could not be used here since successive thermal treatments induce crosslinking. This change modifies the crystallization parameters and introduces more errors than using experimental melting points obtained at the same heating rate. 3. Optical and scanning electron microscopy The cast films are transparent over the whole composition range. Owing to the uncertainty in the value of the refractive index of PVAI [22], this observation is not by itself proof of miscibility but supports this hypothesis when related to the other 22o

P

200-

t'E 180

--

160 0

I t0

I 20

I 30

I 40

I 50

P A A w e i g h t (%)

Fig. 6. Melting temperature of PAA/PVAI blends as a function of composition.

The shift of the i.r. absorptions, the melting point depression and the large negative value found for B indicate strong intermolecular interactions between PVA1 and PAA. These interactions could favour miscibility mainly in the range of compositions near the equimolar region. Figure 2 deafly shows that, when H-bonding interactions of pure PAA and PVAI are replaced by interpolymer H-bonding in the blend, the OH absorption band shifts to higher frequency indicating weaker H-bonds in the blend than in the individual components. Such a shift to higher frequency has already been reported [4-6] and discussed recently by Painter et al. [23]. Two situations can justify miscibility in systems where intermolecular bonds are less stable than the self-association interactions in the individual components. If the initial number of more stable H-bonds is replaced in the blend by a larger number of intermolecular bonds of slightly lower stability, the resulting enthaipy of mixing AHm could be negative. In the case of a positive AHm, mixing can occur because of favourable non-combinatorial entropic factors usually ignored in simple theories of mixing. The large negative value obtained for B and X by measuring the melting point depression needs further comment. Indeed, as pointed in Section 2, determination of B using equation (5) implies that blending is performed not far from crystallization-melting equilibrium conditions. In the present work, the blends have been obtained in quite different conditions, by slow evaporation of solutions at room temperature. Although B values have often been obtained for films prepared from solutions, they can only be considered as an order of magnitude revealing strong intermolecular interactions between the components. Such strong interactions are still more Table 3. Interaction energy density and parameter for various binary blends with specificinteractions Blend B(cal/cm3) Xl: PVF2/PMMA [18] -2.1 -0.2 PVF2/PEMA [13] -2.8 -0.34 PS/PPO [14] - 10.4 -0.17 PCL/Saran [19] - 12.2~ - 5.0 - 1.06--, -0.55 Cellulose/PeAl[20] -18 ---,-10 -1.9 --, -I.1 PVP/PVAI [6] -7.6 -0.69

Intermolecular interactions in blends of PVA1 and PAA directly obvious in blends of PVAI and poly (methacrylic acid). Indeed, association of these polymers gives stable complexes which are well known to precipitate upon mixing solutions of the two components. Blends of P A A with PVAI prepared by evaporation of aqueous solutions have recently been studied by high resolution solid state ~3C-NMR [24]. This method allows determination of the scale of miscibility of the blend, a property which is not accessible with the methods used in the present work. PVA1 and P A A blends of composition 1/2 and 2/1 in monomer units were shown to be homogeneous on a scale of 2 0 - 3 0 n m but heterogeneous on a smaller scale. Blends of equimolecular composition in PVAI and P A A units are homogeneous on a scale of 2-3 nm and the crystallinity is completely destroyed. The observed changes in the N M R spectra of the blend are assigned, in agreement with the present i.r. measurements, to the replacement of H-bonding in P A A and PVAI, by intermolecular H-bonding between the components.

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