Miscibility Behavior and Specific Interaction of
Phenolic Resin with Poly(acetoxystyrene) Blends
Shiao Wei Kuo, Feng Chih Chang*Institute of Applied Chemistry, National Chiao Tung University, Hsin Chu, Taiwan, Republic of China Fax: 886-3-5723764; E-mail: [email protected]
Keywords:blends; FTIR; miscibility; polystyrene; resins;
Introduction
The miscibility of polymer blends has attracted signifi-cant attention in polymer science. For nonpolar polymer blends, the miscibility can be roughly estimated using the Flory–Huggins polymer solution theory presented in Equation (1): DGN RT ¼ U1 N1 ln U1þ U2 N2 ln U2þ U1U2v12 ð1Þ
where U and N denote the volume fraction and the num-ber of segments, respectively, v represents the so-called Flory–Huggins interaction parameter calculated by Hil-derbrand’s solubility parameter, and subscripts 1 and 2 refer to the blend compounds. In the case of high-molecu-lar-weight polymers, N1, N2 S 1, and consequently, the
first two entropy terms become vanishingly small and the miscibility becomes increasingly dependent on the nature of the contribution of the enthalpic term. To enhance the
formation of a one-phase miscible system in polymer blends, it is necessary to ensure that favorable specific intermolecular interaction exists between the two base components of the blend. Ideally, one polymer possesses donor sites and the other possesses acceptor sites on the chain. The most commonly observed interactions are the general acid–base type, i. e., hydrogen bonding,[1 – 4]
ion-dipole, or charge-transfer interaction.
Painter and Coleman[5]suggested adding an additional
term to account for the free energy of the hydrogen bond-ing formation into the simple Flory–Huggins expression for the free energy of mixing of two polymers, as in Equation (2): DGN RT ¼ U1 N1 ln U1þ U2 N2 ln U2þ U1U2v12þ DGH RT ð2Þ
Here DGHdenotes the free-energy change contributed
by the hydrogen bonding between two components, Full Paper:The miscibility behavior and specific
interac-tion of phenolic resin with poly(acetoxystyrene) (PAS) blends were examined using differential scanning calori-metry (DSC), fourier-transform infrared (FT-IR) spectro-scopy and solid-state NMR. This phenolic/PAS blend is fully miscible, as indicated by a single glass-transition temperature, due to the formation of inter-hydrogen bond-ing between the hydroxyl group of the phenolic resin and the carbonyl group of PAS. The DSC study indicates that this phase-separation exothermic peak area is closely related to the interaction between the components. Furthermore,13C solid-state NMR and FT-IR
spectrosco-pies were used to study the extent of specific interaction with various compositions and degrees of inter- and intra-molecular hydrogen bonding. Moreover, the inter-associa-tion equilibrium constant and the related enthalpy of this phenolic/PAS blend were determined and used to predict the free energy and fraction of the hydrogen bonding according to the Painter–Coleman association model.
Macromol. Chem. Phys. 2002, 203, No. 5/6 iWILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1022-1352/2002/0504–0868$17.50+.50/0
The fraction of free carbonyl groups versus temperature for the phenolic/PAS = 50 : 50 blend.
which can be estimated from FT-IR spectroscopy. This equation neglects the change in free volume and other complications.[6]
Phenolic resin contains a high density of hydroxyl groups that can interact with numerous other polymers through hydrogen bonding. Hydrogen bonding serves as a physical crosslink in phenolic resin and increases the glass-transition temperature (Tg). The Novolac-type
phe-nolic resin possesses a higher Tgthan other materials with
a similar molecular weight because of the high density of its hydrogen bonds. When phenolic resin is blended with poly(acetoxystyrene) (PAS), the resultant Tg is
signifi-cantly lower than the value predicted with the Fox rule (see later), indicating that the blend system must involve some special interaction. This work investigates the inter-action between phenolic resin and PAS caused by the association of hydroxyl and carbonyl groups.
The carbonyl stretching vibration has been proven to be an excellent indicator of molecular interaction for a number of polymers.[7 – 13]The repeated PAS unit inhibits
the self-association of the phenolic hydroxy groups that causes the Tgof the phenolic/PAS blends to be lower than
predicted.
This work employs the association parameter of the Painter–Coleman association model (PCAM) to investi-gate the thermodynamic properties of the phenolic/PAS blends and predict the free energy and fraction of hydro-gen bonding.
Experimental Part
Materials
The phenolic resin was synthesized with sulfuric acid via a condensation reaction producing average molecular weights M—n = 500 and M
—
w = 1 200. The chemical structure of the
Novolac-type phenolic resin was determined from the solu-tion 13C NMR spectrum, and found to contain 0.15 wt.-%
free phenol, consisting of phenol rings bridge-linked ran-domly by methylene groups with 19% ortho–ortho, 57% ortho–para, and 24% para–para methylene bridges.[14,15]
The phenolic resin does not contain any reactive methylol group which is capable of causing crosslinking on heating.
PAS was obtained through radical polymerization of the p-acetoxystyrene monomer using 2,29-azoisobutyronitrile (AIBN) initiator (1 wt.-% based on the monomer) at 60 8C under a nitrogen atmosphere. The product was dissolved in benzene, and the solution was poured into the vigorously stirred methanol to precipitate the polymer. The polymer was then characterized using FT-IR spectroscopy, differen-tial scanning calorimetry (DSC), and gel permeation chroma-tography (GPC). The synthesized PAS is characterized by Tg
at 122 8C, M—n = 21 500 and M
—
w 35 000. The chemical
struc-tures of phenolic resin and PAS and their atom-numbering schemes are illustrated in Figure 1.
Blend Preparation
Blends of phenolic/PAS with various compositions were pre-pared by solution blending. Tetrahydrofuran solution con-taining 5 wt.-% polymer mixture was stirred for 6–8 h, and the solution was allowed to evaporate slowly at room tem-perature for 1 d. The film of the blend was then dried at 50 8C for 2 d.
Characterizations DSC
Tgs of blend films were determined by using a DSC from
Du-Pont (DSC-9000) at scan rate of 20 8C N min– 1and a
tem-perature range of 30–125 8C. The measurement was taken using a 5–10-mg sample on a DSC sample cell after the sam-ple was quickly cooled to –50 8C from the melt of the first scan. The Tgoccurs at the midpoint of the heat capacity
tran-sition between the upper and lower points of deviation from the extrapolated liquid and glass lines.
FT-IR Spectroscopy
FT-IR spectra were measured using a Nicolet Avatar 320 FT-IR spectrophotometer and 32 scans were collected with a spectral resolution of 1 cm– 1. Samples containing hydroxyl
groups are water sensitive, so a pure nitrogen flow was used to purge the IR optical box to prevent the sample film from being exposed to moisture. IR spectra recorded at elevated temperatures were obtained using a cell mounted inside the temperature-controlled compartment of the spectrometer. For the solution sample, an adequately sealed cell with NaCl windows and 0.05-mm sample thickness was used. A single optical path was used to study the inter-association equili-brium constant between model compounds of 2,4-dimethyl-phenol and p-tolyl acetate. All model compound solutions in the absorption range obey the Beer–Lambert law. Cyclohex-ane was selected as the solvent because the specific confor-mation of the cyclohexane is favorable in this study.
Solid-State NMR Spectroscopy
High-resolution solid-state13C NMR experiments were
car-ried out on a Bruker DSX-400 Spectrometer operating at resonance frequencies of 399.53 and 100.47 MHz for1H and 13C, respectively. The13C CP/magic-angle sample spinning
(MAS) spectra were measured with a 3.9-ls 90 8 pulse, a 3-s pulse delay time, an acquisition time of 30 ms and 2 048
Figure 1. Chemical structures of the phenolic polymer (left) and PAS (right) and their atom-numbering schemes.
scans. All NMR spectra were taken at 300 K using broad-band proton decoupling and a normal cross-polarization pulse sequence and a MAS rate of 5.4 kHz was used to avoid absorption overlapping.
Results and Discussion
Thermal Analyses
In a polymer blend, a single, composition-dependent glass transition indicates full miscibility with a dimension in the order of 20–40 nm. Single Tgbehavior represents
macroscopic evidence of the full miscibility of the blend. The DSC analyses of Figure 2 reveal the monotonic Tg
behavior from all compositions as would be expected for a miscible blend; Tgs of pure phenolic resin and pure PAS
occur at 66.3 8C and 122.3 8C, respectively. Various equa-tions have been designed to predict the variation of the Tg
of a random copolymer or miscible blend as a function of composition. The most widely used equation is the Kwei equation:[16]
Tg¼
W1Tg1þ kW2Tg2
W1þ kW2
þ qW1W2 ð3Þ
where W1and W2denote weight fractions of the
composi-tions, Tg1 and Tg2 represent the Tgs of the corresponding
blend components, and k and q are fitting constants. The Kwei equation can be applied to miscible polymer blends with a specific interaction. Figure 3 shows plots of the Tg
of the blend versus its composition for cases where the Gordon–Taylor[17]
and Fox equation[18]
do not fit well from the experimental data. However, the Kwei equation can correlate well with the experimental data. Based on the non-linear least squares “best fit”, k = 1 and q = –245
are obtained. Here q is a parameter corresponding to the strength of hydrogen bonding in the blend, reflecting a balance between the breaking of the self-association and the forming of the inter-association hydrogen bonding. The q value of the blend should depend on the entropy change corresponding to the change in the number of hydrogen bonding interactions. In this study, a negative q of “–245” was obtained, which indicates that the inter-molecular hydrogen bonding is weaker than the intra-molecular ones. The observed reduction in Tg levels in
these phenolic/PAS blends is caused by the partial removal of the self-association of the intra-hydrogen bonding. Consequently, a special interaction must exist between these two base polymers to reduce the phenolic intra-molecular hydrogen bonding from DSC analyses.
FT-IR Analyses
Figure 4 displays the infrared spectra in the region 1 680 cm– 1
to 1 820 cm– 1
for various phenolic/PAS blend compositions measured at 25 8C. The carbonyl-stretching frequency is split into two bands at 1 760 cm– 1 and
1 730 cm– 1, corresponding to the free and the
bonded carbonyl groups, respectively. The hydrogen-bonded fraction of the carbonyl group increases with the phenolic content. The band can be easily decomposed into two Gaussian peaks, with areas corresponding to the hydrogen-bonded carbonyl (1 730 cm– 1
) and free carbo-nyl (1 760 cm– 1). Provided the respective absorption
coef-ficients are known, the relative fractions of free and hydrogen-bonded carbonyl groups can be calculated. The fraction of the hydrogen-bonded carbonyl groups[5]
can be calculated from Equation (4):
Figure 2. The DSC scans of phenolic/PAS blends with differ-ent compositions.
Figure 3. Tgversus composition curves from experimental data and the different fitting equations.
fbC¼O¼ Ab=1:5 Ab=1:5þ Af
ð4Þ where Abdenotes the peak area of the hydrogen-bonded
carbonyl absorption and Afrepresents the peak area of the
free carbonyl absorption. The conversion coefficient 1.5 is the ratio of these two bands, free and hydrogen-bonded carbonyl groups, in an ester group.[5]Table 1 summarizes
the results from curve fitting, indicating that the hydro-gen-bonded fraction of the carbonyl group increases with the increase of the phenolic content.
The hydroxyl-stretching region of the phenolic/PAS blends is examined next. Figure 5 shows infrared spectra for the 2 700 cm– 1to 4 000 cm– 1regions of the pure
phe-nolic polymer, pure PAS, and various phephe-nolic/PAS blends. The pure phenolic polymer exhibits two bands in the hydroxyl-stretching region of the infrared spectrum. A very broad band centered at 3 350 cm– 1is attributed to
the wide distribution of the hydrogen-bonded hydroxyl group while a narrower shoulder band at 3 525 cm– 1 is
caused by the free hydroxyl group.
Figure 5 also indicates that the intensity of free hydro-xyl absorption (3 525 cm– 1) decreases gradually as the
PAS content of the blend is increased from 10 to 90 wt.-%. The band due to the hydrogen-bonded hydroxyl in the phenolic polymer tends to shift into a higher fre-quency with increasing PAS content in the vicinity of 3 384 cm– 1. This change results from the switch from the
hydroxyl–hydroxyl bond to the hydroxyl–carbonyl bond. Therefore, it is reasonable to assign the band at 3 384 cm–1to the hydroxyl group that is bonded to the carbonyl
group. The frequency difference between the free and the hydrogen-bonding hydroxyl has been used to investigate the average strength intermolecular interaction in our pre-vious study.[19 – 22]
In this study, the hydroxyl–carbonyl inter-association (Dm = 139 cm– 1) is weaker than that of
Figure 4. FT-IR spectra for different phenolic/PAS blends recorded at room temperature in the 1 820 cm– 1–1 680 cm– 1 region.
Table 1. Curve-fitting results of the phenolic/PAS blend at 25 8C. Phenolic=PAS
wt:-%
free C2O H-bonded C2O fb
m cmÿ1 W1=2 cmÿ1 Af % m cmÿ1 W1=2 cmÿ1 Af % 90 : 10 1 761.7 18.45 12.70 1 731.2 37.50 87.30 82.10 75 : 25 1 762.6 17.87 18.51 1 732.1 39.13 81.49 74.59 60 : 40 1 763.8 17.56 25.47 1 732.4 38.49 74.53 66.12 50 : 50 1 763.8 18.33 31.06 1 736.8 39.70 68.94 59.68 40 : 60 1 763.1 17.35 37.88 1 738.2 38.92 62.12 52.23 25 : 75 1 760.9 19.60 53.41 1 735.9 39.63 46.59 36.77 10 : 90 1 758.3 21.77 78.74 1 728.6 43.87 21.26 15.26
Figure 5. FT-IR spectra for different phenolic/PAS blends recorded at room temperature in the 4 000 cm– 1–2 700 cm– 1 region.
the self-associated hydroxyl of the phenolic polymer (Dm = 175 cm– 1
) and this result is consistent with the negative q value according to the Kwei equation.
Solid-State NMR Analyses
The phenolic polymer is a proton donor possessing easily accessible hydroxyl groups while PAS is a proton accep-tor containing one carbonyl group on each side chain. Solid-state NMR spectroscopy can provide a tool to clar-ify the phase behavior and morphology of polymer blends involving the hydrogen bond formation. Figure 6 displays the13C CP/MAS spectra of selected phenolic/PAS blends
of various compositions, where the line (a) shows the peak assignments of four major peaks. The hydroxyl sub-stituted carbon atom in the phenolic ring (C1OH) has a resonance peak at 152.1 ppm, while the peaks at 115.8 ppm and 129.4 ppm correspond to the ortho-unsub-stitituted carbon atom and the other carbon atoms in the phenol ring, respectively. Meanwhile, the other resonance at 35 ppm corresponds to the methylene carbon atoms. The PAS spectrum is line (g) in Figure 6, showing eight resonance peaks; Figure 1 presents the corresponding car-bon atoms.
The chemical shift of the carbonyl carbon atom of the PAS increases with the phenolic content (Figure 7). The variation in chemical shift can be interpreted as the extent of the intermolecular specific interaction between the components.[23 – 28]
As illustrated in Figure 7, the carbonyl carbon atom of the PAS component shifts downfield as the phenolic content in the blend increases, implying the existence of intermolecular hydrogen-bonding interac-tion. The bond angle and interchain distance between the nearest neighbor are generally expected to change after a specific interaction is formed. Such a change creates a different chemical environment for the correlated carbon atom, thus changing the magnetic shielding and hence the chemical shift. The observed shift of the carbonyl carbon
atom in this phenolic/PAS = 60 : 40 blend is 0.61 ppm higher than that of the pure PAS, a value which is com-parable to other hydrogen-bonded miscible blends. Fig-ure 7 also reveals a downfield shift of 2. 1 ppm in the phe-nolic/PAS = 60 : 40 blend relative to the pure phenolic polymer, indicating strong intermolecular hydrogen bonding between the phenolic polymer and PAS.
Estimation of Inter-Association Equilibrium Constant (KA)
According to the PCAM equation (Equation (2)), the DGH/RT term is defined as the free-energy contribution
owing to the change in hydrogen bonding upon mixing. A set of equilibrium constants, including self-association, inter-association, and other thermodynamic properties, must be determined to accurately predict the phase dia-gram, miscibility windows, and maps of polymer-blend systems involving specific interactions. The self-associa-tion of phenolic polymer involves the usual hydroxyl– hydroxyl interaction characteristics and requires two equilibrium constants, K2and KB to account for the
for-mation of the hydrogen-bonded dimer and multimer, respectively. The typical interaction scheme that was con-sidered in the PCAM based on the competing equilibrium is described in Equation (5) and (6)
phenolic + phenolic agggsKB, K2 phenolic–phenolic (5) phenolic + PAS agggsKA
phenolic–PAS (6)
The inter-association equilibrium constant KA reflects
the extent of hydrogen bonding between phenolic
poly-Figure 6. 13C CPMAS spectra at room temperature for differ-ent phenolic/PAS blends.
Figure 7. Composition dependence of the chemical shift of carbonyl group (0), the hydroxyl-substituted carbon (f) in the phenolic/PAS blends.
mer and PAS. This study attempts to use PCAM to pre-dict thermodynamic properties of this phenolic/PAS blend where the phenolic polymer contains a high density of hydroxyl groups.
To confirm the credibility of these association param-eters of the phenolic polymer used in PCAM, the PAS has been chosen as an appropriate modifier to blend with the phenolic polymer. The p-tolyl acetate contains the carbonyl group that was selected as the model compound of PAS. The inter-association constant KAof the PCAM
describes the hydrogen-bonding interaction between the phenolic polymer and the PAS unit. The “free” hydroxyl bond absorptions of 0.02 M 2,4-dimethylphenol solution containing various concentrations of p-tolyl acetate in cyclohexane are used herein for quantitative measure-ments. Notably, the FT-IR from various concentrations of p-tolyl acetate solutions used as a background must be prescanned before addition of the 2,4-dimethylphenol. Figure 8 shows hydroxyl absorption of the 2,4-dimethyl-phenol in cyclohexane containing different concentra-tions of p-tolyl acetate, where the intensity of the free hydroxyl absorption at 3 620 cm– 1decreases with
increas-ing p-tolyl acetate concentration. The absolute intensity of the free hydroxyl group at 3 620 cm– 1is assumed to be
a measurement of the amount of free hydroxyl in the mix-ture. Figure 8 displays that the frequency of the asso-ciated hydroxyl band shifts from the free hydroxyl at 3 620 cm– 1 to 3 510 cm– 1 as the concentrations of p-tolyl
acetate increases, because of the formation of inter-asso-ciation hydrogen bonding between 2,4-dimethylphenol and p-tolyl acetate. The method proposed by Coggeshall and Saier,[29] involving the calculation of the
hydrogen-bonding association constant, Ka (in L N mol– 1) is
expressed by the following Equation (7):
Ka¼ 1ÿ f OH m fOH m ðCAÿ ð1 ÿ fmOHÞðCBÞ ð7Þ
Where CAand CB denote the concentrations of p-tolyl
acetate and 2,4-dimethylphenol in mol N L– 1, respectively,
while fOH
m represents the fraction of free hydroxyl of the
2,4-dimethylphenol. Table 2 lists the data on the level of fOH
m for the 2,4-dimethylphenol containing various
con-centrations of p-tolyl acetate and the resulting Ka. The intrinsic inter-association constant Ka of (10.67 L N mol–1)
is obtained by extrapolating the p-tolyl acetate concentra-tion of zero. Ka must be modified into KAby dividing the
molar volume of the phenolic repeated unit (0.083 L N mol– 1 at 25 8C).[30] The inter-association equilibrium
constant, KA, yielded through this procedure is 128.60.
Figure 8 shows that the absorption frequency in the shift from the free hydroxyl (3 620 cm– 1
) to the hydrogen-bonding hydroxyl (3 510 cm– 1) in the mixture of
2,4-dimethylphenol with p-tolyl acetate is concentration inde-pendent. The frequency difference between a free
hydro-xyl and a hydrogen-bonding hydrohydro-xyl group can be roughly estimated from the enthalpy of the association and average strength in the polymer blend. In our pre-vious study, we have proposed an equation to calculate the enthalpy of the association in the phenolic blend as given by the absorption shift, expressed here in Equa-tion (8):[30]
–DH = 2.564 + 0.0122 DmOH(cm– 1) (8)
The enthalpy of the association between 2,4-dimethyl-phenol and p-tolyl acetate was calculated to be –3.88 Kcal N mol– 1. However, the K
Avalue obtained from
model compounds is not exactly the same as that from the true polymer blend due to the intramolecular screen-ing effect and functional group accessibility,[31 – 36]
as well as the chain stiffness and connectivity in miscible poly-mer blends. Therefore, this investigation attempted to approximate another method proposed by Coleman et
Figure 8. FT-IR spectra of the hydroxyl-stretching region of 0.02 m 2,4-dimethylphenol containing various p-tolyl acetate concentrations.
Table 2. fOH
m and Ka of 2,4-dimethylphenol in cyclohexane solution with various p-tolyl acetate concentrations.
Conc. of p-tolyl acetate mol N Lÿ1 Intensity of IR Absorption fOH Inter-associa-tion equili-brium constant (Kamodel) 0 0.0321 1.0000 – 0.08 0.0185 0.5757 10.30203 0.1 0.0179 0.5571 8.7223 0.2 0.0128 0.3983 8.0341 0.25 0.0113 0.3517 7.7767 0.4 0.0097 0.3019 5.9900 0.5 0.0085 0.2645 5.7285
al.[37] to obtain the K
A value, expressed by Equation (9)
and (10) UB¼ UB1 1ÿ K2 KB þ K2 KB 1 ð1 ÿ KBUB1Þ 2 1þ KAU0A r ð9Þ UA¼ U0Aþ KAU0AUB1 1ÿ K2 KB þ K2 KB 1 ð1 ÿ KBUB1Þ ð10Þ Where UAand UB are volume fractions of
non-self-associated species A and self-associating species B, respectively, U0Aand UB1are the corresponding volume
fractions of isolated PAS and phenolic segment, respec-tively, and r is the ratio of molar volume, VA/VB.
Self-association equilibrium constants, KBand K2, describe the
formation of multimers and dimers, respectively. Finally, KAis the equilibrium constant describing the association
of A with B. In addition, KBand K2of the phenolic
poly-mer were set at 52.31 and 23.29, respectively, at 25 8C.[30]
Employing these previously determined values of KBand
K2and a given value for KA, together with the appropriate
value of r, we can calculate and obtain the root (UB1)
numerical for a given value of UBover the whole
compo-sition range. The fraction of hydrogen-bonded carbonyl groups as a function of the volume fraction of phenolic polymer is then simply given by 1–(U0A/UA). According
to Table 1, the value of KAis employed to determine the
best fit of the experimental data for the phenolic/PAS blends at 25 8C using a least-squares method. Figure 9 displays the theoretical curves derived from the best-esti-mated value of KA= 64.6.
The equilibrium constant, enthalpy of the phenolic/ PAS blend, molar volume, molecular weight, and solubil-ity parameters of the phenolic/PAS blend are summarized in Table 3. The observed KA= 128.6 or 64.6 (model
com-pound or polymer blend) is found to be much higher than the K2 = 23.29 from the hydroxyl dimer formation and
the KB = 52.31 from the hydroxyl multimer formation,
implying that the tendency of the phenolic resin to form the hydrogen bonding with PAS dominates over the self-association forming the intra-hydrogen bonding of the phenolic resin in the mixture.
Prediction of Degree of Hydrogen Bonding
Figure 9 shows plots of the experimental data and theore-tical predicted curve as a function of composition at 25 8C, and demonstrates the ability of PCAM to predict the degree of hydrogen bonding on the carbonyl group. Figure 9 shows that the experimental values are generally
Table 3. Summary of the self-association and inter-association parameters of the phenolic/PAS blend. Equilibrium constant K 258C Enthalpy DH kcal N molÿ1 Self-associationa) Dimer formation K2 23.29 –4.246 Multimer formation KB 52.31 –6.110
Inter-association from model compound 128.60 –3.875 Inter-association from polymer blend 64.6 –3.875
Polymer Molar volume mL N molÿ1 Molecular weight g N molÿ1 Solublity parameter ðcal N mLÿ1Þ0:5 Degree of polymerization Phenolica) 84 105 12.05 6 PASb) 128.60 162.20 10.29 132 a) Ref.[30]
b) Estimated using a group contribution method proposed by Coleman et al.[5]
Figure 9. Fraction of the hydrogen-bonded carbonyl group ver-sus composition: (f) FT-IR data, (- - -) theoretical values from model compounds and (—) theoretical values from polymer blend calculated at 25 8C.
lower than the predicted values based on model com-pounds (KA = 128.6) due to two of the more important
chain connectivity effects, intramolecular screening and functional group accessibility. However, the experimental values show excellent agreement with the predicted values from the polymer blend (KA= 64.6) for the
hydro-gen bonded carbonyl at 25 8C. Therefore, the inter-asso-ciation constant KA= 64.4 for the phenolic/PAS blend is
considered to be valid. The inter-association equilibrium constant between polymer blend and model compound should therefore be different after taking into account the intramolecular screening and functional group accessibil-ity effects.
Prediction of Free Energy
The calculation follows the ‘Miscibility Guide and Phase Calculator’ software package.[5]The path to correlate the
hydrogen-bonding equilibrium concentration to the free-energy values has been described by Coleman et al.[5]
Fig-ure 10 illustrates that the predicted free energy (DGm) is
negative for all compositions at 125 8C. DGm reached a
minimum of –3.47 cal N cm– 3when the PAS content was
around 66 wt.-%. The theoretically predicted values were compared with the experimental results at 125 8C because this temperature exceeds the Tgs of pure phenolic polymer
and PAS, and because these DSC thermograms were obtained by quenching from this temperature. It is there-fore reasonable to assume that equilibrium conditions can be attained at this temperature. Although the original Painter–Coleman association model has been modified in recent papers,[6, 31 – 36] we can also roughly consider the
phenolic/PAS blend as being miscible due to negative free energy and the positive second derivative of the weight fraction.
Phase Separation
Figure 2 presents DSC traces from all compositions heated above their Tgs. All blends show an exothermal
peak above their Tgs, which can be assigned to
decom-plexation or phase separation.[38 – 46]The exothermic peak
temperature and its enthalpy depend on the DSC heating rate as shown in Figure 11 for the phenolic/PAS = 50 : 50 blend. Figure 12 displays the effects of heating rate on this exothermic peak temperature and its enthalpy, reveal-ing that both increase with increased heatreveal-ing rate. Extra-polation of the demixing-peak temperature to zero heat-ing rate (thermodynamic equilibrium condition) should produce the corresponding pseudobinodal or “cloud point” in the phase diagram[43]
which is at 129.5 8C (Fig-ure 12). On the other hand, the extrapolation to large heating rates is related to the spinodal temperature[43]
which is at 169.5 8C (Figure 12).
Heat of Demixing
At high heating rates, the plateau value for the area of the exothermal peak can be considered to be the demixing heat for the blend, that is 5.88 cal N g– 1. In a
nonequili-brium state, the blend is forced to transform from a single phase to a two-phase system, and the heat release becomes a measure of the two components’ interaction.
Figure 10. Calculated free energy of mixing against phenolic/ PAS blend composition at 125 8C.
Figure 11. The DSC thermograms of the phenolic/PAS = 50 : 50 blend under different heating rates.
Figure 12. Exothermic peak temperature (f) and its heat enthalpy (0) of the phenolic/PAS = 50 : 50 versus different heat-ing rates.
Indeed, the plateau value of the peak area reflects the nat-ure of the interaction of the components.
If the calculation is repeated at different temperatures for a single mixture (50 : 50) and the classic Gibbs–Helm-holtz expression is employed by Equation (11):
DHM¼ qðDGM=TÞ=qð1=TÞ ð11Þ
Then the enthalpy of mixing (DHM) of a polymer pair
can be calculated over a range of temperatures. Using the free energy of between 140 8C and 180 8C the mixing enthalpy can be calculated. Figure 13 plots DGm/T versus
1/T for the phenolic/PAS = 50 : 50 blend following the expression of Gibbs–Helmholtz. In the temperature range investigated herein, the relationship is linear, and the enthalpy of mixing of –4.08 cal N cm– 3can be determined
by its slope. A reasonable estimate of the specific volume for a 50 : 50 mixture of phenolic resin and PAS is 1.257 cm3N
g– 1
which gives an enthalpy of mixing of –5.12 cal N g– 1. This obtained enthalpy of mixing is close
to the value experimentally obtained by DSC
(–5.88 cal N g– 1) for the phenolic/PAS blend. Figure 14
shows the enthalpy of demixing against blend composi-tion for phenolic/PAS blends based on results from the Gibbs–Helmholtz approach and from the phase separa-tion in DSC. Figure 14 reveals that demixing of phenolic/ PAS blends is exothermic and the trend of enthalpy of demixing determined by DSC is similar to the calculated from the Gibbs–Helmholtz approach based on PCAM. Apparently, the enthalpy of demixing measured by DSC can be accepted as the real enthalpy of demixing for blends. Also shown in Figure 14, the enthalpy of demix-ing from the Gibbs–Helmholtz approach and from DSC results in a minimum value at about 66 wt.-%, exhibiting the same trend as in Figure 10.
Evidence of Phase Separation via FT-IR Analyses Figure 15 illustrates the FT-IR spectra of the phenolic/ PAS = 50 : 50 blend measured at different temperatures in the range 25 8C to 180 8C. The carbonyl-stretching fre-quency splits into two bands at 1 760 cm– 1 and
1 730 cm– 1
, corresponding to the free and hydrogen-bonded carbonyl groups, respectively. The intensity of the hydrogen-bonded carbonyl group decreases with increasing temperature. As a result, the quantity of the DGH/RT term of Equation (2) will decrease with
increas-ing the temperature, and when the measured temperature reaches a critical level, this negative chemical term, DGH/
RT will be unable to overcome the positive physical term, v12U1U2and results in positive free energy of mixing and
phase separation, according to Equation (2). Figure 16 shows the corresponding fraction of free carbonyl groups versus temperature. The fraction of the free carbonyl
Figure 13. DGm/T versus 1/T for the phenolic/PAS = 50 : 50 blend.
Figure 14. Heat of demixing versus blend composition for var-ious phenolic/PAS blends derived from Gibbs–Helmholtz and DSC results.
Figure 15. FT-IR spectra recorded at various temperatures at 1 820 cm– 1
–1 680 cm– 1
group approaches a plateau, an indication that the phase separation is approaching completion.
Conclusions
Phenolic resin and PAS are completely miscible over the entire composition range. The Kwei equation can accu-rately predict Tgs from the experimental results owing to
the existence of hydrogen bonding between the hydroxyl group in the phenolic resin and the carbonyl group in PAS.
The inter-association equilibrium constant (KA) for the
phenolic/PAS blend is higher than the self-association equilibrium constants (K2and KB), implying that the
ten-dency toward hydrogen bonding of the phenolic resin and PAS dominates the self-association (intra-hydrogen bond-ing) of the phenolic resin in the mixture. However, the average strength of the inter-hydrogen bonding is weaker than that of the self-associated hydroxyl groups of the phenolic resin. The relatively weak interaction from the hydrogen bonding causes the observed Tgnegative
devia-tion compared with the Kwei equadevia-tion.
The phase behavior of this miscible polymer blend has been analyzed by DSC and the enthalpy of demixing has been obtained, which agrees with the Gibbs–Helmholtz expression where the free energy is derived from the Pain-ter–Coleman association model. Above the lower critical solution temperature, the phase separation occurs due to gradual decrease of the inter-hydrogen bonding at higher temperatures that can be characterized by FT-IR analyses.
Acknowledgment: The authors would like to thank the National Science Council, Taiwan, Republic of China, for finan-cially supporting this research under Contract Nos. NSC-90-2216-E-009–026.
Received: June 22, 2001 Revised: November 2, 2001 Accepted: November 26, 2001
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