Ind. Eng. Chem. Res. 2004, 43, 1561-1567
1561
Thermal Degradation of Poly(vinyl acetate) and Poly(E-caprolactone) and Their Mixtures in Solution G. Sivalingam and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore-12
The thermal degradation of the poly(-caprolactone) (PCL), poly(vinyl acetate) (PVAC), and their mixtures in solution has been investigated. The thermal degradation of PCL-PVAC mixtures with different PVAC concentrations (0, 35, 50, 70 wt %) was investigated at various temperatures (463-523 K) in an inert solvent (diphenyl ether). The molecular weight distributions of the polymers were determined using gel permeation chromatography (GPC). The experimental results indicated significant increase in the degradation rates of PVAC and a decrease in the degradation rates of PCL. This can be attributed to the proton-accepting nature of PCL and proton-donating nature of PVAC. A model based on continuous distribution kinetics was developed considering the interaction between the polymers through hydrogen abstraction. The degradation rate coefficients of the polymers and their mixtures were determined numerically by fitting the model to the experimental data. Introduction Polymer blends, having poly(-caprolactone) (PCL) as a constituent, have been investigated extensively in recent years.1 PCL shows better miscibility on a molecular scale in physical mixtures with various polymers, such as poly(vinyl chloride),2 poly(styrene-co-acrylonitrile),3 chlorinated polyethylenes,4 poly(vinyl methyl ether),5 and poly(vinyl acetate).6 The polymer mixtures containing carboxyl group-bearing polymers, such as poly(lactic acid) (PLA),7 poly(hydroxybutyrate) (PHB),8 poly(ethylene oxide) (PEO),9 and poly(-caprolactone) (PCL)6-10 with poly(vinyl acetate), showed synergistic enhancement in mechanical properties and significant reduction in the biodegradability of the former polymers. The phenomenon that causes these effects was attributed to the proton-accepting and proton-donating nature of the carboxyl groups of PLA, PHB, PCL, and R-hydrogen of PVAC, respectively.6-10 A mixture of polyethylene and polystyrene shows strong thermal interaction leading to enhanced degradation of a hydrogen-donor polymer (polyethylene) under isothermal holding at high temperatures of 390440 °C.11,12 Addition of polystyrene to phenol novoloc leads to the enhanced degradation rate of phenol novoloc.13 The degradation rate of polystyrene increased 8-fold in the presence of poly(methyl acrylate) and poly(butyl acrylate) at 430 °C, while the degradation rates of the other polymer in the mixture decreased by 8-fold14 due to polymer interaction by hydrogen abstraction. Photodegradation of polystyrene and poly(vinyl acetate) mixtures is mildly influenced by the interaction of radicals from both the polymers, leading to increased degradation of polystyrene.15 The thermal degradation of a mixture of polystyrene and poly(vinyl acetate) in solution showed a significant increase in the degradation rate of poly(vinyl acetate) with a decrease in the degradation rate of polystyrene.16 The degradation rate of polystyrene decreases in the presence of a hydrogen donor (tetralin).17 * To whom the correspondence should be addressed. Tel: 091-80-2932321. Fax: 091-80-3600683. E-mail: Giridhar@ chemeng.iisc.ernet.in.
The type of interaction in a PCL and PVAC mixture is also by a hydrogen-donor and -acceptor mechanism.6 Recently, a strong reduction in the enzymatic degradation of PCL in the physical mixture of 90/10 PCL/PVAC was observed, as compared to the enzymatic degradation of pure PCL, and attributed to the interaction between the polymers.10 Because of better miscibility and interaction due to hydrogen abstraction, this mixture showed synergistic enhancement in the tensile strength, ultimate strength, and percentage elongation at breakage.6 Under pyrolytic conditions with dynamic heating, this mixture behaved ideally over all the composition range, indicating no interaction.6 However, the degradation rates are influenced marginally under isothermal holding at higher temperatures.6 The pyrolytic degradation of polymers normally occurs at high temperatures. In addition, the pyrolytic degradation is associated with the evolution of noxious gaseous products that cause environmental pollution with low yields of desired products and excessive char formation.18 To ameliorate these difficulties, singlephase, contained degradation of polymers in solution has been proposed. In this method, the polymer is dissolved in an inert solvent and thermally degraded. Since the degradation occurs in a single phase, uniform heat transfer would be achieved with better yields.18 Further, polymers in solution degrade faster at lower temperatures with lower activation energies.19 Apart from plastic recycling, solution degradation studies can be applied to investigate the stability20 and decomposition characteristics21 of polymers and their mixtures.16 The degradation of poly(-caprolactone) and poly(vinyl acetate) and their mixtures was investigated in solution (diphenyl ether) at various temperatures (190-250 °C). In the present investigation, a model based on continuous distribution kinetics considering detailed RiceHerzfeld reactions22-24 has been postulated to explain the degradation rates of pure polymers and their mixtures. The interaction parameters for both polymers in the mixture were determined as a function of mixture composition. The activation energies were determined
10.1021/ie034115y CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004
1562 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004
from the temperature dependency of the degradation rate coefficients. Experimental Section Materials. Poly(-caprolactone) (Mn, 80 000; polydispersity, 1.3) and vinyl acetate monomer were purchased from Aldrich Chemicals, U.S.A. Diphenyl ether and tetrahydrofuran (THF) were procured from S. D. Fine chemicals (India). Solvents were distilled and filtered prior to use. Polymer Synthesis and Pretreatment. The vinyl acetate monomer was freed from inhibitor by caustic wash followed by distillation at reduced pressure. The peroxide-based initiator was used for the bulk polymerization of vinyl acetate at 60 °C. Benzoyl peroxide was purified by dissolution in chloroform, followed by precipitation in methanol. To obtain a initial molecular weight distribution distinct from that of PCL, PVAC was fractionally precipitated using a solvent (acetone) and a nonsolvent (hexane). The precipitated polymer was dried at lower temperature over calcium carbonate until constant weight was attained. The number average molecular weight and polydispersity of synthesized PVAC were 15 400 and 1.54, respectively. Degradation Experiments. The thermal degradation of the polymers (PCL, PVAC) and their mixtures in diphenyl ether was studied at various temperatures (190-250 °C) in a three-necked round-bottomed flask. Long vertical condensers ensured the volatiles generated during the operation remained in the reactor. The total concentration of the polymers employed was 2 kg/ m3. The uniformity of the polymer concentration in the reactor was achieved with a magnetic stirrer. The temperature was controlled within ( 1 °C using a PID controller. The reactor was charged with distilled and filtered solvent (diphenyl ether) and heated until it reached the desired reaction temperature. Then preweighed amount of polymers were added to the reactor to start the degradation experiments. The various mass fractions of PVAC in the mixture used in the present study were 30, 50, and 65%. Portions (200 µL) of samples were collected during the reaction at regular intervals and taken for the subsequent analysis. Molecular Weight Determination. The MWD of the samples was determined by gel permeation chromatography (GPC) (Waters, USA) that consisted of an isocratic pump (Waters 501), size- exclusion columns, differential refractrometer (Waters R401) and a data acquisition system. THF was used as eluent at a flow rate of 1 mL/min. The columns (Styragel HR 4, HR 3, and HR 0.5) (300 × 7.5 mm) packed with cross-linked polystyrene-co-divinylbenzene were used in series at 50 °C. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the refractive index was continuously monitored using a differential refractive index detector and stored digitally. The chromatograph was converted to molecular weight distribution using a calibration based on polystyrene standards (Polymer Lab, U.K.). The MW of polystyrene was converted to MW of the individual polymers using the MarkHouwink equation. Further details are available elsewhere.18 The mass of the polymer system was calculated from the area under the gel permeation chromatograph. The calculation indicated that the mass of the reaction system remained constant at all reaction times, confirming that no volatiles were lost from the system. Figure 1 shows the initial molecular weight distribution
Figure 1. Typical molecular weight distribution for the poly(caprolactone) (PCL) and poly(vinyl acetate) (PVAC) mixture and showing the methodology of molecular weight determination of individual polymers. The solid line represents the GPC response for the mixture. The dashed and the dotted line denote the MWD of PCL and PVAC, respectively.
of PCL/PVAC mixtures as obtained from GPC. The first peak corresponds to PCL, and the second peak corresponds to PVAC. The solid line is the GPC response of the mixture, and the dotted lines are the contribution from the individual polymers and are obtained by deconvoluting the mixture peak. Since the MWD for the individual polymers can be obtained by deconvolution, the moments for each polymers can be determined by numerical integration. Theoretical Model The mechanism of polymer degradation is complex and involves several elementary reactions, such as bond fission, hydrogen transfer, and disproportionation. Quantitative modeling of these processes can be done by a Rice-Herzfeld mechanism involving initiation, propagation, and termination reactions. In the case of polymer mixtures, the interaction between the polymers can lead to an alteration in the degradation rate of individual polymers.7-10,16 The present model is similar to the models developed previously.10,16 In these cases,10,16 while both the polymers interact, only one polymer degrades predominantly. This allows the equations to have an analytical solution. However, in this study, both of the polymers undergo degradation, and a numerical solution is used to determine the degradation rates. Mechanism of Degradation of Polymers in Mixtures. The thermal degradation of the pure polymers (PCL, PVAC) and their mixtures occurs by random chain scission and can be assumed to degrade in following four steps (A-D). (A) Initiation and Termination. The initiation is by thermolytic cleavage. The reaction is represented as a reversible reaction with backward reaction, signifying the termination reaction. kfA
PA(x) {\ } RA•(x′) + RA•(x - x′) k tA
kfB
PB(x) {\ } RB•(x′) + RB•(x - x′) k tB
(A) (B)
PA(x), PB(x), and RA•(x), RB•(x) represent the polymers and their radicals, respectively and x is the molecular weight. The subscripts A and B refer to poly(vinyl acetate) (A) and poly(-caprolactone) (B), respectively.
Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1563
kfA and ktA are the rate constants for the forward and backward reactions representing initiation by thermolysis and termination by coupling, respectively. These initiation and termination steps are infrequent compared to the depropagation, and hence, rates of these steps can be neglected. This assumption is known as long chain approximation (LCA) and is commonly used in the polymer degradation.10,16,22-24 (B) Reversible Hydrogen Abstraction. Apart from the radical formation by thermolysis of the polymer, simple hydrogen abstraction can also lead to the generation of the radicals. khA
PA(x) {\ } RA•(x) k
(C)
HA
khA
•
PB(x) {\ } RB (x) k
kh and kH represent the rate coefficients for radical generation by hydrogen abstraction and capping of radicals, respectively. (C) Depropagation. The radicals generated undergo degradation, resulting in the reduction of molecular weight of the polymer. This step is assumed to be irreversible, because the temperature of the reaction is above the ceiling temperature of the polymers. kbA
•
RA (x)98PA(x′) + RA (x - x′)
(E)
kbB
RB•(x)98PB(x′) + RB•(x - x′)
(F)
(D) Interaction between Polymers. Similar to the hydrogen abstraction reactions (C) and (D), hydrogen abstraction can also occur due to the interaction with the other polymer in the mixture. The interaction between the polymers due to hydrogen transfer can be written as16,25,26 kd1
kD2
} Ri•(x + x′) {\ } PB(x) + RA•(x′) (G) RB•(x) + PA(x′) {\ k k D1
∫x∞ rB
(x′, t) Ω(x, x′) dx′ - kd2 pB(x, t)
d2
∫x∞ ri(x′, t) Ω(x, x′) dx′
(2) ∂rA(x, t) ) khA pA(x, t) - kHA rA(x, t) - kbA rA(x, t) + ∂t kbA
∫x∞ rA(x′, t) Ω(x, x′) dx′ - kd2 rA(x, t)∫0∞ pB ∞ (x′, t) dx′ + kD2∫x ri(x′, t) Ω(x, x′) dx′
(3) ∂rB(x, t) ) khB pB(x, t) - kHB rB(x, t) - kbB rB(x, t) + ∂t kbB
∫x∞ rB(x′, t) Ω(x, x′) dx′ - kd1 rB(x, t)∫0∞ pA ∞ (x′, t) dx′ + kD1∫x ri(x′, t) Ω(x, x′) dx′
∂pA(x, t) ) -khA pA(x, t) + kHA rA(x, t) + kbA ∂t
∫0
(x′, t) Ω(x, x′) dx′ - kd1 pA(x, t)
∞
∫x∞ rA
rB(x′,t) dx′ +
∫x∞ ri(x′, t) Ω(x, x′) dx′
kD1
(1)
(4)
When the change in the molecular weights is less than
∂ri(x, t) ) ∂t
∫0x {kd1 pA(x′, t) rB(x - x′) + kd2 pB(x′, t) rA (x - x′)} dx′ - (kD1 + kD2) ri(x′, t) (5)
an order of magnitude, the rate constants can be assumed to be independent of molecular weight.24 The satisfactory fit to the experimental data in this case validates this assumption. Operating moments, defined as p(j)(t) ) ∫∞0 xjp(x, t) dx, in eqs 1-5 yields
r(j) dp(j) A (t) A (j) (j) ) -khAp(j) + k r + k - kd1r(0) A HA A bA B pA + ∂t (j + 1) r(j) i kD1 (6) j+1 r(j) dp(j) B (t) B (j) ) - khBp(j) + k r + k B HB B bB ∂t (j + 1)
•(x
The intermediate complex, Ri + x′), indicates the reversible interaction between the polymer and radical by disproportionation facilitating the formulation of population balance equations. If the intermediate formed is ignored in the reaction (G), the forward and reverse rate coefficients are kd and kD, respectively.25,26 Population Balances of the Polymer and Radical Species. Continuous distribution kinetics provides a straightforward technique to determine the temporal dynamics of molecular weight. The polymer, P(x), of molecular weight x has the distribution p(x, t) by assuming the x as a continuous variable. The molecular weight distribution (MWD) is defined such that p(x, t) dx is the amount of polymer present at any time between the size interval of (x, x + dx). The zeroth moment of p(x, t) is the molar concentration of polymer; the first moment signifies the mass concentration, and the second moment determines the spread of the distribution. The population balances for the equations (C-G) with LCA assumption can be written as
∫0∞ rA(x′, t) dx′ +
kD2
(D)
HA
•
∂pB(x, t) ) -khB pB(x, t) + kHB rB(x, t) + kbB ∂t
kd2r(0) A
p(j) B
r(j) i (7) + kD2 j+1
dr(j) r(j) A (t) A (j) (j) (j) ) khApA - kHArA - kbArA + kbA ∂t (j + 1) r(j) i (j) r + k kd2p(0) (8) B A D2 j+1 dr(j) r(j) B (t) B (j) (j) ) khBp(j) k r k r + k B HB B bB B bB ∂t (j + 1) r(j) i (j) (9) kd1p(0) A rB + kD1 j+1 dr(j) i (t) dt
j
)
(j-k) j Ck{kd1r(j-k) (t)p(k) (t)p(k) ∑ B A (t) + kd2rA B (t)} k)0
(kD1 + kD2)r(j) i (t) (10) j ) 0, 1, 2 corresponds to the zeroth, first, and second moments, respectively. Using the quasi-steady-state assumption (QSSA) that assumes that the change in the radical concentration is 0, the molar concentration (obtained by solving zeroth moment of all the species)
1564 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004
of the polymers is
dp(0) A ) kbAr(0) A dt
(11)
dp(0) B ) kbBr(0) B dt
(12)
The concentrations of the radical intermediates required to solve eqs 11 and 12 are
r(0) A )
r(0) B )
khAp(0) A
+
kD2r(0) i
(13)
kHA + kd2p(0) B khBp(0) B
+
kHB +
(
(14)
kd1p(0) A
)
(0) kd1khB kd2khA p(0) A pB + kHBk2 kHAk1 kD2 kD1 + k1 k2
(15)
The coefficients, k1 and k2, are time-dependent and are (0) equal to 1 + k12 p(0) B and 1 + k21pA , respectively, where k12 and k21 correspond to kd2/kHA and kd1/kHB, respectively. These coefficients represent the hydrogen abstraction capacity of a polymer from the other polymer compared to its own rate of hydrogen abstraction. The higher value of these parameters indicates the reduction in degradation rate of the polymer, thus enhancing the degradation rate of the other polymer in the mixture. Substituting eqs 13-15 in eqs 11 and 12,
( (
dp(1) dp(1) A B ) )0 dt dt
(18)
Equation 18 confirms mass conservation during the degradation as the first moment of the distribution represents the mass concentration of the polymer. Results and Discussion
kD1r(0) i
The intermediate formed by the disproportionation is thus
r(0) i )
dation in the mixture. The rate coefficients, kA and kB, are determined from the degradation of the individual polymers in the absence of the other polymer. The first moment of the molecular weight distribution can be obtained by setting j ) 1 in eqs 6-10, which yields
) )
kA (0) kD2k3p(0) dp(0) A B ) pA 1 + dt k1 khA
(16a)
kD1k3p(0) kB dp(0) B A 1 + ) p(0) dt k2 B khB
(16b)
kA and kB represent the degradation rate coefficients of A and B and are equal to kbAkhA/kHA and kbBkhB/kHB, respectively. The k3 represents the interaction between the polymers through hydrogen abstraction and is equal to (khBkd1/k2kHB + khAkd2/k1kHA) 1/((kD1/k2) + (kD2/k1)). Because kD1 is large compared to kD2 and kd2 is negligibly small, the expressions for k1, k2, and k3 reduce, respectively, to 1.0, 1 + k21 p(0) A and kd1/kD1 khB/kHB. Thus, eqs 16a and 16b can be written as
dp(0) A (0) (0) ) kAp(0) A + kint,ApA pB dt
(17a)
(0) (0) kBp(0) dp(0) B B + kint,BpA pB ) dt 1 + k p(0)
(17b)
21 A
The interaction coefficient, kintA and kintB, for A and B are k3kD2kA/khA and k3kD1kB/khB, respectively. The simultaneous solution of 17a and 17b gives the molecular weight dynamics of the individual polymer degra-
In the presence of one polymer, the degradation rate of the other polymer can decrease (if k1or k2 . kA or kB and k3) or increase (k3 . all other coefficients, and k1 or k2 is 1.0) or remain unaffected (k1 and k2 can be 1.0, and kd1 and kd2 are 0). On the basis of eqs 16 and 17, the degradation rate of poly(vinyl acetate) is a function of poly(-caprolactone) in the mixture through hydrogen abstraction interaction. The degradation rate of PVAC in the absence of PCL can be obtained by substituting p(0) B as 0 in eq 16a and is given by
dp(0) A ) kAp(0) A dt
(19)
Equation 19 can be solved with the initial condition (0) of p(0) A (t ) 0) ) pA0 to get the time variation of the molar concentration of PVAC.
ln
( ) p(0) A
pA0(0)
) kAt
(20)
Since the mass concentration of the polymers is constant over all time (eq 18), eq 20 can be written in terms of (1) the number average molecular weights (p(0) A ) pA /Mn).
ln
( )
MnA0 ) kAt MnA
(21)
Similarly, the degradation rate of poly(-caprolactone) in the absence of PVAC is
ln
( )
MnB0 ) kBt MnB
(22)
The degradation rate coefficients of the individual polymers, in the absence of other polymer, are determined from slope of the linearly regressed lines of the semilog plot of Mn0/Mn against the reaction time (t). To determine the molecular weight distribution of the individual polymer in the mixture, coupled differential equations represented in eqs 17a and 17b are to be solved simultaneously. In the studies reported earlier,10,16 the degradation rate of one of the polymers is negligible and only interacts with the other polymer without undergoing any change. Thus, the molar concentration of one of the polymers in the mixture remains constant, and the solution of simultaneous equations reduces to solving a single differential equation that has a simple analytical solution. In the present case, the
Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1565 Table 1. Rate Coefficients of PCL, PVAC, and Their Mixtures (w/w PCL/PVAC) at Various Temperaturesa kintA, L mol-1 s-1 kintB, L mol-1 s-1 temp kA (×105 kB (×105 (°C) s-1) s-1) 30/70 50/50 65/35 30/70 50/50 65/35 190 210 230 250
0.5 1.8 5.8 11.0
Eact
56
2.9 5.5 10.8 15.0
1.5 2.3 4.0 8.2
2.7 4.5 6.5 9.4
6.0 8.1 11.7 16.2
0.20 0.56 1.02 2.31
Energy of Activation, Eact, kJ/mol 106 57 41 34 80
0.4 1.0 1.6 3.1 65
0.8 1.3 2.6 3.9 56
a
The subscripts, A and B, refer to poly(vinyl acetate) and poly(caprolactone), respectively.
Figure 2. Variation of molecular weight of PVAC as a function of degradation time at various temperatures: (a) 30/70 PCL/PVAC, (b) 50/50 PCL/PVAC, (c) 65/35 PCL/PVAC mixtures. Legends: 9, 190; b, 210; 2, 230; 1, and 250 °C for the degradation of polymers in the absence of the other polymer. 0, 190; O, 210; 4, 230; and 3, 250 °C for the degradation of polymers in the presence of the other polymer. The lines indicate the model fit to the experimental data.
degradation rates of both the polymers are comparable and a numerical solution is required. Mathematica reports an analytical solution in terms of hypergeometric functions that is too complicated for practical use. Therefore, eqs 17a and 17b were solved numerically using the Runge-Kutta technique with an adaptive step-size control. The accuracy of the numerical solution was verified for certain cases with the analytical solution. Figure 2a shows the variation of the molecular weight of PVAC in the mixture containing 30% PCL and 70%
PVAC with time at various temperatures (190-250 °C). The plot also shows the experimental data for the degradation of PVAC in the absence of PCL under identical conditions to quantify the change in degradation rates of PVAC in the mixture. The open symbols in the figures indicate the molecular weight variation of PVAC in the mixture, whereas the solid symbols in the figures represent the degradation of PVAC in the absence of PCL. The lines shown in the figures are the linear fits obtained by regressing the experimental data according to eq 21 for PVAC in the absence of PCL. The model fit for the mixture of polymers is based on the numerical solution of eqs 17a and 17b. The values of rate constants reported in Table 1 correspond to the values obtained by regression of the experimental data, and the variation in the rate constants is within (5%. The experimental data indicates that the degradation rates of PVAC are higher in the presence of PCL. PVAC is a good hydrogen donor, whereas PCL is a hydrogen acceptor,6-9 leading to more hydrogen abstraction from PVAC and enhanced degradation of PVAC. The value of k1 is close to unity, indicating no hydrogen abstraction from the PCL, and kA remained invariant in both the degradation of the mixture and pure PVAC. The enhancement in the degradation rate is due to kintA () kD2k3kA/khA). The kinetic parameters for the degradation of the polymer in both the presence and the absence of the other polymer are given in Table 1. The rate coefficients in the table indicate that the interaction between the polymers increases as temperature increases. Figure 2b,c shows the evolution of the number average molecular weight with time for different mixture compositions of 50/50 PCL/PVAC and 65/35 PCL/ PVAC. The degradation rate coefficients are listed in Table 1. The extent of degradation of PVAC increases with the amount of PCL in the mixture. Figure 3a-c depicts the variation of the molecular weight of PCL with time at various temperatures (190250 °C) for mixture compositions 30/70, 50/50, and 65/ 35 PCL/PVAC, respectively. The degradation rates of the PCL decreases in the mixture compared to the degradation rate of PCL in the absence of PVAC. This is due to the proton-accepting nature of the PCL, leading to the capping off of the radicals and reduced degradation. Since PCL has the tendency to abstract hydrogen from the PVAC chain, the disproportionation reactions are much pronounced, leading to the higher values of k21, as compared to the interaction group. Although k21 is independent of temperature, it is strongly dependent on the composition of the blend. The values of k21 are determined to be 63, 18, and 12 L mol-1 s-1 in the 30/ 70, 50/50, and 65/35 mixtures of PCL/PVAC, indicating that the values decrease with increasing concentrations of PCL. The values of the interaction parameter, kintB, are also determined and presented in Table 1.
1566 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004
Figure 4. Arrhenius plot for the pure polymers for interaction parameters for the determination of activation energies. Legends: [, kA × 105; ], kB × 105; kintA for the 30/70 (9), 50/50 (b), and 65/35 (2) PCL/PVAC. kintB for the 30/70 (0), 50/50 (O), and 65/35 (4) PCL/PVAC.
decrease with increasing PCL content in the mixture. The decrease in the activation energy with an increase in the PCL concentration can be attributed to the increased hydrogen interaction nature of the polymers in the mixture. It can be seen from the Table that k21 is not a strong function of the temperature, although it is a strong function of the composition, that is, hydrogen abstraction capacity, and is in agreement with the observation in the literature.27 Conclusions The thermal degradation of poly(vinyl acetate), poly(-caprolactone), and their blends was studied in solution at various temperatures. The degradation of the mixtures showed enhancement in the degradation rates of PVAC and reduction in the degradation rate of PCL. The enhancement in the degradation rate of PVAC increased with the increase in PCL concentration. Similarly, the degradation rate of PCL reduced with the increase in PVAC concentration in the mixture. This was attributed to the hydrogen-donating and hydrogenaccepting nature of PVAC and PCL, respectively. A model based on such interactions has been proposed and could explain the experimental degradation of a polymer in the binary blends. Figure 3. Variation of molecular weight of PCL as a function of degradation time for various (a) 30/70 PCL/PVAC, (b) 50/50 PCL/ PVAC, and (c) 65/35 PCL/PVAC mixture compositions at various temperatures. See Figure 2 for legends.
Figure 4a shows the Arrhenius plot for degradation rate coefficients of PCL, PVAC, and interaction rate coefficients of PVAC. The activation energies, determined from the Arrhenius plot, for all the rate coefficients are shown in Table 1. The activation energies for the degradation of the individual polymers in the absence of the other polymer are 56 and 106 kJ/mol for PVAC and PCL, respectively. The activation energies for the interaction coefficient of PVAC (kintA) in 30/70, 50/50, and 65/35 PCL/PVAC mixtures show that the activation energies decrease with increasing PCL content in the mixture. Similarly, the activation energies for the interaction coefficient of PCL (kintB) in 30/70, 50/ 50, and 65/35 PCL/PVAC mixtures indicate that they
Acknowledgment The authors thank the Department of Science and Technology, India, for the financial support to carry out this work. The first author thanks the General Electric Company, U.S.A., for a Fellowship. Literature Cited (1) Yam, W. Y.; Ismail, J.; Kammer, H. W.; Schmidt, H.; Kummerlowe, C. Polymer Blends of Poly(-caprolactone) and Poly(vinyl methyl ether) - Thermal Properties and Morphology. Polymer 1999, 40, 5545. (2) Khambatta, F. B.; Warner, F.; Russell, T.; Stein, R. S. SmallAngle X-ray and Light Scattering Studies of the Morphology of Blends of Poly(-caprolactone) with Poly(vinyl chloride). J. Polym. Sci.: Polym. Phys. Ed. 1976, 14, 1391. (3) Schulze, K.; Kressler, J.; Kammer, H. W. Phase Behavior of Poly(-caprolactone)/(Polystyrene-ran-acrylonitrile) Blends Exhibiting Both Liquid-Liquid Unmixing and Crystallization. Polymer 1993, 34, 3704.
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Received for review September 6, 2003 Revised manuscript received January 9, 2004 Accepted January 21, 2004 IE034115Y
