Thermal Degradation of Poly(vinyl acetate) andPoly(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(ε-caprolactone) (PCL), poly(vinyl acetate) (PVAC), and theirmixtures in solution has been investigated. The thermal degradation of PCL-PVAC mixtureswith 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 thepolymers were determined using gel permeation chromatography (GPC). The experimentalresults indicated significant increase in the degradation rates of PVAC and a decrease in thedegradation rates of PCL. This can be attributed to the proton-accepting nature of PCL andproton-donating nature of PVAC. A model based on continuous distribution kinetics wasdeveloped considering the interaction between the polymers through hydrogen abstraction. Thedegradation rate coefficients of the polymers and their mixtures were determined numericallyby fitting the model to the experimental data.

IntroductionPolymer blends, having poly(ε-caprolactone) (PCL) as

a constituent, have been investigated extensively inrecent years.1 PCL shows better miscibility on a molec-ular scale in physical mixtures with various polymers,such as poly(vinyl chloride),2 poly(styrene-co-acryloni-trile),3 chlorinated polyethylenes,4 poly(vinyl methylether),5 and poly(vinyl acetate).6 The polymer mixturescontaining carboxyl group-bearing polymers, such aspoly(lactic acid) (PLA),7 poly(hydroxybutyrate) (PHB),8poly(ethylene oxide) (PEO),9 and poly(ε-caprolactone)(PCL)6-10 with poly(vinyl acetate), showed synergisticenhancement in mechanical properties and significantreduction in the biodegradability of the former polymers.The phenomenon that causes these effects was at-tributed to the proton-accepting and proton-donatingnature of the carboxyl groups of PLA, PHB, PCL, andR-hydrogen of PVAC, respectively.6-10

A mixture of polyethylene and polystyrene showsstrong thermal interaction leading to enhanced degra-dation of a hydrogen-donor polymer (polyethylene)under isothermal holding at high temperatures of 390-440 °C.11,12 Addition of polystyrene to phenol novolocleads to the enhanced degradation rate of phenol novo-loc.13 The degradation rate of polystyrene increased8-fold in the presence of poly(methyl acrylate) and poly-(butyl acrylate) at 430 °C, while the degradation ratesof 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 ofradicals from both the polymers, leading to increaseddegradation of polystyrene.15 The thermal degradationof a mixture of polystyrene and poly(vinyl acetate) insolution showed a significant increase in the degrada-tion rate of poly(vinyl acetate) with a decrease in thedegradation rate of polystyrene.16 The degradation rateof polystyrene decreases in the presence of a hydrogendonor (tetralin).17

The type of interaction in a PCL and PVAC mixtureis also by a hydrogen-donor and -acceptor mechanism.6Recently, a strong reduction in the enzymatic degrada-tion of PCL in the physical mixture of 90/10 PCL/PVACwas observed, as compared to the enzymatic degrada-tion of pure PCL, and attributed to the interactionbetween the polymers.10 Because of better miscibilityand interaction due to hydrogen abstraction, this mix-ture showed synergistic enhancement in the tensilestrength, ultimate strength, and percentage elongationat breakage.6 Under pyrolytic conditions with dynamicheating, this mixture behaved ideally over all thecomposition range, indicating no interaction.6 However,the degradation rates are influenced marginally underisothermal holding at higher temperatures.6

The pyrolytic degradation of polymers normally occursat high temperatures. In addition, the pyrolytic degra-dation is associated with the evolution of noxiousgaseous products that cause environmental pollutionwith low yields of desired products and excessive charformation.18 To ameliorate these difficulties, single-phase, contained degradation of polymers in solutionhas been proposed. In this method, the polymer isdissolved in an inert solvent and thermally degraded.Since the degradation occurs in a single phase, uniformheat transfer would be achieved with better yields.18

Further, polymers in solution degrade faster at lowertemperatures with lower activation energies.19 Apartfrom plastic recycling, solution degradation studies canbe applied to investigate the stability20 and decomposi-tion characteristics21 of polymers and their mixtures.16

The degradation of poly(ε-caprolactone) and poly(vinylacetate) and their mixtures was investigated in solution(diphenyl ether) at various temperatures (190-250 °C).In the present investigation, a model based on continu-ous distribution kinetics considering detailed Rice-Herzfeld reactions22-24 has been postulated to explainthe degradation rates of pure polymers and theirmixtures. The interaction parameters for both polymersin the mixture were determined as a function of mixturecomposition. The activation energies were determined

* To whom the correspondence should be addressed. Tel:091-80-2932321. Fax: 091-80-3600683. E-mail: Giridhar@chemeng.iisc.ernet.in.

1561Ind. Eng. Chem. Res. 2004, 43, 1561-1567

10.1021/ie034115y CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 03/03/2004

from the temperature dependency of the degradationrate coefficients.

Experimental Section

Materials. Poly(ε-caprolactone) (Mn, 80 000; polydis-persity, 1.3) and vinyl acetate monomer were purchasedfrom Aldrich Chemicals, U.S.A. Diphenyl ether andtetrahydrofuran (THF) were procured from S. D. Finechemicals (India). Solvents were distilled and filteredprior to use.

Polymer Synthesis and Pretreatment. The vinylacetate monomer was freed from inhibitor by causticwash followed by distillation at reduced pressure. Theperoxide-based initiator was used for the bulk poly-merization of vinyl acetate at 60 °C. Benzoyl peroxidewas purified by dissolution in chloroform, followed byprecipitation in methanol. To obtain a initial molecularweight distribution distinct from that of PCL, PVAC wasfractionally precipitated using a solvent (acetone) anda nonsolvent (hexane). The precipitated polymer wasdried at lower temperature over calcium carbonate untilconstant weight was attained. The number averagemolecular weight and polydispersity of synthesizedPVAC were 15 400 and 1.54, respectively.

Degradation Experiments. The thermal degrada-tion of the polymers (PCL, PVAC) and their mixturesin diphenyl ether was studied at various temperatures(190-250 °C) in a three-necked round-bottomed flask.Long vertical condensers ensured the volatiles gener-ated during the operation remained in the reactor. Thetotal concentration of the polymers employed was 2 kg/m3. The uniformity of the polymer concentration in thereactor was achieved with a magnetic stirrer. Thetemperature was controlled within ( 1 °C using a PIDcontroller. The reactor was charged with distilled andfiltered solvent (diphenyl ether) and heated until itreached the desired reaction temperature. Then pre-weighed amount of polymers were added to the reactorto start the degradation experiments. The various massfractions of PVAC in the mixture used in the presentstudy were 30, 50, and 65%. Portions (200 µL) ofsamples were collected during the reaction at regularintervals and taken for the subsequent analysis.

Molecular Weight Determination. The MWD ofthe samples was determined by gel permeation chro-matography (GPC) (Waters, USA) that consisted of anisocratic pump (Waters 501), size- exclusion columns,differential refractrometer (Waters R401) and a dataacquisition system. THF was used as eluent at a flowrate of 1 mL/min. The columns (Styragel HR 4, HR 3,and HR 0.5) (300 × 7.5 mm) packed with cross-linkedpolystyrene-co-divinylbenzene were used in series at 50°C. Samples were injected in a Rheodyne valve with asample loop of 50 µL, and the refractive index wascontinuously monitored using a differential refractiveindex detector and stored digitally. The chromatographwas converted to molecular weight distribution using acalibration based on polystyrene standards (PolymerLab, U.K.). The MW of polystyrene was converted toMW of the individual polymers using the Mark-Houwink equation. Further details are available else-where.18 The mass of the polymer system was calculatedfrom the area under the gel permeation chromatograph.The calculation indicated that the mass of the reactionsystem remained constant at all reaction times, con-firming that no volatiles were lost from the system.Figure 1 shows the initial molecular weight distribution

of PCL/PVAC mixtures as obtained from GPC. The firstpeak corresponds to PCL, and the second peak corre-sponds to PVAC. The solid line is the GPC response ofthe mixture, and the dotted lines are the contributionfrom the individual polymers and are obtained bydeconvoluting the mixture peak. Since the MWD for theindividual polymers can be obtained by deconvolution,the moments for each polymers can be determined bynumerical integration.

Theoretical Model

The mechanism of polymer degradation is complexand involves several elementary reactions, such as bondfission, hydrogen transfer, and disproportionation. Quan-titative modeling of these processes can be done by aRice-Herzfeld mechanism involving initiation, propa-gation, and termination reactions. In the case of polymermixtures, the interaction between the polymers can leadto an alteration in the degradation rate of individualpolymers.7-10,16 The present model is similar to themodels developed previously.10,16 In these cases,10,16

while both the polymers interact, only one polymerdegrades predominantly. This allows the equations tohave an analytical solution. However, in this study, bothof the polymers undergo degradation, and a numericalsolution is used to determine the degradation rates.

Mechanism of Degradation of Polymers in Mix-tures. The thermal degradation of the pure polymers(PCL, PVAC) and their mixtures occurs by randomchain scission and can be assumed to degrade infollowing four steps (A-D).

(A) Initiation and Termination. The initiation isby thermolytic cleavage. The reaction is represented asa reversible reaction with backward reaction, signifyingthe termination reaction.

PA(x), PB(x), and RA•(x), RB

•(x) represent the polymersand their radicals, respectively and x is the molecularweight. The subscripts A and B refer to poly(vinylacetate) (A) and poly(ε-caprolactone) (B), respectively.

Figure 1. Typical molecular weight distribution for the poly(ε-caprolactone) (PCL) and poly(vinyl acetate) (PVAC) mixture andshowing the methodology of molecular weight determination ofindividual polymers. The solid line represents the GPC responsefor the mixture. The dashed and the dotted line denote the MWDof PCL and PVAC, respectively.

PA(x) \kfA

ktARA

•(x′) + RA•(x - x′) (A)

PB(x) \kfB

ktBRB

•(x′) + RB•(x - x′) (B)

1562 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

kfA and ktA are the rate constants for the forward andbackward reactions representing initiation by thermoly-sis and termination by coupling, respectively. Theseinitiation and termination steps are infrequent com-pared to the depropagation, and hence, rates of thesesteps can be neglected. This assumption is known aslong chain approximation (LCA) and is commonly usedin the polymer degradation.10,16,22-24

(B) Reversible Hydrogen Abstraction. Apart fromthe radical formation by thermolysis of the polymer,simple hydrogen abstraction can also lead to the gen-eration of the radicals.

kh and kH represent the rate coefficients for radicalgeneration by hydrogen abstraction and capping ofradicals, respectively.

(C) Depropagation. The radicals generated undergodegradation, resulting in the reduction of molecularweight of the polymer. This step is assumed to beirreversible, because the temperature of the reaction isabove the ceiling temperature of the polymers.

(D) Interaction between Polymers. Similar to thehydrogen abstraction reactions (C) and (D), hydrogenabstraction can also occur due to the interaction withthe other polymer in the mixture. The interactionbetween the polymers due to hydrogen transfer can bewritten as16,25,26

The intermediate complex, Ri•(x + x′), indicates the

reversible interaction between the polymer and radicalby disproportionation facilitating the formulation ofpopulation balance equations. If the intermediate formedis ignored in the reaction (G), the forward and reverserate coefficients are kd and kD, respectively.25,26

Population Balances of the Polymer and Radi-cal Species. Continuous distribution kinetics providesa straightforward technique to determine the temporaldynamics of molecular weight. The polymer, P(x), ofmolecular weight x has the distribution p(x, t) byassuming the x as a continuous variable. The molecularweight distribution (MWD) is defined such that p(x, t)dx is the amount of polymer present at any timebetween the size interval of (x, x + dx). The zerothmoment of p(x, t) is the molar concentration of polymer;the first moment signifies the mass concentration, andthe second moment determines the spread of thedistribution. The population balances for the equations(C-G) with LCA assumption can be written as

When the change in the molecular weights is less than

an order of magnitude, the rate constants can beassumed to be independent of molecular weight.24 Thesatisfactory fit to the experimental data in this casevalidates this assumption. Operating moments, definedas p(j)(t) ) ∫0

∞ xjp(x, t) dx, in eqs 1-5 yields

j ) 0, 1, 2 corresponds to the zeroth, first, and secondmoments, respectively. Using the quasi-steady-stateassumption (QSSA) that assumes that the change in theradical concentration is 0, the molar concentration(obtained by solving zeroth moment of all the species)

∂pB(x, t)∂t

) -khB pB(x, t) + kHB rB(x, t) + kbB∫x

∞rB

(x′, t) Ω(x, x′) dx′ - kd2 pB(x, t)∫0

∞rA(x′, t) dx′ +

kD2∫x

∞ri(x′, t) Ω(x, x′) dx′ (2)

∂rA(x, t)∂t

) khA pA(x, t) - kHA rA(x, t) - kbA rA(x, 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)∂t

) khB pB(x, t) - kHB rB(x, t) - kbB rB(x, 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′ (4)

∂ri(x, t)∂t

) ∫0

xkd1 pA(x′, t) rB(x - x′) + kd2 pB(x′, t) rA

(x - x′) dx′ - (kD1 + kD2) ri(x′, t) (5)

dpA(j)(t)∂t

) -khApA(j) + kHArA

(j) + kbA

rA(j)

(j + 1)- kd1rB

(0)pA(j) +

kD1

ri(j)

j + 1(6)

dpB(j)(t)∂t

) - khBpB(j) + kHBrB

(j) + kbB

rB(j)

(j + 1)-

kd2rA(0) pB

(j) + kD2

ri(j)

j + 1(7)

drA(j)(t)∂t

) khApA(j) - kHArA

(j) - kbArA(j) + kbA

rA(j)

(j + 1)-

kd2pB(0)rA

(j) + kD2

ri(j)

j + 1(8)

drB(j)(t)∂t

) khBpB(j) - kHBrB

(j) - kbBrB(j) + kbB

rB(j)

(j + 1)-

kd1pA(0)rB

(j) + kD1

ri(j)

j + 1(9)

dri(j)(t)

dt) ∑

k)0

jjCkkd1rB

(j-k)(t)pA(k)(t) + kd2rA

(j-k)(t)pB(k)(t) -

(kD1 + kD2)ri(j)(t) (10)

PA(x) \khA

kHARA

•(x) (C)

PB(x) \khA

kHARB

•(x) (D)

RA•(x)98

kbAPA(x′) + RA

•(x - x′) (E)

RB•(x)98

kbBPB(x′) + RB

•(x - x′) (F)

RB•(x) + PA(x′) \

kd1

kD1Ri

•(x + x′) \kD2

kd2PB(x) + RA

•(x′) (G)

∂pA(x, t)∂t

) -khA pA(x, t) + kHA rA(x, t) + kbA∫x

∞rA

(x′, t) Ω(x, x′) dx′ - kd1 pA(x, t)∫0

∞rB(x′,t) dx′ +

kD1∫x

∞ri(x′, t) Ω(x, x′) dx′ (1)

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1563

of the polymers is

The concentrations of the radical intermediates requiredto solve eqs 11 and 12 are

The intermediate formed by the disproportionation isthus

The coefficients, k1 and k2, are time-dependent and areequal to 1 + k12 pB

(0) and 1 + k21pA(0), respectively, where

k12 and k21 correspond to kd2/kHA and kd1/kHB, respec-tively. These coefficients represent the hydrogen ab-straction capacity of a polymer from the other polymercompared to its own rate of hydrogen abstraction. Thehigher value of these parameters indicates the reductionin degradation rate of the polymer, thus enhancing thedegradation rate of the other polymer in the mixture.Substituting eqs 13-15 in eqs 11 and 12,

kA and kB represent the degradation rate coefficients ofA and B and are equal to kbAkhA/kHA and kbBkhB/kHB,respectively. The k3 represents the interaction betweenthe polymers through hydrogen abstraction and is equalto (khBkd1/k2kHB + khAkd2/k1kHA) 1/((kD1/k2) + (kD2/k1)).Because kD1 is large compared to kD2 and kd2 is negli-gibly small, the expressions for k1, k2, and k3 reduce,respectively, to 1.0, 1 + k21 pA

(0) and kd1/kD1 khB/kHB.Thus, eqs 16a and 16b can be written as

The interaction coefficient, kintA and kintB, for A andB are k3kD2kA/khA and k3kD1kB/khB, respectively. Thesimultaneous solution of 17a and 17b gives the molec-ular weight dynamics of the individual polymer degra-

dation in the mixture. The rate coefficients, kA and kB,are determined from the degradation of the individualpolymers in the absence of the other polymer.

The first moment of the molecular weight distributioncan be obtained by setting j ) 1 in eqs 6-10, whichyields

Equation 18 confirms mass conservation during thedegradation as the first moment of the distributionrepresents the mass concentration of the polymer.

Results and Discussion

In the presence of one polymer, the degradation rateof the other polymer can decrease (if k1or k2 . kA or kBand k3) or increase (k3 . all other coefficients, and k1or 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 functionof poly(ε-caprolactone) in the mixture through hydrogenabstraction interaction. The degradation rate of PVACin the absence of PCL can be obtained by substitutingpB

(0) as 0 in eq 16a and is given by

Equation 19 can be solved with the initial conditionof pA

(0)(t ) 0) ) pA0(0) to get the time variation of the molar

concentration of PVAC.

Since the mass concentration of the polymers is constantover all time (eq 18), eq 20 can be written in terms ofthe number average molecular weights (pA

(0) ) pA(1)/Mn).

Similarly, the degradation rate of poly(ε-caprolactone)in the absence of PVAC is

The degradation rate coefficients of the individualpolymers, in the absence of other polymer, are deter-mined from slope of the linearly regressed lines of thesemilog plot of Mn0/Mn against the reaction time (t). Todetermine the molecular weight distribution of theindividual polymer in the mixture, coupled differentialequations represented in eqs 17a and 17b are to besolved simultaneously. In the studies reported ear-lier,10,16 the degradation rate of one of the polymers isnegligible and only interacts with the other polymerwithout undergoing any change. Thus, the molar con-centration of one of the polymers in the mixture remainsconstant, and the solution of simultaneous equationsreduces to solving a single differential equation that hasa simple analytical solution. In the present case, the

dpA(0)

dt) kbArA

(0) (11)

dpB(0)

dt) kbBrB

(0) (12)

rA(0) )

khApA(0) + kD2ri

(0)

kHA + kd2pB(0)

(13)

rB(0) )

khBpB(0) + kD1ri

(0)

kHB + kd1pA(0)

(14)

ri(0) ) (kd1khB

kHBk2+

kd2khA

kHAk1) pA

(0) pB(0)

kD2

k1+

kD1

k2

(15)

dpA(0)

dt)

kA

k1pA

(0)(1 +kD2k3pB

(0)

khA) (16a)

dpB(0)

dt)

kB

k2pB

(0)(1 +kD1k3pA

(0)

khB) (16b)

dpA(0)

dt) kApA

(0) + kint,ApA(0)pB

(0) (17a)

dpB(0)

dt)

kBpB(0) + kint,BpA

(0)pB(0)

1 + k21pA(0)

(17b)

dpA(1)

dt)

dpB(1)

dt) 0 (18)

dpA(0)

dt) kApA

(0) (19)

ln( pA(0)

pA0(0)) ) kAt (20)

ln(MnA0

MnA) ) kAt (21)

ln(MnB0

MnB) ) kBt (22)

1564 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

degradation rates of both the polymers are comparableand a numerical solution is required.

Mathematica reports an analytical solution in termsof hypergeometric functions that is too complicated forpractical use. Therefore, eqs 17a and 17b were solvednumerically using the Runge-Kutta technique with anadaptive step-size control. The accuracy of the numericalsolution was verified for certain cases with the analyti-cal solution.

Figure 2a shows the variation of the molecular weightof 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 thedegradation of PVAC in the absence of PCL underidentical conditions to quantify the change in degrada-tion rates of PVAC in the mixture. The open symbolsin the figures indicate the molecular weight variationof PVAC in the mixture, whereas the solid symbols inthe figures represent the degradation of PVAC in theabsence of PCL. The lines shown in the figures are thelinear fits obtained by regressing the experimental dataaccording to eq 21 for PVAC in the absence of PCL. Themodel fit for the mixture of polymers is based on thenumerical solution of eqs 17a and 17b. The values ofrate constants reported in Table 1 correspond to thevalues obtained by regression of the experimental data,and the variation in the rate constants is within (5%.The experimental data indicates that the degradationrates of PVAC are higher in the presence of PCL. PVACis a good hydrogen donor, whereas PCL is a hydrogenacceptor,6-9 leading to more hydrogen abstraction fromPVAC and enhanced degradation of PVAC. The valueof k1 is close to unity, indicating no hydrogen abstractionfrom the PCL, and kA remained invariant in both thedegradation of the mixture and pure PVAC. The en-hancement in the degradation rate is due to kintA ()kD2k3kA/khA). The kinetic parameters for the degradationof the polymer in both the presence and the absence ofthe other polymer are given in Table 1. The ratecoefficients in the table indicate that the interactionbetween the polymers increases as temperature in-creases. Figure 2b,c shows the evolution of the numberaverage molecular weight with time for different mix-ture compositions of 50/50 PCL/PVAC and 65/35 PCL/PVAC. The degradation rate coefficients are listed inTable 1. The extent of degradation of PVAC increaseswith the amount of PCL in the mixture.

Figure 3a-c depicts the variation of the molecularweight of PCL with time at various temperatures (190-250 °C) for mixture compositions 30/70, 50/50, and 65/35 PCL/PVAC, respectively. The degradation rates ofthe PCL decreases in the mixture compared to thedegradation rate of PCL in the absence of PVAC. Thisis due to the proton-accepting nature of the PCL, leadingto the capping off of the radicals and reduced degrada-tion. Since PCL has the tendency to abstract hydrogenfrom the PVAC chain, the disproportionation reactionsare much pronounced, leading to the higher values ofk21, as compared to the interaction group. Although k21is independent of temperature, it is strongly dependenton the composition of the blend. The values of k21 aredetermined to be 63, 18, and 12 L mol-1 s-1 in the 30/70, 50/50, and 65/35 mixtures of PCL/PVAC, indicatingthat the values decrease with increasing concentrationsof PCL. The values of the interaction parameter, kintB,are also determined and presented in Table 1.

Figure 2. Variation of molecular weight of PVAC as a functionof 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 polymersin 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 otherpolymer. The lines indicate the model fit to the experimental data.

Table 1. Rate Coefficients of PCL, PVAC, and TheirMixtures (w/w PCL/PVAC) at Various Temperaturesa

kintA, L mol-1 s-1 kintB, L mol-1 s-1temp(°C)

kA (×105

s-1)kB (×105

s-1) 30/70 50/50 65/35 30/70 50/50 65/35

190 0.5 2.9 1.5 2.7 6.0 0.20 0.4 0.8210 1.8 5.5 2.3 4.5 8.1 0.56 1.0 1.3230 5.8 10.8 4.0 6.5 11.7 1.02 1.6 2.6250 11.0 15.0 8.2 9.4 16.2 2.31 3.1 3.9

Energy of Activation, Eact, kJ/molEact 56 106 57 41 34 80 65 56

a The subscripts, A and B, refer to poly(vinyl acetate) and poly(ε-caprolactone), respectively.

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1565

Figure 4a shows the Arrhenius plot for degradationrate coefficients of PCL, PVAC, and interaction ratecoefficients of PVAC. The activation energies, deter-mined from the Arrhenius plot, for all the rate coef-ficients are shown in Table 1. The activation energiesfor the degradation of the individual polymers in theabsence of the other polymer are 56 and 106 kJ/mol forPVAC and PCL, respectively. The activation energiesfor the interaction coefficient of PVAC (kintA) in 30/70,50/50, and 65/35 PCL/PVAC mixtures show that theactivation energies decrease with increasing PCL con-tent in the mixture. Similarly, the activation energiesfor the interaction coefficient of PCL (kintB) in 30/70, 50/50, and 65/35 PCL/PVAC mixtures indicate that they

decrease with increasing PCL content in the mixture.The decrease in the activation energy with an increasein the PCL concentration can be attributed to theincreased hydrogen interaction nature of the polymersin the mixture. It can be seen from the Table that k21 isnot a strong function of the temperature, although it isa strong function of the composition, that is, hydrogenabstraction capacity, and is in agreement with theobservation in the literature.27

Conclusions

The thermal degradation of poly(vinyl acetate), poly-(ε-caprolactone), and their blends was studied in solu-tion at various temperatures. The degradation of themixtures showed enhancement in the degradation ratesof PVAC and reduction in the degradation rate of PCL.The enhancement in the degradation rate of PVACincreased with the increase in PCL concentration.Similarly, the degradation rate of PCL reduced with theincrease in PVAC concentration in the mixture. Thiswas attributed to the hydrogen-donating and hydrogen-accepting nature of PVAC and PCL, respectively. Amodel based on such interactions has been proposed andcould explain the experimental degradation of a polymerin the binary blends.

Acknowledgment

The authors thank the Department of Science andTechnology, India, for the financial support to carry outthis work. The first author thanks the General ElectricCompany, 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(ε-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. Small-Angle X-ray and Light Scattering Studies of the Morphology ofBlends of Poly(ε-caprolactone) with Poly(vinyl chloride). J. Polym.Sci.: Polym. Phys. Ed. 1976, 14, 1391.

(3) Schulze, K.; Kressler, J.; Kammer, H. W. Phase Behaviorof Poly(ε-caprolactone)/(Polystyrene-ran-acrylonitrile) Blends Ex-hibiting Both Liquid-Liquid Unmixing and Crystallization. Poly-mer 1993, 34, 3704.

Figure 3. Variation of molecular weight of PCL as a function ofdegradation time for various (a) 30/70 PCL/PVAC, (b) 50/50 PCL/PVAC, and (c) 65/35 PCL/PVAC mixture compositions at varioustemperatures. See Figure 2 for legends.

Figure 4. Arrhenius plot for the pure polymers for interactionparameters for the determination of activation energies. Leg-ends: [, 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), and65/35 (4) PCL/PVAC.

1566 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

(4) Defieuw, G.; Groeninckx, G.; Reynaers, H. Miscibility andMorphology of Binary Polymer Blends of Polycaprolactone withSolution-Chlorinated Polyethylenes. Polymer 1989, 30, 595.

(5) Luyten, M. C.; Bogels, E. J. F.; Alberda van Ekenstein, G.O. R.; ten Brinke, G.; Bras, W.; Komanschek, B. E.; Ryan, A. J.Morphology in Binary Blends of Poly(vinyl methyl ether) andε-Caprolactone-Trimethylene carbonate diblock copolymer. Poly-mer 1997, 38, 509.

(6) Sivalingam, G.; Karthik, R.; Madras, G. Mechanical Proper-ties and Thermal Degradation of Poly(ε-caprolactone) and Poly-(vinyl acetate) Blends. Polym. Degrad. Stab. 2003, in press.

(7) Gajria, A. M.; Dave, V.; Gross, R. A.; McCarthy, S. P.Miscibility and Biodegradability of Blends of Poly(lactic acid) andPoly(vinyl acetate). Polymer 1996, 37, 437.

(8) Kulkarni, S. Effect of Poly(vinyl acetate) on the CrystallineBehavior of Poly(â-hydroxybutyrate), M.S. Thesis, University ofMassachusetts, Lowell, MA, 1992.

(9) Kalfoglu, N. K. Compatibility of Poly(ethylene oxide)-Poly-(vinyl acetate) Blends. J. Polym. Sci., Polym. Phys. Ed. 1982, 20,1259.

(10) Sivalingam, G.; Chattopadhyay, S.; Madras, G. EnzymeDegradation of Poly(ε-caprolactone), Poly(vinyl acetate) and TheirBlends by Lipases. Chem. Eng. Sci. 2003, 58, 2911.

(11) Koo, J. K.; Kim, S. W.; Seo, Y. H. Characterization ofAromatic Hydrocarbon Formation from Pyrolysis of Polyethylene-Polystyrene Mixtures. Resour. Conserv. Recycl. 1991, 5, 365.

(12) McCaffrey, W. C.; Brues, M. J.; Cooper, D. G.; Kamal, M.R. Thermolysis of Polyethylene-Polystyrene Mixtures. J. Appl.Polym. Sci. 1996, 60, 2133.

(13) Sato, Y.; Kodera, Y.; Goto, J.; Matsui, Y. Effect of Poly-styrene Addition on the Monomer Recycling of Phenol Novolac.Polym. Degrad. Stab. 2002, 78, 315.

(14) Gardner, P.; Lehre, R.; Turner, D. Polymer DegradationModified by Blending with Polymers Chosen on the Basis of TheirΦ Factors. J. Anal. Appl. Pyrolysis 1993, 25, 11.

(15) Kaczmarek, H. Photodegradation of Polystyrene and Poly-(vinyl acetate) Blends. I. Irradiation of PS/PVAc Blends byPolychromatic Light. Eur. Polym. J. 1995, 31, 1037.

(16) Karmore, V.; Madras, G. Degradation Kinetics for PolymerMixtures in Solution. Ind. Eng. Chem. Res. 2001, 40, 1306.

(17) Madras, G.; McCoy, B. J. Effect of Hydrogen Donors onPolymer Degradation. Catal. Today 1998, 40, 321.

(18) Sivalingam, G.; Madras, G. Kinetics of Degradation ofPolycarbonate in Supercritical and Subcritical Benzene. Ind. Eng.Chem. Res. 2002, 41, 5337.

(19) Sivalingam, G.; Madras, G. Thermal Degradation of Poly-(ε-caprolactone). Polym. Degrad. Stab. 2003, 80, 11.

(20) Hawkins, W. L. Polymer Degradation and Stabilization;Springer-Verlag: New York, 1984.

(21) Flynn, J. H.; Florin, R. E. Degradation and PyrolysisMechanisms. In Pyrolysis and GC Polymer Analysis; Leibbeman,S. A., Levy, E. S., Eds.; Dekker: New York, 1985.

(22) Nigam, A.; Fake, D. M.; Klein, M. T. Simple ApproximateRate Law for Both Short Chain and Long Chain Herzfeld Kinetics.AIChE J. 1994, 40, 908.

(23) Gavalas, G. R. The Long Chain Approximation in FreeRadical Reaction Systems. Chem. Eng. Sci. 1966, 21, 33.

(24) Madras, G.; McCoy, B. J. Discrete and Continuous Modelsfor Polymerization and Depolymerization. Chem. Eng. Sci. 2001,56, 2831.

(25) Madras, G.; McCoy, B. J. Continuous Distribution Kineticsfor Polymer Mixtures Degradation. Ind. Eng. Chem. Res. 1999,38, 352.

(26) Kodera, Y.; McCoy, B. J. Continuous-Distribution Kineticsof Radical Mechanisms for Polymer Decomposition and Depolym-erization. AIChE J. 1997, 43, 3205.

(27) Kim, Y. C.; McCoy, B. J. Degradation Kinetics Enhance-ment of Polystyrene by Peroxide Addition. Ind. Eng. Chem. Res.2000, 39, 2811.

Received for review September 6, 2003Revised manuscript received January 9, 2004

Accepted January 21, 2004

IE034115Y

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1567