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Thermal degradation of poly (ε-caprolactone)
Polymer Degradation and Stability 80 (2003) 11–16 www.elsevier.com/locate/polydegstab
Thermal degradation of poly (e-caprolactone) G. Sivalingam, Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore-12, India Received 27 August 2002; accepted 16 November 2002
Abstract The thermal degradation kinetics of poly (e-caprolactone) (PCL) in solution was investigated at various temperatures (170–245 C). The degradation was also investigated by pyrolysis at various temperatures (280–330 C). The time evolution of molecular weight distribution (MWD) was obtained using gel permeation chromatography. It was observed that in pyrolysis, PCL degrades by specific chain end scission by unzipping of monomers from the hydroxyl end of the polymer chains. In contrast, the degradation of PCL in solution is by random chain scission. A continuous distribution kinetics model is proposed to explain the observed behaviour under both conditions. The activation energy, determined from the temperature dependence of rate coefficients, was 61 kJ/mol and 201 kJ/mol for the degradation in solution and by pyrolysis, respectively. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Poly (e-caprolactone); Random chain scission; Specific chain scission; Pyrolysis; Degradation in solution; Continuous distribution kinetics
1. Introduction The study of degradation of polymers is important in understanding their usability and recycling. The common methods of polymer degradation are biodegradation, pyrolysis, oxidative degradation, photo degradation, and catalytic degradation [1–3]. In spite of its limitations like high melt viscosity, poor heat transfer and excessive vapour release, pyrolysis has been the common method for polymer degradation. To ameliorate these difficulties, degradation in a single phase has been proposed to lower the degradation temperature by better heat transfer [4]. However the mechanism of degradation of polymers is still not completely understood. Biodegradability of poly (e-caprolactone) has been investigated in detail [5– 8], but very little information is known about its thermal stability. Recently, the thermal degradation of PCL was investigated [9,10], but the mechanism is debatable. Persenaire et al. [9] proposed a two-stage degradation mechanism of random cleavage through cis-elimination and specific chain end scission by unzipping from the hydroxyl end of the polymer chain. Aoyagi et al. [10] * Corresponding author. Fax: +91-80-360-0683. E-mail address: [email protected] (G. Madras).
proposed a single step degradation mechanism, where the polymer degrades by specific removal of monomer from the end groups. However, there is no study on the solution degradation of PCL. To the best of our knowledge, this is the first study on solution degradation of PCL. The thermal degradation of PCL was investigated in bulk (pyrolysis) and solution and different mechanisms were observed. A continuous degradation kinetics model was proposed for determination of the rate coefficients in both the processes. The activation energy was determined from the temperature dependence of the rate coefficients.
2. Experimental Poly (e-caprolactone) (Aldrich) of number-average molecular weight of 80,000 and polydispersity of 1.15 was used for the degradation study. The degradation of PCL in bulk was conducted using a Thermogravimetric analyser (TGA) (Perkin-Elmer, Pyris). The degradation was carried out with a heating rate of 50 K/min to the specified temperature and then isothermally holding the temperature for the specified time. Degraded samples were dissolved in tetrahydrofuran (THF) and analysed by gel permeation chromatography to obtain the time
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00376-2
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G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
evolution of molecular weight distribution. In solution degradation, the solvent (diphenyl ether) was heated in a three-necked 250 mL flask by a heating mantle controlled with a PID controller ( 2 C) with constant stirring to the required temperature. PCL was then added to the solution to obtain a concentration of 5 kg/ m3. Samples of 0.5 mL were collected at regular intervals for analysis by Gel permeation chromatography. 2.1. GPC analysis The GPC (Waters, USA) consisted of an isocratic pump, three size exclusion columns, differential refractometer (Waters R401) and a data acquisition system. THF was used as eluent at the flow rate of 1 ml/min. The columns (Styragel HR 4, HR 3, and HR 0.5) (3007.5 mm) packed with cross-linked polystyrene– divinylbenzene were used in series at 50 C. Samples were injected by a Rheodyne valve with a sample loop of 50 ml and the refractive index was continuously monitored using a differential refractive index detector and stored digitally. The chromatogram was converted to the molecular weight distribution using calibration with polystyrene standards. Fig. 1 shows the calibration obtained with polystyrene standards (Polymer Lab, UK) of various molecular weights. The molecular weight (MW) of polystyrene was converted to MW of PCL using the Mark–Houwink equation (KPS=1.25104 dl/g, PS=0.707; KPCL=1.09103 dlL/g, PCL=0.60 in the [Z]=KMa). The calibration (Fig. 1) with polystyrene standards is log10 (MW)=9.9131–0.0046t, where t is in seconds.
2.2. Degradation study The samples of PCL degraded in solution (diphenyl ether) were injected into the GPC. Because diphenyl ether covered the peaks below MW of 200, a conclusion regarding specific products could not be reached. The outlet of the GPC was analysed in a GC–MS and no monomers or oligomers of caprolactone were found. In order to confirm this conclusion, further experiments were conducted in benzene in a Parr reactor at 250 C and 50 atm. Samples were collected at regular intervals and were injected into a GPC system using dichlorobenzene as eluent. This GPC system is equipped with an ELSD (Evaporative Light Scattering Detector) to evaporate the eluent and solvent. This ensured that the low molecular weight products would be clearly visible in the chromatograph. No specific chain end products were detected, indicating that PCL degrades by random chain scission in solution. This is further confirmed by injecting the caprolactone monomer, which gave a distinct peak thus confirming that, in solution, PCL degrades by random chain scission. In contrast, the degradation of the PCL in pyrolysis is by chain-end scission resulting in the monomer. 2.3. Theoretical model 2.3.1. Model for degradation by pyrolysis The chain end scission of PCL of molecular weight x, P(x) to form monomer species Q(xs) can be written as, ks
PðxÞ ! Pðx xs Þ þ Qðxs ÞðAÞ
Fig. 1. Calibration plot for GPC using polystyrene standards. log10 (MW)=9.9131–0.0046t.
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Assuming x to be a continuous variable, the instantaneous concentration of polymeric and specific chain end product is given by p(x,t) and q(t). The population balance for the individual species [11] is ð1 @pðx; tÞ ¼ ks pðx; tÞ þ ks pðx0 ; tÞðx; ðx0 xs ÞÞdx0 ð1Þ @t xs @qðtÞ ¼ @t
ð1
ks pðx0 ; tÞðxs ; x0 Þdx0
ð2Þ
xs
where ks is assumed to be independent of molecular weight. Applying the moment operation f ðjÞ ðtÞ ¼ Ð1 j 0 x ½fðx; tÞ dx to Eqs (1) and (2) yields j X dpðjÞ ðtÞ ¼ ks pðjÞ ðtÞ þ ks j Ci ðxs Þi pðjiÞ ðtÞ dt i¼0
ð3Þ
dqðjÞ ðtÞ ¼ ks pð0Þ xjs ð4Þ dt Eqs. (3) and (4) show the time evolution of polymer and specific product moments. The zeroth and first moment represent the molar and mass concentration of the polymer and, obtained by substituting j=0 and 1 in Eq. (3), are
@pðx; tÞ ¼ kd ðxÞpðx; tÞ @t ð1 þ 2 kd ðx0 Þpðx0 ; tÞOðx; x0 Þdx0
13
ð9Þ
x
The stoichiometric kernel (x,x0 ) in Eq. (9) determines the distribution of scission products and kd(x) is the degradation rate coefficient. For random scission of polymers, the distribution of degraded products [11], (x,x0 ) is 1/x0 and assuming a linear dependence of kd on x yields kd(x)=kdx. Thus Eq. (9) becomes [11], ð1 @pðx; tÞ ¼ kd xpðx; tÞ þ 2 kd pðx0 ; tÞdx0 ð10Þ @t x Applying the moment operation to Eq. (10), dpðjÞ j1 kd pðjþ1Þ ðtÞ ¼ jþ1 dt
ð11Þ
The zeroth and first moments are obtained from Eq. (11) by substituting j=0 and 1, respectively, dpð0Þ ¼ kd pð1Þ ðtÞ dt
ð12Þ
dpð1Þ ¼0 dt
ð13Þ
ð0Þ
dp ¼0 dt
ð5Þ
dpð1Þ ¼ ks xs pð0Þ dt
ð6Þ
Eq. (5) suggests that molar concentration of polymer species remains constant, pð0Þ ðtÞ ¼ pð0Þ 0 , during the specific chain end scission of polymer. Thus the Eq. (6) can be integrated with initial condition at t ¼ 0; pð1Þ ¼ pð1Þ 0 . ð0Þ pð1Þ ¼ pð1Þ 0 ks xs p0 t
ð7Þ
Because the number-average MW, Mn, is defined as, ð1Þ Mn ¼ ppð0Þ , the above equation can be rearranged to yield, Mn ¼ Mn0 ks xs t
ð8Þ
Eq. (8) suggests that the variation of Mn with time is linear with the slope of the rate coefficient ks and xs is the molecular weight of monomer, 139 g/mol. 2.3.2. Model for solution degradation As explained earlier, PCL undergoes degradation by random chain scission in solution. The binary random fragmentation along the chain can be represented as kd
Pðx0 Þ ! PðxÞ þ Pðx0 xÞ
ðBÞ
The rate of disappearance of reactant species can be obtained by population balance as [11],
Eq. (13) indicates the total mass concentration of the polymer is constant. p(1)=p(1) 0 (at t=0). Solving Eq. (12) with initial condition, p(0) (t=0)=p(0) and invariant 0 mass concentration, as given by Eq. (13), yields ð1Þ pð0Þ ðtÞ pð0Þ 0 ¼ k d p0 t
ð14Þ
Further rearrangement gives Mn0 1 ¼ kd Mno t ¼ ksol t Mn
ð15Þ
Eq. (15) suggests that the time variation of 1/Mn is linear with the slope equal to the rate coefficient, ksol. This is in contrast to Eq. 8 that suggests that the variation of Mn with time is linear with the slope of the rate coefficient ks.
3. Results and discussion The degradation of poly (e-caprolactone) was investigated in bulk and solution. The degradation of PCL in solution was investigated at 170–245 C while the degradation in bulk was carried out at 280–330 C. As discussed earlier, PCL degrades by specific chain end scission in bulk pyrolysis and degrades by random chain scission in solution. Fig. 2 shows the time variation of the number-average molecular weight for the reactions in bulk. The lines in the figure for various temperatures are nearly linear indicating the validity of Eq. (8) and implying that bulk
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G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Fig. 2. Time variation of number average molecular weight in bulk degradation. Legend: &, 280 C; *, 300 C; ~, 310 C; !, 330 C.
degradation occurs by specific chain end scission. As can be seen from the figure, the starting molecular weight for each temperature is different. This is because of transient dynamic heating at 50 K/min till the desired temperature is reached. Aoyagi et al. [10] also observed unzipping mechanism of e-caprolactone from PCL from the hydroxyl end of polymer chain for bulk
degradation. The degradation rate coefficients of poly (e-caprolactone) in bulk conditions were determined from the slopes of Fig. 2 and were in the range of 0.12102 to 4.64102 s1 for the temperature range of 280–330 C. Fig. 3 depicts the variation in the number-average molecular weight with time when the reactions were
Fig. 3. Time variation of number average molecular weight in solution degradation. Legend: &, 170 C; *, 205 C; ~, 225 C; !, 245 C.
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
15
Fig. 4. Arrhenius plot for rate coefficients in bulk.
conducted in solution. It shows the validity of Eq. (15) and thus that the polymer degrades by random degradation. The degradation rate coefficients for solution degradation in the temperature range of 170–245 C were in the range of 0.15104 to 1.54104 s1. The degradation by solution showed degradation at lower temperatures compared bulk pyrolytic degradation. Figs. 4 and 5 show the Arrhenius plot of the degradation rate coefficients of PCL in bulk and solution. The activation energies, obtained from slopes, are 201 and 61 kJ/mol respectively. The activation energy for the degradation in bulk is significantly higher than that for degradation in solution. This also confirms that the
degradation mechanisms of PCL are different when degraded in solution and bulk.
4. Conclusions The thermal degradation of PCL was investigated in bulk and solution. The polymer degrades by random chain scission and specific chain end scission in solution and bulk, respectively. Models based on continuous distribution kinetics were proposed to evaluate the rate coefficients. The activation energy of the processes, determined from the temperature dependency of the rate coefficients, was found to be significantly higher than the degradation in solution.
Fig. 5. Arrhenius plot for rate coefficients in solution.
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G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Acknowledgements The authors thank the Department of Science and Technology, India for financial support.
References [1] Reich L, Stivala SS. Elements of Polymer Degradation. New York: McGraw Hill; 1971. [2] Jellinek HHG. Degradation and Stabilization of Polymers. Amsterdam: Elsevier; 1983. [3] Jellinek HHG. Degradation of Vinyl Polymers. New York: Academic Press; 1955. [4] Sato S, Murakata T, Baba S, Saito Y, Watanabe S. J Appl Polym Sci 1990;40:2065.
[5] Kobhayashi S, Umaya H, Takamoto T. Biomacromolecules 2000;1:3. [6] Narayan R. Biodegradable plastics. In: Opportunities for innovation: biotechnology [NIST GCR 93-633]. Gaithersburg (MD): National Institute of Standards and Technology; 1993. [7] Vert M, Feijen J, Albertson AC, Scott G, Chiellini E. In: Biodegradable polymers and plastics. Redwood Press, Melksham (UK): Royal Society Chemistry; 1992. [8] Sivalingam G, Chattopadhyay S, Madras G. Polym Degrad Stab [in press]. [9] Persenaire O, Alexandre M, Degee P, Dubois P. Biomacromolecules 2001;2:288. [10] Aoyagi Y, Yamashita K, Doi Y. Polym Degrad Stab 2002;76:53. [11] Kodera Y, McCoy BJ. Distribution kinetics of reversible mechanisms: reversible polymer decomposition. AIChE J 1997; 43:3205.
Thermal degradation of poly (e-caprolactone) G. Sivalingam, Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore-12, India Received 27 August 2002; accepted 16 November 2002
Abstract The thermal degradation kinetics of poly (e-caprolactone) (PCL) in solution was investigated at various temperatures (170–245 C). The degradation was also investigated by pyrolysis at various temperatures (280–330 C). The time evolution of molecular weight distribution (MWD) was obtained using gel permeation chromatography. It was observed that in pyrolysis, PCL degrades by specific chain end scission by unzipping of monomers from the hydroxyl end of the polymer chains. In contrast, the degradation of PCL in solution is by random chain scission. A continuous distribution kinetics model is proposed to explain the observed behaviour under both conditions. The activation energy, determined from the temperature dependence of rate coefficients, was 61 kJ/mol and 201 kJ/mol for the degradation in solution and by pyrolysis, respectively. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Poly (e-caprolactone); Random chain scission; Specific chain scission; Pyrolysis; Degradation in solution; Continuous distribution kinetics
1. Introduction The study of degradation of polymers is important in understanding their usability and recycling. The common methods of polymer degradation are biodegradation, pyrolysis, oxidative degradation, photo degradation, and catalytic degradation [1–3]. In spite of its limitations like high melt viscosity, poor heat transfer and excessive vapour release, pyrolysis has been the common method for polymer degradation. To ameliorate these difficulties, degradation in a single phase has been proposed to lower the degradation temperature by better heat transfer [4]. However the mechanism of degradation of polymers is still not completely understood. Biodegradability of poly (e-caprolactone) has been investigated in detail [5– 8], but very little information is known about its thermal stability. Recently, the thermal degradation of PCL was investigated [9,10], but the mechanism is debatable. Persenaire et al. [9] proposed a two-stage degradation mechanism of random cleavage through cis-elimination and specific chain end scission by unzipping from the hydroxyl end of the polymer chain. Aoyagi et al. [10] * Corresponding author. Fax: +91-80-360-0683. E-mail address: [email protected] (G. Madras).
proposed a single step degradation mechanism, where the polymer degrades by specific removal of monomer from the end groups. However, there is no study on the solution degradation of PCL. To the best of our knowledge, this is the first study on solution degradation of PCL. The thermal degradation of PCL was investigated in bulk (pyrolysis) and solution and different mechanisms were observed. A continuous degradation kinetics model was proposed for determination of the rate coefficients in both the processes. The activation energy was determined from the temperature dependence of the rate coefficients.
2. Experimental Poly (e-caprolactone) (Aldrich) of number-average molecular weight of 80,000 and polydispersity of 1.15 was used for the degradation study. The degradation of PCL in bulk was conducted using a Thermogravimetric analyser (TGA) (Perkin-Elmer, Pyris). The degradation was carried out with a heating rate of 50 K/min to the specified temperature and then isothermally holding the temperature for the specified time. Degraded samples were dissolved in tetrahydrofuran (THF) and analysed by gel permeation chromatography to obtain the time
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00376-2
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G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
evolution of molecular weight distribution. In solution degradation, the solvent (diphenyl ether) was heated in a three-necked 250 mL flask by a heating mantle controlled with a PID controller ( 2 C) with constant stirring to the required temperature. PCL was then added to the solution to obtain a concentration of 5 kg/ m3. Samples of 0.5 mL were collected at regular intervals for analysis by Gel permeation chromatography. 2.1. GPC analysis The GPC (Waters, USA) consisted of an isocratic pump, three size exclusion columns, differential refractometer (Waters R401) and a data acquisition system. THF was used as eluent at the flow rate of 1 ml/min. The columns (Styragel HR 4, HR 3, and HR 0.5) (3007.5 mm) packed with cross-linked polystyrene– divinylbenzene were used in series at 50 C. Samples were injected by a Rheodyne valve with a sample loop of 50 ml and the refractive index was continuously monitored using a differential refractive index detector and stored digitally. The chromatogram was converted to the molecular weight distribution using calibration with polystyrene standards. Fig. 1 shows the calibration obtained with polystyrene standards (Polymer Lab, UK) of various molecular weights. The molecular weight (MW) of polystyrene was converted to MW of PCL using the Mark–Houwink equation (KPS=1.25104 dl/g, PS=0.707; KPCL=1.09103 dlL/g, PCL=0.60 in the [Z]=KMa). The calibration (Fig. 1) with polystyrene standards is log10 (MW)=9.9131–0.0046t, where t is in seconds.
2.2. Degradation study The samples of PCL degraded in solution (diphenyl ether) were injected into the GPC. Because diphenyl ether covered the peaks below MW of 200, a conclusion regarding specific products could not be reached. The outlet of the GPC was analysed in a GC–MS and no monomers or oligomers of caprolactone were found. In order to confirm this conclusion, further experiments were conducted in benzene in a Parr reactor at 250 C and 50 atm. Samples were collected at regular intervals and were injected into a GPC system using dichlorobenzene as eluent. This GPC system is equipped with an ELSD (Evaporative Light Scattering Detector) to evaporate the eluent and solvent. This ensured that the low molecular weight products would be clearly visible in the chromatograph. No specific chain end products were detected, indicating that PCL degrades by random chain scission in solution. This is further confirmed by injecting the caprolactone monomer, which gave a distinct peak thus confirming that, in solution, PCL degrades by random chain scission. In contrast, the degradation of the PCL in pyrolysis is by chain-end scission resulting in the monomer. 2.3. Theoretical model 2.3.1. Model for degradation by pyrolysis The chain end scission of PCL of molecular weight x, P(x) to form monomer species Q(xs) can be written as, ks
PðxÞ ! Pðx xs Þ þ Qðxs ÞðAÞ
Fig. 1. Calibration plot for GPC using polystyrene standards. log10 (MW)=9.9131–0.0046t.
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Assuming x to be a continuous variable, the instantaneous concentration of polymeric and specific chain end product is given by p(x,t) and q(t). The population balance for the individual species [11] is ð1 @pðx; tÞ ¼ ks pðx; tÞ þ ks pðx0 ; tÞðx; ðx0 xs ÞÞdx0 ð1Þ @t xs @qðtÞ ¼ @t
ð1
ks pðx0 ; tÞðxs ; x0 Þdx0
ð2Þ
xs
where ks is assumed to be independent of molecular weight. Applying the moment operation f ðjÞ ðtÞ ¼ Ð1 j 0 x ½fðx; tÞ dx to Eqs (1) and (2) yields j X dpðjÞ ðtÞ ¼ ks pðjÞ ðtÞ þ ks j Ci ðxs Þi pðjiÞ ðtÞ dt i¼0
ð3Þ
dqðjÞ ðtÞ ¼ ks pð0Þ xjs ð4Þ dt Eqs. (3) and (4) show the time evolution of polymer and specific product moments. The zeroth and first moment represent the molar and mass concentration of the polymer and, obtained by substituting j=0 and 1 in Eq. (3), are
@pðx; tÞ ¼ kd ðxÞpðx; tÞ @t ð1 þ 2 kd ðx0 Þpðx0 ; tÞOðx; x0 Þdx0
13
ð9Þ
x
The stoichiometric kernel (x,x0 ) in Eq. (9) determines the distribution of scission products and kd(x) is the degradation rate coefficient. For random scission of polymers, the distribution of degraded products [11], (x,x0 ) is 1/x0 and assuming a linear dependence of kd on x yields kd(x)=kdx. Thus Eq. (9) becomes [11], ð1 @pðx; tÞ ¼ kd xpðx; tÞ þ 2 kd pðx0 ; tÞdx0 ð10Þ @t x Applying the moment operation to Eq. (10), dpðjÞ j1 kd pðjþ1Þ ðtÞ ¼ jþ1 dt
ð11Þ
The zeroth and first moments are obtained from Eq. (11) by substituting j=0 and 1, respectively, dpð0Þ ¼ kd pð1Þ ðtÞ dt
ð12Þ
dpð1Þ ¼0 dt
ð13Þ
ð0Þ
dp ¼0 dt
ð5Þ
dpð1Þ ¼ ks xs pð0Þ dt
ð6Þ
Eq. (5) suggests that molar concentration of polymer species remains constant, pð0Þ ðtÞ ¼ pð0Þ 0 , during the specific chain end scission of polymer. Thus the Eq. (6) can be integrated with initial condition at t ¼ 0; pð1Þ ¼ pð1Þ 0 . ð0Þ pð1Þ ¼ pð1Þ 0 ks xs p0 t
ð7Þ
Because the number-average MW, Mn, is defined as, ð1Þ Mn ¼ ppð0Þ , the above equation can be rearranged to yield, Mn ¼ Mn0 ks xs t
ð8Þ
Eq. (8) suggests that the variation of Mn with time is linear with the slope of the rate coefficient ks and xs is the molecular weight of monomer, 139 g/mol. 2.3.2. Model for solution degradation As explained earlier, PCL undergoes degradation by random chain scission in solution. The binary random fragmentation along the chain can be represented as kd
Pðx0 Þ ! PðxÞ þ Pðx0 xÞ
ðBÞ
The rate of disappearance of reactant species can be obtained by population balance as [11],
Eq. (13) indicates the total mass concentration of the polymer is constant. p(1)=p(1) 0 (at t=0). Solving Eq. (12) with initial condition, p(0) (t=0)=p(0) and invariant 0 mass concentration, as given by Eq. (13), yields ð1Þ pð0Þ ðtÞ pð0Þ 0 ¼ k d p0 t
ð14Þ
Further rearrangement gives Mn0 1 ¼ kd Mno t ¼ ksol t Mn
ð15Þ
Eq. (15) suggests that the time variation of 1/Mn is linear with the slope equal to the rate coefficient, ksol. This is in contrast to Eq. 8 that suggests that the variation of Mn with time is linear with the slope of the rate coefficient ks.
3. Results and discussion The degradation of poly (e-caprolactone) was investigated in bulk and solution. The degradation of PCL in solution was investigated at 170–245 C while the degradation in bulk was carried out at 280–330 C. As discussed earlier, PCL degrades by specific chain end scission in bulk pyrolysis and degrades by random chain scission in solution. Fig. 2 shows the time variation of the number-average molecular weight for the reactions in bulk. The lines in the figure for various temperatures are nearly linear indicating the validity of Eq. (8) and implying that bulk
14
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Fig. 2. Time variation of number average molecular weight in bulk degradation. Legend: &, 280 C; *, 300 C; ~, 310 C; !, 330 C.
degradation occurs by specific chain end scission. As can be seen from the figure, the starting molecular weight for each temperature is different. This is because of transient dynamic heating at 50 K/min till the desired temperature is reached. Aoyagi et al. [10] also observed unzipping mechanism of e-caprolactone from PCL from the hydroxyl end of polymer chain for bulk
degradation. The degradation rate coefficients of poly (e-caprolactone) in bulk conditions were determined from the slopes of Fig. 2 and were in the range of 0.12102 to 4.64102 s1 for the temperature range of 280–330 C. Fig. 3 depicts the variation in the number-average molecular weight with time when the reactions were
Fig. 3. Time variation of number average molecular weight in solution degradation. Legend: &, 170 C; *, 205 C; ~, 225 C; !, 245 C.
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
15
Fig. 4. Arrhenius plot for rate coefficients in bulk.
conducted in solution. It shows the validity of Eq. (15) and thus that the polymer degrades by random degradation. The degradation rate coefficients for solution degradation in the temperature range of 170–245 C were in the range of 0.15104 to 1.54104 s1. The degradation by solution showed degradation at lower temperatures compared bulk pyrolytic degradation. Figs. 4 and 5 show the Arrhenius plot of the degradation rate coefficients of PCL in bulk and solution. The activation energies, obtained from slopes, are 201 and 61 kJ/mol respectively. The activation energy for the degradation in bulk is significantly higher than that for degradation in solution. This also confirms that the
degradation mechanisms of PCL are different when degraded in solution and bulk.
4. Conclusions The thermal degradation of PCL was investigated in bulk and solution. The polymer degrades by random chain scission and specific chain end scission in solution and bulk, respectively. Models based on continuous distribution kinetics were proposed to evaluate the rate coefficients. The activation energy of the processes, determined from the temperature dependency of the rate coefficients, was found to be significantly higher than the degradation in solution.
Fig. 5. Arrhenius plot for rate coefficients in solution.
16
G. Sivalingam, G. Madras / Polymer Degradation and Stability 80 (2003) 11–16
Acknowledgements The authors thank the Department of Science and Technology, India for financial support.
References [1] Reich L, Stivala SS. Elements of Polymer Degradation. New York: McGraw Hill; 1971. [2] Jellinek HHG. Degradation and Stabilization of Polymers. Amsterdam: Elsevier; 1983. [3] Jellinek HHG. Degradation of Vinyl Polymers. New York: Academic Press; 1955. [4] Sato S, Murakata T, Baba S, Saito Y, Watanabe S. J Appl Polym Sci 1990;40:2065.
[5] Kobhayashi S, Umaya H, Takamoto T. Biomacromolecules 2000;1:3. [6] Narayan R. Biodegradable plastics. In: Opportunities for innovation: biotechnology [NIST GCR 93-633]. Gaithersburg (MD): National Institute of Standards and Technology; 1993. [7] Vert M, Feijen J, Albertson AC, Scott G, Chiellini E. In: Biodegradable polymers and plastics. Redwood Press, Melksham (UK): Royal Society Chemistry; 1992. [8] Sivalingam G, Chattopadhyay S, Madras G. Polym Degrad Stab [in press]. [9] Persenaire O, Alexandre M, Degee P, Dubois P. Biomacromolecules 2001;2:288. [10] Aoyagi Y, Yamashita K, Doi Y. Polym Degrad Stab 2002;76:53. [11] Kodera Y, McCoy BJ. Distribution kinetics of reversible mechanisms: reversible polymer decomposition. AIChE J 1997; 43:3205.