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Blends of poly(ε-caprolactone) and poly(vinyl acetate): mechanical properties and thermal degradation
Polymer Degradation and Stability 84 (2004) 345e351 www.elsevier.com/locate/polydegstab
Blends of poly(3-caprolactone) and poly(vinyl acetate): mechanical properties and thermal degradation G. Sivalingam, R. Karthik, Giridhar Madras) Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India Received 1 December 2003; received in revised form 5 January 2004; accepted 6 January 2004
Abstract Poly(3-caprolactone) [PCL] and poly(vinyl acetate) [PVAC] were blended by melt blending through single screw extruder and solution blending with tetrahydrofuran (THF) as solvent. Scanning electron microscope [SEM] in conjunction with thermogravimetry [TG] confirmed the complete miscibility of the polymers. Results obtained from the mechanical tensile testing of blends indicated a significant increase in the Young’s modulus, ultimate strength and percentage elongation at breakage over pure polymers suggesting synergism in the polymer blend. The thermal degradation of the pure and blend polymers were studied in nitrogen environment in TG analyzer. The TG, DTG profiles showed the validity of simple linear additive rule suggesting the absence of interaction between the polymers during the pyrolytic degradation. This indicates that though the mechanical properties of the blend exhibit the synergism, the thermal degradation of the polymer blend shows no interaction. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Degradation; Mechanical properties; Blends; Poly(3-caprolactone); Poly(vinyl acetate); biodegradable polymers; Pyrolysis
1. Introduction Biodegradable polymers receive increased attention because of their biodegradability and thermoplastic properties. Poly(3-caprolactone) [PCL] is popular as a biodegradable polymer because of its good mechanical properties and its compatibility with varieties of the polymers. Apart from the biomedical use, it has been used as mould release agent, adhesive, pigment dispersant, synthetic wood dressing as a replacement for plaster of Paris in splints and as a material for orthopedic casts [1]. Poly(vinyl acetate) [PVAC] is normally used as adhesives, binders, and paper finishing agents. One of the most widely employed methods to modify the polymer resins is to blend them [2]. Physical blending is one such technique that has been reported to greatly influence the mechanical properties of polymers [2,3]. PVAC has been used as blending polymers for most
) Corresponding author. Tel.: C91-80-309-2321; fax: C91-80-3600683. E-mail address: [email protected] (G. Madras). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.01.011
of the polymers such as poly(lactic acid) (PLA) [2], poly(ethylene oxide) (PEO) [3], and poly(hydroxybutyrate) (PHB) [4]. It has been proposed that the carboxyl groups from PLA and PHB interact with the a-hydrogens of PVAC due to their proton accepting and proton donating properties, respectively. Such a favorable interaction between the polymers can lead to miscible system [2,4]. Such a complete miscibility and interactions lead to the rapid reduction in the enzymatic degradation of the 95/5 PLA/PVAC blend compared to degradation of PLA [2]. In solution degradation catalyzed by lipases, a strong reduction in the degradation of the 90/10 PCL/ PVAC compared with degradation of PCL was observed and attributed to the interaction between the polymers [5]. In completely miscible systems PLA/PVAC [2], PHB/ PVAC [4] and PEO/PVAC [3], a definite increase in the tensile strength has been observed with smaller addition of PVA ranging from 5 to 30 PVAC with PLA, PHB, and PEO. These suggest that there is synergism in the mechanical properties for such a combination and was attributed to the increased surface tension of the materials due to PVAC addition [2]. One would thus expect
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the blend of PCL and PVAC might also exhibit similar behavior. But, there are no details on the thermal stability or mechanical properties of these blends. The objective of the present study is to study the thermal stability, mechanical properties (tensile and impact strength), and miscibility of the PCL/PVAC. The mode of blending (mechanical blending through single screw extruder and solution blending in THF) on the thermal stability of the polymer was also investigated. The interaction between the polymers was quantified using simple ideal additive rule.
drive motor, amplifier and recorder and storage system. The dumb-bell specimens were also tested on an impact strength tester (Model CS-183T1-079) by following the standard ASTM D1822-93 method. 2.4. Differential scanning calorimetry The glass transition temperatures of various blends were recorded in a Differential Scanning Calorimeter (Perkin Elmer, DSC-2). The experiments were carried out in inert atmosphere (N2) from 100 (C to 100 (C in an aluminum pan. The glass transition temperatures were determined by differentiating the DSC signal.
2. Experimental section 2.5. Scanning electron microscopy 2.1. Materials Poly(3-caprolactone) (Mnd80,000, polydispersityd 1.3, Aldrich) and Poly(vinyl acetate) (Mnd53,500, polydispersityd1.6, prepared by bulk polymerization) were used. The solvent tetrahydrofuran (THF) was procured from S. D. Fines (India).
The miscibility of the PCL and PVAC was tested using SEM (Cambridge instruments, S360), which was operated at 20 kV. The samples were coated with graphite using vacuum evaporator (JOEL, JEE-4X) to aid the conduction during scanning. The magnification was 500!. 2.6. Thermogravimetric analysis
2.2. Preparation of polymer blends The blends of PCL with PVAC were prepared by mechanical blending and solution blending [6,7]. In solution blending, polymers were dissolved in THF in predetermined amounts and the mixture was stirred vigorously to obtain homogeneity. The solvent was removed by evaporation leaving the polymer as a thin film and the polymer was further dried at 40 (C for 72 h. In mechanical blending, the pre-proportionated amount of polymers were mixed in china clay crucible and ground to powders. The physical mixture was charged into a single stage blendereextruder machine operated at a temperature of 125 (C. The once through blend was charged into the feeder cup several times to obtain uniformity. The blending time was kept minimal to avoid the thermal degradation during the blending. To check whether oxidative degradation of polymers took place during sample preparation, TGA studies were carried out for pure polymer prepared by the above blending methods. The TG profiles for both cases were the same, indicating the absence of oxidative degradation of samples during sample preparation. 2.3. Mechanical testing The dumb-bell specimens for mechanical testing were prepared by extruding the pure polymer and homogeneously mixed blends in a Minimax molder (Custom Scientific Instruments, NJ, USA, Model CS-183MMX). The tensile strength of specimens was determined using Minimax tensile tester following the ASTM D1708-93 method. The tensile tester consisted of tensile tester,
Thermal decomposition studies were carried out in a TGA (Perkin Elmer, Pyris) under inert flowing nitrogen (150 mL/min) under non-isothermal heating mode at a heating rate of 5 K/min [8]. The reference material was a-alumina and the mass of the samples analyzed was in the range of 25e30 mg in a platinum crucible. All the runs were carried out in the range of 50 (Ce600 (C. 3. Results and discussion 3.1. Miscibility of the blends The miscibility of poly(3-caprolactone) with poly (vinyl acetate) has been studied using a differential scanning calorimeter. Fig. 1a shows the first derivative of heat flow rate during the measurement for pure polymers (PCL and PVAC) and their physical blends (90/10, 70/30, 50/50 PCL/PVAC). The glass transition temperatures (Tg) measured from the peak of the plot (corresponding to the midpoint glass transition temperature) are shown in Fig. 1b. A single glass transition temperature for PCL, PVAC, 90/10, 70/30 and 50/50 PCL/PVAC blends was observed and are 56.5, 47.0, 32.0, 16.5 and 37.0 (C, respectively. The Fox equation for the prediction of the glass transition temperature of a completely miscible physical polymer blends is 1 w1 w2 ¼ C Tb T1 T2
ð1Þ
In Eq. (1), Ti denotes the glass transition temperature of polymer ‘i’ in Kelvin, wi represents the weight fraction
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Fig. 2. SEM of mechanical blend of (a) 50% PCLe50% PVAC and (b) 10% PCLe90% PVAC with a magnification of 500!.
Fig. 1. a. Plot showing the first derivative of the DSC signal to show the glass transition temperature of various blends. The composition of the blend is marked on the figure. b. Variation of the glass transition temperature with the weight fraction of PCL in the PCLePVAC blend.
of corresponding polymer and Tb is the glass transition temperature of the polymer blend. The glass transition temperature estimated using Fox equation for the 90/10, 70/30 and 50/50 blends are 49, 35 and 18 (C, respectively, which are close to the measured experimental values. In Fig. 1b, the symbols denote the experimental data and the solid line denotes the prediction with the Fox equation. Fig. 2a and b shows the SEM plots of the polymer blends 50/50 PCL/PVAC and 90/10 PCL/ PVAC blend, respectively. TG experiments were repeated several times from the different portions of the solution and mechanical blends and the results are reproducible within G 2%. SEM studies in conjunction with TGA study suggest the complete miscibility of the polymers. PCL was semi-crystalline (around 50%
crystallinity based on DSC) and PVAC is amorphous. In the blends, the degree of crystallinity of PCL remains relatively constant for blends having a large amount of PCL (O90%) but as the PVAC content in the blend increases, the degree of crystallinity of the blend decreases in proportion to the weight fraction of PVAC. 3.2. Mechanical testing The tensile strength, ultimate strength, percentage elongation at breakage and impact strength were determined for the pure polymers and physical blends made by mechanical blending and solution blending. Mechanical properties determined for both mechanically blended and solution blended polymers are found to be invariant suggesting the usability of either method for making the blend. Fig. 3a shows the variation in the Young’s modulus (YM) with composition. The Young’s modulus of the blends is higher than that for pure polymers with the Young’s modulus of 90% PCL in the blend being significantly higher than that of pure PCL. The increase in the mechanical strength can be attributed to the synergism in the PCL/PVAC blend possibly due to the interaction of carboxyl groups in PCL with
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the a-hydrogen atoms of PVAC chain. This synergism is consistent with the earlier observation [2] for the blend of poly(lactic acid) (PLA) and PVAC. Similar trends have also been reported in literature for PEO/PVAC [3] and PHB/PVAC [4] blends. It has been hypothesized that the carboxyl groups of PLA or PHB can interact with the a-hydrogen atoms of PVAC chain leading to some kind of interaction due to their proton accepting and proton donating properties [2,4]. The surface tension measurement of PLA/PVAC blend showed drastic change in the surface tension values between PLA and 95/5 PLA/PVAC blend [2]. Fig. 3b shows the variation of ultimate strength with composition, which also indicates the synergistic behavior of the polymer blends. The percentage elongation at breakage of the blends is also found to be significantly higher than the pure polymers, which further confirms the synergism in the tensile properties. Contrary to the observations of tensile strength of the blends, the impact strength of blend is in between the pure polymers as shown in Fig. 3c. This
indicates that more than 50% PVAC in the blend reduces the impact strength significantly. Fig. 3d shows that presence of 70% PVAC in the blend significantly reduces the percentage elongation at breakage. The above experimental data on mechanical properties can also explain the recent observations on the enzyme degradation of pure polymers and solution blends by lipases [5,9e11]. The enzymatic degradation [9e11] of the pure polymers PCL and PVAC degrades by specific scission leading to the formation of chain scission products of molecular weights w500 and w700 g/mol, respectively [5]. The degradation of PCL, PVAC and their blends in solution by enzyme (Novozyme 435) at 60 (C showed a drastic reduction [5] in the degradability of the 90/10 PCL/PVAC blend compared to the degradation of PCL. This observation can be corroborated to the current observation of increased mechanical properties and the complete mixing indicating some interaction between the polymers [2,3,5].
Fig. 3. Variation of (a) Young’s modulus, (b) ultimate strength, (c) impact strength, and (d) percentage elongation at break with composition of the blend.
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The next objective of the present investigation is to determine the thermal degradation behavior of the physical polymer blends. The TGA profiles of PVAC, PCL and their blends made by mechanical blending and solution blending are shown in Fig. 4a and b, respectively. There was no mass loss up to 250 (C for all polymers. PVAC degrades thermally in two stages [6]. In the first stage (around 325 (C), known as deacetylation, the acetate radicals are generated which in turn abstracts hydrogen from the polymer leading to the formation of polyolefinic backbone and in the second stage (around 450 (C), the structural degradation of the polyene backbone occurs, leading to the evolution of benzene, toluene, and naphthalene. The major reactions occurring in the second stage are cisetrans isomerization,
aromatization and cross-linking [6,12]. On the other hand, PCL degrades in a single stage (around 385 (C) involving simultaneous occurrence of two types of reactions viz, random chain scission and unzipping from hydroxyl end leading to the formation of 3-caprolactone [13,14]. The figure indicates that the blends uniformly degrade between the pure polymers. To quantify the interaction between pure polymers during thermal degradation, differential thermogravimetry profiles (DTG) were used. Fig. 5a and b shows the DTG profiles for mechanical blends and solution blends, respectively. It can be seen from the figures that there are three peaks. The first peak corresponds to deacetylation of PVAC, the second peak corresponds to PCL decomposition and the third smaller peak corresponds to structural breakage of polyolefinic backbone of PVAC. The DTG was divided into two sections with
Fig. 4. Thermogravimetry curves for various blend compositions for blends made by (a) mechanical blending and (b) solution blending. (1), 100% PCL; (2), 90% PCL; (3), 70% PCL; (4), 50% PCL; (5), 30% PCL; (6), 20% PCL; (7), 10% PCL; (8), 0% PCL (PVAC).
Fig. 5. Differential thermogravimetry curves for various blend compositions for blends made by (a) mechanical blending and (b) solution blending. (1), 100% PCL; (2), 90% PCL; (3), 70% PCL; (4), 50% PCL; (5), 30% PCL; (6), 20% PCL; (7), 10% PCL; (8), 0% PCL (PVAC).
3.3. Thermal degradation
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first stage (section A) being the pure deacetylation in the temperature range of 275 (Ce350 (C and the second stage (section B) covers the PCL degradation and the structural degradation of PVAC in the temperature range of 350 (Ce500 (C. Since the PCL decomposition and polyolefinic breakage were merging, the second stage is taken as the sum of the above two processes. These plots indicate that the decomposition temperatures of the blends and pure polymers are not significantly different. The weight loss regime plots for section A and section B for the blends made by mechanical and solution blending are shown in Fig. 6a and b, respectively. The theoretical prediction is obtained by assuming interaction between the individual polymers in the blend. This is P obtained by using the additive rule [6,15] Yobs ¼ i Xi Yi , where Yi is the DTG response of ith polymer in the blend with mass fraction Xi at any temperature. The
Fig. 6. Weight loss regime plots for deacetylation (section A) and nondeacetylation (section B) regime for blends made by (a) mechanical blending and (b) solution blending.
above rule is applied to individual sections A and B. The solid lines indicate the prediction using a simple additive rule as mentioned above for each section and the data points are the area calculated from the DTG of the polymer blends. Thus, Fig. 6a and b implies that the blend behaves close to the prediction indicating no interaction between PCL and PVAC in thermal degradation. It further implied that the method of preparing polymer blend do not play important role on the thermal degradation. Fig. 7a and b shows the deacetylation temperatures and PCL decomposition temperatures of the blends, respectively, for the blends made by above methods. The variation in the decomposition temperatures is within G 5 (C confirming the absence of interaction between the polymers during thermal degradation. The activation energies, determined by
Fig. 7. a. Decomposition temperature plots in deacetylation regime for solution blend (1) and mechanical blend (2) as a function of PVAC weight percent. b. Decomposition temperature plots in PCL decomposition regime for solution blend (1) and mechanical blend (2) as a function of PVAC weight percent.
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Friedman technique [6], of the decomposition were also similar and did not vary with the composition. The activation energy for the PVAC decomposition by deacetylation was in the range of 215e228 kJ/mol while the activation energy for the PCL decomposition was 230e250 kJ/mol. 4. Conclusions The thermal degradation, mechanical properties and enzyme degradability of PCL, PVAC and their blends made by solution and mechanical blends were investigated. A significant increase in the Young’s modulus, ultimate strength and percentage elongation at breakage have been observed for the blends over the pure polymers indicating some kind of synergism in the blend. The impact strength lies between the pure polymers. The thermal degradation studies indicated the absence of interaction between the polymers suggesting the absence of interaction between polymers.
Acknowledgements The authors thank the Department of Science and Technology for financial support and Dr. R. R. N. Sailaja for helpful discussions. The first author thanks
the General Fellowship.
Electric
Company
(GE)
for
a
References [1] Crescenzi V, Manzini G, Calzolari G, Borri C. Eur Polym J 1972; 8:449. [2] Gajria AM, Dave V, Gross RA, McCarthy SP. Polymer 1996;37: 437. [3] Kalfoglu NK. J Polym Sci Polym Phys Ed 1982;20:1259. [4] Kulkarni S. M.S. thesis. Univ. of Massachusetts Lowell; 1992. [5] Sivalingam G, Chattopadhyay S, Madras G. Chem Eng Sci 2003; 58:2911. [6] Sivalingam G, Karthik R, Madras G. Ind Eng Chem Res 2003;42: 3647. [7] Gupta MC, Viswanath SG. Ind Eng Chem Res 1998;37:2707. [8] Sivalingam G, Madras G. Polym Degrad Stab 2003;80:11. [9] Sivalingam G, Chattopadhyay S, Madras G. Polym Degrad Stab 2003;79:413. [10] Chattopadhyay S, Sivalingam G, Madras G. Polym Degrad Stab 2003;80:477. [11] Kobayashi S, Uyama H, Takamoto T. Biomacromolecules 2003;1:3. [12] Jamieson A, McNeill IC. J Polym Sci Polym Chem Ed 1976;14: 603. [13] Sivalingam G, Karthik R, Madras G. J Anal Appl Pyrol 2003; 70(2):631. [14] Persenaire O, Alexandre M, Degee P, Dubois P. Biomacromolecules 2001;2:288. [15] Ishida H, Lee YH. Polymer 2001;42:6971.
Blends of poly(3-caprolactone) and poly(vinyl acetate): mechanical properties and thermal degradation G. Sivalingam, R. Karthik, Giridhar Madras) Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India Received 1 December 2003; received in revised form 5 January 2004; accepted 6 January 2004
Abstract Poly(3-caprolactone) [PCL] and poly(vinyl acetate) [PVAC] were blended by melt blending through single screw extruder and solution blending with tetrahydrofuran (THF) as solvent. Scanning electron microscope [SEM] in conjunction with thermogravimetry [TG] confirmed the complete miscibility of the polymers. Results obtained from the mechanical tensile testing of blends indicated a significant increase in the Young’s modulus, ultimate strength and percentage elongation at breakage over pure polymers suggesting synergism in the polymer blend. The thermal degradation of the pure and blend polymers were studied in nitrogen environment in TG analyzer. The TG, DTG profiles showed the validity of simple linear additive rule suggesting the absence of interaction between the polymers during the pyrolytic degradation. This indicates that though the mechanical properties of the blend exhibit the synergism, the thermal degradation of the polymer blend shows no interaction. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Degradation; Mechanical properties; Blends; Poly(3-caprolactone); Poly(vinyl acetate); biodegradable polymers; Pyrolysis
1. Introduction Biodegradable polymers receive increased attention because of their biodegradability and thermoplastic properties. Poly(3-caprolactone) [PCL] is popular as a biodegradable polymer because of its good mechanical properties and its compatibility with varieties of the polymers. Apart from the biomedical use, it has been used as mould release agent, adhesive, pigment dispersant, synthetic wood dressing as a replacement for plaster of Paris in splints and as a material for orthopedic casts [1]. Poly(vinyl acetate) [PVAC] is normally used as adhesives, binders, and paper finishing agents. One of the most widely employed methods to modify the polymer resins is to blend them [2]. Physical blending is one such technique that has been reported to greatly influence the mechanical properties of polymers [2,3]. PVAC has been used as blending polymers for most
) Corresponding author. Tel.: C91-80-309-2321; fax: C91-80-3600683. E-mail address: [email protected] (G. Madras). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.01.011
of the polymers such as poly(lactic acid) (PLA) [2], poly(ethylene oxide) (PEO) [3], and poly(hydroxybutyrate) (PHB) [4]. It has been proposed that the carboxyl groups from PLA and PHB interact with the a-hydrogens of PVAC due to their proton accepting and proton donating properties, respectively. Such a favorable interaction between the polymers can lead to miscible system [2,4]. Such a complete miscibility and interactions lead to the rapid reduction in the enzymatic degradation of the 95/5 PLA/PVAC blend compared to degradation of PLA [2]. In solution degradation catalyzed by lipases, a strong reduction in the degradation of the 90/10 PCL/ PVAC compared with degradation of PCL was observed and attributed to the interaction between the polymers [5]. In completely miscible systems PLA/PVAC [2], PHB/ PVAC [4] and PEO/PVAC [3], a definite increase in the tensile strength has been observed with smaller addition of PVA ranging from 5 to 30 PVAC with PLA, PHB, and PEO. These suggest that there is synergism in the mechanical properties for such a combination and was attributed to the increased surface tension of the materials due to PVAC addition [2]. One would thus expect
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the blend of PCL and PVAC might also exhibit similar behavior. But, there are no details on the thermal stability or mechanical properties of these blends. The objective of the present study is to study the thermal stability, mechanical properties (tensile and impact strength), and miscibility of the PCL/PVAC. The mode of blending (mechanical blending through single screw extruder and solution blending in THF) on the thermal stability of the polymer was also investigated. The interaction between the polymers was quantified using simple ideal additive rule.
drive motor, amplifier and recorder and storage system. The dumb-bell specimens were also tested on an impact strength tester (Model CS-183T1-079) by following the standard ASTM D1822-93 method. 2.4. Differential scanning calorimetry The glass transition temperatures of various blends were recorded in a Differential Scanning Calorimeter (Perkin Elmer, DSC-2). The experiments were carried out in inert atmosphere (N2) from 100 (C to 100 (C in an aluminum pan. The glass transition temperatures were determined by differentiating the DSC signal.
2. Experimental section 2.5. Scanning electron microscopy 2.1. Materials Poly(3-caprolactone) (Mnd80,000, polydispersityd 1.3, Aldrich) and Poly(vinyl acetate) (Mnd53,500, polydispersityd1.6, prepared by bulk polymerization) were used. The solvent tetrahydrofuran (THF) was procured from S. D. Fines (India).
The miscibility of the PCL and PVAC was tested using SEM (Cambridge instruments, S360), which was operated at 20 kV. The samples were coated with graphite using vacuum evaporator (JOEL, JEE-4X) to aid the conduction during scanning. The magnification was 500!. 2.6. Thermogravimetric analysis
2.2. Preparation of polymer blends The blends of PCL with PVAC were prepared by mechanical blending and solution blending [6,7]. In solution blending, polymers were dissolved in THF in predetermined amounts and the mixture was stirred vigorously to obtain homogeneity. The solvent was removed by evaporation leaving the polymer as a thin film and the polymer was further dried at 40 (C for 72 h. In mechanical blending, the pre-proportionated amount of polymers were mixed in china clay crucible and ground to powders. The physical mixture was charged into a single stage blendereextruder machine operated at a temperature of 125 (C. The once through blend was charged into the feeder cup several times to obtain uniformity. The blending time was kept minimal to avoid the thermal degradation during the blending. To check whether oxidative degradation of polymers took place during sample preparation, TGA studies were carried out for pure polymer prepared by the above blending methods. The TG profiles for both cases were the same, indicating the absence of oxidative degradation of samples during sample preparation. 2.3. Mechanical testing The dumb-bell specimens for mechanical testing were prepared by extruding the pure polymer and homogeneously mixed blends in a Minimax molder (Custom Scientific Instruments, NJ, USA, Model CS-183MMX). The tensile strength of specimens was determined using Minimax tensile tester following the ASTM D1708-93 method. The tensile tester consisted of tensile tester,
Thermal decomposition studies were carried out in a TGA (Perkin Elmer, Pyris) under inert flowing nitrogen (150 mL/min) under non-isothermal heating mode at a heating rate of 5 K/min [8]. The reference material was a-alumina and the mass of the samples analyzed was in the range of 25e30 mg in a platinum crucible. All the runs were carried out in the range of 50 (Ce600 (C. 3. Results and discussion 3.1. Miscibility of the blends The miscibility of poly(3-caprolactone) with poly (vinyl acetate) has been studied using a differential scanning calorimeter. Fig. 1a shows the first derivative of heat flow rate during the measurement for pure polymers (PCL and PVAC) and their physical blends (90/10, 70/30, 50/50 PCL/PVAC). The glass transition temperatures (Tg) measured from the peak of the plot (corresponding to the midpoint glass transition temperature) are shown in Fig. 1b. A single glass transition temperature for PCL, PVAC, 90/10, 70/30 and 50/50 PCL/PVAC blends was observed and are 56.5, 47.0, 32.0, 16.5 and 37.0 (C, respectively. The Fox equation for the prediction of the glass transition temperature of a completely miscible physical polymer blends is 1 w1 w2 ¼ C Tb T1 T2
ð1Þ
In Eq. (1), Ti denotes the glass transition temperature of polymer ‘i’ in Kelvin, wi represents the weight fraction
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Fig. 2. SEM of mechanical blend of (a) 50% PCLe50% PVAC and (b) 10% PCLe90% PVAC with a magnification of 500!.
Fig. 1. a. Plot showing the first derivative of the DSC signal to show the glass transition temperature of various blends. The composition of the blend is marked on the figure. b. Variation of the glass transition temperature with the weight fraction of PCL in the PCLePVAC blend.
of corresponding polymer and Tb is the glass transition temperature of the polymer blend. The glass transition temperature estimated using Fox equation for the 90/10, 70/30 and 50/50 blends are 49, 35 and 18 (C, respectively, which are close to the measured experimental values. In Fig. 1b, the symbols denote the experimental data and the solid line denotes the prediction with the Fox equation. Fig. 2a and b shows the SEM plots of the polymer blends 50/50 PCL/PVAC and 90/10 PCL/ PVAC blend, respectively. TG experiments were repeated several times from the different portions of the solution and mechanical blends and the results are reproducible within G 2%. SEM studies in conjunction with TGA study suggest the complete miscibility of the polymers. PCL was semi-crystalline (around 50%
crystallinity based on DSC) and PVAC is amorphous. In the blends, the degree of crystallinity of PCL remains relatively constant for blends having a large amount of PCL (O90%) but as the PVAC content in the blend increases, the degree of crystallinity of the blend decreases in proportion to the weight fraction of PVAC. 3.2. Mechanical testing The tensile strength, ultimate strength, percentage elongation at breakage and impact strength were determined for the pure polymers and physical blends made by mechanical blending and solution blending. Mechanical properties determined for both mechanically blended and solution blended polymers are found to be invariant suggesting the usability of either method for making the blend. Fig. 3a shows the variation in the Young’s modulus (YM) with composition. The Young’s modulus of the blends is higher than that for pure polymers with the Young’s modulus of 90% PCL in the blend being significantly higher than that of pure PCL. The increase in the mechanical strength can be attributed to the synergism in the PCL/PVAC blend possibly due to the interaction of carboxyl groups in PCL with
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the a-hydrogen atoms of PVAC chain. This synergism is consistent with the earlier observation [2] for the blend of poly(lactic acid) (PLA) and PVAC. Similar trends have also been reported in literature for PEO/PVAC [3] and PHB/PVAC [4] blends. It has been hypothesized that the carboxyl groups of PLA or PHB can interact with the a-hydrogen atoms of PVAC chain leading to some kind of interaction due to their proton accepting and proton donating properties [2,4]. The surface tension measurement of PLA/PVAC blend showed drastic change in the surface tension values between PLA and 95/5 PLA/PVAC blend [2]. Fig. 3b shows the variation of ultimate strength with composition, which also indicates the synergistic behavior of the polymer blends. The percentage elongation at breakage of the blends is also found to be significantly higher than the pure polymers, which further confirms the synergism in the tensile properties. Contrary to the observations of tensile strength of the blends, the impact strength of blend is in between the pure polymers as shown in Fig. 3c. This
indicates that more than 50% PVAC in the blend reduces the impact strength significantly. Fig. 3d shows that presence of 70% PVAC in the blend significantly reduces the percentage elongation at breakage. The above experimental data on mechanical properties can also explain the recent observations on the enzyme degradation of pure polymers and solution blends by lipases [5,9e11]. The enzymatic degradation [9e11] of the pure polymers PCL and PVAC degrades by specific scission leading to the formation of chain scission products of molecular weights w500 and w700 g/mol, respectively [5]. The degradation of PCL, PVAC and their blends in solution by enzyme (Novozyme 435) at 60 (C showed a drastic reduction [5] in the degradability of the 90/10 PCL/PVAC blend compared to the degradation of PCL. This observation can be corroborated to the current observation of increased mechanical properties and the complete mixing indicating some interaction between the polymers [2,3,5].
Fig. 3. Variation of (a) Young’s modulus, (b) ultimate strength, (c) impact strength, and (d) percentage elongation at break with composition of the blend.
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The next objective of the present investigation is to determine the thermal degradation behavior of the physical polymer blends. The TGA profiles of PVAC, PCL and their blends made by mechanical blending and solution blending are shown in Fig. 4a and b, respectively. There was no mass loss up to 250 (C for all polymers. PVAC degrades thermally in two stages [6]. In the first stage (around 325 (C), known as deacetylation, the acetate radicals are generated which in turn abstracts hydrogen from the polymer leading to the formation of polyolefinic backbone and in the second stage (around 450 (C), the structural degradation of the polyene backbone occurs, leading to the evolution of benzene, toluene, and naphthalene. The major reactions occurring in the second stage are cisetrans isomerization,
aromatization and cross-linking [6,12]. On the other hand, PCL degrades in a single stage (around 385 (C) involving simultaneous occurrence of two types of reactions viz, random chain scission and unzipping from hydroxyl end leading to the formation of 3-caprolactone [13,14]. The figure indicates that the blends uniformly degrade between the pure polymers. To quantify the interaction between pure polymers during thermal degradation, differential thermogravimetry profiles (DTG) were used. Fig. 5a and b shows the DTG profiles for mechanical blends and solution blends, respectively. It can be seen from the figures that there are three peaks. The first peak corresponds to deacetylation of PVAC, the second peak corresponds to PCL decomposition and the third smaller peak corresponds to structural breakage of polyolefinic backbone of PVAC. The DTG was divided into two sections with
Fig. 4. Thermogravimetry curves for various blend compositions for blends made by (a) mechanical blending and (b) solution blending. (1), 100% PCL; (2), 90% PCL; (3), 70% PCL; (4), 50% PCL; (5), 30% PCL; (6), 20% PCL; (7), 10% PCL; (8), 0% PCL (PVAC).
Fig. 5. Differential thermogravimetry curves for various blend compositions for blends made by (a) mechanical blending and (b) solution blending. (1), 100% PCL; (2), 90% PCL; (3), 70% PCL; (4), 50% PCL; (5), 30% PCL; (6), 20% PCL; (7), 10% PCL; (8), 0% PCL (PVAC).
3.3. Thermal degradation
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first stage (section A) being the pure deacetylation in the temperature range of 275 (Ce350 (C and the second stage (section B) covers the PCL degradation and the structural degradation of PVAC in the temperature range of 350 (Ce500 (C. Since the PCL decomposition and polyolefinic breakage were merging, the second stage is taken as the sum of the above two processes. These plots indicate that the decomposition temperatures of the blends and pure polymers are not significantly different. The weight loss regime plots for section A and section B for the blends made by mechanical and solution blending are shown in Fig. 6a and b, respectively. The theoretical prediction is obtained by assuming interaction between the individual polymers in the blend. This is P obtained by using the additive rule [6,15] Yobs ¼ i Xi Yi , where Yi is the DTG response of ith polymer in the blend with mass fraction Xi at any temperature. The
Fig. 6. Weight loss regime plots for deacetylation (section A) and nondeacetylation (section B) regime for blends made by (a) mechanical blending and (b) solution blending.
above rule is applied to individual sections A and B. The solid lines indicate the prediction using a simple additive rule as mentioned above for each section and the data points are the area calculated from the DTG of the polymer blends. Thus, Fig. 6a and b implies that the blend behaves close to the prediction indicating no interaction between PCL and PVAC in thermal degradation. It further implied that the method of preparing polymer blend do not play important role on the thermal degradation. Fig. 7a and b shows the deacetylation temperatures and PCL decomposition temperatures of the blends, respectively, for the blends made by above methods. The variation in the decomposition temperatures is within G 5 (C confirming the absence of interaction between the polymers during thermal degradation. The activation energies, determined by
Fig. 7. a. Decomposition temperature plots in deacetylation regime for solution blend (1) and mechanical blend (2) as a function of PVAC weight percent. b. Decomposition temperature plots in PCL decomposition regime for solution blend (1) and mechanical blend (2) as a function of PVAC weight percent.
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Friedman technique [6], of the decomposition were also similar and did not vary with the composition. The activation energy for the PVAC decomposition by deacetylation was in the range of 215e228 kJ/mol while the activation energy for the PCL decomposition was 230e250 kJ/mol. 4. Conclusions The thermal degradation, mechanical properties and enzyme degradability of PCL, PVAC and their blends made by solution and mechanical blends were investigated. A significant increase in the Young’s modulus, ultimate strength and percentage elongation at breakage have been observed for the blends over the pure polymers indicating some kind of synergism in the blend. The impact strength lies between the pure polymers. The thermal degradation studies indicated the absence of interaction between the polymers suggesting the absence of interaction between polymers.
Acknowledgements The authors thank the Department of Science and Technology for financial support and Dr. R. R. N. Sailaja for helpful discussions. The first author thanks
the General Fellowship.
Electric
Company
(GE)
for
a
References [1] Crescenzi V, Manzini G, Calzolari G, Borri C. Eur Polym J 1972; 8:449. [2] Gajria AM, Dave V, Gross RA, McCarthy SP. Polymer 1996;37: 437. [3] Kalfoglu NK. J Polym Sci Polym Phys Ed 1982;20:1259. [4] Kulkarni S. M.S. thesis. Univ. of Massachusetts Lowell; 1992. [5] Sivalingam G, Chattopadhyay S, Madras G. Chem Eng Sci 2003; 58:2911. [6] Sivalingam G, Karthik R, Madras G. Ind Eng Chem Res 2003;42: 3647. [7] Gupta MC, Viswanath SG. Ind Eng Chem Res 1998;37:2707. [8] Sivalingam G, Madras G. Polym Degrad Stab 2003;80:11. [9] Sivalingam G, Chattopadhyay S, Madras G. Polym Degrad Stab 2003;79:413. [10] Chattopadhyay S, Sivalingam G, Madras G. Polym Degrad Stab 2003;80:477. [11] Kobayashi S, Uyama H, Takamoto T. Biomacromolecules 2003;1:3. [12] Jamieson A, McNeill IC. J Polym Sci Polym Chem Ed 1976;14: 603. [13] Sivalingam G, Karthik R, Madras G. J Anal Appl Pyrol 2003; 70(2):631. [14] Persenaire O, Alexandre M, Degee P, Dubois P. Biomacromolecules 2001;2:288. [15] Ishida H, Lee YH. Polymer 2001;42:6971.