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poly(ethylene glycol) blends via direct pyrolysis mass spectrometry
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Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry Esra Ozdemir a , Teoman Tinc¸er a,b , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 15 June 2016 Received in revised form 12 August 2016 Accepted 20 September 2016 Available online xxx Keywords: Poly(lactide) Poly(lactide)/poly(ethylene glycol) blends Thermal degradation Pyrolysis mass spectrometry
a b s t r a c t In this study, melt blended poly(lactic acid) and poly(ethylene glycol), (PLA)/PEG samples involving 10, 15 and 20 wt% PEG were prepared and characterized by direct pyrolysis mass spectrometry technique in addition to classical techniques such differential scanning calorimetry, thermogravimetric analyses and mechanical tests. The incorporation of PEG resulted in consistent and significant decrease in the tensile strength and modulus, and reduction in endothermic melting peak of PLA due to the plasticizing effect of PEG with the virgin PLA matrix. Both TGA and DP-MS analyses pointed out that the thermal decomposition of the blend occurred mainly in two steps. In addition, the pyrolysis mass spectrometry analyses indicated presence of chains generated by the interactions of ether linkages of PEG and COOH groups of PLA, present either as end groups or due to reactions with water, during the blending and/or pyrolysis process. As a consequence, decrease in the thermal stability of PEG chains was detected. Analyses of the blends prepared by solution mixing for variable periods confirmed that these interactions took place mainly during the blending process. In addition, increase in the thermal stabilities of both components for the blends prepared by stirring for prolonged times was detected and associated with generation of a crosslinked structure. © 2016 Published by Elsevier B.V.
1. Introduction Among all bio-based biodegradable polymers, poly(lactic acid) (PLA), a promising polyester, has drawn significant attention not only due to its comparable properties with conventional polymers but also relatively better properties than other biodegradable polymers [1–7]. Even though PLA presents environmentally friendly properties like biocompatibility, it still shows limited thermal, mechanical and barrier properties for many applications [6,8–12]. Mechanical and thermal properties of PLA can be improved by copolymerization, blending with other polymers, plasticization using biocompatible plasticizer and incorporation of filler materials. Among these, being a simple and more economic way, blending of PLA with various polymers has been investigated by many authors [13–15]. Poly(ethylene glycol), (PEG), due to its miscibility, biodegradability, and food contactable application, is almost the most suitable material which has been used as an impact modifier for PLA [16–20].
∗ Corresponding author at: Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey. E-mail address: [email protected] (J. Hacaloglu).
It has been determined that both the molecular weight and the amount of PEG has significant effects on the properties of PLA/PEG blends [17,21–25]. A limit of miscibility and brittleness characteristics depending upon the plasticizer content and the molecular weight were detected [17,21]. The plasticizing efficiency was increased with decreasing molecular weight of PEG [23,24]. Sheth et al. found that PLA/PEG blends varied from completely miscible to partially miscible, depending upon the PEG concentration [22]. Kim and coworkers showed that in the presence of 40% high molecular weight PEG percentage of elongation at break is improved significantly [23]. PEG, being a plasticizer, shows plasticization effect, gives rise to increase in PLA chain mobility, allows the rearrangement of the chains. PEG by disturbing the intermolecular forces, decreases the glass transition temperature, Tg of PLA [21–26]. Increasing the amount of PEG leads to decrease in Tg and improvement in crystallization [21]. Ahmed and coworkers determined that the incorporation of PEG as a plasticizer reduces Tg and melting point, Tm of neat PLA significantly, as expected [25]. In a recent research, Stoehr and coworkers examined the effect of PEG amount and recorded that PEG content affects Tg and cold crystallization temperature, Tcc [26].
http://dx.doi.org/10.1016/j.jaap.2016.09.010 0165-2370/© 2016 Published by Elsevier B.V.
Please cite this article in press as: E. Ozdemir, et al., Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.09.010
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Several studies on thermal degradation characteristics of PLA and PEG were also appeared in the literature [27–34]. However, there is insufficient information about thermal degradation behaviors of PLA/PEG blends. Therefore, the main objective of this research was to focus on both thermal properties and degradation mechanisms of plasticized PLA. For this purpose we also applied direct pyrolysis mass spectrometry, DP-MS that has been shown as a valuable tool for analyses of polymers [35–39]. The applications of DP-MS for thermal characterization involve investigation of thermal stability, degradation products and decomposition mechanism of homopolymers, copolymers, polymer blends and composites. The technique has great potential for identification of additives and structure in complex polymer matrices without time consuming extractions or derivatizations, as components are separated as a function of their volalities and/or thermal stabilities [35]. 2. Experimental 2.1. Materials Polylactide, PLA, (number average molecular weight, Mn ∼ 190000), and poly (ethylene glycol) (Mn ∼ 8000), were purchased from Cargill Dow. PLA and PEG were dried at 60 ◦ C overnight under reduced pressure prior to the mixing processes. The mixtures of PLA/PEG with appropriate blend ratios, (90/10, 85/15 and 80/20 wt/wt) were melt-compounded using a DSM Xplore twin screw micro compounder at 190 ◦ C, with a screw speed 100 rpm for 8 min. PLA/PEG 80/20 wt/wt blends were also prepared by solution mixing by magnetic (1000 rph) stirring for 1, 12 and 24 h in chloroform and dried at 60 ◦ C overnight under reduced pressure. Samples of PLA and PLA/PEG blends were dried in vacuum at 60 ◦ C before mechanical testing. The dog-bone shaped specimens of PLA and its blends (dimension 50.0, 7.6, 2.0 mm length, width and thickness respectively) were obtained by Daca injection molding instrument at a barrel temperature of 190–195 ◦ C and mold temperature of 40 ◦ C. The extruded samples were forced to mold cavities at 8 bar pressure. The dog bone shaped specimens obtained after injection molding, were dried again at 65 ◦ C overnight under vacuum before tensile testing.
Table 1 Mechanical properties of PLA and PLA/PEG blends prepared by melt blending.
PLA PLA/PEG 90/10 PLA/PEG 85/15 PLA/PEG 80/20
Tensile Strength (MPa)
% Elongation at Break
Young’s Modulus (MPa)
64.5 ± 1.3 48.9 ± 0.7 31.5 ± 2.6 16.1 ± 1.6
11.0 ± 0.5 12.1 ± 2.1 8.1 ± 0.8 43.1 ± 10.4
1180 ± 30 950 ± 30 650 ± 40 330 ± 30
break, as brittleness is inversely proportional to elongation at break. The incorporation of PEG resulted in consistent and significant decrease in the tensile strength and modulus, due to the plasticizing effect of PEG with the virgin PLA matrix. The blend prepared at PLA/PEG ratio of 80/20, exhibited a decrease in tensile strength to the tune of 75.0% and tensile modulus to 72.0% respectively, as compared with the pristine PLA matrix. The variation in elongation at break for the blends involving 10 and 15 wt% PEG, showed no constituency with addition of PEG, however, it seems that there existed almost no significant change in elongation within the experimental errors. Finally, the elongation at break was increased noticeably only for the blend containing 20 wt% PEG, indicating significant decrease in brittleness of PLA. The blends with high elongation at break were characterized by relatively weak impact values. 3.2. Differential scanning calorimetry and thermogravimetry analyses DSC images of the melt blended PLA/PEG samples involving 10, 15 and 20 wt% PEG prepared by melt blending, are shown in Fig. 1. The endothermic melting peak of PLA at around 172.3 ◦ C was shifted to 165.7, 166.7 and 165.9 ◦ C with the addition of 10, 15 and 20% PEG blends respectively, mainly due to the plasticizing effect of PEG and indicated generation of a compatible system [12]. The cold crystallization temperature (Tc) of PLA appeared at 86.2, 77.3 and 88.4 ◦ C with addition of 10, 15 and 20 wt% PEG respectively, The TGA curves of PLA, PEG and the melt blended PLA/PEG samples are shown in Fig. 2 and the relevant data are listed in Table 2. Thermal stability of PEG was noticeably higher than that of PLA. Thermal degradation of PLA/PEG blends occurred in two steps. The thermal decomposition of PLA was shifted to lower temperatures in the presence of PEG. The second step of decomposition associ-
2.2. Characterization The universal testing machine (Lloyd LR30K) as per ASTMD638, at crosshead speed of 50 mm/min and cell load of 5 kn was used for tensile tests. Differential Calorimetry, (DSC) and Thermogravimetry Analyses (TGA) were performed on a Perkin Elmer Instrument STA6000 under nitrogen atmosphere at a flow rate of 20 mL/min and a heating rate of 10 ◦ C/min. Direct pyrolysis mass spectrometry (DP-MS) analyses were performed using 5973 HP quadruple mass spectrometry system coupled to a JHP SIS direct insertion probe pyrolysis system. 70 eV EI mass spectra, at a rate of 2 scan/s, were recorded. Samples (0.10 mg) in the flared glass sample vials were heated to 450 ◦ C at a rate of 10 ◦ C/min. Experiments were repeated at least twice to ensure reproducibility. 3. Results and discussions 3.1. Mechanical properties The variation of mechanical properties of PLA and PLA/PEG blends as a function of PEG loading are collected in Table 1. The neat PLA is typically brittle and rigid. Thus, it has high tensile strength and elastic modulus (Young’s modulus) but very low% elongation at
Fig. 1. DSC curves of PLA, PEG and PLA/PEG blends.
Please cite this article in press as: E. Ozdemir, et al., Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.09.010
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Fig. 2. TGA curves of PLA, PEG and PLA/PEG blends.
Table 2 TGA data for PLA, PEG and PLA/PEG blends prepared by melt blending.
PLA PEG PLA/PEG 90/10 PLA/PEG 85/15 PLA/PEG 80/20
T5%
T10%
Tmax1
Tmax2
%char yield
329.06 338.36 315.82 308.82 310.48
336.96 354.94 326.91 320.78 320.63
366.91
– 395.3 – 394.26 397.68
0.81% 1.18% 0.91%
358.31 358.58 351.04
ated with the degradation of PEG became more pronounced as the amount of PEG incorporated was increased. 3.3. Pyrolysis mass spectrometry analyses The total ion current, TIC, curves (the variation of total ion yield as a function of temperature), of PLA, PEG and PLA/PEG blends, prepared by melt blending, and the mass spectra recorded at peak maxima are given in Fig. 3. The pyrolysis mass spectrum of PLA is dominated by series of peaks due to fragments with general formula (C2 H4 CO2 )x C2 H4 CO (m/z = 72x − 88 for x = 2–12), generated by elimination of the neutral molecules CO2 and acetaldehyde during ionization of the cyclic oligomers, in addition to low mass fragments produced by random chain homolysis reactions [27–31]. A second, much less pronounced series of peaks with m/z = 72x + 73 for x = 2–12 are associated with fragments produced by cis-elimination reactions with general formula (C2 H4 CO2 )x H. It is known that thermal degradation of PEG mainly occurs via random chain cleavages of C O bonds followed by H transfer reactions [32–34]. Products due to dissociation of C C bonds are also generated but are less abundant. Nevertheless, several end groups such as methyl, (C2 H4 O)x CH3 (m/z = 44x + 15 for x = 2–12), ethyl, (C2 H4 O)x C2 H5 (m/z = 44x + 29 for x = 2–12), hydroxyl, (m/z = 44x + 17 for x = 2–12), aldehyde (C2 H4 O)x C2 H3 O (m/z = 44x + 43 for x = 0–17), and unsaturated groups (C2 H4 O)x C2 H3 (m/z = 44x + 27 for x = 0–17) are generated. As a consequence of further dissociation during ionization, peaks due to low mass fragments are more intense. Among these, the most abundant product is C2 H4 OH fragment (m/z = 45 Da) and the yields of the products involving hydroxyl end groups are more intense.
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The TIC curves of the blends show two peaks, an intense one associated with the decomposition of PLA and a weak one attributed to the degradation of PEG. In general, the evolution of thermal degradation products of PLA was detected almost in the same temperature region as the neat PLA. On the other hand, the high temperature peak associated with the decomposition of PEG was shifted to low temperature regions with the increase in the amount of PEG incorporated. Unfortunately, due to the similarities in the repeating units of PLA and PEG, some of the thermal degradation products of PLA and PEG have the same m/z values; i.e. CO2 H from PLA and C2 H4 OH from PEG have m/z value 45 Da and OC2 H4 CO2 H from PLA and (C2 H4 O)2 H from PEG have m/z value 89 Da. However, the thermal degradations of PLA and PEG occur at two distinct temperature regions. Thus, the trends in single ion pyrograms have significant importance in understanding the thermal behaviors. Single ion evolution profiles of some representative characteristic products of PLA and PEG detected during the pyrolysis of PLA, PEG and 90/10 wt% PLA/PEG blend are depicted in Fig. 4. The evolution of products generated by cis-elimination reactions, such as (C2 H4 CO2 )2 H (145 Da) and (C2 H4 CO2 )3 H (217 Da) and those produced by trans-esterification reactions such as (C2 H4 CO2 )2 C2 H4 CO (200 Da) and (C2 H4 CO2 )4 C2 H4 CO (344 Da) showed almost identical trends during the pyrolysis of PLA and PLA/PEG blend. In contrast, noticeable changes in the evolution profiles of low mass products such as CO2 H (45 Da) and C2 H4 CO (56 Da) were detected. In general, the relative intensities of the thermal degradation products diagnostic to PEG were less abundant as expected. A sharp peak at around 366 ◦ C appeared in the single ion pyrograms of thermal degradation products of PEG. One possibility is the contribution of PLA based products with the same m/z values. However, the evolution profiles of the diagnostic products PEG such as (C2 H4 O)2 CH2 CHO (131 Da), (C2 H4 O)3 CH2 CHO (175 Da), and (C2 H4 CO)2 CH3 (103 Da) having m/z values different than those of the characteristic products of PLA, also showed two peaks, a sharp one with a maximum at around 366 ◦ C and a broad peak with a maximum at around 442 ◦ C. Elimination of new products that were not recorded during the pyrolysis of the neat components was also noticed upon inspection of pyrolysis mass spectra carefully. These relatively weak products (i.e. fragments with m/z values 231 and 303 Da) showed a single peak at around 366 ◦ C. The single ion evolution profiles of these products are also included in the figure for comparison. In order to get a better in sight, the pyrolysis data for PLA/PEG blends involving 10, 15 and 20 wt% PEG were studied (Fig. 5). The most abundant products were due to the thermal degradation of PLA chains via trans-esterification reactions. However, the relative yields of low mass products and the products generated by ciselimination reactions decreased as the amount of PEG in the blend increased. Increase in the relative yields of PEG based products was detected as expected. In addition, the relative yields of products showing a single sharp peak in their evolution profiles were also increased noticeably. The trends in the single ion evolution profiles of PLA based products revealed that the evolution of major thermal degradation products of PLA generated by trans-esterification and cis-elimination reactions were shifted slightly to high temperature regions as the amount of PEG involved in the blend was increased. A shift to high temperature regions was also recorded for the sharp peak at around 366 ◦ C in the same order. In contrast, the peak at around 437 ◦ C associated with degradation of PEG was shifted to low temperatures with the increase in PEG wt% in the blend. It is clear that thermal behaviors of PEG and PLA were changed upon incorporation of PEG into PLA matrix. It may be proposed that during the mixing process via melt blending at 190 ◦ C or during the pyrolysis of the blend, the PLA chains or fragments
Please cite this article in press as: E. Ozdemir, et al., Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.09.010
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Fig. 3. TIC curves and pyrolysis mass spectra recorded at peak maxima recorded during the pyrolysis of PLA, PEG and PLA/PEG blends.
involving COOH end groups, leads to the cleavage of the ether groups of PEG as shown in Scheme 1. As a consequence, lower mass chains of PEG bearing OH end groups and PLA chains linked to ethylene oxide units should be generated. Thermal degradation of PLA chains with ethylene oxide units may generate series of products involving both repeating units. Considering the thermal degradation mechanisms of PEG and PLA, it may be thought that
the decomposition of these chains yields products with general formula (C2 H4 CO2 )x (C2 H4 O)y H, (C2 H4 CO2 )x (C2 H4 O)y C2 H3 , and (C2 H4 CO2 )x (C2 H4 O)y C2 H4 H (C2 H4 CO2 )x (C2 H4 O)y C2 H3 O (C2 H4 CO2 )x (C2 H4 O)y CH3 , by the cleavage of ether linkages followed by H-transfer reactions (Scheme 2). As discussed before some of the pyrolysis products of PLA, PEG or both have the same m/z values with these products. However, products with
Please cite this article in press as: E. Ozdemir, et al., Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.09.010
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Fig. 4. Single ion evolution profiles of some selected products recorded during the pyrolysis of PLA, PEG and 85/15 PLA/PEG blend.
Fig. 5. Single ion evolution profiles of some selected products recorded during the pyrolysis of PLA blend involving 10, 15 and 20 wt% PEG prepared by melt blending.
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Scheme 1. Acid decomposition of ether linkages.
m/z values 231, 303 and 319 Da can directly be associated with (C2 H4 CO2 )(C2 H4 O)3 C2 H3 , (C2 H4 CO2 )2 (C2 H4 O)3 C2 H3 , and (C2 H4 CO2 )2 (C2 H4 O)3 C2 H3 O, respectively and the significant increase in the relative abundances of these fragments with the increase in the amount of PEG incorporated into PLA matrix can be regarded as a direct evidence for the proposed chemical interactions between PLA and PEG. Actually, as the pyrolysis experiments were carried under the high vacuum conditions of the mass spectrometry, the degradation products were removed rapidly away from the heating zone and thus, secondary reactions were almost totally eliminated [32].
However, in order to clarify whether the pyrolysis or the melt blending process was effective in acid-ether reactions, pyrolysis mass spectrometry analyses of PLA/PEG 80/20 blends prepared by solution mixing for 1, 12 and 24 h were also performed. Single ion evolution profiles of selected thermal degradation products detected during the pyrolysis of the PLA/PEG 80/20 blends prepared by melt blending and solution mixing, are shown in Fig. 6. The elimination of the products associated with the degradation of the chains generated by the interactions of COOH groups present in the PLA matrix as end groups with the ether linkages of PEG were detected to a certain extend during the pyrolysis of the blend prepared by solution mixing for only 1 h. The relative yields of these products increased upon increasing the mixing period to 24 h. This corroborates the idea that the acid-ether reactions mainly took place during the mixing processes. The increase in the thermal stability of the blends prepared by solution mixing for 13 and 24 h may be associated with generation of a cross-linked structure increasing the thermal stability of both PLA and PEG chains. The TGA and DTGA curves for PLA/PEG 80/20 blends prepared by solution mixing for 24 h and by melt blending shown in Fig. 7, also
Scheme 2. Possible thermal degradation products of PLA chains involving ethylene oxide units as the end groups.
Fig. 6. Single ion evolution profiles of some selected products recorded during the pyrolysis of 80/20 PLA/PEG blends prepared melt blending or by solution mixing.
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Fig. 7. TGA curves of PLA/PEG blends prepared melt blending or by solution mixing for 24 h.
confirmed increase in thermal stability of both components upon prolonged mixing periods. 4. Conclusions In this study, melt blended PLA/PEG samples involving 10, 15 and 20 wt% PEG were prepared and characterized. Addition of 20 wt% PEG decreased the tensile strength and tensile modulus of PLA while improving its ductility. DSC measurements revealed that the addition of PEG into PLA matrix reduced the endothermic melting peak of PLA due to the plasticizing effect of PEG and the generation of a miscible system. Both TGA and DP-MS analyses pointed out that the thermal decomposition occurred mainly in two steps. In addition, pyrolysis mass spectrometry analyses indicated presence of chains generated by interactions of COOH groups of PLA and ether linkages of PEG during the blending and/or pyrolysis process. As a consequence, decrease in the thermal stability of PEG chains was detected. Analyses of the blends prepared by solution mixing for variable periods confirmed that the interactions took place mainly during the blending process. Increase in thermal stabilities of both components for the blends prepared by stirring for 13 and 24 h was detected and associated with generation of a crosslinked structure upon stirring for prolonged periods. Acknowledgment This work is partially supported by TUBITAK Research Fund 112T493. References [1] V. Siracusa, P. Rocculi, S. Romani, M.D. Rosa, Biodegradable polymers for food packaging: a review, Trends Food Sci. Technol. 19 (12) (2008) 634–643. [2] L.S. Nair, C.T. Laurencin, Biodegradable polymers as biomaterials, Prog. Polym. Sci. 32 (8–9) (2007) 762–798. [3] D. Garlotta, A literature review of poly(lactic acid), J. Polym. Environ. 9 (2) (2002) 63–84. [4] L.T. Lim, R. Auras, M. Rubino, Processing technologies for poly(lactic acid), Prog. Polym. Sci. 33 (8) (2008) 820–852. [5] K.M. Nampoothiri, N.R. Nair, J.R. Pappy, An overview of the recent developments in polylactide (PLA) research, Biores. Technol. 101 (2012) 8493–8501. [6] F. Carrasco, P. Pagès, J. Gámez-Pérez, O.O. Santana, M.L. Maspoch, Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties, Poly. Degrad. Stabil. 95 (2) (2010) 116–125.
[7] A.K. Mohanty, M. Misra, G. Hinrichsen, Biofibres biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng. 276–277 (2000) 1–24. [8] M. Drieskens, R. Peeters, J. Mullens, D. Franco, P.J. Lemstra, D.G. Hristova-Bogaerds, Structure versus properties relationship of poly(lactic acid): I. Effect of crystallinity on barrier properties, Polym. Phys. Part B 47 (2009) 2247–2258. [9] Y.J. Li, H. Shimizu, Toughening of polylactide by melt blending with a biodegradable poly(ether)urethane elastomer, Macromol. Biosci. 7 (2007) 921–928. [10] L. Yu, K. Dean, L. Li, Polymer blends and composites from renewable resources, Prog. Polym. Sci. 32 (2006) 576–602. [11] E. Lezak, Z. Kulinski, R. Masirek, E. Piorkowska, M. Pracella, K. Gadzinowski, Mechanical and thermal properties of green polylactide composites with natural fillers, Macromol. Biosci. 8 (2008) 1190–1200. [12] B.K. Chen, C.C. Shih, A.F. Chen, Ductile PLA nanocomposites with improved thermal stability, Composites Part A Appl. Sci. Man. 43 (2012) 2289–2295. [13] N. Bitinis, R. Verdejo, P. Cassagnau, Structure and properties of polylactide/natural rubber blends, Mater. Chem. Phys. 129 (2011) 823–831. [14] X. Ma, J. Yu, N. Wang, Compatibility characterization of poly(lactic acid)/poly(propylene carbonate) blends, J. Polym. Sci. Part B Polym. Phys. 44 (2006) 94–101. [15] M. Llana-Ruíz-Cabello, S. Pichardo, N.T. Jimenez-Morillo, P. Abad, E. Guillamon, F.J. Gozalez-Vila, A.M. Camean, J.A. Gonzalez-Perez, Characterization of a bio-based packaging containing a natural additive from Allium spp. using analytical pyrolysis and carbon stable isotopes, J. Anal. Appl. Pyrol. 120 (2016) 334–340. [16] S. Jacobsen, H.G. Fritz, Plasticizing polylactide – the effect of different plasticizers on the mechanical properties, Polym. Eng. Sci. 39 (1999) 1303–1310. [17] I. Pillin, N. Montrelay, Y. Grohens, Thermo-mechanical characterization of plasticized PLA: is the miscibility the only significant factor? Polymer 47 (2006) 4676–4682. [18] M. Baiardo, G. Frisoni, M. Scandola, M. Rimelen, D. Lips, K. Ruffieux, E. Wintermantal, Thermal and mechanical properties of plasticized poly(L-lactic acid), J. Appl. Polym. Sci. 90 (2003) 1731–1738. [19] Z. Kulinski, E. Piorkowska, Crystallization, structure and properties of plasticized poly(L-lactide), Polymer 46 (2005) 10290–10300. [20] A.K. Mohapatra, S. Mohanty, S.K. Nayak, Effect of PEG on PLA/PEG blend and its nanocomposites: a study of thermo-Mechanical and morphological characterization, Polym. Compos. 35 (2014) 283–293. [21] B.S. Park, J.C. Song, D.H. Park, K.B. Yoon, PLA/chain-extended PEG blends with improved ductility, J. Appl. Polym. Sci. 123 (4) (2012) 2360–2367. [22] M. Sheth, R.A. Kumar, V. Dave, R.A. Gross, S.P. McCarthy, Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 1495–1505. [23] K.S. Kim, I.J. Chin, J.S. Yoon, H.J. Choi, D.C. Lee, K.H. Lee, Crystallization behavior and mechanical properties of poly(ethylene oxide)/poly(L-lactide)/poly(vinyl acetate) blends, J. Appl. Polym. Sci. 82 (2001), 3618–366. [24] Y. Hu, M. Rogunova, V. Topolkaraev, A. Hiltner, E. Baer, Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity, Polymer 44 (2003) 5701–5710. [25] J. Ahmed, S.K. Varshney, R. Auras, S.W. Hwang, Thermal and rheological properties of L-polylactide/polyethylene glycol/silicate nanocomposites films, J. Food Sci. 75 (2010) N97–N108.
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G Model JAAP-3817; No. of Pages 8 8
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[26] N. Stoehr, B. Baudrit, E. Haberstroh, M. Nase, P. Heidemeyer, M. Bastian, Properties and weldability of plasticized polylactic acid films, J. Appl. Polym. Sci. 131 (2014) 40394. [27] I.C. McNeill, H.A. Leiper, Degradation studies of some polyesters and polycarbonates 2. Polylactide – Degradation under isothermal conditions, thermal degradation mechanism and photolysis of the polymer, Polym. Degrad. Stabil. 11 (1985) 309–326. [28] F.D. Kopinke, M. Remmler, K. Mackenzie, M. Milder, O. Wachsen, Thermal decomposition of biodegradable polyesters 2. Poly(lactic acid), Polym. Degrad. Stabil. 43 (1996) 329–342. [29] M.P. Arrieta, F. Parres, J. Lopez, Development of a novel pyrolysis-gas chromatography/mass spectrometry method for the analysis of poly(lactic acid) thermal degradation products, J. Anal. Appl. Pyrol. 101 (2013) 150–155. [30] V.R. Taubner, R. Shishooi, Influence of processing parameters on the degradation of poly(L-lactide) during extrusion, J. Appl. Polymer Sci. 79 (2001) 2128–2135. [31] Y. Wang, B. Steinhoff, C. Brinkmann, I. Alig, In-line monitoring of the thermal degradation of poly(L-lactic acid) during melt extrusion by UV–vis spectroscopy, Polymer 49 (2008), 1257–126. [32] M.M. Fares, J. Hacaloglu, S. Suzer, Characterization of degradation products of polyethylene oxide by pyrolysis mass spectrometry, Eur. Polym. J. 20 (1994) 845–850.
[33] R.P. Lattimer, Mass spectral analysis of low-temperature pyrolysis products from poly(ethylene glycol), J. Anal. Appl. Pyrol. 56 (2000) 61–78. [34] K. Pielichowski, K. Flejtuch, Non-oxidative thermal degradation of poly(ethylene oxide): kinetic and thermoanalytical study, J. Anal. Appl. Pyrol. 73 (2005) 131–138. [35] J. Hacaloglu, Direct insertion probe mass spectrometry of polymers, Adv. Polym. Sci. 248 (2012) 69–104. [36] T. Uyar, A.E. Tonelli, J. Hacaloglu, Thermal degradation of poly(carbonate), poly(vinyl acetate) and their blends, Polym. Deg. Stabil. 91 (2006) 2960–2967. [37] Z. Dolezal, R. Kubinec, J. Sedlacek, V. Pacakova, J. Vohlidal, Characterization of substituted polyacetylene microstructure by pyrolysis gas chromatography, J. Separ. Sci. 30 (2007) 731–739. [38] F. Samperi, C. Puglisi, T. Ferreri, R. Messina, G. Cicala, A. Recca, C.L. Restuccia, A. Scamporrino, Thermal decomposition products of copoly(arylene ether sulfone)s characterized by direct pyrolysis mass spectrometry, Polym. Deg. Stabil. 92 (2007) 1304–1315. [39] K. Kiersch, J. Kruse, K.U. Eckhardt, A. Fendt, T. Streibel, R. Zimmermann, G. Broll, P. Leinweber, Impact of grassland burning on soil organic matter as revealed by a synchrotron- and pyrolysis-mass spectrometry-based multi-methodological approach, Org. Geochem. 44 (2012) 8–20.
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Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry Esra Ozdemir a , Teoman Tinc¸er a,b , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 15 June 2016 Received in revised form 12 August 2016 Accepted 20 September 2016 Available online xxx Keywords: Poly(lactide) Poly(lactide)/poly(ethylene glycol) blends Thermal degradation Pyrolysis mass spectrometry
a b s t r a c t In this study, melt blended poly(lactic acid) and poly(ethylene glycol), (PLA)/PEG samples involving 10, 15 and 20 wt% PEG were prepared and characterized by direct pyrolysis mass spectrometry technique in addition to classical techniques such differential scanning calorimetry, thermogravimetric analyses and mechanical tests. The incorporation of PEG resulted in consistent and significant decrease in the tensile strength and modulus, and reduction in endothermic melting peak of PLA due to the plasticizing effect of PEG with the virgin PLA matrix. Both TGA and DP-MS analyses pointed out that the thermal decomposition of the blend occurred mainly in two steps. In addition, the pyrolysis mass spectrometry analyses indicated presence of chains generated by the interactions of ether linkages of PEG and COOH groups of PLA, present either as end groups or due to reactions with water, during the blending and/or pyrolysis process. As a consequence, decrease in the thermal stability of PEG chains was detected. Analyses of the blends prepared by solution mixing for variable periods confirmed that these interactions took place mainly during the blending process. In addition, increase in the thermal stabilities of both components for the blends prepared by stirring for prolonged times was detected and associated with generation of a crosslinked structure. © 2016 Published by Elsevier B.V.
1. Introduction Among all bio-based biodegradable polymers, poly(lactic acid) (PLA), a promising polyester, has drawn significant attention not only due to its comparable properties with conventional polymers but also relatively better properties than other biodegradable polymers [1–7]. Even though PLA presents environmentally friendly properties like biocompatibility, it still shows limited thermal, mechanical and barrier properties for many applications [6,8–12]. Mechanical and thermal properties of PLA can be improved by copolymerization, blending with other polymers, plasticization using biocompatible plasticizer and incorporation of filler materials. Among these, being a simple and more economic way, blending of PLA with various polymers has been investigated by many authors [13–15]. Poly(ethylene glycol), (PEG), due to its miscibility, biodegradability, and food contactable application, is almost the most suitable material which has been used as an impact modifier for PLA [16–20].
∗ Corresponding author at: Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey. E-mail address: [email protected] (J. Hacaloglu).
It has been determined that both the molecular weight and the amount of PEG has significant effects on the properties of PLA/PEG blends [17,21–25]. A limit of miscibility and brittleness characteristics depending upon the plasticizer content and the molecular weight were detected [17,21]. The plasticizing efficiency was increased with decreasing molecular weight of PEG [23,24]. Sheth et al. found that PLA/PEG blends varied from completely miscible to partially miscible, depending upon the PEG concentration [22]. Kim and coworkers showed that in the presence of 40% high molecular weight PEG percentage of elongation at break is improved significantly [23]. PEG, being a plasticizer, shows plasticization effect, gives rise to increase in PLA chain mobility, allows the rearrangement of the chains. PEG by disturbing the intermolecular forces, decreases the glass transition temperature, Tg of PLA [21–26]. Increasing the amount of PEG leads to decrease in Tg and improvement in crystallization [21]. Ahmed and coworkers determined that the incorporation of PEG as a plasticizer reduces Tg and melting point, Tm of neat PLA significantly, as expected [25]. In a recent research, Stoehr and coworkers examined the effect of PEG amount and recorded that PEG content affects Tg and cold crystallization temperature, Tcc [26].
http://dx.doi.org/10.1016/j.jaap.2016.09.010 0165-2370/© 2016 Published by Elsevier B.V.
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Several studies on thermal degradation characteristics of PLA and PEG were also appeared in the literature [27–34]. However, there is insufficient information about thermal degradation behaviors of PLA/PEG blends. Therefore, the main objective of this research was to focus on both thermal properties and degradation mechanisms of plasticized PLA. For this purpose we also applied direct pyrolysis mass spectrometry, DP-MS that has been shown as a valuable tool for analyses of polymers [35–39]. The applications of DP-MS for thermal characterization involve investigation of thermal stability, degradation products and decomposition mechanism of homopolymers, copolymers, polymer blends and composites. The technique has great potential for identification of additives and structure in complex polymer matrices without time consuming extractions or derivatizations, as components are separated as a function of their volalities and/or thermal stabilities [35]. 2. Experimental 2.1. Materials Polylactide, PLA, (number average molecular weight, Mn ∼ 190000), and poly (ethylene glycol) (Mn ∼ 8000), were purchased from Cargill Dow. PLA and PEG were dried at 60 ◦ C overnight under reduced pressure prior to the mixing processes. The mixtures of PLA/PEG with appropriate blend ratios, (90/10, 85/15 and 80/20 wt/wt) were melt-compounded using a DSM Xplore twin screw micro compounder at 190 ◦ C, with a screw speed 100 rpm for 8 min. PLA/PEG 80/20 wt/wt blends were also prepared by solution mixing by magnetic (1000 rph) stirring for 1, 12 and 24 h in chloroform and dried at 60 ◦ C overnight under reduced pressure. Samples of PLA and PLA/PEG blends were dried in vacuum at 60 ◦ C before mechanical testing. The dog-bone shaped specimens of PLA and its blends (dimension 50.0, 7.6, 2.0 mm length, width and thickness respectively) were obtained by Daca injection molding instrument at a barrel temperature of 190–195 ◦ C and mold temperature of 40 ◦ C. The extruded samples were forced to mold cavities at 8 bar pressure. The dog bone shaped specimens obtained after injection molding, were dried again at 65 ◦ C overnight under vacuum before tensile testing.
Table 1 Mechanical properties of PLA and PLA/PEG blends prepared by melt blending.
PLA PLA/PEG 90/10 PLA/PEG 85/15 PLA/PEG 80/20
Tensile Strength (MPa)
% Elongation at Break
Young’s Modulus (MPa)
64.5 ± 1.3 48.9 ± 0.7 31.5 ± 2.6 16.1 ± 1.6
11.0 ± 0.5 12.1 ± 2.1 8.1 ± 0.8 43.1 ± 10.4
1180 ± 30 950 ± 30 650 ± 40 330 ± 30
break, as brittleness is inversely proportional to elongation at break. The incorporation of PEG resulted in consistent and significant decrease in the tensile strength and modulus, due to the plasticizing effect of PEG with the virgin PLA matrix. The blend prepared at PLA/PEG ratio of 80/20, exhibited a decrease in tensile strength to the tune of 75.0% and tensile modulus to 72.0% respectively, as compared with the pristine PLA matrix. The variation in elongation at break for the blends involving 10 and 15 wt% PEG, showed no constituency with addition of PEG, however, it seems that there existed almost no significant change in elongation within the experimental errors. Finally, the elongation at break was increased noticeably only for the blend containing 20 wt% PEG, indicating significant decrease in brittleness of PLA. The blends with high elongation at break were characterized by relatively weak impact values. 3.2. Differential scanning calorimetry and thermogravimetry analyses DSC images of the melt blended PLA/PEG samples involving 10, 15 and 20 wt% PEG prepared by melt blending, are shown in Fig. 1. The endothermic melting peak of PLA at around 172.3 ◦ C was shifted to 165.7, 166.7 and 165.9 ◦ C with the addition of 10, 15 and 20% PEG blends respectively, mainly due to the plasticizing effect of PEG and indicated generation of a compatible system [12]. The cold crystallization temperature (Tc) of PLA appeared at 86.2, 77.3 and 88.4 ◦ C with addition of 10, 15 and 20 wt% PEG respectively, The TGA curves of PLA, PEG and the melt blended PLA/PEG samples are shown in Fig. 2 and the relevant data are listed in Table 2. Thermal stability of PEG was noticeably higher than that of PLA. Thermal degradation of PLA/PEG blends occurred in two steps. The thermal decomposition of PLA was shifted to lower temperatures in the presence of PEG. The second step of decomposition associ-
2.2. Characterization The universal testing machine (Lloyd LR30K) as per ASTMD638, at crosshead speed of 50 mm/min and cell load of 5 kn was used for tensile tests. Differential Calorimetry, (DSC) and Thermogravimetry Analyses (TGA) were performed on a Perkin Elmer Instrument STA6000 under nitrogen atmosphere at a flow rate of 20 mL/min and a heating rate of 10 ◦ C/min. Direct pyrolysis mass spectrometry (DP-MS) analyses were performed using 5973 HP quadruple mass spectrometry system coupled to a JHP SIS direct insertion probe pyrolysis system. 70 eV EI mass spectra, at a rate of 2 scan/s, were recorded. Samples (0.10 mg) in the flared glass sample vials were heated to 450 ◦ C at a rate of 10 ◦ C/min. Experiments were repeated at least twice to ensure reproducibility. 3. Results and discussions 3.1. Mechanical properties The variation of mechanical properties of PLA and PLA/PEG blends as a function of PEG loading are collected in Table 1. The neat PLA is typically brittle and rigid. Thus, it has high tensile strength and elastic modulus (Young’s modulus) but very low% elongation at
Fig. 1. DSC curves of PLA, PEG and PLA/PEG blends.
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Fig. 2. TGA curves of PLA, PEG and PLA/PEG blends.
Table 2 TGA data for PLA, PEG and PLA/PEG blends prepared by melt blending.
PLA PEG PLA/PEG 90/10 PLA/PEG 85/15 PLA/PEG 80/20
T5%
T10%
Tmax1
Tmax2
%char yield
329.06 338.36 315.82 308.82 310.48
336.96 354.94 326.91 320.78 320.63
366.91
– 395.3 – 394.26 397.68
0.81% 1.18% 0.91%
358.31 358.58 351.04
ated with the degradation of PEG became more pronounced as the amount of PEG incorporated was increased. 3.3. Pyrolysis mass spectrometry analyses The total ion current, TIC, curves (the variation of total ion yield as a function of temperature), of PLA, PEG and PLA/PEG blends, prepared by melt blending, and the mass spectra recorded at peak maxima are given in Fig. 3. The pyrolysis mass spectrum of PLA is dominated by series of peaks due to fragments with general formula (C2 H4 CO2 )x C2 H4 CO (m/z = 72x − 88 for x = 2–12), generated by elimination of the neutral molecules CO2 and acetaldehyde during ionization of the cyclic oligomers, in addition to low mass fragments produced by random chain homolysis reactions [27–31]. A second, much less pronounced series of peaks with m/z = 72x + 73 for x = 2–12 are associated with fragments produced by cis-elimination reactions with general formula (C2 H4 CO2 )x H. It is known that thermal degradation of PEG mainly occurs via random chain cleavages of C O bonds followed by H transfer reactions [32–34]. Products due to dissociation of C C bonds are also generated but are less abundant. Nevertheless, several end groups such as methyl, (C2 H4 O)x CH3 (m/z = 44x + 15 for x = 2–12), ethyl, (C2 H4 O)x C2 H5 (m/z = 44x + 29 for x = 2–12), hydroxyl, (m/z = 44x + 17 for x = 2–12), aldehyde (C2 H4 O)x C2 H3 O (m/z = 44x + 43 for x = 0–17), and unsaturated groups (C2 H4 O)x C2 H3 (m/z = 44x + 27 for x = 0–17) are generated. As a consequence of further dissociation during ionization, peaks due to low mass fragments are more intense. Among these, the most abundant product is C2 H4 OH fragment (m/z = 45 Da) and the yields of the products involving hydroxyl end groups are more intense.
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The TIC curves of the blends show two peaks, an intense one associated with the decomposition of PLA and a weak one attributed to the degradation of PEG. In general, the evolution of thermal degradation products of PLA was detected almost in the same temperature region as the neat PLA. On the other hand, the high temperature peak associated with the decomposition of PEG was shifted to low temperature regions with the increase in the amount of PEG incorporated. Unfortunately, due to the similarities in the repeating units of PLA and PEG, some of the thermal degradation products of PLA and PEG have the same m/z values; i.e. CO2 H from PLA and C2 H4 OH from PEG have m/z value 45 Da and OC2 H4 CO2 H from PLA and (C2 H4 O)2 H from PEG have m/z value 89 Da. However, the thermal degradations of PLA and PEG occur at two distinct temperature regions. Thus, the trends in single ion pyrograms have significant importance in understanding the thermal behaviors. Single ion evolution profiles of some representative characteristic products of PLA and PEG detected during the pyrolysis of PLA, PEG and 90/10 wt% PLA/PEG blend are depicted in Fig. 4. The evolution of products generated by cis-elimination reactions, such as (C2 H4 CO2 )2 H (145 Da) and (C2 H4 CO2 )3 H (217 Da) and those produced by trans-esterification reactions such as (C2 H4 CO2 )2 C2 H4 CO (200 Da) and (C2 H4 CO2 )4 C2 H4 CO (344 Da) showed almost identical trends during the pyrolysis of PLA and PLA/PEG blend. In contrast, noticeable changes in the evolution profiles of low mass products such as CO2 H (45 Da) and C2 H4 CO (56 Da) were detected. In general, the relative intensities of the thermal degradation products diagnostic to PEG were less abundant as expected. A sharp peak at around 366 ◦ C appeared in the single ion pyrograms of thermal degradation products of PEG. One possibility is the contribution of PLA based products with the same m/z values. However, the evolution profiles of the diagnostic products PEG such as (C2 H4 O)2 CH2 CHO (131 Da), (C2 H4 O)3 CH2 CHO (175 Da), and (C2 H4 CO)2 CH3 (103 Da) having m/z values different than those of the characteristic products of PLA, also showed two peaks, a sharp one with a maximum at around 366 ◦ C and a broad peak with a maximum at around 442 ◦ C. Elimination of new products that were not recorded during the pyrolysis of the neat components was also noticed upon inspection of pyrolysis mass spectra carefully. These relatively weak products (i.e. fragments with m/z values 231 and 303 Da) showed a single peak at around 366 ◦ C. The single ion evolution profiles of these products are also included in the figure for comparison. In order to get a better in sight, the pyrolysis data for PLA/PEG blends involving 10, 15 and 20 wt% PEG were studied (Fig. 5). The most abundant products were due to the thermal degradation of PLA chains via trans-esterification reactions. However, the relative yields of low mass products and the products generated by ciselimination reactions decreased as the amount of PEG in the blend increased. Increase in the relative yields of PEG based products was detected as expected. In addition, the relative yields of products showing a single sharp peak in their evolution profiles were also increased noticeably. The trends in the single ion evolution profiles of PLA based products revealed that the evolution of major thermal degradation products of PLA generated by trans-esterification and cis-elimination reactions were shifted slightly to high temperature regions as the amount of PEG involved in the blend was increased. A shift to high temperature regions was also recorded for the sharp peak at around 366 ◦ C in the same order. In contrast, the peak at around 437 ◦ C associated with degradation of PEG was shifted to low temperatures with the increase in PEG wt% in the blend. It is clear that thermal behaviors of PEG and PLA were changed upon incorporation of PEG into PLA matrix. It may be proposed that during the mixing process via melt blending at 190 ◦ C or during the pyrolysis of the blend, the PLA chains or fragments
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Fig. 3. TIC curves and pyrolysis mass spectra recorded at peak maxima recorded during the pyrolysis of PLA, PEG and PLA/PEG blends.
involving COOH end groups, leads to the cleavage of the ether groups of PEG as shown in Scheme 1. As a consequence, lower mass chains of PEG bearing OH end groups and PLA chains linked to ethylene oxide units should be generated. Thermal degradation of PLA chains with ethylene oxide units may generate series of products involving both repeating units. Considering the thermal degradation mechanisms of PEG and PLA, it may be thought that
the decomposition of these chains yields products with general formula (C2 H4 CO2 )x (C2 H4 O)y H, (C2 H4 CO2 )x (C2 H4 O)y C2 H3 , and (C2 H4 CO2 )x (C2 H4 O)y C2 H4 H (C2 H4 CO2 )x (C2 H4 O)y C2 H3 O (C2 H4 CO2 )x (C2 H4 O)y CH3 , by the cleavage of ether linkages followed by H-transfer reactions (Scheme 2). As discussed before some of the pyrolysis products of PLA, PEG or both have the same m/z values with these products. However, products with
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Fig. 4. Single ion evolution profiles of some selected products recorded during the pyrolysis of PLA, PEG and 85/15 PLA/PEG blend.
Fig. 5. Single ion evolution profiles of some selected products recorded during the pyrolysis of PLA blend involving 10, 15 and 20 wt% PEG prepared by melt blending.
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Scheme 1. Acid decomposition of ether linkages.
m/z values 231, 303 and 319 Da can directly be associated with (C2 H4 CO2 )(C2 H4 O)3 C2 H3 , (C2 H4 CO2 )2 (C2 H4 O)3 C2 H3 , and (C2 H4 CO2 )2 (C2 H4 O)3 C2 H3 O, respectively and the significant increase in the relative abundances of these fragments with the increase in the amount of PEG incorporated into PLA matrix can be regarded as a direct evidence for the proposed chemical interactions between PLA and PEG. Actually, as the pyrolysis experiments were carried under the high vacuum conditions of the mass spectrometry, the degradation products were removed rapidly away from the heating zone and thus, secondary reactions were almost totally eliminated [32].
However, in order to clarify whether the pyrolysis or the melt blending process was effective in acid-ether reactions, pyrolysis mass spectrometry analyses of PLA/PEG 80/20 blends prepared by solution mixing for 1, 12 and 24 h were also performed. Single ion evolution profiles of selected thermal degradation products detected during the pyrolysis of the PLA/PEG 80/20 blends prepared by melt blending and solution mixing, are shown in Fig. 6. The elimination of the products associated with the degradation of the chains generated by the interactions of COOH groups present in the PLA matrix as end groups with the ether linkages of PEG were detected to a certain extend during the pyrolysis of the blend prepared by solution mixing for only 1 h. The relative yields of these products increased upon increasing the mixing period to 24 h. This corroborates the idea that the acid-ether reactions mainly took place during the mixing processes. The increase in the thermal stability of the blends prepared by solution mixing for 13 and 24 h may be associated with generation of a cross-linked structure increasing the thermal stability of both PLA and PEG chains. The TGA and DTGA curves for PLA/PEG 80/20 blends prepared by solution mixing for 24 h and by melt blending shown in Fig. 7, also
Scheme 2. Possible thermal degradation products of PLA chains involving ethylene oxide units as the end groups.
Fig. 6. Single ion evolution profiles of some selected products recorded during the pyrolysis of 80/20 PLA/PEG blends prepared melt blending or by solution mixing.
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Fig. 7. TGA curves of PLA/PEG blends prepared melt blending or by solution mixing for 24 h.
confirmed increase in thermal stability of both components upon prolonged mixing periods. 4. Conclusions In this study, melt blended PLA/PEG samples involving 10, 15 and 20 wt% PEG were prepared and characterized. Addition of 20 wt% PEG decreased the tensile strength and tensile modulus of PLA while improving its ductility. DSC measurements revealed that the addition of PEG into PLA matrix reduced the endothermic melting peak of PLA due to the plasticizing effect of PEG and the generation of a miscible system. Both TGA and DP-MS analyses pointed out that the thermal decomposition occurred mainly in two steps. In addition, pyrolysis mass spectrometry analyses indicated presence of chains generated by interactions of COOH groups of PLA and ether linkages of PEG during the blending and/or pyrolysis process. As a consequence, decrease in the thermal stability of PEG chains was detected. Analyses of the blends prepared by solution mixing for variable periods confirmed that the interactions took place mainly during the blending process. Increase in thermal stabilities of both components for the blends prepared by stirring for 13 and 24 h was detected and associated with generation of a crosslinked structure upon stirring for prolonged periods. Acknowledgment This work is partially supported by TUBITAK Research Fund 112T493. References [1] V. Siracusa, P. Rocculi, S. Romani, M.D. Rosa, Biodegradable polymers for food packaging: a review, Trends Food Sci. Technol. 19 (12) (2008) 634–643. [2] L.S. Nair, C.T. Laurencin, Biodegradable polymers as biomaterials, Prog. Polym. Sci. 32 (8–9) (2007) 762–798. [3] D. Garlotta, A literature review of poly(lactic acid), J. Polym. Environ. 9 (2) (2002) 63–84. [4] L.T. Lim, R. Auras, M. Rubino, Processing technologies for poly(lactic acid), Prog. Polym. Sci. 33 (8) (2008) 820–852. [5] K.M. Nampoothiri, N.R. Nair, J.R. Pappy, An overview of the recent developments in polylactide (PLA) research, Biores. Technol. 101 (2012) 8493–8501. [6] F. Carrasco, P. Pagès, J. Gámez-Pérez, O.O. Santana, M.L. Maspoch, Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties, Poly. Degrad. Stabil. 95 (2) (2010) 116–125.
[7] A.K. Mohanty, M. Misra, G. Hinrichsen, Biofibres biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng. 276–277 (2000) 1–24. [8] M. Drieskens, R. Peeters, J. Mullens, D. Franco, P.J. Lemstra, D.G. Hristova-Bogaerds, Structure versus properties relationship of poly(lactic acid): I. Effect of crystallinity on barrier properties, Polym. Phys. Part B 47 (2009) 2247–2258. [9] Y.J. Li, H. Shimizu, Toughening of polylactide by melt blending with a biodegradable poly(ether)urethane elastomer, Macromol. Biosci. 7 (2007) 921–928. [10] L. Yu, K. Dean, L. Li, Polymer blends and composites from renewable resources, Prog. Polym. Sci. 32 (2006) 576–602. [11] E. Lezak, Z. Kulinski, R. Masirek, E. Piorkowska, M. Pracella, K. Gadzinowski, Mechanical and thermal properties of green polylactide composites with natural fillers, Macromol. Biosci. 8 (2008) 1190–1200. [12] B.K. Chen, C.C. Shih, A.F. Chen, Ductile PLA nanocomposites with improved thermal stability, Composites Part A Appl. Sci. Man. 43 (2012) 2289–2295. [13] N. Bitinis, R. Verdejo, P. Cassagnau, Structure and properties of polylactide/natural rubber blends, Mater. Chem. Phys. 129 (2011) 823–831. [14] X. Ma, J. Yu, N. Wang, Compatibility characterization of poly(lactic acid)/poly(propylene carbonate) blends, J. Polym. Sci. Part B Polym. Phys. 44 (2006) 94–101. [15] M. Llana-Ruíz-Cabello, S. Pichardo, N.T. Jimenez-Morillo, P. Abad, E. Guillamon, F.J. Gozalez-Vila, A.M. Camean, J.A. Gonzalez-Perez, Characterization of a bio-based packaging containing a natural additive from Allium spp. using analytical pyrolysis and carbon stable isotopes, J. Anal. Appl. Pyrol. 120 (2016) 334–340. [16] S. Jacobsen, H.G. Fritz, Plasticizing polylactide – the effect of different plasticizers on the mechanical properties, Polym. Eng. Sci. 39 (1999) 1303–1310. [17] I. Pillin, N. Montrelay, Y. Grohens, Thermo-mechanical characterization of plasticized PLA: is the miscibility the only significant factor? Polymer 47 (2006) 4676–4682. [18] M. Baiardo, G. Frisoni, M. Scandola, M. Rimelen, D. Lips, K. Ruffieux, E. Wintermantal, Thermal and mechanical properties of plasticized poly(L-lactic acid), J. Appl. Polym. Sci. 90 (2003) 1731–1738. [19] Z. Kulinski, E. Piorkowska, Crystallization, structure and properties of plasticized poly(L-lactide), Polymer 46 (2005) 10290–10300. [20] A.K. Mohapatra, S. Mohanty, S.K. Nayak, Effect of PEG on PLA/PEG blend and its nanocomposites: a study of thermo-Mechanical and morphological characterization, Polym. Compos. 35 (2014) 283–293. [21] B.S. Park, J.C. Song, D.H. Park, K.B. Yoon, PLA/chain-extended PEG blends with improved ductility, J. Appl. Polym. Sci. 123 (4) (2012) 2360–2367. [22] M. Sheth, R.A. Kumar, V. Dave, R.A. Gross, S.P. McCarthy, Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 1495–1505. [23] K.S. Kim, I.J. Chin, J.S. Yoon, H.J. Choi, D.C. Lee, K.H. Lee, Crystallization behavior and mechanical properties of poly(ethylene oxide)/poly(L-lactide)/poly(vinyl acetate) blends, J. Appl. Polym. Sci. 82 (2001), 3618–366. [24] Y. Hu, M. Rogunova, V. Topolkaraev, A. Hiltner, E. Baer, Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity, Polymer 44 (2003) 5701–5710. [25] J. Ahmed, S.K. Varshney, R. Auras, S.W. Hwang, Thermal and rheological properties of L-polylactide/polyethylene glycol/silicate nanocomposites films, J. Food Sci. 75 (2010) N97–N108.
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[26] N. Stoehr, B. Baudrit, E. Haberstroh, M. Nase, P. Heidemeyer, M. Bastian, Properties and weldability of plasticized polylactic acid films, J. Appl. Polym. Sci. 131 (2014) 40394. [27] I.C. McNeill, H.A. Leiper, Degradation studies of some polyesters and polycarbonates 2. Polylactide – Degradation under isothermal conditions, thermal degradation mechanism and photolysis of the polymer, Polym. Degrad. Stabil. 11 (1985) 309–326. [28] F.D. Kopinke, M. Remmler, K. Mackenzie, M. Milder, O. Wachsen, Thermal decomposition of biodegradable polyesters 2. Poly(lactic acid), Polym. Degrad. Stabil. 43 (1996) 329–342. [29] M.P. Arrieta, F. Parres, J. Lopez, Development of a novel pyrolysis-gas chromatography/mass spectrometry method for the analysis of poly(lactic acid) thermal degradation products, J. Anal. Appl. Pyrol. 101 (2013) 150–155. [30] V.R. Taubner, R. Shishooi, Influence of processing parameters on the degradation of poly(L-lactide) during extrusion, J. Appl. Polymer Sci. 79 (2001) 2128–2135. [31] Y. Wang, B. Steinhoff, C. Brinkmann, I. Alig, In-line monitoring of the thermal degradation of poly(L-lactic acid) during melt extrusion by UV–vis spectroscopy, Polymer 49 (2008), 1257–126. [32] M.M. Fares, J. Hacaloglu, S. Suzer, Characterization of degradation products of polyethylene oxide by pyrolysis mass spectrometry, Eur. Polym. J. 20 (1994) 845–850.
[33] R.P. Lattimer, Mass spectral analysis of low-temperature pyrolysis products from poly(ethylene glycol), J. Anal. Appl. Pyrol. 56 (2000) 61–78. [34] K. Pielichowski, K. Flejtuch, Non-oxidative thermal degradation of poly(ethylene oxide): kinetic and thermoanalytical study, J. Anal. Appl. Pyrol. 73 (2005) 131–138. [35] J. Hacaloglu, Direct insertion probe mass spectrometry of polymers, Adv. Polym. Sci. 248 (2012) 69–104. [36] T. Uyar, A.E. Tonelli, J. Hacaloglu, Thermal degradation of poly(carbonate), poly(vinyl acetate) and their blends, Polym. Deg. Stabil. 91 (2006) 2960–2967. [37] Z. Dolezal, R. Kubinec, J. Sedlacek, V. Pacakova, J. Vohlidal, Characterization of substituted polyacetylene microstructure by pyrolysis gas chromatography, J. Separ. Sci. 30 (2007) 731–739. [38] F. Samperi, C. Puglisi, T. Ferreri, R. Messina, G. Cicala, A. Recca, C.L. Restuccia, A. Scamporrino, Thermal decomposition products of copoly(arylene ether sulfone)s characterized by direct pyrolysis mass spectrometry, Polym. Deg. Stabil. 92 (2007) 1304–1315. [39] K. Kiersch, J. Kruse, K.U. Eckhardt, A. Fendt, T. Streibel, R. Zimmermann, G. Broll, P. Leinweber, Impact of grassland burning on soil organic matter as revealed by a synchrotron- and pyrolysis-mass spectrometry-based multi-methodological approach, Org. Geochem. 44 (2012) 8–20.
Please cite this article in press as: E. Ozdemir, et al., Characterization of polylactide/poly(ethylene glycol) blends via direct pyrolysis mass spectrometry, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.09.010