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poly(lactic acid) microfibrillar composites
Polymer Testing 60 (2017) 166e172
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Material Behaviour
Effect of halloysite nanotubes on the thermal degradation behaviour of poly(ε-caprolactone)/poly(lactic acid) microfibrillar composites Adriaan S. Luyt a, *, Ivan Kelnar b a b
Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar m. 2, 162 06 Praha, Czechia Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsk eho na
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2016 Received in revised form 5 January 2017 Accepted 24 March 2017 Available online 27 March 2017
This paper reports on the thermal degradation behaviour and kinetics of halloysite nanotubes containing microfibrillated poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blends. It was found that the nanotubes probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with halloysite nanotubes (HNT) catalysis. Drawing to form microfibrillated nanocomposites had little influence on the degradation behaviour of these materials, but pre-mixing of the HNT with PLA or PCL prior to melt-mixing and extrusion-drawing of the blends did influence the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had an influence on the degradation mechanism, as evidenced through a Fourier-transform infrared (FTIR) analysis of the degradation products. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Poly(lactic acid) Poly(ε-caprolactone) Halloysite nanotubes Nanocomposites Thermal degradation
1. Introduction Microfibrillar composites (MFC) are polymerepolymer systems prepared by melt- or cold-drawing of suitable polymer blends. In other words, MFC are a special case of polymer blends with the dispersed minority phase in the form of micron-sized fibrils instead of spherical inclusions [1]. The application of various nanofillers has a strong potential to eliminate the basic disadvantage of MFC arising from limited parameters of polymer fibres [2]. The range of nanofiller modification in MFC is very broad. In addition to the well-known enhanced barrier properties, the thermal stability, flammability, reinforcement [3], structure [4e7] and melt drawing process [8,9] are also strongly influenced. Nanofillers significantly affect coalescence [10] during melt-drawing and thus the fibrils dimensions. Its migration between the polymer phases in the course of drawing is especially important in MFC systems with different component polarities, like HDPE/PA6 [11,12]. The MFC concept is promising for upgrading biodegradable and biocompatible PCL suffering from weak mechanical properties [13], because its preferable blending with more rigid polyesters is limited by low compatibility and the necessity of compatibilization
* Corresponding author. Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar. E-mail address: [email protected] (A.S. Luyt). http://dx.doi.org/10.1016/j.polymertesting.2017.03.027 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
[14,15]. Reinforcement of PCL using nanofillers is especially effective in the case of complicated in situ polymerization methods [16], whereas the promising application of natural fibres or those based on biodegradable polymers, like poly(lactic acid) (PLA), is limited by their bad dispersion [2,17]. Recently, it has been found that wellbalanced properties can be achieved in the case of the combination of blending and nanocomposite concepts [4e7]. Our recent results indicate a significant improvement in poly(ε-caprolactone) (PCL)-based material parameters in the case of nanofiller-aided MFC [9,18]. For example, the preparation of MFC based on the promising PCL/PLA combination was impossible due to its very unstable extrusion, resulting in highly variable extrudate dimensions, and it was therefore impossible to draw the nanofillerfree blend. We found that organically modified montmorillonite (oMMT) and HNT caused the extrusion to be more stable and supported melt drawing [9,18]. As a result, a unique biodegradable PCL matrix-based material with dual in situ formed PLA fibres and nanofiller reinforcement was obtained. Slightly better mechanical properties of HNT-modified MFC, in spite of the lower reinforcing effect of HNT on single components, indicated a complex effect of nanofiller in a drawn blend [11]. A further important effect of nanofillers, i.e. the suppression of polymer degradation, was studied for plate-like nanofillers like oMMT [19e21]. However, the effect on thermal degradation was studied much less extensively in the case of tubular HNT that are
A.S. Luyt, I. Kelnar / Polymer Testing 60 (2017) 166e172 Table 1 Samples investigated in this paper. Sample
Description
PCL/PLA-3HNT-u PCL/PLA-3HNT-dr6 PCL/PLA-4HNT-u PCL/PLA-4HNT-dr6 PCL/PLA-7HNT-u PCL/PLA-7HNT-dr6 (PCL-3HNT)/PLA-dr6 (PCL-3HNT)/PLA-u PCL/(PLA-3HNT)-dr5 PCL/PLA-3HNT-u-2x
80/20 w/w PCL/PLA þ3 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 3 phr HNT (draw rate 6) 80/20 w/w PCL/PLA þ 4 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 4 phr HNT (draw rate 6) 80/20 w/w PCL/PLA þ 7 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 7 phr HNT (draw rate 6) 80/20 w/w (PCL þ 3% HNT)/PLA (draw rate 6) 80/20 w/w (PCL þ 3% HNT)/PLA (undrawn) 80/20 w/w PCL/(PLA þ 3% HNT) (draw rate 5) 80/20 w/w PCL/PLA þ 3 phr HNT (undrawn/2x extruded) 80/20 w/w PCL/PLA þ 3 phr HNT (draw rate 6/2x extruded) 80/20 w/w PCL/PLA (undrawn)
PCL/PLA-3HNT-dr6-2x PCL/PLA-u
easily dispersible in polar polymers without modification [22]. Our previous work evaluated the effect of PLA fibrils and oMMT on the thermal stability of biodegradable MFC [23]. The present work deals with a study of the thermal degradation of analogous PCL/PLA micro-fibrillated composites reinforced with unmodified halloysite nanotubes. 2. Materials and methods 2.1. Materials Poly(lactic acid) (PLA): Ingeo 2002D (NatureWorks) with a Disomer content of 4.3%, Mw of 2.53 105 g mol1, melt flow index of 6 g/10 min (190 C/2.16 kg), and density of 1.24 g cm3. Poly(3caprolactone) (PCL): CAPA6800 (Perstorp) Mn of 80 104 g mol1, and density of 1.145 g cm3. Halloysite nanotubes (HNT) were purchased from Sigma Aldrich (USA). The diameter and length evaluated from transmission electron microscopy (TEM) [18] were 20e40 nm and 200e500 nm, respectively. 2.2. MFC preparation The preparation of the micro fibrillated composites was described in a previous paper [18] where the same samples,
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investigated in this work, were prepared and evaluated in terms of morphology and mechanical properties. The samples analysed in this paper are listed in Table 1. Briefly, the mixing and extrusion were carried out in a co-rotating segmented twin-screw extruder (L/D 40) Brabender TSE 20 at 400 rpm, and the temperatures of the respective zones (from feeding to die) were 170, 170, 170, 170, 175, and 180 C. The bristle with the 80/20 w/w PCL/PLA composition was melt-drawn using an adjustable take-up device. In addition to the one-step feeding of all the components, pre-blends of PCL/HNT and PLA/HNT were also used. Dumbbell shaped specimens were prepared in a laboratory micro-injection moulding machine (DSM). The barrel and mould temperatures were 137 C and 30 C, respectively. 2.3. Characterization A Perkin-Elmer TGA 4000 thermogravimetric analyser (TGA) was used to analyse the thermal degradation behaviour of the samples. The analyses were done from 30 to 600 C at a heating rate of 10 C min1 under nitrogen flow (20 ml min1). The sample masses were 15e20 mg. The thermal degradation volatiles were analysed in a Perkin-Elmer Frontier Fourier-transform infrared (FTIR) spectrometer, connected to the TGA through a PerkinElmer TL 8000 FT-IR-GCMS interface. The samples for the thermal degradation kinetics were heated at 3, 5, 7, 9 and 11 C min1, and the TGA's integrated kinetics software (based on the Flynn-WallOzawa method [24,25]) was used to calculate the kinetic parameters. TGA analyses were repeated three or four times for each composition, and the results from two, three or four analyses (depending on the reproducibility of the TGA results for each composition) were averaged to give the TGA and derivative TGA curves presented in Figs. 1 and 2. 3. Results and discussion Halloysite (Al2Si2O5(OH)4$2H2O) is a two-layered aluminosilicate, with a predominantly hollow tubular structure in the submicron range and chemically similar to kaolin [26]. The neighbouring alumina and silica layers, and their waters of hydration, curve and form multilayer tubes [27] due to a packing
Fig. 1. (a) TGA and (b) derivative TGA curves of seven comparable samples [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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Fig. 2. (a) TGA and (b) derivative TGA curves of six comparable samples [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
disorder. Chemically, the outer surface of the halloysite nanotubes has properties similar to SiO2, while the inner cylinder core is related to Al2O3. The charge (zeta potential) behaviour of halloysite particles can be described by a superposition of the mostly negative (at pH 6e7) surface potential of SiO2, with a small contribution from the positive Al2O3 inner surface [28]. The positive (below pH 8.5) charge of the inner lumen promotes loading of halloysite nanotubes with negative macromolecules, which are at the same time repelled from the negatively charged outer surfaces. This halloysite morphology may have an influence on the mass loss of polymers during thermal degradation, either through immobilization of the polymer chains and chain radicals, and/or through delaying the diffusion of volatile degradation products out of the HNT containing polymers. In the presentation of the results in this paper the samples have been grouped into two groups. The first group consists of the undrawn 80/20 w/w PCL/PLA control sample (PCL/PLA-u) and the undrawn and drawn 80/20 w/w PCL/PLA samples containing respectively 3, 4 and 7 phr HNT (PCL/PLA-3HNT-u, PCL/PLA-3HNTdr6, PCL/PLA-4HNT-u, PCL/PLA-4HNT-dr6, PCL/PLA-7HNT-u and PCL/PLA-7HNT-dr6). The second group consists of the undrawn 80/ 20 w/w PCL/PLA control sample (PCL/PLA-u), the undrawn and drawn samples where PLA and PCL were respectively pre-mixed with HNT before mixing with the other polymer ((PCL-3HNT)/ PLA-dr6, (PCL-3HNT)/PLA-u and PCL/(PLA-3HNT)-dr5), and the undrawn and drawn two-times extruded 80/20 w/w PCL/PLA samples containing 3 phr HNT (PCL/PLA-3HNT-u-2x, PCL/PLA3HNT-u-2x). Although there are different viewpoints on how thermal stability should be defined, it is commonly accepted that there is not necessarily a direct relation between mass loss temperatures as determined during TGA analyses and the thermal stabilities of the respective samples, since there are a number of factors (i.e. rate of diffusion of volatiles out of the sample, interaction between volatiles and filler particles) that may influence the release of volatiles from the degrading sample. Since degradation of the sample may start long before any mass loss is observed, we shall use the term ‘apparent thermal stability’ and relate it to mass loss events as observed through thermogravimetric analysis. Fig. 1 and Table 2 show that the apparent thermal stability of the blend slightly decreased in the presence of 3 and 4 wt% HNT, but quite significantly when 7 wt% HNT was present. The TGA
curves show two clear mass loss steps corresponding well with the respective amounts of polymer initially mixed into the blend. It is known that PLA decomposes before PCL [23], and Fig. 1 shows that the mass loss of both PLA and PCL moved to lower temperatures. This is especially interesting because our previous paper, where the morphology and mechanical properties of the same systems were investigated [18], showed that the HNT was predominantly located in the PCL phase and on the interface, especially in the case of the samples containing 7 wt% HNT. The HNT on the interface must have played a significant role in catalyzing the PLA degradation process, and the free radicals formed during the degradation of PLA must have initiated the early degradation of PCL. Our previous paper [18] showed a reduction in the sizes of the PLA inclusions and in the crystallinity of PLA after addition of the HNT. If HNT catalyzed the degradation of PLA, then the degradation will be more effectively
Table 2 Data from the TGA and dTGA curves of all the investigated samples. Sample
Tmax(1)/ C
Tmax(2)/ C
% Residuea
PCL/PLA-3HNT-u PCL/PLA-3HNT-dr6 PCL/PLA-4HNT-u PCL/PLA-4HNT-dr6 PCL/PLA-7HNT-u PCL/PLA-7HNT-dr6 (PCL-3HNT)/PLA-dr6 (PCL-3HNT)/PLA-u PCL/(PLA-3HNT)-dr5 PCL/PLA-3HNT-u-2x PCL/PLA-3HNT-dr6-2x PCL/PLA-u
358.0 360.0 357.5 358.5 352.5 357.5 353.5 359.5 359.0 364.0 358.0 368.0
414.0 416.5 414.0 414.0 408.5 400.5 412.0 411.5 414.5 417.0 412.0 416.5
5.1 4.3 5.8 4.7 8.7 7.9 4.2 4.2 4.2 2.7 4.2 1.0
(4.0) (3.3) (4.7) (3.6) (7.7) (6.8) (3.2) (3.2) (3.2) (1.6) (3.2)
PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded. Tmax(1): temperature of peak maximum of first peak in dTGA curve; Tmax(2): temperature of peak maximum of second peak in dTGA curve. a Values between brackets in last column are the measured % residue minus the % residue for the PCL/PLA blend without any HNT, and should be an indication of the amount of HNT in the investigated samples.
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Fig. 3. FTIR intensity profile (a) and spectra of PCL/PLA-3HNT-u (b), PCL/PLA-3HNT-dr6 (c), PCL/PLA-4HNT-u (d), PCL/PLA-4HNT-dr6 (e), PCL/PLA-7HNT-u (f), PCL/PLA-7HNT-dr6 (g), and PCL/PLA-u (h) [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/ 20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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Fig. 4. FTIR intensity profile (a) and spectra of (PCL-3HNT)/PLA-dr6 (b), (PCL-3HNT)/PLA-u (c), PCL/(PLA-3HNT)-dr5 (d), PCL/PLA-3HNT-u-2x (e), PCL/PLA-3HNT-dr6-2x (f), and PCL/ PLA-u (g) [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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initiated if the inclusions are smaller and more PLA is in contact with the HNT on the interface. It is quite possible that the HNT located in the PCL played a role in initiating the early degradation of PCL, but this is not directly evident from our results. Drawing after extrusion clearly improved the apparent thermal stability of all the composites presented in Fig. 1, which is probably the result of the increased crystallinity of the drawn samples [18]. If one looks at the % residue values in Table 2, there are generally good correlations between these values and the amount of HNT initially mixed into the blends, and the values are basically the same for the undrawn and drawn samples. There is, however, a significant variation in the values for (PCL-3HNT)/PLA-dr6, (PCL3HNT)/PLA-u, PCL/(PLA-3HNT)-dr5, PCL/PLA-3HNT-u-2x and PCL/ PLA-3HNT-dr6-2x, and the differences between the measured residues and the amounts of HNT initially mixed into the samples could not be related to differences in the pre-mixing and postmixing conditions. When comparing the apparent thermal stabilities of the undrawn and drawn samples with 3 wt% HNT mixed into the PCL/ PLA blend (PLA/PCL-3HNT-u and PCL/PLA-3HNT-dr6) with the undrawn and drawn samples where PCL was pre-mixed with 3 wt% HNT ((PCL-3HNT)/PLA-u and (PCL-3HNT)/PLA-dr6), and where PLA was pre-mixed with 3 wt% HNT (PCL/(PLA-3HNT)dr5), it was found that pre-mixing PCL with HNT increased the thermal stability of PLA in the undrawn sample, but decreased the thermal stability of PCL, compared to the undrawn sample where the HNT was directly mixed into the PCL/PLA blend. This is probably because pre-mixing caused the HNT to be located more inside the PCL phase and less on the interface between the two polymers. The HNT therefore primarily catalyzed the degradation of PCL. This explanation can, however, not be applied to the drawn samples. In this case the PCL/(PLA-3HNT)-dr5 sample was found to be slightly less thermally stable than the PCL/PLA-3HNTdr6 sample, but much more thermally stable than the (PCL3HNT)/PLA-dr6 sample (Fig. 2 and Table 2). During extrusion mixing of the PCL/(PLA-3HNT)-dr5 sample the HNT probably diffused towards the interface between the two polymers, and the drawing probably caused a significant amount of the HNT to re-locate inside the PCL phase, which caused the morphology of this sample to be similar to that of PCL/PLA-3HNT-dr6, but with slightly more HNT on the interface between the two polymers, which would have contributed towards catalyzing the PLA degradation and decreasing the apparent thermal stability. The reason for the much lower thermal stability of the (PCL-3HNT)/ PLA-dr6 sample is not clear, since one would expect most of the HNT to be located inside the PCL phase, unless the drawing has forced a significant amount of the HNT out of the PCL phase into the interphase. The compositions of the following samples were identical: PCL/ PLA-3HNT-u, PCL/PLA-3HNT-u-2x, PCL/PLA-3HNT-dr6, PCL/PLA3HNT-dr6-2x. The only differences were in their extrusion processing, where the PCL/PLA-3HNT-u sample was extruded once without drawing, the PCL/PLA-3HNT-u-2x sample was extruded twice without drawing, the PCL/PLA-3HNT-dr6 sample was extruded once with a draw ratio of 6, and the PCL/PLA-3HNT-dr62x sample was extruded twice with a draw ratio of 6. The reason for the double extrusion of the two samples was to see if the HNT dispersion could not be improved. In our previous paper it was, however, found that neither the dispersion nor the mechanical properties were improved [18]. The results in Table 2 show that the apparent thermal stabilities of these samples show the following trend: PCL/PLA-3HNT-u z PCL/PLA-3HNT-dr6-2x < PCL/PLA3HNT-u-2x z PCL/PLA-3HNT-dr6. This trend in the apparent thermal stabilities cannot be directly linked to differences in the
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morphologies of the different samples, but the observed differences will be the result of a complex, interrelated combination of the following factors: (1) Differences in the distribution of the nanotubes between the PCL phase and the interphase; (2) Differences in the respective crystallinities of the PLA and PCL phases in the blends; (3) Degradation of one or both polymers because of multiple extrusions. A kinetic analysis of the degradation process was performed on each of the two steps observed in the TGA curves. We assumed that the degradation of each step progressed according to first-order (n ¼ 1) kinetics, which is usually the case for the degradation of polymers. Unfortunately, the results from this part of the investigation did not reveal much, and the apparent activation energy values for both degradation steps varied between 140 and 210 kJ mol1, with no specific relationship between these values and the presence or amount of HNT, or between these values and the post-mixing treatments. This is evidence of the well-known complex nature of polymer degradation, which is made worse in this case by possible interactions between the two polymers and the filler and by differences between the morphologies of the different samples. Figs. 3 and 4 show the intensity profiles and FTIR spectra for the volatiles released during the degradation of the different samples. The spectra were recorded (i) just before the first intensity profile peak, (ii) at the maximum of the first intensity profile peak, (iii) at the minimum between the two intensity profile peaks, (iv) at the maximum of the second intensity profile peak, and (v) immediately after the second intensity profile peak. The presence of two peaks in all the intensity profiles corresponds with the presence of two degradation steps. The spectra show peaks at 3573 cm1 (OH), 2947 and 2735 cm1 (CH3 and CH2), 2345 cm1 (CO2), 2177 and 2111 cm1 (CO), 1758 cm1 (C]O), and 1478-896 cm1 (heavily overlapped C-O-C, C-O, CH and CH2 peaks). In the first two spectra of each series these peaks are characteristic of the lactide molecules, oligomeric rings, acetaldehydes and carbon monoxide, that are the previously determined degradation products of PLA [29]. The spectrum obtained between the two peaks in each intensity profile is clearly indicative of a mixture of PLA and PCL degradation products, while in the last two spectra of each series the peaks are characteristic of 5-hexanoic acid, ε-caprolactone, carbon dioxide, water, and methyl pentanoate [30] that are the typical thermal decomposition products for PCL. The weak CO peaks were probably the result of small amounts of CO from the degradation of PLA. There were no real differences between the related spectra for the different samples, indicating that the presence of HNT and the different mixing modes and post-mixing treatments had little influence on the nature of the volatile products formed during the degradation of respectively PLA and PCL. The differences in the positions of the peaks in the intensity profile curves in Figs. 3(a) and 4(a) correspond well with the differences in the positions of the different degradation steps in Figs. 1 and 2. In our previous research on the same type of microfibrillated nanocomposites with organically modified montmorillonite as filler [23], we found that the Cloisite 30B filler caused an apparent increase in the thermal stability of the PCL/PLA blends, and we attributed it to the interaction of the degradation volatiles with the -OH groups in the structure of the surface modifier present in this clay, which delayed the diffusion of these volatiles out of the sample. Although HNT contains -OH groups and one would expect these groups to interact with the oxygen-containing degradation volatiles, this obviously did not happen in this setup, and the release of volatiles occurred unhindered for all the samples during the degradation process.
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4. Conclusions This investigation was an extension of already reported research on the influence of the presence and amount of halloysite nanotubes and drawing after mixing (to form microfibrillated blends and nanocomposites) on the morphology and physical properties of 80/20 w/w PCL/PLA blends. This paper reports on the thermal degradation behaviour and kinetics as investigated through TGA and TGA-FTIR. It was found that the nanotubes on the interface between PLA and PCL probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with HNT catalysis, because HNT was primarily located in the PCL phase of the blend. Drawing to form microfibrillated nanocomposites improved the apparent thermal stability of these materials, and pre-mixing of the HNT with PLA or PCL prior to meltmixing of the blends influenced the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had a significant influence on the degradation kinetics (as evidenced through the apparent activation energy values) or on the nature of the volatile products formed during each of the two mass loss steps. Acknowledgements This work was supported by Czech Science Foundation (Grant No 13-15255S), while the Center for Advanced Materials at Qatar University provided the infrastructure for conducting the experimental work. References [1] S. Fakirov, D. Bhattacharyya, R.J. Shields, Nanofibril reinforced composites from polymer blends, Colloid Surf. A 313 (2008) 2e8, http://dx.doi.org/ 10.1016/j.colsurfa.2007.05.038. [2] J. Chen, L. Lu, D. Wu, L. Yuan, M. Zhang, J. Hua, J. Xu, Green poly(3-caprolactone) composites reinforced with electrospun polylactide/poly(3-caprolactone) blend fibre mats, ACS Sustain. Chem. Eng. 2 (2014) 2102e2110, http://dx.doi.org/10.1021/sc500344n. [3] S.N. Bhattacharya, R.K. Gupta, M.R. Kamal, Polymeric Nanocomposites, Hanser, Munich (, 2008. [4] M.Y. Gelfer, H.H. Song, L. Liu, B.S. Hsiao, B. Chu, M. Rafailovich, M. Si, V. Zaitsev, Effects of organoclays on morphology and thermal and rheological properties of polystyrene and poly(methyl methacrylate) blends, J. Polym. Sci. Part B 41 (2003) 44e54, http://dx.doi.org/10.1002/polb.10360. , Toughening of recycled [5] I. Kelnar, V. Sukhanov, J. Rotrekl, L. Kapr alkova poly(ethylene terephthalate) with clay-compatibilized rubber phase, J. Appl. Polym. Sci. 116 (2010) 3621e3628, http://dx.doi.org/10.1002/app.31905. jka, L. Kapr , A. Zhigunov, J. Hroma dkov [6] J. Rotrekl, L. Mate alkova a, Epoxy/PCL nanocomposites: effect of layered silicate on structure and behavior, eXPRESS Polym. Lett. 6 (2012) 975e986, http://dx.doi.org/10.3144/expresspolymlett. 2012.103. [7] S.S. Ray, S. Pouliot, M. Bousmina, L.A. Utracki, Role of organically modified layered silicate as an active interfacial modifier in immiscible polystyrene/ polypropylene blends, Polymer 45 (2004) 8403e8413, http://dx.doi.org/ 10.1016/j.polymer.2004.10.009. lkov dkov [8] I. Kelnar, I. Fortelný, L. Kapra a, J. Hroma a, Effect of nanofiller on fibril formation in melt-drawn HDPE/PA6 microfibrillar composite, Polym. Eng. Sci. 55 (2015) 2133e2139, http://dx.doi.org/10.1002/pen.24055. lkova , J. Kratochvíl, B. Angelov, M. Nevoralov [9] I. Kelnar, I. Fortelný, L. Kapra a, Effect of layered silicates on fibril formation and properties of PCL/PLA microfibrillar composites, J. Appl. Polym. Sci. 133 (2016) 43061, http:// dx.doi.org/10.1002/app.43061.
[10] S. Fakirov, D. Bhattacharyya, R.J.T. Lin, C. Fuchs, K. Friedrich, Contribution of coalescence to microfibril formation in polymer blends during cold drawing, J. Macromol. Sci. Phy. 46 (2007) 183e193, http://dx.doi.org/10.1080/ 00222340601044375kkk. lkov [11] I. Kelnar, L. Kapra a, J. Kratochvíl, J. Kotek, L. Kobera, J. Rotrekl, dkov J. Hroma a, Effect of nanofiller on the behavior of a melt-drawn HDPE/PA6 microfibrillar composite, J. Appl. Polym. Sci. 132 (2015) 41868, http:// dx.doi.org/10.1002/app.41868. , J. Kratochvíl, Z. Padovec, M. R , [12] I. Kelnar, L. Kapr alkova u zi cka, J. Hrom adkova Effect of layered silicates and reactive compatibilization on structure and properties of melt-drawn HDPE/PA6 microfibrillar composites, Polym. Bull. (2016), http://dx.doi.org/10.1007/s00289-015-1570-6. [13] M.A. Woodruff, D.W. Hutmacher, The return of a forgotten polymer: Polycaprolactone in the 21st century, Prog. Polym. Sci. 35 (2010) 1217e1256, http://dx.doi.org/10.1016/j.progpolymsci.2010.04.002. [14] Y.H. Na, Y. He, X. Shuai, Y. Kikkawa, Y. Doi, Y. Inoue, Compatibilization effect of poly(ε-caprolactone)-b-poly(ethylene glycol) block copolymers and phase morphology analysis in immiscible poly(lactide)/poly(ε-caprolactone) blends, Biomacromolecules 3 (2002) 1179e1186, http://dx.doi.org/10.1021/bm020050r. [15] F. Tuba, L. Olah, P. Nagy, Characterization of reactively compatibilized poly(D,L-lactide)/poly(epsilon-caprolactone) biodegradable blends by essential work of fracture method, Eng. Fract. Mech. 78 (2011) 3123e3133, http:// dx.doi.org/10.1016/j.engfracmech.2011.09.010. [16] K. Chrissafis, G. Antoniadis, K.M. Paraskevopoulos, A. Vassiliou, D.N. Bikiaris, Comparative study of the effect of different nanoparticles on the mechanical properties and thermal degradation mechanism of in situ prepared poly(εcaprolactone) nanocomposites, Compos. Sci. Technol. 67 (2007) 2165e2174, http://dx.doi.org/10.1016/j.compscitech.2006.10.027. [17] D. Ju, L. Han, F. Li, S. Chen, L. Dong, Crystallization, mechanical properties, and enzymatic degradation of biodegradable poly(ε-caprolactone) composites with poly(lactic acid) fibres, Polym. Compos. 34 (2013) 1745e1752, http:// dx.doi.org/10.1002/pc.2257. lkova , A. Zhigunov, V. Khunova , [18] I. Kelnar, J. Kratochvíl, I. Fortelný, L. Kapra , Effect of halloysite on structure and properties of melt-drawn M. Nevoralova PCL/PLA microfibrillar composites, eXPRESS Polym. Lett. 10 (2016) 381e393, http://dx.doi.org/10.3144/expresspolymlett.2016.36. [19] Q. Zhou, M. Xanthos, Nanosize and microsize clay effects on the kinetics of the thermal degradation of polylactides, Polym. Degrad. Stab. 94 (94) (2009) 327e338, http://dx.doi.org/10.1016/j.polymdegradstab.2008.12.009. [20] J.W. Gilman, Flammability and thermal stability studies of polymer layeredsilicate (clay) nanocomposites, Appl. Clay Sci. 15 (1999) 31e35, http:// dx.doi.org/10.1016/S0169-1317(99)00019-8. [21] H. Qin, Q. Su, S. Zhang, B. Zhao, M. Yang, Thermal stability and flammability of polyamide 66/montmorillonite nanocomposites, Polymer 44 (2003) 7633, http://dx.doi.org/10.1016/j.polymer.2003.09.014HNT, 7538. [22] M. Du, B. Guo, D. Jia, Thermal stability and flame retardant effects of halloysite nanotubes on poly(propylene), Eur. Polym. J. 42 (2006) 1362e1369, http:// dx.doi.org/10.1016/j.eurpolymj.2005.12.006. [23] J.P. Mofokeng, I. Kelnar, A.S. Luyt, Effect of layered silicates on the thermal stability of PCL/PLA microfibrillar composites, Polym. Test. 50 (2016) 9e14, http://dx.doi.org/10.1016/j.polymertesting.2015.12.004. [24] T. Ozawa, A new method of analyzing thermogravimetric data, Bull. Chem. Soc. Jpn. 38 (1965) 1881e1886. [25] J.H. Flynn, L.A. Wall, Quick direct method for the determination of activation energy from thermogravimetric data, Polym. Lett. 4 (1966) 323e328. [26] http://www.sigmaaldrich.com/materials-science/nanomaterials/nanoclaybuilding/halloysite-nanotubes.html(accessed 9 August 2016). [27] E. Joussein, S. Petit, J. Churchman, Halloysite clay minerals e A review, Clay Miner. 40 (2005) 383e426, http://dx.doi.org/10.1180/0009855054040180. [28] G. Tari, I. Bobos, C.S.F. Gomes, J.M.F. Ferreira, Modification of surface charge properties during kaolinite to halloysite-7Å transformation, J. Colloid Interface Sci. 210 (1999) 360e366, http://dx.doi.org/10.1006/jcis.1998.5917. [29] I. McNeill, H. Leiper, Degradation studies of some polyesters and polycarbonates. 2. Polylactide: Degradation under isothermal conditions, thermal degradation mechanism and photolysis of the polymer, Polym. Degrad. Stab. 11 (1985) 309e326. [30] J.P. Mofokeng, A.S. Luyt, Morphology and thermal degradation studies of meltmixed poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) biodegradable polymer blend nanocomposites with TiO2 as filler, Polym. Test. 45 (2015) 93e100, http://dx.doi.org/10.1016/j.polymertesting.2015.05.007.
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Material Behaviour
Effect of halloysite nanotubes on the thermal degradation behaviour of poly(ε-caprolactone)/poly(lactic acid) microfibrillar composites Adriaan S. Luyt a, *, Ivan Kelnar b a b
Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar m. 2, 162 06 Praha, Czechia Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsk eho na
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2016 Received in revised form 5 January 2017 Accepted 24 March 2017 Available online 27 March 2017
This paper reports on the thermal degradation behaviour and kinetics of halloysite nanotubes containing microfibrillated poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blends. It was found that the nanotubes probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with halloysite nanotubes (HNT) catalysis. Drawing to form microfibrillated nanocomposites had little influence on the degradation behaviour of these materials, but pre-mixing of the HNT with PLA or PCL prior to melt-mixing and extrusion-drawing of the blends did influence the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had an influence on the degradation mechanism, as evidenced through a Fourier-transform infrared (FTIR) analysis of the degradation products. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Poly(lactic acid) Poly(ε-caprolactone) Halloysite nanotubes Nanocomposites Thermal degradation
1. Introduction Microfibrillar composites (MFC) are polymerepolymer systems prepared by melt- or cold-drawing of suitable polymer blends. In other words, MFC are a special case of polymer blends with the dispersed minority phase in the form of micron-sized fibrils instead of spherical inclusions [1]. The application of various nanofillers has a strong potential to eliminate the basic disadvantage of MFC arising from limited parameters of polymer fibres [2]. The range of nanofiller modification in MFC is very broad. In addition to the well-known enhanced barrier properties, the thermal stability, flammability, reinforcement [3], structure [4e7] and melt drawing process [8,9] are also strongly influenced. Nanofillers significantly affect coalescence [10] during melt-drawing and thus the fibrils dimensions. Its migration between the polymer phases in the course of drawing is especially important in MFC systems with different component polarities, like HDPE/PA6 [11,12]. The MFC concept is promising for upgrading biodegradable and biocompatible PCL suffering from weak mechanical properties [13], because its preferable blending with more rigid polyesters is limited by low compatibility and the necessity of compatibilization
* Corresponding author. Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar. E-mail address: [email protected] (A.S. Luyt). http://dx.doi.org/10.1016/j.polymertesting.2017.03.027 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
[14,15]. Reinforcement of PCL using nanofillers is especially effective in the case of complicated in situ polymerization methods [16], whereas the promising application of natural fibres or those based on biodegradable polymers, like poly(lactic acid) (PLA), is limited by their bad dispersion [2,17]. Recently, it has been found that wellbalanced properties can be achieved in the case of the combination of blending and nanocomposite concepts [4e7]. Our recent results indicate a significant improvement in poly(ε-caprolactone) (PCL)-based material parameters in the case of nanofiller-aided MFC [9,18]. For example, the preparation of MFC based on the promising PCL/PLA combination was impossible due to its very unstable extrusion, resulting in highly variable extrudate dimensions, and it was therefore impossible to draw the nanofillerfree blend. We found that organically modified montmorillonite (oMMT) and HNT caused the extrusion to be more stable and supported melt drawing [9,18]. As a result, a unique biodegradable PCL matrix-based material with dual in situ formed PLA fibres and nanofiller reinforcement was obtained. Slightly better mechanical properties of HNT-modified MFC, in spite of the lower reinforcing effect of HNT on single components, indicated a complex effect of nanofiller in a drawn blend [11]. A further important effect of nanofillers, i.e. the suppression of polymer degradation, was studied for plate-like nanofillers like oMMT [19e21]. However, the effect on thermal degradation was studied much less extensively in the case of tubular HNT that are
A.S. Luyt, I. Kelnar / Polymer Testing 60 (2017) 166e172 Table 1 Samples investigated in this paper. Sample
Description
PCL/PLA-3HNT-u PCL/PLA-3HNT-dr6 PCL/PLA-4HNT-u PCL/PLA-4HNT-dr6 PCL/PLA-7HNT-u PCL/PLA-7HNT-dr6 (PCL-3HNT)/PLA-dr6 (PCL-3HNT)/PLA-u PCL/(PLA-3HNT)-dr5 PCL/PLA-3HNT-u-2x
80/20 w/w PCL/PLA þ3 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 3 phr HNT (draw rate 6) 80/20 w/w PCL/PLA þ 4 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 4 phr HNT (draw rate 6) 80/20 w/w PCL/PLA þ 7 phr HNT (undrawn) 80/20 w/w PCL/PLA þ 7 phr HNT (draw rate 6) 80/20 w/w (PCL þ 3% HNT)/PLA (draw rate 6) 80/20 w/w (PCL þ 3% HNT)/PLA (undrawn) 80/20 w/w PCL/(PLA þ 3% HNT) (draw rate 5) 80/20 w/w PCL/PLA þ 3 phr HNT (undrawn/2x extruded) 80/20 w/w PCL/PLA þ 3 phr HNT (draw rate 6/2x extruded) 80/20 w/w PCL/PLA (undrawn)
PCL/PLA-3HNT-dr6-2x PCL/PLA-u
easily dispersible in polar polymers without modification [22]. Our previous work evaluated the effect of PLA fibrils and oMMT on the thermal stability of biodegradable MFC [23]. The present work deals with a study of the thermal degradation of analogous PCL/PLA micro-fibrillated composites reinforced with unmodified halloysite nanotubes. 2. Materials and methods 2.1. Materials Poly(lactic acid) (PLA): Ingeo 2002D (NatureWorks) with a Disomer content of 4.3%, Mw of 2.53 105 g mol1, melt flow index of 6 g/10 min (190 C/2.16 kg), and density of 1.24 g cm3. Poly(3caprolactone) (PCL): CAPA6800 (Perstorp) Mn of 80 104 g mol1, and density of 1.145 g cm3. Halloysite nanotubes (HNT) were purchased from Sigma Aldrich (USA). The diameter and length evaluated from transmission electron microscopy (TEM) [18] were 20e40 nm and 200e500 nm, respectively. 2.2. MFC preparation The preparation of the micro fibrillated composites was described in a previous paper [18] where the same samples,
167
investigated in this work, were prepared and evaluated in terms of morphology and mechanical properties. The samples analysed in this paper are listed in Table 1. Briefly, the mixing and extrusion were carried out in a co-rotating segmented twin-screw extruder (L/D 40) Brabender TSE 20 at 400 rpm, and the temperatures of the respective zones (from feeding to die) were 170, 170, 170, 170, 175, and 180 C. The bristle with the 80/20 w/w PCL/PLA composition was melt-drawn using an adjustable take-up device. In addition to the one-step feeding of all the components, pre-blends of PCL/HNT and PLA/HNT were also used. Dumbbell shaped specimens were prepared in a laboratory micro-injection moulding machine (DSM). The barrel and mould temperatures were 137 C and 30 C, respectively. 2.3. Characterization A Perkin-Elmer TGA 4000 thermogravimetric analyser (TGA) was used to analyse the thermal degradation behaviour of the samples. The analyses were done from 30 to 600 C at a heating rate of 10 C min1 under nitrogen flow (20 ml min1). The sample masses were 15e20 mg. The thermal degradation volatiles were analysed in a Perkin-Elmer Frontier Fourier-transform infrared (FTIR) spectrometer, connected to the TGA through a PerkinElmer TL 8000 FT-IR-GCMS interface. The samples for the thermal degradation kinetics were heated at 3, 5, 7, 9 and 11 C min1, and the TGA's integrated kinetics software (based on the Flynn-WallOzawa method [24,25]) was used to calculate the kinetic parameters. TGA analyses were repeated three or four times for each composition, and the results from two, three or four analyses (depending on the reproducibility of the TGA results for each composition) were averaged to give the TGA and derivative TGA curves presented in Figs. 1 and 2. 3. Results and discussion Halloysite (Al2Si2O5(OH)4$2H2O) is a two-layered aluminosilicate, with a predominantly hollow tubular structure in the submicron range and chemically similar to kaolin [26]. The neighbouring alumina and silica layers, and their waters of hydration, curve and form multilayer tubes [27] due to a packing
Fig. 1. (a) TGA and (b) derivative TGA curves of seven comparable samples [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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Fig. 2. (a) TGA and (b) derivative TGA curves of six comparable samples [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
disorder. Chemically, the outer surface of the halloysite nanotubes has properties similar to SiO2, while the inner cylinder core is related to Al2O3. The charge (zeta potential) behaviour of halloysite particles can be described by a superposition of the mostly negative (at pH 6e7) surface potential of SiO2, with a small contribution from the positive Al2O3 inner surface [28]. The positive (below pH 8.5) charge of the inner lumen promotes loading of halloysite nanotubes with negative macromolecules, which are at the same time repelled from the negatively charged outer surfaces. This halloysite morphology may have an influence on the mass loss of polymers during thermal degradation, either through immobilization of the polymer chains and chain radicals, and/or through delaying the diffusion of volatile degradation products out of the HNT containing polymers. In the presentation of the results in this paper the samples have been grouped into two groups. The first group consists of the undrawn 80/20 w/w PCL/PLA control sample (PCL/PLA-u) and the undrawn and drawn 80/20 w/w PCL/PLA samples containing respectively 3, 4 and 7 phr HNT (PCL/PLA-3HNT-u, PCL/PLA-3HNTdr6, PCL/PLA-4HNT-u, PCL/PLA-4HNT-dr6, PCL/PLA-7HNT-u and PCL/PLA-7HNT-dr6). The second group consists of the undrawn 80/ 20 w/w PCL/PLA control sample (PCL/PLA-u), the undrawn and drawn samples where PLA and PCL were respectively pre-mixed with HNT before mixing with the other polymer ((PCL-3HNT)/ PLA-dr6, (PCL-3HNT)/PLA-u and PCL/(PLA-3HNT)-dr5), and the undrawn and drawn two-times extruded 80/20 w/w PCL/PLA samples containing 3 phr HNT (PCL/PLA-3HNT-u-2x, PCL/PLA3HNT-u-2x). Although there are different viewpoints on how thermal stability should be defined, it is commonly accepted that there is not necessarily a direct relation between mass loss temperatures as determined during TGA analyses and the thermal stabilities of the respective samples, since there are a number of factors (i.e. rate of diffusion of volatiles out of the sample, interaction between volatiles and filler particles) that may influence the release of volatiles from the degrading sample. Since degradation of the sample may start long before any mass loss is observed, we shall use the term ‘apparent thermal stability’ and relate it to mass loss events as observed through thermogravimetric analysis. Fig. 1 and Table 2 show that the apparent thermal stability of the blend slightly decreased in the presence of 3 and 4 wt% HNT, but quite significantly when 7 wt% HNT was present. The TGA
curves show two clear mass loss steps corresponding well with the respective amounts of polymer initially mixed into the blend. It is known that PLA decomposes before PCL [23], and Fig. 1 shows that the mass loss of both PLA and PCL moved to lower temperatures. This is especially interesting because our previous paper, where the morphology and mechanical properties of the same systems were investigated [18], showed that the HNT was predominantly located in the PCL phase and on the interface, especially in the case of the samples containing 7 wt% HNT. The HNT on the interface must have played a significant role in catalyzing the PLA degradation process, and the free radicals formed during the degradation of PLA must have initiated the early degradation of PCL. Our previous paper [18] showed a reduction in the sizes of the PLA inclusions and in the crystallinity of PLA after addition of the HNT. If HNT catalyzed the degradation of PLA, then the degradation will be more effectively
Table 2 Data from the TGA and dTGA curves of all the investigated samples. Sample
Tmax(1)/ C
Tmax(2)/ C
% Residuea
PCL/PLA-3HNT-u PCL/PLA-3HNT-dr6 PCL/PLA-4HNT-u PCL/PLA-4HNT-dr6 PCL/PLA-7HNT-u PCL/PLA-7HNT-dr6 (PCL-3HNT)/PLA-dr6 (PCL-3HNT)/PLA-u PCL/(PLA-3HNT)-dr5 PCL/PLA-3HNT-u-2x PCL/PLA-3HNT-dr6-2x PCL/PLA-u
358.0 360.0 357.5 358.5 352.5 357.5 353.5 359.5 359.0 364.0 358.0 368.0
414.0 416.5 414.0 414.0 408.5 400.5 412.0 411.5 414.5 417.0 412.0 416.5
5.1 4.3 5.8 4.7 8.7 7.9 4.2 4.2 4.2 2.7 4.2 1.0
(4.0) (3.3) (4.7) (3.6) (7.7) (6.8) (3.2) (3.2) (3.2) (1.6) (3.2)
PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded. Tmax(1): temperature of peak maximum of first peak in dTGA curve; Tmax(2): temperature of peak maximum of second peak in dTGA curve. a Values between brackets in last column are the measured % residue minus the % residue for the PCL/PLA blend without any HNT, and should be an indication of the amount of HNT in the investigated samples.
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Fig. 3. FTIR intensity profile (a) and spectra of PCL/PLA-3HNT-u (b), PCL/PLA-3HNT-dr6 (c), PCL/PLA-4HNT-u (d), PCL/PLA-4HNT-dr6 (e), PCL/PLA-7HNT-u (f), PCL/PLA-7HNT-dr6 (g), and PCL/PLA-u (h) [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/ 20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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Fig. 4. FTIR intensity profile (a) and spectra of (PCL-3HNT)/PLA-dr6 (b), (PCL-3HNT)/PLA-u (c), PCL/(PLA-3HNT)-dr5 (d), PCL/PLA-3HNT-u-2x (e), PCL/PLA-3HNT-dr6-2x (f), and PCL/ PLA-u (g) [PCL/PLA: 80/20 w/w blend of PCL and PLA; (PCL-3HNT)/PLA: 80/20 w/w blend of (PCL pre-mixed with 3 phr halloysite nanotubes) and PLA; PCL/(PLA-3HNT): 80/20 w/w blend of PCL and (PLA pre-mixed with 3 phr halloysite nanotubes); xHNT: x ¼ 3, 4 or 7 phr halloysite nanotubes mixed into 80/20 w/w PCL/PLA blend; u: undrawn; dr5: drawn at a rate of 5; dr6: drawn at a rate of 6; 2x: two times extruded].
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initiated if the inclusions are smaller and more PLA is in contact with the HNT on the interface. It is quite possible that the HNT located in the PCL played a role in initiating the early degradation of PCL, but this is not directly evident from our results. Drawing after extrusion clearly improved the apparent thermal stability of all the composites presented in Fig. 1, which is probably the result of the increased crystallinity of the drawn samples [18]. If one looks at the % residue values in Table 2, there are generally good correlations between these values and the amount of HNT initially mixed into the blends, and the values are basically the same for the undrawn and drawn samples. There is, however, a significant variation in the values for (PCL-3HNT)/PLA-dr6, (PCL3HNT)/PLA-u, PCL/(PLA-3HNT)-dr5, PCL/PLA-3HNT-u-2x and PCL/ PLA-3HNT-dr6-2x, and the differences between the measured residues and the amounts of HNT initially mixed into the samples could not be related to differences in the pre-mixing and postmixing conditions. When comparing the apparent thermal stabilities of the undrawn and drawn samples with 3 wt% HNT mixed into the PCL/ PLA blend (PLA/PCL-3HNT-u and PCL/PLA-3HNT-dr6) with the undrawn and drawn samples where PCL was pre-mixed with 3 wt% HNT ((PCL-3HNT)/PLA-u and (PCL-3HNT)/PLA-dr6), and where PLA was pre-mixed with 3 wt% HNT (PCL/(PLA-3HNT)dr5), it was found that pre-mixing PCL with HNT increased the thermal stability of PLA in the undrawn sample, but decreased the thermal stability of PCL, compared to the undrawn sample where the HNT was directly mixed into the PCL/PLA blend. This is probably because pre-mixing caused the HNT to be located more inside the PCL phase and less on the interface between the two polymers. The HNT therefore primarily catalyzed the degradation of PCL. This explanation can, however, not be applied to the drawn samples. In this case the PCL/(PLA-3HNT)-dr5 sample was found to be slightly less thermally stable than the PCL/PLA-3HNTdr6 sample, but much more thermally stable than the (PCL3HNT)/PLA-dr6 sample (Fig. 2 and Table 2). During extrusion mixing of the PCL/(PLA-3HNT)-dr5 sample the HNT probably diffused towards the interface between the two polymers, and the drawing probably caused a significant amount of the HNT to re-locate inside the PCL phase, which caused the morphology of this sample to be similar to that of PCL/PLA-3HNT-dr6, but with slightly more HNT on the interface between the two polymers, which would have contributed towards catalyzing the PLA degradation and decreasing the apparent thermal stability. The reason for the much lower thermal stability of the (PCL-3HNT)/ PLA-dr6 sample is not clear, since one would expect most of the HNT to be located inside the PCL phase, unless the drawing has forced a significant amount of the HNT out of the PCL phase into the interphase. The compositions of the following samples were identical: PCL/ PLA-3HNT-u, PCL/PLA-3HNT-u-2x, PCL/PLA-3HNT-dr6, PCL/PLA3HNT-dr6-2x. The only differences were in their extrusion processing, where the PCL/PLA-3HNT-u sample was extruded once without drawing, the PCL/PLA-3HNT-u-2x sample was extruded twice without drawing, the PCL/PLA-3HNT-dr6 sample was extruded once with a draw ratio of 6, and the PCL/PLA-3HNT-dr62x sample was extruded twice with a draw ratio of 6. The reason for the double extrusion of the two samples was to see if the HNT dispersion could not be improved. In our previous paper it was, however, found that neither the dispersion nor the mechanical properties were improved [18]. The results in Table 2 show that the apparent thermal stabilities of these samples show the following trend: PCL/PLA-3HNT-u z PCL/PLA-3HNT-dr6-2x < PCL/PLA3HNT-u-2x z PCL/PLA-3HNT-dr6. This trend in the apparent thermal stabilities cannot be directly linked to differences in the
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morphologies of the different samples, but the observed differences will be the result of a complex, interrelated combination of the following factors: (1) Differences in the distribution of the nanotubes between the PCL phase and the interphase; (2) Differences in the respective crystallinities of the PLA and PCL phases in the blends; (3) Degradation of one or both polymers because of multiple extrusions. A kinetic analysis of the degradation process was performed on each of the two steps observed in the TGA curves. We assumed that the degradation of each step progressed according to first-order (n ¼ 1) kinetics, which is usually the case for the degradation of polymers. Unfortunately, the results from this part of the investigation did not reveal much, and the apparent activation energy values for both degradation steps varied between 140 and 210 kJ mol1, with no specific relationship between these values and the presence or amount of HNT, or between these values and the post-mixing treatments. This is evidence of the well-known complex nature of polymer degradation, which is made worse in this case by possible interactions between the two polymers and the filler and by differences between the morphologies of the different samples. Figs. 3 and 4 show the intensity profiles and FTIR spectra for the volatiles released during the degradation of the different samples. The spectra were recorded (i) just before the first intensity profile peak, (ii) at the maximum of the first intensity profile peak, (iii) at the minimum between the two intensity profile peaks, (iv) at the maximum of the second intensity profile peak, and (v) immediately after the second intensity profile peak. The presence of two peaks in all the intensity profiles corresponds with the presence of two degradation steps. The spectra show peaks at 3573 cm1 (OH), 2947 and 2735 cm1 (CH3 and CH2), 2345 cm1 (CO2), 2177 and 2111 cm1 (CO), 1758 cm1 (C]O), and 1478-896 cm1 (heavily overlapped C-O-C, C-O, CH and CH2 peaks). In the first two spectra of each series these peaks are characteristic of the lactide molecules, oligomeric rings, acetaldehydes and carbon monoxide, that are the previously determined degradation products of PLA [29]. The spectrum obtained between the two peaks in each intensity profile is clearly indicative of a mixture of PLA and PCL degradation products, while in the last two spectra of each series the peaks are characteristic of 5-hexanoic acid, ε-caprolactone, carbon dioxide, water, and methyl pentanoate [30] that are the typical thermal decomposition products for PCL. The weak CO peaks were probably the result of small amounts of CO from the degradation of PLA. There were no real differences between the related spectra for the different samples, indicating that the presence of HNT and the different mixing modes and post-mixing treatments had little influence on the nature of the volatile products formed during the degradation of respectively PLA and PCL. The differences in the positions of the peaks in the intensity profile curves in Figs. 3(a) and 4(a) correspond well with the differences in the positions of the different degradation steps in Figs. 1 and 2. In our previous research on the same type of microfibrillated nanocomposites with organically modified montmorillonite as filler [23], we found that the Cloisite 30B filler caused an apparent increase in the thermal stability of the PCL/PLA blends, and we attributed it to the interaction of the degradation volatiles with the -OH groups in the structure of the surface modifier present in this clay, which delayed the diffusion of these volatiles out of the sample. Although HNT contains -OH groups and one would expect these groups to interact with the oxygen-containing degradation volatiles, this obviously did not happen in this setup, and the release of volatiles occurred unhindered for all the samples during the degradation process.
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4. Conclusions This investigation was an extension of already reported research on the influence of the presence and amount of halloysite nanotubes and drawing after mixing (to form microfibrillated blends and nanocomposites) on the morphology and physical properties of 80/20 w/w PCL/PLA blends. This paper reports on the thermal degradation behaviour and kinetics as investigated through TGA and TGA-FTIR. It was found that the nanotubes on the interface between PLA and PCL probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with HNT catalysis, because HNT was primarily located in the PCL phase of the blend. Drawing to form microfibrillated nanocomposites improved the apparent thermal stability of these materials, and pre-mixing of the HNT with PLA or PCL prior to meltmixing of the blends influenced the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had a significant influence on the degradation kinetics (as evidenced through the apparent activation energy values) or on the nature of the volatile products formed during each of the two mass loss steps. Acknowledgements This work was supported by Czech Science Foundation (Grant No 13-15255S), while the Center for Advanced Materials at Qatar University provided the infrastructure for conducting the experimental work. References [1] S. Fakirov, D. Bhattacharyya, R.J. Shields, Nanofibril reinforced composites from polymer blends, Colloid Surf. A 313 (2008) 2e8, http://dx.doi.org/ 10.1016/j.colsurfa.2007.05.038. [2] J. Chen, L. Lu, D. Wu, L. Yuan, M. Zhang, J. Hua, J. Xu, Green poly(3-caprolactone) composites reinforced with electrospun polylactide/poly(3-caprolactone) blend fibre mats, ACS Sustain. Chem. Eng. 2 (2014) 2102e2110, http://dx.doi.org/10.1021/sc500344n. [3] S.N. Bhattacharya, R.K. Gupta, M.R. Kamal, Polymeric Nanocomposites, Hanser, Munich (, 2008. [4] M.Y. Gelfer, H.H. Song, L. Liu, B.S. Hsiao, B. Chu, M. Rafailovich, M. Si, V. Zaitsev, Effects of organoclays on morphology and thermal and rheological properties of polystyrene and poly(methyl methacrylate) blends, J. Polym. Sci. Part B 41 (2003) 44e54, http://dx.doi.org/10.1002/polb.10360. , Toughening of recycled [5] I. Kelnar, V. Sukhanov, J. Rotrekl, L. Kapr alkova poly(ethylene terephthalate) with clay-compatibilized rubber phase, J. Appl. Polym. Sci. 116 (2010) 3621e3628, http://dx.doi.org/10.1002/app.31905. jka, L. Kapr , A. Zhigunov, J. Hroma dkov [6] J. Rotrekl, L. Mate alkova a, Epoxy/PCL nanocomposites: effect of layered silicate on structure and behavior, eXPRESS Polym. Lett. 6 (2012) 975e986, http://dx.doi.org/10.3144/expresspolymlett. 2012.103. [7] S.S. Ray, S. Pouliot, M. Bousmina, L.A. Utracki, Role of organically modified layered silicate as an active interfacial modifier in immiscible polystyrene/ polypropylene blends, Polymer 45 (2004) 8403e8413, http://dx.doi.org/ 10.1016/j.polymer.2004.10.009. lkov dkov [8] I. Kelnar, I. Fortelný, L. Kapra a, J. Hroma a, Effect of nanofiller on fibril formation in melt-drawn HDPE/PA6 microfibrillar composite, Polym. Eng. Sci. 55 (2015) 2133e2139, http://dx.doi.org/10.1002/pen.24055. lkova , J. Kratochvíl, B. Angelov, M. Nevoralov [9] I. Kelnar, I. Fortelný, L. Kapra a, Effect of layered silicates on fibril formation and properties of PCL/PLA microfibrillar composites, J. Appl. Polym. Sci. 133 (2016) 43061, http:// dx.doi.org/10.1002/app.43061.
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