Molecular weight changes during photo-oxidation of polyethylene nanocomposites

Transcription

1 Korea-Australia Rheology Journal Vol. 22, No. 3, September 2010 pp T.O. Kumnayaka, R. Parthasarathay*, M. Jollands and I. Ivanov Rheology and Materials Processing Centre, School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Vic 3001, Australia (Received December 1, 2009; final revision received April 6, 2010; accepted May 28, 2010) Abstract The environmental degradation of synthetic polymers is enhanced by blending them with biopolymers such as starch, pro-oxidants (metal complexes, such as cobalt stearate and manganese stearate) and inorganic fillers. Recent studies have shown that nano-clay used to enhance the mechanical properties of polymers such as polyethylene also helps in polymer degradation (Lee et al., 2005; Qin et al., 2003). The present work aims to investigate the effect of montmorillonite nano-clay (MMT) on the photo-oxidation of polyethylene. The molecular weight changes of polyethylene in MMT/polyethylene nanocomposites upon UV irradiation were evaluated using rheological measurement and gel permeation chromatography. The molecular weight decrease for nanocomposites is found to be greater than that for the pure polyethylene. Furthermore, the increase in clay concentration also enhances the rate of photo-oxidation of the polymer. These results show that clay is acting as a self pro-oxidant during the photo-oxidation. The dynamic shear measurements show that both cross-liking and chain scission occur during the photo-oxidation but the latter seems to be dominant. Keywords : polyethylene, nanocomposites, photo-oxidation, shear rheology, molecular weight distribution (MWD) 1. Introduction *Corresponding author: rchrp@rmit.edu.au 2010 by The Korean Society of Rheology Polyethylene (PE) nanocomposites received a lot of attention in the past decade from both academia and industry mainly because of their enhanced mechanical strength, barrier and other important properties as compared to that of pure polyethylene (Lee et al., 2005). However, the substitution of PE nanocomposite for PE does not reduce the amount of solid plastic waste generated. But some recent studies have shown that PE nanocomposites have a higher degree of photo degradation as compared to PE (Qin et al., 2005; Kumanayake et al., 2009). In spite of these studies, it cannot be claimed that the knowledge on photo degradation of PE nanocomposites is complete and the mechanism is clearly understood. The present study focuses on the photo degradation of PE and its nanocomposites using the changes in the polymer molecular structure which are investigated using the rheological properties of the polymer. Many studies have shown that degradation initiated by UV radiation commences with free radical formation followed by chain propagation, chain branching and termination. The direct consequence of these changes at the molecular level is the deterioration of polymer mechanical properties. However, the rheological properties of the polymer are also likely to change due to the changes in molecular structure, average molecular weight (MW), long chain branching (LCB), and molecular weight distribution (MWD) (Hussein 2007; Qin et al., 2005; Fechine et al., 2009). The present work investigates the changes in polymer rheology and uses them to study the changes in molecular structure during UV irradiation. Although several authors have used changes in rheological properties to evaluate the morphology of nanocomposites (Yang et al., 2006), not many have used it to investigate the changes in molecular structure. The findings of this research will fill the gap in knowledge on photo degradation of polyethylene nanocomposites and the changes in their molecular structure during degradation. 2. Experimental 2.1. Materials and film preparations The materials used in this investigation were: low density polyethylene (LDPE, LDD203) supplied by Qenos, Australia, maleic anhydride-grafted polyethylene (PEgMA, Fusabond MX110D) supplied by Dupont which was used as a compatibliser, and organo-modified clay (Cloisite 15A) supplied by Southern Clay Products. Cloisite 15A is a natural montmorillonite (MMT) exchanged with Korea-Australia Rheology Journal September 2010 Vol. 22, No

2 T.O. Kumnayaka, R. Parthasarathay, M. Jollands and I. Ivanov Table 1. Composition of PE nanocomposites used Sample name LDPE PE-gMA MMT PE PE-2 wt% clay (2C) PE-5 wt% clay (5C) PE-7 wt% clay (7C) quaternary ammonium salt with interlayer spacing of 31.5 Å and a cation exchange capacity (CEC) of 95 meq/ 100 g clay. A Brabender twin screw extruder was used for compounding of LDPE, PE-gMA and Cloisite 15A. PE films with 65~75 µm thickness were prepared using a lab-scale blown film unit manufactured by Strand Palst Maskiner, Sweden. Compositions of different nanocomposites prepared in this study are shown in Table 1. Before the melt mixing, the clay was dried in a vacuum oven at 80 o C for at least 16 hours Photo-oxidation Ultra violet (UV) exposure tests were carried out in an accelerated QUV weathering tester (Q-lab Corp., U.S.A.). It reproduces the damage that can occur in outdoor environment by several environmental factors such as sunlight, temperature, humidity and rain. Spectral irradiance in the tester was according to ASTM standard G154. Nanocomposites films were cut into mm strips and subjected to 0.68 W/m 2 /nm intensity at 340 nm at the black panel temperature of 60 o C. The 12-hours weathering cycle used include 8-hours continuous radiation followed by 4- hours condensation of de-mineralized water on the surface of the film at 50 o C. Samples were allowed to dry for 1 hour after each testing cycle. The aged films were sealed and stored under 10 o C to minimize further oxidation Gel permeation chromatography (GPC) Molecular weight distribution of PE and PE nanocomposites samples was monitored by high temperature GPC analysis performed at 140 o C. The instrument used was a Waters Alliance GPCV 2000 chromatographer equipped with differential refractive index (DRI) and viscometer detectors. 1, 2, 4-trichlorobenzene (TCB) was used as the solvent at a flow rate of 1.00 ml/min. The system of three Styragel HT (4, 5 and 6) columns was calibrated with polystyrene standards with average molecular weight ranging form 1,000 to 5,000,000. PE and its nanocomposites samples were dissolved in TCB and filtered through a 0.5 µm polytetrafluoroethylene (PTFE) filter to remove the solid particles. Universal calibration was applied and the chromatograms Fig. 1. Shear rheology data for non-irradiated samples (a) complex viscosity η* (b) storage moduli G'. were processed using Millennium software Rheological measurements Viscoelastic behaviour of PE and its nanocomposites films were analysed by a strain-controlled dynamic oscillatory rheometer (ARES). The tests were carried out at 170 o C, using 25 mm parallel plate geometry with 1 mm gap. The complex viscosity (η*) and storage modulus (G ) of PE and its nanocomposites were recorded as a function of angular frequency in the range of rad sec -1 with a constant strain of 5% (liner viscoelastic region). 3. Results and Discussion 3.1. Changes in rheological data The complex viscosity, η* and elastic modulus, G' for non-irradiated PE and its nanocomposites are shown in Figs. (1a) and (1b), respectively. According to the X-ray diffraction pattern of nanocomposite samples, except for the sample with 2 wt% clay, all other samples display characteristic peaks at angles less than that for pure clay indicating the formation of an intercalated morphology (results are not shown here). From Fig. (1a) and (1b), it can be seen that PE nanocomposites exhibit a higher degree of pseudoplasticity as compared to pure PE. The Newtonian plateau that can be 174 Korea-Australia Rheology Journal

3 Fig. 2. Shear rheology data of samples irradiated for 10 days at 0.68 W/m 2 /nm (a) complex viscosity η* (b) storage moduli G'. observed for PE in η* curve (Fig. 1a) at low frequencies is less evident for the nanocomposites, especially for those with higher concentrations of clay (5C and 7C). Similarly, it can be seen that the slope of G' curves (Fig. 1b) in the terminal region (low frequencies) decreases significantly with increasing clay concentration indicating more solidlike behaviour, especially for 5C and 7C. All PE-clay nanocomposites used in this study exhibit shear-thinning behaviour, i.e. the complex viscosity decreases with increase in frequency (Fig. 1a). The pseudoplasticity of nanocomposites increases significantly with increase in clay concentration such that the complex viscosities of nanocomposites are comparable to that of pure PE at higher frequencies. These observations suggest that the silicate layers of nano-clay are oriented towards the flow direction at high shear rates and the shear thinning behaviour observed for nanocomposites is dominated by the pure polymer (Bhattacharya et al., 2007). This finding also suggests that the presence of clay does not affect the processing of PE nanocomposites as it usually occurs at high shear rates. The complex viscosity, η* and elastic modulus, G' for irradiated PE and its nanocomposites are shown in Figs. (2a) and (2b), respectively. There are significant differences in the trends of η* and G' values for irradiated samples from those for non-irradiated samples. The η* values of irradiated PE and its nanocomposites are lower than those of non-irradiated samples suggesting that these samples have undergone degradation with significant molecular weight reduction. Although the decrease in η* is observed for all samples at all ω, the decrease is quite at low ω. The η* values of irradiated nanocomposites at low ω are lower than those of non-irradiated samples by two orders of magnitude whereas those of irradiated PE are lower than the non-irradiated PE by one order of magnitude. The G' values of irradiated samples shown in Fig. (2b) also exhibit a behaviour which is similar to that of η* values. As compared to G' values of non-irradiated samples, G' values of irradiated samples decrease at all ω. The decrease in G' values of irradiated samples is significant at all ω. It is more than two orders of magnitude for nanocomposites, particularly for for 5C and 7C at high ω. It can be seen that, at low ω, both η* and G' values decrease for all irradiated samples with increasing clay concentration due to molecular weight reduction of polymer during photo-oxidation. The decrease in molecular weight of the polymer is attributed to the occurrence of chain scission reactions during photo-oxidation. According to above results, it is clear that montmorillonite has enhanced the photo-oxidation process in PE nanocomposites. According to previous studies on photo-oxidation of polyolefin nanocomposites, this enhancement could be due to several reasons. Naturally present iron (Fe 3+ ) ions in MMT affect the rate of degradation of polymeric matrix by reducing to ferrous ions (Fe 2+ ) by redox catalysis of the hydroperoxide decomposition reaction since clay mineral can act as an electron acceptor and/or electron donor (Solomon 1968). The electron acceptor sites are aluminium atoms at crystal edges and the transition metals in the higher valency states in the silicate layers, and the electron donor sites are transition metals in the lower valency state. The POO and PO radicals in polymer enhance the alkyl radical and peroxide formations in the polymer matrix, which enhance the photo-oxidation process according to the reaction scheme shown below (Scott and Wiles, 2001). POOH + Fe 3+ G POO +H + +Fe 2+ POOH + Fe 2+ G PO +OH - +Fe 3+ POO + PH G POOH + P G POOH PO + PH G POH + P G POOH (PH-hydrocarbon polymer, POOH-polymer hydroperoxide) Korea-Australia Rheology Journal September 2010 Vol. 22, No

4 T.O. Kumnayaka, R. Parthasarathay, M. Jollands and I. Ivanov Fig. 3. Molecular weight distribution of irradiated PE and its nanocomposites, irradiation period = 10 days Changes in molecular weight distribution The molecular weight distribution (MWD) curves for 10 days irradiated PE and its nanocomposites samples are shown in Fig. 3. It can be seen clearly that the molecular weight distribution curves for nanocomposites have shifted towards low molecular weight scale with an increase in clay concentration. The curves are also narrower for higher clay concentration samples, which indicate the occurrence of higher rate of chain scissions rather than crosslinkings. Fig. 4 shows the changes in the mass average molecular weight of the nanocomposites with irradiation time. The molecular weights for PE nanocomposites decrease significantly compared with the molecular weight of pure polymer. The molecular weight of PE decreases by 65% after 14 days of irradiation whereas that of 7C nanocomposite decreases by 94%. All the above results suggest that nano-clay plays an important role during the photo-oxidation in the reduction of molecular weight of the polymer thereby making the polymer vulnerable to microbial attack (Albertsson et al., 1987). 4. Conclusions Fig. 4. Weight-average molecular weights (Mw) of non-irradiated and irradiated PE and its nanocomposites. During the photo-oxidation, PE is degraded due to a combination of cross-linking and chain-scission reactions. But the large extent of reduction in η* values seen in Fig. (2a) suggests that chain scission reactions are dominant over cross linking reactions during UV irradiation in the present case. The G' data of irradiated samples suggest that the chain scission rate or the radical formation rate is higher in PE nanocomposites as compared to PE. The above results, therefore, suggest that the presence of MMT clay in PE helps to enhance the photo-oxidation. It is also clear that the pseudoplasticity of PE increases as compared to that of non-irradiated PE during the photo-degradation. However the pseudoplasticity of nanocomposites decreases with irradiation. The reasons for this behaviour can be explained by considering the molecular weight and its distribution data obtained from GPC analysis. This work demonstrates the potential use of nano-clay in enhancing the photo-oxidation of polyethylene which is not otherwise easily degradable due to its high molecular weight and hydrophobic nature. The rate of photo-oxidation of PE nanocomposites is found to be significantly greater than that of pure PE due to the catalytic action of nano-clay (MMT). Results obtained from oscillatory rheological testing and GPC analysis have shown that, upon exposure to UV radiation, the polymer matrix undergoes chain scissions and end up with low molecular weight species which are more susceptible to biodegradation. References Albertsson, A.-C., S.O. Andersson and S. Karlsson, 1987, The mechanism of biodegradation of polyethylene, Polymer Degradation and Stability 18, Bhattacharya, S.N., R.K. Gupta and K. Musa R, 2007, Polymeric nanocomposites, theory and practice, Hanser Gardner. USA. Fechine, G.J.M., D.S. Rosa, M.E. Rezende and N.R. Demarquetter, 2009, Effect of UV radiation and pro-oxidant on PP biodegradability, Polymer Engineering and Science 49, Hussein, I.A., 2007, Rheological investigation of the influence of molecular structure on natural and accelerated UV degradation of linear low density polyethylene, Polymer Degradation and Stability 92, Kumanayaka, T. O., R. Parthasarathy and M. Jollands, 2009, Accelerating effect of montmorillonite on oxidative degradation of polyethylene nanocomposites, Polymer degradation and stability 95, Lee, J.-H., J. Daeseung, H. Chang-Eui, Y., K.Y. Ree and G. S. 176 Korea-Australia Rheology Journal

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