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Communication: Polymer/layered silicate nanocomposite technology is not only suitable for the significant improvement of mechanical and various other materials properties of virgin polymers, it is also suitable to enhance the rate of biodegradation of biodegradable polymers such as polylactide. The biodegradability of polylactide in nanocomposites completely depends upon both the nature of pristine layered silicates and surfactants used for the modification of layered silicate, and we can control the biodegradability of polylactide via judicious choice of organically modified layered silicate.
Biodegradation of neat PLA and various PLA/OMLS nanocomposites recovered from compost with time.
Control of Biodegradability of Polylactide via Nanocomposite Technology Suprakas Sinha Ray,1 Kazunobu Yamada,2 Masami Okamoto,*1 Kazue Ueda2 1
Advanced Polymeric Materials Engineering, Graduate School of Engineering, Toyota Technological Institute, Hisakata 2-12-1, Tempaku, Nagoya 468 8511, Japan Fax. þ81 52 809 1864; E-mail:
[email protected] 2 Unitika Ltd., Kozakura 23, Uji, Kyoto 611 0021, Japan
Keywords: biodegradable; nanotechnology; organically modified layered silicates; polylactide
Introduction Advanced technology in petrochemical based polymers has brought many benefits to mankind. However, it becomes more evident that the ecosystem is considerably disturbed and damaged as a result of non-degradable plastic materials for disposable items. The environmental impact of persistent plastic wastes is growing a more global concern, and alternative disposal methods are limited. Incineration of these materials produces a large amount of carbon dioxide and sometimes generates toxic gases, makes global pollution, and satisfactory landfill sites are also limited. Also, the petroleum resources are finite and are becoming limited. For this reason there is an urgent need to develop renewable source based environmental benign polymeric materials, especially in short-term packaging and disposable applications that would not involve the use of toxic or noxious components in their manufacture, and could allow Macromol. Mater. Eng. 2003, 288, No. 3
for composting to naturally occurring degradation products. Accordingly, polylactide (PLA) is of increasing commercial interest since it is completely made from renewable agriculture products with excellent properties comparable to many petroleum-based plastics and readily biodegradable.[1,2] High molecular weight PLA is generally produced by the ring opening polymerization of lactide monomer,[3,4] which in turn is obtained from the fermentation of sugar feed stocks, corn etc.[5] Even when burned it produces no nitrogen oxide gases and only one-third the combustible heat generated by polyolefin, and does not damage the incinerator and thus, provide significant energy savings.[6] So PLA is a promising polymer for various enduse applications,[7] and currently, there is increasing interest in using PLA for disposable degradable plastic articles.[8] However, some of the other properties such as flexural properties, heat distortion temperature (HDT), gas
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permeability, impact factor, melt viscosity for further processing etc. are frequently not suitable for various enduse applications.[9] To improve these properties we have applied polymer/ layered silicate nanocomposite technology[10] in case of PLA, and materials properties were concurrently improved after nanocomposites preparation with organically modified layered silicates using simple melt intercalation technique.[11–13] The main objective of this Communication is to describe that the polymer/layered silicate nanocomposite technology is not only suitable for the concurrent improvement of materials properties of pristine polymers, it is also useful for the nanoscale control of the biodegradability of biodegradable polymer like PLA and this is very important property improvement for PLA because the degradation rate of PLA is still slow compared to the waste accumulation rate.
Experimental Part Materials PLA with a D-content of 1.1–1.7% (Tg ¼ 60 8C and Tm ¼ 168 8C) was supplied by Unitika Co. Ltd., Japan, and was dried under vacuum at 60 8C, and then kept under dry nitrogen gas for one week prior to use. Three different types of organically modified layered silicates (OMLS) used in this research were synthesized by the ion exchange reaction between Naþ in different layered silicates (CEC ¼ 90–120 mequiv./100 g) and different types of alkylammonium cations. In Table 1 we present the detail specifications of three different types of OMLS used in this research. Nanocomposites Preparation Nanocomposites were prepared by melt extrusion. OMLS (powder) and PLA (pellets) were first dry-mixed by shaking them in a bag. The mixture was melt extruded using a twinscrew extruder (PCM-30, Ikegai machinery Co.) operated at 210 8C (screw speed ¼ 100 rpm, feed rate ¼ 120 g min1) to yield various nanocomposite strands. The abbreviations of various types of nanocomposites prepared by using three different types of OMLS are also presented in Table 2. The strands were pelletized and dried under vacuum at 60 8C for 48 h to remove water. Dried nanocomposites pellets were then
converted into sheets with a thickness of 1 mm by pressing with 1.5 MPa at 190 8C for 3 min. The molded sheets were then quickly quenched between glass plates and then annealed at 110 8C for 1.5 h to crystallize isothermally before being subjected to WAXD analyses and TEM observations, and biodegradability study. Characterization Glass transition (Tg), melting (Tm) temperatures as well as degree of crystallinity (wc) of neat PLA and various nanocomposites were determined by a temperature-modulated differential scanning calorimeter (TMDSC; MDSCTM, TA2920, TA instruments), operated at a heating rate of 5 8C/ min1 with a heating/cooling cycle of the modulation period of 60 s and the amplitude of 0.769 8C. For the measurement of wc prior to DSC analysis, the extra heat absorbed by the crystallites formed during heating had to be subtracted from the total endothermic heat flow due to the melting of the whole crystallites, as was described previously by us.[14] By considering the melting enthalpy of 100% crystalline poly (L-lactide) as 93 J g1 m1,[15] wc was estimated for neat PLA and various nanocomposites (see Table 2). The number-average (M n) and weight-average (M w) molecular weights of PLA before and after nanocomposites preparation were determined by means of gel permeation chromatography (GPC; LC-VP, Shimadzu Co.), using polystyrene standards for calibration and tetrahydrofuran (THF) as the carrier solvent at 40 8C with a flow rate of 0.5 ml min1. For GPC measurements first PLA or nanocomposites were dissolved in CHCl3 and then diluted with THF. GPC results are presented in Table 2. WAXD analyses were performed for the three different types of OMLS powders and various types of PLA/OMLS nanocomposites on a MXlabo X-ray diffractometer (MAC Science Co., 3 kW, graphite monochromator, Cu Ka radiation (wavelength, l ¼ 0.154 nm), operated at 40 kV/20 mA). Samples were scanned in a fixed time (FT) mode with a counting time of 2 s under the diffraction angle 2Y in the range of 1 to 108. Internal structure of various nanocomposites was investigated by means of TEM (H-7100, Hitachi Co.), operated at an accelerating voltage of 100 kV. The ultra thin sections (the edge of the sample sheet perpendicular to the compression mold) with a thickness of 100 nm were microtomed at 80 8C using a Reichert Ultra cut cryo-ultramicrotome without staining.
Table 1. Specifications of OMLS used in this research. OMLS codes
Pristine LS
Particle length
CEC
nm
mequiv./100 g
ODA
Montmorillonite [Na1/3(Al5/3Mg1/3)Si4O10(OH)2]
150
110
SBE
Montmorillonite [Na1/3(Al5/3Mg1/3)Si4O10(OH)2]
100
90
MAE
Synthetic Fluorine Mica [NaMg2.5Si4O10F2]
300
120
Organic salts used for the modification of LS
Suppliers
Octadecylammonium cation Trimethyloctadecyl ammonium cation Dimethyldioctadecyl ammonium cation
Nanocor Inc., USA Hojun Yoko Co., Japan CO-OP Chemicals Co., Japan
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Table 2.
Characteristic parameters of various nanocomposites.
Nanocomposite codea)
Type of composite
Mw
M w/M nb)
103 PLA/ODA4 PLA/SBE4 PLA/MAE4 a) b)
Ordered intercalated nanocomposite Disordered intercalated nanocomposite Conventional composite
161 163 162
1.58 1.61 1.57
Tg
Tm
wc
8C
8C
%
58 58 61
169.8 169.3 169.3
49 65 45
‘‘4’’ indicates amount of OMLS in wt.-%. M w and M w/M n of extruded (at 210 8C) PLA are 177 k and 1.58, respectively.
Biodegradability of neat PLA and various nanocomposites were studied on a homemade compost instrument at 58 2 8C. We intentionally used little high temperature because the biodegradability of PLA is very slow at ambient temperature.[1,5] The used compost was prepared from food waste and was supplied by Japan Steal Works Ltd., Japan. The initial shape of the test specimen were 3 10 0.1 cm3 and were crystallized at 110 8C for 1.5 h before performing the degradation test.
Results and Discussion WAXD patterns of various OMLS powders and corresponding nanocomposites in the range of 2 Y ¼ 1–108 are presented in Figure 1a. In Figure 1b we show bright-field TEM images of various nanocomposites. WAXD patterns and TEM observations respectively established that wellordered intercalated nanocomposite was formed in case of PLA/ODA4 system and disordered intercalated nanocomposite was formed when SBE clay was used for nanocomposite preparation, while conventional composite was formed in case of PLA/MAE4 system. Despite the considerable amount of reports concerning the enzymatic degradation of PLA[16] and various PLA blends,[17] there remain very few reports regarding the compost degradation of PLA.[1,5,13] Figure 2a shows the real picture of the recovered samples of PLA and various PLA/OMLS nanocomposites from the compost with time. The decreased M w and residual weight percentage (Rw) of the initial test samples with time also reported in Figures 2b and 2c respectively. Obviously, the biodegradability of neat PLA is significantly enhanced after nanocomposite preparation with SBE clay. Within one month, both extent of M w and the extent of weight loss are almost same level for both PLA and PLA/SBE4 nanocomposite. However, after one month, a sharp change occurs in weight loss of PLA/ SBE4 nanocomposite, and within two months, it is completely degraded in compost. The compost degradation of PLA occurs by a two-step process. During the initial phases of degradation, the high molecular weight PLA chains hydrolyze to lower molecular weights oligomers. This reaction can be accelerated by acids or bases and is also affected by both temperature and moisture. Fragmentation of the plastic occurs during this step at a point where the M n decreases to less than about 40 000. At about this same M n,
microorganisms in the compost environment start the degradation process by converting these lower molecular weight components to CO2, water and humus.[18] Therefore, any factor, which increases the hydrolysis tendency of PLA-matrix, which ultimately control the degradation of PLA. The Mw and M n of PLA in pure state and in OMLS filled systems are presented in Table 2. As anticipated, the incorporation of OMLS fillers into the PLA-matrix resulted in a little reduction in the molecular weight of the matrix. Decreased molecular weights of PLA in nanocomposite systems may be explained either from the high temperature shears mixing of PLA and OMLS or due to the presence of modified salt both results a certain extent of hydrolysis at high temperature. It is well known that PLA of relatively lower molecular weight may show higher rates of enzymatic degradation due to, for example, the high concentration of accessible chain end groups.[19] However, in this case the rate of molecular weight change of neat PLA and PLA in nanocomposites is almost same (Figure 2b). So initial molecular weight is not a main factor here to control the biodegradability of nanocomposites. Another factor, which controls the biodegradability of PLA is the degree of crystallinity (wc in %), because the amorphous phase is more easily to degrade than the crystalline phase.[16a] In Table 2, we show that the wc value of neat PLA is lower than that of nanocomposites. We expect that two factors are responsible for the significant enhancement of biodegradability of PLA in PLA/SBE4 as compared to that of pristine and other nanocomposite systems: one is the presence of terminal hydroxylated edge groups of the silicate. In case of PLA/ SBE4 nanocomposite, the stacked (4 layers) and disordered intercalated silicate layers are homogeneously dispersed in the PLA-matrix (see TEM image, Figure 1b)[13c] and these hydroxy groups start heterogeneous hydrolysis of the PLA-matrix after absorbing moisture from compost. This process takes some time to start. For this reason, the weight loss and degree of hydrolysis of neat PLA and PLA/ SBE4 is almost same up to one month (Figures 2b and 2c). However, after one month there is a sharp weight loss in case of PLA/SBE4 compared to that of PLA (see Figure 2c). That means one month is a critical value to start heterogeneous hydrolysis, and due to this type of hydrolysis
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Figure 1. (a) WAXD patterns of various OMLS and corresponding nanocomposites. The dashed line in each panel indicates the location of silicate (001) reflection of OMLS. (b) Bright field TEM images of various PLA/OMLS nanocomposites. The dark entities are the cross section of intercalated or stacked OMLS layers and bright fields are the matrix.
matrix becomes very small fragments and disappear with compost. This assumption was confirmed by conducting the same experiment with PLA/MAE4 nanocomposite prepared with MAE clay which has no terminal hydroxylated edge group (because original clay is synthetic fluorine mica, see Table 1), and the degradation tendency is almost same with that of neat PLA (Figure 2a).
Another factor, which controls the biodegradability of PLA is the dispersion of the intercalated OMLS in the PLAmatrix. When intercalated OMLS are nicely distributed in the matrix that means maximum parts of the matrix are in contact with the clay edge and surface, there is a great tendency to fragment more rapidly and hence ultimate degradation, which we observed in case of PLA/SBE4
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Figure 2. (a) Photographs demonstrating the biodegradation of neat PLA and various PLA/OMLS nanocomposites recovered from compost with time. The initial shape of the crystallized samples was 3 10 0.1 cm3. Time dependence of (b) change of matrix M w and (c) residual weight, Rw of neat PLA and various PLA-OMLS nanocomposites under compost.
system. This assumption is also confirmed by conducting same experiment with PLA/ODA4 where the dispersed OMLS has hydroxylated edge groups but poorly dispersed in the matrix.[11] For this reason PLA/ODA4 nanocomposite shows degradation tendency better than PLA/MAE4 but lower than PLA/SBE4. In conclusion, we described a novel nanocomposite approach for polylactide that results in a nanoscale control of the biodegradability of polylactide under compost.
Acknowledgement: Thanks are due to the Japan Society for the Promotion of Science for awarding a postdoctoral fellowship and a research grant to S. Sinha Ray (No. P02152).
Received: October 21, 2002 Accepted: January 10, 2003
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