Biodegradable materials based on poly (L,L-lactide) and montmorillonite: Ageing and thermal degradation of poly(ethylene glycol)-plasticized nanocomposites Marie-Amélie Paul, Michael Alexandre, Marius Murariu and Philippe Dubois Laboratory of Polymeric and Composite Materials, Materia Nova Research Center & University of Mons-Hainaut, 20 Place du Parc, 7000-Mons, Belgium, email:
[email protected]
Introduction The market for biodegradable polymers is growing every year and important demands can be expected for those applications where biodegradability offers clear advantage for customers and environment. In this context, polylactide (PLA) is undoubtedly one of the most promising candidates; it is not only biodegradable but also produced from renewable resources, like sugar beets or corn starch [1-4]. The property profile and price of PLA can be changed through its combination with other materials (fillers, additives, other polymers). Therefore, some mineral fillers (clays, calcium carbonate, calcium phosphate, hydroxyapatite etc.), either surface-modified or not, can be incorporated to PLA by melt-blend compounding in order to obtain a nanocomposite or a microcomposite, where the filler has dimensions in the order of some nanometers to several microns or more, respectively. Polymer-based nanocomposites represent a radical alternative to conventional-filled polymers or polymer blends. In contrast to the conventional systems where the reinforcement is in the order of microns, discrete constituents in the order of a few nanometers characterize polymer nanocomposites [5,6]. PLA-based nanocomposites [2-4,7] with enhanced properties, especially in term of mechanical, thermal and gas barrier behavior, could find new applications in the field of flexible packaging. Considerable efforts have been made to improve ductility and processing properties of PLA, so as to compete with low cost and flexible commodity polymers, by using adequate plasticizers: lactide, lactic acid oligomers, glycerol esters, citrates, poly (ethylene glycol) (PEG). It has been reported that most of the plasticizers tend to migrate at the material surface with time and therefore influence the crystallinity and thermal behavior of the polyester matrix [7-9]. Although PEG appears to be an effective plasticizer for PLA, there is evidence that the blends between these two components are not stable. So, as the storage of plasticized PLA (pPLA) compositions above its glass transition temperature (Tg), leads to structural changes of the matrix, one can expect that ageing will promote some modifications of the physical properties of pPLA-based nanocomposites. Moreover, there is also a strong interest to study to which extent the aluminosilicate fillers may modify the ageing process. In the field of research for new PLA grades with improved properties through the nanocomposite technology, the plasticized compositions can represent a real interest for a great number of applications. In this objective, PLA has been plasticized with PEG by melt-bending using an internal mixer, in the presence of organo-modified montmorillonite (MMT). This communication aims at discussing selected aspects concerning the thermal degradation (during processing or resulting from thermogravimetric analyses (TGA) and the ageing of some plasticized nanocomposites based on PLA matrix. Materials and characterization methods PLA with 100% of L,L-lactide units (number average molecular weight (M n): 82,000, polydispersity index: 1.9) was kindly supplied by Galactic sa. PEG 1000 (molecular weight: 1000 g/mol) used as plasticizer for PLA, was purchased from SigmaAldrich (Fluka Div.). Three different clays (Table 1) were used and were supplied by Southern Clay Products (Texas, USA). Both polyester matrix and fillers were dried in order to limit partial PLA degradation induced by water at the processing temperature (PLA was dried overnight at 60°C under vacuum; clays were dried overnight in a ventilated oven at 40°C). The nanocomposites prepared by melt blending PLA with nanofillers (1- 10 wt-%) and PEG 1000 (20 wt%) were obtained by mixing all the components in the presence of 0.3 wt-% of Ultranox 626 stabilizer (GE Specialty Chemicals) in a Brabender counter-rotating mixer. Blending of PLA with clay particles and PEG was performed at the temperature of 180°C, with a rotation speed for premixing of 20 rpm for 4 min, then at 60 rpm for 3 min. To obtain specimens for characterization, the obtained blends were then pressed at 180°C using a hydraulic press Agila PE20, during 8 minutes. Table 1 (Organo-modified) MMTs Filler
Ammonium cation (organic fraction in wt-%)* Cloisite 30B (C18+H35)-N+(C2H4OH)2CH 3 Cloisite 25A (C18H37)-N [CH2-CH(C 2H5)-C3H8](CH3)2 Cloisite 20A (C18H37)2N+ (CH3)2 *Determined by TGA
(20.1) (26.9) (29.2)
Interlayer distance (Å) 18.4 20.4 23.6
Code CL 30B CL 25A CL 20A
Molecular weight determination of PLA by size exclusion chromatography (SEC) was carried out after eliminating the clay and the catalyst residues by filtration of samples previously dissolved in chloroform. Details about the used procedures can be found in the references [4,7]. SEC measurements were performed in THF at 35°C using an Agilent liquid chromatograph equipped with a RI detector. Molecular weights and molecular distributions were calculated by reference to a universal calibration curve relative to1.0486 PS standard, and using the Kuhn-Mark-Houwink equation for P( L-LA) in THF: Mn(PLA) = 0.4055 x Mn(PS) . TGA were performed by using a Hi-Res TGA 2950 thermogravimetric analyzer from TA Instruments with a heating ramp of 20°C/min under air flow from room temperature to 600°C. Thermal behavior (differential scanning calorimetry-DSC) has been measured with a DSC Q 50 from TA Instruments, with a heating and cooling ramp of 10°C/min from –50°C to 200°C under nitrogen flow. The values were generally recorded during the second heating scan. In the ageing study, the thermal behaviors were measured using a Temperature Modulated DSC 2920 (TMDSC) from TA Instruments. Measurements were performed with a heating ramp of 3°C/min using a modulation period of 40s and modulation temperature amplitude of 0.318°C. About one year after their preparation by melt blending, the effect of ageing (1 year at room conditions) on the structural and physical properties of pPLA nanocomposites based on organo-modified MMT has been studied. Results and discussion Three different (organo-)clays have been dispersed within a PLA matrix plasticized with 20 wt-% PEG 1000. Indeed, it has been reported that, at this level of plasticizer, the large decrease in T g allows to obtain a PLA matrix with good flexural properties [4]. The influence of the interlayer cations on the morphological and thermal properties of the composites has been studied for different amounts of filler (1-10 wt-% aluminosilicates, where the filler content is expressed as the amount in inorganics). A special attention has been paid to the thermal degradation that the PLA matrix could undergo upon processing and upon ageing. Relation between clay-based nanocomposite structures and thermal degradation Several studies on layered silicate nanocomposites based on different types of polymer matrices have demonstrated that intercalation or exfoliation of clay nanoplatelets can induce a shift of the matrix thermal degradation towards higher temperature [7,10]. Optimal shifts were usually obtained for a filler content of ca. 2.5 to 5 wt-%. In terms of clay dispersion during melt blending, the most interesting results, for the pPLA, have been obtained by using 3-5 wt-% CL 30B. In this case, intercalated to partially exfoliated structures were observed by morphological analyses (transmission electron microscopy-TEM, wide-angle X-ray scattering-WAXS). When comparing TGA and TEM results, one can report a direct correlation between the structure of the polymer nanocomposite and its thermal behavior. Intercalated to partially exfoliated structures have been demonstrated by TEM pictures for pPLA /CL 30B nanocomposite with content of 3 wt-% of filler (Figure 1) and were confirmed by WAXS analyses.
Figure 1(left) TEM picture of a nanocomposite based on pPLA (with 20 wt-% PEG 1000) and 3 wt-% in CL 30B showing good distribution of clay Figure 2 (right) TGA traces of pPLA (a) and pPLA filled with various amounts of CL 30B, from 1 (b), 3 (c), 5 (d) to 10 wt-% (e) (heating ramp of 20°C/min. under air flow) Conventional TGA performed on the resulting blends shows an increase of the thermal stability of the polymer matrix when filled with a small amount of nanoclay, i.e. 3-5 wt-% in inorganics (Figure 2). It comes out that an increase in thermal stability with the clay content is observed, with a maximum obtained for a
loading of 5 wt-% in clay. In order to explain this phenomenon, the quality of filler exfoliation and dispersion, together with the nature of dispersed nanoplatelets has to be taken in account. At low filler content, exfoliation dominates but the amount of exfoliated particles is not high enough to promote an increase in the thermal degradation temperature through the formation of insulating and incombustible char. When increasing the filler content, a large number of exfoliated particles are formed, char forms more easily and displaces the thermal degradation of the nanocomposite towards higher temperature until 3-5 wt-% of clay is reached. When further increasing the filler content, a decrease in thermal stability is however noticed. This behavior, also observed for unplasticized PLA/CL 30B nanocomposites, can be explained by the limitation to have exfoliated nanoplatelets dispersed homogeneously in the material at filler content above 5 wt-%. Indeed at such a loading level, intercalated nanocompositions dominate, which are presumed less thermally stable than exfoliated ones. At higher levels, equilibrium between exfoliation and intercalation is drawn towards intercalation, and even if char is still formed in high quantity, the morphology of the nanocomposite does not allow for maintaining a good thermal stability. Depending on the nature of the alkylammomium cations (length of the alkyl chain or functionality attached onto the ammonium cation), this effect is more or less pronounced. Degradation upon processing As reported, the degradation of PLA during processing is mainly caused by intramolecular transesterification reactions leading to cyclic oligomers and lactide that can be volatized [7]. Moreover, hydrolysis of ester linkages may also occur and depends on residual water content into the polymer matrix leading to a reduction of the molar masses. Minimizing moisture content can help to reduce losses and preserve molecular weight. The maximum percentage of water for PLA processed by injection, extrusion or for fibre production seems to be limited to 100-250 parts per million (ppm) [11]. Figure 3 shows that during first processing and whatever the clay nature, the molecular weight of the PLA matrix (initially Mn (PLA): 82,000) dropped down, as determined by SEC, with the increase of filler content. Such a molecular weight decrease might appear dramatically high but it is not that much surprising knowing the hydrolytic sensitivity of PLA chains at high temperatures and shearing. Decrease of the molecular weights of nanocomposites Mn has been reported [2,7] and may be explained by either 60000 the shear arising from the mixing of PLA and fillers or 50000 by the presence of ammonium cations, both being able to produce a certain extent of ester cleavage at high 40000 temperature. It is necessary to precise that it is difficult 30000 to limit the water content in organo-modified clays, because water elimination occurs at temperatures 20000 higher than the one at which the clay ammonium salts 10000 usually begin to degrade. Theoretically, molecular weight of PLA matrix, 0 after nanocomposites preparation, may depend also pPLA pPLA pPLA pPLA pPLA pPLA pPLA upon the nature of ammonium cation used for the 1wt% 10w t% 1wt% 10w t% 1wt% 10w t% CL20A CL20A CL25A CL25A CL30B CL30B modification of MMT but, this aspect has not been proved by the experimental results. Figure 3 Modification of Mn upon processing for plasticized PLA matrix and resulting nanocomposites In parallel to the melt blending procedure, which unavoidably leads to a decrease of the molar mass (M n) of the matrix at high temperature, it can be deduced from Figure 3 that the incorporation of a high amount of nanofillers increases the matrix degradation upon processing. Amongst the various factors contributing to the Mn decrease, the most important one seems to be the amount of clay in the materials, independently to the ammonium cation nature. Degradation after ageing PLA matrix degradation has been evaluated by characterizing every “aged” sample in term of molecular parameters, crystallinity, thermal stability and morphology [12]. One of the main characteristics of PLA matrix, which is currently responsible for many of its applications, stands in its easiness to degrade by enzymatic or hydrolytic way. The hydrolytic degradation of PLA is a wellknown process. It happens mainly in the bulk of the material and not on its surface. The hydrolytic chains cleavage proceeds preferentially in amorphous regions, leading therefore to an increase of the polymer global crystallinity. Figure 4 shows Mn values after ageing and after reprocessing by melt-blending compared to initially processed samples. A large reduction in M n is observed and can be most probably explained by the natural hydrolysis of PLA by atmospheric water during the one-year ageing. DSC analyses for pPLA/CL 30B compositions – Figure 5, have first of all shown that the incorporation of PEG 1000 leads to a decrease of the matrix T g from 55°C to ca. 15°C, giving evidence of the PLA plasticization. In addition, it has been demonstrated (the results are not shown) that the nature and percentage of the clay (CL 20A, CL 25A, CL 30B) does not affect Tg.
T g, °C Mn
Initial Tg, °C Tg after ageing, °C
60 Before ageing
60000
After ageing
50
50000
40
40000
30000
30
20000
20
10000
10
0 pPLA
pPLA pPLA 1wt% 10wt% CL 20A CL 20A
pPLA pPLA 1wt% 10wt% CL 25A CL 25A
pPLA pPLA 1wt% 10wt% CL 30B CL 30B
0 PLA
pPLA
pPLA 1 wt% CL30B
pPLA 3 wt% CL30B
pPLA 5 wt% CL30B
pPLA 10 wt% CL30B
Figure 4 (left) Modification of Mn for plasticized PLA matrix and resulting nanocomposites, after ageing Figure 5 (right) Tg of pPLA and pPLA filled with increasing amounts of CL30B, before and after ageing, as measured by DSC After ageing, one may remark a very important increasing of T g due to structural reorganization of PLA matrix and to slow crystallization of PEG from the homogeneous blend. Depletion of PEG increases T g and is not precluded by a higher percentage of the dispersed nanofiller. The ageing process has proven to promote noticeable changes in the structure of PLA matrix, while keeping intercalated nanocomposite structures. It has been shown that the structural reorganization of PLA matrix was additionally promoted by the plasticizer, while the presence of layered silicate filler restricted its extent. In the plasticized compositions, reorganization of the PLA structure upon ageing was combined with the migration of some amount of the plasticizer towards sample surface. The systems exhibited therefore modified mechanical properties, particularly in the temperature region of the glass transition for both the PEG component and PLA matrix. Conclusions Plasticised poly(L,L-lactide) based nanocomposites were prepared by melt blending the matrix with both 20 wt-% of PEG 1000 and nanofillers based organo-modified MMT. The best results in term of thermal stability and dispersion, was brought by the MMT organo-modified by bis-(2-hydroxyethyl)methyl tallowalkyl ammonium cations (Cloisite 30B) melt blended with the plasticized matrix. A new confirmation for the existent correlation between the structure of the polymer nanocomposite and the thermal behavior arises from TGA and TEM analyses since the greater thermal stability improvement was indeed obtained for the intercalated to partially exfoliated nanocomposites for which, the main process of degradation is shifted towards higher temperature by 40°C. During first processing and whatever the clay nature, the molecular weight of the PLA matrix dropped down, as determined by SEC, when increasing the percentage of filler. Knowing that changes in PLA crystallinity may appear due to both the degradation of the polyester matrix and the diffusion of the plasticizer at the material surface, the effect of ageing process on pPLA-based nanocomposites, involving initially amorphous PLA matrix, has been studied. A high reduction of molecular weights and an important increase of glass transition temperatures have been reported. Acknowledgements The authors are very grateful to ‘Région Wallonne” and European Community for the financial support in the frame of interregional project MABIOLAC –“Production of biodegradable composite materials based on lactic acid”. M.-A. Paul thanks the F.R.I.A. (Fonds pour la Recherche Industrielle et Agricole) for her PhD grant. M. Alexandre is very grateful for the financial support from “Region Wallonne” and European Community (FEDER, FSE) in the frame of “Pole d’Excellence Materia Nova”. This work was partly supported by the Belgian Federal Government Office of Science Policy (SSTC-PAI 5/3). References Vink E.T.H., Rabago K.R., Glassner D.A, Gruber P.R, Polym Degrad. and Stability, 3, (2003) 403 Ray S., Okamoto M., Macromol. Rapid Commun. 24, (2003) 815 Degee Ph., Dubois Ph.,“Recent advances in polylactide chemistry and materials science”, submitted for publication, (2003) Paul M.-A., Alexandre M., Degée Ph., Henrist C., Rulmont A., Dubois Ph., Polymer, 44, (2003) 443 Murariu M., Gilman J.W., Bourbigot S., Davis R., “Thermoplastic Layered Silicate Nanocomposites for Technical Applications. Thermal Studies for Choice of Best Condition for Processing of Polymer Clay Nanocomposites”, The 2nd NIST-KIPS Symposium on Polymer Science, NIST, Gaithersburg, Maryland, USA, March 20-21, (2003) 6. Vaia R., AMPTIAC Newsletter, 6, (2002) 17 7. Paul M.-A., “New nanocomposites materials based on poly (L-lactide) and organomodified montmorillonites”, PhD, December 15, (2003) 8. Martin O., Averous L., Polymer, 42, (2001) 6209 9. Hu Y., Hu Y.S., Topolkaraev V., et. al., Polymer, 44, (2003) 5711 10. Liu L., Qi Z., Zhu X., J. Appl. Polym. Sci., 71, (1999), 1133 11. ***”PLA Polymer 3000D - Injection Molding Process Guide”, Cargill Dow LLC, Oct. (2002) 12. Pluta M., Paul M.A., Alexandre M., Dubois Ph., New plasticized polylactide/clay nanocomposites II. The effect of aging on structure and properties in relation to the filler content and the nature of its organo-modification, submitted for publication, (2004)
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