Morphology and properties of polylactide modified by thermal treatment, filling with layered silicates and plasticization
Introduction
Polylactide (PLA) is known for its satisfying physicochemical properties, possibility of producing from annually renewable resources, degradability to natural products in a short period of time (0.5–2 years) in contrast to conventional plastics like PS, PE, etc. needed 500–1000 years [1]. For these reasons, PLA is friendly for environment. For some applications, however, PLA requires modification. High brittleness and stiffness of PLA observed at ambient temperature and below it (because of relatively high glass transition temperature about 55 °C) can be substantially decreased by blending with plasticizer [2], [3], [4], [5]. The enhancement of thermal stability, mechanical and barrier properties become possible by filling of PLA with layered silicate. This leads to PLA-based silicate layered nanocomposites with intercalated or mixed, semi-intercalated and semi-exfoliated nanostructures [6], [7], [8], [9]. The nanostructure type can be regulated by the preparation method and/or the nature of organomodification of layered silicate used. The PLA/silicate layered nanocomposites considered in the papers [6], [7], [8], [9] were prepared by melt blending. The change of the procedure of nanocomposite preparation can substantially modify its nanostructure. For instance, in situ intercalative coordination—insertion polymerization of l,l-lactide directly in the interlayer space of the organomodified montmorillonite leads to the formation of the exfoliated nanocomposite without intercalation effect in extent detectable by X-ray diffraction (XRD) method [10].
This paper deals with the modification of physical properties of PLA by filling with different fillers as well as with the determination of the resulting structure on different levels from the molecular to supermolecular ordering. Organomodified montmorillonite, not modified Na+—montmorillonite were used as fillers and poly(ethylene glycol) as a plasticizer. Also unfilled PLA processed in these same conditions and neat (unprocessed) PLA were prepared for elucidating the role of thermo-mechanical history. Two groups of PLA-based systems featured by initially amorphous PLA matrices or initially semi-crystalline PLA matrices were studied. The crystalline structure, thermal behavior, thermo-optical properties and viscoelastic response during periodic deformation were investigated as a function of temperature in relation to sample composition and initial structure of PLA matrix.
Section snippets
Materials
PLA from Dow–Cargill, Inc. (Mw=166,000, I=2.0) containing 96% lactoyl repeating units of l-configuration and 4.1% d-configuration and a residual lactide content of 0.27% was used as the polyester matrix. Poly(ethylene glycol) 1500 (PEG, Mw=1500) from Sigma-Aldrich (Fluca div.) was taken as a plasticizer for PLA. Two types of layered silicates from Southern Clay Products (Texas, USA) were used: sodium montmorillonite—Cloisite Na+ and montmorillonite Cloisite 25A organically modified with
Results and discussion
Before the investigations, all samples were stored in sealed bags at ambient temperature for about 3 months for stabilization of the material. The stabilization was accompanied by the ageing process that occurs in PLA even at ambient temperature, i.e. below its Tg and it involves some changes in the amorphous phase. Different aspects of this phenomenon have been already investigated for amorphous-non-crystallizable PDLLA [13], for amorphous-crystallizable PLA [14], for plasticized PLA with
Conclusions
It has been shown that melt filling of PLA by organomodified nanoparticles leads to nanocomposite characterized by intercalated nanostructure. The intercalated nanostructure was also formed in plasticized nanocomposites. Molecules of PEG participate or stimulate the intercalation process. In turn, unmodified particles of clay form with PLA matrix classical microcomposite system. It was revealed that thermo-mechanical processing of PLA enhances its ability to crystallization by heating up from
Acknowledgements
The author gratefully acknowledge the Cargill–Dow Polymers, LLC, Minnetonka for supplying PLA.
The financial support from the State Committee for Scientific Research (Poland) grant No. 7 TO8E 027 19 is acknowledged. XRD measurements were carried out by M.P. at the Physics Department of Max-Planck-Institut für Polymerforschung, Mainz (Germany) during a visit in Prof. T. Pakula group.
M.P. thanks Dr Michael Alexandre from UMH, Belgium, for assistance in preparation of the nPLA and mPLA composite
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