Morphology and properties of polylactide modified by thermal treatment, filling with layered silicates and plasticization

doi:10.1016/j.polymer.2004.09.057

Polymer

Volume 45, Issue 24, November 2004, Pages 8239-8251

https://doi.org/10.1016/j.polymer.2004.09.057 Get rights and content

Abstract

Polylactide (PLA) was modified physically by filling with inorganic additives—organomodified particles of clay, unmodified particles of clay as well as with a plasticizer—poly(ethylene glycol). The PLA-based systems were prepared by melt blending of PLA with other components. Combination of PLA with organomodified clay particles formed nanocomposite with intercalated nanostructure, while introducing microparticles led to microcomposite, respectively. The clay concentration was maintained at 3 wt% in the both systems. Additionally unfilled PLA, plasticized PLA and plasticized nanocomposite were prepared under the same blending conditions. For plasticization, 10 wt% of poly(ethylene glycol) was used. Two groups of the PLA samples and PLA-based systems featured by starting amorphous structure or semicrystalline structure were considered, respectively. The structure, thermal behavior, thermo-optical properties and dynamic mechanical response of the PLA-based systems were investigated and compared with unfilled PLA of the same thermo-mechanical history and with the neat (unprocessed) PLA. The influence of the composition and thermal treatment on the structure and physical properties of the considered samples was determined. The effect of aging on the structure and crystallization ability of the PLA material from its glassy amorphous state was evaluated as well. Temperature modulated differential scanning calorimetry, thermo-optical analysis, X-ray diffraction technique, dynamic mechanical analysis (DMTA) were used.

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. (M_w=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, M_w=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 T_g 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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Morphology and properties of polylactide modified by thermal treatment, filling with layered silicates and plasticization

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Polymer

Polymer

Polymer

Eur Polym J

Polymer

Polymer

JMS—Pure Appl Chem

J Appl Polym Sci

Appl Polym Sci

Macromol Rapid Commun