Polylactide (PLA)-clay nanocomposites prepared by melt compounding in the presence of a chain extender
Introduction
A significant attention has been devoted to biodegradable and biocompatible polymers in recent years, both from ecological and biomedical perspectives. The predominant biopolymers are polycaprolactone (PCL), polylactic acid (PLA), polyglycolic acid (PGA) and polyhydroxyalkanoate (PHA), among which PLA is the most promising candidate since it is made from renewable agricultural sources and can keep its transparency after processing [1], [2], [3]. Although biocompatibility, biodegradability, and bioresorbability of PLA make it an appropriate candidate for packaging end-use applications, there are, however, some issues such as a low drawability [1], [4], [5], insufficient toughness [6], [7] and limited gas barrier properties [8], [9] that should be properly overcome. Copolymerization, blending and filling techniques are generally used to prevail over these drawbacks [10]. However, the incorporation of a filler into the PLA matrix has attracted the most attention since it pairs low cost with good results. Recently, there have been several attempts to broaden the end-use properties of PLA by developing PLA/clay nanocomposites [1], [5], [6], [7], [9], [11]. Three main techniques can be distinguished for nanocomposites preparation based on thermoplastic matrices: in-situ polymerization, solution intercalation and melt intercalation. The melt intercalation method is the most useful approach for industrial applications due to the absence of solvent, and compatibility with current industrial compounding and processing techniques [12].
The delamination of natural hydrophilic clay in the hydrophobic polymer matrix is a crucial issue [13]; hence, the modification of the clay surface with a surfactant is required to make it organophilic and compatible with common hydrophobic polymers. Cloisite® 30B is an organo-modified montmorillonite having two hydroxyl groups. The reaction occurring between its hydroxyl groups and the carboxyl groups of PLA makes this clay favorable for producing PLA-clay nanocomposites [1]. On the other hand, many attempts have been made to enhance PLA physical and mechanical properties through modification of the polymer and increased molecular weight [14], [15]. It has been shown that using a chain extender could make the molecular weight increase and improve properties [14], [16]. The effect of different chain extenders such as: polycarbodiimide (PCDI), tris (nonylphenyl) phosphite (TNPP) and Joncryl ®ADR 4368F on the thermal degradation of PLA and PLA-clay nanocomposites was investigated in our previous work [17]. Joncryl was found to be the most efficient chain extender, strongly influencing the molecular weight of PLA and rheological properties of PLA and PLA/clay nanocomposites.
The aim of the present work is to investigate the effect of processing conditions on clay dispersion in the presence of the chain extender Joncryl. The impact on the resulting morphology, rheological, mechanical, and barrier properties of PLA/clay nanocomposites is examined. Different strategies for the incorporation of the chain extender into the PLA nanocomposites are investigated.
Section snippets
Materials
The polylactide (PLA) used in this study was purchased from NatureWorks Co. (USA). The selected grade, PLA 4032 D, is a semi-crystalline material in pellet form with an L-lactide: D-lactide ratio of 98:2. The glass transition temperature Tg and the melting point Tm are 60 and 170 °C, respectively, as reported by the manufacturer. The organo-modified nanoclay used was Cloisite® 30B (Southern Clay Products Inc., USA). Finally, Joncryl® ADR-4368F, an epoxy-based chain extender supplied by BASF,
Morphology
The structure of the nanocomposites obtained using the various compounding strategies (Table 1) was first characterized on the compression molded samples using X-ray diffraction. The XRD patterns of Cloisite 30B, PLA and Joncryl-based nanocomposites are presented in Fig. 1. The diffraction pattern of the organo-clay (Cloisite 30B) reveals a sharp reflection peak at 2θ = 4.73° that corresponds to a mean interlayer space of 1.86 nm. The insertion of polymer chains inside the clay gallery in the
Conclusion
In this work the effects of a chain extender (Joncryl) and processing conditions on the clay dispersion and the final properties of PLA-clay nanocomposites were examined. PLA-nanoclay without Joncryl and Joncryl-based nanocomposites were prepared by melt compounding using different strategies. The morphological observations and quantification of clay dispersion revealed that an increased and homogeneous dispersion of clay was achieved in Joncryl-based nanocomposites prepared by the second
Acknowledgments
Financial support from NSERC (Natural Science and Engineering Research Council of Canada) in the context of the NRC-NSERC-BDC Nanotechnology Initiative is gratefully acknowledged. The authors would like to gratefully thank Ms. W. Leelapornpisit who prepared the SEM and TEM micrographs.
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