(751d) Crystallization and Biodegradation of Poly(lactic acid)/Clay Nanocomposites Prepared by Solid-State Shear Pulverization

Bioplastics such as polylactic acid (PLA) offer a sustainable alternative to traditional polymers from petroleum derivatives, from both the source (bio-based) and disposal (biodegradable) standpoints [1].  The ability for PLA to degrade [2] under certain natural conditions is ideal for short lifetime applications such as packaging and consumer products.  Neat PLA, however, exhibits some undesirable physical characteristics, mainly due to its hydrophilicity and low glass transition and melting temperatures.  Incorporating small amounts of nanoscale fillers, such as layered silicates (clay), into the PLA matrix can improve mechanical, thermal, and other physical properties to make them more viable in commercial applications.  Previous studies on PLA/montmorillonite clay nanocomposites have shown significant property enhancement when high levels of exfoliation and dispersion of the clay sheets are achieved [3-5]. Common processing methods, such as in situ polymerization and twin-screw extrusion (TSE) processing, have been employed to used to prepare PLA-clay nanocomposites. 

We have recently introduced solid-state shear pulverization (SSSP) as an alternative processing technique to exfoliate and disperse clay nanofillers in PLA; SSSP processes polymer nanocomposites without the need for chemical modification, additives, and solvents/monomers, and is simple and industrially applicable.  SSSP has proven to successfully compatibilize immiscible polymer blends [6] and exfoliate sheet-like fillers in more traditional polymer-clay nanocomposites [7,8].

In this paper, commercial grade PLA and pristine and organically-modified clay were processed under different TSE and SSSP conditions, and characterized in terms of PLA crystallization and biodegradation kinetics.  Both of these rates are expected to be closely tied to the molecular weight of PLA and the level of exfoliation and dispersion of the clay fillers, both of which are controlled by the processing parameters.  The structural characterization of the materials was conducted by way of gel-permeation chromatography and X-ray diffraction.  Differential scanning calorimetry and dynamic mechanical analysis, as well as polarized optical microscopy, were employed for crystal development monitoring.  Biodegradation study was conducted using an in-house instrument based on the ISO 14855 standard method. The results demonstrate the versatility and tunability of SSSP processing parameters in PLA nanocomposites.

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