The Green Production of Hybrid Poly(lactide) / Layered Double Hydroxide Nanocomposites

 

E.D. McCarthy, J.W. Gilman, P. Maupin1, P. Trulove2, M. Zammarano, Y.S. Kim, J. Haghpanah3, J. Howarter, J. Montclare3, S. Hudson, M. Waters

 

1 US Department of Energy, Germantown, MD.

2 Department of Chemistry, US Naval Academy, Annapolis, MD.

3 New York University, Brooklyn Polytechnic, New York, NY.

 

 

Abstract

 

Poly(lactic acid), PLA, has assumed greater commercial importance in the last decade as a sustainable crop-sourced substitute for petroleum polymers such as polyesters and poly(ethylene-terephthalate)s in applications such as blown films, bottles and soft packagings. It is currently produced as neat polymer by ring opening polymerisation of lactide monomer, the cyclic dimer of natural lactic acid which in turn is produced from the fermentation of plant starch.

PLA is also being studied as a base matrix for many composite materials featuring montmorillonites, smectites and other clay-based mineral fillers including layered double hydroxides. Typically such composites are produced by means of melt-blending and extrusion of existing polymer with clay.

The original polymerisation process for the production of PLA relied on the polycondensation of lactic acid to produce the polymer, but this process was unsatisfactory due to the difficulties of efficiently removing water generated during the process, which hydrolyses the polymer. The current, dominant industrial process for the production of PLA is ring opening polymerisation, (ROP), which uses metal salt catalysts, such as tin II octoate, with an alcohol co-initiator to induce polymerisation of lactide. This process has the advantage of tuning the structure of PLA to include varying ratios of the two lactide isomers, l- and d- lactide so as to vary the crystallinity and other properties of the final PLA polymer.

In the present work we discuss the bulk ROP polymerisation of lactide monomer in situ by layered double hydroxide clay, LDH, in the absence of conventional tin catalyst and without added solvent, so that improved dispersion and intercalation of lactide molecules is accompanied by their simultaneous polymerisation and grafting to clay. The ultimate goal for the production of such a grafted PLA-LDH composite is to not only produce a viable commercial composite, hybrid material with improved application properties but also to demonstrate the ability of LDH as a sole polymer initiator in its own right for other lactone polymers and copolymer systems and to gain a deeper understanding of the structure-property relationship of these materials.