High molecular weight poly (L‐lactic acid) clay nanocomposites via solid‐state polymerization

Abstract

AbstractWe propose here, a novel technique to synthesize high molecular weight (MW) poly (L‐lactic acid)‐clay nanocomposite (PLACN), via solid state polymerization (SSP). We synthesize prepolymer of PLACN (pre‐PLACN) from both,L‐lactic acid andL‐lactide, as starting materials. Synthesis of pre‐PLACN fromL‐lactic acid is carried out viain situmelt polycondensation (MP) ofL‐lactic acid oligomer, followed by SSP, to achieve high MW PLACN (Mw∼ 138,000 Da). In case ofL‐lactide as the starting material, we prepareL‐lactide–clay intercalated mixture which yields moderate MW pre‐PLACN during subsequent ring opening polymerization (ROP). Interestingly, ROP is performed by using hydroxyl functionalized ternary catalyst system (L‐lactide–Sn(II) octoate–oligo (L‐lactic acid) complex), which provides the terminal hydroxyl end‐groups, required for step‐growth SSP. Pre‐PLACN MW is now increased toMw∼ 127,000 Da, by the subsequent SSP process.1H NMR analyses confirm that these end‐groups, are indeed consumed during SSP. During SSP, the PLACN also achieves up to 90% crystallinity, which may be due to the synchronization of the slow step‐growth SSP of poly(L‐lactic acid) (PLA) with the crystallization kinetics. Optical purity of PLACNs is similar to that of neat PLA, whereas the thermal stability of PLACNs is significantly superior. As evidenced by wide‐angle X‐ray scattering/small‐angle X‐ray scattering analyses and in line with the literature, both, intercalated and exfoliated PLACN morphologies, have been synthesized, by suitable selection of clays. We also verify the correlation between the PLA semicrystalline morphology and the PLACN morphology, which is consistent with those of PLACN synthesized by other techniques. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers