Optimization of Melt-mixing Transesterification of Polylactide by Polyethylene Glycol Employing Response Surface Methodology

Abstract

A process for preparing polylactide-block-polyethylene glycol (PLA-b-PEG) copolymers has been developed and optimized by employing a transesterification reaction in a melt-mixing process. The reaction was carried out in a counter-rotating mixer unit, using polyethylene glycol (PEG) and tetrabutyl titanate (TBT) catalyst. The effects of PEG weight percentage, catalyst contents, temperatures, and rotor speeds on chemical structures of the resulting PLA-b-PEG were examined, in terms of number average molecular weight (Mn), weight average molecular weight (Mw), in-chain PEG contents (wt.% PEG), and specific mechanical energy (SME) imparted by the mixer. The process parameters were optimized using a central composite rotatable design (CCRD) response surface methodology (RSM). The CCRD was designed with four variables at three levels of variations (-1, 0,-1), four replicates center point, and a redundancy factor (α) of 2.000. The responses (Mn, Mw, wt.% PEG, and SME) from 28 experimental trials were analyzed by a multiple linear regression fitting, a second-order equation, and the RSM model. Mn and Mw of the products were determined by gel permeation chromatography (GPC). The in-chain PEG content was examined by nuclear magnetic resonance (1H-NMR) spectroscopy. The results show that the PEG weight percentage and the reaction temperature significantly affect (P< 0.05) Mw and Mn of the products, which are drastically decreased with an increase in the PEG weight percentage and temperature. A quadratic interaction is observed between the PEG weight percentage and temperature, indicating that high reaction temperature leads to lower PEG conversions, due to undesirable competing thermal-oxidative degradations of PEG in the presence of the catalyst. Optimum operating conditions on the PEG weight percentage, catalyst contents, temperatures, and rotor speed for obtaining high Mw with high wt.% PEG was identified. Although the optimal conditions are observed at the boundary level, the model serves as a platform for effective preparation of PLA-b-PEG copolymers with designed molecular weight and chemical structures. Further optimization of the model may be conducted by extending the range of independent factor levels. The resulting flexible PLA-b-PEG copolymers, with tunable structures and properties, have high potential for use as single-component degradable bioplastics with excellent mechanical properties, plasticizers, or toughening agent for enhancing PLA’s toughness.