Preparation And Properties Of Bacterial Cellulose/Polylactic Acid Composites

Polylactic acid(PLA),which is derived from corn and other plants,is one of the most widely used bio-based polymers,and the demand for PLA is increasing worldwide.PLA has excellent biodegradability,biocompatibility,good mechanical properties and safety.However,in the preparation of PLA,there are problems such as low synthesis yield of intermediate product propyl cross ester,which makes it difficult to increase the molecular weight of PLA;and low heat deflection temperature and poor toughness due to slow crystallization rate of PLA itself.These problems have a direct impact on the use of PLA.In this paper,we prepared the intermediate product propyleneglyceride in high yield by single-factor experiments;designed orthogonal experiments to optimize the process conditions for synthesizing PLA;and improved the problems such as poor toughness of PLA by introducing graft-modified bacterial cellulose(BC-g-PLA)into the PLA matrix.Firstly,L-lactic acid with an optical purity of 99.96% and zinc oxide nanoparticles as catalysts were used to explore the univariate variables to arrive at the optimal experimental conditions for the polymerization stage in the preparation of propyl cross-ester(LLA):vacuum of-0.06 MPa,polymerization temperature of 140 ℃,polymerization time of 3 h,catalyst addition of 1%,and the highest crude yield of LLA of 97%.The effect of polymerization temperature,polymerization time,catalyst addition and LLA purification times on the viscosity-average molecular weight of PLA was investigated by designing orthogonal experiments.The optimized process conditions were:polymerization temperature of 130 ℃,polymerization time of 9 h,the ratio of catalyst to monomer substance of 1:500,and the number of LLA purification times of 5.The orthogonal experiment was able to obtain 150,000 viscosity-averaged molecular weight PLA with a milky white color.BC-g-PLA composites were obtained by in situ ring opening of LLA using bacterial cellulose(BC)as the substrate,and the grafting product was added to PLA as a toughening agent,and the composite films were prepared by the solution casting method.The reaction efficiency of the solution grafting method was higher than that of the melt grafting method,and the grafting rate could reach 76.60%.The structural tests of the grafting products by Fourier transform infrared spectroscopy,nuclear magnetic resonance spectroscopy and X-ray diffractometer confirmed the successful grafting of PLA to the BC surface.The highest degree of homogeneous refinement of spherical crystals was observed by 0.6 wt% addition of BC-g-PLA as a heterogeneous nucleating agent;the mechanical property tests revealed that the elongation at break could be increased by 175% and the tensile strength by 22.7%after PLA film toughening modification;the crystallinity of the composite film material was increased from 2.53% to 13.26% from unmodified by differential scanning calorimetry.