Study On Preparation And Properties Of Polylactide/Nanocellulose Composites

Polymers derived from renewable resources are now considered as promising alternatives to traditional petroleum-based polymers as they fulfill current environmental concerns in terms of environmental pollution, greenhouse gas emissions and the depletion of fossil resources. One such polymer is polylactide(PLA), a biodegradable and biocompatible, aliphatic thermoplastic polyester. Though PLA has so many advantages, its low crystallization capacity and poor heat resisting property restrict its application. In this paper, we aim to fabricate PLA/nanocellulose nanocomposites to improve PLA’s crystallization capacity and heat resisting property.Firstly the cellulose nanocrystals were fabricated by sulfuric acid hydrolysis of cellulose microcrystalline(MCC). Single factor experiment method was used to optimize process conditions of acid hydrolysis. The optimal process parameters were: the ratio of H2SO4 to MCC was 11.0; H2SO4 concentration was 63%; reaction temperature and time were 45 ℃ and 60 min. The yield of CNC was 30.3%. TEM image exhibited that CNCs were needle-like colloidal particles with about 20 nm in width and 200 nm in length. CNCs could disperse stably in water for more than a month and the dispersion was transparent with blue light. XRD showed a higher crystallinity of CNCs than MCC which meant hydrolysis occurred in amorphous regions. TGA results revealed thermal stability of CNCs decreased.Two different methods were applied to fabricate PLA/CNC composites. One is melting-blending. The content of sulfate ester groups was measured by conductometric titration. Then CNCs were modified by Hexadecyl trimethyl ammonium Bromide(CTAB) which decreased the polarity and improved thermal stability of CNCs for that sulfate ester groups were taken action with CTAB. PLA/modified CNCs composites were prepared by twin screw extrusion, but crystallization capacity and flexural strength and modulus decreased compared with neat PLA for aggregation of CNCs happened. The other method is Pickering emulsion. The dynamic light scattering(DLS) was used to evaluate the colloidal stability(zeta potential and size) of CNC suspensions as function of NaCl. Stable emulsion was fabricated when the concentration of NaCl was 160 mmol. Differential scanning calorimetry(DSC) revealed an enhanced crystallization capacity of PLA/CNC composites, but the flexural strength and modulus decreased for CNC’s low compatibility with PLA.Compared with melting compounding, Pickering emulsion method has obvious advantages such as that there’s no modification process. What’s more, the ability to form strong networks by hydrogen bonding is preserved. Nanofibrillated celluloses(NFC) has lower surface tension than CNC which means NFC owns a better compatibility with PLA than CNC. Therefore, we employed NFCs as emulsifier to stabilize CH2Cl2 solution of PLA in water. After evaporation of CH2Cl2, filtration, drying and hot-pressing, composites were obtained with good NFCs dispersion. Measurements of differential scanning calorimetry(DSC) revealed an enhanced crystallization capacity of NFC/PLA composites. Thermogravimetric analysis(TGA) indicated an increase of onset degradation temperature. Storage modulus of the composites increased throughout the temperature range tested, especially in the high temperature region(70 ℃) of NFC/PLA composites. The Tg peak slightly shifted toward higher temperatures with increasing NFC content indicating that the motion of the PLA polymer chains had been physically restricted due to the entangled NFC network. The flexural modulus and strength also increased.Cellulose nanocrystals(CNCs)-coated polylactide(PLA) microspheres were successfully fabricated via a Pickering emulsion route. Optical microscopy, scanning electron microscopy, laser diffraction particle size analyzer, and thermogravimetric analysis were used to characterize the microspheres in terms of their size, dispersibility in aqueous media, morphology of the microspheres, and PLA/CNCs weight ratio. The results indicated that the microspheres were successfully prepared and the size of which could be controlled by fabrication parameters(PLA concentration in CH2Cl2, CNC concentration in water, water/oil mass ratio and homogenizing time). PLA/CNCs composite could be useful in drug delivery, tissue engineering and so on.