Preparation And Properties Of Cellulose Nanocrystal/Poly (Lactic Acid) Composites

With increasing environmental concerns, people pay more attention to the development and application of green products. Polylactic acid (PLA) receives extensive attention for its excellent biocompatibility, biodegradability and deriving entirely from renewableresources. However its low strength, poor thermal stability and brittleness limit its application. In this paper, nanocrystalline cellulose (NCC) with a high strength, high specific surface area and renewability was used as a reinforced phase to prepare NCC/PLA composites. Both methods of grafting modification of NCC and addition of compatibilizer were used to improve the interfacial compatibility between NCC and PLA. The influence of the two modification methods on structure, physico-mechanical properties and biodegradability of the composite was investigated. The main research contents and results are as follows:First, NCC was prepared by the acid hydrolysis of medical absorbent cotton with 64% sulfuric acid. Effects of swelling time andacid hydrolysistime on the yield, morphology and structure of NCC were studied by transmission electron microscope (TEM), fourier transfer infrared spectroscopy (FT1R), wide angle X-ray diffraction (WAXD) and thermo-gravimetric analysis (TG). TEM measurements show that the NCC particles are rod-like, with diameter in the range of 10-20nm, length in the range of 40-400nm and an aspect ratio (length:diameter) of about 10 to 100. Swelling pretreatment of cottonprior to acid hydrolysis raises the yield of NCC. A high NCC yield of 75.88% was obtained when the cotton was acid hydrolyzed for 1.5h at 50? using 64% sulfuric acidafter swelling pretreatment in water for 4 hours at 90?. The NCC prepared with a swelling pretreatment has a higher crystallinity in comparison with the NCC prepared without a swelling treatment.The esterified NCC (ENCC) was prepared through surface esterification of NCC using maleic anhydride (MAH) as esterification reagent and pyridine as catalyst. Then the grafting modification of ENCC was carried out using methyl acrylate (MA) and butyl acrylate (BA) as monomers, benzoyl peroxide (BPO) as initiator. The structure and properties of the grafted products were characterized using FTIR, WAXD and TG.The acrylate monomers are successfully grafted onto the surface of NCC through the process of esterifying first then grafting. Compared with the unmodified NCC, the graft modified NCC (ENCC-g) shows a lower thermal stability and crystallinity.The mPLA copolymers were prepared through solution grafting using BPO as initiator, MAH and BA as monomers. The structure and properties of the grafting copolymers were characterized by FTIR, WAXD, TG and differential scanning calorimetry (DSC). Themonomer graft degree on PLA first increases and then decreases with increasing mass ratio of BA to MAH or monomer content. It reaches the highest value of 1.96% when the mass ratio of BA to MAH and MAH to PLA are 1:1 and 3:100 respectively. The mPLA copolymer also exhibits a lower crystallinity and thermal stability compared with the pure PLA.The modified NCC/PLA (ENCC-g/PLA) composites and NCC/modified PLA/PLA (NCC/mPLA/PLA) composites were prepared through solution casting. The structure and properties of the composites were studied by WAXD, TG, DSC, SEM and mechanical test. The tensile strength of ENCC-g/PLA composites first increases and then decreases with increasing ENCC-g content, and reaches the highest value when the ENCC-g content is 2%, which increases about 32.02% compared with that of the NCC/PLA composite. ENCC-g/PLA composites also show a higher crystallinity. For NCC/mPLA/PLA composites, the tensile strength also presents a tendency of increasing first and then decreasing with increasing mPLA content, reaching the highest value when mPLA content is 8%, which increases about 30.20% in comparison with that of the NCC/PLA composite. The addition of mPLA into the composite makes a contribution to a higher crystallinity. The results of composites' degradation in soil and phosphate buffer solution indicate that the addition of mPLA or ENCC-g accelerates the degradation speed of the composite.