Modification Of Nanocellulose And The Properties Of Poly(Lactic Acid) Composites

Nanoellulose(NC)has been found to be a good candidate to produce fully organic bionanocomposites because NC can provide wide raw material resources,light weight,biogegrability,large specific surface area,excellent mechanical properties and so on.However,intrinsic hydrophilicity and inferior thermal stability limit their application in the reinforcement for composites.In this work,physical adsorption,chemical grafting and enzyme catalytic modification of NC were studied in order to enhance the compatibility between modified NC and PLA matrix.The modified effect and enhancement mechanism for PLA compoties were discussed.NC was isolated from cotton using sulfuric acid hydrolysis and were used to prepare PLA composites.The influence of NC addition on properties of the composites was analized.Results of XRD shows that the unmodified NC present crystalline structure of cellulose I.Results of dynamic contact angle measurement indicates the initial NC present hydrophilic property with an initial contact angle of 40°.According to the redults of TG,the initial degradation temperature of NC was 150 °C,and the remaining carbon was 30 wt.%,which indicates inferior thermal stability of NC.Aggregation of unmodifed NC and carbonization of initial NC occurred during the matrials processing.Transoarency,thermal properties and toughness of the composites were decreased with the addition of unmodified NC,indicating poor compatibility with PLA matrix.NC was oxidized with TEMPO/Na Br/Na Cl O,followed by modification through electrostatic adsorption with benzyltrimethylammuniun chloride(BTAC),diallyldimethyl-ammunium chloride(DTAC)and stearyltrimethylammunium chloride(STAC).The modification effect of the quaternary ammonium salts with different chemical structures on the properties of modified NC.Hydrophilicity of the NC modified with STAC decreased significantly with an initial contact angle of 55°.Furthermore,initial decomposition temperature of NC-STAC was 210 °C,and its remaining carbon was 7 wt.%,indicating the best thermal stability in comparison with NC-DTAC and NC-BTAC.Properties of PLA composites reinforced with quaternary ammonium salts-modified NC were also discussed.PLA composites reinforced with NC-STAC showed better properties including transparency,thermal and mechanical properties in comparison with NC-DTAC and NC-BTAC.According to a preliminary inspection,NC modified with hydrophobic flexible molecular chains are more favorable to improve the properties of PLA composites.However,the modification effect was not ideal because of the weak interaction of electrostatic adsorption.In order to improve the modification effect of NC,triazine derivatives were synthesized through the reaction between 2,4,6-trichloro-1,3,5-triazine(TCT)and n-butylamine(BA),hexylamine(HA)and dodecylamine(DA).Subsequently,NC were grafted with the triazine derivatives.Results of TEM revealed that the morphology and size of triazine derivatives-modified NC were not seriously disturbed.Results of dynamic contact angle measurement showed that NC-TCT-DA presented the most hydrophobic property with an initial contact angle of 110°.NC-TCT-DA showed the best thermal stability with initial decomposition temperature of 300 °C and the remaining carbon of less than 3 wt.%.Compatibitliy between NC-TCT-DA and PLA matrix was significantly improved according to the results of SEM,DSc and transparency measurements.However,hydrolysis and complex side reaction of triazine occurred easily during the reaction,which made it necessary to reaserch other methods for the modification of NC.γ-aminopropyltriethoxysilane(APS)and poly(ethylene glycol)(PEG)were used to modify NC.Firstly,three-dimensional network structure of silane was grafted on NC surface.Subsequently,PEG was grafted on NC-APS.Modification effects of NC was improved by the synergy effect of silane and PEG.The modified NC dispersed uniformly throughout the PLA matrix.In addition,the transparency,thamel and mechanical properties of the composites were improved with the addition of modified NC.It’s necessary to research another modification method because the high price of epoxy-terminated PEG.In order to make the most of synergy of silane and other grafted chains,methacryloxypropyltrimethoxylsilane(MPS)was grafted on NC surface,followed by the grafting of poly(n-butyl acrylate)(PBA).Hydrophobic NC was obtained because of the replacenment of hydrophilic hydroxyl grouos on NC surface.Thermal stability of NC was improved since the coverment of grafted chains for the sulfuric acid covering on the surface of NC.The transparency,thermal and mechanical properties of the composites were improved with addition of the modified NC.However,grafting density and grafting ratio via grafting onto was low because accessibility was decreased due to the steric hindrance induced by the PBA chains grafted on NC surface.Tailored functionalization of NC was performed by grafting PBA on their surface via activators regenerated by electron transfer(ARGET)atom transfer radical polymerization(ATRP).Size exclusion chromatography(SEC)was performed to characterize molecular weight and its distribution of PBA chains grafted from NC surface.FT-IR and XPS were used to characterize the chemical structure of modified NC.Dynamic contact angle measurement showed that NC modified with PBA with DPtarget of 400 presented the biggest initial contact angle.Results of TG revealed improved thermal stability of modified NC.SEM images showed that the modified NC dispersed uniformly throughout PLA matrix,and thread-like adhesions were observed in the cross section of the composites.Transparency,thermal and mechanical properties of the composites improved significantly since the enhanced interfical interaction between the modified NC and PLA matrix.Enzym was applied for the modification of NC based on the results of physical and chemical modification methods.Cellulose laurate were prepared from NC and lauric acid(LAA),with the addition of Novozyme 435 as the lipase product.Chemical structure of the modified NC was analized with FT-IR and 13 C NMR.XRD was performed to analyze the crystalline structure of the modified NC.Results of dynamic contact angle measurement showed that the initial contact angle of NC-LAA was 115°.The initial decomposition temperature of NC-LAA was 270 °C.SEM images revealed homogenous dispersion of NC-LAA in PLA matrix.Transparency,thermal and mechanical properties of the composites were improved.