| Poly(lactic acid) (PLA) is a kind of full biodegradable eco-friendly composite, however, the flaws of high brittleness and weak tenacity restrict its applications. In this paper, moso bamboo cellulose nanowhisker (BCNW) was extracted and characterized from the abundant bamboo industrial waste, and BCNW was introduced into PLA matrix as toughness. In addition, silane coupling agents were employed to surface modify the BCNW in order to improve compatibility between PLA and BCNW. High-strength and high-modulus ultrafine bamboo-char (UFBC) was considered as reinforcement for fabricating of PLA/BCNW/UFBC ternary composites. The microstructure and property of PLA/BCNW and PLA/BCNW/UFBC composites were investigated, respectively. The interfacial bonding, strengthen and toughen mechanism were clarified.1) BCNW was extracted from bamboo residues using sulfuric acid hydrolysis. Changes of bamboo cellulose at different stages of processing, as well as the roles of acid concentrations (55 and 65 wt%) and hydrolysis times on characteristics of nanowhiskers were comprehensively investigated by SEM, TEM, TGA-FTIR, synchrotron radiation WAXS, composition analysis and BET analysis. Results showed that both rod-like and network-like whiskers were existing in nanowhiskers. Alkaline pretreatment removed impurities and part of hemicellulose. Cellulose content enormously increased to nearly 85% and specific surface area improved as well after bleaching. Nanowhiskers had an average length of 455 nm, diameter of 12 nm, aspect ratio of about 37. Cellulose I was the dominant composition in bamboo cellulose, transformation of cellulose Iα to cellulose Iβ was observed. Nanowhiskers presented greater crystallinity and crystallite size than those cellulose without hydrolysis, but lower thermal stability than that of the cellulose.2) To improve the mechanical and thermal properties of PLA composites, PLA/BCNW biofilms were fabricated by solution casting with varying BCNW loadings. Characteristics of the biofilm was investigated by optical observation, SEM, mechanical measurements, synchrotron radiation WAXS, and differential scanning calorimetric (DSC) analysis. Results showed that the transparency of films decreased with the increasing of BCNW loadings. The maximum tensile modulus and elongation at break of 427.72 MPa and 22.27% were reached at 2.5% and 1.0% loadings, respectively. Heterogeneous nucleation effect was confirmed by WAXS. Both homogeneity and stereocomplexed crystallites were noticed. After the addition of BCNW, the size of crystallites remarkably increased. DSC illustrated that PLA/BCNW composites had a higher glass transition temperature and larger PLA crystallinity when 2.5% BCNW was loaded. However, morphological analysis illustrated relative low compatibility between the two phases.3) To improve the compatibility,4 kinds of silane coupling agents namely triethoxyvinylsilane (A-151), aminopropyltriethoxysilane (KH-550),3-methacryloxypropyltrim-ethoxysilane (KH-570), and (3-mercaptopropyl) trimethoxysilane (KH-590), were employed to surface modify the BCNW with 5 concentrations between 1-16 wt%. FTIR analysis showed that coupled reaction between coupling agents and cellulose were confirmed, and both hydrogen-bond and covalent-bond of Si-O-C interaction were existing. Cellulose was alkylated uniformly so that hydroxyl was comparatively reduced than that of the untreated. Thermal degradation characteristic of BCNW decreased when A-151 and KH-570 were used, whereas KH-550 treatment showed the highest thermal stability. KH-590 treatment reduced thermal degradation temperature ranges with the increasing of concentrations. The alkylated BCNW maintained original morphology. When increasing the treating concentration, it became smooth due to the coating of coupling agents, however, when 16 wt% used, it presented rough surface again. The C/O ratio decreased after alkylation. KH-570 treatment showed the weakest compatibility.4) Alkylated BCNW toughened PLA composite films were further fabricated via solution casting. With the increasing of treating concentrations, elongation at break showed inversely proportional with the tensile strength and tensile modulus. The best tensile tenacity was found to be 250.8% after KH-590 treated comparing to that of 12.35% with untreated composite. All samples treated by A-151, KH-570 and KH-590 exhibited elongation at break values above 200%. Alkylated BCNW had strong hydrogen-bond interaction with carboxyl terminal on PLA. The two phases showed physical interlocks and chemical interaction. Wiredrawing phenomenon was noticed on the fractural surface. Thermal degradation stability decreased with the increase of alkylation concentrations. Toughening mechanism was mainly due to the strong chemical bonds and good surface compatibility, made tensile stress transmit well when the system under tension. Macromolecular chain, segment, microcrystalline of PLA and some asymmetric filler tended to be orderly deformation and arrangement along with tensile direction.5) UFBC was introduced into PLA/BCNW composites as reinforcement. The transparency sharply decreased when UFBC content exceed 1.0 wt%. Maximum tensile strength and modulus of 18.87±0.65 MPa and 272.24±30.48 MPa, similar to that of PLA, were reached when 0.25 wt% UFBC added, at which the composites also showed a relative high tensile tenacity. Thermal degradation stability decreased with addition of UFBC. The ternary compounding system showed lower crystallization capacity but higher degree of crystallinity comparing to PLA/BCNW system. The strengthening effect was conducted by the core-shell structure of the dispersed phase during tension. |
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