Biobased Polymer Composites Enhanced by Nano-reinforcements

Abstract

High performance biodegradable polymer composites based on the polylactide and bio-based polyurethane (BPU) were successfully developed and their properties were extensively investigated in this research to alternate the petroleum-based polymers which were used for micro injection molded polymeric parts, packaging materials, and polyurethane products. In the research of polylactide/carbon nanotube (CNT) composites, CNT reinforced polylactide composites were prepared by a melt compounding process and then injection molded into a mold with micro needle patterns. The isothermal crystallization kinetics of polylactide/CNT composites according to Avrami’s theory, were analyzed and the incorporation of CNT improved effectively the crystallization rate of polylactide/CNT composites through heterogeneous nucleation. The rheological properties of the polylactide/CNT composites were also investigated. Furthermore, the tensile strength/modulus and thermal stability of polylactide/CNT composites were enhanced when a very small quantity of CNT was added. The improved level of replication quality of the microfeatures in the polylactide/CNT composites has been achieved by elevating holding pressure and injection speed, which increases the polymer melt flow. The effects of CNTs on the surface properties, crystallization, thermal behavior, and biodegradation of micro injection molded polylactide composites were investigated. An analysis of crystallinity and thermal behavior indicated that the CNTs promoted the unique α’ to α crystal transition of polylactide, leading to an enhancement of surface modulus and hardness, as measured using a nanoindentation technique. The specific interaction between polylactide and CNTs was characterized using an equilibrium melting point depression technique. Furthermore, the CNTs increased the activation energy for thermal degradation of polylactide due to the physical barrier effect and also accelerated the biodegradation of polylactide because of both the larger interspace area and an enzyme-CNT conjugate effect. In the research of polylactide/cellulose nanocrystal (CNC) composites, the CNC was prepared by acid hydrolysis and incorporated into the polylactide matrix. A small quantity of CNC resulted in improvement of complex viscosity, storage modulus, and mechanical properties of the polylactide/CNC composites. However, the thermal stability of the polylactide/CNC composites was decreased with CNC content because of sulfate ions on the surface of CNC. In order to enhance the dispersion of CNC in the non-polar environment and the compatibility with polylactide, the surface alkylation of CNC was effectively developed by a nucleophilic substitution reaction with alkyl bromide to convert hydrophilic groups on the CNC into alkyl group and the degree of substitution was quantitatively determined. The resultant alkylated CNC exhibited improved dispersion in non-polar environment and increased hydrophobicity as compared with unmodified and acetylated CNCs. The polylactide composites reinforced by unmodified and modified CNCs were prepared by solution casting method and the effects of reinforcements on the thermal stability, mechanical properties, morphology, and barrier property have been investigated. In addition, the mechanical property modeling is evaluated to simulate the moduli of the polylactide composites and compared with the experimental values. As a result, polylactide composites reinforced by alkylated CNC revealed superior properties in terms of thermal stability, tensile strength, Young’s modulus, and barrier property because of its uniform dispersion and strong interfacial adhesion between filler and matrix. In the research of BPU composites and electrospun hybrid nanofibers, the BPU was successfully synthesized using castor oil and castor oil/poly(ε-caprolactone) diol (PCL) hybrid polyol. The CNC was added during synthesis of BPU in the preparation of the BPU/CNC composites to form covalent boding between BPU and CNC, leading to improvement of interfacial adhesion. The tensile strength and modulus of the CNC reinforced BPU composites were significantly improved, as compared with the neat BPU. The elongation at break of the BPU/CNC composites decreased with increasing CNC content, indicating that CNC made the BPU stiff and rigid. Dynamic mechanical analysis showed that the storage modulus was increased and the tan ? peak was shifted toward higher temperatures by incorporation of CNC. This was due to the increased cross-linking density of the rubber network resulting from the strong interaction between CNC and the BPU matrix. Furthermore, the CNC enhanced the thermal stability of the BPU composites. The activation energy for thermal decomposition was evaluated using the Horowitz-Metzger method. The BPU was also synthesized using a castor oil/PCL hybrid polyol to develop the soluble BPU in organic solvent and the variations in the properties of BPU with castor oil contents were investigated. The BPU hybrid nanofibers containing silica nanoparticles or antibacterial agent were developed by electrospinning process for industrial realization of BPU materials. The modified silica nanoparticle (m-silica) was successfully prepared before the incorporation into BPU matrix to realize hydrophobic surfaces. The rheological analysis revealed that a network structure existed between the BPU and m-silica, which led to a remarkable improvement in the mechanical properties and thermal stability. A morphological change in the nanofibers on incorporation of the m-silica nanoparticles was also observed: the average fiber diameter of the hybrid nanofibers decreased with increasing m-silica content. Furthermore, the m-silica nanoparticles resulted in a change in the effective surface wettability of the BPU nanofibers resulting in a change from hydrophilic to hydrophobic behavior. Triclosan (TR) as an antibacterial agent was compounded with cyclodextrin (CD) to achieve the TR-CD formations. The three types of α-CD, β-CD, and γ-CD were used for the formation of TR-CD and the results indicated that the β-CD and γ-CD successfully formed TR-CD while α-CD could not form the TR-CD formation as confirmed by chemical structure and crystallinity analyses. The BPU/TR-CD hybrid nanofibers exhibited superior releasing property and antibacterial activity, as compared to the BPU/TR nanofibers. The high performance and biorenewable composites developed in this researth are expected to expand the utilization of biopolymers by replacing the petroleum-based polymers which were used in practical applications such as micro injection molded polymeric parts, packaging materials, and polyurethane products for an industrial perspective.