Renewable polymer nanocomposites with optimal mechanical properties

Abstract

Polyhydroxyalkanoate (PHA) bionanocomposites which consist of biopolymers and nanoparticles (1-100nm) are particularly interesting since they can offer renewable alternatives to the most widely used petroleum-based polymers with equal or better properties and enable new applications. However, the absence of structure-property relationships and the dearth of cost-effective methods for controlling the dispersion of the nanoparticles in a polymer matrix are the two major stumbling blocks to the large-scale production and commercialization of nanocomposites. Therefore, the main objective of this work is to enable the design of PHAs with desirable properties, which can be achieved by tailoring the biopolymer microstructures and the nanoparticle shape and volume fraction. Specifically speaking, it consists of the following two aims: (1) discover how to control the morphologies of the biopolymers by characterizing the effects of cooling rate, and shape and volume fraction of nanoparticles on the structures of the biopolymer matrix; (2) use this knowledge to better understand the structure-property relationships. To this end, the influence of cooling rate on the thermal behavior and solid-state morphologies of neat PHAs are investigated. The thermal behavior and spherulitic morphologies of PHAs studied are observed to depend strongly on cooling rate. However, there is little influence of cooling rate on the crystal structures. The knowledge and the process-dependent multiple-length scale structural information thus obtained can be used for the development of multiscale models that predict mechanical properties of PHAs. Furthermore, the SiO2/PHBHx and SiO2 fiber/PHBHx bionanocomposites are prepared by a “fast evacuation” method. Their mechanical properties and the structures and morphologies of the fillers and the matrix are studied. The results show that at a very low filler loading (1 wt%), the nanoparticles are well dispersed in the matrix. However, the nanoparticles aggregate at higher filler loadings (3 wt% and 5 wt%). A simultaneous improvement of both stiffness and toughness is observed at 1 wt% loading of SiO2 spheres or SiO2 fibers for the higher molecular weight matrix. The SiO2 fibers have a stronger toughening effect than the SiO2 spheres. When the loading is 3 wt% and above, the Young’s modulus continues to increase, but the strain at break and toughness decrease. The ultimate strength does not change for all the nanocomposites compared with the unfilled polymer. It is found that a high molecular weight of the polymer matrix, debondings at the particle-polymer interfaces due to weak interfacial adhesion, and a good dispersion of the nanofillers are necessary to improve toughness and stiffness simultaneously. It is shown that the findings in this thesis provide new design rules for making PHAs and other renewable polymer nanocomposites with optimal mechanical properties.