Renewable polymer nanocomposites with optimal mechanical properties
Authors
Xie, Yuping
Issue Date
2008-12
Type
Electronic thesis
Thesis
Thesis
Language
ENG
Keywords
Chemical engineering
Alternative Title
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.
Description
December 2008
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY