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Chiral poly(beta-hydroxyalkanoates): an adaptable helix influenced by the alkane side-chain
R H Marchessault1, C J Monasterios, F G Morin
1Department of Chemistry, McGill University, Montreal, Quebec, Canada.
International Journal of Biological Macromolecules
|April 1, 1990
Summary
Bacterial poly(beta-hydroxyalkanoates) (PHA) with longer side chains crystallize differently, forming elastic materials. Side-chain packing influences crystallization, affecting properties like melting point and crystallinity.
Area of Science:
- Polymer Science
- Materials Science
- Biochemistry
Background:
- Bacterial polyesters, poly(beta-hydroxyalkanoates) (PHA), are biodegradable polymers with tunable properties.
- Understanding the crystallization behavior of PHA is crucial for their application.
Purpose of the Study:
- To investigate the crystallization and structural properties of bacterial copolyesters of poly(beta-hydroxyalkanoates) (PHA) with varying side-chain lengths.
- To correlate structural features with macroscopic properties like elasticity and thermal behavior.
Main Methods:
- X-ray diffraction was used to analyze the crystalline structure and determine the fibre repeat (c dimension).
- Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were employed to study thermal properties.
- 13C-nuclear magnetic resonance (NMR) spectroscopy was utilized to assess chain dynamics and estimate crystallinity.
- Stress-strain analysis was performed to evaluate mechanical properties.
Main Results:
- PHAs with average side-chain lengths of C5-C7 crystallize in a 21 helix within an orthorhombic lattice, exhibiting a reduced fibre repeat (c dimension) compared to PHB and PHV.
- Crystallization is influenced by side-chain packing, leading to ordered sheets with polyethylene-like domains.
- Thermal properties (melting point, glass transition temperature) and fibre repeat show regular changes with increasing side-chain length.
- Melt-crystallized PHA showed a maximum crystallinity of approximately 25% after several hours.
- Stress-strain curves indicated elastomeric behavior, distinct from P(HB-co-HV) polyesters.
- 13C-NMR revealed high motional freedom in side-chains and slower motion in backbone carbons, with crystallinity estimates aligning with X-ray data.
- Conformational analysis suggested two possible crystalline helix models: 'herringbone' and 'comb-like'.
Conclusions:
- Side-chain packing significantly influences the crystallization of PHA, leading to unique structural and mechanical properties.
- The observed elastomeric behavior and tunable thermal properties make these PHAs promising for various applications.
- Further investigation into the helical models can provide deeper insights into PHA structure-property relationships.