Elastic Properties of Poly(hydroxybutyrate) Molecules

Abstract

Elasticity of various poly(hydroxybutyrate) (PHB) molecules of regular and irregular conformational structure was examined by the molecular mechanics (MM) calculations. Force - distance functions and the Young's moduli E were computed by stretching of PHB molecules. Unwinding of the 2(1) helical conformation H is characterized at small deformations by the Young's modulus E = 1.8 GPa. The H form is transformed on stretching into the highly extended twisted form E, similar to the beta-structure observed earlier by X-ray fiber diffraction. The computations revealed that in contrast to paraffins, the planar all-trans structure of undeformed PHB is bent. Hence, a PHB molecule attains the maximum contour length in highly straightened, but slightly twisted conformations. A dependence of the single-chain moduli of regular and disordered conformations on the chain extension ratio x was found. The computed data were used to analyze elastic response of tie (bridging) molecules in the interlamellar (IL) region of a semi-crystalline PHB. A modification of the chain length distribution function of tie molecules tau(N) due to secondary crystallization of PHB was conjectured. The resulting narrow distribution tau(N) comprises the taut tie molecules of higher chain moduli prone to overstressing. The molecular model outlined is in line with the macroscopically observed increase in the modulus and brittleness of PHB with storage time.

Influence of cooling rate on the thermal behavior and solid‐state morphologies of polyhydroxyalkanoates

“…These simulations were conducted using results from diffraction experiments to provide a molecular level insight into the structure-property relationship for polylactide (PLA) and PHA materials. 26 Spitalsky and Bleha 27 have utilized molecular mechanics calculation to examine the deformation response of regular and irregular conformations of PHB. Sato et al 4 used quantum chemical calculation as well as infrared and Raman spectroscopy to explore the structure and thermal behavior of PHB and PHBHx.…”