Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 38

An enhancement in the elastic response of PHO, and poly(b-hydro-
xyoctanoic-co-undecylenic acid) (PHOU), was achieved using peroxide
crosslinking both with and without multifunctional co-agents and curing
thermally under vacuum to obtain cross-linked PHOU. Differential scanning
calorimetry (DSC) showed that crosslinking could eliminate all crystallinity.
The elastic response was improved and in general, the crosslinked materials
exhibited a decrease in tensile modulus, and a very low tensile strength and
tear resistance. 106 In another work, the enhancement in mechanical prop-
erties of PHOUs containing varying amounts of unsaturation was attempted
through sulfur vulcanization. DSC showed that crosslinking could eliminate
all crystallinity. The elastic response was near ideal with less than a 5%
tensile set after 200% elongation. In general, sulfur-vulcanized materials
exhibited a decrease in tensile modulus, tensile strength and tear resistance,
which did not appear to vary with crosslink density. Unlike the case of
peroxide crosslinking, material integrity was left intact and mechanical
property results are reported. The network structure was elucidated through
the determination of the molecular weight between crosslinks. 107 PHOU was
crosslinked using gamma-irradiation. The advantage of this process is that it
would allow sterilization and crosslinking simultaneously. 108
Epoxidation of different bacterial polyesters containing unsaturated side
chains in the repeating units with m-chloroperbenzoic acid, as a chemical
reagent, led to quantitative conversions of the unsaturated groups into epoxy
groups with no side reactions observed on the macromolecular chain by
molecular weight measurements. It has been possible to produce new
functional bacterial polyesters containing terminal epoxy groups in the side
chains, in variable proportions up to 37%. 109 Epoxidation of the unsaturated
side chains in PHOU, by reacting the polymer with m-chloroperbenzoic acid
(MCPBA) in homogeneous solution, was readily carried out. The acidic
conditions used for the epoxidation reaction did not result in a significant
decrease in molecular weight of the PHOU. The epoxidized PHOUs were
completely soluble, indicating that cross-linking did not occur, but trans-
formation of the vinyl groups in PHOU into epoxide groups caused a de-
crease in the melting temperature and the enthalpy of melting. In contrast,
the glass transition temperatures increased in a linear manner with epoxide
group content in the product polymer, and the thermal degradation be-
havior of the epoxidized polymer was considerably different from that of the
initial polymer. 110
The synthesis of PHAs containing pendant diol groups was carried out by
the chemical modification of unsaturated PHA using KMnO 4 in cold alkaline
solution (pH 8-9) at 20 1C without a severe reduction in molecular weight. 111
The degree of hydroxylation increased to approximately 60% after 3 h of
reaction, but there was no further increase for the longer reaction times, and
the degree of hydroxylation of products was almost constant at 50-60% after
3 h, irrespective of the unsaturated unit content of the original PHOU or of
the KMnO 4 /unsaturated unit molar ratios, which varied from 0.7 to 2.0. The
polymers, which were 40-60% hydroxylated, were completely soluble in

d n 2 r 4 n g | 8

.