Modification of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 171

weight of the polymer, crystallinity, monomer composition, porosity and
roughness of the polymer surface. 173 In order to measure the rate of deg-
radation, changes in the weight loss of the PHA film over time can be
monitored. While the biopolymer chains could be completely degraded into
monomers and dimers of 3-hydroxyalkanoic acids by PHA depolymerase, 174
this process is very slow. Williams et al. 175 measured the molecular weights
at the surface versus the interior of the PHO implants but no significant
differences were detected. This showed that homogeneous hydrolytic
breakdown of the polymer occurred inside the mice. 176

d n 2 r 4 n g | 1

7.4.5 PHA Functionalization
Previously, applications of lipase had been limited because the enzyme was
thought to work effectively only in environments with a high water content,
and would rapidly lose its activity in organic solvents. 177 Later, the enzyme
was found to remain catalytically active in organic solvents, and this gen-
erated huge interest among researchers and industries as most of the in-
dustrial substrates are hydrophobic in nature. Utilization of an organic
solvent as a reaction medium helps to improve hydrophobic substrate
solubility and simultaneously elevates the rate of the reaction. In addition, a
micro-aqueous environment enables reactions that are impossible in water,
such as esterification, to be carried out. 178,179 It has been suggested that
organic solvents may play a role in prolonging enzyme activity by replacing
the molecules of water on the enzyme with solvent molecules. 177 Reactions
in water usually caused downstream diculties in the separation of soluble
enzyme and products. However, in organic solvent media, the enzyme is
insoluble and this facilitates product recovery. 180
The functionalization of polymers may involve chemo-enzymatic synthetic
steps. This can be seen in the esterification of poly (e-caprolactone)
with hydrophilic moieties in the presence of CAL B as a biocatalyst where
tetrahydrofuran, dichloromethane and dioxane were used as reaction
media. 149,181 Longer chains of PHA pose a steric hindrance for the enzyme to
bind effectively with the polymer substrate at the carboxyl end terminus for
transesterification. To overcome this limitation, enzyme catalyzed hydrolysis
of the poly (e-caprolactone) was carried out prior to chemical transester-
ification. 181 The enzymatic step not only facilitates the subsequent step but
also helps to generate more carboxyl end terminals for transesterification. 81
In another study, Gumel et al. 182 studied the use of two organic solvents viz.
dimethylsulfoxide (DMSO) and chloroform at a 1 : 4 ratio to dissolve reaction
substrates with contrasting solubilities i.e. sucrose and PHA. Since a small
amount of DMSO was sucient to dissolve enough sucrose, and then mix
with PHA in chloroform, a system that behaves largely like a single-phase
system could be achieved. Consequently, the effects of interfacial resistance
can be avoided. In this system, the researchers demonstrated a successful
functionalization of a medium-chain-length PHA with sucrose thereby im-
proving the modified polymer film's biodegradability.

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