Chemistry Reference
In-Depth Information
The terminal carboxyl groups of PHAs are open to react with chitosan
amine functions and cellulose hydroxyl functions.
99
For the synthesis of
PHB-g-chitosan graft copolymers, chitosan solutions in dilute acetic acid are
treated with different molar ratios of reduced-molecular-weight PHB. The
partially polymerized PHB samples can be prepared either in situ or before
use by dissolving the PHB in a mixture of acetic acid-DMSO (1 : 50, v/v) and
stirring for 16 h at ambient temperature.
77
Although neither of the parent
polymers is water soluble, the PHA-chitosan derivatives form opaque, vis-
cous solutions in water.
10,77
Upon drying of such solutions, strong elastic
films can be prepared. The T
m
of PHB shifts from 175 to about 150 1C for
PHB-g-chitosan. At the same time, the endotherm of chitosan also decreased
from 116 to 105 1C.
PHO, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and linoleic
acid can be grafted onto chitosan via condensation reactions between
carboxylic acids and amine groups.
78
The percentage of microbial polyester
grafted onto the chitosan backbone varied from 7 to 52 wt% as a function
of the molecular weight of PHA, namely as a function of steric effect.
The plasticizer effect of PHO in PHO-g-chitosan lowered the T
m
of the graft
copolymer to 80 1C depending on its PHO content. Thermal analysis of PHO-
g-chitosan graft copolymers indicated the plasticizer effect of PHO by
showing melting transitions, T
m
s, at 80, 100, and 113 1C or a broad T
m
s
between 60.5-124.5
d
n
2
r
4
n
g
|
8
1C and 75-125
1C while pure chitosan showed a
sharp T
m
at 123 1C.
100
Poly(ester urethane)s with P3HB as the hard and hydrophobic segment
and poly(ethylene glycol) (PEG) as the soft and hydrophilic segment were
synthesized from telechelic hydroxylated PHB (PHB-diol) and PEG using 1,6-
hexamethylene diisocyanate as a non-toxic coupling reagent.
101
The PHB
segment and PEG segment in the poly(ester urethane)s formed separate
crystalline phases with lower crystallinity and a lower melting point than
those of their corresponding precursors, except that no PHB crystalline
phase was observed in those with a relatively low PHB fraction. Thermo-
gravimetric analysis showed that the poly(ester urethane)s had better ther-
mal stability than their precursors. Water contact angle measurements and
water swelling analysis revealed that both the surface hydrophilicity and
bulk hydrophilicity of the poly(ester urethane)s were enhanced by in-
corporating the PEG segment into PHB polymer chains. Regarding mech-
anical properties, it was found that the poly(ester urethane)s were ductile,
while natural source PHB is brittle. The Young's modulus and the stress at
break increased with increasing PHB segment length or PEG segment
length, whereas the strain at break increased with increasing PEG segment
length or decreasing PHB segment length.
101
Hao and Deng (2001) prepared semi-interpenetrating networks (IPNs)
hydrogels based on bacterial P3HB and net-poly(ethylene glycol) (net-PEG)
by the UV irradiation technique. Net-PEG-based hydrogels all show higher
equilibrium water contents (EWCs), the crystallinity of PEG segments is
noticeably decreased by cross-linking and would drop further with
.
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