Chemistry Reference
In-Depth Information
At elevated temperature of polymer hydrolysis (70 °C) as well as at
physiological temperature 37 °C we have demonstrated again that the
PHBV films are the stablest because by 95th day they lost only 4 wt.%.
The enhanced stability of PHBV relative to the PHB has been confirmed
by other literature data [21]. Here it is worth to remark that during biosyn-
thesis of the PHBV two opposite effects of water sorption acting reversely
each other occur. On the one side, while the methyl groups are replaced
by ethyl groups, the total hydrophobicity of the copolymer is enhanced,
on the other side, this replacement leads to decrease of crystallinity in the
copolymer [22]. The interplay between two processes determines a total
water concentration in the copolymer and hence the rate of hydrolytic deg-
radation. Generally, in the case of PHBV copolymer (HB/HV = 4:1 mol.
ratio) the hydrophobization of its chain predominates the effect of crystal-
linity decrease from 75% for PHB to ~60% for PHBV.
5.3.2 CHANGE OF MOLECULAR WEIGHT FOR PHB AND
PHBV
On exposure of PHB and PHBV films to buffer medium at physiological
(37 °C) or elevated (70 °C) temperatures, we have measured both their
total weight loss (Section 1) and the change of their MW simultaneously.
In particular, we have shown the temperature impact on the MW decrease
that will be much clear if we compare the MW decrements for the samples
at 37° and 70 °C. At 70 °C the above biopolymers have a more intensive
reduction of MW compared to the reduction at 37 °C (see Fig. 5.2). In par-
ticular, at elevated temperature the initial MW (= 350 kDa) has the decre-
ment by 7 times more than the MW decrement at physiological condition.
Generally, the final MW loss is nearly proportional to the initial MW of
sample that is correct especially at 70 °C. As an example, after the 83-days
incubation of PHB films, the initial MW= 170 kDa dropped as much as 18
wt.% and the initial MW= 350 kDa has the 9.1 wt.% decrease.
The diagrams in Fig. 5.2 shows that the sharp reduction of MW takes
place for the first 45 days of incubation and after this time the MW change
becomes slow. Combining the weight-loss (Section 1) and the MW deple-
tion, it is possible to present the biopolymer hydrolysis as the two-stage
process. On the initial stage, the random cleavage of macromolecules and
the MW decrease without an significant weight-loss occur. Within this
