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5.3.3 CRYSTALLINITY OF PHB AND PHBV
We have above revealed that during hydrolytic degradation, PHB and
PHBV show the MW reduction (Section 2) and the total weight decrease
(Section 1). Additionally, by the X-ray diffraction technique (XRD) we
have measured the crystallinity degree of PHB and PHBV that varied de-
pending on time in the interval of values 60-80% (see Fig. 5.3a). We have
noted that on the initial stage of polymer exposition to the aqueous buf-
fer solution (at 37 °C for 45 days) the crystallinity degree has slightly
increased and then, under following exposition to the buffer, this char-
acteristic is constant or even slightly decreased showing a weak maxi-
mum. When taken into account that at 37 °C the total weight for the PHB
films with MWs equal 350, 500 and 1000 kDa and the PHBV film with
MW equals 1050 are invariable, a possible reason of the small increase in
crystallinity is recrystallization described earlier for PLA [26]. Recrystal-
lization (or additional crystallization) happens in semicrystalline polymers
where the crystallite portion can increase using polymer chains in adjoin-
ing amorphous phase [22].
At higher temperature of hydrolysis, 70 °C, the crystallinity increment
is strongly marked and has a progressive trend. The plausible explanation
of this effect includes the hydrolysis progress in amorphous area of bio-
polymers. It is well known that the matrices of PHB and PHBV are formed
by alternative crystalline and noncrystalline regions, which determine both
polymer morphologies and transport of aggressive medium. Additionally,
we have revealed recently by H-D exchange FTIR technique that the
functional groups in the PHB crystallites are practically not accessible to
water attack. Therefore, the hydrolytic destruction and the weight decrease
are predominantly developed in the amorphous part of polymer [22, 27].
Hence, the crystalline fraction becomes larger through polymer fragment
desorption from amorphous phase. This effect takes place under the strong
aggressive conditions (70 °C) and does not appear under the physiological
conditions (37 °C) when the samples have invariable weight.
Owing to the longer lateral chains in PHBV, copolymerization modi-
fies essentially the parent characteristics of PHB such as decreasing in
crystallinity, the depression of melting and glass temperatures and, hence,
enhancing ductility and improvement of processing characteristics [14,
28, 29]. Additionally, we have founded out that the initial crystallinity of
PHB films is a monotonically increased function of initial MW (see Fig
5.3b). For samples with relatively low molecular weight it is difficult to
