Chemistry Reference
In-Depth Information
with 60/40% ratio and then to 168 °C for the blend with 40/60% content.
Therefore, the SPEU molecules prevent the crystallization completion of
PHBV and, hence, reduce both the quality of structural organization and
the crystallinity degree.
2.3 DYNAMICS OF ESR PROBE MOBILITY AND H-D
EXCHANGE KINETICS IN D
2
O
Recently we have studied the behavior of spin probes (TEMPO and TEM-
POL) in high crystalline PHB at room and elevated temperatures [14].
Water-temperature effects on molecular mobility of the ESR probe (TEM-
PO) in PHBV were studied at two different temperatures: 40 °C, close to
physiological temperature, and 70 °C adopted in a number of studies as the
standard temperature for accelerated testing of hydrolytic stability of bio-
polymers [15, 16]. Figure 2.2 shows that the heating of polymer composi-
tions in water for 4 h at ~40 °C does not significantly reduce the molecular
mobility of the probe in the mixture, as well as does not change the PHBV
crystallinity that we have shown above by DSC technique. For samples of
PHBV and its blends, the impact of the aquatic environment under more
rigorous conditions, that is, at 70 °C, is accompanied by the probe mobility
increase in comparison with the mobility in the initial polymer systems.
These results obtained from ESR spectra reveal plasticizing effect of wa-
ter molecules, so that segmental mobility of macromolecules in the inter-
crystalline space is increased and hence the relaxation processes proceeds
faster that lead to an increase in the rotation velocity of the probe TEMPO.
The supplemental feature of SPEU behavior in the blends is a FTIR
band intensity decrease for the -NH- fragments of urethane groups, which
can be represented as a series of kinetic curves. The curves reflect the H-D
exchange rate in the PHB/SPEU blends immersed into heavy water. Only
those -NH- groups can exchange proton for deuterium that are accessible
to the attack by molecules of D
2
O [17].
