Page 248 - Biodegradable Polyesters

226 9 Environment-Friendly Methods for Converting Biodegradable Polyesters

~(—C—O—)~ Polyesters:
n
PET
O PETG (copolyester)
Poly(buthylene terephthalate)
Poly(lactic acid)
H Poly(hydroxy butyrate)
Poly(caprolacton)
O Poly(glycolic acid)
—)~
~(—CH—CH 2
n
PVA
Figure 9.8 Hydrogen bonding between polyesters and poly(vinyl alcohol).

immiscible blends, is one of the major achievements during the past 20 years of
polymer science, as stated in a recent review on hydrogen-bonding in polymer
blends [31].
Coming back to our particular systems, the blends of PVA with various
polyesters, one can assume that one deals with partial solubility leading to a
good compatibilization of the two polymers owing to the formation of H-bonds
between them (Figure 9.8). Existence of complete solubility (thermodynamic
miscibility) is excluded because in such a case one will obtain a one-phase
melt. On the contrary, the partial solubility provoked by the H-bonding is the
driving force for penetrating a small amount of the dominating component
(PVA) in the dispersed particles of the minor component (e.g., PLA), forming
a structure consisting of two co-continuous phases. After the extraction of this
small amount of finely dispersed PVA, a 3-D network of micro- or nanopores is
formed (Figures 9.5b, 9.6, 9.7b, and 9.10b). This assumption [25] is supported
by the observation that in a ternary blend of poly(vinyl butyral)/PVA/PA 6, a
thermodynamic miscibility of 0.4–0.6 volume fraction of vinyl alcohol has been
found [32]. The SEM inspection of this blend after selective extraction of the
dissolved component revealed a network of micropores [32, 33].
For a system much closer chemically and compositionally to PVA/PLA, Park
and Im [34] reported that PLA/poly(vinyl acetate) (PVAc) blends were miscible
systems for the entire composition range, but for the blends with even 10%
hydrolyzed PVAc copolymer, the phase separation and double glass transition
could be observed. Another thorough study [35] on miscibility and phase
structure of binary blends of poly(L-lactic acid) (PLLA) and PVA indicated
that PLLA and PVA were immiscible in the amorphous regions. However, the
data of the differential scanning calorimetry analysis still demonstrated that
some degree of compatibility related to block composition existed in the blend
systems. Furthermore, the formation of interpolymer hydrogen bonding in the
amorphous region which is regarded as the driving force leading to some degree
of component compatibility in these immiscible systems has been confirmed by
13
FTIR and further studied by C solid-state NMR analysis [35].