A study of poly(3-hydroxybutyrate)/epoxidized natural rubber blends
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Date
2008
Authors
Lee, Heng Kah
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Abstract
A polymer blend system comprising poly(3-hydroxybutyrate) (PHB) and 50
mol% epoxidized natural rubber (ENR-50), prepared by solvent casting technique,
has been investigated to reveal its reactive nature and characteristic thermal and
physical properties. The compositions studied are 90/10, 80/20, 70/30, 60/40, 50/50,
40/60, 30/70, and 20/80 (wt/wt) PHB/ENR-50. The reaction involves opening of the
epoxide group of ENR-50 with the carboxyl group formed in situ via thermal
degradation of the PHB chains. Controlled reactions were carried out by annealing
the blends at various temperatures in the range between 184 and 199 C for specific
times using differential scanning calorimetric (DSC) technique. The melt reaction is
a consecutive first order reaction comprising thermal chain scission of PHB and
consumption of the carboxyl group by the epoxide ring opening reaction. The
proposed mechanism gives rise to PHB-grafted ENR-50. The structural change in the
reacted blends was confirmed by the NMR and FTIR results. Kinetics study shows
temperature and composition dependent overall rates of the reaction. The gradual
inward shift of Tg with increasing annealing time evidenced occurrence of melt
reaction which induces homogenization between immiscible PHB and ENR-50
phases leading to formation of single homogenous phase after half-time of the
reaction (t0.5,mr). The blends that showed single homogenous phase (ta = t0.5,mr)
crystallized at slower overall rate (1/t0.5,cr) compared to the partially miscible blends
that consist of the PHB-rich and ENR-50-rich phases (ta = 1 min). Avrami analysis of
the isothermally crystallized samples showed that the crystallization rate constant
(KA) of PHB in the blends decreases with increasing annealing time. Thermal
behaviour of neat PHB was apparently unaffected by annealing when the polymer
was annealed up to 10 min at 190 C, further reinforcing the role of the melt reaction.
Changes in the crystallization and melting behaviour are mainly due to narrowing of
the undercooling (ΔT) with increasing annealing time. Equilibrium melting
temperature (Tm) of PHB in the blend was drastically reduced with increasing
annealing time. This resulted in lower free energy of crystallization in the blends
annealed for t0.5,mr compared to one minute. The melt reaction reduced crystallinity
(Hm,PHB) of PHB in the blends which is attributed to the reduced fraction of
crystallizable PHB segments that fulfil the critical length required for a stable
nucleus that determines the final degree of crystallization. POM micrographs showed
slower linear growth of the PHB spherulite in the blends after subjected to annealing.
The result indicated that ENR-50 was rejected to the interlamellar and interfibrillar
region and the amorphous chains obstruct the regular arrangement of the ring banded
spherulite. Analysis based on Lauritzen-Hoffman (LH) theory showed occurrence of
regime II-III transition in the blends, not in the neat PHB, which is attributed to the
effect of reaction. It also revealed trend of reducing surface free energy and work of
chain folding indicating a decreasing probability of folding and reentering the crystal
attributed to decreasing sequence length of crystallizable PHB due to structural
change. For mechanical blends, obvious diffused interface and plastically deformed
structure were observed indicating occurrence of homogenization process which
enhanced interfacial adhesion between the PHB and ENR-50 phases. Higher
modulus and greater elongation at break at 100% strain were observed following
moulding at the same temperatures and durations studied for the melt reaction,
indicating modified viscoelastic behaviour of the blends due to structural change.
Description
PhD
Keywords
Industrial technology , Natural rubber blends , Thermal properties