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.
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PhD
Keywords
Industrial technology , Natural rubber blends , Thermal properties
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