Poly[(R)-3-hydroxybutyrate] (PHB) is a bacterial storage polyester that is genuinely biodegradable, but its brittleness hampers its wide applicability as a biodegradable plastic. Most attempts to improve the mechanical properties of PHB have focused on incorporating comonomers, but at the expense of production costs. Strikingly, it appeared possible to toughen PHB homopolymer using a simple annealing treatment. The present study attributes this remarkable phenomenon to a change in the lamellar morphology, which reduces the amorphous-crystalline interface area and thus the constraint imposed on the amorphous chains by the crystals. This elevates the relaxation strength of the amorphous regions, giving rise to the favourable fracture behaviour.

ABSTRACT:The morphology of homogeneous copolymers of ethene and 1-octene synthesized using a V-based Ziegler-Natta catalyst was studied as a function of the short chain branching content (SCBC) and the molar mass. Linear polyethylenes (LPE) were used as reference material. For the linear samples an increase in molar mass results in an increase of the long period and the crystalline lamella thickness. A decrease of cooling rate results in an increase of the melting temperature, the long period and the crystalline lamella thickness and an evolution from spherulitic structures to perfectly stacked lamellae. For the branched samples, increasing the SCBC results in a decrease of the melting and the crystallization temperature, crystallinity, spherulite radius, the long period, and the crystalline lamella thickness. The two latter tend to a limiting value on reaching a SCBC of 20CH 3 /1000C. On the other hand, an increase of the a axis and to a lesser extent the b axis of the unit cell is observed. Decreasing the cooling rate affects only the crystallinity of the least branched samples. Furthermore decreasing the cooling rate results in smaller spherulites, has a minor influence on the lamellar parameters and reduces the dimensions of the basal plane of the unit cell. Increasing the molar mass of the branched samples results in a drop of the crystallinity, a deterioration of the superstructure, enlarges the amorphous layer thickness and the dimensions of the basal plane. All these observations can be accounted for by the different crystallization regimes being applicable when different molar masses, SCBC and cooling rates are used.

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