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Toughness, strength, and microstructure of poly(butylene terephthalate)/BT copolymer blends

Peter Chang, University of Massachusetts Amherst


Percolation concepts successfully correlate the effect of crystallinity on the mechanical properties of the poly(butylene terephthalate) (PBT)/BT copolymer blends (miscible and immiscible); the degree of miscibility depends on the hard block length of the BT copolymer. The dependence of the modulus (E) and the yield stress ($\sigma\sb{\rm y}$) on crystallinity can be modelled as the deformation of a percolative crystalline network. It is proposed that tie molecules between the crystals resist the deformation. Yielding and drawing of semi-crystalline polymers is shown to be compatible with a stress induced melting (decrystallization) model. In addition, the pressure dependence of the yield stress for several semi-crystalline polymers agrees with predictions from the Clausius-Clapeyron equation for the effect of pressure on a phase transformation. The stress induced melting model supports and incorporates the proposed percolation model. A new test procedure, the "zero ligament" method, to measure the toughness (J$\sb{\rm c}$) of ductile polymers was developed that does not require identification of the initiation event nor measurement of the extent of crack growth. J$\sb{\rm c}$ obtained by this technique was in good agreement with the toughness (J$\sb{0.2}$) measured according to the proposed ASTM J-integral protocol for polymers. The toughness of the blends, determined by the "zero ligament" method, was shown to be primarily from the energy dissipated by the $\alpha\to\beta$ transformation for the miscible blends, and from copolymer enhanced shear yielding for the immiscible blends, in which the energy dissipation mechanisms was identified with the aid of percolation concepts. Also, fractal analysis of the fracture surface showed that the fractal dimension (D) depended on the fracture mechanism only, and the scaling factor (K) correlated with fracture toughness and crack propagation resistance.

Subject Area

Mechanical engineering|Materials science|Plastics

Recommended Citation

Chang, Peter, "Toughness, strength, and microstructure of poly(butylene terephthalate)/BT copolymer blends" (1992). Doctoral Dissertations Available from Proquest. AAI9233042.