In addition to homopoly(butylene terephthalate) (homo-PBT), the microhardness technique was applied to its multiblock copolymers for examination of the stress-induced polymorphic transition. Drawn and annealed at 170°C for 6 h with fixed ends in a vacuum, bristles of poly(ether ester) (PEE) having PBT as hard segments and poly(ethylene oxide) (PEO) with molecular weight 1000 as soft segments (PBT/PEO=57/43 wt%) were characterized with respect to their microhardness H at various stages of tensile deformation. A sharp H decrease (by 20%) in a narrow deformation interval (2-4%) due to the stress-induced α ⇔ β polymorphic transition is observed followed by an increase and decrease of H. The transition is registered at much higher relative tensile deformations (between 25% and 30%) compared with the case of homo-PBT. This is explained by the presence of a very soft amorphous phase of PEO, which deforms first. The same phase, distinguished by low viscosity in which the PBT crystallites are "floating," is the reason for the very low H values measured (between 25 and 35 MPa, depending on the amount of α- and β-modifications). The stress-induced polymorphic transition in copolymers of PBT with PEO is demonstrated for the first time, also supporting the recently proposed concept of the microhardness depression effect.

Grateful acknowledgment is due to DGICYT (Grant PB94-0049), Spain, for the support of this investigation. The partial support of NATO (Grant No. CRG 920985 and No. NIG 95 1394) is also highly appreciated. It is a pleasure for S. F. to acknowledge the tenure of a sabbatical grant from DGICYT, Spain.

13 pags., 4 figs., 1 tab.