Effect of Flexible Fibers on Rheological Properties of Poly(Lactic Acid) Composites under Elongational Flow

Rheological properties of polymer composites with flexible fine fibers are studied employing poly(lactic acid) PLA containing a small amount of polymeric fibers of poly(tetrafluoroethylene) PTFE, poly(butylene terephthalate) PBT, and poly(4-methyl-1-pentene) PMP. Both PBT and PMP fibers are prepared by hot-stretching of the blends with PLA, whereas PTFE fibers are obtained by simple mixing of PLA and PTFE powder at the temperature between Tm's of both polymers. The polymer composites with PTFE and PBT fibers whose diameters are smaller than 1 µm show marked strain-hardening behavior in elongational viscosity, i.e., rapid increase in the transient elongational viscosity with time or strain. On the contrary, the composite with PMP fibers with a diameter of ca. 2 µm shows no strain-hardening, although the elongational viscosity is enhanced in the measured strain/time range irrespective of the strain rate.