TRANSCRYSTALLINITY IN PTFE FIBER PP COMPOSITES

Measurements have been made using a polarized optical microscope equip ped with hot stages in order to investigate the transcrystallization o f polypropylene (PP) on polytetrafluoroethylene (PTFE) fibers. Based o n the theory of heterogeneous nucleation, the interfacial free energy difference function, Delta sigma, of PP on PTFE fibers was determined and compared with that in the bulk matrix. It has been found that Delt a sigma(PTFE) = 0.75 +/- 0.12 erg/cm(2), and Delta sigma(bulk) = 1.23 +/- 0.07 erg/cm(2). From a thermodynamic point of view, crystallizatio n of PP is most likely to take place on PTFE fiber rather than in the bulk. Moreover, a simple model, based on the thermal-stress-induced cr ystallization and the morphology of fiber surface, is proposed to acco unt for the development of transcrystallinity from a molecular point o f view. Effect of the thickness of transcrystalline layers on the inte rfacial strength has been investigated using a single-fiber pull-out t est. To generate transcrystalline layers with different thickness, two different methods were applied. One is to allow the isothermal crysta llization to proceed to completion at various temperatures. The other is to let specimens crystallize at 140 degrees C first for various tim es, and then quench there to room temperature to complete the crystall ization. Values of adhesive fracture energy and the frictional stresse s in the debonded region were deduced. Results show that the presence of transcrystallinity does not promote the level of adhesion. However, the frictional stresses at the debonded fiber/matrix interface are in creased for specimens crystallized at a higher temperature where a thi cker transcrystalline layer is developed.