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Abstract
Measurements have been made using a polarized optical microscope equipped with hot stages in order to investigate the transcrystallization of polypropylene (PP) on polyletrafluoroethylene (PTFE) fibers. Based on the theory of heterogeneous nucleation, the interfacial free energy difference function, Δ[sgrave], of PP on PTFE fibers was determined and compared with that in the bulk matrix. It has been found that Δ[sgrave]PTFE = 0.75 ± 0.12 erg/cm2, and Δ[sgrave]bulk = 1.23 ± 0.07 erg/cm2. From a thermodynamic point of view, crystallization 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 crystallization and the morphology of fiber surface, is proposed to account for the development of transcrystallinity from a molecular point of view.
Effect of the thickness of transcrystalline layers on the interfacial strength has been investigated using a single-fiber pull-out test. To generate transcrystalline layers with different thickness, two different methods were applied. One is to allow the isothermal crystallization to proceed to completion at various temperatures. The other is to let specimens crystallize at 140°C first for various times, and then quench there to room temperature to complete the crystallization. Values of adhesive fracture energy and the frictional stresses 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 debonde fiber/matrix interface are increased for specimens crystallized at a higher temperature where a thicker transcrystalline layer is developed.