|Ph.D Thesis||Department of Chemical Engineering|
|Supervisors:||Prof. Cohen Yachin|
|Dr. Dmitry Rein|
|Full Thesis text|
In recent years there has been much interest in “self-reinforced” composite materials due to their unique properties. Such composites, in which both the reinforcing fibrous phase and the matrix between them are made of the same polymer, are currently fabricated by subjecting oriented fibers to pressure and temperature in a well - controlled processing scheme. In such processes a matrix phase is generated with molecular continuity to the reinforcing phase.
The overall objective of this research was to characterize the microstructure and morphology of composite materials made of polymeric fibers, and to describe the relevant melting and recrystallization processes under pressure. In order to gain this aim, the research focused mainly on ultra high molecular weight polyethylene (UHMWPE) fibers as an "ideal" system composed of fully extended chains in a highly crystalline state. In addition a system of oriented polypropylene (iPP) tapes was studied.
In order to elucidate the nature of the transformations that lead from fibers to a monolithic composite, In-situ X-ray diffraction (XRD) studies with a pressure cell were performed using synchrotron radiation. The mesomorphic hexagonal phase was metastable at a relatively low pressure during both melting and recrystallization. The existence and metastability of the mesomorphic hexagonal phase can explain the basis of compaction technology of UHMWPE fibers to monolithic composites.
Using microbeam XRD and scanning electron microscopy, it was observed that melting occurs both on the surface of the fiber as well as in its internal regions. The recrystallized phase was nucleated on the fiber surface, and retained the initial highly oriented structure. XRD microbeam measurements did not show any significant core-shell structure.
Apart from UHMWPE fibers, oriented iPP tapes were also examined under pressure by heating and cooling during In-situ XRD measurements using synchrotron radiation. Melting and recrystallization of the monoclinic iPP tape assembly resulted in the appearance of orthorhombic iPP, even after two cycles. However, the amount of orthorhombic iPP after the second run was lower compared to its value after the first run. In addition, the content of orthorhombic iPP was not negligible after the second run. Moreover, it was surprisingly evident that the effect of the compaction pressure on orthorhombic iPP content was not monotonic.
To sum up, this research indicates the direct link between the phase transitions occurring during melting and recrystallization under pressure and the resultant composite materials. Such understanding enables intelligent usage of compaction processing parameters for fabricating self-reinforced composites.