Doctoral Dissertations

Date of Award

8-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

P.J. Phillips

Abstract

Work involves determining what improves fiber/matrix adhesion in fiber reinforced, semicrystalline thermoplastic composites. Isotactic polypropylene is used as a matrix, in conjunction with pitch and PAN carbon fibers, to investigate factors involving the polymer morphology, immediately surrounding and further from the fibers, that affect fiber/matrix adhesion. All samples for comparisons are prepared at the same temperature and any variation in morphology is accomplished by either solution treating fibers with a nucleating agent or by compounding a different nucleating agent into the matrix. Gold is used to suppress transcrystalline growth on pitch fibers. The degree of transcrystalline growth upon different types of carbon fibers is determined by the fiber's surface structure and is explained using a newly developed heterogeneous nucleation theory. The theory also explains different literary accounts of the temperature dependent, epitaxial and transcrystalline polymer morphology and is unique to polymers because it involves lamellar thicknesses. Epitaxy is used as a tool for selecting polymer nucleating agents and to determine the orientation of polypropylene upon the basal planes comprising graphitic pitch fiber surfaces. By using an adaptation of the single fiber fragmentation test, fragments and morphology are investigated using optical transmission/reflection microscopy. Results show that suppressing transcrystalline morphology initiated upon pitch fibers and placing a nucleating agent upon otherwise non-nucleating PAN fibers decreases fragment length. Results also show that matrix compression during polymer solidification likely affects fiber/matrix adhesion in a manner similar to that of ceramic composites and that 0.04 mm thick composites do not generate as much compression upon fibers as 1.6 mm composites. Questions are raised concerning the possible differences between the type of transcrystalline growth initiated at pitch fiber surfaces and initiated upon fiber surfaces treated with nucleating agents.

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