School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Characterisation, processing and mechanical modelling of a thermotropic main-chain liquid crystalline polymer
Author: DIMITRIADIS, Klisthenis
Supervisor(s): Costas Galiotis
Completely aromatic polyesters have rigid molecular chains. During flow these polymers retain a rod-like configuration aligned in the flow direction. They are known as Liquid Crystal Polymers (LCPs) because of the highly ordered structure they assume even in the melt state. Structurally, LCPs are identified as Main-Chain, Side-Chain, Combined and Crosslinked. They are also classified in Lyotropics (processed in solution) and Thermotropics (processed in the melt).
This study aims to bridge the existing theoretical gap concerning dynamic rheology of LCPs and molecular orientation development during flow. For this purpose, Mechanical Modelling was used focusing on Thermotropic Main-Chain LCPs. The originality of this approach lies on the fact that it is based on the analogy between LCPs and dilute suspensions of rigid-rod particles. Vectra A950, an unfilled LCP, was the reference material.
A physico-mechanical investigation of Vectra was conducted, including WAXD, TEM, Specific Gravity, Hardness, Tensile, Flexural, High Impact, and Fatigue tests.
Vectra was injection moulded using a specially designed mould which induced certain flow regimes. The molecular orientation at specific locations of the moulding was measured using FTIR-ATR.
A flow apparatus was constructed which included a transparent flow cell, geometrically similar to the mould used for injection moulding of Vectra. The flow of a suspension of rod-like particles through the flow channel was visualised using a laser-beam set-up and then video-recorded. Appropriate selection of the flow parameters ensured dynamic similarity between the suspension and the LCP melt. The orientation of the particles at specific locations of the channel (corresponding to equivalent locations on the moulding) was measured and averaged using Image Analysis equipment.
Comparison of the Mechanical Modelling results to the FTIR-ATR analysis permitted the evaluation of theories attempting to explain the complex phenomenon of Liquid Crystallinity in polymers. The experimental findings were finally verified by the Mechanical Characterisation results.