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theses

In this research, structure-properties-processing relationships of melt-processed CNT-PA 66 composites are investigated. Using a novel high-shear mixing device, efficient dispersal of CNTs in molten PA 66 is achieved, forming a well-bonded composite that displays enhanced mechanical properties. Post-fabrication heat treatment of some graphite-polymer composites also changes properties in new and unexpected ways. Observations by SEM and TEM indicate good bonding between CNTs and PA 66. DSC measurements show a second crystallization peak in the cooling curve, as the CNT content in PA 66 matrix increases from 1 to 5 wt. %. This new type of crystallization becomes a dominant feature in the range of 6-8 wt. % CNTs. Such interactions correlate with changes in elastic modulus and impact strength. FTIR analysis shows a steady increase in peak intensities for 1-5 wt. % CNT-PA 66 composites, until at 6 wt. % there is a sudden decrease in Amide I and II stretching peak intensities, accompanied by a shift to lower wave numbers. This behavior may be attributed to transformation of the original matrix structure to a new C-C rich bonded crystalline state. Three-point bend tests show a decrease in elastic modulus up to 3 wt. % CNTs in PA 66 due to minor degradation of the polymer matrix. At higher CNT concentrations, covalent bonding between fractured-ends of CNTs and polymer matrix occurs, which increases elastic modulus via a load transfer mechanism. At even higher CNT concentrations, a bridging effect occurs between adjacent covalently-bonded regions that increases impact strength. Eventually this results in a fully cross-linked structure of high stiffness and low impact resistance. As might be expected, peak stress at fracture shows an inverse relationship with stiffness. A two-step heat treatment is used to modify the structure and properties of 35 wt. % graphite-PEEK composite. It involves heating the composite at 200°C for 20 hours and then at 380°C for 18 hours. Examination of samples by TEM and XPS showed some polymer has gasified and the remaining polymer is a well-crystallized PEEK. 35 wt. % graphite-PA 66 composite was heat treated at 280°C for 24 and 48 hours. A new highly crystalline phase led to increase in elastic modulus was investigated by DSC and FTIR. This opens new opportunities to enhance properties and performance of injection-molded components or parts.

ETD_8218

Ph.D.

Includes bibliographical references

by Nofel Zuhier Whieb

doi:10.7282/T30K2CQP

ETD doctoral

The author owns the copyright to this work.