Date of Award

5-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Natural Resources

Major Professor

Siqun Wang

Committee Members

David P. Harper, Kevin M. Kit, Timothy M. Young

Abstract

Contact resonance force microscopy (CR-FM) is a valuable technique for evaluating the interphase of natural fiber-reinforced polymer composites and for characterizing the elastic properties of cell wall layers of natural fibers. The nanoscale spatial resolution of CR-FM, combined with its ability to provide quantitative modulus images, makes it possible to investigate the mechanical properties of interphases as narrow as 30 nm in NFRPCs and thin cell wall layers in natural fibers. The nanoscale characterization of interphase and its effects on the bulk mechanical properties in this study shows that an increased interphase thickness is very essential for the improved tensile strength in lyocell/polypropylene (PP)/maleic anhydride grafted polypropylene (MAPP) composites. An optimum amount of MAPP increase the interphase thickness to the maximum of 100 nm and further addition only decreased the interphase thickness and adversely affected the strength properties. The average impact strength was found to decrease with the increasing concentration of MAPP and our results showed that matrix properties were also a determinant factor on the impact strength. After comparing the results obtained from CR-FM, tensile testing, and dynamic mechanical analysis (DMA), it was quite clear that β transition was not a strong indicator of the filler –matrix interaction within these composites. For lyocell/PP/maleic anhydride grafted styrene-ethylene/butylene-styrene (MA-SEBS) composites, tensile strength was not a direct reflection of interfacial bonding. The impact strength was found to increase with addition of MA-SEBS. Interphase region showed gradient of modulus values that ranged between the modulus values of the fiber and the matrix for both lyocell/PP/MAPP and lyocell/PP/MA-SEBS composites. The interphase region showed a gradient in modulus that could be described to first order by a linear fit, with a gradual decrease in modulus from fiber to matrix. Also, it was quite evident that the interphase thickness accounts for the majority of property variations within the interphase for different treatments. This result defies the earlier perception of a flexible interphase with low modulus than the matrix formed by the elastomers in composites.

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