Date of Award

Summer 2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

Marino Xanthos

Second Advisor

Marios Avgousti

Third Advisor

Reginald Farrow

Fourth Advisor

Pradyot Patnaik

Fifth Advisor

Kun S. Hyun

Sixth Advisor

S. Mitra

Seventh Advisor

Kwabena A. Narh

Abstract

In this study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium based cation were evaluated as potential plasticizers and lubricants for polylactic acid (PLA) and also for their effects on its hydrolytic and thermal degradation. Both ILs at 5wt% were well dispersible and partly miscible with PLA as evidenced by scanning electron microscopy/energy dispersive x-ray scattering, SEMIEDX, and glass transition temperature, Tg, characterization, as well as from solubility parameters calculations.

The effects of the IL containing a decanoate anion were more pronounced on lubrication and degradation as evidenced by reduced melt viscosities and accelerated thermal/hydrolytic degradation; this IL could also considered as a more effective plasticizer based on Tg suppression results although the degree of plasticization could also be affected by the observed excessive molecular weight, MW, degradation during melt processing. The IL containing the tetrafluoroborate anion increased the thermal stability of PLA as also confirmed from rate constants calculated from random chain scission statistics and activation energies from thermogravimetric analysis, and accelerated its hydrolytic degradation, but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with a model compound.

Hydrolytic chain scission of ester bonds catalyzed by acid groups are the suggested mechanisms of hydrolytic degradation based on results of pH and acid number changes, although it appears that the presence of the phosphonium cation contributes also to accelerated degradation. Degradation results were confirmed by optical and SEM examination and experiments in "as received" and sterilized soil. The different surface characteristics of PLA/ILs were reflected in differences in coefficients of friction and contact angles. The tetrafluoroborate anion containing IL reduced the flexural modulus and strength of PLA, affecting much less its ductility than the decanoate anion containing IL.

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