Thesis defense of Ph.D. candidate Erica Redline
Dept of Chemical Engineering and Materials Science
IPrime Coatings Process Fundamentals Program
Friday, September 30, 2011
Walter Library 402, University of Minnesota
Within the past decade, amphiphilic block copolymers have shown great potential as toughening additives for epoxy thermosets. When properly designed, these polymers will self-assemble into discrete, nano-sized phases at low loadings (5 wt. % or less). Previous work has focused primarily on utilizing these additives in bulk materials which cure using a step-growth polymerization mechanism. This work was expanded upon by investigating the potential for block copolymers to toughen thermosets which cure using a chain-growth mechanism. Additional work was directed on the concentration effect of block copolymer additives in bulk, step-growth polymerized materials with an emphasis on further understanding the relevant toughening mechanisms. The final aspect of this dissertation used knowledge gained from studying bulk materials and applied it to coatings made from step-growth polymerized materials where further considerations (e.g. interfaces, two-dimensional constraints, and stresses due to thermal expansion mismatch) may influence block copolymer self-assembly and the resultant ability to toughen the material.
The purpose of this thesis is to expand upon existing knowledge in the area of block copolymer-toughened epoxies in order to better understand how to maximize the potential of the additives. First, the effect of thermoset polymerization mechanism and resulting network morphology on block copolymer self-assembly and mechanical properties is investigated. The materials used in these experiments underwent a chain-growth polymerization mechanism and were compared to prior work with block copolymer-modified thermosets prepared using a step-growth polymerization. While self-assembly of the block copolymer is similar in the two systems, a dramatic difference in the toughenability of chain-growth thermosets versus step-growth thermosets is noted. Several possible explanations for this behavior are explored.
The remainder of the thesis focuses on step-growth polymerized thermosets. In one case, the effect of block copolymer concentration on critical strain energy release rate, G1c, is explored. In this study, two diblock copolymers consisting of the same "epoxy-philic block'' but different "epoxy-phobic'' blocks were investigated; one of the diblock copolymers formed spherical micelles with glassy cores and the other formed spherical micelles with rubbery cores. Additionally, concentration effect on modulus and glass transition temperature is reported. Finally, to understand the potential influence of the "epoxy-philic'' block on these properties, the homopolymer of the "epoxy-philic'' block possessing similar molecular weight to the block copolymers was added in concentrations to mimic the amounts within the diblock copolymers.
The next research topic utilizes the epoxy materials from the previous study. Extensive fractography analysis is performed in order to understand the underlying toughening mechanism of block copolymer additives. Surprisingly little work has been published in this area, and the proposed mechanism of nanocavitation-induced shear yielding is unable to explain why glassy-core polymers are capable of toughening thermosets, albeit not to the degree of those with a rubbery core. From the research conducted herein, two new toughening mechanisms are proposed.
Finally, the preliminary work in moving from block copolymer-toughened bulk epoxy thermosets to epoxy coatings is reported. Until now, bulk thermosets have been the primary focus of block copolymer-toughened materials, despite coatings being one of the largest end-use areas of the epoxy market. The impact of thickness on modulus and glass transition temperature are reported at three different concentrations and compared to bulk materials. Furthermore, the attempts to make quantitative comparisons of the fracture resistance of coatings to one another and to the bulk are chronicled.