Michelle M. Mok

Behavior of gradient copolymers at liquid/liquid interfaces.

The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated by using drop shape analysis to measure the interfacial tension. Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water. Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through control of the drop volume. In this way, we were able to independently determine the interfacial pressure and area modulus of the adsorbed layer. Gradient copolymers showed the largest interfacial pressure, a result that is attributed to kinetic factors associated with the nature of the micellar aggregates that form in the chloroform phase. The area modulus of the adsorbed layer depended on the processing history and was not directly related to the interfacial pressure. This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed.

Ellipsometry measurements of glass transition breadth in bulk films of random, block, and gradient copolymers

Bulk films of random, block and gradient copolymer systems were studied using ellipsometry to demonstrate the applicability of the numerical differentiation technique pioneered by Kawana and Jones for studying the glass transition temperature (Tg) behavior and thermal expansivities of copolymers possessing different architectures and different levels of nanoheterogeneity. In a series of styrene/n -butyl methacrylate (S/nBMA) random copolymers, Tg breadths were observed to increase from ∼ 17°C in styrene-rich cases to almost 30°C in nBMA-rich cases, reflecting previous observations of significant nanoheterogeneity in PnBMA homopolymers. The derivative technique also revealed for the first time a substantial increase in glassy-state expansivity with increasing nBMA content in S/nBMA random copolymers, from 1.4×10-4 K-1 in PS to 3.5×10-4 K-1 in PnBMA. The first characterization of block copolymer Tg ’s and Tg breadths by ellipsometry is given, examining the impact of nanophase-segregated copolymer structure on ellipsometric measurements of glass transition. The results show that, while the technique is effective in detecting the two Tg ’s expected in certain block copolymer systems, the details of the glass transition can become suppressed in ellipsometry measurements of a rubbery minor phase under conditions where the matrix is glassy; meanwhile, both transitions are easily discernible by differential scanning calorimetry. Finally, broad glass transition regions were measured in gradient copolymers, yielding in some cases extraordinary Tg breadths of 69- 71°C , factors of 4-5 larger than the Tg breadths of related homopolymers and random copolymers. Surprisingly, one gradient copolymer demonstrated a slightly narrower Tg breadth than the S/nBMA random copolymers with the highest nBMA content. This highlights the fact that nanoheterogeneity relevant to the glass transition response in selected statistical copolymers can be comparable to or exceed that observed in moderately phase-segregated gradient copolymers.