Molecular design of ordering transitions in block copolymers

Thesis (Ph.D.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2000.Vita.Includes bibliographical references (p. 201-216).The tendency of block copolymers (BCP's) to microphase separate at the molecular level, producing a wide array of ordered nanostructures, is of particular interest from an engineering standpoint due to the unique mechanical, optical or electrical properties that ensue. Upon considering the potential applications of these materials, however, one limitation arises from the lack of control over bulk thermodynamics and the appearance of order/disorder (solid-like/liquid-like) transitions in these materials. To address this problem, this thesis aims to, firstly, develop a more quantifiable understanding of the molecular factors governing BCP phase behavior, and, secondly, use that knowledge to molecularly engineer new BCP's with enhanced processibility. While most BCP's microphase separate upon cooling through an upper disorder-to-order transition (UDOT), polystyrene-block-poly n-butyl methacrylate, PS-b-PBMA, undergoes ordering upon heating through a lower disorder-to-order transition (LDOT). Preliminary studies on this material revealed a unique pressure sensitivity of this ordering transition. By applying pressure, this material could be forced into the segmentally mixed liquid state, implying "baroplasticity", a highly attractive property from a processing standpoint. To better understand the molecular origin of this behavior, the bulk thermodynamics of a family of BCPs formed from styrene and a homologous series of n-alkyl methacrylates (PS-b-PnAMA, n ranging from 1 to 12) was investigated, both as a function of pressure and temperature. The results of this study reveal an unexpected, though systematic, dependence of the phase behavior of these BCP's on monomer architecture. In short, over a certain range of alkyl side chain length, PS-b-PnAMA block copolymers are marginally compatible and exhibit unexpectedly large pressure coefficients for the ordering transition, ranging from 60 to 150°C/kbar. In an attempt to identify molecular parameters responsible for these thermodynamic trends, as well as those displayed by other systems reported in the literature, combined group contribution/lattice fluid model calculations of the cohesive properties of the corresponding homopolymers are performed. Based on this analysis, the homopolymer mass density is proposed as a macroscopic parameter that appears to govern phase behavior in weakly interacting block copolymers or polymer blends. Using this new criterion, a simple tool for the molecular design of phase behavior into weakly interacting BCP's is identified, which is successfully used to engineer "baroplastic" behavior into several new systems of commercial relevance, including elastomers and adhesives based on styrene and low Tg acrylates. In light of the improved understanding of BCP phase behavior emerging from these studies, a simple phenomenological free energy expression is proposed for compressible polymer mixtures, that can be extended to block copolymers. Its ability to predict qualitative phase diagrams for the systems investigated in this thesis as well as many other polymer pairs is demonstrated. Using this expression, basic principles regarding polymer thermodynamics are outlined.by Anne-Valérie G. Ruzette.Ph.D