Influence of Segregation Strength on the Local Glass Transition of Lamellar Diblock Copolymers

By selectively incorporating a pyrene-labeled monomer into the hard block of a model rubbery/hard block copolymer, forming a self-assembled microdomain structure, local measurements of the glass transition temperature (T_g) are performed as a function of segregation strength. The temperature dependence of the fluorescence emission of pyrene enables the determination of the location-specific T_g. The average position of a pyrene segment relative to the microdomain interface is estimated via fluctuation-corrected self-consistent field theory (SCFT) composition profiles. At weak and intermediate segregation strengths, lamellar poly(butyl methacrylate -b- methyl methacrylate) diblock copolymers, PBMA-PMMA, show gradients in T_g vs. this average label position of 22 K/nm and 14 K/nm, respectively, from the microdomain interface into the PMMA-rich domain. The role of the relative softness of the confining and confined blocks is demonstrated by a comparison of the observed T_g to values calculated based the Lodge-McLeish model of self-concentration. At the domain interface, labeled PMMA segments at both segregation strengths exhibit a 30 K depression in T_g relative to the calculated value.