(122b) Effects of Monomer Structure and Chain Architecture On Glass Transition Breadth In Homopolymers, Random Copolymers, Gradient Copolymers and Copolymer Blends

Recently, gradient copolymers have attracted significant interest as potential vibration or acoustic damping materials due to the extremely broad glass transition (T_g) response of these materials, in some cases approaching 100 K in breadth.  This unique behavior was attributed to the large degree of compositional heterogeneity present in the material as a result of a sinusoidal composition profile in nanophase separated systems.  However, we have discovered that specific homopolymers and random copolymers also exhibit large T_g breadths due to heterogeneities present in the repeat unit structure.  We also determined that blending a strongly-segregating styrene/4-vinyl pyridine (S/4VP) gradient copolymer with a selective plasticizer significantly increases the T_g breadth of the resulting material, up to approximately 100 K.

Here, we present the T_g breadth of a range of homopolymers, primarily focused on the series of n-alkyl methacrylates.   We determined that T_g breadth increases with increasing side chain length in the series of n-alkyl methacrylates as measured by differential scanning calorimetry (DSC), ellipsometry, and dynamic mechanical analysis.  We conclude that these repeat unit structures exhibit significantly broad T_gs, up to 42 K for poly(n-hexyl methacrylate), due to heterogeneities corresponding to incompatible main and side chain interactions.  We also incorporated these repeat units in random copolymers and determined that random copolymer T_g breadth is a strong function of copolymer composition, with upper and lower bounds dictated by breadths of the corresponding homopolymers.

Additionally, we show that blending oligomeric polystyrene into S/4VP gradient copolymers results in a dramatic shift in the T_g response of the bulk material.  Oligomeric polystyrene is expected to selectively plasticize the S-rich regions of the copolymer, shifting the glass transition response of these regions to lower temperatures while leaving the 4VP-rich regions relatively unchanged.  For a S/4VP gradient copolymer initially exhibiting T_g breadth of 35 K, addition of 20 weight percent oligomeric polystyrene yielded a T_g breadth of roughly 100 K.

We also blended weakly-segregating styrene/n-butyl acrylate (S/nBA) random copolymers of different compositions in an effort to approximate a sinusoidal composition profile as found in gradient copolymers.  We demonstrate the range of composition differences in random copolymer blends to maintain a single continuous glass transition, and the resulting increase in T_g breadth.  We also show that S/nBA gradient copolymer blends are able to maintain a single, broad, and continuous T_g with greater composition differences than that of random copolymer blends.  We conclude that the presence of the full range of compositions in each gradient copolymer sample is extremely important in determining the compatibility of two copolymers.