Impact of Monomer Sequence and Interaction Parameter on Polymer Glass Transition Temperature.

Control of the glass transition temperature () is a major goal in polymer engineering as is a key determinant of mechanical behavior, barrier properties, and material processability. In copolymers of nonpolar monomers, the Fox equation can provide an approximate description of the dependence of on copolymer composition (monomer ratio), based on a harmonic weighted average of values for the individual homopolymers. However, the Fox equation does not consider the influence of intermonomer interactions, nor does it account for self-concentration effects. Here, we explore changes in by altering copolymer sequence and the interaction parameter (χ) between monomer units, with the Fox equation as a benchmark. We synthesized styrene/isoprene random copolymers with varying sequence at a 50:50 wt % overall styrene:isoprene composition and a molecular weight of approximately 100 kg/mol. The sequence was altered from an entirely random copolymer by incorporating short (∼5-10 kg/mol) homopolymer blocks of either polystyrene (PS) or polyisoprene (PI) at either the end or center of a random copolymer chain. Fully random styrene-isoprene copolymers show only a slight negative deviation (∼3 °C) from the Fox equation. Incorporation of a short homopolymer block of either PS or PI into the chain resulted in an additional depression of the (by ∼2 °C). This depression is not influenced by the location of the block but instead reflects self-concentration of the blocks. Additionally, we hydrogenated the isoprene units to increase χ and observed even larger negative deviations from the Fox equation (by ∼13 °C). These results demonstrate that both sequence and monomer interactions are polymer design parameters that can be used to manipulate bulk .