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

Control of the glass transition temperature (<i>T</i>_g) is a major goal in polymer engineering as <i>T</i>_g 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 <i>T</i>_g on copolymer composition (monomer ratio), based on a harmonic weighted average of <i>T</i>_g 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 <i>T</i>_g 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 <i>T</i>_g (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 <i>T</i>_g.