(641c) Modifying Glass Transition Temperature through Block-Random Copolymer Sequence

The glass transition temperature (T_g) of amorphous polymers plays an important role in material processing conditions and mechanical behavior. Therefore, control and modification of these chain dynamics is a highly sought-after goal in polymer engineering. A standard method for tuning T_g is copolymerization. The T_g of a copolymer is commonly estimated by the Fox equation, which calculates T_g as a harmonic weighted average of T_g for the individual homopolymers. Therefore, by varying the ratio of the two monomer units within the copolymer, it is possible to interpolate to any value between the T_g of the two individual components. However, the Fox equation often overpredicts T_g for copolymers of highly incompatible monomers. Here, we explore changes in T_g and deviations from the Fox equation through copolymer sequence at fixed composition. Anionic polymerization was used to synthesize styrene/isoprene copolymers with varying sequence at a 50:50 wt% composition of styrene:isoprene and a molecular weight of 100 kg/mol. The sequence was altered from an entirely random copolymer by incorporating small (~5 kg/mol) homopolymer blocks of either polystyrene (PS) or polyisoprene (PI) at the end of a random copolymer chain. The χN for all polymers studied here was sufficiently small as to not microphase separate. We find that random copolymers have ~10 K negative deviation from the Fox equation. This negative deviation can be increased further (by ~2 K) by changing the sequence to include a small homopolymer of either PS or PI block at the end of the chain. This counterintuitive result was explored further through cooperative movement and self-concentration dynamics between rubbery and glassy domains. Polymers with a PI homopolymer block show a large decrease in the characteristic length scale of cooperative movement compared to random copolymers or PS-b-random copolymers. This is attributed to the rubbery nature of the PI block which softens before the random copolymer block. Conversely, we find that a small block of PS does not significantly alter chain dynamics; instead the glass transition behavior is dominated solely by the lower-T_g random copolymer segment.