Which factor is most likely to increase the glass transition temperature (Tg) of a polymer, thereby enhancing its suitability for high-temperature applications like automotive components?

The factor that most often raises a polymer’s glass‑transition temperature is making its chains stiffer, for example by adding bulky or rigid side groups or aromatic rings that limit how easily the chains can move. When the chains are less able to slide past one another, the material stays in a glassy, rigid state up to a higher temperature, so the Tg climbs. This reasoning is the same as why a polyethylene with large phenyl groups has a higher Tg than plain polyethylene. For instance, replacing a small methyl side chain with a bulky tert‑butyl group in a polycarbonate keeps the polymer rigid at higher temperatures, making it suitable for engine‑cooling parts. Thus, increasing chain rigidity through bulky or aromatic side groups is the key factor.