The “hard” or “soft” classification is related to the glass transition temperature of the monomer. The glass transition temperature, Tg, is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. For example, the Tg of styrene and butadiene is 100 °C and -80 °C, respectively. The styrene to butadiene ratio will determine the Tg of the polymer. A low Tg means that there is more butadiene present in the polymer, whereas a higher Tg has more styrene present.