Composition and structure
The elastomeric ligature and chains are polyurethanes, which are thermosetting polymers possessing a –(NH)-(C=O)-O-stural unit formed by condensation polymerization. The cross linking between the chains must be relatively few to facilate large extension with the failure of primary bonds. The glass transition temperature of biomedical polyurethanes range from -50 to -80 deg. C The difference in energy between the rigid and rubber states corresponds to increase in the amount of molecular motion experienced by the polymer after undergoing the glass transition temperature www.indiandentalacademy.com