(i) Glass transition temperature:-
When plastic or rubber is cooled up to certain temperature, it becomes so hard and brittle that it breaks into pieces the glass, on application of stress. The temperature below which the polymer becomes hard, brittle and glassy and above which it is softener and flexible, is known as glass transition temperature (TG).
Signification of Tg - molecular weight- GTT (glass transition temperature) is directly proportional to molecular weight of polymer.
Cross links- GTT is directly proportional to degree of cross links
Flexibility- GTT is inversely proportional to flexibility.
Plasticizer – GTT is inversely proportional to plasticization.
Inter molecular force – GTT is directly proportional to intermolecular force.
Side groups – GTT is directly proportional to number of side groups
When a polymer is heated beyond Tg, it passes from glassy state to rubbery state. Further heating much above, causes melting of the polymer and it starts flowing and it starts flowing.
The temperature below which the polymer is in rubbery state and above which it is a liquid is called melting temperature of the polymer, while the temperature at which the polymer starts flowing is known as its flow temperature.
(ii) Polymers in medicine and surgery
Bio materials can be implanted in the body or used in diagnostic, surgical and therapeutic applications without causing adverse effect on blood and other tissues. It is developed from metal ceramics and polymers .Its acceptability is mainly due to its versatility and the fact that they can be modified to suit specific body functions. They can be sterilized easily and do not show allergic reactions towards cellular elements.
(iii) Conducting Polymers
Organic polymers are generally used as insulators in the electrical connections due to their poor electrical conductivity.
However electrical conductivity of the organic polymers like polyacetylene can be increased by the factor of 1012 to about 103 Siemens/cm when it doped with the electron donor such as an alkaline metal ion or an electron acceptor such as arsenic penta-fluoride.
Other conducting polymers are polyaniline, polydiacetyline, polythiosphere, polypyrrole etc. such polymers which are made conducting are called as advanced polymeric is materials or conducting polymers.
An organic polymer with highly delocalized pie electron system having conductance of the order of a conductor is called a conducting polymer.
Application - as cathodes and solids electrolytes in batteries for automotive and other uses, in variable transmission windows, electrochromic displays, sensors, L.E.D. and non-linear optical materials, conductive paints, low-current wires etc.