The Term Of Crystallinity In Polymers
Polymers have very complex moleculer structures. Even this complex structures that they have, they show crystalline or non-crystalline behaviours like metals and ceramics. So temperature characteristics of polymers affected from this molecular characteristics. In here, we explained the thermal behaviours and characteristics of polymer structures briefly.
As we stated above, the crystalline structure or amorphous structure of polymer materials give specific properties. For crystalline polymers, melting point is very high, and with the increasing temperature, it gets higher and higher. Also change of physical structure of material from solid to liquid state, is very abrupt.
But for amorphous structure of polymers, there is no abrupt change of physical structure like crystalline one. When the temperature decreases from liquid to solid state, amorphous polymers show gradually changing pyhsical appearence from liquid to solid. Thermal expansion of amorphous polymer lowers and lowers with the decreasing temperature. There is a specific point called as ‘glass-transition temperature(Tg)’. In this point, the thermal expansion of amorphous polymer lowers abruptly upon lowering temperature.
In level of crystallinity between these two extremes, polymers show thermal characteristics between these extremes again. Above melting point(Tm), polymers show viscous characteristics and, between Tg and Tm, they show viscoelastic properties.
As you know from formation of crystaline structures in materials, upon slow cooling, crystalline structure formation is induced. It is valid for polymers also. With the increasing crystalline structure, Tm also increases.
If we take a look at the thermal characteristics of thermosets and elastomers, they show amorphous characteristics by decreasing temperature from liquid state. But with the decrasing temperature, cross-linkings also occured. These cross-linkings inhibits the formation of crystalline structure. Also they can not be melted and liquified again because of these cross-linked structures.
The thermal characteristics of polymers can be explained like above briefly. Do not forget to leave your comments and questions below about thermal characteristics of polymers below. Your feedback is very important for us.