Physical properties of polymers in bulk

By definition, polymers in bulk are materials consisting exclusively of polymers, and they appear in liquid, solid, or viscous form depending on temperature. 

The liquid polymer state is characterized by a viscosity which strongly increases with the molecular mass. On the other hand, the response of the polymer liquid to a mechanical constraint, at the beginning is of the elastic type, the viscous flow appearing only after a time delay which increases with the molecular mass. The behaviour of such a material is called viscoelastic. 

For a large number of polymer species, the passage from the liquid state to the solid state by decrease of temperature is a continuous process. The structure of the solid is amorphous. For instance, this is what happens for atactic polystyrene and polydimethylsiloxane, materials whose structure in solution will be studied later on. 

The transition between the viscous state and the solid state is characterized by a temperature Tg called glass transition temperature. This temperature corresponds to a discontinuity of physical parameters and in particular of the thermal expansion coefficient. The discontinuity observed when T decreases seems to result from the freezing, for T Tt, of rotation and of large motions of chain elements. 

We also note that certain polymer melts crystallize partially upon cooling. The transition occurs at a well-defined temperature rf. Crystallization takes place only if the polymer has a perfect linear structure for instance, it must not contain any asymmetric carbon. However, this tacticity condition is not sufficient, since polydimethylsiloxane, which is perfectly periodical, does not crystallize under normal conditions. On the contrary, polyethylene and isotactic polyethylene crystallize easily. In general, these polymers contain a large amorphous fraction. This is why they are called semi-crystalline. In certain conditions, it is possible to prepare polymer samples that are perfect crystals, in particular by polymerization in situ of a crystal made of monomers (polydiacetylene and polyoxyethylene). 

---