[Physics] Why polymeric solids are said to be intermediate between crystalline solids and amorphous solids


Crystalline solids have ordered arrangement and amorphous solids do not. Polymeric solids are simply formed by the joining of some monomeric units. It has nothing to do with ordered or not ordered then why they are called intermediates.

Best Answer

A general highly simplified overview: Generally macromolecules/polymers exhibit very complex structures. The completely disordered/amorphous state is on the one hand to be expected from purely configurational entropy considerations, on the other hand there are cases where the chains or side-chains would energetically favor to be aligned, but this can almost never be fully achieved as polymers tend to form networks, by getting entangled onto one another (e.g. covalently, thus very robust networks can result). Thus the emergent properties of a mixture of polymers in a frozen state will generally lie between that of the crystalline and amorphous solids, where some regions are semi-crystalline, some amorphous. See e.g. this schematic drawing from wikipedia: ("Spherulite2" by Materialscientist - Own work (transfer my upload from en.wiki). Licensed under CC BY-SA 3.0 via Commons)

enter image description here

Note that alignment of side chains does not always come with entropy reduction, rather often it leads to more efficient packing of the chains, thus leading to higher rotational/translational mobility, where the effects of topological limitations are minimized. There are many other factors that play important/competing roles, e.g. the boundary attraction, solvent properties, homogeneity in number density distribution of monomers, and so on. All of which means that at the end, through free energy minimization, highly non-trivial equilibrium structures will emerge.

Related Question