ABSTRACT
The physical states in which a polymer can exist may be idealized by considering first a very long, regular polymer chain. Many of the physical properties and phase transitions of polymers can be described using physics concepts that ignore the actual chemical constitution of the chain, be it for example polyethylene, polystyrene (PS), or poly(methyl methacrylate) (PMMA). The reason is that these properties are dominated by the extreme length and chain-like structure of these molecules and not by their actual chemical composition. For more complex structures such as branched, cross-linked, or block copolymers for which the chains are inhomogeneous, new physical models need to be introduced to describe the general properties of these structures. The chemical composition and configuration of polymers play, however, a crucial role on how the molecules will pack in their pure state. Atactic PS, for example, is an amorphous solid (existing in a condition called a glassy state) at room temperature, whereas isotactic and syndiotactic PS exist in a crystalline state. Even though isotactic and syndiotactic PS have the same chemical composition as atactic PS, their regular configurations (see Section 2.3) enable their molecules to fit together in such a way that intermolecular (chemical) attractions stabilize the chains in a regular lattice, despite the weak intermolecular barriers to rotation around single bonds. As the temperature is lowered, intermolecular barriers to rotation become greater, and the crystal is more stable. However, as the temperature is increased, the ease of intermolecular rotation increases and the crystal structure eventually melts.
