Polymer Crystallinity and Structure

Physical Transitions in Polymers

• Polymers can range from cloudy to opaque depending on the degree of crystallinity.
• Non-crystalline (amorphous) polymers have a glass transition temperature (Tg) and crystalline polymers have a crystalline melting temperature (Tm).

Glass Transition Temperature (Tg)

• Tg is the main transition temperature found in amorphous polymers.
• At Tg, frozen backbone sequences begin to move.
• Tg is determined by the main chain architecture and its immediate surroundings.
• Chain ends and low-MW plasticizers lower the Tg of a polymer, while a sufficiently large number of cross-links increase Tg.
• Above Tg, a few carbon atoms in each chain can still move with relative freedom, but below Tg, nearly all carbon atoms become fixed.

Crystalline Melting Temperature (Tm)

• Crystalline polymers have a melting transition at a temperature called Tm.
• Tm and Tg are important parameters that define the upper and lower temperature limits for numerous applications.

Relationship between Tg and Tm

• For unsymmetrical chains, Tg ≈ 0.5-0.7Tm.
• For symmetrical chains, Tg ≈ 0.8-0.9Tm.

Crystallinity

• The degree of crystallinity is an important factor affecting polymer properties.
• Crystallinity in plastics consists of thousands of small “islands” of crystalline regions surrounded by unordered or amorphous material.
• The crystalline areas are known as crystallites.
• High MW, narrow MWD, and linearity in the polymer backbone can yield high crystallinity.
• The crystallization rate is diminished by the presence of impurities such as catalysts, fillers, and pigments.
• Stretching a film orients the crystallites and realigns other molecules or segments of molecules, causing the total crystallinity of the film to increase.

Effect of Crystallinity on Polymer Properties

• Transparency of unfilled plastics is a function of crystallinity, with non-crystalline polymers having excellent transparency.
• The physical properties of a thermoplastic polymer depend on the values of Tm and Tg relative to room temperature.
• If both Tm and Tg lie below room temperature, the polymer is a liquid.
• If room temperature lies between Tm and Tg, the polymer is either a very viscous supercooled liquid or a crystalline solid.
• If both Tm and Tg are above room temperature, an amorphous polymer is glassy in nature, tending to be brittle.

Factors Affecting Tg

• Bulky side groups decrease the mobility of the chain and raise Tg.
• Molecular symmetry tends to lower Tg.
• Tg values are low for flexible, linear polymers such as PEs, and relatively high for stiff chain polymers such as PET and PC.