Property

Crystallization

Glass Transition

Physical effects
(1, 2, 4, 6, 7) ^A

Becomes stiff (hard) but not necessarily brittle

Becomes stiff and brittle

Temperature-volume relation
(1, 2, 3, 4, 5, 8)

Significant decrease in volume

No change in volume, but definite change in coefficient of thermal expansion

Latent heat effect (4, 5, 8)

Heat evolved on crystallization

Usually no heat effect, but definite change in specific heat

Rate (2, 4, 6, 7, 8)

Minutes, hours, days, or even months may be required. In general, as temperature is lowered, rate increases to a maximum and then decreases with increase in deformation. Rate also varies with composition, state of cure, and nuclei remaining from previous crystallizations, or from compounding materials such as carbon black.

Usually rapid; takes place within a definite narrow temperature range regardless of thermal history of specimen. May be limited rate effect (2)

Temperature of occurrence
(4, 5, 7, 8

Optimum temperature is specific to the polymer involved.

Very wide limits, depending on composition

Effect on molecular structure
(1, 2, 5, 6, 8)

Orientation of molecular segments; random if unstrained, approaching parrallelism under strain

Change in type of motion of segments of molecule

Materials exhibiting
properties (5, 7, 8)

Unstretched polymers including natural rubber (low sulfur vulcanizates), chloroprene, Thiokol A polysulfide rubber, butadiene copolymers with high butadiene content, most silicones, some polyurethanes. Butyl rubbers crystallize when strained. Straining increases rate of crystallization of all of the above materials.

All