Glass Transition and Thermal Transitions

The glass transition is the temperature region in which a polymer's amorphous regions change between a rigid glass and a mobile rubber, and along with melting and secondary relaxations it defines the temperatures at which a polymer can be processed and used.

Definition

The glass transition is the reversible change in an amorphous polymer between a hard, glassy state and a soft, rubbery state, occurring over a temperature range characterized by the glass transition temperature; thermal transitions more broadly include this transition, crystalline melting, and secondary relaxations.

Scope

This topic covers the glass transition temperature and its molecular origin in segmental motion and free volume, the factors that raise or lower it (chain stiffness, side groups, molar mass, plasticizers, crosslinking), its measurement by calorimetry and dynamic mechanical analysis, the crystalline melting transition, and secondary sub-glass relaxations associated with localized motion.

Core questions

• What molecular motion is activated at the glass transition?
• Which structural factors raise or lower the glass transition temperature?
• How is the glass transition distinguished from melting experimentally?
• Why does the glass transition depend on cooling rate and thermal history?

Key theories

Free-volume theory

The glass transition is reached when free volume falls below the amount needed for cooperative segmental motion; this picture explains the rate dependence of the transition and the lowering of the glass transition by low-molar-mass plasticizers.

Williams-Landel-Ferry relation

Above the glass transition, relaxation times depend on temperature through the WLF equation, which collapses viscoelastic data measured at different temperatures onto a single master curve and links the transition to segmental dynamics.

Mechanisms

Below the glass transition, large-scale segmental motion is frozen and the amorphous polymer is a rigid glass; on heating through the transition, enough free volume becomes available for cooperative rearrangement of chain segments, and the modulus drops by orders of magnitude into the rubbery plateau. Stiff backbones, bulky or polar side groups, hydrogen bonding, and crosslinking raise the transition by hindering motion, while flexible chains, plasticizers, and free chain ends lower it. Because the transition is a kinetic, not equilibrium, event, its apparent temperature shifts with cooling rate and thermal history.

Clinical relevance

The glass transition temperature determines whether a polymer is used as a rigid plastic or a flexible elastomer at room temperature, sets the upper service temperature of amorphous materials, and defines processing windows for molding and film forming. Plasticization to tune the transition is exploited in flexible PVC, and understanding sub-glass relaxations helps explain impact toughness.

History

The viscoelastic shifting captured by the WLF equation was formulated in 1955, free-volume interpretations of the glass transition were developed in the 1950s and 1960s, and differential scanning calorimetry became the routine method for measuring the transition, together establishing the modern understanding of polymer thermal transitions.

Key figures

• John Ferry
• Malcolm Williams
• Robert Landel
• Walter Kauzmann

Seminal works

• sperling2006
• hiemenz2007

Frequently asked questions

What happens to a polymer at its glass transition temperature?

Its amorphous regions change from a hard, brittle glass to a soft, flexible rubber as chain segments gain enough mobility to rearrange. The stiffness can drop by a factor of a thousand or more across the transition.

Why does adding a plasticizer lower the glass transition?

Small plasticizer molecules increase free volume and space the chains apart, making segmental motion easier at lower temperatures. This is how rigid PVC is turned into flexible products such as cable insulation and tubing.