307 - Thermomechanical properties

Glass transition temperature (Tg)

The temperature range where the amorphous phase changes from a rigid glassy state to a more mobile rubbery state.

Glassy state

The rigid state of a polymer below Tg where molecular motion is strongly restricted.

Rubbery state

The more flexible state above Tg where significant molecular mobility occurs.

Melting temperature (Tm)

The temperature at which crystalline regions melt.

Glass transition region

The temperature range over which significant changes in molecular mobility and mechanical properties occur.

Rubbery plateau

The region above Tg where the polymer retains significant stiffness due to entanglements or crosslinks.

Viscous flow region

The high-temperature region where polymer chains can move past one another and flow.

Effect of molecular weight on Tg

Increasing molecular weight generally increases Tg towards a limiting value because longer chains have fewer chain ends per unit mass.

Effect of free volume on Tg

Increasing free volume generally decreases Tg because chains have more space to move.

Effect of branching on Tg

Branching can increase free volume and generally lower Tg.

Effect of side groups on Tg

Large or rigid side groups can restrict backbone rotation and increase Tg.

Effect of crosslinking on Tg

Increasing crosslinking restricts molecular motion and generally increases Tg.

Effect of secondary bonding on Tg

Stronger intermolecular bonding restricts molecular motion and generally increases Tg.

Effect of aromatic rings on Tg

Aromatic rings increase rigidity and restrict chain rotation, generally increasing Tg.

Effect of plasticisers on Tg

Plasticisers increase molecular mobility and generally decrease Tg.

Elastic behaviour

Behaviour where deformation is recoverable and energy is stored.

Viscous behaviour

Behaviour where deformation involves molecular flow and energy dissipation.

Back stress

A restoring stress generated by the polymer's tendency to return towards its original configuration.

Recovery

The return of a polymer towards its original shape after removal of a load.

Permanent set

Strain remaining after a polymer has been unloaded.

Hysteresis

Energy loss during loading and unloading due to viscoelastic molecular motion.

Mechanical damping

Dissipation of mechanical energy as heat.

Creep

Time-dependent increase in strain under constant applied stress.

Creep test

A test where constant stress is applied and strain is measured with time.

Creep strain

Strain that develops over time under constant stress.

Creep compliance (J)

A measure of strain response relative to applied stress during creep.

Creep recovery

Recovery of strain after a creep load is removed.

Residual strain

Strain remaining after removal of the applied load.

Irrecoverable creep

Creep deformation that does not recover after removal of the load.

Retardation time

Characteristic time associated with molecular rearrangement during creep.

Stress relaxation

Decrease in stress with time when a polymer is held at constant strain.

Stress relaxation test

A test where a fixed strain is applied and the resulting stress is measured over time.

Relaxation time

Characteristic time required for a polymer to redistribute internal stresses.

Relaxation modulus

Time-dependent modulus describing the reduction in stiffness during stress relaxation.

Stress decay

Reduction in stress with time during stress relaxation.

Isometric

A condition of constant strain.

Isochronous

A condition involving a constant time.

Linear viscoelasticity

Viscoelastic behaviour where the response is proportional to the applied deformation within the linear range.

Spring

Model element representing elastic behaviour.

Dashpot

Model element representing viscous behaviour.

Maxwell model

A spring and dashpot arranged in series, commonly used to model stress relaxation.

Kelvin-Voigt model

A spring and dashpot arranged in parallel, commonly used to model creep.

Standard Linear Solid

A viscoelastic model combining springs and a dashpot to represent both instantaneous and time-dependent behaviour.

Burgers model

A combination of springs and dashpots used to represent more complex creep behaviour.

Time constant

A characteristic time describing how quickly a viscoelastic response occurs.

Dynamic Mechanical Analysis (DMA)

A technique that applies oscillating deformation and measures the resulting stress response to determine viscoelastic properties.

Storage modulus (E′)

The elastic component of the response representing energy stored during deformation.

Loss modulus (E″)

The viscous component representing energy dissipated as heat.

Complex modulus (E*)

The combined elastic and viscous response.

Phase angle (δ)

The phase difference between applied strain and resulting stress.

Loss tangent (tan δ)

The ratio of loss modulus to storage modulus.

Damping

The dissipation of mechanical energy through internal molecular motion.

Time-Temperature Superposition (TTS)

A method for predicting long-term polymer behaviour by shifting data obtained at different temperatures onto a common curve.

Thermorheologically simple

A material for which different relaxation mechanisms can be represented using the same temperature-dependent shift factor.

Aging effect on modulus

Modulus generally increases as the polymer ages.

Aging effect on damping

Damping generally decreases with aging.

Aging effect on creep

Creep generally decreases with aging.

Aging effect on stress relaxation

Stress relaxation generally becomes slower as the polymer ages.

Thermoreversibility

Heating above Tg can remove the effects of physical aging.