ENME307 - Polymers

Thermoplastic bonding

Secondary covalent intermolecular forces.

Thermoset bonding

Primary covalent linkage.

Thermoplastic structure

Linear or branched chains held together by intermolecular forces.

Thermoset structure

3D condensed network.

Functional group

Group responsible for the chemical characteristics of a polymer.

Functional group polarity

Changes chemical properties and structure depending on the site of polarity.

Examples of functional groups

Amide, carboxylic acid, ether, etc.

Addition polymerisation

Polymerisation that does not produce a by-product and has faster growth.

Condensation polymerisation

Polymerisation where functional groups react, producing a by-product such as H₂O, with slower growth.

Degree of polymerisation (DP)

Average number of repeat units in a polymer.

Lower DP

Shorter polymer molecules.

Isotactic

Polymer arrangement where groups are on the same side.

Syndiotactic

Polymer arrangement where groups regularly alternate sides.

Atactic

Polymer arrangement where groups are randomly positioned/opposite sides.

Gauche

Rotation around a single bond.

Configuration

Spatial arrangement of atoms that requires bonds to be broken to change.

Conformation

Arrangement of atoms that can change through rotation around bonds.

Dipole–dipole

Forces caused by differences in charge between permanent dipoles.

Induction

Formation of an induced dipole in neighbouring atoms due to a permanent dipole.

Dispersion

Momentary dipole formed by movement of electrons.

Hydrogen bonding

Bonding involving a positive charge and F, O or N when an H atom is present.

Effect of increasing temperature on bonding

Higher temperature increases molecular interaction and chain separation, weakening the bonds.

Effect of stereochemistry on polymer properties

Less favourable chain stereochemistry lowers strength and Tg.

Tg (glass transition temperature)

Temperature separating the glassy and rubbery behaviour of a polymer.

Below Tg

Polymer is glassy.

Above Tg

Polymer is rubbery.

Amorphous polymer

Polymer with a completely random structure, some entanglement and no crystal structure.

Semi-crystalline polymer

Polymer containing both amorphous and crystalline regions.

Resilience

Energy stored and released by a material; related to degree of elasticity.

Effect of entanglement on resilience

More entanglement gives higher resilience.

Linear polymer

Polymer with secondary bonding and higher structure/packing.

Branched polymer

Polymer with secondary bonding and lower packing.

Cross-linked polymer

Polymer with cross-links that restrict chain movement and crystallisation.

Network polymer

Polymer with 3D covalent bonding and no crystallisation.

Crystallinity

Degree to which polymer chains form an ordered crystalline structure.

Effect of entanglement on crystallinity

Fewer entanglements increase crystallinity.

Thermoset crystallinity

Thermosets generally have lower crystallinity.

Thermoset molecular structure

3D molecular network.

Crystallisation process

Nucleation, lamella formation, chain folding and spherulite formation.

Nucleation

Formation of sites around which crystals can grow.

Lamella

Thin, ordered crystalline regions formed by folded polymer chains.

Chain folding

Polymer chains fold back and forth to form crystalline regions.

Spherulites

Larger spherical crystalline structures formed from crystalline lamellae.

Effect of molecular weight on crystallinity

The notes indicate that increased molecular weight may increase crystallinity.

Effect of cooling rate on crystallinity

Slow cooling gives more time for crystallisation, while fast cooling gives less time.

Slow cooling

Lower entanglement and increased crystallinity.

Fast cooling

Less time for crystallisation.

Effect of nucleation sites on crystal size

More nucleation sites produce smaller crystals; fewer nucleation sites produce larger crystals.

Small crystals

Have more grain boundaries.

Large crystals

Have fewer grain boundaries.

Effect of crystallinity on density

Higher crystallinity increases density.

Effect of crystallinity on stiffness and strength

Higher crystallinity increases stiffness and strength.

Effect of crystallinity on brittleness

Higher crystallinity increases brittleness.

Crystal defects

Defects in crystals can reduce strength.

Small crystal defects

Smaller crystals have more boundary defects.

Slip

Movement of polymer chains past one another that causes deformation.

Chain movement in crystalline polymers

Crystalline material has limited chain movement.

Effect of crystal thickness on melting temperature

Smaller thickness provides more surface area and higher energy.

Effect of cooling rate on melting temperature

Faster cooling decreases melting temperature.

Post-process crystallisation

Formation or growth of crystalline regions after processing.

Effect of processing on crystallisation

Less processing can result in less crystallisation.

Plasticisers

Additives that increase free volume in a polymer.

Effect of plasticisers

Increase free volume.

Effect of large side groups

Large side groups increase free volume.

Effect of temperature on molecular motion

Higher temperature increases molecular motion.

Molecular weight average

Average molecular weight of polymer molecules.

Weight-average molecular weight

Molecular weight average weighted according to molecular mass.

Importance of molecular weight

Molecular weight affects the structural properties and behaviour of polymers.

Lower molecular weight

Produces shorter molecules.

Higher molecular weight

Produces longer molecules.

Molecular weight and viscosity

Molecular weight affects polymer viscosity.

GPC

Gel permeation chromatography.

Gel permeation chromatography

Technique that separates polymer molecules based on molecular size.

GPC separation

Larger particles/chains move faster through the column, while smaller particles move more slowly.

DSC

Differential Scanning Calorimetry.

DSC operation

Measures the difference in heat flow between a polymer sample and a reference as temperature changes.

DSC output

Heat absorption or release plotted against temperature.

DSC purpose

Determine thermal transitions and thermal properties.

DSC determination of Tg

Tg is identified by a change in heat flow.

DSC determination of Tc

Tc is identified by an exothermic peak during cooling.

DSC determination of Tm

Tm is identified by an endothermic peak during heating.

Tg

Glass transition temperature.

Tc

Crystallisation temperature.

Tm

Melting temperature.

Degree of crystallinity from DSC

Percentage of the polymer that is crystalline.

Thermoplastic DSC analysis

Used to determine Tg, Tc and Tm.

Second heating in DSC

Used when analysing thermoplastics.

Thermoset DSC analysis

Used to analyse thermal decomposition and curing.

Curing exothermic peak

Exothermic peak associated with curing of a thermoset.

Onset of cure

Point where curing begins.

Degree of cure

Measure of how much curing has occurred in a thermoset.

TMA

Thermomechanical Analysis.

TMA operation

Measures dimensional change in a sample as a function of temperature.

TMA probe

Measures displacement of the sample as temperature changes.

TMA purpose

Determines dimensional and mechanical changes with temperature.

TMA uses

Shrinkage, dimensional stability, residual stress and polymer behaviour with temperature.

TMA determination of Tg

Tg is identified from a change in dimension or change in slope.

TMA can determine

Thermal expansion coefficient, softening behaviour, residual stress and dimensional stability.

TMA shrinkage analysis

Used to troubleshoot and understand polymer shrinkage.

TMA dimensional stability

Determines how stable polymer dimensions are with temperature.