307 - Polymer Structure

Aliphatic

A polymer structure based primarily on non-aromatic carbon structures.

Aromatic

A structure containing aromatic rings, such as benzene rings, which generally increase rigidity.

Dispersion forces

Attractions caused by temporary fluctuations in electron distribution.

Induction forces

Attractions where a permanent dipole induces a dipole in a neighbouring molecule.

Dipole-dipole interaction

Attraction between molecules containing permanent dipoles.

Hydrogen bonding

A relatively strong secondary interaction involving hydrogen bonded to an electronegative atom.

Physical entanglement

Interlocking of polymer chains without covalent bonds between them.

Chemical crosslinking

Formation of covalent bonds between polymer chains.

Chain entanglement

Physical interlocking of polymer chains that restricts their movement.

Critical molecular weight (Mc)

The approximate molecular weight above which significant chain entanglement occurs.

Tie molecule

A polymer chain that connects different crystalline regions and helps transfer mechanical loads.

Lamella

A thin, ordered crystalline layer formed by polymer chains.

Spherulite

A roughly spherical crystalline structure that grows radially from a nucleation site.

Shish-kebab structure

A crystalline structure consisting of an aligned central chain or "shish" with lamellae growing from it.

Tacticity

The arrangement of substituent groups along a polymer backbone.

Configuration

The spatial arrangement of atoms or groups that requires bonds to be broken to change.

Conformation

A particular arrangement of a polymer chain that can change through rotation around bonds without breaking bonds.

Isotactic

A polymer in which substituent groups are arranged on the same side of the backbone.

Syndiotactic

A polymer in which substituent groups regularly alternate sides of the backbone.

Atactic

A polymer in which substituent groups are randomly arranged.

Cis

A configuration where relevant groups are on the same side.

Trans

A configuration where relevant groups are on opposite sides.

Gauche

A molecular configuration produced by rotation around a single bond.

Helix

A twisted molecular configuration formed by a polymer chain.

Number-average molecular weight (Mn)

Molecular weight calculated by weighting molecules according to their number.

Weight-average molecular weight (Mw)

Molecular weight calculated by weighting molecules according to their mass.

Viscosity-average molecular weight (Mv)

A molecular-weight average related to polymer solution viscosity.

Molecular-weight distribution (MWD)

The distribution of molecular weights within a polymer sample.

Polydispersity index (PDI)

A measure of the breadth of molecular-weight distribution.

Molecular-weight average relationship

Mw > Mv > Mn

Low molecular-weight fraction

Short polymer chains that can dilute the entanglement network and reduce some mechanical properties.

Crystallinity

The degree to which polymer molecules are arranged in an ordered structure.

Degree of crystallinity

The fraction of a polymer existing in the crystalline state.

Chain folding

The process where a polymer chain folds back into a crystalline region.

Fringe micelle

A structure where polymer chains connect crystalline and amorphous regions.

Crystal perfection

The degree to which a crystalline structure is ordered and defect-free.

Crystallisation

The process by which polymer chains form ordered crystalline regions.

Homogeneous nucleation

Nucleation occurring throughout the polymer without preferential nucleation sites.

Heterogeneous nucleation

Nucleation occurring preferentially at specific sites such as impurities or surfaces.

Nucleation energy barrier

The energy required to form a stable crystal nucleus.

Undercooling

Cooling below the equilibrium crystallisation temperature to provide the driving force for crystallisation.

Epitaxial growth

Crystal growth along an existing ordered surface.

Crystallisation temperature (Tc)

The temperature associated with the maximum rate of crystallisation.

Melting temperature (Tm)

The temperature at which crystalline regions melt.

Inoculant

An additive that promotes nucleation and can produce finer crystals.

Post-process crystallisation

Crystallisation that continues after processing.

Spherulite

A spherical crystalline structure formed by radial crystal growth.