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Glossary

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active center: Location of the unpaired electron on a free radical where reactions take place. (See section on polymer synthesis)
alignment layer: A layer and/or surface treatment applied to the boundary of a liquid crystal cell to induce a particular director orientation. For example, a layer of polyimid buffed in one direction induces alignment parallel to the buffing direction, or a surfactant may be polymerized on a boundary surface to induce perpendicular alignment.
amorphous: Irregular; having no discernible order or shape. In the context of solids, the molecules are randomly arranged, as in glass, rather than periodically arranged, as in a crystalline material.
amorphous polymers: A glass-like structure with tangled chains and no long-range order. (See Polymer Morphology section)
amphiphilic: A molecule with a hydrophilic head and a hydrophobic tail. That is, a molecule that has one end which attracts water and one end which repels water.
anisotropic: Having properties which vary depending on the direction of measurement. In liquid crystals, this is due to the alignment and the shape of the molecules. See also: isotropic
aromatic: A compound containing a series of benzene (6 Carbon) rings; so named because many have a distinctive odor.
atactic polymer: See stereoregular polymer
axial ratio: The ratio of the length of the molecule to the diameter of the molecule.
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backbone: The main structure of a polymer onto which substituents are attached.
biaxial: Possess two directions along which monochromatic light vibrating in any plane will travel with the same velocity. The optic axis lies just between these directions.
bilayer: A double layer of amphiphilic molecules, arranged such that either the nonpolar ends are on the inside screened by the polar ends or the polar ends are on the inside screened by the nonpolar ends, depending on whether the solvent is polar or nonpolar.
birefringence: Also called double refraction. The property of uniaxial anisotropic materials in which light propagates at different velocities, depending on its direction of polarization relative to the optic axis. A wave with polarization perpendicular to the optic axis will exhibit an "ordinary" index of refraction, n_o (this is often referred to as the ordinary ray). In contrast, a wave with polarization parallel to the optic axis exhibits an "extraordinary" index, n_e (the extraordinary ray). The ordinary index, n_o, is isotropic with respect to direction of propagation while the extraordinary, n_e, varies depending on the direction of propagation with a maximum value for light traveling perpendicular to the optic axis and, of course, polarized parallel to it. The difference Dn = n_e - n_o is also referred to as the birefringence or the optical anisotropy.
block polymers: Polymers composed of two or more connected sequences (blocks) of homopolymers. In the simplest case, the AB diblock consists of two homopolymers, A and B, joined together.
bond orientational order: Describes a line joining the centers of nearest-neighbor molecules without requiring a regular spacing along that line. Thus, a relatively long-range order with respect to the line of centers but only short range positional order along that line. (See discussion of hexatic phases in a text such as Chandrasekhar, Liquid Crystals)
branched polymer: A polymer with a chemical side chain extending from the main backbone.

buffing: To give the inner glass surfaces of a liquid crystal cell a texture so as to align the liquid crystal molecules in a certain direction parallel to the surfaces.
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chain polymer

A polymer in which the repetition of units is linear. The monomers are linked end to end forming a single straight polymer.

chain axis

The straight line parallel to the directions of chain extension, connecting the centers of mass of successive blocks of chain units, each of which is contained within an identity period.

Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers

chain polymerization

a chain reaction in which the growth of a polymer chain proceeds exclusively by reaction(s) between monomer(s) and reactive site(s) on the polymer chain, with regeneration of the reactive site(s) at the end of each growth step.

[Pure & Appl. chem. 1994, 66, 2483 (Basic Classification and Definitions of Polymerization reactions IUPAC Recommendations 1994)]

chiral molecule

A molecule that is not identical to its mirror image. This gives a chiral substance its characteristic twisted shape, due to the fact that its molecules do not line up when combined.

cholesteric liquid crystals

Also known as Chiral Nematic. Similar to the nematic phase, however, in the cholesteric phase, molecules in the different layers orient at a slight angle relative to each other (rather than parallel as in the nematic). Each consecutive molecule is rotated slightly relative to the one before it. Therefore, instead of the constant director of the nematic, the cholesteric director rotates helically throughout the sample. Many cholesterol esters exhibit this phase, hence the name cholesteric. See also chiral.

See the Liquid Crystal Phases section.

cholesteric mesophase

Nematic liquid crystals with chiral centers form in two dimensional nematic-like layers with directors in each layer twisted with respect to those above and below so that the directors form a continuous helix about the layer normal. Many cholesteric esters exhibit this phase, hence the name cholesteric. This mesophase exhibits circular dichroism and is optically active.

chromophoric groups

Chemical groups which have distinctive colors.

circular dichroism

See dichroism.

circular polarization

A condition caused by two waves whose electric field components are 90 degrees out of phase, causing an effective rotation of the electric field about an axis in direction of propagation.

cis (configuration)

A polymer configuration in which adjacent bonds are coplanar and on the same side of the carbon-carbon double bond.

columnar phase

A liquid crystal phase characterized by disc-shaped molecules that tend to align themselves in vertical columns.

colligative properties

Are those that depend on the number of species present rather than their kind.

combination

A type of termination reaction which involves a free radical joining with an active growing polymer to form a stable chain and end the growth.

configuration

The geometrical arrangement in polymers arising from the order of atoms determined by chemical bonds.

configurational unit

a constitutional unit having one or more sites of defined stereisomerism.

configurational base unit:

a constitutional repeating unit, the configuration of which is defined at one or more sites of stereoisomerism in the main chain of a polymer chain.

configurational repeating unit:
the smallest set of one, two or more successive configurational base units that prescribes configurational repetition at one or more sites of stereoisomerism in the main chain of a polymer molecule.

Pure and Appl. chem, 1981, 53, 733 IUPAC Macromolecular Nomenclature for Stereochemical Definitions Relating to Polymers

conformation

The geometrical arrangement in polymers arising from rotation about adjacent carbon-carbon single bonds.

conformation repeating unit

The smallest unit of a polymer chain with a given conformation that is repeated along that chain through symmetry operations.

Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers 1989 IUPAC

contour length

The maximum end-to-end distance of a linear polymer chain. For a single-strand polymer molecule, this usually means the end-to end distance of the chain extended to the all-trans conformation. For chains with complex structure, only an approximate value of the contour length may be accessible.

copolymer

Polymers that are derived from more than one species of monomer. Polymers having monomeric units differing in constitutional or configurational features but derived from a single monomer, are not regarded as copolymers.

statistical copolymers: are copolymers in which the sequential distribution of the monomeric units obeys known statistical laws; e.g., the monomer sequence distribution may follow Markovian statistics of zeroth (Bernoullian), first, second, or a higher order. Statistical copolymers are named: poly (A-stat-B)

random copolymers: is a special case of a statistical copolymer. It is a statistical copolymer in which the probability of finding a given monomeric unit at any given site in the chain is independent of the nature of the neighboring units at that position (Bernoullian distribution). Random copolymers are named: poly(A-ran-B)

Alternating copolymers: a copolymer comprising two species of monomeric units distributed in alternating sequence: the arrangement -ABABABAB- or (AB)n represents an alternating copolymer. Alternating copolymers are named:poly(A-alt-B)

periodic copolymers: copolymers where the monomeric units appear in an ordered (for example, ABC) sequence. Periodic copolymers are named: poly(A-per-B-per-C)

block copolymers: a polymer comprising molecules in which there is a linear arrangement of blocks, a block being defined as a portion of a polymer molecule in which the monomeric units have at least one constitutional or configurational feature absent from the adjacent portions. Block copolymers are named: polyA-block-polyB.

graft copolymers: a polymer comprising molecules with one or more species of block connected to the main chain as side chains, these side chains having constitutional or configurational features that differ from those in the main chain. In a graft copolymer, the distinguishing feature of the side chains is constitutional, i.e., the side chains comprise units derived from at least one species of monomer different from those which supply the units of the main chain. An example:
-AAAAAAAXAAAAAAAAA-

B
B
B
B
and the corresponding name is polyA-graft-polyB where the monomer named first (A in this case) is the backbone (main chain, while that named second (B) is the side chains.

Pure and Appl. chem, 1985, 57, 1427 IUPAC Macromolecular Nomenclature Source-based Nomenclature for Copolymer 1985 IUPAC

cross-linking: A process in which bonds are formed joining adjacent molecules. At low density, these bonds add to the elasticity of the polymer and at higher densities, eventually produce rigidity in the polymers.
crystallinity: The presence of three-dimensional order on the level of atomic dimensions. In polymers, the range of order may be as small as about 2 nm in one (or more) crystallographic direction(s) and is usually below 50 nm in at least one direction. Polymer crystals frequently do not display the perfection that is usual for low-molecular mass substances. Polymer crystals that can be manipulated individually are often called polymer single crystals.
Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature
crystallization: the process of forming crystals from the melt or solution.

primary crystallization: the first stage of crystallization, considered to be ended when most of the spherulite surfaces impinge on each other. In isothermal crystallization, primary crystallization is often described by the Avrami equation.

secondary crystallization: crystallization occurring after primary crystallization, usually proceeding at a lower rate.

recrystallization: reorganization proceeding through partial melting. Recrystallization is likely to result in an increase in the degree of crystallinity, or crystal perfection or both.

reorganization: the molecular process by which (i) amorphous or poorly ordered regions of a polymer specimen become incorporated into crystals, or (ii) a change to a more stable crystal structure takes place, or (iii) defects within the crystals decrease.

Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers

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defects: A local break in the translational or orientational symmetry of the material.
deformation: The condition where the director in a liquid crystalline material changes its orientation from one molecule to the next.
degree of polymerization: The number of monomeric units in a molecule of a polymer. The DP determines many physical characteristics of the system.
Pure and Appl. chem, 1989, 61, 211 IUPAC Macromolecular Terms relating to individual macromolecules, their assemblies and dilute polymer solutions 1989 IUPAC
dendrimers: Highly branched molecules that have several layers of branching. These molecules exhibit a characteristic spherical shape.
diastereomers: Stereoisomers that are not mirror images of each other. They have similar chemical properties but may have vastly different physical properties.
dichroism: Refers to the selective difference in absorption between the two orthogonal components of the polarization state of light propagating in a given direction in an anisotropic medium. Generally applied to linearly polarized light. This is how a sheet polarizer works with one direction of polarization transmitted and the perpendicular directions absorbed. Circular dichroism describes the corresponding effect for circularly polarized light where one "handedness" of circularly polarized light is absorbed more strongly than the opposite "handedness." Solutions of chiral molecules will produce this effect as will cholesteric liquid crystals for wavelengths well removed from the pitch value.

dielectric anisotropy: A condition arising when the dielectric constant parallel to the length of a mesogen, e_{| |}, is different from that perpendicular to it, e_^. This occurs when the charge distribution along the molecule responds differently to the parallel component of the local electric field than the distribution perpendicular to the length does to the perpendicular component, yielding a difference in dielectric constants. Positive anisotropy is defined as the case e_{| |} > e_^, while a negative anisotropy corresponds to e_^ > e_{| |}.
dipole: Two equal electric or magnetic charges of opposite sign, separated by a small distance. In the electric case, the dipole moment is given by the product of one charge and the distance of separation. Applies to charge and current distributions as well. In the electric case, a displacement of charge distribution produces a dipole moment, as in a molecule.
director: The molecular direction of preferred orientation in liquid crystalline mesophases.
disclination: Line defects arising from singularities in orientational order in a director field.
discotic liquid crystal: The component disc-shaped molecules self-assemble in a way that resembles stacks of coins. The discs are stacked on top of each other to form columns, which in turn are packed on a two-dimensional, usually hexagonal, lattice.
dislocations: Line defects arising from singularities in translational order in a crystalline lattice.
disproportionation: Disproportionation is a method of terminating chain polymerization which involves a free radical stripping a hydrogen atom from another free radical, so that one chain stabilizes with the hydrogen, and the other with a double bond.
double refraction: See birefringence.
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elastomers: A class of polymers that have some degree of cross linking and are rubbery. Elastomers possess memory, that is, they return to their original shape after a stress is applied.
electro-optical materials: Materials whose optical properties are changed under the application of an electric field.
emulsion: A mixture of two mutually insoluble liquids such that one is dispersed in the other in droplets which often cause the solution to be cloudy or translucent.
enantiomers: Molecules which exist in two nonsuperimposable mirror images, analogous to human hands. Chiral molecules are perfect physical and chemical models of each other with the exception of their rotation of polarized light and those interactions that involve other chiral systems, such as chiral molecular recognition. A racemic mixture contains equal amounts of two enantiomers and thus produces no rotation of the plane of polarization of light.
enantiotropic liquid crystal: Exhibit the liquid crystal state both when the temperature rises from the solid state side or when it falls from the liquid state. Monotropic liquid crystals exhibit the liquid crystalline state only when the temperature changes in one direction.
entanglement: A crossing of polymer chains, which, when subjected to a strain, remains intact and hence mechanically active.
enthalpy: A measure of the heat energy in a system; thermodynamic systems tend to react in ways that decrease their enthalpy
entropy: A measure of the disorder in a system; thermodynamic systems tend to react in ways that increase their entropy.
epitaxial growth: An ordered growth of material as it is deposited on an ordered substrate, in registry with the substrate (for example, preserving elements of the crystalline order of the substrate).
extraordinary ray: See birefringence
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ferroelectric material: One that produces domains of spontaneous polarization whose polar axis can be reversed in an electric field directed opposite to the total dipole moment of the lattice.
Freedericksz transition: The point at which a liquid crystal changes from an aligned to a deformed state under the influence of an external electric or magnetic field.
fibers: Characterized by having a high length to diameter ratio, that is, the length is enormously greater than the diameter. As a result of the spinning process, most of the polymer molecules are oriented along the fiber axis. In regions where polymer molecules are perfectly oriented along the axis, crystals will be formed. Nearly perfect uniaxial orientation of the macromolecules represents the basic requirement for the preparation of strong polymer fibers.
free radical: A molecule with an unpaired electron, making it highly reactive.
free radical polymerization: The synthesis of a polymer involving the chain reaction of free radicals with monomers.
fringed-micelle model: A model of crystallinity in which the crystallized segments of a macromolecule belong predominantly to different crystals. Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers; [ Back to Top ]

gauche: A carbon-carbon bond in the main chain of a polymer that is rotated 120 degrees out of the plane. gauche(+) denotes a rotation of +120 degrees and gauche(-) denotes a rotation of -120 degrees--both as defined by the "right hand" rule. Also see the conformation movie in the Polymer Structure section.
glass transition temperature: Tg, can be defined as the temperature at which the specific volume vs temperature plot has a change in slope. Chain motion and other local molecular motions are greatly reduced below this temperature, which is also dependant on the rate of cooling.
Grandjean texture: A specific pattern of defects found in chiral nematic liquid crystals that is caused by the chiral nature of the crystal. Named for the French scientist F. Grandjean who worked with chiral nematic liquid crystals.; [ Back to Top ]

helix: The molecular conformation of a spiral nature, generated by regularly repeating rotations around the backbone bonds of a macromolecule.
Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers
helix residue: The smallest set of one or more successive configurational base units that generates the whole chain through helical symmetry.
Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers
homeotropic texture: A mesogen configuration in which the molecules are aligned normal to the boundary surfaces, as at the faces of a liquid crystal cell, as illustrated. Consequently the director will be normal to the surface. This orientation is generally obtained by the application of an electric field normal to the surface but can be achieved through surface treatment.
homogeneous: An uniform structure or composition throughout. Having or possessing the same properties.

homogeneous (planar) texture: A mesogen configuration in which the molecules are aligned parallel to the boundary surfaces, as at the faces of a liquid crystal cell.
homopolymer: A polymer that is constructed of identical monomers.
hybridization: The combination of atomic orbitals on the same atom. (Example: sp2, the composite of the "s" and two "p" orbitals.)
hydrophilic: "Water loving"; describes a molecule which is attracted to water.
hydrophobic: "Water fearing"; describes a molecule which is repelled by water.
hypertext: Text that includes pointers to other text, pictures, movies, etc. "Clicking" on these links takes the reader to the object that it is pointed to. Links may point to documents on other computers connected on the Internet.
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indentity period: The shortest distance along the chain axis for translation repetition of the chain structure. The chain identity period is usually denoted by c. Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers
index of refraction: Ratio of the phase velocity of electromagnetic radiation in free space divided by the phase velocity in a given medium. It is greater than one except for rather special cases.
initiation reaction: The first step in chain polymerization. Initiation involves the formation of a free radical.
initiator: A relatively unstable molecule that decomposes into a free radical. Used to "initiate" a polymer growth reaction.

isomer: A molecule which has an identical molecular formula to another molecule, but has a different structure.
isomorphism: Statistical co-crystallization of units having different repeating constituents, which may either belong to the same copolymer chains (copolymer isomorphism) or originate from different homopolymer chains (homopolymer isomorphism). Isomorphism is a general term; in the strict sense, the crystal structure is essentially the same throughout the range of compositions.
Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers
isotactic polymer: See stereoregular polymer
isotropic: Having properties that are the same regardless of the direction of measurement. In the isotropic state, all directions are indistinguishable from each other. See also anisotropic.
ITO: Indium Tin Oxide (indium oxide doped with tin). A transparent conductive material very commonly used for electrodes in displays or other applications which require conductivity along with light transmission.
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lipophilic: describes a molecule which is attracted to hydrocarbons.
liquid crystal: a thermodynamic stable phase characterized by anisotropy of properties without the existence of a three-dimensional crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase, hence the term mesophase.
living polymerization: Chain growth polymerizations proceeding in the absence of chain breaking terminations. This process can be used to produce essentially monodisperse polymers.

lyotropic: materials in which liquid crystalline properties appear induced by the presence of a solvent, with mesophases depending on solvent concentration, as well as temperature. See also: thermotropic
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macromolecule: A very large molecule. Many polymers are composed of hundreds of thousands of atoms, and are thus characterized as macromolecules.
macroscopic molecular orientation: The property that allows the orientation of liquid crystals to be seen only on a large scale and cannot seen if a single layer is compared to any other single layer.
main-chain polymer liquid crystals: Polymer liquid crystals in which the mesogenic cores are a part of the main chain or backbone of the polymer.
melting transition temperature: The temperature at which the substance loses its translational and orientational order, changing from a solid phase to a liquid phase.
mesogen: Rigid rodlike or disclike molecules which are components of liquid crystalline materials.
mesogenic cores: The mesogenic core is the primitive structural unit of a polymer having the requisite anisotropic shape and attractive interactions to establish long range intermolecular order in its liquid phase. That is, it is what gives a polymer liquid crystal its liquid crystal properties.
mesomorphic substance: Another term for a liquid crystal material.
mesophase: Equilibrium liquid crystalline phases formed with order less than three dimensional(like crystals) and mobility less than that of an isotropic liquid. Parallel orientation of the longitudinal molecular axes is common to all mesophases (long-range orientational order) See the Liquid Crystal Phases section
micelle: A spherical formation caused by an amphiphilic substance in a solution. The lyophilic end of the molecule tends to orient itself toward the outside of the sphere while the lyophobic end tends to orient itself toward the inside of the sphere.
molar mass: M, is the mass divided by the amount of the substance. Molar mass is usually expressed in g/mol or kg/mol units. The g/mol unit is recommended in polymer science.
molecular weight: The ratio of the average mass per formula unit of a substance to 1/12 of the mass of an atom of nuclide ₁₂C.
molecular weight distribution: Represents the distribution of number of polymer chains with a given length and molecular weight M.
monochromatic light: Light composed of only one specific wavelength.
monolayer: A layer of amphiphilic molecules on the surface of a solvent, arranged such that the ends attracted to the solvent are in contact with it and the other ends point into the air.
monomer: The simple chemical unit which, when many are joined together, form a polymer.
monotropic: A type of material which exhibits the liquid crystalline state only when the temperature changes in one direction. This is generally a result of the liquid crystal phase being below the melting temperature of the solid, where the liquid crystal phase is only observed if the liquid is supercooled below the melting point. See also enantiotropic liquid crystal.
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nematic mesophase: Liquid crystals are characterized by long-range orientational order and the random disposition of the centers of gravity in individual molecules. Nematics may be characterized as the simplest spontaneously anisotropic liquids. Nematic phases are composed of rod-shaped molecular aggregates that are arranged with parallel but not lateral order.; See Liquid Crystal section.
number-average molecular weight: The number of molecules of length i, multiplied by the molecular weight of an ith unit divided by the total number of molecules present.
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optical activity: The plane of vibration of linearly polarized light rotates as it propagates through a medium. This rotation can occur in either a right or left handed direction. Since linearly polarized light can be regarded as the sum of right and left hand circularly polarized components, this optical activity corresponds to different indices of refraction for the two circular components(circular birefringence).
optic axis: In a uniaxial material, a single direction of propagation along which double refraction does not occur. The index of refraction for both polarization directions is n_oalong this axis. This axis lies along the director for nematic liquid crystal.
order parameter: S describes the orientational order of liquid crystalline material, allowing for the individual orientational deviation of the molecules from the director, which represents the average over the collection. Typically, S ranges from 0.3 to 0.9, depending on the temperature, with a value of unity for perfect order. See Introduction to Liquid Crystal phases section.
ordinary ray: See birefringence
orientational order: Measure of the tendency of the molecules to align along the director on a long-range basis. See order parameter.
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perturbations: Disturbances from an equilibrium condition.
plastics: A large group of polymers that has properties between elastomers and fib
ers. As such, plastics have a wide range of properties such as flexibility and hardness and can be synthesized to have almost any combination of desired properties.
plasticizer: Material added to a polymer to improve its processability and/or flexibility. These are low molecular weight substances which, when mixed with a polymer, lower its glass transition temperature, T_g.
pitch: The distance it takes for the director of a cholesteric liquid crystal to go through one complete rotation of 360 degrees. (See LC Phase Section)
polarizability: Relates the induced electric dipole moment, p, of an atom or molecule to the local electric field it experiences as a = p / E_local, hence depending on the displacement by the field of the electronic charge from its equilibrium position in the atom or molecule.
polarizer (linear): A device, which in the transmission of electro-magnetic radiation, confines the vibration of the electric and magnetic field vectors to one plane.

polydisperse polymer system: A polydisperse polymer system is one in which there is a distribution of molecular weights present. A monodisperse system would have only one molecular weight present.
polydispersity: The ratio of weight average molecular weight to number average molecular weight.
polymers: Long chains of covalently bonded atoms.
polymer liquid crystals: Polymers that contain mesogen units and thus have liquid crystal properties.
polymerization: The act of joining simple molecules(monomers)into giant ones (polymers) to form plastics, fibers, elastomers and non-structural resins.
polymorphism: The ability to exist in more than one crystal structure.
positional order: The extent to which the position of an average molecule or group of molecules shows translational symmetry.
propagation reaction: The middle step in chain polymerization where successive monomers are attached to the growing chain.
prolate: Elongated at the poles.
propagation (in polymer growth): The irreversible repetitive addition of monomers to the growing chain.
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racemic mixture: See enantiomers
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relaxation: A term used to mean all irreversible processes which bring a system back to equilibrium after it has been perturbed by some external force. For instance, if an electric field is applied to a fluid of polar materials a polarization will be induced, but this will disappear after the field is removed, because of the randomization of the molecular orientations produced by Brownian motions.
reorganization: the molecular process by which (i) amorphous or poorly ordered regions of a polymer specimen become incorporated into crystals, or (ii) a change to a more stable crystal structure takes place, or (iii) defects within the crystals decrease.
Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers
resonance: A method of stabilizing a bond by delocalizing the electrons around the molecule.
rotatory power: Measure of the capability of a twisted liquid crystal layer to rotate the angle of incident polarized light.
rotational symmetry: Consider the example of an unmarked billiard ball. Rotation of any amount about any axis through its center will take the ball into itself so it has complete rotational symmetry. However, the stitching on a baseball places several restrictions on the axes about which it can be rotated into itself.
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schlieren texture: The texture that appears in the optical microscopy of nematic and related smectic C phases under crossed polarizers when the planarity of the phase is interrupted by defects. The schlieren, dark streaks or brushes, form in the liquid crystal, connecting the defect points. The dark streaks or brushes that are characteristic of this texture may also appear along disclinations in a liquid crystal.
self-assembly: The aggregation of molecular moieties into more ordered structures that are thermodynamically stable and involve noncovalent bonds. Crystallization is an example of such self-assembly. Self-assembly is used to build nanostrucures such as inorganic clusters and lattices, nanotubes and channels, host-guest complexes, monolayers, hydrogen-bonded networks and systems of intertwined molecules.
side chain polymer liquid crystal: A polymer liquid crystal in which the mesogens are attached to the side of the main chain and not a part of the main chain itself.
smectic mesophase: The molecules organize themselves into layers. The smectic phases form a one dimensional periodic lattice in which the individual layers are two dimensional liquids. Now 12 different smectic phases have been identified.; [J. Patel and J. Goodby, Optical Eng 26 (5) 373 (1987)].; (See Liquid Crystal Phases section.)
spacer: Flexible section of polymer chain between two mesogens or the mesogen and the backbone of a polymer.
stability: A measure of the reactivity of a given molecule.
star-branching: A type of polymerization in which a branched polymer is formed as branches emanating from a single point.
stereoisomers: Isomers that differ only in the way their atoms are oriented.
steric hindrance: A condition when the rotation of a given group is restricted due to the size of neighboring groups.
stereoregular polymer: A regular polymer, the molecules of which can be described in terms of only one species of stereorepeating unit in a single sequential arrangement.

isotactic polymer: a regular polymer, the molecules of which can be described in terms of only one species of configuration base unit (having chiral or prochiral atoms in the main chain) in a single sequential arrangement. In an isotactic polymer, the configurational repeating unit is identical with the configurational base unit.

syndiotactic polymer: a regular polymer, the molecules of which can be described in terms of alternation of configurational base units that are enantiomeric. In the syndiotactic polymer, the configurational repeating units consist of two configurational base units that are enantiomeric.


Isotactic		Syndiotactic

atactic polymer: a regular polymer, the molecules of which have equal numbers of the possible configurational base units in a random sequence distribution.

Pure and Appl. chem, 1981, 53, 733 IUPAC Macromolecular Nomenclature for Stereochemical Definitions Relating to Polymers.

stereorepeating unit: a configurational repeating unit having defined configuration at all sites of stereoisomerism in the main chain of a polymer molecules.

enantiomeric: the configurational unit that corresponds to the same consitutional units if they are non-superposable mirror images.

diastereoisomeric: two non-superposable configurational units that correspond to the same constitutional unit (if they are not mirror images).

[Pure and Appl. chem, 1981, 53, 733 IUPAC Macromolecular Nomenclature for Stereochemical Definitions Relating to Polymers]

stereoselectivity: The chemical selectivity to polymerize only one stereoisomer (enantioselectivity and diastereoselectivity)
stereospecific polymerization: polymerization in which a polymer of a specific tacticity is formed. A polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific.
Pure and Appl. chem, 1981, 53, 733 IUPAC Macromolecular Nomenclature for Stereochemical Definitions Relating to Polymers
step-growth polymerization: Often termed condensation, it is defined as a polymerization in which all molecular species in the system can react with each other ,and the growth of polymer chains proceeds by condensation reactions between molecules of all degrees of polymerization.
super critical fluids: A substance above its critical point on the temperature/pressure phase diagram. Above the critical point, the fluid is neither a gas nor a liquid but possesses properties of both. The viscosity of a supercritical fluid is at least one order of magnitude higher than the viscosity in the gaseous state, but is one or two orders of magnitude less than in the liquid state.
surfactants: Surface active agents. Organic compounds consisting of two parts: a water-attracting (hydrophilic) portion and a water-resistant (hydrophobic) portion. Detergents may contain more than one kind of surfactant. The hydrophobic ends attach themselves to the soil particles or to the fabrics being washed while the hydrophilic ends are attracted to the water. The surfactant molecules surround the soil particles, break them up, force them away from the surface of the fabric, then suspend the soil particles in the wash water. Surfactants are classified by their ionic (electrical charge) properties in water.

symmetry: The invariance of some properties of the object being investigated with respect to all the transformations considered.
syndiotactic polymer: See stereoregular polymer.
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tacticity: The orderliness of the succession of configurational repeating units in the main chain of a polymer molecule. (See stereoregular polymer)
[ Pure and Appl. chem, 1981, 53, 733 IUPAC Macromolecular Nomenclature for Stereochemical Definitions Relating to Polymers]
tensile strength: A measure of the ability of a polymer to withstand pulling stresses.
termination reaction: The final step in chain polymerization where the growth of a polymer is stopped.
terpolymerization: Polymerization involving three types of monomers.
thermoplastics: Linear plastics of finite molecular weight that can be fabricated into complex shapes by melting and injection molding.
thermosets: A type of plastic that must be cured, forming network-like structures that do not soften at high temperatures.
thermotropic: Liquid crystal molecules which exhibit temperature dependent liquid crystalline behavior. See also: lyotropic
threading: When a nemaparallelepipedtic liquid crystalline material shows a tangled, thread-like appearance when observed between crossed polaroids in the optical microscope.
topology: A branch of mathematics concerned with those properties of geometric configurations which remain unaltered under very general kinds of elastic deformations (transformations such as stretching or twisting) where length, angles, and shapes are changed.

torsion: The rotation about a single bond which joins two atoms.
translational order: A condition when molecules have some arrangement in space. Crystals have three degrees of translational order (each molecule is fixed in space with an x, y, and z coordinate) and liquids have no translational order.
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uniaxial materials

Possess only one direction along which monochromatic light vibrating in any plane will travel with the same velocity. This direction is know as the optic axis.

unit cell

The smallest, regularly repeating material portion contained in a parallelepiped from which a crystal is formed by parallel displacements in three dimensions. Unlike the case of low-molar mass substances, the unit cell of polymer crystals usually comprises only parts of the polymer molecules, and the regularity of the periodic repetition may be imperfect.

[Pure and Appl. chem, 1989, 61, 769 IUPAC Macromolecular Nomenclature for Crystalline polymers]

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van der Waal's forces: Forces which act between molecules that are caused by small random fluctuations in the polarity of the molecules.
viscosity: The internal resistance to flow existing between two liquid layers when they are moved relative to each other. This internal resistance is a result of interaction between liquid molecules in motion.
vulcanization: A process by which a network of crosslinks is introduced into an elastomer to strengthen it.
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weight average molecular weight: In the distribution of molecular weights represented in a polymer sample, the weight average is determined by summing the product of the number of molecules of length i and the square of the molecular weight of a molecule of length i and then dividing by the total weight. See also number average
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