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Terms and Definitions

1 Chemistry/Polymer Chemistry

Terms and definitions important in the context of this ebook or the dental literature are explained in accordance with the IUPAC definitions [80-83] or with the literature [29-32, 85, 86].

Additive: Any type of substance that is added in very small quantities to a monomer, oligomer or polymer to improve, alter, and stabilize or to change its properties in any requested direction.

Antioxidant: A substance that inhibits or reduces the oxidation of other molecules or macromolecules, respectively. Primary and secondary antioxidants are differentiated. Primary antioxidants (mostly sterically hindered phenols or amine derivatives of higher molecular mass) are radical scavengers but secondary are not. Secondary antioxidants (sterically hindered phenols of lower molecular mass, organic phosphites or organic sulfides) decompose hydroperoxides to form stable alcohols and, thereby, chain branching can be avoided. It is the common purpose of all antioxidants to hinder or to diminish polymer degradation due to oxidative processes and to preserve the polymer’s properties.

Catalyst: Atoms, molecules or ions which diminish the activation energy with the result that a specific chemical reaction can occur. The catalyst does not participate in the reaction but exists before and after the reaction in the same chemical condition.

Comonomer: A second monomer added to the main monomer.

Constitutional unit: A species of atoms or atomic groups in a macromolecule, polymer or oligomer.

Composite resin/composite plastic: A resin/plastic that contains organic and/or inorganic fillers in all kinds of shapes (fibers, splinters, platelets, crystals, spheres, ligaments, etc.).

Copolymer: A polymer derived from more than one species of monomer.

Copolymerization: Polymerization of more than one species of monomer in which a copolymer is formed.

Cross-linkers: Cross-linkers are multifunctional monomers which form covalent chemical bonds between two separately growing polymeric chains to form a firm polymeric network. For polymerization reaction at least bifunctional monomers are requested, for polyaddition and polycondensation the monomers must be trifunctional at least.

Degree of crystallinity: The percentage of crystalline amount in a thermoplastic polymer.

Degree of conversion: The percentage of monomers that polymerize and form the polymer.

Degree of cross-linking: Relates to the number of groups that interconnect two materials. It is generally expressed in mole percent (mol%).

Degree of polymerization: The number of monomeric units/repeat units in a macromolecule, an oligomer or chain. For homopolymers the number of monomeric units corresponds with the number of repeat units. For copolymers this is not always true and sometimes the degree of polymerization is defined as the number of repeat units. Considering polyamide 66 (PA 66), for instance, the repeat unit consists of two monomeric units (-NH-(CH₂)₆-NH-OC-(CH₂)₄-CO-) with the result that a chain of two thousand monomeric units have only one thousand repeat units.

Functional group: A group of atoms in a molecule which significantly determines the reactivity or properties of the molecule (e.g. double bonds, triple bonds, aromatic compounds and hydroxyl or carboxyl groups).

Homopolymer: A polymer derived from only one specific monomer.

Inhibitor = Stabilizer: A molecule which deactivates radicals to inhibit a premature or unintended free radical polymerization. Inhibitors/stabilizers act similar to primary antioxidants.

Initiator: One or more molecules or ions forming radicals under the influence of energy and, thereby, start the free radical polymerization. The initiator takes part in the reaction and is consumed. In case the energy involved is light the initiator is called photoinitiator or light-initiator, in case it is heat it is called thermal or heat initiator, and in case it is “chemical” energy it is called redox initiator.

Ligand: Atom, molecule, ion or radical chemically bonded to a central atom.

Macromolecule/polymer molecule: A molecule of high relative molecular mass, the structure of which derives essentially of the multiple repetitions of molecule units with relative low molecular mass.

Macroradical: A macromolecule which is a radical.

Matrix resin: Unpolymerized monomer/oligomer blend or polymerized material that may contain different types of fillers (organic or inorganic), initiators, catalysts, stabilizers, pigments or various types of other additives.

Molecule: Two or more identical or different atoms chemically bonded to each other.

Monomer molecule, functionality: It is differentiated between mono-, bi-, tri-, tetra- or penta-functional monomer molecules. Monofunctional molecules have one reactive group, bifunctional have two, trifunctional have three and so on reactive groups to run a polyreaction. Monomers with more than one functional group are also called multifunctional or higher functional monomers; they function as cross-linkers.

Monomer molecule: A molecule which can polymerize and contributes a constitutional unit to the structure of a macromolecule. In other words: the smallest molecule which repeats oneself during a polymerization to form a polymer/macromolecule.

Monomer: A substance composed of molecules each of which can provide one or more constitutional units to a polymer.

Monomeric unit/monomer unit: The largest constitutional unit contributed by a single monomer molecule in a polymerization process to the structure of a macromolecule or oligomer molecule.

Oligomer molecule: A substance of intermediate relative molecular mass composed of a few or more constitutional units repetitively linked to each other. The properties of an oligomer vary with the addition or removal of one or a few of the constitutional units.

Oligomer: A substance composed of oligomer molecules.

Polymer: A substance composed of macromolecules.

Polyaddition: The process of converting a monomer or a mixture of monomers into a polymer by polyaddition reaction.

Polycondensation: The process of converting a monomer or a mixture of monomers into a polymer by polycondensation reaction.

Polymerization: The process of converting a monomer or a mixture of monomers into a polymer by free radical, anionic or cationic polymerization reaction.

Polymerization rate: Can be measured and mathematically expressed. The polymerization rate describes the kinetics/growth rate of the chain propagation.

Polymerization shrinkage (often only called shrinkage): (a) Volumetric Shrinkage: The percentage of volumetric change of the unpolymerized monomer, oligomer or substance during the polymerization. (b)Linear Shrinkage: The percentage of linear change of the unpolymerized monomer, oligomer or substance during the polymerization. The randomly distributed monomer or oligomer molecules move towards each other when polymerized (density increases) and, therefore, need less room. In other words: The density/specific weight of the polymer is higher than of the monomer.

Polyreaction: Any type of process converting a monomer or a mixture of monomers into a polymer.

Pre-polymer molecule: A macromolecule or oligomeric molecule that provides reactive groups for further polymerization and contributes more than one monomeric unit to at least one chain of the final macromolecule.

Pre-polymer: A substance composed of macromolecules or oligomer molecules having reactive polymerizable groups.

Radical (often called: free radical): An atom or molecule that contains an unpaired electron. Mostly, radicals are very reactive substances. They are usually formed when a covalent bond breaks to leave an unpaired electron on each of the two species created by the bond breaking. The symbol is “R•”; the dot symbolizes the free unpaired electron.

Relative molecular mass (often only called molecular mass, obsolete is molecular weight): The sum of the relative atomic masses of all atoms forming a molecule.

Residual monomer: Monomeric molecules of the same or of different species that do not participate in the polymerization but remain in the polymer in their original state.

Residual monomer content: Percentage of monomer that did not participate in the polymerization but remains unpolymerized in the polymer.

Resin/plastic: No consistent and comprehensive definition was found in the literature or the internet. Therefore, it is tried to combine what was found and to formulate a definition to meet the needs of this book and of dental material science. Resin/plastic materials are polymeric materials whose main components are organic or silicon organic macromolecules. These macromolecules can be made synthetically or by transformation of natural products. To become a resin or a plastic material the macromolecule/polymer contains additional ingredients as for instance additives (stabilizers, UV-stabilizers, plasticizers, antioxidants), pigments, dies or fillers. The differentiation between resin/plastic and polymer is not always precise because also homogeneous polymers (e.g. polyethylene, polypropylene, polyvinylchloride or polyaryletheretherketone) are called resins or plastics in case their technical application and thus their properties as a product are considered. Organic macromolecular compounds have a carboncarbon backbone but the silicon macromolecular ones, the so called polysiloxanes (trivial names: silicones, silicon rubbers) have a silicon-oxygen-silicon backbone with organic ligands or side chains.

Synergist = Accelerator: A substance that increases the polymerization rate. More generally: a substance that increases the rate of a chemical reaction. The synergist/accelerator takes part in the reaction.

UV-stabilizer: A substance that protects the polymer against UV-light.

2 Radiometry

Radiometry is the science and technology of radiation measurement of all wavelengths within the optical spectrum. The radiometric terminology is important to understand the performance of light-curing devices. The most important radiometric terms and their definitions for dental curing-lights are taken from [87-90].

Illuminance: is measured in lux [lx] and describes the total luminous flux incident on a surface per unit area.

Irradiance: Is expressed in Watt per square centimeter [W cm^-2] and describes the radiant power [W] an object receives per unit area [cm^-2].

Radiant energy: Is expressed in Joule [J] and describes the energy from the light source delivered per unit time [W s^-1].

Radiant exitance/radiant emittance: Is expressed in Watt per square centimeter [W cm^-2] and describes the radiant power [W] emitted from a known surface area of a light source.

Radiant exposure: Is expressed in Joule per square centimeter [J cm^-2] and describes the energy an object receives per unit area [cm^-2].

Radiant power/radiant flux: Is expressed in Watt [W] and describes the radiant energy delivered per unit time [J s^-1].

Spectral irradiance: Is expressed in Watt per square centimeter and nanometer [W cm^-2 nm^-1] and describes the irradiance [W cm^-2] at each wavelength [nm] of an electromagnetic spectrum.

Spectral radiant power: Is expressed in Watt per nanometer [W nm^-1] and describes the radiant power [W] at each wavelength [nm] of an electromagnetic spectrum.