Chapter 8 Flashcards
Define: Polymer
Compound consisting of long-chain molecules with each molecule made up of repeating units connected together.
Define: Thermoplastic
Are solid materials at room temperature but that become viscous liquids when heated to temperatures of only a few hundred degrees. Can be repeatedly heated and cooled without significant degradation to the polymer.
Define: Thermoset
Cannot tolerate repeated heating cycles. Once initially heated , they soften and flow for molding but the elevated temperatures produce a chemical reaction that hardens the material into an in-fusible solid.
Define: Elastomer
Rubbers; polymers that exhibit extreme elastic extensibility when subjected to relatively low mechanical stress.
Define: Monomer
The small units that make up the chain like structures (macromolecules) in polymers. Generally simple unsaturated organic molecules such as ethylene, C2H4.
Define: Macromolecule
The combination of many small molecules (monomers) into large molecules that possess a chain-like structure.
Define: Chain Polymerization
The process by which long chains of repeating “mers” are formed. It is initiated by a chemical catalyst (initiator) to open the carbon double bond in some of the monomers. with unpaired electrons, these monomers capture other monomers to begin forming chains that are reactive.
Define: Degree of Polymerization
The number of repeating “mers” in polymerization is known as “n” and the mean value of “n” is the degree of polymerization for the batch. Higher DP increases mechanical strength but also increases viscosity in the fluid state.
Define: Molecular Weight
In a polymer, is the sum of the molecular weights of the “mers” in the molecule (“n” times the molecular weight of one “mer”)
Define: Linear Polymer
When polymers form in chain-like structures, they are classified as linear polymers. Is a characterization of a thermoplastic polymer.
Define: Cross-Linked Polymer
When multiple linear polymers are connected by branches and other molecules at certain connection points (spider web)
Define: Degree of Crystallinity
The proportion of crystallized material in the mass. As crystallinity increases in a polymer, density, stiffness, strength, and heat resistance also go up.
Define: Glass-Transition Temperature
The point when cooling molten polymer in which the thermal expansion of the amorphous material becomes lower (change in slope). Below this temperature, the material is hard and brittle.
Define: Fillers
Solid materials added to a polymer usually in particulate or fibrous form to alter its mechanical properties or to simply reduce material cost.
Define: Plastizers
Chemicals added to a polymer to make it softer and more flexible and to improve its flow characteristics during forming.
