Term
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Definition
| A polymer is a compound comprised of a long-chain molecules that consist of repeating units, called mers, connected end to end |
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Term
| What are the three basic categories of polymers? |
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Definition
| The categories are: (1) Thermoplastics, (2) thermosetting polymers, and (3) elastomers |
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Term
| How do the properties of polymers compare with those of metals? |
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Definition
| In general, polymers have lower strength, hardness, stiffness, density, and temperature resistance compared to metals. In addition, polymers have low electrical and thermal conductivity |
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Term
| What does the degree of polymerization indicate? |
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Definition
| The degree of polymerization indicates the average number of mers or repeating units in the polymer molecule |
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Term
| What is cross-linking in a polymer, and what is its significance? |
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Definition
| Cross-linking is the formation of connections between the long-molecules in a polymer. It causes the polymer structure to be permanently altered. If the amount of cross-linking is low, the polymer is transformed into an elastomer, if cross-linking is significant, the polymer is transformed into a thermoset |
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Term
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Definition
| A copolymer is a polymer made up of two different types of mers, such as ethylene and propylene. |
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Term
| Copolymers can possess four different arrangements of their constituent mers. Name and briefy describe the four arrangements. |
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Definition
| The four possible arrangements of the mers along the chain are (1) alternating, in which the mers repeat every other position; (2) random, in which the mers are in random order; (3) block, in which mers of each type group themselves into long segments along the chain; and (4) graft, in which mers of one type are attached as branches to a main backbone of mers of the other type. |
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Term
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Definition
| A terpolymer is a polymer with three different mer types. An example is ABS (acrylonitrile-butadiene-styrene) plastic. |
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Term
| How are a polymer's properties affected when it takes on a crystalline structure? |
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Definition
| Density, stiffness, and melting temperature increase |
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Term
| Does any polymer ever become 100% crystalline? |
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Definition
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Term
| What are some of the factors that influence a polymer's tendency to crystallize? |
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Definition
| The factors that influence a polymer's tendency to crystallize are the following: (1) only linear polymers can form crystals; (2) copolymers do not form crystals; (3) stereoregularity - isotactic polymers always form crystals, atactic polymers never form crystals, and syndiotactic polymers sometimes form crystals; (4) slow cooling from the molten states promotes crystal formation; (5) plasticizers inhibit crystal formation; and (6) stretching the polymer tends to promote crystallization. |
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Term
| Why are fillers added to a polymer? |
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Definition
| Fillers are added to increase strength or simply to reduce the cost of the polymer |
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Term
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Definition
| A plasticizer is a chemical added to the polymer to make it softer and more flexible. It is often added to improve the polymer's flow characteristics for shaping |
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Term
| In addition to fillers and plasticizers, what are some other additives used with polymers? |
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Definition
| Other additives include (1) lubricants - to reduce friction and improve flow; (2) flame retardents; (3) colorants; (4) cross-linking agents, (5) antioxidants, and (6) ultraviolet light absorbers |
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Term
| Describe the difference in mechanical properties as a function of temperature between a highly crystalline thermoplastic and an amorphous thermoplastic. |
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Definition
| A highly crystalline TP retains rigidity during heating until just before its melting temperature is reached. An amorphous TP shows a significant drop in deformation resistance as its T_g as temperature is reached; it becomes increasingly like a liquid as temperature continues to increase. |
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Term
| What is the chemical formula of ethylene, the monomer for polyethylene? |
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Definition
| The chemical formula of ethylene is C2H4. |
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Term
| What is the basic difference between low-density and high-density polyethylene? |
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Definition
| LDPE has a branched structure and is amorphous. HDPE is linear and highly crystalline. These differences account for HDPE, having higher density, stiffness, and melting point |
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Term
| How do the properties of thermosetting polymers differ from those thermoplastics? |
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Definition
| Thermosets are more rigid, brittle, capable of higher service temperatures, and cannot be remelted |
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Term
| Cross-linking (curing) of thermosetting plastics is accomplished by one of three ways. Name the three ways. |
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Definition
| The three ways are (1) temperature-activated systems, in which elevated temperatures accomplish curing; (2) catalyst-activated systems, in which small amounts of a catayst cause cross-linking; and (3) mixing-activated systems, in which two reactive components are mized and curing occurs by their chemical reaction. |
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Term
| Elastomers and thermosetting polymers are both cross-linked. Why are their properties so different? |
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Definition
| Elastomers are lightly cross-linked, whereas thermosets are highly cross-linked. Light cross-linking allows extensibility; a highly cross-linked structure makes the polymer rigid. |
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Term
| What happens to an elastomer when it is below its glass transition temperature? |
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Definition
| An elastomer becomes hard and brittle when its temperature is below its T_g |
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Term
| How do thermoplastic elastomers differ from conventional rubbers? |
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Definition
| TPE's are different in two basic ways: (1) they exhibit thermoplastic properties, and (2) their extensibility dervices from physical connections between different phases in the polymer rather than cross-linking. |
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Term
| Briefly describe why polymers have low electrical and thermal conductivity, compare with metals? |
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Definition
| Polymers generally have very low thermal conductivity. The thermal conductivity and the electrical conductivity of materials is dictated primarily by the type of chemical bonds present in the material. In the case of polymers, they have mostly covalent bonds where pairs of electrons are shared between atoms. This bonding limits the movement of the electrons, therefore limiting the conductivity of the material. In the case of metals, they have metallic bonds, where electrons can move freely in all directions (electron cloud) in a metal, leading to high thermal conductivity. |
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Term
| Give three examples of Polymers. Write the molecular formula for the monomer and the repeating unit. Write the generic formula for the macromolecule. |
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Definition
1. Polyethylene is a very good example of a polymer. It is the most widely used plastic. The molecular formula for the monomer is Ch2=CH2 and the repeating unit is -(CH2-CH2)-. The generic formula for the macromolecule is CH3-(CH2-CH2)n-CH3 where n is the number of repeating units.
2.Polypropylene is also a good example as it is used in a wide variety of products and industrial applications. The molecular formula for the monomer is CH2-CH-CH3 and the repeating unit is -(CH2-CH-CH3)-. The generic formula for the macromolecule is CH3-(CH2-CH-CH3)n-CH3 where n is the number of repeating units.
3. Polyvinyl Chloride is the most widely used thermoplastic after polyethylene. The molecular formula for the monomer is CH2=CH-Cl and the repeating unit is -(CH2-CH-Cl)-. The generic formula for the macromolecule is CH3-(CH2-CH)n-CH3 where n is the number of repeating units |
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Term
| Discuss why a semi crystalline polymer is most likely opaque. |
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Definition
| A semi crystalline polymer is most likely opaque due to the fact that the crystalline portion of the polymer structure will most likely have a different refractive index than the amorphous (non-crystalline) part. This effect results in the scatter of light, making the material cloudy or opaque. |
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Term
| Briefly discuss why polymers can help reduce the weight of automotives. Discuss the design changes that may be needed, to take advantage of polymers and more important reinformed polymer composites. |
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Definition
Polymers have very good mechanical properties; however their properties are not as high as metals. However, when weight is considered, polymers become extremely attractive. Polymers exhibit very high specific strength and specific stiffness (strength/weight and modulus/weight ratio). This results in a weight savings to a structural part that is expected to withstand a particular load. When designing polymer or composite components, there are several changes or considerations that should be addressed. In particular, a polymeric part will most likely need a larger cross sectional area than its metal counterpart, so part size becomes a consideration. Another, perhaps ore important consideration would be the termal expansion coefficient of the materials. Differences in thermal expansion coefficient could result in differences in thermal expansion which could lead to failure of the design or asthetic issues. |
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Term
| Describe the polymerization reaction to form PVC. |
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Definition
In the case of PVC, which is a thermoplastic, the polymerization reaction is called Addition or Chain. It is a process where heat is applied to an initiator, which then breaks down to free radicals (with unpaired electrons). When these free radicals meet the unstaurated monomers (which have an extra internal bond) they break the extra bond and this leads to linking up with other monomers to form the repeating chain. This process sometimes leaves low molecular weight material unless the reaction is carried out to nearly full completion. |
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Term
| Describe the polymerization reaction to form polyurethane. |
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Definition
In the case of polyurethane, the reaction is called Step Growth polymerization. This process is characterized by the stepwise reaction between functional groups of monomers. Step growth polymers increase in molecular weight at a very slow rate and reach moderately high molecular weights only at high conversion. Low molecular weight material disappears early in the reaction. |
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Term
| Discuss the main differences between addition (chain) and step (condensation) polymerization |
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Definition
| In the addition polymerization the chain grows adding repeating units, while in the step process any monomer can react with any other. In step polymerization the molecules grow slowly, in the addition process, there are long molecules early in the reaction. In addition monomer decreases steadily, in chain, monomer decreases early. |
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Term
| Discuss the differences between thermoplastics and thermoset materials. |
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Definition
| Thermoplastic materials are also known as thermosoftening materials, are polymers that turn to a liquid when heated and freezes to a very glasses state when cooled sufficiently. The main difference with thermoset materials is that thermoplastics can be remelted and remolded. Thermosetting material are materials that irreversbily cure. |
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Term
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Definition
| Elastomer is a polymer with the property of elastiticity. Elastomers are amorphous polymers exisiting above their glass transition temperature, so that segmental motion is possible. their elasticity coes from extremely long chains' ability to reconfigure themselves to distribute an applied stress. The covalent cross-linkages ensure that the elastomer will return to its original configuration when the stress is removed. |
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Term
| What is the difference between a blend of polyethylene and polypropylene and a copolymer of polyethylene and polypropylene? |
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Definition
| The copolymer will have covalent bonds between the polyethylene and polypropylene chains. If these polymers are blended, the end result will be a mixture of these polymers however the chains will not be chemically bonded together. |
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Term
| Briefly describe how polymers crystallize |
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Definition
| Polymers can crystallize either during polymerization or crystallization can be included by orientation. This is controlled by the chains folding on each other. Thus the simpler the molecule the easier it is to crystallize. Molecules can be part of more than one crystalline region. |
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Term
| Which polymer will crystallize easier, linear or branched polyethylene and why? |
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Definition
| Linear polyethylene will crystallize easier than branched polyethylene due to the fact that the molecules can align with each other much more easily. Where as in the branched polyethylene, the structure would tend to be more amorphous. |
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Term
| Polymer A has 25% crystallinity and polymer B has 60% crystallinity. Which one will have a larger specific gravity an elastic modulus? |
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Definition
| An increase in crystallinity leads to an increase in density, stiffness, strength, toughness, as well as properties. Since polymer B has a higher crystallinity, it will have a larger specific gravity and elastic modulus. |
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Term
| Why do we need to define molecular weight averages in the case of polymers? How are they difference than simple liquids, say water? |
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Definition
| The average molecular weight represents the weight of the molecules of a polymer. In consequence it expresses the size of the polymer and it helps to represent the distribution of the chain lengths in the polymer. The average is taken because there are molecules with different length. The ratio of the number average molecular weight and the weight average molecular weight is called the polydispersity index, used to express the width of the molecular weight distribution. |
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