Answer. Density, stiffness, and melting temperature increase.
8.12 Does any polymer ever become 100% crystalline?
Answer. No.
8.13 What are some of the factors that influence a polymer's tendency to crystallize?
Answer. Factors are: (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.
8.14 Why are fillers added to a polymer?
Answer. Fillers are added to increase strength or simply to reduce the cost of the polymer.
8.15 What is a plasticizer?
Answer. 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.
8.16 In addition to fillers and plasticizers, what are some other additives used with polymers?
Answer. Other additives include: lubricants - to reduce friction and improve flow; flame retardents;
colorants; cross-linking agents, antioxidants, and ultraviolet light absorbers.
8.17 Describe the difference in mechanical properties as a function of temperature between a highly crystalline thermoplastic and an amorphous thermoplastic.
Answer. A highly crystalline TP retains rigidity during heating until just before its Tm is reached.
An amorphous TP shows a significant drop in deformation resistance at its Tg as temperature is raised; it becomes increasingly like a liquid as temperature continues to increase.
8.18 What is unique about the polymer cellulose?
Answer. Cellulose is a polymer that grows in nature. Wood fiber contains about 50% cellulose and cotton fiber is about 95% cellulose.
8.19 The nylons are members of which polymer group?
Answer. Polyamides.
8.20 What is the chemical formula of ethylene, the monomer for polyethylene?
Answer. C2H4
8.21 What is the basic difference between low density and high density polyethylene?
Answer. LDPE has a branched structure and is amorphous. HDPE is linear and highly crystalline.
These differences account for HDPE higher density, stiffness, and melting point.
8.22 How do the properties of thermosetting polymers differ from those of thermoplastics?
Answer. Thermosets are more rigid, brittle, capable of higher service temperatures, and cannot be remelted.
8.23 Cross-linking (curing) of thermosetting plastics is accomplished by one of three ways. Name the three ways.
Answer. The three ways are: (1) temperature-activated systems, in which elevated temperatures accomplish curing; (2) catalyst-activated systems, in which small amounts of a catalyst cause cross-linking; and (3) mixing-activated systems, in which two reactive components are mixed and curing occurs by their chemical reaction.
8.24 Elastomers and thermosetting polymers are both cross- linked. Why are their properties so different?
Answer. 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.
8.25 What happens to an elastomer when it is below its glass transition temperature?
Answer. An elastomer is hard and brittle below its Tg. 8.26 What is the primary polymer ingredient in natural rubber?
Answer. Polyisoprene.
8.27 How are thermoplastic elastomers different from conventional rubbers?
Answer. TPEs are different in two basic ways: (1) they exhibit thermoplastic properties, and (2) their extensibility derives from physical connections between different phases in the polymer.
Multiple Choice Quiz
There are a total of 25 correct answers in the following multiple choice questions (some questions have multiple answers that are correct). To attain a perfect score on the quiz, all correct answers must be given, since each correct answer is worth 1 point. For each question, each omitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the number of answers required reduces the score by 1 point. Percentage score on the quiz is based on the total number of correct answers.
8.1 Of the three polymer types, which one is the most important commercially? (a) thermoplastics, (b) thermosets, or (c) elastomers.
Answer. (a)
8.2 Which one of the three polymer types is not normally considered to be a plastic? (a) thermoplastics, (b) thermosets, or (c) elastomers.
Answer. (c)
8.3 Which one of the three polymer types does not involve cross- linking? (a) thermoplastics, (b) thermosets, or (c) elastomers.
Answer. (a)
8.4 As the degree of crystallinity in a given polymer increases, the polymer becomes denser and stiffer, and its melting temperature decreases: (a) true or (b) false.
Answer. (b) Melting temperature increases with higher degree of crystallinity.
8.5 Which of the following is the chemical formula for the repeating unit in polyethylene? (a) CH2, (b) C2H4, (c) C3H6, (d) C5H8, or (e) C8H8.
Answer. (b)
8.6 Degree of polymerization is which one of the following? (a) average number of mers in the molecule chain; (b) proportion of the monomer that has been polymerized; (c) sum of the molecule weights of the mers in the molecule; or (d) none of the above.
Answer. (a)
8.7 A branched molecular structure is stronger in the solid state and more viscous in the molten state than a linear structure for the same polymer: (a) true or (b) false.
Answer. (a)
8.8 A copolymer is a mixture consisting of macromolecules of two different homopolymers: (a) true or (b) false.
Answer. (b)
8.9 As temperature of a polymer increases, its density (a) increases, (b) decreases, or (c) remains fairly constant.
Answer. (b)
8.10 Which answers complete the following sentence correctly (more than one): As the temperature of an amorphous thermoplastic polymer is gradually reduced, the glass transition temperature Tg is indicated when (a) the polymer transforms to a crystalline structure, (b) the coefficient of thermal expansion increases markedly, (c) the slope of specific volume versus temperature changes markedly, (d) the polymer becomes stiff, strong, and elastic, or (e) the polymer solidifies from the molten state.
Answer. (c) and (d).
8.11 Which of the following plastics has the highest market share? (a) phenolics, (b) polyethylene, (c) polypropylene, (d) polystyrene, or (e) polyvinylchloride.
Answer. (b)
8.12 Which of the following polymers are normally thermoplastic (more than one): (a) acrylics, (b) cellulose acetate, (c) nylon, (d) polychloroprene, (e) polyethylene, or (f) polyurethane.
Answer. (a), (b), (c), and (e).
8.13 Polystyrene (without plasticizers) is amorphous, transparent, and brittle: (a) true or (b) false.
Answer. (a)
8.14 The fiber rayon used in textiles is based on which of the following polymers: (a) cellulose, (b) nylon, (c) polyester, (d) polyethylene, or (e) polypropylene.
Answer. (a)
8.15 The basic difference between low density polyethylene and high density polyethylene is that the latter has a much higher degree of crystallinity: (a) true or (b) false.
Answer. (a)
8.16 Among the thermosetting polymers, the most widely used commercially is which of the following:
(a) epoxies, (b) phenolics, (c) silicones, or (d) urethanes.
Answer. (b)
8.17 Polyurethanes can be which of the following (more than one): (a) thermoplastic, (b) thermosetting, or (c) elastomeric.
Answer. (a), (b), and (c).
8.18 The chemical formula for polyisoprene in natural rubber is which of the following: (a) CH2, (b) C2H4, (c) C3H6, (d) C5H8, or (e) C8H8.
Answer. (d)
8.19 The leading commercial synthetic rubber is which of the following: (a) butyl rubber, (b) isoprene rubber, (c) polybutadiene, (d) polyurethane, (e) styrene-butadiene rubber, or (f) thermoplastic elastomers.
Answer. (e)