Chapter 15

Question 1

elastomers

Answer 1

a polymeric material that may experience large and

reversible elastic deformations

Question 2

viscoelasticity

Answer 2

a type of deformation exhibiting the mechanical

characteristics of viscous flow and elastic deformation.

Question 3

relaxation modulus

Answer 3

time-dependent elastic modulus for viscoelastic polymers

Question 4

leathery or glass transition region

Answer 4

deformation will be time dependent and not totally recoverable on release of an applied load

Question 5

viscoelastic creep

Answer 5

time-dependent deformation when the stress level is maintained constant

Question 6

craze

Answer 6

• regions of very localized plastic deformation, which lead to the formation of small and interconnected microvoids
• can support a load across its face

Question 7

fatigue limit

Answer 7

a stress level at which the stress at failure becomes independent of the number of cycles

Question 8

tear strength

Answer 8

energy required to tear apart a cut specimen that has a standard geometry

Question 9

drawing

Answer 9

a deformation technique in which polymer fibers are strengthened by elongation

Question 10

entropy

Answer 10

• measure of the degree of disorder within a system

- entropy increases with increasing disorder

Question 11

criteria for a polymer to be an elastomer

Answer 11

-it must not easily crystallize; elastomeric materials are amorphous, having molecular chains that are
naturally coiled and kinked in the unstressed state
-chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force
-for elastomers to experience relatively large elastic deformations, the onset of plastic deformation must be delayed

Question 12

vulcanization

Answer 12

a nonreversible chemical reaction involving sulfur or another suitable agent in which crosslinks are formed between molecular chains in rubber materials. The rubber’s modulus of elasticity and strength are enhanced

Question 13

three phenomena that are important with respect to the design and processing of polymeric
materials

Answer 13

crystallization, melting, and the glass transition

Question 14

crystallization

Answer 14

process by which, upon cooling, an ordered solid phase is produced from a liquid melt having a highly random molecular structure

Question 15

melting

Answer 15

reverse process that occurs when a polymer is heated

Question 16

glass transition

Answer 16

occurs with amorphous or noncrystallizable polymers that, when cooled from a liquid melt, become rigid solids yet retain the disordered molecular structure that is characteristic of the liquid state

Question 17

melting temperature

Answer 17

the temperature at which, upon heating, a solid (and crystalline) phase transforms into a liquid

Question 18

glass transition temperature

Answer 18

temperature at which the polymer experiences the transition from rubbery into rigid states

Question 19

plastics

Answer 19

materials that have some structural rigidity under load and are used in general-purpose applications

Question 20

fiber

Answer 20

• any polymer, metal, or ceramic that has been drawn into a long and thin filament
• have a high tensile strength (over a relatively wide temperature range) and a high modulus of elasticity, as well as abrasion resistance

Question 21

coatings

Answer 21

are applied to:

• protect the item from the environment, which may produce corrosive or deteriorative reactions
• improve the item’s appearance
• provide electrical insulation

Question 22

latex

Answer 22

• common coating

- stable suspension of small, insoluble polymer particles dispersed in water

Question 23

adhesive

Answer 23

• a substance used to bond together the surfaces of two solid materials
• bonds: mechanical and chemical

Question 24

mechanical bonding

Answer 24

there is actual penetration of the adhesive into surface

pores and crevices

Question 25

chemical bonding

Answer 25

``` involves intermolecular forces between the adhesive and adherends (the two surfaces) ```

Question 26

films

Answer 26

have thicknesses between 0.025 and 0.125 mm are fabricated and used extensively as bags for packaging food products and other merchandise, as textile products

Question 27

foams

Answer 27

plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles

Question 28

polymerization

Answer 28

process by which monomers are linked together to generate long chains composed of repeat units

Question 29

addition polymerization

Answer 29

- monomer units are attached one at a time in chainlike fashion to form a linear macromolecule - initiation, propagation, & termination

Question 30

condensation polymerization

Answer 30

- formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species - usually a low-molecular-weight by-product such as water that is eliminated

Question 31

additives

Answer 31

intentionally introduced to enhance or modify many of these properties and thus render a polymer more serviceable

Question 32

fillers

Answer 32

added to polymers to improve tensile and compressive | strengths, abrasion resistance, toughness, & dimensional and thermal stability

Question 33

plasticizers

Answer 33

- improve flexibility, ductility, and toughness of polymers | - produce reductions in hardness and stiffness

Question 34

stabilizers

Answer 34

additives that counteract deteriorative | processes

Question 35

colorants

Answer 35

impart a specific color to a polymer

Question 36

flame retardants

Answer 36

enhance the flammability resistance of the remaining combustible polymers

Question 37

fabrication of thermosetting polymers

Answer 37

- first comes the preparation of a linear polymer (sometimes called a prepolymer) as a liquid having a low molecular weight - the second stage, termed curing, may occur during heating and/or by the addition of catalysts and often under pressure

Question 38

molding

Answer 38

shaping a plastic material by forcing it, under pressure and at an elevated temperature, into a mold cavity

Question 39

compression molding

Answer 39

the mold is closed, and heat and pressure are applied, causing the plastic to become viscous and flow to conform to the mold shape

Question 40

transfer molding

Answer 40

molten material is injected into the mold chamber, the pressure is distributed more uniformly over all surfaces

Question 41

injection molding

Answer 41

- material is fed into cylinder by a ram - pushed forward into heating chamber, which melts it - impelled through tube into mold - very fast

Question 42

extrusion

Answer 42

-a mechanical screw or auger propels the pelletized material through a chamber, where it is successively compacted, melted, and formed into a continuous charge of viscous fluid -molten mass is forced through a die orifice

Question 43

blow molding

Answer 43

-a parison, or length of polymer tubing, is extruded -while still in a semimolten state, the parison is placed in a two-piece mold -the hollow piece is formed by blowing air or steam under pressure into the parison, forcing the tube walls to conform to the contours of the mold

Question 44

spinning

Answer 44

the process by which fibers are formed. A multitude | of fibers are spun as molten or dissolved material is forced through many small orifices

Question 45

melt spinning

Answer 45

- material is first heated until it forms a relatively viscous liquid - pumped through a plate called a spinneret, which contains numerous small, typically round holes - as the molten material passes through each of these orifices, a single fiber is formed, which is rapidly solidified by cooling with air blowers or a water bath

Question 46

metal vs polymers

Answer 46

Metal: higher ultimate tensile strength and modulus of elasticity Polymers: higher level of ductility