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MSE 2034 > Chapter 15 > Flashcards

Chapter 15 Flashcards

1
Q

What are the three types of elasticities of polymers?

A
  • Brittle
  • Plastic
  • Elastic
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2
Q

What type of stress-strain behavior do brittle polymers exhibit?

A

Fractures during elastic deformation

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3
Q

What type of stress-strain behavior do plastic polymers exhibit?

A

Deformation is initially elastic followed by yielding and plastic deformation until eventual fracture

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4
Q

What type of stress-strain behavior do elastic polymers exhibit?

A

Completely elastic deformation, but still will eventually fracture

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5
Q

Where is the yield point on a stress-strain curve for a plastic polymer?

A

The local max of the curve where plastic deformation begins

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6
Q

What is tensile strength?

A

The amount of stress at which fracture occurs

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7
Q

What is yield strength?

A

The amount of stress at the onset of plastic deformation

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8
Q

What mechanism of deformation occurs with crosslinked and network polymers?

A

Slight elongation of polymer chains then brittle failure

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9
Q

What is the order of deformation in semicrystalline (plastic) polymers?

A
  1. Undeformed
  2. Elongation of amorphous regions
  3. Crystalline regions align (necking begins)
  4. Crystalline block segments separate
  5. Fibrillar structure forms
  6. Plastic failure
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10
Q

What is the purpose of pre-deformation by drawing a plastic material?

A
  • Increases the elastic modulus (stiffness) and tensile strength in the stretching direction
  • Decreases ductility
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11
Q

How does drawing affect the structure of a plastic material?

A
  • Stretches the polymer before use
  • Aligns chains in the stretching direction
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12
Q

How does annealing affect a drawn semicrystalline polymer?

A
  • Decreases chain alignment
  • Reduces elastic modulus and tensile strength in the stretching direction
  • Increases ductility
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13
Q

What is the order of deformation in elastomers?

A
  1. Amorphous chains are kinked and cross-linked
  2. Chains are straighter, elongated, and cross-linked
  3. Failure
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14
Q

What is a thermoplastic?

A
  • A polymer that melts into a soft, pliable form above a certain temperature and solidifies upon cooling
  • Can be reshaped/melted an indefinite amount of times
  • Often in pellets
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15
Q

What is a thermoset?

A
  • A polymer that will remain in a permanent solid state after being cured (melted and cooled) one time
  • Cannot be reshaped/melted
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16
Q

What are some characteristics of thermoplastics?

A
  • Little crosslinking
  • Ductile
  • Soften with heating
    Ex: polyethylene, polypropylene, polycarbonate, polystyrene
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17
Q

What are some characteristics of thermosets?

A
  • Significant crosslinking (10-50% of units)
  • Hard and brittle
  • Do not melt/soften when heated, instead degrades
    Ex: vulcanized rubber, epoxies, polyester/phenolic resin
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18
Q

How does increasing temperature or strain rate affect thermoplastics?

A

As temperature decreases or strain increases: the elastic modulus (stiffness) increases, tensile strength increases, and ductility decreases

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19
Q

How does chain stiffness affect melting and glass temperatures?

A

Increasing chain stiffness increases Tm and Tg

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20
Q

What increases chain stiffness?

A
  • Bulky side groups
  • Polar groups or side groups
  • Chain double bonds and aromatic chain groups
21
Q

What is the relaxation modulus?

A

The ratio between the time-dependent stress and a constant strain value

22
Q

What is the stress relaxation test?

A

A material is strained in tension to a predetermined, low-strain level. The stress needed to maintain the strain decreases over time due to molecular relaxation.

23
Q

How does the relaxation modulus relate to viscosity?

A

Low for rigid solids and high for viscous liquids

24
Q

What happens during craze formation in a fracturing thermoplastic polymer?

A
  • The plastic deformation of spherulites
  • Formation of micro voids and fibrillar bridges
  • Crack/fracture
25
Q

What are the two types of polymerization?

A
  • Addition/chain polymerization
  • Condensation/step polymerization
26
Q

What are the steps of addition/chain polymerization?

A
  1. Initiation: a free radical is added to a monomer, opening up the double bonds
  2. Propagation: more repeating units are added with single bonds
  3. Termination: Polymerixation stops through combination (combining with another chain) or disproportionation (free radical moves)
27
Q

What is condensation/step polymerization?

A

A form of step-growth polymerization where smaller molecules or monomers react with each other to form larger structural units (usually polymers) while releasing by-products.

28
Q

What are the different types of polymer additives?

A
  • Fillers
  • Plasticizers
  • Stabilizers
  • Lubricants
  • Colorants
  • Flame retardants
29
Q

What do fillers do?

A
  • Improve tensile strength, abrasion resistance, toughness, and decrease cost
    Ex: carbon black, silica gel, wood flour, glass, limestone, talc
30
Q

What do plasticizers do?

A
  • Reduce the glass transition temperature
  • Turns brittle polymers into ductile polymers
    Ex: commonly added to PVC to increase ductility
31
Q

What do stabilizers do?

A
  • Counteract deterioration processes
    Ex: antioxidants, UV protectants
32
Q

What do lubricants do?

A
  • Reduce friction for easier processing (in dies)
    Ex: sodium stearate
33
Q

What do colorants do?

A
  • Add color to materials
    Ex: dyes and pigments
34
Q

What do flame retardants do?

A
  • Increase flammability resistance
    Ex: substances with chlorine, fluorine, and boron
35
Q

What are the different types of polymer processing techniques?

A
  • Compression
  • Injection
  • Extrusion
  • Blown-film
36
Q

What is compression molding?

A
  • Used for thermoplastics and thermosets
    1. Polymer and additives are placed in a mold
    2. Mold is heated and hydraulic pressure is applied
    3. Polymer assumes the shape of the mold
37
Q

What is injection molding?

A
  • Used for thermoplastics and some thermosets
    1. A ram forces pellets into a heating chamber
    2. Pellets melt while moving around a spreader
    3. Molten plastic is injected into a mold where it cools and takes the shape of the mold
38
Q

What is extrusion?

A
  • Used for thermoplastics
    1. Plastic pellets drop from a hopper onto a screw
    2. Pellets melt as the screw pushes them forward
    3. Molten plastic is extruded through a die to form the extrudate
39
Q

What is blown-film extrusion?

A
  1. Plastic pellets drop from a hopper onto a screw
  2. Pellets melt as the screw pushes them forward
  3. Molten plastic is extruded through a tubing die
  4. Air is blown through the die forming an air bubble
  5. Bubbled polymer is collected into pinch rolls
  6. Film roll is formed
40
Q

What are the different types of polymers?

A
  • Fibers
  • Coatings
  • Adhesives
  • Films
  • Foams
41
Q

What are fibers used for?

A

Primarily textiles

42
Q

What are the characteristics of polymers?

A
  • High tensile strengths
  • High degrees of crystallinity
  • Structures containing polar groups
43
Q

How are fibers formed?

A
  • Formed by spinning
    1. Extrude polymer through a spinneret
    2. Spun fibers are drawn under tension
    3. Results in highly aligned chains/fibrillar structure
44
Q

What are coatings?

A

Thin polymer films applied to surfaces
- Protect against corrosion/degradation
- Decorative
- Electrical insulation

45
Q

What are adhesives?

A

Bonds two solid materials (adherends) using either van de Walls forces (secondary bonding) or penetration into pores/crevices (mechanical)

46
Q

What are foams?

A

A polymer that has been made porous/spongelike through the incorporation of gas bubbles

47
Q

What are films?

A

Thin polymer films produced by blown-film extrusion

48
Q

What are the characteristics of ultra-high-molecular-weight polyethylene?

A
  • High molecular weight
  • High impact strength
  • Resistance to wear and abrasion
  • Low coefficient of friction
  • Self-lubricating surface
  • Used in bullet-proof vests, golf ball covers, and joint implants

MSE 2034 (15 decks)

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