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Packaging Polymers > Units 1 & 2 > Flashcards

Units 1 & 2 Flashcards

1
Q

What gives a polymer its properties?

A

Chemical structure + Physical structure

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2
Q

Polymers differ considerably in what?

A

amount of crystallinity

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3
Q

Crystallinity requires the polymer molecules to be able to….

A

pack into a regular repeating array

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4
Q

Building Block Unit

A

Mer

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5
Q

_____________ can significantly limit potential crystallinity.

A

Structural irregularities

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6
Q

The higher the degree of irregularity, the ____ the polymer is able to crystallize.

A

less

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7
Q

All polymers have a ________ over which they can crystallize.

A

limited range of temperature

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8
Q

A polymer cannot crystallize ____ its melting temperature, or ____ its glass transition temperature.

A

above

below

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9
Q

Why can’t a polymer crystallize above its melting point?

A

the polymer molecules have too much energy to “settle down”

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10
Q

Define melting temperature

A

the breakup of crystallites

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11
Q

Why can’t a polymer crystallize below its glass transtion temperature?

A

the polymer molecules do not have enough energy to move sufficiently to pack into a crystallite

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12
Q

For some plastics, the range for crystallization to occur is ______ than Tm to Tg.

A

considerably narrower

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13
Q

What happens if a plastic is cooled very quickly from the melt to a temperature below its crystallization temperature range? 1

What happens to the same plastic if held within the crystallization temperature range for an extended period of time? 2

A
  1. It will develop little or no crystallinity

2. It may become highly crystalline

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14
Q

Chemical Regularity - Branches

A

at branch points the polymer won’t “fit” into the crystal lattice

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15
Q

Chemical Regularity - Cross-links

A

not only interfere with the fit, but also restrict the ability of molecular segments to move in order to assume a regular order

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16
Q

What is the linear homopolymer of ethylene?

A

HDPE

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17
Q

What is the branched homopolymer of ethylene?

A

LDPE

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18
Q

What is the crystallinity of HDPE?

A

65-90%

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19
Q

What is the crystallinity of LDPE?

A

40-60%

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20
Q

HDPE vs LDPE

By decreasing the crystallinity, the presence of the branches results in….

A

less efficient packing of the molecules into space, decreasing the density of the plastic

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21
Q

In the vast majority of materials, the spacing between adjacent molecules is _____ in crystalline areas than in amorphous areas.

A

smaller (water is an exception)

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22
Q

The structure affects how much crystallinity ___ develop. Processing affects how much crystallinity ___ develops.

A

can

actually

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23
Q

For a given polymer, increasing crystallinity increases….

A

Sealing temperature
Tensile strength
Stiffness

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24
Q

For a given polymer, increasing crystallinity decreases…

A

Elongation
Permability
Impact strength
Tear strength

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25
Q

In general, significant structural irregularity will prohibit crystallinity. Why is LDPE an exception?

A

it has very small side groups (hydrogens only)

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26
Q

HDPE, LDPE, and LLDPE are all _____ polymers formed by what?

A

addition

free radical polymerization reactions from a monomer with a double bond

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27
Q

HDPE - Initiation step

A

formation of the free radical and the addition of the first monomer

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28
Q

HDPE Polymerazation - How is the initial generation of a free radical usually accomplished?

A

by the decompostion of an initiator, which is a relatively unstable compound such as peroxide, hydroperoxide or azo compound

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29
Q

Sometimes _____ or _____ are used to generate free radicals.

A

UV radiation

High temperatures

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30
Q

HDPE Polymerazation - What happens after the initiator free radical is formed?

A

the addition of the first monomer unit occurs

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31
Q

How does branching occur?

A

the free radicals participate in chain transfer reactions

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32
Q

Chain transfer reaction are favored by….

A

high energy states in the free radicals (high temperature)

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33
Q

The rate of the propagation reaction generally _____ as temperature increases.

A

increases

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34
Q

How is LDPE made?

A

under conditions of high temperature and high pressure

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35
Q

Why does LDPE have lots of short branches and some long ones?

A

the free radical is most likely to collide with a nearby part of the same molecule

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36
Q

How is HDPE made?

A

under milder conditions than LDPE, where chain transfer reactions are much less likely to occur
*requires a catalyst

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37
Q

In most cases, two different monomeric units _____ into the same crystalline arrangment.

A

will not fit

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38
Q

The presence of a comonomer ______ crystallinity.

A

disrupts

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39
Q

Isomorphous

A

when 2 different structures will fit into the same crystalline latice

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40
Q

LLDPE is a _______ copolymer, with comonomers of ______

A

linear polyethylene

butene, hexene or octene

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41
Q

How is LLDPE made?

A

essentially the same conditions as HDPE

using a catalyst to provide a lower energy route for polymerization

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42
Q

How are HDPE and LLDPE different?

A

differences in crystallinity

43
Q

How are LLDPE and LDPE similar?

A

equal density

equal percent crystallinity

44
Q

How are LDPE and LLDPE different?

A

molecular weight and molecular weight distribution

45
Q

What is required for molecules to pack into a crystal lattice?

A

Stereochemical regularity and structural regularity

46
Q

What does stereochemistry refer to?

A

the arrangement of molecules in space

47
Q

Chiral Carbon

A

a carbon attached to 4 different groups, therefore it can exist in two mirror image forms, which cannot be interconverted without breaking primary bonds

48
Q

What are the different stereochemical forms?

A

Isotactic
Syndiotactic
Atactic

49
Q

What refers to the pattern of bonding within the molecule - what atoms are attached to what, and in what position?

A

Configuration

50
Q

What refers to the current arrangement in space?

A

Conformation

51
Q

_____ changes every time there is rotation about a bond.

A

Conformation

52
Q

______ cannot be changed without disrupting and reforming primary chemical bonds.

A

Configuration

53
Q

In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, the position of the methyl group alternates from one side of the chain to the other.

A

Syndiotactic

54
Q

In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, all the methyl groups are on the same side of the chain.

A

Isotactic

55
Q

In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, the position of the methyl group is random

A

Atactic

56
Q

______ polymers are generally amorphous.

A

Atactic

57
Q

What is the PP we use in packaging?

A

linear isotactic

58
Q

_____ PP is a soft, weak material which is not useful to us as a packaging material.

A

Atactic

59
Q

Sources of structural irregularity:

A
  • branching (or, even worse, cross-linking)
  • departure from the “normal” head-to-tail pattern of monomer addition
  • presence of comonomers (except if we have an alternating copolymer)
60
Q

_____ may be present if we have chiral carbons in our polymer, even if the polymer is chemically (structurally) regular.

A

Stereochemical irregularity

61
Q

A chiral carbon in a polymer molecule can be recognized as one that has _____ side groups attached to it.

A

2 different

62
Q

How does crystallinity affect polymer properties?

A

Increases tensile strength, stiffness, and barrier

63
Q

The coordinated crankshaft-type motion of 10-50 atoms.

A

Segmental mobility

64
Q

What is the basic requirement for segmental mobility?

A

that there be rotation around single bonds in the polymer backbone

65
Q

Which conformation has the lowest energy state?

A

Staggered conformation, where all the side groups are, on average, farthest apart

66
Q

Why is the eclipsed conformation at a higher energy level than the staggared conformation?

A

As there is rotation around the single C-C bound causes side groups to move closer to each other

67
Q

Polymers with double bonds in the backbone -

If we look at rotation about the single bond adjacent to the double bond, there is ________ with rotation

A

less interference

68
Q

Polymers with double bonds in the main chain tend to have ____ Tg’s, and are _______.

A

low

soft and flexible

69
Q

What group are elastomers usually found?

A

polymers with double bonds

70
Q

The presence of a double bond means that the material …

A

retains some reactivity, permitting subsequent cross-linking and other modification, if desired.

71
Q

What do ring structures in the backbone of a polymer do?

A

put rigid links but provide no appreciable effect of easing rotation around adjacent single bonds like double bonds do

72
Q

Polymers containing _____tend to be quite stiff, and have high Tg and Tm.

A

backbone rings

73
Q

What does the presence of O or N in the main chain tend to do?

A

increase the flexibility of a polymer structure, lowering its Tg (and Tm, if applicable)

74
Q

What must we consider whenever we have atoms other than C and H in the polymer?

A

the effects of the magnitude of secondary forces on properties

75
Q

Below Tg, there is ______ mobility of the polymer molecules.

A

no significant

76
Q

What happens to the polymer above Tm?

A

we have mobility everywhere (liquid flow)

77
Q

Large side groups ______ the mobility (conformational flexibility) of the molecule.

A

decrease

78
Q

What are the 3 main types of primary bonds?

A

covalent, ionic and metalic

79
Q

Which type of bonds involve sharing of electrons between the 2 atoms being bonded.

A

covalent

80
Q

Which type of bonds involve the transfer of electrons from one atom to another?

A

ionic (the molecular fragments are then joined together by electrostatic attraction)

81
Q

_____ bonds occur when electrons are very loosely held, and are therefore relatively free to wander away from their originating atom.

A

metallic

82
Q

Covalent bonds can be further subdivided into…

A

non-polar and polar bonds

83
Q

Non-polar Bonds

A

electron sharing is equal

84
Q

Polar Bonds

A

electron sharing is unequal

85
Q

Bonds involving _______ are generally polar.

A

C and O, N, Cl and F

86
Q

What is the prime determinant of the chemical reactivity of the polymer?

A

the strength of the primary bonds

87
Q

What determines the basic spacing of the atoms in the molecule?

A

the bond length

88
Q

If a substance had no secondary bonds….

A

it would behave like an ideal gas

89
Q

Types of secondary bonds

A

dispersion forces
induction forces
dipole forces
hydrogen bonds

90
Q

Forces due to electostatic attractions between atoms with equal and opposite electric charges.

A

Dipole

91
Q

_______ forces occur when a polar molecule induces dipoles in surrounding nonpolar molecules.

A

Induction

92
Q

_______ forces are the attraction between neighboring molecules that results from instantaneous configurations of electrons and nuclei.

A

Dispersion

93
Q

Intermolecular attractions are proportional to ____

A

r^-6, where r is the distance between the atoms

94
Q

One of the ways in which crystallinity influences polymer properties is by achieving a _______ packaging of the atoms, and hence _____.

A

tighter

increasing intermolecular forces

95
Q

In order for segmental mobility to occur, not only must we have enough energy for ______, we must also have enough energy to ______.

A

rotation within the polymer backbone

disrupt the secondary bonds

96
Q

In general, lower conformational flexibility, for instance, associated with side groups, would tend to ____.

A

increase Tm

97
Q

Increasing the spacing (in the crystal framework) acts to _______ the strength of secondary bonds in the cystallite, which acts to _____ Tm.

A

decrease

lower

98
Q

_______ is a measure of the strength of secondary forces.

A

cohesive energy density

99
Q

Polymers which have intermolecular hydrogen bonds are often ________ by the presence of water.

A

strongly affected

example: PVOH, which interacts so strongly that it is soluble in water

100
Q

Polymers with significant hydrogen bonding are usually ________ when dry than in high moisture conditions.

A

better barriers

ex: nylon and ethylene vinyl alcohol

101
Q

Primary bonds are inportant in determining properties such as ___________ and _________.

A

thermal stability

chemical reactivity

102
Q

Most of the performance properties that we care about are determined predominantly or exclusively by the ______.

A

secondary bonds

primary bonds can have an important indirect effect

103
Q

The more polar groups we have (or the more polar the groups are), the _________ the resulting intermolecular forces.

A

stronger

Packaging Polymers (1 decks)

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