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Chemistry for the Analysis of Trace Evidence > Polymers > Flashcards

Polymers Flashcards

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1
Q

What are the three types of polymers?

A
  • Natural
  • Synthetic
  • Semi-synthetic
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2
Q

What are some examples of natural polymers?

A
  • Proteins
  • Polysaccharides
  • Resins/gums
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3
Q

What are some examples of synthetic polymers?

A
  • Plastics
  • Elastomers
  • Rubber
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4
Q

What is a polymer?

A
  • long chain molecule
  • constructed from many monomers that are covalently bonded together
  • can be linear, branched, cross-linked or colloidal
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5
Q

What is a homopolymer?

A

Comprised of all the same monomers

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6
Q

What is a co-polymer?

A

Made up of different monomers

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7
Q

How do polymers end?

A
  • there are end groups
  • the nature of end groups allows method of synethsis to be analysed
  • it is possible for there to be no end groups = rings
  • useful for identifying a material
  • ratio of end groups to in-chain groups allows measuring of polymer length
  • reactive end groups allow further modification to control properties or add additional functionality
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8
Q

Why are end groups useful for identifying a material?

A
  • type of end group is unique and will give a unique signature
  • ususally more reactive so can give a unique signature
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9
Q

What are the 3 main types of polymer when classed by bulk properties?

A
  • Thermoset
  • Thermoplastic
  • Elastomer
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10
Q

What are the properties of thermoset polymers?

A
  • burns when heated
  • irreversibly hardened once shaped
  • generally inflexible
  • chemical process called curing required
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11
Q

Examples of thermoset polymers

A
  • epoxy resins
  • silicones
  • polyurethanes
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12
Q

What are the properties of thermoplastic polymers?

A
  • melts when heated
  • can be remelted and reshaped
  • generally more flexible
  • no chemical curing required
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13
Q

Examples of thermoplastic polymers

A
  • polystyrene
  • nylon
  • polycarbonate
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14
Q

What are the properties of elastomer polymers?

A
  • can be thermoplastic or thermoset
  • has viscosity (can flow) and eslasticity (return to orignial shape)
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15
Q

Examples of elastomer polymers

A
  • rubbers
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16
Q

HDPE

A
  • high density polyethylene
  • high crystalline content
  • less transparent than LDPE
  • stiffer and harder than LDPE
  • higher density
  • very low/no branching
  • less gas permeable
17
Q

LDPE

A
  • low density polyethylene
  • low crystalline content
  • more transparent than HDPE
  • forms good films
  • lower density
  • highly branched
  • more gas permeable than HDPE
18
Q

Define crystallinity

A

The regions of atomic ordering where intramolecular folding/stacking of adjacent chains occur

19
Q

What properties does crystallinity affect?

A
  • crease
  • thermal behaviour
  • transparency
  • stiffness
  • impact resistance
  • tensile strength
  • young’s modulus
20
Q

What does a degree of 0 and 1 mean in crystallinity?

A
  • 0 = completely amorphous
  • 1 = completely crystalline
21
Q

What is tensile strength

A

how much something will stretch before it breaks
* for polymers this increases with polymer chain length and crosslinking
* easier to stetch smaller things - less matter to move = less force required

22
Q

How do we measure tensile strength?

A

-ε = l/L
* ε = strain (use this instead of stretch in solids
* I = fractional change after stretching
* L = initial starting length

23
Q

For a solid how do we work out the force used to stretch it?

A

-σ = F/A
* σ = stress (used instead of force)
* independant of sample size
* F = force
* A = distance stretched

24
Q

What does Young’s Modulus describe?

A

How stiff a solid is

25
Q

How to measure Young’s Modulus?

A

E = -σ/ε
* plot stress over strain and the gradient is the answer
* Divide the gradient by 1000 to be in GPa units
* To be in N/m^2 multiply the gradient by 1000000

26
Q

What is the force for a spring?

A

F = -kx
* k = force/spring constant
* x = distanced stretched (i.e. I)

27
Q

How do you work out k in F = -kx?

A
  • plot force (y-axis) by distanced stretched (x-axis)
  • the gradient of the line is k

Chemistry for the Analysis of Trace Evidence (16 decks)

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