Polymers

Question 1

What is a crystal?

Answer 1

Anisotropic, homogenous body consisting of a 3D periodic ordering of atoms, ions or molecules

Question 2

What is the difference between an amorphous and a crystalline polymer?

Answer 2

Question 3

What factors influence polymer crystalinity?

Answer 3

Molecular structure

Crystalline state is lowest energy

Speed + temp. of solidification

• when a melted polymer cools or a solution dries
• rapidly = amorphous
• slowly = crystalline

Question 4

How do polymer crystal form?

Answer 4

Question 5

What is nucleation?

Answer 5

Tiny crystals form at points of molecular contact

Crystalline regions grow outwards from nucleation sites - as polymer chains rearrange

Question 6

Polyethylene can be either amorphous or crystalline.

Name which types of polyethylene are under.

Answer 6

High density = up to 90% crystalline

Low density = amorphous

Semicrystalline both have amorphous domains + crystalline domains

Question 7

Fill in the gaps

Answer 7

Question 8

Draw a typical viscoelastic diagram, identifying the 5 regions of viscoelastic behaviour of an amorphous or semicrystalline polymer

Answer 8

T_g = glass transition temperature

Why does it happen?

There is a decrease in mol. contact, inter-molecular interactions + entanglement, which increases freedom for chain to undergo thermal movement = creates internal space, stretching = elastic behaviour

Question 9

What are plasticisers?

Answer 9

Small molecules that dissolve in the polymer and interact with it

Question 10

Describe the polymer properties in the 5 regions.

Answer 10

Glassy

Polymer chains held rigidly together by large numbers of inter-chain interactions

Transition

At the ‘glassy transition temp’ the polymer chains gain some molecular freedom

Deformations can be accommodated by chain movement

Lots of chain-chain interactions, so retains strength

Tough solid to flexible leathery. Not brittle.

Rubbery

Chains held by fewer contact points, long parts of polymer chain can extend like springs

Elastic solid - extends like a rubber band

Rubbery flow

Fewer contact points so some molecules can flow past each other

Elastic withi viscous flow

A solid that plastically deforms under pressure - like blue tack’

Liquid

losing any strong inter-chain bonds

Polymer is melting + flows

Sticky liquid

Question 11

Discuss speed of deformation

Answer 11

Question 12

What is the effect of crosslinking on polymer?

Refer to the viscoelastic diagram

Answer 12

Except the polymer only reaches rubber solid/gel as polymer chains cannot flow

Question 13

How does crystallinity affect the viscoelastic properties of a polymer?

Answer 13

Question 14

What is the melting point of a polymer?

Answer 14

It is when the polymer goes from crystalline to amorphous

Question 15

Define the mechanisms of transport

Answer 15

Surface mechanisms e.g. drugs + IV bags

• adsorption to surfaces
• partitioning

Microchannels (fast) e.g. tablet coatings

• cracks/pores in polymer film
• rate of movement depends on:
  
  • number of cracks
  • size of cracks
  • pathlength
  • shape of cracks

Diffusion

• molecules move from regions of high concentration to low concentration
• packaging needs to be effective

Question 16

What are the factors affect diffusion through polymers?

Answer 16

Molecular density of the polymer

• a more open molecular structure = more diffusion

Crystalline/amorphous ratio

• diffusion rate drops markedly as crystallinity increases

Similarity of diffusing molecule to polymer

• similar side chains = poly (vinyl alcohol) ; permeable to water vaopour not to organics
• similar polarity = LDPE (hydrocarbon); permeable to non-polar solvents
• strong interaction e.g. plasticisers = diffuse readily

Plasticisers

Fillers

Pigments

Opacifiers

UV absorbers

Solid particles in a polymer slow down transport by increasing the pathlength for diffusion

Question 17

Name the key pharmaceutical examples where polymeric materials were chosen for specific applications

Answer 17

Packaging plastics

Tablet film coating

Question 18

Name the physicochemical properties which make packaging plastics good for their use

Answer 18

Amorphous polymers e.g. Brown plastic tablet bottle

• Glassy - good transparency
• Hard but may be brittle below Tg
• Flexible and rubbery above Tg
• More permeable to water & organic vapour

Crystalline polymers e.g. standard white tablet container

• tough + stiff
• less permeable
• chemically more inert

Combining the two e.g. PVC/PVDC for tablet blister packs

• PVC layer - amorphous (see through)
• PVDC layer - crystalline (provides high degree of protection against water vapour)
  *

Question 19

Explain why the physicochemical properties which make tablet film coating good for their use

Answer 19

Ideal phase = transition (strong + tough)

Spray coat as liquid (spreads well

Rubbery flow/rubbery - sticky: so keep it moving, may need powder as a separating agent

Transition = strong and tough (ideal)

Glassy = too brittle (use plasticiser)

Coating = HPMC (hydroxymethylcellulose)

Question 20

Why don’t we coat tablets with gelatin?

Answer 20

Film frows strong crystalline regions

Coat shrinks as the polymer desifies

Coat put massive stress on core

Causes exploding tablets

Question 21

What polymer do we use to tablet coat?

Answer 21

HPMC (hydroxymethylcellulose)

Irregular polymer structure

Made out of sugar (glucose)

Amorphous

No crystal growth in film with time

Water penetrates + dissolves coat easily