Polymers Flashcards
What is a crystal?
Anisotropic, homogenous body consisting of a 3D periodic ordering of atoms, ions or molecules
What is the difference between an amorphous and a crystalline polymer?
What factors influence polymer crystalinity?
Molecular structure
Crystalline state is lowest energy
Speed + temp. of solidification
- when a melted polymer cools or a solution dries
- rapidly = amorphous
- slowly = crystalline
How do polymer crystal form?
What is nucleation?
Tiny crystals form at points of molecular contact
Crystalline regions grow outwards from nucleation sites - as polymer chains rearrange
Polyethylene can be either amorphous or crystalline.
Name which types of polyethylene are under.
High density = up to 90% crystalline
Low density = amorphous
Semicrystalline both have amorphous domains + crystalline domains
Fill in the gaps
Draw a typical viscoelastic diagram, identifying the 5 regions of viscoelastic behaviour of an amorphous or semicrystalline polymer
Tg = 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
What are plasticisers?
Small molecules that dissolve in the polymer and interact with it
Describe the polymer properties in the 5 regions.
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
Discuss speed of deformation
What is the effect of crosslinking on polymer?
Refer to the viscoelastic diagram
Except the polymer only reaches rubber solid/gel as polymer chains cannot flow
How does crystallinity affect the viscoelastic properties of a polymer?
What is the melting point of a polymer?
It is when the polymer goes from crystalline to amorphous
Define the mechanisms of transport
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
What are the factors affect diffusion through polymers?
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
Name the key pharmaceutical examples where polymeric materials were chosen for specific applications
Packaging plastics
Tablet film coating
Name the physicochemical properties which make packaging plastics good for their use
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)
*
Explain why the physicochemical properties which make tablet film coating good for their use
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)
Why don’t we coat tablets with gelatin?
Film frows strong crystalline regions
Coat shrinks as the polymer desifies
Coat put massive stress on core
Causes exploding tablets
What polymer do we use to tablet coat?
HPMC (hydroxymethylcellulose)
Irregular polymer structure
Made out of sugar (glucose)
Amorphous
No crystal growth in film with time
Water penetrates + dissolves coat easily
