Polymers Flashcards
4 polymeric features
- never in gaseous state
- 1st stage of dissolution is swelling
- higher viscosity than low mwt compounds of the same conc
- at certain conditions they can show elasticity and reversible deformations
What are the main differences between polymers and small molecules
1- aggregate state- small molecules exist in 3 states solid liquid and gas but polymers are composed of large macromolecules and so only exist in 2 aggregate states- solid and liquid - no gas - too large to evaporate
2- dissolution- small molecules dissolve quickly in water, polymers swell and form a gel then dissolve
3- viscosity - with polymers the liquid will flow slowly as it is viscous - more viscous than small molecules at the same conditions and conc
4- mechanical properties- if you apply force to small molecules they will break but with polymers they can experience reversible deformations so go back to original shape
How do we get from a small molecule to a polymer ?
Addition of molecules / repeat units together - increasing the mwt - turn into a polymer when they no longer change physical properties with each addition of a repeating unit - but below that specific mwt for a polymer they would still be an oligomer
Equation for molecular weight of polymer
Mwt = molecular weight of repeating unit X degree of polymerisation
Polydispersity index
Weight average and number average - can calculate the polydispersity index to show the mwt distribution in a sample
Mw / Mn
Different configurations a polymer (homopolymer) can take
Linear Dendrimer Ladder Star Branched Cross linked
What are the types of confirmations of co polymers?
Statistical (random)
Alternating co polymers
Block co polymers
Graft co polymers
2 types of synthetic polymers
Thermoplastic - eg polyethylene - material that’s plastic/deformable, melts into a liquid when heated and freezes to a brittle glassy state when cooled
Thermosetting plastics - phenolformaldehyde thermosets polymers that cure through addition of energy to a stronger form
What is the crystalline state?
Highly ordered array of molecules & atoms
Anisotropic properties eg depends on direction
Definite MP
What is the amorphous state ?
Randomly orientated molecules and atoms
Isotopic properties eg doesn’t depend on direction
No definite mp
(Some properties of it are typical of liquids but would flow slower)
After evaporation of water - what is left from-
Solution of Kcl which is an inorganic low molecular weight salt
Solution of chitosan polymer
Kcl low mwt - would have crystals - solid left after crystallisation which depends on the nature of the material
Chitosan polymer - films form !!
What are the degree of crystallinity ranges and what does it depend on
0% amorphous
<90% crystalline
Depends on chemical structure- temp, mwt , processing conditions
Can’t really get 100% crystallinity just degrees of it
What are the 3 physical states of amorphous polymers
Glassy state
Rubbery state (soft) -
Liquid state
Elongation occurs as transition to these states
What happens in the rubbery state to a polymer
Can stretch so undergo reversible deformations
Different polymers have different glass transition temps depending on structures - polyethylene has a tg of -120 C ; what does this tell us about the material ?
At around room temp it will be a soft material (rubbery state) because this temp is higher than the glass transition temp
Define Tg
Temp at which polymers will change from hard (glassy) to soft (rubbery)
PVC has a Tg of +87 C - what does this mean?
How can we modify it?
Means that it will probably be in the glassy state and this not soft or rubbery as the Tg is really high before it changes to soft
We could add small molecular weight additives so it can undergo plasticisation - reducing the tG and allowing it to become soft
How can we decrease Tg
Small molecular weight additives to decrease Tg and thus make it soft e.g plasticisers
Important consideration though when thinking of drugs - need to ensure that the drug doesn’t get plasticiser or equally the drug plasticise the polymer
Why are polymers elastic eg what is the mechanism of the elasticity ?
If we add stress - every macromolecule stretches - but they stay elongated due to the sliding effect- they slide over eachother and won’t come back to their original length (amorphous)
Above tG- polymers are?
Soft and elastic
Below Tg polymers are?
Hard and glassy
What are the 4 methods to synthesise polymer s
1- addition polymerisation
2- step growth polymerisation
3- ring opening pol
4- modification of polymers to form new polymers
What’s addition polymerisation?
Monomer with DOUBLE BONDS form macromolecules chain - double bond opens
Eg vinyl acetate - PVA
What is step growth polymerisation (poly condensation)
Monomers have at LEAST TWO FUNCTIONAL GROUPS - requires several combinations of polymers - need end groups of monomers that are reactive and then release the macromolecule and a small molecule eg h20
Used in bandages dressings, breast implants and contact lenses
What is ring opening polymerisation
Monomer is AN UNSTABLE RING - bond can break and form a polymer
Eg caprolactone —> polycaprolactone
This is a biodegradable polymer so good for drug delivery
What is modification of polymers
Take the polymer from nature and modify its properties eg
Deacetylation of chitin:
Chitin —> chitosan
Chitin is very insoluble, so treat with NaOH - acetate linkages disappear and form amino group - form chitosan which is very soluble in water under slightly acidic conditions and is an excipient in pharmaceutics
(Same with deacetylation of poly(vinylacetate)
What are 5 factors that affect the solubility of polymers
- polymer solvent interactions - chemical nature
- presence of cross links
- moleular weight
- temperature
And also groups on the polymer can affect solubility
How does temp affect polymer solubility
(phase diagrams);
For things like agar/gelatin - they are insouble at cold temperatures and are in two phases/immiscible. As you increase temp, they start to form 1 phase as miscible past the upper critical solution temp. If cool down, they start to form gel/precipitate as they go back into1 phase
Methylcellulose in water is more soluble in cold water. At high temps, 1phase/ immisible so will be a gel/precipitate. As you cool, starts to form 2 phases and forms a cloudy solution - past the lower critical solution temp.
How does polymer solvent interactions affect solubility of polymers
If the polymer and solvent interact well- good solubility - al to do with chemical similarity e.g polar polymers dissolve better in polar solvents and visa versa
How does presence of cross links affect solubility
Covalently cross linked polymers wont dissolve, water cannot get into the tight structure, crystalline materials and strong intramolecular interactions act as physical cross links
how does molecular weight affect solubility
Smaller macromolecules dissolve easier - less solvent needed to dissolve the molecule due to short c chain
How can the groups on the polymer dictate solubility
Hydrophilic groups are water soluble so polar - swell with water e.g charged molecules V soluble - OCH3, NO2, CHO, COOH, NH2, COO-, NH3+, OH
Whereas hydrophobic groups will reduce solubility e.g CH3, C6H5, Cl, F,Br, OCH2CH3, N(CH3)2
Name some synthetic water soluble non ionic polymers
PEG
Polyvinyl alcohol
Polacrylamide PAAM
Poly N- vinyl pyrrolidone PVP
Also block co polymers of PEG and PPG - pluronics (PPH is poly propyl so addition of a C)
Looking at the groups on cellulose e.g OH it should be soluble but why isnt it?
It is semi crystalline and has cross linking - strong intermolecular hydrogen bonding.
How would you increase solubility of cellulose?
Introduction of hydrophobic groups - this prevents crystallisation so makes it more soluble . formation of cellulose ethers such as methylcellulose (soluble in cold - LCST), hydroxyethylcellulose - soluble in all
Name some cellulose ethers and solubilities
Methylcellulose - only soluble in cold LCST
Hydroxyethylcellulose - soluble in any conditions - no gelation or LCST
Hydroxypropylmethylcellulose
SOdium carboxymethylcellulose - soluble, replaced the OH groups with charged carboxylic groups
What are polyelectrolytes and name the 3 common ones
Polymers which have ionisable functional groups (charged)
Cationic polybases - positive charge
Anionic polyacids-negative charge
Polyampholytes - both negative and positive charges
Give examples of anionic polyacids
Weak and strong based on ability to produce charged electolytes e.g
weak - poly(acrylic acid) / Poly(methacrylic acid)
Strong - Heparin charged groups - increase pH shifts to ionised
Examples of cationic polybases
Weak - chitosan - NH2 groups can get protonated to form NH3+ soluble in water
- Polyethylene imine - weak cationic
Strong - Poly[2-(methylacryloyloxy)ethyl] trimethyl ammonium chloride
GIve examples of polyampholytes
Gelatin, proteins, carboxymethyl chitosan - Have isoelectric points
Define IEP
pH at which the effective net charge on the macromolecules is zero = neutral. Polyampholytes precipitate at the IEP
Below IEP - the amino groups are charged - protonated groups NH3+ (positive)
Above IEP- the carboxyl groups are charged (negative) - COO-
