Degradation of Polymers

significance of understanding degradation

- can enhance/improve the device's properties (more cell proliferation/triggers rxn)
- mech properties can change at various stages (may not go as expected)
- success/failure of implant
- lifespan and recyclability
- shelf-life (storing response)
- sterilization

bulk erosion degradation mechanism

- characterized by breakage of chemical bonds in the polymer chain at the center of the material
- the result is an empty shell but maintains its size for a considerable amount of time
- hydrolytic mechanism used in tissue engineering
- chemical bonds break, size of material remains the same

surface erosion degradation mechanism

- characterized by loss of the material from the surface only, resulting in very predictable mass loss profiles
- materials get smaller but keep their original geometric shape
- more useful for drug delivery applications (more predictable)

oxidative degradation

- involves the disintegration of macromolecules by the action of oxygen on the substrate
- free radicals are formed which react with oxygen-producing oxy- and peroxy- radicals (these can react with each other or remove hydrogen from polymer chains)
- can occur from UV light or heat
- occurs with pharmaceuticals, polyethylene, PP, or PS
- changes material apperance

photooxidation

- when UV light is present with oxygen
- occurs at the surface of the polymers
- reduces the MW of the polymer: strength dec, workability inc
- since the mech properties and utility of the polymer are significantly reduced, this is due to secondary cross-linking and disruption of molecular chains which may lead to softening of the surface of the polymer (cross-linking may cause excessive embrittlement of the material)

thermal oxidation

- heat causes bonds to break
- an interaction with oxygen with the polymer breaks down bonds on the main macromolecule
- occurs when the thermal energy is higher than the energy required to break the bond
- takes place as a result of the interaction of oxygen with the polymer and high temps, can extend throughout the material

hydrolytic degradation

- opposite of condensation polymerization
- hydrolysis is the cleavage of bonds in functional groups by rxn with water
- occurs mainly in polymers that take up a lot of moisture and that have water-sensitive groups in the polymer backbone
- some synthetic polymers that degrade when exposed to moisture include polyesters, polyanhydrides, polyamides, polyethers, and polycarbonates
- hydrophilic polymers are more susceptible to hydrolysis compared to hydrophobic
- natural polymers: polysaccharides
- surface moisture absorption (permeability) and the rate of diffusion of the water within the polymer are the two most critical factors

hydrolytic degradation process steps

1. gradual diffusion of water solution into the polymer matrix (breaks bonds of polymer chain, geometry affects rate of diffusion)
2. dec in MW, porosity inc, strength dec, start to attack amorphous bonds
3. polymer matrix collapses and there is a dramatic dec in MW (crystallinity dec)

hydrolysis of semi-crystalline polymers

- first stage: degradation occurs by diffusion of water into amorphous regions with subsequent hydrolysis
- second stage: starts when moisture penetrates and degrades the crystalline regions
- a bimolecular weight distribution is observed at any time during these two stages
- the bulk erosion rate usually inc over time due to dec crystallinity and MW as well as inc water solubility which turns the polymer into a porous body with only minor dimensional changes
- surface erosion occurs at a constant rate and does not affect the MW but produces dimensional changes

affect of crystallinity on polyester hydrolytic degradation

- material properties known to influence polyester hydrolytic degradation are interrelated
- when present, crystallinity may be a primary contributor
- water diffusion is more restricted within the crystalline lamellae of semi-crystalline spherulites vs within the amorphous tie chain regions

polyesters- bulk or surface erosion?

- erosion number (E) is based on the comparative rates of bond degradation and water diffusion
- L is half the thickness of the material
- lambda is how fast the material will degrade (more reactance = higher lambda)
- Deff: constant of diffusion
- Na is avogadros #
- N is deg of polymerization
- p is polymer density
- if hydrolysis occurs more rapidly than water diffusion, surface erosion is expected to occur (E>1)
- if rate of water diffusion exceeds the rate of hydrolysis bulk erosion would occur (E<1)

how to control hydrolysis

- copolymerization (may be used to accelerate hydrolysis
- cross-linking: depending on what you use to cross-link
- surface area to volume ratio (a high ratio of exposed surface to volume can inc the rate of hydrolysis (alka seltzer)
- temperature
- carbonyl bonds (C=O) is the most hydrolytically susceptible among commonly used medical plastics

affect of pH of material's environment

- acid or base can act as a catalyst/greatly accelerate the degradation process
- the water uptake and oligomer/monomer water solubility is particularly important for polymers that degrade by bulk erosion
- these factors depend on the polarity of the polymer (low polarity tends to dec rxn rate which causes the hydrolytic stability to inc in the same order as hydrophobicity

rate of hydrolysis

- depends on hydrophilicity, type of func group, backbone structure, morphology (more crystallinity-longer to degrade), pH (more acidic=faster degradation), additives or impurities, geometry, manufacturing
- strong covalent bonds with no hydrolyzable groups take longer to degrade

enzymatic degradation

- may follow a surface erosion mechanism, especially for a highly crystalline and hydrophobic homopolymers
- due to their relatively large size, the enzyme molecules cant penetrate the tightly packed structure of certain polymers
- enzymatic catalysis occurs at the polymer-enzyme interface
- as the degrading surface becomes roughened or fragmented, enzymatic action may be enhanced as a result of inc SA
- typically only surface erosion
- depends on chemical comp, deg of homogeneity, processing technique
- coatings can affect surface erosion
- modeling can predict how degradation will occur over time

treatments for enzymatic degradation

- copolymerization
- cross-linking
- additives
- coatings

characterization techniques

- how you know how to alter a material
- infrared spectroscopy
- x-ray spectroscopy
- contact angle measurements (determimes surface roughness)
- SEM and AFM to look at surface roughness

bulk analysis of polymers

- weight loss: can get weight loss trend over time
- MW: gel permeation chromatography
- mechanical prop wrt time (compression or tension testing)
- crystallinity: dynamic scanning calorimetry

hydrolytic vs enzymatic degradation

- for some drug delivery applications: you want enzymatic
- bioresorbable screws: hydrolytic
- sutures: hydrolysis is used
- drug-delivery systems (liposomes): enzymatic degradation is used to provide controlled release