Degradation and Characterization of Polymers Flashcards
What does degradation effect? (5)
- enhance device properties - cell proliferation; trigger a reaction
- mechanical properties can change at various stages
- success/failure of implant
- lifespan and recyclability
- sterilization
What are the two degradation mechanisms?
- bulk erosion - breakage of chemical bonds in the polymer chain at the center of the material; results in empty shell that maintains its size for considerable amount of time
- surface erosion - loss of material from surface only; results in predictable mass loss profiles; materials get smaller but keep original geometric shape, more useful for drug delivery!
What are the three types of degradation?
- oxidative
- enzymatic
- hydrolytic
Oxidative degradation involves the ______________ of _______________ by the action of oxygen on the substrate.
disintegration of macromolecules
oxy- and peroxy- radicals are formed when ____________. They can react with __________ or _______ hydrogen from polymer chains.
free radicals react with oxygen;
they can react with each other or remove hydrogen from polymer chains
Oxidative degradation occurs from __________ or _______.
UV light or heat.
Oxidative degradation occurs with _________ (4)
- pharmaceuticals
- polyethylene
- polypropylene
- polystyrene
Photo-oxidation
when UV light is present with oxygen
Where does photo-oxidation occur?
at the surface of polymers
What does photo-oxidation cause and how?
causes: reduced molecular weight of the polymer = decreased strength
how: secondary cross-linking and disruption of molecular chains, which can lead to softening of the polymer surface; cross-linking can cause embrittlement of the material.
What is hydrolysis?
cleavage of bonds in functional groups by reaction with water
What type of polymers does hydrolysis usually occur? Examples include __________.
polymers that take up a lot of moisture (hygroscopic) and have water-sensitive groups in the polymer backbone (hydrophilic)
Ex. polyesters, polyanhydrides, polyamides, polyether, and polycarbonates
What are two critical factors of polymers when considering hydrolytic degradation? And what mechanism depend on these factors as well?
- surface moisture absorption (permeability)
- rate of diffusion of water within the polymer
* bulk vs. surface erosion is dependent on these two factors as well.
First stage of hydrolytic degradation. What are the little and big changes that occur in the polymer during this phase?
most significant -> gradual diffusion of water solution into the polymer matrix -> leads to increased uptake of water.
- little changes occur in the sample mass, the molecular weight and tensile strength.
- main changes related to surface = increased roughening, free energy, chemical changes
Second stage of hydrolytic degradation. What is decreased at this phase?
decrease in the polymer molecular weight = result of chain scission and by diffusion of large molecular weight fragments to the solution -> leads to increased weight loss and porosity of the matrix and loss of tensile strength.
Third stage of hydrolytic degradation. What is the results of the polymer structure and properties?
advanced stage = collapse of polymer matrix and dramatic decrease in molecular weight and an increase in weight loss -> crystallinity disappears and marked decline in mechanical properties.
Describe the two stages of hydrolysis for semi crystalline polymers. Include examples of semi crystalline polymers.
Ex. esters, amides, anhydrides
1. degradation = diffusion of water into amorphous region with subsequent hydrolysis
2. moisture penetrates and degrades the crystalline regions
The bulk erosion rate for semi crystalline polymers _________ over time due to __________ crystallinity and MW and increased ___________________ which turns the polymer into a _________ body with ________ dimensional changes.
bulk erosion rate INCREASES over time due to DECREASED crystallinity and MW and increased WATER SOLUBILITY which turns the polymer into a POROUS body with MINOR dimensional changes.
The surface erosion occurs _________________ and does _____ affect the MW, but produces ____________ changes.
The surface erosion occurs at a CONSTANT EROSION RATE and does NOT affect the MW, but produces DIMENSIONAL changes.
Water diffusion is more _________ in the crystalline lamellae of semi crystalline spherulites versus in the amorphous tie chain regions.
restricted in crystalline lamellae
Material properties known to influence polyester hydrolytic degradation are ______________. What is the primary contributer?
interrelated; crystallinity
List the 4 ways to control hydrolysis:
- copolymerization -> used to accelerate hydrolysis
- cross-linking
- surface area to volume ratio -> high ratio can increase rate of hydrolysis
- temperature
What is the most hydrolytically susceptible bond? And what is it usually attached to?
carbonyl bonds
- attached to oxygen or nitrogen = bonds that could be esters, amides, carbonates, and urethanes
Rank the carbonyl polymers that display the highest hydrolysis rates (3)
- anhydrides
- esters
- carbonates
