Lecture 8 Flashcards
What is a polymer?
Substances made of large molecules (macromolecules) composed of many repeated subunits (monomers).
Characteristic features of a macromolecule
- Chemical composition of the monomers
- Size of the macromolecule (number of monomers)
- Microstructure of the macromolecule (composition, architecture, tacticity)
- Macromolecular functionality
- Structure/Conformation of the macromolecule
Degree of polymerization (DP):
the number N of repeating units. Usually 10^3 - 6
The size of a polymer
Bulk polymers (esp. synthetic) are often composed of a distribution of macromolecules of different lengths.
Dispersity in chain size
# average mol. weight, weight average mol. weight. Disparity index = Mw/Mn
Trough the motion of its segments ___, a macromolecule can adopt different shapes ___
(monomers), (conformations)
The number and nature of macromolecule conformations are governed by __
interactions between monomers.
Biological tissues are composed of polymers, ex:
ECM: proteins (collagen, fibronectin, laminin)
pollsaccharides and proteoglycans
Macromolecules of biological tissues, proteins:
amino acids
The biological, chemical, physical activity of a macromolecule depends on its
conformation
Glycoproteins
Protein chain + grafted oligosaccharide (glycans) chains. Ex: fibronectin
Proteoglycans
Protein chain densely grafted by long linear glucose chains (glycosaminoglycans: GAG)
One macromolecule in a melt or solution have an extremely ___. Its shape is incessantly changing, exploring many possible configurations.
large number of degrees of freedom.
The persistance length Lp
If chain length = Nb < Lp it will be stiff. If chain length = Nb»_space; Lp it will be flexible
Nb = # of monomers
In the absence of long range interactions between monomers
long polymer chains (L»lP) have a “random coil” conformation.
How to describe a long flexible chain ?
L»lP
Mean end-to-end distance (difficult), so radius of gyration
radius of gyration
The root mean square distance of chain segments from
the chain center of mass.
Probability distribution of obeys
Gaussian statistics
Validity of the ideal chain model
- In real chains, monomers are not freely articulated.
2. Two monomers cannot be on a same position
For chains much longer than the persistence length,
the characteristic chain size in a melt or solution is much smaller than the chain length.
Many biological macromolecules are not ideal chains because
their conformation is governed by interactions
between segments far from each other
Stiffness of the spring
- increases linearily with T
- decreases with increasing chain length
Elastomers
Networks of cross-linked polymer melts.
Soft solids with an elastic behavior up to very large deformations (several times their size).
What is the effect of solvent on a cross-linked macromolecular network ?
Swelling: polymer chains of the network diffuse into the solvent to allow solvatation and mixing. Swelling reaches a plateau.
Swelling of polymer gels
- a decrease in mixing energy
* an increase in stored elastic energy
At high enough temperature, a bulk polymer material behaves like
very viscous liquid with a peculiar elasticity
Polymer melts: Entanglements. Above a ___
Above a critical chain length, macromolecules are entangled.
Ne : Number of monomer units between entanglements
N < Ne is not entangled.
N > Ne is entangled.
N < Ne
A chain “swims” in a viscous medium formed by the other chains. (Rouse model)
N > Ne
Disentanglement by diffusing in a “tube” (Reptation model). Much slower process than Rouse regime.
In entangled polymer melts, entanglements have a characteristic
lifetime te (te depends on T)
For t
the nodes of a temporary network
Characteristics of elastomers
- Macromolecular chains are permanently (covalently) attached.
- The cross-linked network behaves like a very soft elastic solid.
- Can withstand large deformations (x100%)
Below a certain temperature (Tg), thermal agitation does not provide enough energy for
macromolecular rearrangements and equilibration. Transition from a liquid to a glass like behavior.
Crystallization and semi-crystalline polymers. Before reaching Tg and at a critical temperature Tc
some macromolecules can crystallize (partially)
Crystals are ___ than amorphous regions.
denser
Crystals are much _____ than amorphous regions E ~ 100GPa (along chain axis)!
stiffer
Crystalline regions act as __ and ___
cross-links and stiffening fillers.
Features of semi-crystalline polymers
• high elastic modulus (~0.1-10GPa)
• often ductile (plasticity mechanisms in the crystals).
• often opaque (refraction index fluctuations in the semi-crystalline structure).
• better resistance to solvent than amorphous polymers (thanks to higher density of crystals)
• better thermal resistance than amorphous polymers (below melting T of crystals)
- Example: nylon, polyethylene
Polymer behavior depends strongly on ___ and ___
time and temperature
Terminal regime: chains can disentangle
(1/w > te).
The melt behaves like a very viscous liquid.
G”
viscous modulus
G’
elastic modulus
Elastic or Rubbery plateau: chains cannot disentangle
(1/w < te). the melt behaves like a soft elastic solid.
In many medical applications, great interest for biomaterials with limited in vivo residence time.
- Surgical aids
- Implants
- Drug delivery devices
- Scaffolds for tissue engineering
- Biomarkers and contrast agents
How do bioabsorbed materials exit the body?
Exhalation through lungs, addition to body as biomass, Excretion through kidneys
Bioresorbability
is the ability to be eliminated in vivo through natural pathways either because of simple filtration of degradation by-products or after their metabolization.
Design approaches to bioresorbable implants
- Using dissolution in body fluids (bioabsorbability)
- Using chemical degradation (corrosion, hydrolysis…)
- Using cell-mediated degradation (biodegradability)
Methods of breaking down bioresporbable materials
dissolution, chemical degradation, biodegradation
Challenges in the design of bioresorbable devices
- Resorption kinetics compatible with the application
- Mechanisms and by-products non-toxic
- Compatible with sterilization
Strategies to tune biodegradability
- Change the quantity or surface/volume ratio
- Add/Remove defects
- Change the density in degradable functions
- Change the accessibility to degradable functions
A paradigm for tissue engineering
Designing artificial matrices (scaffolds) with the appropriate bioactivity and biodegradability. [Ex: Nerve regeneration in collagen tubes, Decreasing the degradation speed of the collagen scaffold
(by increasing cross-linking)]
