Lecture 19 - Polymers (A) Flashcards
Polymer Structure
- Can be amorphous, crystalline, or semi-crystalline
- Comprised of repeat units called mers
Monomer
Molecule containing only one mer
Oligomer
Molecule containing 2-10 mers
Repeat Units
Can be saturated:
- Each carbon in mer bonded to 4 other atoms
Can be unsaturated:
- Two of carbons in mer bonded with double bonds
Degree of Polymerization (n)
- Length of polymer chain
- Ex: [PP]14 where n=14
Number-Average Molecular Weight (M_n)
- M_n = [sum]_i (x_i * M_i)
- x_i = N_i / ([sum]_i N_i)
- N_i = number of chains with the molecular weight M_i
- x_i = number fraction of molecular weight
- M_i = an average molecular weight for the chosen molecular weight range
Weight-Average Molecular Weight (M_w)
- M_w = [sum]_i (w_i * M_i)
- w_i = W_i / ([sum]_i W_i)
- W_i = number of chains with the molecular weight W_i (also = N_i * W_i)
- w_i = mass fraction of molecular weight
- M_i = an average molecular weight for the chosen molecular weight range
Polydispersity
- M_n treats all polymer chains equally
- M_w gives more emphasis to larger chains
- PI = M_w / M_n
Size Exclusion Chromatography
Measures molecular weight and PI
Mer Conformation
- Each macromolecule can take on a wide variety of different shapes
- C-C-C bonds in backbone are not strictly linear
- C can rotate along 109.5 bond allowing polymer to bend and fold back upon itself
- Chemistry of backbone and size of side groups affects conformation
Isotactic
All side groups evenly spaced on the same side of polymer backbone
Syndiotactic
Side groups uniformly spaced on either side of polymer backbone
Atactic
Side groups arranged in random manner
Cis
Side groups on same side of C/C double bond
Trans
Side groups on opposite sides of C/C double bond
Linear
Bifunctional mer
Branched
Trifunctional mer
Crystallinity
- Key physical properties of polymers
- Percent Crystallinity dependent on chemical structure of mer and polymer configuration
Side Groups
Size:
- If large and bulky, prevents neighboring chains from coming close enough to form crystalline structure
Location:
- Prevents neighboring chains from folding in ordered fashion
Percent Crystallinity
- Crystalline regions more dense (up to 20%)
- Calculate density of sample (ps)
- Known density in fully amorphous (pa)
- Known density in almost fully crystalline (pc)
- % crystallinity = [pc * (ps-pa) * 100] / [ps * (pc-pa)]
Lamella
- Basic unit of crystal structure
- Larger than unit cell
- Polymer chains folded back on themselves
- Highly idealized
Spherulite
- Crystallization from melt
- 3D aggregates of lamella
Examples of Biomedical Spherulites
- Polycaprolactone (PCL)
- Polylactic Acid (PLA)
Functionality
- Bifunctional —> may bond with mers on both ends
- Trifunctional —> possess three active bonds that can bind to other mers
