Ch 2 - Chemical Structure of Biomaterials Flashcards
Crystalline
- Periodic pattern of atoms (Long-Range Order)
* i.e. metals, ceramics, polymers
Amorphous
- Lacking systemic atomic arrangement (like liquid)
* i.e. ceramics, polymers
Structure of metals
- Non-directional metallic bonding
* Crystal structures (where atoms are located e.g. BCC/FCC, HCP)
Unit cell
Config. of atoms that is repeated in all 3 dimensions to form final material
Coordination number (CN)
nearest neighbor atoms
Atomic Packing Factor (APF)
APF (per unit cell) = V_atoms/V_total
• BCC = 0.68
• FCC/HCP = 0.74
Face-centered cubic (FCC)
- a = 2r*√(2)
- APF = 0.74
- CN = 12
Body-centered cubic (BCC)
- a= 4r/√(3)
- APF = 0.68
- CN = 8
- e.g. Ti β-phase = improved \ductility
Hexagonal-close packed (HCP)
- APF = 0.68
* e.g. Titanium α-phase
Ductility
Plastic deformation before fracture
Lattice structures
- Cartesian representation
- Defines unit cell by \lattice parameters e.g. lengths of edges (a,b,c) and angles b/w axes (α, β, γ)
- \lattice points = vertices of unit cell
Crystal system
Unique combinations of lattice parameters (a,b,c) and (α, β, γ) • BCC 1 . Cubic (3 same lengths) 2. Tetragonal (2 same length) 3. Orthorhombic (all diff lengths) 4. Rhombohedral (//) • HCP 1 . Hexagonal (2 same length) 2. Monoclinic (~rhombohedral) 3. Triclinic (no edges/angles equal)
Miller indices
Coordinate system to indicate location of points and orientation of planes (i.e. cubic crystals)
1 . Determine plane intersection of x, y, and z axes (if // to axis, "intercept" is ∞) 2. Reciprocal of intercepts 3. Clear fractions (LCD) 4. Record as "(h k l)" 5. Indicate any negative #s w/ bar over integer
Defects
- \point defects i.e. vacancies & self-interstitials
* \impurities i.e. solid solutions (alloys) & liquid solutions
Point defects
- Gen’lly occur b/c of thermodynamics of crystal growth
- Creation of defects is favorable b/c it ↑ entropy of system (thermodynamically favorable)
- e.g. \vacancies & \self-interstitials
Vacancy
Missing atom (expected at lattice site)
Self-interstitial
- Atom is crowded into interstitial space b/w 2 adjacent atoms
- Occupying what should be “empty” space
Why form crystalline structures?
Balancing thermodynamic need to form bonds (crystal) and creation of defects (↑ entropy, also ↑ \strain)
Lattice strain
- Strains in local lattice struc., caused by both vacancies and interstitials
- esp. interstit. defects in metals b/c of large atoms v. small space
Solid solution
[metals/ceramics]
• Normal crystal structure is maintained + addition of impurity atoms
• e.g. metal alloys (impurity atom improves prop’s of host material)
Weight % composition
Weight_elem/W_total
Atom % composition
Moles_elem/Moles_total
Liquid solution
[metals/ceramics]
• \solute (impurity) mixes in \solvent (host)
• e.g. \interstitial OR \substitutional solutions
* Ceramics: must not affect electroneutrality (solute ion must be similar in size/charge to solvent ion AND simultaneous diffusion for BOTH species)
Interstitial solution
[metals/ceramics]
• Impurities fill spaces BETWEEN solvent atoms
• Gen’lly when solute smaller than solvent (↓ lattice strain)
Substitutional solution
[metals/ceramics]
• Impurity/solute atoms REPLACE solvent atoms
• Gen’lly favored if \Hume-Rothery rules are satisfied
* Typ. for solute anions b/c too large for interstitial space
Hume-Rothery rules
- Diff. in size of atomic radii < 15% (min. lattice strain)
- EN are similar (same/bond lengths/strengths)
- Valence charges are similar (same bond lengths/strengths)
- Crystal structures are identical (only if large ~50% solute) → otherwise forms 2 interpenetrating crystals (complex)
Structure of ceramics
• Crystal structure (composed of ions, rather than atoms)
* Must be electrically neutral
• Optimal stability when cations have max # anions (and vice versa)
AX crystals
- Cation (A) and anion (X) have EQUAL charge
* Must have equal # of both to be stable ceramic
[A]_m*[X]_p crystals
• Cation (A) and anion (X) have DIFFERENT charges
* # m, p must balance charges to maintain electroneutrality
[A]_m[B]_n[X]p crystals
- 2 cations (A,B) and 1 anion (X)
* e.g. ZSCAP and FECAP ceramics
Carbon-based materials e.g. graphite
- Sometimes classified as ceramic (loose defin.)
- Crystalline structure, but no standard unit cell
- Ability to adsorb gases e.g. CV devices
Schottky defect
[ceramics]
• Vacancies of BOTH cation and anion (must balance ratio to maintain electroneutrality)
• Created based on same thermo. principles for ceramics as in metals (↑ entropy)
Frenkel defect
[ceramics]
• Vacancy/interstitial pair is created to maintain electroneutrality
• Typ. only w/ cations b/c anions are too large for interstitial spaces (→ lattice strain)
Macromolecules
- Scale: xE5-xE6 g/mol
- e.g. polymeric mat’ls
- Typ. CH covalent bonds = main constituent
Mer
\repeat unit (fixed # of atoms) of polymers
Monomer
1 mer
Oligomer
2-10 mers
Polymer
“many” mers
Saturated
∀ carbon in the mer has 4 other atoms
Unsaturated
- <=3 atoms per carbon, allows for double bonds
* May affect X-talinity and crosslinking
Bifunctional
\repeat units can bond w/ mers on BOTH ends (most polymers)
Trifunctional
\repeat units can bond w/ THREE (3) other mers → polymer network
Degree of polymerization
“n” = # repeat units in polymer
Number-average molecular weight
[ M_n = (ΣNM)/(ΣN) ]
• N = # chains of single MW
• M = avg. molec weight for chosen MW range
* Treats all polymer chains equally
Weight-average molecular weight
[ M_w = (ΣNM^2)/(ΣNM) ]
- Weight larger chains as larger contribution to final value
Polydispersity Index (PI)
[ PI = M_w/M_n ]
- Min. value = 1 (all polymers have same MW = \monodispersed; ideal for predicting prop’s)
- Shows MW distrib. (PI ↑ as MW broadens)
Conformation
- ROTATION of single (σ) bonds
• Impaired by bulky side groups
• Frozen by rigid C==C (double bonds)
Configuration (tacticity)
- BREAKING/REFORMING primary bonds
• \isotactic, \syndiotactic and \atactic config.’s
Isotactic configuration
R groups on same side of chain
Syndiotactic configuration
R groups on alternate sides of chain
Atactic configuration
R groups randomly distributed
Special case: repeat unit contains C==C bond
- \cis = constituents on same side
* \trans = constituents on opposite sides
Linear polymers
\repeat units joined end-to-end
Branched polymers
Synthesis conditions produce side reactions, which produce chains that branch off main polymer chain
Crosslinked polymers
• (“ladder”) adjacent chains joined at certain points via covalent bonds → 3D polymer network
• May be induced during synthesis or afterwards via nonreversible chemical rxn
• ↑ MW of polymer chains as they are bonded together
* ↓ X-talinity when crosslinked (b/c need movement)
Polymerization
- Synthesis of polymers through repeated chemical rxns, which join individual mer units into a chain
- \addition & \condensation
Addition polymerization
• “chain reaction” - bifunctional monomers required; product contains same chemical structure as mer unit
1 . \initiation - initiator species activates monomer (radical or ionic species)
2. \propagation - monomers successively join polymer; active site con’t transfer to new monomer
3. \termination - destruction of active site via rxn (of 2 propag. chains, free radical or ionic solvent)
Free radical polymerization
[addition polymerization]
1 . \initiation: free radical activates monomer
2. \propagation: monomers join polymer chain
3. \termination: free radical reacts w/ active carbon
Ionic polymerization
[addition polymerization] * Gen’lly less polydisperse
1 . \initiation: cat/an-ionic species activates monomer
2. \propagation: monomers join polymer chain
3. \termination: charged active site reacts w/ solvent/water or side reactions
Condensation polymerization
- “step reaction” involving mult. monomer species
- Occurs thru elim. of one molec. (typ. water) ∴ product does not have same chem. formula as either mer
- Need long reaction times and near depletion of monomer → high MW
- PI values similar to addition polym.
Polymer synthesis via genetic engineering
- Potential for greater control over polymer weight distrib./geom.
- Expression of gene within host (protein polymer of interest, PPOI) → isolated → introduced into host
Copolymers
- Mult. repeat unit types
- Formed by addition polym. or condensation polym., using a blend of monomer types as reactant species
- \random, \alternating, or \block
Homopoolymers
1 type of repeat unit
Random copolymers
2 mer units distrib. along chain w/o specif. pattern
Alternating copolymers
2 mer units alternate
Block copolymers
Each type of repeat unit is clustered (blocks)
Graft copolymer
Homopolymer chains attached as side chains to main homopolymer chain of different repeat unit
Polymeric crystal structures
• More complex unit cells and contain more atoms
• Depends on tactility and degree of branching (e.g. more branching/bulky side groups reduces X-talinity of polymeric material)
∴ most polymers are semicrystalline
Polymeric point defects
- Vacancies = spaces b/w chain ends
- Impurities may be intentional e.g. copolymers
- Less impact in polymers (than metals/ceramics)
Spectroscopy
- Excitation of electrons (absorption of energy)
* Measures how compounds differ in % absorp.
Chromatography
• Physical separation of molec’s based on chem char
• e.g. MW or charge
* Does not indic chem composition of mat’l
Mass spectrometry
• Determines atomic/molec mass of species in mat’l
1 . \ionization chamber (high energy particles)
2. \mass analyzer: Magnetic fieldm
3. Deflection based on mass (lighter = more deflection, only want target mass to hit \detector)
• Can control magnetic field to direct ions of specif. mass to detector
• Computer plot: relative intensity/absorption v. mass
* Highest molec ion ≈ MW of entire molec (all else = fragments of molec)
** App: chem compos of polymers and relative strength/stability of bonds
Size-exclusion chromatography (SEC)
- Column w/ beads w/ pores inside
- Small particles can enter the pores, whereas large particles will float around the pores
- Larger species elute first b/c they cannot be trapped; smaller particles = longer (\retention time)
- Pores in beads allow mobile phase to pass thru
- i.e. either gravity feed (large columns) or pump (small columns) → circulates \mobile phase
Stationary phase
= column + polymer beads (or small porous silica)
Mobile phase
= liquid solvent + dissolved sample
Retention time
• Time retained in porous structure, before elution
* Anything larger than pores cannot enter ∴ only smaller analyte can penetrate network
Gel filtration chromatography (GFC)
(polar) aqueous solvents + hydrophilic stationary beads
Gel permeation chromatography (GPC)
(nonpolar) organic solvents + hydrophobic stationary phase
High performance liquid chromatography (HPLC) SEC instrumentation
1 . Pump (circulation of mobile phase)
- Injector (of sample into mobile phase)
- Column (separates molec’s based on retention time)
- Detector/spectrophotometer (converts amount of analyte in mobile phase to electrical signal)
- Processor/computer (converts electrical signal to graph)
- Must match column w/ expected MW (start w/ broad range of beads, then more distinct beads)
- App: used to determ. MW of polymers (compared w/ \reference mat’ls to produce \standards of MW v. elution time)
