Ch 3 - Physical Properties of Biomaterials Flashcards
Phys prop’s of metals/ceramics
- Determined by interxns of mult. CRYSTALS
* i.e. amount & type of dislocations (within or between crystals)
Phys prop’s of polymers
- Determined by interxns of MERS to create crystalline/amorphous regions
- i.e. % X-talinity of polymer (impacts mech & degrad. prop’s of final material)
Dislocations
- Cause localized lattice strains
* \slip plane (high atomic density) contains both \Burger’s vector and \dislocation line
Linear defects: Edge dislocations
- Linear (1D) defect [metals/ceramics]
- Extra half-plane of atoms terminates in a crystal
- May occur due to improper crystal growth, internal stresses from other defects, or interxn of resident dislocations during plastic deformation
- BV ⊥ \dislocation line
Dislocation line
⊥ line which defines end of extra half-plane
Burger’s vector
Magnitude and direction of the lattice distortion resulting from a dislocation in a crystal lattice
Linear defects: Screw dislocations
- Result of shear forces (repeat symbol) only on part of material
- Helical pattern (BV // dislocation line)
Mixed dislocation
= edge + screw disloc. qualities
dislocation line neither ⊥ nor // to BV
Dislocation glide
- Plastic (permanent) deformation due to con’t single atomic movement of dislocations (caterpillar analogy)
- Lattice strain = thermodynamic driving force for movement of linear defects
- e.g. if shear stress τ is applied to a crystal → dislocation glide until edge dislocation exits the crystal → plastic deformation
- Occurs more easily on planes w/ smaller steps or “higher atomic density” (= \slip plane)
Slip
- Plastic deformation in slip plane
- Occurs only if dislocation’s geometric plane coincides w/ crystallographic \slip plane (plane w/ highest atomic density)
- Must achieve \critical resolved shear stress to initiate
Slip plane
Contains:
• Burger’s vec + dislocation line (oriented to absorb some of force imparted)
• High atomic density (= smaller steps for dislocation glide)
Slip system
• Crystallographic plane (through which slip can occur)
• # of directions slip can take place along plane (higher # = more ductile/defor.)
* Similar for metals/ceramics, but ceramics must maintain electroneutrality in slip ∴ BV longer and more brittle → less slip systems (less plastic defor.)
Planar defects: Surface tension
• Atoms on surf. ≠ bonded to max. possible # of nearest neighbors (valence is not filled)
∴ higher energy = surface free energy per unit area
• Thermo. unstable → try to min. surface energy → driving force for chem rxns w/ proteins/water
Planar defects: Grain boundaries
• Interface b/w grains/spherulites (crystals)
∴ not bonded to max # neighbors (≠ optimal CN) → extra energy → chemical reactivity
• e.g. metals: corrosive attack starts at GB (↑ GB/vol = stronger, but more susceptible to corrosion)
• Total interfacial energy ↓ in mat’l w/ grain size ↑ b/c fewer GB areas
Volume defects: Precipitates
- Long-range order (LRO) is lost
* Clusters of substitutional or interstitial impurities
Volume defects: Voids
- Accidental 3D aggregates/clustering of vacancies (point defects)
- Control of voids aka "pores” can alter biological response to biomaterial
Porogens
• Solid: solid when mixed, but can be extracted w/ solvent
• Gaseous: bubble gas through polymer while cooling (nothing to extract)
* Amt of porogen affects \porosity and shape affects geometry of pores
Pores
• 3D aggregates/clustering of vacancies (point defects)
• Allow for exchange of fluids/gases, encouraging tissue ingrowth & implant anchoring
• But also ↓ mech. prop’s and alters biodegradation/corrosion prop’s of implant
* Reduce CSA ∴ E_YM ↓ and σ_y, strength ↓, stress raiser ↑
% crystallinity (X-talinity)
- Fraction of crystalline areas (key for polymers!)
- Denser than amorphous b/c chains closely packed
- Reduced by:
- Bulky side groups (prevent packing)
- Chain branching (prevents alignment)
- Atacticity (prevents packing)
- Random copolymers
Calculate: % crystallinity
% crystallinity = ρ_c(ρ_s−ρ_a )/(ρ_s (ρ_c−ρ_a ) )∗100
Chain-folded model
- \lamella = basic unit of crystalline struc. (several polymer chains folded within itself)
- Crystalline regions = inside lamellae, separ. by amorphous regions (chain folds)
- \spherulites = 3D spherical aggregates of lamellae
- \tie molec’s bind lamellae together
Defects in polymeric crystals
• (linear) covalent bonds within polymer chains are longer than 2 ° forces b/w chains ∴ slip typ. ALONG axis of polymer chains
• Planar/volume
- Surfaces also have additional energy
- Boundaries b/w spherulites similar to GB b/w metals/ceramics
- Porogens can be added to form voids (volume defects)
Thermal transition temps
• [metals/crystalline ceramics] \melting point
• [amorphous ceramics} \glass transition temp
* Polymers have both
Melting point
[metals/crystalline ceramics]
* “T_m” temp. above which atomic mvmt large enough to break highly-ordered structure
• Above: liquid, viscous
• Below: solid w/ crystal structure & GB
- Amorphous ceramics become INCREASINGLY viscous w/ decr. temp, until solid (do not instantly solidify at T_m)
- Factors which influence 2 ° bonds also impact T_m
Glass transition temperature
[amorphous ceramics/polymers]
“T_g” temp. below which mat’l is glassy/solid
• Above: enough atoms/chains vibrating w/ sufficient energy to result in translational motion → overcomes 2 ° bonds (rubbery polymer)
• Below: polymer is brittle (no plas. defor. ∴ no mvmt around polymer backbone)
- Semicrystalline polymers can have both T_m and T_g (both crystalline and amorphous prop’s)
Factors which impact T_m
[same factors that influence 2 ° bonds]
• ↑ branching, ↓ packing, ↓ VDW ∴ ↓ T_m
• ↑ MW → less polymer chain ends → more energy to produce chain motion ∴ ↑T_m (but also ↓ X-talinity)
Factors which impact T_g
- ↑ chain flexibility → molec. motion → less energy necessary to achieve mvmt ∴ ↓ T_g
• Bulky side groups reduce mvmt ∴ ↑ T_g
• Polar side groups promote chain interxn ∴ ↑ T_g
• ↑ MW ∴ ↑ T_g
• Crosslinking reduces motion ∴ ↑ T_g
Crystallization temperature
- “T_c” = temp of crystallization (just above T_g, enough to move to highly-ordered state)
- \cold crystallization (T_g < T_c < T_m) ↑ chain alignment, ↑ X-talinity
Degree of crystallinity
X(t) = 1 - e^(-k(t^n))
Differential Scanning Calorimetry (DSC)
- Provides info. on T_g/T_m, % X-talinity of polymeric mat.
- NOT about chemical prop’s of polymer
- Difference in heat flow (sample v. ref.) recorded as func. of temp. while they’re exposed to controlled temp. ramp
1 . Furnace (heats sample/ref)
- DSC sensors (record power)
- Processor (controls ramp temp.
* Labeled endothermic up: bump = T_g, min = T_c, and max = T_m
Power-compensated DSC
• Sample & ref. cells heated by indiv. heaters until temp. diff. ≈ 0
• Meas. power necessary to maintain equal temp’s
• Plot: Heat flow (Power/mass) v. temp.
* \endothermic (req energy/power to maintain temp) v. \exothermic (releases energy)
