Ch 3 - Physical Properties of Biomaterials

Question 1

Phys prop’s of metals/ceramics

Answer 1

• Determined by interxns of mult. CRYSTALS

* i.e. amount & type of dislocations (within or between crystals)

Question 2

Phys prop’s of polymers

Answer 2

• Determined by interxns of MERS to create crystalline/amorphous regions
• i.e. % X-talinity of polymer (impacts mech & degrad. prop’s of final material)

Question 3

Dislocations

Answer 3

• Cause localized lattice strains

* \slip plane (high atomic density) contains both \Burger’s vector and \dislocation line

Question 4

Linear defects: Edge dislocations

Answer 4

• 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

Question 5

Dislocation line

Answer 5

⊥ line which defines end of extra half-plane

Question 6

Burger’s vector

Answer 6

Magnitude and direction of the lattice distortion resulting from a dislocation in a crystal lattice

Question 7

Linear defects: Screw dislocations

Answer 7

• Result of shear forces (repeat symbol) only on part of material
• Helical pattern (BV // dislocation line)

Question 8

Mixed dislocation

Answer 8

= edge + screw disloc. qualities

dislocation line neither ⊥ nor // to BV

Question 9

Dislocation glide

Answer 9

• 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)

Question 10

Slip

Answer 10

• 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

Question 11

Slip plane

Answer 11

Contains:
• Burger’s vec + dislocation line (oriented to absorb some of force imparted)
• High atomic density (= smaller steps for dislocation glide)

Question 12

Slip system

Answer 12

• 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.)

Question 13

Planar defects: Surface tension

Answer 13

• 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

Question 14

Planar defects: Grain boundaries

Answer 14

• 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

Question 15

Volume defects: Precipitates

Answer 15

• Long-range order (LRO) is lost

* Clusters of substitutional or interstitial impurities

Question 16

Volume defects: Voids

Answer 16

• Accidental 3D aggregates/clustering of vacancies (point defects)
• Control of voids aka "pores” can alter biological response to biomaterial

Question 17

Porogens

Answer 17

• 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

Question 18

Pores

Answer 18

• 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 ↑

Question 19

% crystallinity (X-talinity)

Answer 19

• 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

Question 20

Calculate: % crystallinity

Answer 20

% crystallinity = ρ_c(ρ_s−ρ_a )/(ρ_s (ρ_c−ρ_a ) )∗100

Question 21

Chain-folded model

Answer 21

• \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

Question 22

Defects in polymeric crystals

Answer 22

• (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)

Question 23

Thermal transition temps

Answer 23

• [metals/crystalline ceramics] \melting point
• [amorphous ceramics} \glass transition temp
* Polymers have both

Question 24

Melting point

Answer 24

[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

Question 25

Glass transition temperature

Answer 25

[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)

Question 26

Factors which impact T_m

Answer 26

[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)

Question 27

Factors which impact T_g

Answer 27

• ↑ 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

Question 28

Crystallization temperature

Answer 28

• “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

Question 29

Degree of crystallinity

Answer 29

X(t) = 1 - e^(-k(t^n))

Question 30

Differential Scanning Calorimetry (DSC)

Answer 30

• 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)

2. DSC sensors (record power)
3. Processor (controls ramp temp.
   * Labeled endothermic up: bump = T_g, min = T_c, and max = T_m

Question 31

Power-compensated DSC

Answer 31

• 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)