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CHAPTER
33Physical Propertiesof Biomaterials
3.1 Introduction: From Atomic Groupings to Bulk Materials
Metals and Ceramics: Polycrystalline materials (interactions of multiple crystals) Amount and type of dislocations
Polymers: Crystalline and amorphous regions (% Crystallinity)
Thermal transition of physical properties
3.2 Crystallinity and Linear Defects
point defects, linear defects, planar defects
3.2.1. Dislocations (1) Edge dislocations
half-planedislocation line
magnitude and direction of atomic displacement
atomic circuit drawing Burger’s vector
(2) Screw and mixed dislocations
Screw dislocation: shear force ---- helical pattern Mixed dislocation: Edge + Screw
(3) Characteristics of dislocations
a) localized lattice strains b) relationship between the Burger’s vector and the dislocation line c) invariant Burger’s vector d) termination of dislocation e) slipping of dislocations (slip planes)
3.2.2. Deformation
plastic (permanent) deformation [dislocation glide] dislocation glide: planes with higher atomic density slip and slip plane
dislocation’s geometry plane = crystallographic slip plane
slip system: crystallographic planes x # of slip directions high --- more deformable (ductile), low --- little deformation (brittle)
Ceramics limited movement
electroneutrality requirement longer Burger’s vector less slip --- brittle ceramics
3.3. Crystallinity and Planar Defects
planar defects: surface and grain boundaries
3.3.1. External surface
atoms at the surface --- no maximum coordination --- higher energy [surface tension]---- thermodynamic instability ---- chemical reaction at the surface
3.3.2. Grain boundaries
metals and ceramics: polycrystalline atoms at grain boundary --- no optimal coordination
---- higher energy ---- higher chemical reactivity
total interfacial energy: low in materials with larger grains
Two types of grain boundaries (1) small-angle grain boundary
tilt boundary (edge dislocations), twist boundary (screw dislocations) (2) high-angle grain boundary
severe misalignment [atomic mismatch ---- energy increase]
cf) twin boundary
3.4 Crystallinity and Volume Defects
volume defects: precipitates and voids voids (pores): 1) accidental formation, 2) creation with porogens and fibers
porogens: 1) solid porogens [salts, gelatin (collagen), waxy materials (lipids or paraffin)]
---- extraction --- pore formation extraction methods amount and shape --- porosity and pore geometry 2) gaseous porogens
N2, CO2 / liberation and bubbling amount, rate, timing of gas introduction --- porosity and pore geometries
fibers: fiber size and packing density --- porosity and pore geometry
advantages: 1) exchange of fluids and gases, 2) tissue ingrowth & implant anchoring 3) tissue engineering applications disadvantages: 1) decrease in mechanical strength,
2) altering biodegradation and corrosive properties
% porosity must be optimized
3.5 Crystallinity and Polymeric Materials
physical property of polymer ---- % crystallinity
3.5.1. % Crystallinity chemical structure of mer and polymer’s configuration
factors: 1) mer side groups 2) chain branching3) tacticity 4) regularity of mer placement
in copolymer side groups:
large and bulkybranched vs. linear
location of side groupstacticityblock copolymer
% crystallinity : density 비교
3.5.2. Chain-folded model of crystallinity
Basic unit of polymer crystalline structure: Lamella structure cf.) polymeric crystal’s unit cell
Real situation1) several polymer chains per each lamella2) single chain between lamella structure and interface 3) amorphous regions separating lamellae 4) intermingled chains
Spherulite formation three dim. radial arrangement of lamellae impingement upon growth
3.5.3. Defects in Polymer Crystals
(1) Linear defects (2) Planar and Volume defects
planar defects: boundaries between spherulites volume defects: void formation
3.6 Thermal Transition of Crystalline and Non-crystalline Materials
thermal transition of biomaterials ---- viscosity and material deformation
3.6.1. Viscous flow
crystalline materials --- plastic deformation non-crystalline materials --- viscous flow
rate of deformation & applied stress
viscosity: material’s ability to resist deformation (handle-ability)
water; caramel; glass
3.6.2. Thermal transition (1) Metals and crystalline ceramics
T > Tm: liquid and viscous flow T < Tm: solid --- crystal structure and grain boundaries 유지
(2) Amorphous ceramics (Glasses) T>Tm: liquid stateTm: temp with viscosity of 100 PTw: temp with glass viscosity of 104 P T<Tg: solid state (glass)
(3) Polymers liquid (rubbery solid) & glass Tm and Tg
Crystalline polymers T>Tm: random ordering of chains with no repeating structure
[translational motions] Tm>T: highly ordered crystals
secondary bonds and Tm1) degree of branching --- Tm 감소 2) molecular weight --- Tm 증가
Amorphous polymers T>Tg: rubbery elastic materialsT<Tg: glassy and brittle polymer [Tg<Tm] --- 1.4 < Tm/Tg < 2.0 for polymer
chain vibration and rotation1) chain flexibility2) chemical constituents [bulky side groups, polar groups, high mol. wt., X-linking]
Polymers to be crystallizable Tg< Tc <Tmtemp increase --- polymer chains with energy
--- highly ordered crystalline state [exothermic process] --- disruption of the crystal structure
polymer annealing
degree of crystallinity
3.7 Techniques: Introduction to Thermal Analysis
Temp analysis; measurement of the physical properties of a material as a function of temperature
TGA (thermogravimetric analysis) DMA (dynamic mechanical analysis) DSC (differential scanning calorimetry)
3.7.1. Differential Scanning Calorimetry (1) Basic principles
power-compensated DSC heat-flux DSC
(2) Instrumentation
furnace/DSC sensors/ processor
(3) Information provided
Tg: heat capacity Tm: peak temp% crystallinity