Test 1 Flashcards
why is it easier to heal bone than cartilage
bone has a higher metabolism than cartilage (it has a higher turnover rate)
ceramics bond type?
ionic bonds
why are ceramics used for osseointegration
they have calcium phosphate in them which allows it to integrate into the bone
metal bond type?
metallic bonds
metallic bond characteristics?
highly mobile electrons
hydrogel characteristics
swell many times when put in water but won’t dissolve
why don’t hydrogels dissolve when put in water
crosslinked
physical hydrogels vs chemical hydrogels
physical –> held together with entanglements
chemical –> held together with covalent bonds
FDA doesn’t approve __
it doesn’t approve materials, it approves devices
synthetic vs. natural debate
- natural polymers have similar chemical properties to the tissues they are replacing
- natural are more easily integrated
- natural has lower mech. properties
- difficult to remove pathogens from chemical
- sythetic do not interact in an active manner
- synthetic can be specifically tailored
distance of surface properties
1nm
surface properties dictate __
biological response; where proteins will attach
anisotropy meaning
different mechanical properties in different directions
chemical vs. physical surface properties
chemical –> hydrophobicity
physical –> roughness
bonding in metal is __
nondirectional
coordination number
number of nearest neighboring atoms
FCC
APF = .74
4 atoms in each unit
BCC
APF = 0.68
2 atoms in each cell
HPC
APF .70
in Titanium, which is the more ductile crystal structure
BCC is more ductile than HCP
describe process for miller indicies
- determine where it crosses the xyz axis
- tape reciprocal of intercepts
- multiple by integers to clear fractions
- write integers in parentheses, no commas (h k l)
dimension of point defects
zero dimensional
examples of point defects
vacancies and interstitial
both vacancies and interstitial cause __
lattice strain
vacancies and interstitial are caused by __
entropy during formation
types of impurities
interstitial and sustitutional
hume-Rothery rules for substitutional solids
- want size of replacement atom to be same as original
- electronegativities are similar
- valence charge are similar
- same crystal structure
types of solid state diffusion
vacancy diffusion and interstitial diffusion
crystal structure of ceramics
AmXp crystal structure
why do individual point defects not occur in ceramics
it would affect the electoneutrality of the material
two types of defect in ceramics
schottky defects -> vacancies in both cation and anions
frenkel defects -> vacancy/interstitial pair
why do frenkel defects only occur with cations
frenkel defects –> vacancy/ interstitial pair
anions are too big to reside in interstitial space
_ materials have the longest burgers vectors
ceramics
what is degree of polymerization
number of repeat units in a polymer
configuration vs. conformation
configuration –> break bonds
conformation –> bond rotation
is configuration or conformation harder to change
configuration is harder to change
isotactic syntactic atactic
isotactic –> R groups on the same side
syntactic –> R groups on alternating sides
atactic –> R groups are random
crosslinking __ crystallinity
crosslinking decreasing crystallinity
it prevents the alignment fo chains so it is harder to form lamellae
different types of polymer synthesis methods
Addition polymerization and condensation polymerization
addition polymerization?
initiation -> activation of monomer using radical
propagation -> monomers join the polymer chain
termination -> end of polymerization
condensation polymerization
polymerization occurs through the elimination of water
molecular reasoning for decreasing crystallinity
anything that prevents chain alignment will prevent crystallinity
things the influence crystallinity
SBTR 1. Side groups 2. Branching 3. tacticity 4.Reguality of copolymers ...... more randomness leads to less crystallinity
Longer polymer chains have __
higher mechanical properties
__ is more indicative of its final properties than __
Mw than Mn
Polydispersity Index
Mw/Mn
smallest possible value is 1
what is IR spectroscopy used for
to determine what kind of bonds are in a material
As wavelength __ energy __
as wavelength increases, energy decreases
IR radiation interacts with what kinds of bond
dipoles
how does IR radiation work
IR radiation hits the bonds, which increases their amplitude (not frequency) which results in absorption of IR at that frequency
Fourier transform IR spectroscopy
allow the sample to be bombarded with many wavelengths simultaneously; allows for more scans in a small time
How does liquid chromotography work
separation by size
gel permeation chromatography
mobile phase is non-polar/organic.
Stationary phase is hydrophobic
what are the one-dimensional defects
Edge, screw, mixed
burger’s vector in edge dislocations
burgers vector is perpendicular to the dislocation line
__ is the driving force for deformation
lattice strain
burger’s vector in screw dislocation
burger vector is parallel to dislocation line
what is the defining characteristic of mixed dislocations
the burger’s vector is neither perpendicular or parallel to the dislocation line
__ develops as a result of dislocations
lattice strain
__ does not change for a given defect
burger’s vector
a dislocation must end on__
on the crystal surface, on itself, or on another dislocation
plastic deformation occurs due to __
dislocation glide
dislocation allow __ deformation
plastic deformation
how does dislocation glide work
a dislocation can glide one atomic spacing at a time until it exists the crystal by forming and breaking one bond at a time
dislocation glide occurs on ___ becaue __
planes of highest atomic density becuase less energy is required to break and form each bond
plane of highest atomic density is the same as _
slip plane
slip only occurs if__
slip only occurs if the crystal’s dislocation plane(defined by the burger’s vector) coincides with the plane of highest atomic density (slip plane)
ductile materials have a higher number of __
slip systems
why are ceramics brittle
they have a low number of slip planes
linear defects are limited due to electoneutrality
in polymers, __ do not play a large role in mechanical properties
linear dislocations
what is surface tension
atoms at surface are not bonded to the max number of atoms, so they have higher energy
surface tension is considered a __
planar defect
__ are examples of two dimensional defects
(planar defect)
surface tension
grain boundaries
grain boundaries deal with the __, not the surface
the bulk, not the surface
how are grain boundaries similar to surface tension
atoms in grains do not have optimal coordination number, so they have higher energy
why does corrosion happen at grain boundaries
atom at grain boundaries have higher energy because they are not bound to the man number of atoms
tilt boundaries are composed of __
edge dislocations
twist boundaries are composed of
screw dislocations
examples of three dimensional defects
voids and precipitates
what are precipitates
they are clusters of substitutional or interstitial impurities
what are voids
they are aggregates of vacancies
why can pores be useful
they allow the exchange of fluids and gasses within the material
what are 2 examples of porogens
salts –> disslove in water
wax –> melt away at high temperature
what is the chain folded model
polymer chains fold back on themselves at the faces
why are polymers not 100% crystalline
the face of the lamella is amorphous
chain folded model
what are spherulites
they are 3D aggregates of lamellae
why are spherulites more prone to chemical attacks
they are like grain boundaries (metals and ceramics)
burger vectors of linear defects are much longer in
polymers due to the size of the unit cell
sliding in polymers usually takes place __
along the axis of polymer chains due to the covalent bonds within the chain
what is viscous flow
deformation in non-crystalline materials
what is melting point
temp above which atomic movement is big enough to break the highly ordered structure
there is not distinct __ in amorphous ceramics (glasses)
melting temperature
amorphous ceramics are also known as
glasses
polymer have a distinct __
melting temperature
factors the influence polymer melting temperature
- branching -> decrease
- increase MW -> increase
- bulky side groups –>increase glass transition because they reduce movement around the backbone
cold crystallization is a __ process
exothermic
why does cold crystallization increase crytallinity
the increased temp allows the polymer chains to move into a more ordered state
what influences melting temperature
anything that affects its ability to crystallize
i.e. branching
why does increasing MW increase Tm
per volume, higher MW have fewer chain ends. the chain ends are the first to vibrate since they are free
more energy under DSC curve mean_
higher crystallinity
Tg is lower if __
there is easier rotation around the backbone
differential scanning calorimetry tells us what_
the percent crystallinity
power compensated DSC
sample and reference are heated to the same temp and the power needed to maintain that temp is recorded
what are the instrumentation for tensile testing
- grips
- load cell
- extensometer - records elongation
- computer- generates stress stain curve
units of strain
dimensionless
why is the force in compression testing negative
because it is in the opposite direction of tensile testing
besides force, __ is also negative for compressive testing
strain
in tensile testing, force is applied __ to the cross-section
perpendicular
slope of stress strain curve is __
modulus of elasticity
larger slope of stress strain curve mean __
siffer material
the linear region of a stress stain curve represents __
elastic deformation
why do materials with stronger bonds deform less
they require more energy to deform
values of E are greater for __ than __
E is greater for ceramics than metals
mechanical properties in polymeric materials are highly __
direction dependent
why are mechanical properties highly directional dependant
along the chain axis, there are primary (covalent) bonds
in other directions, there are secondary bonds
plastic deformation occurs after the __
elastic region
plastic deformation is the point where stress strain curve __
doesn’t follow hooke’s law
yield strength
stress at the end of the elastic region
yield point strain
strain at the end of the elastic region
after yielding, there is a __ in stress required to continue plastic deformation
increase in stress
the max stress is known as
tensile strength
after max stress, __ occurs
necking occurs
__ is used as a key design parameter, since by the time it has reach its tensile strength, it has become too deformed
yield stress
brittle materials have __ ductility
low
how is plastic deformation different in ceramics than metals
necking occurs in both metals and polymers, but when necking occurs in polymers, the chains orient with the load
just before fracture in polymers, why is there an increase in stress
more strength is needed to overcome the primary bonds
how is necking different in polymers and metals
in metals, elongation is confined to the original necked region
in polymers elongation occurs by growth in length of the necked region
metals and crystalline ceramics undergo deformation due to
dislocation glide along a slip plane
why is plastic deformation favorable
it is thermodynamically favorable to have dislocation glide along defects all the way through the crystal structure
what is the critical resolved shear stress
stress needed for slip to occur
need __ to be oriented in the proper direction for slip to occur
plane of highest atomic density
why is BCC more ductile than HCP
because it has more slip systems
why are ceramics brittle
they don’t have many slip systems because of electroneutrality
why are polycrystalline materials strong
the presence of grain boundaries hinder slip
in polymers, increasing __ or decreasing __ causes a reduction in E
increasing temp or decreasing strain rate
below Tg, there is no __
rotation around the backbone
why are polymers brittle at high strain rates
at high strain rates, the polymers don’t have enough time to reorient with the axis of loading
in semi-crytalline polymers, spherulites contain __ regions that radiate from the center
lamellar regions
what are the stages of semi-crystalline deformation
- tie chains expane
- lamella reorient
- blocks of lamella separate from each other
- blocks and tie molecules orient along the loading axis
on a molecular level, how do you strength polymers
prevent chain sliding
strength a polymer also means to increase __
yield strength
why does increasing percent crystallinity in polymers strengthen it?
it reduces amorphous regions where most of the chain sliding occurs
what are the four ways to strengthen polymers
- thermal treatment
- Increasing MW
- crosslinking polymers
- adding filler material
how does cold crystallization increase polymer strength
heating up the polymer allows the chains to reorient which increases percent crystallinity
why does cooling from melt increase polymer strenght
at high temps, spherulites are formed
spherulites are crystalline regions
why does cooling a polymer quickly make it weaker
spherulites are formed at higher temperature, so cooling it quickly means less time for spherulites to develop which leads to less percent crystallinity
why does increasing MW strength polymers
more entanglement
makes sliding harder
why does crosslinking polymers make it stronger
adjacent chains are covalently bonded to each other
this requires more energy for sliding to occur
why does adding fillers increase polymer strength
fillers form secondary bonds with the polymer which prevents sliding
elastomers have large __ at low __
large strain at low stress
why don’t elastomers undergo plastic deformation
coiled chains are crosslinked which prevents them from slipping past each other
why do elastomers return to their coiled state
it is thermodynamically favorable to be in the more disordered state
what is creep
plastic deformation of a sample under constant load over time
what is primary creep
increase in strain with time
why does creep rate decrease over time
it is due to the repositioning of dislocations over time
what is secondary creep
linear relationship between creep strain and time
the slope of the secondary creep is __
steady state creep
what is tertiary creep
defects appear within the material and leads to failure
creep rate increases with __ temperature and stress
increasing temp and stress
higher creep rate = __ time to rupture
lower time to rupture
what are the molecular causes of creep in metals
grain boundaries sliding or migration of vacancies
what is stress induced vacancy diffusion
cause of creep in metals
vacancies move to the face parallel to the stress
dislocation climb
cause of creep
dislocation move one atomic spacing
why are ceramics resistant to creep
electroneutrality constrictions
what is the molecular causes of creep in polymers
creep is due to viscous flow of chains in amorphous regions
what influences creep in polymers
crstyallinity and temperature
creep __ as crystallinity increases in polymers
decreases
why is there no creep in polymers below Tg
there is no rotation of flow of chains
what is stress relaxation
decrease in stress over time when under constant strain
what is an example of stress relaxation
a rubber band will relax over time when put around a fat stack of bills
what are the molecular causes for stress relaxatoin
movement of chains in the amorphous regions of the polymer
what is ductile fracture
if a material undergoes plastic deformation before breaking
why is ductile fracture the preferred mode of fracture
because there is visible warning in the form of plastic deformation before failure
what is brittle fracture
there is no plastic deformation before breaking
why do ceramics undergo brittle fracture
because they have less slip
what is a stress concentrator
stress can be amplified at the tip of a flaw
why is the presence of a stress concentrator more significant in brittle than ductile materials
plastic deformation reduces the localized stress in the area around the flaw
there is not plastic deformation in brittle materials
why is more energy need to ductile fracutre
the crack initiation can be stabilized by the ductile material, meaning you need more energy to propagate the fracture
why can fatigue occurs at significantly lower loads than predicted by tensile testing
during fatigue testing, repeated stress increases the number of dislocation and creates more imperfections in the structure
stages of fatigue failure
- initiation
- propagation
- failure
why does strengthen a material make it more brittle
you are reducing plastic deformation
what is the molecular cause of strengthening metals and ceramics
reduce the movement of dislocations
why do smaller grains increase strength
smaller grains have more grain boundaries
grain boundaries discourage dislocation
fatigue failure is always
brittle
fatigue life is measured at a specific __ value
stress
equation for fatigue life
Nf = Ni + Np
Nf = cycles til failure Ni = cycles to inititation Np = cycles to propagate
three things that affect fatigue life (Nf)
- stress risers
- biodegradable materials
- environment
cracks will start (initiate) at
stress risers
stress riser do what__
amplify the stress locally
how does biodegradable materials affect fatigue life
flaws form as degradation occurs
susceptible to failure at end of degradation
how does the environment affect fatigue life
salts and proteins can interact with the material
a reason why the environment can lead to a shorter fatigue life
corrosion fatigue
what is corrosion fatigue
corrosion due to cyclic stress and chemical attack
__ is specifically the degradation of metals in the body
corrosion
why can materials fail even though the general conditions in the body are not hard
inflammation can lead to the release of acids and oxidizing agents around the implant
what causes corrosion on a molecular level
the presence of anodic and cathodic regions
corrosion is an electrochemical process that involves the transfer of __
electrons
oxidation occurs at the __
anode
reduction occurs at the
cathode
in corrosion, the material will dissolve at the __
anode
on a molecular level, how do you prevent corrosion
prevent transfer of electrons from anode to cathode
how can you slow degredation
- slow the rate of either the oxidation or reduction
2. add an insulative layer
what does the nernst Equation tell
how active a material is a particular pH
what is galvanic corrosion
when two different types of metals are placed in the body, the body acts like a wire connecting the two electrodes
the __ material acts as the anode and dissolve
the more active material
what does the Pourbaix diagram tell
regions of non-corrosion as a function of cell potential and pH
what are the 3 regions of a pourbaix diagram
immunity, passivation, corrosion
pourbaix diagram cannot be used to tell __
the rate of a reaction
in a pourbaix diagram, anything not in the __ or __ regions is susceptible to corrosion
immunity and passivation region
what is meant when a material is in the passivation region of a pourbaix diagram
the material has an insulating layer due to surface oxidation
corrosion in the passive layer is also dependent on __
pH
that dashed lines on a pourbaix diagram represent _
the stability of water
if a material is not within the dashed lines on a pourbaix diagram, why do we not care about it
outside of the dashed lines do not represent body-like conditions
what is the potential reason behind crevice corrsion
no oxygen in the crevice sets up an anode region
what is pitting corrosion
small defects on the surface of the material acts as anodes
what is intergranular corrosion
corrosion due to grain boundaries
grain atoms have more energy and are susceptible to chemical attack
how do you prevent intergrandular corrosion
create larger grain sizes
but this will make the material weaker
why is a material under stress more susceptible to corrosion
it is in a higher energy state
what is stress corrosion cracking
(different than fatigue corrosion)??
corrosion that occurs due to metal under stress and corrosive environments?
what is fatigue corrosion
continual loading cna remove the passive layer from a material
why can protein attachment to the surface of a material lead to corrosion
proteins can remove the passivating layer
what are 2 things than can change the equilibrium on a pourbaix diagram
protein attachment
bacteria
what are the two ways polymers degrade
swelling and chain scission
what is swelling
the polymer will absorb water and become more ductile by reducing secondary bonds between the chains
unlike swelling, chain scission breaks __ bonds
primary
what are the two types of chain scission
hydrolysis and oxidation
how does chain scission by hydrolysis work
water cleaves primary bonds
rate of scission by hydrolysis increases due to __
- reactivity of the groups
- lower number of bonds to break
- water can penetrate faster
how to you slow scission by hydrolysis
- increase crystallinity makes it harder for water to penetrate
- increase hydrophobicity
what is chain scission by oxidation
radicals attack and break covalent bonds
because chain scission by oxidation involves radicals, it involves
initiation, propagation, termination
chain scission by oxidation is caused by __ after an implant is placed in the body
inflammation
how to you speed up the rate of chain scission by oxidation
- increase the amount of bonds susceptible to chain scission
- lower MW -> less bonds to break
how to you prevent oxidation when you have to use radicals to crosslink a polymer
you heat the polymer to allow radicals to combine and dissipate
how can pores increase degredation
- they acts are stress raisers
2. they increase surface area which allows them to be cleaved by environmental factors
why do polycrystalline ceramics degrade faster than ceramics made from a single crystal?
they have grain boundaries
what do biodegradable mean?
chemical breakdown of material mediated by bioenvironment
__ materials degrade by hydrolysis whereas __ materials degrade by __
synthetic materials degrade by hydrolysis
natural materials degrade by enzymes
why is hydrolysis a more consistent degredation
enzymes can vary between patients
why are natural materials used for localized drug delivery
enzymes can be localized, which means the natural material will only degrade where the enzyme is present
bulk degradation vs. surface degradation
bulk degration –> water enters the polymer very fast
surface degradation –> degrades just the surface
why don’t you want implants to have bulk degradation
bulk degradation causes the implant to collapse and reduce mechanical properties
in surface degradation, __ is not lost
mechanical properties
