Polymers and Composites W5 Flashcards
What are some applications of polymers in Dentistry
Resin based composities, impression materials, dissolvable sutures, luting cements
Monomers joined together by covalent bonds form ___ in a process called ___
Polymers (Macromolecules)
Polymerisation
The 2 major classes of polymers are…
- Biopolymers ex. peptides, proteins, carbs, alginate, formed through condensation
- Synthetic ex. Nylon, LDPE, PVC
What are the 2 main ways in which synthetic polymers are formed, explain the differences…*
- Addition polymerisation
Whole monomer becomes part of polymer! ie. no atoms lost
ie. reactive end group reacts with monomers to propagate chain length - regenerating reactive end group - Condensation polymerisation
a small molecule is condensed out of the reaction (as a byproduct) - usually less H20
Explain step and chain growth*
Chain Growth: monomers are added one at a time to the growing polymers (usually addition)
Step Growth: polymer may grow from both ends ie. growing polymer, monomers and oligomers may react with eachother (usually condensation)
What are initiators vs radicals
Initiators: source of radials/substance that can produce a radical species - unexcited state
Radical: species with single unpaired electron - highly reactive, excited state
Denoted as either I* or R*
What are examples of initators commonly used in dentistry
Benzoyl peroxide
Camphorquinone (CQ)
Can be heat or UV-light initiated to = radicals
With regard to radical chain growth, draw the 3 steps employing the initiator, benzoyl peroxide*
- initiation BP -> 2 x R*
- propagation R* + CH2=CH2 -> R-CH2-CH2*
- Termination R-(CH2-CH2)n* + R* -> R-(CH2-CH2)n-R (dead polymer)
What part of methyl acrylate undergoes polymerisation
the C=C is the polymerisable group
Draw Methacrylate polymerisation*
Draw Methyl Methacrylate polymerisation*
How can branching occur and what does branching lead to?
The active site (radical) on a growing polymer can…
1. transfer somewhere else on the chain (intramolecular) = “Chain transfer to polymer” => BRANCHING
2. jump from one polymer to another (intermolecular transfer) = 1 dead polymer + 1 new reactive radical species
May lead to crosslinked network
The degree of branching has important consequences for physical properties, what affects the degree of branching?
Steric hinderances (bulky groups) (less)
Flexibility of polymer (less)
Synthesis conditions
Describe the compositon of synthetic polymers with regard to crystallinity*
Lack any degreee of long range order therefore considered amorphous…
But Local regions (domains) of crystallinity
A polymer can be either -___ (3 degrees of branching…)
- linear (amphorous - no order)
- branched
- cross-linked
What is a copolymer
Formed from 2 or more types of monomers
Can be regular, random, graft or block configuration
What does MWD stand for
Molecular weight distribution of a polymer
As not all chains in the polymer are the same length we graph the number/fraction of polymers with that molecular weight
How do you calculate polymer molar mass (what is the difference)*
Using averages due to varying chain lengths…
- Number average molar-mass (Mn)
(number of polymers x/total number of all polymers in sample) - Weight average molar-mass (Mw)
(total mass of polymer x in sample/mass all polymers in sample)
What is the polydispersity index
Ratio of the Mw to Mn
Relates to the breadth of molar mass distribution
Ex. If all polymers are the same length PDI = 1, (Mw=Mn) such as in biopolymers
PDI cannot be 1 for synthetic polymers (Mw>Mn)
What is the degree of polymerisation
Average number of monomers per polymer
= Mn/m (Mn: mm of polymer/m: mm of monomer)
How do we seperate polymers
Fractionating polymers by size exclusion chromatography
Seperates based on size, large polymers come through the column first
Synthetic polymers have varying degrees of amorphus and crystallinity.
How does the percentage (degree) of crystallinity effect the properties?
High crystallinity
= high melting point, dentistry, stiffness and yield stress
LDPE (low crystallinty) vs HDPE (high crystallinity)
What does Tm Stand for and how does it change with crystallinity
The melt transition temperature
Crystalline regions (polymer) melts from solid at Tm
Higher crystallinity = higher Tm, therefore improved strength, stiffness, pack well, strong intermolecular forces (H bonding, D-D)
What does Tg stand for
The glass transisiton temp (softening temp)
Temp at which amphorous polymer go from hard glass to rubbery elastomeric material ie. soften
As synthetic polymers have both amphorous (glassy polymers) and crystaline regions, where would they sit in terms of the Tm and Tg graph*
Between the middle of Tm and Tg lines/curves ie. is not completely amorphous or crystalline
Why is Tg important from dental composites
Tg (softening temp) is the region between rubbery state and glassy state (on cooling) and glassy to rubbery (on heating)
Tg of a polymer in a composite restorative resin MUST exceed the max temp encountered in the oral cavity. Otherwise restoration compromised/fail
ie. Tg > temp in oral cavity
What affects the Tg**
- Flexibility of polymer chains (reduced flexibility = higher Tg)
- Intermolecular forces (strong = higher Tg)
- Molar mass polymer (increasing Mn = higher Tg)
- Crosslinking/branching (more = higher Tg)
- Plasticisers - hold polymers apart therefore cant pack as well (more = lower Tg)
- Degree of crystallinity (increases = higher Tg)
- Co-polymerisation (depends on copolymers)
Dental restoration polymers are not used on their own but composited to other materials. What comprises a cured resin-based composite?
Highly crosslinked copolymer matrix
+
reinforcing filler particles (dispursed and bound in matrix using silane coupling agents)
+
activators/initiators (promote reaction)
+
inhibitors (increase shelf life and working time)
How are silica-based fillers bound to the copolymer matrix of resin-based composites
Silane coupling agents - bind the filler to the composite copolymer matrix (resin)
Ex. MPS
One end, methacrylate group which binds to polymer, other end binds silica (SiOH) to filler particle
What is the definition of a composite?
Solid containing 2 or > distinct phases, combined to produce superior properties to the individual constituents.
Ex. Filler particles dispersed in synthetic polymer matrix
What are some requirements for a resin-based composite
mechanically strong
chemically stable
easy to mix, shape, cure
Tg > temp in oral cavity
excellent aesthetic qualities
biocompatible
non-toxic
Copolymersation of acrylate monomers forms highly crosslinked structures as to increase the Tg, how is this acheieved?
At least one of the comonomers has >1 acrylate group per monomer = lots of active sites where polymerisation can take place
Most dental composites combine a base acrylate monomer with an acrylate comonomer to form a copolymer. What are the common ones used in dentistry?
Base monomer: BisGMA (methacrylate)
- intermolecular H bonding and rigid, high viscosity
Co-monomer: TEGDMA (dimethacrylate) - allows crosslinking network to develop, flexible, low viscosity
BE ABLE TO DRAW**
What is the role of fillers in resin
- improved physical and mechanical strength (stops crack propagation)
- minimises composite shrinkage/contraction on curing (more filler = less shrinkage)
- reduces CTE (coefficent of thermal expansion) - when temp changes occur
- decreased water sorption
- enable radiographic contrast (increases radiopacity - absorbance of X rays)
What are resin inhibitors, how do they work, give an example
Prolong storage time (and working time) by minimising spontaneous accidental polymerisation because inhibitor reacts with free radicals at a faster rate than the monomer
Once all inhibitor is consumed, polymerisation of monomer can occur
Ex. BHT
Polymerisation of composite resins may be activated…(2 ways)
- Chemically
- Photochemically
BOTH PRODUCE RADICALS
How do self curing resins (chemically activated) work, give an example*
Two parts - initiator & activator - mixed + monomer and filler
The activator speeds up the formation of radicals (from the initiator)
Ex. Benzoyl peroxide (initator) & aromatic tertiary amine (activator)
How do light curing resins (photochemically activated) work, give and example*
Single tube containing photosensister and initiator. The CQ is hit with blue light -> excited state (CQ) - reacts with amine initiator DMAEMA (requires 400nm blue light) = radicals (DMAEMA - which can undergo polymerisation)
Ex. Herculite Ultra
1. photosensitiser - CQ (camphorquinone)
2. amine initiator - DMAEMA
and bisGMA
What wattage is the lamp intensity for curing
460-480nm (usually 468nm) of blue light
The composite (CQ) absorbs the light
What is the Degree of conversion (DOC)*
Measure of the % of C=C in acrylate resin that have been converted to C-C during polymerisation
Determined from change in absorbance of IR radiation of methacrylate double bond (C=C) between cured and uncured resin
ie. any C=C left = absorbance, should see a difference in absorbance from cured to uncured.
What happens when parts of the composite are unpolymerised
Leach into oral cavity, microcracks, secondary caries, adverse tissue reaction, water sorption, early restoration failure
What is the preferred method of curing?
Dual cure resin
- Both chemically (self cure- Benzoyl peroxide initiator and activator -tertiary amine = radical)
and photochemically activated (CQ + DMAEMA (initiator) and BISGMA (monomer), blue light 440nm excites CQ = radicals).
continuing to cure when light is removed = high degree of curing
What are some other common dental polymers*
- Lactomer polymers - condensation polymer (biodegradable and absorbable)
- PEEK polymers (polyetheretherketone) - condensation polymer (similar to bone)
- Polycarbonate - step growth polymer (high impact and tensile strength)
- Epoxies resins (root canal sealers)
- Ring opening polymers - low shrink restoratives (used to counter shrinkage)
On the Tg Tm Graph at the Tg Tm what happens?*
When cooling the amorphous solid it turns from a liquid to rubber to glass (at Tg)
When cooling the crystalline solid it undergoes crystallisation suddently (liquid straight to a crystal at Tm).
