Polymers and Composites W5

Question 1

What are some applications of polymers in Dentistry

Answer 1

Resin based composities, impression materials, dissolvable sutures, luting cements

Question 2

Monomers joined together by covalent bonds form ___ in a process called ___

Answer 2

Polymers (Macromolecules)
Polymerisation

Question 3

The 2 major classes of polymers are…

Answer 3

1. Biopolymers ex. peptides, proteins, carbs, alginate, formed through condensation
2. Synthetic ex. Nylon, LDPE, PVC

Question 4

What are the 2 main ways in which synthetic polymers are formed, explain the differences…*

Answer 4

1. 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
2. Condensation polymerisation
   a small molecule is condensed out of the reaction (as a byproduct) - usually less H20

Question 5

Explain step and chain growth*

Answer 5

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)

Question 6

What are initiators vs radicals

Answer 6

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*

Question 7

What are examples of initators commonly used in dentistry

Answer 7

Benzoyl peroxide
Camphorquinone (CQ)

Can be heat or UV-light initiated to = radicals

Question 8

With regard to radical chain growth, draw the 3 steps employing the initiator, benzoyl peroxide*

Answer 8

1. initiation BP -> 2 x R*
2. propagation R* + CH2=CH2 -> R-CH2-CH2*
3. Termination R-(CH2-CH2)n* + R* -> R-(CH2-CH2)n-R (dead polymer)

Question 9

What part of methyl acrylate undergoes polymerisation

Answer 9

the C=C is the polymerisable group

Question 10

Draw Methacrylate polymerisation*
Draw Methyl Methacrylate polymerisation*

Question 11

How can branching occur and what does branching lead to?

Answer 11

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

Question 12

The degree of branching has important consequences for physical properties, what affects the degree of branching?

Answer 12

Steric hinderances (bulky groups) (less)
Flexibility of polymer (less)
Synthesis conditions

Question 13

Describe the compositon of synthetic polymers with regard to crystallinity*

Answer 13

Lack any degreee of long range order therefore considered amorphous…
But Local regions (domains) of crystallinity

Question 14

A polymer can be either -___ (3 degrees of branching…)

Answer 14

1. linear (amphorous - no order)
2. branched
3. cross-linked

Question 15

What is a copolymer

Answer 15

Formed from 2 or more types of monomers
Can be regular, random, graft or block configuration

Question 16

What does MWD stand for

Answer 16

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

Question 17

How do you calculate polymer molar mass (what is the difference)*

Answer 17

Using averages due to varying chain lengths…

1. Number average molar-mass (Mn)
   (number of polymers x/total number of all polymers in sample)
2. Weight average molar-mass (Mw)
   (total mass of polymer x in sample/mass all polymers in sample)

Question 18

What is the polydispersity index

Answer 18

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)

Question 19

What is the degree of polymerisation

Answer 19

Average number of monomers per polymer
= Mn/m (Mn: mm of polymer/m: mm of monomer)

Question 20

How do we seperate polymers

Answer 20

Fractionating polymers by size exclusion chromatography
Seperates based on size, large polymers come through the column first

Question 21

Synthetic polymers have varying degrees of amorphus and crystallinity.
How does the percentage (degree) of crystallinity effect the properties?

Answer 21

High crystallinity
= high melting point, dentistry, stiffness and yield stress
LDPE (low crystallinty) vs HDPE (high crystallinity)

Question 22

What does Tm Stand for and how does it change with crystallinity

Answer 22

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)

Question 23

What does Tg stand for

Answer 23

The glass transisiton temp (softening temp)
Temp at which amphorous polymer go from hard glass to rubbery elastomeric material ie. soften

Question 24

As synthetic polymers have both amphorous (glassy polymers) and crystaline regions, where would they sit in terms of the Tm and Tg graph*

Answer 24

Between the middle of Tm and Tg lines/curves ie. is not completely amorphous or crystalline

Question 25

Why is Tg important from dental composites

Answer 25

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

Question 26

What affects the Tg**

Answer 26

1. Flexibility of polymer chains (reduced flexibility = higher Tg) 2. Intermolecular forces (strong = higher Tg) 3. Molar mass polymer (increasing Mn = higher Tg) 4. Crosslinking/branching (more = higher Tg) 5. Plasticisers - hold polymers apart therefore cant pack as well (more = lower Tg) 6. Degree of crystallinity (increases = higher Tg) 7. Co-polymerisation (depends on copolymers)

Question 27

Dental restoration polymers are not used on their own but composited to other materials. What comprises a cured resin-based composite?

Answer 27

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)

Question 28

How are silica-based fillers bound to the copolymer matrix of resin-based composites

Answer 28

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

Question 29

What is the definition of a composite?

Answer 29

Solid containing 2 or > distinct phases, combined to produce superior properties to the individual constituents. Ex. Filler particles dispersed in synthetic polymer matrix

Question 30

What are some requirements for a resin-based composite

Answer 30

mechanically strong chemically stable easy to mix, shape, cure Tg > temp in oral cavity excellent aesthetic qualities biocompatible non-toxic

Question 31

Copolymersation of acrylate monomers forms highly crosslinked structures as to increase the Tg, how is this acheieved?

Answer 31

At least one of the comonomers has >1 acrylate group per monomer = lots of active sites where polymerisation can take place

Question 32

Most dental composites combine a base acrylate monomer with an acrylate comonomer to form a copolymer. What are the common ones used in dentistry?

Answer 32

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**

Question 33

What is the role of fillers in resin

Answer 33

- 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)

Question 34

What are resin inhibitors, how do they work, give an example

Answer 34

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

Question 35

Polymerisation of composite resins may be activated...(2 ways)

Answer 35

1. Chemically 2. Photochemically BOTH PRODUCE RADICALS

Question 36

How do self curing resins (chemically activated) work, give an example*

Answer 36

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)

Question 37

How do light curing resins (photochemically activated) work, give and example*

Answer 37

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

Question 38

What wattage is the lamp intensity for curing

Answer 38

460-480nm (usually 468nm) of blue light The composite (CQ) absorbs the light

Question 39

What is the Degree of conversion (DOC)*

Answer 39

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.

Question 40

What happens when parts of the composite are unpolymerised

Answer 40

Leach into oral cavity, microcracks, secondary caries, adverse tissue reaction, water sorption, early restoration failure

Question 41

What is the preferred method of curing?

Answer 41

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

Question 42

What are some other common dental polymers*

Answer 42

- 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)

Question 43

On the Tg Tm Graph at the Tg Tm what happens?*

Answer 43

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).