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Chemistry > Polymers and Composites W5 > Flashcards

Polymers and Composites W5 Flashcards

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1
Q

What are some applications of polymers in Dentistry

A

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

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2
Q

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

A

Polymers (Macromolecules)
Polymerisation

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3
Q

The 2 major classes of polymers are…

A
  1. Biopolymers ex. peptides, proteins, carbs, alginate, formed through condensation
  2. Synthetic ex. Nylon, LDPE, PVC
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4
Q

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

A
  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
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5
Q

Explain step and chain growth*

A

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)

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6
Q

What are initiators vs radicals

A

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*

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7
Q

What are examples of initators commonly used in dentistry

A

Benzoyl peroxide
Camphorquinone (CQ)

Can be heat or UV-light initiated to = radicals

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8
Q

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

A
  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)
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9
Q

What part of methyl acrylate undergoes polymerisation

A

the C=C is the polymerisable group

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10
Q

Draw Methacrylate polymerisation*
Draw Methyl Methacrylate polymerisation*

A
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11
Q

How can branching occur and what does branching lead to?

A

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

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12
Q

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

A

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

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13
Q

Describe the compositon of synthetic polymers with regard to crystallinity*

A

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

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14
Q

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

A
  1. linear (amphorous - no order)
  2. branched
  3. cross-linked
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15
Q

What is a copolymer

A

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

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16
Q

What does MWD stand for

A

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

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17
Q

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

A

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)
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18
Q

What is the polydispersity index

A

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)

19
Q

What is the degree of polymerisation

A

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

20
Q

How do we seperate polymers

A

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

21
Q

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

A

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

22
Q

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

A

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)

23
Q

What does Tg stand for

A

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

24
Q

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

A

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

25
Q

Why is Tg important from dental composites

A

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

26
Q

What affects the Tg**

A
  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)
27
Q

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

A

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)

28
Q

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

A

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

29
Q

What is the definition of a composite?

A

Solid containing 2 or > distinct phases, combined to produce superior properties to the individual constituents.

Ex. Filler particles dispersed in synthetic polymer matrix

30
Q

What are some requirements for a resin-based composite

A

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

31
Q

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

A

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

32
Q

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

A

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**

33
Q

What is the role of fillers in resin

A
  • 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)
34
Q

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

A

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

35
Q

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

A
  1. Chemically
  2. Photochemically

BOTH PRODUCE RADICALS

36
Q

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

A

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)

37
Q

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

A

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

38
Q

What wattage is the lamp intensity for curing

A

460-480nm (usually 468nm) of blue light

The composite (CQ) absorbs the light

39
Q

What is the Degree of conversion (DOC)*

A

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.

40
Q

What happens when parts of the composite are unpolymerised

A

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

41
Q

What is the preferred method of curing?

A

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

42
Q

What are some other common dental polymers*

A
  • 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)
43
Q

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

A

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).

Chemistry (7 decks)

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