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2011-12-01 23:28:59
Materials M2 Polymers

Materials 2 M2 Polymers
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  1. Requisites for Dental Polymers
    • Biological compatibility
    • -Tasteless, odorless, nontoxic, nonirritating
    • -Insoluble in saliva, impermeable to oral fluids
    • -Good bond to tooth structure
    • Physical properties
    • -Adequate strength and resilience, dimensionally stable
    • -Resistance to biting/impact force and excessive wear
    • Manipulation
    • -No toxic fumes or dust during handling
    • -Easy to mix, insert, shape, and cure
    • Aesthetic properties
    • -Sufficient translucency or transparancy
    • -Easy of being tinted or pigmented
    • Economic consideration
    • -Processing with simple and inexpensive equipment
    • -Easy to repair with low cost
  2. Polymerization
    Chemical reaction that links small compounds (monomer) into long chains of repeating monomer units
  3. High degree of polymerization
    • Fewer polymer chains
    • Longer polymer chains
    • More rigid, less soluble
  4. Low degree of polymerization
    • More polymer chains
    • Shorter polymer chains
    • Less stiff, more soluble
  5. Degree of Conversion
    • Each monomer has at least one chemical group that participates in the polymerization reaction
    • Unreacted residual monomer
    • -Not all monomers may be able to react completely
    • The extent to which all monomer is polymerized
    • -Degree of conversion
    • High degree of conversion
    • -Fewer residual monomers
  6. Condensation polymerization
    • Two molecules (not usually the same) react to form a larger molecules
    • Production of low molecular weight byproducts-Water, alcohols, halogen acids, ammonia
    • Dimensional shrinkage
    • Condensation silicone, polysulfide rubber
  7. Additional polymerization
    • Two molecules (either the same or dissimilar) react to form a larger molecule without the elimination of a smaller molecules (no byproduct)
    • No change in composition
    • Most dental resins
    • Free-radical polymerization
    • Most commonly used in dentistry
    • -Most prosthodontic polymers, direct restorative materials
    • Carbon-carbon double bond
    • -Polymerization reaction sites
    • No byproduct
  8. Stages in Additional Polymerization
    • Activation-create free radical(unpaired electron)
    • Initiation
    • Propagation
    • Termination
  9. Activation
    • Formation of free radicals
    • Reactive chemical species possessing an unpaired electron (●)
    • Initiators
    • -Free-radical producing molecules
    • -Benzoyl peroxide (BPO)
    • -Camphorquinone (CQ)
    • Activation of initiators
    • -Heat
    • -Chemical
    • -Light
  10. Heat cured
    • Heat > 65 °C
    • Decomposition of peroxide bond (O-O)
    • Most denture base resins
  11. Chemically cured
    • Tertiary amine
    • Decomposition of benzoyl peroxide at room temperature
    • Denture repairs, temporary restorations
  12. Chemical Activation BPO
    N,N-dihydroxyetheyl-p-toluidine (DHPT)
  13. Light Activation
    • Blue light (λ = 470 nm)
    • Light sensitive initiator
    • -Camphoroquinone (CQ) as the initiator in combination with dimethaminoethyl methacrylate (DMAM) accelerator
    • Direct restorative materials
  14. Initiation
    • Free radicals formed on initiators react with the carbon-carbon double bond of the first monomer
    • Convert the monomer itself into a free radical at the expense of the double bond
  15. Propagation
    • The new free radical reacts with a second monomer, forming still another free radical
    • Polymer chain propagates until no more monomer is available
  16. Termination
    • When the monomer is depleted, the last free radical terminates
    • Direct coupling of two free radical chain ends
    • Exchange of a hydrogen atom from one growing chain to another
  17. Ring Opening Polymerization
    • Monomer contains a ring structure, reactant (amine) causes the ring to open and cross-linking polymerization to occur
    • -Additional polymerization
    • -No by-products
  18. Epoxy resin
    • Ring opening polymerizatoin
    • Dies from rubber impression
    • Monomers with epoxide group + Hardener (Triethylenetetramine, TETA)
    • Water reacts with epoxide -> agar and alginate impressions are incompatible
  19. Cross-linking Polymers
    • Each polymer chain is covalently linked together by a cross-linking agent
    • -Triethyleneglycol dimethacrylate (TEGDMA)
    • -Two double bonds
    • Physical and chemical properties
    • -More rigid, more temperature resistant, less soluble
    • -Wear resistance, susceptibility to organic solvents
    • -Acrylic denture teeth, direct esthetic restorative materials
  20. Plasticizer
    • Low Mw substance is added to a polymer to modify the physical properties of the polymer
    • Used to reduce the brittleness of cross-linked polymers
    • Do not participate in the polymerization reaction
    • Do not become part of the polymer chains
  21. Plasticized polymer Physical properties
    • Reduce the forces of attraction between the polymer chains
    • Brittle polymer -> Soft, flexible, tough polymer
  22. Dental plasticizer
    • PMMA + dibutyl phthalate -> Denture soft liners
    • No leaching out of the polymer to oral tissues
    • -Low vapor pressure
    • -Low diffusion rate
  23. Inhibitor
    • Minimize or prevent spontaneous or accidental polymerization of monomers
    • free radical reacts with the carbon double bond of the inhibitor
    • Oxygen is main inhibitor for most system
    • Additional inhibitors
    • -React with the free radical faster than free radical can react with the monomer
    • -Hydroquinone (HQ): very efficient but causes discoloration
    • -Monomethyl Ether of Hydroquinone (MEHQ): excellent
    • -Butylated hydroxytoluene (BHT): less discoloration
    • inhibitor delays the polymerization process, so that we can increase work time
  24. Dental Resins Desired properties
    • Dimensional stability (minimum shrinkage)
    • Strong, hard, not brittle
    • Chemical stability (insoluble)
    • Easy to process
    • Inexpensive
  25. Acrylic resins
    • Derivatives of ethylene contain a vinyl (-C=C-) group, the simplest molecule for additional polymerization
    • 1. Acrylic acid -COOH, dental cements
    • 2. Methacrylic acid- methy group, central building block of most dental resins
    • 3. Methyl methacrylate
  26. Dimethacrylate
    • Bis-GMA
    • Triethyleneglycol dimethacrylate (TEGDMA)
    • Urethane dimethacrylate (UDMA)
  27. Methyl methacrylate (MMA)
    • Esterification of polyacids (Products of the dehydration of a carboxylic acid and an alcohol)
    • e.g. Methacrylic acid + methanol  water + ester (methyl methacrylate)
    • Methacrylate esters
    • R can be any organic or inorganic radical
    • Thousands of different acrylic resins
  28. Tg
    • Softening temperature
    • Poly(methyl methacrylate): hardest resin, highest Tg 125 Celsius
  29. Copolymer
    • Polymers containing two or more types of monomer in their polymer network
    • Vary in the molecular sequence and arrangement among the repeating random block and graft units
    • Modification of physical and mechanical properties
    • MMA + Butyl methacrylate -> more fracture resistance denture bases
    • MMA + Octyl methacrylate -> soft and flexible (denture soft liner)
    • MMA + Hydroxyethyl methacrylate -> wettability ↑ (xerostomia; dry mouth)
  30. Methyl methacrylate
    • The most common methacrylate polymer
    • -Transparent liquid at room temperature
    • -Dentures, trays, removable orthodontic appliances, temporary crowns, etc.
    • Shrinkage during polymerization
    • -21 volume % (7% linearly)
    • -Pre-polymerized methyl methacrylate beads are mixed the monomer to reduce the shrinkage (3.5 % linearly)
  31. Bis-GMA
    • Most commonly used monomer for dental resins
    • Derived from the reaction of Bisphenol-A and Glycidyl Methacrylate
    • Bisphenol-A -> Core structure of most high strength materials
    • Terminal methacrylate groups -> Sites for free radical polymerization
    • Two aromatic rings -> stiffness
    • High molecular weight
    • Less shrinkage
  32. Bis-GMA High Viscosity
    • Two phenol rings -> stiff central backbone
    • Two –OH groups -> hydrogen bonds b/w the monomers
  33. Urethane dimethacrylate (UDMA).
    • Linear structure in the middle based on urethane linkages
    • Lower viscosity compared to bis-GMA
  34. Diluent
    • Low molecular weight monomer
    • Bis-GMA needs to be diluted with low viscous monomers
    • Methyl methacrylate
    • Triethylene glycol dimethacrylate (TEGDMA)
    • Linear backbone structure -> Lower viscosity
    • Dental composite: 70% high MW monomer + 30% low MW monomer (TEGDMA)
    • Shrinkage: 70:30 < 50:50 (high MW: low MW)
    • Polymerization shrinkage ↔ Reaction of its double bonds