Polymers in Dentistry - A University of Oslo Presentation PDF

Polymers in dentistry Hanna Tiainen Learning objectives What are polymers and how do they form? What gives polymers their characteristic properties? What is crosslinking and how does it affect polymer properties? What is the difference between addition and condensation polymerisation?...

Polymers in dentistry Hanna Tiainen Learning objectives What are polymers and how do they form? What gives polymers their characteristic properties? What is crosslinking and how does it affect polymer properties? What is the difference between addition and condensation polymerisation? What does degree of conversion mean? How to avoid shrinkage stress in dental composites? What are the most important monomers in dental composite resins? What are polymers? Definition polymers, or macromolecules, are long typically organic molecules that are built by repetitive covalent linking of many smaller units called monomers can be of natural or synthetic origin → e.g. plastics, rubbers, proteins, and DNA are all polymers Dental polymers Denture bases and teeth (polymethyl methacrylate, polycarbonate) Filling resins, resin cements and bonding agents (acrylic resins) Orthodontic brackets and aligners (polycarbonate, polyoxymethylene, polyurethane, polymethyl methacrylate) Biteguards(bittskinner) and mouthguards for sports (polymethyl methacrylate) Impression materials (alginate, polyvinyl siloxane, polyether) Root canal fillers and sealants (guttapercha, epoxy resins) polyurethane poly(methyl methacrylate) Polymer structure Typically organic macromolecules mostly based on C, H, O, N → can also be based on an inorganic –Si–O–Si– H backbone, H e.g. silicones Monomers polymerise into long chainlike H C molecules HH C H Strong and directional covalent bondsHbetweenHthe monomers H H H H ethylene CH2 = CH2 + CH2 = CH2 H C H C C O C H Opolyethylene H H H H H H HH H C C C C H HC O C O O O A bit of polymer chemistry homopolymer copolymer terpolymer alternating random block-copolymer linear branched grafted crosslinked lineær forgrenet kryssbundet Interchain bonding in polymers Atoms making up the polymer chains bond covalently → valence electrons shared between atoms Polymer chains held together by physical bonding chain entanglement and weak physical bonds (van der Waals, H-bonds) → low interchain bonding strength → polymer chains can slide past each other Results in materials with: low melting point & thermal stability low stiffness but high ductility poor wear resistance chain entanglement poor thermal and electrical conductivity Effect on physical properties physical property molecular weight Physical and mechanical properties of a polymer are highly dependent on chain length, branching, and crosslinking → affects stiffness, strength, melting temperature, and resistance to dissolution Effect on physical properties low density polyethylene high density polyethylene ultra high molecular weight polyethylene (LDPE) (HDPE) (UHMWPE) stiffness and strength increases Why polymers? Critical question: What is the main advantage of using polymers in dental applications? Especially for impression materials, fillings, cements, bonding agents? Liquid-to-solid transition application Viscous behaviour easy to mix and/or extrude flow freely and wet the tissue sufficient viscosity → no dripping adequate work time before setting Transition from fluid to solid happens within a few minutes takes places in physiological condition without toxic by-products ideally on-demand activated Elastic behaviour appropriate stiffness for application setting resists plastic deformation good dimensional stability in moist environment (low water uptake) Viscous flow Viscosity defines a fluid’s resistance to flow stronger the interchain bonding forces, higher the viscosity → high MW and increased chain entanglement increase viscosity Filling and impression materials should allow viscous flow when applied and transform to elastic solids in a matter of minutes Chemical crosslinking(kryssbinding) Linking of polymer chains into viscous elastic 3D molecular networks Rapid increase in MW → increases resistance to viscous flow Crosslinks act as anchor points between chains macromers crosslinker, crosslinked (polymer chains catalyst or polymer sliding of polymer chains prevented or monomers) activator network → restrict viscous flow → material becomes elastic or → viscoelastic solid flexible polymer chains between crosslinks have freedom to move → low E-mod & high reversible strains no loading loading Chemical crosslinking(kryssbinding) covalent ionic between reactive functional between charged functional groups groups and reactive crosslinkers (e.g. –COO-) and multivalent ionic or between difunctional monomers crosslinker (e.g. Ca2+ or Al3+) strong and durable chemical bond relatively weak physical bond irreversible reversible elastomeric impression materials, alginate impression materials, glass resin cements ionomer cements covalent bonds crosslinked dimethacrylate resin crosslinked alginate  Effect of crosslinking on physical properties stiffness viscosity molecular weight melting temperature solvent/water uptake plasticity and flowability crosslinking density low crosslinking density high crosslinking density Degree of crosslinking filling materials impression materials Needs to withstand load after setting Needs to be removed after setting minimal elastic strain allowed large elastic strain recovery needed high elastic modulus low elastic modulus long-term exposure to moisture no long-term exposure to moisture low water absorption necessary low water absorption not essential → high crosslink density → low crosslink density A bit more polymer chemistry Synthetic polymers can be classified depending on how the polymerisation process takes place → chain-growth, or addition, polymers polyethylene → step-growth, or condensation, polymers catalyst n H2O polyamide (Nylon) Condensation polymerisation Occurs by reaction of two or more different monomers with loss of a small molecule (e.g. water or alcohols) → monomers need to be at least difunctional (reactive group in either end)  Condensation polymerisation Reaction proceeds in a stepwise fashion from a monomer to dimer to trimer, and so on, until larger oligomers form polymers → polymerisation tends to stop before giant molecules are formed due to loss of → → chain mobility and reactive partners 1. Monomers consumed in formation of small dimers, trimers, tetramers, etc. monomers 2. Small fragments combine to oligomers 3. Long polymer chain Addition polymerisation Occurs by repetitive addition of a monomer into a growing chain → monomers are activated one at time by reaction of a C=C bond → no reaction by-products are formed Most dental polymers are formed by addition polymerisation Two main reaction types: free-radical and ring-opening polymerisation catalyst Free-radical polymerisation Ring-opening polymerisation Addition polymerisation Polymerisation reaction typically occurs in three stages: 1. initiation (or induction). +. 2. propagation 3. termination..................................... Free-radical polymerisation: initiation Reaction initiated by free radical (unpaired electron ∙) radical-producing molecules activated by heat, UV or visible light, or another chemical initiation efficiency can be improved with co-initiators Light- and chemical initiation used when heat cannot be applied (e.g. composite fillings) λ = 460 nm heat. chemical. benzoyl peroxide (BPO) camphorquinone (CQ) Light curing(lysherding) Polymerisation in most dental composite resins is light-initiated (photoactivation) → light curing resins (LC) allows unlimited working time before hardening → polymerisation reaction not initiated until resin is exposed to blue light limited depth of cure (depth in which free radicals are formed) only materials accessible for light exposure can be light-cured Light curing(lysherding) Use correct type of curing light and exposure time for each product curing time typically 10-20 s, can be reduced to 1-3 s with high power lights higher power lights deliver same amount of energy faster (10 J/cm2 energy) darker shades need longer curing time required wavelength depends on the used photoinitiator wavelength spectrum of most modern curing light units optimised for CQ multiwave LEDs can be used to cover both blue and violet spectrum Ivocerin Lucerin TPO Light curing(lysherding) Minimise distance between filling and light source to deliver maximum illumination power on exposed surface (irradiance) intensity of delivered energy per area declines as distance from the light source increases reduction in irradiance can vary among different light curing units Always follow recommendations / instructions from the producer of light curing unit material you intend to cure Nor Tannlegeforen Tid, 2016, 126, 848–56 Chemical curing(kjemisk herding) Chemically activated polymerisation is initiated by mixing of two reactants to generate free radicals (self curing resins) limited working time and slower reaction curing occurs independent on volume Combination of the two initiations: dual curing resins (DC) Degree of conversion(omsetningsgrad / konversjonsgrad) Not all monomers react → incomplete polymerisation reaction Degree of conversion (or polymerisation) describes the efficiency of the polymerisation reaction → determined as the ratio between reacted and unreacted C=C bonds 𝐶 = 𝐶 𝑏𝑜𝑛𝑑𝑠 𝑖𝑛 𝑐𝑢𝑟𝑒𝑑 𝑟𝑒𝑠𝑖𝑛 𝐷𝐶(%) = 100 − × 100 𝐶 = 𝐶 𝑖𝑛 𝑢𝑛𝑐𝑢𝑟𝑒𝑑 𝑟𝑒𝑠𝑖𝑛 Residual monomers(restmonomerer) Unreacted monomers that may leach out from the resin → potential to cause irritation, inflammation, allergic reactions Inverse correlation to degree of conversion → the higher the DC, the fewer unreacted monomers remain Most monomers in composite resins are difunctional with a reactive C=C bond at each end of the polymer → not all unreacted C=C correspond to residual monomers dimethacrylate Oxygen inhibition(oksygeninhibisjon) Oxygen highly reactive with free radicals → radical scavenging R. R. Surface of the filling material exposed to. R R. atmospheric oxygen (O2) in air tacky layer of uncured resin on the surface of cured composite layer allows bonding between composite layers when building up the filling top-most layer of the filling surface must be removed during finishing unreacted monomers can cause staining and adverse reactions if left on the surface Polymerisation shrinkage(polymerisasjonskontraksjon) Polymerisation always results in a certain degree of shrinkage elimination of physical space between the molecules that form the polymer smaller monomers (lower MW) result in more shrinkage desired length of polymer chain low molecular weight monomers polymerised chain high molecular weight monomers polymerised chain Polymerisation shrinkage(polymerisasjonskontraksjon) Polymerisation process results in a certain degree of shrinkage up to 2-3% volume reduction in most common dental composites maximising DC also maximises polymerisation shrinkage causes internal stresses that pull the resin away from the margins Polymerisation shrinkage(polymerisasjonskontraksjon) Polymerisation process results in a certain degree of shrinkage up to 2-3% volume reduction in most common dental composites maximising DC also maximises polymerisation shrinkage causes internal stresses that pull the resin away from the margins Shrinkage stress can result in the formation of a gap between the restoration and tooth can lead to e.g. microleakage, secondary caries, marginal staining Summary: polymer science Physical properties of polymers depend on their chemical structure, such as: which monomers the polymer chains are composed of organisation of monomers in the polymer chain (copolymers) structure of the polymer chains (linear, branched, crosslinked) degree of crosslinking Two main polymerisations reactions: addition (chain-growth) polymerisation and condensation (step-growth) polymerisation Polymerisation shrinkage results from elimination of physical space between the molecules that form the polymer network results in reduction in volume, and thereby, internal contraction stress polymerisation shrinkage and resulting polymerisation stress increased by: increasing degree of conversion (DC) low molecular weight of monomers (more polymerisation reactions needed) Resins for dental composites Resin matrix in dental composites Resin matrix forms a continuous phase around filler particles Composed of fluid mixture of monomers that are converted to rigid polymer by (photo)chemical activation resin is the chemically active component in dental composites allows for shaping and adaptation to cavity before hardening glass / ceramic particles polymer matrix aka “resin” Resin matrix in dental composites Resin matrix in most dental composites restorations is based on dimethacrylate(dimetakrylat) monomers (acrylic resins) difunctional monomers with two reactive groups methacrylate group dimethacrylate monomer Acrylic resins: chemistry Difunctional monomers can link with up to four other monomers Dimethacrylate-based resins form highly crosslinked polymer networks upon curing 1 2 → increase rigidity and wear → resistance → pendant unreacted chains act as → plasticisers 4 3 Crosslinking dramatically increases MW → increase in size reduce mobility and → eventually stops polymerisation Acrylic resins: monomers rigid segment Most commonly used monomer in dental composite resins is bisphenol-A glycidyl methacrylate (bis-GMA) → high viscosity due to high MW and hydrogen bonding between –OH groups → needs to be blended with other monomers to allow flowable resins Acrylic resins: monomers EGDMA TEGDMA Flexible low molecular weight monomers such as ethylene glycol dimethacrylate (EGDMA) and triethylene glycol dimethacrylate (TEGDMA) are blended into the resin → reduce viscosity and provide useful consistency and manipulation properties → lower MW of these monomers contributes to higher polymerisation shrinkage Acrylic resins: monomers urethane group UDMA rigid segment bis-EMA Most current resins also contain urethane dimethacrylate (UDMA) and/or ethoxylated bisphenol-A dimethacrylate (bis-EMA) monomers → high MW but reduced viscosity in comparison to bis-GMA (no –OH groups) → UDMA has more flexible backbone due to lack of stiff aromatic ring structure Desired resin properties What are the desired properties of a dental composite resin? application flows and adapts to cavity geometry viscosity easy to apply and manipulate low shrinkage and high DC hardening high strength high wear resistance low water absorption Desired properties: viscosity youtu.be/7cKGp7ob0Ao Dentsply Sirona Restorative Desired properties: viscosity Large monomers with rigid sections increase resin viscosity appropriate ‘flowability’ can be tuned by the use of flexible low MW monomers Not the only factor influencing viscosity of composite resins filler content and particle size distribution also affect viscosity of composites Minimising polymerisation stress Minimising shrinkage stress Incremental build-up of restoration Incremental placement and curing of the composite reduces the shrinkage stress experienced at the marginal interface Minimising shrinkage stress Monomer design youtu.be/g1gShvXwG5o 3M ESPE – Filtek Bulk Fill Posterior Non-acrylic resins? Critical question: Do all dental composite filling materials contain acrylic monomers? Can the resin be made of something else? Non-acrylic resins Not all polymers are based on organic hydrocarbon chains → can also be based on an inorganic –Si–O–Si– backbone, e.g. silicones silorane Non-acrylic resins oxirane Silorane-based resins siloxane Low shrinkage silorane monomer Ring-opening polymerisation → addition polymerisation, light-initiated → cationic polymerisation reaction → no oxygen-inhibited unreacted layer since → initiation not based on radicals Non-acrylic resins Silorane-based resins Ring-opening polymerisation gains space and counteracts some of the volume reduction during polymerisation Less volumetric shrinkage does not necessarily mean less stress → filler loading, E-modulus, shrinkage rate and degree of conversion all play a role acrylic-based composite silorane-based composite Polymerisation shrinkage ~2-3% polymerisation shrinkage < 1% 3M ESPE – Filtek Silorane Non-acrylic resins oxirane Silorane-based resins siloxane Low shrinkage silorane monomer Ring-opening polymerisation → addition polymerisation, light-initiated → cationic polymerisation reaction → no oxygen-inhibited unreacted layer since → initiation not based on radicals Comparable strength to acrylic resins Less often used in clinical practice vs acrylic resin composites modern stress-reducing acrylic monomers have reduced the advantage gained with silorane 3M ESPE, 2007 Non-acrylic resins ORMOCER® resins ORganically MOdified CERamic “all-ceramic direct fillings” No monomers..? NTFs Tidende, 6/2022 Non-acrylic resins ORMOCER® resins Inorganic-organic copolymer resin consists of organic reactive species with C=C bonds for polymerisation, which are bound to an inorganic -O-Si-O- network → addition polymerisation reaction unreacted inorganic-organic oligomers have similar viscosity to bis-GMA → TEGDMA used as viscosity controller Limited shrinkage & excellent aesthetics Filler particles typically required for achieving sufficient strength ORMOCER oligomer Non-acrylic resins Learn more about the science of ORMOCERs youtu.be/RMc3PWcMyH8 Summary: dental composite resins Organic resin allows flow and shaping of composite materials into cavities before polymerisation Composite resin transformed into a rigid solid by blue light initiated free radical polymerisation of the resin monomers Acrylic resins are based on dimethacrylate monomers most commonly used monomers: bis-GMA, UDMA, TEGDMA, bis-EMA large and rigid monomers increase viscosity and flexible low MW monomers are blended into resins to improve flowability new modified dimethacrylate monomers developed to reduce polymerisation shrinkage and resulting shrinkage stresses Non-acrylic resins developed largely to reduce problems caused by polymerisation shrinkage (e.g. silorane and ormocer) acrylic resins are still the most dominant materials for composite restorations Do you still remember..? What are polymers and how are they formed? very large macromolecules built by repeating covalent linking of monomers What gives polymers their characteristic properties? MW and degree of crosslinking determine the physical properties of polymers larger chain length (MW) results in increased degree of chain entanglement high crosslinking density results in purely elastic behaviour but turns a polymeric material very stiff and brittle low crosslinking density results in materials with low elastic modulus that allow large elastic deformations (rubber-like behaviour) What is the difference between addition (or chain-growth) and condensation (or step-growth) polymerisation? addition polymers are made by adding monomers together without elimination of any reaction by-products condensation polymer is formed by reaction of two different functional groups with loss of a small molecule What does degree of conversion mean? percentage of C=C bonds that reacted during polymerisation Definisjoner Polymerisasjon: kjemiske reaksjon der relativt små molekyler (monomerer) reagerer ved å binde seg sammen til mye større molekyler. Høymolekylære stoffene som dannes, kalles polymerer. homopolymerisasjon: bare én enkelt monomer (A) inngår i reaksjonen, polymeren har formen AAA… kopolymerisasjon: to eller flere forskjellige monomerer (f.eks A og B) polymeriserer sammen, polymeren har for f.eks formen ABABAB... eller en annen gruppering, som f.eks AAABBB... Kryssbinding: kjemisk binding mellom polymerkjeder; jo flere kryss- bindinger som dannes, jo stivere blir polymeren Addisjonspolymerisasjon: like eller ulike monomerer knytter seg sammen i kjeder uten å avspalte biprodukter friradikal-polymerisasjon: reaksjonen initieres ved bruk av en initiator som danner frie radikaler R˙, avsluttes ved at to voksende polymerradikaler møtes og reagerer med hverandre (avbrudd, terminering), eller ved at radikalet tilriver seg et atom fra et annet molekyl som derved blir et radikal som kan gi opphav til vekst av et nytt polymermolekyl (kjedeoverføring) Kondensasjonspolymerisasjon: ulike monomerer reagerer med hverandre samtidig med at det blir avspaltet et lavmolekylært stoff (vanligvis vann)