Polymer Chemistry Overview

Resin Composites

• Good aesthetics, mechanical strength, bonds to tooth structure using bonding agents.
• Technique sensitive, discoloration over time, polymerization shrinkage, does not bond directly to tooth structure.

Polymers

• Distinct repeating group of atoms called monomers.
• Monomers can be gases or liquids, converted to solid polymers after polymerization.
• Polymers are light, versatile, cheap and easy to handle materials.

Addition Polymerization

• Two molecules react to produce a larger molecule without the elimination of a smaller molecule or by-product.
• Activation: process of producing free radicals by light, heat or chemicals.
• Initiation, Propagation, Termination.

Free Radical Polymerization

• Free radicals are provided by highly reactive chemicals (initiators) with unpaired electrons.
• Benzoyl peroxide is a commonly used free radical producer (initiator).

Condensation Polymerization

• Two molecules react to produce a larger molecule with the elimination or production of a smaller molecule.

Factors Controlling Polymer Properties

• Molecular structure of repeating units.
• Molecular weight and chain length.
• Degree of chain branching.
• Cross linking.
• Filler or plasticizer.

Polymer Structure

• Crosslinked polymers form a three-dimensional network by joining individual chains (Makes structure more rigid).

Glass Ionomer Cement

• Powder: Fluoro-alumino-silicate glass and Na-alumino-silicate glass and CaF. Contains pigments.
• Liquid: Poly (acrylic)+ (maleic) or (itaconic) acid, Tartaric acid and distilled water.
• Less technique sensitive, bonds directly to tooth structure, releases fluoride, good clinical performance when used correctly.
• Relatively poor mechanical strength, weak bonding to tooth structure, not the best for aesthetics.

Conventional Dental Amalgam

• High compressive strength, good clinical track record, less technique sensitive.
• Contains mercury, does not bond to tooth structure, poor aesthetics.

Resin Composites & GIC Bonding

• Resin composites are bonded using adhesives (bonding agents).
• GIC is bonded directly to tooth tissue.

Polymers in Dentistry

• Resin-based restorative composites, impression materials, dissolvable sutures, prosthodontics.

Polymerization

• Chemical reaction joining monomers to form polymers.
• All polymers consist of a large number of repeating monomeric units.

Plastics

• Polymers that can be formed into various shapes by heat and/or pressure.
• Thermoplastics can be heated to a liquid and reshaped.
• Thermosetting plastic is fixed into shape by an irreversible reaction, heat drives the reaction or it may be exothermic.
• Polymers are substantially cross-linked covalently, often in 3-D networks.

Polymer Classification 1: Based on Origin

• Biopolymers (Carbs, proteins, DNA). Most important biopolymers are formed by a condensation process eliminating water.
  • Alginate is a naturally derived biopolymer used in synthesis of dental impression materials.
• Synthetic polymers (LDPE, PVC, nylon...).
  • Two synthetic methods are condensation (nylon) and addition (LDPE) polymerization.

Polymer Classification 2: Based on Thermal Behaviour

• Thermoplastic (nylon, PMMA, LDPE)
• Thermoset (BisGMA, TegDMA)
  • Cannot be reshaped due to irreversible hardening/cross-linking upon curing.

Polymer Classification 3: Based on Structure

• Linear
• Branched
• Hyperbranched
• Polysaccharides built from glucose with different structures that relate to their physiological and biochemical functions.
• Most synthetic polymers have structures that are linked to function.

Polymer Classification 4: Based on Chemistry of Monomer Structural Unit

• Resin: a synthetic organic polymer system used in many plastics.
• Acrylic resins (PMMA, bis-GMA)
• Polyamide resins

Surface Energy

• Molecules/atoms at a surface are at a higher energy state (not as stable) as those below the surface (bulk).
• Liquids can adjust their shape to minimize their surface energy, but solids can't.
• Surface energy is affected by intermolecular forces, functional groups present and for metals, crystal planes presenting at surface.

Crystal Planes

• Surface energy of a metal plane depends on the number of stabilizing nearest neighbors "removed" to form the surface.

Surface-sorption

• Surfaces are important to dentistry. At the surface physisorption (not chemically bonded) (loose association van der waals) and chemisorption (chemically bonded) (bonded covalent or ionic) can occur.

Surface Wetting

• Effective wetting of surfaces is related to surface energy/ surface tension of solid/ liquid and rheology of liquid/suspension.
• Wetting is critical in many dental procedures.
• Surface contamination lowers solid surface energy, decreasing wettability.
• Cleaning a surface raises surface energy of solid, improving wettability.

Copolymers

• Formed from 2 (or more) different types of monomer, A and B.
• Block: each monomer clustered into blocks.
• Graft: linear backbone of one composition and randomly distributed branches of a different composition.
• Comonomer: a minor component in copolymer, bonds with the principal monomer e.g. cross-linking agent.

Acrylate Monomers

• Monomers with the acrylate functional group are common in many dental resins.

Resin Copolymers

• Heraeus Kulzer Charisma - bisGMA/TEGDMA - 22% polymer by weight. Contains comonomer viscosity diluent and cross-linking agent.
• Heraeus Kulzer Durafill - bisGMA/UDMA - 40% polymer by weight.
• 3M filtek P60 - bisGMA/UDMA/bisEMA.
• The relative amounts depend on the polymer, synthesis used and its conditions.

Polyethylene

• Polyethylene crystallinity shows a dependance on average molar mass and degree/length of side branching (LDPE M=10^4^ g/mol vs HDPE M=10^6^ g/mol).
• LDPE: radical addition synthesis (high P>1000 bar).
• HDPE: Ziegler-Natta catalyst (low P\1

Resin Copolymers and Tg

• Polymerisation substantially increases the glass transition temperature, well above that of the monomer.
• Copolymerisation may increase the Tg.
• Maximum oral cavity temperatures experienced with eating and drinking is (55-58 degrees Celsius).
• If the resin copolymer temperature exceeds Tg, it will soften and fall.

Cardiovascular System

• Consists of blood vessels and heart.
• Conductive system: automatic, initiates electrical impulses, distributes throughout heart, ensures all cells depolarize and muscles contract in a coordinated manner.
• Transports blood around body for:
  • Oxygen diffusion
  • CO2 removal
  • Transport of waste
  • Immune function

Cardiac Muscle

• Chamber muscles contract (changes blood volume) as one: functional syncytium.
• Unique cell structure: contractile muscle cells and excitatory and conductive cells.
• Its own signal generator.
• Its own conducting system.

Contractile Muscle Cell Membrane

• Intercalated: each cell is interconnected, meaning one action potential is sent over multiple cells.
• Desmosomes: fuse cells together – prevention of separation of muscle cells.
• Gap junctions: share ions between cells.

Functional Syncytium

• Atrial and ventricular syncytium split in a way that electrically isolates the atria and ventricles.
• Allows atria to contract slightly before ventricles.

Cardiac Contractile Muscle Action Potential

• Phase 0: Na+ open.
• Phase 1: Na+ close, fast K+ open.
• Phase 2: Ca 2+.K+ open/close.
• Phase 3: Ca2+ close, slow K+ open.
• Phase 4: all closed.

Cardiac Pacemaker Cells

• Set rhythm of heart through impulses at approximately 60-100 times/min.
• SA node usually dominates (Pacemaker).
• Sequence:
  • SA node: fastest depolarization in heart.
  • SA node is connected to adjacent myocardial cells which send impulse.
  • AV node: delays impulse conduction from atria to ventricles (slow).
    • Pace set by AV is junctional rhythm 40-60 beats/min.
  • AV bundle (bundle of His): only electrical connection between atria and ventricles.
  • Bundle branches.
  • Endocardial network (Purkinje fibres).
• Contraction: atria then ventricles.

Pacemaker Cell Action Potential

• Phase 4: slow inward Na+ current.
• Phase 0: fast inward Ca2+ current.
• Phase 3: outward K+ current.

Electrocardiograms

• Currents from detected with electrodes.
• Number of leads determines sensitivity of signal (minimum 3, standard 11-12).
• Normal heart rhythm has a distinct sequence.
• Abnormal sequence = heart problem.

Cardiac Cycle

• All events occurring from beginning of one heartbeat to the beginning of next.

Normal Sequence

• P-R interval: atrial excitation to ventricular excitation. Can also be P-Q interval.
• S-T segment: ventricular myocardium is depolarized.
• Q-T interval: ventricular depolarization to ventricular repolarization.
• R-R interval: ventricular cycle. Used for heart rate.

Cardiac Output

• Cardiac output = stroke volume x heart rate.
• Cardiac output = venous return.
• Cardiac output = arterial pressure/total peripheral resistance.
• The amount of blood pumped out depends on:
  • How many times the heart beats.
  • How much blood is in the ventricles.
  • How much pressure is developed in the ventricles.
  • How much pressure there is in the arteries.

Venous Return

• Most important factor in stroke volume.
• How much blood comes in through major veins (vena cava/pulmonary veins).
• Most blood in circulation is in veins.

Frank-Starling Law of the Heart (Pre-load)

• More blood in the heart means more stretch.
• More stretch = more contraction.
• Stronger contraction = more blood ejected.

Stroke Volume

• Amount of blood pumped out of the ventricle in 1 beat (50-120mL)
• Calculated from the end of diastolic volume - the end of systolic volume of the ventricle.
• Affecting factors include: preload, contractility and afterload.

Preload

• Preload up, stroke volume up.
• Quantity of blood flowing from veins into the right atrium each minute.
• Increased by: exercise and blood volume increase.
• Decreased by: dehydration, blood loss.