Dental Polymers: Polymerization

• Materials used in dentistry.

Polymerization

• This is a chemical reaction where many molecules join to form one large molecule.
• A mer is a single molecule.
• A polymer is a reaction resulting in one large, high molecular weight molecule.

Polymer Settings

• Acrylic resins solidify during polymerization.
• Additional polymerization describes linked molecules forming chains without byproducts, such as PVC acrylic.
• Condensation polymerization forms a byproduct like water or alcohol, as seen in nylon polyester epoxies.

Steps in Polymerization

• Initiation requires heat, light, or chemical energy to create free radicals.
• Free radicals are molecules with one or more unpaired electrons.
• High energy radicals result from energy input, like in self-curing mixtures.
• Propagation involves the polymer expanding its chain length.
• Termination occurs through direct coupling with another radical and monomer.

Oral Environment Considerations

• Average biting force is about 200 pounds, 55 in the anterior, and 247 in the molars, thus materials must resist fracture.
• Materials must have low expansion and contraction values because sudden temperature changes can fracture or crack teeth.
• Maintaining a pH balance near 7 in the oral cavity keeps materials from tarnishing and corroding.

Microleakage

• Fluids, microorganisms, and debris can penetrate restoration margins.
• It may cause tooth sensitivity, marginal discoloration, and pulpal pathology (pulp inflammation).

Galvanism

• Electrical currents are conducted (galvanic currents).
• It can irritate the pulp and cause sharp pain.
• Opposing restorations and saliva can form a battery.

Types of Restorations

• Fixed restorations include crowns, bridges, implants, and inlays.
• Removable restorations include full dentures, partial dentures, and orthodontic appliances.
• Fixed/removable options consist of overdentures, fixed dentures, ortho appliances, combinations, and temps.

Solids

• They can be crystalline with atoms held in place by interatomic forces.
• Most crystalline dental materials form a cubic lattice.
• Crystalline structures are rigid at room temperature below their melting points.
• Amorphous solids have isotropic properties due to uniform atom distribution.
• They lack a sharp melting point, softening over a temperature range and tend to be less rigid and more flexible than crystalline solids.

Surface Contact Angle

• The contact angle occurs where a liquid droplet meets a solid surface and it quantifies the wettability of the surface by liquid.
• Low contact angles (<90 degrees) indicate good wettability where the liquid spreads, typical of hydrophilic surfaces.
• High contact angles (>90) indicate poor wettability where the liquid beads up, which is typical for hydrophobic surfaces.

Mechanical Properties

• Strain is the amount of deformation.
• Stress is force per unit area.
• Strain is the measurement of deformation.

Stress Types

• Tensile stress results from a load that tends to elongate a structure, such as a ortho wire.
• Tensile stress accompanies tensile strain.
• Compressive stress opposes a load compressing a structure.
• Shear stress refers to deformation where one part slides parallel to another.

Elasticity

• Elasticity is the property of a body that deforms under load and returns to its original shape when unloaded.
• The maximum stress a structure can withstand without permanent deformation is its elastic limit.
• The proportional limit is when the structure no longer returns to its original shape after force removal.
• It also occurs at the same time as the elastic limit, yield point, or strength.
• Modules of elasticity define the rigidity of a material under stresses below the elastic limit.

Retention

• This is the ability to maintain position and resist displacement.
• Mechanical retention involves undercuts.
• Chemical retention involves glass ionomer cements.

Abrasion Resistance

• This is the material's ability to resist being worn away by rubbing or friction and maintain its surface integrity and functionality over time.
• Harder materials and smoother surfaces typically have better abrasion resistance.
• Composition, chemical, and physical properties can influence abrasion resistance.

Rheology

• Rheology studies the flow and deformation of materials.
• Thixotropic materials become less viscous under shear stress over time and then regain their original viscosity when stress is removed.
• Enamel conducts little to no heat and so it well insulates the pulp against temp changes.

Coefficient of Thermal Expansion (CTE)

• The CTE measures how much a material expands or contracts with temperature changes.
• Energy is stored in the intermolecular bonds between atoms during heat transfer.
• Stored energy can alter the molecular bond length, leading to solid expansion.
• Types of thermal expansion includes linear, area, and volumetric.