DENTAL POLYMERS IN PROSTHODONTICS & CROWN AND BRIDGE

IT INCLUDES ALL ABOUT POLYMERIC STRUCTURE.

1. PRESENTED BY : ASTHA PALIWAL JR 1 DENTA L POLYMERS

2. D EFINITION Dental polymers are formed through chemical reactions that convert large number of low molecular weight molecules known as Monomers into large and high molecular weight long chain m acro molecules .

3. Resins Resins are compositions of either monomers or macromolecules. blended with other components to provide a material with a useful set of properties. Resins are of 2 types – 1:Monomer Resins 2:Synthetic Polymer Resins

4. Monomer Resins Monomer resins are useful in dentistry because they can be shaped and molded and then transformed to a solid to take on a permanent shape when they polymerize .

5. Synthetic Polymer Resins Synthetic polymer resins are often called plastics, which are substances that, although dimensionally stable in normal use, can be permanently reshaped by irreversible deformation .

6. Dental Uses Of Polymeric materials and Resins - Prosthodontics: denture bases and teeth, soft liners, custom trays, impression materials, core buildup materials, temporary restoratives, cementing/luting materials, and maxillofacial prostheses Operative Dentistry: dentin bonding agents, cavity fil lings, resin and glass-ionomer cements, pit and fissure sealants, splinting materials, and veneers .

7. Orthodontics: brackets, bracket bonding resins and cements, and spacers • Endodontics: gutta-percha points, root canal sealants, and rubber dams • Equipment: mixing bowls and spatulas, mouth guards (sports equipment), and protective eyewear .

8. CHAIN BRANCHING & CROSSLINKING In addition to linear macromolecules, polymer chains are often connected together to form a nonlinear, branched, or crosslinked polymer . Branching is analogous to extra arms growing out of a polymer chain thus, the probability of entangled, physical connections among chains increases.

9. FIGURE : Effect of polymer chain length, branching, and crosslinking on mechanical and physical properties. Rigidity, strength, and melting temperature increase as polymer chain length grows and molecular weight increases.

10. Key points of Crosslinking The entangled interchain connections formed by chain branches are temporary that they can be disentangled with relatively low-energy. Crosslinks that are chemical bond connections between chains and require a relatively high energy to break. Because of interlinking a large number of chain backbones, a highly crosslinked polymeric material can consist of just a few giant molecules or even a single giant molecule. Crosslinking forms bridges between chains and dramatically increases molecular weight. Consequently, physical and mechanical properties vary with the composition and extent of crosslinking for a given polymer system. Crosslinking of a low-molecular-weight polymer increases the softening temperature, known as the glass-transition temperature ( Tg ), compared with that of a high-molecular-weight polymer .

11. FIGURE :Schematic diagrams of linear, branched, and crosslinked polymers

12. Copolymer Structures Polymers that have only one type of repeating unit (mer) are homopolymers Those with two or more types of mer units are known as copolymers.

13. There are 3 different types of copolymers :- • Random copolymer—No sequential order exists among the two or more units along the polymer chain . • Block copolymer— Identical monomer units occur in relatively long sequences (blocks) along the main polymer chain. • Graft or branched copolymer—Sequences of one type of mer unit (B) are “grafted” onto a backbone chain of a second (A) type of mer unit to form a branched configuration .

14. FIGURE : Schematic diagram of polymers that contain only amorphous intermolecular and intramolecular organization (left) and combinations of both amorphous and crystalline regions (shaded areas on right).

15. Performance criteria for Dental Resins Mechanical and Physical Properties :- Dental resins should have sufficient strength and resilience to resist the forces developed by biting, chewing, and impact and sufficient toughness as well as fracture and fatigue resistance to maintain form and function for many years. When used as a denture base for maxillary dentures, a resin should also have a low density to ensure a light weight, and it should have good thermal conductivity to maintain the patient’s ability to detect temperature changes.

16. Manipulation properties :- The resin should not produce toxic fumes or dust during handling and manipulation. It should be easy to mix, insert , shape, and cure, and it must have a relatively short setting time and be insensitive to variations in these handling procedures. Clinical complications—such as oxygen inhibition, saliva contamination, and blood contamination—should have little or no effect on the outcome of any handling .

17. Esthetic Properties :- The material should exhibit sufficient translucency or transparency so that it can be made to match the appearance of the oral tissues it replaces. The resin should be colorless and capable of being tinted or pigmented, and there should be no change in color or appearance of the material subsequent to its fabrication .

18. Chemical stability Property :- Conditions in the mouth are highly demanding, and only the most chemically stable and inert materials can withstand such conditions without deterioration.

19. Biological Compatibility :- Polymers and resins should be tasteless, odorless, nontoxic, nonirritating, and otherwise not harmful to the oral tissues. To fulfill these requirements, a resin should be completely insoluble in saliva or in any other fluids taken into the mouth ,and it should be impermeable to oral fluids to the extent that the resin does not become unsanitary or disagreeable in taste or odor. If the resin is used as a filling or cementing material, it should set fairly rapidly and bond to tooth structure to prevent microbial ingrowth along the tooth-restoration interface.

20. Economic C onsideration The cost of the resin and its processing method should be relatively low, and processing should not require complex and expensive equipment.

21. Mechanical & Physical properties of Polymers DEFORMATION AND RECOVERY :- Plastic strain is irreversible deformation that cannot be recovered and results in a new, permanent shape as the result of slippage (flow) among polymer chains .

22. Elastic strain : It is reversible deformation and will be quickly and completely recovered when the stress is eliminated, as the result of polymer chains uncoiling and then recoiling. Viscoelastic strain : It is a combination of both elastic and plastic deformation, but only the elastic portion is recovered when the stress is reduced Also, recovery is not instantaneous and occurs over time because the elastic recovery process is impeded by the viscous flow resistance among chains. The amount of deformation that is not recovered at the moment the stress is eliminated is known as plastic deformation.

23. Rheometric Properties Plastic flow : Irreversible strain behavior that occurs when polymer chains slide over one another and become relocated within the material, resulting in permanent deformation. Branching and crosslinking impede plastic flow.

24. Elastic recovery Reversible strain behavior that occurs in the amorphous regions of polymers when the randomly coiled chains straighten and then recoil, like springs that return to their original locations without sliding past one another when the applied force is removed .

25. FIGURE : Elastic recovery: spring like behavior (rapid and reversible). Chains uncoil, but they do not slip past one another because of crystalline regions, entanglements, or crosslinks. Thus, they recoil completely when unloaded.

26. FIGURE : Viscoelastic recovery: chains stretch and uncoil and also slip past one another, producing plastic, irreversible, permanent distortion and partial recovery when unloaded.

27. Solvation and Dissolution Properties :- Polymers are usually slow to dissolve, and are seldom clearly either soluble or insoluble in any particular liquid. Also , their solvation characteristics are very sensitive to Mw, M w /Mn (polydispersity), crosslinking, crystallinity, and chain branching .

28. Polymer solvation characteristics relevant to dental use :- • The longer the chains (the higher the molecular weight),the more slowly a polymer dissolves. • Polymers tend to absorb a solvent, swell, and softe n rather than dissolve. Any dissolution occurs from the swollen state. • Crosslinking prevents complete chain separation and retards dissolution; thus, highly crosslinked polymers cannot be dissolved.

29. • Elastomers swell more easily and to a greater extent than do plastics. • Absorbed molecules (e.g., water) spread polymer chains apart and facilitate slippage between chains. This lubricating effect is called plasticization. • Swelling of dental polymeric devices can adversely affect fit—as, for example, with full and partial dentures

30. Thermal Properties :- The physical properties of a polymer are influenced by changes in temperature and environment and by the composition, structure, and molecular weight of the polymer. In general, the higher the temperature, the softer and weaker the polymers become. Polymers can be formed into many desired shapes, using processes that depend on whether the polymeric material is a “thermoset” or a “thermoplastic” type .

31. Thermoplastic resins: • Soften on heating, and harden on cooling • Can be reprocessed by heating and cooling • Undergo a reversible reorganization among the molecular chains upon heating, as in denture-base resins

32. Addition Polymerization Most dental resins are polymerized by a mechanism in which monomers add sequentially to the end of a growing chain. Addition polymerization starts from an active center, adding one monomer at a time to rapidly form a chain.

33. STAGES IN THE ADDITION POLYMERIZATION OF VINYL MONOMERS There are four stages in the addition polymerization chain reaction : 1) I nduction 2) P ropagation 3)chain transfer 4)termination.

34. Induction Two processes control the induction stage—activation and I nitiation . For an addition polymerization process to begin, a source of free radicals, R•, is required. Free radicals can be generated by the activation of radical-producing molecules using a second chemical, heat, visible light, ultraviolet light, or energy transfer from another compound that acts as a free radical .

35. FIGURE : Activation (heat or chemical) of benzoyl peroxide (BPO). During activation, the bond between the two oxygen atoms (–O–O– or O:O) is broken and the electron pair is split between the two fragments. The dot adjacent to the oxygen atom (O•) symbolizes the unpaired electron of the free radical.

36. FIGURE : Initiation of a methyl methacrylate molecule. As the unpaired electron of the free radical approaches the methyl methacrylate molecule (A and B), one of the electrons in the double bond is attracted to the free radical to form an electron pair and a covalent bond between the free radical and the monomer molecule (C and D). When this occurs, the remaining unpaired electron makes the new molecule a free radical (D).

37. Propagation The resulting free radical-monomer complex then acts as a new free radical center when it approaches another monomer to form a dimer, which also becomes a free radical. This reactive species, in turn, can add successively to a large number of ethylene molecules so that the polymerization process continues through the propagation of the reactive center.

38. FIGURE : Propagation and chain growth. As the initiated molecule approaches other methyl methacrylate molecules, the free electron interacts with the double bond of the methyl methacrylate molecule and a new, longer free radical is formed

39. Chain Transfer In this process the active free radical of a growing chain is transferred to another molecule (e.g., a monomer or inactivated polymer chain) and a new free radical for further growth is created. For example, a monomer molecule may be activated by a growing macromolecule in such a manner that termination occurs in the latter .

40. Thus , a new nucleus for growth results. In the same manner, an already terminated chain might be reactivated by chain transfer, and it will continue to grow .

41. FIGURE :Chain transfer occurs when a free radical approaches a methyl methacrylate molecule and donates a hydrogen atom to the methyl methacrylate molecule. This causes the free radical rearrangement to form a double bond and become unreactive and the methyl methacrylate monomer to form a free radical that can participate in a chain-propagation reaction.

42. FIGURE : Another type of chain transfer can occur when a propagating chain interacts with the passivated segment that was formed . During this interaction, the passive segment becomes active while the active segment becomes passive

43. Termination Although chain termination can result from chain transfer , addition polymerization reactions are most often terminated either by direct coupling of two free radical chain ends or by the exchange of a hydrogen atom from one growing chain to another.

44. FIGURE : Termination occurs when two free radicals interact and form a covalent bond.

45. Ring Opening Addition Polymerization This typically involves monomers with one or more three-atom end- group rings, which open and then join with other broken rings to form single bonds, similar to the way carbon-carbon double bonds open and link up to form single bonds. Ring-opening monomers containing more than three atoms in the ring are known but as yet have no application as dental materials. Two types of three-atom ring monomers have found significant usage in dentistry: imines, with two carbons and a nitrogen, and epoxies, with two carbons and an oxygen.

46. Step Growth/Condensation Polymerization The reactions that produce step-growth polymerization can progress by any of the chemical reaction mechanisms that join two or more molecules to produce a simple nonmacromolecular structure. The primary compounds react, often with the formation of by-products such as water, alcohols, halogen acids, and ammonia. The formation of these by-products is the reason step-growth polymerization often is called condensation polymerization.

47. Copolymerization Two or more chemically different monomers, each with some desirable property, can be combined to yield specific physical properties of a polymer. As defined earlier, the polymer formed is a copolymer, and the process of formation is known as copolymerization .

48. FIGURE : Copolymerization is exemplified here by the reaction between butyl methacrylate and methyl methacrylate. Because the butyl methacrylate molecules increase the equilibrium distance between polymer chains the intermolecular interactions decrease, which causes the glass transition temperature to decrease.

49. Acrylic Dental Resins The acrylic resins are derivatives of ethylene and contain a vinyl (–C C–) group in their structural formula . There are at least two acrylic resin series of dental interest. One series is derived from acrylic acid, CH2 CHCOOH, and the other from methacrylic acid, CH2 C(CH3)COOH. Both polymerize by addition.

50. Although the polyacids are hard and transparent, their polarity, related to the carboxyl group, causes them to imbibe water. Water tends to separate the chains and cause a general softening and loss of strength, making them generally unsuitable for dental use. The esters of these polyacids, however, are widely used for a variety of dental applications.

51. Methylmethacrylate The degree of polymerization varies with conditions of polymerization such as temperature, method of activation, type of initiator, initiator concentration, and purity. A volumetric shrinkage of approximately 21% occurs during the polymerization of pure methyl methacrylate, which can be a problem for accuracy of fit.

52. Poly ( Methylmethacrylate) (PMMA) PMMA is a transparent resin of water-like clarity that is transparent to light in the visible and ultraviolet range down to a wavelength of 250 nm. It is a hard resin with a Knoop hardness number of 18 to 20 KHN. It has a tensile strength approximately 60 MPa, a density of 1.19 g/ cm3, and a modulus of elasticity of approximately 2.4 GPa. It is also extremely stable: it does not discolor in ultraviolet light and it exhibits remarkable aging properties. It is chemically stable to heat below 125 °C, softens at 125 °C, and can be molded as a thermoplastic material.

53. CONCLUSION Dental polymers are widely used in dentistry due to their versatile properties, including excellent aesthetics, ease of manipulation, cost-effectiveness, and customizable mechanical properties , making them a valuable option for various restorative and preventive applications, such as tooth fillings, crowns, dentures, and adhesives.

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