Basics in dental material MCQs 3

Basics in dental material (Polymers in dental material ; Definition , Types of bonds , Dental application, Basic nature, types of Copolymer, Spatical structure, Molecular weight, polymerizaion process, Stages of polymerization Reaction, Inhibition of polymerization, thermal behavior, Crystallinity l  MCQs in dental material for dental students

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Polymers in dental material

• Polymers: long chain molecules made up pattern of many repeating units connected together.
• Monomer: the smallest repeating unit in the polymer chain.

Types of bonds :

1. Covalent bonds : polymers are solid molecules bond by a strong primary covalent bonds bt. The monomers along the polymer chain ( inter-atomic or intramolecular ).
2. Polar bonds ( Secondary bond ) : between chains ( Inter-molecular ).

• NB: as these polar bonds are weak they are responsible for 1. Low strength. 1. High water sorption.

Dental applications of polymers:

• Impression materials ( Agar & alginate & Elastomer ) 4- acrylic denture base & teeth.
• Composite filling materials & Pits , fissure sealant 5- Soft lining materials
• Cements ( Glass Inomer & Zincpolycarboxylate) .

Basic nature of polymers:

• The term polymer denotes a molecule that is made up of many (poly) parts (mers).
• The mer-ending represents the simplest repeating units from which the polymer is formed.
• Co-polymers : is Polymer molecules formed of repeated mixture of monomers units.
• Homo-polymers : is a polymer formed of one type of monomer units
• Ex: Poly-methyl metgacrylate (PMMA) is a polymer having chemical structural units derived from methyl meth-acrylate .

Types of Co-polymers:

Random copolymer :

• Two monomers enter in a relatively random manner along the chain. ( M-A-M-M-M-A-A-M-A)

Alternating copolymer :

• Alternating sequence of different monomers. ( M-A-M-A-M-A-M-A-M )

Block copolymer :

• A linear copolymer with one or more long uninterrupted sequences of each polymeric species. ( M-M-M-M-M-M-A-A-A-A-A-A )

Graft copolymer :

• Is a branched copolymer with one monomer chin to which one or more side chains of another monomer are attached to it .

 

Spatial structure :

• There are 3 basic types of structures: linear & branched & cross-linked.

Linear

• Structural units : Monomers connected to another in a linear & adjacent sequence.
• Molecules : are separated and discrete
• Ex. Poly-ethyle, Naylon

Branched

• Structural units : Side branch chain are connected to the main one
• Molecules : same a linear

Cross-linked

• Structural units : adjacent linear chain are joined one to another in a various positions by covalent bond.
• Molecules : form 3-dimensional network
• Ex. Rubber.

Note Book :

• Main chain of the polymer is called back-bone chain.
• Side chain of the polymer is called pendants.

Effect of cross linking :

1. Increases the strength.
2. Increases the hardness .
3. Increases the brittleness (extensive cross-linking may lead to brittleness of polymer)
4. Increases the glass transition temperature (Tg).
5. Decrease the water sorption
6. Decrease the solubility ( the material show better resistance to organic solvents as alcohol and monomer ‘’ crazing resistance’’ ).
7. Decrease the ductility.

Molecular weight:

• The M.W of the polymers has an effect on the physical properties of polymer.
• The higher M.W the higher melting range , stiffer the plastic .

Polymerization process :

• Its a process consist of the monomers become chemically linked together To form high molecular weight molecules.

Clinical Significance of Polymerization process :

• Polymerization is never entirely completed, But there remain an amount of monomer incompletely cooked called residual monomer, Which is the cause of irritation of the soft tissues of the mouth, and hypersensitivity . (i.e. 100% polymerization is not present ).
• NB: ratio of polymerization is measured by degree of conversion which their maximum value about 75%.

Types :

Addition polymerization :

• The reaction between two molecules to form larger molecule Without the elimination of a smaller molecule
• Occurs as result of Free-radical or Ring-opening or ionic reaction

Condensation polymerization :

• The reaction between two molecules to form larger molecule with the elimination of a smaller molecule such as water or alcohol as a by-product.
• Occurs as result of by product

Steps of Polymerization reaction :

( activation ---> Initiation ---> Propagation ---> termination ---> Inhibition )

Activation & Initiation (induction):

• Initiator (Benzoyl peroxide or Camphorquinone) is activated by activator ( Chemical or Light or Heat )
• Activated Initiator generates free radical ( highly reactive unpaired electron ).
• Free radical will attack a double bond of monomer and pairs with one of the electrons, leaving the other member of pair free electron to form anew free radical.

Propagation:

• Theoretically; the chain reaction continues until all the monomer convert into polymer.

Termination: - chain process is terminated either by

1. Direct coupling of two radical chain ends .
2. By the exchange of a hydrogen atom from one growing chain to another.

• N.B: actually the reaction is never complete but there are residual monomers remain.

 

Inhibition of polymerization :

Cause:

• Any impurity in the monomer will retard the polymerization to prevent further growth.

Example of polymerization inhibitors:

1. Hydroquinone .
2. Large amounts of oxygen .
3. Eugenol .

Significance:

• Inhibitors are added to the monomers to prevent polymerization during storage and give adequate shelf life .

 

Notes on Ceramic

Q: why ceramic is brittle material ( difficult to shear plastically) or zero creep at room temperature ??

• Answer: due to minimum number of dislocation and its nature ionic bond ( if dislocation occurs +Ve come opposite to each other leading to catastrophic failure ‘’ brittle fracture’’ )

1. The stress at crack tip is higher many times than the stress in the material away from the crack tip resulting in stress concentration which weak the ceramic considerably .
2. Ceramic has low tensile strength, high compressive strength, so ceramic is weak under tension, strong under compression ( i.e crack increased by tension, reduced by compression )

Notes on Dental Composite Resin

• It’s a material used directly to restore the missed tooth structure esthetically & functionally.

Consist of :

1. Organic phase : polymer matrix.
2. Inorganic phase : ceramic filler that added to reduce the disadvantages of polymer which are abrasion & polymerization shrinkage .

Note Book :  

• Use of polymer only will not achieve a sufficient mechanical propertied and stability.
• Ceramic particles can’t be used alone as they are not condensable or packable, So polymer matrix is to bine the particles to enable the composite to used as a paste

Activation:

• It’s activated by light cure to adhesive first ,then to composite.