Polymers are composed of molecules of molecular weight from about 10^3 to 10^7. A polymer is made up of repeated basic units produced from monomers. Polymers can be synthesized from various types and combinations of monomers to yield unusual properties, both physical and chemical.
Classification of Polymers
Polymers are difficult to classify because there are so many interacting relationships among many polymeric materials.
Four methods of classification are frequently used.
- Physico-chemical structure
- Mode of preparation
- Physical properties
- Technical Applications
1. Physico-chemical structure of polymers
1.1 Functionality:- The ultimate result of coupling monomers to produce polymers is wholly dependent on the functionality of the monomer. A molecule is called a mono, bi, tri, or polyfunctional, depending on the number of reactive bonds or groups available for coupling during the polymeric reaction.
- Unifunctional - Example: CH3CHOOH where are carboxyl (COOH) groups are the only chemically reactive groups. Obviously, a polymeric chain cannot be made from this acid alone.
- Bifunctional - Example:- CH2 = CH2 is the double bond splits from coupling with another molecule containing a double bond, not necessarily ethylene. If only ethylene were present the polymer would be.
- Trifunctional - Example:- Trimethylol phenol made from phenol and formaldehyde.
1.2 Physical Structure and Functionality
- Linear polymers - Formed from bifunctional groups only.
- Cross-linked polymers - formed from bi- and trifunctional groups,
- Branched-chain polymer - an intermediate case where branches grow from the parent chain without cross-linking.
1.3 Polymeric Chemical Reactions
- Polycondensation Reaction - monomers react by repeatedly splitting off H2O, NH3, CH2O, NaCl, or other small molecules in a step-by-step reaction to build up the molecular size.
- Additional reaction - occurs among molecules containing double or triple bonds or with oxo-ring compounds; no small molecules are liberated and the reaction is the rapid chain type.
2. Modes of Polymerization
Polycondensation methods differ appreciably from those used in addition to polymerization and therefore the discussion will be divided into two sections.
2.1 Condensation Polymerization Methods
- Roughly follows the procedure used for low molecular weight group reactions.
- Stoichiometric proportions of reactants chosen for desired final properties.
- Can be processed with or without solvent additional.
- Mixed-in batch reaction vessels with catalyst addition and heat control to avoid premature gelation.
- The major problem is to remove condensation products such as H2O to yield a higher DP vacuum processing or azeotropic distillation schemes used.
2.2 Additional Polymerization Methods
These factors are carried out via controlled heat and catalyst conditions under these classifications.
2.2.1 Homogenous polymerization
- Bulk-monomer is only feed that can be gas, liquid and solid.
- Solution-monomer is completely dissolved in a solvent.
2.2.2 Heterogeneous polymerization
- Emusion-monomer is emulsified in an aqueous media as micelles.
- Suspension-monomer is held in an aqueous or other type of media as larger droplets in suspension.
2.2.3 Bluk Polymerization
(i) Gas phase type-ethylene or propylene monomers with BF, catalyst.
(ii) Liquid phase type
- Most common.
- Problems of effective heat dissipation of exothermic reactions as viscosity of polymers increases with DP.
- Products are optically clear, pure polymer.
- Most frequently a batch-wise process.
- Continuous bulk polymerization is now being done by carrying out an early stage in a tower reactor, and then atomization of the polymer in an insert gas spray chamber where polymerization is completed and a solid powder is obtained. polystyrene power is made by this process or method.
2.2.4 Solution Polymerization
- Has the advantage of better heat control.
- Soluvent slows down reaction and causes considerable chain termination and low molecular weight polymer.
2.2.5 Emulsion Polymerization
- The most used method of polymerisation.
- Overcomes difficulties of two previous methods, heat control and low DP.
- Monomer + H2O emulsified with a suitable reagent to yield spherical monomer particles in the 1-10 μ range, called micelles.
- Heat is controlled by transfer to the aqueous phase with little change in the viscosity of the emission.
- The reaction proceeds rapidly in each particle with some shrinkage to form a stable emulsion.
- The disadvantage of emulsion polymerization-occludes emulsifiers, catalysts and products; has poor capacity and low electrical resistance which are sometimes critical properties.
3. Physical Properties of Polymerization
The physical properties of polymers are an important subject, but one which can't be treated in detail in this brief outline of the entire polymer field.
3.1 Important Physical Properties List
(i) Optical Properties
- Odor
- Hardness
- Density
- Weatherability
(ii) Mechanical Properties
- Tensile strength
- Compressive strength
- Flexural strength
- Shear strength
- Impact resistance
- Rigidity and brittleness
- Ductility
- Creep
- Fatigue
- Dimensional stability
- Sound transmission
(iii) Thermal Properties
- Thermal Diffusivity
- Heat resistance
- Coefficient of expansion
- Flammability
(iv) Electrical Properties
- Resistivity
- Dielectric
- Power Factor
- Arcing resistance
(v) Solvent Properties
- Solubility
- Plasticizer compatibility
- Viscosity
(vi) Chemical Resistance to;
- Chemical solutions
- Oils and fats
- Solvent
4. Technical Properties of Polymer
4.1 Adhesives Polymers
Polymers of the resin type are used for bonding where some water resistance is necessary. plywood laminate glueing is major use. Cellulose adhesive comprises cellulose derivatives dissolved in a solvent.
4.2 Coating and Film Polymers
This application probably accounts for the largest production of polymers. Free films of the polyethene and cellulose type are encountered daily. Protective and decorative coating such as synthetic resin solvent mixtures, lacquers and varnishes are products from a major industry.
4.3 Fibers Polymers
Synthetic polymeric fibres are linear molecules formed by extrusion or spinning into small diameters or thin sections, with weight per unit length and economic criterion.
4.4 Solid Shapes Polymers
Polymers as resins, can be molded, cast, laminated or extruded into a variety of shapes rather easily. Finishing operations such as machining, grinding, and polishing require little additional effort expense.