Unit 3- Class 11- High Polymers (Structure property relationship of polymers)

Structure property relationship of polymers:
The fundamental properties, which influence the structure property relationship are molecular mass, polarity, crystallinity, molecular cohesion, the nature of the polymeric chains and stereochemistry of the molecule.

Impact and tensile strength of polymers and molecular mass: Density, melt viscosity, impact and tensile strength are a few mechanical properties of a polymer. Tensile strength and impact strength increases with molecular mass up to a certain point and then become constant. The melt viscosity of the polymer initially shows a gradual increase with the molecular mass and steep increase at higher molecular masses. For polymer to be commercially useful it should have low melt viscosity, high tensile and impact strength.


Crystallinity: Any polymer will contain a definite percentage of crystalline part and amorphous part. The degree of crystallinity depends on how best the polymer chain can be closely packed. Crystalline regions of a polymer are formed when their individual chains are linear (without branching), contain no bulky substituents and are closely arranged parallel to each other. The chains of polymer may be held together by vander wall’s force, hydrogen bonding or polar interaction. A polymer with high degree of crystallinity has high tensile strength, impact and wear resistance, high density and high fusion temperature, it has high Tg, and melt viscosity.
Crystallinity of a polymer also depends on the stereo regular arrangement. Polymers like HDPE, isotactic and syndiotactic polypropylene etc are highly crystalline. On the other hand atactic polypropylene, polystyrene, PVC which have their substituents in a random arrangement are less crystalline.


Elasticity: Elasticity of a polymer material is mainly because of the uncoiling and recoiling of the molecular chains on the application of force. For a polymer to show elasticity the individual chains should not break on prolonged stretching. Breaking takes place when the chains slip past each other and get separated. In rubber this is avoided by molecular engineering such as 1. Introducing cross- link at suitable molecular position 2.
Avoiding bulky side group such as aromatic and cyclic structure in the repeat unit 3. Introducing more non- polar groups in the chains so that the chains do not separate on stretching. The structure should be amorphous, this can be brought about by introducing plasticizer molecule in the polymer chain by co-polymerization or by7 compounding the rubber with a suitable plasticizer liquid.


Elastic deformation( rheology) of polymer: This can be studied by applying stress on the polymer material and finding the deformation caused. Polymers, since they contain both crystalline and amorphous regions, exhibit a complex behavior. The deformation depends upon on the degree of crystallinity, degree of cross- linking and the glass transition temperature.

 
Chemical resistivity: If a polymer is attacked by a reagent it undergoes softening, swelling and loses strength. Chemical resistivity of polymers depends on number of factors like presence of polar and non- polar groups, the molar mass, degree of crystallinity, extent of cross linking.
Polymers with non- polar groups undergo a welling and dissolution in non-polar solvents like benzene, toluene and carbon tetra chloride etc. Polar polymers containing –OH group or –COOH group are soluble in polar solvents like water, alcohol etc. Polymer containing ester group (polyester) undergo hydrolysis with strong alkalis at high temperature. Polyamide like nylon containing –NHCO- group, NHCOO group can be hydrolysed  by using strong acids or alkali.

Polyalkenes, PVC, Flourocarbon are some polymers, which have high degree of chemical resistance. For a given polymer resistivity increase with increase in molar mass. Linear polymers have lower resistivity than branched chain and cross- linked polymers.


This is all about Structure property relationship of polymers.  In our next class we will learn about  some commercial polymers.