Properties of polymers

physical and chemical properties of polymers, mechanical properties toughness, strength, elasticity,

1. Properties of polymers By CH. SAI SRI RAMA CHANDRA MURTHY

2. Properties of polymers: Polymers possess both physical and mechanical properties. Physical properties of polymers are: Solubility Effect of heat on polymer Diffusion and permeability Mechanical properties of polymers are: Toughness Strength Elasticity

3. Physical properties: 1. Solubility: When a polymer is dissolved in a suitable solvent, the polymer absorbs water and swells in size. Slowly polymer goes into solution a viscous polymer solution which is heterogeneous. Where as a non-polymer like NaCl when added to water, ionizes and form homogeneous solution. Like this polymer behave differently towards the solvents.

4. 2. Effect on heat on polymer: At room temperature all polymers are solids existing in rubber or visco elastic state. As the temperature increases the polymer melts and this is called the melting point of the polymer ( T f ) where the material exist in visco fluid state. As the temperature further increases to boiling (T b ) the polymer decomposes to monomers which are in vapour phase, they exist in solid and liquid phase depending on the temperature. When the temperature is lowered below zero degree and below zero degree a temperature comes where the polymer becomes hard, brittle and glassy and the polymer behaves like glass. This temperature is called glass transition temperature ( T g ).

5. The polymer remains soft flexible and rubbery above T g an below T g the polymer becomes hard brittle and glassy. For Eg : T g of Rubber = -73ºC Glass state (brittle) Rubber Visco elastic state Degradation of polymer to monomer Viscofluid state T g T b T f Room temperature Temperature

6. 3. Diffusion and permeability: Crystalline polymer resist diffusion of molecules through voids or other gaps and tends to resist diffusion because of much greater degree of molecule packing. The diffusion rates are higher in rubber.

7. Mechanical properties: Polymers behave differently towards the following mechanical properties. 1. Toughness : The impact strength is measured as toughness. Below glass transition temperature ( T g ) the polymer break with a brittle fracture and they become tougher when the temperature proceeds from room temperature to T g . The size of the crystalline structure also determines the toughness. The small spherical crystalline masses increases toughness.

8. 2. Strength : The strength of polymer depends on the length of polymer chain, branching or cross-liking which are controlled by elastomeric properties, crystallinity and branching of chain. The strength of a polymer increases with increase in molecular weight. The inter molecular forces of attraction, presence of polar groups and chain length increases the strength of the polymer. Four types of stress-strain curves ( The line ends where the sample breaks)

9. The strength of a polymer is determined by stress-strain test. Typical stress-strain curves for different types of polymers are explained. Crystalline region cannot contribute to increase in tensile strength, where as amorphous region contribute to increase in tensile strength. Elasticity: Elasticity is the property of recovering original shape after the removal of deforming stress. Natural rubber possess high elasticity due to coiled helix structure of poly-isoprene.

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