Properties

Properties of Introduction

1. Strength:  

   1. Strength of polymers increases with the chain length or the molecular weight of the polymer.No molecular polymer are soft or gummy and they are also brittle at low temperatures. The mechanical strength of the polymers increases rapidly when the degree of polymerization is more than 150. High molecular weight polymers have more strength and more resistance to heat. The mechanical strength of any polymer also depends upon the nature of inter molecular forces present. 

   2. Mechanical strength of polymers can be increased by introduction of more polar groups. Ex: OH, COOH, NH2 which are capable of hydrogen bonding. 

2. Plastic deformation is observed in thermoplastics. During the deformation the intermolecular forces between the different chains get easily broken upon application of heat or mechanical strength. This property is utilized for molding and reshaping of plastic materials. The polymeric chains which can slip one over another can undergo easy deformation. 

Ex : reversible plastic deformation is a common property of linear chain polymers. 

3. Physical State: Polymeric materials maybe crystalline or amorphous based upon their  

   1. repeating units  

   2. Cross linking 

   3. Packing properties 

   Long repeating units generally impart a very low degree of symmetry to the polymers and these kind of polymers do not crystallize easily. 

   Ex : polystyrene, pmma 

   Crystal nature of the polymer is responsible for denser packing of the molecules, thus increasing the intermolecular forces of attraction. Crystalline polymers are characterized by sharp and high softening temperature.  

   Irl,  nothing is 100% crystalline or amorphous. 

   Polymers are generally made up of cyrstallite beads embedded in amorphous matrix. 

   Elastomers are elastic in nature and are highly entangled. 

4. Chemical Resistance: Densely packed polymers have very little gaps between the polymeric chains, hence it becomes very difficult for the solvent of chemical molecules to penetrate and attack the polymer. 

   1. Polythene can be dissolved in Carbon tetra chloride / benzene / became (non polar) 

   2. Imp points: 

      1. Tendency to swell decreases with molecular weight  

      2. High molecular weight polymers when dissolved, produce highly viscous solutions. 

      3. More the crystalline nature of the polymer, more will be the resistance towards chemical attack. 

      4. More the cross linking, more will be the resistance to solvent attack. 

5. Effect of heat:  

   1. Amorphous polymers do not have any melting point but they are characterized by a softening point. At low temperature amorphous polymers are brittle like glass. When heated, they soften at a particular temperature called softening point and become rubber like material.  

      1. Glass =>(softening point) rubber => gum => liquid  

   2. With crystalline polymers there is a transition glass? temperature beyond which they are converted into thermoplastics. This thermoplastic when heated beyond its melting point gets converted into liquid.  

      1. Crystalline => thermoplastics => liquid