Side Chain Effects on Polymer Properties
Effect of Side Chains on Polymer Properties
Hydrogen as a Side Chain
In hydrogen polymers, the properties are determined by C-H bonds. Thus, these polymers are:
- Water-resistant
- Soluble in organic solvents
- Resistant to chemical attack
- Sensitive to free radical attack
Hydrogen as a side group does not provide any steric hindrance to the torsional mobility of the main chain. Therefore, such polymers have low glass transition temperatures. The small size of hydrogen favors the packing of chains; hence, the polymers possess a high degree of crystallinity.
Alkyl Group
Alkyl side groups enhance the hydrocarbon character of the polymer. This makes the polymer resistant to water and increases its solubility in organic solvents. The side group in isotactic polypropylene makes it a crystalline thermoplastic. The two terminal methyl groups in polyisobutylene enhance the flexibility of the polymer.
Aryl Group
Phenyl rings are bulky, rigid, and water-repellent in nature. As side groups, they create steric hindrance and impart stiffness to the chain. For example, the glass transition temperature of polystyrene is almost 100°C higher than that of polypropylene and 200°C higher than that of polyethylene.
Fluorine as a Side Group
Fluorine attached as a side group greatly affects almost all the properties of the polymer. Due to this reason, fluoropolymers are treated as a separate class of polymer. Fluorine imparts water repellency and water insolubility to the polymer. It imparts thermal and oxidative stability and makes the polymer resistant to any type of solvent. For example, polyvinyl chloride and Teflon.
Chlorine as a Side Group
Chlorine as a side group increases the polymer’s resistance to chemical attack, for example, polyvinyl chloride and polyvinylidene chloride. The presence of chlorine as a side group reduces the solubility in solvents, which is important for many applications.
Cyano as a Side Group
The cyano group is polar and hydrophilic in nature. It changes the solubility behavior of hydrocarbon polymers, making them insoluble in nonpolar organic solvents and soluble in polar organic solvents like dimethylformamide, dimethylacetamide, etc. Thus, copolymers containing acrylonitrile monomers are used in solvent-resistant elastomers. The cyano group imposes chain stiffness through dipole-dipole interaction; therefore, it is a crystalline polymer. Cyanopolymers have moderate thermal stability. Polyvinylidene cyanide has excellent stability towards water.
Hydroxyl Group as a Side Chain
Hydroxyl groups are hydrophilic and form strong hydrogen bonds. Intramolecular hydrogen bonding restricts torsional movement. Polyvinyl alcohol, although hydrophilic, does not dissolve in water at room temperature; however, it dissolves in hot water. This is because hydrogen bonds are broken in hot water. The glass transition temperature of polyvinyl alcohol is high due to the effect of the -OH group on chain mobility.
Amide Side Group
The amide group contains both NH2 and C=O. Both of these groups form hydrogen bonds with water. Thus, such polymers are hygroscopic. They undergo hydrolysis in both alkaline and acidic media. Polyacrylamide is an example of such a type of polymer.
Ester Side Chain Group
The commercially important polymers of this type are esters of polyacrylic acid, polymethylacrylic acid, and esters of polyvinyl alcohol. The ester group in the side chain favors solubility in organic solvents and makes the polymer insoluble in water.