Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of these polymers involve cross linkages? (I) Bakelite (II) Polythene (III) Melmac (IV) Vulcanized rubber (a) I, III, IV (b) II only (c) III, IV (d) I, IV

Step-by-Step Solution

Answer

(a) I, III, IV

Step 1: Understand the Problem

The problem asks which of the given polymers involve cross-linkages. Cross-linking is a process where polymer chains are linked together by covalent bonds, which often leads to enhanced mechanical properties like increased strength and elasticity.

Step 2: Analyze Bakelite

Bakelite is a thermosetting plastic, which means it hardens permanently once set and does not melt. It is known to contain cross-linked polymers, giving it a rigid, three-dimensional structure.

Step 3: Evaluate Polythene

Polythene, or polyethylene, is a common thermoplastic. It consists of linear or branched chains without cross-linkages, allowing it to be remolded.

Step 4: Examine Melmac

Melmac is a brand name for a type of thermosetting plastic made from melamine resin, which involves cross-linking through polymerization, resulting in a stable structure.

Step 5: Consider Vulcanized Rubber

Vulcanized rubber is rubber treated with sulfur or other curatives, forming cross-linkages between polymer chains. This cross-linking enhances its elastic properties and resistance to deformation.

Step 6: Determine Solution

Based on the analysis, Bakelite (I), Melmac (III), and Vulcanized rubber (IV) all exhibit cross-linkages. Therefore, the correct polymers are part of option (a): I, III, IV.

Key Concepts

Thermosetting Plastics

Thermosetting plastics, like Bakelite and Melmac, are a fascinating type of plastic polymer. Once they are molded and set, they cannot be remolded or reheated to soften. This final setting is due to the formation of cross-linked chains within their structure. Initially, thermosetting plastics start out similar to any other liquid resin or soft solid but undergo a chemical reaction when cured with heat or a catalyst. This reaction forms covalent bonds that create a rigid, three-dimensional network. Once set, these plastics become incredibly strong and resistant to heat and many chemicals. This makes them ideal for applications requiring durability and heat resistance, such as electrical insulators, kitchenware, and adhesives.

• They exhibit excellent thermal stability.
• They are not as flexible as thermoplastics.
• Once hardened, reshaping or melting them is impossible.
• They tend to be brittle, making them suited for specific applications.

Covalent Bonds in Polymers

Covalent bonds are responsible for the formation of long chain-like structures in polymers. In polymers, these bonds occur when atoms share electrons, ensuring that the polymer chains are held together strongly. The strength of these bonds contributes significantly to the overall stability and durability of the material. Cross-linking takes this process further by creating bridges between the chains through additional covalent bonds. This enhancement creates a three-dimensional lattice that can alter the polymer's properties.

• Covalent bonds in polymers offer high strength and stability.
• They are crucial in forming both thermosetting and thermoplastic polymers.
• Cross-links in polymers result from additional covalent bonding between molecular chains.
• This process is key for producing materials with high resistance to deformation and thermal stability.

Vulcanization of Rubber

Vulcanization is a chemical process that enhances the properties of natural rubber by forming cross-links between individual polymer chains. Introduced by Charles Goodyear, this vital process typically involves adding sulfur or other agents to rubber and heating the mixture. This creates bridges or cross-links between the long chains of polyisoprene (the principal constituent of rubber). Through vulcanization:

• The rubber becomes more elastic and less sticky.
• Its tensile strength increases, enhancing durability.
• It gains improved heat resistance.
• The material resists degradation more effectively over time.

Vulcanized rubber is pivotal in a myriad of everyday applications, including tires, shoe soles, hoses, and conveyor belts, due to these enhanced properties.