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

The correct answer is (a) I, III, IV.

Step 1: Understanding the Problem

The question is asking which polymers have cross linkages. Cross-linked polymers have a structure where the polymer chains are linked together by covalent bonds, increasing their stability and rigidity.

Step 2: Analyzing Option I: Bakelite

Bakelite is a thermosetting plastic, a type of polymer that is heavily cross-linked. It is known for its rigidity, heat resistance, and widespread use in electrical insulators. Therefore, Bakelite does involve cross linkages.

Step 3: Analyzing Option II: Polythene

Polythene, also known as polyethylene, is a thermoplastic polymer. It usually does not have cross linkages in its basic form, as it consists of linear or branched polymer chains which can be melted and reshaped.

Step 4: Analyzing Option III: Melmac

Melmac is a brand name often used for melamine-formaldehyde plastics. These are thermosetting materials, indicating that they are cross-linked during curing, increasing rigidity and heat-resistance.

Step 5: Analyzing Option IV: Vulcanized Rubber

Vulcanized rubber involves a chemical process where sulfur cross-links are added to the natural rubber. This process enhances the elasticity, strength, and durability of the rubber, clearly involving cross linkages.

Step 6: Conclusion from the Analysis

Based on the analysis, Bakelite (I), Melmac (III), and Vulcanized rubber (IV) involve cross linkages. Thus, the answer involves these polymers.

Key Concepts

Thermosetting plastics

Thermosetting plastics are a fascinating class of polymers that undergo a chemical transformation when heated. Unlike thermoplastics, which can be melted and reshaped multiple times, thermosetting plastics form irreversible bonds. This transformation is due to the formation of cross-linked structures.
During the curing process, the polymer chains connect through covalent bonds, locking the structure in place.
This process gives thermosetting plastics their characteristic high mechanical strength and heat resistance.

• They do not soften upon reheating, making them ideal for high-temperature applications.
• Common examples include Bakelite, used in electrical insulators, and Melmac, found in dishes and kitchenware.

Thermosetting plastics are thus essential in applications requiring materials that maintain their shape and structure under stress, making them prevalent in the automotive, electronic, and industrial sectors.

Vulcanization

Vulcanization is a critical process in the production of rubber that involves adding sulfur or other curatives to natural rubber. This process creates cross-linkages between the rubber's polymer chains, dramatically changing its properties.
Before vulcanization, natural rubber tends to be sticky and lacks structural integrity, limiting its uses. Vulcanized rubber, on the other hand, becomes much more resilient:

• The elasticity is greatly improved, allowing the rubber to stretch and return to its original shape.
• The strength and durability are enhanced, making it suitable for products like tires, hoses, and shoe soles.

The process of vulcanization was invented by Charles Goodyear in the 19th century and has since been fundamental in the manufacturing of various rubber goods worldwide.

Polymer chemistry

Polymer chemistry is the branch of chemistry focused on the synthesis and chemical properties of polymers. These large molecules are made up of repeating units called monomers, which can link into long chains.
Polymer chemistry explores how the arrangement and bond types of these monomers determine the polymer's characteristics. In the case of cross-linked polymers, the polymer chains are interconnected via covalent bonds:

• This interconnection enhances the mechanical strength and thermal stability.
• Cross-linking can be achieved through various chemical reactions, typically during a curing process.

Understanding how polymers are made and modified is essential for developing new materials that meet specific industrial or commercial requirements, thereby making polymer chemistry a key field in materials science.