Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following polymers does not involve cross linkages? (a) melmac (b) bakelite (c) polythene (d) vulcanized rubber

Step-by-Step Solution

Answer

(c) polythene

Step 1: Understand Cross-Linkages

Cross-linkages in polymers refer to the presence of chemical bonds that connect different polymer chains together. These bonds create a network structure, significantly altering the physical properties of the polymer by making it more rigid and durable.

Step 2: Analyze Each Polymer

Let's examine each option: (a) **Melmac** - A type of melamine formaldehyde that involves cross-linkages. (b) **Bakelite** - A phenolic resin known for its widespread cross-linked structure. (c) **Polythene** - A polymer that typically does not have cross-linkages. (d) **Vulcanized Rubber** - Contains sulfur-induced cross-links.

Step 3: Identify Polymer Without Cross-Linkages

Among the listed polymers, **polythene** is a polymer that does not typically involve cross-linkages in its standard form. It consists of linear or branched non-crosslinked chains, making it flexible and less rigid compared to the others.

Key Concepts

Cross-linkages

In the world of polymer chemistry, cross-linkages play a vital role in determining the characteristics of polymers. These chemical bonds connect several polymer chains, forming a complex network that affects the polymer's properties.

Why Cross-linkages Matter

• Strength and Durability: They increase the strength and make the material more robust.
• Heat Resistance: Polymers with cross-linkages can withstand higher temperatures without melting.
• Elasticity: Some cross-linked polymers like rubber retain their shape after deformation.

Without cross-linkages, polymers remain more flexible. They might lack rigidity which is preferable in certain uses, such as plastic bags and food wraps.

Polyethylene

Polyethylene, commonly known as polythene, is one of the most prevalent plastics used globally. It is a type of polymer that is typically created through the polymerization of ethylene molecules.

Characteristics of Polyethylene

• Structure: It generally consists of long, linear chains or branches without cross-linkages.
• Flexibility: Due to the absence of cross-linkages, polyethylene is highly flexible.
• Everyday Use: Found in plastic bags, bottles, toys, and other household items.

Despite being less durable than cross-linked polymers, its versatility makes it invaluable in the manufacturing sector.

Polymer Chemistry

Polymer chemistry is the branch of chemistry that focuses on the synthesis and chemical properties of polymers. Understanding polymer chemistry is essential to innovating new materials and improving existing ones.

Key Concepts in Polymer Chemistry

• Monomers and Polymers: Polymers are large molecules made by joining smaller units known as monomers.
• Types of Polymers: Based on structure and properties, polymers can be linear, branched, or network-linked.
• Applications: From medical devices to automotive parts, polymers play a role in various industries.

Polymer chemists aim to mix and match monomers and different bonding arrangements to produce materials with desired traits.

Thermosetting Polymers

Thermosetting polymers are a fascinating category of polymers characterized by their unique ability to form irreversible bonds upon heating.

Understanding Thermosetting Polymers

• Permanently Set: Once they are set or cured, they cannot be remelted, unlike thermoplastics.
• Cross-links: Their cross-linked structure makes them hard and heat-resistant.
• Common Examples: Include bakelite, melmac, and some epoxies used in electronics and adhesives.

Due to their rigid structure, thermosetting polymers do not dissolve or deform easily, making them suitable for products requiring high durability and stability.