Nylon: Properties, Preparation & Uses | Class 8 Chemistry

Plastics (polymers) are materials composed of extremely long molecular chains made from repeating units called mers, with their properties determined by molecular structure including chain arrangement (linear, branched, or cross-linked), crystallinity, and side group composition; these factors influence whether polymers behave as thermoplastics (melting when heated) or thermosets (decomposing when heated), and determine key mechanical properties like stiffness, strength, and ductility, which vary significantly with temperature around the glass transition temperature.

Thermoplastics become soft and moldable when heated and harden when cooled, with this process repeatable multiple times. They undergo only physical changes, not chemical changes. Examples include polythene, PVC, and polystyrene. Thermosetting plastics harden permanently when heated and cannot be reshaped. They undergo chemical changes forming cross-links. Examples include Bakelite and melamine. Thermoplastics have linear or branched structures held by weak van der Waals forces, while thermosetting plastics have cross-linked structures.

Thermoplastics and thermosetting plastics exhibit fundamentally different behaviors under heat and have distinct molecular structures. Thermoplastics undergo reversible phase changes when heated, losing strength temporarily but regaining it upon cooling, allowing reshaping. They have 2D structures without cross-links, lower strength, and lower melting points. Thermosetting plastics undergo irreversible degradation when heated, with permanent loss of structural integrity. They possess 3D cross-linked networks providing higher strength and higher melting points. Thermoplastics are manufactured via addition polymerization without byproducts, while thermosetting plastics use condensation polymerization producing water as a byproduct. These differences determine their respective applications in engineering and manufacturing contexts.

Plastics are polymers that can be easily molded. They are divided into thermoplastics (soften on heating, can be remolded, soluble in organic solvents, non-brittle) and thermosetting plastics (become hard lumps when heated, cannot be remolded, insoluble, brittle). Thermoplastics use addition polymerization (linear structures), while thermosetting plastics use condensation polymerization (3D cross-linked structures). Examples: thermoplastics include PVC, nylon, polyethylene; thermosetting include bakelite, melamine, epoxy.

Thermoplastics and thermosetting plastics differ fundamentally in their properties: thermoplastics soften on heating, consist of linear polymers, are prepared by addition polymerization, are soft and weak with weak intermolecular forces, can be reclaimed and reshaped, and are soluble in organic solvents (examples: polythene, PVC, polystyrene); while thermosetting plastics do not soften on heating, consist of three-dimensional network polymers, are prepared by condensation polymerization, are hard and brittle with strong intermolecular forces, cannot be reclaimed or reshaped, and are insoluble in organic solvents (examples: bakelite, urea formaldehyde, melamine).

Synthetic fibers like polyester are essentially plastic that never biodegrade. Mixed fibers such as polycotton cannot be recycled. The speaker explains that wearing plastic next to the skin can cause microplastics to shed and potentially affect hormones. This environmental and health concern led the speaker to establish a fabric shop that only offers 100% natural fibers with no synthetics or greenwashing.

Washing synthetic fiber clothing releases microplastics less than 5mm in length into waterways. Studies suggest synthetic fibers contribute more than one-third of all ocean microplastics. Additionally, polyester clothing doesn't biodegrade in landfills, rivers, or oceans. Recycling polyester is challenging because it's often mixed with cotton, spandex, chemicals, and dyes that must be separated first.

Plastic fibers such as polyester, nylon, and acrylic, even when recycled or recyclable, create microplastic pollution every time they are washed. After disposal, these fibers typically end up in landfills where they continue to create pollution. This makes synthetic fibers environmentally problematic despite recycling capabilities.

Washing synthetic fiber clothing releases microplastics (less than 5mm in length) into waterways. Studies suggest synthetic fibers contribute more than one-third of all microplastics in the ocean. Additionally, recycling polyester is challenging because it is often mixed with cotton, spandex, chemicals, and dyes that must be separated before recycling, making the process complex and expensive.

Synthetic fabrics like acrylic and polyester are problematic environmentally because they're made from plastic, use fossil fuels, release microplastics into air and waterways, and won't biodegrade. They also perform poorly by tending to pill and trapping sweat. Blends combining natural and synthetic fibers are worse because they're almost impossible to recycle, require more cleaning, quickly look washed out, and contain microfibers that pollute the environment. Natural fiber blends (cotton and wool) are biodegradable but still involve chemical treatments.

Nylon-6,6 is formed by condensation polymerization between hexanediamine (H2N-(CH2)6-NH2) and adipic acid (HOOC-(CH2)4-COOH). During the reaction, one H from the amine group and one OH from the carboxylic acid group are eliminated as water, forming an amide linkage (-CONH-). This process repeats to form the polymer chain. The bond forms between the nitrogen of the amine group and the carbon of the carboxylic acid group.

This section covers condensation polymerization (step growth polymerization) where monomers react with elimination of small molecules like water. Nylon is presented as the classic example, formed from diamine and dicarboxylic acid monomers. The instructor explains that Nylon got its name from New York and London, where it was developed. The section details Nylon 6,6 structure, formed from hexanediamine (6 carbons) and adipic acid (6 carbons), with the repeating unit -NH-(CH2)6-NH-CO-(CH2)4-CO-. The amide linkage (-CONH-) connects the monomers. The section also covers Nylon 6, formed from caprolactam through ring-opening polymerization.

Nylon 6,6 is formed through condensation polymerization between hexamethylenediamine and adipic acid. The reaction involves nucleophilic attack by the amino group on the electron-deficient carbonyl carbon, forming a tetrahedral intermediate that collapses to expel water and form an amide bond. This step-growth mechanism allows chains of different lengths to react with each other, making molecular weight control more challenging compared to addition polymerization.

Nylon 6,6 is a polyamide formed by condensation polymerization of hexamethylenediamine (a six-carbon diamine) and adipic acid (a six-carbon dicarboxylic acid). The reaction eliminates water molecules as the amide linkages form. The name 'nylon 6,6' indicates that both monomers contain six carbon atoms each. The polymerization requires high temperature and high pressure conditions. The resulting polymer has amide linkages (-CO-NH-) connecting the repeating units, characteristic of polyamides.

Condensation polymerization is a reaction where two different monomers react to form a polymer with the elimination of a small molecule (usually water). The general equation is: nA + nB → (A-B)n + nH2O. The resulting polymer is called a copolymer. Nylon-6,6 is formed by condensation polymerization between hexanedioic acid (adipic acid) and 1,6-diaminohexane (hexanediamine): nHOOC-(CH2)4-COOH + nH2N-(CH2)6-NH2 → [NH-(CH2)6-NH-CO-(CH2)4-CO]n + 2nH2O. The resulting polymer has alternating amide linkages.