Polymers (Properties And Biodegradable Polymers)

Molecular Mass Of Polymers

Unlike small molecules, polymers consist of a large number of monomer units, and the degree of polymerization (number of repeating units, 'n') can vary among different polymer molecules in a sample. This leads to a distribution of molecular masses rather than a single, fixed molecular mass.

Understanding Polymer Molecular Mass:

The molecular mass of a polymer sample is typically expressed as an average value. The most common ways to represent this average are:

• Number Average Molecular Mass ($M_n$): This is the total mass of all polymer molecules in a sample divided by the total number of polymer molecules.

  If a polymer sample contains $N_1$ molecules of mass $M_1$, $N_2$ molecules of mass $M_2$, ..., $N_i$ molecules of mass $M_i$, then:

  $M_n = \frac{\sum N_i M_i}{\sum N_i}$

  This value is sensitive to the number of molecules, regardless of their size.

• Weight Average Molecular Mass ($M_w$): This average gives more weight to larger molecules. It is calculated by summing the product of the molecular mass and its weight fraction, or equivalently, by summing the product of the square of the molecular mass and the number of molecules, then dividing by the sum of the product of molecular mass and the number of molecules.

  $M_w = \frac{\sum N_i M_i^2}{\sum N_i M_i}$

  For most polymer samples, $M_w > M_n$. The ratio $\frac{M_w}{M_n}$ is known as the Polydispersity Index (PDI) or dispersity. A PDI of 1 indicates a monodisperse sample (all molecules have the same molecular mass), which is rare for synthetic polymers. Typical synthetic polymers have PDIs ranging from 2 to 10.

Significance of Molecular Mass: The molecular mass of a polymer significantly affects its physical and mechanical properties, such as:

• Tensile Strength: Generally increases with molecular mass.
• Melting Point: Increases with molecular mass.
• Viscosity: Increases significantly with molecular mass.
• Brittleness: Can increase at very high molecular masses.

Techniques like gel permeation chromatography (GPC) or size exclusion chromatography (SEC) are used to determine the molecular mass distribution of polymers.

Biodegradable Polymers

Biodegradable polymers are polymers that can be decomposed by the action of biological agents (like bacteria, fungi, and algae) into simpler substances such as carbon dioxide, water, and biomass under specific environmental conditions.

The increasing concern over plastic waste pollution has led to a greater interest in biodegradable polymers as sustainable alternatives to conventional petroleum-based plastics.

Types of Biodegradable Polymers:

Biodegradable polymers can be broadly categorized into:

• Naturally Occurring Biodegradable Polymers: These are derived from renewable resources and are inherently biodegradable.
  • Polysaccharides: Cellulose, starch, chitosan. Used in packaging, biodegradable films, and biomedical applications.
  • Polypeptides: Proteins like silk, wool, collagen. Used in sutures, tissue engineering, and drug delivery.
  • Natural Rubber: Can degrade over time, though at a slower rate.
• Synthetic Biodegradable Polymers: These are synthesized from monomers, often designed to break down under specific conditions.
  • Polylactic Acid (PLA): Derived from lactic acid (produced by fermentation of sugars). Biodegrades into lactic acid, which is metabolized. Used in packaging, disposable cutlery, and biomedical implants.
  • Polyhydroxyalkanoates (PHAs): A family of polyesters produced by microorganisms. They are fully biodegradable in soil and marine environments. Used in packaging, medical devices.
  • Polycaprolactone (PCL): A synthetic polyester. Biodegrades slowly and is used in drug delivery and medical devices.
  • Aliphatic Polyesters: Such as polybutylene succinate (PBS) and polyethylene succinate (PES).

Factors Affecting Biodegradation:

The rate and extent of biodegradation depend on several factors:

• Polymer Structure: Presence of ester or amide linkages that can be hydrolyzed by enzymes.
• Molecular Weight: Lower molecular weight polymers generally degrade faster.
• Crystallinity: Amorphous regions degrade faster than crystalline regions.
• Environmental Conditions: Temperature, humidity, pH, presence of specific microorganisms.
• Additives: Presence of plasticizers or fillers can affect degradation rates.

Applications of Biodegradable Polymers:

Biodegradable polymers are finding increasing use in:

• Packaging: Films, containers, disposable cutlery.
• Agriculture: Mulch films, controlled-release fertilizers.
• Biomedical Applications: Sutures, drug delivery systems, tissue engineering scaffolds, implants.
• Textiles: Biodegradable fibers.

Polymers Of Commercial Importance

Polymers have revolutionized modern life due to their versatility, low cost, and adaptability. Here are some polymers of significant commercial importance:

1. Polyethylene (PE):

• Monomer: Ethene ($CH_2=CH_2$).
• Types and Properties:
  • Low-Density Polyethylene (LDPE): Flexible, tough, transparent. Formed by high-pressure addition polymerization, resulting in branched chains.
  • High-Density Polyethylene (HDPE): Stiffer, stronger, opaque. Formed by low-pressure polymerization using catalysts (Ziegler-Natta), resulting in linear chains with less branching.
• Uses: Films, bags, bottles, pipes, containers, insulation for wires.

2. Polyvinyl Chloride (PVC):

• Monomer: Vinyl chloride ($CH_2=CHCl$).
• Properties: Rigid and durable, but can be made flexible with plasticizers. Good electrical insulator.
• Uses: Pipes, window frames, flooring, cable insulation, raincoats, medical tubing.

3. Polypropylene (PP):

• Monomer: Propene ($CH_2=CHCH_3$).
• Properties: Similar to HDPE but lighter, more rigid, and has a higher melting point. Good resistance to chemicals and heat.
• Uses: Packaging films, containers, automotive parts, fibers (carpets, ropes), laboratory equipment.

4. Polystyrene (PS):

• Monomer: Styrene ($CH_2=CHC_6H_5$).
• Properties: Rigid, transparent, brittle. Can be foamed (styrofoam). Good electrical insulator.
• Uses: Disposable cups and cutlery, packaging foam, insulation, CD cases, toys.

5. Polyesters (e.g., PET - Polyethylene Terephthalate):

• Monomers: Ethylene glycol and terephthalic acid (condensation polymerization).
• Properties: Strong, rigid, good clarity, good barrier properties.
• Uses: Bottles for beverages, fibers (Dacron, Terylene) for clothing and textiles, films.

6. Polyamides (e.g., Nylon):

• Monomers: Diamine and dicarboxylic acid (e.g., hexamethylenediamine and adipic acid for Nylon-6,6) (condensation polymerization).
• Properties: High tensile strength, abrasion resistance, elasticity, resistance to chemicals.
• Uses: Fibers for clothing, carpets, ropes, fishing nets, engineering plastics in automotive parts, electrical components.

7. Bakelite (Phenol-Formaldehyde Resin):

• Monomers: Phenol and formaldehyde (thermosetting condensation polymer).
• Properties: Hard, rigid, excellent electrical insulator, heat resistant.
• Uses: Electrical insulators, handles for cookware, radios and telephone casings.

8. Natural Rubber and Vulcanized Rubber:

• Monomer: Isoprene.
• Properties: Elasticity (natural rubber); improved strength, elasticity, and durability (vulcanized rubber).
• Uses: Tires, footwear, hoses, gloves, seals.