Polymer Science: Introduction to Polymers

Polymer Science & Technology CHEE 3204 Objectives

• Develop an understanding of the principles and concepts behind polymer science and its engineering applications.
• Understand the properties of polymeric materials and how they are applied.
• Develop understanding of polymerization process.
• Identify current and emerging polymer technologies.

Introduction to Polymers

• Polymer comes from the Greek words "Poly" (many) and "Meres" (parts).
• A polymer is a long chain of repeating units with an identical structure.
• Natural polymers include proteins, cellulose, and silk.
• Synthetic polymers include polystyrene, PVC, and nylon.
• Cellulose nitrate was synthesized in 1868, marking the beginning of the plastics industry.
• John Hyatt created cellulose nitrate/celluloid by mixing cotton and HNO3.

Development and Significance of Plastics

• Dr. Leo Baekeland developed the first man-made plastic which was phenol formaldehyde.
• Phenol formaldehyde is used in electrical plugs, electric iron and cookware handles, and grinding wheels.
• Cellulose acetate is used in toothbrushes, combs, cutlery handles, and eyeglass frames.
• Urea formaldehyde is used in buttons and electrical accessories.
• Polyvinyl chloride is used in flooring, upholstery, shower curtains, and cable insulation.
• Nylon is used in toothbrush bristles, stockings, and surgical structures.
• Synthetic fibers like nylon and polyesters can replace naturally occurring polymers such as cotton, wool, and silk.

Polymerization

• Vinyl chloride monomer has a double bond.
• Free radicals attach to C when a double bond breaks.
• CH2-CHCl, is the repeating unit and the C atoms become active.
• Conversion of monomer to polymer includes electron rearrangement and the addition of repeating units.

Polymer Classification

• Polymers can be classified based on source, structure, chemical microstructure, tacticity, molecular forces, and polymerization reaction.
• Source: Natural, semi-synthetic, and synthetic polymers
• Structure: Linear, branched chain, crosslinked, or network polymers
• Chemical Microstructure: Homopolymers, random copolymers, alternating copolymers, block copolymers and graft copolymers
• Tacticity: Isotactic and syndiotactic
• Molecular Forces: Elastomers, fibers, thermoplastic polymers, and thermosetting polymers
• Polymerization Reaction: Addition, condensation, copolymerization, and ring-opening polymerization

Natural vs. Synthetic Fibers

• Fibers are classified into natural, semi-synthetic, and wholly synthetic fibers.
• Natural fibers are filaments from solutions of modified cellulose or organic substances. The solutions are passed through and orifice, resulting in solidification.
• Nylon is superior to wool in tensile strength, stiffness, water absorbency, elastic recovery, strength, and heat resistance; it resists moths and mildew. A disadvantage of nylon is that is can cause allergic effects.
• Semi-synthetic fibers are a mixture of natural and synthetic fibers.
• Cotton is (C6H9O4OH)n, and Rayon is (C6H9O4OH)3n.

Comparison of Natural and Synthetic fibers

• The mechanical properties of fibers are influenced by their degree of orientation, crystallinity, and average chain length.
• Three spinning procedures include melt, dry, and wet spinning.

Classification Based on Structure

• Linear polymers consist of long, straight chains (e.g., HDPE, PVC).
• Branched chain polymers contain linear chains with some branches (e.g., LDPE, polypropylene).
• Crosslinked or network polymers consist of strong covalent bonds between various linear polymer chains via bifunctional and trifunctional monomers (e.g., polyglycerol sebacate, melamine, etc.).

Classification Based on Chemical Microstructure

• Homopolymers have all carbon atoms along their backbone.
• C chain polymers with only single bonds along the backbone are called Polyalkylenes (e.g., Polystyrene, Polypropylene, PVC).
• C chain polymers with double bonds along the chain (e.g., Polyacetylene).
• Heterochain polymers have more than one atom type in their backbone, grouped by atom and chemical group type.
• Polysiloxanes (-Si-O-) are heterochain polymers with different groups attached to Silicon.
• Carbon-Oxygen polymers include Polyethers (-C-O-), Polyesters (-O-C=O), Polyanhydrides of carboxylic acid (O=C-O-C=O), and Polycarbonates (-O-C-O-).
• Carbon-Sulfur polymers include Polythioethers (-S-C-) and Polysulfones (-S-C-).
• Carbon-Nitrogen polymers include Polyamines (-C-N-), Polyimines (-C=N-), Polyamides (-C-N-), and Polyureas (-N-C-N-).

Sub-classification of Chemical Microstructure

• Homopolymers: Polymers from a single monomer (PE, PP).
• Random Copolymers: Two randomly distributed monomers (e.g., Poly(acrilonitrile-ran-butadiene)). The chain is [A-B-B-A-B-A-A-B]
• Alternating Copolymers: Sequentially incorporated monomers (e.g., Poly(styrene-alt-maleic anhydride)), where the chain is (A-B-A-B-A-B-A-B).
• Block Copolymers: Linear arrangement of blocks with higher molecular weight (e.g., Polystyrene-b-polybutadiene-b-styrene). The chain is (-A-A-A-A-B-B-B-B-).
• Graft Copolymers: Differing backbone and side-chain monomers (e.g., Poly (isobutylene-graft-butadiene)). The backbone chain is -A-A-A-A-A-A-A-A and the sidechain is B-B-B-B-.

Polymer Tacticity

• Isotactic: Asymmetric substituent groups (R) lie on the same side of the extended-chain backbone, creating isotactic polymers.
• Syndiotactic: Substituent groups alternate from one side of the plane to the other, termed syndiotactic polymers.
• Tactic polymers are partially crystalline, while atactic polymers are amorphous by nature.

Molecular Forces in Polymers

• Elastomers: Polymers displaying elastomeric properties (stretch/contract upon applying/releasing stress), with polymer chains held together by the weakest intermolecular forces.
• Fibers: Thread-forming solids with high tensile strength and high modulus, attributed to strong intermolecular forces like hydrogen bonding, which imparts crystallinity.
• Thermoplastic Polymers: Linear or branched polymers melted upon heating, then molded and remolded using conventional techniques. They regain properties when applying or removing heat/pressure.
• Thermosetting Polymers: Heavily crosslinked polymers consisting of a dense 3-D molecular network that are rigid and intractable.

Polymerization Reactions

• Addition Polymers: Formed by the repeated addition of monomer molecules possessing double or triple bonds (e.g., polythene from ethylene). Polymers created from a single monomeric species are known as Homopolymers. Polymers created using 2 monomers are known as Copolymers.

• Condensation/Step-Growth Polymers: Formed by repeated condensation reactions between two different bi-functional or tri-functional units. Small molecules like water, alcohol, or HCl are eliminated. Poly(glycerol-sebacate) and Nylon 6,6 are examples.

Polymer Structure

• Plasticity is the property of a material to undergo permanent deformation under stress.
• Elasticity is the property of a material to deform under stress yet revert to its original shape.
• Plastics are artificial (synthetic) materials that are plastic during manufacturing, and shaped with heat and pressure.
• Molecular weight classifications include Number average molecular weight (Mn) and Weight average molecular weight (Mw).
• The Polydispersity Index (PDI) is expressed as the equation Mw/Mn.

Synthetic Resins

• Synthetic resins are materials synthesized chemically from simple organic raw materials and are used to make plastics.
• Synthetic resins are classified into Thermosetting and Thermoplastics
• Thermoplastics: Linear or branched polymers that melt when heated and can be molded and remolded.
• Thermosetting plastics: Heavily crosslinked polymers consisting of dense 3-D molecular networks, which makes them rigid and intractable.
• Elastomers: Polymers displaying elastomeric properties (stretch/contract upon applying/releasing stress).
• Crystalline polymers: Thermoplastic materials with a regularly arranged molecular structure with key properties such as Tg, Tc, and Tm
• Amorphous polymers: Thermoplastic materials with irregularly arranged molecular structures and higher amorphous regions. They solidify into a random coily structure with poor molecular packing/structure.

Properties of Polymers

• Liquid crystalline polymers have stiff, rod-like molecules arranged in large parallel arrays or domains in both melt and solid states.
• Organic polymers are formed by reacting organic monomers (C-C-C).
• Inorganic polymers are formed by reacting inorganic monomers (Si-O-Si).
• Examples of Thermoplastics: Cellulose acetate, Ethyl cellulose, Nylon 6,6, Nylon 6, Polyurethanes, polyethylene, polypropylen, PTFE, PS, PVC
• Examples of Thermosetting polymers: Alkyds, Epoxies, Phenolics, Silicones, Casein

Polymer Terminology

• Chain: A linear polymer formed by covalently linking monomeric units.
• Backbone: The main chain through which other branches or grafts are attached.
• Crosslink: A structure bonding two or more chains together.
• Network: A 3-dimensional polymer structure with interconnected chains.
• Copolymer: Polymers derived from more than one species of monomer.
• Block: A portion of a polymer molecule.
• Graft copolymer: A combination of two or more chains.
• Polymer blend: An intimate combination of polymers not chemically bonded.
• Star polymer: Three or more chains linked at one end through a central moiety.
• Star block copolymer: Three or more chains linked at one end through a central moiety.

Chain Length and Polymer Properties

• Polymer properties are influenced by chain length where increases to molecular weight raise the melting point.
• At room temperature, polymers with very short chains (100 g/mol) exist as liquids.
• Polymers with molecular weights around 1000 g/mol are waxy solids and soft resins.
• Solid polymers range between 10,000 and several million g/mol.
• Properties are affected by molecular weight and crosslinking (Elastic modulus, strength, stiffness etc).

Degree of Polymerization (DP)

• DP measures the number of repeating units strung together in the polymer chain. It is expressed by the expression n (CH2 =CH2) -------→ --(CH2—CH2--)n

• Polymerization occurs via sequential reactions of these monomers, which can form dimers that react with additional monomers to become trimer as they travel on to tetramer.

• Homopolymers are made up of one monomer

• Copolymers are made up of two monomers

• Terpolymers are made up of three monomers

• Tetrapolymers are made up of four monomers.

• Low molecular weight polymerization products (dimers, trimers, tetramers) are Oligomers.

• Molecular weight (polymer) = DP x Mw (repeat unit)

• The molecular weight of Polypropylene is DP multiplied by Mw (repeat unit) (3x12+ 6x1 = 42). Polypropylene with a DP of 3x10^4 would have a molecular weight = 1.26 x 10^6 g/mol or Da.

• Molecular weight distribution: During polymerization, chains grow to varying lengths leading to differences in Mw.

• Two average values from experiment include Mn (number average) and Mw (weight average) molecular weights.

Degree of Polymerization Equations

• Number average Molecular weight (Mn) is the weight of the sample divided by the number of molecules in the sample, written through an equation as Μn = Σ niMi/ Σ ni
• Weight average molecular weight (Mw): The equation takes account of average distribution of molecular weights of species rather than the number of molecules, written through an equation as Mw = Σ niM2i/ Σ ni Mi
• Polydispersity index (PDI): A measure of molecular distribution in a given polymer sample, calculated as Mw/Mn.