M7 OC6 Polymers

IQ6. What are the properties and uses of polymers?

6.1 Addition polymers:

a) Polyethylene.

b) Polyvinyl chloride.

c) Polystyrene.

d) Polytetrafluoroethylene.

6.2 Condensation polymers:

a) Nylon.

b) Polyesters.

Some terms you must know:

• Monomer: A relatively simple molecule from which a polymer can be made.
• Polymer: A large molecule in which one or more monomers are repeated.
• Polymerisation: The reaction of monomers to form a polymer.

There are a number of properties which need to be taken into account when we are selecting a polymer for a particular use. These include:

• Melting or softening point
• Stability to heat and/or light
• Chemical stability
• Mechanical strength
• Flexibility or rigidity

These properties are affected by the structure of the polymer which is characterised by

• Average molecular weight: the longer the chain, the higher the melting point and the harder the polymer
• Chain branching: unbranched chains tend to intertwine and line up very closely producing a highly crystalline arrangement, which increases density, melting point and hardness. More branching produces a more amorphous (less structured) macromolecule with lower m.p., low density, softness and flexibility.
• Chain stiffening: addition of a large side group into a chain reduces its flexibility. Addition of chlorine (PVC) or benzene (polystyrene) produces more rigidity in the polymer.
• Point of interest - don't need for this course Crosslinks: joining of two or more linear chains to increase the hardness of the polymer. Vulcanised rubber has sulfur bridges between the chains for strength.

There are two types of polymerisation processes: addition polymerisation and condensation polymerisation.

Addition polymerisation

Addition polymerisation occurs between monomers of the type CH2=CHX where these add together by breaking one of the double bonds and forming long chains. There are no atoms lost in the process.

eg. n(CH2=CHX) → —(CH2-CHX)n—

Note that while the monomer is unsaturated, the polymer is saturated. Some commercially significant examples of addition polymers include: polythene (polyethene), polystyrene (polyethylbenzene), PVC (polychloroethene), polyacrylonitrile (polycyanoethene), teflon (PTFE or polytetrafluoroethene) and polypropylene (polypropene).

• Ethylene can act as a monomer that polymerises to form polyethylene. Polyethylene is an addition polymer, as it is the only product when ethylene undergoes polymerisation.
• The double bond in ethylene breaks open to form single bonds with neighbouring molecules, forming the long saturated (alkane) polyethylene chain.
• NOTE CAREFULLY: It is called polyethylene from its monomer, even though the long chain polymer is all single bonds.

There are two industrially important polymers made from polyethylene. Contrast the structure and properties for each polymer

1. Low Density Polyethylene, LDPE:

Uses include: Wrapping materials, Carry bags, Milk bottles, Squeeze bottles

2. High Density Polyethylene, HDPE: has a high strength-to-density ratio

Uses include: Kitchen utensils, Toys, Grocery carry bags, Building materials. , HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber

VIEW videos

• Ethylene https://youtu.be/6nCP54Em5tI 10.43
• What is an addition polymer https://youtu.be/FesZ8CzNs4Y 7.25
• POM 4 https://www.youtube.com/watch?v=yr6hZ81yYho&index=5&list=PLeFSFSJ9WqSA1rv6SqTeF0Uz_4ObE3bE2 6.51
• POM 6 https://www.youtube.com/watch?v=4TSjGAZbRmU&index=7&list=PLeFSFSJ9WqSA1rv6SqTeF0Uz_4ObE3bE2 6.10

• Ethylene monomers can be modified by adding functional groups, which replace hydrogen atoms. For polyvinyl chloride, a chlorine atom replaces a hydrogen on one carbon (IUPAC name is chloroethene).

Vinyl choride (or chloroethene) is the monomer used in the production of polyvinyl chloride (PVC).

Polyvinylchloride (PVC) [-(-CH₂ -CHCl-)n-] is one of the three most important polymers currently used worldwide. This is because PVC:

• is one of the cheapest polymers to make
• has a large range of properties so can be used to make hundreds of products

Uncompounded PVC is colourless and rigid and possesses poor stability towards heat and light. However, the use of additives/stabilisers enables us to change the properties of the PVC to how we desire.

Advantages

PVC is water resistant and flame resistant and therefore it is used in the manufacture of electrical insulation, appliance leads, sewerage and drain pipes and garden hoses.

PVC comes in two basic forms: rigid (sometimes abbreviated as RPVC) and flexible. The rigid form of PVC is used in construction for pipes, and in doors and windows. It is also used in making bottles, non-food packaging, food-covering sheets, and cards (such as credit cards or membership cards). It can be made softer and more flexible by the addition of plasticisers. In this form, it is also used in plumbing, electrical cable insulation, imitation leather, flooring, signage, inflatable products, and many applications where it replaces rubber. By adding to cotton or linen, it is used to make canvas.

PVC polymers are linear and are strong. The monomers are mainly arranged head-to-tail, meaning that there are chlorides on alternating carbon atoms. PVC consists of polar molecules which are attracted to each other by dipole-dipole interactions due to electrostatic attractions of a chlorine atom in one molecule to a hydrogen atom in another atom: These considerable intermolecular attractions between polymer chains make PVC a fairly strong material.

Disadvantages

• In PVC, the C-Cl bond can be attacked by UV light and needs to be protected, usually by applying a thin layer of coating material.
• Heating to high temperatures can also be a problem, especially in fires, since PVC releases HCl and dioxins, both poisonous.

VIEW videos

• Vinyl chloride and styrene https://youtu.be/vpdXG38KXjs 10.21

Polystyrene (polyphenylethene) is a synthetic aromatic (benzene based) hydrocarbon polymer made from the monomer styrene (phenylethene).

Polystyrene can be solid or foamed. General-purpose polystyrene is an inexpensive resin per unit weight.

Polystyrene is one of the most widely used plastics. Polystyrene uses include protective packaging (eg foods, eggs), containers, lids, bottles, trays, tumblers, disposable cutlery, and in the making of models.

As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room temperature but flows if heated above about 100 °C. It becomes rigid again when cooled. This temperature behaviour is exploited for extrusion (as in Styrofoam) and also for moulding and vacuum forming, since it can be cast into moulds with fine detail.

Polystyrene is slow to biodegrade. It is accumulating as a form of litter in the outdoor environment, particularly along shores and waterways, especially in its foam form, and in the Pacific Ocean. It is a poor barrier to oxygen and water vapour and has a relatively low melting point.

PTFE is a fluorocarbon solid, as it is a high molecular weight compound consisting wholly of carbon and fluorine.

PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate reduced London dispersion forces due to the high electronegativity of fluorine. There is no Hydrogen bonding, as there are no Hydrogen atoms in the molecule.

NOTE: The carbon–fluorine bond is a polar covalent bond between carbon and fluorine. It is the fourth strongest single bond in organic chemistry—behind the B-F single bond, Si-F single bond and the H-F single bond, and relatively short—due to its partial ionic character. The bond also strengthens and shortens as more fluorines are added to the same carbon on a chemical compound. As such, fluoroalkanes like tetrafluoromethane (carbon tetrafluoride) are some of the most unreactive organic compounds.

PTFE is nonreactive, partly because of the strength of carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals.

PTFE has one of the lowest coefficients of friction of any solid, so it is used

• as a non-stick coating for pans and other cookware, because it is also quite heat resistant
• as a lubricant, PTFE reduces friction, wear, and energy consumption of machinery
• as a graft material in surgical interventions and as coating on catheters; this interferes with the ability of bacteria and other infectious agents to adhere to catheters and cause hospital-acquired infections.

Condensation polymerisation reactions occur between monomers each having two or more complementary functional groups. During the reaction a small molecule such as water is eliminated (released).

The formation of cellulose is an example of a condensation polymerisation reaction:

n(HO–C6H10O4–OH) → H–(O–C6H10O4)n–OH + (n–1)H2O

In the formation of cellulose shown above, n glucose molecules are combined to form the cellulose chain and (n–1) molecules of water.

When two glucose monomers react, a hydroxyl group from each monomer combines to release a water molecule. This leaves an oxygen atom linking the two monomers. This process repeats to form the cellulose chain.

Cellulose is therefore a polymer made up of many glucose monomers. When glucose units combine one must be inverted (upside down) relative to the other. The geometry of the glucose rings produce a polymer molecule which is very linear in structure. Cellulose is a major component of biomass. Cellulose contains the basic carbon-chain structures needed to build petrochemicals.

VIEW videos

• Condensation polymer https://youtu.be/SPThAjz2V_E 6.33
• POM 9 https://www.youtube.com/watch?v=zlCryvdxjM4 15.55

Another important condensation polymer is nylon. Nylon was first produced in 1938 and marketed in toothbrushes. The following year “nylon stockings were exhibited to the public at the New York World’s Fair by creator Du Pont, where they were touted as being ‘as strong as steel, as fine as a spider’s web’. On 15 May 1940, nylon stockings went on sale throughout the USA, and in New York City alone four million pairs were sold in a matter of hours. Sixty-four million pairs sold in the first year” (Selinger & Barrow, 2017, p. 250).

One form of nylon is nylon-6,6. It is formed through the condensation polymerisation of 1,6-hexanediamine and adipoyl dichloride (see diagram below). The critical bond formed during the polymerisation process is an amide bond, with HCl the small molecule which is released in the process.

Nylon is a thermoplastic silky material that can be melt-processed into fibres, films, or shapes. It is made of repeating units linked by amide links similar to the peptide bonds in proteins. Nylon polymers can be mixed with a wide variety of additives to achieve many different property variations. Nylon polymers have found significant commercial applications in fabric and fibres (apparel, flooring and rubber reinforcement), in shapes (moulded parts for cars, electrical equipment, etc.), and in films (mostly for food packaging).

https://www.youtube.com/watch?v=c7ihpZhCj6k How to make nylon [4.41]

Polyesters have great strength, elasticity and electrical resistance and are widely used. The most common type of polyester is polyethylene terephthalate. When it is used to manufacture bottles, it is marketed as PET; when it is sold as a fibre, it goes by the name of Terylene or Dacron. (Selinger & Barrow, 2017, p. 222). Polyesters may be used in textile manufacture due to their low water absorption and minimal shrinkage after washing. (Thickett, 2018, p. 178).

In PET, there are two monomers, just as there are for nylon, however this time, they are terephthalic acid (benzene-1,4-dicarboxylic acid) and ethylene glycol (ethan-1,2-diol).

An ester bond is formed when an acid and alcohol are combined to release a water molecule.

The majority of the world's PET production is for synthetic fibres (in excess of 60%), with bottle production accounting for about 30% of global demand. In the context of textile applications, PET is referred to by its common name, polyester, whereas the acronym PET is generally used for packaging. Plastic bottles made from PET are widely used for soft drinks. PET sheet can be thermoformed to make packaging trays and blister packs.