CHEMISTRY IN INDUSTRY AND
THEORY OF KNOWLEDGE Advances in what we know about
food, agriculture, manufacturing, communications, sanitation, medicine and transport have been responsible for dramatic changes in how people live and work. Because of this, what is valued in society infl uences what research and development is valued and funded in science.
For example, when society values the need to fi nd alternatives for fossil fuels, wants ways to recycle or extend the life of batteries and know for sure if nano-particles are bad for your health, research in these areas will take place. In addition, economic and political forces infl uence the direction of science, what research will be carried out and what new technologies will be developed and used.
You are on the selection committee of an organization that offers grants for scientifi c research into new technologies.
What sort of questions would you ask someone who is applying for funding for medical research into a drug-delivery system that uses nano-particles to control the release of antibiotics to the site of an infection?
1 Defi ne nanotechnology and explain, in terms of metres, how small the nano-scale is.
2 Describe the role of scanning tunnelling microscopes in the development of nanotechnology.
3 Explain how atoms can be manipulated by both physical and chemical means.
4 a Describe the structure of a carbon nanotube.
b List the special properties of carbon nanotubes.
5 Discuss the safety implications of using nanotechnology, and the validity of the statement, ‘Each type of nanoparticle may be as deadly as asbestos.’
Earlier in this chapter, we looked at addition polymers. Now we will examine the other important class of polymers—condensation polymers. We will also look more closely at the mechanisms by which low-density polyethene (LDPE) and high-density poylethene (HDPE) are manufactured.
In contrast to addition polymerization, monomers used to form condensation polymers such as polyester and nylon do not need to contain a carbon–carbon double bond; instead, they must contain reactive functional groups. A
condensation reaction is one in which larger molecules join covalently.
Each new bond causes the formation of a small molecule, normally water. An example is ethanoic acid and ethanol undergoing an esterifi cation reaction to form an ester, ethyl ethanoate, and water:
CH3COOH(l) + CH3CH2OH(l) H SO (l)2 4 CH3COOCH2CH3(l) + H2O(l) If monomers were available that each contained two functional groups, aligned at opposite ends of the molecules, the process could be used to form long chains.
If a monomer with two hydroxyl groups reacted with a monomer with two carboxyl groups, the esterifi cation reaction continues and polyester is formed.
In situations such as this, where two different monomers are used to form the polymer chain, the product is called a copolymer.
C C
3.6 MORE ABOUT POLYMERS
HLC.8.1
Distinguish between addition and condensation polymers in terms of their structures.
© IBO 2007
Condensation polymers
C.8.2
Describe how condensation polymers are formed from their monomers. © IBO 2007
CHAPTER 3 CHEMISTRY IN INDUSTRY AND TECHNOLOGY Natural examples of condensation polymers include proteins and cellulose.
In addition polymers, the backbone of the long chain is composed of carbons linked by single carbon–carbon bonds. In contrast, the condensation polymer has a backbone of mostly carbon, broken up at regular intervals by the link formed in the condensation reaction that formed it. Commonly, this is an ester, ether or peptide linkage.
Polyurethane foam is formed from a two-part mixture. The fi rst part contains a diol or triol (most commonly glycerol), a catalyst, a blowing agent (low boiling point liquid) and a silicone surfactant. The other part contains a diisocyanate.
The polymerization reaction is rapid, and the water causes some of the isocyanate to decompose, forming carbon dioxide that produces the foam.
Unfoamed polyurethane is spun into Spandex fi bres and sold as Lycra. An example synthesis pathway is shown in fi gure 3.6.2.
CH2 N
C
O N C O
n + n
CH2OH
CHOH
CH2OH
CH2 N
C N C O
O H H O
–CH2CHOHCH2–O
n diphenylmethane diisocyanate
Figure 3.6.2 The formation of polyurethane.
Polyethylene terephthalate (PET or PETE, polyethylene terephthalic ester) is a type of polyester used in clothing, fi bres and drink bottles. It is one of the fi ve most-produced polymers in the world. PETE fi bres are called Dacron and Fortrel; PETE fi lm is called Mylar. To make PETE, the diol ethylene glycol (1,2-ethanediol) is reacted with the diester dimethyl terephthalate
(CH3OOCC6H4COOCH3) or terephthalic acid (TPA). The latter is more commonly used. In the TPA process, the intermediate bis-hydroxyethyl terephthalate is fi rst formed in aqueous solution then polymerized using reduced pressure, heat and a catalyst.
HOOC COOH
terephthalic acid (TPA)
+ 2CH2OHCH2OH
ethylene glycol
HOCH2CH2OOC COOCH2CH2OH
bis-hydroxyethylterephthalate
+ 2H2O
HOCH2CH2OOC COOCH2CH2OH OOC COOCH2CH2 + n CH2OHCH2OH
n n
Figure 3.6.4 The formation of polyethylene terephthalate.
Synthesis of nylon 6–10
Figure 3.6.3 Polyurethane is used in footwear, automobile parts and insulation.
Phenol–methanal plastics, also called Bakelite, are the oldest synthetic polymers. They are unusual in that the monomers each contain only one functional group. Phenol (C6H5OH) joins with methanal (CH2O) to form the copolymer. The reaction relies on the fact that the hydrogens in the ortho position (i.e. in the position next to the hydroxyl group) in phenol may react with the methanal, with the formation of a water molecule. The polymer formed is strong, black and resistant to heat, so is often used in electrical applications.
Figure 3.6.5 The formation of phenol–methanal polymers.
OH
n
phenol
C O
H
H + n
methanal
nH2O +
OH
CH2
n
OH
CH2
OH
CH2
OH
OH
CH2
OH OH
CH2
CH2
C O
H
H crosslinking occurs
As described in section 3.2, the properties of polymers will depend on the strength of their intermolecular forces, which rely on chain length, degree of branching and polarity, and the presence or absence of cross-links. Bulky side groups will inhibit the molecules from approaching too closely and will decrease the amount of crystallinity in the structure.
Due to its amide linkages, polyurethane chains are able to hydrogen bond to each other. This increases the strength of their intermolecular forces, compared to a polymer such as PETE. Similarly, Kevlar™ chains are able to hydrogen bond to each other. Like polyurethane, they also contain amide linkages between monomers in the chain. The hydrogen bonding between Kevlar™ chains causes them to line up in a regular fashion. The strong bonding within the polymer molecules, the strong bonding between the chains, and the regular arrangement of the chains in the material combine to make Kevlar™ incredibly strong. It is used for ropes, fi reproof material, bulletproof vests and sports gear. Cross-linking greatly increases the strength of a polymer, and changes a thermoplastic
polymer into a thermosetting one. In the initial formation of phenol–methanal Structure and properties
C.8.3
Describe and explain how the properties of polymers depend on their structural features.
© IBO 2007
CHAPTER 3 CHEMISTRY IN INDUSTRY AND TECHNOLOGY
Figure 3.6.6 The structure of Kevlar™.
Figure 3.6.7 Cross-links in a phenol–methanal polymer.
CH2
The modifi cation of addition polymers has previously been described. This section describes a few more ways in which polymers may be modifi ed. Many additives are currently available, including UV absorbers, dyes, antimicrobials, fl ame retardants, plasticizers, antistatic agents, discolourant inhibitors,
antioxidants and cross-linking agents.
Polyurethane has the toughness of steel and better elasticity than rubber.
Polyurethane foam is widely used, but can be susceptible to ageing effects.
To combat this, additives such as the UV stabilizer hydroxybenzotriazole and antioxidants such as polymeric hindered phenols are employed. The bubbles of gas in the foam improve the insulation capacity. Normal blowing agents are water (which reacts to form carbon dioxide) and methylene chloride. If air seeps into the foam, it reduces its insulation capacity.
Polyethyne belongs to a class of polymers called conducting polymers. Due to the conjugated double bonds in the structure, it is able to conduct electricity.
It was discovered accidentally when a student added 1000 times too much catalyst in a reaction. The red cis form is unstable, but the silver-blue trans form is stable.
doped polyethyne: iodine removes an electron to form I3– and a carbocation H
Figure 3.6.8 The forms of polyethyne.
Doping involves adding an agent that either adds electrons to (reduces) the polymer or takes electrons from (oxidizes) the polymer. It has been discovered that doping polyethyne with iodine oxidizes the polymer, increasing its conductivity from 10–3 S m–1 to 3000 S m–1. The iodine attracts pi-bond
electrons, creating ‘holes’ in the structure. This makes electrons jump from one end to the other to fi ll the ‘holes’. This discovery earned Alan MacDiarmid, Alan Heeger and Hideki Shirakawa the 2000 Nobel Prize in Chemistry.
The popular material polyester is strong, but has some drawbacks. It is
diffi cult to dye, and feels warm and heavy against the body. High temperatures and pressures are needed to print and dye polyester. To address these
problems, polyester is usually blended with other fi bres, such as cotton at a 60% cotton, 40% polyester ratio. It is also sometimes blended with wool.
Techniques have been developed to produce more comfortable polyester fi bres, so 100% polyester clothing is not uncommon, despite its lack of breathability.
Polymers overtook natural materials for many uses over 50 years ago. They are readily produced and their properties can be designed to match a particular Modifying polymers
C.8.4
Describe ways of modifying the properties of polymers.
© IBO 2007
Advantages and disadvantages of polymers
C.8.5
Discuss the advantages and