Polymers

Chemistry in Action (Polymers)

Polymers •

Polymers are macromolecules formed by repeated joining of many small molecules.

•

Polymerisation is the process of joining together many small molecules repeatedly to form very large molecules like polymers.

•

Monomers are compounds that join together repeatedly to form a polymer in the process of polymerisation.

Polymers can be natural or synthetic. The natural polymers covered include proteins, polysaccharides and nucleic acids The most important naturally occurring polymers are:  Proteins  Polysaccharides (e.g. cellulose, starch)  Nucleic acids (e.g. DNA, RNA)  Rubber Synthetic polymers are produced commercially on a very large scale. They have a wide range of properties and uses. Plastics are all synthetic polymers Synthetic polymers can be made from monomers by two basic polymerisation processes: (a) addition polymerisation which produces addition polymers (b) condensation polymerisation which produces condensation polymers Well-known examples of synthetic polymers are:  Polyethene (PE)  Polystyrene (PS)  Polyvinyl chloride (PVC)  Nylon  Urea-methanal

Page 1

Chemistry in Action (Polymers)

Natural Polymers Amino acids and protein Introduction Amino acid are bifunctional compounds containing both the amino (-NH2) and carboxylic (-COOH) groups. NH2

COOH

Classification of amino acids 1. Neutral amino acids: Number of amino groups = number of carboxylic groups E.g. Glysine NH2CH2COOH 2. Basic amino acids Number of amino groups > number of carboxylic groups E.g. Lysin H2N

NH2 COOH

3. Acidic amino acids Number of amino groups < number of carboxylic groups E.g. Aspartic acid HOOC COOH H2N

Stereochemistry of Amino Acids All amino acids except aminoethanoic acid contain an asymmetric atom and exhibit optical isomerism. Example: Alanine

Page 2

Chemistry in Action (Polymers)

They are optical isomers, but optical inactive, since they are racemic mixture. Laboratory synthesized amino acids are ONLY optically inactive because of the formation of race mixture. Physical properties of Amino Acids The dipole moments of the amino acids are very large. For example, + NH3CH2COO- CH3CH2COOH CH3(CH2)2CH2NH2 Dipole Moment 14D 1.7D 1.4D Ionic Compound

Acid

Base

In fact, in the solid state and in solution, amino cids exist as internal ionic salts, called Zwitterions.

So, Amino acids are high melting point solids.  e.g. Glycine melts at 235℃ They are very soluble in water, but they only dissolove slightly in organic solvents. They have a very large dipole moment.

  

Chemical properties of Amino Acids Amphoteric nature of amino acids At some intermediate pH value, a dipolar (zwitterions) form is produced.

The hydrogen ion from the carboxyl group is trasnsferred to the basic amino group within the molecule.

+

H3N – CH – COOH | R

OH− H+

+

OH−

H3N – CH – COO− Page 3 | H+ R

H2N – CH – COO− | R

Chemistry in Action (Polymers)

The existence of the zwitterionic form can be explained in terms of acid-base theory: 1. –NH2 is a stronger base than –COO2. –COOH is a stronger acid than –NH3+ Further evidence for zwitterions formation is electrophoresis. Methods to separate a mixture of amino acids.

Paper chromatography will be used to separate amino acids. There is a thin film of water on the chromatography paper. The amino acids distribute themselves between the stationary phase (water on the paper) and the moving phase (the solvent/eluent) To make the amino acid spots visible to naked eyes, spray chromatography paper with ninhydrin solution which reacts with amino acids to give purple coloured compounds. (also accept using UV radiation/ iodine vapour to detect the amino acid spots.) Reactions of Amino acids Two main types reaction of the Amino Acids 1. reaction of the carboxyl group 2. reactions of the amino group

Page 4

Chemistry in Action (Polymers)

Dil. NaOH(aq)

SOCl2 or PCl5 Showing acidic Properties Fusing with soda lime

H NH2 CCOOH R Showing basic Properties

H NH2 CCOO-Na+ R HO NH2 C-C-Cl R H NH2-C-H R

CH3OH/H+

+

Dil HCl

-

HO NH3 C-C-OCH3 R

H Cl NH3 CCOOH R

CH3COCl

+

OHH CH3-C-N-C-COOH R

Peptides, Polypeptides and protein Dipeptide The (-NH2) group of one amino acid can react with the (-COOH) group of another to form an amide. The resultion molecule is a dimmer containing two amino acid units which is describes as a dipeptide. In the process, the two amino acid molecules are joined by the condensation reaction. A water molecule is eliminated.

Page 5

Chemistry in Action (Polymers)

Amino acids are linked together by peptide linkage. Polypeptide Amino acids undergo condensation polymerization to form long-chain polyamide molecules.

dipeptide

Further reaction of each end Polypeptide/protein

Page 6

Chemistry in Action (Polymers)

(1) If n< about 50, the product is a polypeptide. (2) If n> about 50, the product is a protein.

Structure of proteins Protein structure is describe at 4 levels: 1∘, 2∘, 3∘ & quaternary. Being polyamides, both proteins and nylon can be hydrolysed and are thus broken down to their constituent amino acids. For example,

Polypeptide

Dipeptide

Page 7

Chemistry in Action (Polymers)

Amino acids

The peptide linkages in a protein molecule can be broken by hydrolysis using mineral acids or some enzymes. On complete hydrolysis, the protein is broken down into amino acids. By analyzing the resulting amino acids, the composition of the protein molecule may be deduced.

Carbohydrates Monosaccharide, disaccharide and polysaccharide Sugar, starch and cellulose are carbohydrates. Carbohydrates are important in the diet as a source of energy. They are compounds containing carbon, hydrogen and oxygen with the general formula CxHyOz. Carbohydrates may be divided into three groups,  Monosaccharides  Disaccharides  Polysaccharides The simplest carbohydrates are the sugars. (glucose, fructose and ribose) Monosaccharides The monosaccharides consist of a single polyhydroxyaldehyde or polyhydroxyketone. Monosaccharides are a group of sweet, soluble crystalline molecules with relatively low molecular masses. They cannot be hydrolyzed into simpler compounds. The monosaccharides commonly found in food have the general formula C6H12O6. Two most important examples are glucose and fructose. They are found in many fruits and in honey. Glucose is also found in the blood Page 8

Chemistry in Action (Polymers)

of animals (including humans) Each monosaccharides molecule contains one carbonyl group. All the other carbon atoms are bonded to hydroxyl groups. There are aldose and ketose, for which the carbonyl group is and is NOT terminal respectively.

aldehyde ketone

Open chain and ring structures of glucose and fructose Glucose can exist in acyclic and cyclic forms: 0.02%

36%

64%

Page 9

Chemistry in Action (Polymers)

Glucose contains an aldehyde group in its acyclic form. Glucose is an aldohexose Most of the reactions of glucose in aqueous solutions are due to presence of the free aldehyde group of the acyclic form.These reactions include its reducing action Fructose can exist as acyclic form, as well as cyclic forms of 6-membered rings and 5-membered rings

Fructose contains a keto group in its acyclic form  fructose is an ketohexose Most of the reactions of fructose in aqueous solutions are due to:  presence of the free keto group of the acyclic form Disaccharides Page 10

Chemistry in Action (Polymers)

Disaccharides are sweet, soluble and crystalline. They have the general formula: C12H22O11 Disaccharides can be formed from the condensation reaction of two monosaccharide molecules a water molecule is eliminated Common disaccharides include Sucrose (Source: sugar cane), Maltose (Source: malt) and Lactose (Source: milk) Glycosidic Linkage in Carbohydrates Common disaccharides are formed from the condensation reaction between two monosaccharide molecules and a water molecule is eliminated The bond formed between two monosaccharides is called a glycosidic linkage A sucrose molecule is formed by the condensation reaction of a glucose molecule and a fructose molecule

A maltose molecule is formed by the condensation reaction of two glucose molecules Polysaccharides Polysaccharides are polymers of monosaccharides General formula: (C6H10O5)n where n is a large number (up to thousands) Examples of polysaccharides: starch and cellulose Starch is commonly found in rice, bread and potatoes Cellulose is found in fruits, vegetables, cotton and wood Page 11

Chemistry in Action (Polymers)

The condensation process can be repeated to build up giant molecules of polysaccharides e.g. Starch

Cellulose

Adjacent chains of cellulose molecules are linked up by hydrogen bonds. Page 12

Chemistry in Action (Polymers)

These cellulose chains intertwine into fibrils of considerable strength.

DNA as Nucleic acid Nucleic acids are the molecules that preserve hereditary information, transcribe and translate it in a way that allows the synthesis of all the various proteins of a cell Nucleic acid molecules are long polymers of small monomeric units called nucleotides. The monomers of nucleic acids, called nucleotides, are formed from the following units: 1. A phosphate unit 2. A five carbon sugar 3. A nitrogen – containing organic base. Two kinds of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) The sugar component of RNA is ribose, whereas that in DNA is deoxyribose.

Deoxyribos e

Ribose

The following nitrogen bases are found in DNA and RNA:

Page 13

Chemistry in Action (Polymers)

(in DNA) (in RNA) DNA is the nucleic acid that most genes are made of DNAs have four different kinds of nucleotides as the building blocks All the four kinds of nucleotides have deoxyribose as their sugar component They differ in their nitrogen-containing bases Adenine (A) and guanine (G)  have double-ring structures  known as purines Cytosine (C) and thymine (T)   

 

have single-ring structures known as pyrimidines

Formation of the nucleotide of a DNA molecule

Page 14

Chemistry in Action (Polymers)

The nucleotides within a DNA molecule are joined together through condensation reactions between the sugar of a nucleotide and the phosphate group of the next nucleotide in the sequence long chain (i.e. a polymer) of alternating sugar and phosphate groups is formed Two hydrogen bonds are formed between A in one chain and T in the other Three hydrogen bonds are formed between G in one chain and C in the other Hydrogen bonding between complementary base pairs. The hydrogen bonds are responsible for formation of the double stranded helical structure of DNA.

Page 15

Chemistry in Action (Polymers)

DNA replication The original DNA double helix partially unwinds, and new nucleotides line up on each strand in a complementary manner. Hydrogen bonds help align the new nucleotides with the original DNA chain. When the new nucleotides are joined by condensation reactions, two identical double helix DNA molecules result.

Page 16

Chemistry in Action (Polymers)

Synthetic Polymers Addition Polymers Formation and Uses of Addition Polymer Addition polymerization is a chemical process in which monomer molecules are joined together to form a polymer without elimination of small molecules. The resulting polymer will therefore have the same percentage composition as the reactant monomers. 1. Polyethene, or Polyethylene [PE] Polyethene has many useful properties: – it is easily moulded; – – – – –

it is an excellent electrical insulator; it does not corrode; it is tough; it is not affected by the weather; it is durable.

Ethene is the monomer that is used to synthesize polyethene Depending on the manufacturing conditions, two kinds of polyethene can be made  low density polyethene (LDPE)

• • • •



Molecular mass between 50 000 and 3 000 000 Light, flexible Low melting point Used to make soft items (e.g. wash bottles, plastic bags and food wraps)

high density polyethene (HDPE) Page 17

Chemistry in Action (Polymers)

• • • •

Molecular mass up to 3 000 000 Tougher Higher melting point Used to make more rigid items (e.g. milk bottles and water buckets)

Polyethene is a thermoplastic It softens at a high temperature Uses of polyethene Insulate telephone line Its unique electrical properties were essential during the development of radar. Plastic bags It used in supermarket for packing various food product. milk bottles and water buckets Hard and rigid, not poisonous Mechanism for the addition polymerization: Free Radical Addition Polymerization of Ethene The reaction mechanism consists of three stages:  chain initiation  chain propagation  chain termination Chain initiation

diacyl peroxide molecule as a initiator

Page 18

Chemistry in Action (Polymers)

Chain propagation

Chain termination steps

Addition polymers formed from these substituted ethenes (H2C=CHX) have a range of properties predictable based on the structure of –X (non polar substituent). –X group like –CH3 or –C6H5 are soluble in organic solvents like acetone or propanone. 2. Polypropene (PP) With the use of Ziegler-Natta catalyst, propene can be polymerized to Page 19

Chemistry in Action (Polymers)

polypropene

Polypropene can exist in different configurations depending upon the orientation of the methyl groups in the polymer. The properties of polypropene can be modified by adjusting the manufacturing conditions In isotactic polypropene, all methyl groups are on the same side of the carbon chain. Using Ziegler Natta Catalyst, the methyl groups all arrganged on one side of the carbon chain.

In atactic polypropene the methyl groups are randomly arranged

Isotactic Polypropene More symmetrical Molecules pack together closely. High melting point Greater strength 

Atactic polypropene Less symmetrical Poor packing Low melting point sticky

It is more rigid than HDPE and used for moulded furniture Page 20

Chemistry in Action (Polymers)

 

High mechanical strength and strong resistance to abrasion. It is used for making crates, kitchenware and food containers Spun into fibres for making ropes and carpets especially useful for making athletic wear. They do not absorb water from sweating as cotton does

3. Polystyrene (PS) Styrene is made from the reaction of benzene with ethane, followed by dehydrogenation.

The styrene produced is polymerized by a free radical mechanism into polystyrene at 85 – 100°C using dibenzoyl peroxide as the initiator

Mechanism:

Page 21

Chemistry in Action (Polymers)

Polystyrene is transparent, brittle and chemically inert. It used to make toys, specimen, containers and cassette cases By heating polystyrene with a foaming agent,expanded polystyrene can be made. Expanded polystyrene is extremely light, white solid foam. It mainly used to make light-weight ceiling tiles in buildings, and food boxes and shock absorbers for packaging 4. Polyvinyl Chloride (PVC) PVC is produced by addition polymerization of the choroethene monomers in the presence of a peroxide catalyst (e.g. hydrogen peroxide at about 60°C)

Page 22

Chemistry in Action (Polymers)

(a)

(b)

Presence of the polar C – Cl bond is considerable dipole-dipole interactions exist between the polymer chains makes PVC a fairly strong material The large size of Cl atom means the chains cannot easily be moved over one another. This result in the polymer being rigid and brittle. PVC is hard and brittle and used to make pipes and bottles

Products made of PVC without plasticizers When plasticizers are added, the effectiveness of the dipole- dipole interactions is reduced. PVC becomes more flexible • Used to make shower curtains, raincoats and artificial leather • Used as the insulating coating of electrical wires

Page 23

Chemistry in Action (Polymers)

Products made of PVC with plasticizers

Despite its extensive uses, one problem with the use of PVC is its disposal. PVC is non-biodegradable, normally not disposed of by land-filling. Incineration of PVC produces HCl (g) This HCl (g) produced is usually absorbed by the wet scrubber filled with an alkali such as Na2CO3 (aq). Also, monomer of PVC is a carcinogen.

5. Polytetrafluoroethene (PTFE) PTFE is produced through addition polymerization of the tetrafluoroethene monomers under high pressure and in the presence of a catalyst

Page 24

Chemistry in Action (Polymers)

C—F bonds are exceptionally strong & resistant to heat and chemicals PTFE has a relatively high melting point and is chemically inert Its non-stick properties make it an ideal material for the coating of frying pans ⇒Since fluorine is highly electronegative atom an evenly distributed layer of negative charge is developed on the surface of PTFE ⇒ Layers of negative F atoms repel almost all other materials ⇒ Thus preventing them from adhering to PTFE ⇒ PTFE has very slippery surface To make Teflon stick to the surface of a newly made cooking pan, Teflon is heated to a very high temperature pressed firmly onto the surface of the item. This film pressing increases the area of contact between Teflon and the surface of the pan Increasing the strength of van der Waals’ force Another technique: Teflon with oxygen-containing group is polar This helps strengthening the attraction between Teflon and the surface. Other polarized group: methyl methacrylate can also be incorporated into the Teflon polymer chains to achieve the above purpose. Whatever the technique, there is still the chance of Teflon peeling off from cooking ware after extended use, because ⇒Teflon decompose at high temperature ⇒Its coefficient of expsion is different from that of the material of the cooking surface 6. Polymethyl Methacrylate (Perspex) (PMMA) PMMA is formed by the free radical addition polymerization of methyl methacrylate in the presence of an organic peroxide at about 60°C

Page 25

Chemistry in Action (Polymers)

PMMA is a dense, transparent and tough solid makes it a good material for making safety goggles, advertising sign boards and vehicle light protectors However, it has poor scrtching resistance and can be dissolved by a number of organic solvents.

Condensation Polymers Formation and Uses of Condensation Polymer Condensation polymerization is a chemical process in which monomer molecules are joined together to form a polymer with elimination of small molecules such as water, ammonia and hydrogen chloride

Ethanoic acid

Ethanol

Ethyl ethanoate (An ester)

Each monomer molecule must have at least two functional groups 1. Polyamide 1. Nylon Nylon 6,6

Page 26

Chemistry in Action (Polymers)

faster

When a solution of hexane-1,6-dioyl dichloride in hexane is poured gently onto a solution of 1,6-diaminohexane in water, a white film of nylon is formed at the interface between the two layers. The film can be pulled up as a string and wound onto a stirring rod. Used for making carpets, thread, cords and various kinds of clothing from stockings to jackets Advantages:  drips dry easily  not easily attacked by insects  resists creasing

Page 27

Chemistry in Action (Polymers)

There are interchain hydrogen bond so Nylon6, 10 is expected to have a lower tensile strength than nylon6,6. There is decrease in the number of hydrogen bond per unit length, as a result of the longer carbon chain in nylon 6,10 3. Kevlar Aramid is a synthetic poly amide Aliphatic hydrocarbon unit within the polmer chain has been replaced by an aromatic unit in Aramid • Kevlar is an aromatic polyamide • The structure of Kevlar is similar to nylon-6,6 • The two monomers of Kevlar are benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene Both monomers are bifunctional

In Kevlar, the starting material was modified to create straighter chains in the Page 28

Chemistry in Action (Polymers)

polymer. A polyamide was produced with the heat resistance of asbestos. Strength was much greater than steel. In Kevlar the aliphatic hydrocarbon chain parts of the poly amide are replaced by benzene rings. These parts of the polymer chain make the chains inflexible due to delocalized bonding. Some of this delocalization extends beyond the benzene rings and onto part of the amide link resulting in long, rigid molecules that do not easily flex or twist.

This extended delocalization also leads to enhanced intermolecular hydrogen bonging between the adjacent Kevlay polymer chains. This hydrogen bonding network causes the chains to interlock each other, forming a sheet structure. All the C = O and – N – H groups in the polymer chains are on opposite sides. This makes the chains highly symmetrical. The regular structure of the polymer chains allows them to interlock with each other.

Page 29

Chemistry in Action (Polymers)

Applications: (a) Kevlar is an unusual polymer with fire resistant properties and also great strength. It is found in the crash helmets of Formula I racing drivers as well as in the suits of racing motorcyclists. (b) The hull of this offshore racing craft is also reinforced with Kevlar. (c) Kevlar is used in making bullet proof vests. A more recent innovation is to use carbonanotubes to make fibres for these bullet proof vests. These new bullet proof vests can be made 30 % lighter, but 1.5 times more bullet resistant than conventional Kevlar vests.

Page 30

Chemistry in Action (Polymers)

(b)

(a) (c) 2. Dacron Formed by repeated condensation reactions of benzene-1,4-dicarboxylic acid (also called terephthalic acid) and ethane-1,2-diol (also called ethylene glycol) in the presence of a catalyst and at a low pressure and moderate temperature (about 250°C) The two monomers of Dacron are:

The polymerization begins with the formation of an ester A water molecule is eliminated

Due to polarization of the carbonyl groups C=O, Dacron chains are crossPage 31

Chemistry in Action (Polymers)

linked by strong dipole-dipole attractions

⋅ ⋅⋅ ⋅⋅ ⋅

⋅ ⋅⋅ ⋅⋅ ⋅

⋅ ⋅⋅ ⋅⋅ ⋅

⋅ ⋅⋅ ⋅⋅ ⋅

Properties of Dacron:  High tensile strength  High resistance to stretching  Low absorption of moisture Garments made of Dacron:  are tough  can resist wrinkling  can be washed and dried easily and quickly Excellent for making trousers and skirts, sheets and boat sails Can be used alone or blended with cotton to make it absorb sweat better 3. Urea-methanal Produced by the condensation polymerization of urea and methanal under heat and pressure When an urea molecule joins up with a methanal molecule, water molecule is eliminated In the presence of excess methanal, further condensation reactions between the polymer chains and methanal occur.Cross-linkages between the polymer chains are formed. A rigid structure of urea-methanal is produced

Page 32

Chemistry in Action (Polymers)

• Urea-methanal is a thermosetting plastic and cannot be softened or melted again by heating once they have been set hard • Excellent electrical insulator • Resistant to chemical attack Effect of Structure on Properties of Polymers • Polymers are long-chain giant molecules • The final form and the properties of the polymers depend on how these • • • • •

long polymer chains are packed together If the polymer chains do not have a specific arrangement but are loosely packed together the polymer is said to be amorphous Amorphous polymers are generally transparent, flexible and less dense When the polymer chains are regularly packed together, the polymer is said to be crystalline Polymers with a high degree of crystallinity are translucent or opaque, harder and denser The attractive forces holding polymer chains together also affect the properties of polymers Page 33

Chemistry in Action (Polymers)

• Polymer chains containing carbon and hydrogen atoms only are held together by weak van der Waals’ force • slow melting points • low mechanical strength • If polymer chains are held together by stronger van der Waals’ forces or hydrogen bonds, the mechanical strength of the polymers would be stronger • If cross-linkages are present between polymer chains, the polymers would be mechanically stronger, more elastic or more rigid ,depending on the extent of cross- linkages in the polymer Low Density Polyethene and High Density Polyethene High Density Polyethene • When Ziegler-Natta catalysts are used, the polymer chains produced are long molecules with very little branching. The polymer chains can pack closely together into a largely crystalline structure Thus, the polymer has a higher density • Compared with LDPE, HDPE  is harder and stiffer  has a higher melting point  has greater tensile strength  has strong resistance to chemical attack  has low permeability to gases blow-molded objects: bottles for milk, soft drinks, shampoos, bleaches and so on

Page 34

Chemistry in Action (Polymers)

Low Density Polyethene

• When ethene is polymerized at 200°C and 1000 atm using peroxide as the catalyst, low density polyethene (LDPE) is made • Under these reaction conditions, highly branched polymer chains are formed • The branches prevent the polymer chains from getting close to each other. The polymer chains do not pack together well and creates a significant proportion of amorphous regions in the structure • Thus, the polyethene made has a low density Low density polyethene is a  Waxy  Semirigid  Translucent material  Low melting point Nylon and Kevlar • Nylon is a group of polyamides • It contains a relatively large number of crystalline regions arranged in a Page 35

Chemistry in Action (Polymers)

random manner • When nylon is spun into fibres and is drawn • the crystalline regions are aligned • leads to an increase in the tensile strength

Non-aligned crystalline region

Aligned crystalline region

In the stretched or drawn nylon, the polymer chains line up and are parallel to each other. The amide groups on adjacent chains form strong hydrogen bonds with each other These hydrogen bonds hold the adjacent chains together making nylon thread strong

The structure of Kevlar is basically the same as nylon-6,6 When molten Kevlar is spun into fibres, the polymer has a crystalline arrangement and the polymer chains oriented parallel to each other

Page 36

Chemistry in Action (Polymers)

• Kevlar is much stronger than nylon • The difference in their strength is due to the orientation of the amide groups along the polymer chains • In nylon, between the amide groups are the carbon chains the  C = O and  N  H groups can be on opposite sides or on the same side • When the  C = O and  N  H groups are on the same side, the polymer chain would not be straight and the number of hydrogen bonds formed between adjacent chains would be less • Kevlar has a regular structure the polymer chains interlock with each other Kevlar fibres are very strong used for making reinforced rubbers and bullet-proof vests Vulcanization of Polymers Natural rubber is a polymer of the monomer 2-methylbuta-1,3-diene (isoprene)

2-Methylbuta-1,3-diene

• Poly(2-methylbuta-1,3-diene) or polyisoprene can exist in two isomeric forms • Natural rubber is the cis-form

Page 37

Chemistry in Action (Polymers)

Part of a polymer chain of natural rubber Natural rubber is not a useful polymer because it is too soft and too chemically reactive. The long chain molecules can be coiled twisted and interwined with one another Vulcanization of natural rubber is the chemical process that confers crosslinkage among the polymer chains of rubber, turning natural rubber into a flexible elastic material. • In the process of vulcanization,1–3 % by mass of sulphur is added to natural rubber and the mixture is heated • Short chains of sulphur atoms (i.e. cross-linkages) are formed between the polymer chains • The sulphur changes rubber into a thermosetting polymer by cross linking the polymer chains through reaction at some of the double bonds as shown:

This makes the rubber harder and reduces its susceptibility to oxidation or other chemical attrack. • When vulcanized rubber gets hot, the polymer chains cannot slip across one another since they are still held together by short chains of sulphur atoms • That is why vulcanized rubber does not melt when heated and does not become brittle when cooled Page 38

Chemistry in Action (Polymers)

The extent of the cross-linkages formed between the polymer chains affects the properties of vulcanized rubber • If the rubber has few cross-linkages, the rubber is softer, more flexible and more elastic • If the rubber has many cross-linkages, it is stiffer, less flexible and less elastic • Car tyres are made of vulcanized rubber • Because of the presence of cross-linkages among the polymer chains, the rubber does not melt when it gets hot • That is the reason why car tyres do not melt when drivers drive really fast

Car tyres are made of vulcanized rubber

Degradable Plastics • Natural polymers (e.g. wood and paper) are biodegradable Micro-organisms in water and in the soil use them as food • Synthetic polymers (e.g. plastics) are nonbiodegradable can remain in the environment for a very long time • Nowadays, plastic waste constitutes about 7 % of household waste • In Hong Kong, plastic waste is buried in landfill sites it remains unchanged for decades more and more landfill sites have to be found Uses of plastics in Hong Kong (an approximation) (a) Hong Kong is a highly densely populated city (b) The volume of domestic waste generated daily is very great. (c) Plastic waste contributes to the main bulk of our domestic waste. (d) Few sites are left to be used for landfill (e) The building / operation cost of incineration plants is high and recycling of plastics also involves very tedious procedures. Page 39

Chemistry in Action (Polymers)

In order to tackle the pollution problems caused by the disposal of plastic waste, degradable plastics have been invented Several types of degradable plastics: • biopolymers • photodegradable plastics • synthetic biodegradable plastics 1. Biopolymers • Polymers made by living micro-organisms (e.g. paracoccus, bacillus and spirullum) • e.g. The biopolymer poly(3-hydroxybutanoic acid) (PHB) is made by certain bacteria from glucose • When PHB is disposed, the micro-organisms found in the soil and natural water sources are able to break it down within 9 months • However, PHB is 15 times more expensive than polyethene (a) Paracoccus

(b) Bacillus

(c) Spirullum

2. Photodegradable Plastics • Photodegradable plastics have light-sensitive functional groups (e.g. carbonyl groups) incorporated into their polymer chains • These groups will absorb sunlight use the energy to break the chemical bonds in the polymer to form small fragments Page 40

Chemistry in Action (Polymers)

3. Synthetic Biodegradable Plastics • Made by incorporating starch or cellulose into the polymers during production Micro-organisms consume starch or cellulose and the plastics are broken down into small pieces • The very small pieces left have a large surface area greatly speeds up their biodegradation • Drawbacks of this method: • the products of biodegradation may cause water pollution • the rate of biodegradation is still too low for the large quantity of plastic waste generated • They are much more expensive than ordinary materials. • When buried in landfill, they will not be exposed to sunlight light and may therefore remain unchanged for many years. • Their long term effects on the environment are unknown of any residues. • They may encourage a ‘throwaway is OK’ culture. • They interfere with the present recycle program.

END

Page 41