Topic 9 Crystals, macromolecules, polymers, plastics
Updated 2008-09-27
Please send comments to: J.Elfick@uq.edu.au
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Table of contents
3.1.0 Crystals
3.2.0 Water of crystallization
2.31.0 Hygroscopic, deliquescent and efflorescent chemicals
3.4.0 Polymers and plastics
3.1.0 Crystals
3.54 Grow crystals from solutions
3.54.1 Grow salt crystals from sea water
3.54.2 Grow crystals from a melt
3.54.3 Grow crystals with different shapes
3.54.4 Grow crystals from a mixture of salts
3.54.5 Grow large crystals
3.54.6 Grow clusters of crystals
3.54.7 Split crystal
3.1.4 Prepare aspirin crystals
3.1.10 Prepare sugar crystals from brown sugar
3.1.10.1 Prepare sugar crystals from sugar cane juice
12.18.6 Prepare sodium thiosulfate crystals, "hypo", Na2S2O3.5H2O
12.18.6a Reactions of sodium thiosulfate crystals, Na2S2O3.5H2O
12.20.1 Prepare boric acid crystals
3.2.0 Water of crystallization
3.31 Hygroscopic, deliquescent and efflorescent chemicals
3.31.1 Expose different salts to the air
3.31.2 Expose sodium carbonate decahydrate, washing soda, to the air
3.31.3 Tests for water with cobalt (II) chloride
3.2.1 Heat copper (II) sulfate crystals
3.2.2 Heat iron (II) sulfate-7-water crystals
3.2.3 Heat different crystals with water of crystallization, test for the presence of water with blue cobalt (II) chloride paper.
3.2.4 Heat magnesium sulfate-7-water crystals
3.2.5 Secret writing inks (invisible ink) with cobalt (II) chloride, sucrose, starch, ammonium iron (II) sulfate, sodium chloride
4.19 Invisible inks (Primary)
3.2.6 Make fibrous plaster board with plaster of Paris
3.67 Strength of plaster of Paris
3.4.0 Polymers and plastics
3.95 Break down starch to sugars
3.96 Break down of ethanol to ethene (ethylene)
3.97 Break down of polymers with heat
3.99 Gases from wood
3.100 Prepare plastic from milk casein
3.101 Prepare urea formaldehyde resin
3.102 Burning tests for plastics
3.4.01 Glass transition temperature (Tg)
3.4.02 Chemical sources of polymer materials
3.4.03 Names of polymers
3.4.04 Super ball
3.4.1 Natural polymers
3.4.1.1 Stretched rubber band
3.4.2 Addition polymers
3.4.2.1 Polyethene polymer (polyethylene, Polythene)
3.4.2.1a Push pencils through a polythene bag
3.4.2.2 Polyethylene terephthalate (PET)
3.4.2.3 Polyvinyl chloride polymer (PVC, poly-chloroethane)
3.4.2.4 Polystyrene polymer (PS, polyphenylethene)
3.4.2.5 Polymethyl methacrylate polymer (acrylic resin, Perspex, Lucite, Plexiglas, PMMA)
3.4.2.6 Polyacrylonitrile polymer (acrylonitrile monomer) (acrylic resin, Acrilan)
3.4.2.7 Polypropenonitrile
3.4.2.8 Polypropene polymer (polyprene, polypropylene, PP)
3.4.2.9 Teflon, Polytetrafluoroethene polymer (PTFE)
3.4.2.10 Polyurethane polymers
3.4.3 Condensation polymers
3.4.3.1 Epoxy resin polymers
3.4.3.2 Polyacetals, polyoxymethylene resin
3.4.3.3 Polycarbonates
3.4.4 Break polymers into small molecules
3.4.4.1 Action of propanone on expanded polystyrene beads
3.4.5 Milk casein
3.4.6 Urea formaldehyde (urea methanal) resin
3.4.6.1 Formaldehyde resorcinol resin
3.4.7 Prepare nylon polymer
3.4.8 Prepare rayon, copper (II) sulfate with ammonia solution, "regenerated fibre", "artificial silk"
3.4.8.1 Prepare rayon, basic copper carbonate with ammonia solution
3.4.9 Prepare Bakelite plastic, phenol / methanal polymerization
3.4.10 Tests for plastics
3.4.11 Slime ball, "Silly putty", silicone polymer to amuse children
3.4.12 Alginate polymer
3.5.0 Polymer terminology
3.5.1 Polymer foam
3.5.2 "Silly string"
3.1.0 Crystals
Many solids are crystalline. Their particles exist in an ordered arrangement. Crystallization involves the formation of the pure solid from its solution. Many crystals contain water of crystallization, e.g. copper (II) sulfate crystals, CuSO4.5H2O. However, anhydrous copper (II) sulfate, CuSO4, contains no water of crystallization.
3.1.4 Prepare aspirin crystals
See diagram 16.3.4.11
Dissolve an aspirin table in methylated spirit. BE CAREFUL! Filter if necessary. Heat the solution.
3.1.10 Prepare sugar crystals from brown sugar
See also 11.0: Activated carbon [commercial information]
Activated carbon decolorizes and refines brown sugar solution by adsorbing coloured impurities on it. After filtering, concentrating and cooling you can obtain the white (refined) sugar crystals.
1. Put 5-10 g of brown sugar in a small beaker. Dissolve the sugar in 40 mL water by heating. Add 0.5-1.0 g of activated carbon while constantly stirring. Filter the suspension when it is still hot to obtain a colourless solution. If the filtrate appears yellow, add a little more activated carbon, heat and filter the suspension again until the filtrate becomes colourless. By heating, concentrate the filtrate in a small beaker on a water bath until you reduce the volume of the solution to about 1 / 4. White sugar crystals separate out of the liquor after cooling it naturally.
2. Dissolve 100 g of brown sugar in 90 mL water. Add calcium hydroxide solution until the solution turns red litmus blue. Filter the solution then heat the filtrate with absorbent charcoal while it is still hot. Evaporate at reduced pressure at 50 to 65oC. Put the syrup in a refrigerator for several days to form crystals.
3.1.10.1 Prepare sugar crystals from sugar cane juice
In commercial production of cane sugar, the solution is clarified by adsorbing impurities on bone char, a type of carbon made by heating bones in the absence of air.
Add calcium hydroxide to sugar cane juice or sugar beet juice until red litmus turns blue. Leave it to stand overnight. Filter by suction. Evaporate in reduced pressure at 50- 65oC until it becomes a brown syrup that is so viscous that it scarcely flows. Put the syrup in a refrigerator for several days to form crystals. Use a centrifuge to separate crystals from the liquor.
3.2.0 Water of crystallization
Hydration occurs when water molecules become orientated around ions. The oxygen atoms move closer to cations and the hydrogen atoms move closer to anions. Many inorganic compounds form crystals in which water is part of the crystal lattice. Water of crystallization (water of hydration) exists when water becomes part of a crystal of a hydrated ionic compound, e.g. CuSO4.5H2O.
3.2.1 Heat copper (II) sulfate crystals
The experiment can be done with other crystalline salts.
Heat copper (II) sulfate crystals to make it lose its water of crystallization and leave anhydrous copper (II) sulfate as a white powder. The lost water appears as drops on the inner surface of the upper part of the test-tube. Test the drops for the presence of water with blue cobalt (II) chloride paper. Transfer the anhydrous copper (II) sulfate to another test-tube and add a drop of water. The blue hydrated salt forms again.
3.2.2 Heat iron (II) sulfate-7-water crystals
The water in the crystal is necessary for the shape and colour of the crystal. Put crystals in a test-tube. Heat gently and note any reaction. The crystals lose water vapour that condenses as liquid in the cool upper region of the test-tube. The crystals lose their shape and colour.
3.2.3 Heat crystals that have water of crystallization
Heat crystals of bluestone CuSO4.5H2O, borax Na2B4O7.10H2O, common salt NaCl, Epsom salts MgSO4.7H2O, Glauber's salts Na2SO4.10H2O, green vitriol FeSO4.7H2O, hypo Na2S2O3.5H2O, washing soda Na2CO3.10H2O, and white vitriol ZnSO4.7H2O. They all contain water of crystallization except common salt. Tests for the presence of water with blue cobalt (II) chloride paper, CoCl2.
CoCl2(s) + 6H2O(l) <---> CoCl2.6H2O(s)
3.2.4 Heat magnesium sulfate-7-water crystals
Relative molecular mass = 246.47. A similar experiment can be done with copper (II) sulfate crystals.
Weigh the hydrated crystals, W1. Heat the crystals and weigh again. Continue heating and weighing until the weight does not change, W2. Weight of a water of crystallization = W1 - W2. Number of molecules of the crystal, n = W1 ÷ 246.47. Number of molecules of water in each molecule of crystal = (W1 - W2) ÷ n = 7
3.2.5 Secret writing inks, invisible inks
See also 4.19: Invisible inks (Primary)
A safe way to see secret writing by heating the paper is to wrap the paper over a 100 watt light globe or to iron it with a hot iron, not a steam iron. The experiments below can also be done on a clean egg. After writing on the egg, wait to allow the chemical to sink in, then wash the egg leaving the secret writing chemical still on the inside of the eggshell. Then treat the egg as below.
1. Alum solution, K2SO4Al2(SO4)3.2H2O
Let the ink dry on the paper then heat the paper over a warm stove until the dry invisible letters become visible as dark carbonized areas. The alum dehydrates the cellulose in the paper by acting as a proton donor to form H2O from the -OH groups of the cellulose.
2. Cobalt (II) chloride
See also 12.05a.1 (4.) Properties of cobalt chloride
Novelty weather indicators contain cobalt (II) chloride that turns blue to show that the weather is fine and turns pink to show that the weather is wet.
Dissolve crystals of cobalt (II) chloride-6-water and use for ink. Use a small paint brush or a chewed end of a match to paint the ink on the paper, then leave to dry. To reveal the secret message, heat the paper over a candle flame without burning the paper. The secret message appears as blue writing. When you paint the message with water, it disappears again. Some spies use a mixture of cobalt chloride and glycerine.
Dilute cobalt salt solutions are almost invisible. If you write on paper with the invisible solution, you can later see the writing by heating the paper over a flame. However, if you put the paper aside, the cobalt salt absorbs water from the atmosphere and the writing becomes invisible again. So there is no chemical reaction - just a dilution or evaporation effect.
3. Cane sugar or milk
Use a solution of cane sugar, sucrose in water or milk. Write on a piece of cardboard. Read the secret message by burning paper and rubbing the ash on the cardboard.
4. Starch, cornstarch suspension
See 5.4.8: Iodine Solution
Use liquid starch for ink. First test the paper with iodine solution to be sure that it does not contain starch. Read the secret message by dipping the paper in iodine solution. The writing appears dark blue on light blue paper.
5. Ammonium iron (II) sulfate or ammonium chloride
Use a solution of ammonium iron (II) sulfate or ammonium chloride for ink. On heating, the secret message appears as brown black or yellow brown writing.
6. Sodium chloride
Use a saturated solution of sodium chloride for ink. Rub the dry paper with a soft lead pencil. The secret message appears as a darker pencil mark where the pencil scrapes on the salt crystals.
7. Vinegar, or lemon juice, or squeezed onion juice
These juices convert paper to substances similar to cellophane with ignition temperature lower than paper. When you gently heat the paper, the parts written on turns brown. Most organic liquids will char on heating as some of the organic molecules are reduced to carbon with loss of water. Also, you can add dilute iodine solution to see white writing on a light blue background. Also, you can use red cabbage water to make the secret writing appear red.
8. Phenolphthalein (uncoated laxative tablets)
Sprinkle sodium bicarbonate or washing soda solution on the secret writing to make it appear pink.
3.2.6 Make fibrous plaster board with plaster of Paris
Plaster of Paris is made by heating the mineral gypsum (calcium sulfate-2-water) to remove some of its water of crystallization. This process is called calcination or calcining. Plaster of Paris is partly dehydrated gypsum 2CaSO4.H2O(s). Gypsum as a hemihydrate is shown as CaSO4.½H2O. Plaster of Paris is used for making casts, e.g. of the shape of a shell or for keeping broken bones in place.
CaSO4.½H2O. 2CaSO4.2H2O(s) <---> 2CaSO4.H2O(s) + H2O(l) gypsum <---> plaster of Paris + water
Mix wet Plaster of Paris with fibres. As the mixture dries, gypsum crystals form again by taking in water. The setting plaster gives out heat and expands slightly, but it does not dry because of evaporation.
3.4.0 Polymers
See also 31.1.02: Electrostatic series, triboelectric series
BE CAREFUL! Do these experiments in a fume cupboard and cover the beaker. Keep all chemicals away from the skin. Use safety glasses and nitrile chemical-resistant gloves!. Do not inhale vapours.
Polymers are large molecules built up by repetition of small, simple chemical units called monomers. For example (+)glucose is the unit of starch, and ethylene (ethene) is the unit of polythene.
Synthetic polymers are formed by chemical reactions for making plastics where monomers are joined by polymerization and condensation.
Homopolymers are formed from the same monomer, e.g. ethene (CH2CH2) is polymerized to form polyethene (polyethylene).
Heteropolymers (copolymers) are made from different monomers, e.g. ABS plastics are made form acrylonitrile, butadiene, styrene copolymers. ABS is the generic name for plastics from acrylonitrile, butadiene and styrene units
Synthetic resins are polymer compounds before curing.
Thermoplastic polymers have long-chain molecules which soften on heating and harden on cooling. This plastic is heated to soften it, then squeezed to fuse it and to shape it by viscous flow. Most polymers are thermoplastics. The weak Van der Waals bonds between molecule chains can be broken by heating.
Thermoset polymers are extensively cross-linked to form 3-D networks that does not soften much on heating. A network of strong covalent bonds forms during the initial curing so the material cannot be remoulded.
Regular polymers normally crystallize and may be cooled from the melt so quickly to allow crystallization, e.g. polyethylene terephthalate film is amorphous owing to cooling at a high rate.
3.4.01 Glass transition temperature (Tg)
Glass transition temperature is the temperature when the material changes from glass to rubber properties. The viscosity drops suddenly. Amorphous polymers below Tg are hard and stiff but above Tg are rubbery. The glass transition process is gradual. Semi-crystalline polymers have less change at Tg and above Tg may not be brittle. An amorphous polymer has molecular chains in the irregular arrangement at Tg are homogeneous and transparent because they do not contain crystals to scatter the light.
Natural chewing gum comes from chicle based on gutta-percha plasticized by triterpenes. Commercial chewing gums are based on poly (vinyl acetate) PVA. If chewing gum is stuck to a carpet, you can freeze it with ice to bring its temperature down to its Tg, then remove it as a solid.
Plastic containers made from recycled plastic has raised Tg because the mix of materials slows molecular movement. In cold climates, below Tg the containers become brittle bins and crack easily if dropped. Cotton is a cellulose polymer with Tg of 225oC. It absorbs water because the water molecules can slip in between the polymer molecules plasticizes the cotton and lowers the Tg. When ironing cotton fabric, you can sprinkle with water or use a steam iron to increases the plasticizing effect then remove by heat to raise the Tg and set the fabric in a new shape. However, nylon fibres have Tg of 50oC and polyester has Tg of 69oC, so lower temperature ironing is needed! Steam ironing of wool breaks the disulfide bonds that keep wool fibres in shape and then allows them to reform.
3.4.02 Chemical sources of polymer materials
acrylonitrile-butadiene copolymer, acrylonitrile-butadiene styrene (ABS) acrylonitrile-styrene-butyl acrylate (ASA) cellulose acetate butyrate (CAB) chlorinated polyethylene (CPE) ethylene vinyl acetate (EVA) ionomer resins, melamine-formaldehyde resin, phenol-formaldehyde resin, styrene-acrylonitrile (SAN) urea-formaldehyde resin, polymer alloys
Polyacrylics (inc. PMMA): amide (e.g. nylon, 66) butadiene (high cis) elastomer, butadiene-styrene elastomer, butadiene-styrene resins, butylene terephthalate (PBT) carbonate, epichlorahydrin, ester, thermoplastic) ester (thermoset) ethylene (LD) ethylene (LLD) ethylene (HD) oxymethylene (polyacetal) phenylene oxide (e.g. Naryl) propylene copolymer, propylene homopolymer (PP) styrene (PS) " urethane-prepolymer, urethane-thermoplastic, vinyl acetate (PVA) vinyl chloride (PVC) vinyl chloride copolymer Poly-diallylcarbonate (Columbia resin CR39, CR64, EX80) is used for plastic optical lens, embedding and casting.
3.4.03 Names of polymers
Acetate: Generic name for fibres from cellulose (21 / 2) acetate
Acrilan: Trade name for fibres from poly(acrylonitrile)
Acryl: Generic name for fibres >85% acrylonitrile units
Alcantara: Synthetic suede from polyester fibres in a polyurethane matrix
Araldite: Epoxy resins
Aramide: Generic name for fibres from aromatic polyamides
Bakelite: Thermoset from phenol and formaldehyde
Balata: Natural trans-1,4-polyisoprene
Cellophane: Films from regenerated cellulose
Celluloid: Cellulose nitrate plasticized with camphor
Dacron: Fibres from polyethylene terephthalate
Diolen: Fibres from poly(ethylene terephthalate)
Dralon: Fibres from poly(acrylonitrile)
Grilen: Fibre from poly(ethylene terephthalate)
Guttapercha: Natural trans-1.4-polyisoprene (latex of Palaquium oblongifolium)
Hostalen: Polyethylene thermoplastic
Hostalen PP: Polypropylene thermoplastic
Hostalit: Poly(vinyl chloride) thermoplastic
Hostaphan: Films from poly(ethylene terephthalate)
Igelit: Poly(vinyl chloride) thermoplastic
Lupolen: Polyethylene thermoplastic
Luran: Thermoplastic from styrene and acrylonitrile
Marlex: Thermoplastic polyethylene
Moltopren: Cellular polymer from polyurethanes
Mylar: Films from polyethylene terephthalate
Natural rubber: cis-1.4-polyisoprene (latex of Hevea brasiliensis)
Neoprene: Elastomeric polymers and copolymers from chloroprene
Nylon: Generic name for polyamides
Orlon: Fibre from polyacrylonitrile
Perlon: Fibre from polycaprolactam
Plexiglas: Thermoplastic poly(methyl methacrylate)
Polyester: Generic name for fibres from polyesters >85% terephthalic acid and ethylene glycol units
Polyester unsaturated: Thermosets from maleic acid / ethylene glycol polymers cross linked with polystyrene
Rayon: Generic name for fibres from regenerated cellulose
Silicone: Generic name for polymers with a siloxane chain
Spandex: Generic name for elastic fibres from polymers >85% segmented polyurethane
Styrofoam: Cellular plastic from polystyrene Lustron: Thermoplastic polystyrene
Terital: Fibres from poly(ethylene terephthalate)
Terlenka: Fibres from poly(ethylene terephthalate)
Terylene: Fibres from poly(ethylene terephthalate)
Trevira: Fibres from poly(ethylene terephthalate)
Triacetate: Generic name for fibres from cellulose triacetate
Tricel: Fibres from cellulose triacetate
Vestolit: Poly(vinyl chloride)
Viscose: Generic name for fibres from regenerated cellulose
Zytel Various aliphatic polyamides
3.4.04 Super ball
Super ball is made from polybutadiene with small amounts of sulfur to reinforce the material and serve as a vulcanizing agent. The ball is moulded under very high pressure and temperature and is said to have a 92% resiliency, about three times the resiliency of a tennis ball. It can continue to bounce for about a minute after being dropped from a short height.
3.4.1 Natural polymers, natural rubber
See also elasticity 34.5.1: Hooke's law, elastic limit, deforming force, stress and strain, elasticity
Natural polymers occur as brittle glassy gums and resins in plants, e.g. conifers, and as polysaccharides, e.g. starch. Natural rubber is a polymer of isoprene CH2=C(CH3)CH=CH2 in which all the -CH=CH-= groups are cis. The polymer chains in natural rubber are elastic in the sense that the chains can be unravelled without coming apart, i.e. the rubber can stretch. Elasticity was improved by cross-linking with sulfur, using the Goodyear process to produce vulcanized rubber. Stretching aligns the random chains, and temporarily crystallizes and toughens rubber so that rubber tyres do not form cracks. Rubber is not very elastic in the Hooke's law sense of stress being proportional to strain. The transisoprene polymer, trans-1.4-polyisoprene, occurs in tropical trees in the latex of Palaquium oblongifolium and is called gutta-percha.
Collect, examine and describe different addition polymers, e.g. plant gums, natural starch.
3.4.1.1 Stretched rubber band
1. Stretch a thick rubber bandit quickly against your lips and note the change in temperature. Hold it stretched, allow it to cool back to room temperature. Then let it suddenly contract against your lips to its original length and note the temperature change. Polymers behave in an opposite way to metals in that rubber contracts on heating and expands on cooling.
2. Use a hair dryer to heat a stretched rubber band with a weight on the end.
3. Cool a rubber band. Stretch a wide rubber band between the index finger of your two hands. Let the rubber band touch your lips. Stretch the rubber band (not so far that it breaks!) then slowly release the tension. You can feel heat in your lips when the rubber band stretches because of friction between the rubber molecules. The stretched rubber band feels cooler when you release the tension.
4. Suspend a 100 g mass from a rubber band. Use a vertical ruler to measure the length of the suspended rubber band. Bring a heat sources. e.g. a lighted match close to the middle of the stretched rubber band. Note that the heated rubber band contracts.
5. Observe the thermal properties of rubber. Hang a 1 kg mass from four rubber bands so it touches the table. Heat with a radiant heater for 20 seconds and the mass will lift. Enclose a rubber tube in a copper cylinder and heat with a Bunsen burner. The rubber tubing contracts as it is heated. Stretch and unstretch rubber bands on the lips to feel the changes in temperature.
3.4.2 Addition polymers
See diagram 3.4.2: Vinyl polymers
Addition polymerization occurs when identical monomers link under high temperature and pressure. The reactive group is the carbon to carbon double bond [C=C]. For example many units of ethylene (ethene [CH2=CH2] combine to form polyethene (polythene) [-CH2-CH2-CH2-CH2-]. That is the only product of the reaction and it has no definite chemical formula. Addition polymers are called thermoplastic (thermosoftening) polymers because they melt easily. They may be recycled because they can be melted and used again but, when they burn, they may form poisonous gases.
Polymers called polyelectrolytes that can absorb up to 1,000 times their weight in water, gel capacity, are known as "Super Absorbents" and "Water Crystals". When dry, the polymer is a white powder and when in gel form it is a transparent gel. It is used in diapers, bed pads, fire control, spray drift control, seed germination, soil conditioning,period pads, and hydroponics. It is sensitive to the salts in hard water so dissolved minerals decrease absorption capacity.
3.4.2.1 Polyethene polymer (polyethylene, Polythene) is made from the monomer ethylene (ethene, CH2=CH2). Polyethylene is used for plastic bags, bin liners, laboratory wash bottles, insulators, moulded objects. Polyethylene has a waxy feel and is acid resistant. Polyethylene (LDPE) is opaque, white, soft, flexible, impermeable to water vapour, unreactive towards acids and bases, absorbs oils and softens, melts at 100-1250C, does not become brittle until -100oC, oxidizes on exposure to sunlight, subject to cracking if stressed in presence of many polar compounds. Polyethylene (HDPE) is similar to LDPE, more opaque, denser, mechanically tough, more crystalline and rigid.
3.4.2.1a Push pencils through a polythene bag
Half fill a polythene bag with water and tie it shut with one end of a long string. Suspend the bag by raising the other end of the string. Very quickly push a sharpened pencil through both polythene bag walls and enclosed water. Leave the pencil in place and push other pencils through the bag. Withdraw the pencils and the polythene bag returns to its former shape. Polythene consists of web-like matrix of molecules such that the polythene is easily stretched and then can return to its former shape without tearing.
3.4.2.2 Polyethylene terephthalate (PET) is transparent, high impact strength, impervious to acid and atmospheric gases, not subject to stretching. PET is used for light-weight, shatter proof soft drink bottles. The bottles used to have a second black polyethylene as a cup on the bottom for a strong base but now have a bottom with five convolutions. PET is a birefringent 2-dimensional orientated plastic.
3.4.2.3 Polyvinyl chloride polymer (PVC, poly-chloroethane) [(CHCl.CH3)n] is made from the monomer is vinyl chloride (chloroethene) [CH2=CHCl]. PVC is used for gramophone records, upholstery, rain coats, garden hose, swimming pool liners and plastic tubing for burners. Polyvinyl chloride (PVC) is rigid, thermoplastic, impervious to oils and most organic materials, transparent, high impact strength.
3.4.2.4 Polystyrene polymer (polyphenylethene) is made from the monomer phenylethene (styrene). Styron thermoplastic, Styropor cellular thermoplastic. Polystyrene is used for "rigid foams", heat insulation, packaging, moulded objects, electrical insulation, balls for atomic models, and disposable petri dishes. Polystyrene is glassy sparkling clarity, rigid, brittle, easily fabricated, upper temperature use 90oC, soluble in many organic materials.
3.4.2.5 Polymethyl methacrylate polymer (acrylic resin, Perspex, Lucite, Plexiglas, PMMA) Poly(methyl methacrylate) thermoplastic transparent polymer (poly [methyl 2-methylpropenoate]) Perspex is made from the monomer methyl methacrylate. Perspex is used as a glass substitute for lenses and blocks for light experiments, safety glass, windscreen glass, rod for electrostatics experiments, burner tubing, and super glue.
3.4.2.6 Polyacrylonitrile polymer (acrylonitrile monomer) (acrylic resin, Acrilan, polypropenonitrile, acrylonitrile-butadiene-styrene). Polypropenonitrile is made from the monomer propenonitrile (acrylonitrile, vinyl cyanide) [CH2=CHCN].
3.4.2.7 Polypropenonitrile is used to make acrylic fibres (Orlon knitted fabrics, imitation fur and carpets, acrilan textile fibres). Mixed polymer SAN made from styrene and acrlonitrile used for latex paints and plastic plates. Mixed polymer ABS made from acrylonitrile, butadiene and styrene, used for telephone sets, shoe soles, car parts.
3.4.2.8 Polypropene polymer (polyprene, polypropylene) is made from the monomer propene (propylene) [CH3CH=CH2] is used for chairs, buckets, bowls, tubing connectors, carpets and heavy duty bottles. Polypropylene is opaque, high melting point (160-170oC) high tensile strength and rigidity, lowest density commercial plastic, impermeable to liquids and gases, smooth surface with high lustre.
Acrylic acid is manufactured from propylene (from catalytic cracking of petroleum) in two steps via acrolein in a gas phase using special catalysts.
CH2=CHCH3 (propylene) + O2 --->CH2=CHCHO (acrolein) + H2O
CH2=CHCHO + 1/2O2 --->CH2=CHCO2H (acrylic acid)
3.4.2.9 Teflon, Polytetrafluoroethene polymer (PTFE) the monomer tetrafluoroethene [CF2=CF2] Polyprene is used for non-stick surfaces of frying pans and non-lubricated bearings, non-stick cooking pan lining, gaskets, chemical resistant films.
Polyprene is a thermosetting, thermoplastic.
3.4.2.10 Polyurethane polymers contain the urethane group [-NH-CO-O-]. Polyurethanes form tough materials and are used for paints, rubber, foam plastics, tough linings and car body parts. Collect, examine and describe different addition polymers. Molecules containing the isocyanate group, -NCO, can react with molecules containing an -OH group to give a urethane which is similar to the amide bond in nylons. Heated polyurethanes produce unpleasant vapours that may contain nitric acid, HNO3, nitrogen dioxide (NO2) and hydrogen cyanide (HCN). So a burning pillow made of polyurethane foam may produce dense toxic fumes. Polyurethanes can be formulated to make plastics with either low or high glass transition temperatures for the packaging industry and shoe soles.
3.4.2.5.1 Sodium polyacrylate, acrylic sodium salt polymer, ASAP
Fibres + sodium hydroxide, then polyacrylic acid --> sodium polyacrylate, a crosslinked acrylic acid polymer sodium salt. It has a very high molecular weight, is very soluble in water and forms a linear anion polymer. The monomer is: -CH2-CHCOONa-. Sodium polyacrylate is used as a thickening agent, in urine test kits, in baby diapers (nappies) in tampons for menstruation (but may cause toxic shock syndrome if not changed daily) as "water crystals" to store water in soils, as "magic snow powder" novelty and movie set decoration and as "ghost crystals" because they become invisible in water, having the same refractive index.
1. Attach a pin to a crystal of sodium polyacrylate. Tie a string around the crystal and lower it into water. The crystal disappears but the pin remains suspended in water.
2. Dissolve some powder or gel form of sodium polyacrylate in alcohol. The solution turns a deep magenta colour until the alcohol evaporates. (Magenta is a brilliant red aniline dye derived from coal tar.)
3.4.3 Condensation polymers
Condensation polymers need two, usually different, reactive molecules to link, with the loss of a small molecule such as water, to form a polymer. The reaction is similar to the formation of esters and starches. These reactions may be thermoplastic (softening on heating) or thermosetting (do not soften on heating). You cannot recycle thermosetting plastics.
3.4.3.1 Epoxy resin polymers are formed by polymerizing epoxide compounds [R1COCR2] with phenols [C6H5O-] epichlorhydrin and bisphenol-A. They are used as surface coatings and embedding electronic components because of resistance to chemicals. They are also used as adhesives that have a two pack system of a resin and a hardener.
3.4.3.2 Polyacetals, polyoxymethylene resin, acetyl resin, polyformaldehyde, POM, "Kematal", "Deirin" are abrasion-resistant and resist organic solvents and water. This moulding polymer is widely used in engineering products, e.g. gear wheels, plumbing to replace brass or zinc and in pens. It may be copolymerized with etylene oxide to increase stability and not become depolymerized.
3.4.3.3 Polycarbonates retain their dimensions and resistance to impact and wide range of temperatures. The functional carbonate group is -O-CO-O-. It is widely injection mould to make baby' bottles, bus windows, fire masks, interior of aircraft, helmets, battery cases
Collect, examine and describe different addition polymers, e.g. nylons, polyurethanes (urea formaldehyde) polyesters (Terylene, fibre glass) and epoxy resins.
3.4.4 Break polymers into small molecules
See diagram 3.4.4
Put very small pieces of perspex or polystyrene in a hard-glass test-tube. Connect a delivery tube to a receiving test-tube that must be cooled thoroughly with cold water because the fumes are harmful. Heat the test-tube containing the perspex gently. The polymer melts and forms vapours collected in the receiving tube. Control the heating to enable all the fumes to condense in the receiving tube. Heat breaks down the polymer to smaller molecules.
3.4.4.1 Action of propanone on expanded polystyrene beads
Pour 50 mL of propanone into a beaker full of expanded polystyrene beads used a packing material or into a polystyrene coffee cup in a beaker. The expanded polystyrene fizzes and shrinks to form a sticky gel. The expanded polystyrene does not dissolve in he propanone but just loses the gas that had puffed it out.
3.4.5 Milk casein
Casein is a phosphoprotein thermoplastic polymer that may be used to make insulators, buttons, handles, adhesives and artist's priming paint. You can make casein from the reactions of skimmed milk with ethanoic acid (acetic acid).
Calcium caseinate + 2H+ ---> casein + Ca2+.
3.4.6 Urea formaldehyde (urea-methanal) resin
See diagram 3.4.6: Urea-formaldehyde methanal condensation polymerization | See diagram 9.4.9 Urea formaldehyde
Urea formaldehyde resin is thermosetting, has pale colour so it can be dyed, is non-inflammable, and is used in fibre glass and adhesives. Urea-formaldehyde is sold as a moulding powder with alpha-cellulose (wood pulp) filler. These resins are less water resistant and less heat resistant than the phenol-formaldehyde resins. If melamine is substituted for urea, the melamine formaldehyde resin can withstand temperatures above 100oC and is used for light coloured dinner ware.
Condensation polymerization with the elimination of water
(NH2).CO.(NH2) + CH2O ---> NH-CO-NH-CH2 + H2O
urea + formaldehyde ---> urea formaldehyde
3.4.6.1 Formaldehyde resorcinol resin
Add 2 g of resorcinol to 5 mL of 45% formaldehyde solution in a small beaker and stir the mixture. Add drops of concentrated hydrochloric acid and stir the mixture. The mixture suddenly hardens as molecules build up into larger molecules. Take out this condensation polymer resin and wash it thoroughly.
3.4.7 Prepare nylon polymer
See diagram 3.4.7: Wind the polymer onto a glass rod
Nylons have a structure like a long protein. Nylons are formed by condensation between the amino group (-NH2) of one molecule and the carboxylic acid group (-COOH) of another molecule. Nylon-66 ("Bri nylon") is [-NH-(CH2)6-NH-CO-(CH2)4-CO-NH-(CH2)6-NH-]. Nylon-66 forms by condensation of hexane-1,6-diamine (1,6-diaminohexane) (NH2.[CH2]6.NH2) and hexanedioic acid (adipic acid) (CH2.CH2[COOH]2). Nylon-66 is used for nylon thread, rope, toothbrush bristles, cog wheels, shirts, combs. It is thermoplastic
3.4.7.1 Prepare two solutions: Solution 1. Add 2 g of decanedioyl dichloride (sebacoyl chloride) to 20 mL of dichloromethane IRR (methylene chloride, methylene dichloride). Add phenolphthalein to make it more visible. Solution 2. Add 2 g of hexane-1,6-diamine (1,6-diaminohexane) to 20 mL of 1 M NaOH solution. Slowly pour Solution 2. into the Solution 1. Do not mix the solutions. Grab the film of polymer between the two solutions with forceps. Wind the polymer onto a glass rod. Wash the nylon thread in water.
3.4.7.2 Prepare two solutions: Solution 1. Dissolve 2.0 mL of decanedioyl dichloride (sebacoyl chloride) in 50 mL of n-hexane (hexane). Solution 2. Dissolve 3 g of hexane-1,6-diamine and 1 g of sodium hydroxide in 50 mL of deionized water. Add phenolphthalein to make it more visible. Slowly pour Solution 1. as a second layer on Solution 2. Use forceps to grasp the polymer film that forms at the interface of the two solutions. Pull it gently from the centre of the beaker. Wind it round a glass stirrer or a cotton reel. Wash it thoroughly in 50% ethanol then in water until moist red litmus paper does not turn blue.
3.4.7.3 Prepare two solutions: Solution 1. Dissolve 2.2 g 1,6-diaminohexane in 50 mL of deionized water, i.e. 1.4 mol per litre. Solution 2.: Dissolve 1.5 g of decanedioyl dichloride in 50 mL cyclohexane, C6H12, i.e.15 mol per litre. Pour 5 mL Solution 1. into a beaker. Pour 5 mL of Solution 2. on top of Solution A using a glass rod so that the two solutions do not mix. A grey film of nylon forms a the interface of the two solutions.
3.4.8 Prepare rayon, copper (II) sulfate with ammonia solution, "regenerated fibre", "artificial silk"
BE CAREFUL! Concentrated ammonia gives off choking fumes that badly stink your eyes. Do this experiment in a fume cupboard.
In commercial manufacture, cellulose filaments pass through solutions that then coagulate them or cellulose ethanoate (cellulose acetate) ethyl cellulose or cellulose nitrate is dissolved in a solvent. Rayon contains about 270 glucose units per molecule, but cotton contains 2,000 to 10,000 units per molecule. The solutions are forced through fine nozzles to form rayon or acetate rayon fibre. Commercial production of rayons uses treat cellulose from wood pulp with sodium hydroxide and carbon disulfide to produce xanthate that is squeezed to produce threads or cellophane.
Dissolve finely shredded paper in a saturated solution of copper (II) sulfate in concentrated ammonia solution. Put the solution in a plastic syringe and squirt into 1 M sulfuric acid. A blue thread forms that slowly runs white. The acid solution slowly turns blue.
3.4.8.1 Prepare rayon, basic copper carbonate with ammonia solution
Add 10 g of basic copper carbonate to 100 mL of 880 ammonia solution. Stir then pour the blue solution containing tetraamminecopper (II) ions into a second beaker. Slowly add 1.5 shredded cotton wool or filter paper or newspaper and stir for up to an hour until the solution becomes a gel, called viscose. Put viscose into a hypodermic syringe and inject it under 500 mL of 1 mol per litre sulfuric acid. The extruded blue fibre turns white as the as the acid neutralizes the tetraamminecopper (II) solution.
3.4.9 Prepare Bakelite plastic, phenol / methanal polymerization 1
See diagram 3.4.9: Phenol-methanal condensation polymerization
Bakelite is a trade name for phenol-formaldehyde resins, or phenolics. It is used for light bulb holders, electrical fittings and saucepan handles. Phenolic resins are used in varnishes and lacquers. Phenols, hydroxybenzenes are aromatic compounds with the hydroxyl group attached to the benzene nucleus. They react as alcohols and as weak acids to from salts. Phenol (carbolic acid, C6H5O10) is used to make Nylon, phenolic resins and epoxy resins. It is a strong disinfectant. As it is a weak acid, it can ionize: C6H5OH --> C6H5O- + H+. The phenol group C6H5- is the organic group in benzene, C6H6. Resorcinol [1,3-dihydroxybenzene, C6H4(OH)2] is a dihydric phenol used with formaldehyde (methanal, HCHO) to make cold-setting adhesives, also used to make plasticizers, resins and fluorescein dyes
BE CAREFUL! Teacher demonstration only! Do the experiment in a fume cupboard. Use safety glasses and nitrile chemical-resistant gloves.
1. Make a mould in Plasticine by pushing an object into it, e.g. a key. Put resorcinol in a beaker and add the formalin solution. Stir the solution until it is clear. Add 1 mL of dilute hydrochloric acid while stirring and then quickly pour the mixture into the mould. Leave the plastic to harden for a day or two. Remove plastic from mould and wash with water.
2. Add 30 mL concentrated sulfuric acid to 30 mL water. Be careful! Pour slowly and keep stirring! Then leave to cool to room temperature. Pour 25 mL of formalin into a disposable container and add 55 mL of glacial ethanoic acid (glacial acetic acid). Add 20 g of phenol (C6H5OH) and stir with a disposable glass stirring rod until the phenol dissolved. Add 60 mL dilute sulfuric acid (diluted above) and keep stirring. The mixture turns pale yellow then opaque pink, especially around the stirring rod. Heat is given off. Discard the milky liquid then take out the pink polymer, Bakelite, and heat it with a Bunsen burner flame. The polymer chars but does not melt because it is a thermosetting plastic. Discard the disposable container and the stirring rod.
3.4.10 Tests for plastics
This experiment is based on: Selinger, Ben, 1991, "Chemistry in the market place", Harcourt Brace Jovanovich Publishers, ISBN 0 7295 0334 8, Experiment 13.13.
1. Density test: Polyethylene, polypropylene, styrene- butadiene and some types of nitrile will float after you wet it thoroughly push it below the surface of water then release. You cannot test foam plastics in this test.
2. Feel test: Only polyethylene and polytetrafluoroethrylene have a waxy feel. Before the test, clean the surface to remove grease or plasticizers.
3. Copper wire test
BE CAREFUL! Hold small samples with tongs in a fume cupboard or well ventilated place.
Hold the burner at an angle at an angle. Stick a copper wire in a cork. Heat the wire with a Bunsen burner until any yellow, green or red colour disappears. Press the end hot wire into the plastic sample, then put the end with molten plastic on it back in the flame. Observe the colour in the flame, usually yellow, easy or difficult to ignite, melting, residue, fumes and odour. Remove the burning plastic from the flame and note whether it still burns. A green colour indicates that the plastic contains a halogen, e.g. chlorine in poly(vinyl chloride) (PVC) or poly(vinylidene chloride) (PVDC). Cyanide, e.g. from Orlon, may give a positive result. Before repeating the test with another plastic, again heat the copper wire until the green colour disappears.
3.4.11 Slime ball, "Silly putty", silicone polymer to amuse children
See also 7.2.6: "Silly putty"
Slime flows like a liquid under normal conditions but bounces on impact. Commercial slime is a water-soluble polymer sold by chemical supply firms or novelty firms to amuse children. The long molecules which comprise slime can slide over and around one another and cover the entire bench if left unguarded. They can also form temporary cross-linking bonds which affect the viscosity of the slime. Polymers are very large molecules made by linked monomers. Polymer molecules can cross-link with weak and strong chemical bonds. A strand of PVA, molecular weight 78, 000, may contain 1800 monomer units.
1. Prepare glue slime with 50 mL of 4% poly(vinyl alcohol) (PVA), or use pre-dissolved PVA, and 10 mL of 4% borax solution.
2. Add 1 mL of 4% borax solution to the 50 mL of 4% poly(vinyl alcohol). With each successive addition of 1 mL of 4% borax solution, observe any changes when you stir the gel slowly, stir the gel rapidly, pour the gel into another container, roll the gel in the hands to form a ball of gel, leave the ball of gel to stand, pull on a ball of gel.
2. Dissolve 2 tablespoons of borax in half a cup of warm water with a spoon. Add a drop of food colouring. Pour some PVA glue into a bowl. Put an equal amount of the coloured water into the bowl and mix.
3. The "viscosity builder" grade vegetable guar gum is used as a colloid stabilizer in foods, e.g. salad, dressing and ice cream. Slime is a non-Newtonian fluid. Dissolve 1 g of "viscosity builder" grade vegetable guar gum in 20 mL of water. Boil 60 mL of water and add the 20 mL suspension of guar gum while stirring. Dissolve 0.75 g of borax in 20 mL of water and add to the still warm solution while stirring. Leave to cool as a green gum and store in a closed container to prevent drying.
4. At the boiling point of water, poly(vinyl acetate) PVA, breaks down to give a lower molecular mass polymer. Dissolve 3% solution of poly(vinyl acetate) in boiling water then cool rapidly. Add 10 mL of saturated borax solution and food colouring, then stir slowly.
5. Funny worms, magic octopus: Thermoplastic polymers may be treated to form substances to amuse children. They may be plasticized and "tackified" so that when thrown against a clean wall they stick to the wall and fall slowly, so appearing to crawl down the wall.
3.4.12 Alginate polymer
Medicines to relieve heartburn, e.g. Gaviscon, may contain sodium alginate as well as sodium bicarbonate and calcium carbonate. An alginate is a polycsaccharide from aseaweed, mainly D-mannuronic acid and L-guluronic acid subunits. Alfinates are also used a food thickeners: E400 Alginic acid (from seaweed) (vegetable gum, thickener, emulsifier) (used in flavoured milk, ice blocks, yoghurt), E401 Sodium alginate (vegetable gum) (as in E400).
Pour a stream of the heartburn medicine, or 2 g of the sodium salt of alginic acid in 100 ml of deionized water, into a solution of 1g of calcium chloride in 100 mL of deionized water. Worm-like masses form as the polymer becomes cross-linked. As soon as they form, lift out some worm-like masses, feel their texture, and put them in a saturated sodium chloride solution in a beaker. Shake the beaker to see the worm-like masses dissolve to form a cloudy solution. The cross-linking in the presence of Ca2+ has disapperared as Na+ replaces the Ca2+.
3.5.0 Polymer terminology
elastomer: A polymer material with elastic properties, namely the ability to snap back to the original dimensions after distortion.
initiator: A substance used to start a polymerization reaction, e.g. a free radical.
isomerization: Rearrangement of the geometry of a molecule without changing its overall formula.
monomer: A simple molecule that is joined to others to form a dimer, trimer, or polymer.
neoprene: A chlorinated synthetic rubber, made from 2-chloro-1,3-butadiene, when vulcanized is very resistant to oils, chemicals, sunlight, ozone, and heat.
plasticizers: This is an additive that makes a polymer material more flexible or less rigid.
polyester embedding resin: a viscous liquid of a polymerized unsaturated polyester dissolved in styrene monomer. The methyl ethyl ketone peroxide catalyst cross-links the styrene to the double bonds in the polyester.
polymerization: The process by which single units, monomers, are joined to form a giant molecule, polymer. A linear polymer has links only in one dimension forming a chain. A cross-linked polymer has cross-links between chains.
polymorphism: Substance occurs in distinct solid forms.
thermoplastic: Polymer that softens or melts on heating. Thermosetting: Polymer that once formed does not melt on heating, charring instead. Its structure has cross-linking from one chain to the next.
"Polaroid" sheet is a piece of birefringent material that absorbs light in one of the directions strongly, while transmitting much of the other. Polaroid is made from plastic by adding a long chain molecule dye before stretching so that the dye gets orientated as well as the plastic.
Vinyl specialized polymers
Polymer poly (vinyl acetate) (PVA) (vinyl acetate monomer) used for adhesives and latex paints
Poly (vinylidene chloride) (vinylidene chloride monomer) used for cling wraps, freezer bags, Saran: Films and fibres from polymers >80% vinylidene chloride units
3.5.1 Polymer foam
The latex of natural rubber has a low glass transition temperature so foam rubber is always flexible. Polystyrene has a high glass transition temperature so its foams are rigid. Polyurethane foams can be flexible or rigid depending on the formulation so they can be used for soles of shoes. The foaming agent may be air or a "blowing agent" that decomposes to a gas on heating.
3.5.2 "Silly string"
It is reported to be a mixture of polyisobutyl methacrylate, sorbitan trioleate and a solvent that evaporates in the air contained in an aerosol can. Release of pressure causes a thin spurt to shoot out and harden in the air as a string. The product is a nuisance, but it is supposed to be useful for detecting hidden wires in dark places, e.g. trip wires.