Polymer

• 1. Introduction

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  A polymer is a substance made of enormous molecules, and more than 330 million tons of the synthetic kinds were manufactured every year as of 2015. That figure counts only what chemists build on purpose. The same word also covers molecules assembled inside living things, and both families follow one blueprint of small units linked again and again.

  This documentary asks how a single idea produces such different results. Why does one chain of atoms behave like glass while another stretches like rubber? How did people come to understand these materials at all? And what happens when a polymer, trusted to carry fuel or water, quietly gives way?

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• 2. Made by Cells and Chemists

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  Hemp, shellac, amber, wool, silk and natural rubber have all served as materials for centuries, and every one of them is a natural polymer. Cellulose, the main constituent of wood and paper, belongs on the same list.

  Polysaccharides, polypeptides and polynucleotides are the three main classes of biopolymer. Inside living cells, enzymes drive their assembly. DNA polymerase, for example, catalyzes the formation of DNA. Making a protein takes several enzyme-mediated steps. Genetic information is first transcribed from DNA into RNA, then translated into a chain of amino acids. The finished protein may be modified further so that it takes up the right structure and does its job. Rubber, suberin, melanin and lignin are further biopolymers.

  Hemoglycin, previously called hemolithin, is a polymer that arrived from space. It is the first polymer of amino acids ever found in meteorites.

  Nitrocellulose shows how chemists reworked nature's chains. It forms when nitric acid reacts with cellulose. Vulcanized rubber comes from heating natural rubber in the presence of sulfur. Cotton, starch and rubber were familiar for years before polyethene and perspex reached the market. Many commercially important polymers are still made by modifying natural ones, through oxidation, cross-linking or end-capping.

  Polymerization is the name for joining many small monomers into a covalently bonded chain or network. PET polyester shows the bookkeeping. Its monomers are terephthalic acid and ethylene glycol, but the repeat unit equals the two combined minus two molecules of water.

  Chemists sort synthetic methods into step-growth and chain polymerization. Polystyrene grows by chain polymerization, with monomers joining the chain one at a time. Polyester grows by step-growth, where whole chains can link to each other directly. Step-growth divides further into polycondensation, which releases a small by-product at every step, and polyaddition. Plasma polymerization fits neither category. Reactions may run with or without a catalyst, and building proteins in the laboratory remains an area of intensive research.

  Polyethylene, called polythene in British English, has a backbone made mostly of carbon atoms, and its monomer is ethylene. Oxygen also turns up in backbones, as in polyethylene glycol, in the glycosidic bonds of polysaccharides and in the phosphodiester bonds of DNA. Silicon forms the silicones, which include Silly Putty and waterproof plumbing sealant.

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• 3. Letters in the Chain

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  Polystyrene is built only from styrene units, so it counts as a homopolymer. A polymer with two or more types of repeat unit is a copolymer, and one with three types is a terpolymer. Ethylene-vinyl acetate is a copolymer. Polyethylene terephthalate comes from two monomers but forms just one repeat unit, so it is usually treated as a homopolymer. DNA, by contrast, is composed of four types of nucleotide subunits.

  Styrene-butadiene rubber strings together styrene and 1,3-butadiene units in any order, which makes it a random copolymer. Copolymers are classified as statistical, alternating, block, graft or gradient. The equimolar copolymer of styrene and maleic anhydride, formed by free-radical chain-growth polymerization, is a clean example of the alternating type. Nylon 66 can also be seen as strictly alternating between diamine and diacid residues, though it is often described as a homopolymer.

  The chain of vinyl chloride and vinyl acetate is truly random, because each position is independent of its neighbours. Block copolymers have long runs of different units. Two blocks make a diblock, three blocks a triblock, and three blocks of three different species make a triblock terpolymer. Graft copolymers carry branches of a different composition added onto a preformed main chain. Any copolymer with a controlled arrangement of monomers is called a sequence-controlled polymer.

  Tacticity adds a three-way distinction by describing how substituents sit along the chain. In isotactic polymers they all lie on the same side. In syndiotactic polymers they alternate, and in atactic polymers their placement is random.

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• 4. Order and Tangle

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  Atactic polymers, highly branched polymers and random copolymers all freeze into amorphous, glassy structures in the solid state. In the melt or in solution, their chains form a constantly changing statistical cluster. Hooking and entanglement create a kind of mechanical bond between neighbouring chains.

  Star polymers, comb polymers, polymer brushes, dendronized polymers, ladder polymers and dendrimers are all branched architectures. Two-dimensional polymers, called 2DP, are built from topologically planar repeat units. Architecture changes solution viscosity, melt viscosity, solubility, the glass-transition temperature and the size of single coils in solution. Living polymerization is one technique for producing such architectures on demand.

  Wide-meshed cross-linking between main chains gives elastomers, while close-meshed cross-linking gives thermosets. Branched and unbranched polymers are usually thermoplastics. Heating a cross-linked polymer does not melt it; it only decomposes it. Thermoplastic elastomers are the exception, because their cross-links are physical and reversible. In these block copolymers a hard segment tends to crystallize and a soft segment stays amorphous.

  Linear polymers with a periodic structure, low branching and regular stereochemistry become semi-crystalline. In polyethylene the chains lie in a zigzag conformation and pack into dense crystallites called lamellae. These are often only about 10 nm thick, far thinner than the chains are long. Amorphous regions sit between them, and tie molecules can bridge two lamellae. Several lamellae form a spherulite, often between 0.05 and 1 mm across.

  Isotactic polypropylene curls into a helix, which also allows dense packing. In p-aramid, hydrogen bonds between chains make the interactions especially strong. Compared with amorphous structures, semi-crystalline ones give higher stiffness, density, melting temperature and resistance.

  The degree of crystallinity runs from zero for a fully non-crystalline polymer to one for a theoretical fully crystalline one. Few synthetic polymers are entirely crystalline. Microcrystalline regions make a polymer tougher and more impact-resistant. Polymers near zero or near one tend to be transparent, while those at intermediate crystallinity look opaque because crystalline or glassy regions scatter light.

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• 5. Length, Heat and Flow

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  Lengthen a polymer chain tenfold and its melt viscosity can rise more than 1000 times. Below a threshold called the entanglement molecular weight, viscosity grows in direct proportion to weight-average molecular weight. Above it, viscosity grows with that weight raised to the power 3.4. Longer chains also move less, grow stronger and tougher, and raise the glass-transition temperature. Van der Waals attractions and entanglements hold the chains in place against stress and heat.

  The degree of polymerization counts the monomers in a chain. Because synthesis gives a statistical spread of lengths, chemists report the number-average molecular weight, Mn, and the weight-average molecular weight, Mw. Their ratio, Mw over Mn, is the dispersity, which describes how wide the spread is.

  Reptation names the way a single macromolecule moves. Entanglements with neighbours confine each chain to a virtual tube. The theory explains polymer dynamics and viscoelasticity. Diffusivity, a transport property, matters for films and membranes.

  A rubber band with higher tensile strength holds a greater weight before it snaps. Tensile strength generally rises with chain length and with cross-linking. Young's modulus measures elasticity and depends strongly on temperature. Viscoelasticity adds a time-dependent response that shows hysteresis in the stress-strain curve. Dynamic mechanical analysis, or DMA, measures it by oscillating the load.

  Kevlar, also sold as Twaron, owes its strength and high melting point to hydrogen bonds. The partly positive hydrogen of an N-H group on one chain is attracted to the partly negative oxygen of a C=O group on another. Urethane and urea linkages behave similarly. Polyesters rely on weaker dipole-dipole bonding, so they melt lower but flex more. Polyethylene interacts only through weak Van der Waals forces and melts at lower temperatures still.

  Below the glass-transition temperature, Tg, every polymer is brittle and glassy because molecular motion is frozen. Above it, polymers turn rubbery and viscous. Crystallization happens between Tg and the melting temperature, Tm, and in polymers it means a change between two solid states rather than solid to liquid. The glass transition shows a jump in heat capacity like a second-order transition, but it is generally not considered a thermodynamic transition between equilibrium states.

  Unplasticized PVC, or uPVC, goes into pipes because pipes must stay strong and heat-resistant. Plasticized PVC goes into clothing for its flexibility, and plasticizers are also added to some cling film. Plasticizers are small molecules chemically similar to the polymer. They open gaps between chains and lower Tg.

  Mixing is hard for polymers because the driving force is entropy, and big molecules gain little of it. Concentrated polymer solutions are therefore far rarer than those of small molecules. In a good solvent the chain swells. In a poor solvent it contracts. At the theta condition, also called the Flory condition, it behaves as an ideal random coil. Most small-molecule solutions separate on cooling, at an upper critical solution temperature. Polymer mixtures commonly separate on heating instead, at a lower critical solution temperature.

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• 6. Everyday Uses

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  Parachutes, sails, tents, skis, snowboards, footballs and billiard balls all appear on the source list of polymer products. Low density, low cost, good thermal and electrical insulation, high resistance to corrosion, low-energy manufacture and easy processing explain the spread. Combining a polymer with other materials, as in composites, can tune its properties further.

  Lighter cars and planes and thermally insulated buildings save energy. Packaging protects food and drinking water. Synthetic fibres save land and lower the use of fertilizers. Coatings preserve other materials, and hygiene and medical products protect and save lives.

  Blood bags, syringes, rubber gloves, surgical suture, contact lenses and prostheses are polymer products. So are controlled drug delivery systems and matrices for cell growth. Diapers rely on superabsorbent polymers. Toothbrushes, cosmetics, shampoo and condoms are also on the list.

  Polyethylene is a classic electrical insulator, yet polymers containing pi-conjugated bonds, such as the polythiophenes, behave as semiconductors. That opened the field of organic electronics, with organic field effect transistors, light emitting diodes and solar cells. Compact discs, photoresists and holography also appear on the list.

  PMMA and HEMA:MMA serve as matrices in solid-state dye lasers. Their high surface quality and transparency let the laser dye dominate the behaviour. These lasers yield very narrow linewidths, which suits spectroscopy and analytical work. For these polymers the refractive index shifts by about minus 1.4 times ten to the minus four per kelvin between 297 and 337 kelvin.

  Bulletproof vests, space suits, ropes, polymer banknotes, payment cards, toys such as the Rubik's Cube and 3D printing complete a list the source calls non-exhaustive. Tires, bumpers, windshields, windscreen wipers, fuel tanks and car seats are polymer car parts too. A fuel tank is exactly where a failing polymer becomes dangerous.

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• 7. When Chains Break

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  Heat, light, oxygen, enzymes and certain chemicals can all degrade a polymer. The change shows up in tensile strength, colour, shape or molecular weight. Usually the cause is chain scission, the breaking of bonds in the backbone, at the chain ends or at random positions.

  A copolymer of polylactic acid and polyglycolic acid is used in hydrolysable stitches that slowly degrade after being applied to a wound. Biodegradation and recycling also depend on controlled breakdown, in order to prevent pollution. For sorting waste, the Society of the Plastics Industry developed resin identification codes that label the type of plastic.

  Epoxies and chains with aromatic groups are especially vulnerable to UV light. Polyesters are vulnerable to hydrolysis. Polymers with an unsaturated backbone suffer ozone cracking. Carbon-based polymers degrade with heat more easily than polydimethylsiloxane, so they are poor choices for most high-temperature work.

  Polyethylene heated above 450 degrees Celsius breaks down into a mixture of hydrocarbons by random scission. When breaks happen at chain ends, monomers are released in a process called unzipping or depolymerization. Polymers whose repeat units carry no substituent, or a single small one, generally decompose by random-chain scission.

  Chlorine-induced cracking of acetal resin plumbing joints and polybutylene pipes caused many serious floods in homes, especially in the US in the 1990s. Traces of chlorine in the water supply attacked the polymers, faster where parts were poorly extruded or injection molded. The acetal joints failed through faulty molding, cracking along the threads where stress concentrates.

  Ozone cracking is one of the oldest known failure modes. Natural rubber and nitrile rubber have double bonds in their repeat units, and ozone cleaves them. Cracks in a fuel line can reach the bore and leak fuel. In an engine compartment, an electric spark can then ignite the gasoline and start a serious fire. Polymer oxidation has also caused accidents with medical devices.

  Nylon 66 is susceptible to acid hydrolysis. In one accident a fractured fuel line spilled diesel onto a road, leaving a deposit as slippery as black ice. The diesel also dissolved the asphaltenes in the asphalt, damaging the surface and the structural integrity of the road itself.

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• 8. Proving the Chain

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  The latex sap of caoutchouc trees reached Europe from South America in the 16th century. The Olmec, Maya and Aztec had long used it for balls, waterproof textiles and containers. Ancient societies also made garments from linen fibres and paper from paper reed, both cellulose.

  Jöns Jacob Berzelius coined the word polymer in 1833, though his definition differed from the modern IUPAC one. Chemists of the 19th century manipulated polymers without understanding them. Thomas Graham's theory treated them as colloidal aggregates of small molecules held together by unknown forces.

  Braconnot, Parkes, Ludersdorf, Hayward and many others modified natural polymers and produced celluloid, galalith, parkesine, rayon and, later, Bakelite. These materials entered factories quickly and reached homes as fabrics, buttons, crockery and decorative items. The promise of cheap, accessible materials was grasped immediately, despite the missing theory.

  In 1920 Hermann Staudinger published his seminal work "Über Polymerisation". He argued that polymers were long chains of atoms linked by covalent bonds. He then spent the next decade finding experimental evidence. The scientific community debated the idea at length before accepting it, and Staudinger received the Nobel Prize in 1953.

  After the 1930s came a golden age in which new polymers were discovered and quickly given commercial uses, replacing naturally sourced materials. An industrial sector with a strong economic drive fuelled this. A broad academic community supported it with syntheses of monomers from cheaper raw material, more efficient polymerisation processes, better characterisation techniques and deeper theory.

  Six Nobel prizes have been awarded in polymer science since 1953, not counting research on biological macromolecules. In 1980 Lord Todd summarised the field this way: "I am inclined to think that the development of polymerization is perhaps the biggest thing that chemistry has done, where it has had the biggest effect on everyday life".

  Staudinger's picture rested on covalent bonds. An emerging area now studies supramolecular polymers, whose units are held together by non-covalent links instead.

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