Introduction to Polymer Science and Technology Introduction

1.3.3 Reduction in weight

Weight, particularly in the context of improvements in strength and stiffness-to-weight ratios, has had the most enormous effects. For example, in aircrafts and other means of transport, in conventional structures, in oil platforms, etc. Improved fuel economy in cars, trucks and aeroplanes due to lighter- weight bodywork (e.g., sheet moulding-compound GRP and glass-mat thermoplastics (GMT) panels in Lotus sports car and in various truck cabs and advanced polymer-matrix composites in structural parts for aircrafts) must account for billions of pounds worth of fuel saving and the associated reduction in atmospheric pollution from exhaust fumes.

The special demands of water-based sports, e.g., competition boat hulls, can only be met by the employment of composite materials. Most types of hulls rely on polymer/glass fibre, often with Kevlar or carbon fibres for extra toughness and strength. A good racing hull, for example, may typically consist of a sandwich construction based on alternate layers of glass fibre mat and Kevlar woven fabrics bonded with a suitable core. The core material is a cellular polymer and provides lightness without loss of stiffness.

Decreases in weight will also continue to occupy the efforts of bicycle manufacturers, particularly for racing bicycles. The Japanese have recently announced the first all paper bicycle! The frame of this bike is constructed from hand-laid-up paper and epoxy resin. The resulting cellulose fibre alignment provides a strength which is 60% of that of carbon fibre (CF) composites, no mean feat! The resulting frame has a mass of only 1.3 kg. A thin plastic coating encases the paper to ensure that the bike does not collapse into a soggy heap in the rain!

Americans developed a bullet-proof vest for the Vietnam War from a laminate of ceramic plate backed with fibre glass-polymer composite «60 kg/m²! These days much lighter body armours are produced from Kevlar or Dyneema.

1.3.4 Resistance to corrosion

Plastics replace metals in many applications because they do not rust. Figure 1.3 shows an area of a swimming-pool plant room where the use of sodium hypochlorite solution, a strong oxidant, as water purifying disinfectant accelerates the rusting of metal pipes and valves. During maintenance periods, the practice is to replace corroded metal pipes with plastic ones. However, it should also be recognised that plastics can suffer discolouration, crazing, cracking, loss of properties and melting or dissolution in the presence of energy sources, radiation or chemical substances.

Introduction to Polymer Science and Technology

Introduction

Figure 1.3An example of metal corrosion and replacement of a length of corroded metal pipe with a plastic alternative

1.3.5 Electrical insulation/conduction

The electrical insulating quality inherent in most polymers has long been exploited to constrain currents flowing along chosen paths in conductors and to sustain high electrical fields without breaking down. Polymers have also been employed in more demanding applications, for instance, polyethylene insulation in coaxial cables for radar and television. Polymers also provide high-performance thin films for capacitors.

Fluorinated polymers (a permanently polarised dielectric material) are used as very low-conductive materials in electret microphones.

Polymers are good insulators, but a lightweight, readily mouldable conducting plastic would also be desirable. Thus carbon black mixed polymers are used commonly as a conductive medium. Even a slight degree of conduction which allows charges to leak away to earth would be desirable to alleviate static charges from manufactured articles.

All the above listed desirable/attractive features of polymeric materials are due to their versatility.

1.4 What makes polymers versatile?

Polymers offer a diversity of molecular structures and properties and thus lend themselves to be employed in a variety of applications. They increasingly replace or supplement more traditional materials such as wood, metals, ceramics and natural fibres. Ordinary polymers offer sufficient scope for most applications, however technological progress and concerns over environmental pollution (often translated into legislation) and health and safety at work introduce further demands to improve/modify existing polymers and synthesise new ones.

Introduction to Polymer Science and Technology

Introduction

Polymers possess extensive structural features, some of which are delineated below.

1.4.1 Intra-molecular features (single molecules)

Polymers are organic materials and consist of chain-like molecules,which are the most salient feature of polymers. A macromolecule is formed by linking of repeating units through covalent bonds in the main backbone. The size of the resultant molecule is indicated as molecular weight (degree of polymerization). The monomers or the repeating unitsin the chain are covalently linked together. Rotation is possible about covalent bonds and leads to rotational isomerism, i.e., conformations,and to irregularly entangled, rather than straight molecular chains, see Figure 1.4.

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Figure 1.4The third carbon may lie anywhere on the circle shown (i.e., the locus of the points that are a fixed distance away from a given point). In this case the locus is the circle at the base of the cone, which forms by revolving C₂-C₃ bond around the C,-C₂ axis, maintaining the valence

angle of 109.5°.

Introduction to Polymer Science and Technology Introduction

Transand gauche conformationsare exhibited as rotation occurs about С - С single bonds, e.g., in a butane molecule consider each molecular segment (- CH₂ - CH₃) being placed on a disk such that а С atom is placed at the centre of the disk , and the two hydrogen atoms and the methyl group are distributed evenly around the circumference. The rotation of one of the disks over the other produces eclipsed (highest repulsive energy between the methyl molecules when they overlap) and progressively staggered conformations (gauche being where the methyls are in a closest stagger and trans where methyls are furthest apart and experience minimum repulsive energy).

Configurationsand/or stereoisomersdescribe the different spatial arrangement of the side chemical elements or groups of elements about the backbone molecular chains. Unlike conformations, the configurations cannot be changed by rotation about the covalent bonds and are established during polymerisation, when the monomer units are combined to form chains. Configurations (cis and trans)describe the arrangements of identical atoms or groups of atoms around a double bond in a repeat unit, e.g., cis- and trans-polyisoprene. Natural rubber contains 95% cis-1, 4-polyisoprene.

Stereoregularity(tacticity) describes the arrangement of side elements/groups around the asymmetric segment of the vinyl-type repeat units, - CH₂ - CHR -, consequently, three different forms of polymer chain results from head-to-tail addition of the monomers: atactic, isotacticand syndiotactic.Stereoregularity and configurations influence crystallisation and the extent of crystallinity in polymers. It is worth noting that by remembering specific chemical formulae for the general term "R", one can easily reproduce the chemical expressions for the repeat units of various well-known thermoplastic polymers: e.g., when R becomes H, CH₃, Cl, CN or a benzene ring then, respectively, the formula represents PE, polypropylene, PVC, polyacrylonitrile and polystyrene.

Conjugated chainscontain sequences of alternating single and double bonds (unsaturation). Highly crystalline, stereoregular conjugated polymers exhibit appreciable electrical conductivity. A conductivity of 0.1 S/m has been obtained with a thin film of trans-polyacetylene (- CH = CH -)_n. The conductivity can be magnified by doping.

The terms and concepts covered in this section are explained in detail in the polymer science dictionary by Alger (1989) and in text books such as Fried (1995) and Young (1991).

Branched chainsconsist of a linear back-bone chain with pendant side chains. Branching occurs quite readily where the functionality(f) of the monomers > 2. It can also occur during the polymerisation of monomers with f = 2 by free radicals abstracting hydrogens from a formed polymer chain, thereby generating new radicals along the backbone which initiates side chains. The presence of branches reduces the ability of the polymer to crystallise, and also affect the flow behaviour of molten polymer. Branching can be controlled by using specific catalysts.

Molecular massindicates the number of repeat units in a polymer molecule, see the box below. The molecular mass must reach a certain value for the development of polymer properties.

Introduction to Polymer Science and Technology Introduction

Examples of different numbers (n) of (- CH₂-) repeat units in petroleum products.

- Monomer (ethene, ethylene) CH₂= CH₂

- Repeat unit - CH₂-CH₂-

- Fuel gas (propane, butane) CH₃- CH₂- CH₃, CH₃- (CH₂)₂- CH₃

- Gasoline CH₃-(CH₂)_n-CH₃ (n=6-12)

- Paraffin wax CH₃-(CH₂)_n-CH₃ (n=25-100)

- Poly(ethylene) CH₃-(CH₂)_n-CH₃ (n=100-100,000)

- UHMWPE CH₃-(CH₂)_n-CH₃ (n= 1,000,000)

Polymerisation produces chains of different lengths, thus the molecular mass is expressed as an average value (e.g., M_n , M _w), and the distribution of the molecular mass is indicated by M _w/ M_n A narrow distribution, e.g., in polyethylenes, gives better impact strength and low-temperature toughness whilst a broad distribution gives better moulding and extrusion characteristics.

Aromatic polymers(e.g., polycarbonate (PC) and polyether ether ketone (PEEK)) are identified by backbone chains which contain benzene rings and/or its derivatives; they are so called because of the strong odour and fragrance of the associated chemicals such as benzene. By contrast, in aliphatic polymers(e.g., PE and polyvinyl chloride (PVC)) the elements along the backbone chain are arranged in a linear manner. Aromatic polymers have good thermal stability, which can be further improved by heterocyclic arrangements. Heterocyclic polymers(e.g., polyimides) have both aromatic (benzene) and non-aromatic ring arrangements along the backbone chain. These are rigid materials with high-temperature resistance (high softening and melting points) and conductive properties. Some aromatic polymers remain crystalline in solution and in a molten state, i.e., they are "liquid crystalline polymers". Mechanical stiffness and thermal stability of both aliphatic and aromatic polymers can be considerably increased by achieving ladder-likemolecular structures along the backbone chains.

The intra-molecular features influence final material properties and the transition temperatures (e.g., the glass-transition temperature (T ), secondary T and melting point, T_m), which indicate the temperature limits in applications. T indicates the temperature at which a rigid (glass-like) material becomes flexible (rubber-like) as it is being heated. The bulk structure and the behaviour of polymers are also dictated by the intra-molecular features, for example, the functionality and the frequency of the reactive sites along the backbone chain of macromolecules result in thermoplastics (TP), thermosets (TS) or elastomers. Depending on the stereoregularity and polarity along the backbone chain, crystallinity or amorphousness predominate in thermoplastics.

1.4.2 Intermolecular features (molecules in bulk)

Thermoplasticsconsist of a large number of independent and intertwined molecular chains. When heated these chains can slip past one another and cause plastic flow. In some thermoplastics as the polymer melt solidifies, the chains of molecules form into an orderly arrangement. These are semi-crystallinethermoplastics (e.g., PE, polypropylene (PP) and polyamide (PA)). The term semi-crystalline is used because the crystalline structure does not exist throughout the polymer.

The regions where the molecules do not form crystallites are referred to as amorphous,i.e., without morphology/shape. Non-

Introduction to Polymer Science and Technology Introduction

crystalline polymers are more readily swollen by solvents and therefore more susceptible to solvent crazing (minute cracking). Some thermoplastics (e.g., PC, polymefhyl- methacrylate (PMMA) and, atactic polystyrene (PS)) are normally totally amorphous.

The crystalline structure comprises of unit cells(dimensions <lnm) and lamellae(i.e., approximately 10-20 nm thick platelets that are formed by an orderly packing of folded chain segments). Lamellae grow from nucleiin a radial fashion into a larger morphological unit, known as the spherulite(approximately 1-100 цт radius). Spherulite size and its uniformity influence mechanical and optical properties. During the blow moulding of PET (polyethylene terephthalate) bottles, the processing conditions are controlled to suppress spherulite formation while orientation and crystallisation occurs. Spherulites will reduce the transparency of the bottles, which is not desirable for marketing the product and also large spherulites embrittle the material.

Amorphous thermoplastics (in the absence of light scattering crystalline entities) are transparent and can be used as glass replacement, e.g., PVC glazing for skylight, acrylic ware in chemistry laboratories, PMMA front and rear car lenses or light clusters (here lower weight is also an advantage over inorganic glasses), PC headlamps and PC riot and anti-vandal shields.

Thermosetsshould be considered where polymers with higher rigidity (i.e., higher elastic modulus) are required. However, they suffer from being brittle and as a result are often used in a reinforced form as load-bearing solids. Thermoset (TS) formation requires that at least one of the monomers (reagents) must be trifunctional or greater. Thermosets (e.g., phenol formaldehyde resins (PF), epoxy resins, polyurethane (PU)) differ from TP in that their molecular chains are crosslinked together by primary bonds (covalent) and they are wholly amorphous. A characteristic common with most elastomers, with the important distinction that the crosslink densityis much lower in elastomers. Varying crosslink density allows control of, in particular, mechanical and chemical properties. The generic term network polymerincludes both elastomers and thermosets.

Date: 2015-12-11; view: 728