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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

Polymeric Materials

Introduction(2009-Bt-Chem-21)

The polymers are a diverse group of engineering materials. They are the main components of   plastics, rubbers, resins, adhesives and paints. These materials have distinctive

microstructures built from

macromolecular chains and

networks of carbon and other light

elements.

A polymer is a very largemolecule in which one or two

small units is repeated over and

over again The small repeating

units are known as monomers

Imagine that a monomer can be

represented by the letter A. Then

a polymer made of that monomer 

would have the structure:

-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-

Polymers are high molecular mass substance consisting of large number of repeating

structural units. As polymers are single, giant molecules i.e. big size molecules, they are also

called macromolecules. Simple molecules which combine to form polymers are called

monomers. Process of formation of polymers from respective monomers is called

 polymerization.

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

Most polymer materials are of organic composition that is they contain carbon, and arecomposed of large molecules (macromolecules) each built of many atoms. They include

materials such as polyethylene, poly (vinyl chloride), polyamide and epoxy resins. Most polymer materials are of organic composition that is they contain carbon, and are composed

of large molecules (macromolecules) each

  built of many atoms. They includematerials such as polyethylene, poly (vinyl

chloride), polyamide and epoxy resins. Themolecular structure of polymers is

responsible for many of the intriguing  physical properties which lie behind their 

various applications. Polymers arecomposed of very large molecules

(macromolecules) which consist of smaller 

units, called monomers, tightly bonded

together with (strong) covalent bonds, for 

the case of a linear polymer chain. The

chemical formula is of the type ± (A) n± where A represents the monomer and the integer number n, called the degree of polymerisation or polymerisation index, is the number of 

monomers composing the chain. The length of the polymer chain (and the molar mass) is

 proportional to n.

A linear polymer consists of a long linear chain of monomers. A branched polymer comprises

a long backbone chain with several short side-chain branches covalently attached. Cross-

linked polymers have monomers of one long or short chain covalently bonded with

monomers of another short or long chain. Cross-linking results in a three-dimensional

molecular network; the whole  polymer is a giant

macromolecule.Another useful classification of 

  polymers is based on thechemical type of the monomers

homopolymers consist of monomers of the same type;

copolymers have different

repeating units. Furthermore,

depending on the arrangement of 

the types of monomers in the

  polymer chain, we have the

following classification: the different repeating units are distributed randomly (random

copolymer) or there are alternating sequences of the different monomers (alternating

copolymers) in block copolymers long sequences of one monomer type are followed by long

sequences of another type; and graft copolymers consist of a chain made from one type of monomer with branches of another type.

-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

Classification of polymers(2009-Bt-Chem-21)

The molecular structure of a fully polymerized polymer can be classified according to one of 

three major types: linear, branched , or crosslinked polymers.

Linear polymersLinear chain does not imply ³straight´ molecules.They can curl, twist or fold. Linear molecules have

higher densities) e.g. high density PE. In additionlinear molecules have higher tensile strengths,

higher stiffness, and higher softening temperature.

Branched polymersPolymerization process

may produce more complex structures ³branched´.

Branched molecules have more voids, lesser density, are

more flexible & more permeable to gases and solvents thanlinear molecules

Cross-linked polymersThe bonding

  between two chains is called crosslinking occurs whenmonomers have more than have more than one double

 bond. Cross links make the sliding of polymer moleculesvery difficult. The polymer becomes very stiff and is

very hard to deform. Fully cross-linked polymers are

thermoset and do not show creep or relaxation, areusually brittle, and do not deform with heat. Theydecompose at high temperatures and fairly resistant.

Polymers are generally classified according to their properties, and use as thermoplastic,

thermosetting, and elastomers.

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

ThermoplasticsThermoplastics consist of flexible linear molecular chains that are

tangled together like a plate of spaghetti or bucket of worms. As the name indicates, theysoften when heated.

ThermosetsThermosets remain rigid when heated and usually consist of a highly

cross-linked, three-dimensional network.

ElastomersElastomers consist of linear polymer chains that are lightly cross-linked.

Stretching an elastomer causes the chains to partially untangle but not deform permanently

(like the thermoplastics).

Crystalline polymersAreas in polymer where chains packed in regular way. Both

amorphous and crystalline areas in same polymer. Crystalline regular chain structure no

 bulky side groups. More crystalline polymer stronger and less flexible.

Engineering properties of 

polymers(2009-Bt-Chem-21)

Polymer materials are generally softer and weaker than metals and ceramics, but have

distinctive engineering properties of great practical value.

Mechanical properties

Elastic, viscous and viscoelastic responseThe

strain response of a material over the passage of time due to the application of a constant load

is called creep. A purely elastic material responds instantaneously to the load and the strain

remains constant; furthermore, it will recover its initial shape instantaneously upon the

removal of the load. On the contrary, a viscous liquid will deform as long as the load

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

continues to be applied. Upon removal of the load, the fluid does not return to its initial  position. Fluids show a characteristic resistance to movement (flow), which is called

viscosity. Viscosity results in a frictional energy loss, which dissipates in the fluid as heat.Polymeric materials behave both as viscous fluids and elastic solids. They are viscoelastic

materials. The most important characteristic of viscoelastic materials is that their mechanical

 properties depend on time. The response of a viscoelastic material is intermediate betweenthe solid and the liquid. There is usually an instantaneous elastic response followed by a

delayed elastic response that could be followed by a purely viscous response. The creeprecovery which follows upon the removal of the load starts with the immediate recovery of 

the instantaneous elastic response followed by the slow and gradual recovery of the delayedone; the viscous part does not recover. Creep and recovery depend on the applied load,

molecular characteristics, microstructure and temperature.

Uniaxial tensile testing

       Amorphous thermoplastics are stiff and strong at temperatures below the glass

transition temperature.       Thermosets, which are highly cross-linked networks, exhibit similar mechanical

 behaviour to glassy thermoplastics. At higher temperatures they also soften but never reach the state of viscous flow due to the cross-links which sustain cohesiveness.

       Semi-crystalline polymers at room temperature are usually above glass transition and

 below the melting temperature, and for these reasons they are less stiff but tougher.

       Elastomers above the glass transition (which is usually below 08C) are characterised

  by low elastic modulus but their main characteristic is the extremely high strains

which can be attained. Recoverable deformations of 1000% are not unusual before

strain hardening and failure. Although they never become completely viscous, in thevicinity and above glass transition, elastomers exhibit time-dependent behaviour and

so-called retarded elasticity, i.e. viscoelasticity. At sufficiently low temperatures, they

 become glassy, i.e. stiff and brittle.

CreepAt high stresses the creep response of a polymeric material becomes non-linear and

the creep compliance or modulus becomes a function of strain. Ultimately, after a period of 

creep (which can be very long) the polymer fails. Polymers can fail in various ways. Brittle

fracture is usual for stiff/rigid and strong thermoplastics and thermosets while ductile yielding

is the mechanical failure mode for semi-crystalline polymers. It has to be stressed again,

though, that the behaviour can vary from these two extremes with temperature and strain rate

in a dramatic fashion. Consequently, the mechanical properties are a function of temperature

and time/frequency of deformation.

Molecular scale origins of mechanical

 behaviour The time-dependent behaviour (creep, recovery, stress relaxation) is a

direct consequence of the macromolecular character of the polymer molecules and the weak 

 physical interactions between them (weak attractive van der Waals forces). The long polymer 

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

chains move/relax at slower rates compared to simple liquids. The mobility of long chains isdue to a relatively slow serpent-like movement called reputation. The mechanical response of 

a material depends on the time it takes for its individual molecules to respond in the imposeddeformation/stress, i.e. the molecular relaxation time. Crazing is a process which occurs only

in polymer materials and is a direct consequence of their macromolecular nature.

Friction and wear resistanceFriction and wear are more

complex materials properties. Abrasion/wear resistance in particular, which is directly

associated to friction and strength, has great practical significance for durability. High

molecular weight, semi crystalline polymers (e.g. high-density polyethylene (HDPE))

 possesses toughness which usually provides excellent abrasion resistance. Rubbers generate

high frictional forces (coefficient of friction 1±3) but they are susceptible to wear and they

have to be reinforced with microscopic organic (e.g. carbon black) or inorganic (e.g. silica)

  particles in order to be used in engineering applications. On the other extreme,

 polytetrafluoroethylene (PTFE) exhibits very low friction (_0.03±0.15). In many cases the

friction coefficient is a function of sliding speed.

Thermal propertiesPolymers tend to expand readily as the temperature rises, and for unfilled polymers the linear 

thermal expansivity, _l, is generally an order of magnitude larger than for metals andceramics. The thermal conductivity, _, can be very low because energy transfer between

  polymer chains or through polymer networks is inefficient. For this reason polymers arewidely used for thermal insulation, especially in fibre or foam form. Specific heat capacity

does not range widely.

PermeabilityMost polymers are not porous (unless designed with a porous structure) and consequently show excellent

 barrier properties to gases, vapours and liquids. However, when polymers are used in the form of thin films

and surface coatings, they cannot be considered impermeable.

Environmental resistance and durabilityThe environmental resistance which ultimately determines the durability of a component is a significant

  property of materials for engineering applications. The conjoint action is a crucial point because, for example, oxygen at moderate temperatures or sunlight induces thermal or photooxidation while at hightemperatures the result is combustion. Water induces hydrolysis and heat alone produces pyrolysis. The

combination of atmospheric oxygen, water and sunlight results in weathering and aging, while solventsand organic fluids induce softening and ultimately dissolution; the effect is exacerbated by heat.

Fire propertiesMost polymeric materials are susceptible to relatively easy ignition

above a critical temperature which leads to combustion, a rapid oxidation process which often involves the production of a flame.

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Biological attack In contrast to natural polymers (cellulose, casein), most synthetic

 polymers are not susceptible to microbial microorganism (bacteria, fungi) attack. In the case of plasticisedPVC, it has been shown that the biological attack does not result from the polymeric material but is due to

the plasticiser used.

ToxicityWhile solid polymers are not usually toxic at normal use temperature, their constituent

monomers can be highly toxic and should be handled with care. Furthermore, potentially toxic monomers

and other toxic gaseous substances can be released as products of pyrolysis and combustion. Moreover,many low molecular weight additives can present toxicity problems which have to be taken into accountwhen polymers are to be used, e.g. in contact with potable water.

Applications of polymeric

materials(2009-Bt-Chem-06)

Construction is one of the largest markets for polymer materials. A huge variety of plasticsand rubbers find a multitude of uses. Among the most important are in pipes, geosynthetics,

coatings and adhesives.

Polymer materials do not generally compete with the main established metallic and ceramic

load-bearing materials but, nonetheless, in recent decades have become indispensable in

construction engineering. They offer a great range of valuable material properties, are

generally softer in behaviour and able to tolerate large strains. Polymer-based materials have

durability and performance attributes which contrast sharply with those of metals and

ceramics. They are generally resistant to damage by water, but are prone to air oxidation and

have poor fire performance. Here we survey briefly their uses in civil engineering.

Structural plastics and compositesApart

from pipes, large load-bearing components of unreinforced solid polymers are rarely found

 because of the low stiffness of these materials. Amorphous thermoplastics (PMMA, PC and

PVC-U) are used to form roofing and cladding panels and as glazing.Polymer materials arealso used for building panels, often of multi-layer sandwich construction to provide rigidity

and thermal insulation.

Coatings 

Surface coatings (paints) are widely used in the construction industry to

 protect timber surfaces and to prevent or reduce the corrosion of metals. Coatings may be

applied on site or during manufacture or fabrication. In almost all these applications, polymer 

materials provide the coating film and bind the functional or decorative pigments which may

  be present. If the film is unpigmented and more or less transparent, the coating is often

described as a varnish.

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Department of Chemical Engineering, University of Engineering and Technology, KSK Campus, Lahore-Pakistan

Adhesives and sealantsPolymers are the basis of all engineering

adhesives High-performance adhesives is formulated to set and develop strength by chemical

reactions and so are thermoset materials. There are therefore similarities between adhesives

and the matrix component of composite reinforced plastics (and indeed also betweenadhesives and paints). Thus structural glulam laminated timber (or indeed plywood) may beregarded as an adhesive bonded wood composite

Ropes and barsPolymer materials achieve maximum stiffness when

drawn to extend and orient the primary macromolecular chains. Some stiffening can also be

achieved by stretching in two directions at right angles (biaxial orientation). Many

semicrystalline thermoplastics are good fibre formers, notably the polyamides (PA), some

 polyesters, polyacrylonitrile (PAN) and polypropylene (PP).

Pipework Large-diameter polymer pipes are used widely for water and gas

distribution, drainage and sewerage, and for handling industrial effluents and

slurriesMembranes and geosyntheticsPolymers are readily formed into continuous

membranes, sheets, meshes and textiles, the use of which in civil engineering has been an

important recent area of innovation. Thermoplastic textiles and meshes used primarily as soil

reinforcement (geotextiles). There is also expanding use of polymer materials in tensile and

air-supported roofing.

Expansion bearings and antivibration

mountsA minor but technically demanding use of polymers is in expansion bearings

for bridge and pipeline construction. These may be fabricated either with a durable synthetic

rubber such as polychloroprene CR (neoprene) or with a low-friction thermoplastic.Chemical

grouts for soils Polymers based mainly on water-soluble acrylamide monomer have played a

minor but long-established role in the stabilisation of soils or in reducing water permeability.

To a large extent this technology has been superseded by the use of geosynthetics. Chemical

grouts have also been used to control water infiltration in sewers.