Linear, Branched, and Cross-linked Polymers

Linear structure – chain-like structure

Characteristic of thermoplastic polymers

Branched structure – chain-like but with side branches

Also found in thermoplastic polymers

Cross-linked structure

Loosely cross-linked, as in an elastomer

Tightly cross-linked, as in a thermoset

Linear structure

Structure of a thermoplastic polymer

Branched structure

Includes side branches along the chain

Loosely cross-linked

Bonding occurs between branches e.g. elastomers

Tightly cross-linked or network structure

Thermosetting polymers

Effect of Branching on Properties

Thermoplastic polymers- Always possess linear or branched structures, or a mixture

of the two

Branches increase disorder among the molecules,which makes the polymer: Stronger in the solid state More viscous at a given temperature in the plastic or

liquid state

Effect of Cross-Linking on Properties

Thermosets possess a high degree of cross-linking,while elastomers possess a low degree of cross-linking

Thermosets are hard and brittle, while elastomers areelastic and resilient

Cross-linking causes the polymer to become chemicallyset The reaction cannot be reversed The polymer structure is permanently changed; if heated, it

degrades or burns rather than melt

Mechanical Properties of Thermoplastics

Low modulus of elasticity (stiffness)-lower than metalsand ceramics

Low tensile strength, hardness than metals or ceramics Greater ductility on average

Important thermoplastic resins

Commercially imp thermoplastic resins include naturalresins and resins derived from biopolymers andsunthetic resins

Natural Resins- resins such as copal, amber, rosin,kauri, dammar and mastic, used in varnishes.

When the plants and trees are wounded they exuderesins known as balsams as a protective measure.

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Natural resin polymers are hard with low thermalconductivity and low dielectric constant

They find use in electrical insulation and binders

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Cellulose derivatives

Naturally occurring cellulose- is a polymer ofglucose, a polyhydroxy molecule represented bygeneral formula (C6H10O5)n

Most widely used cellulose derivatives Cellulose esters- cellulose nitrate and cellulose acetate Cellulose ethers -methyl cellulose and ethyl cellulose

-

These derivatives have film forming capability but thefilms are not adherent. They also tend to wrinkle

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POLY VINYL CHLORIDE (PVC)

When vinyl chloride is heated in an autoclave underpress in the +nce of initiator like H2O2 to yield PVC

nCH2=CH → -(-CH2-CH-)n-

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ClCl ClC C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

Cl Cl

PVC is colorless and odorless powder,chemically inert, non-inflamable, exhibit highresistance to light, atmospheric oxygen, acidsand alkalis

It is soluble in chlorinated HC (ethyl chloride),tetrahydrofuran (THF) and cyclohexanone

Commercial PVC known under different tradename tygon, vinylite, velon, geon etc is a hardand stiff amorphous plastic attributed to itsstrong intermolecular forces but brittle

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Pure Polyvinyl Chloride powder

POLY VINYL CHLORIDE (PVC)

Manufacture of pipes, tubes, tank linings, safetyhelmets, refrigerator components, light fittings, trays,cycle and motor cycle mudguards

In the manufacture of flexible films or sheets ofvarying thickness (0.1-8mm) required for makingtable clothes, curtains, raincoats

Also used for wire coating and insulation of eletriccables and manufacture of upholstery

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POLY VINYL CHLORIDE (PVC)

Polytetrafluoroethylene (PTFE)

PTFE is a synthetic fluoropolymer oftetrafluoroethylene that finds numerous applications

The most well known brand name of PTFE is Teflon

PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbonsdemonstrate moderate dispersion forces due to thehigh electronegativity of fluorine.

Has the lowest coefficients of friction against any solid

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Teflon Monomer

C C

F

F

F

F

tetrafluoroethylene

Teflon Polymer

n

PTFE

PTFE is used as a non-stick coating for pans and othercookware.

It is very non-reactive because of the strength of C–Fbonds, and so it is often used in containers and pipeworkfor reactive and corrosive chemicals.

It is used as lubricant- reduces friction, wear, and energyconsumption of machinery.

Has high resistance to wear and deformation under load

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PTFE

Already has high strength compressive strength of PTFE products can also be further

enhanced by addition of fillers such as asbestos, glass fibers,graphite etc

This polymer has exceptionally high softening point (M.P. 327 °C)

A good electrical insultor used in wires, cables, motors

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POLYCARBONATE20

PC are thermoplastic polysters having functional groups linkedtogether by carbonate groups (-O-(C=O)-O) in a long molecularchain and commercially known as MERLON, LEXAN.

Most common type of polycarbonate plastics are obtained bydiphenyl carbonate with bis-phenol A (2,2-bis-(4-hydroxyphenyl)-propane) to give bis phenol A groups linked by carbonate groups

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POLYCARBONATE

PC have high tensile and impact strength over a widerange of temperatures

Polymers are highly transparent to visible light andhave better light transmission characteristics than manytypes of glasses

They are soluble in organic solvents and alkalis

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POLYCARBONATE

Synthesis of Polycarbonate23

BPA-Bisphenol A Phosgene Polycarbonate

From BPA and PHOSGENE:

Synthesis of Polycarbonate Alternative Route

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From BPA and diphenyl carbonate:

(HOC6H4)2CMe2 + (C6H5O)2CO → -[OC(OC6H4)2CMe2]-n

Diphenyl carbonate Polycarbonate

+

Bisphenol A (BPA)+ 2 C6H5OH

APPLICATIONS

PC are used for making MOULDED domestic wares,housing for apparatus and electrical insulation

Used in manufacture of sunglasses/eyeglass lenses

CR-39 is a specific polycarbonate plastic with good optical andmechanical properties

CDs, DVDs, nalgene bottles and food storagecontainers

They can be laminated to make bullet proof glass

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PHENOLIC RESINS (PHENOPLAST)

PHENOLIC RESINS are derived from the condensation reactionbetween phenol or phenolic derivatives (e.g. resorcinol) andaldehydes (formaldehyde and furfural)

Commercially imp. As well as the erliest phenolic resins known asbakelite is obtained by condensation pol. of phenol withformaldehyde

Linear poly. As well as highly crosslinked 3-D network structure canbe obtained by varying the proportion of phenol andformaldehyde as well as the nature of catalyst

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If the phenol to formaldehyde (P/F) ratio is greaterthan 1, a linear polymer is obtained with an acidcatalyst

On the other hand with P/F ratio less than 1 and with abasic catalyst a 3D network polymer is obtained

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Phenol reacts with formaldehyde initially to form methylolderivatives in the o- and p- positions which undergo polymerizationin the presence of acid catalyst to yield methylene bridged linearpolymeric resin called NOVOLAC resin with m.w. in the range ofabout 1000 corresponding to about 10 phenyl residues

NOVOLAC resin can undergo further polymerization to yield 3Dpolymers only when access formaldehyde is added and a basiccatalyst is used

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Resole resin is obtained by condensing phenol with access offormaldehyde (P/F < 1) in the presence of basic catalyst

The reaction is exothermic and hence require cooling

Viscosity of the mix increases indicating the formation ofpolymer

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Condensation product water is removed by suction to yield athermoplastic A- stage resin soluble in organic solvents

A- stage resin is powdered and necessary fillers, colorants,lubricants are added then additional amount of formaldehyderequired is added in the form of hexamethylenetetramine whichdecomposes to form HCHO and NH3 during the final step

NH3 acts as a curing catalyst

Mixture is rolled in heated mixing equipments to facilitate thecontinuation of the polymerization reaction to yield B-stage resin

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Resin is cooled and cut into required form B-stage resin is nearly insoluble in organic sovents but

can be fused with the application of heat and pressure B-stage resin is moulded into the final desired finished

product and during moulding the highly crosslinked C-stage polymer (bakelite is formed)

Finished product is removed from the mould and cantbe reshaped or reclaimed in any manner as thepolymer has been permanently cured by heat settings

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Properties and applications

Phenolic Resins have good adhesive and bonding properties

After heat setting exhibit high resistance to heat, flammability, abrasion, water, chemicals and solvents

However they are susceptible to attack by alkalis They are hard and infusible with good dielectric

properties Fillers are used during molding of these resins to

reduce cost and also to impart specific properties

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Common fillers- wood powder, asbestos, cotton ragsetc

Phenolic resins find use in the manufacture of electric insulation parts such as switches, plugs, switch

boards, handles for electrical appliances Molded parts for automobiles, PCBs and consumer

electronics and bearing in propeller shafts for rollingmills and paper mills

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