Polymers

polymers - definition

family of synthetic materials made up of repetition of high weight molecules in the form of flexible chain

→ soft and molduble during manufacture + can be formed in lots of different shapes

→ wide application range

polymers - structure

made of monomers (= smaller molecules whose atoms strongly bonded to one another)

they undergo polymerization = monomers linked by chains together to form large polymers

chains may branch off in various ways or may even cross link → linear & branched chain polymers are flexible & weak in nature BUT cross-linked & network polymers = + rigid

when degree & type of polymerization = controlled we can obtain polymers with wide range of properties

types of polymerization

addition polymerization

condensation polymerization

addition polymerization

= straightforward addition of monomers of same kind

→ homogeneous type A+A → A-A-A or copolymer type A+B+A+B → A-B-A-B

ex: polyethylene

Condesation polymerization

polymerization reaction btwn 2 monomers with expulsion of simple by-product (water, hydrogen chloryde)

→ not recyclable because can’t break the chain due to reaction

A+B → A-B + by product

general properties of polymers

→ lightweight

→ corrosion resistant (like steel corrosion)

→ low in strength (depens on polymer but genrally not quite strong)

→ not suitable for high temperature (toxic fumes)

→ relatively inexpensive

→ readily formed in variety of shapes

→ some = transparent

→ elec insulators

→ low coeff of friction

→ some can convert light to elctricity

polymers classification

• thermoplastics

• thermosets

• elastomers

thermoplastics polymers

= branched chain polymers which are usually obtained by addition polymerization

can be softened, hardened, resoftened by application of heat → easily recyclable

ex: polyethylene, PVC, cellulose, nylon…

charasteristics of thermoplastic polymers

• anisotropic

• soften & melt upon heating

• ductile

• high fracture energy

• more easily repaired than thermosets

• good environmental resistance

• high melt viscosities

• no chemistry during fabrication (excluding polymerization)

• can b recyclable

• indefinite shelf life

thermosets polymers

= three dimensional cross linked / network polymers → stronger & harder

usually products of condensation polymerization → once they are set + hardeneed they cannot be reproduced (not moldable)

ex: polyesters (fiberglass), epoxies …

characteristics of thermosets polymers

• isotropic

• don’t melt upon heating

• decreased stiffness at high T

• brittle (epscially in tension)

• low fracture nrj

• sensitive to damage (hard to repair)

• environmentally stable (more than thermoplastics)

• limited shelf lie (depends on catalyst)

Elastomers or rubbers

posseses linear polymer structures with some cross-linking between molecules → unique elastic properties. Exhibit intermediate behaviour when comes to their structure

→ low elastic moduli so can deform easily

SIlicones = special type of elastomers → inorganic polymers with high dimensional & thermal stabiltiy, chemical inertness, low toxicity & good elc & anti-adhesive properties.

Elastomers characteristics

• elasticity (can strech 9-10 times original dimension)

• flexibility (almost as cloth)

• toughness

• impermeability

• resistance to corrosion

• plasticity and ease of molding

Functionality of polymers

another way to classify polymers

defined as number of sites at which the new molecule can be attahched to the mer (ex: ethylene → 2 locations where molecules can be attached → bifunctional chain)

Size of polymers molecule

increase in polymer molecule size induces:

→ increased melting point

→ increased strength

→ increased stiffness

thermal conductivity & specific heat

do not vary greatly form one polymer to another

→ usually thermal insulators

permeability of polymers

do not have interconnected pores → very low permeabilty

polymers = excellent material for transmitting amounts of gases / liquids

durability

polymers undergo environmental degredation → physical & chemical changes at microstructure level → lower durability

→ photo-oxidation & thermal oxidation results in cross-linkage of polymer chains

polymers quite resistant to attack by acids, alkalis & aqueous salt solutions

also resistant to bacterial & fungi attack (though not so much for natural polymers)

Additives added to polymers

plasticizers → improve flow, reduce brittleness and lower glass transition temp in amorphous polymers

fillers → reduce cost

reinforcement agents → improve mech properties

stabilzers → improve heat resistance, UV, oxidation

Amorphous polymers

resemble glass

polymer chain mobility decreases with decrease of T

below glass transition temperature (Tg) polymer becomes rigid solid → brittle

above Tg polymer behaves at viscous solids

Crystalline polymers

linear polymers can adopt a structure similar to that of regular lattice arrangement of metallic/ionic crystlaq

also includes some amorphous regions

polymers = crystalline when neighbouring chains become aligned → region are called micelle

Foamed polymers

= low density polymers → can be rigid or flexible/ elastic

1 method of manufacture = to pass steam over plastic beads to produce the foam structure in a die

→ foamed in-place insulation = two component liquid resin combined either within or immediatly outside of spray nozzle & deposited in place → plastic hardens when comes to contact with cold surface & immediatly air cured

have limited resistance

civil eng uses

• structural members

• sheating (replace plywood)

• insulation

• shingles

• pipe

• vapour & air barriers

• coatings

• adhesives & sealants

• polymer impregnated or modified concrete

• concrete reinforcement

• fibre for concrete or other polymers