Polymer Science ( You Can read this lecture in Bengali, Picture not uploaded. just see the hard copy)

Polymer Science

Polymer Science is concerned with a group of chemical substances composed of macromolecules. The field of polymer science includes in multiple disciplines including chemistry, physics, and engineering.

This science comprises three main sub-disciplines:

• Polymer Chemistry, concerned with the chemical synthesis and chemical properties of polymers.
• Polymer Physics, concerned with the bulk properties of polymer materials and engineering applications.
• Polymer Characterization is concerned with the analysis of chemical structure and morphology and the determination of physical properties in relation to compositional and structural parameters.

What are polymers?

Tetrahedral arrangement of C-H                                                                                                                                                                                                                                                                                                                                                                                                                                

Etymology of Polymer

The word polymer is derived from the Greek words poly meaning "many" and meros meaning "part".

The term was coined in 1833 by Jöns Jacob Berzelius.

What Makes Polymers Unique?

Really big molecules (macromolecules) like polymers have very different properties than small molecules

1.            Chain entanglement:  Long polymer chains get entangled with each other.

2.            When the polymer is melted, the chains can flow past each other.

3.            Below the melting point, the chains can move, but only slowly.  Thus the plastic is flexible, but cannot be easily stretched.

4.            Below the glass transition point, the chains become locked and the polymer is rigid

Reasons Why Polymers are Important

—  Plastics can be formed by molding into intricate part shapes, usually with no further processing required

—  Very compatible with net shape processing

—  On a volumetric basis, polymers:

—  Cost competitive with metals

—  Generally require less energy to produce than metals

—  Certain plastics are translucent and/or transparent, which makes them competitive with glass in some applications

General Properties of Polymers

1.            Low density relative to metals and ceramics

2.            Good strength‑to‑weight ratios for certain (but not all) polymers

3.            High corrosion resistance

4.            Low electrical and thermal conductivity

Limitations of Polymers as Engineering Materials

—  Low strength relative to metals and ceramics

—  Low modulus of elasticity (stiffness)

—  Service temperatures are limited to only a few hundred degrees

—  Viscoelastic properties, which can be a distinct limitation in load bearing applications

—  Some polymers degrade when subjected to sunlight and other forms of radiation

Definition of Polymer

q  Definition by Berzelius

Organic compounds which shared identical empirical formulas but differed in overall molecular weight, the larger of the compounds being described as "polymers" of the smallest.

Example: Glucose (C₆H₁₂O₆) would be a polymer of Formaldehyde (CH₂O)

q   Modern Definition

A Polymer is a large molecule (macromolecule) composed of a long, repeating chain of smaller units called monomers.

• Polymers usually have high melting and boiling points.
• Subunits are connected together by covalent chemical bonds.
• Highest molecular weight among any molecules.
• There is virtually no upper end to the molecular weight range since giant three-dimensional networks may produce cross linked polymers of a molecular weight of many millions.

Macromolecule

q  Is a molecule composed of a great number of atoms.

q  Has high relative molecular mass and molecular weight.

q  Is a high reactive molecular mass, the structure of which essentially comprises multiple repetition of uniting mass derived from molecules of low reactive molecular mass.

Physical Properties

Substances, composed of macromolecules, often have unusual physical properties

• Tendency to break easily.  Example, DNA.
• Need for assistance in dissolving into solution (in the form of ions or salts, for example).
• High concentrations of macromolecules in a solution can alter the rates and equilibrium constants of the reactions of other macromolecules.

Example

• Organic macromolecules: bio-polymers (carbohydrates, proteins, lipids), synthetic polymers (plastics, synthetic fiber, rubber).
• Inorganic macromolecules: graphene, carbon nanotube.

 Monomer

q  Is an atom or a small molecule that may bind chemically to other monomers to form a polymer.

Example

• natural: glucose, amino acid
• organic: ethylene, vinyl chloride, etc

Oligomer

Molecules with lower than 10 repeating units, the degree of polymerization is between 2 to 10 i.e. n=2~10 exhibits quite different thermal and mechanical properties compared to the corresponding high molecular weight polymer.

Oligomeric styrene having 7 repeating units (n=7) is a viscous liquid while commercial grade high molecular weight polystyrene is a brittle solid.

Some Common Polymers

Structure of a Polymer

—  Skeletal Structure

·         Linear – a chain with two ends

—  Chemical Structure

·         Branched – have side chains

·         Crosslinked (Networked) – chains are connected to other chains

Chemical Structure

n  Homopolymer – only one monomer (repeating unit)

- A – A – A – A – A – A – A -

n  Copolymer – more than one monomer

·         Alternating

                - A – B – A – B – A – B – A – B -

                - A – A – B – B – A – A – B – B -

•          Block

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

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

·         Graft                    

                   B-B-B-B-B-B-B

                    B

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

                                     B                          

                                  B-B-B-B-B-B

Classification of Polymers

1. by SOURCES:

1.1 Natural polymers (including macromolecules)

Polysaccharide: Examples: starch, glycogen, cellulose, chitin, chitosan

1.2 Synthetic polymers

Examples: plastics, synthetic rubbers, synthetic fibers ect.

2. By skeletal structure

Linear: Monomeric units are joined in the form of long straight chains, such polymers have high densities, high tensile strength and high melting  point with two ends.e. g. Polyethylene, nylons and polyesters.

Branched chain: are mainly linear in nature but also possess some branches along the main chain. E.g. low density polyethene (LDPE). They have densities, lower tensile strength and low melting point

Crossed Linked polymers : Monomeric unit’s are linked together to constitute a three dimensional network. They are hard, rigid, and brittle. e. g. Bakelite, Melamine formaldehyde resin, etc,

By thermal behavior

The most common way of classifying polymers is to separate them into three groups - thermoplastics, thermosets, and elastomers5. The thermoplastics can be divided into two types - those that are crystalline and those that are amorphous

Thermoplastics are

-relatively weak intermolecular forces so that the material softens when exposed to heat and then returns to its original condition when cooled.

-can be repeatedly softened by heating and then solidified by cooling - a process similar to the repeated melting and cooling of metals.

-          Most linear and slightly branched polymers are thermoplastic.

-          are produced by chain polymerization. Thermoplastics have a wide range of applications because they can be formed and reformed in so many shapes.

-          Some examples are food packaging, insulation, automobile bumpers, and credit cards.

Recycled plastics

A thermosetting plastic, or thermoset, solidifies or "sets" irreversibly when heated.  -cannot be reshaped by heating.

-usually are three-dimensional networked polymers in which there is a high degree of cross-linking between polymer chains.

The cross-linking restricts the motion of the chains and leads to a rigid material. A simulated skeletal structure of a network polymer with a high cross-link density is shown at the right. --are strong and durable.

They primarily are used in automobiles and construction. They also are used to make toys, varnishes, boat hulls, and glues.

Elastomers

Elastomers are rubbery polymers that can be stretched easily to several times their unstretched length and which rapidly return to their original dimensions when the applied stress is released.

Elastomers are cross-linked, but have a low cross-link density. The polymer chains still

have some freedom to move, but are prevented from permanently moving relative to each other by the cross-links.

To stretch, the polymer chains must not be part of a rigid solid - either a glass or a crystal. An elastomer must be above its glass transition temperature, Tg, and have a low degree of crystallinity.

—  Rubber bands and other elastics are made of elastomers.

By polymerization reaction (for synthetic polymers)

—  2.1 Condensation polymerization (from unsaturated hydrocarbons or olefins )

—  2.2 Chain-growth polymerization (from unsaturated hydrocarbons or olefins)

3. By composition

3.1 Homopolymers (contain one type of repeat unit)

3.2Copolymers(contain at least 2 repeat units)

5. By application

·         Deformation 

·         Stress 

·         Strain 

Some Special Classes of Concern

Tacticity

q  the way pendant groups are arranged along the backbone chain of a polymer.

q  particularly significant in vinyl polymers of the type -H₂C-CH(R)

Classification

There are 3 types

q  Isotactic Polymers          

o   all the substituent are located on the same side of the polymer backbone           

o   usually semicrystalline

o   often form a helix configuration

o   Example: Polypropylene, formed by Ziegler-Natta catalysis

q  Syndiotactic Polymers

o   the substituents have alternate positions along the chain

o   Syndiotactic polystyrene, made by metallocene catalysis polymerization, is crystalline with a melting point of 161 °C

q  Atactic Polymers

o   the substituents are placed randomly along the chain

o   formed by free-radical mechanisms

o   usually amorphous due to their random nature

o   cannot crystallize and forms a glass instead

o   most industrial polystyrene produced is atactic

o   Example: polyvinylchloride, polystyrene

q  Eutactic polymers

o   substituents may occupy any specific (but potentially complex) sequence of positions along the chain

o   Isotactic and syndiotactic polymers are more general class of eutactic polymers

o   Example: the side-chains in proteins and the bases in nucleic acids          

Polymer Nomenclature

q  generally based upon the type of monomer residues comprising the polymer

q  Source-based name

•  based on name of monomer
•   Poly + name of monomer
• (  ) is used if monomer has a multi-worded name

q  Structure-based name

•  based on structure of repeat unit
•   Poly(name of structure of repeat unit)

Monomer	Polymer	Structure-based name	Source-based name
CH2= CH2		Poly(methylene)	Polyethene, Polyethylene
		Poly(1-hydroxyethylene)	Poly(vinyl alcohol)
		Poly(1-methylethylene)	Polypropylene, Polypropene

IUPAC Recommendation for Source-based Names

q  Homopolymer

•  Addition polymers

Monomer	Polymer	Name	Uses
		Polyethylene (PE)	Bags, wire insulation, squeeze  bottles
		Polypropylene (PP)	Fibers, indoor-outdoor carpets, bottles
		Polystyrene (PS)	Styrofoam, molded objects such as table ware (fork,  knives, spoon), trays, video cassette cases
		Poly(vinylchloride) (PVC)	Clear food wrap, bottles, floor     covering synthetic leather,  water & drain pipe
		Polytetrafluoroethylene  (Teflon)	Nonstick surfaces, plumbing tape, films

Types of Polymerization

Addition Polymerisation

—  A carbon–carbon double bond is needed in the monomer

—  A monomer is the small molecule that makes up the polymer

—  The polymer is the only product

—                  Involves the opening out of a double bond

—                  The conditions of the reaction can alter the properties of the polymer

—                  Reaction proceeds by a free radical mechanism

—                  Oxygen often used as the initiator

The board specifies that you know this addition polymerisation reaction

Monomers can add head-to-tail, head-to-head, or tail-to-tail

Steps of Addition Polymerization

q  Initiation

o   Formation of active site on some monomer.

q  Propagation

o   Chain growth, the addition of an active site to a growing polymer chain, restoring the active site at the chain end.

q  Termination

o   Destruction of the active growing site.

q  Transfer Reactions

o   Removal of the active site from one chain to another molecule, terminating the first and beginning propagation at the second.

Types of Addition Polymerizations

q  Free radical polymerization

o   The initiator is a radical

o   The propagating site of reactivity (*) is a carbon radical

q  Anionic polymerization

• The initiator is a nucleophile
• The propagating site of reactivity (*) is a carbanion

q  Cationic polymerization

• The initiator is an acid
• The propagating site of reactivity (*) is a carbocation

q  Coordination catalytic polymerization

• The initiator is a transition metal complex
• The propagating site of reactivity (*) is a terminal catalytic complex

Condensation Polymers

q  Involves 2 monomers that have different functional groups.

q  They also involve the elimination of water or another small molecule.

q  Hence the term condensation polymer.

q  Monomer A + Monomer B à Polymer + small molecule (normally water).

q  Common condensation polymers include polyesters (the ester linkage) and  polyamides (the amide linkage as in proteins).

Step-growth or Condensation Polymerization

q  Used for monomers with functional groups such as –OH, – COOH etc.

q  It is usually a succession of non-catalysed, chemical condensation reactions associated with the elimination of low molar mass side products, eg., water.

q  Chain growth is exponential

Maximum molecular weight is obtained late in the reacti

q  Uses

q  Used to prepare fiber such as polyester.

q  Used for preparing a class of adhesive and amorphous solids called epoxy resin.

q  Synthetic plastics, which can be reshaped by heating, are prepared. They are called thermoset.

DP of Step-growth Polymerization

Because high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved.

Polyamides

•       These involve the linkage of two monomers through the amide linkage as in proteins (e.g. silk)

Polymerization Processes

Modern classification of polymerization according to  polymerization mechanism

Step growth polymerization: Polymers build up stepwise

Chain growth polymerization: Addition polymerization molecular weights increase successively, one by one monomer

Ring-opening polymerization may be either step or chain reaction

Carothers equation

( N_O : number of molecules

  N : total molecules after a given reaction period.

  N_O – N : The amount reacted

  P : The reaction conversion )

Or, N = N_O(1 -P)

(  DP is the average number of repeating units of all molecules present)

Comparison of Step-Reaction and Chain-Reaction Polymerization

Step Reaction	Chain Reaction
Growth occurs throughout matrix by   reaction between monomers, oligomers,  and polymers	Growth occurs by successive addition of     monomer units to limited number of     growing chains
DPa low to moderate	DP can be very high
Monomer consumed rapidly while   molecular weight increases slowly	Monomer consumed relatively slowly, but     molecular weight increases rapidly
No initiator needed; same reaction mechanism throughout	Initiation and propagation mechanisms different Usually chain-terminating step involved
No termination step; end groups still reactive	Usually chain-terminating step involved
Polymerization rate decreases steadily as   functional groups consumed	Polymerization rate increases initially as      initiator units generated; remains relatively      constant until monomer depleted

Polymerization Techniques

Sometimes for one monomer several techniques of polymerizing are available. Choice of a specific technique depends on a number of factors:

Ø  Kinetic / mechanistic factors related to chain length, chain composition

Ø  Technological factors e.g. heat removal, reaction rate, viscosity of the reaction mixture, morphology of the product

Ø  Economic factors; production costs, environmental aspects, purification steps etc.

Ø  Bulk Polymerization

Ø   Solution Polymerization

Ø   Suspension Polymerization

Ø   Emulsion Polymerization

Ø   Melt Polycondensation

Ø   Solution Polycondensation

Ø   Interfacial Condensation

Ø   Solid & Gas Phase Polymerization

Bulk (Mass) Polymerization

• The simplest method of polymerization where the reaction mixture contains only the monomer and a monomer soluble initiator. 
• Example PMMA
  Polymers through step reactions (nylon 6)

q  General Description

—  Carried out to high conversion

—   Free radical kinetics apply

 The system is homogeneous

Advantage of the high concentration of monomer result in

                1.            High rates of polymerization

                2.            High degree of polymerization

                3.            High purity of product

                4.            High molar mass polymer are produce

Advantages                                                    Disadvantages

                * Pure products                                               * heat control

                * Simple equipment                       * dangerous

                * No organic solvents                     * molecular weight is very disperse

Process Schemes

• Initiator is dissolved in liquid monomer.
•   Chain transfer agent, whenever used to control MW, is dissolved in monomer itself.
•   Reaction mass is heated or exposed to radiation source for initiation & kept under

    agitation for proper mass & heat transfer.

•   Achieve low conversion in large reactor, then prepare slabs or films.
• Uses
• Used for ethylene, styrene, methyl methacrylate to get transparent moulding powders and casting sheetings
• Used for vinyl chloride to get PVC resin.

Solution Polymerization

This method is used to solve the problems associated with the bulk polymerization because the solvent is employed to lower the viscosity of the reaction, thus help in the heat transfer and reduce auto acceleration.

General Description

• Homogeneous, if polymer remains soluble

Ø   vinyl acetate

Ø   acrylonitrile

Ø   esters of acrylic acid

• Heterogeneous, if polymer is insoluble, leading to precipitation (powder or granular) polymerization:

Ø   acrylonitrile in water

Ø   vinyl chloride in bulk

 Free radical kinetics apply

Process Schemes

• Monomer is dissolved in suitable inert solvent
• Free radical initiator dissolved in solvent medium, the ionic & coordination catalysts can either be dissolved or suspended.
• Presence of inert solvent medium helps to control viscosity increase & promote a proper heat transfer.

q  Advantages

o   Solvent acts as a diluent and aids in removal of heat of polymerization.

o   Solvent reduces viscosity, making processing easier.

o   Thermal control is easier than in the bulk

q  Disadvantages

o   Chain transfer to solvent occurs, leading to low molecular weights.

o   Difficult to remove solvent from final form, causing degradation of bulk properties.

o   Environmental pollution due to solvent release

q   Uses

o   Industrial production of polyacrylonitrile by free-radical polymerization

o   Production of polyisobutylene by cationic polymerization

o   Block co-polymers are exclusively made by this technique.

Suspension (Bead) Polymerization

—  This method is used also to solve the problem of heat transfer.

—  It is similar to bulk polymerization where the reaction mixture is suspended as droplets in an inert medium.

—  Monomer, initiator and polymer must be insoluble in the suspension media such as water

—  Water insoluble monomers are dispersed in water.

—  Initiator dissolved in monomer.

—  Stabilization of droplets/polymer particles with non-micelle forming emulsifiers like polyvinylalcohol or Na-carboxymethylcellulose.

—  Equivalent to bulk polymerization, small droplets dispersed in water.

—  Product can easily be separated, particles 0.01-1mm.

—  Pore sizes can be controlled by adding a combination of solvent (swelling agent) and non-solvent.

—   Viscosity does not change much.

General Description

• Droplets are 0.001-1 cm in diameter.
• Kinetics are the same as in the bulk.
• Must have very low monomer solubility in water or polymer will  form in aqueous phase.

Advantages

• Low viscosity due to the suspension
• Easy heat removal due to the high heat capacity of water
• Polymerization yields finely divided, stable latexes and dispersions to be used directly in coatings, paints, and adhesives.

Disadvantages

• Cannot be used for polymers whose glass transition temperature is less than the polymerization temperature, or else aggregation will occur.
• Must separate and purify polymer, or accept contaminated product.

Uses

• Used for
•  Polystyrene beads (from which polystyrene foams are made)
• Styrene-divinyl benzene co-polymer beads (for preparation of ion-exchange resin)                       
• Polyvinyl acetate beads (for conversion into polyvinyl alcohol)

Emulsion Polymerization

This is similar to suspension polymerization except that the initiation is soluble in suspension  media and insoluble in the monomer.

The reaction product is colloidally stable dispersion known as latex.

The polymer particles have diameter in the range of (0.05 - 1  m m) smaller than suspension.

General Description

• Surfactant is aggregated in micelles.
• Monomer is stabilized by surfactant and  dispersed in water.
• Predominant process for vinyl acetate, chloroprene,butadiene/styrene/acrylonitrile

copolymers, various acrylates.

• Used somewhat for methyl methacrylate, vinyl chloride, vinylidene chloride, styrene.

Advantages

• Thermal and viscosity problems are minimized due to the high heat capacity and ease of stirring of the continuous aqueous phase.
• Molecular weight may be increased without decreasing R_P
• Recall that in normal free-radical polymerization R_P and X_n are inversely related:
• The latex may be used directly without purification.       

Melt Polycondensation

q  General Description

o   Homogeneous system

o   Used for polymerization of monomers having at least one solid component & do not decompose around their melting point.

q  Advantages

o   No exotherm.

o   Suitable for producing medium to high molecular weights products.

Product minimum contaminated.

q  Disadvantages

• Requires longer duration for very high conversions.
• Viscosity of the medium increases very rapidly and at high conversions, proper mass transfer becomes difficult.
• Removal of by-product becomes difficult at high conversions.
• Proper heat transfer within the medium becomes difficult at high conversions.

q  Uses

• Production of polyethylene terephthalate from dimethyl terephthalate and ethylene glycol
• Preparation of nylon 6,6

Solution Polycondensation

q  General Description

o   Homogeneous system

o   System becomes heterogeneous, if the polymer is insoluble in the solvent.

o   Viscosity of the medium increases slowly.

q  Advantages

o   Reaction can be carried out at lower temperature and heat and mass transfer process become easier.

o    No exotherm. Heat transfer is uniform.

o   Removal of by-product is comparatively easier if a suitable solvent is chosen as entrainer.

o   Suitable for producing medium  to high molecular weight products.

q  Disadvantages

o   Requires longer duration for very high conversions.

o   Kinetic probability of chain growth is low  and this leads to a reduced rate and a low degree of polymerization.

o   Product usually contaminated with solvent.

o   Uses

o   Liquid polymer resins based on glycols and unsaturated dicarboxylic acid are prepared.

IONIC POLYMERIZATION

Chain polymerization of olefinic monomers which can occur with charged radicals. For example styrene.

Cationic polymerization when the radical is positively charge.

                                CH₂-C⁺H

                                         X

Anionic polymerization when the radical is negatively charge

                                  CH₂-C^--H

                                           X

This polymerization is more monomer specific than free radical polymerization and will proceed only with monomers that have substituent groups which can stabilize the active center.

For cationic polymerization the active center will proceed if X group is able to donate electrons or delocalize the positive charge.

For anionic polymerization the active center will proceed if X group is able to withdraw electrons or delocalize the negative charge.