Wednesday, June 30, 2010

Unit 3- Class 14- High Polymers (poly methyl methaacrylate)

PMMA (poly methyl methaacrylate or plexi glass):

It is obtained by the polymerization of methylmethacrylate using hydrogen peroxide as initiator.


Methyl metheacrylate is obtained by the following method. Acetone with hydrogen cyanide gives acetone cyanohydrin. The hydrolysis of acetone cyanohydrin with sulphuric acid yields methylacrylic acid, which on estrification with methyl alcohol gives methyl methacrylate.

Plexiglass is a linear thermoplastic. It has excellent optical properties. It is highly resistant to heat, chemicals and water. It readily allows UV rays to pass through it. Hence it is used for making lenses, artificial eyes, light fixtures etc. It also finds uses as paints and adhesives. Air craft windows, instruments, dentures (set of artificial teeth).

Unit 3- Class 13- High Polymers (Some commercial polymers - Teflon)

TEFLON (PTFE- polytetrafluoro ethylene):

Teflon is the trade name for PTFE. The monomer TFE is obtained by the following reaction.

                  CHCl3 + 2HF----------->CHClF2 + 2HCl     
                Chloroform                      Chlorodifluoro methane

               2CHClF2--------------->CF2=CF2 + 2HCl    
                                     TFE

Teflon is usually manufactured by emulsion polymerization of TFE using peroxide as initiators.
                              peroxide
          n CF2=CF2---------------->(-CF2 – CF2 -)n
        Teflon

Properties: 1. High degree of crystallinity 2. High m.p 3. High density 4. Insoluble in any solvent 5. High resistance to corrosive chemicals, oxidizing and reducing agents.6. Not wetted by either oil or water.7. Remains slippery over a wide range of temperature (-40 to 3000c). High thermal stability, good mechanical strength 9. Excellent electrical insulating properties.

Applications: 1. It is used for insulation of motors, generator, transformers, coils, capacitors, wires and cables.
2. Teflon is coated on articles such as bakery trays, frying pans and food processing equipments. 
3.Gaskets, industrial filters, widely used in army weapons.
4.Teflon is an ideal lubricant therefore used for non- lubricated bearings and as a dry lubricant.

Unit 3- Class12- High Polymers (Some commercial polymers)

POLYETHYLENE: is obtained by the polymerization of ethylene at high temperature.

              n (CH2=CH2)------------------>(-CH2-CH2-)n
There are two commercial rate of polyethylene 1. LDPE (low density polyethylene) 2. HDPE (high density polyethylene)

LDPE: is obtained when ethylene is polymerized under very high pressure at 150-2500c in presence of oxygen or azocompounds.
                                     150-2500c 
            n (CH2=CH2)------------------->(-CH2-CH2-)n


Properties: It has a linear structure with extensive branch. It has low density i.e (0.91-0.92g/cc), low crystallinity, low softening point (110-1150c) and molecular weight is between 20,000-50,000.

APPLICATIONS: 1.It is used for packing food and textile materials.
2.It is used for moulding articles such as toys, tubes, pipes etc.
3. It has excellent electrical insulation properties. It is used for cables and wire coverings.

Monday, June 28, 2010

Unit 3- Class 11- High Polymers (Structure property relationship of polymers)

Structure property relationship of polymers:
The fundamental properties, which influence the structure property relationship are molecular mass, polarity, crystallinity, molecular cohesion, the nature of the polymeric chains and stereochemistry of the molecule.

Impact and tensile strength of polymers and molecular mass: Density, melt viscosity, impact and tensile strength are a few mechanical properties of a polymer. Tensile strength and impact strength increases with molecular mass up to a certain point and then become constant. The melt viscosity of the polymer initially shows a gradual increase with the molecular mass and steep increase at higher molecular masses. For polymer to be commercially useful it should have low melt viscosity, high tensile and impact strength.


Crystallinity: Any polymer will contain a definite percentage of crystalline part and amorphous part. The degree of crystallinity depends on how best the polymer chain can be closely packed. Crystalline regions of a polymer are formed when their individual chains are linear (without branching), contain no bulky substituents and are closely arranged parallel to each other. The chains of polymer may be held together by vander wall’s force, hydrogen bonding or polar interaction. A polymer with high degree of crystallinity has high tensile strength, impact and wear resistance, high density and high fusion temperature, it has high Tg, and melt viscosity.
Crystallinity of a polymer also depends on the stereo regular arrangement. Polymers like HDPE, isotactic and syndiotactic polypropylene etc are highly crystalline. On the other hand atactic polypropylene, polystyrene, PVC which have their substituents in a random arrangement are less crystalline.


Elasticity: Elasticity of a polymer material is mainly because of the uncoiling and recoiling of the molecular chains on the application of force. For a polymer to show elasticity the individual chains should not break on prolonged stretching. Breaking takes place when the chains slip past each other and get separated. In rubber this is avoided by molecular engineering such as 1. Introducing cross- link at suitable molecular position 2.
Avoiding bulky side group such as aromatic and cyclic structure in the repeat unit 3. Introducing more non- polar groups in the chains so that the chains do not separate on stretching. The structure should be amorphous, this can be brought about by introducing plasticizer molecule in the polymer chain by co-polymerization or by7 compounding the rubber with a suitable plasticizer liquid.


Elastic deformation( rheology) of polymer: This can be studied by applying stress on the polymer material and finding the deformation caused. Polymers, since they contain both crystalline and amorphous regions, exhibit a complex behavior. The deformation depends upon on the degree of crystallinity, degree of cross- linking and the glass transition temperature.

 
Chemical resistivity: If a polymer is attacked by a reagent it undergoes softening, swelling and loses strength. Chemical resistivity of polymers depends on number of factors like presence of polar and non- polar groups, the molar mass, degree of crystallinity, extent of cross linking.
Polymers with non- polar groups undergo a welling and dissolution in non-polar solvents like benzene, toluene and carbon tetra chloride etc. Polar polymers containing –OH group or –COOH group are soluble in polar solvents like water, alcohol etc. Polymer containing ester group (polyester) undergo hydrolysis with strong alkalis at high temperature. Polyamide like nylon containing –NHCO- group, NHCOO group can be hydrolysed  by using strong acids or alkali.

Polyalkenes, PVC, Flourocarbon are some polymers, which have high degree of chemical resistance. For a given polymer resistivity increase with increase in molar mass. Linear polymers have lower resistivity than branched chain and cross- linked polymers.


This is all about Structure property relationship of polymers.  In our next class we will learn about  some commercial polymers.

Unit 3- Class 10- High Polymers (Glass transition temperature (importance of glass transition temperature))

Importance of Tg:

1.    Tg value is a measure of flexibility of a polymer.
2.    Tg gives us idea of the thermal expansion, heat capacity, electrical and mechanical properties of the polymer.
3.    The use of any polymer at any temperature is decided by its Tg value.
4.    Knowledge of Tg is useful in choosing appropriate temperature range for such processing operations. 

Well this is it about glass transition temperature. Next we will learn about Structure property relationship of polymers.

Sunday, June 27, 2010

Unit 3- Class 9- High Polymers (Glass transition temperature (Tg))

Glass transition temperature (Tg): is the temperature, below which a polymer is hard and above which it is soft and flexible is called the glass transition temperature. The hard, brittle state is known as the glassy state and soft flexible state as the rubbery state.

Parameters affecting Tg:

1.    Crystallinity:  Higher the crystallinity, higher is the Tg value of a polymer. In crystalline polymer, the linear or stereo regular chains are lined up parallel to each other and are held by strong cohesive forces. This leads to a high Tg value of the polymer.
Egs: polyethylene has low Tg compared to that of nylon6,6.

2.    Molecular weight: The Tg of all polymers, in general, increases with molecular weight up to 20,000 and beyond which the effect is negligible.
3.    Effect of side group: Poly [a- methyl styrene] has higher Tg value while polystyrene has lower Tg value, is due to the presence of effective methyl side group which hinders the free rotation about carbon-carbon bond of chain back bone, and restricts the chain mobility, thereby increase Tg value.
4.    Intermolecular forces: Presence of large number of polar groups in the molecular chain lead to strong intermolecular cohesive forces which restrict the molecular mobility. This leads to an increase in Tg. Egs: polypropylene has lower Tg compared to nylon6,6 .
5.    Presence of plasticizers: Addition of plasticizers reduces the Tg value. Egs: Diisooctyl phthalate, which is added to PVC reduces its Tg.
6.    Srereoregularity of polymers increases Tg. Thus Tg of a isotactic polymer is greater than that of the syndiotactic polymer whose Tg is greater than atactic polymer.

We will discuss about the importance of glass transition temperature in our next class.

Unit 3- Class 8- High Polymers (Methods of polymerization - Emulsion polymerization)

Emulsion polymerization: In this method emulsion of water insoluble monomer and water is prepared and is stabilized by the addition of surface acting agents (surfactants) such as soap. Polymerization is initiated by the addition of water-soluble initiator such as potassium persulphate. After adding the initiator, the system is kept agitated in the absence of oxygen at 70oc. 

Mechanism: The surfactant has hydrophilic head and hydrophobic tail. The water- soluble initiator links to the hydrophilic end whereas the monomer is linked to the hydrophobic end. At a little higher concentration it gets dispersed. When the concentration of surfactant exceeds critical micelle concentration (cmc), the soap molecule form micelle (aggregation of 50-100 molecules) oriented with tails inwards and head outwards. Now, an initiator molecule at the polar end diffuses into the micelle ti initiate the polymerization process. As the polymerization progresses, there will be depletion in the number of monomers within the micelle. They are replenished by the medium. This continues till the polymer formed is big enough to come out, the process is terminated by combination. The pure polymer is isolated from the emulsion by the addition of de-emulsifier.

Advantages:
1.    The rate of polymerization is high.
2.    Easy heat control.
3.    Avery high molecular weight polymer is obtained
4.    Molecular weight control is possible.
5.    Viscosity build up is low.

Disadvantages: Polymer needs purification.

Application: This method is used for the production of PVC, PVA etc.

These are the 3 methods of polymerization.We will discuss about glass transition temperature in our next class.

Unit 3- Class 7- High Polymers (Methods of polymerization - Suspension (pearl) polymerization)

Suspension (pearl) polymerization: This polymerization occurs in heterogeneous system. The water insoluble monomer is suspended in water as tiny droplets by continuous agitation. The droplets are prevented from coalescing by using small quantities of water- soluble polymers such as polyvinyl alcohol or colloids. The polymerization is made to occur in each droplet of the monomer using a catalyst. The reaction mass is heated ti initiate the polymerization. After the completion of polymerization pearl like polydispersed polymer mixture is obtained.

Advantages:

1.    The viscosity build up of polymer is negligible.
2.    Isolation of product is easy as it needs only filtration and washing.
3.    High purity products.
4.    The process is more economical since water is used.
5.    Isolated products need no further purification.
6.    Efficient thermal control.

Disadvantages:
1.    This method is applicable only for water insoluble monomers.
2.    It is difficult to control particle size.

Application: This technique is used for the production of polyvinyl acetate, poly styrene, styrene-divinyl benzene etc.

Unit 3- Class 6- High Polymers (Methods of polymerization - Solution polymerization)

Solution polymerization: In this technique, monomer, initiator and chain transfer agents are dissolved in an inert solvent. The solvent helps to control viscosity increase and promote proper heat transfer.

Advantages :

1.    The viscosity build up is negligible.
2.    The heat control is easy.
3.    The agitation is easy.
4.    The polymer in solution form is used as adhesives and coatings.

Disadvantages:

1.    It is difficult to get very high molecular weight products since solvent may act as chain transfer agent thereby limit the molecular weight.
2.    The polymer formed will also have to be isolated from the solution either by evaporation of the solvent or by precipitation in a non-solvent.
3.    Removal of their final traces is always difficult.

Applications: This method is used for the production of polyethylene, PVC, polisobutylene, polyacrylonitrile etc.

Saturday, June 26, 2010

Unit 3- Class5- High Polymers (Methods of polymerization)

 In this class, let us learn about the methods of polymerization.

Methods of polymerization: Industrial polymerization can be carried out by different techniques. The principal polymerization methods are

1. Bulk polymerization 2. Solution polymerization 3. Suspension polymerization 4. Emulsion polymerization. 


Bulk polymerization: It is a homogeneous polymerization. The method is used for the liquid monomer. The monomer is taken in the liquid state and the initiator, chain transfer agents are dissolved in it. The reaction mass is heated or exposed to a radiation source for initiating the polymerization and is kept under agitation for proper mass and heat transfer.

Advantages:
1. Bulk polymerization is quite simple.
2. High purity products are obtained.      
3. Products need neither isolation nor purification.

Disadvantages:
1. As the polymerization proceeds the viscosity of the medium increases leading to polymer products with broad molecular weight distribution.
2. Mixing becomes difficult.
3. Reaction is highly exothermic.
3. As the medium gets viscous, the diffusibility of the growing polymer chain becomes restricted, the probability of the chain collision becomes less, termination becomes difficult, active radical sites accumulate and the rate of polymerization increase enormously. This phenomenon is called auto acceleration.

 Applications:
 This technique is used in the free radical polymerization of methyl methacrylate to poly- methyl methacrylate, styrene to polystyrene, vinyl chloride to PVC etc.


This is it for today's class, in the next class we will learn more about other various methods of polymerization. Untill then revise the Polymers that has been done.Take Care.

Unit 3- Class 4- High Polymers (Mechanism of addition polymerization)

Mechanism of addition polymerization:  Free radical mechanism:

The polymerization of ethylene monomer by free radical mechanism proceeds in three distinct stages: 1. Initiation 2. Propagation 3. Termination.

Initiation: It involves two reactions. The first is the production of free radicals usually by the homolytic dissociation of an initiator such as dibenzoyl peroxide to yield a pair of radicals. A free radical is an atomic or molecular species having an odd or unpaired electron. They are highly active species.
                                Heat                              heat                         . 
       (C6H5COO)2--------->2C6H5COO.--------->2CO2 + 2C6H5 (R.)


The second part of initiation involves the addition of this radical to the first monomer molecule to produce the chain initiating species. 
                                                                                               . 
            R. + CH2=CH2-------->R-CH2-CH2    


Propagation: In the propagation, the radical attacks another monomer to produce yet another free radical and the process continues until termination occurs.


 R-CH2-CH2 +   CH2=CH2-------------> R-CH2-CH2-CH2-CH2  
                                                                                 |
 
                                                                                 |    n CH2=CH2  

                                                                                  |       . 
                                                               R-(CH2)n-CH2-CH2      
                                                                      In general


Termination: At some point, the propagating polymer chain stops growing and terminates. Termination is by 1. Coupling or combination i.e. a. the two growing chain may react with each other.
                                  .          .   
        R-(CH2)n-CH2-CH2 +  CH2-CH2-(CH2)n -R

                                   |

                                   |

                R-(CH2)n- CH2-CH2-CH2-CH2-(CH2)n-R

2. Coupling of growing chain with initiator free radical.
                                         .
               R-(CH2)n-CH2-CH2 + R.-------------->R-(CH2)n-CH2-CH2-R      

3. Or by disproportionation in which a hydrogen atom of one radical center is transferred to another radical center. This results in the formation of two polymer molecules one saturated and another unsaturated.
                                               .                                  .
                     R-(CH2)n-CH2-CH2 + R-(CH2)n-CH2-CH2 ------------------------------------->R-(CH2)n-CH2-CH3 +  R-(CH2)n-CH=CH2   
                                                                      Dead polymer

Thursday, June 24, 2010

Unit 3- Class 3- High Polymers (polymerization)

 As said in our earlier class, today we will discuss about the polymerization process.

Polymerization: Is the process of conversion of low molecular weight substances into high molecular weight substances with or without the elimination of by products such as HCl, H2O, NH3 etc.

Types of polymerization:

1.    Addition (chain) polymerization: A polymerization reaction in which monomers containing one or more double bonds are linked to each other without the elimination of any by products, usually in the presence of initiators is called addition polymerization.

        Egs: 1. Formation of polythene.
   
             n CH2= CH2-------------->[- CH2 – CH2 -]n

The main features of addition (chain) polymerization are:
1. Only olefinic compounds can undergo addition polymerization.
2. No elimination of by products.
3. Double bond provides required bonding sites.
4. The addition of monomers takes place rapidly.
5. Linear polymers are produced.
6. The addition polymerization is brought about by free radical, ionic or co-ordination mechanism.
7. The molecular weight of the polymer is an integral multiple of the monomer.
8. The elemental composition of the polymer is same as that of monomer.


Condensation (step) polymerization: A polymerization reaction in which bi or poly functional monomers undergo intermolecular condensation with continuous elimination of by products such as H2O, HCl, NH3 etc. is called condensation or step polymerization.

Egs: 1. Formation of Nylon66
                   n  NH2-(CH2)6-NH2 + n  HOOC- (CH2)4- COOH -------------------------->
   Hexamethylene diamine                           Adipic acid


                          [-NH-(CH2)6-NH-CO-(CH2)4-CO-] n + 2n H2O
                                        Nylon66


2. Formation of polyester

     nOH-(CH2)2-OH + n HOOC-C6H4-COOH              [-O-(CH2)2-O-CO-C6H4-CO-] n               
      Ethylene glycol                   Terephthalic acid                   Polyester       +2nH2O


The main features of condensation polymerization are:

1. The monomers having two or more reactive functional groups can undergo condensation polymerization.   
2. There is elimination of by products.
3. Polymerization proceeds through intermolecular condensation.
4. The polymer chain build up is slow and stepwise.
5. Polymerization is catalysed by acids or alkali.
6. Linear or cross-linked polymers are produced.
7. The elemental composition of the polymer is different from that of the monomers.


in our next class we will study about the Mechanism of addition polymerization.

Wednesday, June 23, 2010

Unit 3- Class 2- High Polymers (Classification)

Classification of polymers:

 
I.    Based on their sources they are classified into
1.    Natural polymers: The polymers, which are obtained from natural sources such as plants and animals, are called natural polymers.
       Egs. Wood, cellulose, Jute, Cotton, Wool, Silk, Proteins, Natural rubber etc.
         
2.    Synthetic polymers: The polymers, which are synthesized from simple molecules, are called synthetic polymers.
      Egs: Nylon66, PVC, Polystyrene, Teflon, Plexiglass, Polyesters, Polyethylene etc.


II.    Based on their thermal behaviour thy are classified into   
1. Thermoplastic polymers: egs: PVC, Polyethylene etc.
2. Thermosetting polymers: egs: Bakelite, Urea-Formaldehyde etc. 



III.   Based on their mechanism of polymerization they are classified into 
 1. Addition polymers: egs: PVC, Polyethylene etc.
2. Condensation polymers: egs: Nylon66, Polyester etc.

 

IV. Based on their properties they are classified into
    
         1. Elastomers egs; Natural rubber
         2. Fibres        egs: Jute, Wood, Silk etc
         3. Resins        egs: Urea- Formaldehyde, Epoxy resins, Phenol- Formaldehyde etc.
         4. Plastics      egs: Plexiglass, PVC, Teflon etc.

This is the brief classification, In the next classes, we will learn about polymerization

Unit 3- Class 1- High Polymers (Introduction)

Friends in this class we will be starting a new unit called high polymers. Initially lets have an introduction to it.

Definition of polymers: A polymer is a large molecule of high molecular weight obtained by the chemical interaction of many small molecules of low molecular weight of one or more type. The process of manufacture of a polymer is called the polymerization.

Monomers: Small molecules of low molecular weight, which combine to give a polymer, are called monomers.

Degree of polymerization: The number of monomers used in the process is called degree of polymerization.

Functionality: The total number of functional groups or bonding sites present in a monomer molecule is called the functionality of the monomer.

in the next post we will learn more on this, we will be learning the the classification of polymers based on various factors

Unit 2- Class 9- CORROSION (Corrosion Control)

CORROSION INHIBITORS:

Using certain chemical substance known as inhibitors can reduce rate of corrosion.  Inhibitors slow down the anodic or cathodic reactions by forming a protective film on these regions inhibitors mostly used to provide protection to systems in which corrosion environment re-circulated or confined for a long time like IC engine, re-circulated water system etc.


 ANODIC INHIBITORS:
These include oxidizing agents like chromates, molybdates, tungstates and nitrates.  These anions react with metal ions formed at anode during oxidation reaction forming sparingly soluble respective salts.  These compound are deposited on the anodic sites forming a protective films so that further preventing the anodic reactions.
    These inhibitors are found to be effective only when sufficient amount of inhibitors is added into corrosion medium.  So that entire anodic surface can covered other wise intense corrosion takes place.

ii) CATHODIC INHIBITIORS:
These can act by inhibiting the cathodic reaction, which involves liberation of hydrogen or absorption of oxygen.
If the cathodic reaction is liberation of H2 type then by adding organic inhibitors like amines, Urea, thiourea, heterocyclic compound.  They form a protective layer on cathodic region so that evolution of hydrogen gets retarded.  Evolution of H2 can also prevent by increasing hydrogen over voltage.
This can be achieved by adding oxides of arsenic or antimony.
           
If the cathodic reaction is oxygen absorption type then by adding certain oxidizing agent like N2H4 (hydrazine), Sodium sulphite, these compounds absorb oxygen as

       
        N2H4 + O2------------->N2 + 2H2O

      2Na2SO3 + O2----------->2Na2SO4

So that cathodic reaction is retarded otherwise by adding inorganic inhibitors like sulphates of Zinc, Mg, Mn etc. They reacts with OH- ions (which are liberated at cathode) forming insoluble hydroxides which form a protective film over cathodic area and hence corrosion rate reduced. Corrosion inhibitors have certain limitations
1. They contaminate environment
2. Many inhibitors are toxic in nature.
3. It can be used only in   closed systems
4. Generally inhibitors lose their efficiencies as conc. And temperature increases.

Cathodic protection:
It is a method of protecting the metal or alloy from corrosion and no part of it is allowed to act as anode.
The technique of offering cathodic protection to a specimen (metal) against corrosion by providing electrons from an external source.
There are two methods for providing electrons for cathodic reactions there are

1. Sacrificial anode method:
 The method involves the use of more active metals as sacrificial anode in contact with specimen (like iron, copper or brass). The active metals like Zn, Mg, Al, and their alloy acts as an auxiliary anode, and undergoes preferential corrosion protecting the metal structure. Here the anode metals are sacrificed to protect the metal, the method is known as sacrificial anode method, exhausted anodes are replaced by new ones as and when required.
Egs: 1. A Mg or Zn block connected to a buried oil tank
        2. Mg bars are connected to ocean going ships
         
Sacrificial anode methods are simple with low installation cost and do not require power supply but involves recurring expenditure of replacement of consumed anodes.
2. Impressed current method:
This is another method to provide cathodic protection by supplying electrons. These can be provided by a source of direct current.
The structure to be protected should be made negative (connected to negative terminal of a D.C source). Resin bonded graphite rod, platinised Ti, Pb are used as anode and connected to the positive terminal. The main structure being cathode does not undergo corrosion, and anode being inert remains unaffected.    

This technique is used to protect marine structure, water storage tanks and other gas or oil pipelines. This method is simple, can protect large metal area with low maintenance cost but expensive, because it needs high current.

Anodic protection:
Certain metals like Ti and alloy like stainless steel, which readily get passivated so that cathodic protection cannot be offered in such cases corrosion process, can be slowed down by use of anodic current. BY passing anodic current in these metals an oxide layer will grow and that oxide layer will protect the metals. The potential that is required to grow the oxide film and to protect the metal can be obtained from potential-current curve.

At a predetermined range of applied potential the changes observed in the potential and the corresponding changes in current are studied. In initial stages (AB) the current increases on increasing potential indicates the dissolution of metal so corrosion takes place. When the current reaches a critical point, (Icrit ), passivation, that is due to development of oxide layer sets in and that potential  ( Ep) is called passivating potential (Ep). Above Ep along BC current flow decreases to a very small value called passivating current ( Ip). It is the minimum protective current density to maintain passivation. At this point (C ) an increase in potential will not corrode the metal since the metal is in highly passive state. The small current (Ip) is sufficient to maintain the passivity so that corrosion rate brings down.
The anodic protection to a structure is applied by using device called potentiostat. It is an electronic device that maintains a metal at a constant potential with respect to a reference auxiliary Pt electrode and a reference calomel electrode.

The metal structure kept in a suitable oxidizing atmosphere acts as the working electrode ( anode). The potential is then slightly increased to Ep (passivating potential) for the initial corrosion to start. The potential is then slightly increased till the current decreases to very small value (Tp) indicating passivation of metal.  Now the potential is kept at constant Ep and the current is maintained at Ip.

    This method has an advantage as it requires very small current, but this method can be adopted to only passive metals like Ti, Ta,Al,Cr etc., and also by using this method corrosion rate cannot be reduced to zero.

    This method usually applied in the transportation of acid.

This ends the corrosion unit. In the next classes, we will start up with polymers. Until that revise the two units that have been done .

Unit 2- Class 8- CORROSION (Corrosion Control)

ORGANIC COATING:
Coating of inert organic materials like paints, and lacquers on metallic surface to protect the metal usually thickness of the organic coating is less than 0.4mm thickness.

 The functions of organic coating are:
i)    The organic coating serves as a barrier between the metal surface and corrosive environment.
ii)    The pigment or drying oils present in the paint often exert and inhibitive action by electrochemical and other means.

 There are certain requirements to have a good organic coating they are:
i)    The organic coating should adhere tenaciously to the metal surface and should improve its physical appearance.
ii)    The film formed should be uniform, continuous and act as a barrier to air and water.
iii)    It should chemically increase, so that atmosphere pollution are not there.
iv)    Should have reasonably long life.
v)    Should easily available, low cost and easy and properly applicable.
               
 The performance of paint depends on the technique employed during application method.

The application of organic coating involves following steps
i)    Surface preparation:  It includes degreasing the surface and removing rust and scale from it.
ii)    Priming:  This is the first coat of primer like phosphate coating which must strongly adhere to the surface so that painting can properly done.
iii)    Filling:  Fillers like nitrocellulose epoxides etc. are applied on well-dried surface in order to improve external appeaence.
iv)    Sanding:  The roughness and irregularities on the surface are smoothened by means of emery paper.
v)    Final finishing coat of organic paints of two or three coat.
                
Organic coating usually applied such as brushing, spraying, dipping etc. Break of organic coating causes severe corrosion.

Unit 2- Class 7- CORROSION (Corrosion Control)

Inorganic coating or surface conversion coating or chemical conversion coating:

Here a surface layer of the metal is converted into a stable compound by chemical or electrochemical reactions, which forms a barrier between the base metal and corrosion environment. These types of coating are different compared to that of metal coating because they are integral part of the metal itself and in addition to corrosion resistance they also provides increased electrical insulation and also physical appearance of the metal. In surface conversion coating two important type are

  1. Anodized coating or anodizing:
Anodizing usually  carried out in non-ferrous metals like Al, Mg, Cr, Ni, etc or their alloys by anodic oxidation process in which base metal is made as anode, in an electrolytic bath of suitable composition and by passing direct current. However commercially only Al and its alloy are anodized for corrosion resistance.
Process: The article to be anodized is degreased properly and polished. Then made as anode and copper or lead is made as cathode. The electrolyte consists of 5- 10 % chromic acid, the temperature is about 350c.  The voltage is programmed to increase from 0-50V (optimum 40v)
So as to maintain an anode current density of 10-20milli amp/cm2. In first ten minutes the potential is increased from 0-40V and at 40V continue anodizing for about 20min after wards the potential is increased from 40 V- 50V and maintained at 50vc for 5min an opaque oxide layer of 2-8micrometer thickness is obtained. If higher thickness is required 10% sulphuric acid is used as an electrolyte at 220c with around 160milliamp/cm2 of current density at 24V a colorless 25micrometer thickness layer is obtained. Afterwards the article should be dyed properly and finally treated with boil water containing cobalt or nickel sulphate or Acetate to improve corrosion resistance.

Phosphating: It is generally obtained on steel surface by converting surface metal atoms into their phosphate by chemical or electrochemical process.
The phosphating bath containing three essential components 1. Free phosphoric acid 2. Primary phosphate like Fe, Mn, or Mg phosphate. 3. Accelerator such as nitrates, nitrites hydrogen peroxide and PH usually in the range of 1.8-3.2 if the process is immersion type.  If the process is spraying type then PH is in an around 3.2 to 7.8 and temperature maintained at 35oc.

    The mechanism of phosphating involves following steps.
1)    First dissolution of the metal as metal ions.
2)    Metal ions reacting with phosphate ions to form a metal phosphate.
3)    Deposition of the metal phosphate on the surface of the metal.
Phosphating is always given as underline (under coating) before the finishing, because phosphating along with corrosion resistance it also imparts the surface a good paint adhesion quality.

Application:  An undercoating before painting of Automotive bodies, refrigerators, washing machines etc.
     One of most important application of phosphating is of galvanized iron which is otherwise difficult to paint.

Tuesday, June 22, 2010

Unit 2- Class 6- CORROSION (Corrosion Control)

In the classes to come, let us learn about corrosion control.



CORROSION CONTROL: Corrosion of a metal is a natural spontaneous process, by which metal is converted into a more stable compound so that corrosion control is more realistic than corrosion prevention. In general preventing the formation of galvanic cells can control corrosion.
The methods used to control corrosion are as follows:

1.    Protective coatings:
Corrosion of metal can be controlled by isolating them from the corrosive atmosphere. This can be done by covering the metal (base metal) with a layer of another metal. This process is known as metal coating.
The Principal type of coatings applied on the metal surface are :
1. Metal coating 2. Inorganic coating 3. Organic coating.     


Metal coating: This coating is the deposition of a protective metal over the surface of the base metal. The method can be applied by electrodeposition, flame spraying, cladding, hot dipping etc.
On the basis of coating there are two classes:

1.    Anodic coating: It is produced by coating a base metal with more active metal which are more anodic with to the base metal for egs: Iron is coated with Zn, Mg, Al etc.,
One of the important properties of this type of coating are that, even if the coating is ruptured, the base metal does not undergo corrosion. The exposed part of the base metal is cathodic with respect to the coating metal and coating metal only undergone corrosion there by protecting the base metal. The protection is there as long as coating is there. Galvanisation is one of the best egs in anodic coating.

Galvanisation: It is a process of coating a base metal (iron) with zinc(Zn) metal. This process usually carried out by hot dipping method.
Process: first the base metal surface is washed properly with organic solvents to remove any organic matter (like oil, grease etc) on the surface afterwards it washed with dil. H2SO4 to remove any inorganic matter (like rust). Finally the base metal is well washed with water and air-dried. The base metal then dipped in a bath of molten zinc maintained at 425-4300c and covered with a flux of NH4Cl to prevent the oxidation of molten zinc. Then excess zinc on the surface is removed by passing through a pair of hot rollers so that a proper thin coating is obtained.
Application: Galvanized articles are mainly used in roofing sheets, fencing wire, buckets, bolts nuts, pipes and tubes etc. but galvanized articles are not used for preparing and storing food stuffs. Since zinc dissolves in dil. Acids and become toxic.

2.    Cathodic coating: These are the coating produced by coating a base metal with more cathodic (noble metal) for egs: iron is coated with tin, nickel, Cr and Cu. But these coatings provides protection only when it is undamaged and absolutely free from gaps otherwise rapid corrosion of the base metal takes place as a result the formation of large cathodic and small anodic area.
Tinning is the best egs to explain cathodic coating.

Tinning: It is a process of coating base metal (iron) with tin (Sn). It can be carried out by hot dipping method. 
            The iron sheet (base metal) first washed thoroughly with organic solvents to remove any organic matters. Then treated with dil. H2SO4, to remove rust and seal deposits, finally it is washed well with water and air-dried. It is then passed through ZnCl2 and NH4Cl flux (molten) so that molten tin can adheres properly on the metal surface then base metal passed through tank that contains molten tin.
Finally passed through a series of rollers immersed in palm oil. So that uniform, undamaged, continuous deposit of tin takes place. Tinning will provide complete protection against corrosion if it covers the surface completely.
Application: tinned articles are largely used in the manufacturing of containers used for storing foodstuffs, copper utensils are coated with tin so that contamination of food with copper can be prevented.


We will discuss of the next control methods in the classes to come

Unit 2- Class 5- CORROSION (Factors related to corrosive environment)

Factors related to corrosive environment:

1.    pH of the medium:  Usually higher acidic nature (low pH) higher is the rate of corrosion. If the pH is greater than 10 corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron.
 If pH is between 10 and 3, then presence of oxygen is essential for corrosion   of iron. If the pH is 3 or lower than 3, severe corrosion occurs in the absence of air due to the continuous evolution of H2 at cathode. However metals like Al, Zn etc undergo fast corrosion in highly alkaline medium.

2.    Temperature: On increasing the temp. Rate of corrosion process also gets increases because on increase of temp. Conductance of the aqueous medium increases hence rate of diffusion also.
In some cases on rise in temp. Decrease the passivity, which again leads to increase in the corrosion rate.

3.    Polarisation at anodic and cathodic area: Polarisation of cathode or anode decreases the rate of corrosion. If anodic polarization takes place due to some reaction, then tendency of metal to undergo oxidation decreases hence dissolution of metals as metal ion decreases. This is usually due to increase in conc. Of ions of the dissolved metals in the vicinity of electrode or also due to the anodic passivity. Cathode polarization decreases the cathodic reaction hence hindering the combination of cathode reactant and electron. For the corrosion to continue both anodic and cathodic reaction should take place simultaneously if any one reaction is slower then the rate of corrosion is slower. Use of depolarizers reduces the polarization effect hence the rate of corrosion reaction increases.

Unit 2- Class 4- CORROSION (Factors affecting rate of corrosion)

 In this class let us study the factors affecting rate of corrosion .

Factors affecting rate of corrosion: 

Several factors affecting the rate of the corrosion, which can be divided into two parts 
1.    Factors affecting on metal (related to the metal)
2.    Factors affecting on corrosive environment.


Factors affecting the metals:
1.    Nature of the metal: The tendency of the metal to undergo corrosion is mainly dependent on the nature of the metal. IN general the metal with lower electrode potential have more reactive and more susceptible for corrosion and metal with high electrode potential are less reactive and less susceptible for corrosion for egs: metals like K, Na, Mg, Zn etc have low electrode potential are undergo corrosion very easily, where as noble metals like Ag, Au, Pt have higher electrode potential, their corrosion rate are negligible but there are few exception for this general trend as some metals show the property of passivity like Al, Cr, Ti, Ta etc.
2.    Surface state of the metal or nature of the corrosion product (passivity):
The corrosion product is usually the oxide of the metal; the nature of the product determines the rate of further corrosion process.
If the oxide layer, which forms on the surface, is stoichiometric, highly insoluble and non-porous in nature with low ionic and electronic conductivity then that type of products layer effectively prevents further corrosion, which acts as a protective film. For egs: Al, Cr, Ti develop such a layer on their surface and become passive to corrosion and some metal like Ta, Zr and Mo not only forms such a protective layers but are capable of self repairing oxide films when it is damaged. Hence these are extremely passive metals.
If the oxide layer forms on the metal surface is non-stochiometric, soluble, unstable and porous in nature and have an appreciable conductivity, they cannot control corrosion on the metal surface for egs: oxide layer formed on metals like Zn, Fe, Mg etc.
3.    Anodic and Cathodic area:
The rate of the corrosion is greatly influenced by the relative sizes of cathodic and anodic areas.
If the metal has smaller the anodic area and larger the cathodic area exposed to corrosive atmosphere, more intense and faster is the corrosion occurring at anodic area because at anode oxidation takes place and electrons are liberated. At the cathode these electrons are consumed. When anode is smaller and cathode region is larger all the liberated electrons at anode are rapidly consumed. This process makes the anodic reaction to takes place at its maximum rate thus increasing the corrosion rate. If the cathode is smaller and reverse process takes place decrease rate of corrosion.
For egs: If tin (Sn) coated on iron (Fe) and in that some area are not covered or some pin holes are left, there forms smaller anodic area and larger cathodic area because tin is cathodic with respect to iron so intense localized corrosion takes place. On the other hand if Zn coated to Fe then if there are some pin holes are there creates larger anodic area and smaller cathodic area because Fe is cathodic with respect to zinc so that rate of corrosion is very less.
4.    Hydrogen over voltage:
A metal with low hydrogen over voltage on its surface is more susceptible for corrosion. When the cathodic reaction is hydrogen evolution type with low hydrogen over voltage, liberation of H2 gas is more easier so that cathodic reaction is very fast, that makes anodic reaction faster hence overall corrosion process is very fast. If the H2 over voltage is high so cathodic reaction is slow hence corrosion reaction also slower.

Monday, June 21, 2010

Unit 2- Class 3- CORROSION (Types of Corrosion)

In this post let us discuss about the corrosion types.

Types of Corrosion: Corrosion on the metals taking place depending on the nature of metals and depending on the types of environment by different mechanisms, giving different types of corrosion.


1. Galvanic corrosion or differential metal corrosion:
This occurs when two dissimilar metals are in contact with each other in a corrosive conductive medium; a potential difference is set up resulting in a galvanic current. The two metals differ in their tendencies to undergo oxidation. The metal with lower electrode potential or more active metal acts as anode and the metal with higher electrode potential acts as cathode. The potential difference is main factor for corrosion to take place. The anodic metal undergoes corrosion where as cathodic metal gets un -attacked.

Egs: When iron contact with copper iron has lower electrode potential acts as anode and undergo oxidation as,
Fe -------> Fe2+ + 2e-
   Where as copper which is having higher electrode potential acts as cathode gets unaffected. The rate of galvanic corrosion depends upon potential difference between anodic and cathodic metals, ratio of anodic and cathodic area and environmental factors and tendency of the metal to undergo passivity etc.

Other egs: When Fe contact with Sn then Fe acts as anode and Sn acts as cathode but when Fe contact with Zn, Fe acts as cathode where as Zn acts as anode.


2. Differential aeration corrosion:
 Differential aeration corrosion occur when metal surface is exposed to the differential air or oxygen concentration, that develops galvanic cell and initiate the corrosion. The part of the metal exposed to lower oxygen concentration acts as anode and the part of the metal exposed to higher concentration acts as cathode, so that poorly oxygenated region undergoes corrosion.
When a metal strip of iron, partially immersed in an aerated solution of NaCl the concentration of O2 is higher at the surface than inside the solution. Since cathodicreaction requires oxygen, hence cathodic area tends to concentrate near the water line so that bottom portion of the specimen acts as anode where corrosion starts.
At anode: Fe-------------->Fe2+ + 2e-
At cathode (near water line): O2 + 2H20 + 4e-   ---------->4OH-


3.Water line corrosion:
This is a case of differential aeration corrosion commonly observed in steel water tanks, ocean going ships etc. in which portion of the metal is always under water
The part of the metal below the water line is exposed to only to dissolved oxygen while the part above the water is exposed to higher concentration of oxygen.
Thus the metal part below the water line acts as anode where as above the waterline acts as cathode and process of corrosion starts. The metal just below the water line is more anodic and the creep (meniscus) is the one which is more oxygenated acts as cathode and unaffected. Amount of creep of the water determines the rate of corrosion, but mass intense corrosion occurring at the water line; there a brown line is also formed due to the deposition of corrosion products. This type of corrosion is commonly observed in ships floating in seawater for a long period of time.




4.PITTING CORROISON:
Pitting corrosion is a localized and accelerated corrosion. When a small particles of dust or water etc are get deposited on a metal (like steel). The portion covered by the dust will not be well-aerated area compared to the exposed surface hence the covered surface becomes anodic with respect to the surface exposed. In presence of an conducting medium (moisture) corrosion starts below the dust part and forming a pit. Once pit is formed the ratio of corrosion increases, because of the formation of smaller anodic and larger cathodic area intense corrosion takes place.
Pitting corrosion is one of the most destructive forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Because of the small sizes of the pits it is highly difficult to identify the pitting corrosion. Pitting corrosion is an autocatalytic process, and once the corrosion products arte formed, it further provides the condition for differential aeration below the corrosion product and surrounding metal parts. The pit grows and ultimately may cause failure of the metal.


5. STRESS CORROISON:
Stress corrosion of the metal formed by the combined effect of a tensile stress and a specific corrosive environment on the metal., during stress corrosion, the metal or alloy is virtually unaffected over most of its surface, while fine cracks progress through it normal to the direction of tensile stress.
The stress on the metal may be internal or external and these stress is due to some mechanical or service conditions. The metal atoms under stress are always at higher energy level so acts as anode and stress free parts of metal acts as cathode under specific corrosive environmental conditions corrosion process starts.
Egs: Brass undergoes corrosion in the presence of ammonia.
        Stainless steel in the presence of Cl- and caustics

But best example for stress corrosion is caustic embrittlement:

Caustic embrittlement: It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.
 The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.

                Na2CO3 + H2O---------------->2NaOH + CO2                  

The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate ( Na2FeO2). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows

     3Na2FeO2 + 4H2O---------------->6NaOH + Fe3O4 + H2

       6Na2FeO2 + 6H2O + O2--------------->12NaOH + 2Fe3O4

So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a required environment. Thus corrosion process develops cracks and making the metal brittle by the deposition of the product.
The corrosion cell can be represented as

 Fe (under stress) / conc. NaOH / dil. NaOH/ Fe (stress free)
      Anode                                                           Cathode

Caustic embrittlement can be prevented by the addition of compounds like sodium sulphite, tannin, lignin, phosphates etc. which blocks the cracks thereby preventing the infiltration of alkali.


This is enough for today's class, we will discuss Factors affecting rate of corrosion in our next class untill then, just go through everything and revise it twice or thrice

Unit 2- Class 2- CORROSION (Electrochemical theory of corrosion)

Electrochemical theory of corrosion:

Most of the corrosion takes place on the basis of electrochemical reactions on the surface of metal such a type of corrosion is known as wet corrosion.

Electrochemical theory of corrosion can be taking iron as an example.
When a metal like iron is exposed to the environment according to electrochemical theory corrosion of metal takes place due to the formation of anodic and cathodic regions on the same metal surface or when the two metals are in contact with each other in a corrosive medium.

These anodes and cathodes are formed due to the heterogeneities at the interfaces of the metal and environment. The heterogeneities on a metal surface could develop due to several factors like
1. On a metal surface if the concentration of the oxygen is different (if in the metal the area which is exposed to more oxygen acts as cathode, the area which is exposed to less oxygen concentration acts as anode).
2. Due to contact of two different metals (egs: if copper and iron are in contact with each other, then Fe acts as anode and copper acts as cathode due to change in electrode potential).
3. If metal surface subjected to stress (area under stress acts as anode).


Thus anodic and cathodic area are formed, in presence of corrosion medium ( like moisture etc.)
At anode oxidation takes place so that metal is converted into metal ions with the liberation of electrons.
M------>Mn+ + ne-

Egs: Fe------->Fe2+ + 2e-


At the cathodic regions, reduction takes place since the metal at cathodic region cannot be reduced further, so some constituents of the corrosive medium take part in the cathodic reaction. Since in the cathodic reaction as the constituents of the corrosion medium are involved, they are more complicated and dependent on the nature of environment. Most common type of cathodic reaction are 1. Liberation of hydrogen 2. Absorption of oxygen.

Let us end this discussion here and take up types of corrosion in next post

Unit 2- Class 1- CORROSION (Definition)

Welcome back friends today we will start with a new unit , that is corrosion.At first lets look at its definition.


Definition:
Corrosion is defined as “the destruction or deterioration and consequent loss of metals or alloys through chemical or electrochemical attack by the surrounding environment”.

In simple corrosion and metal extraction can be regarded as

                            Metal -------------> Metal ore



The primary factors that initiate corrosion on metals are atmospheric air, water and also conducting surface of the metal.
Egs: Rusting of iron, green scales are formed on copper vessels

Corrosion of metal occurs either by direct chemical attacks or by electrochemical attack on the metal by the corrosive environment.
If the corrosion takes place due to direct chemical attack (in the absence of moisture ) that type of corrosion is known as dry corrosion.
If the corrosion of metal takes place due to electrochemical attack in presence of moisture or a conducting medium such corrosion is known as wet corrosion or electrochemical corrosion.


let us discuss about dry corrosion and electrochemical corrosion in next post

Sunday, June 20, 2010

Unit 1- Class 7- Liquid Crystals (Review Questions)

1> What are liquid crystals? Discuss their classification in brief.
2>Define nematic liquid crystal.
3>Discuss about the various effects on liquid crystal.
4> What are the various applications of liquid crystals?
5>How would liquid crystals revolutionize the display systems?

Unit 1- Class 6- Liquid Crystals (Application of liquid crystals)

Applications of liquid crystals

(i) DISPLAYS
All information displays utilizes the ability to control light, in order to function. By controlling what parts of a display are bright and what parts are dark, information is passed to the user. Regardless of the complexity of the display, the basic working principle remains the control of light from small area of the display. This can be done in two ways- active displays and passive displays

Active display
• Each area is equipped with the ability to generate light
• Different areas are made to produce light by hitting only these areas with the electron beam
E.g.: cathode ray tube (CRT), light emitting diode (LED)

Passive display
• Does not generate light by itself
• Controls the amount of light that passes through
• Utilizes ambient light
• Do not consume electrical power in order to generate light
• Ideal for battery operated equipment
E.g.: liquid crystal display (LCD)

In LCDs in order to enhance the difference in the brightness between dark (turned off) and bright (turned on) areas. Dichoric dyes are used to give desired coloures to the display with a good contrast



(ii) THERMOGRAPHY
Liquid crystals can be used to measure temperature utilizing the selective reflection property of chiral nematic liquid crystals when a light is incident on a cholesteric mesophase parallel to the axis of rotation of the helix, radiation of very small wavelength range corresponding .
gets divided into two beams one of the beam is reflected and the other is transmitted. If the wavelength range is in the visible region, the reflection give rise to light colours. The and hence the colour of the reflected light depends on the temperature of the mesophase. Temperature dependence of the colour reflected by liquid crystals has been utilized for application in thermography.



(iii) DETECTION OF AIR POLLUTANTS.
It has been observed that the colour of liquid crystals changes in the presence of impurities. When impurities diffuse in to the cholesteric liquid crystal film, the pitch is altered and hence the colour changes. This property is used in the detection of contaminants in the atmosphere.



(iv) Solitary wave propagation
A high intensity laser beam injected in a liquid crystal can produce a local reorientation of the director molecules. In this way the light produces its own wave-guide and the laser beam will not diffract but stays confined in a narrow beam. The soliton application can lead to an addressable liquid crystal wave-guide to switch light between several optical fibers.

Unit 1- Class 5- Liquid Crystals (Effects on liquid crystals)

ElelctroOptical effects in liquid crystals
Nematic liquid crystals are the simplest forms with rod like molecular structure and align themselves spontaneously along the director.
Dielectric anisotropy ( defined as the difference between the dielectric constant parallel and perpendicular to the director. The optical anisotropy ( is defined as the difference between the refractive indices parallel and perpendicular to the director. These two properties are important for the electro-optic effects in liquid crystals.


Effects of electric fields
The director in a liquid crystal is free to point in any direction. But when a film of liquid crystals is placed between two plates of certain materials, the director is forced to point along a perpendicular direction. When a thin film of liquid crystal is sandwiched between two glass plates, the molecules close to the glass surface are forced to orient themselves parallel to the surface of the glass sheets.
In the absence of an electric field (below a threshold value) the directors at other layers are also aligned parallel to the surfaces giving a homogeneous texture. But when an electric field is applied perpendicular to the molecules near the center of the crystal orient themselves along the applied field. The deformity begins at a threshold vale of the strength of the applied field and increases with increase in the strength of the field. This deformity brings about a significant change in the optical characteristics of the liquid crystals and are made use in liquid crystal displays


Effects of light
When light is incident on two crossed polarizes no light emerges as the light coming out of the first polarizer is completely absorbed by the second polarizer and hence appears dark. When a pair of crossed polarizers is filled with a twisted nematic liquid crystal having a positive dielectric anisotropy, the twisted structure acts like a wave-guide and gradually rotates the plane of polarization of light by 90o . Hence a linearly polarized light incident on the cell emerges linearly polarized but in an orthogonal direction resulting in a bright appearance. The 90o twist in the cell is lost when a sufficiently strong electric field is applied to the cell. Hence the cell appears to dark between the two crossed polarizers. However the following conditions should be met to see the elelctrooptic effects
(i) The plane of polarization of the incident light should be parallel or perpendicular at the surface of the cell
(ii) The product of the optical anisotropy, and the pitch P should be greater than the wavelength of the incident light

in the next class we will discuss the applications of liquid crystals.

Unit 1- Class 4- Liquid Crystals (Classification of liquid crystals)

Discotic or columnar liquid phases
Liquid crystals formed by molecules, which have disc like or plate like structures are classified as discotic liquid crystals. The simplest discotic phase is also called the nematic discotic phase because there is oreintational order, but they lack positional order. There is random motion of the molecules but on an average the axis perpendicular to the plane of each molecule tend to orient along the director. In smectic discotic or columnar phase, in addition to the oreintational order present, most of the molecules tend to position themselves in columns. The columns are arranged in a hexagonal lattice resembling a set .


Chiral smectic
In a similar way to chiral nematics there are chiral forms of smectic phases. The tilted director rotates from layer to layer forming a helical structure.


Banana shaped liquid crystals
Apart from the cigar shaped molecules some more exotic shaped liquid crystals are also reported. Below figure illustrates the stacking of banana shaped liquid crystals.


In the next class we will discuss about various effects on liquid crystals

Unit 1- Class 3- Liquid Crystals (Classification of liquid crystals)

The nematic phase
They exhibit long range molecular ordering but possess no positional ordering. The preferred direction varies from point to point in the medium, but there is a uniform alignment with respect to the long axis- director. The nematic phase is bifringent due to the anisotropic nature. They also exhibit dielectric anisotropy.


The cholesteric phase
In this structure the local molecular ordering is identical to that of the nematic phase, but are exhibited my molecules possessing chiral centers. Chiral nematic liquid crystals are also refereed as twisted nematic liquid crystals. Unlike in nematic phase where all the molecules stay parallel to one another, in chiral nematic phase, the molecules arrange themselves in such a way that a group of molecules align at different angles with respect to their adjacent groups i.e., the director is not fixed in space, but rotates through out the sample forming a helical pattern as it changes its direction. The distance traveled by the director as it completes one full turn is called the pitch of the liquid crystal. The twist present in chiral nematic crystal makes them to exhibit spectacular optical properties, which is made use of in displays. The most striking features of cholesteric mesophase is strong optical activity and selective light reflection. The pitch is temperature dependent and hence cholesteric phase is finding application in thermography.



The nematic phase
Substances that form smectic phases are soap-like. In smectic mesophase, there is a small amount of orientational order and also a small amount of positional order. The molecules tend to point along the director and arrange themselves in layers. Based on the orientation of the director there are many types of smectic phases. The interlayer force of attraction in smectic phase is weak as compared with the lateral forces between molecules. As a consequence the layers are able to slide over one another.

in the next class we will discuss about d or columnar liquid phases

Unit 1- Class 2- Liquid Crystals (Classification of liquid crystals)

Classification of liquid crystals


Liquid crystals are classified into two main categories:
1. Thermotropic liquid crystals
2. Lyotropic liquid crystals


Thermotropic liquid crystals are those, which exhibit liquid crystalline properties as the temperature is varied. The liquid crystal to liquid transition and liquid-to-liquid crystal transitions of a mesogenic material are essentially reversible. The mesophase which are formed by both the heating and cooling cycles are thermodynamically stable are called enatiotropic phases. But in some compounds, their thermotropic phases appears only during the cooling phase from the isotropic phase, but not on the heating process. These mesophases, formed by the super cooling of the material below its melting point, are meta-stable and are called monotropic phases.


Thermotropic liquid crystals can exist in three phases
  1. Nematic phase
  2. Cholesteric or chiral nematic phase
  3. Smectic phase
This will be discussed in our next class

Unit 1- Class 1- Liquid Crystals (What are liquid crystals?)

What are liquid crystals? 
There are three common states of matter, solid, liquid and gas- are different because the molecules in each state have a different degree of order.

Crystalline solids:
  1. Possess rigid arrangement of molecules
  2. Stay in a fixed position and orientation with small amount of variation from molecular vibrations
  3. There are large forces holding the molecules in place
  4. Solids are difficult to deform
Liquid phase:
  1. Molecules lack both position or orientation
  2. They are free to move in a random fashion
  3. Liquid phase has less order than the solid phase
  4. The intermolecular forces are only strong enough to keep the liquid molecules fairly close together
  5. Liquids can be easily deformed


Liquid crystals
A liquid crystal is a fluid like a liquid, but is anisotropic in its optical and electromagnetic characteristics like a solid. When the liquid crystal is formed from the isotropic state some amount of positional or orientation order is gained. It is this order that accounts for the anisotropies of the substance.

Thus liquid crystal may be described as the distinct states of matter in which the degrees of molecular ordering lie intermediate between the ordered crystalline state and the completely disordered isotropic liquid.

In a liquid crystal the molecules possess orientation order, i.e., the molecules tend remain oriented in a particular direction. The direction of preferred orientation in a liquid crystal is called the director. In a liquid crystal phase they spend more time along the director than in any other direction

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