Polymer Science and Technology (CLPE14) Part-I Materials PDF

Polymer Science and Technology (CLPE14) Part – I Materials B.Tech. (Chemical Engineering) III Semester July 2023 Session  Introduction The polymers are the materials of choice now & one of the bigge...

Polymer Science and Technology (CLPE14) Part – I Materials B.Tech. (Chemical Engineering) III Semester July 2023 Session  Introduction The polymers are the materials of choice now & one of the biggest success stories in new materials development over the last century. Polymers are increasingly replacing & has also replaced conventional materials like wood, metals, stone or ceramics in several applications. Medical applications: plenty of applications products are made of polymers, which includes some of the applications where the polymers are not visible from outside. For example, a disposable or a biodegradable suture which you are using for, you are tying, stitching during operations, artificial heart valve, contact lenses, display devices, etc.  Introduction Why polymers are getting importance? Requirement of materials applications: should have high strength to bear large amount of load should be resilient (comfort and soft feeling) should be transparent should be low cost Conventional materials like metal, ceramics, glass, wood, etc couldn’t satisfy these properties. Glass are brittle, Metals are not transparent, have corrosion issues & are heavy.  Introduction Attractive properties of polymers: Durable: resistant to hydrolysis & are stable Light in weight: maximum strength versus weight ratio Design Flexibility: High freedom of design & styling Good thermal & electrical insulators Feedstock flexibility: Petroleum fractions Natural gas Coal Bio-mass  Polymer Synthesis ❑ Carothers (1929) polymer classification: Condensation polymers Addition polymers ❑ Condensation polymers Those polymers in which the molecular formula of the repeat unit of the polymer chain lacks certain atoms present in the monomer from which it is formed ❑ Addition polymers Those polymers in which the loss of small molecule does not take place.  Polymer Synthesis ❑ Polymers are formed by two major kinetic schemes (amended by Flory): 1.Step polymerization (Condensation polymers) 2.Chain polymerization (Addition polymers)  Polymer Synthesis Mechanism of Stepwise Polymerization ❑ Type of products formed is determined by the functionality of the monomers (by the average number of reactive functional groups per monomer molecule) Monofunctional monomers give only low molecular weight products Bifunctional monomers give linear polymers Polyfunctional monomers give branched or cross linked polymers Carbonyl addition – Elimination reaction Most important reaction The addition & elimination at the carbonyl double bond of carboxylic acids and their derivatives ❖ R & R’ – alkyl or aryl groups ❖ X – OH, OR’, NH2, NHR’, OCR’ or Cl ❖ Y – R’O-, R’OH, R’ NH2, or R’COO-  Polymer Synthesis Carbonyl addition – Elimination reaction Direct addition Example: dibasic acid & glycol to form polyester, dibasic acid & diamine to form polyamide Glycol and Ester Reaction Acid Chloride or Anhydride Acid chloride or Anhydride can be reacted with glycol or amine to give polymer Glycol and phthalic anhydride Reaction  Polymer Synthesis Carbonyl addition – Elimination reaction Interfacial condensation Reaction carried out at the interface between two liquid phases Acid halide with glycol or diamine leads to high molecular weight polymer Polyamides, polyurethanes, polyureas, polysulfonamides, etc. Useful for preparing polymes that are unstable at higher temperatures Cyclic product formation Bifunctional monomers may react intramolecularly to produce a cyclic product If the ring contains less than five atoms or more than seven, the product is usually linear polymer.  Polymer Synthesis Chain Polymerization Polymerization of unsaturated monomers into chain reaction Initiated by simple gas phase Susceptible to retardation and inhibition Act of initiation lead to thousands of monomer molecules Chain growth is associated with a single polymer molecule through the addition of many monomer units No intermediate species between monomer & high-molecular-weight polymer are found of several postulated types of active center, three have been found experimentally: cation, anion, and free radical The carbon--carbon double bond is, because of its relatively low stability, particularly susceptible to attack by a free radical The reaction of the double bond with a radical proceeds well for compounds of the type CH2=CHX and CH2=CXY, called vinyl monomers (Monomers in which fluorine is substituted for hydrogen may be included)  Polymer Synthesis Chain Polymerization ❑ Generation of Free Radicals Stability of the radicals vary Primary radicals are less stable, more reactive than secondary radicals Secondary radicals are less stable than tertiary radicals such as the triphenylmethyl, can be isolated in the solid state without decomposition ) The phenyl radical is more reactive than the benzoyl radical the allyl radical is quite unreactive  Polymer Synthesis Chain Polymerization ❑ Mechanism of Vinyl Polymerization High-energy radiation from electrons, gamma rays, x-rays, and slow neutrons are effective in producing free radicals that can initiate polymerization (Dole 1972, Wilson 1974). The reactions involved are varied and nonspecific due to gross damage to molecular structures and transfer of large amounts of energy. Monomers can be polymerized in the solid as well as the liquid and gaseous states (Eastmond 1970). Radicals are also generated as a result of oxidation-reduction reactions (redox initiation).  Polymer Synthesis Chain Polymerization ❑ Step 1: Initiation When free radicals are generated in the presence of a vinyly monomer, the radical adds to the double bond with the regeneration of another radical. If the radical formed by decomposition of the initiator I is designated by R ❑ Step 2: Propagation The chain radical formed in the initiation step is capable of adding successive monomers to propagate the chain  Polymer Synthesis Chain Polymerization ❑ Step 3: Termination Propagation would continue until the supply of monomer was exhausted were it not for the strong tendency of radicals to react in pairs to form a paired-electron covalent bond with loss of radical activity. This tendency is compensated for in radical polymerization by the small concentration of radical species compared to monomers The termination step can take place in two ways: combination or coupling  Polymer Synthesis Chain Polymerization ❑ Step 4: Radical-Molecule Reactions Although the three steps of initiation, propagation, and termination are both necessary and sufficient for chain polymerization, reaction between a radical and a molecule are classified. Chain Transfer It was recognized by Flory (1937) The reactivity of a radical could be transferred to another species, which would usually be capable of continuing the chain reaction. The reaction involves the transfer of an atom between the radical and the molecule. If the molecule is saturated, like a solvent or other additive, the atom must be transferred to the radical  Polymer Synthesis Chain Polymerization ❑ Inhibition and Retardation A retarder is defined as a substance that can react with a radical to form products incapable of adding monomer. If the retarder is very effective, no polymer may be formed which is called as inhibition and the substance involved is inhibitor. The action of a retarder is two fold: It reduces concentration of radicals and shortens their average lifetime  Polymer Synthesis Chain Polymerization ❑ Mechanism of Vinyl Polymerization ❑ Rate of Polymerization Chemical method or isolation and weighing of polymers can be used to measure the rate of polymerization Evacuated and sealed system are used to avoid extraneous effects due to oxygen presence Polymerization rate is followed through changes in a physical property of the system, such as density, refractive index, or ultraviolet or infrared absorption The experiments are usually carried out in region below 5% polymer to avoid deviations from constant polymerization rate due to depletion of reactants and other factors. The dilatometer is placed in a constant-temperature bath to ensure isothermal conditions throughout the experiment  Polymer Synthesis Ionic and co-ordination polymerization 1. Cationic polymerization 2. Anionic polymerization Strength of acid-base electron donating groups and low dielectric constant in media Mechanisms involving coordination compounds among the monomer, the growing chain, and a catalyst, usually a solid Reaction systems are often heterogeneous, involving inorganic catalysts and organic monomers High rate of reaction may lead to form very-high-molecular-weight polymer  Polymer Synthesis Kinetics of ionic polymerization Formation of initiator: The added substance reacts with the monomer or with the solvent to produce the initiating species. Initiation – Propagation – termination Cationic and anionic polymerization Cationic polymerization: Mechanism: ✓Aprotonic acids (Lewis acids, Friedel-Crafts halides) A- catalyst ✓Protonic acids (Bronsted acids) and RH-Co-catalyst ✓Stable carbenium ion salts. Anionic polymerization: Mechanism: First anionic reaction was the D- Donar M- Monomer polymerization of methacrylonitrile by sodium in liquid ammonia at 75°C  Polymer Synthesis Co-ordination polymerization Coordination polymerization is far from simple in terms of mechanism, kinetics, or application. The same catalysts can also initiate polymerization by cationic, anionic, or even free-radical mechanism. Ziegler - Natta Catalysts: Cyclopentadienyl titanium (IV) dichloride (Cp2TiCl2), Aluminium alkly’s – dichloroethyl aluminium (Al(C2H5)2Cl2). - Transition metal AR - Metal alkyl M - Monomer Overall rate Initiation Propagation Transfer with monomer  Polymer Synthesis Ring - Opening Polymerization The addition of monomer (but never of larger units) to growing chain molecules that are not chain reactions in the kinetic sense. As in stepwise polymerization, it continues to increase in molecular weight throughout the reaction. Most of these cyclic compounds polymerize by ionic mechanisms in the presence of strong acids or bases (Exclude water and alcohols).  Polymer Synthesis Co - Polymerization Simultaneous polymerization of two or more Mechanism of copolymerization monomer (Dostal, 1936) Ex., Olefins and diolefins (homopolymers), Vinyl chloride-vinyl acetate (Staudinger,1939) Monomer reactivity ratio: (𝑟1 /𝑟2 ) The rates of disappearance The rate const. for radical adding its own monomer The rate constant for its adding the other monomer If 𝑟1 >1, 𝑀1. is to be added. If 𝑟1