Classification of Polymers

classification of polymers based on molecular forces. structure and classification of polymers based on end use application. lecture notes polymer chemistry pdf free
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Published Date:21-07-2017
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1. POLYMERS 1.1 GENERAL INTRODUCTION AND ITS SCOPE Polymers form a very important class of materials without which the life seems very difficult. They are all around us in everyday use; in rubber, in plastic, in resins, and in adhesives and adhesives tapes. The word polymer is derived from greek words, poly= many and mers= parts or units of high molecular mass each molecule of which consist of a very large number of single structural units joined together in a regular manner. In other words polymers are giant molecules of high molecular weight, called macromolecules, which are build up by linking together of a large number of small molecules, called monomers. The reaction by which the monomers combine to form polymer is known as polymerization 1. The polymerization is a chemical reaction in which two or more substances combine together with or without evolution of anything like water, heat or any other solvents to form a molecule of high molecular weight. The product is called polymer and the starting material is called monomer. 1.2 HISTORICAL DEVELOPMENT OF POLYMERS Polymers have existed in natural form since life began and those such as DNA, RNA, proteins and polysaccharides play crucial roles in plant and animal life. From the earliest times, man has exploited naturally-occurring polymers as materials for providing clothing, decoration, shelter, tools, weapons, writing materials and other requirements. However, the origin of today’s polymer industry is commonly accepted as being the nineteenth century when important discoveries were made concerning the modification of certain natural polymers. In eighteenth century, Thomas Hancock gave an idea of modification of natural rubber through blending with ceatrain additives. Later on, Charles Goodyear improved the properties of natural rubber through vulcanization process with sulfur. The Bakelite was the first synthetic polymer produced in 1909 and was soon followed by the synthetic fiber, rayon, which was developed in 1911. The systematic study of polymer science started only about a century back with the pioneering work of Herman Staudinger. Staudinger has given a 1 Page new definition of polymer. He in1919 first published this concept that high molecular mass compounds were composed of long covalently bonded molecules. 1.3 CLASSIFICATION OF POLYMERS Polymer is a generic name given to a vast number of materials of high molecular weight. These materials exist in countless form and numbers because of very large number and type of atoms present in their molecule. Polymer can have different chemical structure, physical properties, mechanical behavior, thermal characteristics, etc., and on the basis of these properties polymer can be classified in different ways, which are summarized in Table 1.1, whereas, important and broad classification of polymers are described in the next section. Table1.1: Classification of Polymers Basis of Classification Polymer Type Origin - Natural, Semi synthetic, Synthetic Thermal Response - Thermoplastic, Thermosetting Mode of formation - Addition, Condensation Line structure - Linear, Branched, Cross-linked Application and Physical - Rubber, Plastic, Fibers Properties Tacticity -Isotactic, Syndiotactic, Atactic Non crystalline(amorphous), Semi-crystalline, Crystallinity - Crystalline Polarity - Polar, Non polar Chain -Hetro, Homo-chain 1.3.1 Origin On the basis of their occurrence in nature, polymers have been classified in three types 2:- A. Natural polymer:- The polymers, which occur in nature are called natural polymer also known as biopolymers. Examples of such polymers are natural rubber, natural silk, cellulose, starch, proteins, etc.. 2 Page B. Semi synthetic polymer:- They are the chemically modified natural polymers such as hydrogenated, natural rubber, cellulosic, cellulose nitrate, methyl cellulose, etc. C. Synthetic polymer:- The polymer which has been synthesized in the laboratory is known as synthetic polymer. These are also known as manmade polymers. Examples of such polymers are polyvinyl alcohol, polyethylene, polystyrene, polysulfone, etc.. 1.3.2 Thermal Response On the basis of thermal response, polymers can be classified into two groups 3:- A. Thermoplastic polymers:- They can be softened or plasticized repeatedly on application of thermal energy, without much change in properties if treated with certain precautions. Example of such polymers are Polyolefins, nylons, linear polyesters and polyethers, PVC, sealing wax etc.. B. Thermosetting polymers:- Some polymers undergo certain chemical changes on heating and convert themselves into an infusible mass. The curing or setting process involves chemical reaction leading to further growth and cross linking of the polymer chain molecules and producing giant molecules. For example, Phenolic, resins, urea, epoxy resins, diene rubbers, etc. 1.3.3 Mode of Formation On the basis of mode of formation, polymers can be classified as 2:- A. Addition polymers:- They are formed from olefinic, diolefnic, vinyl and related monomers. They are formed from simple addition of monomer molecules to each other in a quick succession by a chain mechanism. This process is called addition polymerization. Examples of such polymers are polyethylene, polypropylene, polystyrene. 3 Page B. Conden nsation poly ymer:- The ey are forme ed from inte ermolecular r reactions b between bifunctio onal or po olyfunctiona al monome er molecule es having rreactive fu unctional groups s such as –OH H, -COOH, -NH , -NCO O, etc.. 2 1.3.4 Lin ne Structure e On the basi is of structu ure, polymer rs are of thr ree types. A. Linear p polymer:- If the mono omer units are joined iin a linear f fashion, polymer is said to b be linear pol lymer. Linear Hom mopolymer Linear Cop polymer B. Branche ed polymer r:- When m monomer un nits are join ned in bran nched mann ner, it is called br ranched pol lymer. Bra anched Hom mopolymer B Branched Co opolymer C. Cross li inked polymer:- A po olymer is s said to be a a cross link ked polyme er, if the monome er units are j joined togetther in a chain fashion. Cross linke ed Homopol lymer Cross linke ed Copolym mer 4 Page 1.3.5 App plication an nd Physicall Propertie es Dep pending on its ultimate form and u use a polymer can be cllassified as 2:- A. Rubber r (Elastome ers):- Rubb ber is high h molecula ar weight p polymer wi ith long flexible chains and d weak inter rmolecular forces. Th hey exhibits s tensile strength in the rang ge of 300-30 000 psi and d elongation n at break r ranging bettween 300-1000% . Example es are natural and synth hetic rubber r. B. Plastics:- Plastics a are relativelly tough substances wiith high mo olecular wei ight that can be m molded with h (or witho out) the app plication of heat. These e are usuall ly much stronger r than rubbe ers. They ex xhibit tensi ile strength ranging be etween 4000-15000 psi and elongation at break ran nging usual lly from 20 0 to 200% o or even high her. The example es of plastics are, polye ethylene, po olypropylene e, PVC, pollystyrene, et tc. C. Fibers:- - Fibers ar re long- ch hain polym mers characterized by highly cry ystalline regions r resulting m mainly from secondary f forces. They y have a mu uch lower e elasticity than plastics and elastomers. T They also ha ave high ten nsile streng gth ranging b between 20,000- 150,000 psi., are light weight and d possess mo oisture abso orption prop perties. 1.3.6 Tac cticity:- It m may be d defined as the geom metric arran ngement (o orientation) of the characterist tic group of f monomer u unit with re espect to the e main chain n (backbone) of the polymers. O On the basis s of structur re, polymer may be cla assified into three group ps:- A. Isotactic c polymer:- It is the ttype of poly ymer in whiich the char racteristic group are arranged d on the sam me side of th he main cha ain. Is sotactic Pol lypropene 5 Page B. Syndiot tactic polym mer:- A po olymer is s said to be s syndiotactic c if the sid de group (characteristic group) are arran nged in an a alternate fash hion. Sy yndiotatic Po olypropene C. Atactic polymer:- A polymer is said to b be atactic, if f the charactteristic grou ups (side group) a are arranged d in irregula ar fashion (r randomness) around the e main chai in. It has proper strength and d more elastiicity. A Atactic Poly ypropene 1.4 BIOCO OMPOSIT TES Com mposite are attractive m materials be ecause they combine m ma aterial prop perties in ways not fo ound in natu ure. Such m materials oft ten result in n lightweigh ht structures s having high stiffne ess and tailo ored proper rties for spe ecific applic cations, the erreby saving g weight and reducing energy needs 4-6 6. Fiber-re einforced pllastic comp posites beg gan with cellulose f fiber in phe enolics in 1908, later r extending g to urea a and melami ine, and reaching co ommodity s status in th he 1940s w with glass fiber in unsa aturated polyesters. From guita ars, tennis ra acquets, and d cars to mi icrolight air rcrafts, elec ctronic comp ponents, and artificia al joints, co omposites ar re finding u use in divers se fields. Com mposite ma aterials deriived from b biopolymer and synth hetic fibers such as glass and c carbon also come unde er biocompo osites. Biocomposites d derived from m plant- derived fibe er (natural/b biofiber) an nd crop/biod derived plas stic (biopoly ymer/biopla astic) are 6 Page likely to be more ecofriendly, and such biocomposites are sometimes termed “green composites” 4. 1.4.1 Natural/Biofibers as Reinforcements in Biocomposites The world’s supply of natural resources is decreasing and the demand for sustainable and renewable raw materials continues to rise. Biofiber-reinforced composites represent a potential nontraditional, value-added source of income to the agricultural community. Jute is from India and Bangladesh; coir is produced in the tropical countries of the world, with India accounting for 20% of the total world production; sisal is also widely grown in tropical countries of Africa, the West Indies, and the Far East, with Tanzania and Brazil being the two main producing countries; kenaf is grown commercially in the United States; flax is a commodity crop grown in the European Union as well as in many diverse agricultural systems and environment throughout the world, including Canada, Argentina, India, and Russia. Flax fiber accounts for less than 2% of world consumption of apparel and industrial textiles, despite the fact that it has a number of unique and beneficial properties. Hemp originated in Central Asia, from which it spread to China, and is now cultivated in many countries in the temperate zone. Ramie fibers are the longest and one of the strongest fine textile fibers mostly available and used in China, Japan, and Malaysia. Most of the polymers by themselves are not suitable for load-bearing applications due to their lack of sufficient strength, stiffness, and dimensional stability. However, fibers possess high strength and stiffness. Unfortunately, they are not suitable for use in load-bearing applications by themselves because of their fibrous structure. In fiber-reinforced composites, the fibers serve as reinforcement by giving strength and stiffness to the structure while the plastic matrix serves as the adhesive to hold the fibers in place so that suitable structural components can be made. A broad classification (nonwood and wood fibers) of natural fibers is represented schematically in Fig. 1.1. Currently several nonwood fibers (e.g., hemp, kenaf, flax, and sisal) are being utilized commercially in biocomposites in combination with polypropylene for 7 Page automotive e application ns. Now fro om need of f society an nd research point of vi iew it is much impo ortant to wor rk on leaf b based nonwo ood fibers 4 4. Fig 1.1: Sc chematic re epresentatio on of reinfo orcing natu ural/biofibe ers classific cation 4 1.5 MECH HANISM OF POLYM MERIZATIO ON The e linking together of f a large n number of small mo olecules ter rmed as monomers with each h other to fo orm a macr romolecule or polymer r molecule through chemical r reactions is s termed as s polymerization. It can also be defined d as the fundamenta al process b by which lo ow molecul lar weight c compounds are conver rted into high molec cular weigh ht compoun nds. In addi ition to the e structural and compo ositional differences between p polymers Flory stresse ed the very y significan nt difference in the mechanism m by which polymer m molecules ar re build up.. Although Flory conti inued to use the te erms "addi ition polym merization" (polymerization by repeated addition processes) and "co ondensation polymeri ization" (p polymerizattion by r repeated condensatio on processes, i. e., w with the e elimination of small molecules) ) in his discussion of polym merization mechanism m. The cu urrent term minology c classifies polymeriza ation into ste ep growth p polymerizati ion and cha ain growth p polymerizat tion 7. The e degree of f polymeriz zation is related to the e molecula arr mass (M) of the polymer an nd is given b by the equattion, 8 Page 1.1 where, m is the mass of the monomeric unit. 1.5.1 Condensation Polymerization or Step-growth Polymerization Condensation Polymerization is a chemical reaction in which polymer is formed and a small molecule of by-product with a lower molecular weight is released. The by-product eliminated is called as condensate. The reaction can take place between two similar or different monomers. It is also called as step-growth polymerization 8-9. 1.5.2 Addition Polymerization or Chain Polymerization In addition polymerization, two or more molecules of monomers attach together to form a polymer. In this polymerization, there is no elimination of any molecule. It is a chain reaction and no by product is released. It is obtained by linking together the monomer molecules by a chain reaction to give a polymer whose molecular weight is exactly an integral multiple of that of the monomer as in the case of polyethylene obtained by polymerization of ethylene. Only a single monomer is involved in addition polymerization and hence the polymer is homopolymer and contains the same monomer units. Addition polymerization reaction is usually induced by light, heat or a catalyst for opening the double bond of the monomer and creating the reactive sites 9. 1.6 CHARACTERIZATION OF POLYMERS It is comparable to the synthesis of organic compound, composites, biocomposites without a subsequent characterization of its various properties Synthesized material characterization is therefore of very great importance. Some of the important aspects related to characterizations have been described in this section. 9 Page 1.6.1 Molecular Weight and its Distribution The molecular weight of a polymer is of prime importance in the polymer’s synthesis and application. It is important because it determines many physical properties. The term molecular weight is a ratio of the average mass per formula unit 12 of substance to 1/12 th of an atom of C. Molecular weight (MW) and its distribution (MWD) has a considerable effect on macroscopic properties of polymer such as toughness, tensile strength, adherence and environmental resistance, etc. 10. A simple chemical has fixed molecular weight but when we discuss about the molecular weight of polymer, we mean something different from that which applies to small sized compounds. Since polymers are mixture of molecules of different molecular weight, the molecular weight is expressed in the term of “average” value. This average molecular weight is basically based on either average number of repeating units, known as number average molecular weight, or average weight, known as weight average molecular weight. The number average molecular weight is obtained from the number n of macromolecules for each degree of polymerization P by taking i i for each degree of polymerization the product of the number of polymer molecules and their degree of polymerization n p and dividing the sum of these product by the i i total number of monomers N : o  n p  i i i1 M n N o 1.2 The weight average molecular weight is obtained in a similar manner M w from mass m of each degree of polymerization P according to the following i i equation:  m p  i i i1 M w M o 1.3 When the molecular weight distribution is very narrow, the number average and weight average molecular weights are essentially equal. When the distribution is broad, the weight average molecular weight is considerably greater than the number 10 Page average mo olecular we eight and b broader the e distributio on, the gre eater the di ifference between th hem as show wn in Figur re 1.2. From m these mollecular aver rages the m molecular M w weight distr ribution is r represented by their rat tio M n Fig1.2: Sc chematic re epresentatiion of the weight av verage and d number average mol lecular w weight dist tribution as a fun nction of the deg gree of poly ymerization 11. Ten nsile and im mpact strenngths incre ease with molecular weight. Th he melt viscosity of the polym mer, howeve er, shows a a different ttrend. At ve ery high m molecular weights, th he melt viscosity risees more st teeply than n at low m molecular w weights. Molecular weight distribution allso affects properties of polymer rs. To kno ow of a polymer pr roperly, we must have e a good kn nowledge o of both the average m molecular weight as w well as its di ispersion pa atterns. 1.6.2 Cry ystallization n Cry ystallization in polymer r has alway ys been the subject of great scient tific and academic i interest, since polyme ers are kno own to exh hibit a varie ety of struc ctures at various len ngth scales, such as un nit cell, lam mella, and s spherulites. It is an int teresting property re elated to ph hase transittion which determines the final p properties o of many technologic cally releva ant and sc cientifically y exciting systems 12, 13. P Polymer 11 Page crystallization controls the structural formation process of polymeric materials and thereby dominates the properties of final polymer product. Polymer crystallization is usually divided into two separate processes : primary nucleation and crystal growth 14. In general, a crystalline phase must primarily nucleate. The fundamental kinetics of nucleation are often complicated to determine because the rate of heterogeneous nucleation at defects, impurities, and surfaces is much faster than the homogeneous rate of nucleation within the pure bulk liquid. After a nucleus forms, the kinetics of crystal growth determines the overall rate of the phase transformation. For growth from solid or liquid solutions, the rate limiting step in the kinetics is often mass diffusion. For crystal growth from a melt, heat transfer plays an important role for metals, ceramics, and semiconductors, but for large molecules and polymers, the microscopic kinetics associated with attaching a large molecule or polymer segment to the growing crystal are typically the most important consideration 14. The crystalline and amorphous components influence polymer properties. Actually, crystallinity of a polymer sample is expressed in terms of that fraction of the sample which is crystalline. The definition of degree of crystallinity is, of course, based on the premise that crystalline and non-crystalline components of a polymer can co- exist''. The highest crystallinity is generally associated with polymers which have a simple unit cell structure and a relatively high degree of molecular order. The overall property (Q) of a partially crystalline polymer can be expressed as a sum of its two components as: 1.4 where Q and Q are contributions of the crystalline and amorphous components of c a the sample, respectively. A number of methods can be used to detect crystallinity and estimate its degree: X-ray diffraction, infrared-absorption spectroscopy, polarized light microscopy, density, differential thermal analysis and nuclear magnetic resonance spectroscopy, etc.. Accurate and undisputed measurement of the volume fraction of crystallinity in a polymer is not easily accomplished because each of the method of measurement mentioned above is concerned with a different physical aspect of material. Nevertheless, the great practical consequences of crystallinity and 12 Page orientation on mechanical properties dictate that at least relative changes in these factors be observed by whatever means are applicable, and that these be correlated with changes in processing and fabrication methods and with end-use behaviour. It was found that the preparation method of samples influence the morphology and crystallization behaviour of blends. 1.6.3 Glass Transition Temperature In the study of polymers and their applications, it is important to understand the concept of the glass transition temperature Tg. The glass transition is a phenomenon observed in linear amorphous polymer. It occurs at fairly well defined temperature when the bulk material ceases to be brittle and glassy in character and become less rigid and more rubbery. The knowledge of Tg is essential in the selection of materials for various applications. Many Physical properties change profoundly at the glass transition temperature, including mechanical properties and electrical properties. All of these are dependent on the relative degree of freedom for molecular motion within a given polymeric material and each can be used to monitor the point at which the glass transition occurs 15-17. 1.7 MECHANICAL PROPERTIES OF POLYMERS To study the necessary set of valuable properties, polymers differing from one another by their chemical structure and properties are usually mixed together, either homogeneously or heterogeneously. The mechanical properties of inhomogeneous mixture are worse than those of individual polymers, while the mechanical properties of homogeneous mixture are good. Polymers can exhibit the features of glassy, brittle solid or an elastic rubber, or that of a viscous liquid, depending on the temperature and time scale of measurement. The studies on mechanical properties of polymer can, therefore, be carried out by subjecting them to some form of mechanical stress either continuous or in a periodic manner at different rate. Some of the important properties of polymers with regard to their use as engineering material are tensile strength, compressive and flexural strength, hardness, creep, fatigue resistance and impact resistance. 13 Page Toughness of a polymer is the ability to absorb mechanical energy without fracturing. The property such as tensile strength is the maximum amount of tensile load per unit area a material can withstand, while the tensile elongation gives the measure of increase in length in response to a tensile load expressed as a percent of the original length. Elongation at break is the maximum elongation the plastic can undergo. Engineering applications of polymers are governed to a great extent by strain hardening considerations. The designer using polymeric materials must, therefore, understand their mechanical behaviour with respect to the maximum permissible strains to avoid failure. As for most materials, a simple tensile stress-strain curve provides a good start towards understanding the mechanical behaviour of a particular polymer. This curve is usually established by continuously measuring the force developed as the sample is elongated at constant rate of extension until it breaks. Portions of the curve in Fig.1.3 represent the stress-strain behaviour of any polymer and are used to define several useful quantities. The initial slope provides a value for Young's modulus (or the modulus of elasticity) which is a measure of stiffness. The curve also gives yield stress, strength and elongation at break. The area under the curve or work to break is a rough indication of the toughness of the polymeric material. The stress at the knee in the curve (known as the yield point) is a measure of the strength of the material and resistance to permanent deformation. The stress at the breaking point, commonly known as ultimate strength, is a measure of the force required to fracture the material completely. A hard, brittle material such as an amorphous polymer far below its Tg, usually has an initial slope indicative of very high modulus, moderate strength, a low elongation at break, and a low area under the stress-strain curve (Fig. 1.4). Polymeric materials showing hard brittle behaviour at room temperature or below are polystyrene, poly (methyl methacrylate) and many phenol-formaldehyde resins. Hard and strong polymers have high modulus of elasticity, high strength, and elongation at break of approximately 5 percent. The shape of the curve often suggests that the material has broken where a yield point might be expected. This type of curve is characteristic of some rigid poly(vinyl chloride) formulations and polystyrene polyblends. Hard, tough behaviour is shown by polymers such as cellulose acetate, 14 Page cellulose n nitrate and nylons; the ey have hig gh yield po oints and h high modulu us, high strengths an nd large elo ongations. M Most polym mers of this s group sho ow cold-dra awing or "necking" d during the s stretching p process. Cold l -drawing is importan nt in synthe etic fiber technology y, and is us sed to deve elop strength. Polymer ric materialls that are soft and tough show w low modu ulus and yie eld values, m moderate strength at br reak, and very high elongation ranging fr rom 20 to 100 per c cent. This ttype of str ress-strain c curve is characterist tic of plastic cized PVC and rubbers s (elastomer rs). The e two mech hanical perf formances c creep and s stress relaxa ation are re elated to each other. In creep, e elongation ttakes place e under the application n of constan nt stress, while in str ress relaxati ion, decreas se in stress o occur when a specimen n is held at constant and essent tially instan ntaneously induced s strain. The varying s stress or s strain is measured a as a function n of time. R Repeated flexing of a sa ample throu ugh a given distance often cause es a sample to fail at a a lower stress than it could for a s single flexu ure. This feature is re eferred to as s fatigue. Fig 1.3: G Generalized d tensile str ress-strain curve for some polym meric mater rials 15 Page Fig 1.4: Te ensile stress s-strain cur rves for fou ur types of polymeric material. Fati igue testing g may be c carried out by subjectting the sam mple to alt ternating tensile and compressiv ve stress. Th he fatigue r resistance u usually decr reases with increase in rigidity. Imp pact strength h is a measu ure of tough hness or resistance to b breakage under high velocity im mpact cond ditions. Fro om this poi int of view w, polymer riic material ls under normal con nditions of use are th hought to b be either brittle or to ouugh. For e example, polystyrene e, poly(met thyl methac crylate) and d unmodifiied, non pllasticized P PVC are usually rat ted as britt tle, breakin ng with a sharp frac cture; plastiicized PV VC's are considered to be toug gh. In generral, polyme eric materia als are eith heer brittle or tough, depending on the tem mperature an nd rate of impact, i.e. rate of de eformation.. Impact strength of f polymers and deriv ved plastics s depends on the pos sition of th he glass transition temperature (T) wiith respect t to room m temperatture and e ease of g crystallizati ion. Far be elow the g glass-transition, amorp phous polym mers break k with a brittle fract ture, but they become tougher as s the service temperatu ure approac ches T .. g Above T , amorphous polymers a are in a rub bbery state s so that the term impac ct ceases g to have any y significanc ce 1.7.1 Mec chanical pr roperties of f a thin film m Polymer The e mechanica al propertie es of any po olymer blen nd and com mposite also o depend on its dime ensional size weather itt is thinner or thicker in appearan nce. The siz ze of the blend make es it usable in various f fields. Toda ay polymers s are used e extensively in many application ns in thin film m form, lik ke film coati ing, adheren nt epoxy un nder fills to localize 16 Page the stress concentration during thermal expansion, in electronic packaging etc.. The mechanical state within the polymer necessarily varies from point to point within the thin film, as can the strength of the adhesion between the polymer and the substrate. In most practical cases, even in thin film applications such as found in electronic packaging applications, the typical dimensions of the polymer component are much larger than the dimensions of individual molecules. At scales much larger than this, the material can be considered as continuum and the usual mechanics can be invoked to model the deformation behavior, and bulk tensile properties (which will vary with resin and processing) can accurately describe the time dependent properties of the thin film. 1.7.2 Microhardness of a Polymer If we discuss the mechanical properties of polymer and biocomposites, hardness of the material is one of the most important aspects in its mechanical characterization; it is generally defined as “resistance of a material to plastic deformation, usually by indentation”. It is the property of a material, which gives it the ability to resist permanent deformation (bend, break, or have its shape changed), when a load is applied. The greater the hardness of the material, the greater is the resistance it has towards deformation. The one of the best available definition is given by Ashby 18, which states “hardness is a measure of the resistance to permanent deformation or damage”. Microhardness is the hardness of a material measured at low loads as determined by forcing an indenter such as Vickers or Knoop into the surface of the material under 5 g to 160 g load. Usually the indentations are so small that they must be measured with a microscope. Micro indenters work by pressing a tip into a sample and continuously measuring applied load, penetration, depth and cycle time. Nano- indentation tests measure hardness by indenting with very small (of the order of 1 nano-Newton) indentation forces and measuring the depth of the indention that is made. The hardness test measures the mean contact pressure, when an indenter is pressed onto the surface of a flat specimen. It provides the simple and non- destructive means of assessing the resistance of material to plastic deformation. In this present work Vickers’s microhardness indentation testing has been utilized to 17 Page study various properties of polymer and polymer biocomposite blends, which has also been utilized by various workers. This typical hardness test involves applying a fixed load to the indenter, and measuring the resultant size of indentation. This has been related empirically to the yield stress of the material. This work presents the results of micro-indentation testing on electrically stressed polymer thin films 19-26. 1.8 ELECTRICAL PROPERTIES OF POLYMER Materials such as glass, ceramics, polymers and biocomposites are non conducting materials. They prevent flow of current through them. When these types of non-conducting materials are placed in an electric field, they modify the electric field and they themselves undergo appreciable changes as a result of which they act as stores of electrical charges. When charge storage is the main function, the materials are called dielectrics. For a material to be a good dielectric, it must be an insulator. As good insulators, polymers posses excellent dielectric properties. Many authors have reported theoretical and experimental work related to these properties 29-52. The common electrical properties of interest are discussed below: 1.8.1 Dielectric polarization Polarization with the application of an electric field, due to the displacement of charge particles inside the material forming dipoles, is known as internal polarization. Some of the internal polarizations are electronic, orientational (dipolar), space charge and barrier polarization. Dielectric can also be charged by direct injection of charge carriers, when high 5 electric field (10 V/cm) is applied between the electrodes in intimate contact with it. Charges get sprayed or deposited due to corona discharge or dielectric breakdown in the thin air gap between the electrode and the dielectric. If the injected charge in the surface is of same sign as that of electrode in contact, it is called homo-charge; if it is opposite in sign then it is called hetero-charge. The total polarization is the resultant of both internal and external polarization. P = P + P 1.5 int ext 18 Page Polymers as dielectrics are known to store charge permanently when subjected to field - temperature treatment; such quasi permanently charged dielectrics are known as Electret. 1.8.2 Dielectric strength It measures the highest current that can be applied to a plastic before it allows current to pass. It is expressed as the voltage just before this happens divided by the thickness of the sample (in volts/m). It is affected by temperature, thickness, how the sample was conditioned, rate of voltage increase, test duration and contamination etc. 1.8.3 Electrical conduction The electrical conductivity of polymer, x, measures the presence of free ions not connected chemically with the macromolecules. It also depends on presence of low molecular weight impurities that can serve as source of ions. The chemical constitution has only an indirect effect on the mobility of the ions. 13191 In glassy state the conductivity of the polymer is approximately 10 to 10 ohm . With increasing temperature, the conductivity of polymer increases according to the exponential law as, URT x = A e 1.6 where, A is a Coefficient mainly dependent on temperature, R is Universal gas constant, and U is the Activation energy 1.8.4 Dielectric constant (or permittivity) ׀ Dielectric constant, ε , indicates how easily a polymer/plastic can be polarized relative to vacuum. It is defined as the ratio of the capacity of an electric capacitor filled with the substance to that of the same capacitor in vacuum, at a definite external field frequency. This dimensionless number which is important in high frequency applications varies with temperature, moisture, frequency and thickness. 1.8.5 Dissipation factor This measures the energy dissipated during rapid polarization reversals, as with an alternating current. It can be seen as the ratio of energy lost as heat to current 19 Page transmitted. It is usually measured at 1 MHz. This factor should be low when polymers are used as insulators in high-frequency applications such as radar and microwave equipments. 1.8.6 Dielectric loss ׀׀ Dielectric loss, ε , measures the part of the energy of an electric field that is dissipated irrecoverably as heat in the dielectric. Dielectric loss in polymeric materials is due to the independent movement of chain sections consisting of large number of monomer units. 1.8.7 Volume resistance A standard measure of conductivity when a direct current potential is applied across a material is volume resistivity (measured as ohm  area of the smaller 8 electrode/ specimen thickness). Materials measuring volume resistance above 10 ohm-cm are insulators. 1.8.8 Surface resistance This expresses how well current flows over the surface of a material between electrodes placed on the same side of a specimen. While volume resistance is a property of the material, surface resistance measures how susceptible a plastic is to surface contamination, especially moisture. It is useful when surface leakage may be a problem but since it is not measurable exactly it should be used with wide margin of safety. 1.9 STATEMENT OF THE PROBLEM Composite materials are attractive because they combine material properties in ways not found in nature. Such materials often result in lightweight structures having high stiffness and tailored properties for specific applications, thereby saving weight and reducing energy needs 4,54-55. Typically, a manmade composite would consist of a reinforcement phase of stiff, strong material, frequently fibrous in nature, embedded in a continuous matrix phase. The advantage of such a coupling is that the high strength and stiffness of the fiber may be exploited. Biocomposites are composite materials comprising one or more phase(s) derived from a biological origin. In terms of the reinforcement, this could include 20 Page

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