Excerpt
Content
INTRODUCTION
1.1 Introduction to polymers
1.2 Orientations of the work
1.3 Scope
RADIATION EFFECTS ON POLYMERS
2.1 Introduction
2.2 Radiation sources
2.3 Microtron
2.4 Terminology and units
2.5 Radiation induced chemical changes in polymer
EXPERIMENTAL METHODS
3.1 Preparation of sample
3.2 Thickness measurement
3.3 Irradiation
3.4 Electrode coating
3.5 X-ray Diffraction
3.6 Calculation of crystallinity
3.7 IR spectra
3.8 Measurements
RESULT AND DISCUSSION
4.1 X-ray Diffraction analysis
4.2 FTIR Analysis
4.3 Conductivity Measurements
CONCLUSION
Bibliography
CHAPTER 1 INTRODUCTION
Over the past decade a great deal of information has been published about intrinsically conducting polymers, more commonly known as ‘synthetic metals’. The basic interest comes from the fact that these new materials combine the physical and chemical attributes of plastics with the electrical, electronic, magnetic, and optical properties of metals or semiconductors. (1)
Poly (vinyl chloride) (PVC) is extraordinarily useful as a commercial material. Among the thermoplastics, it ranks second only to polyolefins in total worldwide production volume. Remarkably, it has achieved this status despite its molecular instability toward heat, an instability that is much more pronounced than those of all of its major competitors. In a technological sense, this difficulty has been overcome to a large degree, for otherwise the usage of PVC would never have reached its current level Commercial interest in PVC was first revealed in a number of patents independently filed in 1928 by Carbide and Carbon Chemical Corporation, Dupont and IG Farben .
Poly (vinyl chloride), PVC, has a polyhalogenated chain with chlorine atoms covalently linked to atoms of carbon, providing thus many points of dipolar interaction along its chain which give rise to strong interchain interactions and consequent rigidity of the polymeric material. Plasticizing additives break interchain dipole interaction providing a material with mobility and flexibility characteristics of a polymer with less interchain interaction. PVC compounded with plasticizers has many applications among them medical devices, packaging material and children’s products. Many of the applications require sterilization by gamma radiation. (2)
When polymeric materials are subjected to irradiation changes are observed on their molecular structure, mainly chain scission which leads to reduction on molar mass, and reticulation which increases molar mass and reduces solubility. These molecular alterations lead to changes on mechanical properties of the material. (3)
1.1 Introduction to polymers
Polyvinyl chloride was accidentally discovered on at least two different occasions in the 19th century, first in 1835 by Henri Victor Regnault and in 1872 by Eugen Baumann. On both occasions, the polymer appeared as a white solid inside flasks of vinyl chloride that had been left exposed to sunlight. In the early 20th century, the Russian chemist Ivan Ostromislensky and Fritz Klatte of the German chemical company Griesheim-Elektron both attempted to use PVC (Polyvinyl Chloride) in commercial products, but difficulties in processing the rigid, sometimes brittle polymer blocked their efforts. In 1926, Waldo semon of B.F Goodrich developed a method to plasticize PVC by blending it with various additives. The result was a more flexible and more easily processed material that soon achieved widespread commercial use (13).
Polymers and polymer composition with Vinyl was first used in electrical applications more than a half century ago as a replacement for rubber insulation. Today, vinyl commands nearly half of the market for electrical applications such as wire insulation and sheathing. That's because of vinyl's reliable durability and outstanding safety record. But the relatively low electrical conductivity is exhibited by the polymers like polyvinyl chloride gained importance during the past decades. The electrical conduction in polymer films has much importance due to the discovery of the memory phenomenon (Kryezewski 1975) and has wide application now a day in thin film devices (Mcad 1961). In recent years, because of the need for electrostatic charges dissipation, electromagnetic shielding etc, new polymers with electrical conductivity have been formulated particularly because of their electrographic and solar cell applications (11). Many synthetic polymers (Kumar et al 1985) like polypyrrloe, polycarbazol and polyacetylene etc have been studied. The behavior of these organic polymers on irradiated by the electron beam of different dose at room temperature. (10)
Polymers, macromolecules, high polymers, and giant molecules are high-molecular-weight materials composed of repeating subunits. These materials may be organic, inorganic, or organometallic, and synthetic or natural in origin. Polymers are essential materials for almost every industry as adhesives, building materials, paper, cloths, fibers, coatings, plastics, ceramics, concretes, liquid crystals, photoresists, and coatings (12). They are also major components in soil, plant and animal life. They are important in nutrition, engineering, biology, medicine, computers, space exploration, health, and the environment.
Natural inorganic polymers include diamonds, graphite, sand, asbestos, agates, chert, feldspars, mica, quartz, and talc. Natural organic polymers include polysaccharides (or polycarbohydrates) such as starch and cellulouse, nucleic acids, and proteins. Synthetic inorganic polymers include boron nitride, concrete, many high-temperature superconductors, and a number of glasses. Siloxanes or polysiloxanes represent synthetic organometallic polymers (13).
Synthetic polymers used for structural components weight considerably less than metals, helping to reduce the consumption of fuel in vehicles and aircraft. They even most metals when measured on a strength-per-weight basis. Polymers have been developed which can also be used for engineering purposes such as gears, bearings, and structural members (7).
Throughout the years, vinyl electrical products have repeatedly met or exceeded requirements set by the National Electrical Code, the three model building codes and Underwriters' Laboratories® for fire, physical and electrical performance (5). Vinyl offers a unique combination of properties that make it a material of choice in the electrical market, including:
a) Superior fire performance . Vinyl electrical products play a key role in preventing fires from starting and in limiting the amount of damage a fire can do, once it starts. PVC burns very slowly, which prevents the fire from spreading and keeps smoke levels low.
b) Easy, economical installation . Installing PVC electrical systems over metallic systems can mean a real cost savings to any project. A series of 1999 studies conducted by IFT, Inc./Fire Cause Analysis in four major geographical areas found that installation of PVC nonmetallic electrical tubing provides substantial savings in labour and installation costs over the comparable metallic electrical systems. The reports from the Boston, Los Angeles and St. Louis, Miss., areas show that vinyl tubing saves between 28 and 32 percent in labor and installation costs. In the Orlando, Fla., area savings range between 22 and 26 percent (6).
c) Durability in all conditions . Vinyl is resistant to chemicals, corrosion, abrasion, UV degradation, extreme temperature, weather variations and general wear, keeping maintenance costs low.
d) Versatility . Vinyl can be easily formulated to meet the requirements of nearly any application. It provides superior flexibility, colorability, formulation and design versatility, and good dielectric behavior.
Nomenclature:
Many polymers have both a common name and a structure-based name specified by the International Union of Pure and Applied Chemistry (IUPAC). Some polymers are commonly known by their acronyms. Some companies use trade names to identify the specific polymeric products they manufacture. For example, Fortrel® polyester is a poly (ethylene terephthalate) (PET) fiber. Polymers are often generically named, such as rayan, polyester, and nylon (4).
Composition:
Polymer structures can be represented by similar or identical repeat units. These are derived from smaller molecules, called monomers, which react to form the polymer. Propylene monomer and the repeat unit it forms in polypropylene are shown below.
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With the exception of its end groups, polypropylene is composed entirely of this repeat unit. The number of units (n) in a polymer chain is called the degree of polymerization (DP). Other polymers, such as proteins, can be described in terms of the approximate repeat unit where the nature of R (a substituted atom or group of atoms) varies (4).
Primary structure:
The sequence of repeat units within a polymer is called its primary structure. Unsymmetrical reactants, such as substituted vinyl monomers, react almost exclusively to give a “head-to-tail” product, in which the R substituents occur on alternate carbon atoms. A variety of head-to-head structures are also possible.
Each R-substituted carbon atom is a chiral center (an atom in a molecule attached to four different groups) with different geometries possible. Arrangements where the substitutes on the chiral carbon are random are referred to as atactic structures. Arrangements where the geometry about the chiral carbon alternates are said to be syndiotactic. Structures where the geometry about the chiral atom has the same geometry are said to be isotactic or stereoregular.(4)
Stereoregular polymers are produced using special stereoregulating catalyst systems. A series of soluble catalysts have been developed that yield products with high stereoregularity and low chain-size disparity. As expected, polymers with regular structures that is, isotactic and syndiotactic structures tend to be more crystalline and stronger.
Polymers can be linear or branched with varying amounts and lengths of branching. Most polymers contain some branching.
Copolymers are derived from two different monomers, which may be represented as A and B. There exists a large variety of possible structures and, with each structure, specific properties. These varieties include alternating, random, block, and graft see (illustration).
Secondary structure:
This refers to the localized shape of the polymer, which is often the consequence of hydrogen bonding. Most flexible to semiflexible linear polymer chains tend toward two structures-helical and pleated sheet/skirtlike. The pleated skirt arrangement is most prevalent for polar materials where hydrogen bonding can occur. In nature, protein tissue is often of a pleated skirt arrangement. For both polar and nonpolar polymer chains, there is a tendency toward helical formation with the inner core having “like” secondary bonding forces.
Tertiary structure:
This refers to the overall shape of a polymer, such as in polypeptide folding. Globular proteins approximate rough spheres because of a complex combination of environmental and molecular constraints, and bonding opportunities. Many natural and synthetic polymer have “superstructures,” such as the globular proteins and aggregates of polymer chains, forming bundles and groupings.
Quaternary structure:
This refers to the arrangement in space of two or more polymer subunits, often a grouping of tertiary structures. For example, hemoglobin (quaternary structure) is essentially the combination of four myoglobin (tertiary structure) units. Many crystalline synthetic polymers form spherulites.
Polymer synthesis:
Polymers are synthesized by three primary methods: organic synthesis in a laboratory or factory, biological synthesis in living cells and organisms, or by chemical modification of naturally occurring polymers.
Organic synthesis:
In 1907, Leo Baekeland created the first completely synthetic polymer, Bekelite, by reacting phenol and formaldehyde at precisely controlled temperature and pressure. Subsequent work by Wallace Carothers in the 1920s demonstrated that polymers could be synthesized rationally from their constituent monomers. The intervening years have shown significant developments in rational polymer synthesis. Most commercially important polymers today are entirely synthetic and produced in high volume, on appropriately scaled organic synthetic techniques.
Laboratory synthetic methods are generally divided into two categories, condensation polymerization and addition polymerization. However, some newer methods such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a catalyst. Efforts towards rational synthesis of biopolymers via laboratory synthetic methods, especially artificial synthesis of proteins, is an area of intense research.
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For polymerization to occur, monomers must have at least two reaction points or functional groups. There are two main reaction routes to synthetic polymer formation—addition and condensation. In chain-type kinetics, initiation starts a series of monomer additions that result in the reaction mixture consisting mostly of unreacted monomer and polymer. Vinyl polymers, derived from vinyl monomers and containing only carbon in their backbone, are formed in this way. Examples of vinyl polymers include polystyrene, polyethylene, polybutadiene, polypropylene (see structure), and poly vinyl chloride.
The second main route is a step-wise polymerization. Polymerization occurs in a step-wise fashion so that the average chain size within the reaction mixture may have an overall degree of polymerization of 2, then 5, then 10, and so on, until the entire mixture contains largely polymer with little or no monomer left. Polymers typically produced using the step-wise process are called condensation polymers, and include polyamides, polycarbonates, polyesters, and polyurethanes (see structures). 3
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Condensation polymer chains are characterized as having a noncarbon atom in their backbone. For polyamides the noncarbon is nitrogen (N), while for polycarbonates it is oxygen (O). Condensation polymers are synthesized using melt (the reactants are heated causing them to melt), solution (the reactants are dissolved), and interfacial (the reactants are dissolved in immiscible solvents) techniques.
Biological synthesis:
Natural polymers and biopolymers formed in living cells may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA and subsequently translate that information to synthesize the specified protein. The protein may be modified further following translation in order to provide appropriate structure and function.
Modification of natural polymers:
Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulfur.
Polymer Structure and Properties:
Types of polymer 'properties' can be broadly divided into several categories based upon scale. At the nano-micro scale properties that directly describe the chain itself. These can be thought of as polymer structure. At an intermediate mesoscopic level are properties that describe the morphology of the polymer matrix in space. At the macroscopic level properties that describe the bulk behavior of the polymer (6).
Structure:
The structural properties of a polymer relate to the physical arrangement of monomers along the backbone of the chain. Structure has a strong influence on the other properties of a polymer. For example, a linear chain polymer may be soluble or insoluble in water depending on whether it is composed of polar monomers (such as ethylene oxide) or nonpolar monomers (such as styrene). On the other hand, two samples of natural rubber may exhibit different durability even though their molecules comprise the same monomers. Polymer scientists have developed terminology to precisely describe both the nature of the monomers as well as their relative arrangement (6).
Monomer identity:
The identity of the monomers comprising the polymer is generally the first and most important attribute of a polymer. Polymer nomenclature is generally based upon the type of monomers comprising the polymer. Polymers that contain only a single type of monomer are known as homopolymers, while polymers containing a mixture of monomers are known as copolymers (4). Poly(styrene), for example, is composed only of styrene monomers, and is therefore is classifed as a homopolymer. Ethylene-vinyl acetate, on the other hand, contains more than one variety of monomer and is thus a copolymer. Some biological polymers are composed of a variety of different but structurally related monomers, such as polynucleotides composed of nucleotide subunits.
A polymer molecule containing ionizable subunits is known as a polyelectrolyte. An ionomer is a subclass of polyelectrolyte with a low fraction of ionizable sub units (4).
Chain linearity:
The simplest form of polymer molecule is a straight chain or linear polymer, composed of a single main chain. The flexibility of an unbranched chain polymer is characterized by its persistence length. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Special types of branched polymers include star polymers, comb polymers, and brush polymers. If the polymer contains a side chain that has a different composition or configuration than the main chain the polymer is called a graft or grafted polymer. A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of crosslinking is referred to as a polymer network. Sufficiently high crosslink concentrations may lead to the formation of an 'infinite network', also known as a 'gel', in which networks of chains are of unlimited extend there is essentially all chains have linked into one molecule(4,6).
Chain size:
Polymer bulk properties may be strongly dependent on the size of the polymer chain. Like any molecule, a polymer molecule's size may be described in terms of molecular weight or mass. In polymers, however, the molecular mass may be expressed in terms of degree of polymerization, essentially the number of monomer units which comprise the polymer. For synthetic polymers, the molecular weight is expressed statistically to describe the distribution of molecular weights in the sample. This is because of the fact that almost all industrial processes produce a distribution of polymer chain sizes. Examples of such statistics include the number average molecular weight and weight average molecular weitht. The ratio of these two values is the polydispersity index, commonly used to express the "width" of the molecular weight(7,8).
The space occupied by a polymer molecule is generally expressed in terms of radius of gyration or excluded volume.
Morphological Properties
Crystallinity:
When applied to polymers, the term crystalline has a somewhat ambiguous usage. In some cases, the term crystalline finds identical usage to that used in conventional crystallography. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for X-ray crystallography(13), may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of angstroms or more.
A synthetic polymer may be described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline (7, 8).
Crystalinity of polymer
Properties of textile fibers are determined by their chemical structure degree of polymerization, orientation of chain molecules, crystallinity, package density and cross linking between individual molecules. Polymer crystallinity is one of the important properties of all polymers. Polymer exists both in crystalline and amorphous form.
Fig 1.1. Shows the arrangement of polymer chain forming crystalline and amorphous regions (13). It can be seen that part of molecules are arranged in regular order, these regions are called crystalline regions. In between these ordered regions molecules are arranged in random disorganized state and these are called amorphous regions.
Image has been removed for copyright reasons
Fig (1.1) Mixed of Amorphous crystalline Macromolecular polymer structure
Crystallinity is indication of amount of crystalline region in polymer with respect to amorphous content.
Crystallinity influences many of the polymer properties some of there are, Hardness, Modulus ,Tensile, Stiffness, Crease, Melting Point So while selecting polymer for required Application its crystallinity plays foremost role.
Bulk Properties:
The bulk properties of a polymer are those most often of end-use interest. These are the properties that dictate how the polymer actually behaves on a macroscopic scale.
Tensil strength:
The tensile strength of a material quantifies how much stress the material will endure before failing. This is very important in applications that rely upon polymer's physical strength or durability. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping. In general tensile strength increases with polymer chain length (14).
Young’s Modulus of elasticity:
This parameter quantifies the elasticity of the polymer. It is defined, for small strains, as the ratio of rate of change of stress to strain. Like tensile strength this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands.
Transport Properties:
Transport properties such as diffusivity relate to how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes.
Pure component phase behavior
Melting point:
The term "melting point" when applied to polymers suggests not a solid-liquid phase transition but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though abbreviated as simply "Tm", the property in question is more properly called the "crystalline melting temperature". Among synthetic polymers, crystalline melting is only discussed with regards to thermoplastics, as thermosetting polymers will decompose at high temperatures rather than melt.
Boiling point:
The boiling point of a polymer substance is never defined due to the fact that polymers will decompose before reaching theoretical boiling temperatures.
Glass transition Temperature (Tg)
A parameter of particular interest in synthetic polymer manufacturing is the glass transition temperature (Tg), which describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid to a brittle, glassy amorphous solid. The glass transition temperature may be engineered by altering the degree of branching or cross-linking in the polymer or by the addition of plasticizer.
Polymer Structure/Property relationships:
Polymer bulk properties are strongly dependent upon their structure and mesoscopic behavior. A number of qualitative relationships between structure and properties are known.
Chain Length:
Increasing chain length tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (Tg). This is a result of the increase in chain interactions such as Vander Waal’s attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.
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