Polymers PDF

Prof. Walid Fathalla  Polymers have been with us from the beginning of time, and form the building blocks of life. Animals, plants- all classes of living organisms - are composed of polymers.  in the middle of the 20th century we began to understand the true nature of polym...

Prof. Walid Fathalla  Polymers have been with us from the beginning of time, and form the building blocks of life. Animals, plants- all classes of living organisms - are composed of polymers.  in the middle of the 20th century we began to understand the true nature of polymers. This understanding came with the development of plastics, which are true man-made materials that are the ultimate tribute to man’s creativity and ingenuity. Subsequently polymers have changed our lives to luxury and comfort.  The word polymer is derived from the greek words; poly and meros, meaning many and parts, respectively. Some scientists prefer to use the word macromolecule, or large molecule, instead of polymer. Polypeptide molecule a residue of proteins main consitituent of meat, skin, silk and wool.  A polymer is a chemical that can be made by joining together lots of individual units (monomers). Monomer is a small molecule that can make polymers. It should contain two or more bonding sites. Therefore monomers have double bonds or two or more functional groups.  Phenol-formaldehyde plastics (phenolics) was developed by Dr. Leo Hendrick Baekeland in 1909 and was used as electric iron and cookware handles, grinding wheels, and electrical plugs. The following table shows the commercial names and nomenclature of some polymers and their commercial uses. Polymer Monomer Uses of Polymer Isoprene (1, 2-methyl 1 – 1, 3- Making tyres, elastic Rubber butadiene) materials BUNA – S (a) 1, 3-butadiene (b) Styrene Synthetic rubber (a) 1, 3-butadiene (b) Vinyl BUNA – N Synthetic rubber Cyanide Non-stick cookware – Teflon Tetra Fluoro Ethane plastics Uses of polymers  (a) Ethylene glycol (b) Glyptal Fabric Phthalic acid Plastic switches, Mugs, Bakelite (a) Phenol (b) Formaldehyde buckets PVC Vinyl Cyanide Tubes, Pipes Melamine (a) Melamine (b) Formaldehyde Ceramic, plastic material Formaldehyde Resin Nylon-6 Caprolactam Fabric Uses of polymers Types of Polymerization Reactions   Addition Polymerization This is also called chain growth polymerization. In this, small monomer units join to form a giant polymer. In each step, the length of the chain increases. For example, polymerization of ethylene in the presence of peroxides.  Addition Polymerization Mechanism of chain growth polymerization In the initiation step we use benzoyl peroxide which is capable of splitting equally (homocleavage) to give two pairs of phenoxy radical called initiator fragment (free radical; neutral fragment but very active). This free radical will attack the C-C double bond of the ethylene monomer to be paired. Consequently a new free radical fragment is formed containing the ethylene monomer. This step will be repeatedly formed in the propagation step and each time an ethylene molecule is added to form a large molecular weight polyethylene free radical.  Addition Polymerization  Polyethylene free radical is still very active and neutral and needs to pair its electron. This happens in the termination step and among the big variety of termination to get rid of this free radical electron by pairing we chose the simplest one; by pairing electrons of two Polyethylene free radical to finally give the final product. Notice that the molecular weight of Polyethylene free radical is doubled in the last step and also there are infinite number of termination steps occur in reality; so different polymers are formed with different molecular weights so we will deal with polymers as commercial materials (no definite molecular weight, not pure compounds) and we use the term average molecular weight. The following scheme shows examples for other termination steps.  polymer monomer Repeating uses unit polyacrylonitrile N N fibers C C HC CH2 CH CH2 polyisobutylene CH3 CH3 Rubber C CH2 C CH2 industry CH3 CH3 polyethylene Bottles, H2C CH2 CH2 CH2 plastics, bags, films Polyvinylchloride Water pipes, (PVC) Cl Cl synthetic HC CH2 CH CH2 leather, floor tiles, garden house Teflon Chemical F F reaction F2C CF2 C C resistant  polystyrene H2C CH CH2 CH Insulators, synthetic rubber polyisopryne Rubber H2C CH CH2 CH H3C C H3 C C CH2 CH2 Polyvinyledine Used as chloride copolymer Cl Cl to PVC to C CH2 C CH2 increase Cl Cl flexibility  Condensation Polymerization  In this type, small molecules like H2O, CO, NH3, are eliminated during polymerization (step growth polymerization). Generally, organic compounds containing bifunctional groups, such as idols, dials, diamines and dicarboxylic acids, undergo this type of polymerization reaction. For example, preparation of nylon -6, 6. Repeating unit and the average molecular weight Repeating unit is the basic unit which repeatedly placed in the polymer chain. It is closely similar to the monomer. The length of the polymer chain is decided by the number of repeating units in the polymer chain. That is called "Degree of polymerization". Polymer end-end distance (length) can be calculated using the following relation Ll m  A plastic sample made of polystyrene with a degree of polymerization 525. How many molecules of poly styrene are present in a 1 gram of thee sample.  The degree of polymerization of isoprene is 125. How many carbon atoms are there in polyisoprene then calculate the molecular weight.  Calculations were made for 1 g of polypropylene and gave 1020 molecules. Calculate the average molecular weight of polypropylene and the degree of polymerization.  If the average length end-end of poly vinyl alcohol was 10 nm. Calculate the degree of polymerization (C-C bond lenghth 0.154 nm). Determine the molecular weight  CH3 OH CH2 CH CH CH2 CH CH2 n n Polyisoprene Polyvinyl alcohol CH3 CH2 CH CH CH2 n Polypropylene polystyrene Polymerization of dienes Polymers composed of only one repeating unit in the polymer molecules are known as homopolymers. Polymers composed of two different repeating units in the polymer molecule are defined as copolymers. As indicated earlier, some polymers such as nylon 6,6 (5) and poly(ethylene terephthalate) have repeating units composed of more than one structural unit. Such polymers are still considered homopolymers. What Is Copolymerization? In this process, two different monomers join to form a polymer. Synthetic rubbers are prepared by this polymerization. For example, BUNA – S.   Stereo regular polymer  In case of polymers having double bonds in their structure two geometrical isomers could be identified as shown in the following structures which effects the physical and chemical properties of the polymer and are considered two different compounds Also in case of polymers containing sp3 hypridized carbon; specially that the carbon atom is attached to four different groups could be found in the form of two isomers called enantiomers and in this case we will have three different types of polymers according to the rearrangement of the repeating units; isotactic, syndiotactic and atactic as follows CH3 H CH3 H CH3 H CH3 H C C C C C C C C H H H H H H H H CH3 H H H CH3 H H H C C C C C C C C H H CH3 H H H CH3 H CH3 H CH3 H CH3 H H H C C C C C C C C H H H H H H CH3 H  Polymer Morphology  In polymer chemistry, morphology is a key factor in describing the distinction between amorphous and crystalline solids.  Polymers with an amorphous morphology have their atoms held together in a loose structure, but this structure is never orderly or predictable, which is why chemists will say that amorphous solids have no long-range order (tangled).  In crystalline polymers, the chains behave differently. They still form folds, but instead of becoming hopelessly tangled, they form orderly stacks of folded chains, known as lamellae. Lamellae bring long-range order to polymers, which is more like the orderly arrangement of atoms in typical crystals.  Degree of Crystallinity  Most crystalline polymers have amorphous regions, which means crystalline polymers are never completely crystalline. Scientists often refer to a polymer’s degree of crystallinity to describe where it sits along this spectrum. Crystallinity can range from 0 percent (entirely amorphous) to 100 percent (entirely crystalline), but most polymers fall somewhere between those extremes. Factors affecting the degree of crystallinity  Strong intermolecular forces increase the DC  Linear and small residue branched polymers have higher DC  Crosslinked polymers decreases the dgree of crystallinity  Dragging the polymer increases the crystallinity  Arrangement of molecules and the presence of stereoarranged polymers increase the DC  The DC is very important and could be used in the explanation of the previus polymer properties and could be further explained in the lecture  Polymer Hardness  The DC increase the hardness increase and the behavior of the polymer is to bear and resist tensions. This is found in nylon fibers and HDPE high density polyethylene  Polymer transparency  Amorphous 100% polymers are transparent and the transparency will decrease by increasing DC of the polymer till reaches completely opaque Classification according to molecular forces Fibers, Plastics, or Elastomers chemical bonds may be classified as either primary or secondary. Primary bonds are of three types: ionic, metallic, and covalent. The atoms in a polymer are mostly, although not exclusively, bonded together by covalent bonds. The forces of attraction responsible for the cohesive aggregation between individual molecules are referred to as secondary valence forces such as van der Waals, hydrogen, and dipole bonds. Since secondary bonds do not involve valence electrons, they are weak (generally hydrogen and dipole bonds are much stronger than van der Waals bonds.) bonds molecules must come together as closely as possible for secondary bonds to have maximum effect. Classification according to molecular forces Fibers The ability for close alignment of molecules depends on the structure of the molecules. Those molecules with regular structure can align themselves very closely for effective utilization of the secondary intermolecular bonding forces. The result is the formation of a fiber. Fibers are linear polymers with high symmetry and high intermolecular forces that result usually from the presence of polar groups. They are characterized by high modulus, high tensile strength, and low extensibilities. Classification according to molecular forces Fibers Classification according to molecular forces Elastomers There are, on the other hand, some molecules with irregular structure, weak intermolecular attractive forces, and very flexible polymer chains. These are generally referred to as elastomers. Chain segments of elastomers can undergo high local mobility. In absence of applied (tensile) stress, molecules of elastomers usually assume coiled shapes. Consequently, elastomers exhibit high extensibility (up to 1000%) from which they recover rapidly on the removal of the imposed stress. Elastomers generally have low initial modulus in tension, but when stretched they stiffen.  Plastics fall between the structural extremes represented by fibers and elastomers. However, in spite of the possible differences in chemical structure, the demarcation between fibers and plastics may sometimes seem to be unclear. Polymers such as polypropylene and polyamides can be used as fibers and as plastics by a proper choice of processing conditions.