Chapter-14 - OUTLINE 3.12 Polymer Structures 3.12.1 Polymer...

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Unformatted text preview: OUTLINE 3.12 Polymer Structures 3.12.1 Polymer Molecules a) Single, Double and Triple Bonds b) Common Polymers 3.12.2 Physical Characteristics of Polymers a) Molecular weight b) Degree of Polymerization c) Molecular Shape d) Molecular Structure d1) Linear polymers d2) Branched Polymers d3) Cross-Linked Polymers d4) Network Polymers 3.12.3 Classification of Polymers a) Plastics a1) Therrnosetts a2) Thermoplastics b) Elastomers c) Fibres 3.12 Polymer Structures 3.12.1 Polymer Molecules Another class of materials, rather different from ceramics and metallics materials, is the class of polymeric materials. In general, it is common to group the polymers in three categories, namely a) natural polymers (wood, rubber, cotton, wool, leather), b) Bio- polymers (proteins, enzymes, starches, cellulose), and c) synthetic polymers (teflon, kevlar). Most of the polymers are organic in nature and are strongly covalent bonded. As a result, polymers are resistant to decay and corrosion, and are poor electrical conductors. a) Single, Double and Triple Bonds Polymer molecules consist of a repetition of a single unit called “mer” a term that derives fi'om the Greek word “meros” which means “part”. Therefore, the word “poly+mer” means “many parts”. The simplest type of polymer is called polyethylene, and the “mer” is called “ethylene”. The ethylene molecule has the following structure (Figure 73). 59 H H \___/ ./—\H Figure 73: Mer unit of ethylene. Let us look into the ethylene molecule. The molecule is composed by carbon and hydrogen. As we have discussed before, carbon has 4 valence electrons who may participate in bonding whereas hydrogen has only one. Thus, in the ethylene molecule each carbon forms a single bond with each hydrogen atom (each atom is sharing one electron), whereas the two carbon atoms form a double bond between themselves (each atom is sharing two electrons) (Figure 74). Let us now look at the molecule Csz (Acetylene) [H-C ? C-H]. What will happen between the two carbon atoms? In fact, a triple bond will form [H-C E C-H]. a) b) H H H l H H H 7 ,le . 8 //: \\: i I demons“, . \lé cleccrons\'\\ C::IC k (.2 /. I l I /2 o, o x}: H H .H \H H H H Figure 74: a) Monomer with a double, or unsaturated, covalent bond. b) Mer, with unsatisfied bonds for each carbon atom. b) Common Polymers As we have discussed previously, a polymer molecule is formed by many of the “mer” units. Thus, it is of interest to look at the formation of polyethylene from ethylene 60 molecules. What is going to happen? Well, in the presence of heat, pressure or a catalyst the double carbon bonds in the ethylene monomer will break. This is possible because the original molecule had unsaturated bonds. Thus by changing the double bond to a single bond, the carbon atoms are still joined but other molecules can now be added. On the other hand, in saturated hydrocarbons, all bonds are single, which means that no new atoms can be added unless some of the existing atoms are removed. Thus, the polyethylene molecule will look like the one in Figure 75. H H H H H l l n H—C—H H H—(Z—H n ll—C—H n n—(lz—n 1-1 H—(Ii—H" | -+ | | -’ | l | - I —(Z-——(2— ~(Z———(Z——C——(;._c.__c_ (I: l I I 1 | H H H H U H IL 1‘] ll! Mun-omcr Mvr l‘ulymu | (I: l H Figure 75: The addition reaction for producing polyethylene molecules from ethylene molecules. The unsaturated double bond in the monomer is broken to produce a mer, which can then attract additional mers to either end to produce a chain. Now let us imagine that we replace all hydrogen atoms by fluorine atoms. What will happen? The resulting polymer will be now called PTFE Polytetrafluroethylene (Teflon) (Figure 76). What about if we replace one hydrogen by one Cl atom? Then we obtain the polymer Polyvinylchloride (PVC). What about doing the same but changing the hydrogen atom by a CH3 methyl group? Then we obtain the polymer Polypropylene (PP). We can further complicate the single “mer” unit if we now change a hydrogen atom by an aromatic ring (structure shown in Figure 77). Then we form the polymer Polystyrene. We can further characterize the types of polymers by observing the repeating unit. Hence, when all the repeating units along a chain are of the same type, the resulting polymer is called homogolmer. However, if chains are composed of two or more different units, the polymer is called a copolymer. In addition, if the different units are arranged in blocks, then the polymer is called a blockc0polymer (Figure 78). 61 % Polyethylene (PE) —(l;_c:_ ll H H H m Polyvinyl chloride (PVC) —(|:_(:;_ H Cl F F m Polylelmnuoronllylcnc (l’TFE) _%_%.. n F F H H m Polypropylene (PP) _(::_(::_ ll CH, H H m —<l:—<':— w Polystyrene (PS) Figure 78: Schematic representation of a) random, b) alternating, 0) block and d) grafi copolymer. 62 3.12.2 Physical characteristics of polymers a) Molecular weight Based on what we have discussed in the former section, during the polymerization process, large molecules are obtained and synthesized from smaller molecules. In this process there is some statistical variation and thus not all the chains will grow to the same length. As a result, there will be a distribution of chain lengths and molecular weights. This distribution can be characterized in two ways: 1) Divide the chains into a series of size ranges and then determine the number fraction of each size range. This is called the average-number molecular weight. It can be expressed as Mn = inM where XI is the number fraction of chains within a certain range and M; is the mean of size range i. Example: —_— _ 0.2 Thus, 171" = 0.2 * 7,500+ 0.6 *12,500+ 02* 17,500 = 12,500 g/ v01 2) Divide the chains into a series of size ranges and determine the weight fi'action of the molecules within that range. This is called the weight-average molecular weight. It can be expressed by Ezzwilvfi where wi is the weight fiaction of chains within a certain range and the other symbols have the same meaning as before. 63 b) Degree of Polymerization Another quantity of interest in the characterization of polymers is the degree of polymerization n, which is the average number of “mer” units in a chain. Depending on the method used to determined the molecular weight, we can have two different degrees of polymerization, namely (number average degree of polymerization) 5, [LE] 5 ILE] (weigh average degree of polymerization) where 77? is the “met” molecular weight. For a copolymer (two or more different mer units) fl is obtained by W = Zfimj where f; is the chain fraction and m is the molecular 'e- weight of mer j. c) Molecular shape Because of energy considerations, the singly bonded carbon atoms form an angle of 109° between the bonds. What is the result of this geometry? The result is that there is a cone of revolution where the atoms can sit and preserve the109°angle (Figure 79). As a consequence, the chain can twist around and look something like spaghetti (Figure 80). What is particular interesting about that? Well, the mechanical and thermal properties of polymers will be a function of the ability of chain segments to experience rotation and twist around. For example, polymer molecules with predominantly double bonds will be quite rigid and difficult to experience rotation. Figure 79: Schematic representation of polymer molecular shape. 64. Figure 80: Single polymer molecule that exhibits various rotations. Notice the difference between the separation distance in between the ends, and the total length of the molecule. d) Molecular structure d1) Linear polymers In these polymers the met units are joined together end-to~end in single chains. The molecules will be Spaghetti-like and characterized by having many Van der Waals bonds between the chains (Figure 81). Some examples are the polymers Polyethylene, Polystyrene and Nylon. d2) Branched polymers In this category, the main chains are connected through branches. As a result, the density of the polymer is lowered (Figure 81). d3) Cross-linked polymers In this case adjacent linear chains are joined one to another at various positions by covalent bonds. This is called cross-linking and the process is accomplished by adding additives, which form the covalent bonds (Figure 81). Examples of this type of polymers are the rubber elastic materials. d4) Network polymers The network polymers form when mer units can bond in 3-dimensions (Figure 81). An example of this type of polymers is the case of epoxies. 65 Figure 81: Schematic representation of a) linear, b) branched, c) crosslinked and (1) network polymer molecular structures. 3.12.3 Classification of Polymers In general, synthetic polymers are normally divided into 3 groups, in particular a) Plastics, b) Elastomers and c) Fibers. This classification is usually based on the mechanical behavior of the various polymers, as shown by the stress-strain behavior depicted in Figure 82. Fkxibfe obslie Figure 82: Stress-strain diagram for various polymer molecular structures. Notice the difference between the elastomers, for which a small stress can cause a large strain, and the fibres, for which a high stress can only induce a small strain. a) Plastics a1) Thermosets Thermosetting materials, if heated above a certain temperature or mixed with an appropriate material will become permanently hard. Why do they get hard? Because, the use of temperature, pressure and the addition of another material will cause cross-linking of the polymer chains. Some of the examples of thermosetts materials are phenolic resins 66 (used in electrical filaments, heat resistant knobs for cooking, buckles), amino resins (lightweight table ware), polyester resins (paints), and epoxy resins (surface coatings, adhesives). a2) Thermoplastic materials These materials are characterized by having a melting temperature (Tm) and a glass transition temperature (TE). Below T8 the polymer is in a glassy state and it will be very britlle. This is what happened to the Space Shuttle “Challenger”, when exploded in space. In that morning, in Cape Canaveral, the temperature was below the normal, and thus the polymer O-rings that seal the two fuel tanks became glassy-type. During the launch, the vibration was sufficient to cause the O-rings to fracture and allow the liquid oxygen and hydrogen oxygen to come into contact. Thermoplastic materials when heated above '1‘g will soften and can be shaped. On cooling will harden again. In addition, thermoplastic materials can be amorphous or partially crystalline depending on the cooling rate upon solidification. Examples of this type of polymer are polyethylene (pipes, toys, bottles), polypropylene (waterpipes, sterilizable equipment), PVC (records), polystyrene (wall tiles, panels) and polycarbonates (safety helmets, cooling fans). As we mentioned above these polymeric thermoplastic materials can be crystalline. However, they are often only partially crystalline. Hence, the material will be composed of crystalline regions dispersed within the remaining amorphous material (Figure 83). Region of high crystallinity N Amorphous ion Figure 83: Structure of a semi-crystalline polymer, showing both crystalline and amorphous regions. The degree of crystallinity can be calculated according to % etystallirzio/ = w * 100 ps(pc - p.) where p5 is the density of a specimen for which the percentage of crystallinity is to be determined, pc is the density of crystalline polymer and pa is the density of amorphous 67 ...
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This note was uploaded on 09/04/2011 for the course ME 311 taught by Professor Meyers during the Spring '08 term at University of Texas at Austin.

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Chapter-14 - OUTLINE 3.12 Polymer Structures 3.12.1 Polymer...

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