Polymer Chemistry - Topic 6 (Slides) PDF

Chemistry 86 Chemistry for Engineers Polymer Chemistry Polymers: Definition Combination of 2 Greek words “poly”, which means many, and “meros”, which means parts or units A polymer is a molecular compound with a high molar mass made up of many repeating chemical uni...

Chemistry 86 Chemistry for Engineers Polymer Chemistry Polymers: Definition Combination of 2 Greek words “poly”, which means many, and “meros”, which means parts or units A polymer is a molecular compound with a high molar mass made up of many repeating chemical units called monomers linked together covalently. monomers polymer Polystyrene polymer short-hand notation Polymers: Classification based on monomer composition When all of the repeating units along the chain are of the same type, the resulting polymer is called a homopolymer. ( CH2 CH2 )n Polytetrafluoroethylene Polyethylene Poly(vinyl choride) (Teflon) PVC  Some uses plastic bags, bottles, toys, electrical insulation bags for intravenous solutions, pipes, tubing, floor coverings nonstick coatings, electrical insulation plastic furniture, gears in machinery and vehicles, packaging for cleaning products packaging, medical, appliances, insulation, foodservice, electronics, automotive  Some uses Automotive (interior and exterior panels, -also known as acrylic, bumpers, fenders), acrylic glass, or plexiglass medical and dental telephones, electrical gadgets, jewelry, saucepan handles manufacture of fibers packaging foods and beverages (soft drinks, juices and water) protective glasses, medical, automotive, building, appliances Polymers: Classification based on monomer composition Copolymer is a polymer made up of two or more different repeat units. ( CH CH2 CH2 CH CH CH2 )n Styrene-butadiene rubber Types of Copolymers Copolymers are often classified based on the order in which the monomers are joined together. Types of Copolymers Mass of a Polymer Chain Sample Problem: Mass of a Polymer Chain The molar mass of the ethylene (C2H2) repeat unit is 28 g/mol. If an individual polyethylene chain in a plastic grocery bag has a degree of polymerization of 7100, what is the molar mass of that particular chain? C2H2 polymerization (–CH2–CH2–)7100 Mpolymer = Mrepeat x n Mpolyethylene = (28 g/mol) (7.1x103) Mpolyethylene = 2.0x105 g/mol Molar Masses of Some Common Polymers Polycarbonate (100 000 g/mol) Poly(ethylene terephthalate) (20 000 g/mol) Polystyrene (300 000 g/mol) Poly(vinyl chloride) (100 000 g/mol) Number Average Molar Mass, Mn In reality, no two polymers have the same degree of polymerization (n). For this reason, polymer chemists use various definitions of average molar mass, and a common one is the number average molar mass, Mn Polymer Length Polymer chains are huge, ranging from between 20,000 and 40,000 individual monomers. This chain length is what gives the polymer most of its desirable characteristics: ductility, tensile strength, hardness. Polymers with very short chains (roughly with 100 g/mol) will exist as liquids. Those with weights of 1000 g/mol are typically waxy solids and soft resins. Solid polymers range between 10,000 and several million g/mol. Polymer Length The long axis of a polymer chain is called its backbone. The length of an extended backbone is simply the number of repeat units (degree of polymerization, n) times the length of each repeat unit (l0). Length of extended chain = n x l0 Sample Problem: Polymer Length The length of an ethylene repeat unit is about 250 pm. The number of repeat units for a grocery-bag polyethylene is 7100. What is its extended length? Length of extended chain = n x l0 Length of extended chain = (7.1x103)(2.5x102 pm) Length of extended chain = 1.8x106 pm The size of the coiled polymer chain is expressed by its radius of gyration, Rg, the average distance from the center of mass of the polymer to the outer edgeof the coil. Polymer Classification by Reaction Type Addition polymers Formed by simply adding monomers together. The monomers of most addition polymers contain an alkene group. Synthetic plastics Condensation polymers Formed by combination by exclusion of a small molecule (usually water) Extensively used by nature Addition Polymers / Chain-growth Polymers Reaction requires an initiator/catalyst (free radical R ) to start the growth of the reaction. Free-radical polymerization of styrene. Initiated by a free radical (R ) that reacts with styrene. The compound that is formed still is a free radical, which can react again. Polystyrene (cont.) polymer short-hand notation Common Addition Polymers and their Monomers ethylene polyethylene propylene polypropylene vinyl chloride poly(vinyl chloride) Condensation Polymers Formed when monomers link by a dehydration-condensation type reaction. Condensation Polymers The formation of nylon by the condensation reaction between hexamethylenediamine and adipic acid. Polymer Classification by Structure Linear Chain Polymers Repeat units are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of “spaghetti”. Extensive van der Waals and H-bonding between the chains; their molecules are closely packed and have high density and tensile strength. Polyethene, PVC, nylons, polyesters, etc. Types of Linear Chain Polymers Branched Polymers Side-branch chains are connected to the main chains. The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of polymer density. Polypropylene, amylopectin and glycogen. Cross-linked Polymers Adjacent linear chains are joined to one another at various positions by covalent bonds. Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains. The polymer molecules cannot slide over each other so easily. This makes materials tougher and less flexible, and they cannot be easily stretched. Cross- linking also gives materials high melting points. Vulcanized Rubber The process of heating natural rubber with Sulphur to improve its properties is called vulcanization of rubber. Vulcanization of rubber makes the rubber hard, strong and more elastic and loses its sticky properties. Thermoplastics / Thermoplastic Polymers Molecules in a thermoplastic are held together by relatively weak intermolecular forces so that the material softens when exposed to heat and then returns to its original condition when cooled – processes that are reversible and repeatable. Most linear and slightly branched polymers are thermoplastic. Thermosets/ Thermosetting Polymers A thermosetting plastic, or thermoset, solidifies or "sets" irreversibly when heated; they cannot be reshaped by heating. The cross-linking restricts the motion of the chains and leads to a rigid material.  Thermoplastic Thermoset Little cross linking Large cross linking Ductile Hard and Brittle Soften with heating Does not soften with heating Formed and reformed in Strong and durable - so many shapes - food automobiles and packaging, insulation, construction, toys, automobile bumpers, and varnishes, boat hulls, and credit cards glues Polyethylene, Vulcanized rubber, Polypropylene, Epoxies, Polyester resin, Polycarbonate, Phenolic resin Polystyrene Elastomers Elastomers are polymers with viscoelasticity (i.e., both viscosity and elasticity). They are rubbery polymers that can be stretched easily to several times their unstretched length and which rapidly return to their original dimensions when the applied stress is released. Elastomers are cross-linked, but have a low cross-link density. The polymer chains still have some freedom to move, but are prevented from permanently moving relative to each other by the cross-links. Polymer Classification by Origin of Source Natural Polymer Polymers which occur in nature Synthetic Polymer Polymers synthesized in the lab Natural Polymers Polymers which occur in nature Also known as biopolymers Examples of such polymers are natural rubber, natural silk, cellulose, starch, proteins, etc. Cellulose Natural Rubber Natural rubber is obtained as latex from rubber trees. The monomer of natural rubber is isoprene. There may be as many as 11000 to 20000 isoprene units in a polymer chain of natural rubber. Protein Protein Synthetic Polymers The fibers obtained by polymerization of simple chemical molecules in laboratory are synthetic polymers. Nylon, polyethene, polystyrene, synthetic rubber, PVC, Teflon. etc. Polymer Crystallinity Crystalline regions - chains which are linearly extended and close in proximity to one another; render a polymer hard and durable Amorphous regions - the non-crystalline regions of a polymer; render a polymer flexible. Polymer Crystallinity Degree of crystallinity and physical properties of a polymer greatly depends on the steric requirements of the substituent(s) present in the repeating unit of the polymer. Linear polyethylene exhibits a high degree of crystallinity. There are no substituents that prevent the chains from closely packing. Polyisobutylene exhibits a low degree of crystallinity. There are two methyl (-CH3) groups that provide steric bulk, preventing the chains from closely packing. Crystalline Polymers Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration. Crystallinity makes a polymer strong, but also lowers their impact resistance. Polymers form lamellar (plate-like) crystals with a thickness of 10 to 20 nm in which the parallel chains are perpendicular to the face of the crystals. Amorphous Polymers Polymer chains with branches cannot pack together regularly enough to form crystals. These polymers are said to be amorphous. Amorphous polymers are softer, have lower melting points, and are penetrated more by solvents Amorphous regions of a polymer are made up of a than are their crystalline randomly coiled and counterparts. entangled chains. Semi-crystalline Polymers Semi-crystalline polymers have both crystalline and amorphous regions. Semi- crystalline polymers can be tough with an ability to bend without breaking. The percentage of the polymer that is crystalline is called the percent crystallinity. The percent crystallinity has an The crystals are small and connected to important influence on the the amorphous regions by polymer properties of the polymer. chains so there may be no sharp well- defined boundaries between the two types of regions. Factors affecting crystallinity Some polymers form more crystalline solids than others. Six factors favor a polymer with a high percent crystallinity: a regular and symmetrical linear chain a low degree of polymerization strong intermolecular forces small and regular pendant groups a slow rate of cooling oriented molecules Structural Regularity Crystallization is favored by a regular arrangement along the polymer chain giving the structure a high degree of symmetry. Linear polyethylene can form a solid with over 90% crystallinity in some cases. This is made possible by the planar zig-zag structure easily assumed by the molecule. Atactic Polystyrene Normal polystyrene is atactic with no regular order in the position of the benzene rings along the chain. The irregularity prevents the chains from packing closely to each other. Atactic polystyrene is amorphous. It is comparatively soft, low melting, and becomes swollen in solvents. Syndiotactic Polystyrene In syndiotactic polystyrene, the benzene rings are on alternate sides of the chain. This allows the chains to pack into crystals. Syndiotactic polystyrene is crystalline. It is rigid, high melting, and not penetrated readily by solvents. Intermolecular Forces Crystallinity is favored by strong interchain forces. The presence of polar and hydrogen bonding groups favors crystallinity because they make possible dipole-dipole and hydrogen bonding intermolecular forces. A polyester, such as poly(ethylene terephalate), contains polar ester groups. Dipole-dipole forces between the polar groups hold the PET molecules in strong crystals. Crystallinity in poly(ethylene terephalate) also is favored by the structural regularity of the benzene rings in the chain. The benzene rings stack together in an orderly fashion Pendant Groups Regular polymers with small pendant groups crystallize more readily than do polymers with large, bulky pendant groups. Poly(vinyl alcohol) (PVA) is made by the hydrolysis of poly(vinyl acetate) (PVAc). hydrolysis PVA crystallizes more readily than PVAc because of the bulky acetate groups in PVAc. The -OH groups in PVA also form strong hydrogen bonds. Mechanical Properties of Polymers The mechanical properties of a polymer involve its behavior under stress. Highly sensitive to the strain rate, temperature, chemical nature of the environment (presence of water, oxygen, organic solvents, etc.) Mechanical Properties of Polymers These properties tell a polymer scientist or engineer many of the things he or she needs to know when considering how a polymer can be used. The mechanical properties of polymers are one of the features that distinguishes them from small molecules. Mechanical Properties of Polymers Tensile Strength % Elongation-to-Break Young's Modulus Toughness Stress and Strain Stress is defined as the force per unit area of a material. where, σ = stress F = force applied A = cross sectional area of the object Units: N/m2 or Pa Strain is defined as extension per unit length. where, ε = strain lo = the original length e = extension = l – lo l = stretched length Strain has no units because it is a ratio of lengths. Stress-strain behavior of common polymeric materials Tensile Strength The tensile strength is the stress needed to break a sample. It is expressed in Pascals or psi (1 MPa = 145 psi). The tensile strength is an important property for polymers that are going to be stretched. Fibers, for instance, must have good tensile strength. % Elongation-to -Break The elongation-to-break (or ultimate elongation) is the strain on a sample when it breaks. This usually is expressed as a percent. Fibers have a low elongation-to-break while elastomers have a high elongation-to-break. Young's Modulus / Modulus of Elasticity / Tensile Modulus Young's modulus is the ratio of stress to strain, or the slope of a stress-strain curve. Stress-strain curves often are not straight-line plots, indicating that the modulus is changing with the amount of strain. In this case, the initial slope usually is used as the modulus. Rigid materials, such as metals, have a high Young's modulus. In general, fibers have high Young's modulus values, elastomers have low values, and plastics lie somewhere in between. Toughness The toughness of a material is the area under a stress-strain curve. The stress is proportional to the tensile force on the material and the strain is proportional to its length. The area under the curve then is proportional to the integral of the force over the distance the polymer stretches before breaking. Strong vs Tough There is a difference between toughness and strength, as is illustrated in the three plots below. A material that is strong but not tough is said to be brittle. Brittle substances are strong, but cannot deform very much. Strong vs Tough For example, general purpose polystyrene (GPPS) is brittle. High impact polystyrene (HIPS), a blend of polystyrene and polybutadiene (a rubbery polymer) is said to be rubber- toughened. GPPS References 1. Silberberg, M. (2013). Principles of Chemistry, 3rd Edition. McGraw Hill. 2. Chang, R. (2010). Chemistry, 10th Edition. McGraw Hill. 3. Klein, D. (2013). Organic Chemistry. 2nd Edition. John Wiley and Sons. 4. Callister, W. and Rethwisch, D. (2014). Materials Science and Engineering. 9th edition. John Wiley and Sons. 5. http://faculty.uscupstate.edu/llever/polymer%20resources/mechanical.htm