Polymeric Materials. Today s Topics

Transcription

1 MME 131: Lecture 29 Polymeric Materials Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Definition of polymers Structure of polymers Generic polymers and their applications Synthesis and processing of polymers Properties of polymers Lec 29, Page 1/21

2 What is a Polymer? Polymer is a large molecule consisting of repeated chemical units ( mers ) joined together, like beads on a string. Usually contain 5 or more monomers, and some may contain 100s or 1000s of monomers in each chain. Extremely large molecular weight (M) Monomer Molecule that combines with others (identical or different) to form a polymer e.g., ethylene Molecular forces in polymers 1. Intramolecular forces Forces between atoms in one chain generally covalent bonds (strong) 2. Intermolecular forces Forces between two chains: van der Waals forces (PE) hydrogen bridges (PS) (stronger) Can become very strong as M increases 3. Entanglements (physical) In the crystalline state, the van der Waals bonds are important. In the rubbery amorphous state, the entanglements are important. Lec 29, Page 2/21

3 ydrocarbon Molecules Most polymers are organic, and formed from hydrocarbon molecules Each C atom has four e- that participate in bonds, each atom has one bonding e- Saturated hydrocarbon molecules all bonds are single ones C C C - methane, C 4 ethane, C 2 6 Unsaturated hydrocarbon molecules contain double and triple bonds between C atoms sharing of two or three electron pairs more reactive C = C C C ethylene, C 2 4 acetylene, C 2 2 Polymer Molecules Polymer molecules are very large: they are macromolecules. Most polymers consist of long and flexible chains with a string of C atoms as a backbone. C atoms are side-bonded to atoms or radicals Double bonds are possible in both chain and side bonds Three ways to represent the structure of polyethylene: (a) a solid three-dimensional model, (b) a three-dimensional space model, and (c) a simple two-dimensional model. Lec 29, Page 3/21

4 Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. O, Cl, F, N, Si, P, and S are other elements found in the molecular makeup of polymers. E.g., Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone. These are considered inorganic polymers. E.g., polysiloxanes (silicones) and polyphosphazenes. Examples of Some Polymers X = Polyethylene (PE) C C X n X = C 3 X = Cl X = Polypropylene (PP) Polyvinylchloride (PVC) Polystyrene (PS) When all mars are the same, the polymer molecule is called homopolymer. When there are more than one types of mars, the molecule is called copolymer. Lec 29, Page 4/21

5 Poly(ethylene terephthalate) PET Poly(carbonate) Poly(methylmethacrylate) PMMA Poly(acrylate) PAA Poly(dimethyl siloxane) Polymer molecular shape The angle between the singly bonded carbon atoms is 109 carbon atoms form a zigzag pattern in a polymer molecule. While maintaining the 109 angle between bonds polymer, chains can rotate around single C-C bonds (double, triple bonds are very rigid). r = l n r = head-toe distance n = no. of steps l = length of step Random kinks and coils lead to entanglement, like in the spaghetti structure Lec 29, Page 5/21

6 Polymer molecular structure Branched polymers Inefficient chain packing causes reduced density Examples: LDPE Linear polymers Van der Waals bonding between chains. Examples: polyethylene, nylon, DPE. Network polymers 3D networks made from trifunctional mers. Examples: epoxies, phenolformaldehyde Cross-linked polymers Chains are connected by covalent bonds. Example: rubbers. DPE Linear, unbranched polymer Linear, branched polymer LDPE Cross-linked, unbranched polymer Cross-linked, branched polymer Branching increases the volume and thus reduces the density of the polymer. Schematic showing linear and branched polymers. Note that branching can occur in any type of polymer (e.g., thermoplastics, thermosets, and elastomers), but not cross-linking. (a) Linear unbranched polymer: notice chains are not straight lines and not connected. Different polymer chains are shown using different shades and design to show clearly that each chain is not connected to another. (b) Linear branched polymer: chains are not connected, however they have branches. (c) Thermoset polymer without branching: chains are connected to one another by covalent bonds but they do not have branches. Joining points are highlighted with solid circles, (d) Thermoset polymer that has branches and chains that are interconnected via covalent bonds. Different chains and branches are shown in different shades for better contrast. Places where chains are actually chemically bonded are shown with filled circles. Lec 29, Page 6/21

7 Polymer molecular weight, M W The average molecular weight can be obtained by averaging the masses with the fraction of times they appear (number-average molecular weight) or with the mass fraction of the molecules (weightaverage molecular weight). Number average: Weight average: M n = x i M i M w = f i M i M i = the mean mol. wt. of range i f i = wt. fraction of chains of length i x i = no. fraction of chains of length i Final molecular weight (chain length) is controlled by the synthesis process of polymers, which is known as the polymerization. Melting / softening temperatures, and tensile strength increase with molecular weight (up to ~ 100,000 g/mol) of polymers. At room temperature, short chain polymers (molar weight ~ 100 g/mol) liquids or gases intermediate length polymers (~ 1000 g/mol) waxy solids long chain polymers ( g/mol) solid Example We have a polyethylene sample containing 4000 chains with molecular weights between 0 and 5000 g/mol, 8000 chains with molecular weights between 5000 and 10,000 g/mol, 7000 chains with molecular weights between 10,000 and 15,000 g/mol, and 2000 chains with molecular weights between 15,000 and 20,000 g/mol. Determine both the number and weight average molecular weights. Solution First we need to determine the number fraction x i and weight fraction f i for each of the four ranges. We can then use the equations to find the molecular weights. M n = x i M i M w = f i M i Lec 29, Page 7/21

8 Polymer Crystallinity Atomic arrangement in polymer crystals is complex. Polymer molecules are often semicrystallined, with partially crystalline regions dispersed within amorphous material. r c (r s r a ) % Crystallinity = x100 r s (r c r a ) crystalline region r c = density of perfect crystalline polymer r a = density of completely amorphous polymer r s = density of partially crystalline polymer sample The degree of crystallinity depends on 1. Rate of cooling (faster cooling, less crystallinity) 2. Type of polymer (simple structure, more crystallinity) 3. Linear polymer more easily form crystals. amorphous region annealing causes crystalline regions to grow % crystallinity density, TS, E with crystallinity Lec 29, Page 8/21

9 The Glass Transition Temperature It is difficult to crystallise polymer. More often they are amorphous, and have properties more like glass. T m and T g temperatures are critical aspects of designing with semicrystalline and amorphous polymers, Modulus of elasticity and other properties change at the Tg. Sample Tg values: Low density polyethylene (LDPE) -110 C igh density polyethylene (DPE) -90 C Polyvinyl chloride +87 C Polystyrene +100 C Polycarbonate +150 C Classification of Polymers There is a bewildering number of polymers and the number increases every year. The four generic polymers are: Linear polymer - Any polymer in which molecules are in the form of spaghetti-like chains. Thermoplastics (or, Plastics) - Linear or branched polymers in which chains of molecules are not interconnected to one another. They softens on heating. E.g., polyethelene. Thermosetting polymers (or, Resins) - Polymers that are heavily cross-linked to produce a strong three dimensional network structure. They will harden when two components (resin and hardener) are heated together. E.g., epoxy. Elastomers (or, Rubbers) - These are either thermoplastics or lightly cross-linked thermosets and that have more than 200% elastic deformation. Lec 29, Page 9/21

10 Thermoplastics Often described as linear polymer with a little branching but NO cross linking; can be amorphous or semicrystalline Reversibly soften with heating - the secondary bond that binds the molecules to each other melts so that it flows like viscous fluid and allowing it to be formed Ductile Example: PE, PP, PTFE (teflon), PS, PVC, PMMA (prespex), nylon Thermosets They are network polymers, with heavily cross linked (10-50% of mars); almost always amorphous structured Do NOT soften with heating secondary bond melt but cross links remain and prevent melting and viscous flow, so thermosets cannot be hot worked; further heating causes decomposition. They are made by mixing two components, a resin and a hardener (catalyst), which react and harden, either at room temperature or on heating Usually hard and brittle Example: polyester resin (fibre glass), epoxies, phenolic resins (ckt board), silicones. Table 1: Generic thermoplastics. Lec 29, Page 10/21

11 Table 2: Generic thermosets. Polymer Synthesis The synthesis of large molecular weight polymer is called polymerization. most of the raw materials for synthetic polymers are derived from coal and petroleum products, which are low molecular weight polymers. low molecular weight monomers are joined over and over. Two types of polymerization reactions: Addition polymerization Process by which polymer chains are built up by adding monomers together without creating a byproduct. Condensation polymerization A small molecule (e.g., water, methanol, etc.) is condensed out as a byproduct. Lec 29, Page 11/21

12 1. Initiation reaction Addition Polymerization R. + C C R C C An active centre (which is an unpaired electron) capable of propagation is formed by a reaction between an initiator or catalyst (R.) and the monomer unit. 2. Rapid propagation (~1000 mer units / 1-10 nanoseconds) R C C + C C R C C C C Linear growth of molecule as monomer units become attached to one another in succession to produce the chain molecules 3. Termination R C C C C + R C C C C R When two active chain ends meet each other, or active chain ends meet with initiator or other species with single active bond, polymerisation ends. Condensation Polymerization The condensation reaction for polyethylene terephthalate (PET), a common polyester. The OC 3 group and a hydrogen atom are removed from the monomers, permitting the two monomers to join and producing methyl alcohol as a byproduct. Lec 29, Page 12/21

13 Degree of Polymerization Degree of polymerization The average molecular weight of the polymer divided by the molecular weight of the monomer. Example Calculate the degree of polymerization of 6,6-nylon having a molecular weight of 120,000 g/mol. [1 mol hexamethylene diamine and 1 mol adipic acid react to form 1 mol 6,6 nylon and 2 mol of water] The molecular weights are 116 g/mol for hexamethylene diamine, 146 for adipic acid, and 18 for water. The repeat unit for 6,6-nylon is: SOLUTION The molecular weight of the repeat unit is the sum of the molecular weights of the monomers, minus that of the two water molecules that are evolved: M repeat unit = (18) = 226 g/mol Degree of polymerization = 120,000/226 = 531 The degree of polymerization refers to the total number of repeat units in the chain. The chain contains 531 hexamethylene diamine and 531 adipic acid molecules. Lec 29, Page 13/21

14 Polymer Processing It is relatively rare for an article to be made from polymer alone. Most are polymer compounds, consisting of a mixture of polymer and various additives. These additives are used to modifying physical, mechanical and chemical properties. Fillers: Solid additives used to improve tensile and compressive strengths, abrasion resistance, toughness, and thermal stability Particle sizes range from very small (10 nm) to large (mm) Examples: Wood flour, silica flour, sand, glass, clay, talk, limestone Reduces the cost of product by replacing more expensive polymer Plasticizers: Small molecules of non-volatile solvents which occupy positions between polymer chains (increase distance and interactions between chains) Used to increases flexibility, ductility, and toughness, and to reduces hardness and stiffness in brittle polymers e.g., PVC Almost 90% of plasticizers are used to lower the Tg value of PVC Examples: phthalate ester (in PVC), Stabilisers/Antioxidants: Compounds that reduce polymer degradation (in exposure to ultraviolet radiation) through intervention in free radical reactions. Examples: antioxidant, ultraviolet stabilizers, emulsifiers & surfactants Colourants: Additives used to change product aesthetics Examples: pigments (soluble colourants), dyestuffs (insoluble additives) Lec 29, Page 14/21

15 Polymer Forming Depends on (1) the type of polymers (2) temperature, (3) atmospheric stability (4) geometry of finished product Forming Processes for Thermoplastics: Extrusion Blow Molding Injection Molding Thermoforming Calendaring Spinning Forming Processes for Thermosetting polymers: Compression Molding Transfer Molding Schematic of an extruder used for polymer processing. Lec 29, Page 15/21

16 One technique by which polymer films (used in the manufacture of garbage bags, for example) can be produced. The film is extruded in the form of a bag, which is separated by air pressure until the polymer cools. Typical forming processes for thermoplastic: (a) extrusion, (b) blow molding, (c) injection molding, (d) thermoforming, (e) calendaring, and (f) spinning. Lec 29, Page 16/21

17 Typical forming processes for thermosetting polymers: (a) compression molding, and (b) transfer molding. Polymer Joining Cementing with a solution of the same polymer in a volatile solvent. The solvent softens the surface, and the dissolved polymer molecules bond them together Monomer-cementing, where surfaces are coated with monomer which polymerise onto the pre-existing polymer chains, creating a bond. Use of epoxy additives and various snap fasteners; must be designed to distribute the fastening load uniformly over a wide area to avoid fracture. Can be friction-welded, by bringing the rotating parts into contact; frictional heat melts the surfaces which are held under static load until they re-solidify. Joining can sometimes be avoided by integral design, in which coupled components are moulded into a single unit. Lec 29, Page 17/21

18 Polymer Properties All polymers show a spectrum of mechanical behaviour: brittle-elastic at low temperature through plastic to viscoelastic or leathery, to rubbery, and finally to viscous at high temperature. Between -20 C and +200 C, polymer can pass through all of these mechanical states modulus and strength can change by a factor of 1000 or more Tensile response of polymers: -- brittle response (aligned, cross linked and networked case) Three typical classes of polymer stress-strain characteristic -- plastic response (semi-crystalline case) Temperature Effect on Stress-Strain Curve Decreasing T... increases E increases TS decreases %EL Behavior w.r.t. glass transition temperature T g (= 3 C for PMMA): T > Tg: plasticity T Tg: brittleness Increasing strain rate causes the same effects as decreasing T. Effect of temperature on stress-strain behavior of PMMA Lec 29, Page 18/21

19 The stress-strain curve for 6,6-nylon, a typical thermoplastic polymer General Characteristics: Summary Modulus of Elasticity may be as low as 7 MPa or has high as 4x10 3 MPa (compare to x10 3 MPa for metals) Tensile Strength about 100 MPa (metals up to 4100 MPa) Elongation often elongate plastically as much as 1000% (compare to metals - rarely over 100%) Temperature Dependence mechanical properties are very T dependent - even close to room T Strain Rate Dependence same behavior as raising temperature Lec 29, Page 19/21

20 Polymeric Materials Data Specifications are not tight as in metals two pieces of 316L stainless steels from two different manufacturers will not differ much but polyethylene made by one manufacturer may be very different from polyethylene made by another this is due to the variations in polymerisation process, which controls (1) molecular length, (2) molecular branching, and (3) degree of crystallinity in the final product all of these can be further changed by a varying degrees of mechanical processing For all of these reasons, data from compilations are approximate. Fore accurate data, use manufacturers data sheet, or conduct your own tests. Room temperature data for generic polymers Polymer Density D (Mg/m 3 ) Young s Modulus E (Gpa) Tensile Strength TS (Mpa) Fracture Toughness K C (MPa. m) Glass Temp. T g (K) Polyethylene (LDPE) Polyethylene (DPE) Polypropylene (PP) PTFE (teflon) Polystyrene (PS) Polyvinyl chloride (PVC) PMMA (perspex) Softening Temp. T s (K) Nylons Epoxies Polyesters Phenol formaldehyde Lec 29, Page 20/21

21 Next Class Lecture 33 Composite Materials Lec 29, Page 21/21