Modern Materials One - Modern Materials 1. Polymers Polymer...

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Unformatted text preview: Modern Materials 1. Polymers Polymer synthesis Polymers: properties Bio-polymers Polymers: uses 2. BioMaterials 3. Ceramics 4. Glass POLYMERS High molecular weight materials formed from many small molecules called monomers. Monomer = repeating unit Polymer Synthesis 1. Addition polymers Monomer contains a double bond polymers form via addition reaction Rubber Cross-linking 2. Condensation polymers polymers form via condensation reaction a small molecule (H2O, HCl, CO2) is eliminated in the reaction Silicone polymers 3. Ring opening Cyclic monomer opens to form a linear chain (eg. polymerization of Sulfur) Biological Polymers Proteins DNA carbohydrates ADDITION POLYMERS ADDITION POLYMERS H C H C H H CH2 CH2 n polyethylene(gladwrap) F H C H C H H C C H H C C H ethylene F C C F CF2 CF2 H H H H H F n Teflon,TFal tetrafluoroethylene ethylene monomers H C H C H CH2 CH n H * C H H C H H C H H C H H C H H H stryrene H C H C Cl vinyl chloride H CH3 C H C O C CH3 polystyrene(Styrofoam) C H * CH2 CH n Cl polyvinyl chloride(PVC) CH3 CH2 C polyethylene n C C H3 O O polyMMA (Plexiglass) O methyl methacrylate MMA Condensation Reactions Rubber: A naturally occurring Addition Polymer 2 molecules join by eliminating a small molecule (such as water). Ethers, Esters, and Amides are formed via condensation reactions: alcohol ROH + alcohol ether + water + H2O + HOR' ROR' carboxylic + acid O R C OH alcohol ester + water O + HOR' C + H2O R OR' carboxylic + amine acid O R C OH amide R O C + water + H N H R + H2O NHR' Condensation Polymers Polymers formed via Condensation reaction Examples Monomers Diol + diacid Polymers polyester Example Dacron nylon Diamine + diacid polyamide Diol is a compound with two --OH groups HO -- R -- OH Diacid is a compound with two --COOH groups HOOC-- R'--COOH Diamine has 2 --NH2 groups H2N --R"--NH2 Mixed monomers also work Example: amino acids H2N--R'--COOH Silicones Silicones: condensation polymers chains of alternating silicon and oxygen atoms with organic substituents on the silicon. Ring Opening Cyclic monomer opens to form a linear chain Monomer: silicon compound with organic groups (R) attached. R2SiCl2 Example: dichlorodimethylsilane [(CH3)2SiCl2] with water (H2O). The silicone formed is dimethylsiloxane. Properties of Polymers Properties depend on: 1) 2) 3) 4) Structure Identity of side groups Chain length Degree of cross-linking Polymer structure Chainlike polymers have a hard time crystallizing Properties of Polymers Interactions between chains of a polymer lend elements of order to the structure of polymers. Amorphous: polymer chains do not line up well Polymer chains tend to be flexible and easily entangled or folded. Degree of crystallinity is the amount of ordering in a polymer. Crystallinity effects optical properties Amorphous polymers (glasslike) are often transparent Polymers that are partly crystalline and partly glassy are translucent. Crystalline polymers are stiffer. Side chains can effect packing: Stretching or extruding a polymer can increase crystallinity. Stretching (or extruding) the polymer chains as they form can increase the amount of order, leading to a degree of crystallinity of the polymer. Properties of Polymers Degree of crystallinity is the amount of ordering in a polymer. Crystalline polymers are harder, more dense. Side chains can effect packing: Packs well; is more crystalline. Plastics will be stiffer: used to make tupperware Properties of polymers Polymers form glasses Amorphous: Doesn't pack well: used to make flexible plastics (garbage bags) Degree of crystallinity is also determined by average molecular mass: low density polyethylene (LDPE) has an average molecular mass of 104 amu (used in plastic wrap) high density polyethylene (HDPE) has an average molecular mass of 106 amu (used in milk cartons). Above Tm: polymer is liquid-like (fluid chains) Below Tm: wants to crystallize, but can't Between Tm and Tg= elastomer Tg = glass transition temperature: substance becomes amorphous solid (glass) Elastomer turns to brittle solid... Properties of Polymers Forces between chains Crosslinking: covalent bonds between chains: increases stiffness, strength Examples: rubber, PVA, Silicone Metal ions can serve as cross-linking agents Example: Sodium alginate Intermolecular forces can hold polymer chains together: LDF hold chains in nonpolar polymers together: Polyethylene, polypropylene H-bonding plays an important role in biological polymers DNA, Proteins Cross-Linking Bonds formed between polymer chains make the polymer stiffer. Natural rubber is too soft and chemically reactive to make a useful material. By vulcanizing the rubber (crosslinking the chains with sulfur) useful materials are made. S8 heat Isoprene (monomer) Natural rubber (gummy) Cross-linked rubber (tough elastomer) Crosslinking Poly(vinylalcohol): An Addition Polymer H C=C H OH heat C H2 Silicones Properties Nontoxic, stable (unreactive) flame resistant hydrocarbon substituents act to repel water. OH CH C H2 H catalyst HO CH vinyl alcohol x poly(vinyl alcohol) x = 103 - 104 linear chain polymer Resulting material is viscoelastic Properties depend on: 1) 2) 3) R-group Chain length Degree of cross-linking No crosslinks: liquid silicone oils high T lubricants, hot oil baths Few cross links; silicone rubber caulking material More cross links: silicone resins coatings and adhesives All cross links: SiO2 = quartz cross linker: borax B4O72- (forms covalent crosslink) Gummy worms: polymer of Sodium alginate Alginate is a polysaccharide found in seaweed: (It's edible!) Used to thicken food (like milkshakes) To make gummy worms: add crosslinker: CaCl2 Ca+2 ion hold chains together. Biological Polymers 1. DNA, RNA Monomers: nucleotides (Figure 25.32) React via condensation DNA and RNA are polyesters (of H3PO4). DNA 2. Proteins Monomers: amino acids(Figure 25.21) React via condensation Proteins are polyamides. amino acid monomers H N H H C R C OH O H H C R C OH O H H C R C OH O + H N + H N monomer protein H * N H C R C N H O H N C R C R O C C * H H O polymer amide linkage or peptide bond DNA Base pairs DNA replication Structure of Proteins Secondary structure of proteins is maintained by Hbonding Tertiary structure (protein folding) is even more complex. This structure is important in protein activity. Uses of Polymers in Smart Sensors Polymers play an important role in modern sensors Polyvinylidene Fluoride (PVDF) is one of the main polymers used to make smart sensors. Monomer: Polymer: H C H C F H F F * H C F C * n When a film of PVDF is stretched the F align on top and the H on the bottom, producing a vertical dipole moment. The dipole moment in PDVF gives it piezoelectric properties. The piezoelectric effect is the ability of a material to produce a voltage when subject to mechanical stress. The stretched PVDF film can be attached to electrodes. Bending the film will produce a voltage by fanning out the dipole moments. Other Piezoelectric Polymers PVDF is not the only polymer that is piezoelectric. Polyvinyl Fluoride (PVF): Monomer: Polymer: H C H C H H H Uses of Piezoelectric Polymers (PVDF) Computer touch screens Infrared sensors Rain intensity gauges Air Bag sensors Measurement of ocean waves F * H C F C * n Polytrifluoroethylene (PTRIF) Monomer: Polymer: H F C F C H * C F F C F * n F Properties of Piezoelectric Polymers The monomer must be polar. The atoms bonded to the polymer backbone must be small. Fluorine is common in piezoelectric polymers because of its size and electronegativity. Biomaterials Biomaterials are any materials that have biomedical applications. For example, the materials that are used to fill teeth are biomaterials. Characteristics of Biomaterials The biomaterials must be biocompatible: The body's immune system must not attack the biomaterial. Physical requirements: Biomaterials must be created for a specific environment. Artificial heart valves must open and close 70 to 80 times per minute. Chemical requirements: Biomaterials must be of medical grade. Polymers are very important biomaterials: beware of fillers, stabilizers, etc. Polymeric Biomaterials The degree to which the body tolerates foreign materials depends on the nature of the atomic groups in the material. Naturally occurring biomaterials are polymers of sugars (polysaccharides), nucleotides (RNA, DNA) and amino acids (proteins, enzymes, etc.). Types of Polymers Plastic: materials that can be formed into shapes. Thermoplastic: materials that can be shaped more than once. (Used as replacements for blood vessels.) Thermosetting: materials that can only be shaped once (Used in dental devices, and orthopedics such as hip replacements.) Elastomer: material that is elastic in some way. If a moderate amount of deforming force is added, the elastomer will return to its original shape. Used as catheters, and for covering leads on implanted electronics, like pacemaker. Examples of Biomaterial Applications Heart Replacement and Repairs: Polyethylene terephthalate, called DacronTM, is often used in the manufacture of artificial heart valves. DacronTM is used because tissue will grow through a polyurethane mesh. Examples of Biomaterial Applications Artificial Tissue: Artificial skin, which is grown in the laboratory, is used to treat patients with extensive skin loss. The challenge with growing artificial skin is getting the cells to align properly. Therefore a scaffold must be used for the cells. The most successful scaffold is lactic acidglycolic acid copolymer. Vascular grafts: A vascular graft is the replacement for a piece of blood vessel. DacronTM is used for large arteries. Polytetrafluoroethylene, is used for smaller vascular grafts. F * C F F C F * Hip Replacements: About 200,000 total hip replacements are performed each year. A metal ball, a cobalt chromium alloy, is often used in a hip replacement. This alloy is attached to a titanium alloy and cemented using a tough thermoset polymer. The acetabulum, which accommodates the femur, is lined with a polyethylene layer Ceramics Inorganic, nonmetallic solids Typically: brittle, stable at high temperatures less dense than metals, more elastic than metals high melting. Ceramics Applications of Ceramics Used in cutting tool industry. Used to make tiles on the space shuttle. Used in electronic industry (semiconductor integrated circuits usually made of alumina). Piezoelectric materials (generation of an electrical potential after mechanical stress) used in watches and ultrasonic generators. Highly resistant to heat, corrosion, and wear. Ceramics do not deform under stress. They are much less dense than metals, and so are used in their place in many high-temperature applications Can be covalent network and/or ionic Crystalline: oxides Al2O3, ZrO2,BeO Carbides SiC Silicates Aluminosilicates Al2O3 + SiO2 + Other metal oxides Mica, Talc, Clays Amorphous (glasses) Silicon (Si) Second most abundant element Always found in nature in combination with O e.g. SiO44- silicate minerals SiO2 sand quartz with Al alumino-silicates (clay) Structure: Tetrahedra e.g SiO44- Zircon, ZrSiO4 b) Chains or sheets asbestos: chains talc: sheets 3-D: complete cross-linking quartz Silicates SiO44- units Zircon ZrSiO4 Infinite Chain (SiO3)x2Asbestos Positive ions to balance charge Hold chains (sheets together) Silicates Infinite Sheet: (Si2O5)x Talc Fused Silica Glass GLASS: made from SiO2 Excellent materials properties: Transparent in visible and ultraviolet High index of refraction Low thermal conductivity Low thermal expansion coefficient But Tg = 1200C for fused silica (too high) Additives lower processing temperature Usually oxides like Na2O, CaO, B2O3, etc. Break up covalent ring network with ionic bonds Lower Tg (and therefore processing temp.) E.g., soda-lime glass (window glass), contains Na2O, CaO Softens at 600C 3-D infinite network: SiO2 Quartz, sand CEMENT Glass Soda-lime glass: SiO2, Na2O, CaO Soda lime glass + CoO deep blue (cobalt glass) Use K2O instead of Na2O harder glass, higher mp Using PbO instead of CaO denser "leaded" glass greater refractive index (bends light differently) Pyrex Glass "borosilicate Contains B2O3 in addition to SiO2, Na2O Has low thermal expansion coefficient (doesn't crack when rapidly heated or cooled) Glass with AgCl or AgBr added photochromic (dark when exposed to light, clear with little or no light) A "modern" glass H2O Ca3SiO5 Ca2SiO4 cement crystalline silicates (powders) Ca3Si2O73H2O glassy, ca. 70% of + Ca(OH)2 (crystalline) Cement is a composite material (glassy + crystalline phases heterogeneous mixture) Aluminosilicates Al2O3 + SiO2 + Other metal oxides Clays Replace Si4+ in sheets with Al3+ Ropes/sheets are more negative. Held together more strongly Clay (ceramic) hardening Cement Zeolites Zeolites Crystalline aluminosilcates Well-defined structures Porous (cage-like) Naturally occurring and man-made Molecular sieves Variety of pore sizes and uses Zeolites Uses Catalyst: reactions occur in cavities Example: petrochemical cracking ion-exchange water softening and purification separation of gases Used to separate O2 from N2 removal of gases and solvents; Example: radioactive Sr Ceramics Composites Composite: two or more materials making up a ceramic. Result: tougher ceramic. Most effective method: add fibers to ceramic material. Example: SiC fibers added to aluminosilicate glass. Fiber must have a length 100 times its diameter. Carbon nanotubes in polymer resins: to create high strength materials ...
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This note was uploaded on 07/23/2008 for the course CHEM 112 taught by Professor Vandersluys,lorschmid,kylem during the Fall '07 term at Pennsylvania State University, University Park.

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