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Modern Materials I - Polymers and Biomaterials • Penn State has a rich tradition in materials research. • First school to offer a materials curriculum in 1960. • Developed the first interdisciplinary materials laboratory in 1962. • Penn State is ranked #1 in the US in materials research Materials You Know Ceramics (inorganic, nonmetalic solids) Material: Zeolites for water softening. Na+ ions in aluminosilicate cages are easily exchanged for Ca2+ Sodium Zeolite A Polymers Material: Kevlar for bullet proof vests. Hydrogen bonding between polymer chains produces a strong material. Kevlar Materials You Know Semiconductors Material: Light Emitting Diodes in traffic lights. A diode is a semiconducor device that only allows current to flow in one direction only. LEDs are diodes The chemical makeup of the LED determines the wavelength (color) of the photon. Materials You Know Mr. McGuire: Are you listening? Ben: Yes I am. Mr. McGuire: “Plastics.” Ben: Exactly how do you mean? Mr. McGuire: There's a great future in plastics. Think about it. Will you think about it? Ben: Yes I will. The Graduate 1967 POLYMERS Monomer = repeating unit Polymer = macromolecule made by joining monomers Polymer Synthesis Condensation polymers polymers form by eliminating a small molecule (H2O, HCl, CO2) from monomers Examples: Biological Polymers Proteins DNA carbohydrates Addition polymers Monomer contains a double bond Polymers form by addition reaction Condensation Reactions 2 molecules join by eliminating a small molecule (such as water) Esters and Amides are formed by condensation reactions: carboxylic + alcohol ester + water acid O O + HOR’ + H2O C C R OR' R OH carboxylic + amine acid O + H N H C R OH R amide + water O R C NHR' + H2O Structure of Proteins Secondary structure of proteins is maintained by H-bonding Tertiary structure (protein folding) is even more complex. This structure is important in protein activity. DNA monomer ADDITION POLYMERS ethylene TFE styrene Poly(ethylene) (lunch bags) Teflon Poly(styrene) (styrofoam) 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 Natural rubber (gummy) Cross-linked rubber (tough elastomer) Poly(vinylalcohol): An Addition Polymer H H C=C vinyl alcohol H OH CH ( CH 2 catalyst heat OH OH CH ) CH x 2 poly(vinyl alcohol) x = 10 3- 104 linear chain polymer cross linker: borax (B O 2- ) 4 7 Resulting material is viscoelastic Properties of Polymers •Polymer chains tend to be flexible and easily entangled or folded. Crystalline alignment of chains makes a polymer stiffer. •Stretching or extruding a polymer can increase crystallinity. •Degree of crystallinity determined by chemical structure and polymer chain length: low density polyethylene (LDPE) has an average molecular weight of 104 amu (used in plastic wrap) high density polyethylene (HDPE) has an average molecular weight of 106 amu (used in milk cartons). Biomaterials Characteristics of Biomaterials Biomaterials are any materials that have biomedical applications. For example, the materials that are used to fill teeth are biomaterials. The biomaterials must be biocompatible: The body’s immune system must not attack the biomaterial. Biomaterials Biomaterials Characteristics of Biomaterials 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. Biomaterials 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.). Biomaterials 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. If moderately deformed, the elastomer will return to its original shape. Used as catheters, and for covering leads on implanted electronics, like pacemakers. Biomaterials Examples of Biomaterial Applications Heart Replacement and Repairs: A heart that fails completely must be replaced by a donor organ. About 60,000 people in the US suffer heart failure and only 2,500 donor hearts are available. About 250,000 heart valve replacements are made each year. About 45 % of these valve replacements occur with a mechanical valve. Biomaterials Examples of Biomaterial Applications The replacement valve must be smooth to prevent the destruction of blood vessels. The valve must also be anchored to the inside of the heart. Polyethylene terephthalate, called Dacron™, is used in the artificial heart valves. Dacron™ is used because tissue will grow through a polymer mesh. Biomaterials Examples of Biomaterial Applications Vascular grafts: A vascular graft is the replacement for a piece of blood vessel. Dacron™ is used for large arteries. Polytetrafluoroethylene, -[(CF2CF2)n-]-, is used for smaller vascular grafts. Biomaterials 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 acid-glycolic acid copolymer. Biomaterials Examples of Biomaterial Applications Hip Replacements: About 200,000 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.