Chem 109H Fall08

As you can see, there is no order to the arrangement of the atoms. We call materials like this amorphous. This is the glass that is used for telescope lenses and such things. It has very good optical properties, but it's brittle. For everyday uses, we need something tougher. Most glass is made from sand, and when we melt down the sand, we usually add some sodium carbonate. This gives us a tougher glass with a structure that looks like this:

So is this a polymer or not? Usually it isn't considered as such. Why? Some may say it's inorganic, and polymers are usually organic. But there are many inorganic polymers out there. For example, what about polysiloxanes? These linear, and yes, inorganic materials have a structure very similar to glass, and they're considered polymers. Take a look at a polysiloxane:

So glass could be considered a highly crosslinked polysiloxane. But we usually don't think of it that way. Why not? Probably because even in a highly crosslinked system, you could still trace a polymer chain and see where the crosslinks are. But with glass, it'd be tough to do that.

Other examples of thermoplastics are: Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film, house wrap, agricultural film Polypropylene: carpet fibers, automotive bumpers, microwave containers, external prostheses Polyvinyl chloride (PVC): sheathing for electrical cables, floor and wall coverings, siding, credit cards, automobile instrument panels

These monomers are then chemically bonded into chains called polymers.

The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement.

Polymers are created by the chemical bonding of many identical or related basic units and those produced from a single monomer type are called homopolymers. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the carbon backbone.

In order to achieve a commercial product, the plastic is subject to further treatment and the inclusion of additives which are selected to give it specified properties

Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer properties as flow; to provide product color; and to provide special characteristics such as improved surface appearance or reduced friction.

Types of Additives:     antioxidants: for outside application,      colorants: for colored plastic parts, foaming agents: for Styrofoam cups,      plasticizers: used in toys and food processing equipment

A Thermoset is a polymer that solidifies or "sets" irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form.

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled.

In this method, a separate molding and cooling station on the equipment allows the parison to be continuously formed.  This technique is used mainly for small thin-walled parts ranging up to containers with five gallon capacities.  Parison programming can be used to vary the wall thickness.  Continuous extrusion also allows the use of heat-sensitive materials due to streamlined flow areas and die designs.

This technique is performed in three basic ways --reciprocating, ram accumulator, and accumulator head systems.  All three vary in machine design and the flow of molten resin through the die for parison forming.  However, each system is designed to produce larger, heavier, and thicker parts than continuous extrusion.

Blow moldable grades of material are initially injection molded into preform shapes.  These preforms are then thermally conditioned and then stretched (utilizing pneumatically operated stretch rods) low pressure air, followed by high pressure air up to 40 bar to form axially oriented parts with molded in necks.  The process is used to manufacture PET bottles.

This process utilizes various thermoplastic materials in a solid pelletized state and converts these materials by way of heat, pressure and compressed air into a finished good stat.The pellitized raw material is conveyed to the feed section of a plasticating extruder by way of a vacuum loader or auger screw.  The raw material is then conveyed forward through the extruder and is plastisized to a molten state of between 350 degrees and 500 degrees F. by way of a feed screw and external heating elements.The material in a melt state is then reshaped into a round hollow geometry termed a parison.  This parison is then extruded vertically from the head section of the machine through a round die at various outside and inside diameters.After extrusion of the parison between the two halves of a mold the press section closes encapsulating the parison inside the mold halves.  Upon mold close compressed air is entered into the parison by way of a centrally located air pipe or by piercing air needles.The molds are chilled with cooled water which transfers the hear form the now formed part inside the mold.  Upon complete part cooling the press section opens and the finished product is removed.  The material which is pinched off outside the mold cavity, or the flash, is then fed into a granulator which cops the flash into a granule size which can be fed back to the feed section of the extruder.

To form polypeptides and proteins, amino acids are joined together by peptide bonds (def), in which the amino or NH2 of one amino acid bonds to the carboxyl (acid) or COOH group of another amino acid as shown in (see Fig. 2). Animation showing the formation of a peptide bond.

The actual order of the amino acids in the protein is called its primary structure (def) (see Fig. 3) and is determined by DNA.

it is commonly said that the order of deoxyribonucleotide bases (def) in a gene determines the amino acid sequence of a particular protein. Since certain amino acids can interact with other amino acids in the same protein, this primary structure ultimately determines the final shape and therefore the chemical and physical properties of the protein.

In globular proteins such as enzymes, the long chain of amino acids becomes folded into a three-dimensional functional shape or tertiary structure (def).

In some cases, such as with antibody molecules and hemoglobin, several polypeptides may bond together to form a quaternary structure (def) (see Fig 6).

Polysaccharides make ideal storage molecules for energy for a number of reasons; a) they are large, this makes them insoluble in water and therefore they exert no osmotic or chemical effect on the cell; b) they fold into compact shapes; c) they are easily converted into the required sugars when needed.

Glycogen is a branched polysaccharide found in nearly all animal cells and in certain protozoa and algae.

In humans and other vertebrates it is principally stored in the liver and muscles and is the main form of stored carbohydrate in the body, acting as a reservoir of glucose

Starch is similar to glycogen, however it is found in plant cells, protists and certain bacteria.

The starch granules are made up of two polysaccharides, amylose and amylopectin. Amylose is an unbranched molecule made up of several thousand glucose units, coiled helically into a more compact shape. Amylopectin is also compact but has a branched structure and is made up of twice as many glucose units as amylose.

For example, peptidoglycans, which are a combination of protein and polysaccharide and are found in the cell wall of certain bacteria. Glycolipids, a combination of polysaccharides and lipids are found in the cell membrane.

Apparently, it's difficult to use these nanoclays. So the researchers turned to "high-powered ultrasonics — high-frequency sound waves too high for human hearing – to separate and disperse the platelets."

Example 1:A carboxylic acid monomer and an amine monomer can join in an amide linkage. As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer. Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide.

Example 2:A carboxylic acid monomer and an alcohol monomer can join in an ester linkage. A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding.

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

The 9 atoms that make up the fused rings (5 carbon, 4 nitrogen) are numbered 1-9. All ring atoms lie in the same plane.

The deoxyribose sugar of the DNA backbone has 5 carbons and 3 oxygens. The carbon atoms are numbered 1', 2', 3', 4', and 5' to distinguish from the numbering of the atoms of the purine and pyrmidine rings. The hydroxyl groups on the 5'- and 3'- carbons link to the phosphate groups to form the DNA backbone

A nucleoside is one of the four DNA bases covalently attached to the C1' position of a sugar.

Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and (deoxy)thymidine (dT, or T).

A nucleotide is a nucleoside with one or more phosphate groups covalently attached to the 3'- and/or 5'-hydroxyl group(s).

The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as "phosphodiester" bonds.

DNA is a normally double stranded macromolecule. Two polynucleotide chains, held together by weak thermodynamic forces, form a DNA molecule.

Two DNA strands form a helical spiral, winding around a helix axis in a right-handed spiral The two polynucleotide chains run in opposite directions The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a sprial staircase The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.

Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite strand, and G forms 3 hyrdorgen bonds with C on the opposite strand.

their products have variable physical properties. To this day, the systems are little understood, but the monomers (polymer starting materials) react through a number of reaction sites on the catalyst. Unfortunately, this means the polymer can grow from many sites and at different rates, leading to a very wide distribution in the molecular weight, based on the polymer chain length.

Modern life has demanded more of the humble polymer. We want polymers that are stronger than steel, lighter than aluminium, and can be dyed any colour imaginable

Metallocenes are positively charged metal ions, most commonly Titanium or Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see fig). Their big advantage over the Ziegler-Natta systems is that they catalyse the reaction of olefins through only one reactive site. Due to this “single site” reaction, the polymerization continues in a far more controllable fashion, leading to polymers with narrow ranges of molecular weight and, more importantly, predictable and desirable properties.

it has been found that changing the ligands (functional groups attached to the metal) upon the metallocene molecule can controllably affect the properties of the polymer.

The drawback of metallocene catalysts is that they are unable to polymerize polar molecules, such as common acrylics or vinyl chloride. This is due to the metallocenes’ oxophilicity – their propensity for binding to oxygen.

Catalysts using late transition metals – those metals from groups 6 and higher in the Periodic Table – have become increasingly utilized. These compounds have good polymerization activity, although slightly less than metallocenes. However, crucially they can polymerize reactions with polar monomers.

The most commercially advanced of this type of catalysts are the Brookhart catalysts [6], which are diimine complexes of palladium or nickel (see fig).

Take a piece of copper wiring," says Meddick. "It is encased in plastic - a kind of hydrocarbon material. We release all the hydrocarbons, which strips the casing off the wire." Not only does the process produce fuel in the form of oil and gas, it also makes it easier to extract the copper wire for recycling.

Autofluff is the stuff that is left over after a car has been shredded and the steel extracted. It contains plastics, rubber, wood, paper, fabrics, glass, sand, dirt, and various bits of metal. GRC says its Hawk-10 can extract enough oil and gas from the left-over fluff to run the Hawk-10 itself and a number of other machines used by Gershow.

Because it makes extracting reusable metal more efficient and evaporates water from autofluff, the Hawk-10 should also reduce the amount of end material that needs to be deposited in landfill sites.

Table 7 shows that the composition varies from chains of only monomer A (homopolymer A) to chains containing only monomer B (homopolymer B).

In many of the structures no regularity can be detected, although there will be short sequences of one type of unit, and the copolymer can be regarded as completely random; such copolymers are usually said to be ideal copolymers. These possible copolymer structures are shown schematically in Table 7.

It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1 and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of growing chain end adds on to a monomer of the same structure relative to the rate at which it adds on to the alternative monomer. The copolymer equation can be used to predict chain structure in the three different ways, already mentioned.

An ideal copolymer will tend to form when each type of chain end shows an equal preference for adding on to either monomer. In this case, and the copolymer equation becomes Hence composition depends on the relative amounts of monomer present at any time and the relative reactivities of the two monomers.

Step growth copolymerizations produce ideal (random) copolymers since in this special case r 1 = r 2 = 1.

The main reason for copolymerizing different monomers is to adjust the physical properties of a given homopolymer to meet a specific demand. SBR elastomer, for example (Table 1), based on 24 wt% styrene monomer shows better mechanical properties and better resistance to degradation than polybutadiene alone

A second reason for copolymerization is to enhance the chemical reactivity of a polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers is brought about by forming sulphur crosslinks at or near double bonds in the chain

To show the dramatic effect of copolymer structure on physical properties, consider the change from random SBR copolymer to a block copolymer of exactly the same chemical composition but where the styrene and butadiene parts are effectively homopolymer chains linked at two points: The material behaves like a vulcanized butadiene rubber without the need for chemical crosslinking since the styrene chains segregate together to form small islands or domains within the structure. Such so-called thermoplastic elastomers (TPEs) today form an important growth area for new polymers because of the process savings in manufacture that can be achieved with their use.

call Styrofoam. Polypropylene (PP) is used in dishwasher-safe containers

You'll often find polyethylene terephthalate (PET) in soda bottles, and it is sometimes recycled into fleece, upholstery fabrics, and other useful materials. And then of course there's polyvinyl chloride (PVC)

about 4 percent of the world's annual oil production of some 84.5 million barrels per day is used as feedstock for plastic, and another 4 percent or so provides the energy to transform the feedstock into handy plastic.

drilling. Recycling, however, does cut into energy use. According to the U.S. EPA, manufacturing new plastic from recycled plastic requires two-thirds of the energy used in virgin plastic manufacture. I have more numbers, too: one ton of recycled plastic saves 685 gallons of oil.

A five-carbon sugar (hence a pentose). Two kinds are found: Deoxyribose, which has a hydrogen atom attached to its #2 carbon atom (designated 2') Ribose, which has a hydroxyl group atom there

A nitrogen-containing ring structure called a base. The base is attached to the 1' carbon atom of the pentose. In DNA, four different bases are found:

The combination of a base and a pentose is called a nucleoside.

One (as shown in the first figure), two, or three phosphate groups. These are attached to the 5' carbon atom of the pentose.

The nucleic acids, both DNA and RNA, consist of polymers of nucleotides. The nucleotides are linked covalently between the 3' carbon atom of the pentose and the phosphate group attached to the 5' carbon of the adjacent pentose.

Most intact DNA molecules are made up of two strands of polymer, forming a "double helix". RNA molecules, while single-stranded, usually contain regions where two portions of the strand twist around each other to form helical regions.

It can be terminated--stage three--when any two free radicals combine, thus pairing their unpaired electrons and forming a covalent bond that links two chains together.

Polyethylene molecules made with the free radical initiation process tend to form branches that keep the molecules from fitting closely together. Techniques have been developed that use catalysts, like Cr2O3, to make polyethylene molecules with very few branches.

yielding a high-density polyethylene, HDPE, that is more opaque, harder, and stronger than the low-density polyethylene, LDPE, made with free radical initiation.

Advantages of emulsion polymerization include:[1] High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate. The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate. Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight. The final product can be used as is and does not generally need to be altered or processed. Disadvantages of emulsion polymerization include: Surfactants and other polymerization adjuvants remain in the polymer or are difficult to remove For dry (isolated) polymers, water removal is an energy-intensive process Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.

The first "true" emulsion polymerizations, which used a surface-active agent and polymerization initiator, were conducted in the 1920s to polymerize isoprene.[6][7]

The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps: A monomer is dispersed or emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water. Excess surfactant creates micelles in the water. Small amounts of monomer diffuse through the water to the micelle. A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles. (This characteristic differs from suspension polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart Interval 1. The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet. Monomer in the micelle quickly polymerizes and the growing chain terminates. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart Interval 2. More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react. Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart Interval 3. Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow. The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'.

Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation.

Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern process control schemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.

Colloidal stability is a factor in design of an emulsion polymerization process. For dry or isolated products, the polymer dispersion must be isolated, or converted into solid form. This can be accomplished by simple heating of the dispersion until all water evaporates. More commonly, the dispersion is destabilized (sometimes called "broken") by addition of a multivalent cation. Alternatively, acidification will destabilize a dispersion with a carboxylic acid surfactant. These techniques may be employed in combination with application of shear to increase the rate of destabilization. After isolation of the polymer, it is usually washed, dried, and packaged.

Ethylene and other simple olefins must be polymerized at very high pressures (up to 800 bar).

Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization.

Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility.

Ethylene and other olefins are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers.

Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C.

Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption

Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent.

Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate.

Some grades of poly(vinyl alcohol) and other water soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower surface tension). It is believed that these polymers graft onto growing polymer particles and stabilize them.[12]

Other ingredients found in emulsion polymerization include chain transfer agents, buffering agents, and inert salts. Preservatives are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.

Polymers produced by emulsion polymerization can be divided into three rough categories. Synthetic rubber Some grades of styrene-butadiene (SBR) Some grades of Polybutadiene Polychloroprene (Neoprene) Nitrile rubber Acrylic rubber Fluoroelastomer (FKM) Plastics Some grades of PVC Some grades of polystyrene Some grades of PMMA Acrylonitrile-butadiene-styrene terpolymer (ABS) Polyvinylidene fluoride PTFE Dispersions (i.e. polymers sold as aqueous dispersions) polyvinyl acetate polyvinyl acetate copolymers latexacrylic paint Styrene-butadiene VAE (vinyl acetate - ethylene copolymers)