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Natural polymers (from the Greek poly meaning "many" and meros meaning "parts") are found in many forms such as horns of animals, tortoise shell, shellac (from the lac beetle), rosin (from pine trees), asphalt, and tar from distillation of organic materials. One of the most useful of the natural polymers was rubber, obtained from the sap of the hevea tree. (Rubber was named by the chemist Joseph Priestley who found that a piece of solidified latex gum was good for rubbing out pencil marks on paper. In Great Britain, erasers are still called "rubbers".) Natural rubber had only limited use as it became brittle in the cold and melted when warmed. In 1839, Charles Goodyear discovered, through a lucky accident, that by heating the latex with sulfur, the properties were changed making the rubber more flexible and temperature stable. That process became known as vulcanization.
The first synthetic polymer, a phenol-formaldehyde polymer, was introduced under the name "Bakelite", by Leo Baekeland in 1909. Its original use was to make billiard balls. Rayon, the first synthetic fiber was developed as a replacement for silk in 1911. Although many polymers were made in the following years, the technology to mass produce them was not developed until World War II, when there was a need to develop synthetic rubber for tires and other wartime applications and nylon for parachutes. Since that time, the polymer industry has grown and diversified into one of the fastest growing industries in the world. Today, polymers are commonly used in thousands of products as plastics, elastomers, coatings, and adhesives. They make up about 80% of the organic chemical industry with products produced at approximately 150 kg of polymers per person annually in the United States.
What is a polymer?
Polymer (pÅl'É™mÉ™r), chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bond). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer, and three monomers, a trimer. A structural unit is a group having two or more bonding sites. A bonding site may be created by the loss of an atom or group, such as H or OH, or by the breaking up of a double or triple bond, as when ethylene, H2C-CH2, is converted into a structural unit for polyethylene, -H2C-CH2-. In a linear polymer, the structural units are connected in a chain arrangement and thus need only be bifunctional, i.e., have two bonding sites. When the structural unit is trifunctional (has three bonding sites), a nonlinear, or branched, polymer results. Ethylene, styrene, and ethylene glycol are examples of bifunctional monomers, while glycerin and divinyl benzene are both polyfunctional. Polymers containing a single repeating unit, such as polyethylene, are called homopolymers. Polymers containing two or more different structural units, such as phenol-formaldehyde, are called copolymers. All polymers can be classified as either addition polymers or condensation polymers. An addition polymer is one in which the molecular formula of the repeating structural unit is identical to that of the monomer, e.g., polyethylene and polystyrene. A condensation polymer is one in which the repeating structural unit contains fewer atoms than that of the monomer or monomers because of the splitting off of water or some other substance, e.g., polyesters and polycarbonates (see illustration). Many polymers occur in nature, such as silk, cellulose, natural rubber, and proteins. In addition, a large number of polymers have been synthesized in the laboratory, leading to such commercially important products as plastics, synthetic fibers, and synthetic rubber. Polymerization, the chemical process of forming polymers from their component monomers, is often a complex process that may be initiated or sustained by heat, pressure, or the presence of one or more catalysts.
Cross linked polymer
Cross linked polymer is a type of polymer.
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers (such as proteins). When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of cross-links to promote a difference in the polymers' physical properties. When "crosslinking" is used in the biological milieu, it refers to use of a probe to link proteins together to check protein-protein interactions, as well as other creative cross-linking methodologies.
Cross-linking is used in both synthetic polymer chemistry and in the biological sciences. Although the term is used to refer to the "linking of polymer chains" for both sciences, the extent of crosslinking and specificities of the crosslinking agents vary. Of course, with all science, there are overlaps, and the following delineations are a starting point to understanding the subtleties
Vulcanization is an example of cross-linking.
Examples of cross linked polymer
Crosslinks in synthetic polymer chemistry
When polymer chains are linked together by cross linking,they loose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a soild or gel by cross linking the chains together.
In polymer chemistry, when a synthetic polymer means is said to be crosslinked , it usually means that the bulk of the polymer has been exposed to the crosslinking method, resulting in fairly extensive crosslinking treatment. Crosslinking inhabits close packing of the polymer chains, preventing the formation of crystalline regions. The restricted molecular mobility of a crosslinked structure limits the extension of the polymer material under loading.
(a)thermoplastic-no cross linked
Formation of crosslinks
Crosslinks can be formed by chemical reactions that are initiated by heat ,pressure or radiation. For example, mixing of an unpolymerized or partially polymerized resin with specific chemicals called crosslinking reagents results in a chemical reaction that forms crosslinks. Crosslinking can also be induced in materials that are normally thermoplastic through exposure to a radiation source, such as electron beam exposure , gamma radiation , or UV light. For example ,electron beam processing is used to cross linking the C type of cross linked polyethylene. Other types of cross linked polyethylene are made by addition of peroxide during extruding (type A) pr by addition of a cross linking agent (eg vinylsline) and a catalyst during extruding and then performing a post extrusion curing.
The chemical process of vulcanization is a type of cross linked and it changes the property of rubber to the hard durable material we associate with car and bike tires. This process is often called sulfur curing, and the term vulcanization comes from Vulcan, the Roman god of fire. However, this is a slow process, taking around 8 hours. A typical car tire is cured for 15 minutes at 150Â°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalysing the addition of sulfur chains to the rubber molecules.
Crosslinks are the characteristic property of thermosetting plastic materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave solutions, for example, break and re-form naturally occurring cross-links (disulfide bonds) between protein chains in hair.
High Conductivity Single-ion Cross-linked Polymers for Lithium Batteries and Fuel Cells
APPLICATION OF TECHNOLOGY:
Lithium metal polymer and lithium polymer batteries
PEM fuel cells
High lithium ion conductivity (10-5 S/cm at ambient temperatures)
No concentration polarization, i.e. the transference number is one
Clean grafting and cross-linking chemistry leaves no reactive residues
Materials have uniform, reproducible mechanical properties and electrochemical stability
Polymer backbone and cross-link density and flexibility may be adapted to tune the transport and mechanical properties
Innovative solution to lithium mobility problems promises even higher conductivity
John Kerr and co-workers at Berkeley Lab have developed single-ion cross-linked comb-branched polymer electrolytes with high conductivity for use as membranes in lithium batteries, fuel cells, and electrochromic windows. Solid polymer electrolyte separators are used in lithium batteries instead of common organic solvents because (1) they are non-volatile, (2) they inhibit the growth of dendrites, the tiny metallic snowflake structures in lithium metal electrodes that lead to battery failure, and (3) they can be used in very thin films thereby improving the power performance of the battery and increasing the energy density.
Solid polymer electrolytes have been improved by the creation of single-ion polymer conductors. Single ion conductors, transference number of one, avoid the development of concentration gradients that result in low voltage upon discharge and irreparable damage on charge because the anion is immobilized by covalently connecting it to the polymer comb. Until now, lithium single ion polymer conductors have been plagued with low conductivity, reactivity to lithium, poor cathode compatibility, and mechanical stiffness that leads to poor processing properties. Kerr's new cross-linked polymer electrolytes based on trifluoromethylsulfonylmethide, sulfonate, and fluoroalkylsulfonate and imide anions overcome these limitations.
The controllable method of preparation results in a material that has uniformly excellent mechanical and ion transport properties that appear to be unaffected by the cross-linking density. This allows density to be varied to suit the application. The cross-linked materials achieve much higher lithium ion conductivities than other cross-linked polymers (10-5 S/cm at ambient temperatures) and yet also inhibit dendrite growth due to the mechanical properties. The side chains of the comb-branched structures are long enough to allow for maximum segmental motion so that the polymer can effectively penetrate between the electrode particles and adhere to electrode surfaces while maintaining the amorphous nature that facilitates high ion mobility. This overcomes many of the problems involved in the preparation of good composite electrode structures.
The capabilities, materials, and principles used for developing these polymer electrolytes for lithium batteries can be adapted to develop polymer films for fuel cells and electrochromic windows. Kerr's group is investigating the use of new proton solvating functions on comb branch polyether polyelectrolyte materials to provide water-free membranes that can operate at high temperatures for fuel cells.
Crosslinker use in protein study
The interactions or mere proximity of proteins can be studied by the clever use of crosslinking agents. For example, protein A and protein B may be very close to each other in a cell, and a chemical crosslinker could be used to probe the protein-protein interaction between these two proteins by linking them together, disrupting the cell, and looking for the crosslinked proteins.
A variety of crosslinkers are used to analyze subunit structure of proteins, protein interactions and various parameters of protein function by using differing crosslinkers often with diverse spacer arm lengths. Subunit structure is deduced since crosslinkers only bind surface amino residues in relatively close proximity in the native state. Protein interactions are often too weak or transient to be easily detected, but by crosslinking, the interactions can be captured and analyzed.
Examples of some common crosslinkers are the imidoester crosslinker dimethyl suberimidate, the NHS-ester crosslinker BS3 and formaldehyde. Each of these crosslinkers induces nucleophilic attack of the amino group of lysine and subsequent covalent bonding via the crosslinker. The zero-length carbodiimide crosslinker EDC functions by converting caboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water soluble analog, Sulfo-SMCC, are commonly used to prepare antibody-hapten conjugates for antibody development.
In-vivo crosslinking of protein complexes using photo-reactive amino acid analogs was introduced in 2005 by researchers from the Max Planck Institute In this method, cells are grown with photoreactive diazirine analogs to leucine and methionine, which are incorporated into proteins. Upon exposure to ultraviolet light, the diazirines are activated and bind to interacting proteins that are within a few angstroms of the photo-reactive amino acid analog (UV cross-linking).
Uses for crosslinked polymers
Synthetically crosslinked polymers have many uses, including those in the biological sciences, such as applications in forming polyacrylamide gels for gel electrophoresis. Synthetic rubber used for tires is made by crosslinking rubber through the process of vulcanization. Also most rubber articles are cross-linked to make them more elastic. Hard-shell kayaks are also often manufactured with crosslinked polymers.
Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.
Application of cross-linked beta-cyclodextrin polymer for adsorption of aromatic amino acids
Beta-cyclodextrin (beta-CyD) was cross-linked by hexamethylene diisocyanate and the polymer was investigated for adsorption of aromatic amino acids (AAA) from phosphate buffer. High adsorption rates were observed at the beginning and the adsorption equilibrium was then gradually achieved in about 45 min. The adsorption of AAA decreased with the increase of initial concentration and also temperature. Under the same conditions, the adsorption efficiencies of AAA were in the order of L-tryptophan (L-Trp) > L-phenylalanine (L-Phe) > L-tyrosine (L-Tyr). Much higher adsorption values, up to 52.4 and 43.0 mg/g for L-Trp and L-Phe, respectively, at 50 mmol/L and 3.2 mg/g for L-Tyr at 2 mmol/L, were obtained with the beta-CyD polymer at 37 degrees C. It was shown that the adsorption of AAA on the beta-CyD polymer was consistent with the Freundlich isotherm equation. The adsorption of mixed aromatic amino acids and branched-chain amino acids (BCAA) showed that AAA were preferentially adsorbed with adsorption efficiencies 10-24%, while those of BCAA were lower than 2%. It seems that the structure and hydrophobicity of amino acid molecules are responsible for the difference in adsorption, by influencing the strength of interactions between amino acid molecule and the polymer.