Sheath polymer

Species Rigid sacculus S-layer (sheath) Polymer... 

A number of polymer and fiber modifications have been devised to overcome this problem, although none has been successful enough to allow acrylics to compete in successfully easy-care apparel markets. The fibers may be treated with compounds such as ammonium sulfide [433], hydrazine derivatives [434,435], thiosemicarbazides [436], silicone oils [437], and emulsions of polysiloxane and polyepoxide [438]. Some success has been achieved by incorporating comonomers that increase the wet glass transition temperature of the polymer or make the copolymer more water-resistant [439-444]. Sheath-core fibers have been reported [445] in which the core polymer is stable under hot-wet conditions and the sheath polymer is used to compensate for deficiencies in dyeability. 

Effect of solution concentration on nanofiber diameter (a) effect of sheath polymer concentration (sheath gelatin core PCL), (b) effect of core polymer concentration (sheath collagen core PCL). ... 

Some materials are rendered nonelectrospinnable due to their low molecular weight, limited solubihty, unsuitable molecular arrangement, or lack of the requisite viscoelastic properties. Conductive polymers, metals or some natural polymers that cannot form hbers by themselves for these reasons can be spun using this approach and can hnd applications in the electronics, optics and biomedical helds. The obvious requirements for this approach are that the sheath polymer be effectively electrospinnable by itself and possess appropriate viscosity. ... 

The first study, conducted by Sun et al., demonstrated the feasibility of the approach by producing hbers from a polymer poly(dodecylthiophene) (PDT) and a metal salt [palladium (II) diacetate (Pd(OAc)2)], either of which can form hbers by themselves. A subsequent study, by Yu et al. showed that ultra-hne hbers can be produced using a highly dilute solution of a polymer in the core, which would otherwise produce droplets due to jet break-up when spun alone. This concept was illustrated by the use of dilute solutions of PAN (3 and 5 wt%) in the core wrapped in PAN-co-PS. Removal of the sheath after spinning resulted in very hne PAN hbers (100 nm or less) with narrow and unimodal distribution. More recently, in a study based on silk and PEO (core/sheath) polymers, with water as the solvent for both, it was reported that the crystallized silk hbers (converted from the random coil I conformation to the stable y -sheet silk II structure) could be produced after anneaUng at high humidity (90% RH at 25°C) and subsequent water extraction of the PEO sheath. ... 

Li et ai adopted the same approach to prepare nanohbers using conjugated polymers having conductive properties for use in electronics and semiconductor apphcations.lhe polymers used were poly[2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthio-phene) (PHT) which could not be electrospun into hbers due to Umited solubility. The sheath polymer chosen was PVP which was later extracted using ethanol. The results show that using co-axial spinning technology. 

Dasgupta D, Kamar Z, Rochas C, Dahmani M, Mesini P, Guenet JM. Design of hybrid networks by sheathing polymer fibrils with self-assembled nanombules. Soft Matter 2010 6 3573-3581. 

A second type of sheath/core is filled fibers consisting of a core polymer produced from recycled material, conductive material or other material that is covered by a sheath that possesses desired esthetics or other properties. A third type of sheath/core is a sheath polymer containing expensive additives or other attributes primarily useful on the surface of filaments. 

More recentiy, melt-spun biconstituent sheath—core elastic fibers have been commercialized. They normally consist of a hard fiber sheath (polyamide or polyester) along with a segmented polyurethane core polymer (11,12). Kanebo Ltd. in Japan currentiy produces a biconstituent fiber for hosiery end uses called Sideria. 

Bicomponent fibers have also provided a route to self-texturing (self-crimping) fibers. The crimp results from the length differential developed during processing caused by differential shrinkage in the two polymers in side-by-side or eccentric core—sheath configurations (50). 

Difluoroethanol is prepared by the mercuric oxide cataly2ed hydrolysis of 2-bromo-l,l-difluoroethane with carboxyHc acid esters and alkaH metal hydroxides ia water (27). Its chemical reactions are similar to those of most alcohols. It can be oxidi2ed to difluoroacetic acid [381-73-7] (28) it forms alkoxides with alkaH and alkaline-earth metals (29) with alkoxides of other alcohols it forms mixed ethers such as 2,2-difluoroethyl methyl ether [461-57-4], bp 47°C, or 2,2-difluoroethyl ethyl ether [82907-09-3], bp 66°C (29). 2,2-Difluoroethyl difluoromethyl ether [32778-16-8], made from the alcohol and chlorodifluoromethane ia aqueous base, has been iavestigated as an inhalation anesthetic (30,31) as have several ethers made by addition of the alcohol to various fluoroalkenes (32,33). Methacrylate esters of the alcohol are useful as a sheathing material for polymers ia optical appHcations (34). The alcohol has also been reported to be useful as a working fluid ia heat pumps (35). The alcohol is available ia research quantities for ca 6/g (1992). 

The amount of plasticizer added to the polymer in question varies, depending on the magnitude of the effect required. For example, a small addition of plasticizer may be made simply to improve the workabiUty of the polymer melt. This contrasts with larger additions made with the specific intention of completely transforming the properties of the product. For example, PVC without a plasticizer, ie, unplasticized PVC (PVC-U), is used in appHcations such as pipes and window profiles with plasticizer added, articles such as PVC food film, PVC cable insulation, and sheathing and PVC floorings are formed. 

Bicomponent technology has been used to introduce functional and novelty effects other than stretch to nylon fibers. For instance, antistatic yams are made by spinning a conductive carbon-black polymer dispersion as a core with a sheath of nylon (188) and as a side-by-side configuration (189). At 0.1—1.0% implants, these conductive filaments give durable static resistance to nylon carpets without interfering with dye coloration. Conductive materials such as carbon black or metals as a sheath around a core of nylon interfere with color, especially light shades. 

To make these polymers have the desired properties, certain additives (chemicals), as noted below, are added to the sheathing compound in specific ratios. The additives act like Hame retardants and diminish the igniiability of the insulation by lowering the temperature of the cable, delaying ignition and resisting the spread of fire in the insulation and ihe polymeric compounds... 

Extruded applications of nylon, other than film and monofilcunent, are less commonly encountered because of the low melt viscosity of the polymers. Uses include cable sheathing which requires resistance to abrasion and/or chemical attack, flexible tubing for conveying petrol and other liquids, piping for chemical... 

Polytetrafluoroethylene and fluorinated ethylene-propylene are the only resins composed wholly of fluorine and carbon. The polymer consists of fluorine atoms surrounding the carbon chain as a sheath, giving a chemically inert and relatively dense product from the strong carbon-fluorine bonds. Polytetrafluoroethylene must be molded at high pressure. Fluorinated ethylene-propylene c.m be injection molded and extruded as thin fdm. Both plastics have exceptional heat resistance... 

Hron P. Hydrophilisation of silicone rubber for medical applications. Polym Int, 2003, 52, 1531-1539. Tcholakian RK and Raad. Durability of anti-infective effect of long term silicone sheath catheters impregnated with antimicrobial agents. Antimicrob Agents Chemother, 2001, 45(7), 1990-1993. 

In in sitn STM experiments the tip is immersed in the cell electrolyte. Electrochemical reactions can occnr on all the exposed metal snrface of the tip. It is important to snppress these electrochemical currents to valnes that are considerably less than the tunnehng current. This is done by sheathing all bnt a few micrometers at the end of the tip, with an insnlating layer of glass or polymer (Bach et al., 1993). 

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