Polysulfones are a family ofhigh performancethermoplastics. Thesepolymers are known for their toughness and stability at high temperatures. Technically used polysulfones contain anaryl-SO2-aryl subunit. Due to the high cost of raw materials and processing, polysulfones are used in specialty applications and often are a superior replacement forpolycarbonates.
Three polysulfones are used industrially: polysulfone (PSU), polyethersulfone (PES/PESU) andpolyphenylsulfone (PPSU). They can be used in the temperature range from -100 to +200 °C and are used for electrical equipment, in vehicle construction andmedical technology.[1] They are composed ofpara-linked aromatics,sulfonyl groups andether groups and partly alsoalkyl groups. Polysulfones have outstanding resistance to heat and oxidation, hydrolysis resistance to aqueous and alkaline media and good electrical properties.[2]
The term "polysulfone" is normally used for polyarylethersulfones (PAES), since only aromatic polysulfones are used commercially. Furthermore, since ether groups are always present in these polysulfones, PAESs are also referred to as polyether sulfones (PES), poly(arylene sulfone)s or simply polysulfone (PSU).
The simplest polysulfone poly(phenylene sulfone), known as early as 1960, is produced in aFriedel-Crafts reaction frombenzenesulfonyl chloride:[3]
With a melting point over 500 °C, the product is difficult to process. It exhibits attractive heat resistance, but its mechanical properties are rather poor. Polyarylether sulphones (PAES) represent a suitable alternative. Appropriate synthetic routes to PAES were developed almost simultaneously, and yet independently, from3M Corporation,[4]Union Carbide Corporation in theUnited States,[5] andICI's Plastics Division[6] in the United Kingdom. The polymers found at that time are still used today, but produced by a different synthesis process.
The original synthesis of PAES involvedelectrophilic aromatic substitution of an diaryl ether with the bis (sulfonyl chloride) of benzene. Reactions typically use aFriedel-Crafts catalyst, such asferric chloride orantimony pentachloride:
This route is complicated by the formation of isomers arising from both para- and ortho- substitution. Furthermore, cross-linking was observed, which strongly affects the mechanical properties of the polymer.[7][4] This method has been abandoned.
PAES are currently prepared by apolycondensation reaction of diphenoxide andbis(4-chlorophenyl)sulfone (DCDPS). The sulfone group activates the chloride groups toward substitution. The required diphenoxide is produced in situ from a diphenol andsodium hydroxide. The cogenerated water is removed byazeotropic distillation usingtoluene orchlorobenzene). The polymerization is carried out at 130–160 °C under inert conditions in a polar, aprotic solvent, e.g.dimethyl sulfoxide, forming apolyether concomitant with elimination ofsodium chloride:
Bis(4-fluorophenyl)sulfone can be used in place of bis(4-chlorophenyl)sulfone. The difluoride is more reactive than the dichloride but more expensive. Through chain terminators (e.g.methyl chloride), the chain length can be controlled for melt-processing.
The diphenol is typicallybisphenol-A or1,4-dihydroxybenzene. Such step polymerizations require highly pure monomer and precise stoichiometry to ensure high molecular weight products.[8]
DCDPS is the precursor to polymers known as Udel (from bisphenol A), PES, and Radel R. Udel is a high-performance amorphous sulfone polymer that can molded into a variety of different shapes. It is both rigid and temperature-resistant, and has applications in everything fromplumbing pipes, toprinter cartridges, to automobilefuses. DCDPS also reacts withbisphenol S to form PES. Like Udel, PES is a rigid and thermally-resistant material with numerous applications.
Polysulfones are rigid, high-strength and transparent. They are also characterized by high strength and stiffness, retaining these properties between −100 °C and 150 °C. The glass transition temperature of polysulfones is between 190 and 230 °C.[9] They have a high dimensional stability, the size change when exposed to boiling water or 150 °C air or steam generally falls below 0.1%.[10] Polysulfone is highly resistant tomineral acids,alkali, andelectrolytes, inpH ranging from 2 to 13. It is resistant to oxidizing agents (although PES will degrade over time[11]), therefore it can be cleaned bybleaches. It is also resistant tosurfactants andhydrocarbonoils. It is not resistant to low-polarorganic solvents (e.g.ketones andchlorinated hydrocarbons) andaromatic hydrocarbons. Mechanically, polysulfone has high compaction resistance, recommending its use under high pressures. It is also stable in aqueous acids and bases and many non-polar solvents; however, it is soluble indichloromethane andmethylpyrrolidone.[8]
Polysulfones are counted among thehigh performance plastics. They can be processed byinjection molding,extrusion or hot forming.
Poly(aryl ether sulfone)s are composed of aromatic groups,ether groups andsulfonyl groups. For a comparison of the properties of individual constituentspoly(phenylene sulfone) can serve as an example, which consists of sulfonyl and phenyl groups only. Since both groups are thermally very stable, poly(phenylene sulfone) has an extremely high melting temperature (520 °C). However, the polymer chains are also so rigid that poly(phenylene sulfone) (PAS) decomposes before melting and can thus not be thermoplastically processed. Therefore, flexible elements must be incorporated into the chains, this is done in the form of ether groups. Ether groups allow a free rotation of the polymer chains. This leads to a significantly reduced melting point and also improves the mechanical properties by an increasedimpact strength.[7] Thealkyl groups in bisphenol A act also as a flexible element.
The stability of the polymer can also be attributed to individual structural elements: Thesulfonyl group (in whichsulfur is in the highest possibleoxidation state) attracts electrons from neighboring benzene rings, causingelectron deficiency. The polymer therefore opposes further electron loss, thus substantiating the high oxidation resistance. The sulfonyl group is also linked to the aromatic system bymesomerism and the bond therefore strengthened by mesomeric energy. As a result, larger amounts of energy from heat or radiation can be absorbed by the molecular structure without causing any reactions (decomposition). The result of the mesomerism is that the configuration is particularly rigid. Based on the biphenylsulfonyl group, the polymer is thus durable heat resistant, oxidation resistant and still has a high stiffness even at elevated temperatures. The ether bond provides (as opposed toesters) hydrolysis resistance as well as some flexibility, which leads to impact strength. In addition, the ether bond leads to good heat resistance and better flow of the melt.[12]
Polysulfone has one of the highest service temperatures among all melt-processable thermoplastics. Its resistance to high temperatures gives it a role of aflame retardant, without compromising its strength that usually results from addition of flame retardants. Its high hydrolysis stability allows its use in medical applications requiring autoclave and steam sterilization. However, it has low resistance to some solvents and undergoesweathering; this weathering instability can be offset by adding other materials into the polymer.
Polysulfone allows easy manufacturing ofmembranes, with reproducible properties and controllable size of pores down to 40 nanometers. Such membranes can be used in applications likehemodialysis,waste water recovery, food and beverage processing, and gas separation. These polymers are also used in the automotive and electronic industries. Filter cartridges made from polysulfone membranes offer extremely high flow rates at very low differential pressures when compared withnylon orpolypropylene media.
Polysulfone can be used as filtration media infilter sterilization.
Polysulfone can be reinforced withglass fibers. The resultingcomposite material has twice the tensile strength and three times increase of itsYoung's modulus.[citation needed]
Polysulfone is often used as acopolymer. Recently, sulfonated polyethersulfones (SPES) have been studied as a promising material candidate among many other aromatic hydrocarbon-based polymers for highly durable proton-exchange membranes in fuel cells.[13] Several reviews have reported progress on durability from many reports on this work.[14] The biggest challenge for SPES application in fuel cells is improving its chemical durability. Under oxidative environment, SPES can undergo sulfonic group detachment and main chain scission. However the latter is more dominant; midpoint scission and unzip mechanism have been proposed as the degradation mechanism depending on the strength of the polymer backbone.[15]
Polysulfone food pans are used for the storage, heating, and serving of foods. The pans are made toGastronorm standards and are available in the natural transparent amber colour of polysulfone. The wide working temperature range of -40°C to 190°C allow these pans to go from a deep freezer directly to a steam table or microwave oven. Polysulfone provides a non-stick surface for minimal food wastage and easy cleaning.
Some industrially relevant polysulfones are listed in the following table:
| Structural formula | trade name | Systematic Name | CAS |
|---|---|---|---|
 | Poly(arylene sulfone) (PAS) | ||
 | poly(bisphenol-A sulfone) (PSF) | Poly[oxy-1,4-phenylensulfonyl-1,4-phenylenoxy-1,4-phenylen(1-methylethyliden)-1,4-phenylen] | 25135-51-7 |
 | Polyether sulfone (PES) | Poly(oxy-1,4-phenylsulfonyl-1,4-phenyl) | 25608-63-3 |
 | Polyphenylenesulfone (PPSU) | 25608-64-4 | |
 | Victrex HTA | 121763-41-5 |
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