Polysulfones are a family of thermoplastic polymers. These polymers are known for their toughness and stability at high temperatures. They contain the subunit aryl-SO2-aryl, the defining feature of which is the sulfone group. Polysulfones were introduced in 1965 by Union Carbide. Due to the high cost of raw materials and processing, polysulfones are used in specialty applications and often are a superior replacement for polycarbonates.
Definition and technically used polysulfones
In principle, any polymer containing a sulfonyl group could be called "polysulfone". However, the term "polysulfone" is normally used for polyarylethersulfones (PAES), since only aromatic polysulfones are used in a technical context. Furthermore, since ether groups are always present in the industrially used polysulfones, PAES are also referred to as polyether sulfones (PES), poly(arylene sulfone)s or simply polysulfone (PSU). The three terms (and abbreviations) may therefore be synonyms. As a term for all polysulfones, "poly(aryl ether sulfone)s (PAES)" is preferable because polysulfone (PSU), polyethersulfone (PES) and poly(arylene sulfone) (PAS) are additionally used as a name for individual polymers. These and some other PAES are listed in the chapter Industrially relevant polysulfones (below).
Since this polymer has a melting point of over 500 °C, it is on one side very heat resistant, on the other hand it is very difficult to process. In addition, its mechanical properties are rather poor. Therefore, thermoplastic (melt-processable) polysulfones were researched as an alternative. At that time it was already assumed that polyarylether sulphones (PAES) would be a suitable alternative. Appropriate synthetic routes to PAES were developed almost simultaneously, and yet independently, from 3M Corporation, Union Carbide Corporation in the United States, and ICI's Plastics Division in the United Kingdom. The polymers found at that time are still used today, but produced by a different synthesis process.
The synthesis method used at that time followed an electrophilic synthesis. Not only para- but also ortho bonds were generated, which led to cross-linking in some cases and generally to worse mechanical properties. The syntheses consisted of an electrophilic aromatic substitution of an aryl ether with a sulfuryl chloride using a Friedel-Crafts catalyst (e. g. iron(III)chloride or antimony(V)chloride):
All PAES commercially available nowadays are not synthesized via this route but rather via a nucleophilic synthesis, see chapter Preparation.
Technically, polyethersulfones are prepared by a polycondensation reaction of the sodium salt of an aromatic diphenol and bis(4-chlorophenyl)sulfone. The sodium salt of the diphenol is formed in situ by reaction with a stoichiometric amount of sodium hydroxide (NaOH). The formed reaction water must be removed with an azeotropic solvent (e.g. methylbenzene or chlorobenzene). The polymerization is carried out at 130–160 °C under inert conditions in a polar, aprotic solvent, e.g. dimethyl sulfoxide, forming a polyether by elimination of sodium chloride::
Also bis(4-fluorophenyl)sulfone can be used, it is more reactive than the dichloride but too expensive for commercial use. Through chain terminators (e.g. chloromethane), the chain length can be regulated in a range that a technical melt processing is possible. However, the product shown in the reaction equation still has reactive end groups. To prevent further condensation in the melt, the end groups can be etherified with chloromethane.
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. 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%. Polysulfone is highly resistant to mineral acids, alkali, and electrolytes, in pH ranging from 2 to 13. It is resistant to oxidizing agents (although PES will degrade over time), therefore it can be cleaned by bleaches. It is also resistant to surfactants and hydrocarbon oils. It is not resistant to low-polar organic solvents (e.g. ketones and chlorinated hydrocarbons) and aromatic 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 in dichloromethane and methylpyrrolidone.
Poly(aryl ether sulfone)s are composed of aromatic groups, ether groups and sulfonyl groups. For a comparison of the properties of individual constituents poly(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 increased impact strength. The alkyl groups in bisphenol A act also as a flexible element.
The stability of the polymer can also be attributed to individual structural elements: The sulfonyl group (in which sulfur is in the highest possible oxidation state) attracts electrons from neighboring benzene rings, causing electron deficiency. The polymer therefore opposes further electron loss, thus substantiating the high oxidation resistance. The sulfonyl group is also linked to the aromatic system by mesomerism 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 to esters) 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.
Polysulfone has one of the highest service temperatures among all melt-processable thermoplastics. Its resistance to high temperatures gives it a role of a flame 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 undergoes weathering; this weathering instability can be offset by adding other materials into the polymer.
Polysulfone allows easy manufacturing of membranes, with reproducible properties and controllable size of pores down to 40 nanometres. Such membranes can be used in applications like hemodialysis, 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 with nylon or polypropylene media.
Polysulfone can be used as filtration media in filter sterilization.
Polysulfone is often used as a copolymer. 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. Several reviews have reported progress on durability from many reports on this work. 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.
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