Ionomer-type elastomers

Ionomer-type elastomers, containing small amounts (less than 5%) of metal carboxylate or sulfonate groups, have potential as a new class of thermoplastic elastomers. Carboxylic acid groups are introduced into polymers such as polybutadiene by copolymerization with a monomer such as acrylic or methacrylic acid. 

In addition, the conventional division between thermoplastics and thermosets is often losing distinction, while hybrid materials (ionomers, and elastomers of the SBS block type) are appearing, which behave as thermoplastics at elevated temperatures and as thermosets (most important for elastomers) at ambient temperature. Moreover, many thermoplastics are now also modified by cross-linking (either by chemical or radiation process), so they can serve dual pvuposes—good workability as a thermoplastic is combined with the improved thermal and mechanical stability of a thermoset. 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

A specialty class of carboxyl containing elastomers are the telechelic ionomers. In these systems the carboxyl functionality terminates both ends of the polymer chain. Such polymers range in molecular weight from 1500 to about 6000. These materials can be prepared via several synthetic routes involving anionic or free radical initiated polymeri-zation(32-34). Recently, telechelic sulfonate ionomers of poly-isobutylene have been synthesized(35). These systems offer an unusual opportunity to assess the influence of chain length, chain architecture, cation type, and the Influence of polar additives on ionomer properties. 

To toughen PA, 2-5 wt% of either PO, elastomer, ionomer, acidified or epoxidized copolymer may be added. PA/PO blends of type (2) were developed to improve dimensional stability and to reduce water absorbency of PA. Alloying PA with PO reduces the rate of water migration to and from the blend, but not the inherent water absorption of PA [Utracki and Sammut, 1991, 1992]. The alloying is either a two- or three-step reactive process (1°) acidification of PO, (2°) preparation of a compatibilizer, and (3°) compounding PP, PA, and the compatibilizer. Usually, the reactive blending is carried out in a twin screw extruder [Nishio et al., 1990 Hu and Cartier, 1998], Since it may cause reduction of the blend crystallinity (thus performance), the extend must be optimized. The rigid PA/PP blends usually comprise PA PP =... 

Free radical copolymerization is used to produce ionomers that are used commercially as thermoplastic elastomers. There are two types of TPE ionomers copolymers of ethylene and methacrylic acid, and copolymers of ethylene and acrylic acid. The mole fraction of the acid monomer is typically 5% or less. The property difference between the two types of copolymers is... 

Thermoplastic elastomers based on blends of a silicone rubber (cross-linked during processing) with block copolymer thermoplastic elastomers have also been prodnced (36,37). Other types that have been stndied (38) include graft copol5uners and elastomeric ionomers, but these have not become commercially important. 

Tg measurements have been performed on many other polymers and copolymers including phenol bark resins [71], PS [72-74], p-nitrobenzene substituted polymethacrylates [75], PC [76], polyimines [77], polyurethanes (PU) [78], Novolac resins [71], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer and butyl rubber [79], bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [80], mono and dipolyphosphazenes [81], polyethylene glycol-polylactic acid entrapment polymers [82], polyether nitrile copolymers [83], polyacrylate-polyoxyethylene grafts [84], Novolak type thermosets [71], polyester carbonates [85], polyethylene naphthalene, 2,6, dicarboxylate [86], PET-polyethylene 2,6-naphthalone carboxylate blends [87], a-phenyl substituted aromatic-aliphatic polyamides [88], sodium acrylate-methyl methacrylate multiblock copolymers [89], telechelic sulfonate polyester ionomers [90], aromatic polyamides [91], polyimides [91], 4,4"-bis(4-oxyphenoxy)benzophenone diglycidyl ether - 3,4 epoxycyclohexyl methyl 3,4 epoxy cyclohexane carboxylate blends [92], PET [93], polyhydroxybutyrate [94], polyetherimides [95], macrocyclic aromatic disulfide oligomers [96], acrylics [97], PU urea elastomers [97], glass reinforced epoxy resin composites [98], PVOH [99], polymethyl methacrylate-N-phenyl maleimide, styrene copolymers [100], chiral... 

The word ionomer dates back to 1965, but materials of this type had been synthesized and investigated long before. Examples of early work include the investigation of Littman and Marvel in 1930, synthesis of an elastomer based on butadiene and acrylic acid in the early 1930 s, and the appearance in the 1950 s of du Font s Hypalon (sulfonated, chlorinated, and cross-linked polyethylene) [29]. 

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