Polymeric materials Block copolymers

Piirma I. Synthesis of amphipathic polymeric material block copolymers. In Polymeric Surfactants, Surfactant Science Series. Vol. 42. New York Marcel Dekker, 1992 17-34. 

Usually polymeric substances of appropriate chemical structure and morphology which promote the miscibility of incompatible materials. Block copolymers are especially useful surfactants at the polymer/polymer interface because the two blocks can be made up from molecules of the individual polymers to be mixed. Typical compatibilisers in polymer blends are LDPE-g-PS in PE/PS CPE in PE/PVC acrylic- -PE, -PP, -EPDM in polyolefin/PA and maleic-g-PE, -PP, -EPDM, -SEBS in polyolefin/polyesters. 

The possibility of employing block copolymers as materials that might possess desirable properties was originally considered by Mark In the first period the effort in preparing block copolymers was directed to radical polymerization and it was only in 1956 that Szwarc obtained well-defined block copolymers by anionic polymerization . In block copolymers, the incompatibility between polymeric chains becomes an advantage a phase separation of the blocks occurs leading to the formation of microdomains which are responsible for the ecific properties of block copolymers. For instance, the presence in a molecule of an elastomeric block linked by its ends to thermoplastic blocks generates a polymer in which reversible physical multifunctional cross-links allow the behaviour of conventional vulcanized elastomers at room temperature, but the material remains easily moldable at elevated temperature just as normal thermoplastic resins ° ... 

To ascertain control of the molecular weight, structure and composition, block copolymers are usually synthesized in anionic polymerization. The block copolymers of commercial interest are specifically prepared from monomers that upon polymerization yield immiscible macromolecular blocks, a smaller one rigid and the other flexible. The rigid blocks form physical crosslinks that upon heating above the transition point make the copolymer to flow. Thus, these materials belong to the growing family of thermoplastic elastomers. 

Part I of the present volume includes the fundamentals and developments of the ESR experimental and simulations techniques. This part could be a valuable introduction to students interested in ESR, or in the ESR of polymers. Part II describes the wide range of applications to polymeric systems, from living radical polymerization to block copolymers, polymer solutions, ion-containing polymers, polymer lattices, membranes in fuel cells, degradation, polymer coatings, dendrimers, and conductive polymers a world of ESR cum polymers. It is my hope that the wide range of ESR techniques and applications will be of interest to students and mature polymer scientists and will encourage them to apply ESR methods more widely to polymeric materials. And I extend an invitation to ESR specialists, to apply their talents to polymers. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

The second front originates in the polymer synthesis community. Efforts are mainly directed toward production of monodisperse block copolymers by living polymerizations. These stmctures typically result in microphase separated systems if one block is a high T material and the other is elastomeric in... 

Commercially, anionic polymerization is limited to three monomers styrene, butadiene, and isoprene [78-79-5], therefore only two useful A—B—A block copolymers, S—B—S and S—I—S, can be produced direcdy. In both cases, the elastomer segments contain double bonds which are reactive and limit the stabhity of the product. To improve stabhity, the polybutadiene mid-segment can be polymerized as a random mixture of two stmctural forms, the 1,4 and 1,2 isomers, by addition of an inert polar material to the polymerization solvent ethers and amines have been suggested for this purpose (46). Upon hydrogenation, these isomers give a copolymer of ethylene and butylene. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

Among the different pressure sensitive adhesives, acrylates are unique because they are one of the few materials that can be synthesized to be inherently tacky. Indeed, polyvinylethers, some amorphous polyolefins, and some ethylene-vinyl acetate copolymers are the only other polymers that share this unique property. Because of the access to a wide range of commercial monomers, their relatively low cost, and their ease of polymerization, acrylates have become the dominant single component pressure sensitive adhesive materials used in the industry. Other PSAs, such as those based on natural rubber or synthetic block copolymers with rubbery midblock require compounding of the elastomer with low molecular weight additives such as tackifiers, oils, and/or plasticizers. The absence of these low molecular weight additives can have some desirable advantages, such as ... 

The properties of styrenic block copolymers are dependent on many factors besides the polymerization process. The styrene end block is typically atactic. Atactic polystyrene has a molecular weight between entanglements (Me) of about 18,000 g/mol. The typical end block molecular weight of styrenic block copolymers is less than Mg. Thus the softening point of these polymers is less than that of pure polystyrene. In fact many of the raw materials in hot melts are in the oligomeric region, where properties still depend on molecular weight (see Fig. 1). 

Hazer [20,25] reported on the reaction of a po]y(eth-ylene g]ycol)-based azoester with methacryloyl chloride in the presence of (CH3CH2)3N. In this reaction double bonds were attached to the chain ends of the poly(ester) thus obtaining a macroinimer. Being used for the thermal polymerization of styrene, the material formed an insoluble gel [20]. Probably, both the C=C double bonds and the azo bonds reacted in the course of the thermal treatment. The macroninimer in a later work [25] was used for thermally polymerizing poly(butadiene) thus leading to poly(ethylene glycol-/ -butadiene) block copolymers. 

The reaction of ACPC with linear aliphatic amines has been investigated in a number of Ueda s papers [17,35,36]. Thus, ACPC was used for a interfacia] polycondensation with hexamethylene diamine at room temperature [17] yielding poly(amide)s. The polymeric material formed carried one azo group per repeating unit and exhibited a high thermal reactivity. By addition of styrene and methyl methacrylate to the MAI and heating, the respective block copolymers were formed. 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

The new knowledge and understanding of radical processes has resulted in new polymer structures and in new routes to established materials many with commercial significance. For example, radical polymerization is now used in the production of block copolymers, narrow polydispersity homopolymers, and other materials of controlled architecture that were previously available only by more demanding routes. These commercial developments have added to the resurgence of studies on radical polymerization. 

Block copolymers with materials such as a polyester (PE) (qv) can be prepd by the reaction of diisocyanate-terminated polyesters with hydroxyl-terminated PMMA according to Wilkes and Grezlak (Ref 21). The basic structure was found to be PMMA-PE-FMMA, with a mw of from 7500 to 47000. The purpose of the work was to produce a stronger copolymer (in terms of stress-strain) by tailoring the amt of each monomer used to produce the copolymer Further information on polymerization can be found in Refs 2a 6a... 

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