Copolymers, triblock

The next step concerned the synthesis of the polyester-polyimide copolymers. Triblock copolymers have been prepared by a step-growth copolymerization of stoichiometric amounts of an aromatic diamine and dianhydride (e.g., PMDA and 3FDA, as depicted in Scheme 41a) added with the single oo-amino polyesters as chain end-cappers. Graft copolymers can be prepared as well. In this case, the diamine end-functionalized oligomeric macromonomers are copolymerized with the polyimide condensation comonomers (Scheme 41b). 

Block copolymers themselves are also finding rapidly expanding applications on an industrial scale. A sandwich copolymer (triblock) with an elastomeric core (polybutadiene, polyisoprene, etc.) and plastomeric ends (polystyrene, etc.) represents a physically vulcanizing rubber (plastomeric elastomer). It can be processed above the glass transition temperature of the plastomeric blocks by work-efficient technologies (injection molding, extrusion, etc.). At temperatures below the Tg of the plastic blocks, the copolymer behaves as vulcanized rubber. 

In this two-stage process, B is sequentially polymerized onto A, and then the two chains are coupled to yield an ABBA block copolymer. Triblocks of SBS have been prepared using this method, with methylene dichloride as the coupling agent. The disadvantage is the formation of radical anions, which can lead to contamination of the triblock with multiblock species. 

In addition, dihlock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol. 

Monomers Homopolymers Diblock copolymer Triblock copolymer Fig. 1.1 Architectures of Unear homopolymers and block copolymers... 

Property Physical units Copolymers Multiblock copolymers Triblock copolymers... 

In this two-stage process, B is sequentially polymerized onto A, and then the two chains are coupled to yield an ABBA block copolymer. Triblocks of... 

Block copolymers consist of blocks of two or more different monomeric units. According to the number of blocks, a distinction is made, for example (with binary block copolymers), between two-block copolymers triblock copolymers and multiblock copolymers Multi-... 

The influence of a block copolymer on the droplet breakup and coalescence in model immiscible PEP/PPO polymer blends was investigated by Ramie et fd. [18], who found that the addition of 0.1 wt% or 1.0 wt% of PEO-b-PPO-b-PEO [poly (ethylene oxide)-poly(propylene oxide) copolymer] triblock copolymers facilitated breakup and inhibited coalescence. The steady-state droplet size resulting from breakup was reduced only slightly by the addition of 0.1 wt% copolymer, but more substantially by addition of lwt%. However, the kinetics of coalescence were suppressed effectively even when 0.1 wt% of copolymer was added. In these systems, the copolymer seems to reduce the efficiency of both droplet collision and film drainage and/or rupture. 

Mural and co-workers [57] also optimised the mechanical properties of an rPP and recycled high impact polystyrene (rHIPS) blend at a composition of 70/30 wt%. Consequently, this composition was mixed with a styrene-ethylene-butylene-styrene (SEES) block copolymer triblock copolymer and Cloisite 20A OMMT. Using X-ray diffraction, the samples containing 3 wt% of nanoclay were found to lack the characteristic nanoclay peak, which indicated the mixed intercalated and exfoliated clay layers where the intercalated layers were further pushed toward the interphase [76]. The incorporation of a compatibiliser and nanoclay also improved the thermal stability of the PP/HIPS blend. SEES and nanoclay performed as an interfacial compatibiliser, which led to the reduction in particle size of rHIPS and the promotion of interfacial adhesion. 

A block copolymer contains two or more separate blocks of monomers to form a polymer with the structure poly(A-f -B) [9], where A and B are monomers and b denotes that it is a block. Copolymers can be classified according to how the monomers are arranged in the polymer structure. Linear copolymers consist of a single main chain, whereas branched copolymers consist of a single main chain with one or more polymeric side chains. Examples of branched copolymers are graft copolymers, brush copolymers and comb copolymers [10]. Block copolymers with two, three or four distinct blocks are called diblock copolymers, triblock copolymers and tetrablock copolymers, respectively, (Figure 2.4) [10]. 

Random copolymer Alternating copolymer Diblock copolymer Triblock copolymer Pentablock copolymer Terpolymer AABBBABBAAAB ABABABABABAB AAAABBBBBBB AAAABBBBBAAAA AAABBBBAAABBBBAAAA AABBCCCBBAABBCCCAAA... 

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