Triblock copolymers, conformation

Fig. 57 Schematic comparison of chain conformations of the midblock for ABC and ABA triblocks. ABC triblock terpolymers (a) have bridge conformations only, whereas ABA triblock copolymers (b) have bridge and loop conformations. From [159]. Copyright 2002 Wiley...

Fig. i Schematic representation of chain conformation in micelles from a linear PEO-PBO diblock copolymers, b linear PEO-PBO-PEO triblock copolymers, c linear PBO-PEO-PBO triblock copolymers and d cyclic PEO-PBO diblock copolymers... 

Recent studies on PEO-PPO, PEO-PBO di- and triblock copolymers include the works of Bahadur et al. [121], who examined the role of various additives on the micellization behavior, of Guo et al. [122], who used FT-Raman spectroscopy to study the hydration and conformation as a function of temperature, of Booth and coworkers [ 123], who were mainly interested in PEO-PBO block copolymers with long PEO sequences, and of Hamley et al., who used in situ AFM measurements in water to characterize the morphology of PEO-PPO micelles [56,57]. 

Among other approaches, a theory for intermolecular interactions in dilute block copolymer solutions was presented by Kimura and Kurata (1981). They considered the association of diblock and triblock copolymers in solvents of varying quality. The second and third virial coefficients were determined using a mean field potential based on the segmental distribution function for a polymer chain in solution. A model for micellization of block copolymers in solution, based on the thermodynamics of associating multicomponent mixtures, was presented by Gao and Eisenberg (1993). The polydispersity of the block copolymer and its influence on micellization was a particular focus of this work. For block copolymers below the cmc, a collapsed spherical conformation was assumed. Interactions of the collapsed spheres were then described by the Hamaker equation, with an interaction energy proportional to the radius of the spheres. 

Balazs and Lewandowski (1990) have performed simulations of the adsorption of triblock copolymers onto a planar surface, and examined the conformations of the adsorbed chains. Monte Carlo simulations were performed of the motion of hydrophilic-hydrophobic chains on a cubic lattice. These simulations revealed a complex structure in the interfacial region due to the self-assembly of chains, driven by the solvent-incompatible block, reducing adsorption onto the surface. The influence on the surface coverage of length of the hydrophilic segement, polymer concentration, interaction energy between hydrophilic block and the... 

GPC fractionation and ozonisis of the products, the yield of cyclic polymer was estimated to be ca. 90%. The morphological transition of the cyclic block polymer depends on composition in essentially the same manner as that of the linear triblock copolymers, whereas the domain spacing of polystyrene-block-polyisoprene cyclic block copolymers were all smaller than those of the corresponding SIS linear triblock copolymers, which is attributed to looped chain conformation. 

Fig. 2. Conformation of AB copolymers in solution (a) Random copolymer (b) diblock copolymer (c) triblock copolymer.

The association of flower-like micelles through bridged conformations in an ABA triblock copolymer can be avoided by replacing the ABA triblock by a cyclic diblock copolymer. In this case, sunflower micelles have been observed for example by Borsali and coworkers from the assembly of cyclic and asymmetric polystyrene- >-polvisoprene diblock copolymer (PS-h-PI) and compared to the corresponding linear diblock [46, 47]. 

A block copolymer is expected to be superior to a graft copolymer in stabilizing dispersions of one polymer in another because there will be fewer conformational restraints to the penetration of each segment type into the homopolymer with which it is compatible. Similarly, diblock copolymers might be more effective than triblock copolymers, for the same reason, although tri- and multiblock copolymers may confer other advantages on the blend because of the different mechanical properties of these copolymers. 

Fig. 39. Schematic of the conformation of block copolymers inside lamellae a diblocks and b triblocks. The bridging effect of triblock copolymers is responsible for their better mechanical reinforcement at high copolymer coverage...

The microphase-separated structures formed by the PB-based y-benzyl-L-glutamate containing triblock copolymers appeared to be sensitive to the conformation of the peptide blocks [28]. When instead of y-benzyl-L-glutamate N-carboxyanhydride an equimolar mixture of y-benzyl-L-glutamate AT-car-boxyanhydride and y-benzyl-D-glutamate AT-carboxyanhydride was used for... 

Fig. 7. Transition from a two-domain structure to a three-domain structure in an ABC triblock copolymer lamellar phase. Although state ii is thermodynamically the most stable state, transition to this state from i is hindered because of the high free energy cost in switching the orientation of the bridges and in turning the loops into bridges. Thus, a kinetically more likely process is for the A and C blocks from the bridge conformation to separate laterally, with the loops straddling the interfaces between the A and the C domains.

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