142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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Asymmetric Star Polymers Synthesis and Properties 119 Fig. 11a,b. Bright field TEM micrographs of: a a linear tetrablock copolymer; b a topologically asymmetric miktoarm starblock copolymer of the same composition and molecular weight (see text) (reproduced with permission from [62]) iting a three-fold symmetry. The authors suggested that the PDMS phase was continuous in three dimensions. The other two phases were not distinguishable. Because of the equal fractions of the three components the authors hypothesized that the other two microphases should also be continuous in three dimensions. All the examined simple models failed to describe quantitatively the observed morphology because of the molecular complexity. It has to be mentioned that tilting experiments gave images of other kinds of symmetry, which could not be identified in terms of simple geometrical arrangements. The crystallinity of the PDMS phase was found by DSC to be lower in the miktoarm case than in the PS-PDMS case, due to the increased steric hindrance close to the central branch point. Thomas and coworkers [62] studied the structures formed by strongly segregated compositionally symmetric four-arm inverse star block copolymers and their linear counterparts. These macromolecules having the general formula (PS M-b-PI aM) n-(PI M-b-PS aM) n where M~20,000, n=1, 2 and a=1, 2, 4 (a is the ratio of the outer block molecular weight to that of the inner block, see Fig. 1c) can be considered as topologically asymmetric star copolymers. Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) were used to characterize the microdomain morphology of these materials. Samples with n= 1, 2 a=1, 2 formed highly ordered lamellae structures. For a=4 (sample SB4) a transition from a lamellar structure for n=1 to a triconnected cubic structure (n=2) was observed (Fig. 11). Comparison of TEM data with simulated images
120 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos Fig. 12. Schematic representation of proposed arrangement of A/B junctions on a triply periodic IMDS of constant mean curvature for SB-4 miktoarm copolymer of Fig. 11 (reproduced with permission from [62]) of three dimensional models of level surfaces confirmed that the cubic continuous structure is best described as an ordered bicontinuous double diamond (OBDD) structure. The area per junction point was found to increase with molecular weight in the star architecture compared to the linear tetrablocks. From d spacing measurements it was concluded that as “a” increases the ability of the shorter interior blocks to bridge the domains decreases and more loop conformations are formed. These loop conformations force the A/B junctions further apart on average and the long exterior blocks can assume less stretched conformations (Fig. 12). For the SB-4 sample the short interior blocks would be too overcrowded as loops on flat intermaterial dividing surface (IMDS). In order to avoid overcrowding the IMDS becomes curved, allowing the peripheral junctions to spread further apart than on a flat interface and the microdomain structure becomes triply periodic. Floudas and coworkers [88] investigated the static and kinetic aspects of the order-disorder transition in SI 2 and SIB miktoarm stars using SAXS and rheology. At temperatures above the order-disorder transition (ODT) the mean field theory describes the experimental results quite well. Near the ODT, SAXS profiles gave evidence for the existence of fluctuations. Both samples separated into cylindrical microdomains below the ODT. The ODT was determined on shear oriented samples and found, by SAXS, to be 379 K in both cases. This was confirmed by rheology. The discontinuities in SAXS peak intensity and in the storage modulus near the ODT were more pronounced for the miktoarm stars than for the diblocks. The cN values, where c is the interaction parameter and N the
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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Poly(macromonomers), Homo- and Copo
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Dendrimers and Dendrimer-Polymer Hy
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Author Index Volumes 101-142 233 Ka
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Author Index Volumes 101-142 235 Og
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Author Index Volumes 101-142 237 Su
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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