Molecular chains segregated

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

Block copolymers of polystyrene with rubbery polymers are made by polymerizing styrene in the presence of an unsaturated rubber such as 1,4 polybutadiene or polystyrene co-butadiene. Some of the growing polystyrene chains incorporate vinyl groups from the rubbers to create block copolymers of the type shown in Fig. 21.4. The combination of incompatible hard polystyrene blocks and soft rubber blocks creates a material in which the different molecular blocks segregate into discrete phases. The chemical composition and lengths of the block controls the phase morphology. When polystyrene dominates, the rubber particles form... 

Takagi, S., Tsumoto, K. and Yoshikawa, K. (2001) Intra-molecular phase segregation in a single polyelectrolyte chain. J. Chem. Phys., 114, 6942-6949. 

All the results above lead consistently to the conclusion that the negative-resistance effect is a common property of mixed-chain 1 1 charge-transfer complexes and segregated-chain 1 1 charge-transfer salts, the only requirement being the dimerization of the molecular chains in both cases [47]. 

Direct observation of spherulite growth rates during isothermal crystallization yields fundamental information about the kinetics of this process because the results can be compared with pertinent theories and basic parameters can be quantified. However, such experiments are tedious and only a few studies on blends have been reported. Of related interest are a number of reports on the crystallization of blends of fractions of the same polymer but of differing molecular weights. This approach is useful for assessing the role of overall chain mobility and has shown in some instances that molecular weight segregation occurs on crystallization (20). 

Manipulation of the blends of BCs on surfaces and thin films constitutes an interesting alternative to finely tune the self-assembled structures formed in BCs. Studies of BC blends have been carried out from both an experimental and theoretical point of view. Bates and coworkers reported on the morphologies observed in blends composed of either symmetric or asymmetric diblock copolymers with variable molecular weight and compared the structures with the predicted using SCF calculations [181], The blend composition profiles in symmetric blends show non-uniformity in the distribution of the chains with the longer chains segregating to the domain centers and shorter chains being more... 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

The importance of polydispersity is an interesting clue that it may be possible to tailor the weak interactions between polymer brushes by controlled polydispersity, that is, designed mixtures of molecular weight. A mixture of two chain lengths in a flat tethered layer can be analyzed via the Alexander model since the extra chain length in the longer chains, like free chains, will not penetrate the denser, shorter brush. This is one aspect of the vertical segregation phenomenon discussed in the next section. 

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