Molecular weight segregation

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). 

Model Strength Drawing Entanglements Molecular weight Segregation Orienta-... 

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. 

It is well-established that fractionation occurs during crystallization for broad band molecular weight homopolymers [86, 148] that is, segregation occurs such that the lower molecular weight components remain in the melt (or solution) and only the higher molecular weight components become crystalline. The rejected components depend upon the growth conditions, which indicates that fractionation is directly linked to the crystallization process. Hence a study of the conditions... 

Calculated Molecular Weight Distributions. The calculated weight fraction distributions for the micro-mixed, segregated, and micro-mixed reactor with dead-polymer models for Runs 2, 5,... 

The effects of micro-mixing on the molecular weight distribution are much more pronounced than those of segregation. According to Patterson (33) only a small increase in micro-mixing over that of total segregation will yield a pol)nner distribution very similar to that of micro-mixed reactor. 

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

Hashimoto T., Shibayama M., and Kawai H., Ordered structure in block copolymer solution. 4. Scaling rules on size of fluctuations with block molecular weight, concentration temperature in segregation and homogeneous regimes. Macromolecules, 16, 1093, 1983. 

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