Block molecular weights

The properties of styrenic block copolymers are dependent on many factors besides the polymerization process. The styrene end block is typically atactic. Atactic polystyrene has a molecular weight between entanglements (Me) of about 18,000 g/mol. The typical end block molecular weight of styrenic block copolymers is less than Mg. Thus the softening point of these polymers is less than that of pure polystyrene. In fact many of the raw materials in hot melts are in the oligomeric region, where properties still depend on molecular weight (see Fig. 1). 

During the reactions backbone compositions of the siloxane oligomers employed have been varied between all dimethylsiloxane to 50/50 by weight dimethylsiloxane/ diphenylsiloxane and dimethylsiloxane/(trifluoropropyl,methyl)siloxane. Block molecular weights of the siloxane oligomers have been kept between 3,000-5,000 g/mole, whereas the overall level of siloxane incorporation into the copolymers have been varied between 0 and 60 weight percent. In most cases the ratio of terephthalate to... 

Fig. 2. Stress-strain behavior of MDI-based siloxane-urea segmented copolymers as a function of siloxane block molecular weights (PSX-Mn g/mole), [1. PSX-1140, 2. PSX-1770, 3. PSX-2420, 4. PSX-3670] and their comparison with unfilled (curve 7) and silica filled (curves 5 and 6) conventional silicone rubbers51 158,358,359)...

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. 

Stars with 32% PSt, (PSt/15-h-PIB/34)g-C8, showed -26 MPa tensile stress. There was no appreciable difference in the tensile properties of unextracted and MEK extracted star blocks [see (PSt/15-h-PIB/34)g-C8 and (PSt/16-h-PIB/34)g-C8]. The modulus and Shore A hardness were slightly higher for the unextracted star, (PSt/16-h-PIB/34)g-C8, which maybe due to the presence of PSt contamination which acts as a rigid filler. A dramatic difference in the tensile behavior was observed when PSt content was increased. Stars with low PIB block molecular weight and high PSt content (46%), e.g., (PSt/21-h-PIB/25)g-C8, showed plasticlike behavior, i.e., it showed a high modulus, a yield point, and a short draw. Ex-... 

The micellization process was found to be much more strongly dependent on the insoluble block molecular weight than the soluble block molecular weight, as for non-ionic block copolymers. These conclusions were supported by fluorescence measurements by Morishima et al. (1981,1982a,6). 

The synthesis and characterization of these ABA block copolymers of styrene and dienes have been described elsewhere (JO, 11). Since the polystyrene end blocks aggregate into glassy domains which act as network junctions, the elastic center blocks must virtually represent the network chains/ The polystyrene domains should also act as a finely divided filler. Hence it might be expected that the mechanical properties of these materials could depend on the two basic parameters polystyrene content and length of center block ( molecular weight between crosslinks ). 

Chen, Gardella and Kumler106 have studied a series of polydimethylsiloxane-polystyrene block copolymers and examined the surface composition by ATR-FTIR and XPS. For AB-type PS-PDMS diblock copolymers (26) with siloxane block molecular weights of between 38,000 and 99,000, the surface was found to be exclusively polydimethylsiloxane down to a depth of 10 nm by XPS. ATR-FTIR, which samples... 

Table II shows the Bragg spacing and the sum of the unperturbed block dimensions for PPO 3000/50 and 3000/25, < r0 > was estimated from the values for PPO, and poly (oxyethylene oxyterephthaloyl) given in the literature (26). As the average block molecular weight increased, the Bragg spacing also increased and is the same order of magnitude as the unperturbed block dimensions. This can be taken as a clear indication that phase separation of the blocks has occurred in PPO 3000/50 and 3000/25.

Clearly the difference in solubility parameters is consistent with the difference in block molecular weight required to produce phase separation in these two copolymers. 

The sequence of curves in Figures 5-11 shows the stages of partial phase separation that occur in an amorphous multiblock polymer with polydisperse hard blocks. By systematically varying the block molecular weights and copolymer composition, the breadth and shape of the damping peaks changed in a systematic and predictable way. Controlled partial phase separation would appear to be a powerful way to tailoring polymer properties for a particular application. 

Triblock copolymers of ABA-type have basically the same behavior in giving a star-like morphology (Fig. 6a) when dissolving into a selective solvent for the outer A blocks. However, the micellization of ABA-type triblock copolymers in selective solvent for the middle B blocks deals with a much more complex situation [42], A relatively low concentration of the copolymer in the selective solvent and/or a low molecular weight of the A blocks can lead to isolated flower-like micelles (Fig. 6b) with the middle B block being looped - referred to as petals - and with the two outer A blocks taking part of the same micellar core. However, if the copolymer concentration or the A block molecular weight is increased, a micelle association... 

Figure 13.13 Reduced storage modulus G and dynamic viscosity rj = G /w as functions of reduced frequency uto) for a cylinder-forming polystyrene-polybutadiene-polystyrene triblock copolymer with block molecular weights of 7000-43,000-7000. The curves are time-temperature-shifted to a reference temperature of 138°C the open symbols were obtained in the low-temperature ordered state the closed symbols were obtained in the high-temperature disordered state. (From Gouinlock and Porter 1977, reprinted with permission from the Society of Plastics Engineers.)...

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