Bidisperse copolymer solutions

In the previous section, each micellar solution consisted of one copolymer (either the SI diblock or SIS triblock) at a specified composition. Here, we turn our attention to micellar solutions composed of bidisperse mixtures of both the SI and SIS copolymers. Three solution series with fixed copolymer compositions (<1)bc) of 15, 11 and 7 wt% are examined. The concentration of the SI copolymer in each of these blends, expressed relative to the solution, is designated by The maximum value attainable by ( )si is < >bC which occurs in solutions containing only the SI copolymer. To facilitate comparison between the three series, we also introduce wsi as the concentration (in wt%) of SI copolymer in each SI/SIS blend used to prepare a bidisperse copolymer solution. This blend composition is related to < si wsi=100% < )si/(1)bc-... 

In the presence of a selective solvent, ordered block copolymers form micelles that, at sufficiently high copolymer concentrations, serve to stabilize a diree-dimensional network and promote physical gelation. This study examines the steady and dynamic rheological properties of micellar solutions composed of AB diblock, ABA triblock and bidisperse mfactures of AB and ABA copolymer molecules. Of particular interest is the unexpected improvement in network development upon addition of an AB copolymer to an ABA copolymer at constant solution composition. This behavior is observed for ABA/solvent systems above and below the critical gelation concentration, and is interpreted in terms of the volume exclusion that occurs in bidisperse mixture of grafted chains. 

Figure 10. Variation of G with blend composition for bidisperse SI/SIS block copolymer micellar solutions with < )bc of 15 (O), 11 (A) and 7 ( ) wt%. The solid lines connect the data, and the horizontal dashed lines identify G for the solutions composed of only the SIS copolymer. Dotted lines are linear fits to the data beyond G ax- All data have been acquired at ambient temperature.

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