Structures hexagonally packed cylinders

In analogy to linear-block copolymers different cases can be distinguished when blending asymmetric miktoarm (PS-PI)n-PS and H-shaped (PS-PI)3-PS-(PI-PS)3 copolymers with homopolymer PS [122]. If the latter s molecular weight is lower than the respective PS block, a transition from the L structure to hexagonally packed cylinders without observation of an intervening cubic morphology is observed in the case of the (PS-PI)n-PS types. If the H-shaped (PS-PI)3-PS-(PI-PS)3 star-block copolymer is blended with 30% to... 

Fig. 4.17 Small-angle X-ray scattering patterns for 38wt% aqueous solutions of PE04oPB010 (a) BCC phase observed between 5 and 50°C (b) FCC structure between 50 and 75 °C (c) hexagonally-packed cylinder phase above 75 °C (Pople et al. 1997).

In subsequent work, ordering in solutions of the same matched diblock and triblock spanning a broader range of volume fractions, 0.1 < < 0.4, was explored (Hamley et al. 1997). For liquid-like and SAXS showed that there was no inter-micellar order in the liquid. Above a crossover concentration 0.2, ordering of micelles was shown by the presence of a structure factor peak. The ordered micellar structure, identified as hexagonal-packed cylinders for more concentrated solutions, persisted up to an order-disorder transition located from a discontinuity in the... 

Seguela and Prud homme (1989) investigated a PE-PEP-PE triblock copolymer containing 27wt% poly(ethylene) cast from a neutral solvent close to the Tm of PE and well below it. The samples cast above Tm crystallized within the assumed hexagonal-packed cylinder microphase-separated structure. However, SAXS experiments performed on the samples cast at room temperature suggested that crystallization occurred without prior microphase separation in the melt. This path dependence is a general feature of crystallization in block copolymers. 

Crystallization in asymmetric diblocks with compositions = 0.35 and 0.46 was also investigated by Hamley et al. (19966). It was found that a lamellar structure melted epitaxially (i.e. the domain spacing and orientation were maintained across the transition) to a hexagonal-packed cylinder structure in the /PE = 0.35 sample. This is illustrated in Fig. 5.15, which shows SAXS patterns in the solid and melt states, with a schematic of the epitaxial melting process (Hamley et al. 1996a.b). The same epitaxial transition has been observed for a polyethylene oxide)-poly(buty)ene oxide) diblock (Ryan et at. 1997) vide infra). 

Fig. 5.15 (a) SAXS intensity profiles along the lamellar normal for a PE-FEE diblock (M = 81 kgmor. /pE - 0.35) (Hamley el al. 1996b). (b) Schematic of the epitaxial growth of a hexagonal-packed cylinder melt structure from a lamellar solid structure. The direction of incidence of X-rays is along the direction of the arrow. 

In a solvent, block copolymer phase behavior is controlled by the interaction between the segments of the polsrmers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavorable for one block, this can lead to micelle formation in dilute solution. The phase behavior of concentrated solutions can be mapped onto that of block copolymer melts (97). Lamellar, hexagonal-packed cylinder, micellar cubic, and bicontinu-ous cubic structures have all been observed (these are all lyotropic liquid crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 6. 

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