Modulated lamellar structure

Despite intensive research, the structural arrangements of Rf-Rh diblock molecules in their solid sate and LC structures remain largely hypothetical as XRD patterns provide too small a number of reflections to prove unambiguously the proposed structures [66], The present state of knowledge in this field was recently reviewed by Krafft and Riess [66] and therefore will not be discussed in detail here. Only four representative examples of non-modulated lamellar structures are shown in Fig. 6a-d the two models on top were proposed for the LC1 (high temperature phase = HT) and LC2 (low temperature phase = LT) phases of CioH2i-CioF2i [102] and those shown in Fig. 6c,d were obtained by Monte-Carlo... 

The existence of a second class of complex phases, the modulated and perforated layer structures, has largely been explored by Bates and co-workers (Forster et al. 1994 Hamley et al. 1993, 1994 Khandpur et al. 1995 Schulz et al. 1996), who used SANS and TEM to investigate shear oriented structures. The thermally-induced phase transition from the lam to the hex phase in polyolefin diblocks was studied in detail by Hamley et al. (1993, 1994) using SANS, TEM and rheology. Intermediate hexagonal modulated lamellar (HML) and hexagonal perforated layer (HPL) structures were observed on heating PEP-PEE, PE-PEP and PE-PEE diblocks, where PEP is poly(ethylene-propylene), PEE is... 

Hence, there are two options for molecules with unlike end-chains either the incompatible chains mix which retains the lamellar structure, or these chains segregate and layer modulation with formation of ribbon phases (columnar phases)... 

These segregated side groups will either shield conjugated chains from one another, or induce a partial two-dimensional ordering of CP chains (i.e., an incipient lamellar structure). In all cases, interchain interactions will be affected. Substitutions have therefore also been attempted to modulate and control interchain interactions and therefore electrical conductivity [66]. 

Also in bulk block copolymers microphase-separate into ordered liquid crystalline phases. A variety of phase morphologies such as lamellae (LAM), hexagonally ordered cylinders (HEX), arrays of spherical microdomains (BCC, FCC), modulated (MLAM) and perforated layers (FLAM), ordered bicontinuous structures such as the gyroid, as well as the related inverse structures have been documented. The morphology mainly depends on the relative block length. If, for instance, both blocks are of identical length, lamellar structures are preferred. 

Period d of the lamellar structure can be equal to the molecular length (smectics Ai), to the length doubled (bilayered smectics Aq), or to the intermediate value I < d < 21 (smectics A ). The latter subphases are separated by phase transition lines. Sometimes the smectic A phases were observed even with two incommensurate periods of density waves. However, the coexistence of two collinear density waves along the z-direction is not favorable, and modulated structures are often formed with a component of the wave vector lying in the x, y plane. 

Figure 1.91 Selection of 2D modulated ordered lamellar phases that are partially disordered, but exhibit translational ordering in two dimensions. These gel phases exist half way between crystalline Lc states and completely fluid phases such as the Lai and Hn phases (Figs. 1.93 and 1.94). In lipid assembly terms, these represent the equivalent of secondary/tertiary structure formation.

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