Frustrated chain packing

Frustrated chain packing in this approach aromatic substituents are pendantly attached to the main chain rings. 

Compound 189 with a shorter terphenyl rod-like core also has two Colhex phases, formed by cylinders, but with a hexagonal cross sectional area (Fig. 63). In this case a complete segregation of the Rp-chains from the RSi-chains is impossible as it would lead to a packing frustration and therefore formation of cylinders incorporating mixed chains cannot be avoided. Based on simulation results [346], XRD data, and AFM results, the two color tiling with a p6mm superlattice... 

Oligofluorenes studies have been performed by Wegner and coworkers [18,57,86,87] and others [88,89]. The packing frustration evident in the PF2/6 polymer is already present in the chains as short as a trimer [89]. Even at this short length thin films exhibit mesotropic-type phases [89] as opposed to the crystalline monomer samples. 

Use of monomers with bulky side groups, which prevents close packing in the LC mesophases, sometimes referred to as frustrated chain packing. ... 

We note that there are hints in Table I, and structural fluctuation quantities such as Ag(r) discussed elsewhere, that as N or stiffness mismatch increase the excess free energy of mixing also increases and the blend is less stable. Thus, the possibility of entropy-driven phase separation due to packing frustration of dissimilar flexibility chains as N increases beyond 200 remains open based on the simulation studies of Ref. 97. 

Although a few 3D isotropic cubic network phases have already been observed, other cubic network phases have stayed elusive in BCP SA, such as the I-WP [25], Neovius [26], K surface [27], and Lidinoid [28] structures (see Fig. 2). Packing frustration of polymer chains in the network nodes of these structures is a primary hurdle for formation of these phases. However, as shown in the cases of the plumber s nightmare and double diamond structures, BCP co-assembly with additive molecules or ordered local packing of monomer units may open new routes to such cubic structures. 

Completely rigid rod-like molecules such as poly(4-oxybenzoyl) or poly( p-phenylene terephthalate) tend to be highly crystalline and intractable, with melting points above the decomposition temperature of the polymers (>450°C). The problem of thermotropic MCLCP design is to disrupt the regularity of the intractable para-linked aromatic polymers to the point at which mesomorphic behaviour is manifested below the decomposition temperature and the materials can be processed in fluid yet ordered states. The disruption must not, however, be taken to the stage where conventional isotropic fluid behaviour is preferred. These requirements that the polymer must retain some rod-like nature but at the same time be melt-processable below 400-450°C have limited thermotropic MCLCPs mainly to polymers based on the linear ester or ester/amide bonds. With polyester/ polyesteramides, disruption is normally achieved by the th ee copolymerization techniques outlined in Fig. 8.1, i.e. frustrated chain packing, flexible spacers and non-linear links. 

Frustrated chain packing refers to any mechanism which while maintaining essential linearity and chain stiffness makes close and regular correlation into a 3D lattice difficult. Copolymerization with linear unsubstituted phenyl-based residues to give a random copolymer does result in a reduction in Tm, but the reduction is not enough to render the polymers readily melt-processable. Instead three classes of disrupters are used. The first is the incorporation of a parallel-offset or crankshaft monomer such as a naphthalene unit. Units of this type have been found to be very effective at reducing Figure 8.2 shows plots of vs molar... 

The third type of disrupter is based on ring-substituted phenyl groups where chain packing can be frustrated by sequence randomization resulting from the random occurrence of head-to-head and head-to-tail isomerization if the unit is unsymmetricaF or by steric effects such as increased chain separation and decreased coplanarity of adjacent units in the mesogen. ... 

MCLCPs based on kinks rather than frustrated chain packing do not exhibit the j loss process associated with the naphthyl moieties. 

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