Core orientator

With clusters of eight MBF molecules, the orientational freedom of individual molecules becomes restricted, and, as a consequence of this, the distribution of core orientations changes to favour strongly orientations where the molecular core straddles only one polymer chain. 

When suitably doped, MBF can form a surface-stabilised-ferroelectric smectic-C (SSFLC) structure. Using simple assumptions regarding core orientations, Binger and Hanna are able to place an upper limit on the SSFLC cone angle for MBF of 30°. 

Figure E13.2i shows the average velocity at the end of fill, which determines, to a large extent, core orientation [E13.2j], whereas skin orientation, as discussed earlier, is determined by the normal of the advancing front shown in Fig. E13.2k.

Some tests are affected by core orientation and some test properties are more influenced by surface orientation. Since orientation is not uniform, but has a gradient through and along the flow path, it is difficult to predict directly the effects of process conditions on part properties, without a complex model of the part geometry and estimation of flow characteristics in the cavity. 

It is clear from Figures 4 and 5 that the chain dimensions and probabilities have essentially reached their asymptotic values at modest temperatures. We should expect, therefore, that in melts of semiflexible PLCs, unless there are extreme extenuating circumstances (i. e., perfect core orientational ordering), the dimensions of a spacer chain and the conformer probability distributions within a spacer chain should be similar to those quantities in an alkane liquid. The attachment of a spacer at both ends will of course perturb the dimensions and conformer probabilities of the spacer. Clearly the magnitude of this perturbation of the spacer is the key to understanding the role of the spacer in a PLC. 

In addition to skin-core orientational differences, Hudson and co-workers [28] recently indicated the existence of greater disclination defect intensity in the core region compared to the skin. 

Fig. 5.21. (a) A schematic illustration of relative core orientations in members of a covalently connected pair of mesogenic cores for even and odd spacer-chain parities (all-tmns conformation) and (b) dimer (D ) versus monomer (M ) LC core order parameters in the nematic phase (from [62]). 

One of the characteristics of injection molding is that melt orientation imparted by rapid flow is frozen into the final product. Solidification occurs fastest next to the mold surface, with the result that highly oriented skins are often found overlaying a less oriented core. Orientation may be beneficial or detrimental. 

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