Poly planar orientation

Homogeneously planar oriented nematic elastomers have been prepared using a procedure very similar to that presented above. As alignment layers, unidirection-ally rubbed polyimide or rubbed poly(vinyl alcohol) were used [14]. 

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

All bonds can be situated in gauche or trans states and the chain oxygens can be located opposite each other to relieve the orientation of the dipole moment vectors. The conformation may be similar to that of poly-tetrahydrofuran, planar zigzag172,173). 

These results support current interpretations of the bathochromic shifts observed in dialkyl-substituted poly silane. Experimental results for poly(di-n-hexylsilane) indicate that as the temperature is cooled below a transition temperature of roughly -35 °C, the major absorption peak shifts from a broad peak at about 310-320 nm (3.9-4.0 eV) to a narrower peak at about 350-370 nm (3.3-3.5 eV), with the red shift being attributed to a transition from a disordered system with a large population of gauche bond twists in the silicon backbone and in the alkyl substituent to a planar dll-trans backbone conformation (5-8, 15). Results from polarized absorption spectra of stretch-oriented samples for the cooled samples exhibit absorbance only for polarizations parallel to the stretch (and presumably the chain axis) direction (22). 

Crystallinity. Most poly (thiol esters) are quite crystalline. Crystallinity can be lowered by using nonlinear monomers—for example, HSCH2(C2H5)CH-(CH2)4SH (31)—or by copolymerization (29). The x-ray pattern of a uniaxially oriented poly( -thiocaprolactone) (17) indicates that this polymer adopts a planar structure with extended chains. 

Kevlar, or aramid as it is generically known, is a highly oriented polymer - poly(p-phenylene terephthalamide). Kevlar fiber has the interesting property of having a linear TCE in its axial direction of -2 x 10 6/°C. The E-glass used in normal PWBs has a positive linear TCE of +5 x 10-room temperature TCEs in the X-Y plane of around 4 to 8 x 10 6/ C(3) and solder joint cracking is greatly reduced or eliminated(2). Quartz is another reinforcement, available in fabric form, which can be used to control the planar TCEs of a laminate PWB. Quartz has a TCE of +0.54 x 10-6/°C in the axial direction. 

Poly(2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophenecopolymers (pBTTT) were reported [50] with high charge carrier mobilities. This has been attributed to the large crystalline domains formed by the assembly of the planar conjugated backbones into closely pi-stacked sheet-like lamellae, and subsequent orientation of these domains in the plane of the substrate, hence... 

Fig. 9.19. (left) A schematic of the molecular order of the reactive LC molecules in a coated thin film retarder. The order is forced to be planar by the rubbed poly-imide. (right) An image of a uniaxial planar wet-coated retarder viewed in between two crossed polarisers and its orientation with respect to the crossed polarisers. (A) Analyser, (P) polariser, and (R) retarder. 

As an example, we discuss a structural study, by the Rietveld [10] method, of neutral poly thiophene [11]. This polymer is, sofar, the most ordered polyheterocycle, even though no oriented films have been obtained up to now, and so the only available samples are thermally annealed powders. Interestingly, aside from the expected (and found) co-planarity between adjacent rings, the detected 3-D order can be interpreted in terms of a discrete number of preferred arrangements, each of them shifted with respect to another along the chain axis, and co-present in the crystallites. This hypothesis accounts for the relative doping ease of polythiophene. 

In polysilanes, the primary motivation has been to identify the chain conformational sequences and, to a lesser extent, orientation of the side chains. PDF analysis [356] of three atactic poly(methyl- -alklysilanes) in bulk is indicative of a near-planar transoid/ deviant sequencing of the skeletal backbone with relatively low concentrations of either ortho- or gauche-type conformations. Figure 17.24 shows data for atactic poly(methyl-n-propylsilane) and a single-chain model fit. The difference at lower q, <5 A , is due to scattering from interchain correlations. 

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