Liquid crystalline nature of poly(di-n-hexylsilane)

Weber, P.; Guillon, Daniel; Skoulios, A.; Miller, Robert D.

doi:10.1051/jphys:01989005007079300

Cited by 66 publications

(29 citation statements)

References 14 publications

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“…We conclude pointing out that the self-compacting chain model, although a first approximation, represents so far the only viable alternative to the microphase separation concept 17,20,21 to account for the stability with respect to the melt of mesophases of flexible polymers. Examination of the available experimental data hardly supports the microphase separation approach because, due to the high side-group density in macromolecules yielding class 2 mesophases, their intermolecular interactions are bound to always occur essentially between chemically similar side chains.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 97%

Mesomorphic Phases of Flexible Polymers: The Self-Compacting Chain Model

Allegra

Meille

2004

Macromolecules

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Correlations are established between structural, thermal, and flexibility data of crystalline, locally flexible polymers giving rise to mesomorphic phases, which can be usefully organized in two classes. While in all mesophases the chain conformation is substantially disordered, in class 1 chain packing is characterized by orientation-dependent attractive interactions, so that the transition enthalpy ∆HML from the mesophase to the liquid is of the same order of magnitude as the crystal f mesophase transition enthalpy ∆HCM. Conversely, in mesophases belonging to class 2 and in the corresponding melts nonbonded intermolecular interactions and conformational statistics are roughly similar, so that the M f L transition involves very little enthalpy (∆HML ∼ 0). Consistent with the absence of specific interchain interactions, class 2 mesophases exhibit hexagonal packing and density lower than the corresponding crystalline phase. Polymers giving rise to this type of mesophase are characterized by numerous, regularly spaced side groups, often chemically different from the chain skeleton. The key factor stabilizing class 2 mesophases over the melt appears to be a higher entropy of the side groups, making mesogenic groups unnecessary. Optimization of the intrachain arrangement in class 2 mesophases usually requires a local contraction along the chain axis direction with respect to the extension found in the crystal and the confinement of individual chains within tubes that have a diameter D substantially larger than the average diameter D C in the crystal. With a simple statistical model based on continuum elasticity (the self-compacting chain), applicable to flexible polymers, we show that, consistent with the available experimental data, the persistence length P is proportional to D 2 . As a consequence, the aspect ratio P/D, which is normally assumed to control mesophase stability, is proportional to D. According to our approach, we thus may estimate by simple diffraction measurements of D the persistence length and the class 2 mesophase stability of flexible polymers. In the case of systems which experimentally give rise to class 2 mesophases, relatively large values of P are found despite the relatively low T g.

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Section: Discussion and Concluding Remarksmentioning

confidence: 97%

Mesomorphic Phases of Flexible Polymers: The Self-Compacting Chain Model

Allegra

Meille

2004

Macromolecules

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“…Above this transition, the structue has been shown to be hexagonal for a variety of polysilanes with different side chain lengths [12, 15,16], and the same behavior is expected forPHPS. Embs et al [17] observed ordering with respect to the substrate in thin polysilane Langmuir-Blodgett films.…”

Section: Introductionmentioning

confidence: 93%

Surface orientation of liquid crystalline poly(alkylsilanes)

et al. 1995

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Present address: Polymer Division, NIST, Gaithersburg, MD 20899, USA X-ray diffraction measurements on thin films of poly(hexy1-pentylsilane) cast on silicon substrates are presented. The polymer chains pack in a hexagonal lattice which is oriented such that the chains lie parallel to the surface. In addition, a remarkable degree of orientational order was found with the planes containing the neighbor molecules lying in the surface plane. Parallel to the film surface, the average crystallite size is 600 A, while perpendicular to the surface, there is nearly perfect interchain stacking of the polymers throughout the thickness of the film.

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“…Another important peculiarity of the symmetrically substituted poly(din-alkylsily1ene)s is that they form mesomorphic phases [82]. In the mesophase, the polymer backbone and the side chains are conformationally disordered, while the macromolecules remain extended and are packed hexagonally [83].…”

Section: Fibrillar Structurementioning

confidence: 99%

“…Poly(di-n-pentylsilylene) (PDPS) and poly(di-n-hexylsilylene) (PDHS) transform at 65OC and 42OC, respectively, from a rigid crystal state into the hexagonal columnar LC-phase [80,82]. These temperatures are just below the &-transition of PE [89], above which gel crystallized UHMW PE can be drawn to ultrastrong fibres or tapes and blending of minor frac- Figure 21.…”

Section: Fibrillar Structurementioning

confidence: 99%