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G.H. Chen, J. Springer


Low-molecular-weight and polymeric liquid crystals at interfaces.

A study on the effect of surrounding gases (in English)



Polimery 1999, No 7-8, 489


DOI: dx.doi.org/10.14314/polimery.1999.489


Summary

Experimental data are reported on the effect of ambient gases on the surface—molecular behavior of low-M and polymeric liquid crystals (LC). Experiments were carried out in C 02 and N2 atmospheres over a temperature range covering the phase transitions of the LC examined and at pressures up to 15 bar. Time-dependent measurements verified the agreement between the kinetics of gas sorption and the remarkably long-lasting decrease in the surface tension of freshly formed LC surfaces. The characteristic course of the surface tension vs. temperature relationship reveals a gas-induced molecular excess order at the surface of low-M LC such as MBBA, PCH5 and PCH8-CNS and the LC side-group polyacrylate PAC-6, chosen as model substances. This surface excess order in seen to occur both in the nematic and the isotropic phase and can be enhanced by increasing the gas pressure. When the pressure is high enough, this surface order can be stabilized, a fact unusual for low-M LCs, even in the isotropic phase over a wide temperature range. Square-pulse disturbance of the surface area was used to study the relaxation of gas sorption at the surface of PAC-6. The square-pulse test has demonstrated a new possibility of qualitative evaluation of the surface molecular order. The existence of the pretransitional order was confirmed, indicative of an enrichment of the LC side groups at the polymer surface. Measurements at T > 150°C are in progress


Keywords: surface tension, gas sorption, time dependence, temperature dependence, excess order, square-pulse test, liquid crystals, MBBA, PAC-6, carbon dioxide, nitrogen

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