ABSTRACT:In the present work, contact angles formed by drops of diethylene glycol, ethylene glycol, formamide, diiodomethane, water, and mercury on a film of polypropylene (PP), on plates of polystyrene (PS), and on plates of a liquid crystalline polymer (LCP) were measured at 20°C. Then the surface energies of those polymers were evaluated using the following three different methods: harmonic mean equation and geometric mean equation, using the values of the different pairs of contact angles obtained here; and Neumann's equation, using the different values of contact angles obtained here. It was shown that the values of surface energy generated by these three methods depend on the choice of liquids used for contact angle measurements, except when a pair of any liquid with diiodomethane was used. Most likely, this is due to the difference of polarity between diiodomethane and the other liquids at the temperature of 20°C. The critical surface tensions of those polymers were also evaluated at room temperature according to the methods of Zisman and Saito using the values of contact angles obtained here. The values of critical surface tension for each polymer obtained according to the method of Zisman and Saito corroborated the results of surface energy found using the geometric mean and Neumann's equations. The values of surface energy of polystyrene obtained at 20°C were also used to evaluate the surface tension of the same material at higher temperatures and compared to the experimental values obtained with a pendant drop apparatus. The calculated values of surface tension corroborated the experimental ones only if the pair of liquids used to evaluate the surface energy of the polymers at room temperature contained diiodomethane.

RIS analysis of the deuterium quadrupolar splitting data was performed for a,a»-bis [(4,4'cyanobiphenylyl)oxy]alkane dimer liquid crystals having -0(CH2)"0flexible spacers (n = 9 and n = 10 (CBA-10)) between the 4,4'-cyanobiphenylyl ends according to the scheme previously established. The analysis indicates that most of the conformers involved in the range 0 < if i,if 2 < 45°a dopt spatial configurations reasonably consistent with the nematic arrangement of mesogenic cores in both dimer LC systems, where ifi and if 2 denote the inclination angles of the terminal mesogenic cores with respect to the molecular axis. The conformational entropy changes at the crystal-nematic (CN) and nematic-isotropic (NI) interphases estimated on this basis are as follows: CBA-9, AS"nf = 59.6, AS"nf = 13.3; CBA-10, AS"nf = 64.2, AS"nf = 15.6 (J mol-1 K-1 units). The values of the entropies AS™111 thus derived were compared with the constant-volume transition entropies (AStr)" determined by the PVT measurements reported in the accompanying paper: CBA-9, (AScn)i> = 53.9, (ASn;)" = 7.9; CBA-10, (AScn)i) = 62.4, (ASni)u = 13.3 (J mol-1 K-1 units). In view of the uncertainties involved in the estimation of the entropies both in theory and in experiments, the correspondence is quite favorable. The conformation of the spacer Undoubtedly plays an important role in determining the phase behaviors of these main chain liquid crystals. It is pointed out that the discrepancy between the calculation and experiment may be further improved by considering other contributions such as (1) the entropy changes due to the orientation of the anisotropic molecules in the liquid crystalline state and (2) the possibility of the entropy loss during the compression to achieve constant-volume transitions. It is concluded that the observed increase in the quadrupolar and dipolar splittings with decreasing temperature arises mainly from the variation of the order parameter of the molecular axis.

Experimental data for the surface tension of polystyrenes of different molecular weights (3400 -200,000) and different molecular weight dispersities (1-3) and of different polyolefins are compared with the predictions of the Patterson-Rastogi and Dee-Sauer cell theories, which infer the surface tension from pressure-volumetemperature (PVT) data. PVT data for these polymers were obtained from the literature and experimentally and are fitted to the Flory-Orwoll-Vrij equation of state. Both theories predict that the surface tension will decrease linearly with increasing temperature and increase with molecular weight, thereby corroborating the experimental data. However, both theories underestimate the entropy change in the surface formation per unit area at a constant volume for low molecular weight and polydisperse systems and underestimate the effect of molecular weight dispersity on surface tension. Both theories feature two parameters, m and b, that quantify the enthalpic and entropic contributions to surface tension. The theoretical predictions are fitted to the experimental data for monodisperse polystyrene (with a molecular weight above the molecular weight of entanglement), polypropylene, and linear low-density polyethylene to quantify the enthalpic contribution to surface tension. b is then evaluated as a function of molecular weight and molecular weight dispersity and is found to decrease with increasing molecular weight and to increase with increasing molecular weight dispersity, showing that end-group excess at the surface has some effect on surface tension.

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ABSTRACT:The ordering of a dimer liquid crystal, IX,w-bis [(4,4'-cyanobiphenylyl)oxy]decane (CBA-10), was studied by variable-temperature high-resolution solid-state 13 C NMR. A large transitional change of the 13 C chemical shift was observed on going from the isotropic to the nematic phase due to uniaxial alignment of the molecule. The principal values of the 13 C chemical shift tensor have been determined by the Herzfeld-Berger method for CBA-10 itself. The molecular axis is assumed to lie parallel to the line connecting the centers of both mesogenic cores, and the evaluation of the order parameters along the molecular axis, (Szz), mesogene axis (S~~ng) and C,-C, axis (S~,;'-c") was attempted here, using the obtained isotropic 13 C chemical shift and principal values of the 13 C chemical shift tensor, and taking into account the conformational behavior of the flexible spacer in the frame of rotational isomeric state (RIS) approach. The order parameter of the molecule S., increased from 0.79 to 0.89 with decrease of I 7.5°C in temperature from nearby the nematic-isotropic transition point. S~~"• increased from 0.74 to 0.80 with decrease of temperature whereas S~,:--c· and the conformation of the spacer was essentially unaltered in the same temperature range.KEY WORDS High-Resolution Solid-State 13 C NMR / 13 C Chemical Shift Tensor/ Conformation/ Rotational Isomeric State / Order Parameter / Dimer Liquid Crystal / Low molecular weight monomer liquid crystals are composed of rod-like molecules which form an uniaxially oriented mesophase. The chemical shift anisotropy of the 13 C nucleus has proved to be a powerful tool for probing the orientational ordering at the molecular level of such compounds 1 · 2 and for a side-chain liquid-crystal polymer. 3 Though the principal values of the chemical shift tensor needed for the analysis are in some cases assumed to be the same for analogous compounds, this may not be true 2 .4 and may lead to error in constructing models of microscopic configurations from the observed data.In the case of dimer liquid crystals consisting of two mesogenic moieties linked by a flexible alkyl chain, as the model of polymer liquid crystals, the molecules align as a whole in the nematic phase, the conformation of the spacer being quite stable in a large temperature range. 5 Our goal in this work is to prepare a 13 C-labeled dimer liquid crystal, ll (,w-bis[( 4,4' -cyanobiphenylyl)oxy ]decane (CBA-10) and to analyze its ordering behavior through the observation of the 13 C chemical shift anisotropy, determining the principal values of the 13 C chemical shift tensor for CBA-10 itself. This is the first attempt with a dimer liquid crystal in which the conformation of the spacer must be considered in the analysis of ordering being different from simple monomer liquid crystals. Spacer conformation is considered in the frame of the rotational isomeric state (RIS) approach.

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4¾ -n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of diOE erent volumes ranging from 5 to 11 mm3. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.

: The conformation around the bond of the alkyl spacer of a,u-bis[(4,4@-cyanobiphenylyl)oxy] C e ÈC e@ decane (CBA-10) was evaluated in the solid and liquid crystalline states through the detection of the dipolar interaction between and CBA-10 13C-labelled in the d position of the alkyl spacer was prepared and for the C d C d@ . measurement in the solid state a variation of the CarrÈPurcellÈMeiboomÈGill sequence was used. The spectra in the nematic state were measured with the pulse saturation method. It was concluded that in the solid state the trans conformation is assumed whereas in the liquid crystalline state the fraction of the trans conformers is around 0.6.

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