We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

SYNOPSISHighly oriented fibers of Li-, Na-, K-, and CsDNA were prepared with a previously developed wet spinning method. The procedure gave a large number of equivalent fiber bundle samples (reference length, Lo, typically = 12-15 cm) for systematic measurements of the fiber length L in ethanol-water solutions, using a simple mechanochemical set up. The decrease in relative length L / Lo with increasing ethanol concentration at room temperature gave evidence for the B-A transition centered at 76% ( v / v ) ethanol for NaDNA fibers and at 80 and 84% ethanol for Kand CsDNA fibers. A smaller decrease in L / L o of LiDNA fibers was attributed to the B-C transition centered at 80% ethanol. In a second type of experiment with DNA fibers in ethanol-water solutions, the heat-induced helix-coil transition, or melting, revealed itself in a marked contraction of the DNA fibers. The melting temperature T,, decreased linearly with increasing ethanol concentration for fibers in the B-DNA ethanol concentration region. In the B-A transition region, Na-and KUNA fibers showed a local maximum in T,. On further increase of the ethanol concentration, the A-DNA region followed with an even steeper linear decrease in T,. The dependence on the identity of the counterion is discussed with reference to the model for groove binding of cations in B-DNA developed by Skuratovskii and co-workers and to the results from Raman studies of the interhelical bonds in A-DNA performed by Lindsay and co-workers. An attempt to apply the theory of Chogovadze and Frank-Kamenetskii on DNA melting in the B-A transition region to the curves failed. However, for Na-and KDNA the T , dependence in and around the A-B transition region could be expressed as a weighted mean value of T , of Aand B-DNA. On further increase of the ethanol concentration, above 84% ethanol for LiDNA and above about 90% ethanol for Na-, K-, and CsDNA, a drastic change occurred. T , increased and a few percentages higher ethanol concentrations were found to stabilize the DNA fibers so that they did not melt at all, not even at the upper temperature limit of the experiments ( -80°C). This is interpreted as being due to the strong aggregation induced by these high ethanol concentrations and to the formation of P-DNA. Many features of the results are compatible with the counterion-water affinity model. In another series of measurements, T,,, of DNA fibers in 75% ethanol was measured at various salt concentrations. No salt effect was observed (with the exception of LiDNA at low salt concentrations). This result is supported hy calculations within the Poisson-Roltzmann cylindrical cell model.

The high-resolution calorimetric study was carried out on octylcyanobiphenyl liquid crystal (LC) confined to various controlled-pore glass (CPG) matrices with silane-treated surface. The diameter of the voids cross section ranged between 23.7 and 395 nm. The results are compared to those obtained previously on CPG voids nontreated with silane. We found a striking similarity between the shifts in the isotropic to nematic and nematic to smectic-A phase transition temperatures as a function of the void radius in which order parameter variations at the LC-void interface play the dominant role. Weaker temperature shifts are observed in silane-treated samples, where surface ordering tendency is larger. In nontreated samples, a finite-size scaling law in the maximum value of the heat capacity at the nematic to smectic-A transition was observed for void diameters larger than 20 nm. In silane-treated samples, this behavior is considerably changed by surface wetting interactions.

We present the measurements of the proton spin-lattice relaxation time T1 of liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB) confined into randomly oriented approximately 15 nm pores of untreated porous glass. In the low kilohertz range the spin-lattice relaxation rate in the nanoconfined 8CB is about ten times larger than in the bulk. We show that the increase is mainly due to molecular reorientations mediated by translational displacements (RMTD). In the paranematic phase the power law describing the RMTD dispersion, (T1(-1))RMTD proportional, omega(-p), is well characterized by the exponent p=0.5+/-0.06 and suggests an equipartition of diffusion modes with different wavelengths. The largest distance related to the decay of the orientational correlation function is about twice the diameter of the cavity. The situation is different in the nematic phase, where the orientational correlation is eventually lost at approximately 60 nm in the direction along the pore, a distance corresponding roughly to the length of a pore segment in the glassy matrix. The exponent p is between 0.65 and 0.9, depending on the temperature, which implies that in the nematic phase long wavelength modes are relatively more important--a consequence of the uniform director field along the pore. These observations are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. We point out that in strongly confined liquid crystals the proton NMR relaxometry does not provide the evidence of director fluctuations correlated over micrometer distances as was suggested earlier. The local translational diffusion of molecules within the cavities is found about as fast as in bulk.