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Biomedical subjects

B Zalar

Publications and source records attributed to B Zalar.

At least 19 recordsLinked to original sources
PubMed2005

Evidence of supercritical behavior in liquid single crystal elastomers.

Temperature profiles of the first and the second moment of the nematic order parameter distribution function, as determined from the deuteron nuclear magnetic resonance line shapes, as well as heat capacity response, provide support for the supercritical scenario of the nematic-paranematic phase transition in liquid single crystal elastomers. The relative strength of the locked-in internal mechanical field with respect to the critical field can be decreased by swelling the elastomer samples with low molecular mass nematogen. By increasing the concentration of the dopant, critical and below-critical behavior is promoted.

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PubMed2005

Anchoring and structural transitions as a function of molecular length in confined liquid crystals.

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.

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PubMed2004

Field cooled and zero field cooled 207Pb NMR of the relaxor ferroelectric PMN.

A 2.8 kV/cm electric field has been applied parallel to the external magnetic field along the [111] direction of a PMN single crystal and the 207Pb NMR spectra were measured at 9.1 T. Whereas the zero field cooled (ZFC) spectrum exhibits a Gaussian-like line shape, the FC spectrum clearly shows a two peak structure. One of the two peaks coincides with the ZFC spectrum. The other peak is shifted by about 100 kHz towards lower frequencies with respect to the ZFC peak and seems to be characteristic for the ferroelectric state. The ferroelectric shift agrees with the predictions of the spherical random bond-random field model.

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PubMed2001

Smoking in relation to psychosocial risk factors in adolescents.

OBJECTIVE: The study investigated which psychosocial factors distinguish adolescent smokers from non-smokers, and examined the most important factors correlated with smoking. METHOD: A cross-sectional survey carried out by means of a questionnaire which, in addition to items on family, school, healthy habits, drug abuse and suicidal behaviour, also included a scale of depression (Zung's SDS) and of self-esteem (Rosenberg's SES). Sample. Two thousand one hundred and eleven high school students aged from 17.0 to 18.11 (1,083 girls and 1,028 boys), of whom 221 girls and 239 boys were smokers. RESULTS: The smokers of both sexes differed significantly from the non-smokers in frequency of substance abuse, family dysfunction, suicidal ideation and suicide attempts, attitude towards the harmful effects of smoking, truancy from school, and lesser involvement in sports. With regard to smoking behaviour, the most important predictive factors for adolescents of both sexes proved to be substance abuse, truancy, suicide attempts, a lower estimate of the harmfulness of smoking, and infrequent engagement in sports. Among the girls, predictive factors were also sexual and physical abuse, and binge eating. CONCLUSIONS: Smoking behaviour in adolescence may be part of a pattern of problematic behaviour and may be linked to various psychopathological disturbances.

Adolescent↗
PubMed2000

Deuteron NMR study of monolayer thick films of nematogenic molecules

We present a deuteron NMR study of molecular thickness films of a nematogenic material in cylindrical pores. Compared to liquid crystal in bulk or completely filling the pores, a reduced quadrupole splitting is found. It strongly depends on thickness, but only weakly on temperature, even crossing the bulk nematic-isotropic transition. This demonstrates the presence of a 2D-like film that eventually coexists with a bulklike phase suggesting a dewetting behavior. Motionally averaged angular patterns with biaxiallike features reveal unexpectedly fast surface molecular diffusion.

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PubMed2000

Deuteron NMR investigation of a photomechanical effect in a smectic-A liquid crystal

Deuteron quadrupole-perturbed NMR is used to study the perturbation of orientational order in a smectic-A liquid crystal (octylcyanobiphenyl or 8CB) caused by photoinduced trans-to-cis isomerization of a photosensitive dopant (diheptylazobenzene or 7AB). The time and temperature dependences of the orientational order were independently studied for 8CB, 7AB, and their mixtures. Upon UV irradiation that causes trans-to-cis isomerization of 7AB, the orientational order parameter of the smectic-A phase is reduced. Relaxation in the dark exponentially restores the equilibrium value of the order parameter. The characteristic time for this process closely matches the lifetimes of the 7AB excited state. While in the 8CB smectic-A matrix, the cis-isomerized 7AB molecules retain a uniaxial orientational order with the director oriented along the normal to the smectic layers. The highly bent 7AB cis molecules act as a disorienting factor, decreasing the orientational order in the layers and causing a small increase in layer spacing. This disorder-induced increase in layer spacing is much smaller than the actual increase as observed by in situ x-ray experiments on UV-irradiated mixtures of 8CB:7AB. Concomitant with the experimental observation that only a fraction of 7AB molecules are converted to the cis state, this work provides indirect evidence for a nanophase segregation with the 7AB cis-isomers arranged within the interlayer space, thus significantly increasing the smectic layer spacing.

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PubMed2000

Interlayer molecular exchange in an anticlinically ordered chiral liquid crystal

The interlayer molecular exchange has been determined in the antiferroelectric smectic-C(*)(A) phase of alphad(2) deuterated 4-(1-methylheptyloxycarbonyl)phenyl4(')-octyloxybiphenyl-4-carboxylat e via quadrupolar deuteron NMR self-diffusion in the spatially varying electric field gradient produced by the anticlinic smectic layer structure. The interlayer self-diffusion coefficient is here by two orders of magnitude smaller than in synclinically ordered smectic phases. The results support the entropic suppression model of the origin of anticlinic smectic ordering. The applied technique could possibly allow for a new insight into the local structure of the intermediate "clock" phases.

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PubMed2000

Characteristics of suicide attempters in a Slovenian high school population.

The study was carried out on a representative sample of Slovenian high school students (n = 3687) of whom 238 (137 girls and 101 boys) have already attempted suicide. Both suicide attempters and nonattempters were assessed by a questionnaire consisting of Zung's (1965) Depression Scale, Rosenberg's (1965) Self-Esteem Scale, sociodemographic data, information on suicide attempt, suicide ideation, family relations, and substance abuse. Statistical analyses indicated that groups differed in the levels of self-esteem, emotional reaction to family problems, running away from home, and substance abuse. The differences in the level of depression, frequency of suicide ideation, family suicide occurrence, smoking, and alcohol use were significant.

Adolescent↗
PubMed2000

Gender differences in psychosocial risk factors among Slovenian adolescents.

This study investigated psychosocial risk factors in adolescents and assessed gender differences in the frequency of their occurrence. A specially designed questionnaire, which included validated scales for the evaluation of depression (Zung Self-rating Depression Scale) and self-esteem (Rosenberg Self-Esteem Scale), was administered to a representative sample of Slovenian adolescents. The final sample consisted of 4,590 high school students of both sexes, aged 14 to 19 years. Data analysis revealed several significant gender differences. Girls more often than boys reported family conflict, personal problems, physical inactivity, attempts to control body weight, use of psychoactive drugs, suicidal ideation, and suicide attempts. Their level of depression was higher and self-esteem lower when compared with their male counterparts. Boys more often than girls indicated that they watched a great deal of television, were the victims of peer violence, and drank alcohol.

Adolescent↗
PubMed1999

Measurement of the surface-induced order in polymer dispersed liquid crystals: an approach by NMR relaxometry.

A method based on NMR relaxometry is introduced to study surface-induced order in the isotropic phase of confined liquid crystals. We show that the magnitude of the surface order parameter S0 can be obtained from an increase in the deuteron transverse relaxation rate T-12. The increase originates in the molecular diffusion between the weakly ordered surface region and disordered area in the rest of the cavity. No assumptions concerning the residence time of molecules at the surface are needed. We apply the approach to a polymer dispersed liquid crystal and find a temperature independent S0 of magnitude congruent with 0.08. This indicates that short range interactions at the interface dominate the behavior of S0, and that only a partial orientational wetting occurs.

Journal Article↗
PubMed1999

Effects of quenched disorder on the orientational order of the octylcyanobiphenyl liquid crystal.

Deuteron NMR (DNMR) measurements were performed with high-temperature and spectral resolution on the octylcyanobiphenyl (8CB) liquid crystal confined to the randomly interconnected pores of silica aerogel as a function of temperature and silica density. The aerogel density was varied by one order of magnitude and the temperature spanned the isotropic (I), nematic (N), and smectic-A (SmA) phases of 8CB. For all samples, the liquid crystal was confined to pores smaller than the micron-sized magnetic coherence length. Thus the observed line shapes reflect the director pattern, n(r-->), and the orientational order, Q, as dictated by the porous host. The DNMR spectral patterns are consistent with powder line shapes representative of a randomized n(r-->) characterized by a single Q. The nematic domains formed are of finite dimension that likely exceeds the confining size. The weakly first-order nematic-to-isotropic phase transition becomes less discontinuous with decreasing pore size, eventually becoming continuous at an aerogel density between 0.36 and 0.5 g/cm(3). In the most severe confinement, no phase transitions are observed with only a continuous evolution of Q present. The orientational order is suppressed from the bulk's with no enhancement upon the onset of the SmA phase. This indicates a decoupling of nematic and smectic order parameters and a severe suppression of the SmA phase by this porous media.

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