A. S. Paranjpe

Two-dimensional fractal growth by diffusion limited aggregation of copper

Abstract A novel method for obtaining 2-d diffusion limited aggregates of copper from copper sulphate solution using a simple displacement reaction is reported. No external field is applied for accelerating the aggregation process. The aggregates evince scale invariance over a range of solution concentrations.

Structure of the Short Range Ordered Nematic Phases of nBABA (n = 1 to 9

Abstract X-ray diffraction measurements have been carried out on the nematic phase of nBABA (n = 1 to 9). Our results show that this nematic phase is a fibre type one having in addition to the orientational long range order, two types of short range orders, (SROs): (i) one dimensional correlations of the molecular dimers along the nematic director (ň) and (ii) Sc type SRO. The strength of 1-d correlation increases with increasing n for 1 6. The n-dependence of the length of the correlated units shows that the chains are disordered for large n The Sc type order which is length (ξ2) is found to be asymmetric with ξ2∥(∥[ncirc]) > ξ2⊥ (⊥[ncirc]). This asymmetry decreases with increasing n The formation of the observed structures in the SROs has been qualitatively explained taking into account various types of intermolecular interactions.

Thermal and Structural Studies of the Different Phases of HxBPA

Abstract We have investigated the temperatures as well as the heats of transition involving the various crystalline and liquid-crystalline phases of HxBPA (4-n-hexyloxybenzilidene-4′-n-propyl-aniline) using a differential scanning calorimeter. From our observations we have arrived at the scheme of transitions given below.

X-ray and Raman investigations of two smectic phases of HxBPA

Abstract The temperature dependence of certain lattice parameters of HxBPA (4-n-hexyloxybenzylidene-4′-n-propylaniline) has been determined from 250 K to 300 K, in the smectic phases, S m and S H , by X-ray diffraction. There is a marked difference in the Raman spectra for the two phases in the frequency range, 200 to 1100 cm -1 . In the light of our observations, it seems that the S m → S H transition is due to the onset of disorder in the hydrocarbon chains.

Chain Ordering in the Nematic Phase of 40ba

Abstract From the intensity profile of the X-ray diffraction pattern in the nematic phase of 40BA it is inferred that the alkoxy chains have an orientational order.

Chain Ordering in Smectic C Phases

Abstract A model of SC phase has been proposed in which in addition to the ordering of the rigid portions of the molecules, orientational ordering of the end hydro-carbon (H-C) chains is also taken into account. The layer spacing calculated from this model takes into account the tilt directions of both, the rigid portions as also of the H-C chains. Our x-ray diffraction results agree well with the predictions of the model.

Structure of the Nematic Phase of Pbaba

Abstract X-ray diffraction investigations of the nematic phase (191°C - 276°C) of p-n-propyloxybenzylidene-p-amino benzoic acid have shown that this phase is a skewed cybotactic nematic phase. The diffraction patterns, obtained with aligned samples, reveal an additional surprising structural feature, - the presence of one dimensional molecular correlation in a direction parallel to the nematic director. At 192°C, it is found that the molecular unit, thus correlated, is about 37 A in length (dimer length) and the correlation range extends up to about seven such units. This l-d correlation persists up to the N-I transition.

Two-dimensional pattern growth from aqueous glucose solutions on a glass substrate

Abstract Two-dimensional patterns with striking circular morphology are observed during slow precipitation of glucose from a thin layer of its aqueous solution on a glass substrate. The patterns have spatially periodic radial density variations. The interface growth velocity is independent of the radius of the aggregate.

Thermal Investigations of nBABAs

Abstract The phase transitions occurring in nBABAs (n = 1 to 5), especially those involving the nematic phase, have been studied by DSC. Schemes of transition are presented and the chain length (n) dependences of the the transition temperatures and entropies are discussed.

Molecular Packing in Nematics

Abstract In this communication it is demonstrated− that among the different possible molecular packing directions in liquid crystals with molecules containing more than one aromatic ring, x-ray diffraction techniques can be used to pick out the correct one. Our x-ray diffraction measurements on mBABA indicate the packing to be parallel to the para-axes of the benzene rings.