DSC AND X-RAY DIFFRACTION INVESTIGATIONS OF PHASE TRANSITIONS IN HxBABA AND NBABA

Transcription

1 DSC AND X-RAY DFFRACTON NVESTGATONS OF PHASE TRANSTONS N HxBABA AND NBABA K. Usha Deniz, A. Paranjpe, E. Mirza, P. Parvathanathan, K. Patel To cite this version: K. Usha Deniz, A. Paranjpe, E. Mirza, P. Parvathanathan, K. Patel. DSC AND X-RAY DFFRACTON NVESTGATONS OF PHASE TRANSTONS N HxBABA AND NBABA. Journal de Physique Colloques, 1979, 40 (C3), pp.c3-136-c < /jphyscol: >. <jpa > HAL d: jpa Submitted on 1 Jan 1979 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSQUE Colloque C3, supplément au n 4, Tome 40, Avril 1979, page C3-136 DSC AND X-RAY DFFRACTON NVESTGATONS OF PHASE TRANSTONS N HxBABA AND NBABA K. USHA DENZ, A. S. PARANJPE, E. B. MRZA and P. S. PARVATHANATHAN Bhabha Atomic Research Centre Trombay, Bombay , ndia K. S. PATEL Chemistry Department, Sardar Patel University Vallabh Vidyanagar, Gujarat, ndia Résumé. Les transitions des phases et les chaleurs des transformations (A/f) des HxBABA et NBABA, qui sont des membres de la série des composés, p-n-alkoxy Benzylidène-p-Amino Benzoic Acides, sont étudiées entre 100 C et 300 C, par DSC. Un schéma des transitions est proposé pour chacun de ces composés. Les mesures de diffraction des rayons X sont faites dans des phases, smectique C(S C ) et nématique (N) de ces matériaux. Les résultats ont montré que, (1) la phase S c est une phase C 1 dans les deux composés, (2) l'ordre «type S c», est vu dans toute la phase nématique dans HxBABA, tandis que, dans NBABA, il est vu seulement dans le voisinage de la transition, S c -N, (3) la variation, avec la température de l'épaisseur de la couche smectique, d, et celle de l'angle de tilt, d ta, qui est mesuré directement, sont caractéristiques de la nature de transition, S c -N, du premier ordre, (4) il y a un écart assez important, mal compris jusqu'à présent, entre d'une part les valeurs mesurées 6 tà et calculées à partir de d, 0, c de l'angle de tilt et d'autre part leur comportement en fonction de la température. Abstract. The phase transitions and the heats of transformation (AH), of the hexyl (HxBABA) and nonyl (NBABA) members of the series of compounds, p-n-alkoxybenzylidene-p-aminobenzoic Acids, have been studied by DSC in the temperature range, 100 C to 300 C. A scheme of transitions has been proposed for each of the compounds. X-ray diffraction measurements have been done in the smectic C(S C ) and nematic (N) phases of these materials. The results reveal that (1) the S c phase in both compounds is of the CVtype, (2) S c -type order is seen throughout the nematic phase in HxBABA, whereas in NBABA, it is seen only in the neighbourhood of the S c -N transition, (3) the temperature dependence of the smectic layer thickness, d, and of the directly measured tilt angle, 0, d, reflect faithfully the strength of the first order transition, S c -N, and (4) there is a marked difference between the values and the temperature variations of 0 td and 0, c (tilt angle calculated from d) which is not completely understood, at present. 1. ntroduction. The hexyl (HxBABA) and nonyl (NBABA) members of the series of compounds, p-n-alkoxybenzylidene-p-aminobenzoic Acids, are known [1] to exhibit smectic and nematic liquid crystalline phases between 100 C and 300 C. The smectic phase has been identified as an S c phase from our texture work [2]. n the present investigations, the phase transitions occurring in these compounds in the temperature range C < T < 300 C have been studied and the corresponding heats of transformation have been determined by Differential Scanning Calorimetry (DSC). Several hitherto unknown phases have been found to occur above room temperature (RT). A scheme of transitions has been proposed for each of the compounds, based on our measurements. The X-ray diffraction studies have been carried out to obtain the temperature dependence of the smectic layer thickness, d, the intermolecular distance, D, and the molecular tilt angle, 0, (the angle that the long axis of the molecule makes with the planar normal) in the S c and in the skewed cybotatic nematic (N sc ) phases, specially in the vicinity of the transition, S c -N. t is seen that, (1) the transition, S c -N, is characterised by a dramatic increase in d in NBABA and a rapid decrease of 0 td in the vicinity of this transition for HxBABA, and (2) S c -type order, typical of a N^ phase is found in both HxBABA and NBABA. 2. Experimental details. Preliminary DSC scans were carried out using a Dupont 900 Thermal Analyzer, but a Perkin Elmer DSC-1 B differential scanning calorimeter was employed for all later Article published online by EDP Sciences and available at

3 DSC AND X-RAY DFFRACTON NVESTGATONS OF PHASE TRANSTONS C3-137 scans from which the results quoted here have been obtained. The weights of the samples used varied between 3 and 4 mg. The measurements were made at three rates of heating and cooling, viz., 16, 8 and 40C/min. The true transition temperatures were obtained by extrapolation to zero rates of heating and cooling. The X-ray experiments were done, using Nifiltered Cu radiation for the major part of the measurements performed to obtain d and D, and unfiltered Cu radiation for obtaining diffraction photographs from which 0, could be directly measured, and a Laue camera. For experiments requiring well-aligned samples (single crystals), the samples were contained in ckpillaries of diameters less than 0.5 rnm, whereas larger capillary diameters (0.7 mm) were also used when studying polycrystalline samples. Exposure times for getting clear diffraction photographs varied from 15 min to 60 min and the temperature stability was about OC for this time duration. The samples used in these experiments were obtained by recrystallising from ethanol, several times. A well aligned sample (for the X-ray experiments) was obtained in the capillary, by slowly recycling the sample through the (1) P-S, transition, for HxBABA and (2) Sl-S, transition for NBABA, until proper alignment was obtained. A densitometer was used for analyzing the X-ray diffraction photographs. 3. Results and discussion DFFERENTAL SCANNNG CALORMETRY. - The DSC scans obtained at a heating rate of 16 oc/min for HxBABA and NBABA are shown in figure HxBABA. - On fii-st heating this compound from OC, only three transitions are observed (full line curve in Fig. 1). The hump on the transition peak (C-S,) is not observed for the heating rate, dt/dt = 4 OC/min. When cooling through this transition, this hump is not observed for any cooling rate. f the sample is further cooled to RT or to OC and reheated at ( dt/dt 1 = 16 OC/min, a glass transition phenomenon (dashed line in Fig. 1) occurs at 103 OC. The temperature range and the intensity of the glassy transition depends on the thermal history of the sample. The glass phase transforms to the crystalline phase through an exothermic transition. f the sample in the S, phase is cooled to 135 OC and reheated immediately, the hump on the transition peak (C-S,) is not observed. These rest+,cs indicate that during rapid heating, HxBABA loses,cs crystalline order in two steps (involving order 1 and order 2). While cooling the sample from the S, phase, only order 2 is restored at the observed transition and the sample exhibits a partially ordered phase P. The disorder in the P1 phase is frozen on further cooling, leading to the formation of a glassy phase. From our results, we have arrivea at the scheme of transitions shown in figure 2. The true transition temperatures, the corresponding heats of transformation, AH, and the transition entropy, AS(= AHT) are given in table. HxBABA NBABA Ce H3 0 CH= N-@ COOH HxBABA FG Schemes of transitions for HxBABA and NBABA. The boxed in part of the scheme for NBABA is that part about which our results are inconclusive. f C s, st C, H,, o + CH=N+COOH NBABA Heat Transition Nature of Temperature (OC) of transition Entropy transition Heating Cooling (kcal/mole) (cal/mole Deg) P-C(Tg) Not seen - - C-S, Not seen P-% S,-N N TEMPERATURE ('~ NBABA. - The full line curve in figure 1 FG DSC scans for HxBABA and NBABA at a heating rate shows the 4 transitions occurring in this compound of 16 ~C/min. on first heating from OC. When cooled from

4 C3-138 K. USHA DENZ, A. S. PARANJPE, E. B. MRZA AND P. S. PARVATHANATHAN the S, phase at 16 OC/min, a broad transition (extending from 51 to 68 OC) is observed at about 60 OC. This transition temperature is found to be strongly dependent on the cooling rate (changing from 60 OC at 16 oc/min to 79 OC at 4 oc/min). f the sample is cooled from the S, phase to RT and reheated, instead of the single C,-S, transition, one observes two transitions (dashed line curve in the inset of figure 1). The intensity of the peaks corresponding to these transitions is sensitively dependent on the thermal history of the compound. Cooling the sample from the S phase to any temperature below 75 OC and reheating it, gives back these two transitions. However, if the sample in the S, phase is cooled to RT and left at RT for a sufficient length of time (some days), the compound transforms back to the original crystalline state. Our results indicate that when the sample is cooled from the S, phase, it gradually transforms to a mixture of phases, C2 and C,. The transition at 60 OC observed on cooling corresponds to a transformation to a phase C,, which on reheating changes to a mixture of phases, C, and C,. The phase C4 changes into phase C,, the stable crystalline phase, when annealed at RT for a sufficient time duration. The nature of phases C,, C, and C4 are not known. We feel that phase S, might be a highly ordered smectic phase because (1) it supercools, (2) it has a mosaic texture, and (3) the heat of transformation for the transition, Cl-S,, is quite large, showing that a major structural change takes place at this transition. A scheme of transitions for NBABA is shown in figure 2. directly (O,,) from the diffraction photographs [3] of suitably aligned samples or calculated (q,,) from the values of d, using the relation, cos O,,, = d/l, where L was taken to be the length of the dimer (42.4 A for HxBABA and 48.2 A for NBABA), calculated using De Vries model [4]. The intermolecular distance, D, and 1, the average distance between the electron density minima at the ends of the molecule in the classical nematic [5] phase, have been calculated in the manner described by De Vries [4, 51. Certain interesting features are noticeable in our results : (1) in the case of even well-aligned samples, the diffraction spots corresponding to Bragg reflections from the smectic planes, spread out into a ring in the immediate vicinity of the S,-N transition (Tsc-,-0.25 OC) < T < (Tsc-,+0.25 OC), and (2) the diffraction pattern obtained in the nematic phase for HxBABA is typical of that from an Nsc phase [5, 61, almost up to the N- transition (Fig. 3), The true transition temperatures, the corresponding heats of transformation and the transition entrovv are given in table 11, for NBABA. An interestihg point to note is that AS(S,-N) for NBABA is about thrice that for HxBABA, showing that this transition is of a weakly first order nature in the latter. 3.2 X-RAY DFFRACTON MEASUREMENTS. - These measurements were in general confined to the S, and nematic phases of both the compounds. All the results reported here were obtained while heating the samples from the (a) P phase for HxBABA and (b) S, phase for NBABA. The layer thickness, d, in the S, and Nsc phases have been obtained using the Bragg condition. n these phases, the tilt angle could be measured FG. 3. c) T = "C Diffraction photographs of HxBABA in the Nsc phase.

5 DSC AND X-RAY DFFRACTON NVESTGATONS OF PHASE TRANSTONS C3-139 whereas for NBABA the Nsc-type diffraction pattern is seen only in the neighbourhood of the transition, S,-N, and the pattern transforms to a diffuse ring for higher temperatures HxBABA. - The diffraction photographs taken between 165 OC and 168 O C are characterised by patterns typical of both the P and S, phases. Hence the values of d and D have been obtained from our data only for T OC. These are shown as a function of temperature in figure 4. d increases with increasing temperature both in the S, and Nsc phases. D is almost constant in the S, phase, but increases with increasing temperature in the Nsc phase. d decreases and D increases abruptly at TSCpN, but these jumps are quite small. The temperature dependence of the tilt angles, O,, and O,, are shown in figure 5. O,,, decreases continuously from a value of 680 at 1700 C to that of about 200 at TScpN, whereas O,,, decreases from 53O to 480 in the same temperature range. f one assumes that this discrepancy is only due to the value of L used in calculating O,,,, L would have to be taken to be 68 A at 170 OC and 29 A at the S, - N transition to account for the O,,, values. These values of L seem to be unphysical since L = 68 would correspond to the length of a trimer and L = 29 A would imply a large percentage of monomers, which is belied by our results in the NsN phase. t is also seen that O,, - (TSCVN- T)'.'' for (TSE- - 5 OC) < T < (TS;-N- 0.5 OC). HOWever, the exponent, 0.28 should not be taken too seriously, because of the very large errors in the values of O,,, in the neighbourhood of the transition. O,, increases abruptly at TSc-,, but stays almost constant at 550 in the entire nematic phase, where it is found to be only slightly higher than O,,,. This shows that the molecules are mostly in the dimeric form in the N, phase and hence, also in the S, phase NBABA. - Sharp rings are seen in the diffraction photographs of the S, phase of this compound, indicating that this phase, if smectic, is a highly ordered one. The temperature dependence of d, D and of 1 are shown in figure 6. n the S, phase, ,, -Csc7 1 N FG Temperature dependence of d and D in HxBABA. The full line curves are guides to the eye. W - 40'- a 5 30'- - t 20"- N A FG Tilt angle 0, us temperature for HxBABA. The crosses ( x, 8) represent O,,, and the full circles, O,,,. The full line and dashed line curves are guides to the eye FG. 6. -Temperature dependence of d, D and in NBABA. The full line and dashed line curves are guides to the eye. d and D are found to be almost constant between 145 0C and 180 OC, but for higher temperatures, they increase with increasing temperature. At the transition point, an abrupt increase is observed in the case of both d and D, the increase in d being large. n the nematic phase, 1 and D increase with increasing temperature. The calculation of 1 in the manner indicated earlier is not justified, if an S,-type order persists at these temperatures. n any case, to account for the values of l(3 43 A), the molecules have to be, to a large extent, in dimer form in the nematic phase. The tilt angles, O,,, and O,,,, are plotted as a function of temperature in figure 7. They are both nearly constant between 145 0C and 180 OC, but for 180 O C < T < TSc-,, they decrease with increasing temperature. O,,, decreases from to about 300 and O,,, from 51 to 470, in this temperature range. The discrepancy between the values of O,,, and O,,, for 145 O C < T < 180 OC can be accounted for, by assuming either (1) an overlap

6 K. USHA DENZ, A. S. PARANJPE, E. B. MRZA AND P. S. PARVATHANATHAN ; s,~s,-n FG. 7. -Tilt angle 0, us temperature for NBABA. The crosses ( x, +) represent O,,, and full circles, Q,,,. The full line curves are guides to the eye. of the neighbouring layers, to the extent of 4 A, or (2) incomplete dirnerisation of the molecules. For 180 O C < T < TSFmN, a gradual breaking up of dimers, leading to a decrease in the average value of L from 44 A to 37 A could explain this difference, if one assumes that there is no overlap of the neigh- bouring layers. However, this explanation cannot be reconciled with the observation that in the nematic phase. The solitary value for Ot,d(+) in the nematic phase shows that, as in HxBABA, 8,,d jumps to a larger value at the Sc-N transition. he large values of the tilt angle, O, the absence of the S,-S, transition and the presence of the skewed cybotactic nematic phases indicate that the S, phases in HxBABA and NBABA are the C, phases described by De Vries [3]. The difference between the values of 8,,, and 8,,, in the Sc phase, for temperatures well below the transition Sc-N, can perhaps be explained on the basis of the zig-zag model of Bartolino et al. [7]. The main contribution to the diffracted intensity in both HxBABA and NBABA is from the rigid central core of the dimers, the chains contributing roughly a quarter and a third of the intensity respectively. O,, is therefore a measure of the tilt angle of this rigid central core. Assuming this, the contribution to d, due to the chains, is calculated to be 14.1 A and 10.3 A for HxBABA and NBABA. f the chains are more or less oriented perpendicular to the smectic planes [7], these values indicate that the chains are melted to a much larger extent in NBABA than in HxBABA. Near the S,-N transition, the temperature dependence of the tilt angle of the rigid central core and hence of 8,,, would probably be determined to a great extent by the hydrogen bonding processes involving the closed and open dirners (see ref. 181) which are coupled to the fluctuations of rotational order in the S, phase. Since these fluctuations are much larger in HxBABA than in NBABA (because the transition Sc-N is a weakly first order one in the former) one would expect the difference in the temperature dependences of O,,, and O,,, to be much larger in the former. 3.3 CONCLUSONS. - The temperature dependence of d and O,, in both compounds, specially near the transition, S,-N, is determined by the strength of this transition and also by the H bonding processes. The zig-zag model seems to explain the differences observed between O,,, and 8,,,. Acknowledgments. - We are grateful to Drs M. D. Karkhanavala, N. A. Narasimham and Prof. K. C. Pate1 for their encouragement. We are very thankful to Drs V. R. Mamdapur and A. V. Patankar for preparing the compounds for us. t is a pleasure to acknowledge the useful discussions that we have had with Drs V. A. Lingam, C. Manohar, K. V. Muralidharan and U. R. K. Rao. References [l] DAVE, J. S. and PATEL, P. R., Mol. Cryst. 2 (1966) 115. [6] CHSTYAKOV,. G. and CHAKOWSKY, W. M., Mol. Cryst. Liq. [2] USHA DENZ, K., PARANJPE, A. S., MRZA, E. B., PARVATHA- Cryst. 7 (1969) 269. NATHAN, P. S. and PATEL, K. S., TO be published in the [7] BARTOLNO, R., DOUCET, J., DURAND, R., TO be published in Proceedings of the Nuclear Physics and Solid State Annales de Physique, Comptes Rendus de Madonna, V, Physics Symposium, Poona (1977). no 2 (1978). [3] DE VRE?., A., J. Physique Colloq. 36 (1975) C1-1. [8] DELOCHE, B. and CABANE, B., Mol. Cryst. Liq. Cryst. 19 (1972) [4] DE VRES, A,, Mol. Cryst. Liq. Cryst. 11 (1970) [5] DE VRES, A,, Mol. Cryst. Liq. Cryst. 10 (1970) 219.