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CALORIMETRIC AND INFRA-RED STUDY OF THE PHASE SITUATION IN SOLID MBBA
J. Janik, J. Mayer, E. Ściesinska, J. Ściesinski, J. Twardowski, T. Waluga, W.
Witko
To cite this version:
J. Janik, J. Mayer, E. Ściesinska, J. Ściesinski, J. Twardowski, et al.. CALORIMETRIC AND INFRA- RED STUDY OF THE PHASE SITUATION IN SOLID MBBA. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-159-C1-165. �10.1051/jphyscol:1975131�. �jpa-00215908�
Classification Physics Abstracts
7.130
CALORIMETRIC AND INFRA-RED STUDY OF THE PHASE SITUATION IN SOLID MBBA
J. A. JANIK, J. M. JANIK, J. MAYER, E. SCIESINSKA, J. SCIESINSKI, J. TWARDOWSKI, T. WALUGA and W. WITKO Institute of Nuclear Physics and the Jagiellonian University, Krakow, Poland
Abstract. — If MBBA is solidified by decreasing the temperature of the nematic phase, one obtains a metastable solid, whose specific heat vs. temperature curve has been measured by adiabatic calorimetry. This curve shows a sharp rise at melting. It also shows a quasi-second-order type of behaviour at about 210 K. If the metastable form of solid MBBA is kept for a few hours at a temperature not far below melting, it spontaneously transforms into the stable form, whose specific heat vs. temperature curve has also been measured. This curve lies below that of the metastable form and has a monotonic behaviour and a sharp rise at melting, which now occurs at a slightly higher temperature. Infra-red and far-infrared spectra of the two modifications of solid MBBA were also measured. In the internal vibration wave number region the spectra are significantly different in the whole temperature range. The quasi-second-order phase transition discovered in the metastable MBBA by calorimetry does not show up in IR. Differences between the spectra of the two modifi- cations are greatest in the lattice vibration region, i. e. between 20 cm- 1 and 100 cm-1. Preliminary powder X-ray diffraction data, obtained for the two modifications, show patterns typical of crys- talline materials. The X-ray pattern changes significantly when the metastable MBBA transforms into the stable modification.
1. Introduction. — Calorimetric [1], infra-red 2. Measurements. — All measurements were made absorption [2], and far infra-red absorption [3] measu- with an MBBA sample purchased from the firm Riedel- rements published previously by the authors evidenced de Haen AG, Seeke-Hannover.
the occurrence of solid MBBA in one of two mono- Heat capacity vs. temperature measurements were tropic modifications : a metastable or a stable one. carried out on an adiabatic calorimeter [8] in the Such polymorphic behaviour of solid phases of temperature range from 100 K to 340 K. The sample substances forming liquid crystals was reported also for mass amounted to 62.05 g (0.232 1 mole).
PAA [4, 5], OH-MBBA [6], and for the esterpropyl-p- IR absorption spectra were measured on Zeiss UR-10 methoxybenzylidene - p - amino - a - methyl - cinnamic and Unicam SP-1200 spectrophotometers in the wave
acid [7]. number range from 400 cm 1 to 3 600 cm 1, and in the
This paper presents a more detailed analysis of the temperature range from 80 K to 320 K. The resolution phase situation in MBBA and, in particular, a prelimi- was 3 c m- 1 and 1 c m- 1 for the first and second nary investigation of the kinetics of the monotropic spectrophotometer respectively. The sample was transition from the metastable to the stable modifi- enclosed between two flat AgCl plates in the form of a cation. thin layer.
Résumé. — Si on solidifie le MBBA en abaissant la température à partir de la phase nématique, on obtient un solide métastable dont la chaleur spécifique a été mesurée en fonction de la tempéra- ture par calorimétrie adiabatique. La courbe montre une augmentation rapide à la fusion et un comportement à peu près du second ordre aux alentours de 210 K. Si on maintient le solide méta- stable à une température peu inférieure à la fusion pendant quelques heures, il se transforme spontanément en une forme stable dont on a également mesuré la chaleur spécifique en fonction de la température. Cette courbe est située en dessous de celle de la forme métastable, elle est monotone et a une augmentation rapide au point de fusion qui est à une température un peu supérieure.
On a aussi étudié les spectres infrarouge et infrarouge lointain des deux modifications. Pour les nombres d'onde correspondant aux vibrations internes, les spectres sont nettement différents dans toute la zone de température. La transition quasi-second ordre qui apparaissait par calorimétrie dans la phase métastable n'apparaît pas en infrarouge. Les différences les plus notables apparaissent dans la région des vibrations du réseau, c'est-à-dire entre 20 cm- 1 et 100 cm-1.
Des résultats préliminaires de rayons X, obtenus à partir de diagrammes de poudre, montrent que les deux modifications sont des solides cristallins. On observe néanmoins un changement du dia- gramme lorsque la phase métastable se transforme en forme stable.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975131
Cl-160 J. A. JANIK et al.
Far IR absorption spectra were measured on the Grubb-Parsons IRIS D Fourier interferometric spec- trophotometer in the wave number range from 20 cm-' to 400 cm- l and in the temperature range from 80 K to 240 K. The accuracy of temperature measurements was
+ 1 K. The resolution in the range 20-90 cm-' was
-
1 cm-' and in the 90-400 cm-' range it was in some runs 2 cm-' and in others 4 cm-'. Absorption spectra were computed from the interferograms by the ODRA 1204 computer.
FIG. l . - Specific heat vs. temperature dependence for MBBA [l].
per hour) started in the nematic phase led, below the freezing point, to the metastable modification. The same metastable modification was also obtained in the case for which the cooling process was much slower (about 2 K per hour). The metastable modification transforms spontaneously into the stable one in tempe- ratures not far below the melting point. This transfor- mation has an exothermic character, and its rate depends on the previous thermal history. The sponta- neous transformation may be interrupted by fast cool- ing to temperatures below about 260 K. When this is done, we obtain experimental points of the specific heat vs. temperature dependence lying between those for metastable and stable modifications.
The exothermic character of the spontaneous transi- tion between the metastable and stable modifications enabled us to study the kinetics of this process. By the kinetics we understand here the variation of the relative abundance of the two modifications throughout the duration of the transition. A tem- perature increase of the adiabatically isolated calorimeter-sample system was used as a transition detector. We denote by f (t) the sample fraction which during the time t was transformed from the metastable into the stable modification. f (t) we assumed as equal to the ratio of partial heat created during t in the exo- thermic transition process to the total heat created during the complete transition :
C,,, and C, are the heat capacities of the calorimeter Figure l presents the obtained heat capacity vs. a i d of the sample respectively. We make use here of the temperature dependence. In the solid region, the experi- approximation which assumes that C,,, and C, are mental points lie on one of two curves, thk lower one temperature independent (in the region covered by the (- X - X - X ) corresponding to the stable and the upper spontaneous heating process) and that C, does not (- . - . -) to the metastable modification. The latter shows depend on f (t). The accuracy of this approximation we an anomaly with maxima at about 212 K and 217 K. estimate as about 10 %.
Table I presents values of melting temperatures, enthal- In a series of six f (t) measurement runs we accepted pies, and entropies of both modifications. The clearing the following procedure : the liquid (nematic) sample temperature T(,, was (317.0 + 0.2 K) and the clearing
entropy (AS), (0.469 f 0.046) cal/mol K.
Melting temperature ,T, enthalpy AH and entiopy AS of MBBA
Stable Metastable modification modification
The formation each modification is dependent On FIG. 2. - Transformed fraction f of the sample vs. time, for the the chosen thermal history of the sample. In eight ~Xses different temperatures T, at which the sample was kept after studied by us, a rather fast cooling process (about 70 K freezing.
was cooled down, rather quickly, to one of six tempera- tures Tp : 200.6 K , 211..9 K, 208.4 K, 213.8 K,216.7 K, 237.1 K. The (metastable) sample was kept in this temperature during about 15 hours. The sample was then heated in all runs to the same temperature To = 273.3 f 0.1 K, at which the spontaneous transi- tion to the stable modification .was studied. f (t) was determined by means of formula (l), i. e. from the temperature determination of the calorimeter- sample system in equal' time intervals. A noticeable temperature change was observed not earlier than about 15 min after thermal equilibrium at To was reached by the system. Figure 2 presents the, f (t) dependences obtained in this way.
The results were now fitted to the phenomenological Avrami [9] equation :
1 - f (t) = exp(- ktn) , (2) where k denotes the transition rate constant and n > 1 is the transition order: Making use of this equation and of its time derivative, we obtain the formula :
whose left-hand side is a linear function of t. From its slope the transition order n was determined (Fig. 3) and,
FIG. 3. - Eq. (3) dependences for curves of figure 2 (only f values in the interval 0.25-0.75 are shown).
furthermore, from the formula (2), the transition rate constant k a n d finally the normalized rate constant 7%.
These values are assembled in Table 11. One may see that there exists a correlation between the rate of the transition and the temperature T p at which the sample was kept w%en cooled down from the nematic region.
~arameiers describing the kinetics of the transition : metastable-stable modijication
Moreover, the transition rate appears to reach a maxi- mum when T p coincides with the specific heat anomaly of the metastable modification (Fig. 4).
FIG. 4. - Normalized transition rate constant vs. Tp. For comparison the specific heat curve is also shown.
FIG. 5. - IR' absorption spectrum for MBBA : (a) nematic phase, (b), (c) meta&table, modification, (d),, (e) stable modifi-
cations [2].
Cl-162 J. A. JANIK et al.
Figure 5 presents IR absorption spectra for the metastable and stable modifications in the 400 cm-'- 1 300 cm-' region. Exactly as in calorimetric measure-
FIG. 6 . - IR spectrum variation in the 540-570 cm-1 interval during the transition.
545 cm-' 1
FIG. 7. - f ( t ) dependence as determined from formula (4) for two Tp values ; (a) for 545 cm-l, (6) for 560 cm-'.
ments, the metastable modification was obtained by the freezing of the nematic phase, and the stable one by a spontaneous transition from the metastable modifica- tion. The differences appearing in the spectra of the two modifications stimulated an attempt at a study of the spontaneous transition kinetics. Namely, again and again during the transition process we measured the spectra, which we decided to study in the temperature To = 275 K , very close to that of our calorimetric experiments. (One spectrum measurement lasted about 30 min.) Although quite a few bands (545-560 cm-', 805-845 cm-', 1 030 cm-', 1 100-1 200 cm- l) changed clearly during the transition, we chose for further processing the 545-560 cm-' one for which the changes were most significant (Fig. 6).
The transformed fraction of the sample f ( t ) we assumed equal to :
where : A(t) is the absorbance at the chosen wave number at the time t and A, and A, are the absor- bancies at the same wave number of the pure metastable
T-105K metastable
FIG. 8. -Far IR absorption spectrum in the 80-400cm-1 interval for MBBA : (a) metastable modification, (b) stable
modification.
and stable modifications respectively. Absorbance A was determined from the formula A = log To/T where T and To are transmittances of the effect and back- ground respectively. The background was chosen as a straight linear guess interpolated from transmittances registered below and above the studied band.
Figure 7 presents the f ( t ) dependences determined from formula (4) for two wave numbers 545 cm-' and 560 cm-' and for the two temperatures T, at which the sample was kept after freezing : 214 K and 227 K.
Figure 8 presents the far IR absorption spectra obtained for the metastable and stable modifications at temperatures 105 K and l l l K respectively, in the wave number region 80-400 cm-', and figure 9 presents the
It should be pointed out that, for temperatures about 2 K below the transition, stable modification-nema- tic phase pretransition effects were observed, as an apparent increase in the absorbance. Figure 10 presents two spectra illustrating this effect.
RC. 10. - Far IR spectrum for MBBA showing the pretransi- tion effect. Solid line 292 K, dotted line 293.5 K.
(b)
T = 100 K stable
FIG. 9. -Far IR absorption spectrum in the 20-80cm-1 interval for MBBA : (a) metastable modification, (6) stable modification. Different lines correspond to different runs and
show the noise effect.
spectra for the wave number region 20-80 cm-' for slightly different temperatures (90 K for the metastable and 100 K for the stable modification). This latter region (20-80 cm-') shows quite significant instru- mental noise, no doubt caused by the low intensity of the spectrometer light source in that region. Absorption bands appearing on this noise background are detec- table only in the metastable phase.
3. Discussion. - Independently of our experiments, the specific heat measurements for MBBA were made by Shinoda et al. [10]. The authors suggest the exis- tence of three solid modifications as shown in figure l l.
In the 170 K-230 K region their specific heat values, for modifications denoted by them as Solid I and Solid 111, coincide within a few per cent with our values obtained for the metastable and stable modi- fications respectively. A small and no doubt not essential difference exists in the position of the metas-
FIG. 11. - Specific heat vs. temperature for MBBA [IO].
Cl-164 J. A . JANIK et al.
table modification specific heat anomaly : about 206 K in [l01 and 212 K in our measurements [l] (the lower of the two peaks of the double peak anomaly). Such a difference may perhaps be caused by the slight diffe- rences in impurities. On the basis of paper [Ill we may estimate that the sample usedin [10], having the clearing point T, = 320.137 K, was very pure, whereas our sample could have had impurities not exceeding 0.3 %. We cannot comment on the Solid I1 evidence suggested in [l01 as wed id not observe it.
Sorai and Seki [l21 published the DTA measure- ments for MBBA and suggest an interpretation of the phase situation different from ours. Namely, the authors suggest that the fast cooling (a cooling rate greater than 10 K/min) leads to the transition : super- cooled nematic-glassy form, occurring at - 72 OC.
A glass type transition is observed at the same tempera- ture also when the substance is heated and then leads to the irreversible change to the crystalline MBBA. This result is not in contradiction with our observations because, we believe, two different phenomena are observed in the two studies, Our metastable MBBA modification is no doubt crystalline (not glassy) in view of the character of the IR and far 1R spectra and the well-defined melting point. Additional support is given by the two modifications obtained by us for the powder X-ray diffractograms (Fig. 12). Their character is typi- cal for crystalline substances and the differences observed indicate structural differences between the metastable and stable modifications.
(a) MBBA mst/nem T-l?3K bw' W.-'
Lampo CO
3fso' V16'
uro'
4556'
u[L
43%'I I I I I I
M 4 5 4 0 & f i 4 9 5
M BBA */m$+ TT- 1T3K *M Lampa CO
sa'
J I 1 AA l I I I I I l
E Q 4 5 4 0 3 5 M 2 % 2 0 ~ ~ 0 5
FIG. 12. -X-ray powder diffractograms for the two modifi- cations. of MBBA : (a) metastable form, (b) stable form.
(For. <( Lampa Go >> read CO-Iamp)..
Andrews [l31 suggested recently an interpretation of the thermodynamic differences between the two modifi- cations. He connects the entropy excess of the meta- stable modification in relation to the stable one (2.59 cal/mol K [l] near the melting point) with a rotational disorder of the end groups of the MBBA molecules in the metastable modification., He suggests that at the lower maximum of the anomaly (217 K) reorientational jumps of CH,O groups about the long molecular axis start. These jumps occur between two equilibrium positions with respect to the benzene ring.
The other maximum of the anomaly (212 K) is, accord- ing to Andrews, connected with an Snaldgous increase in'rotational conformations of the'butyl chain.
Andrews' idea seems to be an interesting tentative hypothesis, which of course needs confirmation on the basis of a detailed X-ray diffraction study. We should however point out that, for instance .in n-p-chloro- benzylidene-p'-chloroaniline, there exist similar, i. e.
metastable and stable, modifications [14], with struc- tures proved by X-ray studies. In this case the meta- stable disorder is connected with a random orientation of CH=N groups in neighbouring molecules. More- over, in the metastable*'modification the two benzene rings of one molecule are coplanar, whereas in the stable modification one is ,tilted in relation to the other.
It is possible that the above-mentioned disorder of CH=N groups exists also in the metastable MBBA in addition to the disorder of the end groups suggested by Andrews. We should point out here that the sponta- neous transition between the two modifications in MBBA is a rather long-lasting process (hours !), whereas the transition based upon the Andrews hypo- thesis should, we feel, be much shorter because only reorientations about the long molecular axis are involved in it. The disorder of additional CH=N groups, on the other hand, fieeds the additional occur- rence of translatory diffusion jumps, when the sub- stance transforms from the metastable to the stable modification, which certainly lengthens the transition time.
However, neither this hypothesis nor that of ~ n d r k w s explains why the far IR spectra of the stable modifica- tion and the nematic phase are similar, whereas one would rather expect a similarity of the nematic phase and the metastable solid.
Calorimetric measurements of the transition kinetics described in Sec. 2, although preliminary, seem to prove that an important process occurs in the sample when it is kept for a certain time at the temperature T,.
This process, especially effective in the vicinity of the anomaly at 212 K-217 K, significantly accelerates the transition.
IR measurements of the transition kinetics, described in Sec. 2, are still more preliminary than the calori- metric ones. It is possible to compaxe the results only roughly, and this will be done by comparing the times necessary for changing f from 0;1 to -0.9. The results
for T,, = 227 K and for To = 275 K are roughly the is possible that the geometry of the sample - different same : transition times obtained by both (i. e. calori- in the two methods - is responsible for the above dis- metric and IR) methods are about 100 min. For agreement. We must repeat, however, that the preli- T, = 214 K and To = 275 K, however, the calori- minary character of the IR kinetic measurements metric method gives for the transition time about does not allow any conclusions concerning the compa- 60 min, whereas the IR method gives about 100 min. It rison to be drawn.
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