PHASE TRANSITIONS OBSERVED ON WARMING FAST-QUENCHED MBBA

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PHASE TRANSITIONS OBSERVED ON WARMING FAST-QUENCHED MBBA

J. Lydon, J. Kessler

To cite this version:

J. Lydon, J. Kessler. PHASE TRANSITIONS OBSERVED ON WARMING FAST-QUENCHED MBBA. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-153-C1-157. �10.1051/jphyscol:1975130�.

�jpa-00215907�

Classification Physics Abstracts

7.130

PHASE TRANSITIONS OBSERVED ON WARMING FAST-QUENCHED MBBA

J. E. LYDON

The Astbury Department of Biophysics, The University, Leeds LS2 9JT, England J. O. KESSLER

Physics Department, University of Arizona, Tucson, Arizona 85721, U. S. A.

Abstract. — When the nematic phase of MBBA is rapidly cooled by exposure to liquid nitrogen a glassy solid is formed which, X-ray diffraction studies indicate, has the nematic ordering quenched- in. The sequence of events which occur upon reheating this glass was monitored using X-ray diffrac- tion and a differential thermal technique. A transition to a metastable crystalline phase occurs at about — 14 °C and this phase converts into the nematic phase via some other phase, possibly the stable crystalline phase or a smectic mesophase.

We propose the name anotropic for monotropic transitions that occur on heating.

1. Introduction. — Itjs easy to demonstrate that the Some preliminary experiments demonstrating the nematic phase of ra-(p-methoxybenzylidene)-/?-butyl- existence of the quenched nematic phase of MBBA and aniline (MBBA) can be quenched-in by rapid cooling, delineating some of its properties have been reported If a microscope slide carrying a drop of this material at previously [7]. A variety of techniques were used in this room temperature is plunged into a bath of liquid earlier study : X-ray diffraction, optical microscopy, nitrogen, held there for a few seconds and then electron microscopy (freeze fracturing) and thermal withdrawn for inspection, it can be seen that the mate- conductivity. The present paper reviews the X-ray rial has a clear glassy appearance. If it is allowed to evidence for the existence of the nematic glassy phase warm to room temperature, it retains this appearance and continues with an examination of the progression until it reaches a temperature of about — 14 °C when it of phases obtained by allowing the specimens to warm rapidly becomes white and opaque as a phase change from 80 K to 300 K, as monitored both by X-ray occurs. This phase, which we shall provisionally refer to diffraction and by a differential thermal technique, as a « crystalline » phase, then reverts to the nematic Thermal investigations of the warming of quenched phase at room temperature. MBBA have been reported by Mayer et al. [8] and by The formation of glassy states by supercooling Petrie et al. [9]. Note that the quenching rates used by mesophases to below the glass transition temperature these workers vary considerably from ours, and their has been reported by a number of workers [l]-[5]. results may therefore not be strictly comparable.

Furthermore, the freeze fracturing technique for elec- In this preliminary investigation the warming rate tron microscopy depends on the suppression of phase was approximately exponential (with a time constant of changes by very rapid cooling. In particular this the order of minutes), being determined by the tempe- technique has been found to be viable for the study of rature difference between the specimen and the thermotropic mesophases [6]. ambient. In subsequent investigations we intend to

Résumé. — Si on refroidit rapidement la phase nématique du MBBA à la température de l'azote liquide, on obtient un solide vitreux où la structure nématique a été gelée, ainsi que le montre la diffraction des rayons X. On réchauffe ensuite le MBBA et on étudie son évolution par diffraction des rayons X et par analyse thermique différentielle. Un solide cristallin apparaît vers — 14 °C, qui se transforme ensuite en la phase nématique par l'intermédiaire d'une autre phase qui pourrait être soit la phase cristalline stable, soit une phase smectique.

On propose d'appeler anotropiques les transitions monotropiques qui apparaissent quand on chauffe.

JOURNAL DE PHYSIQUE Colloque C l , supplément au n° 3, Tome 36, Mars 1975, page Cl-153

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975130

Cl-154

3. E. LYDON AND J. 0.

repeat these investigations using controlled warming rates.

2. X-ray diffraction.

The material used in these experiments was purchased from Eastman Kodak Distillation Products .and from Aldrich Chemical Co.

There was no attempt at purification. For the X-ray experiments the material was placed on glass micro- scope slides cleaned with a solvent but not rubbed.

A Philips PW 1050125 powder diffractometer was used for this investigation. This apparatus accepts samples spread on the surface of a microscope slide.

The temperature was measured by means of a thermo- couple probe which was immersed in the sample itself.

The diffraction pattern given by the room temperature nematic phase of MBBA is shown in figure 1. (CuKa

FIG.

The diffraction patterns (using CuKa X-rays) of the room temperature nematic phase of MBBA and of the

phase which forms at about

during the warming of fast-quenched material. The diffraction pattern of the fast- quenched material appeared to be identical to that of the room

temperature mesophase.

M B B A

radiation was used throughout.) The diffraction pattern of the nematic glass formei by rapid cooling of this phase was investigated as a series of short overlapping segments, each being recorded for material immedia- tely after extraction from the quenching bath. It appeared that, to within the experimental error, the trace was the same as that of the room temperature material. The diffraction pattern of the crystalline

phase formed on warming the nematic glass is also shown in figure 1. Note the strong peak at 2 0

5.20 which corresponds to a repeat distance of 17 h. This is to be compared with the molecular length of 19 h and suggests that in this << crystalline

phase, the molecules are aligned more or less normally in a layered arran- gement.

Because the << crystalline

phase gave strong peaks at 2 6'

5.20 and 19.50 these settings were used for observing the warming up process. The diffractometer was set to record how the diffracted intensity at these angles varied as the quenched material warmed.

The form of traces which we expected were simple

5.

.

-

c 3 -

-

2 z - A

1 -

step functions ; in the case of the 5.2O setting a low initial background intensity rising sharply to a plateau which remains level until the transition to the nematic mesophase where it drops to its original value. The expected trace for the 19.5O setting was similar except that the initial and final levels should be considerably higher because the mesophase (and quenched meso- phase) gave an appreciable diffracted intensity at this angle. The experimental traces are shown in figure 2.

They are rather more complex than the expected profile and note particularly the extra peak in the 5.20 trace which occurs just before the transition to the

1 2 3 4 5 5.5

1

1

-145 - 7 2 -14 I

0 7 14 ('C)

Temperature

5" 10" 15' 20"

Bragg Angle

2

---Rcrystall~ne' 19 '5

- - - -

-

5 2 O

quenched nematrc

FIG. 2.

The diffractometer traces for the warming of quenched MBBA monitored at the

0

5.2O and 19.5O settings. The gross features were reproducible from one run to the next

the details, especially the height of the peak at

varied. Note the change in abscissa scale at T

A liquid nitrogen cold

reference junction was used for the thermocouple.

I,

nematic phase. The possible origins of this peak will be discussed later.

We investigated the effect of requenching the sample at two stages in the warming up process. The results are indicated in figure 3. The sample was removed from the diffcactometer when it was at a stage in more or less the centre of the crystalline range (point A), it was plunged into the cooling bath again, held there for a few seconds and then rapidly replaced in the apparatus. The diffracted intensity (monitored at 2 0

5.20) traced -out curve Q,. It can be seen that this remained at essentially its original high level up to the point of requenching-indicating that the molecular ordering in this crystalline phase had been quenched-in. On warming beyond, this stage, the recorded intensity retraced the path of the previous run.

The effect of a similar requenching was investigated when the sample had reached a temperature of 15.5 OC and was in the state which gave the enhanced peak (i. e.

point B in Fig. 3). In this instance very different results were seen. There was a dramatic increase in the diffracted intensity (curve Q,). There are at least two possible explanations for this phenomenon which we shall discuss later.

/ \

I \

PHASE TRANSITIONS ON WARMING FAST-QUENCHED MBBA Cl-155

FIG. 3.

Diffractometer traces showing the effect of requench- ing the sample at two stages in the warming-up process. The dashed line is the initial,curve as in figure 4. QA is the trace obtained upon requenching the specimen at point A.

is the

trace after requenching at point B.

3. Thermal investigation. - A qualitative diffe- rential warming curve technique was used to search for the major thermodynamic features of the observed phase transitions. The apparatus consisted of fine thermocouples (38 gauge) immersed in the quenched

Temperature

FIG. 4. - Differential warming curves for quenched MBBA.

These curves were obtained with fine thermocouples

gauge) immersed in quenched specimens of MBBA and silicone oil, and connected differentially. The ordinate A is d(AT) dt, where AT is the temperature difference between the specimens.

MBBA and in silicone oil connected differentially.

This method is very sensitive but qualitative rather than quantitative because the heat transfer conditions were not sufficiently well defined. The principal result obtained in this way was the large exothermic peak shown in figure 4 at about

14 OC, the temperature where the intensity of the 2 8 = 5.20 X-ray diffraction begins to rise. We interpret this exothermic feature as arising from the crystallisation of the glassy phase. In some of the many runs made with this technique small peaks and dips were apparent at lower temperatures but we found it impossible to obtain completely repro- ducible behaviour.

4. Discussion.

The X-ray diffraction pattern o f the quenched material indicates that the quenching process actually freezes-in the molecular ordering of the nematic phase. It appears that the phase which appears at - 14 OC is the same cr metastable crystalline solid >>

as that observed by Mayer et al. [S] and Petrie et al. [9].

I t may, however, be significant that the overall profile of the diffraction pattern of this phase is similar to that of a smectic B phase in that it contains one sharp outer reflection and one sharp inner reflection with a weak second order reflection (see for example ref. [lO]). The two major events which take place on warming fast- quenched MBBA appear to be the formation and melt- ing of this metastable solid. The latter event is not a simple one-stage process. The additional peak in figure 3 occurring just before the formation of the nematic phase probably indicates the transient pre- sence of a further phase. The enhanced intensity of the 5.20 peak observed on warming the sample requenched at this point poses an interesting problem and we offer two possible explanations.

4.1 A metastable crystalline phase must have a lower melting temperature than the corresponding stable crystalline phase. It is therefore possible that what we have seen is a monotropic metastable crystal -, nematic transition followed by the formation of the stable crystalline form (which then undergoes a transition to the nematic phase at 20 OC). A similar explanation was offered by Petrie et al. [9] for the sequence of end0 ; exo ; end0 peaks in the D. S . C . trace in this temperature range. This strongly suggests that we have observed the same sequence of transitions.

If the heating rate was such that not all of the mate- rial was able to crystallise to the stable solid before the temperature reached 20 OC, quenching at point B would then be expected to produce a mixture of glass and stable solid. On warming, the glassy regions would pass through the same sequence of events as before but the stable crystalline regions would undergo no transition until 20 OC. If the stable crystalline solid gives a stronger diffraction peak at 2 8

5.20 than the metastable solid, this would appear to explain all of the features of the curve Q , (in particular the subsidiary peak at about

18 OC) for the second requenching experiment.

C1-156 J.

AND J. 0. KESSLER

1 m = m ~ 8 ' 8 8 ' 8 1 8 ' 1 ' 8 8 8 ~

-150 -100 -50 0 50 "C

I

exo

- ^sc

1

...

... ... ^{: : :}

:

... ...

A

-

10 hrs.

"metastable* Mayer et al.

I ^A

-

-

-

_L

-150 -100 -50 0 50 Present work

8 . 1 . . . . I

8 1 1 1 8 1 1 8

FIG.

- The comparison of pubIished data of Mayer

at.

and Petrie

al. 191 with those of the present work.

4.2 Alternatively, it may be possible that there is a monotropic smectic phase intermediate between the crystalline phase formed from the glassy nematic, and the room temperature nematic phase. The increase of the diffracted intensity (at 2 O.= 5.20) at 15 OC would then arise' from this mesophase and the phenomena observed for the sample requenched in this state could result from reorientation or texture change of this quenched smectic phase during the re-warming process.

This results in more of the specimen material existing in an orientation where its diffraction peak is detected by the instrument. An X-ray investigation of the mate rial produced by the second quenching process should resolve this #question and this is currently in progress.

A compqrison of our data with those of Mayer et al. [8] and Petrie et al. [9] is given in figure 5. The greatest discrepancy between our results and those of other.investigators appears to be concerning the tempe- rapre of formation of the metastable crystalline solid.

We interpret our data in terms of a glass

crystal transition at about - 14 OC. A number of explanations are possible. The glass transition temperature may be bighly dependent on the rate of warming or perhaps the actual structure of the glass may differ because of different quenching rates.

As an aid in the discussion of these phenomena, we

suggest the use of a three-dimensional free energy ; temperature ; structure diagram, shown in perspective in figure 6. The rr Structural Parameter

embraces the translational and angular relationships between each molecule and its neighbours and should itself be poly- dimensional. It is a similar concept to the cr reaction coordinate

used in the discussion of chemical reaction

FIG. 6.

Schematic phase diagram illustrating energy, tem- perature and structure relationships for the stable, metastable and quenched phases. Isothermal sections are shown on the right.

Note that the area QN representing a perfectly quenched nematic phase is an iso-energetic projection of the section from which the specimen is cooled. The arrow labelled

10 hrs.

from Xz to Xi at the bottom of the figure represents the slow direct transition

observed by Mayer et al.

See also figure 5.

PHASE TRANSITIONS ON WARMING FAST-QUENCHED MBBA Cl-157

i i i

FIG. 7. - The application of the schematic phase diagram to illustrate the two alternative models offered for the warming of the quenched material and for the second requenching experiment. The sequences of

-14 0C

phases formed in these models are respectively

(i) Quenched nematic (QN)- metastable crys-

1 5 oC 20

oc

talline solid ( X 2 ) 4 (via nematic) stable crystalline solid (XI)--+ nematic. (ii) Quenched nematic

14 0C 1 5 0C 2 0 oC

( Q N ) ~ metastable crystalline solid (X&--+ monotropic smectic

nematic. The arrows labelled QB indicate the course of the second requenching. In model (i) the material a t the moment of quench- ing is a mixture of (supercooled) nematic and stable crystalline solid (XI). The latter component remains unchanged as the sample warms until it reaches 20 OC. The quenched nematic component passes through the same sequence of events as before. In model (ii) the material is in the smectic phase a t the instant of requenching and as the quenched material warms it produces the solid (Xz) in an alignment so that more of

the sample is in an orientation which produces a diffraction recorded by the instrument.

kinetics. It may be taken as an arbitrarily selected dimension through a polydimensional space so as to most usefully illustrate the difference in structure between the phases. The greater the distance along this axis between two phases, the greater is the extent of the molecular re-arrangement required to transform one to the other. Diagrams of this kind illustrate the interplay between the two factors which determine a sequence of phase changes ; the free energy and the accessibility of one phase from another. The two alternative series of events suggested for the warming process are illustrated in figure 7.

Studies of this type indicate that interesting and otherwise unobservable phenomena occur during the warming of quenched mesophases. Such transitions are clearly monotropic since they occur during a tempera- ture change in one direction only. They are, however, distinct from the well-known type of monotropic

transitions which occur only on cooling. It may be desirable to distinguish pliase changes of this new type.

We therefore propose that they be described as ano- tropic (from the Greek anoJ meaning upward). Normal monotropic transitions would correspondingly be called cathotropic.

Acknowledgments. - We wish to thank Dr. G. W.

Gray for helpful discussion (which has led to our increasing caution with respect to the existence of the monotropic smectic phase of MBBA). We appreciate the help of Mr. A. Croxon and Messrs.

R. Wing and S. Kessler. One of us (5. 0. K.) would also like to express his thanks to the Science Research Council and the University of Leeds Physics and Bio- physics Departments and the University of Arizona for their help and support which made much of this work possible.

References

[l] JAMES, P. G. and LUCKHURST, G. R., MO[. Phys. 19 (1970) [7] KESSLER, J. 0. and LYDON, J. E., Liquid Crystals and Ordered

489.

Vol. 2, Edited by Johnson, J. F. and Porter, R. S.

[2] FACKLER, J. P. and SMITH, J. A., J. Amer. Chem. Soc. 92 (Plenum Press, New York and London) 1974, p. 331.

(1970) 5787.

[3] PETRIE, S. E. B., Paper presented at the 162nd National MAYER, J.3-WALUGA, T- and JANIK, J. ^A.,

Meeting of the Amer. Chem. Soc., Washington D. C., (1972) 102.

1971. Abstract No. Phys. 210. [9] PETRIE, S. E. B., BUCHER, H. K., KLINGBIEL, R. T. and

FERGASON, J. L., Appl. Opt. 7 (1968) 1729. ROSE, P. J., Eastman Organic Chemical Bulletin 45 [5] SORAI, M. and SEKI, S., Bull. Chem. Soc. Japan 44 (1971)

2887. (1973) 1.

[6] LYDON, J. E: and ROB~NSON, D., Biochem. Biophys. Acta [l01 DOUCET, J., LEVELUT, A. M. and LAMBERT, M., Phys. Rev.

260 (1972) 298.