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Memory effects in liquid crystal elastomers
C. Legge, F. Davis, G. Mitchell
To cite this version:
C. Legge, F. Davis, G. Mitchell. Memory effects in liquid crystal elastomers. Journal de Physique II, EDP Sciences, 1991, 1 (10), pp.1253-1261. �10.1051/jp2:1991131�. �jpa-00247588�
Classificafion Physics Abstracts
61.30 61.40K 64.70
Memory effects in liquid crystal elastomers
C. H. Legge, F. J. Davis and G. R. Mitchell (*)
Polynier Science Centre, University of Reading, W%teknights, Reading RG6 2AF, U-K-
(Received 8 May 1991, accepted in final form 18 June 1991)
Abswact.- Free-standing monodoInain liquid crystal elastoIner saInples are shown to have a
complete meInory of the orientational configuration at the time of cross-linking. This memory is demonstrated through saInples in which the parent polymer system is first aligned in a Inagnetic field prior to cross-linking- These films show reversible neInatic-isotropic phase transitions and x-
ray scattering pattems characteristic of neInatic phases. The1iquid crystal elastomer films exhibit
a remarkable memory effect, in that the sample1nay be held at temperatures well above the
neInatic-isotropic transition for extended periods (~2 weeks), but on cooling into the liquid crystal phase region, both the original director alignment and the degree of preferred orientation
are recovered. It is demonstrated that these novel memory ef§ects are equilibrium in nature. The
origins of this phenomena in terms of coupling between the mesogenic side-chains and the
polymer network are discussed.
Introduction.
Side-chain liquid crystal polymers exhibit complex phase behaviour and structure as a result of the interactions between the mesogenic side chains and the polymer backbone ii, 2]. The inherent connectivity of the polymer system inhibits the phase separation of these normally incompatible components and leads to some coupled orientational behaviour [3-6] in which the polymer chain is ordered with respect to the liquid crystal director [7, 8]. The introduction of interrnolecular cross-links into such a system results in materials [9] in which this coupling
manifests itself in a number of unusual properties including stress-induced molecular switching [10,11] and shifts in phase transitions [12,13], piezoelectricity [14,15] and
electrically induced shape changes [16,17]. Wamer and co-workers have extended their theoretical treatment of side-chain liquid crystal polymers [2, 4] to include the efsects of cross-
linking [18]. They predict that the state of order at the time of cross-linking will effect the transition temperatures of the resultant cross-linked films ; this has been recently confirmed [13]. In this work we extend these concepts to consider the consequences of the state of the
director distribution at the time of cross-linking- We will show that the material has a
complete memory of both the state of order and of the director distribution in the sample. The magnitude of this memory effect will be dependent upon the level of coupling between the
mesogenic side-chains and the polymer network. For the acrylate based liquid crystal polymer
(*) Author for correspondence.
1254 JOURNAL DE PHYSIQUE II M 10
system considered here, small-angle neutron scattering studies have shown that the polymer
backbone tends to lie parallel to the liquid crystal director or mesogenic side-chains [7].
Consequently, the chain paths in an aligned sample must be arranged in an anisotropic
manner, vith, in this case the radius of gyration 10 9b larger parallel to the director than in the perpendicular direction [7]. The introduction of permanent chemical cross-links into the
sample, favours this anisotropic arrangement. This imposed preference for configurational
order on the polymer leads to the possibility of a memory effect. Consider a sample which had been cross-linked in the liquid crystal state but subsequently held in the isotropic phase, in which all long range orientational order is absent. In principle on cooling to the liquid crystal phase, the particular configuration at the time of cross-finking is recovered since that must in the liquid crystal phase present the lowest energy state. In otherwords the system should have
a memory of both director pattem and the level of orientational order.
Expedmental.
Fundamental to these studies is the ability to introduce permanent cross-linking into a side- chain liquid crystal polymer system after the material has been aligned in a magnetic field.
From the methods available, we have selected a chemical route in which the slow reaction rate of the cross-linking agent allows a monodomain to be created using a moderate magnetic field, long before the gel point is reached. A copolymer of I containing 6mol 6b of II,
prepared using standard procedures [19] was used as the base material for these studies. It
exhibited a nematic phase with TNj=123A°C and T~~33°C ~Perkin Elrner DSC-
2,10°Cmin-I). The molecular weight of the sample was M~= 5.llx10~ (gpc (RAPRA Ltd.) using THF as the solvent and polystyrene standards).
/~
CH2~~CH~C, / , / ,
O-(CH2) O
~
C02~
CN/~
CHHCH~C,
O-(CH2)2~-OH
11
Throughout this study we have utilised as a test structure a monodomain texture in which the director alignment is uniform throughout the sample. Cross-linked monodomain polymer
films were prepared using the following procedure [13]. A solution of the copolymer in dichloromethane (106bw/v) with diisocyanatohexane (4mo16b in monomer units of the copo1ynler) and a small quantity of triethylamine was cast onto a Kapton sheet and the solvent
removed by evaporation under reduced pressure at room temperature. Since at room
temperature the extent of the cross-linking reaction is essentially zero [13], this technique
allows the diisocyanatohexane to be dispersed in the polymer film without reaction. A monodomain was created by holding in a unifonn magnetic field of 0.6 T at 108 °C for 30 min in a protective nitrogen atmosphere. In contrast, the cross-linking reaction required
some 24 to 36 hours. To ensure a complete cross-linking reaction the monodomain sample
was left under those conditions for 72 hours, although typically it is completed in 24 hours.
The sample was cooled to room temperature at the end of this period. A number of films, thickness 0.5 mm, were prepared in this manner. All showed essentially the same physical and thermal properties, and the same level of molecular orientation. Samples prepared in this
manner possessed an effective cross-link density in terms of monomer repeat units of
~
l 6b as obtained from modulus measurements in the isotropic phase.
The director alignment and the orientation parameters were evaluated using wide-angle x-
ray scattering techniques. These allowed relatively tllick samples to be employed and hence
remove any ambiguity which might arise in surface dominated thin films. The x-ray scattering
intensity data were measured using a computer controlled 3-circle syrnmetrical transmission difsractometer [20], equipped with an incident-beam monochromator and pinhole collimator
over a scattering vector range s (1), 0.2-6.21-1 and
over a azimuthal variation a of 0° to 360[ The angle a is defined as the angle between the sample axis and the normal to the plane containing the incident and scattered paths. Figure I shows the scattered intensity recorded for a monodomain liquid crystal elastomer sample prepared using the procedures described
-#
D
Se~ion along A
m
~ +
a
c
_
w
az ~ t
- ~
~' Section alongB
«
0 1
S I kl ) S I ll
Fig. I.-The scattered x-ray intensity function I(s,a) measured at room teInperature for a
monodomain sample of a crosslinked side-chain liquid crystal elastoIner. The alignment direction D is parallel to the magnetic field direction applied during the preparation of the sample.
above. The intense peak at s~ I.4A-I arises from correlations between neighbouring mesogenic units and it intensifies normal to the director and hence we may use this to locate the director orientation. The spread of intensity as a function of
a is a measure of the global
orientation (P~) [21], that is the combination of the variation in director orientation
(P~)° and the order parameter S of the nematic phase. These quantities are related by
jp~j =
sjp~jD (1)
(~) Is
= 4 w sin all where 2 8 is the angle between the scattered and incident beaIns, A the incident wavel~ngth.
1256 JOURNAL DE PHYSIQUE II M 10
We have utilised x-ray scattering procedures [20, 2Ii and the scattering at s ~
IA I-I to give
a measure of (P~). Although, as is detailed elsewhere [2Ii, the absolute levels of orientation
thus obtained, if interpreted as the order parameter, are slightly in error, this approach gives repeatable and reliable parameters. All x-ray scattering measurements were made at room
temperature on samples of the liquid crystal elastomer which had been rapidly cooled from the indicated temperatures. As will be seen later the time scale for structural changes is
sufficiently long that this procedure introduces no significant errors.
Results.
Cross-linked liquid crystal polymer films prepared in the above manner exhibited a reversible nematic-isotropic transition at 123.6°C and a glass transition at 33 °C. Polaridng optical microscopy showed the films to contain a monodomain texture, in that complete and uniform
extinction was observed on rotating the sample with respect to crossed polarisers in a
polarising microscope. On heating the films above the nematic-isotropic transition, the field of view was completely black indicating no retention of any level of orientation, the
birefringence being below the measurable limit of 10-5. lvhen the films were cooled into the nematic phase region, a liquid crystal texture reappeared. Rather surprisingly this texture
was still monodomain and moreover the direction of alignment was identical to that determined prior to heating into the isotropic state. Since the sample was not held in a
magnetic field during this cycling, clearly it possessed some memory of the initial liquid crystal configuration.
To determine the extent of this « memory effect » we undertook a series of measurements in which a sample prepared as detailed above was heated into the isotropic state at 150 °C, I.e.
well above the nematic-isotropic transition for increasing periods of time. The samples were
then cooled into the liquid crystal phase at 108 °C for a second period of time, and finally quenched to room temperature. All these subsequent heating and cooling cycles were performed in the total absence of a magnetic field. At room temperature the direction and spread of orientation were determined by the quantitative x-ray scattering procedures described above. Monodomain cross-linked liquid crystal polymer films showed values of
(a) (b)
. o .
. . . o
~ " ° o
~ o . .
. . . D
o . . .
. o o . o . .
Fig. 2. Polar plots of the intensity function I(a ) at s
= IA A-1 Treasured at
room temperature for (a) a sample rapidly cooled after cross-linking in a magnetic field as described in the text, (b) the same
sample as (a) after holding in the isotropic phas~ for 150 hours at 150 °C and cooling through the liquid crystal phase to room temperature. The alignment direction D is parallel to the magnetic field direction
applied during the preparation of the sample.
(P2) of
~ 0.5 after removal from the magnetic field, a similar value was recorded for uncross- linked samples subjected to the same treatment.
The first and most striking observation of these measurements was that in all cases the same direction of preferred orientation retumed on cooling into the liquid crystal phase range, even though samples were held in the isotropic state for up to 373 hours (~17 days). Figure 2
shows the azimuthal variation of the scattered intensity at s
=
I.4i~~ for
a sample as prepared and the scattering recorded after the same sample has been held in the isotropic phase for 150 hours and then cooled through the liquid crystal phase to room temperature.
The fact that the symmetry axes of the two polar plots superimpose, indicates incontrovertibly
that the director alignment is identical for the two measurements. In otherwords, even though the sample has been held in the isotropic phase for 150 hours, on cooling to the liquid crystal phase the monodomain structure is recovered. Optical measurements show that in the
isotropic phase the level of orientation is zero (An
~ 10~ ~). In comparison an uncross-linked
sample of the same copolymer subjected to the same magnetic field alignment treatment as the elastomer, shows no retention of the global orientation after holding momentarily (~ 60 s)
in the isotropic state at 150 °C. Of course, on cooling the uncross-linked sample shows local
alignment (as seen in the optical microscope) commensurate with the presence of a nematic liquid crystal phase, however none of the global orientation induced by the magnetic field is
recovered, (P~) is zero.
Figure 3 shows the results of holding the sample in the liquid crystal phase at l18 °C for
increasing periods of time, after the sample had been held in the isotropic phase (150 °C) for
163 hours. As figure 3 shows, the level of global orientation rises until after some 30-40 hours it reaches the same value as shown by the sample directly after alignment in the magnetic
field. To summarize, a liquid crystal elastomer sample, cross-linked after alignment of the parent polymer in a magnetic field, is able to recover, in the complete absence of that
magnetic field, completely the direction and level of order in that monodomain after being
as
, .
§~ o.4
ii I
I
o.3
Sample held Sample held
~f at 150°C II) at I18°C (LC)
~ o.2
_g~
.j o i
o
o 5o loo iso 2oo mo 3oo 3so
[me in hours
Fig. 3. A plot of the measured orientation parameter (P2) for « monodomain » sample of the liquid crystal elastomer as a function of the time/temperatur~ cycle indicated. Th~ data Inarked (m) was
obtained by x-ray scattering procedures, the data1narked (O) was obtained froIn optical measurements.
1258 JOURNAL DE PHYSIQUE II M lo
o.7
06
)~ ,
~~
~
~
~)
DA
~~
~
j O.3
02
_fl
~
O-I
°
o.9 o.92 o 94 o.96 o-w i
Temperature/Transition Temperature
Fig. 4. A plot of the1neasured orientation parameters as a function of the reduced temperature for monodomain samples of a) a cross-linked liquid crystal polymer (.) ; b) an uncross-linked liquid crystal polymer (O). The data were obtained using x-ray scattering procedures on samples rapidly cooled to
room teInperature.
held in the isotropic phase for hundreds of hours. T%is memory effect is not shown by the
equivalent uncross-linked polymer.
The preceding experiments have explored the « memory effect » through holding the liquid crystal elastomer films at temperatures above the nematic-isotropic transition. Measurements of the orientation average (P~) for a liquid crystal elastomer film held at temperatures in the
nematic phase range for increasing periods of time in the absence of a magnetic field showed that there was no reduction in the level of orientational order at that temperature even for times in excess of1000 hours (55 days). Figure 4 shows the values of (P~) recorded for a liquid crystal elastomer film as a function of holding temperature. At each temperature the
sample was held for increasing periods of time, until the value of (P~) reached a steady state.
Typically this required 150 hours and this emphasizes the sluggish response of these cross- linked liquid crystalline materials and hence the time taken to reach equilibrium. The slow
response underlines the validity of performing the actual measurements of (P~) at room
temperature on samples rapidly cooled from the liquid crystal phase. The data in figure 4 shows that the orientational order increases with reducing temperature as is expected. The values obtained are similar to those recorded for a uncross-linked monodomain sample.
Discussion.
It is important to distinguish between memory effects which arise purely as a consequence of slow dynamics and those which result from equilibrium structural considerations. Examples of the first category abound in polymer science, inherent in the slow relaxations of long chain
molecules. Among liquid crystal polymers such situations have been detailed for both main- chain [22] and side-chain [23, 24] systems. In this study the memory effect in terms of the
director orientation and level of orientation is a static equilibriuTn property. This is shown most forcibly in figure 3 for the level of (P~) increases with holding in the liquid crystal phase
as the system drives towards the equilibrium state. The effect is clearly related to the introduction of chemical cross-links since the uncross-linked parent copolymer does not
display these static memory effects. We assert that this memory effect, in terms of recovering the orientational configuration at cross-linking, is a direct result of the interactions between the mesogenic side-chains and the polymer network. It is a striking manifestation of the
coupling between the polymer backbone trajectory and the orientation of the mesogenic side- chain which has been found in neutron scattering studies of suitably labelled mixtures [7, 8].
For the polyacrylate based system considered here we have found that in both elastomer and uncross-linked systems, the polymer backbone tends to lie parallel to the liquid crystal director or mesogenic side chains and the anisotropy of the radius of gyration of the polymer chain was found to be 10 6b f. As a consequence of this coupling, the chain trajectories in
an aligned sample (for example through a magnetic field) must also be arranged in an
anisotropic manner. The introduction of permanent cross-links into such a sample means that this anisotropy is fixed into the sample. Wamer et al. [2] have shown that cross-linking in this additional order should result in an increase of the nematic-isotropic transition temperature
proportional to the concentration of cross-link points. This has been confirmed [13]. The enhanced stability of the nematic phase arises from the fact that, at the phase transition, the network must deform to an isotropic coil. We attribute the origin of the memory effect demonstrated here to a similar mechanism. Cross-linking in the liquid crystal phase defines a favoured orientational configuration. Any deviation away from that configuration, either in terms of the director pattem or tl~e state of orientational order of the side-chains, must involve a distortion of the network. The probability of that deviation occurring will be related to the energy balance of the system. The energy required for that distortion win clearly be
proportional to the cross-link density and to the temperature. Of course, re-establishment of that configuration may require substantial time to allow for diffusion of the chains and
junction points, and that is indeed the case observed. Such a model which involves the change of the polymer backbone trajectories from an anisotropic to isotropic distribution at the
nematic-isotropic transition should, in a monodomain sample, result in a macroscopic shape change similar in principle to that induced electrically [16, 17]. However, careful examination, using quantitative optical microscopy, of a sample prepared as detailed above, revealed no
significant shape changes during the nematic-isotropic transition. Several samples with areas
~
9 x 10~~m~ were measured in
an optical microscope with a linear dimension resolution of
~
0.5 6b. The sample dimensions were measured after holding in the isotropic phase at 145 °C for 120 hours and then after holding at 102 °C in the liquid crystal phase for a further 120 hours. Measurements of the dimensions parallel and perpendicular to the director showed
no systematic complementary changes within the resolution of 0.5 6b. In contrast, small-angle
neutron scattering measurements have shown a difference of 10 6b in the radius of gyration
measured parallel and perpendicular to the director [7]. The absence of significant dimensional changes suggests that, even when the elastomer undergoes a phase change the
global distribution of cross-link points remains unaltered.
In essence the polymer network acts as an intemal field in a similar manner to that which
would result from an extemal electric, magnetic or stress field. However, here the zero state of the field corresponds to the temperature of cross-linking (T~j). Thus the field is
proportional to T T~i) and should change sign at T
= T~j. We would expect therefore, that the orientational order at temperatures less than T~j to be depressed and that at temperatures
above T~j to be enhanced with respect to an uncross-linked sample. The orientational parameters obtained for both cross-linked and uncross-linked monodomain samples as a
function of temperature are shown in figure 4. Although the differ~nces are small there appears to be a systematic trend in that the two sets of data cross at a temperature of T/TNI
~
0.95. The reduced temperature for the cross-linking reaction was 0.96. The fact that
