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The effect of the spacer length on the nature of coupling in side chain liquid crystals polymers and elastomers
G. Mitchell, M. Coulter, F. Davis, W. Guo
To cite this version:
G. Mitchell, M. Coulter, F. Davis, W. Guo. The effect of the spacer length on the nature of coupling in side chain liquid crystals polymers and elastomers. Journal de Physique II, EDP Sciences, 1992, 2 (5), pp.1121-1132. �10.1051/jp2:1992190�. �jpa-00247696�
Classificaiion
Physics Abstracts
61.20 61.40K 64.70
The effect of the spacer length on the nature of coupling in side chain liquid crystals polymers and elastomers
G. R. Mitchell, M. Coulter, F. J. Davis and W. Guo
Polymer Science Centre, University of Reading, Whiteknights, Reading RG62AF, G-B- (Received 29 October 1991, accepted in final form 17 January 1992)
Abstract. Side chain liquid crystal polymers and elastomers exhibit a rich phase behaviour which arises from the antagonistic influences of the entropically disordered polymer chain
configuration and the long range orientational ordering of the mesogenic units. This competition
arises since the natural macroscopic phase separation is inhibited by the inherent chemical
connectivity of the system. At the heart of this connectivity is the spacer link and we consider here its influence on the phase behaviour. In particular we consider a series of elastomers in which the number of alkyl units in the spacer is systematically varied from 2 to 6. The lengthening of the
coupling spacer is accompanied by an altemation of the sign of coupling between the polymer
chain and the mesogenic unit. These results demonstrate the dominating influence of the so-called
hinge effect in determining the phase behaviour. In addition to the altemation of the sign there is
some decrease in the magnitude of the coupling with increasing spacer length.
Introduction.
Side-chain liquid crystal polymers and elastomers are composed of several structural components, namely, mesogenic units coupled through « spacers » to polymer backbones
(Fig. I). In the case of the elastomers, tile backbones are chemically crosslinked to form a
_/ Polymer Backbone, Coupling Chain
I
Mesogenic Unit
Perpendicular (-ve) parallel (+ve)
Coupling Coupling
Fig. I. Schematic representation of the components of side-chain liquid crystal polymers illustrating
the basic modes of coupling.
network. In the liquid crystal phase these individual components have antagonistic tendencies since the polymer chain is entropically driven to a random coil type configuration, while the
mesogenic units stabilise with long range orientational ordering. As individual units the
polymer chain and the mesogenic units would phase separate, however, the inherent chemical
connectivity of the side-chain system inhibits this. As a result, there is a coupling of the behaviour of the polymer chain and the mesogenic units. Much of the general behaviour of these systems has been set out theoretically by Warner et al. [1-4], Khokhlov et al. [5, 6] and others [7-9]. In the absence of macroscopic phase separation, the polymer chain configuration
is coupled to the orientational ordering of the mesogenic units through two mechanisms. The first relates to the mean nematic field of the mesogenic units. The strength of this coupling will depend upon the relative ordering temperatures of the two components [2, 10]. The second mechanism depends upon the constraints imposed by the nature of the chemical coupling of the mesogenic units to the polymer backbone. In the work of Warner et al. [1-4], variation in this term is seen principally as arising from the strength of the steric crowding of the side-
chains. The interplay between these competing terms provides, at least in theory, a system rich in phase behaviour. Warner identified three basic nematic phases Nj, NIJ and Nj1J. In the latter, the polymer chain lies preferentially parallel (positive coupling) to the mesogenic units, for Nj and NIJ the arrangement is perpendicular (negative coupling). We have assigned No to a nematic phase with zero coupling [10, iii-
In this work we have to set out to explore the role of the coupling chain in determining the nature of the resultant liquid crystal phase especially in terms of the sign of coupling. We have chosen to examine a system in which the number density of side-chains remains constant, while the length of the coupling chain, or more strictly the number of alkyl units comprising
that chain, is systematically varied.
Determination of sign of coupling.
Direct observation of the anisotropy of the backbone configuration in a side-chain liquid crystal polymer by small-angle neutron scattering (SANS) requires both the synthesis of
selectively deuterium labelled chains and the preparation of monodomain samples usually through the application of a magnetic field [12]. Evaluation of the strength of the coupling requires a knowledge of the order parameter of the mesogenic units and for such samples this may be most easily obtained by wide-angle x-ray scattering [12-14]. We have carried out such procedures for one member of the series under investigation [12]. However the synthesis and
experimental requirements of SANS are most restrictive for a systematic study, and as a result
we have chosen to determine the sign of the coupling from the orientation-strain relationships
for liquid crystal elastomers [15-18]. Consider a liquid crystal elastomer prepared with a random or « polydomain » texture. The application of an extensional strain produces some
level of anisotropy of the polymer backbones which leads to a monodomain texture as a
consequence of the coupling between the side-groups and the polymer backbones. The sign of
coupling is determined from the direction of alignment of the liquid crystal director. The strain required to achieve this monodomain structure is dependent upon both the magnitude
of the coupling and upon the rate of orientation development of the backbone chains with strain. The latter component is clearly related to the cross-link density. We have utilised x-ray
scattering techniques to determine the sign and magnitude of the global orientation of the
side-groups [13-14]. These procedures may be easily applied to « thick » samples which eliminate the problems of surface interactions.
Materials.
The basic materials used in these studies were copolymers made from a
« non-mesogenic »
monomer, hydroxyethylacrylate (I) and mesogenic monomers (II). The only difference among the
« mesogenic » monomers is the number, n, of CH~ groups in the coupling chain which is varied from 2 to 6.
~ O
CHpmCHPC~
'O-(CH~)r-OH
(1)
~~~~~'~~~~~~~~
IO~~~-fl-cN
(II)
The hydroxyl groups of (I) provide functionality for introducing chemical crosslinking after
polymerization [19]. The acrylate monomers were prepared by established routes [19-2 Ii. The monomdr (I) was obtained from Aldrich and distilled under reduced pressure before use. The
polymers were prepared using 10 9b solutions in chlorobenzene at 55 °C with mo19b AIBN as
initiator [19]. The feedstock for each polymerization was 6mo19b of monomer (I) and
94 mo19b monomer (II). The copolymers were purified by repeated precipitation of a solution of the polymer in dichloromethane into diethylether. The purity and the composition of the
resultant polymers were determined using NMR and infrared spectroscopy. The molecular weights of these copolymers were measured by GPC (Rapra Ltd) using tetrahydrofuran as the
solvent and polystyrene as standards. The polymers so prepared have the same number
density of mesogenic side-chains. The characteristics of this copolymer series are listed in table I.
Liquid crystal elastomers were prepared from a concentrated solution of the copolymer in
dichloromethane (~ 25 9b w/v) containing 4 molflb equivalent of monomer repeat units of 1,6-
diisocyanatohexane and a small amount of triethylamine (~ l mo19b). The solution was cast on a Kapton film inside a petridish and the cast film was held at 105 °C for 72 h in order to
provide as complete a reaction as possible. The elastomeric films produced in this manner
were 0.I to 0.2 mm in thickness. The films were cut to strips, 3 mm x10 mm, for the
mechanical measurements. The cross-link densities of the elastomers were evaluated through
the measurement of the modulus of each material in the isotropic phase. The modulus, G, of each elastomer was related to the cross-link density through the relationship G
= pRT/M~,
where p is the bulk density and M~ the average molecular weight between junctions. Although
this equation and the associated model may not be capable of describing the complete
behaviour of these complex materials, especially close to the isotropic-nematic transition
point, it does provide a simple method of evaluating cross-link densities. The characteristics of the elastomers prepared for this study are shown in table II. Inspection of this table shows that the cross-link densities of the elastomers decrease with increasing spacer length. This may result from the shielding effects of the longer side chain on the cross-link sites as was found for elastomers formed from polymers of the type P2 and P6 albeit by a different cross-linking procedure [19].
Table I. Characteristics of copolymers.
Code Spacer Fraction of I :
Length Fraction of II M~ (D.P. ) (2) T~ (°C) (3) T~j (°C) (3)
(n) (mo19b)
P2 2 6 : 94 7.8 x 104 (241) 80 102
P3 3 6 : 94 8.4 x 104 (249) 64 100
P4 4 6 : 94 9.3 x 104 (265) 52 120
P5 5 6 : 94 1.0 x 105 (275 45 II9
P6 6 6 : 94 8.6 x 104 (228) 35 125
(2) Determined using gpc with tetral~ydrofuran as solvent and polystyrene standards.
(3) Determined using a Perkin Elmer DSC II with a scanning speed of 20 °C min~l
Table II. Characteristics ofelastomers.
Code Spacer Length (n) T~ (°C) (4) T~j (°C) (4) Cross-link density (9b) (5)
E2 2 82 102 4.3 ± 0.3
E3 3 65 102 3.8 ± 0.4
E4 4 54 122 2.7 ± 0.3
E5 5 45 122 0.7 ± 0.2
E6 6 35 123 1.0 ± 0.2
(4) Determined using a Perkin Elmer DSC II with a scanning rate of 20 °C min-1 (5) Determined from mechanical modulus measurements at Tm + 15 °C.
Experimental.
THERMAL ANALYSIS. All the samples were examined by differential scanning calorimetry
using a Perkin-Elmer DSC-II at a heating rate of 20 °C min~l. Sample weights of
~ 10 mg
were used and three runs were conducted for each experiment. The glass transition
temperature was taken as the mid point of the transition.
MECHANICALLY INDUCED GLOBAL ORIENTATION. In these studies all the elastomers in
the series were mechanically extended at a strain rate of 10~2s~l using a computer
controlled mini-tensometer equipped with an oven. The samples were extended at selected
temperatures to a particular strain and then held for 2 min before being quickly cooled to room temperature in order to preserve the induced macroscopic orientation. At room temperatures all of the samples exhibited glassy phase.
The deformed samples were examined using a computer controlled symmetrical trans-
mission 3-circle x-ray diffractometer [15] equipped with an incident beam monochromator and pinhole collimation. CuKa radiation was used for the experiment which provided a scattering vector range s =
0.2 to 6.2 h~ (s
=
4 w sin 0/A and 2 0 is the angle between the incident and scattered beams, A the wavelength of the beam). Intensity data were recorded at a fixed scattering vector, s = 1.4 h~
~, as a function of the angle a between the symmetry axis of the sample and the normal to the plane containing the incident and scattered beams from 0°
to 90° degrees in steps of 9° degrees. The scattering pattems show a diffuse maxima at s~ IA h~~ which corresponds principally to short-range spatial correlations between the
mesogenic side-chains. The scattered x-ray data at s
=
IA h~~
were used to obtain the orientation order parameter for the mesogenic units (P~) where (P~)
= (3 cos~
a 1)/2, using procedure described elsewhere [13-15].
Results.
PHASE BEHAVIOUR.- For all the copolymers and elastomers, DSC in conjunction with
optical microscopy and X-ray scattering measurements confirmed the existence of only
nematic and isotropic phases. Inspection of table I shows that the glass transition temperature of the series decreases, while the nematic to isotropic transition temperature generally
increases as the spacer length is increased from 2 to 6. As a consequence the temperature range of the liquid crystal phase for each copolymer increases with increasing spacer length.
There is some suggestion of an odd-even effect in the nematic-isotropic transition tempera- tures, in that the transition temperatures for the odd numbered members of the series are
lower than expected for a simple « linear » increase. Table II shows that the variations in
phase transition temperatures of the parent copolymers are broadly transferred to the crosslinked elastomers.
We have previously studied both uncross-linked and cross-linked side-chain liquid crystal systems with n = 6 using small-angle neutron scattering techniques [12]. Monodomain
samples were prepared by both magnetic and mechanical fields. The results showed an
anisotropic chain configuration which was elongated in the direction parallel to the aligned mesogenic units. In other words the system with n = 6 exhibits a Nj1J phase. The extent of the
anisotropy in the radius of gyration is
~
10 9b and this gives a ratio of the anisotropy of the chain to that the mesogenic side chains of 0.01 [10].
MECHANICAL DEFORMATION. -The development of macroscopic orientation of the
mesogenic units with strain is a function of the deformation temperature. At temperatures very close to the glass transition the level of global orientation which can be reached is limited
by the viscosity controlled slow response, while at high temperatures close to the nematic-
isotropic transition the samples show a greater tendency to tear. However, the alignment
direction of the mesogenic units with respect to the extension axis is found to be independent
of the extension temperature within the nematic phase. Figure 2 shows plots of the
orientation parameter obtained as a function of extensional strain for the series of elastomers
deformed at a temperature of ~10°C above the glass transition temperature of each
particular system. The orientation behaviour is universal, in that the macroscopic orientation increases quickly in the initial stage and reaches a plateau after about an extension of 40 9b.
However, there is marked variation in the direction of alignment as the spacer length is
changed. Positive orientation parameters are observed for elastomers with even numbers of
z
o D
o ~ a
~
~ , ' ~
A ~ o ~ °
~~v i o ~
v ,
m m
, h
h
m . . .
.
~ ~ 6 6 AA
I 1,2 1,4 1.6 1.8 2 2.2 2,4
Extension Ratio
Fig. 2. Plots of the orientation parameter (P~) against the extension ratio for a series of elastomers.
(U) Elastomer El at 90 °C, (m) elastomer E3 at 75 °C. (o) Elastomer E4 at 60 °C, (6) elastomer E5 at 55 °C and (Al elastomer E6 at 45 °C.
alkyl units in the coupling chain. The positive orientation parameter indicates a preferential alignment of the side-chains parallel to the extension axis. In contrast, odd numbers of CH~
groups in the spacer result in a perpendicular arrangement. Clearly the sign of the alignment
direction of the liquid crystal director in these elastomers shows a regular altemation with the spacer length. The positive alignment of the material with n
=
6 shown in figure 2 conforms to
the observations made by SANS [12].
The data shown in figure 2 have two particular features of interest. The plateau orientation parameter is effectively the order parameter of the liquid crystal phase. The critical extension ratio at which the plateau orientation parameter is achieved is a measure how rapid the
preferential alignment can be reached and hence perhaps provides an indicator of the strength
of coupling between the polymer backbone and the mesogenic units. In figure 3, the plateau orientation parameter obtained for each elastomer is plotted against the spacer length. The
regular altemation of the sign of alignment is clear. The plateau orientation parameter shows only a slight variation as the coupling chain length is changed although the data presented relates to differing reduced temperatures. Inspection of figure 2 shows that the critical extension ratio is rising with the increasing spacer length, indicating increasing difficulty in
macroscopically aligning the mesogenic units in elastomers having a long spacer.
Discussion.
The chemical coupling of the mesogenic units to the polymer backbone inhibits macroscopic phase separation. This allows the potential describing the ordering S~ of the polymer chain with respect to the nematic director to be written as [9, 10] :
Uc " P21fa lla Sa + (1 fa) AcScl (I)
where S~ is the order parameter for the mesogenic units, f~ is the fraction of mesogenic units in the system, p~ is the coupling between the mesogenic units and the polymer backbone, and A~ is the coupling between polymer chains. These coupling coefficients are expressed as a
ratio of the mesogen-mesogen interaction parameter. For a large ordering of the polymer
chain an additional term relating to the bending of the backbone should be included [I]. The
coupling coefficient p~ arises from two terms :
l~a ~ l~l+ l~) (2)
O.8
j
~~ ~.
~ ;
#
~ ~ ". .~
'. ,J
g = ;~ i_ I
li '. ./ ". ;~
i O.2
~; ;~ ". .~
.c .. ', I
O ". .~
~. ,z
E '. ;" ". ~
i ~
~, ? ~, ,"
i~ '= ,J ._ .~
~
O.2 ~. ,~ ._ ;"
~ ~
O.4
2 3 4 5 6 7
Number of Alkyl Unils in Coupling Chain (n)
Fig. 3.- The «plateau» orientation parameter (P~) for each member of the elastomer series obtained from figure 2 plotted against the number (n) of alkyl units in the spacer or coupling chain.
Note these data were obtained for differing values of the reduced temperature T*.
where the superscript n denotes interactions of the polymer chain with the mean nematic field and h indicates the constraints of the hinge or coupling unit. It is easy to envisage geometries
or chemical configurations of the spacer chain which might impose rigid constraints on the hinge, for example directly coupled mesogens [22]. Such rigid constraints, arising from the geometry of the attachment, may require either a parallel or perpendicular arrangement as
shown schematically in figure 4. However, if a flexible spacer chain is used such geometrical
constraints must surely diminish as the spacer length increases (Fig. 4). There is also the
possibility of constraints arising from the density of side-chains since the attachment points
are at chemically fixed closely spaced points. Such effects are dominant, for example, in semi-
crystalline polymer configurations [23]. Three modes of behaviour may be expected. In terms of equation (2), the first term may dominate and therefore the coupling is always positive
irrespective of the spacer length. A second view is that the hinge effect diminishes with
increasing length and hence the coupling may change from negative to positive with greater spacer length. The third view is that the hinge factor in equation (2) is dominant and therefore the sign of coupling will depend upon the precise nature of the geometry of the spacer unit.
Neutron scattering measurements on nematic side-chain liquid crystal polymers with spacers of six alkyl units [12, 24] appear to provide support for the first and second of these
expectations. Electro-optic investigations of the effect of spacer length on the curvature
elasticity in side-chain liquid crystal polymers provide additional support for the first
possibility [25].
There are two aspects to the coupling coefficient p~ the sign and the magnitude. The results shown in figures 2 and 3 demonstrate that there is regular altemation in the sign of the
