Castelletto V

Gleeson AJ

Hamley IW

Mykhaylyk OO

I. W. Hamley

V. Castelletto

O. O. Mykhaylyk

A. J. Gleeson

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J. Appl. Cryst. (2004). 37, 341±344 DOI: 10.1107/S0021889804001268 341
laboratory notes
Journal of
Applied
Crystallography
ISSN 0021-8898
Received 17 October 2003
Accepted 16 January 2004
# 2004 International Union of Crystallography
Printed in Great Britain ± all rights reserved
Mesoscopic crystallography of shear-aligned soft
materials
I. W. Hamley,a* V. Castelletto,a O. O. Mykhaylyka and A. J. Gleesonb
aDepartment of Chemistry, University of Leeds, Leeds LS2 9JT, UK, and bSynchrotron Radiation Source, CLRC
Daresbury Laboratory, Warrington WA4 4AD, UK. Correspondence e-mail: I.W.Hamley@chem.leeds.ac.uk
The development of a novel device for examining the three-dimensional
structure of shear-aligned soft materials using small-angle X-ray scattering is
reported. Following alignment via oscillatory shear, the shear plates are ®xed
together and the whole assembly mounted on a miniature goniometer placed
within the oven of the rheometer. It is then possible to perform two-axis
crystallography, rotating around the shear direction, and tilting with respect to
this axis. Preliminary results are presented that highlight the information on
aligned block copolymer nanostructures that can be accessed using the
goniometer. Diffraction patterns obtained from a shear-aligned hexagonal
close-packed micellar structure formed in an aqueous solution of an amphiphilic
diblock are discussed, as well as those of the metastable hexagonal perforated
lamellar structure formed in a diblock copolymer melt.
1. Introduction
Soft materials such as polymer melts and gels, lyotropic liquid crystals
and colloidal suspensions, are generally subjected to shear ¯ow
during processing. This ¯ow often leads to orientation of the nano- or
microstructures, and understanding the ultimate state of alignment
and the pathway to it is presently the focus of much activity. Small-
angle scattering of X-rays or neutrons is a powerful non-invasive
technique to probe shear-induced orientation at the nanoscale.
The present paper describes a novel device that enables crystal-
lographic studies of soft solids aligned by oscillatory shear. The
sample is ®rst subjected to large-amplitude oscillatory shear (LAOS)
using a shear sandwich tool (Fig. 1a) within a Rheometrics RSA II
rheometer, modi®ed to allow simultaneous small-angle X-ray scat-
tering (SAXS) experiments (Pople et al., 1997; Hamley, Pople et al.,
1998). The X-rays are incident along the shear gradient (rv) direc-
tion so that scattering patterns in the (v, e) plane are obtained. The
central oscillating tool is then detached from the actuator, and the
whole assembly containing the shear-aligned sample is ®xed together
with a plate (Fig. 1b). This assembly is then placed on a mini-goni-
ometer stage within the rheometer. The sample may be rotated
around the shear direction (’ rotation, up to 60, with reasonable
transmission) with the possibility also to tilt (angle !, 25). In this
way, the structure of shear-aligned `crystals' can be examined in three
dimensions. We have previously employed this technique in small-
angle neutron scattering (SANS) studies of shear-induced alignment
in a cubic micellar phase formed by an amphiphilic diblock co-
polymer (Hamley, Pople et al., 1998) in a collaboration with
researchers at Risù National Laboratory, who developed a system
(also based on the Rheometrics RSA II rheometer) suitable for
neutron scattering (Vigild et al., 1998). At Risù, a shear cell mounted
on a goniometer was used to probe the crystal structure of the
bicontinuous gyroid structure in a diblock copolymer melt (Vigild,
1997). Because the characteristic length scale of the structures probed
by small-angle scattering is typically 5±100 nm, we previously termed
this method `mesocopic crystallography' (Hamley, Mortensen et al.,
1998), although perhaps `nanoscale crystallography' would be more
appropriate nowadays.
Here, we describe the design of a shear cell/goniometer device for
SAXS studies of soft materials. Representative preliminary results
obtained for two ordered block copolymer structures (respectively
face-centred cubic and hexagonal perforated layer) are included.
These illustrate the insights into the shear-aligned structure that can
only be provided when planes other than the (v, e) plane can be
accessed.
2. Experimental
Small-angle X-ray scattering experiments were performed on station
16.1 at the Synchrotron Radiation Source, Daresbury Laboratory,
UK. The wavelength  was 1.41 AÊ . Data were collected on a two-
dimensional gas-®lled area detector (RAPID). The wavevector scale
q (= 4 sin/, where  is the scattering angle) was calibrated using a
specimen of wet collagen (rat-tail tendon).
The modi®ed Rheometrics RSA II rheometer is described in detail
elsewhere (Pople et al., 1997; Hamley, Pople et al., 1998), and for
convenience is illustrated in Fig. 2(a). The sample is subjected to
oscillatory shear in a shear sandwich device (Fig. 1a) mounted within
the rheometer oven (Fig. 2a). To enable the transmission of X-rays,
the tools were made from polycarbonate. No peaks are observed in
the small-angle region for this material. The transmission is inevitably
reduced, by a factor of approximately two for each additional 1 mm
of polycarbonate. Following shear, the central shear tool is detached
from the actuator by unscrewing the L-shaped mount (Figs. 1a and
2b). The three plates in the shear sandwich are then ®xed together via
a ®xing plate screwed into pre-drilled holes in the side of the poly-
carbonate tools (Fig. 2c). To enhance transmission, the ®xing plate
was made from polyimide ®lm of thickness 0.5 mm. After ®xing the
plates, the assembly was removed from the rheometer. The actuator
laboratory notes
342 I. W. Hamley et al.  Shear-aligned soft materials J. Appl. Cryst. (2004). 37, 341±344
post was removed and a miniature goniometer was mounted onto the
transducer post. The shear cell assembly was then attached (inverted)
onto a two-circle goniometer (Huber, custom built) via a grub screw.
The tilt angle on the miniature cradle (Fig. 2c) must be set prior to
closing the oven doors. The rotation angle ’ can be set using the scale
collar outside the oven.
3. Results
3.1. Twinned face-centered cubic structure
We have previously studied shear alignment of a close-packed
structure formed in an aqueous gel of diblock P94E316 (Castelletto et
al., 2001). Here P denotes propylene oxide, E denotes ethylene oxide
and the subscripts are numbers of repeats. This sample was mounted
into the rheometer and prior to shear the diffraction pattern showed
slight orientation due to compression of the sample when mounting
into the shear sandwich. The ®rst-order peak was located at q* =
0.0235 AÊ ÿ1, corresponding to a cell parameter a = 463 AÊ [in good
agreement with the value reported previously (Castelletto et al.,
2001)]. LAOS was then applied. Fig. 3(a) shows the diffraction
pattern obtained following shear (T = 298 K, ! = 100 rad sÿ1, A =
45%) and shows essentially sixfold symmetry of re¯ections (higher
orders were also observed, but are not shown for convenience). This
sample has previously been shown to correspond to a (defective)
face-centred cubic (f.c.c.) structure (Castelletto et al., 2001). The shear
tools were then detached from the rheometer and ®xed on the
goniometer as described above. The SAXS pattern obtained prior to
rotation (i.e. ’ = 0) is shown in Fig. 3(b). Some loss of orientation
due to the manual handling of the sample is apparent; nevertheless,
the main features are the same as those in Fig. 3(a). Rotation of the
shear-aligned sample around the shear direction provides additional
information on the symmetry of the structure. Fig. 3(c) shows the
pattern obtained at a rotation angle ’ = (50  5). It shows four
re¯ections at 1.15q*, close to the position expected for the 200-type
re¯ections of an f.c.c. structure (space group Fm3m). The angle
selected is close to ’ = 54.7, which corresponds to the angle between
the 111 and 100 directions of a cubic structure. Therefore, these two
diffraction patterns are consistent with a structure that is primarily
cubic. The presence of sixfold symmetry along the 111 direction
indicates that the structure does not consist solely of ABCABC . . .
stacked hexagonal close packed (h.c.p.) layers, but must contain some
ABAB . . . stacking [as discussed previously (Daniel et al., 2000;
Castelletto et al., 2001)]. It should be noted that diffraction patterns
with features intermediate between those shown in Fig. 3(a) and 3(b)
were obtained at intermediate angles, but these are not shown for
reasons of clarity.
3.2. Hexagonal perforated layer structure
The hexagonal perforated layer (HPL) structure is known to be a
non-equilibrium structure in diblock copolymer melts. However, it
Figure 2
(a) The Rheometrics RSA II rheometer in situ at the beamline, Synchrotron
Radiation Source, Daresbury Laboratory, UK. (b) Detail of shear tools, prior to
shear. (c) Inverted shear tool assembly mounted on the goniometer (the ®xing plate
is hidden behind the shear tools).
Figure 1
(a) Schematic of shear sandwich tools, made from polycarbonate to allow
transmission of X-rays. (b) Following shear, the tools are inverted and placed on a
miniature goniometer within the rheometer oven (see Fig. 2).
can be highly metastable, as illustrated by our prior work on PEP-
PDMS diblocks where the growth of the equilibrium gyroid phase
from the HPL phase can be extremely slow (hours or even days). In
the present work, we study a blend very similar to one previously
investigated (Vigild et al., 1998), for which the HPL phase is found to
be metastable at room temperature (where the lamellar phase is
stable) (Vigild, 1997). The binary blend consists of two PEP-PDMS
diblocks [PEP = poly(ethylene propylene), PDMS = poly(dimethyl-
siloxane)], one with a volume fraction fPEP = 0.65 and the other with
fPEP = 0.70. A 3:1 (by weight) mixture of these polymers has fPEP =
0.67. In the `one component' approximation (Matsen & Bates, 1995;
Shi & Noolandi, 1995), we expect the phase behaviour of this blend of
two closely matched diblocks to be equivalent to that of a single
diblock of the same composition.
The PEP-PDMS diblock blend was subjected to LAOS (A = 80%,
! = 100 rad sÿ1) at T = 333 K, corresponding to a temperature at
which the HPL phase is highly metastable. Following this, the sample
was cooled to 303 K, at which temperature the shear plates were ®xed
and mounted onto the goniometer stage. The sample was rotated
from ’ = 0 to 60 at various angles. Diffraction patterns corre-
sponding to high-symmetry projections only are included in Fig. 4.
Fig. 4(a) shows the pattern obtained following shear (at ’ = 0). The
pattern consists of two strong equatorial re¯ections, in addition to
broad rings at the same value of q* and at 1.1q*. Inside the ®rst ring
are four weak re¯ections (57 with respect to the equator) that arise
from perforations within the lamellae. The origin of these re¯ections
has been discussed in detail elsewhere (Hamley et al., 1993, 1999;
FoÈ rster et al., 1994; Vigild et al., 1998; Zhu et al., 2003), and is not
reiterated here. The pattern obtained upon rotation to 60 (shown in
Fig. 4b), however, has not, to our knowledge, previously been
reported. It contains ten re¯ections at q*. A pattern with ten
re¯ections at q* in a shear-aligned diblock is often considered to be
the signature of a gyroid structure (space group Ia3d) (FoÈ rster et al.,
1994; Zhao et al., 1996; Hamley, 1998; Vigild et al., 1998; Wang &
Lodge, 2002). However, most of the angles between the re¯ections in
Fig. 4(b) are not consistent with allowed angles between 211 re¯ec-
tions (these are the ®rst re¯ections for space group Ia3d). Therefore,
we discard the possibility that this pattern arises from a gyroid
structure; rather it corresponds to a projection of a HPL structure. It
has been shown elsewhere that the HPL structure corresponds to
coexisting trigonal (R3m) and hexagonal (P63/mmc) structures (with
ABCABC . . . and ABAB . . . stacking of hexagonally perforated
layers respectively) (FoÈ rster et al., 1994; Vigild et al., 1998; Zhu et al.,
2003). We therefore ascribe the pattern in Fig. 4(b) to such a struc-
ture. Further details of the indexation of the re¯ections will be
provided elsewhere. Fig. 4(c) shows the pattern obtained upon
rotation back to ’ = 0. A pattern similar to the starting pattern is
recovered, although the four weak re¯ections inside the inner ring
appear to have disappeared. The ratio between the positions of the
two diffraction rings is 1:1.15q*.
4. Conclusion
In summary, we have developed a novel device for crystallographic
studies of soft materials, following alignment by large-amplitude
oscillatory shear. The data presented from trial experiments are
certainly preliminary, but we hope they illustrate the potential of the
technique, which could also be used to study orientation in liquid-
crystal polymers, biopolymer gels, etc.
This work was supported by the Engineering and Physical Sciences
Research Council, UK (grant GR/N22052). We are very grateful to
Mike Wardell and David Coleman for their help in constructing the
goniometer.
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J. Appl. Cryst. (2004). 37, 341±344 I. W. Hamley et al.  Shear-aligned soft materials 343
laboratory notes
Figure 4
SAXS patterns obtained from a shear-aligned (A = 80%, ! = 100 rad sÿ1 for 10 min
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returning to ’ = 0.
Figure 3
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mounted on the goniometer with a rotation angle (about the shear direction v) ’ =
0. (b) Pattern obtained for ’ = 50 .
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Mesoscopic crystallography of shear-aligned soft materials

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Mesoscopic crystallography of shear-aligned soft materials

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