Photon Factory Activity Report 2022 #40 (2023)

Effects of Layer Deformation on Mechanical Properties ofGlassy Smectic Liquid Crystal Polymer Fibers

BL-6A/2021G040

Katsuki Kawahara, Ryoji Suzuki, and Masatoshi Tokita*
Department of Chemical Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro, Tokyo 152-8550, Japan
1 Introduction
Stacked layers are one of the typical molecular assemblies
found in solid polymers, including semicrystalline
polymers and block copolymers (BCPs). When stackedlayers are elongated along the layer normal direction, thelayers undulate and form chevrons by bending at limitedsites to avoid the increase in spacing.[1–3] These structuralchanges are associated with stress responses, such as stressyielding. In these studies, the layers were flat and large.
Once deformed layers have rarely been examined for
deformation and stress response under another sample
deformation.
In this study,[4] we examine the effects of layer
deformation on the stress response for a smectic liquid
crystal (LC) composed of a main-chain BB-5(3-Me)
polyester comprising biphenyl moieties connected withalkyl spacers via ester linkages (Fig. 1).[5–7] Fibrous
specimens prepared by stretching the isotropic melt hadlayers stacked along the fiber axis. Elongating this fiber inthe LC state deformed the layers, yielding fibers with twotypes of layer morphologies: layers stacked along a
direction tilted from the fiber axis and layers divided andstacked along the fiber axis. Fibers with different layer
deformation types differed in their stress response upon
tensile deformation in a glassy state, as determined usingsynchrotron-radiation X-ray diffraction (XRD). The
results demonstrate that dividing the layers significantly
increased the values of Young’s modulus and yield stressof the fiber, respectively, by three and 2.5 times. Thisagrees with strengthening layer-structured polymericmaterials by dividing the layers, which was recently
reported for a lamellar block copolymer and asemicrystalline polymer.[8,9] This study will lead to thedeveloping of a new method for strengthening polymer
solids other than those with polymer chain or crystal
orientation. However, other layer-structured materials,
such as metallic alloys and MAX-phase ceramics, havebeen strengthened by kinking the layers.[10,11]Fig. 1. (a) Chemical structure of a BB-5(3-Me) polyesterand (b) smectic LC structure formed by the polyester.
Mesogenic biphenyl moieties represented with filled
ellipsoids form layers by segregating from the alkyl
spacers represented with the sold lines.

2 Experiment
Materials: The BB-5(3-Me) polyester was prepared by themelt condensation of dimethyl 4,4′-biphenyl dicarboxylate
and 3-methyl-1,5-pentanediol.[5,6] The number-averagemolecular weight (Mn = 48 kg mol−1) and thepolydispersity index (PDI = 1.7) were determined by sizeexclusion chromatography (SEC) calibrated withpolystyrene standards. This polyester had a glass transitiontemperature Tg = 33 °C and an isotropization temperature
Ti = 153 °C, measured by differential scanning calorimetry(DSC) at a heating rate of 10 °C min−1.
Methods: Synchrotron-radiation XRD measurements wereperformed at the BL-6A beamline at Photon Factory,
Tsukuba, Japan, using a Dectris Pilatus3 1M detector.[12]The X-ray radiation wavelength (λX) and the sample-todetector distance were 0.15 nm and 0.25 m, respectively.
A film sample at a predetermined temperature wascontinuously stretched using a Linkam TST350 tensile
stage at a crosshead speed of 1.3 μm s−1 (nominal strain
rate: 5% min−1). The test sample length and cross-sectional
dimensions were 1.5 mm and 0.4–0.6 × 0.2–0.4 mm2,respectively. The XRD patterns were recorded at an
exposure time of 30 s.
3 Results and Discussion
3.1. Smectic layer deformations
Stretching the fiber in the LC state deformed the smecticlayers. This enabled us to prepare three types of glassy
smectic LC fibers with deformed layers: fibers A, B, andC in order of increasing ε. Fiber A was the nondeformedfiber (ε = 0%) with smectic layers stacked along the fiberaxis (Fig. 2a). This fiber displayed the layer reflection on
the meridional line, indicating that the smectic layers werestacked along the fiber axis.
Fiber B, prepared by elongating the fiber up to ε =30% at a strain rate of 1.25% min−1, exhibited smectic layer
reflection with intensity maxima at off-meridional
positions (Fig. 2b). These intensity maxima indicated thatthe layer stacking direction was tilted from the fiber axis.
This four-point reflection is usually associated withchevrons comprising layers periodically bent along thelayers.[2,3,8,13,14] However, such periodic layer bending
was not detected by Raman spectroscopy and polarizing
microscopy.[4] We speculate that the tilted layers in fiber
B alternated the layer stacking direction through thethickness direction of the fiber with a rectangular crosssection. Besides the four-point reflection, the fiber left areflection intensity on the meridian.

Photon Factory Activity Report 2022 #40 (2023)

Fig. 2. XRD patterns measured for fibers (a) A, (b) B, (c)C, and (d) D at 25 °C. The fiber axis is vertical.
Fiber C, prepared by elongating the fiber up to ε =300% at a rate of 5% min−1, displayed the layer reflection
on the meridian (Fig. 2c). This reflection was elongated in
the horizontal direction parallel to the layers, indicatingthat the layers were divided and shortened.[15]The divided layers recovered the layer length uponannealing fiber C at an LC temperature of 110 °C for 12 h.
This annealed fiber, designated as fiber D, showed an XRD
pattern that resembled that of fiber A (compare Fig. 2d to2a), indicating that fiber D comprised long (or large-area)
smectic layers like fiber A.
3.2. Stress Response of Fibrous Glassy Smectic LC Fibers
with Predeformed Layers
The four types of BB-5(3-Me) fibers were examined forthe stress response upon elongation in a glassy LC state at
25 °C. These fibers had rectangular cross sections. The
cross-sectional dimensions of fibers A and B were 0.6 mmin width and 0.4 mm in thickness, and those of fibers C andD were 0.4 mm in width and 0.2 mm in thickness.
These fibers displayed different σ–ε curves (Fig. 3).
These σ–ε curves allowed us to determine the values ofYoung’s modulus (E), yield stress and strain (σy and εy),and strain at fiber break (εb) as follows. Fiber A with longlayers stacked along the fiber axis showed E = 0.33 GPaand σy = 33 MPa at εy = 27% and broke at εb = 30%. Fiber
B comprising layers tilted from the fiber axis showed E =0.38 GPa and σy = 20 MP at εy = 23%. This fiber showed along plateau on the σ–ε curve and increased σ slightly at ε= 200%, which was sustained up to the ε of the apparatus
limit (300%). Fiber C with divided layers showed thehighest E = 0.97 GPa and σy = 82 MPa among the fourtypes of fibers, which were 3 and 2.5 times greater thanthose of fiber A. Fiber D showed two times greater E thanfiber A and a similar σy to that of fiber A. Therefore,
although the fibers were spun from the same polymer, theyexhibited significantly different σ-ε curves at the same
temperature and strain rate.
These differences in the σ–ε curves can be associatedwith different layer structures. Tilted layers make the fiber
ductile as in fiber B. Divided layers increase E and σy, as
shown for fiber C. This mechanical strengthening is
associated with smaller layer dimensions. Annealing fiber
C in the LC state recovered layer dimensions to becomparable to those in fiber A and decreased σy, as
measured for fiber D. Consequently, the glassy smectic LC
fiber, which is fragile in tensile deformation along the layer
stacking direction, is effectively strengthened by dividing
the layers and made more ductile by tilting the layers.

Fig. 3. σ–ε curves measured for fibers A, B, C, and D at
25 °C at a strain rate of 5% min−1. The part at smaller ε isenlarged and shown in the inset.
3.3. Deformation of Layers in Glassy Smectic LC Fibers
The smectic layer deformation and stress response of thefibers were examined by measuring their synchrotronradiation XRD patterns simultaneously with σ–ε curves.
The patterns demonstrate that the predeformations ofsmectic layers affected the structural deformation as wellas the stress response.
The tensile deformation of fiber A increased the layer
spacing with increasing σ. The XRD patterns showed thatthe fiber elongation shifted the layer reflection toward
smaller diffraction angles as ε increased (Fig. 4a–d). Whenthe fiber broke, the reflection recovered its position (Fig.
4e). Thus, the increase in the layer spacing was
proportional to σ (Fig. 4f), indicating that the layer dilation
stored the mechanical energy of the tensile deformation.

Fig. 4. (a–e) XRD patterns measured for fiber A elongatedat 25 °C at the strains indicated below each panel. The fiber
axis is vertical. The horizontal dashed line is shown tocompare the position of the second-order layer reflection
with that of the undeformed fiber. (f) Smectic layer
spacings determined from the XRD patterns (black circles)
and the measured strain (red curve), plotted against thefiber strain.
Fiber B comprised layers tilted at a larger angle withincreasing ε (Fig. 5a–e). This fiber displayed layer
reflection with four intensity maxima at the positions awayfrom the meridional line. As ε increased, these maxima
shifted away from the meridional line toward larger
diffraction angles, decreasing the d-spacing (circularmarks in Fig. 5f). This layer deformation maintained thepolymer chain axis direction at 30° from the fiber axis, asPhoton Factory Activity Report 2022 #40 (2023)

confirmed by Raman spectroscopy. The layer tilting,which decreased the d-spacing, is associated with sliding
polymer chains along the chain axis. This polymer chain
sliding disrupts the smectic layers composed of biphenylmoieties connected via aliphatic spacers; as a result, thelayer reflection disappears. In addition, chain sliding isassociated with the stress plateau at ε > 50%, making thefiber ductile.
Besides, fiber B displayed a layer reflection intensity
on the meridional line. This meridional reflection
increased the d-spacing with increasing ε up to 50%(triangle marks in Fig. 5f). The layer spacing became
constant as the increase in σ decreased at ε > 50%.

Fig. 5. (a-e) XRD patterns measured for fiber B stretchedat 25 °C at the strains indicated below each panel. The fiber
axis is vertical. (f) Smectic layer spacings determined fromthe XRD patterns (black marks) and the measured stress(red curve), plotted against the fiber strain. Two spacings
were determined from the reflections on the meridionalline (triangles) and from the off-meridional line intensity
maximum (circles) on the patterns.
In case of fiber C, the spacing of short layers
increased with the increasing ε. The XRD patterns include
the layer reflection elongated in the horizontal direction(Fig. 6a–e). The diffraction angle of the layer reflection
decreased as σ increased, demonstrating that tensile
deformation influenced the layer spacing. The increase inlayer spacing is comparable to that observed for theelongated fiber A (Fig. 6f). The layer spacing at ε = 20%
measured for fiber C was 1.77 nm, similar to that obtainedfor fiber A (1.75 nm). Further, at the same ε, fiber C had σ= 68 MPa, 2.3 times greater than that obtained for fiber A(30 MPa).
This greater σ for fiber C is ascribed to the progressof layer division. As ε increased, fiber C displayed wider
layer reflection along the horizontal direction, indicatingthat fiber C, with divided smectic layers at the unloaded
state, further divided the layers upon tensile deformation.
The reflection intensities along the horizontal line wereevaluated, and the half width at half maximum (HWHM)
values are plotted against ε (Fig. 6g). The HWHM
increased with increasing σ. The HWHM increased more
steeply at ε > 20%, which seems to correspond to thesteeper increase in σ at ε > 10%. The division of smecticlayers can dissipate the mechanical energy applied bytensile deformation, allowing fiber C to display greater Eand σy than fiber A when considering the same layer
dilation.

Fig. 6. (a–e) XRD patterns measured for fiber C elongatedat 25 °C at the strains indicated below each panel. The fiber
axis is vertical. (f) Smectic layer spacings obtained fromthe XRD patterns and (g) half width at half maximum(HWHM) values of the smectic layer reflection intensities
measured from the XRD patterns along the horizontal
direction plotted against the fiber strain. In (f) and (g), thestress values simultaneously measured with the XRD
patterns are also shown (red curve).
Fiber D had long layers stacked along the fiber axis
and displayed an XRD pattern similar to that measured forfiber A (compare Fig. 7a with Fig. 4a). The tensile
deformation of the fiber with ε up to 4.5% shifted the layer
reflection at a smaller angle (Fig. 7b–d). Then, at ε = 10%,which was over the yielding point, the layer reflection
appeared at a greater diffraction angle than that measured
at ε = 4.5% (Fig. 7e). Thus, the increase in the layer spacing
was proportional to σ, as for fibers A and C.

Fig. 7. (a–e) XRD patterns measured for fiber D elongatedat 25 °C at the strains indicated below each panel. The fiber
axis is vertical. The horizontal dashed line is shown tocompare the position of the second-order layer reflection
with that of the undeformed fiber. (f) Smectic layer
spacings determined from the XRD patterns (black circles)
and the measured strain (red curve), plotted against thefiber strain.
Glassy smectic LC fibers with the layer stacked along
the fiber axis improve the ductility and toughness by tilting
and dividing the layers, respectively. The high σy value offiber C is certainly due to the divided layers. Fiber D,prepared by annealing fiber C in the LC state, recovered
long layers and decreased σy to 25 MPa, comparable to thatmeasured for fiber A. Fiber C under tensile deformation
increased the smectic layer spacing as much as in fiber A,
whereas the layers were more divided to dissipate
mechanical energy, allowing the fiber to bore moresignificant stress than fiber A.

Photon Factory Activity Report 2022 #40 (2023)

Acknowledgement
4,4′-dimethyl biphenyldicarboxylate was kindly supplied
by Ihara-Nikkei Chemical Industry Co. Ltd. This study
was supported by JSPS KAKENHI for Scientific Researchon Innovative Areas "MFS Materials Science (Grant
Number JP21H00095)".
References[1]
M. Krumova, S. Henning, and G.H. Michler,
Philos. Mag., 86, 1689–1712 (2006).
[2]
Y. Cohen, R.J. Albalak, B.J. Dair, M.S. Capel,and E.L. Thomas, Macromolecules, 33, 6502–6516 (2000).
[3]
Y. Cohen, M. Brinkmann, and E.L. Thomas, J.
Chem. Phys., 114, 984–992 (2001).
[4]
K. Kawahara, R. Suzuki, and M. Tokita,Macromol. Chem. Phys., 224, 2300032 (2023).
[5]
K. Osada, M. Koike, H. Tagawa, M. Tokita, andJ. Watanabe, Macromol. Chem. Phys., 205, 1051–1057 (2004).
[6]
M. Tokita, K. Tokunaga, S.I. Funaoka, K. Osada,and J. Watanabe, Macromolecules, 37, 2527–2531 (2004).
[7]
M. Tokita and J. Watanabe, Polym. J., 38, 611–638 (2006).
[8]
S. Yagi, M. Oguro, and M. Tokita, Macromol.
Chem. Phys., 223, 2100399 (2022).
[9]
T. Murayama, E. Abe, and H. Saito, Polymer,236, 124343 (2021).
[10]
K. Hagihara, Z. Li, M. Yamasaki, and Y.
Kawamura, Acta Mater., 163, 226–239 (2019).
[11]
D. Matsui, K. Morita, D. Terada, K.I. Ikeda, andS. Miura, Mater. Trans., 63, 1055–1064 (2022).
[12]
N. Shimizu, T. Mori, N. Igarashi, H. Ohta, Y.
Nagatani, T. Kosuge, and K. Ito, J. Phys. Conf.
Ser., 425, 202008 (2013).
[13]
J. Kuribayashi, R. Ishige, M. Hayashi, and M.
Tokita, Macromol. Chem. Phys., 221, 2000042(2020).
[14]
S. Sakurai, S. Aida, S. Okamoto, K. Sakurai, and
S. Nomura, Macromolecules, 36, 1930–1939(2003).
[15]
N. Kasai and M. Kakudo, “X-Ray Diffraction byMacromolecules”, Springer Berlin Heidelberg,Berlin, Heidelberg (2005).
*tokita.m.aa@m.titech.ac.jp