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Structure-regulated tough elastomers of liquid
crystalline inorganic nanosheet/polyurethane
nanocomposites†
Toki Morooka,a Yutaka Ohsedo,*b Riki Katoc and Nobuyoshi Miyamoto*aLiquid crystalline nanosheet/polyurethane composite elastomers with superior mechanical properties
were successfully synthesized using the newly developed in situ polymerization technique. A layered
clay mineral fluorohectorite was fully exfoliated into single-layer nanosheets and formed a nematic
liquid crystalline (LC) state in the N,N 0 -dimethylformamide/water mixture added with the prepolymer,
isocyanate-terminated low-molecular-weight poly(ethylene oxide). The composite elastomer fibers wereobtained by injecting this LC mixture into a solution of a four-functional crosslinker molecule,triethylenetetramine, followed by solvent removal. In the composite fibers, the single-layer LC
nanosheets were macroscopically aligned along the long axis of the fibers as revealed by polarizing
Received 7th October 2020,
Accepted 26th December 2020

microscope and small angle X-ray scattering. Tensile tests showed that the composite fibers have

DOI: 10.1039/d0ma00768d

polyurethane fibers without the nanosheets. The outstanding improvements were due to the physicalrsc.li/materials-advancescrosslinks at multiple points between polymer chains and the single-layer nanosheets that aremacroscopically ordered.

superior mechanical strength while maintaining a large breaking strain compared to the pristine

Introduction
Fabrication of nanocomposites with polymers and inorganic
nanoparticles is quite an eﬀective way to achieve extraordinary
functions and properties of materials as exemplified by natural
nacre, which is a CaCO3/polypeptide nanocomposite exhibiting
toughness and light weight. Since the pioneering work on
nylon-6/clay nanocomposites,4–6,8,9 inorganic nanosheets or
stacked nanosheets obtained from layered crystals have beenutilized as excellent nanofillers for nanocomposites due to theirlarge specific surface area, large aspect ratio, low cost, andavailability of a variety of materials with desired electric andoptical properties.10,11 Their mechanical strength,7,9,12 gas barrier properties,5,13,14 thermal stability,6,7,15 and thermal
conductivity16–20 are improved and their electrical and opticala

Department of Life, Environment and Applied Chemistry, the Graduate School ofEngineering, Fukuoka Institute of Technology, 3-30-1, Wajirohigashi, Higashiku,
Fukuoka 811-0295, Japan. E-mail: miyamoto@fit.ac.jpbDepartment of Computer Science and Clothing Environment, Faculty of Human
Life and Environment, Nara Women’s University, Kitauoyahigashi-machi,
Nara 630-8506, Japan. E-mail: ohsedo@cc.nara-wu.ac.jpcDepartment of Material Science and Production Engineering, the Graduate Schoolof Engineering, Fukuoka Institute of Technology, 3-30-1, Wajirohigashi, Higashiku,
Fukuoka 811-0295, Japan† Electronic supplementary information (ESI) available: Supplementary figures.
See DOI: 10.1039/d0ma00768d

2021 The Author(s). Published by the Royal Society of Chemistry

properties are also tunable when even a small amount of alayered crystal is composited with a polymer material.
However, when a layered crystal is not fully exfoliatedinto single layers and is not dispersed homogeneously incomposites, improvements of the materials’ properties arelimited; maximized properties along a certain direction andother smart properties are expected when the orientation andspatial distribution of the nanosheets are precisely controlled.
Nevertheless, fabrication of such ideal nanocomposites is still
very challenging.
In the most conventional synthesis of nanosheet/polymer
nanocomposites,21 layered clay mineral powders intercalatedwith alkylammonium ions, organo-clays, are kneaded in molten
polymers. Polymerization of monomers added with organo-clay
crystals (in situ intercalative polymerization) is another general
method. In these classical methods, however, the layered crystal
is usually not fully exfoliated and the control of orientation andspatial distribution of nanosheets is not possible. Layer-by-layer
deposition of thin composite films22 is a sophisticated techniqueto fabricate precisely designed heterostructures; however,
only ionic polymers can be used and large scale synthesis
is not possible. Doctor-blading,23 spray coating,24 vacuum
filtration,25,26 or simply casting27 of polymer-coated nanosheetsor polymer/nanosheet solution on a substrate and formation ofregulated structures upon solvent evaporation have been also
recently reported. In these methods, however, the structure

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formation depends on the solvent evaporation process so thatprecise structural control is not straightforward; kinetic control
in the complicated situation is necessary in which the intrinsic
phase separation of the polymer and nanosheet occurs due tothe depletion eﬀect,28 while partial isotropic–nematic transitionis driven by the excluded volume effect29 and the viscosity
increases.
Considering the above-mentioned situation, for the fabricationof ideal bulk nanocomposite materials with highly regulated
structures, splitting of the synthesis process into (1) superstructure formation process of the fillers and (2) fixation processes byin situ polymerization, starting from nanosheet/prepolymer mixture, should be an eﬀective strategy. In line with this strategy, we
have been highlighting the structured liquid crystalline colloidal
nanosheets since early 2000s.30 Due to the anisotropic shapes ofnanosheets, the free rotational motion is restricted and an LC
phase is formed when the nanosheet concentration is higher as
explained by Onsager’s theory.29 By tuning this entropic interaction as well as attractive and repulsive interactions between
charged nanosheets, orientational30 and positional31 orderingof nanosheets is precisely controllable on mesoscopic scales in
the colloidal state. Furthermore, electric,32 magnetic,33 andshear30 fields are eﬀective to achieve a macroscopic alignmentof these mesophases, leading to hierarchical structural control.
After a regulated hierarchical superstructure of single-layer
nanosheets is obtained, a monomer dissolved in the LC colloid
is photo-polymerized to finally obtain a structured nanocomposite gel.34–36 In these gels, not only a great improvement in
the mechanical strength but also anisotropic optical and molecular transport properties were demonstrated. However, it has
been difficult to apply this synthetic technique to industrially
important plastic or elastomer materials mainly composed ofhydrophobic polymers because inorganic nanosheets are generally hydrophilic and not dispersed in organic solvents.7,37
In this study, we demonstrate a novel synthetic method to
obtain LC single layer nanosheet/polyurethane composite elastomers
in which exfoliated nanosheets are embedded, retaining theregulated superstructure of the original LC colloid. The new
synthetic method (Fig. 1a) makes use of an isocyanateterminated low-molecular-weight prepolymer (Fig. 1b) which issoluble in a water/polar organic solvent mixture that is also
compatible with the LC nanosheets. The composite elastomer
fiber was successfully obtained by extruding the nanosheet/
prepolymer mixture into a four-functional crosslinker solution(Fig. 1c and Fig. S1, ESI†). Although, in general, an isocyanate
moiety is hydrolyzed easily, it is more reactive with amino groupsof the crosslinker triethylenetetramine than water so that thecrosslinking reaction predominantly proceeds.38 Due to theshear stress in the extrusion process, the macroscopic orientation of the nanosheets was strongly induced, giving rise tomechanical properties. The present process is also characterized
as a new-type of a wet-spinning system where a chemicallycrosslinked network polymer is obtained in situ in contrast to
the conventional system where polymers or nanosheets are
physically crosslinked by coaggregation with Ca2+ 39–41 or solidified by precipitation of a polymer in a poor solvent.

1036 | Mater. Adv., 2021, 2, 10351042

Materials Advances

Experimental
Preparation of the nanosheet colloid
The aqueous sol of the synthetic clay mineral fluorohectorite(NHT-B2 SOL) was supplied by Topy Industries. The as-received
aqueous sol was centrifuged at 15 000 rpm for 1 h to remove
impurities and diluted to 1 wt%, according to the previous
report.42 Counter cations of the nanosheets were exchanged from
Na+ to NH4+ by passing the nanosheet colloid through the column
filled with ion exchange resin (Amberlite IR120B, Organo) thatwas pre-treated with 1 M NH4Cl aq. This aqueous colloid wasadded with four times volume of N,N-dimethylformamide (DMF,
Tokyo Chemical Industry [TCI]) and centrifuged at 15 000 rpm for
1 h, followed by removing the supernatant, to finally obtain aconcentrated NH4-fluorohectorite nanosheet/water/DMF colloid.
During the exfoliation process, mechanical treatment such as
ultrasonication was not necessary. Diﬀerent from van der Waals
type layered crystals like BN and MoS2,43,44 it is known that ionictype layered crystals such as Na-fluorohectorite are spontaneously
exfoliated into single layers in water under mild conditions without hard mechanical treatment because of osmotic swelling.42
This was also the case for the present case: NH4-exchanged
fluorohectorite in a DMF/water mixture.
Preparation of the nanosheet/polyurethane composite fiber
Polyurethane composite fibers were synthesized by the following
method. First, 1.90 g (1.90 mmol) of poly(ethylene glycol) (PEG,
Mw: 1000, Sigma-Aldrich) was placed in a N2-purged flask anddissolved in 4 mL of super dehydrated DMF. Then, 0.784 g(4.00 mmol) of 1,3-bis(isocyanatomethyl)cyclohexane (TCI) and
0.027 g (0.043 mmol) of dibutyltin dilaurate (TCI) were added andstirred at 60 1C for 2 h to obtain the solution of the prepolymer
which have isocyanate groups at both terminals of the PEG(Fig. 1b). This prepolymer solution (1.00 g) was added to thenanosheet colloid (0.0941, 0.491, or 1.04 g) and extruded into a
DMF solution (3.54 M) of the cross-linking agent triethylenetetramine (TCI) by using a syringe pump at a flow rate of 8 mL min1(Fig. S1, ESI†). At this time, the beaker containing the crosslinking
agent solution was rotated by a turntable at 70 rpm. The crosslinking reaction spontaneously proceeded (Fig. 1c and Fig. S1,
ESI†) upon the extrusion and the swollen cross-linked nanosheet/polyurethane composite fiber was obtained. The fiber was stored
in the crosslinker solution for 1 day to allow completion of thecrosslinking reaction. After the reaction, the composite fiber wasstored in DMF for 1 day to remove the unreacted prepolymer andcross-linking agent and was dried in a vacuum oven at 70 1C for 2
hours to finally obtain nanosheet/polyurethane elastomer composite fiber samples.
Characterization
For atomic force microscopy (AFM; AFM5000II, HITACHI)
observation of the nanosheets, the nanosheet colloid diluted
to 0.001 wt% was added dropwise and dried on a mica
substrate. A polarized optical microscope (POM; OLYMPUS
BX51) with crossed polarizers and a wave plate (530 nm) wasused to observe the nanosheet colloid and the prepolymer

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Materials AdvancesFig. 1 (a) Conceptual scheme of the present study and chemical reaction formula for the synthesis of (b) the prepolymer and (c) the crosslinked
polyurethane.

solution enclosed in a cell having a thickness of 1 mm. The
composite fibers were also observed using the same device.
Small-angle X-ray scattering (SAXS) was performed using a
Rigaku NANOPIX equipped with a CCD-type two-dimensional
detector. For the SAXS observation, the nanosheet colloid andthe prepolymer solution were enclosed in a glass capillary having
a thickness of 0.01 mm and an optical path length of 2 mm,
while the composite fiber was measured without a capillary.
Field emission scanning electron microscopy (SEM) observationand energy dispersive X-ray analysis (EDS) were performed on a
JEOL JSM-7100F with acceleration voltages of 1 and 10 kV,

2021 The Author(s). Published by the Royal Society of Chemistry

respectively. The thermogravimetric-diﬀerential thermal analysis(TG-DTA) of the composite fiber was carried out with a Rigaku
TG8120 with a temperature increase rate of 2 1C min1. The
tensile test of the composite polymer was measured with a
Shimadzu EZ-L at a tensile speed of 100 mm min1.

Results and discussion
The nanosheets were completely exfoliated in a DMF/water
mixture (8 : 2) and an LC phase was formed even in the presence

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Paperof the prepolymer. In the AFM image (Fig. S2, ESI†), many
nanosheets with a thickness of 1 nm are observed and nostacked nanosheets or aggregates are found, confirming thecomplete exfoliation into single layers. Based on the AFM
images, the average particle size of the nanosheets was evaluatedto be around 2.5 mm. In the POM observation (Fig. S3a, ESI†),aggregated particles were not observed, while permanent birefringence and optical textures were observed, indicating that thenanosheets form an LC phase that occurs only when the degreeof exfoliation is very high. Even after the addition of the
prepolymer, good dispersion and the liquid crystal phase of
the nanosheets were maintained (Fig. S3b, ESI†). In contrast, nobirefringence was observed in the prepolymer solution without
nanosheets (Fig. S3c, ESI†).
As shown in Fig. 2A, long fibers of the composite gel andcomposite elastomer were obtained continuously. The synthesized

Materials Advances
composite gel fiber (left image of Fig. 2A) was transparent andtough enough to be held with a tweezer, while it turned white
and turbid after the evaporation of the solvent (right side imageof Fig. 2A). The change in the sample appearance by solvent
evaporation is explained by the formation of macroscopic
wrinkles on the fiber surface as well as the formation of submicrometer scale heterogeneous domains of polyurethane/
fluorohectorite intercalation compounds as described later.
In the POM images of the composite fibers (right side images inFig. 2B), blue and yellow interference colors were observed when
the long axis direction of the fiber is parallel and perpendicular
to the wave plate, respectively. No aggregated particles wereobserved. This indicates that the nanosheets maintained awell-dispersed state and the superstructures of the original
LC phase without aggregation even after being embedded inpolyurethane and that the nanosheets are oriented along the

Fig. 2 The structural characterization of the (a) pristine polyurethane fiber without nanosheets and nanosheet/polyurethane nanocomposite fibers
loaded with (b) 1, (c) 5, and (d) 10 wt% of the nanosheet. (A) Shows the photographs of the composite gel fiber before (left) and after (right) drying.
(B) shows the photographs and POM images of the dried samples. (C) shows the SAXS patterns (I–q profiles on the left and 2D patterns on the right) of the(a) pristine polyurethane, (b)–(d) nanocomposites and (e) the nanosheet (2.2 wt%)/water/DMF colloidal sol; the profiles (b)–(d) were obtained after
subtraction of the pristine polyurethane scattering (a) as the background. (D) and (E) are the schematic structures of the nanosheet/polyurethane
nanocomposite fibers containing smaller and larger amount of the nanosheets, respectively.

1038 | Mater. Adv., 2021, 2, 10351042

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Materials Advanceslong axis of the fiber. To further characterize the structure, thecross-section of the fibers was observed using a POM (leftimages of Fig. 2B). According to this observation, thenanosheets are oriented along the outer surface of the fiber.
In the sample with 10 wt% of nanosheets, several domains with
diﬀerent orientational directions were formed inside the fiber.
SEM observations of the composite elastomer fiber further
confirm that the nanosheets are well-dispersed in the polymer
matrix without aggregation with the alignment flat along thefiber surface. In the cross-sectional SEM image (Fig. S4a–c,
ESI†), rough surfaces with thin streaks of several mm length areobserved. The corresponding EDS mapping image (Fig. S4e andf, ESI†) shows that Si due to nanosheets and C due to thepolymer are uniformly distributed without any distinct texture,
confirming that the nanosheets are well-dispersed in the polymer matrix without aggregation. The side surface of the composite fiber (Fig. S4d, ESI†) is covered with a platy object with asize of several mm. In contrast, in the case of elastomers without
the nanosheets (Fig. S5, ESI†), flatter cross-sections and sidesurfaces are observed.
SAXS results (Fig. 2C) also revealed that the nanosheets
maintained ideal dispersion and orientation in the obtained
composite fiber, whereas they partly formed intercalation compounds with the polymer chains when the nanosheet concentration is high. In the nanosheet/polyurethane composites(Fig. 2C(b–d)) as well as LC colloids (Fig. 2C(e)), an anisotropic
2D SAXS pattern was observed and the anisotropy increased withincreasing nanosheet concentration in contrast to the pristinepolyurethane fiber without the nanosheets (Fig. 2C(a)) that show
isotropic scattering. Note that the scattering due to nanosheets ismuch stronger than that due to polymers so that the scattering
pattern of the composites gives information about the nanosheet
superstructure but not about the polymer structure. These
results suggest that the degree of orientation of the nanosheets
increases as the concentration increases. In the I(q) vs. q scattering profiles, broad peaks were found in the wide-angle side
for the samples with 5 wt% and 10 wt% of nanosheets(Fig. 3d and e). From the peak position (q = 4.6 nm1), thed value is calculated to be 1.35 nm according to the relationshipof d = 2p/q. This value is slightly larger than the reported value(1.23 nm) for Na-fluorohectorite.45 The gallery height between
the nanosheet is calculated to be 0.37 nm by subtracting the
thickness of the nanosheet (0.98 nm). Since the thickness ofthe polyurethane chain is estimated to be 0.2–0.3 nm, it isconsidered that the polyurethane chains are intercalated
between the nanosheets. Furthermore, the slope of the profiles(Fig. 2C(b–e)) in the range of 0.1 o q o 1 was 2, which is
ascribed to the form factor of the exfoliated nanosheets. Thus,
most of the nanosheets retain a good dispersed state without
aggregation even when compounded in the polyurethane.
Considering the above-mentioned results, the schematic
structures are deduced. The nanosheets in nematic order witha weak positional order are macroscopically aligned along thelong axis of the fiber and maintain a fully exfoliated state.
While they are mostly dispersed as single layers at low concentrations (Fig. 2D), part of the nanosheets form layered2021 The Author(s). Published by the Royal Society of ChemistryPapercompounds intercalating polyurethane chains at higher
nanosheet concentrations (Fig. 2E).
The composite elastomer showed improved thermal stability
as characterized by TG-DTA (Fig. 3A and B). The pristinepolyurethane without nanosheets showed two-step weightloss characterized by the two DTG peaks at around 300 1C
and 460 1C accompanied by exothermal DTA peaks. In thepresent system, that is a network polymer crosslinked with
an n-functional crosslinker, one PEG chain ideally has two
terminal groups and 2/n crosslinker molecules. Thus, theweight ratio of the PEG chain to the total polymer weight, wPEG,
is calculated as wPEG = MPEG/[MPEG + 2Mter + (2/n)Mcro], where
MPEG, Mter and Mcr are the molecular weights of the PEG chain(1000), terminal group (196), and crosslinker (146), respectively.
With n = 4, wPEG is calculated as 68% and this value roughly
corresponds to the weight loss at the first DTG peak (82%).
Thus, DTG peaks at 300 1C and 460 1C are ascribed to the
combustion of PEG and other part of the polymer network,
respectively. In the nanosheet/polyurethane nanocomposites,a new DTG peak appeared at the intermediate temperature(360–390 1C), replacing the original peak at 300 1C. This newpeak can be ascribed to the combustion of the PEG moiety
interacting with the nanosheets because the DTA peak as wellas the DTG peak shifted to higher temperatures as thenanosheet concentration increased. Thus, it is probable thatthe presence of the nanosheets in the polyurethane matrix iseffective to increase the combustion temperature due to heatabsorption by the nanosheets as well as lowering of polymer
chain motion through nanosheet–polymer interactions. Meanwhile, the residue weight at 900 1C corresponds to the weight ofnanosheets in the present composite elastomer. This value
increased with the increase of the loading amount of thenanosheet: 0.0, 0.2, 2.7, and 7.1 wt% for the samples loadedwith 0, 1, 5 and 10 wt%. The slight difference between themeasured value and the loaded amount is in the range ofmeasurement error (3 wt%), while it is also possible that apart of the loaded nanosheet escaped into the solution phaseduring the crosslinking process.
Due to the regulated structure and good dispersion of thenanosheets, the present nanocomposite elastomers showed
outstanding mechanical properties as revealed by tensile tests(Fig. 3D). Pristine polyurethane fiber showed an elastic modulus of 0.0053 MPa and a stress–strain (S–S) curve typical of
rubber materials,46 while it was broken at a stress of 1.6 MPa
and a strain of 542%. With 1 wt% of the nanosheets, the elastic
modulus, breaking stress, and breaking strain increased to
0.018 MPa, 4.1 MPa, and 519%, respectively. As the nanosheet
concentration increased to 5 wt%, the elastic modulus
increased to 0.14 MPa, while the breaking stress was 9.3 MPaand the breaking strain was almost unchanged (548%). With
10 wt% of the nanosheets, the S–S curve shows a yielding point
at a strain of 8.4%, followed by stress relaxation. The elastic
modulus and breaking stress were largely increased to
0.69 MPa and 5.2 MPa, respectively, while the breaking
strain remained similar to other cases (520%). Thus, it isconfirmed that the elastic modulus increases as the nanosheet

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Fig. 3 (A) TG, (B) DTG, (C) DTA, and (D) tensile stress–strain curves of the polyurethane nanocomposites loaded with (a) 0, (b) 1, (c) 5, and (d) 10 wt% ofnanosheets. (E) Shows the comparison of the mechanical reinforcement factors of the present system (red filled square) with the clay/polymer
composite elastomers reported in the literatures1–7 (open circles). The schematic representations of the reinforcement mechanism by (F) weak and (G)strong physical crosslinks in the nanosheet/polyurethane fiber are also shown.

concentration increases, while the breaking stress showed the
maximum at a nanosheet concentration of 5 wt%.
Compared with previously reported clay/polymer nanocomposite materials, the above-mentioned mechanical reinforcement in the present system is outstanding. In the present
system, the increase of the breaking stress and elastic modulus
reach up to 5.9- and 26.8-fold, respectively. In addition, thismechanical improvement was achieved without a decrease of
breaking strain. In contrast, in usual polymer/clay composites,
the elastic modulus increases with the increase of clay loadingwhereas the breaking strain and breaking stress significantly
decrease. This means that the material becomes harder but
more brittle.11 In some good cases of nanocomposite

1040 | Mater. Adv., 2021, 2, 10351042

elastomers,1,4–7,47,48 the breaking strain only slightly decreases
or even increases; however, as shown in Fig. 3E, the factor ofincrease of breaking stress is only around 2 or lower, which ismuch smaller than in the present system.
The mechanical reinforcement in the present nanocomposite elastomer is explained by weaker and stronger
physical crosslinks between the polyurethane chains andthe nanosheets. Due to the lack of chemically modifiablegroup such as –OH on the surface of the clay mineralnanosheets, the formation of covalent chemical bonds is notplausible. Because the reinforcement is by physical crosslinks,
not by covalent bonds, the crosslinks can slide and form
again and again, avoiding reduction of breaking strain. The

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weaker crosslink is due to the hydrogen bonds between
negatively charged nanosheets and polar polyurethane
chains49 (Fig. 3F). Although the interaction is weak, verylarge surface area of the single layer nanosheets and theinteractions at multiple points on the polymer chain andnanosheet surface make the physical crosslinks very effective.
The mechanical strength improvement at lower nanosheet
loading is mainly explained by this model. The stronger
one is due to the formation of an intercalation compound inwhich polyurethane chains are sandwiched between
nanosheets (Fig. 3G). It is considered that the steep increaseof the tensile stress in the low strain region (Fig. 3D(d)) of thesample with 10 wt% nanosheets is due to the breaking of thisstronger physical crosslink. After these strong physical crosslinks are mostly broken at the yielding point, the reinforcementby the weaker physical crosslink only works. Furthermore, thereinforcement by these physical crosslinks is maximized by themacroscopic ordering of the nanosheets. If the nanosheet plane
were perpendicular to the stretching direction, that is thelong axis of the fiber, the polymer chains should have been
disturbed by the nanosheets so that the reinforcement effect isreduced.

Conclusions
Single-layer nanosheets that form an LC phase with highly
regulated superstructures were successfully composited witha polyurethane elastomer by using the newly developedin situ polymerization technique for the first time. The LC
nanosheet/polyurethane composite fibers showed superior
thermal and mechanical properties due to good exfoliationand the regulated superstructure of the nanosheets originatedform the LC phase. Since the present nanocomposite elastomer
is synthesized by simple contact of the two solutions, production of thin films or small spherical particles as well as
fibers would be possible. Due to the superior properties ofthe elastomer, a wide range of industrial applications inmedical materials, smart fabrics, flexible electronics, anddielectric elastomer-based power generation devices are
expected.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This research was supported by the Research Center for
Materials and Energy Devices of Fukuoka Institute of Technology (FIT-ME) (Strategic Research Foundation Grant-Aided
Project for Private University [#S1511036L] from MEXT),
KAKENHI (#24104005 and #15K05657), Network Joint ResearchCenter for Materials and Devices (#20191350), and theElectronics Research Laboratory of Fukuoka Institute ofTechnology.

2021 The Author(s). Published by the Royal Society of Chemistry

Paper

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