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Anisotropic Mechanical Responses of Poly(Ethylene Oxide)‐Based Lithium Ions Containing Solid Polymer Electrolytes

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  • TIFR Center for Interdisciplinary Sciences, TIFR, Hyderbad

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Development of mechanically deformable solid state devices is receiving tremendous attention, and high ionic conductivity solid polymer electrolytes (SPEs) are highly sought after for their development. The process history‐induced polymer alignment anisotropy can lead to anisotropic conductivity to the SPEs. Here, a Li ion SPE membrane developed using poly(ethylene oxide) (PEO) and LiClO4 is demonstrated for its microstructure variations while applying external stress and the corresponding variations in the ionic conductivity are also calculated. The microstructural evolution shows that larger strain values induce large dislocations in the crystallites of PEO leading to the formation of larger amorphous regions which soften the matrix. The anisotropic mechanical responses are observed while applying cyclic strain to thicker SPEs, where the compressive measurements show softening of the matrix while tensile measurements harden the matrix. The ionic conductivities of the softened matrix are found to be enhanced while those of toughened matrix are found to be decreased. This detailed mechanical analysis along with the in situ ionic conductivity studies of PEO‐based Li ion SPE show that along with the thermal history of the polymers, process history and the anisotropic mechanical responses of the polymers also need to be considered while developing SPEs for flexible devices. Tunability in the ionic conductivity of a solid polymer electrolyte made of polyethylene oxide and lithium perchlorate is shown with the application of external stress, and anisotropic ionic conductivity variations are shown with softening and hardening of the polymer matrix. A detailed dynamic mechanical analysis along with ionic conductivity studies give a mechanistic insight in to the observed phenomenon.
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Full PaPer
Anisotropic Mechanical Responses of Poly(Ethylene
Oxide)-Based Lithium Ions Containing Solid Polymer
Electrolytes
Sudeshna Patra,* Munaiah Yeddala, Piyush Daga, and Tharangattu N. Narayanan*
S. Patra, Dr. M. Yeddala, P. Daga, Dr. T. N. Narayanan
Tata Institute of Fundamental Research – Hyderabad
Sy. No. 36/P Serilingampally Mandal, Gopanapally Village, Hyderabad
500 107, India
E-mail: tnn@tifrh.res.in; patrasudeshna@gmail.com
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/macp.201900348.
DOI: 10.1002/macp.201900348
2D vermiculite sheet has been found to
enhance the ionic conductivity as well as
mechanical strength along the alignment
direction.[4,5] Further, many such SPEs
retain their thermal history of processing
in their properties such as in ionic con-
ductivity.[6] Apart from the development
of safer batteries, stretchable and foldable
storage devices are receiving tremendous
attention and SPEs are highly sought after
for these applications.[7] Hence, apart from
the thermal history, mechanical or stress/
strain history of SPEs in their ionic con-
ductivity is also an important parameter
in developing solid state devices. Recently,
a few such studies on the dynamic strain-
induced micro-structural transformations
in SPEs are conducted and these studies
show ionic conductivity variations due
to macroscopic external stress.[8–10] But,
a detailed understanding on this stress
induced micro-structural variations and
the effect of microscopic polymer align-
ment in ionic conductivity variations are
not attained so far.
Recently, the authors have reported
an SPE made of polyethylene oxide (PEO) and lithium salt
(LiClO4), and it is shown that the crystalline to amorphous con-
tent can be varied with Li+ content.[11] It is shown by the dielec-
tric spectroscopy that interface between crystalline and amor-
phous phases exist in such a semi-crystalline system, giving
large interfacial polarization. Further, the segmental motion of
the PEO can be tuned by this external salting. It is also known
from the previous studies that molecular orientation of polymer
chains in a cast SPE film will be along the plane of the film.[12]
Mechanical properties of polymers are potentially dependent
on their underlying microstructures such as crystalline mor-
phology and the degree of molecular orientation.[13] Due to
their processing history, anisotropic distribution of molecular
orientation in the solid polymer can cause direction-dependent
mechanical properties.[14,15] Further, the deformation history
during polymer processing is dominated by chain mobility
as well as their alignment.[16] In some crystalline polymers,
owing to their chain like structure, constituent molecules tend
to become oriented in the direction of deformation and ulti-
mately this state of favorable orientation gets preserved in the
final product upon rapid cooling of the material. This preferred
molecular orientation spurs the phenomena of mechanical
Development of mechanically deformable solid state devices is receiving
tremendous attention, and high ionic conductivity solid polymer electrolytes
(SPEs) are highly sought after for their development. The process history-
induced polymer alignment anisotropy can lead to anisotropic conductivity
to the SPEs. Here, a Li ion SPE membrane developed using poly(ethylene
oxide) (PEO) and LiClO4 is demonstrated for its microstructure variations
while applying external stress and the corresponding variations in the ionic
conductivity are also calculated. The microstructural evolution shows that
larger strain values induce large dislocations in the crystallites of PEO leading
to the formation of larger amorphous regions which soften the matrix. The
anisotropic mechanical responses are observed while applying cyclic strain
to thicker SPEs, where the compressive measurements show softening of the
matrix while tensile measurements harden the matrix. The ionic conductivi-
ties of the softened matrix are found to be enhanced while those of tough-
ened matrix are found to be decreased. This detailed mechanical analysis
along with the in situ ionic conductivity studies of PEO-based Li ion SPE
show that along with the thermal history of the polymers, process history and
the anisotropic mechanical responses of the polymers also need to be consid-
ered while developing SPEs for flexible devices.
1. Introduction
Commercialization of lithium ion (Li+) batteries revolutionized
the world of portable electronics and biomedical point of care
devices, and presently, the need of “safer batteries” demands
the development of solid electrolyte (SE) counter parts of com-
mercialized liquid ones.[1,2] Semi-crystalline polymer-based
solid polymer electrolytes (SPEs) are emerging as unique ones
in SEs due to their amenability of changing the crystalline and
amorphous content with external filling factors (fillers) and
temperature.[3] For example, properly aligned ceramic filler like
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anisotropy, which is reported by a few of the researchers in the
past. For example, F. F. Rawson et al. showed the variation of
tensile and compressive yield stresses and Young’s modulus of
oriented poly(vinyl chloride) sheet with direction and degree of
orientation.[17] Furthermore, Tim B. Van Erp and co-workers
have studied the processing-induced crystalline orientation
on the mechanical performance of injection molded polypro-
pylene, where the shearing force induced alignment of poly-
mers was happened.[18] Hence the deformation and process his-
tory of the polymers are very much prevalent and this can affect
the properties of the SPEs anisotropically.
The role of external fillers in polymers can affect the
dynamic mechanical properties of the filled elastomers.[19–21]
It is shown that variety of materials, from rubber materials to
biological tissues, exhibit cyclic stress softening (Mullin’s effect)
and stress hysteresis as a result of a transformation of the hard
phase into the soft phase.[22–25] Contrary to this softening, some
biopolymers and shape memory alloys exhibit cyclic hardening
response after repetitive shearing to a certain strain ampli-
tude.[26,27] Though the exact mechanism of Mullin’s effect and
hardening due to cyclic mechanical deformation can depend
on the microscopic features of the concerned system, it is gen-
erally agreed that polymer–polymer, polymer–filler, and filler–
filler interactions play significant roles in dictating the resultant
behavior of the composite matrix.[28,29]
In PEO-based lithium ion SPEs, Li+ is interacting with the
C = O functional groups in the PEO backbone and their inter-
segmental hopping aids the transport of Li+ through the SPE
matrix.[30] The segmental motion of PEO will be highly facile in
an amorphized matrix, while the melting transition tempera-
ture (T
m) of PEO is approximately in the range of 62–67 °C.[31]
It is shown that the addition of salt can lower the T
m leading
to the formation of large crystalline and amorphous interfaces
within the SPE. Further, it was also shown by other studies
that forced alignment of PEO helices in a direction can cause
enhancement in the ion conductivity in that direction. For
example, a recent study showed the microstructural transfor-
mation along the direction of tensile deformation and the sub-
sequent augmented ion transport in strained direction.[32]
Here we explore the anisotropic mechanical responses of
the PEO-LiClO4-based SPE system, which is found to be unex-
plored, though highly important since PEO-LiClO4 has been
identified as an important SPE. Dynamic mechanical analyzer
(DMA) based studies are conducted, and a cyclic stress hard-
ening in tensile strain and stress softening in compressive
strain are observed. Creep studies are conducted to confirm
these anisotropic mechanical responses from the two different
modes of DMA, and the results show the polymer alignment
induced anisotropy. In situ impedance spectroscopy coupled
with tensile dynamic mechanical analysis is carried out on thin
films of these SPEs, and the conductivity values are also probed
to unravel the anisotropic responses of the SPEs.
Initial part of this manuscript discusses the tensile mechan-
ical properties of the PEO/LiClO4 thin film (LiPEO10_1, the
details of the naming is explained later, thickness range (dif-
ferent samples): 0.10–0.16 mm), where the static mechanical
response of the LiPEO10_1 and the yielding strain are deter-
mined. The microstructure of this SPE is studied using dif-
ferential interference contrast microscopy (DIC) and scanning
electron microscopy (SEM) and the deformation of the micro-
structure with higher strains are also studied along with the
corresponding ionic conductivity values (in-plane conductivity).
Later, thick cylindrical SPEs (LIPEO10, thickness range (dif-
ferent samples): 3–6 mm and its higher Li containing analogue
LiPEO20 (of the same dimension)) are developed and the ani-
sotropic mechanical as well as ionic conductivity studies are
probed.
2. Experimental Section
2.1. SPE Cylindrical Sample (Thicker Membrane) Preparation
0.5 g of poly(ethylene oxide) (PEO) (Mw 600 000, Sigma Aldrich)
and varying concentrations of LiClO4 (ACS reagent, 99.99%)
were dissolved in 3 mL of anhydrous acetonitrile (99.9%, Sigma
Aldrich). Detailed composition of each precursor was listed in
Table 1. After vigorous mixing for about 10 min, the resultant
blend was transferred into a teflon-based round mold to be
materialized into the desirable shape. The mixture was then
dried at 90 °C for another 5 h. The shape (cylindrical:: height:
3 mm- 6 mm, diameter: 12 mm) confinement was ensured
with the teflon container and a vertical compressive stress was
applied prior to the further mechanical studies of the cylin-
drical samples.
2.2. SPE Thin Film (Membrane) Preparation
1 g of PEO and 0.1 g of LiClO4 were dissolved in 15 mL of ace-
tonitrile and blended thoroughly. The mixture was then mag-
netically stirred with simultaneous heating (90 °C) for about
20 min before the solution was cast on a teflon support. The
solvent was allowed to evaporate slowly overnight. The thick-
ness of the as-prepared solvent free membrane was 0.15 mm.
These membranes were properly cut into rectangular strips of
same width (5.30 mm) and thickness (0.15 mm) for impedance
analyses.
2.3. Characterization
DMA (DMA Q800, TA Instruments) was employed for static
mechanical testing of the SPEs. Dimensions of the sample
specimens for specific DMA tests were measured by a standard
Vernier Calipers. The lengths of the samples between the fixed
and the movable portions of the tensile/compression clamps
were determined by the DMA instrument itself. All the clamps
(position and clamp calibrations) were calibrated prior to the
Macromol. Chem. Phys. 2019, 220, 1900348
Table 1. Detailed composition of different SPEs and their naming.
Sample Studied thickness range LiClO4 [g] PEO
LiPEO10 (cylindrical sample) 3–6 mm 0.05 0.5
LiPEO20 (cylindrical sample) 3–6 mm 0.1 0.5
LiPEO10_1 (thin film) 0.10–0.16 mm 0.1 1
LiPEO10_2 (thick film) 0.7–0.9 mm 0.5 5
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analysis. All the DMA measurements were carried out at 30 °C
after giving 10 min for thermal stabilization before each meas-
urement sequence.
The test samples were mounted using film tension and com-
pression clamps with a clamp compliance of about 0.174 and
0.8 µm N1, respectively. Static cyclic tests in both modes were
conducted in strain rate mode with a preload of 0.1 N and the
strain is swept with a strain rate of 0.5% min1 for 25 cycles.
Multicreep studies were executed in the tensile as well as com-
pression mode using two different stress values of 0.005 and
0.01 MPa. The creep strain and the recovery strain were evalu-
ated as a function of time (tcreep = 10 min, trecovery = 20 min).
2.4. In Situ Impedance Spectroscopy Measurements
The rectangular strips of the SPE were mounted in film ten-
sion clamp where its ends were connected to the metallic leads
for impedance studies. Electrically insulating tape was used
to cover the exposed portion of the clamp toward the elec-
trode lead. The Nyquist plots from the impedance analyses of
the films were obtained as the film is stretched starting from
a strain of 0.1% up to 50%. The measurements were repeated
several times to cross-check the obtained data.
The electrochemical impedance spectroscopy (EIS) studies
were conducted using a Biologic SP300 potentiostat, accom-
plished by applying an AC potential over the frequency range
from 100 MHz to 7 MHz.
2.5. Morphology Studies
Morphology of the strained and unstrained SPEs were studied
using DIC and SEM. DIC experiments were carried out using
a confocal laser scanning microscope, FV3000 from Olympus.
The SEM studies were carried out using the JEOL JSM- 7200F
microscope operated at 0.5 kV.
3. Results and Discussions
Details of the SPE preparation for LiPEO10 and LiPEO20 are
summarized below and the nomenclature is also mentioned in
Table 1.
The LiPEO10_1 (thin film) is subjected to the initial studies
since this is one of the most common geometries explored for
SPE based applications. Figure 1 shows the DIC images of the
membranes—as prepared as well as stretched but unrelaxed
membrane (the details of the straining and its implications are
discussed later).
The as prepared films (unstretched) show the spherulitic
structures of having large crystalline (spherulite) domains
(light) of PEO separated by amorphous (dark) regions.
The crystalline phase has self-aligned ordered PEO chains
(cystallites).[33,34] The edges of the crystallites have disordered
conformation and these amorphous regions link one crystallite
to other.[35–37] While applying a constant strain on this film via
simple hand stretching (but strain much below the failure of
the membrane), DIC imaging show an enlarged amorphous
regions as shown in Figure 1D–F. Here it is to be noted that the
applied strain is beyond the “Hookean region” of stress–stain
response but before the necking zone. The detailed stress–
strain curve indicating different regions is shown in Figure
S1, Supporting Information. For a small strain and the corre-
sponding small stress, polymer chains elongate elastically. It
is hypothesized that the amorphous regions grow and the load
carried by the crystallite is transmitted through the amorphous
part while the crystalline part try to orient in the direction of the
Macromol. Chem. Phys. 2019, 220, 1900348
Figure 1. The DIC images of A–C) unstretched (as prepared) and D–F) stretched but unrelaxed LiPEO10_1. (A, D) 10X magnification, (B, E) 20X
magnification, (C, F) 40X magnification.
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applied load.[38] Though the crystallite responds to this elasti-
cally, the amorphous part has both elastic and viscoelastic com-
ponents. Beyond the yielding stress, the oriented crystallites
experience increased dislocation and the growth of amorphous
regions and the necking happens just before the complete dis-
entanglement of the polymer chains.
Figure 2A shows the static stress–strain curve using a
strain controlled DMA measurement. The elastic region of
the LiPEO10_1 is found to be upto 15% strain and then it is
entering in to the plastic region. The stress–strain curve for
the higher strain is shown in the Figure S1, Supporting Infor-
mation. It is clear from the analyses that the yielding strain is
50%. In situ impedance analyses are carried out during the
tensile analyses and the data is shown in Figure S2, Supporting
Information, and the conductivity deduced from this analysis is
in Figure 2B. The Nyquist plots are obtained (Figure S2, Sup-
porting Information) for different cycles and the conductivity
values are calculated considering the dimensions of the mem-
brane (the details of the calculation are given in the Supporting
Information). The in-plane conductivity values before and after
the cyclic strain (with elongation at break before the necking
region) show negligible change in the conductivity (within the
error value, Figure 2B). This is in tune with the micro-structural
observation that low strain did not affect the structure much
(Figures 2C (and 2E) and 2D) while high strain values (from
the starting point of necking upto strain to failure) tear apart
the crystallites forming large amorphous regions (SEM image,
Figure 2F). It is observed that in such large strain values, the
conductivity is found to be enhanced from 0.0083(±0.00061)
mS cm1 to 0.126(±0.0029) mS cm1, indicating the strain
induced micro-structural modification aided conductivity varia-
tions, where the amorphous region is considerably grown con-
tributing to the conductivity. Figures 2C,E are the SEM images
of the LiPEO10_1 showing the large (amount) crystallite parts
with smaller amorphous regions. The application of strain
beyond its yielding point (50%, will be discussed later) shows
the growth of amorphous regions (Figure 2F) as discussed
before.
Further to understand the effect of micro-structural variations
of the SPE in the anisotropic mechanical properties, cyclic com-
pressive and tensile measurements on LiPEO10 and LiPEO20
cylindrical samples are conducted using DMA (schematic of
Macromol. Chem. Phys. 2019, 220, 1900348
Figure 2. A) Stress–strain curve of LiPEO10_1 under DMA strain controlled mode. B) The in-plane ionic conductivity (before and after) of LiPEO10_1
membrane during in situ study for low strain values. C) SEM images of LiPEO10_1, as formed, D) stretched at room temperature within 50% strain.
E) SEM images of another set of LiPEO10_1, as formed F) stretched at room temperature beyond the yield point upto breakage.
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the experimental set up is shown in Figures S3A,B, Supporting
Information). The cylindrical nature of LiPEO10 and LiPEO20
ensures their effectiveness in cyclic compressive experiments
without yielding, unlike in the case of films.
Figure 3A illustrates the cyclic compressive loading behavior
of the LiPEO10. The distinctive features of the compressive
cycling can be generalized as follows:
The stress required to generate a given deformation during
the second loading cycle is smaller than that required to pro-
duce the same deformation during the primary loading cycle.
The phenomenon is conventionally referred as cyclic stress
softening or Mullins’ effect.[39] Further, the cyclic loading and
unloading follow different pathways (hysteresis). The stress
drop in the initial cycles is much higher and in large cycles the
variations are found to be insignificant. Further, the compres-
sive experiments on the same samples after a day also show
similar softening effect with much lower stress variation. The
relaxations of these samples are later studied using creep
analyses.
The stress versus time curves shown in Figure 3B clearly
indicate the stress softening behavior of LiPEO10. It is
important to note that all the cyclic loading experiments are
performed within the linear viscoelastic region at a constant
strain rate of 0.5%/min. The storage modulus was found to
be strain insensitive up to 2% strain. Moreover, it has never
reached at the onset of yielding at a given strain of 2%. Pre-
viously it has also been reported that semi-crystalline PEO
exhibits elastic response within a range of 5% strain at room
temperature.[32]
The tensile measurements on the LiPEO10 are shown in
Figure 3C,D. The tensile strain cycles show stress hardening
phenomena within the same strain range (Figure 3C), unlike
the compressive cycling (Figure 3A). The increase in peak
stress with time (Figure 3D) is in tune with the above obser-
vation. Such softening and hardening behavior of cylindrical
LiPEO samples are confirmed with several fresh samples.
To further understand the effect of this anisotropic mechan-
ical response in the ionic transport of LiPEO10, the ionic con-
ductivity (in-plane and through plane) values are evaluated
before and after the mechanical studies using impedance spec-
troscopy. The details of the calculations are given in the sup-
porting information. Figure 4A depicts the Nyquist plots of the
measurement along compressive direction (through-plane),
where the ionic conductivity is found to be increased from
0.00075(±3.3 × 105) mS cm1 to 0.00569(±6.8 × 104) mS cm1
(Figure 4C). In this experiment, the in-plane conductivities of
the sample before and after compressive cycling are found to
be unchanged. But, in tensile elongation experiments, the ionic
conductivity (in-plane) is found to be reduced from 0.0238
(±0.0029) mS cm1 to 0.0062(±4 × 104) mS cm1 (Figure 4B,D).
The through plane conductivity values before and after these
experiments are also found to be unchanged.
It is to be noted here that after the stress softening (com-
pressive), the through-plane conductivity (conductivity in the
Macromol. Chem. Phys. 2019, 220, 1900348
Figure 3. Cyclic loading characteristics of LiPEO10: A) hysteresis curves for 25 cycle under compression mode (inset showing schematic representa-
tion for cyclic loading). B) Stress–time curve for the same mode. C) Hysteresis curve for 25 cycles under tensile mode and D) the corresponding
stress–time curve.
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direction of compression) is found to be enhanced while after
the stress hardening in the tensile mode, the in-plane conduc-
tivity (conductivity in the direction of tension) is found to be
decreased. The softening refers to the formation of extended
amorphous phase and hardening refers the entanglement of
polymers to become more ordered phase. This indicates that
microscopic polymer alignments play a significant role in dic-
tating the macroscopic conductivity. The SEM images of the
LiPEO10 cylindrical samples before and after tensile meas-
urements are shown in Figure S4A,B, Supporting Informa-
tion, and it is to be noted that unlike in LiPEO membranes
(as shown previously), no drastic variation in the surface
microstructure is happening here (while examined using sec-
ondary electron imaging based SEM analyses). This indicates
the 3D (bulk) modification of the polymer alignments in the
thicker cylindrical samples while doing the cyclic strain meas-
urements. Hence the effect of mechanical deformation in the
alignment of polymer chains in SPE can happen in all the
three directions, though the polymers tend to align in the in-
plane (X-Y) direction for thin films while casted on a surface
(X-Y plane).
Viscoelastic polymeric material shows strain rate dependent
mechanical properties.[40,41] It is found that the LiPEO10 also
shows viscoelastic properties (Figure S5A,B), where the max-
imum stress generated is not only a function of strain but a
function of strain rate also. The maximum stress developed
is increasing with increasing strain amplitude (from 0.01 to
1% min1) for both the compression (Figure S5A, Supporting
Information) and tensile mode (Figure S5B, Supporting
Information).
It has been shown in our previous studies that increase in
the Li+ in PEO matrix can enhance the amorphization of the
PEO matrix.[11] To understand the role of enhanced amorphous
phase in the cyclic strain measurements of PEO, LiPEO20 was
subjected to the compressive and tensile strain cycling studies
(Figure 5). It can be seen that the compressive softening and
tensile hardening follow the similar trend as that of LiPEO10.
Although higher salt concentration makes the matrix more
amorphous and compressible, it has been reported earlier
that the physical characteristics and effects of stress softening
behavior for compressible solid materials parallel with that of
their incompressible counterparts although the softening rate
may vary.[42] This hysteresis loss (Figure 5A) and the evolution
of peak stress (Figure 5B) due to microstructure modification
are also higher in the first cycle for LiPEO20, subsequently it
decreases and becomes steady after some cycles. Similar to
LiPEO10, LiPEO20 also showed stress hardening phenomena
in tensile mode (Figure 5C,D).
To investigate the effect of LiClO4 on the cyclic loading
behavior of the polymer electrolytes, a cylindrically shaped pure
PEO sample (without LiClO4) is also made. It can be readily
seen from the Figure S6, Supporting Information, that no spe-
cific trend has been followed for both compressive (S6A) and
tensile cyclic loadings (S6B), affirming the fact that substantial
amount of defects (amorphous) should be there in the matrix
for observing the mechanical anisotropy as discussed in LiPEO
samples.
In this entire study it is seen that tensile cyclic stretching
causes a softening of the LiPEO10 membrane matrix, whereas
for a thick cylindrical sample, cyclic hardening is seen in the
Macromol. Chem. Phys. 2019, 220, 1900348
Figure 4. A,B) Nyquist plots for LiPEO10 as a result of compressive and tensile hysteresis, respectively. C) Comparison of conductivity between before
and after compressive hysteresis. D) Comparison of conductivity between before and after tensile hysteresis. Error bars represent standard deviations
for the repeated measurements.
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Macromol. Chem. Phys. 2019, 220, 1900348
tensile deformation direction. To get more insight in to this, the
cyclic loading experiment is performed in the tensile stretching
direction on LiPEO10_1(thin) and LiPEO10_2 (thick) samples.
Cyclic softening is observed for the LiPEO10_1 (Figure S7A,B,
Supporting Information), while a cyclic hardening is noticed for
the LiPEO10_2 (Figure S7C,D, Supporting Information) which
is in tune with our previous observations. It is already known
that that growth of spherulites are dependent upon the thick-
ness of the film where the spherulites are arranged not only
in the X-Y plane but also in the Z direction.[43,44] For this, the
polymer chain interaction is more prevalent during repetitive
tensile cyclic loading for the thicker samples. This enhanced
stiffness is acting as a driving force for the tensile cyclic hard-
ening whereas for the thin sample ion- polymer interaction
is more predominant in the tensile direction resulting in the
softening of the matrix. The electrolyte stability window before
and after tensile stretching is also tested (Figure S8, Supporting
Information), and the window seems to be unchanged with the
strain applied before the necking region of the stress–strain
curve.
To perform a more detailed investigation on the orientation
of polymer chains as well as network anisotropy and relaxa-
tion, multicreep tests are performed on both the LiPEO10 and
LiPEO20 samples. A creep and recovery test is conducted by
monitoring the response of the SPE toward a constant stress of
0.005 MPa for 10 min, then allowed to recover for next 20 min.
It is obvious that higher stress leads to higher creep strain.
Although this trend is maintained in the compression mode
for both the LiPEO samples, reverse trend is being noticed in
tensile mode. When the applied stress is being swept from
0.005 to 0.01 MPa, the creep strain also increases concomi-
tantly for compression mode due to softening of the polymer
matrix (Figure 6A,C). But in the tensile mode, as the matrix
getting more and more stiffer during the first multicreep
cycle (0.005 MPa preset stress) in the direction of stress, fur-
ther increment in stress (0.01 MPa) cannot enhance the creep
strain (Figure 6B,D). Hence it can be concluded that the already
toughened matrix resists the further deformation, consequently
there is a decrease in creep strain for a stress of 0.01 MPa.
4. Conclusion
In summary, studies on PEO-LiClO4 based SPEs show that
softening (stress) of the polymer matrix can enhance the ionic
conductivity in the strained direction (if it is tensile strain—
in-plane conductivity and in compressive strain—through
plane conductivity) and microstructural analyses show that
softening leads to the growth of amorphous regions of the
semi-crystalline polymer matrix. Present study shows that PEO
segmental alignments in thin membranes are in the X-Y plane
of the casting substrate while thicker SPEs can have PEO crys-
tallites oriented in the Z-direction too, introducing anisotropic
mechanical responses to the PEO based SPEs, though such
anisotropies are not visible in less defective pristine PEO based
membranes. The salt -LiClO4 induces defects in PEO and these
defected matrices are responsible for anisotropic mechanical
responses. Studies on LiPEO10 and LiPEO20 showed that com-
pressive cyclic straining soften the matrix while tensile cyclic
loading can harden the matrix, affecting the ionic conductivi-
ties in the respective directions. These anisotropic responses
are studied via different types of stress–strain relaxation studies
including creep studies and correlations are obtained. This
study on a well researched SPE system—PEO-LiClO4, shows
Figure 5. Cyclic loading characteristics of LiPEO20: A) Hysteresis curves for 25 cycle under compression mode and B) stress–time curve. C) Hysteresis
curve for 25 cycles under tensile mode and D) stress–time curve.
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Macromol. Chem. Phys. 2019, 220, 1900348
that along with the processing history of the SPEs, which has
been extensively studied in the past, the thickness induced ani-
sotropic alignments of the polymers can also affect the mechan-
ical responses (anisotropic) of the SPEs and hence their ionic
conductivities, and this needs to be considered while preparing
SPE based next generation stretchable and foldable solid state
devices. There can be region between the amorphous and crys-
talline parts of the PEO-LiClO4 matrix too,[33] and role of such
region in ion transport is also not understood well. Existence of
such region and its role in ion transport can be studied by tem-
perature dependent dielectric spectroscopy based studies. This
will be conducted in the near future.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank Tata Institute of Fundamental Research (TIFR),
Hyderabad for the financial support.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
anisotropy, cyclic stress hardening, cyclic stress softening, poly(ethylene
oxide)
Received: August 15, 2019
Revised: September 16, 2019
Published online: October 3, 2019
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... 23 The microstructure imaging of the membrane (one such Li incorporated membrane (LiPEO10)) using scanning electron microscope (SEM) is shown in figure 8A, where the details of the experiment is described later. 24 The spherulitic parts represent the crystalline portions and it can be seen that the PEO has large number of spherulites with a very small separation among them. The same film is then subjected to a tensile stress beyond its yielding point (in the necking region, the non-linear region in stress-strain curve before the film failure) and further imaging of the same film is shown in figure 8B, details are published elsewhere. ...
... The same film is then subjected to a tensile stress beyond its yielding point (in the necking region, the non-linear region in stress-strain curve before the film failure) and further imaging of the same film is shown in figure 8B, details are published elsewhere. 24 It can be seen that spherulitic parts got separated and an intermediate amorphous region has been developed. ...
... In other words, the polymer relaxes to the original structure soon after the removal of external stress and hence is found to be not affecting the ionic conductivity of the film. 24 The dynamic modulus of the film, as discussed before for T m measurements, is measured with an oscillatory stress value of 0.01 MPa and the pre-load applied was 0.001 N. ...
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