Facile construction of a double network cross-linked luminescent supramolecular elastomer by hydrosilylation and pillar[5]arene host-guest recognition

Article abstract of DOI:10.1039/d0cc02214d

Reticulated copolymer host pillar[5]arene cross-linked with poly(dimethylsiloxane) (PDMS) was synthesized for the facile construction of a double network cross-linked elastomer upon noncovalently cross-linking with tetraphenyethylene (TPE)-based tetratopi

Full text of DOI:10.1039/d0cc02214d

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Facile construction of double network cross-linked luminescent
supramolecular elastomer from hydrosilylation and pillar[5]arene
host–guest recognition
Received 00th January 20xx,
Accepted 00th January 20xx
Rong Fu,a Junying Zhang,a Shaojie Liu,a Xing-Dong Xu*a and Shengyu Feng a
DOI: 10.1039/x0xx00000x
Reticulated copolymer hosts pillar[5]arene crosslinked with strategy has rarely been used to enhance the mechanical
poly(dimethylsiloxane) (PDMS) were synthesized for the facile properties of elastomers, especially silicone elastomers.7
construction of double network crosslinked elastomer upon A seminal approach to fabricate double-network elastomers is
noncovalently cross-linking with tetraphenyethylene (TPE)-based the introduction of non-covalent bonds such as
tetratopic guests through host-guest interactions. The obtained supramolecular macrocycle-based host-guest chemistry, which
sample strips represented better mechanical property and could induce significantly enhancement of toughness of
luminescent capability.
materials due to the host-guest interactions between
macrocycles and guest molecules.8 Pillar[n]arene, as an
important class of macrocyclic hosts first reported by Ogoshi
and coworkers in 2008, are attracting more and more
attention.9 Among the pillar[n]arene family, pillar[5]arene take
up a lot of advantages: 1) bind neutral molecules in organic
solvents due to the electron-rich cavity; 2) could provide even
10 functionalized reaction sites; and 3) rigid structure donated
stiff component in the fabrication of functional materials, and
have showed potential in sensors, catalysts, supramolecular
gels, separation and storage. 10
Furthermore, a luminous macrocycle guest would bring silicon
elastomers with luminescent capability. Tetraphenylethylene
(TPE) is a well-known AIEgen due to its quite prominent
aggregation-induced emission (AIE) properties and facile
preparation.11 Some TPE-based materials with efficient AIE
luminescence characteristics have been reported with broad
applications in stimuli-responsive systems. Therefore, they are
very advantageous for the fabrication of various AIE systems
and devices for a wide range of applications in different
aggregate forms.12 Herein, based on our previous studies in
luminescent materials13 and organosilicon materials14, we
designed and synthesised a pillar[5]arene-PDMS copolymer,
denoted as P5-PDMS from hydrosilylation reaction as the first
cross-linking network. TPE-based tetratopic guests with cyano-
group, that is TPECN, are employed as binders to assemble
with the as-prepared polymers via host-guest interactions,
fabricating a soft component of noncovalent bonds. This
supramolecular polymer networks system works by covalent
and non-covalent bonds together to form a double-network
cross-linked elastomer and committed to make a luminescent
film for potential applications.
Polymeric materials have increasingly found extensive
applications, under high quality standard conditions for such
as aerospace and organisms which put high demands on the
thermal stability and biocompatibility of the materials.1
Polydimethylsiloxane (PDMS) is a typical rubbery material that
consists of soft and flexible polymer chains with cross-linking
points and possesses the merits of tunable mechanical
strength, high chemical stability, great biocompatibility and
inherent flexibility at broad temperature ranges.2 Through
reasonable crosslinking design, it has been attracted great
attention in the applications of electronic skins, artificial
muscles, and sensors, especially for stretchable devices, etc.3
However, the mechanical properties still needs to be further
enhanced when compared to some load-bearing tissues.4
Inspired by the fact that biological processes frequently
involve a supramolecular self-assembly event followed by the
formation of a covalent bond between two building blocks,
recent research efforts have focused on the development of a
new strategy by combining supramolecular self-assembly with
covalent chemistry for the construction of artificial
architectures that maybe able to solve the above mentioned
problem.5 For example, Suo and co-workers6 reported the
extremely stretchable and tough hydrogels by mixing two
types of crosslinked polymer forming ionically and covalently
crosslinked networks. To the best of our knowledge, this
a. National Engineering Research Center for Colloidal Materials, Key Laboratory of
Special Functional Aggregated Materials of Ministry of Education, Shandong
University, Jinan 250100, Shandong, China. E-mail: xuxd@sdu.edu.cn.
Electronic Supplementary Information (ESI) available: Details of synthesis and
characterization, and additional emission spectra. See DOI: 10.1039/x0xx00000x
To elucidate the AIE properties of the newly designed
molecule TPECN, fluorescence spectra were measured as
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shown in Fig. 1. In the fluorescence measurement, TPECN 2, the complexation is a slow-exchanging system on the proton
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DOI: 10.1039/D0CC02214D
exhibited an archetypal AIE property. In a good solvent THF, it NMR time scale. The resonance of protons in benzene H1,
exhibits a clear solution state without fluorescence emission.
methylene and Vinyl H2, H3, H4, H5, H6 from P5, the aromatic
The reason is that the rotations of the peripheral phenyl rings unit Ha and Hb of TPECN were clear broadened and displayed
against the olefinic stator around the single-bond axes a slightly downfield shift, due to the deshielding effect on
consume the excited-state energy. And the aggregation caused protons exposed outside the electron-rich cavity of P5. While
by the addition of H2O (Fig. 1a) and PDMS (Fig. 1b) clearly the proton signals of Hc, Hd, He, Hf and Hg of TPECN were
increased the fluorescence intensity to its maximum at 506 nm. broadened, and partly shifted upfield remarkably, due to the
When the volume of H2O or PDMS in the mixture reaches 95% shielding effect on protons drilling into the electron-rich
by degrees, the excited state energy consumption by cavities of P5. It is worth noting that there is part of the proton
nonradiative pathways is blocked as a result of the effect of signals of Hc, Hd, He, Hf and Hg almost unchanged, suggesting
the restricted intramolecular rotation (RIR), vitalizing strong that both complexation and uncomplexation between TPECN
15
emission to the maximum.11,
With the addition of poor and P5 exist at the same time.
solvent, it can be clearly observed from Fig. 1c that the
fluorescence intensity at 506 nm showed a trend of linear
change. Among them, the peak of Fig. 1b between 400-450 nm
is due to the weak luminescent of PDMS which is an
interference that can be eliminated. Therefore, TPECN is a
typical AIE molecules whose irradiative mechanism is based on
the theory of restriction of intramolecular rotations (RIR),
which could be well regulated through the combine of
supramolecular macrocycles and AIEgens.
Fig.1 Fluorescence spectra of TPECN (10 μM, ex = 380 nm) in
mixed solvent of THF/H2O(a), and THF/PDMS(b), and a plot of
fluorescence intensity of TPECN (10 μM) at 506 nm in
THF/H2O(PDMS) mixtures with different H2O(PDMS) fractions.
(d) Fluorescent picture
With the newly designed building blocks in hand, the double
network cross-linked supramolecular elastomer from covalent
and non-covalent bonds was investigated. In order to
construct the covalent bond network which bears a hard
segment that acts as a skeleton, P5 and H-terminated PDMS
were reacted through efficient hydrosilylation reaction, which
was widely used in industry. The adequacy of cross-linking
could be demonstrated by FT-IR spectra of P5-PDMS and P5-
TPECN-PDMS elastomers which are shown in Fig. S1 in ESI. In
FT-IR spectra, peaks at 1649cm-1, 3016.8 cm-1 and 3077.88 cm-
1 are not observed, which indicated that the ethylene bonds on
P5 on the P5-PDMS elastomer has disappeared, proved that
P5 and PDMS are almost completely crosslinked by covalent
bonds. On the other hand, we envision that the AIEgens guests
we introduced to covalent polymers systems can not only help
achieve high emission efficiency, but also improved the
flexibility of the elastomer by weak non-covalent bonds.
Scheme 1 Schematic illustration of the construction of
fluorescent supramolecular silicon polymers.
Pillar[5]arene, as a typical supramolecular macrocycles, is
capable of binding toward neutral guest molecules in organic
solvents, such as cyano derivatives (TPECN) which were
selected as neutral guest molecules due to their high binding
affinity toward pillar[5]arenes in certain organic solvents.16 In
this system, the host-guest properties of P5-PDMS and TPECN
The thermal stability of the elastomer is of vital importance in
the application since their decomposition leads to a decrease
in device performance. Therefore their thermal stabilities were
1
were studied in CDCl3 by H NMR spectroscopy using P5 and a
TPECN as model host and guest compounds. As is shown in Fig.
2 | J. Name., 2012, 00, 1-3
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measured by thermogravimetric analysis (TGA) at a heating shown in Fig. 3. The mechanical property was enhanced
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rate of 100°C/min under N2. The thermogravimetric weight dramatically upon the addition of TPECN into the P5-PDMS
loss curves for all compounds show that the decomposition of network (P5-TPECN-PDMS). As we can see from the
the complexes progresses similarly from Fig. 3(a). The TGA strain−stress curve in Fig. 3b, from the elastomer P5-PDMS to
curves indicate that PDMS, P5-PDMS and P5-TPECN-PDMS P5-TPECN-PDMS, the maximum stress increased from 240 KPa
decompose above 346°C, 404°C and 402°C, respectively, which to 375 KPa, while the maximum strain increased from 100% to
means that the thermal property was scarcely influenced with 215%. The enhanced mechanical properties could be explained
the addition of guest molecules. The weight loss step may be by the fact that in addition to the covalent bonds of the P5-
due to the decomposition of the organic groups and the PDMS network, the non-covalent cooperativity of host-guest
breaking of the polysiloxane main chain.
interaction plays a considerable role.
The internal evolution in loading–unloading cycles is
investigated further to search out the elastomers’ fatigue
resistance and resilience capabilities. Typical tensile (20
mm/min) and compressive (5 mm/min) loading–unloading
tests are implemented to achieve this purpose. In the
consecutive tensile mode in Fig. 3c, the elastomer P5-TPECN-
PDMS shows favourable resilience and fatigue resistance
where each cycle nearly coincides with others, indicating
energy dissipation was rare during deformation. Even
increasing the number of cycles to 10 of the force, the samples
still show good anti-fatigue behaviour. In the compressive
tensile mode in Fig. 3d, elastomers P5-PDMS and P5-TPECN-
PDMS both show small hysteresis loop, furthermore, P5-
TPECN-PDMS has better compression strength (2.25 MPa)
compared to P5-PDMS (1.75 MPa).
Fig. 2 1H NMR spectra (CDCl3, 400 MHz, 298 K) of (a) 20 mM P5;
(b) 5 mM TPECN and 20 mM P5; (c) 5 mM TPECN.
Fig. 4 SEM micrographs of P5-PDMS (a) and P5-TPECN-PDMS
(b); (c) Contact angles of the elastomers P5-TPECN-PDMS. (d)
film display.
Fig. 3 (a) TGA of PDMS, P5-TPECN-PDMS and P5-TPECN-PDMS.
(b) Tensile tests measured at room temperature for
Homogeneous surface morphologies of the upper surface of
the silicone elastomers were presented in both P5-PDMS (a)
and P5-TPECN-PDMS (b) from SEM micrographs which could
further confirm the adequacy of cross-linking, as is shown in
Fig. 4. Likewise, bonding TPECN to the silicone elastomer does
not affect the morphology, that is, phase separation did not
occur when TPECN were incorporated. On this homogeneous
surface, static contact-angle analysis was performed to study
the effect of crosslinking on the silicone elastomer hydrophilic
performance. Silicone elastomer is typically a hydrophobic
elastomers
P5-PDMS
and
P5-TPECN-PDMS,
test
speed=20mm/min. (c) Typical successive loading–unloading
tensile tests for ten runs at room temperature for elastomers
P5-TPECN-PDMS, test speed=20mm/min. (d) Typical Loading–
unloading compressive tests measured at room temperature
for elastomers P5-PDMS and P5-TPECN-PDMS, test
speed=5mm/min.
The mechanical property of the elastomeric samples which
was conducted by tension experiment at room temperature is
material with
a
water contact angle of approximately
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109±0.1.17 The contact angles of the P5-TPECN-PDMS
elastomers were tested by a contact angle analyzer with
distilled water (Fig. 4c), suggesting the novel luminescent
elastomers obtained have similar contact angles compared
with traditional silicone rubber. The as-prepared material can
be made into a shape-processable film which shows strong
luminescence performance (Fig. 4d the left two). The LED bulb
(395-400nm) is purple to the naked eye (Fig. 4d the middle
one). And then we apply the sample film to the surface of the
small bulb, the small bulb coated with the sample film emits
green fluorescence excited by the ultraviolet light of the small
LED bulb, which is shown at the far right of the Fig. 4d.
In summary, a newly designed luminescent supramolecular
elastomer was successfully fabricated based on a double
network crosslink strategy. The fluorescence properties of the
elastomer were achieved via host-guest chemistry based on
the classical RIR mechanism. And in the elastomer network,
the covalent bond of P5-PDMS acts as a hard part to act as a
high-strength skeleton, simultaneously, the soft non-covalent
bond imparts flexibility to the elastomer, this double network
crosslink strategy extremely improved the mechanical
properties of silicone elastomers. We believe that the
marriage of AIE molecules (TPECN) and P5-PDMS copolymers
will not only impart supramolecular polymer materials with
stable structure and thermodynamic properties, mechanical
behavior, and remarkable fluorescent properties, but may also
guide a new way to enhance mechanical properties for silicone
elastomers with fascinating structures, functions, and
applications.
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This work was supported by the National Natural Science
Foundation of China (Grant No. 21602124), Natural Science
Foundation of Shandong Province (Grant No. ZR2016BQ11),
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