13 research outputs found
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Photoinduced StrainâAssisted Synthesis of a StiffâStilbene Polymer by RingâOpening Metathesis Polymerization
Developing a novel strategy to synthesize photoresponsive polymers is of significance owing to their potential applications. We report a photoinduced strainâassisted synthesis of mainâchain stiffâstilbene polymers by using ringâopening metathesis polymerization (ROMP), activating a macrocyclic Ďâbond connected to a stiffâstilbene photoswitch through a linker. Since the linker acts as an external constraint, the photoisomerization to the Eâform leads to the stiffâstilbene being strained and thus reactive to ROMP. The photoisomerization of Zâform to Eâform was investigated using timeâdependent NMR studies and UV/Vis spectroscopy. The DFT calculation showed that the Eâform was less stable due to a lack of planarity. By the internal strain developed due to the linker constraint through photoisomerization, the Eâform underwent ROMP by a second generation Grubbs catalyst. In contrast, Zâform did not undergo polymerization under similar conditions. The MALDIâTOF spectrum of Eâform after polymerization showed the presence of oligomers of >5.2â
kDa
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SelfâHealable and Recyclable Tactile Force Sensors with PostâTunable Sensitivity
It is challenging to postâtune the sensitivity of a tactile force sensor. Herein, a facile method is reported to tailor the sensing properties of conductive polymer composites by utilizing the liquidâlike property of dynamic polymer matrix at low strain rates. The idea is demonstrated using dynamic polymer composites (CB/dPDMS) made via evaporationâinduced gelation of the suspending toluene solution of carbon black (CB) and acidâcatalyzed dynamic polydimethylsiloxane (dPDMS). The dPDMS matrices allow CB to redistribute to change the sensitivity of materials at the liquidâlike state, but exhibit typical solidâlike behavior and thus can be used as strain sensors at normal strain rates. It is shown that the gauge factor of the polymer composites can be easily postâtuned from 1.4 to 51.5. In addition, the dynamic polymer matrices also endow the composites with interesting selfâhealing ability and recyclability. Therefore, it is envisioned that this method can be useful in the design of various novel tactile sensing materials for many applications
Modification of Capacitive Charge Storage of TiO2 with Nickel Doping
For practical deployment of supercapacitors characterized by high energy density, power density and long cycle life, they must be realized using low cost and environmentally benign materials. Titanium dioxide (TiO2) is largely abundant in the earth's crust; however, they show inferior supercapacitive electrochemical properties in most electrolytes for practical deployment. In this paper, we show that nickel doped TiO2 (Ni:TiO2) nanowires developed by electrospinning showed five times larger capacitance (âź200 F gâ1) than the undoped analogue (âź40 F gâ1). Electrochemical measurements show that the Ni:TiO2 nanowires have 100% coulombic efficiency. The electrodes showed no appreciable capacitance degradation for over 5000 cycles. The superior charge storage capability of the Ni:TiO2 could be due to its high electrical conductivity that resulted in five orders of magnitude higher ion diffusion as determined by cyclic voltammetry and electrochemical impedance spectroscopy measurements
Design, Synthesis and Characterization of Vitrimers with Low Topology Freezing Transition Temperature
Vitrimers are crosslinked polymeric materials that behave like fluids when heated, regulated by the kinetics of internal covalent bond-exchange that occurs rapidly at or above the topology freezing transition temperature (Tv) of the vitrimer, making these materials readily reprocessable and recyclable. We report two novel multiphase vitrimeric materials prepared by the cross-linking of two polymers, namely poly(triethylene glycol sebacate) and poly(2-hydroxyethyl acrylate), using zinc acetate or tin(II) 2-ethylhexanoate as catalysts, which exhibit significantly low Tv temperatures of 39 °C and 29 °C, respectively. The transesterification reactions allow rapid and pronounced stress relaxation at high temperatures, following the Arrhenius law. The lower Tv of these vitrimers could be attributable to the flexible long chains of these polymers and the significant excess of OH moieties present along the main chain of the polymer. The design of such multiphase vitrimers is not only useful for the practical application of vitrimers to reduce plastic waste but could also facilitate further development of functional polymer materials that can be reprocessed at low temperatures
Topochemical AzideâAlkyne Cycloaddition Reaction in Gels: Size-Tunable Synthesis of Triazole-Linked Polypeptides
Though topochemical reactions are
attractive, the difficulty associated
with crystallization such as low yield, unsuitability for large-scale
synthesis, etc. warranted the exploitation of other self-assembled
media for topochemical reactions. We synthesized a dipeptide gelator
decorated with azide and alkyne at its termini, N<sub>3</sub>-Ala-Val-NHCH<sub>2</sub>-CîźCH, which is designed to self-assemble through intermolecular
hydrogen bonds to β-sheets thereby placing the azide and alkyne
motifs in proximity. As anticipated, this peptide forms gels in organic
solvents and water via hydrogen-bonded β-sheet assembly as evidenced
from IR spectroscopy and PXRD profiling. The microscopic fibers present
in organogel and hydrogel have different morphology as was evident
from scanning electron microscopy (SEM) imaging of their xerogels,
XG<sub>h</sub> (xerogel made from hydrogel) and XG<sub>o</sub> (xerogel
made from organogel). Heating of xerogels at 80 °C resulted in
the topochemical azideâalkyne cycloaddition (TAAC) polymerization
to 1,4-triazole-linked oligopeptides. Under identical conditions,
XG<sub>o</sub> produced larger oligopeptides, and XG<sub>h</sub> produced
smaller peptides, as evidenced from MALDI-TOF spectrometry. We have
also shown that degree of TAAC polymerization can be controlled by
changing gel fiber thickness, which in turn can be controlled by concentration.
SEM studies suggested the morphological intactness of the fibers even
after the reaction, and their PXRD profiles revealed that both XG<sub>h</sub> and XG<sub>o</sub> undergo fiber-to-fiber oligomerization
without losing their crystallinity. In contrast to crystals, the xerogels
undergo TAAC polymerization in two distinct stages as shown by DSC
analyses. Interestingly, XG<sub>h</sub> and XG<sub>o</sub> undergo
spontaneous TAAC polymerization at room temperature; the latter shows
faster kinetics. This is not only the first demonstration of the use
of xerogels for thermally induced topochemical polymerization but
also the first report on a spontaneous topochemical reaction in xerogels
A Spontaneous Single-Crystal-to-Single-Crystal Polymorphic Transition Involving Major Packing Changes
4,6-<i>O</i>-Benzylidene-Îą-d-galactosyl
azide crystallizes into two morphologically distinct polymorphs depending
on the solvent. While the Îą form appeared as thick rods and
crystallized in <i>P</i>2<sub>1</sub> space group (monoclinic)
with a single molecule in the asymmetric unit, the β form appeared
as thin fibers and crystallized in <i>P</i>1 space group
(triclinic) with six molecules in the asymmetric unit. Both the polymorphs
appeared to melt at the same temperature. Differential scanning calorimetry
analysis revealed that polymorph Îą irreversibly undergoes endothermic
transition to polymorph β much before its melting point, which
accounts for their apparently same melting points. Variable temperature
powder X-ray diffraction (PXRD) experiments provided additional proof
for the polymorphic transition. Single-crystal XRD analyses revealed
that ι to β transition occurs in a single-crystal-to-single-crystal
(SCSC) fashion not only under thermal activation but also spontaneously
at room temperature. The SCSC nature of this transition is surprising
in light of the large structural differences between these polymorphs.
Polarized light microscopy experiments not only proved the SCSC nature
of the transition but also suggested nucleation and growth mechanism
for the transition
SelfâHealable and Recyclable Tactile Force Sensors with PostâTunable Sensitivity
It is challenging to postâtune the sensitivity of a tactile force sensor. Herein, a facile method is reported to tailor the sensing properties of conductive polymer composites by utilizing the liquidâlike property of dynamic polymer matrix at low strain rates. The idea is demonstrated using dynamic polymer composites (CB/dPDMS) made via evaporationâinduced gelation of the suspending toluene solution of carbon black (CB) and acidâcatalyzed dynamic polydimethylsiloxane (dPDMS). The dPDMS matrices allow CB to redistribute to change the sensitivity of materials at the liquidâlike state, but exhibit typical solidâlike behavior and thus can be used as strain sensors at normal strain rates. It is shown that the gauge factor of the polymer composites can be easily postâtuned from 1.4 to 51.5. In addition, the dynamic polymer matrices also endow the composites with interesting selfâhealing ability and recyclability. Therefore, it is envisioned that this method can be useful in the design of various novel tactile sensing materials for many applications
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Light-regulated growth from dynamic swollen substrates for making rough surfaces
Natural organic structures form via a growth mode in which nutrients are absorbed, transported, and integrated. In contrast, synthetic architectures are constructed through fundamentally different methods, such as assembling, molding, cutting, and printing. Here, we report a photoinduced strategy for regulating the localized growth of microstructures from the surface of a swollen dynamic substrate, by coupling photolysis, photopolymerization, and transesterification together. Photolysis is used to generate dissociable ionic groups to enhance the swelling ability that drives nutrient solutions containing polymerizable components into the irradiated region, photopolymerization converts polymerizable components into polymers, and transesterification incorporates newly formed polymers into the original network structure. Such light-regulated growth is spatially controllable and dose-dependent and allows fine modulation of the size, composition, and mechanical properties of the grown structures. We also demonstrate the application of this process in the preparation of microstructures on a surface and the restoration of large-scale surface damage
Crystal-to-Crystal Synthesis of Triazole-Linked Pseudo-proteins via Topochemical AzideâAlkyne Cycloaddition Reaction
Isosteric
replacement of amide bond(s) of peptides with surrogate
groups is an important strategy for the synthesis of peptidomimetics
(pseudo-peptides). Triazole is a well-recognized bio-isostere for
peptide bonds, and peptides with one or more triazole units are of
great interest for different applications. We have used a catalyst-free
and solvent-free method, viz., topochemical azideâalkyne cycloaddition
(TAAC) reaction, to synthesize pseudo-proteins with repeating sequences.
A designed β-sheet-forming l-Ala-l-Val dipeptide
containing azide and alkyne at its termini (N<sub>3</sub>-Ala-Val-NHCH<sub>2</sub>CîźCH, <b>1</b>) was synthesized. Single-crystal
XRD analysis of the dipeptide <b>1</b> showed parallel β-sheet
arrangement along the <i>b</i>-direction and head-to-tail
arrangement of such β-sheets along the <i>c</i>-direction.
This head-to-tail arrangement along the <i>c</i>-direction
places the complementary reacting motifs, viz., azide and alkyne,
of adjacent molecules in proximity. The crystals of dipeptide <b>1</b>, upon heating at 85 °C, underwent crystal-to-crystal
polymerization, giving 1,4-triazole-linked pseudo-proteins. This TAAC
polymerization was investigated by various time-dependent techniques,
such as NMR, IR, DSC, and PXRD. The crystal-to-crystal nature of this
transformation was revealed from polarizing microscopy and PXRD experiments,
and the regiospecificity of triazole formation was evidenced from
various NMR techniques. The MALDI-TOF spectrum showed the presence
of pseudo-proteins >7 kDa
Crystal-to-Crystal Synthesis of Triazole-Linked Pseudo-proteins via Topochemical AzideâAlkyne Cycloaddition Reaction
Isosteric
replacement of amide bond(s) of peptides with surrogate
groups is an important strategy for the synthesis of peptidomimetics
(pseudo-peptides). Triazole is a well-recognized bio-isostere for
peptide bonds, and peptides with one or more triazole units are of
great interest for different applications. We have used a catalyst-free
and solvent-free method, viz., topochemical azideâalkyne cycloaddition
(TAAC) reaction, to synthesize pseudo-proteins with repeating sequences.
A designed β-sheet-forming l-Ala-l-Val dipeptide
containing azide and alkyne at its termini (N<sub>3</sub>-Ala-Val-NHCH<sub>2</sub>CîźCH, <b>1</b>) was synthesized. Single-crystal
XRD analysis of the dipeptide <b>1</b> showed parallel β-sheet
arrangement along the <i>b</i>-direction and head-to-tail
arrangement of such β-sheets along the <i>c</i>-direction.
This head-to-tail arrangement along the <i>c</i>-direction
places the complementary reacting motifs, viz., azide and alkyne,
of adjacent molecules in proximity. The crystals of dipeptide <b>1</b>, upon heating at 85 °C, underwent crystal-to-crystal
polymerization, giving 1,4-triazole-linked pseudo-proteins. This TAAC
polymerization was investigated by various time-dependent techniques,
such as NMR, IR, DSC, and PXRD. The crystal-to-crystal nature of this
transformation was revealed from polarizing microscopy and PXRD experiments,
and the regiospecificity of triazole formation was evidenced from
various NMR techniques. The MALDI-TOF spectrum showed the presence
of pseudo-proteins >7 kDa