Influence of Charged Groups on the Structure of Microgel and Volume Phase Transition by Dielectric Analysis

Su, Wenjuan; Yang, Man; Zhao, Kongshuang; Ngai, To

doi:10.1021/acs.macromol.6b00809

Cited by 32 publications

(28 citation statements)

References 56 publications

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“…If the polar alignment of the host molecules is responsible for the nanosecond excited‐state geometry relaxation, the time for the dielectric relaxation of the host molecules will be in accordance with that for the emission redshift of the guest. [10a,12] Previous studies using dielectric spectroscopy suggest that the polymer segments undergo motion under a 10 6 Hz alternating current electric field, while the side chain groups become oriented at a frequency of 10 9 Hz. [10b–d] Dielectric spectroscopy of small molecules in a solid has seldom been reported.…”

Section: Resultsmentioning

confidence: 99%

Exciton‐ and Polaron‐Induced Reversible Dipole Reorientation in Amorphous Organic Semiconductor Films

Deng

Zhang

Wang

et al. 2019

Advanced Optical Materials

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The dynamic redshift in the emission spectra of TADF emitters in films has been recently investigated by using timeresolved emission spectroscopy (TRES). Hirata et al. found a large redshift in the delayed fluorescence band (50 ns to 8 ms) relative to the prompt fluorescence band (0-50 ns) in some carbazole/triazine derivatives doped in films and attributed the shift to the excited-state twisting relaxations of these ICT molecules. [3b] Using a more precise spectroscopic technique, Dias et al. found that the redshift in the emission spectrum even occurs in the nanosecond range. The spectral shift was then explained by a double excitedstate mechanism, in which the prompt fluorescence is a combination of the emission from localized excited (LE) and charge-transfer (CT) states, while the redshifted delayed fluorescence contains an increased fraction of CT emission. [3c] However, other researchers believe that solvation relaxation is responsible for the gradually redshift observed by TRES and the decrease in ΔE ST for TADF emitters in solid films, [3a,f,g,j] although the exact mechanism of the so-called solid-state solvation remains unclear. In 2003, Madigan and Bulovic investigated the photoluminescence (PL) of a laser dye dispersed in blended thin films of polystyrene (PS) and the polar small molecule camphoric anhydride (CA) and found that dielectric continuum models applicable in liquids can also be used in solids. [4] Very recently, Delor et al. further investigated a blend system via ultrafast transient absorption and optical Kerr effect spectroscopy and demonstrated that the solvation in the solid system of PS:CA:DCM2 is due to the rotational orientation of the polar small molecule CA on an ultrafast picosecond timescale. [3g] However, whether a molecular organic semiconductor with a molecular weight over 500 can undergo a spatial rotation like CA is still unknown. In 2014, Méhes et al. observed a redshifted TADF spectrum in organic films doped with a spiro-acridine-based TADF guest and ascribed the spectral shift on a microsecond timescale to the aligned dipoles of the host molecules induced by a polarized triplet CT state ( 3 CT). [3a] Conversely, Northey et al. recently ascribed the effect of solid-state solvation to the structural fluctuations of excited TADF emitters in the picosecond range. [3j] In this work, both the reversible and irreversible orientations of the molecular dipole through spatial rotation in an amorphous organic semiconductor are identified by employing TRES and dielectric relaxation spectroscopy. The dipole orientationThe quantum efficiency of organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) highly depends on the S 1 −T 1 energy gaps (ΔE ST ) of the emitters. However, the ΔE ST values determined in solution or even in organic thin films through a static approach continuously fail in predicting device performance. Herein, by systematically investigating the time-resolved emission spectra of several TADF emitters in various matrixe...

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Section: Resultsmentioning

confidence: 99%

Exciton‐ and Polaron‐Induced Reversible Dipole Reorientation in Amorphous Organic Semiconductor Films

Deng

Zhang

Wang

et al. 2019

Advanced Optical Materials

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“…For example, they can be compressed to packing density in excess of random close packing and their increase in viscosity as they approach the liquid-solid boundary shows significant deviations from the behaviour of hard spheres 6 , 7 . Moreover, microgels exhibit a rich phase behaviour 8 – 10 , which can be tailored by their degree of crosslinking 11 , the presence of charges 12 or inherent network inhomogeneities 13 .…”

Section: Introductionmentioning

confidence: 99%

Deswelling and deformation of microgels in concentrated packings

Aguiar

Laar

Meireles

et al. 2017

Sci Rep

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Increasing the particle density of a suspension of microgel colloids above the point of random-close packing, must involve deformations of the particle to accommodate the increase in volume fraction. By contrast to the isotropic osmotic deswelling of soft particles, the particle-particle contacts give rise to a non-homogeneous pressure, raising the question if these deformations occur through homogeneous deswelling or by the formation of facets. Here we aim to answer this question through a combination of imaging of individual microgels in dense packings and a simple model to describe the balance between shape versus volume changes. We find a transition from shape changes at low pressures to volume changes at high pressures, which can be explained qualitatively with our model. Whereas contact mechanics govern at low pressures giving rise to facets, osmotic effects govern at higher pressures, which leads to a more homogeneous deswelling. Our results show that both types of deformation play a large role in highly concentrated microgel suspensions and thus must be taken into account to arrive at an accurate description of the structure, dynamics and mechanics of concentrated suspensions of soft spheres.

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“…We once studied the interaction between the components of the microemulsion and its percolation mechanism by DS . We also obtained the interaction and collapse dynamics information of different PNIPAM‐based microgel systems based on dielectric analysis …”

Section: Introductionmentioning

confidence: 99%

Cononsolvency of poly(N‐isopropylacrylamide) in methanol aqueous solution—insight by dielectric spectroscopy

Yang

Zhao

2017

J Polym Sci B Polym Phys

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Both water and methanol are good solvents for poly(N‐isopropylacrylamide) (PNIPAM), while PNIPAM does not dissolve in their mixed solvents, this phenomenon is called cononsolvency. Cononsolvency is closely related to many phenomena in life but so far, its mechanism is still controversial. In this work, the dielectric behavior of PNIPAM methanol aqueous solution was studied in the frequency of 40Hz–40GHz. From lower frequency to higher frequency, four relaxations were found. They are, respectively, from global chain motion, local motion of backbone, motion of side chain group, and the dipole orientation of the solvent molecule. The solvent dependence of dielectric parameters for the chain motion implied that the PNIPAM chain has undergone the coil‐globule‐coil transition. Dielectric analysis to microwave frequency showed that the volume of the bound solvent units on PNIPAM chain increases with the increasing methanol concentration, which suggested that the structure of solvation units bound on PNIPAM side chains undergo a changing process experience from water to water‐methanol cluster to the ternary methanol cluster. This work reveals the structure and dynamics of the PNIPAM chain and the solvent unit that involved in the solvation of PNIPAM, and provides some new insight into the cononsolvency phenomenon. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1227–1234

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Influence of Charged Groups on the Structure of Microgel and Volume Phase Transition by Dielectric Analysis

Cited by 32 publications

References 56 publications

Exciton‐ and Polaron‐Induced Reversible Dipole Reorientation in Amorphous Organic Semiconductor Films

Exciton‐ and Polaron‐Induced Reversible Dipole Reorientation in Amorphous Organic Semiconductor Films

Deswelling and deformation of microgels in concentrated packings

Cononsolvency of poly(N‐isopropylacrylamide) in methanol aqueous solution—insight by dielectric spectroscopy

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