Molecular rotors to probe the local viscosity of a polymer glass

Mirzahossein, Elham; Grzelka, Marion; Zhang, Pan; Demirkurt, Begüm; Habibi, Mehdi; Brouwer, Albert M.; Bonn, Daniel

doi:10.1063/5.0087572

Cited by 9 publications

(13 citation statements)

References 51 publications

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“…For the purpose of obtaining the self-diffusion coefficient in an ASD according to eq , one requires to determine the viscosity of the medium through which the AI is diffusing. Note that, throughout this work, the term “viscosity” is used to refer to the microscopic viscosity in contrast to the macroscopic viscosity as a property encompassing the chain dynamics of high polymers in the reptation regime. − In the context of the dielectric investigations, the Stokes–Einstein-Debye (SED) equation, relating η to τ α (or τ ε ) was employed to obtain the viscosity of the dispersions as 1 l false( scriptl + 1 false) τ α = D R = k B T 8 π η R H 3 where D R denotes the rotational diffusivity and scriptl is the order of Legendre polynomial ( l = 1 for dielectric spectroscopy) . The hydrodynamic radius required in eqs and has already been determined for IMI to be 1.3 Å, upon combining its zero frequency complex viscosity with the SED equation .…”

Section: Resultsmentioning

confidence: 99%

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid

et al. 2023

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The main goal of this study is to develop an experimental toolbox to estimate the self-diffusion coefficient of active ingredients (AI) in single-phase amorphous solid dispersions (ASD) close to the glass transition of the mixture using dielectric spectroscopy (DS) and oscillatory rheology. The proposed methodology is tested for a model system containing the insecticide imidacloprid (IMI) and the copolymer copovidone (PVP/VA) prepared via hot-melt extrusion. For this purpose, reorientational and the viscoelastic structural (α-)relaxation time constants of hot-melt-extruded ASDs were obtained via DS and shear rheology, respectively. These were then utilized to extract the viscosity as well as the fragility index of the dispersions as input parameters to the fractional Stokes−Einstein (F-SE) relation. Furthermore, a modified version of Almond−West (AW) formalism, originally developed to describe charge diffusion in ionic conductors, was exercised on the present model system for the estimation of the AI diffusion coefficients based on shear modulus relaxation times. Our results revealed that, at the calorimetric glass-transition temperature (T g ), the self-diffusion coefficients of the AI in the compositional range from infinite dilution up to 60 wt % IMI content lied in the narrow range of 10 −18 − 10 −20 m 2 s −1 , while the viscosity values of the dispersions at T g varied between 10 8 Pa s and 10 10 Pa s. In addition, the phase diagram of the IMI-PVP/VA system was determined using the melting point depression method via differential scanning calorimetry (DSC), while mid-infrared (IR) spectroscopy was employed to investigate the intermolecular interactions within the solid dispersions. In this respect, the findings of a modest variation in melting point at different compositions stayed in agreement with the observations of weak hydrogen bonding interactions between the AI and the polymer. Moreover, IR spectroscopy showed the intermolecular IMI-IMI hydrogen bonding to have been considerably suppressed, as a result of the spatial separation of the AI molecules within the ASDs. In summary, this study provides experimental approaches to study diffusivity in ASDs using DS and oscillatory rheology, in addition to contributing to an enhanced understanding of the interactions and phase behavior in these systems.

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Section: Resultsmentioning

confidence: 99%

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid

et al. 2023

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“…10 Use of molecular probes is one of widely-accepted ways to detect the local viscosity of polymers. 11 We selected PVAC because of its Tg above room temperature (20-25 °C) and below 40 °C suitable for the temperature controller we used. Fig.…”

Section: ⅱ Resultsmentioning

confidence: 99%

Second-order Phase Transition Behavior behind Polymer Glass Transition

Ishikawa

Takahashi

Hayashi

et al. 2022

Preprint

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Glass transition has similarity to the second-order phase transition in temperature dependent changes in entropy, non-Arrhenius viscosity, and heat capacity of glass forming materials. However, it has primarily been considered to be not phase transition. Recent single-molecule spectroscopy developments prompted re-investigating glass transition at the nanometer scale probing resolution, showing that glass transition includes phenomena similar to the second-order phase transition. They are characterized by microscopic collective polymer motion and discontinuous changes in temperature dependent relaxation times, the latter of which resembles the critical slowing down of second-order phase transitions, within a temperature window above the polymer calorimetric glass transition temperature. Simultaneous collective motion and critical slowing down occurrences disclose that the second-order phase transition hides behind polymer glass transition.

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“…The non-radiative relaxation of photo-excited molecular rotors involves mechanical internal rotation of the rotor molecules. , Since the rotor’s rotation is influenced by local friction, molecular rotors can be used as probes for measurements of free volume-related properties. Molecular rotors have been successfully applied to measurements of, for instance, viscosities of small molecular fluids, − and physical aging in polymeric materials. − Molecular rotors have also been shown to be useful for probing the microscale spatial variation in free volume in glassy polymers under mechanical deformation …”

Section: Introductionmentioning

confidence: 99%

“…6−8 Molecular rotors have also been shown to be useful for probing the microscale spatial variation in free volume in glassy polymers under mechanical deformation. 9 For the determination of the glass transition temperatures (T g 's) of polymers, however, conventional fluorescent probes (non-rotor fluorophores such as pyrene) have been more commonly used. Fluorescent probes have been particularly useful for the investigation of the local T g properties of nanoconfined polymers.…”

Section: ■ Introductionmentioning

confidence: 99%

“…at T < T g (Figure ). Also of note, an alternative free volume model of Zaccone and Terentjev ( f ( T ) = 1 – ϕ g exp(α( T g – T )) where ϕ g is the “critical monomer packing fraction” at T g , and α is the thermal expansion coefficient (= α L (at T > T g ) or α G (at T < T g )) can capture the temperature-dependent variation of the f in the glassy state. However, this model assumes that ϕ g (= 1 – f g ) is a universal constant (≅ 0.64), which is inconsistent with experimental results; the values of f g (= (α L – α G ) C 2 g in the WLF theory) vary from polymer to polymer.…”

Section: Introductionmentioning

confidence: 99%

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Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume

Kim,

Lee,

Chang Kim

et al. 2023

Macromolecules

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Fluorescent molecular rotors embedded in polymer matrices can probe the local changes in the dynamics of the polymer matrix (such as the glass transition of polymers near interfaces/surfaces) that are not accessible from macroscopic measurements. Yet, there is little consensus as to how the fluorescence data should be analyzed, what property the fluorescence intensity actually represents, and what are appropriate/optimal rotors for glass transition measurements. By experimentation with a model fluorescent rotor, farnesyl-(2carboxy-2-cyanovinyl)-julolidine (FCVJ), and also re-analysis of data available in the literature for other types of molecular rotors, we investigated the correlation between the relaxation processes of photo-excited rotors and free volume changes in matrix polymers. The temperature dependences of the fluorescence intensities from FCVJ-doped polystyrene (PS), poly(vinyl acetate) (PVAc), and poly(isobutyl methacrylate) (PiBMA) materials across their glass transition temperatures (T g ) were successfully modeled using a free volume model which is based on the original theory of Loutfy that relates the fluorescence of a rotor probe to the free volume of the matrix, and the revised definition of the total free volume quantity proposed by Lipson and White (the Locally Correlated Lattice model-based free volume) that includes both the vibrational free volume and the excess free volume. Atomistic molecular dynamics simulation supports that the timescale of the vibrational motion of PS monomers is comparable to the intrinsic timescale of a rotor's internal rotation known in the literature (on the order of picoseconds), and thus the rotation of a rotor is controlled by the friction from the hard-core volume (not by the friction from the vibrational volume) of the monomers. Measurements on FCVJ-loaded polystyrene−poly(ethylene glycol) (PS−PEG) micelles in water suggest that the PS core domain of the PS−PEG micelle has a broad distribution of glass transition temperatures; a quantitative analysis based on the free volume model enabled to estimate the actual range of T g . These results allow us to conclude that fluorescent molecular rotors with a tendency toward rapid non-radiative (rotational) relaxation and bulky rotating subgroups (such as FCVJ) exhibit a strong coupling between the fluorescence of the rotor and the free volume of the matrix and are thus useful probes for T g measurements.

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Molecular rotors to probe the local viscosity of a polymer glass

Cited by 9 publications

References 51 publications

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid

Second-order Phase Transition Behavior behind Polymer Glass Transition

Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume

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