Broadband Dielectric Relaxation Spectroscopy Research Articles

The relaxation dynamics and dielectric properties of cyanobiphenyl-based nematic tripod liquid crystals

We present a detailed investigation of the phase behaviour, molecular relaxation dynamics, rheology and dielectric properties of two cyanobiphenyl-based liquid crystal (LC) tripods, differing only in the length of their spacer units (6 or 9 carbons). These LC tripods combine properties of low molecular weight LCs and LC polymers, resulting in a range of advantageous properties including a wide nematic range (ΔT>90 K) and a large birefringence (0.3 at T−TNI=-80 K for the 6 spacer tripod). Using broadband dielectric relaxation spectroscopy, calorimetry, oscillatory and steady state shear rheology, we identified four molecular relaxation processes: the structural (α) relaxation (defining the glass transition temperature, Tg); the δ relaxation (reorientation of the mesogen unit around its short axis); the β relaxation (reorientation around its long axis); and the γ relaxation (internal tripod arm fluctuations). The β and γ relaxations follow Arrhenius temperature dependencies in the glass, whereas the α and δ relaxations merge above, but near Tg, where they follow a non-Arrhenius VFT behaviour. For higher temperatures, the two relaxations separate and for T>T⁎, where T⁎ marks a dynamic crossover, the α relaxation transitions from VFT to Arrhenius behaviour. For the two tripods, ratios of T⁎/Tg=1.14 and T⁎/Tg=1.13 were observed respectively, consistent with the ratio observed for many side-chain LC polymers (and other LC systems), and consistent with the ratio where a dynamic crossover is typically observed also for non-LC glass-formers. However, for non-LC systems, the transition to Arrhenius behaviour happens at significantly higher temperatures relative to Tg and the dynamic crossover at T⁎ is typically observed as a transition between two different VFT behaviours. We argue that for the tripods these differences arise from differences in the molecular relaxation mechanisms induced by the LC order. Finally, for temperatures where δ and α relaxations are separated, we find that ion conductivity decouples from the α relaxation, instead following the δ relaxation; this demonstrates that the ion transport properties for the tripods can be tuned by the design of the tripod mesogen arm.

Understanding water absorption effect on molecular dynamics, microstructures and relaxation behavior of segmented polyurethane elastomers

The influence of water absorption on molecular dynamics, microstructures and relaxation behaviors of polyurethane (PU) elastomers with polyether (poly(tetramethylene glycol), PTMG) soft segments has been studied. Kinetics of water absorption of PU elastomer can be well fitted by Fick's model. However, non-Fickian absorption behavior was observed in the late stage at 60 °C, which was presumed to be related to the plasticization of some hard domains. Broadband dielectric relaxation spectroscopy was used to investigate the molecular dynamics of PU elastomers. The analysis of α-relaxation and Maxwell-Wagner-Sillars interfacial polarization indicated that the water absorption has a great influence on the molecular mobility of soft segment and the interface regions between soft domains and hard domains. Microphase separation structures of PU elastomers consisted of loose, partially regular and compact hard domains. Water molecules could interact with loose hard domains and lead to the plasticization. The results of stress relaxation behavior indicated that water absorption leaded to remarkable increase in the stress relaxation ratio. The deterioration of relaxation properties caused by water absorption was not only related to the soft segments, but also caused by the plasticization of the interface regions and the hard domains.

Ethaline and related systems: may be not “deep” eutectics but clearly interesting ionic liquids

Abstract Ethaline, the 1:2 molar ratio mixture of ethylene glycol (EG) and choline chloride (ChCl), is generally regarded as a typical type III deep eutectic solvent (DES). However, careful differential scanning calorimetry (DSC) of EG + ChCl mixtures surprisingly revealed that the liquidus lines of the phase diagram apparently follow the predictions for an ideal binary non-electrolyte mixture. Applying broad-band dielectric relaxation spectroscopy to room-temperature solutions of ChCl, and of the related salts choline iodide and chlorocholine chloride, in EG up to saturation, we explored the possible reasons for this conundrum. It appears that in these solutions free ions are rather scarce. Instead, contact ion pairs and larger aggregates predominate.

Hydration of the neurotransmitter [formula omitted]-aminobutyric acid and its isomer [formula omitted]-aminobutyric acid

Room-temperature aqueous solutions of the neurotransmitter γ-aminobutyric acid (GABA) and its isomer α-aminobutyric acid (AABA) were studied with broad-band dielectric relaxation spectroscopy (DRS) and statistical mechanics (1D-RISM and 3D-RISM) calculations. Comparison of the first-shell coordination numbers from 3D-RISM with the total effective hydration numbers from DRS revealed that for both solutes only about half of the H2O molecules in direct contact with the solute are affected in their dynamics. Whereas for GABA ∼10 of them are moderately retarded by a factor of ∼2–3 compared to bulk water and only ∼1–2 effectively frozen at vanishing solute concentration, AABA appears to be more strongly hydrated with only ∼6 moderately retarded but ∼6 H2O frozen by the solute molecule. This is reflected in the effective solute dipole moments.

Why Does Ethaline Apparently Behave as an Ideal Binary Mixture?

Phase diagram mixtures of the salt choline chloride (ChCl) with ethylene glycol (EG) surprisingly seem to behave as ideal binary nonelectrolyte mixtures [Agieienko, V.; Buchner, R. Phys. Chem. Chem. Phys.2022, 24, 5265]. To shed some light on this conundrum, results of broad-band dielectric relaxation spectroscopy (DRS) and quantum-chemical calculations are reported for solutions of ChCl, choline iodide (ChI), and chlorocholine chloride (ClChCl), in EG up to saturation at 298.15 K. The data revealed that all three solutes are only weakly solvated in the sense that on average per equivalent of solute only one EG OH-group is dynamically affected. While contact ion pairs are significant for solute concentrations of ≲1 M, free cation concentrations are rather low. Instead, over the entire concentration range a large fraction of the dipolar cations could not be detected by DRS. We argue that the latter are embedded in large solute aggregates, explaining thus the phase diagram of ChCl + EG and the very low ionicity of all systems.

Influence of Liquid Crystallinity and Mechanical Deformation on the Molecular Relaxations of an Auxetic Liquid Crystal Elastomer.

Liquid Crystal Elastomers (LCEs) combine the anisotropic ordering of liquid crystals with the elastic properties of elastomers, providing unique physical properties, such as stimuli responsiveness and a recently discovered molecular auxetic response. Here, we determine how the molecular relaxation dynamics in an acrylate LCE are affected by its phase using broadband dielectric relaxation spectroscopy, calorimetry and rheology. Our LCE is an excellent model system since it exhibits a molecular auxetic response in its nematic state, and chemically identical nematic or isotropic samples can be prepared by cross-linking. We find that the glass transition temperatures () and dynamic fragilities are similar in both phases, and the -dependence of the relaxation shows a crossover at the same for both phases. However, for , the behavior becomes Arrhenius for the nematic LCE, but only more Arrhenius-like for the isotropic sample. We provide evidence that the latter behavior is related to the existence of pre-transitional nematic fluctuations in the isotropic LCE, which are locked in by polymerization. The role of applied strain on the relaxation dynamics and mechanical response of the LCE is investigated; this is particularly important since the molecular auxetic response is linked to a mechanical Fréedericksz transition that is not fully understood. We demonstrate that the complex Young’s modulus and the relaxation time remain relatively unchanged for small deformations, whereas for strains for which the auxetic response is achieved, significant increases are observed. We suggest that the observed molecular auxetic response is coupled to the strain-induced out-of-plane rotation of the mesogen units, in turn driven by the increasing constraints on polymer configurations, as reflected in increasing elastic moduli and relaxation times; this is consistent with our recent results showing that the auxetic response coincides with the emergence of biaxial order.

Preparation and molecular dynamics study of polyurethane damping elastomer containing dynamic disulfide bond and multiple hydrogen bond

Using damping materials is an effective measure to control vibration and noise, which is widely used. However, it is relatively difficult for polymer damping materials to find a balance between damping and mechanical properties. Herein, a polyurethane is designed containing dynamic disulfide bonds and hydrogen bonds of different strength to address the dilemma. The polyurethane is endowed with good damping and mechanical properties (effective damping temperature range of 117 °C and a tensile strength of 14.98 ± 0.50 MPa). The molecular dynamics were studied by combining dynamic mechanical analysis (DMA) and broadband dielectric relaxation spectroscopy (BDRS). The microphase separation morphology and degrees of separation were used to help explain molecular dynamics. The segmental motion of soft phase becomes difficult in the glass-transition temperature (Tg) and faster in a high temperature with increasing 2, 2′-Dithiodibenzoic acid (DTSA) contents. These results explain the mechanism improving damping performance and provide some references for designing damping materials in the future.

Broadband dielectric response of silk Fibroin/BaTiO3 composites: Influence of nanoparticle size and concentration

In order to advance towards more sustainable electronics generation, natural polymers with tailored dielectric response are essential. In this search, the combination of bio-based materials with active fillers in composite form, suppose one of the most viable alternatives. To achieve it, this work has explored the ability to control dielectric response of Silk Fibroin, a protein polymer by its combination with ceramic barium titanate (BaTiO3) nanoparticles. Both the effect of filler concentration (0, 5, 10, 20 and 40 wt%) and size (100 and 200 nm) has been studied in composites processed by easily scalable techniques. Samples with a homogeneous distribution of nanoparticles have been obtained. Dielectric relaxation processes assessed by broadband dielectric relaxation spectroscopy (BDS) in wide frequency (0.1 Hz–1 MHz) and temperature ranges (- 40 to 220 °C), revealed a dielectric constant increasing with filler content and decreasing with filler size, ranging from 4.4 for SF up to 142 for the SF/BaTiO3 composite with 40 wt %, at room temperature and 1 kHz. Two relaxations processes are observed, the β-relaxation and the conductivity relaxation, both with temperature-dependent behaviour. The activation energy of the conductivity process decreases with increasing nanoparticle content and decreasing size. A Maxwell-Wagner-Sillar process related to the interface between the silk fibroin matrix and the BaTiO3 nanoparticles was also identified.

Molecular dynamics, microstructures and mechanical properties of segmented polyurethane elastomers under gamma irradiation

The influence of gamma irradiation on molecular dynamics, microstructures and mechanical properties of polyurethane (PU) elastomers with polyether (poly(tetramethylene glycol), PTMG) and polyester (poly(butylene adipate), PBA) soft segments has been studied. Dynamical mechanical analysis and broadband dielectric relaxation spectroscopy were used to investigate the molecular dynamics of PU elastomers. Interestingly, the α-relaxation exhibited contrasting tendency with irradiation dose for PTMG-PU and PBA-PU, demonstrating that main chain scission took place for PTMG soft segments, but main chain crosslink for PBA soft segments. The results of small-angle scattering technique indicated that the volume fraction of hard microdomains increase rapidly with irradiation dose. The microstructures of PTMG-PU were more susceptible to gamma irradiation compared with PBA-PU. The deterioration of tensile properties was not only related to irradiation damage of soft segments, but also caused by the destruction of the microstructures of hard microdomains.

Synthesis, Crystallization, Structure Memory Effects, and Molecular Dynamics of Biobased and Renewable Poly(n-alkylene succinate)s with n from 2 to 10

In this article, we synthesize five poly(n-alkylene succinate)s, PnASs, with n = 2, 4, 6, 8, and 10 via multi-step polycondensation methods. Next, we comparatively investigate these renewable and biobased polyesters from the points of view of structure, crystallinity, and molecular mobility, employing 1H nuclear magnetic resonance spectroscopy, size-exclusion chromatography, viscometry, X-ray diffraction, differential scanning calorimetry (DSC, conventional and temperature modulation modes), polarized optical microscopy (POM), and broadband dielectric relaxation spectroscopy (BDS). Next to the successful synthesis of the materials, we evaluate the characteristics of crystallization (temperature and fraction); moreover, we explore for the first time, on the same type of succinic polyesters, the impact of n on the structure memory related to crystal nucleation as well as the changes in the semicrystalline morphology. We demonstrate that the structure/crystal memory is stronger for the lower n (shorter alkylene succinate monomers) because of more chain–chain associations, the result being independent from the overall length of the polymer chain (molar mass, Mn 13–80 kg/mass). The crystalline fraction (CF ∼ 12–34%) increases with n, also independently from Mn; however, the chain length affects directly the nucleation rate as Tc increases with Mn. The direct effects of n, in the inter-/intrachain interactions, as well as the indirect ones, on the CF and distribution of crystallites were found to be responsible for the alternations in the static glass transition temperature in DSC (lowering of Tg with n) and the dynamic glass transition (α, αc relaxations in BDS). For the sum of these PnASs, the molecular dynamics mapping is shown here, also for the first time. With increasing n, segmental dynamics accelerates, whereas, interestingly, the cooperativity drops (elimination for n = 10). Comparing these results with the recorded alternations in the semicrystalline morphology (POM), we conclude spatial confinement to be imposed on the mobile amorphous polymer by the tightly distributed crystallites when n increases. Overall, these data provide proofs for the potential for tuning of the final polymer properties connected with crystallization (mechanical performance and permeability), envisaging future biomedical, packaging, and other application for these PnASs.

Molecular Dynamics in Nanocomposites Based on Renewable Poly(butylene 2,5-furan-dicarboxylate) In Situ Reinforced by Montmorillonite Nanoclays: Effects of Clay Modification, Crystallization, and Hydration.

This study deals with poly(butylene 2,5-furan-dicarboxylate), PBF, a renewable bio-based polyester expected to replace non-eco-friendly fossil-based homologues. PBF exhibits excellent gas barrier properties, which makes it promising for packaging applications; however, its rather low and slow crystallinity affects good mechanical performance. The crystallization of this relatively new polymer is enhanced here via reinforcement by introduction in situ of 1 wt % montmorillonite, MMT, nanoclays of three types (functionalizations). We study PBF and its nanocomposites (PNCs) also from the basic research point of view, molecular dynamics. For this work, we employ the widely used combination of techniques, differential scanning calorimetry (DSC) with broad-band dielectric relaxation spectroscopy (BDS), supplemented by polarized light microscopy (PLM) and thermogravimetric analysis (TGA). In the PNCs, the crystalline rate and fraction, CF, were found to be strongly enhanced as these fillers act as additional crystallization nuclei. The improvements in crystallization here correlate quite well with those on the mechanical performance recorded recently; moreover, they occur in the same filler order, in particular, with increasing MMT interlayer distance (from ∼1 to ∼3 nm). In the amorphous fraction of the polymer, the chain diffusion (calorimetric Tg and dynamic α process) is easier in the PNCs due to their slightly smaller length, while in the semicrystalline state, it decelerates by crystal-induced constraints. The local polymer dynamics (β process, below Tg) was found to be independent of the PNC composition, however, sensitive to structural changes of the matrix. Finally, a filler-induced dynamics was additionally recorded in the PNCs (α* process), arising possibly from the polymer located at the MMT surfaces. α* follows the changes in polymer chain length and decelerates with crystallization, whereas its activation energy decreases with mild hydration. The combined results on α* with the DSC and TGA findings, provide proof for weak MMT-PBF interactions. Overall, our results, along with data from the literature, suggest that such furan-based polyesters reinforced with properly chosen nanofillers could potentially serve well as tailor-made PNCs for targeted applications.

Transient dynamics of cold‐rolled and subsequently thermally rejuvenated atactic‐polystyrene using broadband dielectric spectroscopy

AbstractThe effect of plastic deformation on the molecular dynamics of atactic polystyrene (a‐PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier‐transform infrared spectroscopy (FTIR) and polarized‐light microscopy. Sheets of a‐PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass‐transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α‐ and γ(I)‐relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation‐enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G‐ratio); and (ii) processes III and IV were identified as helix‐inversion processes of T2G2 helices as reported earlier for syndiotactic‐rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized‐light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.

Effect of low‐temperature physical aging on the dynamic transitions of atactic polystyrene in the glassy state

ABSTRACTThe local and cooperative dynamics of atactic PS (a‐PS) were studied by broadband dielectric relaxation spectroscopy (BDRS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR). The a‐PS has been subjected to thermal rejuvenation and subsequent quenching, short‐term aging (6 weeks), and long‐term aging (1 year) at ambient conditions. Where for the rejuvenated sample only an α‐ and a γ‐relaxation is observed, short‐term aging results in an additional β*‐relaxation that merges with the α‐relaxation at longer aging times. The γ‐relaxation is increasing in intensity and activation energy during aging. The α‐process shows no spectral changes and shift in the relaxation time upon aging. This may be attributed to a possible erasure of history of the material during the temperature‐sweep mode measurement. Fourier transform infrared spectroscopy (FTIR) results suggest that the energetically favorable trans‐trans (tt) conformers are increased in population with aging. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1394–1401

New Insight Into the Molecular Dynamics of Epoxy/Aluminum Hydroxide Nanocomposites by Terahertz Time-Domain Spectroscopy

Nanodielectrics have shown many superior properties than conventional dielectrics. Nanocomposites based on epoxy resin and nanoaluminum hydroxide were fabricated to obtain material systems. In this paper, we investigated molecular dynamics of the nanocomposites by the combination of broadband dielectric relaxation spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and thermally stimulated depolarization current (TSDC). We decompose the molecular dynamics into four relaxation processes according to their typical frequency at room temperature. Relaxation process in THz range is regarded as γ-process, which is well represented by Debye equation. At low-frequency range 10−1–106 Hz, α-process and Johari–Goldstein β-process play dominant role. Additionally, a fast β-process is expected to appear at intermediate frequencies. TSDCs results confirm the existence and change of all relaxation process. Each relaxation process changes differently with increasing filler content. Our results suggest that it is molecular dynamics at different time and space scales rather than free volume result in the change of dielectric property. This paper indicates that THz-TDS is extremely beneficial to understand the underlying mechanism of interface region in nanocomposites. We anticipate that THz-TDS is a prospective tool in future development and analysis of nanocomposites.

Molecular relaxation and ionic conductivity of ionic liquids confined in a poly(vinylidene fluoride) polymer matrix: Influence of anion and cation type

Blends of poly(vinylidene fluoride) (PVDF) and ionic liquids (ILs) with different cations and anions have been prepared by solvent casting. The IL content was the same in all blends of the series. Molecular relaxation and ionic conductivity have been systematically studied by broadband dielectric relaxation spectroscopy (BDS) in wide frequency (0.1 Hz–1 MHz) and temperature ranges (−120 to 150 °C) and the results have been analysed in terms of dielectric modulus M*(ω) and conductivity σ*(ω) formalisms. The main relaxation process (β-relaxation) of the amorphous phase of the blend that integrates amorphous polymer chain segments and IL molecules was observed. Significant differences in the Vogel-Fulcher-Tammann (VFT) fitting parameters in the PVDF/IL blends with different anions were detected. The conductivity σ*(ω) formalism shows that it is strongly dependent on the miscibility of the IL with the amorphous PVDF chains and the type of anion. The Barton-Namikawa-Nakajima (BNN) relation σ0 ∼ ωc is fulfilled for all PVDF/IL blends except for that containing 1-ethyl-3- methylimidazolium hydrogen sulfate, [Emim][HSO4]. The activation energy of the ac conductivity, calculated according to the Dyre model, decreases for all PVDF/IL blends with respect to neat PVDF. The structure of the cation of the IL has been found to exert less influence on the dielectric and conductivity properties of the blends.

Cation Hydration and Ion Pairing in Aqueous Solutions of MgCl2 and CaCl2.

Broadband dielectric relaxation spectroscopy (DRS) has been used to investigate aqueous solutions of MgCl2 and CaCl2 up to concentrations of about 1.8 mol L-1 at 25 °C over the frequency range 0.07 ≤ ν/GHz ≤ 89. Detailed analysis of the dominant solvent mode centered at ∼20 GHz showed that both Mg2+ and Ca2+ are strongly solvated, each immobilizing ∼20 water molecules on the DRS timescale. This is consistent with the formation of two well-defined hydration layers around both cations. The hydration shell of Ca2+(aq) was found to be slightly more labile compared with Mg2+(aq). Two or three low-intensity solute-related modes were observed at frequencies ≲10 GHz for MgCl2(aq) and CaCl2, respectively. Two of these modes were attributed to the formation of double-solvent-separated and solvent-shared 1:1 ion pairs. The third mode (observed at very low frequencies and only for some CaCl2 solutions) was thought to be due to an ion-cloud relaxation. No evidence was found for "slow" water or, consistent with the strong cation hydration, for contact ion pairs. The overall association constants for MgCl+(aq) and CaCl+(aq) calculated from the ion-pairing constants were very small but in good agreement with literature values obtained from other techniques.

Experimental and molecular dynamics simulation study on the damping mechanism of C5 petroleum resin/chlorinated butyl rubber composites

In this work, the damping mechanisms of C5 petroleum resin/chlorinated butyl rubber composites were studied by experimentation, molecular dynamics (MD) simulation, and statistical analysis. At the macro level, damping parameters, including glass transition temperature and effective damping temperature region, loss modulus, contact angle, relaxation time, and activation energy were obtained through dynamic mechanical thermal analysis, drop shape analysis, broadband dielectric relaxation spectroscopy, and differential scanning calorimetry. At the micro level, four intermolecular interaction parameters, including binding energy, fractional free volume, mean square radius of gyration, and mean square displacement, were calculated by MD simulation. The quantitative relationships between the damping and intermolecular interaction parameters were obtained by linear regression analysis. The results are expected to provide useful information for understanding damping mechanisms and a quantitative tool for predicting the damping properties of rubber composites.

Molecular dynamics of polysiloxane polar-nonpolar co-networks and blends studied by dielectric relaxation spectroscopy

Polar-nonpolar silicone materials of different morphology and composition were investigated by means of broadband dielectric relaxation spectroscopy in order to elucidate the effect of polar content and nature of the interface on the segmental mobility of polydimethylsiloxane (PDMS) chains. The variation of dielectric parameters with temperature revealed two distinct α-relaxations corresponding to dynamic glass transitions of the components, while another polarization phenomenon was evidenced due to accumulations of charges at the interphases between the two polymers with different electrical properties. It was found that the secondary β-relaxation was favored by the increased content of polar groups, while this parameter had the reverse influence on both αPDMS and αP62, restricting the PDMS chain segment mobility. The activation energy of the interfacial polarization decreased with the polarity, showing increased mobility of charge carriers and less restriction to the interfacial polarization.

Ionic and conformational mobility in poly(vinylidene fluoride)/ionic liquid blends: Dielectric and electrical conductivity behavior

The glass transition dynamics and the charge transport for blends composed of poly(vinylidene fluoride) (PVDF) and the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [Emim][TFSI] have been investigated as a function of different IL content (0, 10, 25 and 40 % wt) by differential scanning calorimetry (DSC), dynamic-mechanical analysis (DMA) and broadband dielectric relaxation spectroscopy (BDS) in wide frequency and temperature ranges (0.1 Hz–1 MHz and −120 to 150 °C, respectively). The inclusion of the IL in the polymer matrix affected the main relaxation process (β-relaxation) of the amorphous phase of the polymer matrix detected with all the techniques employed. It is demonstrated, that the chain segments of PVDF and the IL are mixed at the nanometer range. The blends were homogeneous regardless of the amount of IL and the glass transition temperature (Tg) shifted to lower temperatures as the IL content was increased. A good agreement between the Tg measured by BDS and DSC was observed for all PVDF/IL samples. The conductivity formalism revealed significant contributions of the IL concentration to the conductivity behavior of the blends in that is described by charge motion and electrode polarization effects. The activation energy of all the PVDF/IL samples, calculated by Dyre model, decreased with IL addition with respect to that of neat PVDF.

Structure, nanomechanics, and dynamics of dispersed surfactant‐free clay nanocomposite films

The current work presents a new approach to achieve high quality dispersion of surfactant‐free nanoclay tactoid particles in sub‐micron thin films despite the absence of organic modifier. Natural Montmorillonite particles, Cloisite, were dispersed in thin films of polycaprolactone (PCL) through a flow coating technique assisted by ultra‐sonication. Wide‐angle X‐ray scattering (WAXS), grazing‐incidence wide‐angle X‐ray scattering (GI‐WAXS), and transmission electron microscopy (TEM) were used to confirm the level of natural clay dispersion down to the level of tactoids (sub‐micron scale stacks of clay sheets). These characterization techniques were carried out in conjunction with an analysis of nanomechanical properties via strain‐induced buckling instability for modulus measurements (SIEBIMM), a high‐throughput technique to characterize thin film mechanical properties. The buckling patterns indicate that the natural clay tactoids separate buckling‐enhancing (high‐modulus) crystalline regions and interconnect buckling‐suppressing amorphous (low‐modulus) regions. Due to the tactoid length scale, the glass transition behavior of the composites as characterized by broadband dielectric relaxation spectroscopy was unmodified by the clay. Likewise, the glass transition temperature, Tg, and fragility (slope of relaxation time behavior approaching Tg), remain unaffected, indicating that these dispersed tactoids do not induce pronounced confinement effects on dynamics. POLYM. ENG. SCI., 58:1285–1295, 2018. © 2018 Society of Plastics Engineers