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The molecular mobility of condensed matter confined to nanometer dimensions can be dramatically changed from those of the bulk state in such a way that, when the guest is a drug, it can be advantageously used in pharmaceutical applications. We show by dielectric relaxation spectroscopy that the molecular mobility of the important ibuprofen drug embedded in nanoporous SBA-15 is significantly influenced by the confinement. An evidence of the existence of two families of molecules with different molecular mobilities is provided and investigated in their temperature dependence. One family is due to molecules close to the pores' center with a higher mobility compared with the bulk at low temperatures, and another family with slower dynamics originated from molecules interacting with the pore walls. The work reports the simultaneous manifestation of true confinement and surface effects in this nanostructured silica host for a drug. For future applications in drug delivery systems, the dynamics determined by the guest–host interplay and the one of the bulklike molecules can be tuned to achieve a desired release profile.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. Here, the vibrational density of states (VDOS) for PIM-1, the prototypical polymer with intrinsic microporosity, is investigated by means of inelastic neutron scattering. The results are compared to data measured for a more conventional high-performance polyimide used in gas separation membranes (Matrimid). The measured data show the characteristic low frequency excess contribution to VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. In comparison to the Boson peak of Matrimid, that of PIM-1 is shifted to lower frequencies. This shift is discussed considering the microporous, sponge-like structure of PIM-1 as providing a higher compressibility at the molecular scale than for conventional polymers. For an annealed PIM-1 sample, the Boson peak shifts to higher frequencies in comparison to the un-annealed sample. These changes in the VDOS of the annealed PIM-1 sample are related to changes in the microporous structure as confirmed by X-ray scattering.
Specific heat spectroscopy was used to study the dynamic glass transition of ultrathin poly(2-vinyl pyridine) films (thicknesses: 405–10 nm). The amplitude and the phase angle of the differential voltage were obtained as a measure of the complex heat capacity. In a traditional data analysis, the dynamic glass transition temperature Tg is estimated from the phase angle. These data showed no thickness dependency on Tg down to 22 nm (error of the measurement of ±3 K). A derivative-based method was established, evidencing a decrease in Tg with decreasing thickness up to 7 K, which can be explained by a surface layer. For ultrathin films, data showed broadening at the lower temperature side of the spectra, supporting the existence of a surface layer. Finally, temperature dependence of the heat capacity in the glassy and liquid states changes with film thickness, which can be considered as a confinement effect.
Specific heat spectroscopy in the frequency range typically from 1 Hz to 1 kHz with a sensitivity of pJ/K was employed to study the glass transition behavior of ultrathin poly(vinyl methyl ether) (PVME) films with thicknesses ranging from 218 nm down to 12 nm. The amplitude and the phase angle of the complex differential voltage as a measure of the complex heat capacity were obtained as a function of temperature at a given frequency simultaneously. Both spectra are used to determine the dynamic glass transition temperature as a function of both the frequency and the film thickness. As main result no thickness dependence of the dynamic glass transition temperature was observed down to a film thickness of 12 nm within the experimental uncertainty of ±2 K. Further the width of the glass transition is independent of the film thickness which indicates that the extent of the cooperativity is essentially smaller than 12 nm.
Carbon nanomembranes are synthetic two-dimensional sheets with nanometer thickness, macroscopic lateral dimensions, and high structural homogeneity. They have great application potential in various branches of nanotechnology. Because of their full carbon structure, it is not clear whether macromolecules like poly(methyl methacrylate) (PMMA) can be irreversibly adsorbed on their surface. Here, irreversible adsorption means that the polymer chains cannot be removed by a leaching process, which is assumed in technological transfer processes. However, if polar defects are present on the carbon nanomembranes (CNMs), it may occur that polymers can be irreversibly adsorbed. To address this question, PMMA was spin-coated on top of CNMs, annealed for a specific time at different temperatures, and then tried to be removed by a acetone treatment in a leaching approach. The samples were investigated in detail by atomic force microscopy, X-ray photoelectron spectroscopy, and broadband dielectric spectroscopy, where the latter method has been applied to CNMs for the first time. Unambiguously, it was shown that PMMA can be adsorbed on the surface of CNMs after annealing the sample above the glasstransition temperature of PMMA. The general occurrence of polar defects on the surface of CNMs and the adsorption of polymers open opportunities for advanced innovative hybrid materials combining the properties of the CNM with those of the polymer.
The influence of distinct carbon based nanofillers: expanded graphite (EG), conducting carbon black (CB), thermally reduced graphene oxide (TRGO) and multi-walled carbon nanotubes (CNT) on the thermal, dielectric, electrical and rheological properties of polybutylene terephthalate (PBT) was examined. The glass transition temperature (Tg) of PBT nanocomposites is independent of the filler type and content. The carbon particles act as nucleation agents and significantly affect the melting temperature (Tm), the crystallization temperature (Tc) and the degree of crystallinity of PBT composites. PBT composites with EG show insulating behaviour over the tested concentration range of 0.5 to 2 wt.-% and hardly changed rheological behaviour. CB, CNT and TRGO induce electrical conductivity to their particular PBT composites by forming a conducting particle network within the polymer matrix. CNT reached the percolation threshold at the lowest concentration (<0.5 wt.-%), followed by TRGO (<1 wt.-%) and CB (<2 wt.-%). With the formation of a particle network, the flow behaviour of composites with CB, CNT and TRGO is affected, i.e., a flow limit occurs and the melt viscosity increases. The degree of influence of the carbon nanofillers on the rheological properties of PBT composites follows the same order as for electrical conductivity. Electrical and rheological results suggest an influence attributed to the particle dispersion, which is proposed to follow the order of EG<< CB<TRGO<CNT.
The complex effects of nanoparticles on a thermosetting material based on an anhydride cured DGEBA/Boehmite nanocomposite with different particle concentrations is considered. A combination of X-ray scattering, calorimetry, including fast scanning calorimetry as well as temperature modulated calorimetry and dielectric spectroscopy, was employed to study the structure, the vitrification kinetics and the molecular dynamics of the nanocomposites. For the first time in the literature for an epoxy-based composite a detailed analysis of the X-ray data was carried out. Moreover, the unfilled polymer was found to be intrinsically heterogeneous, showing regions with different crosslinking density, indicated by two separate dynamic glass transitions. The glass transition temperature decreases with increasing nanoparticle concentration, resulting from a change in the crosslinking density. Moreover, on the one hand, for nanocomposites the incorporation of nanofiller increased the number of mobile segments for low nanoparticle concentrations, due to the altered crosslinking density. On the other hand, for higher loading degrees the number of mobile segments decreased, resulting from the formation of an immobilized interphase (RAF). The simultaneous mobilization and immobilization of the segmental dynamics cannot be separated unambiguously. Taking the sample with highest number of mobile segments as reference state it was possible to estimate the amount of RAF.
The molecular dynamics of the triphenylene-based discotic liquid crystal HAT6 is investigated by broadband dielectric spectroscopy, advanced dynamical calorimetry and neutron scattering. Differential scanning calorimetry in combination with X-ray scattering reveals that HAT6 has a plastic crystalline phase at low temperatures, a hexagonally ordered liquid crystalline phase at higher temperatures and undergoes a clearing transition at even higher temperatures. The dielectric spectra show several relaxation processes: a localized gamma-relaxation a lower temperature and a so called alpha-2-relaxation at higher temperatures. The relaxation rates of the alpha-2-relaxation have a complex temperature dependence and bear similarities to a dynamic glass transition. The relaxation rates estimated by hyper DSC, Fast Scanning calorimetry and AC Chip calorimetry have a different temperature dependence than the dielectric alpha-2-relaxation and follows the VFT-behavior characteristic for glassy dynamics. Therefore, this process is called alpha-1-relaxation. Its relaxation rates show a similarity with that of polyethylene. For this reason, the alpha-1-relaxation is assigned to the dynamic glass transition of the alkyl chains in the intercolumnar space. Moreover, this process is not observed by dielectric spectroscopy which supports its assignment. The alpha-2-relaxation was assigned to small scale translatorial and/or small angle fluctuations of the cores.
The neutron scattering data reveal two relaxation processes. The process observed at shorter relaxation times is assigned to the methyl group rotation. The second relaxation process at longer time scales agree in the temperature dependence of its relaxation rates with that of the dielectric gamma-relaxation.
Thin films (200-7nm) of the asymmetric polymer blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) (25/75wt%) were investigated by broadband dielectric spectroscopy (BDS). Thicker samples ([Formula: see text]37 nm) were measured by crossed electrode capacitors (CEC), where the film is capped between Al-electrodes. For thinner films ([Formula: see text]37 nm) nanostructured capacitors (NSC) were employed, allowing one free surface in the film. The dielectric spectra of the thick films showed three relaxation processes ( [Formula: see text] -, [Formula: see text] - and [Formula: see text] -relaxation), like the bulk, related to PVME fluctuations in local spatial regions with different PS concentrations. The thickness dependence of the [Formula: see text] -process for films measured by CECs proved a spatially heterogeneous structure across the film with a PS-adsorption at the Al-electrodes. On the contrary, for the films measured by NSCs a PVME segregation at the free surface was found, resulting in faster dynamics, compared to the CECs. Moreover, for the thinnest films ([Formula: see text]26 nm) an additional relaxation process was detected. It was assigned to restricted fluctuations of PVME segments within the loosely bounded part of the adsorbed layer, proving that for NSCs a PVME enrichment takes place also at the polymer/substrate interface.
Ionic Liquid Crystals are ionic liquids that exhibit liquid crystalline mesomorphism together with ionic conductivity. As known confined liquid crystal mesophases can show an anomalous dynamics and phase behavior. Investigations considering the factors controlling the macroscopic properties of ILCs in confinement are scare in the literature. This study reports the molecular mobility, and the phase transition behavior of a guanidinium based columnar ILC confined in the nanopores of self-ordered anodic aluminum oxide membranes of various pore diameters (25 – 180 nm) using Broadband Dielectric Spectroscopy (BDS), calorimetry and X-ray scattering. It is aimed to reveal in which way the pore size as well as the pore surface wettability (hydrophobic or hydrophilic) alters the molecular dynamics, and phase transition behavior for this system. These properties are crucial for applications. The DSC investigations reveal: (i) the phase transition temperature for the transition from the plastic crystalline to the crystalline-liquid state has non-monotonic dependence versus the inverse pore diameter and (ii) the transition from the liquid crystalline to the isotropic phase is suppressed for all nanoconfined samples. This transition suppressed in the thermal signal was evidenced by BDS and X-ray scattering. It is discussed as a continuous phase transition taking place in the pores instead of a discontinuous first order transition as observed for the bulk. BDS investigations show different relaxation processes for the bulk and the nanoconfined ILC. Molecular origins for various relaxation processes are discussed and suggested. It is further shown that the self-assembly of this ILC is dynamic in nature which might apply for other ILCs too. The obtained results will have implications for the nanoscale applications of ionic liquid crystals.