Dielectric spectroscopy and differential scanning calorimetry (DSC) were applied to study the molecular dynamics and thermal properties of a low-molecular-weight glass-forming liquid, salol (phenyl salicylate), confined in anodic aluminum oxide membranes of different pore diameters (100−13 nm). On increasing the geometrical confinement, the glass transition temperature shifts toward lower temperatures, while at the same time broadening of the shape of the structural relaxation is observed. This was attributed to the interplay between surface and confinement effects leading to the transition from Vogel−Fulcher− Tammann-like to Arrhenius-like dependence of the structural relaxation times. We have noticed that the temperature of such crossover agrees with the endothermic process detected by DSC. Combined dielectric and calorimetric data have indicated that this phenomenon is related to the decoupling of the dynamics of molecules attached to the pore walls and those at the center. The enhancement of the structural relaxation of the core molecules increases with decreasing pore size possibly due to changes in the packing density. This finding gives a new insight into the behavior of glass-forming liquids under confinement and helps in the understand of the characteristic shift of the dynamic glass transition temperature with decreasing of the pore diameter. ■ INTRODUCTIONManipulation with the physicochemical properties of the materials at the nanoscale, for instance confined polymers, gives an opportunity to obtain unique morphologies that can find promising applications in nanotechnology as miniaturized sensors, magnetic labels, tissue implants, and so on. 1−3 Therefore, the effects at the nanoscale is a very active research area. For example, under confinement on the nanometer scale, the properties of various materials are affected mostly by the finite size and their interactions with the interfaces or confining surfaces. Numerous studies have shown that the melting/ freezing temperature, solid−solid transition, surface free energy, glass transition, and molecular mobility 4,5 are strongly affected by one or two-dimensional confinement. These changes are hotly discussed in the context of varying pore sizes 6−8 or film thicknesses. 9,10 In addition, the strength and the type of interactions between the confined molecules and pore walls (or a substrate) play a key role and have an important impact on the basic physical properties of different materials and potential applications. 11 Despite the intensive studies, the behavior of glass-forming liquids under confinement is still very puzzling. It is very difficult to rationalize or generalize it, because of the variety of theoretical concepts and experimental results that scatter a lot depending on the confining environment or surface interactions. 12,13 According to literature data, the glass transition temperature T g can decrease, increase, or even remain unaffected under nanoconfinement. 6,14−16 The influence of the spatial restriction on T g can be discussed in t...

The effect of 2D confinement on the dynamics of the normal mode (chain mobility) and segmental relaxation in poly(propylene glycol) (PPG) has been studied with the use of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). It is shown that both processes become faster with increasing degree of confinement. Interestingly, the crossover from VFT to the Arrhenius-like behavior of chain and segmental dynamics, observed in the examined system, is strictly related to the vitrification of the adsorbed polymers. We also report that the mean relaxation times of the normal, τ NM , and segmental modes, τ α , depend on the thermal history of confined PPG and can be significantly modified using different thermal treatments. It is demonstrated that annealing of the samples below the crossover temperature, T c , leads to a systematic shift of the segmental relaxation and normal mode toward lower frequencies, resulting in an increase in the glass transition temperature of the spatially restricted PPG. Taking into account recent studies, we allude this new experimental observation to the density equilibration: after annealing, a system with higher density characterized by more homogeneous dynamics can be obtained. It is therefore possible to modify and control the properties of the confined material by using different thermal treatment protocols. Our results offer a better understanding of the behavior of the spatially restricted soft matter and the interplay between mobilities at two completely different length scales.

Comprehensive molecular dynamics studies of vitrified and cryogrounded itraconazole (Itr) were performed at ambient and elevated pressure. DSC measurements yielded besides melting and glass transition observed during heating and cooling of both samples two further endothermic events at around T = 363 K and T = 346 K. The nature of these transitions was investigated using X-ray diffraction, broadband dielectric spectroscopy and Density Functional Theory calculations. The X-ray measurements indicated that extra ordering in itraconazole is likely to occur. Based on calculations and theory derived by Letz et al. the transition observed at T = 363 K was discussed in the context of formation of the nematic mesophase. In fact, additional FTIR measurements revealed that order parameter variation in Itr shows a typical sequence of liquid crystal phases with axially symmetric orientational order; i.e. a nematic phase in the temperature range 361.7 K to 346.5 K and a smectic A phase below 346.5. Moreover, dielectric measurements demonstrated that except for the structural relaxation process, there is also slower mode above the glass transition temperature in both vitrified and cryogrounded samples. We considered the origin of this mode taking into account DFT calculations, rod like shape of itraconazole and distribution of its dipole moment vectors. For the dielectric data collected at elevated pressure, evolution of the steepness index versus pressure was determined. Finally, the pressure coefficient of the glass transition temperature was evaluated to be equal to 190 K GPa(-1).

The effect of the chemical modification of poly­(propylene glycol) (PPG) end groups on the molecular dynamics under 2D confinement and the polymer/matrix interactions (including interfacial energies) was investigated by a combination of differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), surface tension and contact angle measurements. The replacement of −OH groups in native PPG allowed to modify the interactions with the hydroxyl groups attached to the pore walls of nanoporous aluminum oxide (AAO) membranes of various pore diameter. It was found that the observed reduction in the glass transition temperature (T g) of the core polymers correlates well with a general trend (the higher the solid–liquid interfacial tension, γSL, the lower T g,confined) reported earlier. Moreover, we demonstrated that although the interfacial solid–liquid energy seems to be almost the same for each studied herein material, a clear change in the crossover temperature (T c), related to the vitrification of the polymers adsorbed to the pore walls, is noted. Interestingly, the shift in T c with respect to the glass transition temperature of the bulk polymer scales well according to the decreasing ability in the formation of H bonds in the order PPG–OH → PPG–NH2 → PPG–OCH3 for the constant γSL. One can add that no such effect is found for the glass transition of the core polymers, where a similar shift of the T g was recorded. This finding has been discussed in the context of various sensitivity of the studied materials to the density fluctuations, equilibration phenomena occurring below T c, etc. We believe that our finding will help in a better understanding of an interplay between interfacial and core molecules and contribute significantly to the discussion on the impact of interfacial interactions on the molecular dynamics of polymers under 2D confinement.

The dynamics and thermodynamics of confined triphenyl phosphite (TPP) were studied using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Geometric confinement in channels having length scales commensurate with the molecular size of TPP causes bifurcation of the dynamics: two populations are observed, distinguished by their reorientational mobilities and glass transition temperatures. Upon cooling, significant changes in the relaxation process and temperature dependence occur due to the slow vitrification of the molecules in close proximity to the interface. Such a kinetic aspect of glass formation is unusual. This surface interaction alleviates constraints on the molecules, allowing their glass transition to shift to lower temperatures. Simultaneously, it was observed that the structural relaxation process shifts to lower frequencies, and the distribution of the relaxation times becomes narrower upon annealing. This effect is especially visible at lower frequencies, indicating the decreasing contribution of those molecules characterized by slower dynamics. In addition, it was found that structural relaxation times, as well as the glass transition temperatures, can be significantly modified by annealing samples over a particular range of temperatures. This work facilitated the understanding of the interplay between different kinds of mobility and its impact on changes in the glass transition temperature for two-dimensional confined materials.

The properties of a molecular liquid confined at the nanometer length scale can be very distinct from the bulk. For that reason, the macro- and the nanoscopic behaviors of glass-forming liquids are regarded as two nonconnected realms, governed by their own rules. Here, we show that the glassy dynamics in molecular liquids confined to nanometer pores might obey the density scaling relation, ρ/T, just like in bulk fluids. Even more surprisingly, the same value of the scaling exponent γ superposes the α-relaxation time measured at different state points in nanoscale confinement and upon increased pressure. We report this remarkable finding for van der Waals liquids tetramethyl-tetraphenyl-trisiloxane (DC704) and polyphenyl ether (5PPE), considered as simple, single-parameter liquids. Demonstrating that the density scaling idea can be fulfilled in both environments opens an exciting possibility to predict the dynamic features of the nanoconfined system close to its glass-transition temperature from the high-pressure studies of the bulk liquid. Likewise, we can describe the viscous liquid dynamics at any given combination of temperature and pressure based on analysis of its behavior in nanopores.

Here, we have studied the effect of spatial restrictions on the molecular dynamics and crystallization behavior of modeled lipophilic drug fenofibrate incorporated into nanoporous aluminum oxide membranes of different pore size. Our measurements demonstrate that, on subsequent cooling, dynamics of confined liquid split up into two distinct fractions, due to the presence of core and interfacial layers. At the temperature, at which vitrification of the interfacial layer takes place (T g_interface), departure from the bulk-like behavior occurs, and molecules in the center of the pores enter quasi-isochoric conditions. Depending on the thermal protocol and pore size, the volume fixed at T g_interface might be a bit different so as the core liquid’s dynamics. Interestingly, below that temperature, the nanoconfined liquid can still obey the fundamental density scaling relation (1/TV γ), just like in the bulk phase, while not necessarily isochronal superposition. This is in contrast to a common observation that the validity of the density scaling in bulk glass-forming systems always goes together with isochronal superposition of the α-relaxation, and vice versa. Finally, our careful analysis of the crystallization kinetics as a function of lowering pore diameter indicates for systematic slowing down crystallization progress, the shift of the maximum crystallization rate toward higher undercooling and decrease in the dimensionality of growing crystals.

In this paper, we have investigated the molecular dynamics above and below the glass-transition temperature of bisphenol-A diglycidyl ether (known as DGEBA, M n = 340 g/mol) infiltrated in nanoporous alumina (AAO) templates of various pore sizes by means of dielectric and Raman spectroscopies. It was found that the temperature dependence of the structural relaxation times is different under confinement with respect to the bulk sample even in the high-temperature regime. Interestingly, below the glass-transition temperature, the slow secondary process (β) was not detected in dielectric loss spectra of confined DGEBA, while the relaxation times of the faster secondary process (γ) were unaffected by the pore size. To explain this phenomenon, two different scenarios, considering either suppression of the motions related to this mobility or enhancement of its dynamics, were taken into account. Additional annealing experiments, which lead to density perturbation, enabled us to recover bulk-like temperature dependence of structural relaxation times for all confined systems. This finding was discussed in view of the outcome of Raman and contact angle measurements that have shown rather weak interactions between DGEBA and the template. It is also worthwhile to add that except for the clear broadening of the fast secondary relaxation peak, the relaxation times of this process varied within experimental uncertainties due to annealing.