How thermal stress alters the confinement of polymers vitrificated in nanopores

Quantifying and Modeling the Crystallinity of Polymers Confined in Nanopores

We propose a methodology to characterize the crystalline content of interfacial polymer layers in systems confined at the nanoscale level in a 2D geometry. Based on the crystallinity data of a set of polymers, we introduce a simple model to describe the gradient in crystallinity introduced by confining polymer chains in nanopores. Our model underscores the pivotal role that interfaces play in crystallization and unequivocally contradicts the existence of interfacial "dead" layers where crystallization cannot take place. Further, we verified that the organization of crystals near the pore walls resembles the macromolecular architecture of adsorbed layers, hinting at a strong interplay between crystallization and adsorption.

Biphasic GaIn Alloy Constructed Stable Percolation Network in Polymer Composites over Ultrabroad Temperature Region

Flexible and adaptable polymer composites with high-performance reliability over wide temperature range are imperative for various applications. However, the distinct filler-matrix thermomechanical behaviors often cause severe structure damage and performance degradation upon large thermal shock. To address this issue, a general strategy is proposed to construct leakage-free, self-adaptive, stable percolation networks in polymer composites over wide temperature (77-473 K) with biphasic Ga35In65 alloy. The in situ micro-CT technology, for the first time, reveals the conformable phase transitions of Ga35In65 alloys in the polymer matrix that help repair the disruptive conductive networks over large temperature variations. The cryo-expanded Ga compensates the disruptive carbon networks at low temperatures, and flowable Ga and melted In at high temperatures conformably fill and repair the deboned interfaces and yielded crevices. As a proof-of-concept, this temperature-resistant composite demonstrates superb electrical conductivity and electromagnetic interference shielding properties and stability even after a large temperature shock (ΔT = 396 K). Furthermore, the superiority of the construction of temperature self-adaptive networks within the composite enables them for additive manufacturing of application-oriented components. This work offers helpful inspiration for developing high-performance polymer composites for extreme-temperature applications.

The glass transition and enthalpy recovery of polystyrene nanorods using Flash differential scanning calorimetry

The glass transition (Tg) behavior and enthalpy recovery of polystyrene nanorods within an anodic aluminum oxide (AAO) template (supported nanorods) and after removal from AAO (unsupported nanorods) is studied using Flash differential scanning calorimetry. Tg is found to be depressed relative to the bulk by 20 ± 2 K for 20 nm-diameter unsupported polystyrene (PS) nanorods at the slowest cooling rate and by 9 ± 1 K for 55 nm-diameter rods. On the other hand, bulk-like behavior is observed in the case of unsupported 350 nm-diameter nanorods and for all supported rods in AAO. The size-dependent Tg behavior of the PS unsupported nanorods compares well with results for ultrathin films when scaled using the volume/surface ratio. Enthalpy recovery was also studied for the 20 and 350 nm unsupported nanorods with evolution toward equilibrium found to be linear with logarithmic time. The rate of enthalpy recovery for the 350 nm rods was similar to that for the bulk, whereas the rate of recovery was enhanced for the 20 nm rods for down-jump sizes larger than 17 K. A relaxation map summarizes the behavior of the nanorods relative to the bulk and relative to that for the 20 nm-thick ultrathin film. Interestingly, the fragility of the 20 nm-diameter nanorod and the 20 nm ultrathin film are identical within the error of measurements, and when plotted vs departure from Tg (i.e., T - Tg), the relaxation maps of the two samples are identical in spite of the fact that the Tg is depressed 8 K more in the nanorod sample.

Physical Aging of Polystyrene Confined in Anodic Aluminum Oxide Nanopores

We investigated the glassy dynamics of polystyrene (PS) confined in anodic aluminum oxide (AAO) nanopores by differential scanning calorimetry. Based on the outcome of our experiments, we show that the cooling rate applied to process the 2D confined PS melt has a significant impact on both the glass transition and the structural relaxation in the glassy state. A single glass transition temperature (T_g) is observed in quenched samples, while slow-cooled PS chains show two T_gs corresponding to a core-shell structure. The former phenomenon resembles what is observed in freestanding structures, while the latter is imputed to the adsorption of PS onto AAO walls. A more complex picture was drawn for physical aging. In the case of quenched samples, we observed a non-monotonic trend of the apparent aging rate that in 400 nm pores, reaches a value almost twice as larger than what is measured in bulk and decreases upon further confinement in smaller nanopores. For slow-cooled samples, by adequately varying the aging conditions, we were able to control the equilibration kinetics and either separate the two aging processes or induce an intermediate aging regime. We propose a possible explanation of these findings in terms of distribution in free volume and the presence of different aging mechanisms.

The Influence of Thermal Treatments on Anchor Effect in NMT Products

The anchor effect in nanomolding technology (NMT) refers to the effect that polymer nanorods in nanopores on metal surfaces act as anchors to firmly bond the outside polymer components onto the metal surface. In this work, the influences of thermal treatments on the anchor effect are studied at microscopic level from the perspective of interfacial interaction by a model system (poly(n-butyl methacrylate) (PBMA) and alumina nanopore composite). The differential scanning calorimeter and fluorescence results indicate that the formation of a dense polymer layer in close contact with the pore walls after proper thermal treatments is the key for a strong interfacial interaction. Such polymer layers were formed in NMT products composed of PBMA and aluminum after slow cooling or annealing, with an up to eighteen-fold improvement of the interfacial bonding strength. The polymer chains near the nanopore walls eliminate the thermal stress induced by the mismatch of thermal expansion coefficients through relaxation over time and remain in close proximity with the pore walls during the cooling process of nanomolding. The above dynamic behaviors of the polymer chains ensure the formation of stable interfacial interaction, and then lead to the formation of the anchor effect.

Investigation of nematic to smectic phase transition and dynamical properties of strongly confined semiflexible polymers using Langevin dynamics

We investigated the nematic to smectic phase transition for strongly confined semiflexible polymer solutions in slit-like confinements using GPU-accelerated Langevin dynamics. We characterized the phase transitions from the nematic to smectic phases for semi-flexible polymer solutions as the polymer density increased. The dependence for the lyotropic nematic to smectic transition can be collapsed by scaling exponents between 0.2 and 0.3. The smectic C phase is found for all the cases with the polymer orientation director tilted with respect to smectic layer lateral alignment. As the chain rigidity increases, the transition density decreases for systems in which the polymer persistence length (P) to slit height (H) ratios are 1.25, 2.5, 3.75, 5 and 25. We also characterized the polymer dynamics for the isotropic-nematic-smectic transitions. The overall polymer diffusivity decreased steadily as the polymer density increased. We observed anomalous polymer diffusion along the nematic director near the isotropic-nematic transition, similar to previously reported behavior for nematic-forming ellipsoids. Polymer diffusivity decreased sharply by two orders of magnitude upon the nematic-smectic transition.

Nanostructures and Dynamics of Isochorically Confined Amorphous Drug Mediated by Cooling Rate, Interfacial, and Intermolecular Interactions

The production and stabilization of amorphous drugs by the nanoconfinement effect has recently become a research hotspot in pharmaceutical sciences. Herein, two guest/host systems, indomethacin (IMC) and griseofulvin (GSF) confined in anodic aluminum oxide (AAO) templates with different pore diameters (25-250 nm) are investigated by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The crystallization of the confined drugs is suppressed, and their glass transition temperatures show an evident pore-size dependency. Moreover, a combination of dielectric and calorimetric results demonstrates that the significant change in the temperature dependence of the structural relaxation time during the cooling process is attributed to the vitrification of the interfacial molecules and the local density heterogeneity under isochoric confinement. Interestingly, compared with the case of IMC/AAO, which can be described by a typical two-layer model, GSF/AAO presents an rare scenario of three glass transition temperatures under fast cooling (40-10 K/min), indicating that there exists a thermodynamic nonequilibrium interlayer between the bulk-like core and interfacial layer. In contrast, the slow cooling process (0.5 K/min) would lead confined GSF into the stable core-shell nanostructure. Using surface modification, the interfacial effect is confirmed to be an important reason for the different phenomena between these two guest/host systems, and intermolecular hydrogen bonding is also suggested to be emphasized considering the long-range effect of interfacial interactions. Our results not only provide insight into the glass transition behavior of geometrically confined supercooled liquids, but also offer a means of adjusting and stabilizing the nanostructure of amorphous drugs under two-dimensional confinement.