Confined Crystallization of Polymers within 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.

Liquid-liquid phase separation induced auto-confinement

Confinement allows macromolecules and biomacromolecules to attain arrangements typically unachievable through conventional self-assembly processes. In the field of block copolymers, confinement has been achieved by preparing thin films and controlled solvent evaporation through the use of emulsions. A significant advantage of the confinement-driven self-assembly process is its ability to enable block copolymers to form particles with complex internal morphologies, which would otherwise be inaccessible. Here, we show that liquid-liquid phase separation (LLPS) can induce confinement during the self-assembly of a model block copolymer system. Since this confinement is driven by the block copolymers' tendency to undergo LLPS, we define this confinement type as auto-confinement. This study adds to the growing understanding of how LLPS influences block copolymer self-assembly and provides a new method to achieve confinement driven self-assembly.

In Situ Growth of MOF-303 Membranes onto Porous Anodic Aluminum Oxide Substrates for Harvesting Salinity-Gradient Energy

As an emerging metal-organic framework (MOF) material in recent years, the MOF-303 membrane has shown great potential applications in seawater desalination, dehydration, and atmospheric water harvesting. Herein, we report on a dense and uniform MOF-303 membrane fabricated by a facile in situ hydrothermal synthesis approach in the presence of an anodized aluminum oxide (AAO) channel membrane acting as the only Al source and substrate. Interestingly, the MOF-303 isomer can be obtained due to an insufficient amount of organic ligand caused by the less hydrophilic and larger pore size of the AAO substrate. The MOF-based composite membranes possessed surface-charge-governed ionic transport behavior. Moreover, the MOF-303/AAO membrane yielded an output power density of 1.87 W/m2 under a 50-fold KCl concentration gradient. Under a 50-fold gradient of artificial seawater and river water, a maximum power density of 1.46 W/m2 can be obtained. After 30 days of stability testing, the composite membrane still maintained the power output, and the power density was higher than 1.20 W/m2. This work provides a facile and effective strategy for constructing Al-based MOF composite membranes and boosts their applications in harvesting salinity-gradient energy.

Ordered and disordered microstructures of nanoconfined conducting polymers

We probe the microstructural differences of conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives under geometrical nanoconfinement using a high-resolution electron microscopy (HRTEM) technique. Highly ordered domains of poly(3,4-ethylenedioxythiophene):tosylate PEDOT:Tos, which is polymerized within alumina nanochannels, are observed. These features are in contrast to those of the polymer blend poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) PEDOT:PSS inserted into the nanopores. The extent of the order-disorder parameter in terms of surface crystallization and the number of ordered domains of the long-chain polymers strongly depends on the dopant environment, processing conditions and structural confinement. Atomic force spectroscopy of individual PEDOT nanochannels highlights counterion-dependent surface adhesive factors. The molecular dynamics (MD) simulation of these systems reveals similar polymer chain configurations and the resulting morphology.

Single-Molecular Poly(propylene oxide) (PPO) Nucleus-Guided Assembly for Hydrophobicity-Dependent Molecular Transport in the Nanopore

By combining DNA nanotechnology and solid-phase nanopore technology, the aggregation behavior of polymer guided by a single-molecular poly(propylene) (PPO) nucleus in a 3D DNA network has been studied. At low temperature, the PPO chain is evenly dispersed in the rigid 3D DNA network; at higher temperature, the PPO chain self-collapses to a single-molecular nucleus; and upon addition of amphiphilic block copolymers below the critical micelle concentration (CMC), the chains tend to aggregate on the isolated hydrophobic nucleus through intermolecular hydrophobic interactions. The process has been characterized by a rheological test and an electrochemical test. This study not only provides a preliminary understanding of the nucleation and growth process of block copolymers but also offers a theoretical basis for the study of protein self-folding and aggregation in the future. On this basis, utilizing this nucleation and growth event, a novel smart nanopore has been developed for hydrophobicity-dependent molecular transport.

Molecular Mobility of Polymers at the Melting Transition

Melting of crystals is an archetypical first order phase transition. Albeit extensive efforts, the molecular origin of this process in polymers is still not clear. Experiments are complicated by the tremendous change in mechanical properties and the occurrence of parasitic phenomena masking the genuine material response. Here, we present an experimental procedure permitting to circumvent these issues by investigating the dielectric response of thin polymer films. Extensive measurements on several commercially available semicrystalline polymers allowed us to identify a genuine molecular process associated with the newly formed liquid phase. In line with recent observations of amorphous polymer melts, we show this mechanism─known as the slow Arrhenius process (SAP)─involves time scales longer than those characteristics of segmental mobility and has the same energy barrier of the flow of the melt.

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.

Steering self-organisation through confinement

Self-organisation is the spontaneous emergence of spatio-temporal structures and patterns from the interaction of smaller individual units. Examples are found across many scales in very different systems and scientific disciplines, from physics, materials science and robotics to biology, geophysics and astronomy. Recent research has highlighted how self-organisation can be both mediated and controlled by confinement. Confinement is an action over a system that limits its units' translational and rotational degrees of freedom, thus also influencing the system's phase space probability density; it can function as either a catalyst or inhibitor of self-organisation. Confinement can then become a means to actively steer the emergence or suppression of collective phenomena in space and time. Here, to provide a common framework and perspective for future research, we examine the role of confinement in the self-organisation of soft-matter systems and identify overarching scientific challenges that need to be addressed to harness its full scientific and technological potential in soft matter and related fields. By drawing analogies with other disciplines, this framework will accelerate a common deeper understanding of self-organisation and trigger the development of innovative strategies to steer it using confinement, with impact on, e.g., the design of smarter materials, tissue engineering for biomedicine and in guiding active matter.

Effects of network junctions and defects on the crystallization of model poly(ethylene glycol) networks

Polymer crystallization drastically changes the physical properties of polymeric materials. However, the crystallization in polymer networks has been little explored. This study investigated the crystallization behavior of a series of poly(ethylene glycol) (PEG) networks consisting of well-defined branched precursors. The PEG networks were prepared by drying gels synthesized at various conditions. The PEG networks showed slower crystallization with lower final crystallinity than uncrosslinked PEGs with amine end groups. Surprisingly, the effect of network formation was not as significant as that of the relatively bulky end-groups introduced in the uncrosslinked polymer. The molecular weight of the precursor PEG, or equivalently the chain length between neighboring junctions, was the primary parameter that affected the crystallization of the PEG networks. Shorter network chains led to lower crystallization rates and final crystallinity. This effect became less significant as the network chain length increased. On the other hand, the spatial and topological defects formed in the gel synthesis process did not affect the crystallization in the polymer networks at all. The crystallization in the polymer networks seems insensitive to these mesoscopic defects and can be solely controlled by the chain length between junctions.

Melting Temperature Depression and Phase Transitions of Nitrate-Based Molten Salts in Nanoconfinement

Hybrids of nitrate-based molten salts (KNO₃, NaNO₃, and Solar Salt) and anodic aluminum oxide (AAO) with various pore sizes (between 25 and 380 nm) were designed for concentrated solar power (CSP) plants to achieve low melting point (<200 °C) and high thermal conductivity (>1 W m^-1 K^-1). AAO pore surfaces were passivated with octadecyl phosphonic acid (ODPA), and the results were compared with as-anodized AAO. The change in phase transition temperatures and melting temperatures of salts was investigated as a function of pore diameter. Melting temperatures decreased for all salts inside AAO with different pore sizes while the highest melting temperature decrease (ΔT = 173 ± 2 °C) was observed for KNO₃ filled in AAO with a pore diameter of 380 nm. Another nanoconfinement effect was observed in the crystal phases of the salts. The ferroelectric phase of KNO₃ (γ-phase) formed at room temperature for KNO₃/AAO hybrids with pore size larger than 35 nm. Thermal conductivity values of molten salt (MS)/AAO hybrids were obtained by thermal property analysis (TPS) at room temperature and above melting temperatures of the salts. The highest increase in thermal conductivity was observed as 73% for KNO₃/AAO-35 nm. For NaNO₃/AAO-380 nm hybrids, the thermal conductivity coefficient was 1.224 ± 0.019 at room temperature. To determine the capacity and efficiency of MS/AAO hybrids during the heat transfer process, the energy storage density per unit volume (J m^-3) was calculated. The highest energy storage capacity was calculated as 2390 MJ m^-3 for KNO₃/AAO with a pore diameter of 400 nm. This value is approximately five times higher than that of bulk salt.