What Happens to Polymer Chains Confined in Rigid Cylindrical Inorganic (AAO) Nanopores

Spatially Confined Fabrication of Polar Poly(Vinylidene Fluoride) Nanotubes

Polar poly(vinylidene fluoride) (PVDF) nanotubes have attracted significant attention due to their excellent piezoelectric and ferroelectric properties, yet a tunable fabrication of homogeneous polar PVDF nanotubes remains a challenge. Here, a simple method is reported to fabricate polar PVDF nanotubes using anodize aluminum oxide (AAO) membranes as templates that are removed by etching in a potassium hydroxide (KOH) solution and then ageing at room temperature. PVDF nanotubes originally crystallized in the AAO membrane are pure α-crystals with very low crystallinity, yet after being released from the templates, the crystallinity of the nanotubes markedly increases with ageing at room temperature, leading to the formation of β-PVDF crystals in a very short time, with the formation of γ crystals after longer ageing times. A large amount of γ crystals formed when the released PVDF nanotubes are heated to ≈130 °C. The formation of polar PVDF nanotubes released from the AAO templates treated with higher concentrations of alkaline solution results from the reaction of the surface of the PVDF nanotubes with the alkaline solution and structure reorganization under confined conditions. This large-scale preparation of β- and γ-PVDF opens a new pathway to produce polar PVDF nanomaterials.

Structure and dynamics of a 1,4-polybutadiene melt in an alumina nanopore: A molecular dynamics simulation

We present results of Molecular Dynamics simulations of a chemically realistic model of 1,4-polybutadiene (PBD)confined in a cylindrical alumina nanopore of diameter 10 nm. The simulations are done at three different temperaturesabove the glass transition temperature T g . We investigate the density layering across the nanopore as well as theorientational ordering in the polymer melt, brought about by the confinement, on both the segmental and chain scales.For the chain scale ordering, the magnitude and orientation of the axes of the gyration tensor ellipsoid of single chainsare studied and are found to prefer to align parallel to the pore axis. Even though double bonds near the wall arepreferentially oriented along the pore walls, studying the nematic order parameter indicates that there is no nematicordering at the melt-wall interface. As for the dynamics in the melt, we focus here on the mean-square-displacement ofthe monomers for several layers across the nanopore as well as the movement of the chain center of mass which bothdisplay a slowing down of the dynamics in the layer at the wall. We also show the strong adsorption of the monomersto the pore wall at lower temperatures.

Molecular simulation of polymer crystallization under chain and space confinement

Polymer crystallization under chain and space confinements is studied by Monte Carlo simulation. The simulation results show that the crystallinity and melting temperature of confined systems increase with the increase of free chain content. Furthermore, the crystallinity and melting temperature of confined systems with larger lateral size are higher than those with smaller lateral size. These findings are in good agreement with the conclusions obtained in some experiments. An important phenomenon that cannot be observed in experiments has been confirmed, that is, the tethering point can be used as the nucleation site. For the confined polymer system with the lateral size of 8 lattice points, with the increase of free chain content, the surface free energy of the nuclei and the diffusion activation energy of the chains decrease due to the combined effects of chain conformation size and chain movement ability, which leads to the enhancement of the nucleation ability of polymers. However, for the confined polymer system with lateral size of 12 lattice points, with the increase of free chain content, the nucleation sites decrease and the critical free energy barrier increases, which are not conducive to nucleation. Moreover, the existence of interfacial interactions can also significantly change the crystallization of confined polymers. Our results indicate the crystallization kinetics of the confined polymer from a microscopic point of view.

Conformation and dynamics of ring polymers under symmetric thin film confinement

Understanding the structure and dynamics of polymers under confinement has been of widespread interest, and one class of polymers that have received comparatively little attention under confinement is that of ring polymers. The properties of non-concatenated ring polymers can also be important in biological fields because ring polymers have been proven to be a good model to study DNA organization in the cell nucleus. From our previous study, linear polymers in a cylindrically confined polymer melt were found to segregate from each other as a result of the strong correlation hole effect that is enhanced by the confining surfaces. By comparison, our subsequent study of linear polymers in confined thin films at similar levels of confinements found only the onset of segregation. In this study, we use molecular dynamics simulation to investigate the chain conformations and dynamics of ring polymers under planar (1D) confinement as a function of film thickness. Our results show that conformations of ring polymers are similar to the linear polymers under planar confinement, except that ring polymers are less compressed in the direction normal to the walls. While we find that the correlation hole effect is enhanced under confinement, it is not as pronounced as the linear polymers under 2D confinement. Finally, we show that chain dynamics far above Tg are primarily affected by the friction from walls based on the monomeric friction coefficient we get from the Rouse mode analysis.

Efficient Separation of Nucleic Acids with Different Secondary Structures by Metal-Organic Frameworks

We report the use of metal-organic frameworks (MOFs) for the selective separation of nucleic acids (DNA and RNA) with different secondary structures through size, shape, length, and capability of conformational transition. Three MOFs with precisely controlled pore environments, Co-IRMOF-74-II, -III, and -IV, composed of Co²⁺ and organic linkers (II, III, and IV), respectively, were used for the inclusion of nucleic acid into their pores from the solution. This was proven to be a spontaneous process from disordered free state to restricted ordered state via circular dichroism (CD) spectroscopy. Three critical factors were identified for their inclusion: (1) size selection induced by steric hindrance, (2) conformation transition energy selection induced by stability, and (3) molecular weight selection. These selection rules were used to extract nucleic acids with flexible and unstable secondary structures from complex mixtures of multiple nucleic acids, leaving those with rigid and stable secondary structures in the mother liquor. This provides the possibility to separate and enrich nucleic acids in bulk through their different structure feature, which is highly desirable in genome-wide structural measurement of nucleic acids. Unlike methods that rely on specific binding antibodies or ligand, this MOF method is capable of selecting all kinds of nucleic acids with similar secondary structure features; therefore, it is suitable for the handling of a large variety and quantity of nucleic acids at the same time. This method also has the potential to gather information about the folding stability of biomolecules with secondary structures.

Polymers grown in cavities: Vesicles and droplets

In synthetic chemistry and biological or biomimetic systems, polymers are often grown in cavities. Polymerizations in microemulsions, biopolymers grown in cells, or in vesicles containing artificial organelles have an influence on the shape of liquid boundaries. We consider confined grand-canonical polymers to address equilibrium properties of annealed polymers. We calculate the concentration profiles established by annealed (star-) polymers inside a confining cavity. Our emphasis is on the description of pressure fields derived from the contact theorem. We further show how the pressure field exerted by a localized annealed polymer (or pair of polymers) deforms the confining vesicle/ microemulsions droplet.

Wettability and Applications of Nanochannels

Wettability in nanochannels is of great importance for understanding many challenging problems in interface chemistry and fluid mechanics, and presents versatile applications including mass transport, catalysis, chemical reaction, nanofabrication, batteries, and separation. Recently, both molecular dynamic simulations and experimental measurements have been employed to study wettability in nanochannels. Here, wettability in three types of nanochannels comprising 1D nanochannels, 2D nanochannels, and 3D nanochannels is summarized both theoretically and experimentally. The proposed concept of "quantum-confined superfluid" for ultrafast mass transport in nanochannels is first introduced, and the mostly studied 1D nanochannels are reviewed from molecular simulation to water wettability, followed by reversible switching of water wettability via external stimuli (temperature and voltage). Liquid transport and two confinement strategies in nanochannels of melt wetting and liquid wetting are also included. Then, molecular simulation, water wettability, liquid transport, and confinement in nanochannels are introduced for 2D nanochannels and 3D nanochannels, respectively. Based on the wettability in nanochannels, broad applications of various nanochannels are presented. Finally, the perspective for future challenges in the wettability and applications of nanochannels is discussed.

Thermally-induced softening of PNIPAm-based nanopillar arrays

The surface properties of soft nanostructured hydrogels are crucial in the design of responsive materials that can be used as platforms to create adaptive devices. The lower critical solution temperature (LCST) of thermo-responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPAm) can be modified by introducing a hydrophilic monomer to create a wide range of thermo-responsive micro-/nano-structures in a large temperature range. Using surface initiation atom-transfer radical polymerization in synthesized anodized aluminum oxide templates, we designed, fabricated, and characterized thermo-responsive nanopillars based on PNIPAm hydrogels with tunable mechanical properties by incorporating acrylamide monomers (AAm). In addition to their LCST, the incorporation of a hydrophilic entity in the nanopillars based on PNIPAm has abruptly changed the topological and mechanical properties of our system. To gain an insight into the mechanical properties of the nanostructure, its hydrophilic/hydrophobic behavior and topological characteristics, atomic force microscopy, molecular dynamics simulations and water contact angle studies were combined. When changing the nanopillar composition, a significant and opposite variation was observed in their mechanical properties. As temperature increased above the LCST, the stiffness of PNIPAm nanopillars, as expected, did so too, in contrast to the stiffness of PNIPAm-AAm nanopillars that decreased significantly. The molecular dynamics simulations proposed a local molecular rearrangement in our nanosystems at the LCST. The local aggregation of NIPAm segments near the center of the nanopillars displaced the hydrophilic AAm units towards the surface of the structure leading to contact with the aqueous environment. This behavior was confirmed via contact angle measurements below and above the LCST.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

Effect of geometric curvature on vitrification behavior for polymer nanotubes confined in anodic aluminum oxide templates

The glass transition behavior of polystyrene (PS) nanotubes confined in cylindrical alumina nanopores was studied as a function of pore diameter (d) and polymer tube thickness (δ). Both the calorimetric glass transition temperature and the microstructure measured by a nonradiative energy transfer method indicated that the polymer nanotube, or concave polymer thin film, exhibited significant differences in vitrification behavior compared to the planar one. A closer interchain proximity and an increased T_{g} were observed for polymer nanotubes with respect to the bulk polymer. T_{g} for polymer nanotubes was primarily dependent on the curvature radius d of the template, while it was less dependent on the thickness δ of the PS tube wall in the range of 11-23 nm. For small nanotubes (d=55nm), the T_{g} increased as high as 18 °C above the bulk value. This vitrified property reverted back to the bulk value when the substrate was chemically removed, which indicated the crucial importance of the interfacial effect imposed by the hard wall with a concave geometry.

Large-Scale Diffusion of Entangled Polymers along Nanochannels

Changes in large-scale polymer diffusivity along interfaces, arising from transient surface contacts at the nanometer scale, are not well understood. Using proton pulsed-gradient NMR, we here study the equilibrium micrometer-scale self-diffusion of poly(butadiene) chains along ∼100 μm long, 20 and 60 nm wide channels in alumina, which is a system without confinement-related changes in segmental relaxation time. Unlike previous reports on nonequilibrium start-up diffusion normal to an interface or into particulate nanocomposites, we find a reduction of the diffusivity that appears to depend only upon the pore diameter but not on the molecular weight in a range between 2 and 24 kg/mol. We rationalize this by a simple volume-average model for the monomeric friction coefficient, which suggests a 10-fold surface-enhanced friction on the scale of a single molecular layer. Further support is provided by applying our model to the analysis of published data on large-scale diffusion in thin films.