Poly(methyl methacrylate) nanotubes in AAO templates: Designing nanotube thickness and characterizing the T-confinement effect by DSC

Replication of Mesoscale Pore One-dimensional Nanostructures: Surface-induced Phase Separation of Polystyrene/Poly(vinyl alcohol) (PS/PVA) Blends

Mesoscale pore one-dimensional (1D) nanostructures, or vertically aligned porous nanostructures (VAPNs), have attracted attention with their excellent hydrophobic properties, ultra-high surface area, and high friction coefficient, compared to conventional vertically aligned nanostructures (VANs). In this study, we investigate the replication of VAPNs produced by the thermal nanoimprint process using anodic aluminum oxide (AAO2) templates (100 nm diameter). Polystyrene/poly(vinyl alcohol) (PS1/PVA) blends, prepared by the advanced melt-mixing process with an ultra-high shear rate, are used to investigate the formation of porosity at the nanometer scale. The results reveal that domain size and mass ratios of PVA precursors in the PS matrix play a dominant role in the interfacial interaction behavior between PS1-PVA-AAO2, on the obtained morphologies of the imprinted nanostructures. With a PVA nanodomain precursor (PS1/PVA 90/10 wt%), the integration of PVA nanodroplets on the AAO2 wall due to the hydrogen bonding that induces the phase separation between PS1-PVA results in the formation of VAPNs after removal of the PVA segment. However, in the case of PVA microdomain precursors (PS1/PVA 70/30 wt%), the structure transformation behavior of PS1 is induced by the Rayleigh instability between PVA encapsulated around the PS1 surfaces, resulting in the PS1 nanocolumns transforming into nanopeapods composed of nanorods and nanospheres.

Raising glass transition temperature of polymer nanofilms as a function of negative interface energy

Based on a thermodynamic approach, glass transition temperature (T_g) of substrate-supported polymer nanofilms (s-PNFs) is investigated for carbon-chain polymers, taking the role of the interface energy into consideration.

How thermal stress alters the confinement of polymers vitrificated in nanopores

Understanding and controlling the glass transition temperature (T_g) and dynamics of polymers in confined geometries are of significance in both academia and industry. Here, we investigate how the thermal stress induced by a mismatch in the coefficient of thermal expansion affects the T_g behavior of polystyrene (PS) nanorods located inside cylindrical alumina nanopores. The size effects and molecular weight dependence of the T_g are also studied. A multi-step relaxation process was employed to study the relationship between thermal stress and cooling rate. At fast cooling rates, the imparted thermal stress would overcome the yield stress of PS and peel chains off the pore walls, while at slow cooling rates, chains are kept in contact with the pore walls due to timely dissipation of the produced thermal stress during vitrification. In smaller nanopores, more PS chains closely contact with pore walls, then stronger internal thermal stress would be generated between core and shell of PS nanorod, which results in a larger deviation between two T_gs. The core part of PS shows lower T_g than bulk value, which can induce faster dynamics in the center region. A complex and important role stress plays is supposed in complex confinement condition, e.g., in nanopores, during vitrification.

Molecular weight dependence of the intrinsic size effect on Tg in AAO template-supported polymer nanorods: A DSC study

Many studies have established a major effect of nanoscale confinement on the glass transition temperature (T_g) of polystyrene (PS), most commonly in thin films with one or two free surfaces. Here, we characterize smaller yet significant intrinsic size effects (in the absence of free surfaces or significant attractive polymer-substrate interactions) on the T_g and fragility of PS. Melt infiltration of various molecular weights (MWs) of PS into anodic aluminum oxide (AAO) templates is used to create nanorods supported on AAO with rod diameter (d) ranging from 24 to 210 nm. The T_g (both as T_g,onset and fictive temperature) and fragility values are characterized by differential scanning calorimetry. No intrinsic size effect is observed for 30 kg/mol PS in template-supported nanorods with d = 24 nm. However, effects on T_g are present for PS nanorods with M_n and M_w ≥ ∼175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T_{g, rod} - T_{g, bulk} = -2.0 to -2.5 °C for PS with M_n = 175 kg/mol and M_w = 182 kg/mol, and T_{g, rod} - T_{g, bulk} = ∼-8 °C for PS with M_n = 929 kg/mol and M_w = 1420 kg/mol. In general, reductions in T_g occur when d ≤ ∼2R_g, where R_g is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2R_g) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the T_g reduction occurs when chain segment packing frustration is sufficiently perturbed by confinement in the nanorods. This explanation is supported by observed reductions in fragility with the increasing extent of confinement. We also explain why these small intrinsic size effects do not contradict reports that the T_g-confinement effect in supported PS films with one free surface exhibits little or no MW dependence.

Facile design of ultra-thin anodic aluminum oxide membranes for the fabrication of plasmonic nanoarrays

Ultra-thin anodic aluminum oxide (AAO) membranes are efficient templates for the fabrication of patterned nanostructures. Herein, a three-step etching method to control the morphology of AAO is described. The morphological evolution of the AAO during phosphoric acid etching is systematically investigated and a nonlinear growth mechanism during unsteady-state anodization is revealed. The thickness of the AAO can be quantitatively controlled from ∼100 nm to several micrometers while maintaining the tunablity of the pore diameter. The AAO membranes are robust and readily transferable to different types of substrates to prepare patterned plasmonic nanoarrays such as nanoislands, nanoclusters, ultra-small nanodots, and core-satellite superstructures. The localized surface plasmon resonance from these nanostructures can be easily tuned by adjusting the morphology of the AAO template. The custom AAO template provides a platform for the fabrication of low-cost and large-scale functional nanoarrays suitable for fundamental studies as well as applications including biochemical sensing, imaging, photocatalysis, and photovoltaics.