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

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.

Toposelective vapor deposition of hybrid and inorganic materials inside nanocavities by polymeric templating and vapor phase infiltration

Selective deposition of hybrid and inorganic materials inside nanostructures could enable major nanotechnological advances. However, inserting ready-made composites inside nanocavities may be difficult, and therefore, stepwise approaches are needed. In this paper, a poly(ethyl acrylate) template is grown selectively inside cavities via condensation-controlled toposelective vapor deposition, and the polymer is then hybridized by alumina, titania, or zinc oxide. The hybridization is carried out by infiltrating the polymer with a vapor-phase metalorganic precursor and water vapor either via a short-pulse (atomic layer deposition, ALD) or a long-pulse (vapor phase infiltration, VPI) sequence. When the polymer-MO _x hybrid material is calcined at 450 °C in air, an inorganic phase is left as the residue. Various suspected confinement effects are discussed. The infiltration of inorganic materials is reduced in deeper layers of the cavity-grown polymer and is dependent on the cavity geometry. The structure of the inorganic deposition after calcination varies from scattered particles and their aggregates to cavity-capping films or cavity-filling low-density porous deposition, and the inorganic deposition is often anisotropically cracked. A large part of the infiltration is achieved already during the short-pulse experiments with a commercial ALD reactor. Furthermore, the infiltrated polymer is more resistant to dissolution in acetone whereas the inorganic component can still be heavily affected by phosphoric acid.

Optical Imaging of the Molecular Mobility of Single Polystyrene Nanospheres

An ultrathin surface layer with extraordinary molecular mobility has been discovered and intensively investigated on thin-film polymer materials for decades. However, because of the lack of suitable characterization techniques, it remains largely unexplored whether such a surface mobile layer also exists on individual polymeric nanospheres. Here, we propose a thermal-optical imaging technique to determine the glass transition (T_g) and rubber-fluid transition (T_f) temperatures of single isolated polystyrene nanospheres (PSNS) in a high-throughput and nonintrusive manner for the first time. Two distinct steps, corresponding to the glass transition and rubber-fluid transition, respectively, were clearly observed in the optical trace of single PSNS during temperature ramping. Because the transition temperature and size of the same individuals were both determined, single nanoparticle measurements revealed the reduced apparent T_f and increased T_g of single PSNS on the gold substrate with a decreasing radius from 130 to 70 nm. Further experiments revealed that the substrate effect played an important role in the increased T_g. More importantly, a gradual decrease in the optical signal was detected prior to the glass transition, which was consistent with a surface layer with enhanced molecular mobility. Quantitative analysis further revealed the thickness of this layer to be ∼8 nm. This work not only uncovered the existence and thickness of a surface mobile layer in single isolated nanospheres but also demonstrated a general bottom-up strategy to investigate the structure-property relationship of polymeric nanomaterials by correlating the thermal property (T_g and T_f) and structural features (size) at single nanoparticle level.

Effect of Nanoscale Confinement on Polymer-Infiltrated Scaffold Metal Composites

Most research on polymer composites has focused on adding discrete inorganic nanofillers to a polymer matrix to impart properties not found in polymers alone. However, properties such as ion conductivity and mechanical reinforcement would be greatly improved if the composite exhibited an interconnected network of inorganic and polymer phases. Here, we fabricate bicontinuous polymer-infiltrated scaffold metal (PrISM) composites by infiltrating polymer into nanoporous gold (NPG) films. Polystyrene (PS) and poly(2-vinylpyridine) (P2VP) films are infiltrated into the ∼43 nm diameter NPG pores via capillary forces during thermal annealing above the polymer glass transition temperature (T_g). The infiltration process is characterized in situ using spectroscopic ellipsometry. PS and P2VP, which have different affinities for the metal scaffold, exhibit slower segmental dynamics compared to their bulk counterparts when confined within the nanopores, as measured through T_g. The more attractive P2VP shows a 20 °C increase in T_g relative to its bulk, while PS only shows a 6 °C increase at a comparable molecular weight. The infiltrated polymer, in turn, stabilizes the gold nanopores against temporal coarsening. The broad tunability of these polymer/metal hybrids represents a unique template for designing functional network composite structures with applications ranging from flexible electronics to fuel cell membranes.

Effect of Nanopore Geometry in the Conformation and Vibrational Dynamics of a Highly Confined Molecular Glass

The effect of nanoporous confinement on the glass transition temperature (T_g) strongly depends on the type of porous media. Here, we study the molecular origins of this effect in a molecular glass, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), highly confined in concave and convex geometries. When confined in controlled pore glass (CPG) with convex pores, TPD's vibrational spectra remained unchanged and two T_g's were observed, consistent with previous studies. In contrast, when confined in silica nanoparticle packings with concave pores, the vibrational peaks were shifted due to more planar conformations and T_g increased, as the pore size was decreased. The strong T_g increases in concave pores indicate significantly slower relaxation dynamics compared to CPG. Given TPD's weak interaction with silica, these effects are entropic in nature and are due to conformational changes at molecular level. The results highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.

Conformational Transitions of Polymer Chains in Solutions Characterized by Fluorescence Resonance Energy Transfer

The critical overlap concentration C* is an important concept in polymer solutions and is defined as the boundary between dilute and semidilute regimes. In this study, the chain conformational changes of polystyrene (PS) with both high (M_n = 200,000 Da) and low (M_n = 13,000 Da) molecular weights in cis-decalin were compared by intrachain fluorescence resonance energy transfer (FRET). The random labeling of donor and acceptor chromophores strategy was employed for long PS chains, whereas chain-end labeling was used for short PS chains. By monitoring the spectroscopic intensity ratio between acceptor and donor, the concentration dependence on chain conformation from dilute to semidilute solutions was determined. Both long and short chains exhibit a conformational transition concentration, above which the polymer chains begin to collapse with concentration significantly. Interestingly, for randomly labeled polymer long chains, such concentration is consistent with C* determined from the viscosity result, below which only slight conformational change of polymer chain takes place. However, for the chain-end labeled short chain, the conformational transition concentration takes place earlier than C*, below which no significant polymer conformation change is observed.

Dramatic Increase in Polymer Glass Transition Temperature under Extreme Nanoconfinement in Weakly Interacting Nanoparticle Films

Properties of polymers in polymer nanocomposites and nanopores have been shown to deviate from their respective bulk properties due to physical confinement as well as polymer-particle interfacial interactions. However, separating the confinement effects from the interfacial effects under extreme nanoconfinement is experimentally challenging. Capillary rise infiltration enables polymer infiltration into nanoparticle (NP) packings, thereby confining polymers within extremely small pores and dramatically increasing the interfacial area, providing a good system to systematically distinguish the role of each effect on polymer properties. In this study, we investigate the effect of spatial confinement on the glass transition temperature ( T_g) of polystyrene (PS) infiltrated into SiO₂ NP films. The degree of confinement is tuned by varying the molecular weight of polymers, the size of NPs (diameters between 11 and 100 nm, producing 3-30 nm average pore sizes), and the fill-fraction of PS in the NP films. We show that in these dense NP packings the T_g of confined PS, which interacts weakly with SiO₂ NPs, significantly increases with decreasing pore size such that for the two molecular weights of PS studied the T_g increases by up to 50 K in 11 nm NP packings, while T_g is close to the bulk T_g in 100 nm NP packings. Interestingly, as the fill-fraction of PS is decreased, resulting in the accumulation of the polymer in the contacts between nanoparticles, hence an increased specific interfacial area, the T_g further increases relative to the fully filled films by another 5-8 K, indicating the strong role of geometrical confinement as opposed to the interfacial effects on the measured T_g values.

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.

Irreversible Adsorption Governs the Equilibration of Thin Polymer Films

We demonstrate that the enhanced segmental motion commonly observed in spin cast thin polymer films is a nonequilibrium phenomenon. In the presence of nonrepulsive interfaces, prolonged annealing in the liquid state allows, in fact, recovering bulk segmental mobility. Our measurements prove that, while the fraction of unrelaxed chains increases upon nanoconfinement, the dynamics of equilibration is almost unaffected by the film thickness. We show that the rate of equilibration of nanoconfined chains does not depend on the structural relaxation process but on the feasibility to form an adsorbed layer. We propose that the equilibration of the thin polymer melts is driven by the slow relaxation of interfacial chains upon irreversible adsorption on the confining walls.

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.

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.