Chain Dynamics and Segmental Orientation in Polymer Melts Confined to Nanochannels6215

We study changes in the dynamics of polymer chains confined to cylindrical nanochannels within aluminum oxide membranes. Specifically, a proton time-domain NMR technique is used to assess the effect of transient wall contacts on the time-averaged orientational order of poly(butadiene) segments in melts with different molecular weights (MW). Previous work has evidenced that the weakly interacting polymer, residing in ∼100 μm long, 20 and 60 nm wide channels, shows no significant confinement-related changes in the segmental (α) relaxation time and only weak (less than a factor of 2) changes in the micrometer-scale diffusivity. In the relevant temperature range above 340 K, we here use samples with pores oriented at different angles with respect to the main magnetic field to study the macroscopic anisotropy of segmental rotations and the effect of slower motions in regimes III and IV of the tube model up to the milliseconds time scale. We show that the pore walls exert a significant orientation effect on the chains, measured in terms of a time-averaged order parameter with a related length scale of one to a few nanometers, coexisting for high molecular weight (MW) inhomogeneously with bulk-like behavior in the pore center. Low MW with fewer than about 10 entanglements as well as low MW liquids exhibit a homogeneous response, with an overall residual orientation that represents a diffusively averaged quantity reflecting the pore geometry. We support our findings by a simulation model based upon one-dimensional curvilinear chain diffusion along the primitive path. The study is complemented by deuterium NMR experiments on a labeled poly(dimethylsiloxane) sample, in which strong surface contacts prevent full diffusive averaging

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

Semicrystalline Block Copolymers in Rigid Confining Nanopores

We have investigated PLLA crystallization in lamellae-forming PS-<i>b</i>-PLLA confined to straight cylindrical nanopores under weak confinement (nanopore diameter <i>D</i>/equilibrium PS-<i>b</i>-PLLA period <i>L</i>₀ ≥ 4.8). Molten PS-<i>b</i>-PLLA predominantly forms concentric lamellae along the nanopores, but intertwined helices occur even for <i>D</i>/<i>L</i>₀ ≈ 7.3. Quenching PS-<i>b</i>-PLLA melts below <i>T</i>_G(PS) results in PLLA cold crystallization strictly confined by the vitrified PS domains. Above <i>T</i>_G(PS), PLLA crystallization is templated by the PS-<i>b</i>-PLLA melt domain structure in the nanopore centers, while adsorption on the nanopore walls stabilizes the outermost cylindrical PS-<i>b</i>-PLLA shell. In between, the nanoscopic PS-<i>b</i>-PLLA melt domain structure apparently ripens to reduce frustrations transmitted from the outermost immobilized PS-<i>b</i>-PLLA layer. The onset of PLLA crystallization catalyzes the ripening while transient ripening states are arrested by advancing PLLA crystallization. Certain helical structure motifs persist PLLA crystallization even if PS is soft. The direction of fastest PLLA crystal growth is preferentially aligned with the nanopore axes to the same degree as for PLLA homopolymer, independent of whether PS is vitreous or soft

How Gold Nanoparticles Influence Crystallization of Polyethylene in Rigid Cylindrical Nanopores

Even high amounts of gold nanoparticles (AuNPs) only moderately influence crystallization of bulk polyethylene (PE). However, under the rigid two-dimensional confinement of aligned cylindrical nanopores in anodic aluminum oxide (AAO) the presence of Au turns nucleation-dominated crystallization of PE at high supercooling into growth-dominated crystallization at lower supercooling. Transmission electron microscopy investigations revealed formation of larger Au crystals from AuNPs by Ostwald ripening. These larger Au crystals apparently acted as heterogeneous nucleation sites initiating PE crystallization in AAO nanopores. Thus, PE/Au composites in AAO exhibited significantly higher crystallization and melting onset temperatures as well as significantly weaker dependence of crystallization half-times on crystallization temperatures. X-ray texture analysis revealed for pure PE in AAO the existence of two copopulations of crystals with different orientations (indicative of nucleation-dominated crystal growth); PE/Au composites showed uniform alignment of the fastest growing PE crystal direction with the AAO nanopore axes (indicative of growth-dominated crystallization). The prevailing alignment of the [020] direction of orthorhombic PE with the AAO nanopore axes suggests that properly oriented crystals may form on pre-existing crystal surfaces by secondary nucleation. These secondary crystals grow along the AAO nanopores if, under the conditions of growth-dominated crystallization, competing crystals clogging the growth path are absent while the confinement of the AAO nanopore walls stabilizes the (020) growth faces

Arrays of Aligned Supramolecular Wires by Macroscopic Orientation of Columnar Discotic Mesophases

Structure formation, phase behavior, and dynamics of mono-bromo hexa-<i>peri</i>-hexabenzocoronene (HBC-Br) are strongly affected by the confinement of cylindrical nanopores with rigid walls. Using self-ordered nanoporous anodic aluminum oxide (AAO)-containing arrays of aligned nanopores with narrow size distribution as a confining matrix, pronounced alignment of the HBC-Br columns along the nanopore axes was found to be independent of the pore diameter. Hence, arrays of one-dimensional supramolecular HBC-Br wires with the columns uniformly oriented along the wire axes on a macroscopic scale were obtained, unlike with discotics bearing smaller cores. The formation of the crystalline herringbone structure is shifted to lower temperatures in nanopores with diameters of a few hundred nanometers, whereas the formation of this low-temperature phase is completely suppressed when the pore diameter is below 20 lattice parameters. Moreover, the cylindrical confinement affects the disk axial dynamics as well as the distribution of relaxation times