Adding Symmetry: Cylindrically Confined Crystallization of Nylon-6

Confined Crystallization of Polymers within Nanopores

Crystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called "fractionated crystallization", i.e., multiple crystallization peaks during cooling.Because the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material.Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the "chain perpendicular to" to the "chain parallel to" the AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth direction, nucleation, and crystal growth rate. An orientation diagram has been established for the PEO/AAO system, considering the cooling condition and pore size.The interfacial polymer layer has different physical properties as compared to the bulk. In poly(l-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.

Coherent crystal branches: the impact of tetragonal symmetry on the 2D confined polymer nanostructure

The symmetry of polymer crystals greatly affects the optical, thermal con-ductivity and mechanical properties of the materials. Past studies have shown that the two-dimensional (2D) confined crystallization of polymer nanorods could produce anisotropic structures. However, few researchers have focused on understanding confined nanostructures from the perspective of crystal sym-metry. In this research, we demonstrate the molecular chain self-assembly of tetragonal crystals under cylindrical confinement. We specifically selected poly(4-methyl-1-pentene) (P4MP1) with a 41 or 72 helical conformation (usually crystallizing with a tetragonal lattice) as the model polymer. We found a coherent crystal branching of the tetragonal crystal in the P4MP1 nanorods. The unusual 45°- and 135°-{200} diffractions and the meridional 220 diffraction (from 45°-tilted crystals) have shown a uniform crystal branching between the a 1-axis crystals and the 45°-tilted crystals in the rod long axis, which originates from a structural defect associated with tetragonal symmetry. Surprisingly, this chain packing defect in the tetragonal cell can be controlled to develop along the rod long axis in 2D confinement.

Tentative Confinement of Ionic Liquids in Nylon 6 Fibers: A Bridge between Structural Developments and High-Performance Properties

A reversible confinement of ionic liquid (IL) among the amide segments has been carried out for the preparation of high-modulus and high-strength aliphatic semicrystalline nylon 6 fibers. In this research work, the suppression or the weakening of the hydrogen bonds during the conventional low-speed melt spinning process is followed by a hot-drawing stage and a subsequent IL extraction of the IL out of the 2% wt IL-confined fibers and an immediate thermal stabilization process for the improvement of the properties of the pristine nylon 6 fibers. The resulted crystal structural developments of the IL-confined fibers are attributed to ultimate molecular orientations, which have contributed to the developments of the overall fiber properties. Here, the influences of the IL on the γ and the α crystal phases, the γ-α transition, the morphological properties, and the tensile properties are investigated. The FTIR reported, experimentally, additional peaks at 1237 cm^-1 for the γ crystal phase and at 1417 and 1476 cm^-1 for the α crystal phase, in conformity with the theoretical computations. The XRD demonstrated that the conventional low-speed melt spinning can successfully be used to prepare as-spun IL-confined fibers having highly improved properties. The so prepared as-spun IL-confined fibers are found to have a γ phase structure that has a small crystal size and high crystal perfections. Fortunately, the γ-to-α crystal phase transition for the IL-confined nylon 6 fibers can be acquired during the hot-drawing stage (stress-induced phase transformation). Furthermore, the IL extraction process followed by a thermal stabilization process, interestingly, has led to significant increases in both of the tensile strengths and the tensile moduli of the reverted nylon 6 fibers. The values that are found are 8.46 cN/dtex for the tensile strength and 39.09 cN/dtex for the tensile modulus. The structure-property relationships between the IL-confined and the reverted nylon 6 fibers have also been discussed.

Hierarchical structure of the triclinic α-phase crystal in nylon 6,12 mediated by two-dimensional confinement

It has been recognized that macromolecular chains can self-assemble into a hierarchical structure from lamellae to spherulites in bulk crystallization. However, little account has been taken of crystal symmetry effects on the hierarchical nanostructure in polymers under cylindrical confinement. In this research, a model polymer, nylon 6,12, most commonly occurring in the triclinic α phase, was chosen in order to demonstrate the effect of triclinic symmetry on the 2D-constrained polymer nanostructure. The self-arranging unit of nylon 6,12 takes various forms, including stems, unit cells, hydrogen-bonded sheets, lamellae and complex spherulites, which is an essential structural feature for investigating hierarchical nanostructure. The rod nanostructure in confinement was examined by cross-checking electron and X-ray diffraction techniques. It is found that thea* axis of the α-phase cell is inclined at about ±6–11° to the rod long axis within thea*b* plane around thecaxis (caxis ⊥ rod long axis). The rotation of thea*b* plane most likely results from the impact of the triclinic symmetry on the molecular chain packing under 2D confinement. A mechanism for thisa*b* plane tilting is proposed.

How Confinement Affects the Nucleation, Crystallization, and Dielectric Relaxation of Poly(butylene succinate) and Poly(butylene adipate) Infiltrated within Nanoporous Alumina Templates

This work describes the successful melt infiltration of poly(butylene succinate) (PBS) and poly(butylene adipate) (PBA) within 70 nm diameter anodic aluminum oxide (AAO) templates. The infiltrated samples were characterized by SEM, Raman, and FTIR spectroscopy. The crystallization behaviors and crystalline structures of both polymers, bulk and confined, were analyzed by differential scanning calorimetry (DSC) and grazing incidence wide angle X-ray scattering (GIWAXS). DSC revealed that a change in the nucleation process occurred from heterogeneous nucleation for bulk samples to homogeneous nucleation for infiltrated PBA and to surface-induced nucleation for infiltrated PBS. GIWAXS results indicate that PBS nanofibers crystallize in the α-phase, as well as their bulk samples. However, PBA nanofibers crystallize just in the β-phase, whereas PBA bulk samples crystallize in a mixture of α- and β-phases. The crystal orientation within the pores was determined, and differences between PBS and PBA were also found. Finally, broadband dielectric spectroscopy was applied to study the segmental dynamics for bulk and infiltrated samples. The glass temperature was found to significantly decrease in the PBS case upon infiltration, while that of PBA remained unchanged. These differences were correlated with the higher affinity of PBS to the AAO walls than PBA, in accordance with their nucleation behavior (surface-induced versus homogeneous nucleation, respectively).