Jialong Shen

Coalesced Poly(ε-caprolactone) Fibers Are Stronger

Melt-spun fibers were made from poly(ε-caprolactone) (PCL) coalesced from stoichiometric inclusion complex crystals formed with host urea. Melting and crystallization behaviors, mechanical properties, and the birefringence of undrawn and cold-drawn fibers were investigated. Undrawn coalesced PCL fibers were observed to have 500-600% higher moduli than undrawn as-received (asr) PCL fibers and a modulus comparable to drawn asr PCL fibers. Drawn coalesced PCL fibers have the highest crystallinity, orientation, and 65% higher moduli than drawn asr PCL fibers. Drawn coalesced PCL fibers have only a 5% higher crystallinity than drawn asr PCL fibers, yet they have 65% higher moduli and lower elongation at break values. Clearly, the intrinsic alignment of the coalesced polymers is the reason for their higher moduli and lower elongation, as confirmed by the birefringence observed in drawn coalesced and asr-PCL fibers. The improved mechanical properties of coalesced PCL fibers make them a better candidate for use in tissue engineering as scaffolds.

Hierarchical multi-component nanofiber separators for lithium polysulfide capture in lithium–sulfur batteries: an experimental and molecular modeling study

Sulfur (S) has been considered as a promising cathode candidate for lithium batteries due to its high theoretical specific capacity and energy density. However, the low active material utilization, severe capacity fading, and short lifespan of the resultant lithium–sulfur (Li–S) batteries have greatly hindered their practicality. In this work, a multi-functional polyacrylonitrile/silica nanofiber membrane with an integral ultralight and thin multi-walled carbon nanotube sheet is presented and it provides a new approach to significantly improve the overall electrochemical performance of Li–S batteries. The experimental results are in agreement with molecular modeling studies based on density functional theory and Monte Carlo simulations. Remarkably, this design is favorable for the fast diffusion of both lithium ions and electrons and the mitigation of the diffusion of polysulfides. As a consequence, a high reversible capacity of 741 mA h g−1 at 0.2C after 100 cycles with excellent cyclability and high-rate performance (627 mA h g−1 at 1C) are achieved even with a high sulfur loading of 70 wt% in the cathode, revealing its great potential for energy storage applications. Moreover, a capacity of 426 mA h g−1 is retained after 300 cycles at a high current density of 2C. These results represent a major step forward in the progress of Li–S battery technologies.

Pyriproxyfen cyclodextrin inclusion compounds

Pyriproxyfen is a pyridine based pesticide which is effective against mosquito larvae. Here we report the inclusion complexes (ICs) between guest pyriproxyfen and host β and γ cyclodextrins, but not with α cyclodextrin, based on WAXD, DSC, FTIR, and TGA characterization results. We show that one pyriproxyfen molecule is covered with between one and two β or γ cyclodextrin molecules when it forms columnar ICs upon precipitation from aqueous solution.Graphical Abstract

Reorganizing Polymer Chains with Cyclodextrins

During the past several years, we have been utilizing cyclodextrins (CDs) to nanostructure polymers into bulk samples whose chain organizations, properties, and behaviors are quite distinct from neat bulk samples obtained from their solutions and melts. We first form non-covalently bonded inclusion complexes (ICs) between CD hosts and guest polymers, where the guest chains are highly extended and separately occupy the narrow channels (~0.5–1.0 nm in diameter) formed by the columnar arrangement of CDs in the IC crystals. Careful removal of the host crystalline CD lattice from the polymer-CD-IC crystals leads to coalescence of the guest polymer chains into bulk samples, which we have repeatedly observed to behave distinctly from those produced from their solutions or melts. While amorphous polymers coalesced from their CD-ICs evidence significantly higher glass-transition temperatures, Tgs, polymers that crystallize generally show higher melting and crystallization temperatures (Tms, Tcs), and some-times different crystalline polymorphs, when they are coalesced from their CD-ICs. Formation of CD-ICs containing two or more guest homopolymers or with block copolymers can result in coalesced samples which exhibit intimate mixing between their common homopolymer chains or between the blocks of the copolymer. On a more practically relevant level, the distinct organizations and behaviors observed for polymer samples coalesced from their CD-ICs are found to be stable to extended annealing at temperatures above their Tgs and Tms. We believe this is a consequence of the structural organization of the crystalline polymer-CD-ICs, where the guest polymer chains included in host-IC crystals are separated and confined to occupy the narrow channels formed by the host CDs during IC crystallization. Substantial degrees of the extended and un-entangled natures of the IC-included chains are apparently retained upon coalescence, and are resistant to high temperature annealing. Following the careful removal of the host CD lattice from each randomly oriented IC crystal, the guest polymer chains now occupying a much-reduced volume may be somewhat “nematically” oriented, resulting in a collection of randomly oriented “nematic” regions of largely extended and un-entangled coalesced guest chains. The suggested randomly oriented nematic domain organization of guest polymers might explain why even at high temperatures their transformation to randomly-coiling, interpenetrated, and entangled melts might be difficult. In addition, the behaviors and uses of polymers coalesced from their CD-ICs are briefly described and summarized here, and we attempted to draw conclusions from and relationships between their behaviors and the unique chain organizations and conformations achieved upon coalescence.

Aliphatic Polyester Nanofibers Functionalized with Cyclodextrins and Cyclodextrin-Guest Inclusion Complexes

The fabrication of nanofibers by electrospinning has gained popularity in the past two decades; however, only in this decade, have polymeric nanofibers been functionalized using cyclodextrins (CDs) or their inclusion complexes (ICs). By combining electrospinning of polymers with free CDs, nanofibers can be fabricated that are capable of capturing small molecules, such as wound odors or environmental toxins in water and air. Likewise, combining polymers with cyclodextrin-inclusion complexes (CD-ICs), has shown promise in enhancing or controlling the delivery of small molecule guests, by minor tweaking in the technique utilized in fabricating these nanofibers, for example, by forming core–shell or multilayered structures and conventional electrospinning, for controlled and rapid delivery, respectively. In addition to small molecule delivery, the thermomechanical properties of the polymers can be significantly improved, as our group has shown recently, by adding non-stoichiometric inclusion complexes to the polymeric nanofibers. We recently reported and thoroughly characterized the fabrication of polypseudorotaxane (PpR) nanofibers without a polymeric carrier. These PpR nanofibers show unusual rheological and thermomechanical properties, even when the coverage of those polymer chains is relatively sparse (~3%). A key advantage of these PpR nanofibers is the presence of relatively stable hydroxyl groups on the outer surface of the nanofibers, which can subsequently be taken advantage of for bioconjugation, making them suitable for biomedical applications. Although the number of studies in this area is limited, initial results suggest significant potential for bone tissue engineering, and with additional bioconjugation in other areas of tissue engineering. In addition, the behaviors and uses of aliphatic polyester nanofibers functionalized with CDs and CD-ICs are briefly described and summarized. Based on these observations, we attempt to draw conclusions for each of these combinations, and the relationships that exist between their presence and the functional behaviors of their nanofibers.