Dynamic covalent networks are polymer networks that contain a dynamic covalent bond which allows them to be reprocessable, remoldable, and recyclable as well as exhibit crack healing or stress-relaxation properties. A key component of these materials is the nature of the dynamic covalent bond, which in addition to chemical composition and architecture can be used to dramatically alter the physical properties of these networks. The aim of this study is to understand the impact of steric hindrance of N-alkyl substituents and network connectivity in poly(alkylurea− urethane) dynamic network films. In these materials, the dynamic bond is the hindered alkyl urea moiety, whose dynamic behavior is dictated by the sterics of the alkyl substituent. Networks were prepared by the noncatalyzed curing reaction of aminoethanol compounds of varying substituents with a trifunctional isocyanate cross-linker and varying amounts of a monofunctional capping agent. Thermomechanical properties and FTIR studies show the impact of hindered urea bond sterics on the reaction conversion, network connectivity, and therefore the relaxation of the dynamic networks. Stress relaxation analysis show the vitrimer-like behavior of these dynamic networks only when the degree of cross-linking is maintained by high reaction conversion (high equilibrium constant of the dynamic bond). These results give some insights into the design and properties of dynamic covalent networks and how the nature of dynamic bonds can be used to impact their properties.

Ionomers are polymers with bonded ionic species that are used under conditions where the salt groups are in a condensed state. This Feature Article discusses the state of our understanding of ionomers and the historical applications of these nanostructured polymers. It also discusses modern methods for synthesizing new ionomers and describes a number of relatively new applications for ionomers and the potential use of these materials in contemporary technologies, including, shape memory and selfhealing materials and supramolecular polymer systems.

One challenge associated with the utilization of block copolymers in nanotechnology is the difficulties associated with alignment and orientation of the self-assembled nanostructure on macroscopic length scales. Here we demonstrate a simple method to generate unidirectional alignment of the cylindrical domains of polystyrene-block-polyisoprene-block-polystyrene, SIS, based on a modification of the commonly utilized solvent vapor annealing (SVA) process. In this modification, cross-linked poly(dimethylsiloxane) (PDMS) is physically adhered to the SIS film during SVA; differential swelling of the PDMS and SIS produces a shear force to align the ordered domains of SIS in the areas covered by PDMS. This method is termed solvent vapor annealing with soft shear (SVA-SS). The alignment direction can be readily controlled by the shape and placement of the PDMS with the alignment angle equal to the diagonal across the rectangular PDMS pad due to a propagating deswelling front from directional drying of the PDMS by a dry air stream. Herman’s (second order) orientational parameter, S, can quantify the quality of the alignment over large areas with S > 0.94 obtainable using SVA-SS.

Dynamic covalent bonds placed within a polymer chain result in stimulus-responsive materials where the breaking/making and/or exchange of the dynamic bonds controls the response. A key attribute to access such properties is the molecular mobility of the dynamic bonds. The focus of this work is to understand how incorporating a dynamic bond, in the form of disulfide bonds, into the hard phase of a polyurethane will impact the properties of the materials. Thus, uncross-linked polyurethanes with aliphatic disulfide-containing hard segments are synthesized via a two-step protocol, using 2,2′-dithiodiethanol and/or 1,4-butanediol in the second step as the chain extenders. Thermomechanical studies show that, if the dynamic bonds are selectively placed in the hard phase of the polyurethane, the dynamic nature of the disulfide bond can be effectively "switched-off" below the melting temperature of the hard phase. As such the dynamic and mechanical properties of the materials can be controlled by tailoring the nature of the hard phase.

A supramolecular multiblock copolymer was synthesized by mixing two telechelic oligomers, α,ω-sulfonated polystyrene, HO 3 S-PS-SO 3 H, derived from a polymer prepared by RAFT polymerization, and α,ω-amino-polyisobutylene, H 2 N-PIB-NH 2 , prepared by cationic polymerization. During solvent casting, proton transfer from the sulfonic acid to the amine formed ionic bonds that produced a multiblock copolymer that formed freestanding flexible films with a modulus of 90 MPa, a yield point at 4% strain and a strain energy density of 15 MJ/m 3 . Small angle X-ray scattering characterization showed a lamellar morphology, whose domain spacing was consistent with the formation of a multiblock copolymer based on comparison to the chain dimensions. A reversible order−disorder transition occurred between 190 and 210°C, but the sulfonic acid and amine functional groups were observed to decompose at those elevated temperatures based on companion optical microscopy and spectroscopy measurements. For high nonlinear strains, the dynamic modulus, G′, decreased by nearly an order of magnitude and the loss modulus, G″, decreased by a factor of 1.4, but both recovered to their original values once the strain was reduced to within the linear response region.