Effect of Sterics and Degree of Cross-Linking on the Mechanical Properties of Dynamic Poly(alkylurea–urethane) Networks

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 <i>N</i>-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

Tailor-Made Fluorinated Copolymer/Clay Nanocomposite by Cationic RAFT Assisted Pickering Miniemulsion Polymerization

Fluorinated polymers in emulsion find enormous applications in hydrophobic surface coating. Currently, lots of efforts are being made to develop specialty polymer emulsions which are free from surfactants. This investigation reports the preparation of a fluorinated copolymer via Pickering miniemulsion polymerization. In this case, 2,2,3,3,3-pentafluoropropyl acrylate (PFPA), methyl methacrylate (MMA), and <i>n</i>-butyl acrylate (nBA) were copolymerized in miniemulsion using Laponite-RDS as the stabilizer. The copolymerization was carried out via reversible addition–fragmentation chain transfer (RAFT) process. Here, a cationic RAFT agent, <i>S</i>-1-dodecyl-<i>S</i>′-(methylbenzyltriethylammonium bromide) trithiocarbonate (DMTTC), was used to promote polymer-Laponite interaction by means of ionic attraction. The polymerization was much faster when Laponite content was 30 wt % or above with 1.2 wt % RAFT agent. The stability of the miniemulsion in terms of zeta potential was found to be dependent on the amount of both Laponite and RAFT agent. The miniemulsion had particle sizes in the range of 200–300 nm. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses showed the formation of Laponite armored spherical copolymer particles. The fluorinated copolymer films had improved surface properties because of polymer–Laponite interaction

Sulfonation Distribution in Sulfonated Polystyrene Ionomers Measured by MALDI-ToF MS

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS) was used to quantify the sulfonation level and sulfonation distribution of sulfonated polystyrene ionomers prepared by homogeneous solution sulfonation. The sulfonation levels obtained by MALDI-ToF MS and acid–base titration were compared, and the sulfonate distributions determined by MALDI-ToF MS were compared with theoretical random distributions. The results indicate that the sulfonation reaction used produces a sample with a random sulfonate distribution

Supramolecular Multiblock Polystyrene–Polyisobutylene Copolymers via Ionic Interactions

A supramolecular multiblock copolymer was synthesized by mixing two telechelic oligomers, α,ω-sulfonated polystyrene, HO₃S-PS-SO₃H, derived from a polymer prepared by RAFT polymerization, and α,ω-amino-polyisobutylene, H₂N-PIB-NH₂, 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 free-standing flexible films with a modulus of 90 MPa, a yield point at 4% strain and a strain energy density of 15 MJ/m³. 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, <i>G</i>′, decreased by nearly an order of magnitude and the loss modulus, <i>G</i>″, 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