This work studies the origin of the so called "strain hardening" observed when comparing the transient stress response of entangled melts to uniaxial extension with that to simple shear. Strain hardening occurs when the transient extensional viscosity measured from a startup uniaxial extension of finite rate deviates upward from the zero-rate transient viscosity. Our theoretical analysis shows that polymer melts would always exhibit strain hardening at sufficient high Hencky rates because the entanglement network can be effectively strengthened during extension and can only be weakened during shear for linear chains. The kinematic difference between simple shear and uniaxial extension has two effects: (a) The force resulting from the startup deformation is measured from an increasingly shrinking area in uniaxial extension instead of a constant area as in simple shear; and (b) the tendency of the entanglement network to yield, i.e., to undergo chain disentanglement is partially suppressed during startup extension at high Hencky rates. In short, the phenomenon of strain hardening reflects the reality that entangled melts are not fluids but temporary solids and that the conventional description of their uniaxial extension in terms of the Cauchy stress contains a geometric condensation factor.

Living anionic polymerization will soon celebrate 70 years of existence. This living polymerization is considered the mother of all living and controlled/living polymerizations since it paved the way for their discovery. It provides methodologies for synthesizing polymers with absolute control of the essential parameters that affect polymer properties, including molecular weight, molecular weight distribution, composition and microstructure, chain-end/in-chain functionality, and architecture. This precise control of living anionic polymerization generated tremendous fundamental and industrial research activities, developing numerous important commodity and specialty polymers. In this Perspective, we present the high importance of living anionic polymerization of vinyl monomers by providing some examples of its significant achievements, presenting its current status, giving several insights into where it is going (Quo Vadis) and what the future holds for this powerful synthetic method. Furthermore, we attempt to explore its advantages and disadvantages compared to controlled/living radical polymerizations, the main competitors of living carbanionic polymerization.

A universal block copolymer pattern transfer method was demonstrated to produce Co nanostructures consisting of arrays of lines or dots from a polystyrene- block -polydimethylsiloxane (PS- b -PDMS) diblock copolymer. Three processes were used: liftoff, a damascene process, and ion beam etching using a hard mask of tungsten, including a sacrificial poly(methyl methacrylate) layer under the PS- b- PDMS for the etch and liftoff processes. The ion beam etch process produced the most uniform magnetic arrays. A structural and magnetic comparison in terms of uniformity, edge roughness and switching field distribution has been reported.

In addition to the glass transition, vitrimers undergo a topological transition from viscoelastic liquid to viscoelastic solid behavior when the network rearrangements facilitated by dynamic bond exchange reactions freeze. The microscopic observation of this transition is elusive. Model polyisoprene vitrimers based on imine dynamic covalent bonds were synthesized by reaction of α,ω-dialdehyde-functionalized polyisoprenes and a tris(2-aminoethyl)amine. In these dynamic networks nanophase separation of polymer and reactive groups leads to the emergence of a relevant length scale characteristic for the network structure. We exploited the scattering sensitivity to structural features at different length scales to determine how dynamical and topological arrests affect correlations at segmental and network levels. Chains expand obeying the same expansion coefficient throughout the entire viscoelastic region, i.e., both in the elastomeric regime and in the liquid regime. The onset of liquid-like behavior is only apparent at the mesoscale, where the scattering reveals the reorganization of the network triggered by bond exchange events. The such determined "microscopic" topological transition temperature is compared with the outcome of "conventional" methods, namely viscosimetry and differential scanning calorimetry. We show that using proper thermal (aging-like) protocols, this transition is also nicely revealed by the latter.