We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts η0,linear to their ring counterparts η0,ring at isofrictional conditions is discussed as function of the number of entanglements Z. In the unentangled regime η0,linear/η0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η0,linear/η0,ring=2. In the entanglement regime, the Z-dependence of rings viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available experimental data in the rather limited range 1

We present an experimental investigation of the curing kinetics and viscoelasticity for a number of "vitrimers" recently developed by Leibler and coworkers. Vitrimers are covalently crosslinked networks that can relax stress at elevated temperatures due to thermoreversible bond-exchange reactions. The chosen formulations are composed of diglycidyl ether of bisphenol A, commercial fatty acid mixtures and an appropriate catalyst. The effects of the catalyst and functionality of the curing agents on the kinetics of the curing reactions were systematically investigated using rheometry. The curing kinetics followed the Arrhenius law and the catalyst drastically accelerated the reactions. Time-temperature superposition was used to construct master curves of the small-strain amplitude oscillatory shear moduli over wide ranges of frequencies for the cured networks. Terminal relaxation was not reached in oscillatory experiments for temperatures up to 130 °C and creep and stress relaxation experiments were used to probe the long-time relaxation. The shift factors displayed a Williams-Landel-Ferry dependence on temperature which could be divided into two regions, one above 70 °C, where the dynamics appeared to be controlled by the catalyst, and one below, controlled by the monomeric friction and the free volume of the network. The moduli of the vitrimers obeyed the classical rubber theory well, indicating that the curing reactions proceeded to completion. Furthermore, we systematically and reproducibly observed a double relaxation behavior for the vitrimers, i.e. next to the rubbery plateau at high frequencies, the storage modulus displayed a secondary plateau at lower frequencies before reaching terminal relaxation at even lower frequencies. Interestingly, 70 °C was found to be the transition point in agreement with the shift factors. To the best of our knowledge, the double relaxation behavior has not been previously reported in experimental works and recent theories do not incorporate an explanation for this behavior. Consequently, future investigations concerning the viscoelasticity of other "vitrimer-chemistries" are important to assess if the double relaxation is a universal fingerprint for vitrimers or if it is specific to the here-investigated formulations based on commercial fatty acid mixtures.

We present a static and quasi-elastic neutron scattering study on both the structure and dynamics of a ring polymer in a ring and linear polymer melt, respectively. In the first case, the ring structure proved to be significantly more compact compared to the linear chain with the same molecular weight. In the mixture, the ring molecules swell as was confirmed by small angle neutron scattering (SANS) in accordance with both theory and simulation work. The dynamical behavior of both systems, which for the first time has been explored by neutron spin echo spectroscopy (NSE), shows a surprisingly fast center of mass diffusion as compared to the linear polymer. These results agree qualitatively with the presented atomistic MD simulations. The fast diffusion turned out to be an explicit violation of the Rouse model.

We investigate the linear rheology of welldefined dendronized polymers (DPs). They consist of polymethacrylate backbones with tree-like branches (dendrons) of different generations, from zeroth to fourth, grafted at each monomer and a methyleneoxycarbonyl spacer between the polymerizable group and the dendritic substituent. The degrees of polymerization for the different generation polymers are almost constant, allowing for systematic studies as a function of generation. Because of the synthetic approach, these macromolecules possess tert-butoxycarbonyl (Boc) groups which promote hydrogen bonding, whereas the benzene groups allow for weaker bonding (π-stacking) as well. The master curves of frequency-dependent storage and loss moduli of these macromolecular structures were obtained via time− temperature superposition of dynamic frequency sweeps at various temperatures. To access slow relaxations, creep measurements were performed at long times and converted to frequency-dependent moduli. For the first generation, it was possible to detect relaxation processes suggesting an approach to the terminal regime (flow). On the other hand, the zeroth and second to fourth generation polymers exhibited a solid-like behavior throughout a wide range of frequencies. The fast relaxations reflect the coupling of segmental friction and hydrogen bonding and render the WLF-type analysis nontrivial. On the basis of the molecular structure of these unique materials as revealed by molecular dynamics simulations and complementary studies with their linear analogues poly(methyl methacrylate) and poly(tert-butyl)methacrylate, we propose that DPs resemble weakly interpenetrating elongated core−shell systems. As generation increases, their enhanced rigidity and intermolecular hydrogen bonding, which occurs primarily toward the outer surface of the DPs, appear to dominate the dynamics. PG0 is not a DP and has an open structure that promotes intermolecular bonding. These results provide design guidelines for ultrahigh-molecular-weight responsive polymers with possibilities for multifunctional substitution and tailoring Frheological response from liquid-like to solid-like.

We present results from very long (on the order of several microseconds) atomistic molecular dynamics (MD) simulations for the density, microscopic structure, conformation, and local and segmental dynamics of pure, strictly monodisperse ring and linear poly­(ethylene oxide) (PEO) melts, ranging in molar mass from ∼5300 to ∼20 000 g/mol. The MD results are compared with recent experimental data for the chain center-of-mass self-diffusion coefficient and the normalized single-chain dynamic structure factor obtained from small-angle neutron scattering, neutron spin echo, and pulse-field gradient NMR, and remarkable qualitative and quantitative agreement is observed, despite certain subtle disagreements in important details regarding mainly internal ring motion (loop dynamics). A detailed normal-mode analysis allowed us to check the degree of consistency of ring PEO melt dynamics with the ring Rouse model and indicated a strong reduction of the normalized mode amplitudes for the smaller mode numbers (compared to the Rouse model scaling), combined with an undisturbed spectrum of Rouse relaxation rates. We have further measured the zero-shear rate viscosity η0 of the PEO-5k and PEO-10k rings at several temperatures and extracted their activation energies. These were compared with the activation energies extracted from the MD simulations via analysis of the temperature dependence of the corresponding Rouse relaxation times of the two rings in the same temperature range.

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Ordering induced by shear flow can be used to direct the assembly of particles in suspensions. Flow-induced ordering is determined by the balance between a range of forces, such as direct interparticle, Brownian, and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, 1D stringlike structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon not observed in Newtonian fluids at similar particle volume fractions. Here we report on the formation of freestanding crystalline patches in planes parallel to the shearing surfaces. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2D crystalline patches along the flow direction. This is a bulk phenomenon because 2D crystals form throughout the whole gap between plates, with the gap thickness being much larger than the particle size. Shear flow may hence be an easy method to drive particles into crystalline order in suspensions with viscoelastic properties. The crystalline structure reported here could be used to design new materials with special mechanical, optical, thermal, or electric properties.

The micellar system composed of Cetylpyridinium Chloride-Sodium Salicylate (CPyCl-NaSal) in brine aqueous solutions has been studied by systematically changing the salt concentration, in order to investigate the rheology of the arising morphologies. In particular, the zero-shear viscosity and the linear viscoelastic response have been measured as a function of the NaSal concentration (with [CPyCl] = 100 mM). The Newtonian viscosity shows a nonmonotonic dependence upon concentration, passing through a maximum at NaSal/CPyCl approximate to 0.6, and eventually dropping at higher salt concentrations. The progressive addition of salt determines first a transition from a Newtonian to a purely Maxwell-like behavior as the length of the micelles significantly increases. Beyond the peak viscosity, the viscoelastic data show two distinct features. On the one hand, the main relaxation time of the system strongly decreases, while the plateau modulus remains essentially constant. Calculations based on the rheological data show that, as the binding salt concentration increases, there is a decrease in micelles breaking rate and a decrease in their average length. On the other hand, in the same concentration region, a low-frequency elastic plateau is measured. Such a plateau is considered as the signature of a tenuous, but persistent branched network, whose existence is confirmed by cryo-transmission electron microscopy images