Commentary on “Solid-like rheological response of non-entangled polymers in the molten state” by H. Mendil etal.

We discuss the rheology experiments on classical and liquid crystal polymer melts by Mendil et al., in the light of the old and new piezorheometry experiments we have carried out on both types of melt. The mechanical behavior we have observed in the linear and non-linear regimes are independent of the melt studied (classical or liquid crystal), and of their nature (siloxane-type, acrylate-type and styrene-type). In the linear regime, the mechanical behavior of the melts presents two components: the first one is the conventional contribution. It is due to polymer chains, and is independent of sample thickness. The second one, which can be observed only when a strong interaction between the compound and the substrate exists, is associated with the glass transition. This component displays an elastic response depending on the sample thickness, and disappears at high temperature. It can be explained by assuming the presence of long-range density fluctuations, which are associated with the glass transition, and frozen at the frequencies used in the experiments. The experiments as a function of the applied strain show that the value of the elastic component decreases when the applied strain increases. This slipping transition occurs progressively, which highlights the heterogeneous nature of the anchoring. The results on the classical polymer by Mendil et al. can be considered to be consistent with ours. In contrast, their results on the liquid crystal polymer differ markedly from ours, showing that the elastic response of this sample has not the same origin.     

Solid-like rheological response of non-entangled polymers in the molten state

We show that non-entangled polymers display an elastic-like behaviour at a macroscopic scale (probed at some 0.100 mm thickness) up to at least hundred degrees above the glass transition temperature. This observation, found under non-slippage conditions, both for side-chain liquid crystalline polymers and ordinary polymers, is in contradiction with the typically found flow behaviour of polymer melt. Our measurements were carried out with a conventional rheometer at thicknesses of several tenths millimetres. Thus, we were probing bulk properties. The observed elasticity supposedly implies that even in the melt the chains experience a cohesive effect of macroscopic distances, involving collective motions over time scales longer than the individual relaxation time of an individual polymer chain. The detection of such a solid-like property of molten non-entangled polymers is of considerable importance for a better understanding of the polymer dynamics.     

Commentary on the observations of solid-like rheological response in unentangled polymer melts by H. Mendil, P. Baroni, L. Noirez, D. Collin, P. Martinoty

Without abstract.         

Reply to the comment by S. Dannefaer et al. on the paper of H. E. Schaefer et al. on “amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

In the original paper by Schaefer et al. the assignment of positron lifetimes to monovacancies and vacancy agglomerates in Si has been discussed in order to interpret the lifetime of 402 ps observed in amorphous hydrogenated Si. In the present reply to the comment of Dannefaer, Mascher and Kerr we will demonstrate that a precise and critical discussion of the available data is indispensible.     

Comments on the papers recently published by M.M. Khandpekar et al.

It is argued that the existence of the new compounds of glycine recently reported by M.M. Khandpekar et al. (Optics Commun. 284(2011) 1583 and Optics Commun. 284(2011) 1578) is dubious. We argue that these compounds are not what the authors propose. In addition, two more similar cases are indicated.     

Commentary on "Mechanical properties of monodomain side-chain nematic elastomers" by P. Martinoty, P. Stein, H. Finkelmann, H. Pleiner and H.R. Brand

We discuss the background to static and dynamic soft elasticity. The evidence in the static case and the symmetry basis for soft and semi-soft elasticity is well understood. By contrast the dynamic analogy is less clear. Lack of clean time scale separation clouds the interpretation of director relaxation keeping up, or not, with imposed strains. However, the reduction in modulus between geometries obtaining at low frequencies and being lost at high frequencies confirms that director reaction indeed determines dynamical semi-softness.     

Commentary on rheology of polymers in narrow gaps

Without abstract.           

Fat fractals in the lung? A comment on a paper by grebogi et al.

An exponent characterising the scaling behaviour of a branching structure occupying volume (an example of a “fat” fractal), introduced recently by Grebogi et al., is examined using data on the arteries, veins and airways of mammalian lungs. The results cast doubt on the usefulness of the exponent in this context.