Evaluation of viscosity and shear stress in a telechelic polymer when various shear rates are applied

Abstract

We present the results of molecular dynamics (MD) simulations of bead and spring models of associating polymers at melt-like densities. We consider telechelic polymers of general formula AB 12 A at different values of shear rate. Under bulk conditions, A···A associations occur, leading to the construction of clusters or micelles that are interconnected by a number of bridging chains. This results in a physical gel or a reversible network, and the corresponding micelles assume different forms depending on the interaction strength. Whenever a low shear deformation is applied, the clusters are broken and there is a phase transition similar to that from a gel to a common non-Newtonian fluid. However, if the applied shear is increased, the chains stretch themselves (in a manner similar to nematic liquid crystal ordering) in a different way from what is seen without shear. In this work, we calculate the stress needed to yield a required deformation.

Quantitative Evaluation of the End‐to‐End Correlation Vector and of the Mean Square Displacement of the Molecules' Center in a Telechelic Polymer under Several Shear Rates Values

“…At last, the application of a shear rate produces the qualitative changing explained in our previous papers: stretching of the chains along the direction of the shear and, as a consequence, the modifying of the relation between msd and time from linear to quadratic, and the anisotropy of cvec. The effects of a particular shear rate are not the same when coupled to the four different A–A interactions: the general law is that along the z direction, which is neutral with respect to the shear applied, the behavior remains the same of the no‐shear condition; along the y one, which is subjected to the shear, the relaxing time decreases and along the x one the behavior can assume a complex form, which influences the total, as reported in Figure .…”

“…All simulations were carried out, as in our previous works, with the coarse‐grained molecular dynamics code COGNAC, using more recent versions. We adopted a standard coarse bead‐and‐spring representation of a generic flexible polymer…”

“…As in our last work we considered six different shear rate values, in reduced units of τ −1 , (5 × 10 −5 , 1 × 10 −4 , 5 × 10 −4 , 1 × 10 −3 , 5 × 10 −3 , 1 × 10 −2 ), in addition to the 0, for a total of 28 case analyzed, being each of them coupled to the four A–A interactions strengths. Always as in our previous works we started studying the homopolymer in bulk condition, and only when this case reached the equilibrium condition we increased the A–A interactions to 4. Subsequently, in a similar manner, we assigned the ε AA values equal to 6 and 7, always when the condition corresponding to the lower values was arrived in a stability state.…”

“…The following studies were about the rheological behavior and, most of all, the qualitative changing of the correlation function for the correlation of the end‐to‐end vector (in the following always contracted in cvec) and the mean square displacement of the barycenter of the molecules (msd shortly) with respect to time…”

“…These polymers have been the focus of a sizeable number of theoretical studies and computer simulations from the middle of the last decade of the previous century . In particular, in the first years of this millennium, the detailed structures of the aggregates and the behavior of the polymers under bulk conditions have been clarified, as when they are confined to a narrow volume between two parallel surfaces by other authors, as in an isotropic volume by us .…”