Saturation of Shear-Induced Isothermal Crystallization of Polymers at the Steady State and the Entanglement−Disentanglement Transition

Abstract

To provide insight into the formation of shear-induced precursor structures, three apparently unrelated subjects are analyzed and discussed: the saturation of crystallization from sheared polymer melts, evaluated with a new shear DTA instrument, the steady state in steady shear, and the entanglement−disentanglement transition. It is shown that the same large strains that saturate crystallization also lead to a reversible steady state in steady shear, where the viscosity of the sheared melt is constant in time, and their magnitude is only determined by the temperature of the sheared melt. Features of the melt morphology at this state are discussed, and their importance is highlighted to understand the possible mechanisms behind the formation of shear-induced precursors. Measurements of the reptation time for unsheared samples, and samples sheared up to the steady state, allowed the quantification of the entanglements loss during the transition between these two states (around 2/3), which is interpreted as an entanglement−disentanglement transition. The relevance of this result on assumptions of flow models, particularly the constant number of topological constraints, convective constraint release process, and chain stretch, is discussed.

Model Development and Validation of Crystallization Behavior in Injection Molding Prototype Flows

“…Some experimental observations have been reported that a saturation limit exists beyond which the rate of flowinduced nucleation becomes zero. [49] Hristova and Peters [50] measured the number of flow-induced nuclei in iPP HD120MO as a function of shear rate in a Linkam shear cell. These data indicate a saturation level of the total number of flow-induced nuclei, N f,max , independent of the shear rate.…”

A Model for Flow‐enhanced Nucleation Based on Fibrillar Dormant Precursors

“…Consequently, the precursor length l resulting from a shear experiment grows fast for increasing shear times when looking at short-term shear experiments, and as the shear times get larger, the length increase for the same increase in shear time gets smaller. This is depicted in Figure 12( [16] Hence neither the rheology of the HMW tail nor that of the whole melt shows a correlation between steady-state behavior and saturation, as suggested by Martins et al [34] A second effect also has a contribution to the saturation effect, albeit smaller than the first one. The factor Àrðn Ã Þ @n à @l in Equation (10) decreases after reaching a maximum at l ¼ 30 nm (see Figure 8, this precursor length corresponds to N p;a ¼ 2  10 13 m À3 as can be seen from Figure 12(b) and (c)).…”

“…The free energy of nucleation DF is given by Equation (3) and schematically depicted in Figure 5. The coefficient of growth diffusion D gr can be presented in the form [4] D gr ¼ D 0 T=T m ð Þ ffiffiffi n p =t gr (34) where D 0 is a constant and t gr is the relaxation time related with self-diffusion of kinetic units subject to aggregation. The factor ffiffiffi n p accounts for the number of sites available for attachment/detachment of a stem on the surface of an n-sized precursor.…”

In situ polymerization of ethylene in the presence of graphene by α‐diimine Nickel (II)/MAO catalyst system: Thermal and rheological study

“…As seen, there is the first Newtonian plateau for neat PE at low frequencies which is common for most polymers and implies to equilibrium state between entanglement and disentanglement of polymeric chains at this range of shear rate. [ 41,42 ] By incorporation of nanoparticles and formation of strong interactions with polymer chains during in situ polymerization, the aforementioned equilibrium is disrupted, and a non‐Newtonian behavior begins at low frequencies. Moreover, the higher viscosity at the whole range of frequencies is the result of the presence of the GNP and filler–matrix interactions for nanocomposite samples.…”