Dario Romano

A Hemi-metallocene Chromium Catalyst with Trimethylaluminum-Free Methylaluminoxane for the Synthesis of Disentangled Ultra-High Molecular Weight Polyethylene

Recently, it has been shown that by using a single-site catalytic system having titanium as a metallic center, it is possible to tailor the entanglement density in the amorphous region of a semi-crystalline ultra-high molecular weight polyethylene (UHMWPE). This route provides the possibility to make high-modulus, high-strength uniaxially and biaxially drawn tapes and films, without using any solvent during processing. In this publication, it is shown that a single-site catalyst having chromium as metallic center, proposed by Enders and co-workers, can also be tuned to provide control on the entanglement density during synthesis of the UHMWPE. However, to achieve the goal some modifications during the synthesis are required. The synthesized polymers can be processed in the solid state below the equilibrium melting temperature, resulting in uniaxially drawn tapes having tensile strength and modulus greater than 3.5 N/tex and 200 N/tex, respectively. Rheological studies have been performed to follow the increase in entanglement density in melt state with time.

Stress relaxation in the nonequilibrium state of a polymer melt

The influence of entanglement density on the constraint renewal time is studied experimentally in transitory nonequilibrium polymer melts. The entanglement density, as quantified by the rubber elasticity, increases as the linear polymer melt transforms into the equilibrium state. The relaxation modulus obtained from linear step-strain deformations, performed at different points during the equilibration, shows an increase in constraint renewal time as the entanglement density increases. The normalized relaxation modulus curves collapse onto a single curve by rescaling the time axis with a factor G′¯(t)0.9 (where G′¯(t) is the normalized instantaneous modulus). These findings suggest that, though the relaxation time increases with the increasing number of entanglements, the mechanism responsible for stress relaxation, after application of step-strain, is similar to that in a fully entangled melt.

A study on the stability of the stress response of nonequilibrium ultrahigh molecular weight polyethylene melts during oscillatory shear flow

Dynamic shear flow and especially large-amplitude oscillatory shear have been a subject of interest to investigate limitations of theoretical approximations working under the assumption of simple shear. These studies have helped in understanding the kinematics involved in phenomena such as stick-slip and shear banding, among others. The nonequilibrium polymer melt, which transforms with time into the equilibrium state, provides a unique opportunity to investigate the influence of the thermodynamic melt state on the validity of simple shear approximations. Here, we show that during oscillatory deformation, the nonlinear rheological response of ultrahigh molecular weight polyethylene melts, having number-average molecular weight greater than one million g/mol, is strongly dependent on the entangled state of the same polymer. At sufficiently large strain amplitude, the stress response of the material departs from a periodic sinusoidal signal, with maximum stress decaying with consecutive cycles of deformatio...