Unprecedented Mechanical Properties in Linear UHMWPE Using a Heterogeneous Catalytic System

Abstract

Several homogeneous postmetallocene complexes are known to produce weakly entangled ultrahigh-molecular-weight polyethylene (disentangled UHMWPE) under controlled polymerization conditions leading to ultimate mechanical properties. Major challenges in using a homogeneous catalyst system exist such as reactor fouling and uncontrolled polymer morphology, which could be addressed by heterogenization of the single-site complex. In such a scenario, a heterogeneous catalytic system that can synthesize ultrahigh-molecular-weight polyethylene with a reduced number of entanglements, to an extent that the mechanical properties are equivalent to those obtained using a postmetallocene catalyst in a homogeneous condition, remains a challenge. Herein, a magnesium chloride based in situ formed activator/support, MgCl x /Et n Al y (2-ethyl-1-hexoxide) z , is employed with a highly active bis­[N-(3-tert-butylsalicylidene)­pentafluoroanilinato] titanium­(IV) dichloride (Cat. 1) for the synthesis of ultrahigh-molecular-weight polyethylene with a reduced number of entanglements. A novel route is adopted to make a nano-support that allows tailoring of the entangled state and control over the resultant morphology without reactor fouling and wall sheeting, thus providing the feasibility of pursuing the polymerization via a continuous process. The synthesized nascent polymer shows the formation of single crystals of linear UHMWPE, identifying the fold surface and crystal thickness and suggesting a low entangled state. The topological differences with the commercial entangled sample are identified by chain diffusion from the noncrystalline to crystalline region via solid-state NMR, following melting kinetics via DSC, and transformation of the nonequilibrium melt into the equilibrium state via rheology. Thus, the obtained disentangled crystals can be compressed in the solid state, without melting, to an extent that the macroscopic forces can be transferred to the molecular level. This allows the desired chain orientation for securing the ultimate tensile modulus (>200 N/tex) and tensile strength (>4.0 N/tex) in the synthesized low entangled linear polyethylene having a weight average molar mass exceeding a million grams per mole. These mechanical properties are equivalent to those perceived using a homogeneous catalytic system and are the first of their kind reported in a polymer synthesized using a heterogeneous catalytic system.

Influence of Molecular Weight and Entanglement Density on the Creep Response of the Uniaxially Drawn Tapes of dis-UHMWPE

“…With an increase in the polymerization time, the synthesized polymers show a shift in the crossover point (where G′ = G′′ ) to lower frequencies (Table S1, runs 1–2), which is indicative of an increase in the tube renewal time with increasing molecular weight. These observations are in agreement with the previously reported findings using the same catalyst and catalytic systems . The employed catalyst has been shown to exclusively produce linear chains (without the presence of branches), thus allowing application of Mead’s model to correlate the viscoelastic function with the M w .…”

“…In a series of publications, it is shown that the low-entangled UHMWPEs synthesized using homogeneous ,, and heterogeneous catalyst systems can be solid-state processed in an eco-friendly way without using any solvent to uniaxially and biaxially oriented objects that have unprecedented high mechanical properties. Systematic solid-state processing was performed using the literature-reported protocol , (with modification wherever necessary) under uniform processing conditions.…”

“…In a series of publications, it is shown that the low-entangled UHMWPEs synthesized using homogeneous ,, and heterogeneous catalyst systems can be solid-state processed in an eco-friendly way without using any solvent to uniaxially and biaxially oriented objects that have unprecedented high mechanical properties. Systematic solid-state processing was performed using the literature-reported protocol , (with modification wherever necessary) under uniform processing conditions. The processing comprises different steps: (a) compression molding of the synthesized polymer at 125 °C, which is below the equilibrium melting temperature (140 °C) of the nascent UHMWPE, into a sheet and (b) subjection of the thus-compacted powder to calendaring at 130 °C using a roll mill while maintaining a constant gap between the rollers.…”

“…The morphology of the nascent polymers was studied using a Thermo Fischer-FEI Teneo VS scanning electron microscope. The nascent polymer samples were mounted on a SEM stub with the help of carbon tape and coated with iridium by sputtering technique, as previously reported …”

An Unconventional Route for Synthesis and Solid‐State Processing of Low‐Entangled Ultra‐High Molecular Weight Isotactic Polypropylene

“…In UHMWPE, the low-entangled state allows the fusion of the polymer particles and translation of the macroscopic force (experienced during the uniaxial as well as biaxial stretching) to the macromolecular length scale of the chains. [7,26] The nascent polymer powder is compressed at 130 °C, rolled at 145 °C, and consequently stretched three times to achieve high draw ratios. During the rolling (or calendaring) process the sample changed into a transparent tape suggestive of the sintering during rolling below-melting point (T peak = 158.3 °C and T onset = 148.9 °C).…”