Solid-State Processing of In Situ Blended Prepolymer with Z–N Synthesized UHMWPE: Role of the Prepolymer

Abstract

Ultrahigh molecular weight polyethylene (UHMWPE) synthesized using single-site catalytic systems, adopting a homogeneous bis(phenoxy-imine) Ti catalyst or half-metallocene Cr catalyst, under controlled polymerization conditions exhibits a unique low-entanglement state that enables solvent-free solid-state processing into strong, uniaxially and biaxially oriented films having unprecedented tensile strength and tensile modulus. The use of bis(phenoxy-imine) titanium catalysts supported on MgCl₂-based dual activator/support systems has been also shown to facilitate the heterogeneous synthesis of low-entangled UHMWPE, offering a promising industrial route. Conversely, commercially viable heterogeneous Ziegler–Natta catalysts yield UHMWPE with a high number of entanglements per chain (en-UHMWPE), necessitating solution spinning for fiber production. This study aims to investigate an industrially viable, solvent-free processing route for en-UHMWPE using commercial Ziegler–Natta catalysts. Herein, we synthesize UHMWPE sample via a one-pot, two-step protocol, incorporating a relatively low molar mass component (prepolymer) into the UHMWPE matrix, thus achieving a molecular blend between low and ultrahigh molar mass polymers. The sample exhibits excellent solid-state processability, achieving a remarkable draw ratio of up to 148× in a narrow temperature window. This resulted in outstanding mechanical properties of 1.6 and 127 N/tex of tensile strength and tensile modulus, respectively, for a Z–N synthesized polymer. Wide-angle X-ray diffraction (WAXD) measurements demonstrate a strong correlation between the draw ratio and the chain orientation, indicating a high degree of molecular alignment at higher draw ratios. In the drawn samples, solid-state nuclear magnetic resonance spectroscopy reveals the presence of a highly mobile amorphous fraction in the prepol/en-UHMWPE blend. The presence of the mobile fraction, arising from the melt-crystallized component in the drawn samples, is further supported by differential scanning calorimetry, WAXD, and small-angle X-ray scattering. On comparing with the low-entangled/disentangled samples synthesized using the single-site catalytic systems, the studies demonstrate that in the Z–N samples investigated here, the low molar mass component acts as an effective consolidant facilitating solid-state processing in a relatively narrow temperature window. The study emphasizes the influence of polymerization conditions and molecular characteristics in pursuing fundamental studies, especially on ultrahigh molar mass polymers.