Leak-free stopper having low breakloose and sustaining forces

A thermoplastic elastomer stopper that meets the desired material properties of a stopper for a syringe assembly is disclosed. The compression set of the thermoplastic elastomer stopper of the present disclosure is ≤50% when measured at 25% compression for 22 hrs at 70 degree C. The hardness of the thermoplastic elastomer stopper of the present disclosure is 40-70 Shore A. The viscosity of a thermoplastic elastomer stopper of the present disclosure is ≥70 Pa·s at 1,000 s−1 shear rate, ≥12.0 Pa·s at 10,000 s−1 shear rate, and ≥3.0 Pa·s at 50,000 s−1 shear rate when measured using a capillary rheometer at 205 degree C. (Die: Roundhole 20 mm length/1 mm diameter/180 degree inlet, Piston: d=15 mm, and melting time=7 min). The present non-lubricated stopper exhibits the required functional performance of a lubricated stopper.