Influence of Polymerization Conditions on Melting Kinetics of Low Entangled UHMWPE and Its Implications on Mechanical Properties

The synthesis of linear ultra high molecular weight polyethylene, with a “pseudoliving” catalyst in various conditions results in samples of different molecular weights (M w ranging between 2 to 35 million g/mol), all having a reduced number of entanglements to an extent that allows the solid-state uniaxial deformation of such high molar masses without melting. The solid-state processing of these materials shows a clear relationship between mechanical properties and molecular weight. For the adopted polymerization conditions, stretching forces required for the uniaxial deformation increase with the increasing molar mass, ultimately limiting the achievable maximum draw ratio in the polymers having M w > 10 million g/mol. The increase in the stretching force is attributed to the increasing number of entanglements between the crystals with the molar mass. The estimation of entanglements is established with the help of melting kinetics involved in the “disentangled” crystals, and rheological response of the polymer melt obtained just after melting of the crystals. In spite of the increase in the stretching forces with the increasing molar mass, tensile modulus increases with the increasing draw ratio and the molecular weight. However, above the molar mass of 10 million g/mol, the stretching force required increases to the level that the uniaxial deformation becomes difficult–thus limiting the tensile strength.