| The slow crack growth behavior of PE with different molecular structures and morphologies has been investigated using the single edge notched tensile specimen under constant load. It was found that the micro-structures of semi-crystalline PE play an important role in this behavior.; For different molecular weights, except ultra-high molecular weight PE, the time to failure (t{dollar}sb{lcub}rm f{rcub}{dollar}) can be expressed as an empirical equation, t{dollar}sb{lcub}rm f{rcub}{dollar} = A (M{dollar}sb{lcub}rm w{rcub}{dollar}-M{dollar}sb{lcub}rm o{rcub}{dollar}) {dollar}sigmasp{lcub}-5{rcub}{dollar} a{dollar}sb{lcub}rm o{rcub}sp{lcub}-2{rcub}{dollar} exp(110,000/RT).; The fracture mechanism and molecular motion are independent of molecular weight. The effect of molecular weight was associated with the increasing number of tie-molecules.; It was found that ultra-high molecular weight polyethylene behaves significantly different from lower molecular weights. Its lifetime is given by t{dollar}sb{lcub}rm f{rcub}{dollar} = B {dollar}sigmasp{lcub}-2{rcub}{dollar} a{dollar}sb{lcub}rm o{rcub}sp{lcub}-1{rcub}{dollar} exp(70,000/RT).; Furthermore, crazing was not observed with the UHMWPE. It seems that the "two phase" model in UHMWPE is not appropriate and it should be envisaged as an amorphous network crosslinked by small crystals.; For different branch densities of PE, t{dollar}sb{lcub}rm f{rcub}{dollar} = C {dollar}sigmasp{lcub}-5{rcub}{dollar} exp(120,000/RT). Here C is a function of branch density, d. The higher the branch density the longer the lifetime. The side branches indirectly increase tie molecules by reducing the lamellar thickness. But the average branch density is not the only branch parameter to control the slow crack growth. It was found that only the homogeneously distributed branches can effectively reduce the lamellar thickness.; The probability of forming tie molecules was calculated based on the premise that in the melt, the end-to-end distance of a random coil, must be greater than a critical distance L in order to form a tie molecule here L = 2 L{dollar}sb{lcub}rm c{rcub}{dollar} + L{dollar}sb{lcub}rm a{rcub}{dollar}. L{dollar}sb{lcub}rm a{rcub}{dollar} and L{dollar}sb{lcub}rm c{rcub}{dollar} are the amorphous and crystalline thicknesses, respectively.; The brittle fracture strength was measured at low temperatures, it was found to be proportional to the fraction of tie molecules in the amorphous region. |
