STUDY OF SHEAR MODIFICATION OF LDPE BY AN NMR RELAXATION TECHNIQUE

A low-density polyethylene was shear modified by repeated extrusions. These operations resulted in no detectable changes in the polymer mole cular weight distribution. The apparent melt viscosity and elasticity decreased after extrusion, as is commonly observed in shear modificati on. These rheological effects were fully reversible by annealing the m elt. Transverse proton relaxation measurements showed the existence of four identifiable different regions in the polymer melt. The range of relaxation times encompassed two orders of magnitude. These reflect t he existence of regions with different degrees of conformational order . Shear modification results in the breakup of a minor proportion of t he most constrained regions, which have the shortest relaxation times. The apparent ''entanglements'' that are inferred from rheological mea surements probably reflect the coexistence of segments of more than on e macromolecule in chain-folded domains, as indicated by molecular dyn amics calculations. These results are evidence that the equilibrium st ate of LDPE is not a chain-entangled, highest entropic condition, but rather a lowest free energy state in which chain folding is favored. T hey are consistent with other longstanding observations, such as the f ailure of experiments to produce amorphous polyethylene and the appare nt lack of mixing of HDPE and LDPE on a molecular level.