An ethylene/alpha-olefin copolymer composition having a density from 0.935 to 0.955 g/cc; a ratio of weight average molecular weight to number average molecular weight, Mw/Mn, of from 3 to 10; a z-average molecular weight, Mz, from 200 kg/mol to 500 kg/mol; and a PENT value of greater than 500 hours at 80° C. and 2.4 MPa; wherein when the composition is formed into a monolayer pipe the pipe has a pipe hydrostatic strength of greater than 100 hours at 20° C. and 12.0 MPa. Also provided is a pipe or pipe fitting comprising the ethylene/alpha-olefin copolymer composition.