On Crystallization in Polypropylene-Polyethylene Blends

Abstract

Blends of poly pro py lene ( PP ) a nd lo w-densi ty poly ethylene ( L DP E) have been crystal li zed t o fo rm o pen st ruct ures(" cl uster spheruli tes" ) where the dropl et s o f the mi no ri ty P P a re bridg ed by PP lam ellae w hich hav e g ro w n in the L DPE-rich ma trix. These are st udied by perma nga ni c etching f ollo w ed by elect ron and optical microscopi es. T wo simila r PP t ypes a re compared, o ne sy nthesi zed w ith Zi eg ler-Na t ta ca talyst and one wi th met allocene cataly st. The metallo cene-ca taly sed ma terial crystalli zed much mo re slo wl y due t o the presence o f regi o defects in the chai ns, even though the ov erall co ncent ratio n of tactici ty def ect s in the Ziegler-Nat ta m aterial is m uch higher. A mechanism i nv olv ing reversa l of helical di rectio n a t the regi o def ect i nterf eri ng wi th the reg ular chai n packi ng i n the crystal is sug gested. Gro w th of " cl uster spherulit es" is f ast er in regi ons where low mol ecula r w eig ht m aterial i s co ncent rated. It i s slo w er w here dro plet s are la rg er, and this is at t ri but ed to com peti ti on bet w een P P lam ella r g row th i n the mat rix and dif fusio n o f PP t o already crystal li zed droplets.