NMR Characterization of Vis‐Broken Heterophasic Ethylene−Propylene Copolymers

Abstract

a narrower molecular weight distribution by effectively removing the high molecular weight tail of the distribution. This results in a more Newtonian shear viscosity behavior permitting the use of lower processing temperatures and higher spinning speeds when processing into fibers or thin films. [8] In the case of HEPCs a similar vis-breaking treatment can be applied for the same reasons. The necessity of this step is due to the incapability of the catalyst system to provide low molecular weight material. While this step is necessary, its effect on the mechanical properties of the resultant impact copolymer is undesirable. [9,10] The effect of the visbreaking process on impact copolymers is not well understood as the chemical structure of these copolymers is highly complex. HEPCs contain varying distributions of ethylene-and propylene-rich regions. This presents challenges in predicting the manner in which the copolymers will be affected by vis-breaking. In contrast, the process and mechanism of vis-breaking for conventional polypropylene is well documented. [8,11-14] Controlled rheology (also referred to as vis-breaking) is based on chain scission induced by free radicals, exploiting the mechanism of oxidative degradation of polypropylene. As polyolefins in general are susceptible to oxidative and photochemical degradation, polymer modifications which make use of these radical reactions need precise decomposition rates depending on the processing conditions. Therefore radical sources with specific thermal decomposition temperatures and thus controlled radical initiation are ideal. The radical source is most often an organic peroxide such as dicumylperoxide, [14-16] 2,5dimethyl-2,5-di-tert-butylperoxyhexane (DHBP), [7,17,18] or cyclic peroxides such as 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane (Trigonox 301) [9] which forms primary radicals following thermal decomposition. The cyclic peroxides have the advantage of not releasing volatile gases as in the case of the acyclic organic peroxides. [7,19] Since the radicals attack in a random fashion the statistical probability that a monomer in a longer chain reacts with a radical is greater since the distribution of chain sizes generally implies that monomers are more likely to occur in longer chains than shorter chains. This leads to break down of the longer chains and decrease of the overall molecular weight distribution. The use of inorganic peroxides has been reported in the literature [8] but has not been implemented industrially due to the high reactivity of these peroxides which require low reaction temperatures. This limits the applicability Solution 13 C nuclear magnetic resonance (NMR) is used in conjunction with in situ solid-state NMR to determine the effect of peroxide treatment on the chemical structure and morphology of ethylene−propylene copolymers. The copolymers contain increasing quantities of ethylene with the lowest ethylene content corresponding to pure isotactic polypropylene. The vis-breaking of heterophasic ethylene-propylene copolyme...