| 摘要 |
The present invention provides a post-polymerization process for modifying the molecular weight distribution of a polymer having a monomodal molecular weight distribution, which polymer normally exhibits a decrease in molecular weight when subjected to high shear mixing in the presence of a free radical initiator, to produce a polymer having a bimodal molecular weight distribution. The process comprises: (a) heating a mixture of the polymer, a polyunsaturated crosslinking agent and, optionally, a free radical initiator, to form a uniform mixture; and (b) subjecting said mixture to high shear mixing conditions until a polymer having a bimodal molecular weight distribution is obtained. In the more preferred embodiment, the shear mixing is conducted in the presence of a free radical initiator such as an organic peroxide and the crosslinking agent is a difunctional compound containing diallyl, divinyl or diethylenic unsaturation. The process results in the development of polymers having a bimodal molecular weight distribution as a consequence of the coupling of some of the free radically degraded polymer chains added through the unsaturated functionality of the crosslinking agent. The high molecular weight species in the bimodal distribution lends a higher melt strength to the polymer and the lower molecular weight species imparts improved processing and melt flow properties. |
