(46e) Polar Functionalization of Polypropylene with Suppressed Molecular Weight Reduction: Near-Ambient Temperature Free Radical Chemistry During Solid-State Shear Pulverization

Solid-state shear pulverization (SSSP) is a novel processing method, which utilizes a modified twin-screw extruder that operates under near-ambient temperature conditions. During SSSP, polymers are processed in their solid state using high compressional and shear forces that cause repeated fragmentation and fusion of the material. Previous work has shown that intimate mixing in polymer blends as well as effective dispersion and exfoliation in polymer nanocomposites can be achieved with SSSP. In the present work, by taking advantage of unique chemistries associated with near-ambient temperature processing, we demonstrate our ability to achieve polar functionalized polypropylene (PP) via SSSP. Furthermore, because of the differences associated with near-ambient temperature free-radical chemistry of polypropylene and high-temperature, melt-state free radical chemistry of polypropylene, we avoid the significant molecular weight reduction of PP that accompanies commercial polar modification of PP during melt-state processing.

The conventional method of functionalizing PP with maleic anhydride (MA) is via melt processing, which results in drastic molecular weight reduction of PP and consequently the loss of material properties—relative to those of neat PP from which the functionalized PP was made. This molecular weight reduction occurs as a result of β-scission, a free-radical chemistry that is highly dependent on temperature; under the high temperature conditions utilized for melt processing (~190 °C) the degree of β-scission is high. Here, we demonstrate the utility of SSSP to achieve MA functionalization of PP while suppressing molecular weight reduction due to β-scission. We have also identified our ability to functionalize PP with carbonyl groups via SSSP; this functionalization is also achieved with suppressed molecular weight reduction. Results from Fourier transform infrared spectroscopy show that carbonyl functionality cannot be obtained using conventional high temperature processing methods (when working with the same feed material as that which was used in SSSP to obtain carbonyl functionalized PP). Our success in these systems is as a result of our ability to take advantage of the unique chemistries associated with the near-ambient temperature conditions under which SSSP is carried out. Samples of MA and carbonyl functionalized PP were reacted with a fluorescent dye, 1- pyrenemethylamine (Pyr-MeNH₂). Fluorescence spectra suggest that both of these samples are reactive with Pyr-MeNH₂ and could potentially be used interchangeably in applications where MA modified PP has current or potential utility.