Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The activity of a solid, polymer-supported catalyst (a semicrystalline polyethylene film containing grafted, sulfonated styrene) was shown to be altered by cold drawing. The catalytic activity was measured by a test reaction, the dehydration of isopropyl alcohol to give propylene. Catalytic reaction rates were measured with variously drawn films clamped in a differential flow reactor operated at 100°C and 1 atm. The catalytic activity increased with the elongation of the polymer up to a draw ratio of 2.5; the activity decreased upon further drawing. The drawn films were characterized by x-ray diffraction, dynamic mechanical measurements, electron microscopy, birefringence, and density measurements. Since no evidence was found for the formation of surface sites by creation of microcracks, the changes in activity are attributed to modifications in the polymer structure induced by drawing. The kinetics of the catalytic reaction and the data giving percentage crystallinity and crystalline and amorphous orientation factors suggest that, in the undrawn polymer, the catalytically active —SO3H groups form a hydrogen-bonded network, which is excluded from the crystalline regions. Initial elongation partially breaks up the network, allowing more —SO3H groups to bond to alcohol and become catalytically engaged. Elongation beyond a draw ratio of 2.5 leads to a separation of —SO3H groups greater than that required for the formation of the reaction intermediate, which involves the alcohol hydrogen bonded to several —SO3H groups. The combined results of the catalytic kinetics experiments and structural characterizations imply that the preliminary deformation response of the semicrystalline polymer occurs predominantly within the noncrystalline regions and is accommodated by the rigid slip and tilting of crystal lamellae. The hydrogen bonding among the —SO3H groups in the noncrystalline regions hinders lamellar breakup and suppresses the formation of highly aligned fibrillar morphologies.
Type of Medium: