Molecular weights of polyethylene and polypropylene by vapor pressure osmometry at elevated temperatures

The Wescan Model 232-A VPO was modified with a 12 volt regulated power supply for supplying bridge current and with thermocouples to allow for direct digital readout of the syringe box and measuring chamber temperatures. The modified instrument was used to measure M _n of standard (NBS) polyethylenes. Agreement with NBS values was good (within 10%) for M _n < 3 × 10⁴ and fair (within 20%) for M _n = 1 × 10⁵. Results on M _n of commercial polypropylenes are also reported.