Calorimetry of Fuming Nitric Acid Treated Polyethylene

Synopsis

Polyethylene drawn at different temperatures to a draw ratio between 7 and 25 and polyethylene rolled at room temperature to a draw ratio about 3 was treated with fuming nitric acid. From the two characteristic maxima of the thermograms, crystallite length and the number of unoxidized tie molecules are calculated. In the drawn sample the crystallite length is about 50A smaller than the long period found by small angle X-rays. This is a consequence of the non-crystalline layer between the lamellae which is first eaten away by the acid. The number of tie molecules is practically constant in samples drawn between 14 and 110°C, but increases nearly linearly with draw ratio λ up to λ = 20 and stays constant at higher draw ratios. The so obtained numbers are compared to the minimum number of tie molecules needed for the explanation of the elastic modulus and the ultimate tensile strength of the same samples before oxidation.