Thermal Properties of Aliphatic Polypeptoids

A series of polypeptoid homopolymers bearing short (C-1-C-5) side chains of degrees of polymerization of 10-100 are studied with respect to thermal stability, glass transition and melting points. Thermogravimetric analysis of polypeptoids suggests stability to >200 degrees C. The study of the glass transition temperatures by differential scanning calorimetry revealed two dependencies. On the one hand an extension of the side chain by constant degree of polymerization decrease the glass transition temperatures (T-g) and on the other hand a raise of the degree of polymerization by constant side chain length leads to an increase of the T-g to a constant value. Melting points were observed for polypeptoids with a side chain comprising not less than three methyl carbon atoms. X-ray diffraction of polysarcosine and poly(N-ethylglycine) corroborates the observed lack of melting points and thus, their amorphous nature. Diffractograms of the other investigated polypeptoids imply that crystalline domains exist in the polymer powder.