| Abstract: | Two crystal modifications are found in α-helical poly(δ-N-carbobenzoxy L -ornithine). In films as cast, the two-dimensional unit cell is pseudohexagonal and contains two chains. This form transforms irreversibly into a pseudotetragonal form at about 140°C. A second-order transition associated with the onset of the side-chain motion is observed at about 30°C for the bulk sample (by dilatometry) and for the crystalline phase (by x-ray diffraction). The dielectric behavior of the side-chain dispersion suggests that the side chains interact with one another. The temperature dependence of the infrared absorbance due to the NH stretching vibration reveals that about half the side chains are associated via hydrogen bonds at room temperature and become dissociated at higher temperature. The enthalpy and the entropy of the hydrogen bond formation is estimated to be ΔH = ?5.0 ± 0.5 kcal mol?1 and ΔS = ?15 ± 1 e.u. mol?1. |
