Even-odd Effect in Aliphatic Polycarbonates with Different Chain Lengths: from Poly (hexamethylene carbonate) to Poly (dodecamethylene carbonate)

Abstract

We have characterized a series of aliphatic polycarbonates synthesized by organocatalysis containing a variable number of methylene groups (nCH2) in their repeat units ranging from nCH2 = 6 to 12. The melting and crystallization behavior and crystalline structures were studied by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and wide-angle X-ray scattering (WAXS). We found a clear even-odd effect in terms of thermal properties and crystalline structure, for nCH2 = 6 to 9, and a saturation of the even-odd effect, for nCH2 = 10 to 12. These results were independent of the crystallization conditions employed: non-isothermal, isothermal and successive self-nucleation and annealing (SSA). The even-odd region showed that the even samples had higher melting temperatures than the odd ones, and a monoclinic unit cell. On the other hand, the odd samples showed an orthorhombic unit cell. Both even and odd samples exhibited a trans-conformation, with a dilution of the impact of carbonyl group as evidenced by the weakening of the crystalline memory effect as nCH2 increases, independently of the even or odd nature of the samples. In the saturation region, the methylene instead of the carbonyl groups dominated the behavior, resulting in thermal properties that changed almost linearly with nCH2. The unit cells were all orthorhombic and the strength of memory effect was similar, as nCH2 increased. Accordingly, the samples showed a shift of the FTIR bands towards a PE-like dominated conformation.