Unexpected structural properties in the saturation region of the odd-even effects in aliphatic polyethers: Influence of crystallization conditions

Abstract

A series of aliphatic polyethers with different chain lengths (nCH2= 6 to 12, and 16) is studied employing differential scanning calorimetry and X-rays scattering. The calorimetric and structural behavior of samples crystallized from the melt is divided into the odd-even and saturation regions. In the odd-even region (nCH2 = 6 to 10), the odd samples (nCH2 = 7, and 9) show enhanced calorimetric properties (e.g., higher transition temperatures) and faster crystallization kinetics than the even ones (nCH2 = 6, 8 and 10). The odd samples crystallize in orthorhombic unit cells and the even ones in monoclinic unit cells. In the saturation region (nCH2 = 11 to 16), the calorimetric properties increase as nCH2 increases without alternation. However, unexpectedly, the nCH2 = 12 displayed a mixed structure (monoclinic + orthorhombic) instead of an orthorhombic one. Thus, a structural saturation effect (i.e., an orthorhombic unit cell) is not reached. This particular structural feature was investigated under varied thermal histories, induced by different cooling rates. The samples as synthesized (i.e., crystallized during precipitation from solution) exhibited a structural saturation effect since both nCH2 = 10 and 12 display an orthorhombic unit cell. But, the nCH2 = 10 exhibits a monoclinic unit cell, and the nCH2 = 12 a mixed structure when the samples crystallize from the melt at different rates. Only the nCH2 = 16 crystallizes in an orthorhombic unit cell, independently of the thermal history. Thus, the complex odd-even effects in these aliphatic polyethers are a function of the cooling rate from the melt and sample preparation procedures (solution or melt crystallization).