Polyethylene containing aliphatic ring and aromatic ring defects in the main chain: Synthesis via ADMET and comparisons of thermal properties and crystalline structure

Three polyethylene containing different non-planar aliphatic ring ether units (1,4-cyclohexylenedioxy, 4,4'-bicyclohexylenedioxy and 4,4'-isopropylidenedicyclohexylenedioxy) were synthesized via acyclic diene metathesis polymerization (ADMET) and subsequent hydrogenation. They were compared with their counterpart polymers containing aromatic ring defects in aspects of chain structural, thermal properties and crystalline structure by investigations with H-1 NMR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Regardless of the trans/cis isomers of cyclohexane moieties in these polymers, the results demonstrated that polyethylene containing non-planar aliphatic ring units showed decreased thermal stability as compared with the corresponding PE containing aromatic ring defects. Moreover, study on crystallization behaviors indicated that in contrast to their aromatic counterparts, introduction of the non-planar aliphatic defects perturbed the corresponding crystal structure more intensively, and larger space is needed to include these defects in the solid phase. As a result, their WAXD reflections shifted to lower scattering angles that correspond to increased d-spacing, and the polymers exhibited lower melting points. SAXS results proved that non-planar aliphatic ring defects result in increased dimension of amorphous region. Calculated by melting points and enthalpy, these polymers had smaller crystalline lamella thickness and lower crystallinity than their aromatic counterparts. (C) 2016 Elsevier Ltd. All rights reserved.