Studies of chain conformation in triblock oligomers and microblock copolymers of ethylene and ethylene oxide

The conformation of ethylene oxide segments has been found to vary when they are incorporated into triblock oligomers [H(CH2)(m)(CH2CH2O)(n)(CH2)(m)H] or microblock copolymers [-(CH2)(m)-(CH2CH2O)(n)-](x) of ethylene (E) and ethylene oxide (EO) depending on the relative length (m vs n) of the sequences. Raman spectroscopy, wide-angle X-ray diffraction, and thermal analysis have been used to investigate these conformational variations. Since both segments crystallize, we have found a profound influence of intermolecular interactions on determining the conformation of the EO segment. In the regime where n < 7 (m = 12-20), the EO segment is found in the planar zigzag conformation similar to that observed in oriented poly(ethylene oxide) (PEO) held under tension. When n > 7 (m = 12-20), the EO segments are found to crystallize in the 7/2 helical conformation similar to that found in PEO. In the case where n = 7 (m = 12-20), both the helical and planar zigzag forms of EO are found depending on the thermal and solution processing history. The helical form can be induced by melt-quenching, but annealing at room temperature for several days produces a gradual return to the planar conformation. A conformational transition caused by solvents also occurs and will be discussed.