wizyta

od 2020-09-20

Title: Molecular dynamics study of polystyrene-b-poly(ethylene oxide) asymmetric diblock copolymer systems

Source: Langmuir

Year : 2017

Abstract:

Two polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers differing in molecular mass (49 and 78 kDa), but possessing the same PEO cylindrical morphology, are examined to elucidate their molecular dynamics. Of particular interest here is the molecular motion of the PEO blocks involved in the rigid amorphous fraction (RAF). Analysis of complementary thermal calorimetry and X-ray scattering data confirm the presence of a microphase-separated morphology, as well as semi-crystalline structure, in each copolymer. Molecular motion within the copolymer systems is monitored by dielectric and nuclear magnetic resonance spectroscopies. Results reported herein reveal the existence of two local, Arrhenius-type processes attributed to non-cooperative local motion of PEO segments involved in fully amorphous and rigid amorphous PEO microphases. In both systems, two structural relaxations governed by glass transition phenomena are identified and assigned to cooperative segmental motion in the fully amorphous phase (the α_c process) and the RAF (the α_c process). We measure the temperature dependence of the dynamics associated with all the processes mentioned above and propose that these local processes are associated with corresponding cooperative segmental motion in both copolymer systems. In marked contrast to the thermal activation of the α_c process as discerned in both copolymers, the α_c process appears to be a sensitive probe of the copolymer nanostructure. That is, the copolymer with shorter PEO blocks exhibits more highly restricted cooperative dynamics of PEO segments in the RAF, which can be explained in terms of the greater constraint imposed by the glassy PS matrix on the PEO blocks comprising smaller cylindrical microdomains.

DOI: 10.1021/acs.langmuir.7b02017   (Pobrane:  2020-10-23)