Direct visualization of nano and microscale polymer morphologies in as-prepared and dialyzed polyampholyte hydrogels by
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Research Letter
Direct visualization of nano and microscale polymer morphologies in as-prepared and dialyzed polyampholyte hydrogels by electron microscopy techniques Xinda Li, Hemant Charaya, and Thuy Nguyen Thanh Tran, Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6 G 1H9, Canada Byeongdu Lee, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA Jae-Young Cho, National Research Council of Canada (NRC), 11421 Saskatchewan Drive NW, Edmonton, Alberta T6 G 2M9, Canada Hyun-Joong Chung, Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6 G 1H9, Canada Address all correspondence to Hyun-Joong Chung at [email protected] (Received 25 May 2018; accepted 24 July 2018)
Abstract The structure of polymer networks in hydrogels determines the properties. In this study, we investigated the structure of a charge-balanced polyampholyte, poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride). From as-prepared samples, nanoscale globules were visualized in polyampholyte hydrogels for the first time. The impact of dialyses processes on polymer structures were also studied. In deionized water, salt ions are leached out, thus polymer chains undergo zipping process to form cellular structure with micrometer-thick polymer walls that allow mechanical toughness to the hydrogel. Samples dialyzed in 6 M potassium hydroxide solution did not show such cellular structure, as in the case of as-prepared samples.
Introduction Electron microscopy (EM) has been actively used in elucidating structures of polymeric materials. Microscopic techniques are especially useful when the polymer has periodic nanostructures (e.g., block copolymers),[1] when the polymer contains inorganic substances[2] or ionic clusters[3] to prevent detrimental electronic charge accumulation, and when the polymer is semicrystalline.[4] Recently, advanced transmission electron microscope (TEM) enabled single-chain imaging of polymers under specific conditions.[5] For hydrogels, nanoscale molecular and supramolecular structures have mostly been probed by indirect methods, such as small-angle neutron scattering, smallangle x-ray scattering (SAXS), and light scattering.[6,7] To visualize the micrometer-scale fibrous network structures of hydrogels, a protocol of cryogenic quenching, followed by freeze-drying for scanning electron microscope (SEM) imaging was established.[8] An advanced EM technique of cryo-TEM and -SEM has also been used for gel samples,[9] but direct EM imaging of hydrogel nanostructure has been available in specific cases, such as thin film gels[10] or ultramicrotomed gels that contain inorganic particles.[11] Polyampholytes, a subclass of polyelectrolytes, are macromolecules carrying ionizable functional groups that dissociate and leave positively and negatively charged groups on the polymer chain under certain conditions, for example, in the presence of water.[12,13] The hydrogel of polyampholyte is formed
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