Direct Imaging of Nanoscopic Plastic Deformation below Bulk Tg and Chain Stretching in Temperature-Responsive Block Copolymer Hydrogels by Cryo-TEM

This work describes the thermoresponsive transition in polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene (PS-block-PNIPAM-block-PS) triblock copolymer hydrogels, as obsd. by both direct and reciprocal space in-situ characterization. The hydrogel morphol. was studied in both the dry and wet state, at temps. below and beyond the coil-globule transition of PNIPAM, using vitrified ice cryo-transmission electron microscopy (cryo-TEM), in-situ freeze-drying technique, and small-angle X-ray scattering (SAXS). The selected PS-block-PNIPAM-block-PS triblock copolymers were intentionally designed in such a mol. architecture to self-assemble into spherical and bicontinuous morphol. with the poly(N-isopropylacrylamide) forming the continuous matrix. The phase behavior in bulk was directly investigated by SAXS as a function of temp., while free-standing polymer thin films of samples quenched from different temps., allowed observing by cryo-TEM the changes in hydrogel microstructure. Finally, sublimation of water via controlled freeze-drying in the TEM column allowed studying systems without the presence of vitrified water, which enables direct imaging of the densely connected phys. cross-linked polymer network. By combining these techniques on samples exhibiting both spherical and gyroidal morphologies, it was demonstrated that (i) PNIPAM form phys. connected networks in spherical structures and bicontinuous morphologies in the gyroidal phase, (ii) in PNIPAM chains strands are strongly stretched above the polymer coil-to-globule transition, and (iii) surprisingly, upon the gel swelling process, the PS domains undergo extensive plastic deformation although temp. is always maintained well below the PS glass transition bulk temp. The possible phys. mechanisms responsible for this plastic deformation can be understood in terms of the dependence of PS glass transition temp. on the size of nanometer-scaled domains. [on SciFinder(R)]