Phase Behavior and Temperature-Responsive Molecular Filters Based on Self-Assembly of Polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene

Abstract

This work describes the synthesis of temperature-responsive polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene triblock copolymers, i.e., PS-b-PNIPAM-b-PS, their self-assembly and phase behavior in bulk, and demonstration of aqueous thermoresponsive membranes. A series of PS-b-PNIPAM-b-PS triblock copolymers were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. The hydrophobic PS end blocks were selected to form the minority component, whereas the temperatureresponsive PNIPAM midblock accounted for the majority component. The self-assembly and phase behavior in bulk of PS-b-PNIPAM-b-PS as well as selected blends with low molecular weight PNIPAM homopolymers were studied using transmission electron microscopy (TEM). Classical lamellar, cylindrical, spherical, and bicontinuous double gyroid morphologies were observed in the dried state. In aqueous solutions, the glassy PS domains act as physical cross-links, and hydrogels were therefore formed. The bulk block copolymer morphology had a strong effect on the degree of swelling in aqueous solutions upon cooling below the coil-globule transition temperature of the PNIPAM midblock. Bulk compositions with spherical PS domains and PNIPAM continuous phase swelled in water up to 58 times by weight, whereas composition having cylindrical PS domains or bicontinous gyroid structure in bulk swelled 20 or 10 times by weight, respectively. Finally, lamellar compositions did not show any swelling. Composite membranes for separation studies were prepared by spin-coating thin films of PS-b-PNIPAM-b-PS on top of meso/macroporous polyacrylonitrile (PAN) support membrane. The permeability was measured as a function of temperature using aqueous mixture of poly(ethylene glycol) (PEG) with several well-defined molecular weights. The permeability showed a temperature switchable on/off behavior, where higher permeability is obtained below transition temperature of PNIPAM, and the molecular cutoff limits for the PEG molecules are surprisingly lowsbetween 108 and 660 g/mol. The results encourage to further develop and optimize these materials for responsive nanofiltration applications.

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

“…This sample is expected to have bicontinuous morphology in thin film before the swelling. 26 The micrograph clearly demonstrates that the bicontinuous structure is present, and it is preserved during the process of hydrogel swelling and freeze-drying. Polystyrene, appearing dark in the micrographs, forms a continuous network throughout the swollen hydrogel, and both 3-fold and 4-fold symmetries can be recognized in many junction points, as it can be more clearly seen in higher magnification image ( Figure 5B).…”

“…The morphologies reported in Figures 2A,B are in agreement with those previously observed by the authors in bulk systems. 26 Use of cryo-TEM was made in order to directly observe the hydrogel thermoresponsive swelling behavior. As the water vitrification process is very rapid, the gel microstructure at the state defined by the environmental chamber conditions at which the samples were exposed and equilibrated prior to vitrification, is expected to be preserved.…”

“…This rough argument, to be corrected by the entropy loss of highly stretched chains, can be used as a rational to understand the differences in water pickup by different hydrogel structures. 26 In order to investigate further the role of self-assembled block copolymer bulk morphologies on the topology of PS-block-PNIPAM-block-PS physically cross-linked hydrogels, sample PN61.106K with 61 wt % PNIPAM was investigated following the same cryo-TEM and freeze-drying procedures used for sample PN77.118K. Figure 5A shows a cryo-TEM image of the freeze-dried sample PN61.106K vitrified from the hydrogel swollen state (T ) 5°C).…”

“…In our previous work we have demonstrated a simple way to form responsive polymer networks by using ABA triblock copolymers, which can self-assemble into spherical, cylindrical, lamellar, or doublegyroid morphologies. 26 By designing the triblock copolymer in such a way to maintain the responsive PNIPAM midblock in the continuous phase, these materials can be swollen by any selective solvent for the midblock. The temperature responsive behavior of various PS-block-PNIPAM-block-PS triblock copolymers gels in water was investigated in detail in our previous paper, by measuring volume expansion and weight pickup in wet environment or by studying the filtration properties of these materials with respect to hydrophilic polymers, below and above the coil-globule transition of the PNIPAM.…”

“…The temperature responsive behavior of various PS-block-PNIPAM-block-PS triblock copolymers gels in water was investigated in detail in our previous paper, by measuring volume expansion and weight pickup in wet environment or by studying the filtration properties of these materials with respect to hydrophilic polymers, below and above the coil-globule transition of the PNIPAM. 26 Nonetheless, a few methods are available to probe directly the changes arising in the morphology of responsive polymers. Small-angle X-ray scattering can be used to explain the changes in stimuli-responsive structure in block copolymer gels in bulk.…”