Geometric and Edge Effects on Swelling-Induced Ordered Structure Formation in Polyelectrolyte Hydrogels

Abstract

In this paper, we developed several kinds of ordered structures in hydrogels with different geometries and sizes by harnessing heterogeneous swelling induced mechanical instability, i.e., surface creasing, which leads to molecular orientations along the tensile direction. These hydrogels were synthesized by polymerization of a cationic monomer, N-[3-( N,N -dimethylamino)­propyl] acrylamide methyl chloride quaternary (DMAPAA-Q) and a chemical cross-linker, in the presence of a small amount of the semirigid polyanion, poly­(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), as dopant. During the swelling process of as-prepared gels, surface creasing occurs and induces formation of a lattice-like periodic ordered structure, which is maintained in the swollen gels due to the formation of strong polyion complex. Besides this structure formed at the central part of gel sheets, PBDTs align parallel to the gel boundary at the edge of gels with a cuboid, disk, or ring shape. The size of the two regions with different structures and the size of each unit of lattice-like pattern are related to the geometry and size of the gels. The formation of different ordered structures was found due to the different mechanical instabilities at different parts of the gel during the heterogeneous swelling. This work presenting the creation of ordered structures in hydrogels by tuning the mechanical instability will pave the way to develop other functional structured materials and merit revealing the formation mechanism of ordered structures in soft biotissues during the nonequilibrium growth.

Control superstructure of rigid polyelectrolytes in oppositely charged hydrogels via programmed internal stress

“…The birefringence of the gel comes from both PBDT that has a positive birefringence and PDMAPAA-Q that has a negative birefringence. Previous study has shown that PBDT has a much stronger birefringence than PDMAPAA-Q, and the net birefringence observed here is related to the orientation of the rigid PBDT molecules [25][26][27] . From the birefringence colour observed with 530-nm-sensitive tint plate (Fig.…”

“…This internal-stressdirected orientation of rigid molecules inside the hydrogels has recently been demonstrated [24][25][26] . When both of the embedded rigid macromolecules and the hydrogels are polyelectrolytes but carry opposite charges, the orientation can be 'memorized' by polyion complex formation by the dialysis of their counterions 25,26 . Therefore, various superstructures of rigid polyelectrolytes could be developed in oppositely charged polymeric hydrogels by harnessing internal-stress-directed orientation of rigid macromolecules.…”

“…All the samples of PBDT-containing PDMAPAA-Q gel exhibited a first-order whitegrey birefringence colour. To identify the molecular orientation direction, a 530-nm-sensitive tint plate was inserted [25][26][27] . PBDT has a positive birefringence and PDMAPAA-Q has a negative birefringence.…”

Spatially Configuring Wrinkle Pattern and Multiscale Surface Evolution with Structural Confinement

“…[8][9][10][11][12][13] The latter perspective has enabled engineering opportunities with self-adaptive/autonomous structures in low dimensions and has implications in many different contexts such as micro-/ nanofluidics, [14][15][16] flexible electronics, [17,18] adhesion, [19,20] organic solar cells, [21] tunable optics, [22][23][24] wettability, [25][26][27] and promising methods for surface patterning. [1,[28][29][30][31] While our scientific/technical understanding has advanced, there remains much to be explored about the control of instability morphology, and in particular how to configure instabilities, such as wrinkling and creasing, to desired patterns with selective distribution covering the surface and bespoke thresholds for the formation and evolution of instabilities.…”

Post-wrinkle bifurcations in elastic bilayers with modest contrast in modulus

“…Such processes arise when an elastic material is compressed beyond a critical strain either by swelling, differential growth, or mechanical forces. In engineering, similar instabilities have long been thought of as nuisances, but they have recently found applications in many different contexts including flexible electronic devices [1][2][3][4], surface patterning methods [5][6][7][8] and materials with tunable optical properties [9][10][11][12], adhesion [13][14][15], and wettability [9,16,17].…”