Friction of Zwitterionic Hydrogel by Dynamic Polymer Adsorption

A simplified model describing the sliding friction of hydrogel on solid surface by dynamic adsorption of the polymer chains is proposed on the basis of polymer adsorption–repulsion theory. This dynamic adsorption model is used to analyze the friction results of zwitterionic hydrogels sliding over glass substrates with different substrate wettability, hydrogel swelling degree, ionic strength, and pH of bath solution. The adsorption time τ_b of polymer strands is found to decrease with the increase in sliding velocity or the Weissenberg number as a result of stretching. The adsorption time τ_b⁰ and the adsorption energy <i>U</i>_ads at stress-free condition, which are characteristic for each friction system, are also estimated. Roughly, a master curve is observed for the normalized adsorption lifetime τ_b/τ_b⁰ and the Weissenberg number, with less dependence on the adsorption energy and the bulk properties of the gels in the observed experimental conditions. Thus, the dynamic adsorption model successfully correlates the frictional behavior of hydrogels with the adsorption dynamics of polymer strands, which gives insight into the molecular design of hydrogels with predefined frictional properties for biomedical applications

Anisotropic Growth of Hydroxyapatite in Stretched Double Network Hydrogel

Bone tissues possess excellent mechanical properties such as compatibility between strength and flexibility and load bearing owing to the hybridization of organic/inorganic matters with anisotropic structure. To synthetically mimic such an anisotropic structure of natural organic/inorganic hybrid materials, we carried out hydroxyapatite (HAp) mineralization in stretched tough double network (DN) hydrogels. Anisotropic mineralization of HAp took place in stretched hydrogels, as revealed by high brightness synchrotron X-ray scattering and transmission electron microscopic observation. The <i>c</i>-axis of mineralized HAp aligned along the stretching direction, and the orientation degree <i>S</i> calculated from scattering profiles increased with increasing in the elongation ratio λ of the DN gel, and <i>S</i> at λ = 4 became comparable to that of rabbit tibial bones. The morphology of HAp polycrystal gradually changed from spherical to unidirectional rod-like shape with increased elongation ratio. A possible mechanism for the anisotropic mineralization is proposed, which would be one of the keys to develop mechanically anisotropic organic/inorganic hybrid materials

Water-Triggered Ductile–Brittle Transition of Anisotropic Lamellar Hydrogels and Effect of Confinement on Polymer Dynamics

We study the effect of dehydration on the structure and mechanical properties of anisotropic lamellar hydrogels, consisting of alternative stacking of several thousands of nanoscale rigid bilayers from amphiphilic poly­(dodecyl glyceryl itaconate) (PDGI) and submicroscale soft hydrogel layers from hydrophilic polyacrylamide (PAAm) networks. We found that the layered microstructure is well preserved with dehydration, and a ductile–brittle transition occurs at the critical water content. This transition is related to the rubbery–glassy transition of the PAAm layers, which occurs at 58 wt % water content and is much higher than 26 wt % of bulk PAAm hydrogels. Such specific behavior of the lamellar hydrogels indicates that the dynamics of the submicroscale PAAm hydrated layer intercalated between the rigid bilayers are very different from its bulk state

Tough and Variable-Band-Gap Photonic Hydrogel Displaying Programmable Angle-Dependent Colors

One-dimensional photonic crystals or multilayer films produce colors that change depending on viewing and light illumination angles because of the periodic refractive index variation in alternating layers that satisfy Bragg’s law. Recently, we have developed multilayered photonic hydrogels of two distinct bulk geometries that possess an alternating structure of a rigid polymeric lamellar bilayer and a ductile polyacrylamide (PAAm) matrix. In this paper, we focus on fabrication of composite gels with variable photonic band gaps by controlling the PAAm layer thickness. We report programmable angle-dependent and angle-independent structural colors produced by composite hydrogels, which is achieved by varying bulk and internal geometries. In the sheet geometry, where the lamellae are aligned parallel to the sheet surface, the photonic gel sheet exhibits strong angle-dependent colors. On the other hand, when lamellae are coaxially aligned in a cylindrical geometry, the gel rod exhibits an angle-independent color, in sharp contrast with the gel sheet. Rocking curves have been constructed to justify the diverse angle-dependent behavior of various geometries. Despite varying the bulk geometry, the tunable photonic gels exhibit strong mechanical performances and toughness. The distinct angle dependence of these tough photonic materials with variable band gaps could benefit light modulation in displays and sensor technologies

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

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, <i>N</i>-[3-(<i><i>N,N</i></i>-dimethylamino)­propyl] acrylamide methyl chloride quaternary (DMAPAA-Q) and a chemical cross-linker, in the presence of a small amount of the <i>semirigid</i> 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

Sliding Friction of Zwitterionic Hydrogel and Its Electrostatic Origin

Polyzwitterionic materials, which have both cationic and anionic groups in each repeating unit of polymer, show excellent antibiofouling properties. In this study, the surface friction of carboxybetaine type zwitterionic hydrogels, poly­(<i>N</i>-(carboxymethyl)-<i>N</i>,<i>N</i>-dimethyl-2-(methacryloyloxy)­ethanaminium, inner salt) (PCDME), against glass substrates were investigated in aqueous solutions. The friction measurement was performed using a rheometer with parallel plate geometry and the sliding interface was monitored during the measurement. The frictional stress on glass was high in water and it showed weak dependence on pressure as long as the two sliding surfaces were in complete contact. The results performed in solutions with varied ionic strength revealed that the high friction on glass substrates has an electrostatic origin. The electrostatic potential measurement revealed that the PCDME gels have an isoelectric point at pH 8.5. Since the glass substrates carrying negative charges in pure water, the gel and the glass have electrostatic attraction in water. Study on the effect of pH has shown that below pH 8.5, attraction between the positively charged gels and negatively charged glass gives high friction, while above pH 8.5, the electrical double layer repulsion between two negatively charged surfaces gives low friction. From these results, it is concluded that although the PCDME gels behave like neutral gels in the bulk properties, their surface properties sensitively change with pH and ionic strength of the medium

Polymer Adsorbed Bilayer Membranes Form Self-Healing Hydrogels with Tunable Superstructure

We report that polymers can support bilayer membranes to form physical hydrogels of self-healing and tunable isotropic/anisotropic structure. The system consists of poly­(dodecyl glyceryl itaconate) (PDGI) which forms lamellar bilayers and polyacrylamide (PAAm) which adsorbs on the bilayer surfaces via hydrogen bond formation. Adsorption of PAAm brings two effects: disturbs the bilayer packing and causes bending of the bilayers; increases the effective thickness of the bilayers and enhances the repulsion between the bilayers due to excluded volume effect. Competition of these two effects brings about sharp superstructure transition from isotropic multilayer foam phase to unidirectionally aligned lamellar phase. Accompanied by this structure transition, the bulk hydrogel exhibits isotropic/anisotropic swelling. The physical gels exhibit high tensile strength and self-healing properties that can be understood by the sacrificial bonds mechanism

Structure Optimization and Mechanical Model for Microgel-Reinforced Hydrogels with High Strength and Toughness

In this work, the mechanical behavior of sparsely cross-linked, neutral polyacrylamide (PAAm) hydrogels containing densely cross-linked polyelectrolyte microgels of poly­(2-acrylamido-2-methylpropanesulfonic sodium) (PNaAMPS) were studied systematically by varying the formulations. The microgel-reinforced (MR) hydrogels have a two-phase composite structure, where the disperse phase is the <i>rigid</i> double-network (DN) microgels, and the continuous phase is the <i>soft</i> PAAm matrix. At the optimal formulation, the MR gels showed high mechanical strength and toughness, comparable to conventional DN hydrogels. The two critical parameters for the substantial enhancement of mechanical strength and toughness of MR gels are the concentration of PNaAMPS microgel and the molar ratio of the PAAm to the PNaAMPS in the microgel phase. Selective dyeing of the embedded microgels in MR gels allowed for visualization of the deformation of microgels, and we found that the local strain of microgels was much smaller than the global strain applied on MR gels; this indicates that isostress model (Reuss’s model) is more suitable than isostrain model (Voigt’s model) for this composite system

Yielding Criteria of Double Network Hydrogels

Double network (DN) gels, consisting of a brittle first and flexible second network, have been known to be extremely tough and functional hydrogels. In a DN gel subjected to force, the brittle first network breaks prior to the fracture of the flexible network. This process, referred to as internal fracture, dissipates energy and increases the energy required to completely fracture DN gels. Such internal fracture macroscopically appears as a yielding-like phenomenon. The aim of this paper is to investigate the relationship between the yield point and the first network molecular structure of DN gels to more deeply understand the internal fracture mechanism of DN gels. To achieve this goal, we synthesized DN gels having a tetra-PEG first network, which is known to be a nearly ideal and well-controlled network gel. We have found that yielding of the DN gels occurs when the first network strands reach their extension limit (finite extensibility), regardless of their deformation mode. This conclusion not only helps by further understanding the toughening mechanism of DN gels but also allows for the design of DN gels with precisely controlled mechanical properties

<i>In Situ</i> Observation of Ca²⁺ Diffusion-Induced Superstructure Formation of a Rigid Polyanion

Diffusion of multivalent metallic ions into aqueous solution of rigid, negatively charged macromolecules of high concentration is an effective approach to prepare macroscopically anisotropic hydrogels. However, the mechanism for superstructure formation is still not clear. By observing the mixing process of a small drop of CaCl₂ solution with solution of a rigid polyanion, poly­(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), under the polarizing optical microscope, the diffusion profile of Ca²⁺ and detailed anisotropic gelation process of PBDT are revealed. Diffusion of Ca²⁺ into the surrounding PBDT solution immediately induces the formation of physical liquid crystalline (LC) gel with concentric alignment of PBDT. The thickness <i>d</i> of this region increases with diffusion time <i>t</i>, obeying the diffusion law <i>d ∼ t</i>^1/2. A thin ring of constant width (∼100 μm) with radial alignment of PBDT appears at the diffusion/reaction front, ahead of the concentric alignment region. When two drops of CaCl₂ fluxes meet, their outside thin rings interact with each other and the PBDT in this contacting region orients ±45° to the midline of the two drops. From these observations, we rationally contend that the internal stress induced by the contraction of gel phase is responsible for the ion diffusion-induced PBDT orientations. This structure formation mechanism gives insight into other diffusion-directed anisotropic gelation systems