Macroscopic phase separation hydrogel microspheres

Development of high mechanical strength, good flexibility and crack hydrogels have great academic and practical significance. Bis researchers typically by designing hydrogels, hydrogel slip ring, nanocomposite hydrogels, hydrogels topology and other ionically crosslinked hydrogels strategy to achieve high strength hydrogels. But the rational design of the next generation of hydrogels also affected by lack of research on the mechanism of strengthening. At present, the mechanism under strong hydrogel obtained is always derived from the microscopic and macroscopic properties of the network structure, i.e. the conformation affect the morphology and mechanical strength of the assembly is still at level assumptions. Therefore, the need to develop an effective technique in scale topography \”observed\” hydrated state and to clarify the role of energy dissipation. Recently, Hong Kong University of Science and Technology TANG Zhong academicians and Jacky WY Lam , the ninth of the University of Paris alba Marcellan (Corporate communication) a mechanically strong temperature-sensitive hydrogel prepared using poly (N- isopropyl acrylamide) (of PNIPAM) and poly (N, N- dimethylacrylamide). found that by using a luminescent agent (AIEgen) as an aggregation-inducing emitted fluorescent indicator, can be directly observed hydrophilic hydrogel – hydrophobic conversion and phase separation micro-on composition. By observing the morphology and mechanical measurements hydrogel, the concept of mechanical form, and set forth a comprehensive mechanism. The study, entitled \”Making Hydrogels Stronger through Hydrophilicity-Hydrophobicity Transformation, Thermoresponsive Morphomechanics and Crack Multifurcation\” published in \” ChemRxiv \” In order to stimulate the morphological changes in response to visual hydrogel , the design of a new and obtained AIEgens (TVPA), which is shown in Figure 1. TVPA structure is a triphenylamine electron withdrawing and electron tetrasubstituted N- (acryloyloxy) ethyl pyridinium ethylene unit is connected to the core, so as to have molecular rotors, D-A structures and ionic characteristics. TVPA emission in dilute aqueous solution of a weak red light, but the emission intensity were increased 87-fold and 54-fold in the viscous glycerol and solid, the quantum yield is also increased from 0.4% were 10.7% and 6.1%. TVPA lyotropic also have significant discoloration. When the solution changed from toluene with methanol, a deep blue color emitted TVPA to red. In the polymer film when the polymer is nonpolar polybutadiene, TVPA emits blue light; when the polar polymer is a polyethylene glycol, TVPA emit red light. It indicates that the charge transfer TVPA having molecules twisted. Therefore, a strong TVPA emissive aggregate states in the high sensitivity to the environment and polarity in response to the window width. Optical properties

肉眼可见的水凝胶微相分离
of FIG. 1 TVPA

By adjusting the polymerization of N- isopropyl acrylamide (of PNIPAM) and poly-N, N- dimethyl acrylamide (the PDMA) Weight three hydrogels (GN2D3, GN3D3 and GN6D6) prepared comprising TVPA ratio. GN and GD hydrogels are prepared from monomeric NIPAM and DMA and TVPA system. Since the gel incorporated TVPA, the authors studied the emission characteristics (FIG. 2) and after the initial heating for 30 minutes three kinds of hydrogels. From 60 to 20 is thermally stimulated deg.] C, the emission photoluminescence (PL) spectrum maximum of 17.4-fold increase, can be observed visually dark to bright orange fluorescence changes reversibly between blue. In GN gels show similar change in fluorescence, whereas no such change GD gel. This phenomenon is verified clew – ball conformational transition, when indicated above in the lower critical solution temperature (LCST), PNIPAM transition from hydrophilic to hydrophobic, TVTA microenvironment of the hydrogel changes as an indicator. In other hydrophilic – hydrophobic gel at different temperatures also exhibit similar emission color change behavior, showed TVPA sensitively detect stimuli-responsive hydrogel of hydrophilic – hydrophobic variation.

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FIG. 2 design principle schematic and hydrophilic – hydrophobic conversion visualization

Analysis by fluorescence microscopy in mesoscopic studies PNIPAM and PDMA thermal response and a component For phase separation (FIG. 3). Due to the different refractive indices PNIPAM / PDMA and water, So all figures multicolor fluorescence images showed Newton\’s rings fringes of different sizes. In thermal stimulation, the hydrogel emission color is switched between orange and blue. Found to influence the composition PDMA / PNIPAM of microphase separated morphology and size observed by a fluorescence spectrum is very large. For the PDMA GN2D3 hydrogel rich rich PNIPAM ball isolated PDMA dispersed in a continuous phase; GN3D3 hydrogel bicontinuous interpenetrating network; GN6D3 in the hydrogel, PDMA-rich phase to form isolated islands dispersed in a continuous phase PNIPAM. further indicate, AIE secondary fluorescence imaging provides a powerful tool for change in response to stimuli direct visualization of the morphology.

肉眼可见的水凝胶微相分离
FIG. 3 microphase separated fluorescence microscopy images

To investigate the effect on the mechanical properties of the hydrogel morphology authors tested at different temperatures of the water gel rheological behavior. The aqueous gel was heated from 30 deg.] C to 40 ℃, a sharp increase in the elastic modulus (G \’), which microphase separation corresponds to the formation and development. At 20 ℃, the performance of hydrogels prepared typical soft type and hard type mechanical behavior. 60 ℃, the mechanical properties of these hydrogels significantly improved, mainly due to higher than the LCST, PNIPAM collapse entangled structure and forming a plurality of non-covalent bonds. 60 ℃, the best mechanical properties GN6D3, because with increasing PNIPAM, the gel cohesive energy increases, the possibility of experiencing the crack path PNIPAM / PDMA interface can be increased, so that the crack branching increases. Prevent crack penetration and ensure the overall integrity of the ability to sacrifice part by enhancing hydrogel is of great significance for materials engineering and practical application.

肉眼可见的水凝胶微相分离
FIG. 4 thermally responsive viscoelastic behavior of large deformation and crack branching Griffith

PNIPAM form hydrogen bonds with water molecules, thus the hydrated water soluble PNIPAM chain. However, the collapse occurs at the LCST of the PNIPAM coils – spherical transition, formed in a plurality of inter-chain / hydrogen bonds, hydrogen bonds, these water previously occupied by a hydrogen bond acceptor, and leads to an insoluble PNIPAM entire region. Since the formation of a large number of non-covalent bonds (mostly hydrogen), significantly enhanced PNIPAM region. Disentanglement in PNIPAM areaProcess, such reinforcing hydrogels undermine noncovalent, strain energy consumed. During highly stretched hydrogel, having a non-covalent polymer network formed is re-orientation and alignment, thereby enhancing the performance in the final stage before rupture. Further, non-uniform phase and the weak strong PNIPAM PDMA phase distribution facilitates multiple bifurcated crack formation, thereby increasing the fracture energy (FIG. 5). In short, the expense of non-covalent or a weak interface between the two phases separated in a delayed fracture failure, maintaining the overall integrity of the mechanical behavior, morphology and mechanical concepts presented exactly.

肉眼可见的水凝胶微相分离
FIG. 5 toughened and thermally responsive mechanism bifurcated Griffith Crack

In AIEgen OF fluorescent indicator, can PNIPAM / PDMA hydrogel microphase separation visualization. Hydrophilic molecules from hydrophobic to form transition between the plurality of chains due to the formation / hydrogen bond within the chain. Materials research and biology will study this concept have a significant impact. And a plurality of non-covalent forming microphase separation also helps to enhance the mechanical strength and thermal response fracture resistance. Crack branching and related micro-phase separation pattern component related, provide additional energy dissipation pathway, enhances the fracture energy. This work is to explore the stimuli-responsive materials mesoscopic provide inspiration and contribute to the development of high strength and biomimetic hydrogel material original link: https: //chemrxiv.org/articles/Making_Hydrogels_Stronger_through_Hydrophilicity-Hydrophobicity_Transformation_Thermoresponsive_Morphomechanics_and_Crack_Multifurcation/12279686

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