Photo-cross-linking, mechanism

For the continuous mode, we utilized a dynamic rheological technique, Fourier transform mechanical spectroscopy (FTMS) (77,75), which provided a powerful method for monitoring, simultaneously, the evolving dynamic moduli at several frequencies during the course of photo-cross-linking. In this technique, an oscillatory strain, y, was applied to the sample, such that... 

Figure 8 Two strategies to cross-link kinases with their substrates, involving either photo-cross-linking groups (a) or a mechanism-based annulation reaction (b).

Free radical mechanisms also serve to explain the photo-cross-linking of various polymers, such as that of polyethylene accomplished with the aid of lightabsorbing additives such as benzophenone, quinone, benzoin, acetophenone, or their derivatives. When electronically excited by light absorption, these additives either directly abstract hydrogen from the polymer or decompose into free radicals capable of abstracting hydrogen, as shown in Schemes 7.10 and 7.11. 

POMaC also applies dual cross-link mechanism, thermal esterification, and photo-cross-linking of double bonds. 

In addition to conformational studies of RLPs, multiple groups have studied the mechanical properties of cross-linked RLPs. As mentioned above, Elvin et al. successfully expressed RLPs in E. coU in soluble form in 2005. This recombinant RLP was cross-linked via Ru(II)-mediated photochemical protocols to establish the formation of dityrosine cross-links in order to produce a cross-linked polymeric solid material at a concentration greater than lOOmgml". Tliis photo-cross-linking method is rapid, quantitative, and controllable, and its successful application suggests opportunities for multiple modes of cross-linking in engineering resilin-based materials. 

In this equation, the standard heat of polymerization is for full conversion, and AH is the area under the exothermic peak (Figure 2(a)). The percent conversion can be calculated from the ratio of AH to AHq. As mentioned earlier, DSC thermal scans can be used to measure the unreacted components in the cured polymers or networks. Figure 2(b) shows the DSC heating curves for a photo-cross-linked PHEMA-EGDMA network. An exothermal peak immediately associated with thermal polymerization of uncured compounds appeared in the first thermal scan. This peak disappeared when the sample is scanned after being kept at elevated temperature for 20 min. The uncured compounds may influence the structural, mechanical, and physical... 

A gel electrolyte consists of an ion conducting polymer, such as polyoxyethylene or a polymer with polyoxyethylene segment, and an ion source. Hexafluorophosphate or tetrafluoroborate salts of lithium are commonly employed as the ion source. Other common components of the gel are liquid electrolytes like ethylene carbonate or propylene carbonate, both of which are used to increase the overall ion conductivity. Cross-linking agents, such as photo cross-linked acrylates or polyurethanes, can be incorporated to improve the mechanical stability of gel electrolytes. 

Figure 2.4 Mechanism of action of ABPs for metallo proteases. Current metallo-protease probes bind in the active site of a target enzyme using a hydro-xamic acid group that coordinates the active site zinc. Once bound in the active site the probe can be permanently cross-linked into the active site using a photo-cross-linking group that forms a reactive oxygen radical upon irradiation of the probe with UV light." " This secondary reaction allows permanent modification of the target protease.

In addition, Wang s group developed a one-step method to synthesize photo-cross-linkable macromers based on copolymers of PEG and PPE and further photopolymerized them to form hydrogels, which were capable of cell encapsulation. Qiu and co-workers have prepared a series of unsaturated PPEs (eqn [14]), which can be cross-linked. They have investigated the cross-linking, mechanical properties, degradation, and biocompatibility of these materials as injectable and biodegradable bone substitutes. 

Fig. 3.1 Panel (a) The three experimental geometries used for characterizing the shear mechanical properties of the NEs. The small and large arrows show the orientation of the director n and of the applied shear, respectively, v is the velocity of the shear. Panel (b), left Chemical composition of the NE obtained by photo cross-linking a NP oriented with an E- or H-field. x = 78.5, y = 18.5, z = 3 are the concentrations in mol%. Panel (b), right Chemical composition of a NE prepared with the two-step cross-linking process and oriented by a mechanical stretching of the network formed after the first cross-linking step. x = 90, y= 10 are the concentrations in mol%. Reprinted and adapted with kind permission of Springer Science + Business Media (Rogez and Martinoty 2011)...

S.2.2 Influence of Swelling on the Mechanical Properties of NEs Prepared by Photo Cross-linking a NP Oriented Either hy an Electric or a Magnetic Field... 

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