Poly gels gelation process

Gels are also obtained by the addition of borate to aqueous poly(vinyl alcohol) solutions. The tetrafunctional crosslinks, consisting of didiol bonds with borate, are of a dynamic nature, with time constants of the order of 0.01 to 10 s, depending on temperature. The heat of crosslink formation varied firom — 10 to — 20 kJ/(mol of crosslinks), which is much le than the values obtained in the gelation process of aqueous poly(vinyl alcohol) solutions, where it varied from — 37 to —168 kJ/mol, depending on the syndiotacticity of the poly(vinyl alcohol) used. 

Cellular structures are observed in poly(dimethylsiloxane) (PDMS)/silica gels when the sol-gel parameters (H2O, HCl concentration and temperature) are adjusted to accelerate the gelation process. PDMS, which contains direct Si—C bonds along its Si—O backbone, can co-condense with TEOS, as shown in equation 9. 

Armes et al. [58] synthesized, via ATRP, biocompatible, thermoresponsive ABA triblock copolymers in which the onter A blocks are comprised of PNIPAm and the central B block is poly(2-methacryloyloxyethyl phosphorylcholine). This novel system is soluble in dilnte aqueous solution but forms free-standing physical gels at 37 °C due to hydro-phobic associations of the PNIPAm segments. The gelation process is reversible and shows potential applications in drug delivery and tissue engineering. 

All these favorable and unfavorable factors and mechanisms are effective to a different extent during cryotropic gelation in both aqueous and organic media. For instance. Fig. 15 shows the bell-like temperature dependence of the gel-fraction yield when poly(styrene) was crosslinked with 4,4 -xylylene dichloride in nitrobenzene (see Fig.Sa for the reaction scheme). At temperatures 30-40 °C lower than room temperature, the efficiency of polymer crossUnking is higher than in unfrozen solutions (curve with open circles in Fig. 15) [27]. Thus, close similarity in the character of such temperature dependences for the processes in frozen aqueous and... 

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