Gel-sol transition temperature

Aqueous solutions of high-PEG block copolymers possess a unique temperature-dependent reversible gel-sol transition. This allows hydrogels to be loaded with bioactive molecules, such as insulin, in an aqueous phase above the gel-sol transition temperature, usually around 45°C. As a sol, the insulin-loaded copolymer system is injectable. Upon exposme to the subcutaneous... 

The gel-sol transition temperature of (PDA) 4BCMU ( 70 C) is independent of polymer concentration. The lowest concentration in toluene where gel formation was observed is 0.06 wt%. Gels of (PDA) 4BCM exhibit optical birefringence which results from a spontaneous nematic alignment in the gel supported by hydrogen bonding [43]. 

The comparison of mixtures with 20wt% PE IB and different solvents shows considerable differences in gel-sol transitions and isotropization temperatures. The lowest gel-sol transition temperature was detected with 2-f-butylphenol at 36 "C and the highest with... 

Kim and co-workers used peptide dendrons to form gel-phase assembled materials at relatively high concentrations (8 wt/vol %) [54], They employed both dendrons and symmetric dendrimers constituted from repeating amide units with peripheral hydrophobic chains (Fig. 15). They observed a negative dendritic effect on the thermal stability of the gel as determined from the gel-sol transition temperature (Tgei) measurements. 

Tm The melting temperature (in degrees eentigrade) the gel-sol transition temperature. 

If such a structure is heated then an enthalpy change can be measured at the gel/sol transition temperature, and consequently the material is said to have an enthalpic structure [121]. The enthalpy changes for agarose and carrageenan can be seen in Figure 9.31 [122]. 

Synthesis of a thermosensitive, biodegradable hydrogel consisting of blocks of poly(ethylene oxide) and poly(L-lactic acid) was carried out (4). Aqueous solutions of these copolymers exhibited temperature-dependent reversible gel-sol transitions. The hydrogel can be loaded with bioactive molecules in an aqueous phase at an elevated temperature (around 45°C), where they form a solution. In this form, the polymer is injectable. On subcutaneous injection and subsequent rapid cooling to body temperature, the loaded copolymer forms a gel that can act as a sustained release matrix for drugs (4). 

Table 13.1 Gel-sol transitions, anisotropy and isotropization temperatures of mixtures of solvents with 20 wt% PE ID and comparison with solubility parameters...

The gel-sol transition can also be monitored by mechanical analysis either by measuring the shear modulus as a frmction of temperature or by compiling a mastercurve from isothermal viscoelastic measurements over a range of frequencies. 

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