Hydrogels swollen temperature

Fig. 3. Swollen temperature- and pH-sensitive hydrogels may exhibit an abrupt change from the expanded (left) to the collapsed (syneresed) state (center) and then back to the expanded state (right) as temperature and pH change.

At pH 4.3, there is no significant transition detected around 32°C. Transition temperatures are detected and increase as pH increases. This is because, at low pH, the aggregation of PMAA decreases the mobility of the PNIPAAm network as well as the water uptake of the IPN, resulting in drastically lowering the temperature sensitivity of the IPN hydrogel. However, at higher pH value, the swollen PMAA allows the PNIPAAm to have a higher mobility, which makes the IPN more temperature sensitive. 

Thermal and pH sensitive heterogeneous copolymer hydrogels which contain silicone rubber domains within a temperature and pH sensitive copolymer of NIPA and acrylic acid have been synthesized by Dong et al. [60]. These materials contained macropores when swollen and collapsed much faster than homopolymers of iV-isopropylacrylamide. Biocatalyst immobilization using copolymers of NIPA and NN - dimethylaminopropylmethacrylamide have also been studied [61]. 

Fig. 12 shows micro-gel particles cross-linked by irradiation of a phase- separated solution in the two differently swollen states, at 25°C in a highly-swollen state and at 40°C at low degree of swelling. For details see (Amdt et al. 2001a). Irradiation of a high concentrated solution results in a bulky hydrogel with typical dimension in the cm-range. The sponge-like stmcture of the formed PVME hydrogel (irradiation of a PVME-solution in the phase-separated state) at different temperatures (swollen above and below the volume phase transition temperature) is shown in Fig. 13. The irradiation dose was 50 kGy.

Fig. 15 FESEM micrographs of PVME filled with (a, b) ferroelectric BaTiOs particles, (c, d) ferromagnetic nickel particles and (e, f) ferroelectric poly(vinylidene fluoride). The figures show the filled hydrogel in the swollen state (a, c, d) low temperature,) and shrunken state (b, d, f) high temperature). The bars correspond to (a-d) 1 pm and (e, f) 500 nm. Reprinted from Theiss et al. (2005), p. 2262. Copyright John Wiley Sons Inc. Reproduced with permission...

Temperature-sensitive hydrogels with actuator properties show aLCST behaviour. They are swollen at low temperatures and shrink by exceeding of the volume phase transition temperature T. The best known hydrogel with LCST behaviour is PNIPAAm (Fig. 9). 

Microvalves are the simplest hydrogel-based components. The gel actuator is directly placed within a valve chamber (Fig. 11a). The thermo-sensitive PNIPAAm is swollen at room temperature and closes the valve. For opening the attached resistive heater has to be activated. Exceeding the volume phase transition temperature of approximately 34 °C the gel shrinks and opens the valve seat. 

The spectral parameters for component A again coincide with those of TEMPO in pure water ( a 48.3 MHz), i.e., this spin probe is located in a strongly hydrated, hydrophilic environment. The observed decrease of ai o by 3.7 MHz for species B ( hnal at 65 °C) is indicative of much more hydrophobic and less hydrated surroundings for these spin probes (comparable to chloroform or tert-hutyl alcohol [83]). At temperatures below the collapse temperature Tq, only the hydrophilic spectral component A is observed since all dendritic units are water-swollen. Above the critical temperature of 33 °C an increasing fraction of hydrophobic species B is observed with increasing temperature. The dehydration of the dendritic units thus leads to a local phase separation with the formation of hydrophobic cavities. Unlike in PNIPAAM hydrogels (Sect. 3.1), here hydrophobic regions are not observed below the macroscopic collapse temperature. 

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