VPTTVolume Phase Transition Temperature
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As every molecular chain of temperature-responsive PU has a double bond at each one of the two chain ends, PU nanohydrogels have uniform cross-linking networks resulting in narrow VPTT range that starts at approximately 35[degrees]C, demonstrating excellent carrying ability and controlled release of water-soluble substances like vitamin C.
This change is due to the disruption of hydrogen bonds at higher temperatures (just above to its VPTT), causing water to act as a poor solvent for the polymer chain [25-28].
Figure 6(c) shows the digital photograph of the prepared sample (NG-1) showing a reversible temperature-dependent dispersion (swollen state)/aggregation (shrunken state) at its VPTT in water.
To obtain VPTT, isochronal dynamic temperature sweep measurements were carried out at a constant frequency of 0.05 Hz and a uniform heating rate of 0.5[degrees]C [min.sup.-1].
To study the effects of cellulose nanoparticle weight fraction and irradiation dose on VPTT (deswelling or binodal temperature), dynamic temperature sweep tests were performed and the obtained results have been shown in Fig.
Heyes and Branka have demonstrated that in microgel systems at temperatures well below VPTT, there is a soft repulsive force between the particles [62].
The temperature at the onset point of the DSC endotherm is referred to the VPTT of the hydrogels [11].
In general, an abrupt decrease of the swelling ratios can be observed around VPTT for the samples, which is ascribed to the coil-globular transition of PNIPAM.
The novel DN hydrogels exhibit the same VPTT around 30[degrees]C as that of the conventional OR hydrogels.
[10] synthesized gold nanoparticles in thermosensitive microgels and measured the VPTT. They found that VPTT was similar for both pure and hybrid microgels.
After the VPTT, the size becomes constant and further increase in temperature has no effect on Rh value.
The hydrodynamic radius of microgel particles also undergo decrease in the swollen state but after the volume phase transition temperature (VPTT) this trend is reversed, that is the Rh increases for higher concentration of cross-linker and decreases for lower concentration in the collapsed state for microgel particles [22].