IV-Isopropyl-acrylamide

The use of a polymer species as a way to control diffusion to the inside of mesoporous silica was also employed by Lopez and coworkers.67 In this work the researchers polymerized iV-isopropyl acrylamide on mesoporous silica by atom transfer radical polymerization, and took advantage of the changes the polymer experiences upon thermal treatment. The authors discovered that the hybrid material could take up more fluorescein than nonfunctionalized material at temperatures above 45°C. At that temperature the polymer is in a collapsed hydrophobic state and partially covers the negatively charged surface of silica that otherwise repels the negatively charged fluorescein dye. At temperatures below 30°C the polymer exists in a hydrated state in which the chains are expanded. Interestingly, the fluorescein loaded hybrid particles were... 

Hsiue, G.-H., Hsu, S.-h., Yang, C.-C., Lee, S.-H., and Yang, I.-K. (2002), Preparation of controlled release ophthalmic drops, for glaucoma therapy using thermosensitive poly-iV-isopropyl acrylamide, Biomaterials, 23(2), 457-462. 

Fig. 23 Examples of amphiphilic polymers a poly(l-vinylimidazole), b poly(iV-isopropyl acrylamide), and c poly(2-ethyl acrylic acid)...

Temperature-responsive behavior was also observed for gels formed by worm-like micelles of poly(styrene)-bZocA -poly(iV-isopropyl acrylamide). Poly(iV-isopro-pyl acrylamide) molecules are known to associate when the ambient temperature is increased above their lower critical solubihty temperature (LCST) of 32 °C [215]. When the gel was heated to a temperature above the LCST of the poly(A-isopropyl acrylamide) block, a nanofibrillar network was formed [167, 177]. The association of poly(A-isopropyl acrylamide) was reversible [215] cooling of the nanofibrillar gel led to dissociation of the polymer network. 

A novel approach to immobilization of enzymes via covalent attachment is the use of stimulus-responsive smart polymers, which undergo dramatic conformational changes in response to small alterations in the environment, such as temperature, pH, and ionic strength [401 03], The most prominent example is a thermo-responsive and biocompatible polymer (poly-iV-isopropyl-acrylamide), which exhibits a critical solution temperature around 32°C, below which it readily dissolves in water, while it precipitates at elevated temperatures due to the expulsion of water molecules from its polymeric matrix. Hence, the biolransformation is performed under conditions, where the enzyme is soluble. Raising the temperature leads to precipitation of the immobilized protein, which allows its recovery and reuse. In addition, runaway reactions are avoided because in case the reaction temperature exceeds the critical solution temperature, the catalyst precipitates and the reaction shuts down. 

Over the last decade, a significant body of work has been undertaken to develop temperature-responsive polymers for biomedical applications [155]. Various temperature-responsive polymers have been reported, including poly(iV-isopropyl)acrylamide (PNIPAM) [156], poly(V-vinylalkylamides) (PNVAA) [157], poly(V-vinylcaprolactam) (PNVC) [158], andcopolymers suchaspoly(L-lactic acid)-poly(ethylene glycol)-poly(L-lactic acid) (PLLA-PEG-PLLA) triblock copolymers... 

A thermochromic hydrogel based on Bragg reflection has been reported (16). The described system consists of crystalline colloidal arrays of monodisperse polystyrene spheres embedded in a poly(iV-isopropyl acrylamide) (PNIPAM) hydrogel, which was prepared by first dispersing highly charged and monodisperse... 

Figure 2.28 is a rare example of a successful observation.It shows how of poly(iV-isopropyl acrylamide) in water changes with the solvent quahty. The solution exhibits an LCST-type phase diagram with T, s 30.5 C. At T T, the solvent is good to the polymer and the chains are swollen. As T rises, the solvent quality becomes poorer and decreases. AtT>T = 32 C, R is nearly independent of temperature. A slight hysteresis was observed. 

Leobandung, W., Ichikawa, H., Fukumori, Y, Peppas, N. A. (2002). Preparation of stable insulin-loaded nanospheres of poly(ethylene glycol) macromers and iV-isopropyl acrylamide. Journal of Controlled Release, 80, 357-363. 

Lee and Hsu reported the swelling behaviour of thermosensitive hydrogel based on iV-isopropyl acrylamide-trimethyl methacryloyloxy ethyl ammonium iodide (TMMAl) copolymers, a nearly continuous volume transition and associated phase transition from low temperature, and a highly swollen gel network to high temperature, a collapsed phase near its critical point between 31 and 35 °C. The gel with little TMMAl content (Table 4.3) showed a good reversibility and, the higher the TMMAl content (2-6%) in the copolymer, the higher the gel transition temperature. 

Yang, J. M., Yang, S. J., Lin, H. T., Chen, J. K. (2007). Modification of HTPB-based polyurethane with temperature-sensitive poly(iV-isopropyl acrylamide) for biomaterial usage. Journal of Biomedical Materials Research Part B Applied Biomaterials, 80,43—51. 

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