A series of thermoresponsive gating membranes, with a wide range of grafting yields, were prepared by grafting poly (N-isopropylacrylamide) (PNIPAM) onto porous poly (vinylidene fluoride) (PVDF) membrane substrates with a plasma-induced pore-filling polymerization method. The effect of grafting yield on the gating characteristics of thermoresponsive gating membranes was investigated systematically. The results showed that the grafting yield heavily affected both the water flux responsiveness coefficient and the thermoresponsivity of the membrane pore size. When the grafting yield was smaller than 2.81%, both the flux responsiveness coefficient and the thermoresponsivity of the membrane pore size increased with an increase in the grafting yield; however, when the grafting yield was higher than 6.38%, both the flux responsiveness coefficient and the thermoresponsivity of the membrane pore size were always equal to 1; i. e., no gating characteristics existed anymore. Diffusional permeation experiments showed that two distinct types of temperature responses were observed, depending on the grafting yield. The diffusional coefficient of a solute across membranes with low grafting yields increased with temperature, while that across membranes with high grafting yields decreased with temperature. To get a desired or satisfactory thermoresponsive gating performance, the membranes should be designed and prepared with a proper grafting yield.

Membranes with porous substrates and functional gates that are responsive to environmental stimuli are attracting increasing interest from various fields. Their permeation properties can be controlled or adjusted by the gates according to the external chemical and/or physical environment, and they may find various applications e.g. in controlled drug delivery, bioseparation, chemical separation, water treatment, tissue engineering, and as chemical sensors. The functional gates for stimuli-responsive membranes serve as chemical valves, and have been reported to act in response to changes in environmental temperature, [1-5] pH, [5-10] ionic strength, [11] glucose concentration, [12, 13] electric field, [14] light, [15] redox properties, [16] or different molecules. [17-19] There are many cases in which environmental temperature fluctuations occur naturally, and in which the temperature stimuli can be easily designed and artificially controlled; therefore much attention has recently been focused on thermoresponsive membranes. [2-1] Up to now, almost all of the thermoresponsive gating membranes have featured positively thermoresponsive characteristics, that is, the membrane permeability increases with increasing environmental temperature, because all of the thermoresponsive functional gates were constructed from poly (Nisopropylacrylamide) (PNIPAM). [1-5] In these cases, the membrane pores change from a "closed" to an "open" state when the environmental temperature increases from below the lower critical solution temperature (LCST) of PNIPAM to above the LCST, as a result of the swelling/shrinking conformational change of the polymer. In certain applications, however, an inverse mode of the thermoresponsive gating behavior of the membranes is preferred. Here we report a novel family of thermoresponsive gating membranes with negatively thermoresponsive gating characteristics, that is, "opening" of the membrane pores is induced by a decrease rather than an increase in temperature.

Microcapsules can encapsulate various chemical substances in their inner spaces, and it is thus possible to attain a controlled permeation of chemicals by using appropriate microcapsules. Because of their characteristics such as small size, huge total surface area, large inner volume, and stable membrane, microcapsules have found many applications in various fields from drug delivery through to the textile, petroleum, and pesticide industries. As the release rate from core-shell microcapsules is generally controlled by the rate of diffusion of the permeants across the thin microcapsule membrane, a faster response of the release rate to environmental stimuli may be expected as compared to crosslinked gels and microspheres. Therefore, core-shell microcapsules are suitable for stimuli-responsive controlled-release systems. Since the 1980s, environmental stimuli-responsive microcapsules have been investigated widely. These microcapsules control the release of their contents according to environmental stimuli. They are considered to be potentially useful as controlled-release systems, and especially so for drug delivery, because the target of a controlled drug delivery system is improved drug treatment (outcome) through rate- and time-programmed and site-specific drug delivery. [1] By encapsulation inside these microcapsules, chemicals or drugs can be released at a desired rate only when and/or where the release is needed. Modern environmental stimuli-responsive microcapsules have been reported to act in response to changes in temperature, [2-9] pH, [10-16] light, [17] external electric field, [18] redox conditions, [19] Ca2-ions, [20] and other stimuli, and these ™smartº capsules continue to gather increasing attention. In order to promote the applications of environmental stimuli-responsive microcapsules, development of microcapsules responsive to other signals remains essential. Herein, we report on the development of a molecular-recognition microcapsule for environmental stimuli- responsive controlled release: The release of the solute vitamin B12 (VB12) from the prepared microcapsules was significantly sensitive to the presence of Ba2+ ions in the environmental solution. In solution, the release of VB12 from the microcapsules was fast but slowed dramatically in the presence of BaCl2. The prepared microcapsules showed satisfactorily reversible and reproducible molecular-recognition stimuli-responsive release characteristics.

Wehave successfully prepared monodispersed thermoresponsive core-shell hydrogel microspheres with a mean diameter of 200-400nm with poly (N-isopropylacrylamide-co-styrene) [P (NIPAM-co-St)] cores and poly (N-isopropylacrylamide) (PNIPAM) shells. The submicrometer-sized monodispersed P (NIPAM-co-St) core seeds were prepared by using a surfactant-free emulsion polymerization method, and the PNIPAM shell layers were fabricated onto the core seeds by using a seed polymerization method. The particle size, morphology and monodispersity, and thermoresponsive characteristics of the prepared microspheres were experimentally studied. In the preparation of P (NIPAM-co-St) seeds, with increasing the initiator dosage, the mean diameters and the dispersal coefficients were almost at the same levels at first; however, when the initiator dosage increased further to a critical amount, the mean diameters decreased drastically and the monodispersity became worse significantly. With increasing the stirring rate, the particle diameter decreased, and when the stirring rate was larger than 600 rpm, the monodispersity became worse obviously. With increasing the phase ratio, the mean diameter became larger simply, and the monodispersity became worse first and then became better again. With increasing the reaction time, the particle sizes nearly did not change, while the monodispersity gradually became better slightly. For the core-shell microspheres, with increasing the NIPAM dosage in the preparation of the PNIPAM shell layers, the mean diameters became larger simply, the monodispersity became better, and the thermoresponsive swelling ratio of the hydrodynamic diameters increased.

Wehave successfully prepared monodispersed thermoresponsive core-shell microcapsules with a mean diameter of about 4μm with a porous membrane and with linear-grafted poly (N-isopropylacrylamide) (PNIPAM) chains in the membrane pores acting as thermoresponsive gates. The preparation was carried out by using a Shirasu porous glass (SPG) membrane emulsification technique to prepare small-sized monodispersed oil-in-water emulsions and using interfacial polymerization to prepare the core-shell microcapsules with porous membranes. Plasma-graft pore-filling polymerization was used to graft linear PNIPAM chains into the pores of the microcapsule membranes. In the SPG membrane emulsification process, theoptimumsurfactant sodium dodecyl sulfate (SDS) concentrationandpoly (vinyl alcohol) stabilizer concentration could be selected by solely considering the monodispersity of the emulsion droplets. However, before the interfacial polymerization process was started, these two oncentrations needed to be monitored to avoid any aggregation of the microcapsules and, if necessary, the appropriate quantity of SDS or Tween 80 needed to be added to prevent the microcapsules from aggregating in the interfacial polymerization stage. The prepared PNIPAM-grafted monodispersed microcapsules with a mean diameter of about 4μm showed satisfactory reversible and reproducible thermoresponsive release characteristics. The release of both NaCl and vitamin B12 (VB12) from the PNIPAM-grafted microcapsules was slow at 25℃ and fast at 40℃, which is due to the closed/open state of the grafted "gates". The "on/off" ratio of the release rate of VB12 from the PNIPAM-grafted microcapsules was much larger than that of NaCl.