Poly(acrylic acid)/azobenzene microcapsules were obtained through distillation precipitation polymerization and the selective removal of silica templates by hydrofluoric acid etching. The uniform, robust, and monodisperse microcapsules, confirmed by transmission electron microscopy and scanning electron microscopy, had reversible photoisomerization under ultraviolet (UV) and visible light. Under UV irradiation, azobenzene cross-linking sites in the main chain transformed from the trans to cis isomer, which induced the shrinkage of microcapsules. These photomechanical effects of azobenzene moieties were applied to the encapsulation and release of model molecules. After loading with rhodamine B (RhB), the release behaviors were completely distinct. Under steady UV irradiation, the shrinkage adjusted the permeability of the capsule, providing a novel way to encapsulate RhB molecules. Under alternate UV/visible light irradiation, a maximal release amount was reached due to the continual movement of shell networks by cyclic trans-cis photoisomerization. Also, microcapsules had absolute pH responsiveness. The diffusion rate and the final release percentage of RhB both increased with pH. The release behaviors under different irradiation modes and pH values were in excellent agreement with the Baker-Lonsdale model, indicating a diffusion-controlled release behavior. Important applications are expected in the development of photocontrolled encapsulation and release systems as well as in pH-sensitive materials and membranes.

Dual-responsive (light and pH) yolk−shell structured drug delivery nanocapsules, each consisting of a movable upconversion nanoparticle (UCNP) core and a shrinkable poly(methacrylic acid) (PMAA) shell, were prepared by distillation precipitation polymerization. Monodispersed NaYF 4 : Yb 3+ /Tm 3+ UCNPs were synthesized and encapsulated in silica templates, followed by coating to form PMAA shells. Subsequently, the silica templates were dissolved to form nanocavities for drug loading. The PMAA shell contains pH and ultraviolet (UV) light sensing moieties, enabling a control release upon the exposure of nanocapsules to these stimuli. The near-infrared (NIR)-to-UV feature of UCNPs allows azobenzene isomerization to be light triggered remotely to control contraction and swelling of PMAA shells. The loading efficiency of the anticancer drug doxorubicin (DXR) was up to 17 wt % due to the unique nanoporous structure of PMAA shells. The values of the diffusion coefficient under different release conditions were determined using the Baker−Lonsdale model to facilitate the design of dual-responsive drug release devices or systems.

SUMMARYIn this paper, an experimental performance evaluation of a direct expansion ground-coupled heat pump (DX-GCHP) system in heating mode is presented. The DX-GCHP uses R134a as the refrigerant, and consists of three single U-tube copper ground heat exchangers (GHEs) placed in three 30 m vertical boreholes. During the on-off operations from December 25, 2007, to February 6, 2008, the heat pump supplied hot water to fan-coil at around 50.41C, and its heating capacity was about 6.43 kW. The energy-based heating coefficient of performance (COP) values of the heat pump and the whole system were found to be on average 3.55 and 3.28 at an evaporating temperature of 3.141C and a condensing temperature of 53.41C, respectively. The second law efficiency on the DX-GCHP unit basis was around 0.36. The exergetic COP values of the heat pump and the whole system were obtained to be 0.599 and 0.553 (the reference state temperature was set equal to the average outdoor temperature of À1.661C during the tests), respectively. The authors also discussed some practical points such as the heat extraction rate from the ground, refrigerant charge and two possible new configurations to simultaneously deal with maldistribution and instability of parallel GHE evaporators. This paper may reveal insights that will aid more efficient design and improvement for potential investigators, designers and operators of such DX-GCHP systems.