The Tentative Investigation On A Thermochromic System For Energy Saying From Poly (N-isopropylacrylamide)

Nowadays, the energy issue has been a hot topic around the world. Energy saving buildings have caused great interests since the buildings spent a large portion of energy consumed in the city.Roof is the important external protection of the residence building. Its color has a great impact on the internal room temperature. Roof with a light color can reflect sunlight, preventing the temperature rising inside. On the contrary, dark roof can absorb more sunlight, which will warm up the room inside. If the roof color can change from dark in winter to light in summer automatically and reversibly, much energy can be saved from warming in winter and cooling in summer.Poly(N-isopropylacrylamide) is a thermosensitive polymer (PNIPAM). Its aqueous solution gives a reversible phase transition at different temperatures. Below the low critical solution temperature (LCST), PNIPAM dissolves into water. When the temperature is above LCST, however, it separates from water turning the solution into latex. In current work the optical properties of PNIPAM aqueous solution varying with temperature were studied first. It was discovered that above LCST the light was scattered by the PNIPAM particles. The lowest transmittance of light depended on the light path and the concentration of the solution. Based on the thermosensitive properties of PNIPAM, a novel idea of designing a thermochromic system for building energy saving was explored. When the outdoor temperature is below LCST(32℃) (as in winter), the PNIPAM aqueous solution is transparent. As a result the sun will reach the black roof. The absorbed sunlight can supply energy for building saving the energy for heating. When the temperature exceeds LCST(as in summer), PNIPAM separates from water in the form of particle turning the system into latex. The roof gives a white color which can reflect the light. Less sunlight will be adsorbed and the cooling energy can be saved.The study of the trial system showed that the reflection ratio increased when the PNIPAM concentration was increased. When the PNIPAM concentration exceeded 0.05g/ml the reflection ratio reached its maximum value of 33.0%. At this concentration, the temperature rising can be reduced by 6.1℃compared with the blank experiment. Besides, it was found the reflection ratio was independent of the molecular weight of PNIPAM. When it was copolymerized with a small amount of acrylic acid, the copolymer gave a higher LCST and the reflection ratio decreased.This work provided a novel idea for developing energy saving materials for building.