Development of smart window with hydroxypropyl cellulose-acrylamide hydrogel: Effect of hydroxypropyl cellulose molecular weight on light scattering property

Reduction in the use of cooling systems can help in the reduction of CO2 emissions. Sunlight, particularly near-infrared light, entering through windows causes an increase in room temperature; therefore, a heat-shielding material that can automatically control the amount of sunlight without the use of electricity is required. Hydroxypropyl cellulose (HPC), a temperature-responsive polymer with a lower critical solution temperature (LCST), changes its optical properties in response to external temperature. This study aimed to prepare hydrogels with acrylamide (AAm) and HPC (HPC/AAm gel) and investigate the effect of HPC with different molecular weights on light transmittance and the state of HPC aggregation in hydrogels. The scattering mechanisms were discussed further. The solar transmittance of HPC/AAm gels below LCST exceeded 80%. Above LCST, the solar transmittance of HPC/AAm gels with large molecular weights decreased to 25%, whereas that with small molecular weight decreased to approximately 20%. The surface of HPC/AAm gels comprising HPC with large molecular weight exhibited numerous large pores following freeze-drying because the skeleton of the AAm and BIS inhibited the shrinkage of HPC above the LCST. In contrast, HPC/AAm gels comprising HPC with small molecular weight have a small number of pores owing to the easy shrinkage of HPC; they scatter sunlight by increasing the frequency of light collisions. Thus, the light transmittance decreases when using the HPC/AAm gels with small molecular weight.