Chemically Interconnected Thermotropic Polymers for Transparency-Tunable and Impact-Resistant Windows

Thermotropic polymers with the capability of thermally tuning transparency are widely applied in smart windows and energy-saving windows, playing a critical role in enhancing comfort level and energy efficiency of indoor spaces. Usually, thermotropic polymer systems are constructed by physically dispersing phase transition materials in transparent hosting materials. However, bad interfaces universally exist in these systems, resulting in poor mechanical properties, weak interfaces to substrates, or bad long-term stability. Herein, we demonstrate a novel chemically interconnected thermotropic polymer, which is obtained by reacting dodecanedioic acid (DDA) with glycerol. In the system, some of DDA molecules were cross-linked to form a polyester network, poly(glycerol-dodecanoate) (PGD). Other grafted but non-cross-linked DDA molecules form semicrystalline domains, which possess a solid-liquid phase transition within the PGD matrix. The phase transition offers the resulting hybrid materials with tunable optical transparency. The PGD-DDA system shows stable performance after 100 heating-cooling cycles. In addition, when applied for window coating, it results in tough interfacial bonding to glass substrates with toughness of >6910 J m^-2 below its transition temperature and >135 J m^-2 above its transition temperature. It increases the impact resistance of the window by multiple times.