Solar heat reflective coating formed of polystyrene chains bearing 4-vinylpyridine-rich end segments

Abstract

Incorporation of a minor dose of 4-vinylpyridine (4-VP) into an end segment of polystyrene (PS) chain by emulsion copolymerization yields a copolymer molecule P(4-VP/S) with weak surface-activity. The spin cast thin films of P(4-VP/S) display enhanced solar light reflectance of R = 80–90% compared with its PS homopolymer counterpart. It is imperative to use dimethylformamide (DMF) as casting solvent because the hydrophilic nature of DMF induces core-to-grain assembling while dried in humid air, leading to a nodular film matrix. A rise of 4-VP dose in P(4-VP/S) causes a breath-figure patterned matrix instead and consequently an obvious reduction of reflectance in the near-infrared region. Mechanistically, P(4-VP/S) chains that bear a short end-segment with rich 4-VP units constitute tiny hydrophilic cores because of entrapment of moisture during drying, surrounding which the long PS tails thus undergo chain coiling to form nanoparticles. They then aggregate to form submicron grains in rosette and the partial coalescence of adjacent grains results in micron lumps. Such hierarchical evolution finally constructs a matrix exhibiting an enhanced solar reflectance.