Mylar is a PET based Polyester typically used in electric applications due to its high electric strength as well as chemical and thermal stability. Since mylar foils typically are transparent I can hardly belive that Mylar is best material for reflecting solar radiation.
About polymer mirror films developed for solar radiation concentration, you may also find useful to check: http://www.nrel.gov/docs/fy11osti/49273.pdf http://www.reflectechsolar.com/pdfs/ESFuelCell2011-54152Paper.pdf http://www.sciencedirect.com/science/article/pii/S1876610214004779 http://www.reflectechsolar.com/product.html