When cooled, crystalline polymers, whose molecules crystallize in a bundle, have their volume reduced compared to noncrystalline polymers, which solidify in a disorderly manner. Because of crystallization, crystalline polymers solidify compactly and have increased heat resistance, reduced chemical infiltration, and decreased gas permeability. Furthermore, since crystalline and noncrystalline parts have different indexes of refraction, crystalline polymers tend to become opaque.