Formation and transformation of smectic polypropylene nanodroplets

Abstract

AbstractA dispersion of isotactic polypropylene (PP) nanoparticles was produced by interfacially‐driven breakup of PP nanolayers. Layer‐multiplying coextrusion was used to fabricate an assembly of 257 alternating PP nanolayers about 12 nm thick sandwiched between thicker polystyrene (PS) layers. Characterization by thermal analysis and wide‐angle X‐ray diffraction (WAXD) confirmed that PP crystallized primarily in the smectic form when confined as nanolayers. When the layered assembly was heated into the melt, the PP nanolayers broke up to form a dispersion of PP droplets in a PS matrix. After solidification, particle size analysis revealed that 90% of the PP was present as 30 nm nanoparticles. The particles were small enough and numerous enough that most did not contain a primary nucleus. When cooled from melt at 10 °C min−1, the droplets crystallized by homogeneous nucleation at 40 °C. The droplets were found to be in the smectic form by WAXD. Because crystallization occurred below the temperature of the smectic to α‐form transformation, the intermediate smectic form was stable and did not convert to the α‐form until heated above 70 °C. This result provided direct evidence for an intermediate smectic phase in the process whereby homogeneous nucleation leads to α‐form crystals in confined nanoparticles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1795–1803, 2006