The observation of interpenetrated spherulites by polarized scanning near-field optical microscopy

Some miscible crystalline/crystalline polymer blends show an interesting phenomenon that one spherulite grows through the interlamellar regions of another spherulite; this process is called interpenetrated spherulite (IPS) formation. For example, poly(butylenes succinate)/poly (ethylene oxide) (PBSU/PEO) is one of the non-simultaneous IPS systems, where PBSU at first crystallizes during rapid cooling and PEO goes on growing and penetrating into PBSU spherulites at isothermal condition even after collision. IPS formation has already been examined using polarizing optical microscopy (POM). The increase in birefringence of the area of PBSU spherulites was the evidence of IPS. However, the detailed crystallization mechanism could not be well understood due to the poor resolution that was restricted by the diffraction limit. In this paper, we report the result of in situ observation of IPS formation by polarized scanning near-field optical microscopy (SNOM), which has higher optical resolution, beyond the diffraction limit, and simultaneous topographic imaging capability. The increase in topographic unevenness and simultaneous retardation increase were directly observed. We speculate that the result was due to the crystalline growth of PEO along PBSU twisted lamellae.