Growth and Disappearance of Nanobubbles during the Foaming of Polycarbonate

New phase formation is a common physical phenomenon for processes such as crystallization, solidification, boiling, phase separation of polymer blends, and foaming. 1,2 Two major mechanisms are known to occur in new phase formation. 3–6 One is spinodal decomposition in which a wavelike concentration fluctuation with periodic wavelength increases over time by uphill diffusion, producing a co-continuous structure. The other is nucleation and growth in which particles are nucleated and then grow spherically over time. In the theory of the process of nucleation and growth, a nucleated particle can grow spontaneously when the particle exceeds a critical size for growth while particles smaller than the critical size are inherently unstable and disappear. 1,2,7–9 These concepts have been derived from the kinetic aspect of the growth of particles at the micrometer level suggested by scanning electron microscopy (SEM) and optical microscopy. So far, however, the structural development of nanometer-scale particles around a critical size has not been elucidated although it is helpful to completely understand the nucleation and growth process. Observation of nanometer-scale particles during nucleation and growth is expected to clarify the question as to whether all initially nucleated nanoparticles grow to larger ones of a micrometer scale, for instance. In this study, to understand the nucleation and growth process of nanobubbles, we investigate the structural development of the bubbles at an early stage of the foaming of polycarbonate by using a field emission scanning electron microscope (FE-SEM). This FE-SEM has a resolution that is one hundred times higher than that of a conventional SEM and is powerful enough to characterize the structure at a nanometer level. The FE-SEM observation let us notice that initially nucleated nanobubbles could be frozen by rapid cooling of the foamed specimen. To clarify the characteristics of nanobubbles, the structural development of nanobubbles is compared with that of micrometer-scale bubbles as observed by an optical microscope. EXPERIMENTAL