Conventional and Nanometric Nucleating Agents in Poly(e-caprolactone) Foaming: Crystals vs. Bubbles Nucleation

The aim of this article was to investigate the nucleating
ability of different nucleating agents for the foaming of
poly(e-caprolactone), a biodegradable, semicrystalline
polymer. In particular, the efficiency of the nucleating
agent in inducing the formation of the gaseous phase
has been compared to the efficiency in inducing the
formation of the crystalline phase. In effect, in foaming
of semicrystalline polymers, bubble nucleation and
crystal nucleation are concurrent and somehow interacting
phenomena. Here, these two aspects have been
evidenced and clarified. Foams were prepared by using
a batch process with the pressure quench method,
with nitrogen and carbon dioxide as the blowing
agents. Conventional and novel nucleating agents were
used: talc has been compared to several novel nanometric
particles of different geometries and dimensions,
such as titanium dioxide and alumina powders,
exfoliated and intercalated clays, and carbon nanotubes.
Foam densities and morphologies, in terms of
number of cells per initial unit volume, were measured
and found to depend both on crystalline phase nucleation
and gaseous phase nucleation. In fact, the different
nucleating agents, depending on shape, dimension,
and surface functionalization, selectively nucleated the
crystallites and/or the bubbles, affecting, respectively,
bubble growth (and, hence, final foam density) and
bubble nucleation (and, hence, cell number density--