Graded structures and nanocellular polymers are two examples of advanced
cellular morphologies. In this work, a methodology to obtain low-density
graded nanocellular polymers based on poly(methyl methacrylate) (PMMA)/
thermoplastic polyurethane (TPU) blends produced by gas dissolution
foaming is reported. A systematic study of the effect of the processing condition is presented. Results show that the melt-blending results in a solid
nanostructured material formed by nanometric TPU domains. The PMMA/
TPU foamed samples show a gradient cellular structure, with a homogeneous nanocellular core. In the core, the TPU domains act as nucleating
sites, enhancing nucleation compared to pure PMMA and allowing the
change from a microcellular to a nanocellular structure. Nonetheless, the
outer region shows a gradient of cell sizes from nano- to micron-sized cells.
This gradient structure is attributed to a non-constant pressure profile in the
samples due to gas desorption before foaming. The nucleation in the PMMA/
TPU increases as the saturation pressure increases. Regarding the effect of
the foaming conditions, it is proved that it is necessary to have a fine control
to avoid degeneration of the cellular materials. Graded nanocellular polymers
with relative densities of 0.16–0.30 and cell sizes ranging 310–480 nm (in the
nanocellular core) are obtained.
