Nanocellular Polymers with a Gradient Cellular Structure Based on Poly(methyl methacrylate)/Thermoplastic Polyurethane Blends Produced by Gas Dissolution Foaming

Abstract

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.

Effect of the Molecular Structure of TPU on the Cellular Structure of Nanocellular Polymers Based on PMMA/TPU Blends

“…Moreover, the three TPUs grades containing a high HS content are classified in the Shore D hardness scale (referred to rigid types of TPU). The commercial TPU used in the two previous PMMA/TPU works [ 29 , 30 ] on nanocellular polymers had a hardness of 83 A. They are classified in the Shore A hardness scale which belongs to more flexible types of TPU (with low HS contents).…”

“…Foaming was carried out in a water bath at 90 °C for 1 min. Foaming conditions were selected to maximize expansion without degeneration of the structure [ 30 ]. The time between the release of pressure and the immersion of samples in the bath was 2 min (time needed to fully release the pressure and open the vessel).…”

“…Some of the foamed samples present a graded cellular structure, as shown schematically in Figure S2 (Supplementary Information) , with a solid skin on the surface (approximately 5 µm in thickness), a transition region where cells are microcellular and a core region in which the cells are nanocellular. To account for this heterogeneity, the density of the foamed samples was determined twice, density of the sample as produced (global density) and after removing the skin and transition regions with a polisher (model LaboPOl2-LaboForce3, Struers, Struers, Denmark) (details can be found elsewhere [ 30 ] and in the Supplementary Information Section S2 ) (core density). The global relative density ( ρ r,g ) was calculated according Equation (3) by dividing the density of the foamed sample by the density of the solid.…”

“…However, they claimed that an additional cooling step before releasing the pressure was necessary to ensure a more homogeneous cellular structure. In our previous work [ 30 ], we use 2 wt% of a commercial TPU polymer to blend with PMMA to obtain graded cellular polymers with core relative densities between 0.16–0.30 and cell sizes ranging 310–480 nm on the nanocellular core. A graded cellular structure is characterized by variable cell size along the sample radius (samples showed a cylindrical shape).…”

“…Thus, nanocellular PMMA/TPU samples are characterized by a nanocellular region in the core that gradually becomes microcellular toward the edge. We showed that the heterogeneous nucleation in the TPU domains is not predominant at all pressures [ 30 ]. Therefore, we hypothesized that the formation of the gradient of cell sizes is due to gradient nucleation in the PMMA/TPU samples, caused by a non-constant gas concentration profile in the samples due to the desorption of gas before the foaming step.…”