Effect of TPU chemistry and concentration on the cellular structure of nanocellular polymers based on PMMA/TPU blends

Abstract

Nanocellular polymers are characterized by cell sizes in the range of tens to hundreds of nanometers and cell densities greater than 1013 cells/cm3. These materials have aroused great interest in the area of Materials Science due to their interesting properties, such as a reduced thermal conductivity, better mechanical properties and the possibility of producing cellular materials with a certain transparency. In this work, nanocellular polymers based on poly(methyl-methacrylate) (PMMA) have been produced by gas dissolution foaming using thermoplastic polyurethane (TPU) as a nucleating agent. Three grades of TPU with a different fraction of hard segments (60%, 70% and 80%) have been used in different contents (0.5 wt%, 2 wt% and 5 wt% in PMMA). First, the morphology of solid materials has been studied, observing that the TPU phase appears as a dispersed phase in the PMMA matrix forming nanometric domains. The PMMA/TPU foamed samples are characterised by a gradient of the cell size from the edge of the sample towards the core. The cells near the edges are micrometric and cells in the core are nanometric. The presence of the nanometric cells in the core is due to the nucleating effect of the TPU. The cellular structure has been characterised in these PMMA/TPU samples. Specifically, the evolution of the cellular parameters (relative density, cell size and cell nucleation density) have been analysed in the nanocellular core as a function of the TPU content and of the fraction of hard segments in the TPU. On the one hand, as the TPU content increases, the nucleation density increases, and the cell size is reduced due to the increment in the number of TPU domains dispersed in the solid. On the other hand, by increasing the fraction of hard segments in the TPU, the cell nucleation density increases, and the cell size is reduced due to a better dispersion of the TPU. As the content of hard segments increases, the molecular weight of the TPU is higher. In addition, as the molecular weight of the TPU approaches the PMMA molecular weight the size of the domains is reduced, increasing the density of domains. Nanocellular polymers have been obtained with a relative density between 0.15 and 0.20 and cell sizes between 220 and 640 nm.