The mechanics of solid-state nanofoaming.

Frederik Van Loock,Miguel Angel Rodríguez Pérez,Norman A Fleck,Victoria Bernardo

doi:10.1098/rspa.2019.0339

Abstract

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO2 as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gas-filled voids during the solid-state foaming process. Diffusion of CO2 within the PMMA matrix is sufficiently rapid for the concentration of CO2 to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO2 is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall tearing and comparisons are made between the predictions of the model and nanofoaming measurements; it is deduced that the failure strain of the cell walls is sensitive to cell wall thickness.

Highlights

Polymeric nanofoams are polymer foams with an average cell size below 1 μm [1]
The average cell size l of the high Mw nanofoams ranges from 20 to 50 nm, and is an order of magnitude smaller than the average cell size of the low Mw nanofoams. These values of l and Nd for the low Mw nanofoams are consistent with the results of Martin-de León et al [9], who conducted solid-state foaming experiments with an identical low Mw polymethyl methacrylate (PMMA) grade
Solid-state nanofoaming experiments are performed on two grades of PMMA of markedly different molecular weight (Mw = 92 500 g mol−1 and Mw = 3 580 000 g mol−1)

Summary

Introduction

Polymeric nanofoams are polymer foams with an average cell size below 1 μm [1]. This new class of porous solids has the potential to offer unique and attractive combinations of thermal, mechanical and optical properties [2,3,4]. The thermal conductivity λ of polymeric nanofoams may be lower royalsocietypublishing.org/journal/rspa Proc.

Results

Discussion

Conclusion