Microcellular Foaming of Layered Double Hydroxide−Polymer Nanocomposites

Micro foaming performance of scCO2-aid glutaraldehyde/hexametaphosphate/thermoplastic starch foams modified by alkali treatment and montmorillonite nano-platelets

The micro foaming performance, moisture resistance and dynamic viscosity of scCO2-aid glutaraldehyde/hexametaphosphate/thermoplastic tapioca starch (GA/SHMP/TOS) foams were considerably improved by proper NaOH treatment. The expansion ratio, resilience rate, dynamic viscosity values of these NaOH modified foams improved to a maximum, as the time for NaOH treatment approached a proper value. The dynamic viscosity, expansion ratio and resilience rate of the scCO2-aid GA/SHMP/TOS foams modified using 110 atm scCO2-pressure, the proper alkali treatment time, SHMP loading and varying montmorillonite (MMT) loadings improved further, as their MMT loadings approached a proper value of 2.5 part per hundred parts of tapioca starch (PHTOS). Relatively large dynamic viscosity (7.1x104 Pa·s), extremely large expansion ratio (∼75), cell density (1.1x109 cells/cm3) and/or resilience rate (∼80%) were acquired for the scCO2-aid GA/SHMP/TOS/MMT foam modified using the proper alkali treatment time and MMT loading. Thermal analyses results showed that crystallization onset temperatures and crystallization rates of scCO2-aid GA/SHMP/TOS/MMT foams modified using the proper alkali treatment time and varying MMT loadings improved to a highest value by adding 2.5 PHTOS of MMT nano-platelets. Possible reasons accounting for the considerably improved micro foaming performance of scCO2-aid GA/SHMP/TOS/MMT foams modified using the proper alkali treatment time and MMT loading are proposed in this study.

Insight into the Influence of Properties of Poly(Ethylene-co-octene) with Different Chain Structures on Their Cell Morphology and Dimensional Stability Foamed by Supercritical CO2

Poly(ethylene-co-octene) (POE) elastomers with different copolymer compositions and molecular weight exhibit quite distinctive foaming behaviors and dimensional stability using supercritical carbon dioxide (CO₂) as a blowing agent. As the octene content decreases from 16.54% to 4.48% with constant melting index of 1, both the melting point and crystallinity of POE increase, due to the increase in fraction of ethylene homo-polymerization segment. the foaming window of POE moves to a narrow higher temperature zone from 20–50 °C to 90–110 °C under 11 Mpa CO₂ pressure, and CO₂ solubility as well as CO₂ desorption rate decrease, so that the average cell diameter becomes larger. POE foams with higher octene content have more serious shrinkage problem due to lower compression modulus, weaker crystal structure and higher CO₂ permeability. As POE molecular weight increases at similar octene content, there is little effect on crystallization and CO₂ diffusion behavior, the foaming window becomes wider and cell density increases, mainly owing to higher polymer melt strength, the volume shrinkage ratio of their foams is less than 20% because of similar higher polymer modulus. In addition, when the initiate expansion ratio is over 17 times, POE foams with longer and thinner cell wall structures are more prone to shrinkage and recovery during aging process, due to more bending deformation and less compression deformation.

Preparation of high-strength and lightweight microcellular polysulfone foam with a segregated CNT network for excellent electromagnetic shielding

Fabrication of microcellular polymer composite foam based on high-performance plastic is a promising strategy for preparing the lightweight, high-strength and multifunctional materials. Herein, we proposed a facile and green method to prepare microcellular polysulfone/carbon nanotube (PSU/CNTs) composite foams with segregated structure by combining solid-phase milling and supercritical carbon dioxide (scCO₂) foaming. The segregated PSU/CNTs foam with as low as 5.0 wt% CNT was provided with a good electrical conductivity of 5.2 S m^-1 and an acceptable electromagnetic interference shielding effectiveness (EMI SE) of 23.7 dB, respectively. Moreover, the segregated PSU/CNT foam exhibited an ultralow percolation threshold of 0.06 vol%. An absorption-dominant shielding feature was observed for segregated PSU/CNT foam, which could be attributed to the synergistic effect of the perfect CNT networks and the microcellular structure in PSU domains. In addition, benefitting from the inherent properties of the PSU matrix, foam density dropped to 0.69 g cm^-3, and the material still possessed a high specific compression strength of 38.8 MPa cm³ g^-1. Therefore, our work provided an insight into the preparation of lightweight, high-strength and multifunctional materials that might have great potential applications in aerospace and military areas.

Multi-Fold Enhancement in Compressive Properties of Polystyrene Foam Using Pre-delaminated Stearate Functionalized Layer Double Hydroxides

Developing an environmentally benign styrene foam is a critical environmental need. Supercritical CO₂ use in foams has proven to be a valuable path. Adding fillers to increase bubble nucleation has been pursued concurrently. A prominent filler used is high surface area fillers, such as smectic clays. However, all studies to date show a limit of 152% in compressive moduli and 260% in the compressive stress. The values, even with such gains, limit structural application. A seminal work in 1987 by Suh and Cotton proved that carbonyl linkages in calcium carbonates and CO₂ interact and impact nucleation efficiency and performance in supercritical CO₂ foams. In this paper, a high surface area clay (layer double hydroxides) which begins in an exfoliated state, then functionalized with a long chain alkyl carboxylate (stearic acid) is synthesized. The result is a remarkable multi-fold improvement to the compressive properties in comparison to polystyrene (PS); a 268% and 512% increase in compressive modulus and strength, respectively. Using a pre-delaminated approach, the higher surface area was achieved in the clays. The presence of the stearate improved the interactions between the clay galleries and PS through hydrophobic-hydrophobic interactions. The glass transition temperature of the nanocomposites was observed to shift to higher values after foaming. The results point to a new path to increase performance using a pre-delaminated clay with functional groups for environmentally benign foams.