Influence of nanoparticle surface chemistry and size on supercritical carbon dioxide processed nanocomposite foam morphology

https://doi.org/10.1016/j.supflu.2009.09.007Get rights and content

Abstract

Creating polymer foams with controlled pore size and pore density is an important part of controlling foam properties. The addition of nanoparticles has been shown to cause heterogeneous nucleation and can be used to reduce pore size. In the current study, the effects of filler size and filler surface chemistry on pore nucleation in silica/PMMA nanocomposites are investigated. It was found that as the nanofiller size decreased, the pore density increased by a factor of 2–3 decades compared to that of unfilled PMMA (pore cell densities above 1012 cells/cm3 were obtained). In addition, fluorination of the silica nanoparticle surface led to decreased pore size without changing the degree of silica aggregation and overall density. By monitoring the pore density as a function of pressure, a qualitative comparison was obtained that showed that fluorination of the nanoparticle reduced the critical free energy of nucleation.

Introduction

Polymeric foams are used in many industrial and commercial applications where weight is an important factor [1]. Their porous structure can be used to control thermal conductivity, dielectric constant and sound dampening behavior [2], [3]. In addition, the strength-to-weight ratio of structural foams is reported to be two to five times greater than that of metals [4]. However, polymeric foams suffer from low mechanical strength, poor surface quality, and low thermal and dimensional stability [5]. In addition, they are either processed with chlorofluorocarbons that deplete ozone [5], [6] or with chemical blowing agents that generate chemical residues in the foamed material and require further processing for purification [7]. These issues can be resolved by using physical blowing agents, which do not create chemical residues and are environmentally benign. Supercritical carbon dioxide (scCO2) is a physical blowing agent and has many advantages such as low cost, non-toxicity, non-flammability, chemical inertness, easily accessible supercritical conditions (Tc = 31 °C, Pc = 7.38 MPa) [8], and tenability of physicochemical properties (such as density and mobility) by varying pressure and temperature [9], [10], [11], [12]. In addition, scCO2 has been shown to create an evenly distributed micron size closed cell structure, which improves the mechanical properties compared to heterogeneous or open cell structures [5].

The final properties of polymeric foams (e.g., thermal conductivity and dielectric constant) depend on the average pore size and size distribution, volume fraction of pores, and distribution of the pores within the matrix [13]. One advantage of using physical blowing agents is that the pore structure can be controlled via processing temperature and pressure. However, to optimize properties, an overall understanding of the diffusion, and nucleation and growth mechanism is required [9], [13], [14]. During nucleation, scCO2 molecules overcome an energy barrier and form stable nuclei. In order to create pores with a uniform size, nucleation must occur at the same time following rapid depressurization. The common approach to solving this problem is to use fillers that act as nucleation agents [15], [16]. Fillers reduce the nucleation free energy activation barrier and force the nucleation to occur at the filler–polymer interface. Because of the lowered activation barrier, higher pore densities (pores/volume) with smaller pore sizes can be achieved. The pore density and size depend on the size of the fillers, filler size distribution, and filler surface chemistry [17], [18], [19]. Nanoparticles offer a potential advantage over micron-sized fillers because at the same loading (by weight), they offer more nucleation centers and higher interfacial area. In a recent study with calcium carbonate filled and unfilled polystyrene, Chiu et al. [17] investigated the effect of fillers on foam density. Addition of calcium carbonate led to an increase in foam density at high temperatures, but no change at intermediate foaming temperatures (120–130 °C). On the other hand, the average pore size was consistent with expectations; filled samples showed lower average pore size than unfilled samples. In a different study, Chang et al. [18] used polyethylene–octene elastomers and nanoclays, and the results showed that upon addition of clay, both foam density and average pore size decreased, however, SEM pictures suggested a broadening of the pore size distribution in the presence of clays. On the other hand, Wee at al. [19] showed that while average cell size decreased, foam density increased with increasing clay content in polystyrene. Clearly, a consistent picture has not yet emerged.

There are only a few studies reporting that modification of the filler surface chemistry changes the nucleation rate [9]. Tethering of CO2-philic polymers such as poly(methyl methacrylate) onto the surface of nanoclays has been shown to double the nucleation density [5]. This is because the poly(methyl methacrylate) reduced the dissolved gas-particle interfacial tension and contact angle, thus favoring the formation of nuclei at the clay/polymer interface.

In the current study, foams of poly(methyl methacrylate), PMMA, and silica/PMMA nanocomposites were prepared by using supercritical carbon dioxide as the blowing agent. Two sizes of silica nanoparticles with two different surface chemistries were used. The average size of silica was measured via dynamic light scattering (DLS); dispersion of silica was assessed via transmission electron microscopy (TEM); surface modification of silica was verified via infrared spectroscopy (IR); surface tethering density of silica nanoparticles was obtained via thermogravimetric analysis (TGA); and scanning electron microscope (SEM) was used to investigate the pore structure after supercritical carbon dioxide assisted foaming. The effect of saturation pressure, silica size and surface chemistry were investigated.

Section snippets

Materials

Poly(methyl methacrylate), PMMA, was kindly provided in pellet form by ExxonMobil Chemical Co. Silane coupling agent tridecafluoro-1,1,2,2-tetrahydrooctyl silane (TFTOS) was purchased from Gelest Inc. (Morrisville, PA). Tetraethylorthosilicate (TEOS) was purchased from Sigma–Aldrich. Ammonia solution was purchased from Fischer Scientific.

Preparation of silica nanoparticles

Nominally 15 and 150 nm diameter silica nanoparticles were synthesized using ammonia, ethanol and TEOS. To make the 15 nm diameter silica nanoparticles, 194 g of

Characterization of nanoparticles

Synthesized SiO2 nanoparticles were analyzed with Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). FTIR spectra for bare and surface modified silica nanoparticles are shown in Fig. 1. The broad stretching band at 3300 cm−1 arose from the surface hydroxyl groups and the weak peak at 1620 cm−1 from molecular water on pure SiO2. Since the surface of bare silica nanoparticles contains hydroxyl groups, even the dried nanoparticles re-adsorbed molecular water

Conclusions

Poly(methyl methacrylate), PMMA, and silica/PMMA nanocomposites containing two different sized silica particles (150 and 15 nm) and two different surface modifications (bare and modified with fluoroalkanes) were foamed using supercritical carbon dioxide at various saturation pressures. Silica nanoparticle size distribution and dispersion in PMMA matrix was investigated with dynamic light scattering and transmission electron microscopy. Pore density and pore size distribution analysis were

Acknowledgement

This material is based upon work supported by the National Science Foundation under Grant No. 0500324.

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