Polymer nanocomposite foams have received considerable attention because of their potential use in advanced applications such as bone scaffolds, food packaging, and transportation materials due to their low density and enhanced mechanical, thermal, and electrical properties compared to traditional polymer foams. In this study, silica nanofillers were used as nucleating agents and supercritical carbon dioxide as the foaming agent. The use of nanofillers provides an interface upon which CO2 nucleates and leads to remarkably low average cell sizes while improving cell density (number of cells per unit volume). In this study, the effect of concentration, the extent of surface modification of silica nanofillers with CO2-philic chemical groups, and supercritical carbon dioxide process conditions on the foam morphology of poly(methyl methacrylate), PMMA, were systematically investigated to shed light on the relative importance of material and process parameters. The silica nanoparticles were chemically modified with tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane leading to three different surface chemistries. The silica concentration was varied from 0.85 to 3.2% (by weight). The supercritical CO2 foaming was performed at four different temperatures (40, 65, 75, and 85°C) and between 8.97 and 17.93 MPa. By altering the surface chemistry of the silica nanofiller and manipulating the process conditions, the average cell diameter was decreased from
In nature, foams are found in the form of bone, natural sponge, coral, and natural cork. Inspired by these materials, processing of polymer foams has received considerable attention [
Foams are divided into various categories depending on their pore morphology (open versus closed) or density (low versus high). Open cell morphology consists of pores (bubbles) that are connected to each other, making the material softer and more absorbent. In the closed cell morphology, the pores are isolated from each other, which makes the foam more rigid. In addition to these categories, polymer foams can also be characterized according to their density, cell size, cell density, and wall thickness, all of which influence the properties of the foam. In general, polymer foams have low thermal conductivity, poor mechanical properties, and poor surface quality due to the underlying porous structure. However, their low density, low thermal conductivity, and sound barrier properties make them highly attractive for a variety of applications. For example, low density foams are primarily used in packaging and insulation applications, and high density foams are used in structural applications [
In general, the foam morphology is formed via the use of foaming agents, which undergo phase transition either due to physical or chemical changes creating a gas phase that expands forming gas bubbles inside the polymer matrix. The nucleation of the bubbles occurs via two different classical mechanisms: homogeneous or heterogeneous nucleation. In the homogeneous nucleation case, concurrent initiation and growth of bubbles are observed leading to a wide cell size distribution in the final foam structure. The heterogeneous nucleation, however, requires the existence of a secondary material that promotes simultaneous growth of bubbles inside the polymer matrix, resulting in a narrow cell size distribution. The addition of inorganic nanoparticles, which act as nucleating agents, induces heterogeneous nucleation and provides a large number of nucleation sites. Furthermore, the presence of micro- or nanosized fillers dramatically decreases the energy barrier for cell nucleation compared to that required for homogeneous nucleation [
The synthesis of nanofillers is of recent interest because they provide a high density of nucleation sites at low concentrations. The highest nucleation efficiency is achieved when nucleation on the filler surface is energetically favorable and the filler is uniformly distributed and dispersed within the polymer matrix. Therefore, heterogeneous nucleation conditions could be controlled to some extent via filler type, geometry (size, aspect ratio, etc.), and surface chemistry [
In addition to filler characteristics, foaming conditions also influence the final foam structure. Conventional foamed products can be produced either by chemical or physical blowing agents. Chemical blowing agents are mixed into polymer matrix and decompose when heated up yielding a gas release. This process requires an additional step to eliminate the residual chemical blowing agent [
Many researchers investigated the silica/PMMA nanocomposite systems foamed with supercritical CO2 and a review article by Chen and coworkers [
Although nanofiller concentration and surface chemistry are known to have strong influence on the final cell size and cell density [
To synthesize silica nanoparticles, tetraethyl orthosilicate (TEOS, 98% reagent grade, Sigma, 131903) and ammonia solution (NH3, 28%, Sigma, 338818) were used as received. Tetrahydrofuran (THF, Fisher Scientific, T397-1) was used to purify the silica nanoparticles. The surface modification of the silica nanoparticles was performed with silane coupling agent tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane (F-TEOS, Gelest, SIT8175). Poly(methyl methacrylate), PMMA, was chosen as the matrix polymer because of its outstanding chemicophysical properties [
The method used in the synthesis of silica nanoparticles is an adaptation of the Stöber’s procedure [
In order to increase the interaction of silica and CO2, silica nanoparticles were modified by tethering fluoroalkane chains (F-TEOS) onto their surfaces. To prepare the surface modified nanoparticles, silica nanoparticles were synthesized as described before and each batch was divided into multiple parts; one was left as is (bare silica) and the other parts were modified with F-TEOS. This ensured that the starting silica nanoparticles on average had the same size. Two different amounts (0.1 and 0.7 g) of F-TEOS were added to two different silica batches to create two different surface tethering densities. The surface modification reaction took 24 hr at room temperature. Unreacted solvent was removed by rotary evaporator at 60°C and 90 rpm for 2 hours, and the samples were dried in a vacuum oven overnight at 60°C. To remove any residual solvent and impurities, each sample was subsequently washed with THF and water, filtered, and dried in vacuum oven overnight at 60°C.
The size of the bare and surface modified nanoparticles was characterized by field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), and image processing. After the synthesis, the bare and surface modified silica nanoparticles were dried and the nanoparticles were first subjected to gold sputtering to form a 10–15 nm layer of gold on the sample surface. SEM images were obtained with a JEOL JSM-6332 using an accelerating voltage of 10 kV. TEM images were collected with a Philips CM12 with an accelerating voltage of 120 kV. Subsequently, both SEM and TEM images were analyzed with ImageJ [
The extent of the surface modification was determined by thermogravimetric analysis (TGA). Each silica nanoparticle sample was analyzed by TA Instruments TGA Q50 by heating up to 900°C at a rate of 20°C/min. The resulting percent weight changes between 200 and 800°C were used to calculate the percent coverage of the silica surface. Both bare and surface fluorinated samples were tested for at least three times.
The surface area of the silica nanoparticles was measured with Quantachrome BET Surface Analyzer Autosorb. Before BET analysis, the samples were degassed at 150°C under nitrogen atmosphere for 24 hrs.
The polymer nanocomposite samples were prepared by melt mixing with a benchtop twin-screw extruder (Haake MiniLab). The extruder was equilibrated for 2 hrs at 220°C before each use. The rotation speed of the screws was set to 60 rpm. Before each operation, 5 g of neat PMMA was passed through the system to clean the screws. The samples with different concentrations of bare and surface modified silica nanoparticles were mixed with PMMA and each sample was cycled within the extruder for 4 min and flushed in 3 min. These parameters were chosen because they were previously shown to disperse and distribute silica nanoparticles effectively [
Although silica was added to PMMA in premeasured amounts, due to the chaotic nature of the twin-screw extruder used, silica concentration in silica/PMMA composites was subsequently measured again by thermogravimetric analysis (TA Instruments). For statistical accuracy, three different samples, each being approximately 40 mg, were analyzed with a TA Instruments TGA Q50 by heating to 900°C at a rate of 20°C/min. The silica concentration was determined from the remaining weight at 800°C, after which the weight remains constant. The melt mixing process led to several composite samples each having slightly different amount of bare or surface modified silica nanoparticles. In order to establish statistical significance, these samples were separated into two groups of “low” and “high” silica concentrations. The low silica concentration group contained 0.85–1.38% silica and the high concentration group contained 2.5–3.2% silica. All silica concentrations are reported as weight percentages. Another 2.9% silica/PMMA nanocomposite sample was prepared with highly surface modified silica nanoparticles. This sample was used specifically to investigate the effect of supercritical CO2 process parameters on foam morphology while eliminating sample variations and effect of surface modification. The nanocomposite samples were labeled according to their silica surface tethering density (B: bare; F: fluorinated with low tethering density; and FF: fluorinated with high tethering density) and silica concentration (low or high). For instance, B-Low indicates a nanocomposite sample containing bare silica nanoparticles at low concentration (0.85%). The neat PMMA control sample was labeled as “PMMA.” This labeling convention was necessitated by the fact that during sample preparation neither the surface tethering density nor the silica concentration could be controlled accurately.
A batch foaming process was performed to prepare polymer nanocomposite foams using supercritical CO2 as the foaming agent. A high-pressure reactor (Parr, 5512, 50 mL) was connected to a Teledyne ISCO high-pressure syringe pump. The polymer nanocomposite extrudes were cut into 1-2 cm long pieces and were placed in the pressure chamber. The samples were then saturated with supercritical CO2 and were kept at predetermined temperature and pressure for 24 hrs (the exact processing conditions are provided in Section
The foam morphology was investigated using field emission scanning electron microscope (SEM, JEOL JSM-6332). Samples were freeze-fractured in liquid nitrogen and the fracture surfaces were sputter-coated with 10–15 nm of gold. The images were then collected under 15 kV accelerating voltage. SEM images of the fracture surfaces, both from the center and near the perimeter, were recorded with SEM for image processing with ImageJ [
The size of the silica nanoparticles was determined by image analysis of transmission electron micrographs (TEM). Figure
Size distribution of (a) bare and (b) surface modified silica nanoparticles. Surface modified silica samples include both F and FF samples.
As explained in Section
(a) The structure of F-TEOS. (b) Possible reaction mechanisms of the surface modifier, F-TEOS, with hydroxyl groups on silica. Three possible attachments of F-TEOS to the silica surface are possible. Mechanisms I and II leave ethoxysilane groups on F-TEOS that might react with neighboring tethered or free F-TEOS groups resulting in multilayered coverage on the silica nanoparticle.
A quantitative analysis of the F-TEOS coverage on the silica nanoparticle surfaces was performed by thermogravimetric analysis (Figure
Thermogravimetric analysis of bare and fluorinated silica nanoparticles. B denotes the untreated silica nanoparticles. F and FF denote the low and high fluorinated samples, respectively. The weight loss between 200 and 800°C for each sample is presented next to the sample label.
Table
Surface coverage of silica nanoparticles according to different reaction mechanisms as calculated from (
Mechanism | I | II | III |
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F | 68.2% | 72.6% | 77.6% |
FF | 112.2% | 119.4% | 127.5% |
The silica nanoparticles prepared in the current study can be summarized as follows. (i) The average size of silica nanoparticles are
The silica concentration of the samples was determined by thermogravimetric analysis and the results are presented in Figure
The concentration of the B (bare), F (~73% surface coverage) and FF (>100% surface coverage) silica nanoparticles in PMMA as measured by TGA. The vertical capped lines through symbols indicate error from TGA measurements.
Polymer nanocomposite samples were processed with supercritical CO2 and the resulting foam morphologies obtained via scanning electron microscopy (SEM) are presented in Figure
SEM images of the foamed samples with varying surface modification and concentration.
Cell density calculations were based on a method described by Kumar and Suh [
The cell density measurements confirmed that neat PMMA, and the polymer nanocomposite samples prepared with bare silica nanoparticles have very low cell densities compared to the foamed samples containing fluorinated silica nanoparticles regardless of the tethering density and silica concentration (Figure
(a) Cell density and (b) average cell size of the neat PMMA, nanocomposites containing bare nanosilica (B), nanocomposites with surface treated silica at low fluorination (F) and high fluorination (FF). Error bars represent standard error of five samples and error bars smaller than symbol size are not shown.
The immediate change in cell density upon surface modification (going from B to F) is quite impressive showing almost a 100-fold increase. This suggests that there is a tremendous effect of surface chemistry on foam morphology. On the other hand, increasing surface tethering density from ~73% to over 100% did not change the cell density values significantly. It is, therefore, important to have the proper surface chemistry but having too much of CO2-philic chemical groups on the nanofiller surface does not further improve cell density values. This result can be explained by the selective placement of CO2 molecules at the filler/polymer interface. Even a modest selectivity achieved through surface modification leads to improved results. Unfortunately tethering densities lower than ~70% were not available in the current study; therefore, it is not possible to state if there is an optimum tethering density that would lead to the best results (high cell density, small average pore size).
The cell sizes of the samples were measured from SEM images. The results of the average cell diameters are presented in Figure
During supercritical CO2 saturation, CO2 also serves as a plasticizer for PMMA [
In a typical experiment, the polymer nanocomposite sample was saturated with supercritical CO2 at a preselected temperature and pressure. In order to understand the effect of processing conditions on foam morphology, several experiments were conducted at 65 and 85°C and at 8.97, 13.45, and 17.93 MPa. For this study only one sample containing highly fluorinated silica nanoparticles at high concentration (FF-High with 2.9% silica content) was used, thereby, eliminating sample variations and the effect of surface modification and silica concentration. Other processing parameters such as saturation time, rate of pressure reduction, bubble fixation temperature, and foaming duration were all kept constant at 24 hrs, 1.5 MPa/s, 62°C, and 1 min, respectively. The resulting SEM micrographs of fracture surfaces are presented in Figure
SEM images of the foamed highly fluorinated, high concentration (FF-High) sample at various pressures and temperatures.
The effect of pressure on cell density and cell size at two different temperatures is presented in Figure
(a) Cell density and (b) cell size of the highly fluorinated, high concentration (FF-High) sample processed at 65 and 85°C at saturation pressures 8.97, 13.45 and 17.93 MPa. The error bars show the standard error of five samples.
The effect of temperature was found to be more complex than the effect of pressure. The effect of temperature at 8.97 MPa is presented in Figure
Cell densities of FF-High samples processed at 8.97 MPa as a function of saturation temperature. Error bars are the standard error for five samples.
Table
Summary of the cell density and cell size measurements for all samples used in the current study.
Sample label | Surface modification (%) | Concentration (wt%) |
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|
Cell Density† (109 cm−3) | Cell Diameter† ( |
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PMMA | — | — | 40 | 8.97 |
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B | — | 0.85 | 40 | 8.97 |
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B | — | 2.50 | 40 | 8.97 |
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F | ~73 | 1.29 | 40 | 8.97 |
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F | ~73 | 3.16 | 40 | 8.97 |
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FF | >100 | 1.38 | 40 | 8.97 |
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FF | >100 | 3.20 | 40 | 8.97 |
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FF* | >100 | 2.9 | 40 | 8.97 |
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FF* | >100 | 2.9 | 65 | 8.97 |
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FF* | >100 | 2.9 | 65 | 13.45 |
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FF* | >100 | 2.9 | 65 | 17.93 |
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FF* | >100 | 2.9 | 75 | 8.97 |
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FF* | >100 | 2.9 | 85 | 8.97 |
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FF* | >100 | 2.9 | 85 | 13.45 |
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FF* | >100 | 2.9 | 85 | 17.93 |
|
|
*These samples were all processed from the same silica synthesis batch.
†Standard deviation is given.
The cell densities and average cell sizes obtained in the current study are compared to those obtained from other studies performed with PMMA in Figure
Comparison of normalized cell densities and normalized average cell sizes of various silica/PMMA nanocomposite foams.
Silica/poly(methyl methacrylate), PMMA, nanocomposites containing bare and fluoroalkane modified silica nanoparticles with an average size of 100 nm were studied under varying supercritical carbon dioxide conditions. The effect of silica nanoparticle concentration and the extent of fluorination was studied systematically. The fracture surfaces of foamed samples were imaged with field emission scanning electron microscope and image processing tools were utilized to obtain cell density and average cell size. Our findings led to the following conclusions.
Surface modification of silica nanoparticles with CO2-philic fluoroalkane molecules significantly improved foam morphology. The average cell size decreased from 9.62
However, the extent of surface coverage (tethering density) of silica nanoparticles had a minor effect on foam morphology. The average cell sizes increased approximately 20% upon going from ~73% surface coverage to over 100% surface coverage.
Increasing silica concentration had a remarkable effect on the average cell size and cell density. Increasing silica concentration from ~1.3% to ~3.2% led to a ~35% decrease in average cell size.
Foam morphology strongly depends on the foaming temperature in reference to the glass transition temperature of the polymer. Although the glass transition of the neat PMMA used in the current study is around 105°C, soaking with supercritical CO2, which acts as a plasticizer, decreased it to approximately 65°C at 8.97 MPa soaking pressure. As a result, the foam morphology below and above the new glass transition temperature was influenced by different viscoelastic properties of the PMMA matrix. Our results suggest that the maximum cell density is reached in the vicinity of the glass transition temperature because the bubble growth was impeded either by high viscosity below the
Deniz Rende is supported by a fellowship from the Scientific and Technological Research Council of Turkey (TUBITAK), 2219 Program. This material is based upon work supported by the National Science Foundation under Grant nos. 0117792, 0500324, and 1003574.