Article pubs.acs.org/IECR
A New Promising Nucleating Agent for Polymer Foaming: Applications of Ordered Mesoporous Silica Particles in Polymethyl Methacrylate Supercritical Carbon Dioxide Microcellular Foaming Jintao Yang,† Lingqi Huang,† Yuefang Zhang,† Feng Chen,† Ping Fan,† Mingqiang Zhong,*,† and Shukai Yeh‡ †
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, P R China Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan, 10608, Republic of China
‡
ABSTRACT: The introduction of inorganic particles to improve the cell morphology of polymeric foams has been studied for decades. To this end, identifying an ideal nucleating agent and understanding the methodology to control nucleation have always been the focus of this field. In this study, spherical ordered mesoporous silica (OMS) particles were synthesized and applied as a potential nucleating agent in polymethyl methacrylate (PMMA) supercritical carbon dioxide (scCO2) microcellular foaming. These particles were modified with a silane containing fluorine to enhance their affinity with scCO2. For comparison, solid silica (SS) particles with almost similar particle size and surface treatment have also been studied. It was found that both of them could be well dispersed in the PMMA matrix, and exhibited excellent heterogeneous nucleation performance during the foaming process. The addition of a nucleating agent greatly increased the cell density and decreased the average cell diameter, and more importantly, there was no increase in the bulk density. Compared to SS particles, OMS particles showed higher nucleation efficiency. The addition of 5.0 wt % of OMS particles reduced the average cell diameter of PMMA foam from 1.62 to 0.66 μm and increased the cell density from 2.3 × 1011 cells/cm3 to 3.7 × 1012 cells/cm3, while for the composite foams with a similar content of SS particles, the cell diameter and cell density are 1.19 μm and 6.12 × 1011 cells/cm3, respectively. We speculated that the mesoporous structure of OMS particles might trap the CO2 forming gas cavities. The pre-existing gas cavities resulted in a lower nucleation energy barrier. The superior nucleation effect of the OMS particles became more significant when the foaming process was conducted at low foaming temperature, low saturation pressure, and/or high pressure drop rate.
1. INTRODUCTION Polymeric foams, which consist of a polymeric matrix formed by mixing solid and gas phases, have been widely used in multifold applications for several decades.1−3 One significant advantage of using these materials is the reduction of raw material cost. Besides this economic issue, the porous structure also offers many superior properties, including high thermal insulation, low dielectric coefficient, and excellent damping properties.4,5 However, the applications of polymeric foam are associated with limitations, such as inferior mechanical strength, poor surface quality, and low thermal and dimensional stability, which reflect in their practical application. It has been well demonstrated that the mechanical properties of the foam are highly dependent on their cell morphology.6 In particular, small cell size, high cell density, and narrow cell distribution are considered to be suitable for realizing high strength−weight ratio and dimensional stability.7,8 To this end, numerous studies have investigated microcellular foams of smaller cell size (109 cells/cm3).9 Polymeric foams are produced by a so-called phase separation process, which is usually caused by a quick change in temperature or pressure to the homogeneous polymer/ blowing agent system. Typically, the formation of cell takes place via three steps, namely, nucleation, cell growth, and cell coalescence.10,11 Here, the nucleation stage has a significant influence on the morphology of the final cell. In principle, the © 2013 American Chemical Society
foaming process can proceed through two different kinds of nucleation mechanism, namely, homogeneous and heterogeneous nucleation. Compared with homogeneous nucleation, heterogeneous nucleation has a lower energy barrier and can promote the simultaneous formation of the embryos, thereby reducing the average cell size and narrowing the cell size distribution.12 In the foaming of neat polymer, heterogeneous nucleation occurs only when some impurities are included during the process. On the contrary, heterogeneous nucleation becomes dominant when particles with high surface area are introduced. In the last few decades, many inorganic materials, including calcium carbonate,13 magnesium silicate,14 and talc,15 have been widely applied in the foaming of different polymers. Recently, it has been demonstrated that well dispersed nanoparticles, such as clay,16 carbon nanotubes,17 carbon nanofibers,18 and graphene19,20 can be efficiently used in the foaming of different polymers, as they offer more nucleation sites than the conventional micrometer-sized inorganic particles. Among these nanoparticles, organically modified clay has been the most extensively studied material owing to the following Received: Revised: Accepted: Published: 14169
June 11, 2013 September 7, 2013 September 10, 2013 September 10, 2013 dx.doi.org/10.1021/ie4018447 | Ind. Eng. Chem. Res. 2013, 52, 14169−14178
Industrial & Engineering Chemistry Research
Article
reasons: First, the flat surface of organically modified clay can act as excellent nucleation site. Second, it can be easily exfoliated and dispersed in most polymers.21,22 It has been widely accepted that the low energy barrier is a critical concern in the mechanism of heterogeneous nucleation. Consequently, every parameter or property associated with this issue can affect the nucleation. On the basis of an analysis of the thermodynamics and kinetics of coupled nucleation and growth, McClurg23 pointed out that an ideal heterogeneous nucleating agent must possess the following characteristics: (1) a nonwetting surface that makes it energetically and kinetically favorable relative to homogeneous nucleation; (2) uniform geometry and surface properties; (3) excellent dispersibility. Thus far, several theoretical and experimental studies have demonstrated that the above-mentioned factors are critical for an ideal nucleating agent.24−26 For example, carbon-based nanoparticles show higher nucleation efficiency than clay because of the lower contact angle between polymer/CO2/ carbon.27 Meanwhile, Leung et al28 also demonstrated that the surface geometry of the nucleating agents is also an important factor that influences the nucleation efficiency. On the basis of the computer simulation, they suggested that nucleating agents with numerous crevices of small semiconical angles are highly desirable for polymeric foaming. To elucidate the mechanism of heterogeneous nucleation, it is necessary to take into consideration the affinity between the blowing agent and the nucleating agents. This was first reported by Macosko29 et al., who compared the nucleation mechanism using different types of A−B diblock copolymers as the nucleating agents and scCO2 as the blowing agent. According to their study, only the copolymers containing poly(dimethylsiloxane) (PDMS) exhibit a nucleation effect due to the increased solubility of carbon dioxide in PDMS. Lately, it has been reported that grafting CO2-philic polymer or copolymer onto the nanoparticles might also be beneficial for improving the nucleation efficiency of the nanoparticles. The promoted nucleation efficiency is caused by the localization of scCO2 around the nanoparticles.30,31 On the basis of these results, it is reasonable to postulate that the higher is the number of blowing agents participating in the heterogeneous nucleation process, the higher is the nucleation efficiency. In principle, the foaming process of polymer is induced by the supersaturation of dissolved CO2 and the bubbles are produced during gas desorption. This is very similar to the formation of effervescence, when carbonated beverages such as sparkling wine, beer and soft drinks are shaken. For the nucleation of bubbles in a solution supersaturated with a gas, it is found that the nucleation at pre-existing gas cavities can occur at lower levels of supersaturation, indicating that the nucleation energy barrier for gas cavities is lower. Thus far, some studies and reviews have been drawn upon this issue.32,33 The concept of a lower energy barrier of pre-existing gas cavities is also applicable to the polymer foaming process. For instance, Chen et al.34 proved the above-mentioned concept of lower energy barrier of pre-existing gas cavities in the foaming of PMMA/multiwalled carbon nanotube nanocomposite by investigating the influence of the aspect ratio of multiwalled carbon nanotubes on the foam morphologies. It was found that nanocomposites loaded with shorter nanotubes exhibited the greater bubble densities compared to nanocomposites with long nanotubes. They ascribed this result to the fact that nucleation preferably occurred at the ends of nanotubes where
the so-called nanoscale cylinder pores structure may trap CO2 forming gas cavities. Ordered mesoporous silica (OMS) is a new form of silica materials with many unique properties, such as ordered porous structure, high surface area, large pore volume, and well-defined and tunable pore size.35,36 Because of these properties, it has attracted a great deal of research interest, and has been widely applied in many fields, such as catalysis, separations, sensors, drug delivery, and optical devices.37,38 Among these applications, the CO2 absorption of OMS is considered to be an important research area, both from industrial and academic perspectives. OMS has numerous mesopores and a high specific area, offering a large space for CO2 absorption.39,40 Taking into account the principles of heterogeneous nucleation mechanism and the major factors affecting the nucleation efficiency, OMS is considered to be an excellent nucleating agent for the scCO2assisted foaming process due to the following two factors: (1) the high specific area and large pore volume make it a reservoir for trapping scCO2 and increasing solubility of the blowing agent; (2) the mesopores may trap CO2 to form gas cavities, which can lower the critical free energy for nucleation. On the basis of these hypotheses, the present study aims to analyze the suitability of OMS as the nucleating agent for PMMA scCO2 microcellular foaming process. For this purpose, spherical OMS particles of average diameter around 220 nm were synthesized, and a silane containing fluorine was chosen to modify the surface and channels of the particles. To elucidate the mechanism underlying the nucleation of these particles, solid silica (SS) particles with almost similar particle size and surface treatment also were used for comparison.
2. EXPERIMENTAL PROCEDURE 2.1. Materials. CTAB (Hexadecyl trimethyl ammonium bromide) was purchased from Jingchun Reagent Co. Ltd., Shanghai, China. 1H,1H,2H,2H-perfluorodecyltriethoxysilane (F1060) was purchased from SICONG Chemical Co. Ltd., Fujian, China. Tetraethoxysiliane (TEOS), ammonium hydroxide, and ethanol are commercial products with chemical purity and used as received. CO2 was purchased from Jingong Gas Co. Ltd., Hangzhou, China. PMMA (No. CM-207) was purchased from Zhenjiang Qi Mei Co. Ltd. 2.2. Synthesis of Spherical OMS and SS Particles. The synthesis of the spherical MCM-41 OMS particles followed a standard procedure, which is modified from the Stöber’s method.41 Roughly 1 g of CTAB was dissolved in 500 mL of deionized water. Then, 0.32 g of NaOH and 60 g of ethanol were added to the solution and stirred for 30 min to form a clear solution. Subsequently, 20 mL of TEOS was added drop by drop while stirring, whereupon a white precipitate appeared. The suspension was stirred at 80 °C for another 6 h. The product was collected by centrifugation, washed with distilled water and ethanol, and dried overnight at 60 °C. The template agent was removed by extracting the samples using a reflux of hydrochloric acid/methanol solution for 24 h. The silica particles were synthesized by the sol−gel method. Briefly, 25 mL of ethanol, 4 g of H2O2, and 7 g of NH3·H2O were put in a flask and stirred using a magnetic bar. In the meantime, a solution containing 25 g of ethanol and 10 g of TEOS was dripped into the former solution at a constant rate. The process took about 30 min, and the temperature was maintained at 30 °C. The product was collected by centrifugation, washed with distilled water, and dried under vacuum to get the solid silica particles. 14170
dx.doi.org/10.1021/ie4018447 | Ind. Eng. Chem. Res. 2013, 52, 14169−14178
Industrial & Engineering Chemistry Research
Article
Figure 1. Typical TEM images of solid silica (SS) particles (a) and spherical ordered mesoporous silica (OMS) particles (b) and OMS modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (c), XRD patterns (d), nitrogen adsorption−desorption isotherms (e), and pore size distribution of OMS particles before and after surface modification.
2.3. Spherical OMS and SS Particles Treated with Silane Containing Fluorine. To modify OMS and SS particles with silane containing fluorine, a conventional graft technique was employed. OMS or SS particles were dried at 100 °C under vacuum for 2 h, and 2 g of dried particles were dispersed in 150 mL of anhydrous toluene. After that, 9 mmol of 1H,1H,2H,2H-perfluorodecyltriethoxysilane was added to the above suspension and stirred at 80 °C for 24 h. The modified particles were collected by filtration and extensively washed with anhydrous toluene and ethanol to remove the remaining physically absorbed silane, and the particles were vacuum-dried at 70 °C for 8 h. 2.4. Preparation of PMMA/OMS and PMMA/SS Composites. The modified silica particles were compounded with PMMA granules using a HAAKE Minilab microcompounder. A quarter gram of modified nanoparticles was compounded with 4.75 g of PMMA pellets for 6 min. The compounding temperature was set at 200 °C with a screw speed of 60 rpm. The samples were then compression molded into plates with 2 mm of thickness at 200 °C using a hydraulic press (Labtech Engineering Company Ltd.) for the DMA test. Neat PMMA sample was also prepared through compounding and compression in the same manner for comparison. 2.5. Foaming of PMMA and Composites by ScCO2. PMMA and their composites were foamed by a high-pressure batch foaming system using scCO2 as the blowing agent under different foaming conditions. The samples were saturated in scCO2 for 24 h to achieve the equilibrium state. The saturation pressure was achieved and monitored by an ISCO D-series pump. The CO2 was released in different release time using different gas reducers. When the pressure was completely released, the chamber was immediately moved into an ice− water bath to fix the morphology of the samples. 2.6. Characterization. The morphologies of the original and modified OMS particles were observed by a Philips-FEI
transmission electron microscope (Tecnai G2 F30 S-Twin, FEI Company, Netherlands) at an acceleration voltage of 300 kV. The samples for TEM observations were prepared by placing a 10 μL solution of the particles in ethanol on copper grids. X-ray diffraction patterns were recorded on X’Pert PRO (PNAlytical, Dutch) using Cu Kα radiation. The powders were placed on a bracket sample holder. The data were recorded with a scanning rate of 1°/min and a scanning scope between 1° and 10°. Nitrogen adsorption/desorption isotherms were measured at −196 °C using a Micromeritics ASAP 2020 analyzer. Prior to the measurements, samples were degassed under vacuum at 150 °C for at least 6 h. The BET method was used to calculate the specific surface area, and the pore size distributions were derived from the Barrett−Joyner−Halenda (BJH) model. The glass transition temperatures of neat PMMA and PMMA/silica composites were measured by differential scanning calorimetry (DSC, TA-Q100) and dynamic mechanical analysis (DMA, Netzsch DMA242 analyzer). For DSC measurement, 10 mg of samples in aluminum crucible were heated from 40 to 180 °C at a rate of 20 °C/min under N2 flow of 10 mL/min. In the case of DMA measurement, the data were obtained by heating the samples from 40 to 160 °C, with a heating rate of 3.0 K/min and under 0.01% deformation with a 1.0 Hz frequency. The morphology of PMMA/SS and PMMA/OMS composites was examined with a transmission electron microscope (JEM1200EX, Japan Electron Optics Laboratory, Tokyo, Japan) at an acceleration voltage of 80 kV. The ultrathin slices (
