Supercritical fluid applications in polymer nanocomposites

Current Opinion in Solid State and Materials Science 7 (2003) 407–412

Supercritical fluid applications in polymer nanocomposites David L. Tomasko *, Xiangmin Han, Dehua Liu, Weihong Gao Department of Chemical Engineering, The Ohio State University, Columbus, OH 43220, USA Received 6 October 2003; received in revised form 14 October 2003; accepted 14 October 2003

Abstract Supercritical fluids have been used to synthesize and foam a variety of polymer nanocomposite materials. There have been significant advances in developing and characterizing nanoscale structures and using supercritical fluids to alter properties at the nanoscale. In this work we summarize these advances and discuss foam properties generated using supercritical carbon dioxide as well as relevant fundamental properties of the system. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Nanocomposite; Supercritical; Carbon dioxide; Foam adsorption; Rheology

Major recent advances Nanoscale reinforced polymeric materials (aka polymer nanocomposites) provide an opportunity for combining the features of supercritical fluids with traditional polymer processing. Recent work emphasizes reducing the domain size of the dispersed component to achieve true nanoscale features. Advances include surface modification to exfoliate nanoclays, controlled synthesis in confined matrices, and better understanding of the role of supercritical CO2 in rheology and interfacial properties.

1. Introduction The Ôtop down’ approach to nanotechnology signifies the modification of microscale processing techniques to produce nanoscale features. Much of the recent work in using supercritical fluids (SCFs) for polymer composite materials falls into this vein and is the primary focus of this review. The term polymer nanocomposite is broadly defined to include a material consisting of two immiscible phases of which one is a polymeric material and one is present as a dispersed phase (or domain) with a nanoscale feature in at least one dimension. For example, the seminal work from Toyota R&D [*1,2] dem*

Corresponding author. Tel.: +1-614-292-4249; fax: +1-614-2923769. E-mail address: [email protected] (D.L. Tomasko). 1359-0286/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2003.10.005

onstrated a nanoclay platelet dispersed in a polymer matrix to improve the mechanical properties of the composite material. The nanoclay is a layered silicate which can be exfoliated into platelets with nanometer thickness and aspect ratios on the order of 103 . The application of supercritical fluids to the preparation of composites is through one of two approaches: in-situ polymerization of the dispersed phase in a SCFswollen polymer matrix, or dispersive mixing of the nanoscale phase into the polymer matrix using the SCF to reduce the matrix viscosity and/or nucleate nanoscale bubbles in the material. While the basic feasibility of the in-situ processes were demonstrated in the mid-1990s [**3,*4,**5,6], recent work has concentrated on reducing the dispersed phase domain size and isolating the nanoscale features in the composite materials.

2. In situ polymerization for polymer/polymer and polymer/inorganic composites In situ processes for producing nanocomposites apply a two-step procedure. Initially, the polymer substrates are infused with the precursor reactants dissolved in the SCF and then a reaction is induced (typically thermally) to generate a new dispersed phase. The SCF functions as a swelling agent for the polymer to enhance diffusion, a solvent for the precursor reactants and byproducts, and nonsolvent for the reaction products. The resulting nanoparticle or nanophase products are restrained from agglomerating by the polymeric continuous phase.

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2.1. Polymer/inorganic nanocomposites In order to obtain desired metal nanoparticles, the organometallic precursors have to be carefully selected or even synthesized to give acceptable solubility in the supercritical phase and easy reduction to pure metal. High thermal decomposition temperatures of the precursors can limit the range of suitable matrix polymers. The advantage of these systems is the relative ease of obtaining dispersed nanoparticles. Even in the initial reports using this technique, platinum particles of <50 nm were produced. Subsequent efforts have produced copper, silver, and iron nanoparticles in a variety of matrices [*7,8]. A practical polymer/inorganic hybrid device would need controllable long-range order of nanoscale metal domains over bulk dimensions. A clever approach is to use a SCF to pressure tune block copolymer selfassembly, where one polymer phase contains a functional group selectively bound to a metal precursor and then acts as a nanoscale template [**9,*10]. A key to this approach is the proper CO2 interaction with and control over the co-polymer self-assembled structures. 2.2. Polymer/polymer nanocomposites Applying a similar technique to produce polymer/ polymer nanocomposites is slightly more challenging due to the larger size of the product (polymer chain vs. atomic metal cluster) and higher mobility of the product due to stronger interactions with the matrix phase. In polymer/polymer nanocomposites several recent advances have been made to confine the polymerized phase to the nanoscale domain and characterize the dispersion more quantitatively. Recent progress demonstrates polymer blends and interpenetrating networks via SCCO2 (mainly polystyrene-based) [11,*13,14–16], some with nanoscale dispersed domains. The advance of note is the convincing demonstration that crystalline domains (or lamellae) in the matrix phase template the polymerization and maintain the dispersion of an amorphous nanophase [**17,**18].

3. Polymer/clay nanocomposites prepared in supercritical CO2 Using clay as filler, two idealized nanocomposites are possible: intercalated and exfoliated. Intercalation results from the penetration of polymer chains into the interlayer region of the clay with preservation of the ordered layer structure. Exfoliation involves extensive polymer penetration and crystallite delamination, with the individual nanometer-thick silicate platelets are randomly dispersed in the polymer matrix. Exfoliated

nanocomposites usually provide the best property enhancement due to the large aspect ratio and surface area of the clay. Depending on the polymer, intercalated and exfoliated polymer/clay nanocomposites can be obtained by means of either mechanically compounding or in-situ polymerization [19,**25]. However, none of these means is easy and straightforward. Therefore, the use of SCFs to both pretreat/delaminate silicate layers and to reduce melt viscosity is an attractive means of generating wellexfoliated polymer/clay nanocomposites for certain systems. Manke et al. [*26,27] recently patented two methods to produce PP/clay nanocomposites by the assistance of SCCO2 . The essential characteristic of the process is saturation of the clay followed by rapid depressurization to forcibly exfoliate the platelets with the expanding CO2 . A similar procedure was applied in extrusion foaming to make clay nanocomposites of high-density polyethylene (HDPE) and poly(trimethyleneterephthalate) (PTT) using SCCO2 [28]. Thirty-three percent increase in the typical clay d-spacing was realized for HDPE/clay nanocomposites with the assistance of SCCO2 , while a 10% increase was observed for PTT/clay nanocomposites.

4. Foaming Foaming comes naturally in SCF processing of polymers and there are several ways in which it is an active research area for nanocomposites. One is exploring the interaction of CO2 with surface-treated nanoparticles to influence composite foam properties and a second is the preparation of nanoporous polymers with well defined open pore structure with novel applications as low-k dielectrics, separation membranes, and drug delivery devices. 4.1. Nanocomposite foams with CO2 Compared to conventional micron-sized filler particles used in the foaming process, the extremely fine dimensions, large surface area, and intimate contact between particles and polymer matrix may greatly alter cell nucleation and growth. Nam et al. [29,30] foamed polypropylene/clay nanocomposites and showed that biaxial flow during cell growth induces the alignment of clay particles along the cell boundary. Han et al. [31,*32,33] prepared both intercalated and exfoliated polystyrene/nanoclay composites by mechanical blending and in situ polymerization respectively. The composites were then foamed by using CO2 as the foaming agent in an extrusion foaming process. The addition of a small amount of intercalated nanoclay greatly reduces

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Fig. 1. Influence of clay surface modification on the polystyrene foam cell morphology: (a) clay surface is modified by a polystyrene (PS) polymer containing one methacrylic group; (b) clay surface is modified by poly(methyl methacrylate) (PMMA) with a cationic ammonium head group. The scale bar is 50 lm.

cell size and increases cell density. Once exfoliated, the nanocomposite exhibits extremely high cell densities and even smaller cell size. Compared with unmodified polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier properties. Recently, Zeng et al. [**25,34] further manipulated the foam cell morphology by adjusting the interaction between CO2 and the clay surface. The clay was exfoliated by an in situ technique grafting PMMA onto the surface using then the modified clay was extrusion blended with polystyrene. This clay exhibited a much higher nucleation efficiency than the one whose surface was modified by polystyrene with a single methacrylic group linker (Fig. 1). A strong affinity between CO2 and the carbonyl group may reduce the gas–particle interfacial tension, which would lead to the reduction in nucleation free energy and a large increase of nucleation rate. Several other researchers also mention the potential applications of nanoclays in various polymer processing operations, including foaming [35–39]. 4.2. Nanometer sized foams with CO2 To reduce the cell size in polymer foams from micrometer to sub-micrometer is nontrivial requiring both a very high nucleation rate and a truncated cell growth stage. In practice, cell sizes of several hundred nanometers have been achieved with a few reports in the sub-100 nm range. Generally this involves working with high Tg materials and using an increase in temperature to initiate foaming without exceeding the Tg of the native polymer. All of these nanofoams are produced by batch processes that limit the size of products. Materials studied include PMMA [40–42], poly(ether sulfone) (PES) and poly(ether imide) (PEI) [43,44], a blend of polysulfone and polyimide [*45], and a series of polyimides for ultralow-k dielectrics [46,47]. A different nanostructured ‘‘foam’’, layered polymers with nanoscale gaps, was created by Zhang and Handa [48]. Polystyrene beads or films were compressed together

first to introduce weak chain-entanglements at the interfaces. After saturation with CO2 , the polymer was heated and layered structures with nanoscale interlayer gaps were formed due to the nucleation and expansion of CO2 in the weak chain-entanglement regions.

5. Role of SCF at polymer/clay interface 5.1. Rheology In many of the applications discussed above the supercritical fluid acts primarily as a diluent/swelling agent for the polymer phase, solvent for monomers or reactants, and a blowing agent to produce foams. During the dispersion of nanoparticles in a polymer matrix, or during the formation of nanofoam bubbles, the viscosity of the polymer/gas system plays a pivotal role. As a result, increased attention has been given to understand the rheological properties of polymer/gas melt or solution systems. In general, viscosity is observed to decrease as CO2 is dissolved into various polymers. Once the nanometer-sized fillers are dispersed in the polymer matrix, the rheological behavior becomes more complex due to synergetic effect of nanofillers and CO2 . Interesting developments include the shear viscosity change of intercalated nanocomposites of PS/montmorillonite (MMT) nanoclay with and without CO2 [*49]. This work observed that the melt viscosity of polystyrene in the presence of CO2 actually decreased with the addition of nanoclay filler in contrast to the typical increase with filler concentration (Fig. 2). They suggest this may be an interfacial effect wherein CO2 concentrations are enhanced at the polymer/clay interface causing a different structure in the polymer phase near the clay nanoparticles. Secondly, Flichy et al. [**50] used a rotational viscometer, a drag-driven device, to study the effect of CO2 on the rheology of poly(propylene glycol) (PPG) and suspensions of nanoporous silica with PPG. This is one of the first published accounts of high pressure stress rheometry.

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Hocker et al. predicted isotherms in one-dimensional slit-pores of different widths [**54]. In their work, the adsorption ‘‘hump’’ shows up in meso- and macropores, but not in micropores (<2 nm width). In fact, they show that the ‘‘hump’’ in the adsorption isotherm is not correlated with a 3-layer micropore but is fit only with the inclusion of a 30 layer macropore ascribed to the clay binder in their zeolite material.

Shear viscosity (Pa.sec)

3500 10 1/sec 20 1/sec 30 1/sec

3000

2500

2000

1500

6. Conclusions 1000 0.0%

2.5%

5.0%

7.5%

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Fig. 2. Effect of montmorillonite clay (20A) concentration on viscosity of polystyrene (PS) containing 3 wt% CO2 .

The rheological properties of nanocomposites with or without CO2 are far from well understood and are receiving increasing attention from researchers. Elastic and viscous moduli, extensional viscosity, and the time dependent behavior are awaiting further study, and depend to a large extent on the design of the rheometer. These properties will improve understanding of the principles behind the dispersion of nanoparticles and the interaction between nanoparticles and the polymer matrix.

The literature now demonstrates the ability to use SCFs for controlled synthesis and processing of composite materials with nanoscale features. Novel combinations of chemistry, polymer architecture, and CO2 thermodynamics are yielding unique structures and processes. Fundamentally, a direct correlation between adsorption in nanopores, rheology of nanocomposites, and final properties of composite materials has yet to be drawn although this is only a matter of time. Polymer nanocomposite systems are drawing significant attention from many different angles and there will be much more progress in the next few years. From this short review it is clear that supercritical fluids will play an important role in the development of manufacturing processes at the nanoscale.

5.2. Adsorption Another fundamental area relevant for nanocomposite research is the behavior of CO2 confined in nanoscale systems. Recent results show that supercritical fluids (CO2 in particular) exhibit unique behavior in nanoscale systems including pores and films. This behavior most often manifests itself near the critical point of CO2 where the isothermal compressibility is at its highest. Sirard et al. showed using ellipsometry that the swelling of thin films of PMMA undergoes an anomalous maximum near the critical point of CO2 [**51]. The high compressibility of CO2 at these conditions results in concentration inhomogeneities near the surface of the film that are of such a magnitude that they only affect observable properties in nanoscale films (thicknesses up to 325 nm were studied). Swelling in bulk PMMA does not exhibit a maximum. A second, related phenomenon is the Ôhump’ in the excess adsorption isotherms on activated carbons and zeolites near the critical point of CO2 [52,*53]. This behavior is also related to the highly compressible state but is difficult to analyze because molecular model parameters necessary to capture the magnitude of the Ôhump’ require an unrealistic number of CO2 layers in the pore (more than can physically fit). For example,

Acknowledgements The authors gratefully acknowledge financial support from the National Science Foundation through grants EEC-0223592 and DMI-0200324 and from the donors of The Petroleum Research Fund, administered by the American Chemical Society.

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