Chem. Mater. 1996, 8, 1735-1738
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Nanocomposites Prepared by Threading Polymer Chains through Zeolites, Mesoporous Silica, or Silica Nanotubes Harry L. Frisch Department of Chemistry, State University of New York at Albany, Albany, New York 12222
James E. Mark* Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, Ohio 45221-0172 Received January 23, 1996. Revised Manuscript Received May 17, 1996X
The purpose of this brief review is to describe some recent, preliminary attempts to prepare nanocomposites in which polymer chains thread through the cavities or channels of several types of inorganic materials, specifically zeolites, a mesoporous hexagonal form of silica, and silica nanotubes. One goal is to determine the effects of the constraining geometry on the properties of the chains, in particular, their glass transition temperatures. Another is the hope that this molecular threading will provide such intimate interactions between the polymer chains and their inorganic environment that novel reinforcement effects will be obtained.
Introduction There has recently been a great deal of interest in the preparation and characterization of organicinorganic nanocomposites, as evidenced by two books devoted entirely to the topic,1,2 and a number of recent review articles.3-28 There has been particular interest in those materials in which an organic phase is constrained in such a way as to produce novel structures, frequently even on the molecular level. In some cases, a polymer is directly introduced into a constraining structure, and in others a constrained monomer is subsequently polymerized to obtain the final composite materials. One example of this approach to nanostructured materials is the intercalation of polymers into the layered structures of some types of clays and related materials, by Okada,29-39 Pinnavaia,40-45 Giannelis,46-52 and others.53-58 Another example involves attempts to prepare “molecular wires” from conducting chains constrained in cavities or in monolayers, by Bein59-62 and others.63-65 Interest in zeolites in a wide variety of applications is evident from a very extensive literature, covering a considerable period of time.66-70 The present review, however, focuses on some specific aspects of zeolites and related silica-based materials which also have wellcharacterized cavities or channels. These aspects are the morphology of zeolite/polymer nanocomposites and the nature of the reinforcement of the polymer provided by the inorganic phases. Simple Mixing of a Polymer and a Zeolite In the simplest study of this type, Al-ghamdi and Mark71 studied reinforcement of poly(dimethylsiloxane) (PDMS) by two zeolites having different pore sizes. The zeolites were a zeolite “3 A” (pore size 3 Å) and a zeolite “13 X” (pore size 10 Å), both with a cubic crystalline structure and particle sizes of 1-5 µm. They were X
Abstract published in Advance ACS Abstracts, July 15, 1996.
S0897-4756(96)00046-4 CCC: $12.00
simply blended into hydroxyl-terminated chains of PDMS which were subsequently end-linked with tetraethoxysilane to form an elastomeric network. These elastomers were studied by equilibrium stress-strain measurements in elongation at 25 °C. The modulus and the energy required for rupture in the sample containing 13.8 wt % of zeolite 3 A were 0.377 N mm-2 and 47 × 10-5 J mm-3 at an elongation of 3.04 (as compared to 0.173 N mm-2 and 8.5 × 10-5 J mm-3 for the unfilled elastomer at an elongation of 1.92). The sample with 12.3% zeolite 13X exhibited a modulus of 3.51 N mm-2 and energy to rupture of 25.7 × 10-5 J mm-3 at an elongation of 2.21. Thus, both zeolites increased the mechanical properties of the PDMS elastomer, but the effect was larger for the zeolite with the larger pore size. Further characterization of the mechanical properties and structure of such zeolite-reinforced PDMS elastomers by Wen and Mark72 also utilized small-angle neutron scattering (SANS) and transmission electron microscopy (TEM). The neutron-scattering profiles of the pure and zeolite-filled PDMS networks were identical, which indicates negligible penetration of the polymer into the zeolite pores. The TEM pictures showed that the zeolite with the larger pore size (zeolite 13 X) had a smaller particle size, and this is probably the origin of its superior reinforcing ability.72 Polymerizing Monomer within Zeolite Cavities A different approach was initiated by Frisch and Xue73 to introduce polymer into the internal pores of the 13 X zeolites. As will be seen in the following section, the approach is very similar to that used by Wu and Bein60-62 to form molecular-scale wires of polyaniline or carbon within nanometer-sized channels. Polystyrene. In the first application of this approach, Frisch and Xue added (liquid) monomeric styrene in various ratios to a weighed amount of zeolite. A desired amount of benzoyl peroxide was then added to © 1996 American Chemical Society
1736 Chem. Mater., Vol. 8, No. 8, 1996
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Figure 1. Excess SAXS scattering intensities for a poly(ethyl acrylate)/zeolite pseudo-interpenetrating polymer network (PIPN).79
the mixture for subsequent generation of free radicals by thermolysis, in the polymerization of the styrene. The polymerization was carried out at 70 °C, either directly in this form or in the presence of the cross-linker ethylene glycol dimethacrylate (EGDM; 0.017 mol/mol of monomer). In the absence of the cross-linker, the linear PS chains and the zeolite were considered to form a hybrid inorganic/organic “pseudointerpenetrating polymer network” (PIPN), while the cross-linked polystyrene/ zeolite composite was considered to have formed a full interpenetrating polymer network (IPN).74-76 Poly(ethyl acrylate). Subsequently, Frisch et al.77-79 were also able to form both PIPNs and IPNs from poly(ethylene acrylate) (PEA) and zeolite 13 X. In this case, the polymerization was carried out at 40 °C using (liquid) ethyl acrylate monomer in the zeolite mixtures, with benzoyl peroxide initiator, in either the absence or presence of EGDM crosslinker. Results on Both PS and PEA. The linear polystyrene PS or PEA chains could be partially or wholly removed from the PIPNs by extraction with suitable solvents. On the other hand, no significant weight loss was found on even extensive solvent extractions of the IPNs. The samples containing initially 50-60 wt % organic polymer were studied by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and solid-state 13C NMR spectroscopy. These studies provided substantial confirmation that the polymer grew within the internal pores. Specifically:73 (1) Removal of about half of the linear polymer by solvent extraction (determined gravimetrically) left individual zeolite crystals containing the remaining polymer. The SEM micrographs showed there was essentially no polymer between or on the zeolite (which
was present before the extraction), and resembled micrographs of the pure zeolite. All linear polymer could be removed on extended solvent extraction. (2) Solid-state 13C NMR showed significant line-width broadening of the quaternary aromatic, methine, and methylene carbons in the linear PS. The effect was strongest in the partially solvent-extracted samples. This is consistent with the speculation that the “internal” PS chains in the pores of the zeolite are more confined by their environment than in the pure PS (without any zeolite). (3) The SAXS scattering curve of the PIPN of the PEA/ zeolite exhibited two maxima, as shown in Figure 1.79 The first rather sharp peak was the same as in the pure zeolite 13 X and corresponds to a Bragg’s law distance of 14.3 Å (the periodic interpore distance in zeolite 13 X). The second (weaker and broader) peak disappears completely in the scattering curve for the fully extracted PIPN (containing no residual PEA polymer). This weaker peak has a maximum which corresponds to the doubled distance (∼28.6 Å) which is interpreted to be due to the presence of PEA filled pores in an arrangement showing some type of regularity. (4) Of greatest relevance were the DSC curves for IPNs and PIPNs of PS/zeolite and PEA/zeolite containing 50-60 wt % polymer (where the zeolite crystals were present during the polymerizations, with some of them being threaded in a matrix of cross-linked or linear polymer). These materials exhibited a glass transition temperature Tg which was the same as that of the corresponding pure polymer. This is demonstrated in Figure 2.73 Partially extracted samples of the PIPNs in which this matrix has been removed exhibited no Tg at all, despite weight measurements indicating that a considerable amount of PS was still present. Presumably, the internal linear polymer chains in the pores of
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Chem. Mater., Vol. 8, No. 8, 1996 1737
Figure 3. Scanning electron microscope picture of a polystyrene/nanotube PIPN in which the pore diameters range from 200 to 8000 Å.80
Figure 2. Differential scanning calorimetry results for some zeolite/polystyrene (PS) nanocomposites and related reference materials:73 (a) linear polystyrene, (b) full IPN of zeolites/PS, (c) pseudo-IPN of zeolite/PS, (d) extracted sample from composite (c), and (e) pure zeolite.
the zeolite 13 X adopt a more extended, one dimensionally oriented conformation and thus exhibit no bulk Tg. In qualitative terms, the constraints prevent the longrange motions that would otherwise occur at the normal glass transition. This interpretation is consistent with the reports of Giannelis et al.46,51,52 that intercalcacated PS chains in layered silicates similarly do not show a Tg characteristic of the bulk material. Polymerizing Monomer within Mesoporous Hexagonal Silica The disappearance or change in Tg of PS in other crystalline and amorphous silicates with cylindrical pores has also been observed.80 Specifically, no bulk Tg was found in a mesoporous hexagonal silica81 having a pore radius of 28 Å.80 These containing effects aparently parallel those in the zeolite, as expected. Polymerizing Monomer within Tubule Channels On the other hand, PS chains in amorphous silica nanotubes82 having radii between 200 and 8000 Å exhibited values of Tg close to that in the bulk.80 A typical composite is shown in Figure 3.80 It seems reasonable to conclude that only those geometries corresponding to diameters of less than approximately 200 Å sufficiently constrain PS chains to the extent of suppressing their glass transition. Concluding Remarks The preliminary results presented here are certainly unusual enough to encourage additional work on these
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