Formation of Nanocomposites by In Situ Intercalative Polymerization of

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 12, 2017 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch022

Chapter 22

Formation of Nanocomposites by In Situ Intercalative Polymerization of 2-Ethynylpyrdine in Layered Aluminosilicates: A Solid-State NMR Study A. L. Cholli , S. K. Sahoo , D. W. Kim , J. Kumar , and A. Blumstein 1

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Center for Advanced Materials and Department of Chemistry, University of Massachusetts at Lowell, Lowell, M A 01854

Novel polymer nanocomposites having potential anisotropic properties have been synthesized by in-situ intercalative polymerization of 2-ethynylpyridine in the lamellar galleries of Ca -montmorillonite. In this paper, we used solid-state C CP/MAS NMR spectroscopy and X-ray diffraction technique to demonstrate the spontaneous intercalative polymerization of 2-ethynylpyridine in the galleries of montmorillonite without the aid of external initiator. Nanocomposites were synthesized with varying amounts of polymers. X-ray diffraction data indicate the increase in the d- spacing of Ca -montmorillonite with the polymer content. The C NMR chemical shifts of various carbons in the in-situ polymerized nanocomposite samples were assigned and compared with the bulk synthesized polymer of poly (2-ethynylpyridine) (Poly (2Epy)) and the nanocomposite obtained after intercalation of the bulk polymer into the Ca -montmorillonite. Analysis of preliminary solid-state NMR data suggests that the structure of the in-situ polymerized sample (Poly (2EPy)) is structurally similar to that of bulk polymerized sample (Poly (2Epy)). In¬ situ and bulk polymerized sample show different C NMR chemical shifts for various carbons mostly due to the variation in residual charge density on the pyridine nitrogen. 2+

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© 2004 American Chemical Society

Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction In the past few years there is resurgence of research activity in the area of polymer-clay nanocomposites even though the first reported literature data on these materials goes back to late 50's and early 60's by Blumstein (/). Today's fast growing activity in inorganic/organic nanocomposites arises from the realization that these materials show exceptional properties when compared to their macroscopic counterparts. Because of their exceptional and improved thermal, mechanical, and physical properties (2), these polymer-based nanocomposites are being considered for a wide range of applications in electronics and automobile industries. Formation of lamellar polymer-clay nanocomposites had opened new avenues for polymer research in a number of areas (3,4) Inorganic layered materials possess ordered interlayer spacings potentially accessible for polymer chains to occupy this space resulting in nanocomposite materials (Figure la). The clays used in the formation of polymer nanocomposites are aluminosilicates with a sheet-like structures consisting of silica S i 0 tetrahedra bonded to alumina A 1 0 octahedra. In the case of naturally occurring clay, montmorillonite ( M M T ) , the ratio of tetrahedra to octahedra is 2:1. The thickness of these sheet like layered structures consisting of tetrahedra and octahedra structures are in the order of 1 nm and the aspect ratios typically are in the range of 100-1500. These sheets and their edges bear a charge on their surfaces depending on the chemical composition. The charge on the surface is normally balanced by the counter ions. These counter ions normally reside in the gallery space between the layered sheets. The gallery space between the silicate layers is normally ca. 1-2 nm. These are truly nanoparticultaes. The molecular interaction leading to nano length scale structures provides generally two classes of nanomaterial in the form of intercalated or exfoliated structures (2). In the case of intercalated structures, the polymer chains are inserted between layers of ordered multilayered aluminosilicates such that gallery space is expanded to accommodate the presence of polymer chains in way that the layers still maintain well-defined spatial relationship to each other (Figure lb). In the case of exfoliated structures, the layers do not maintain their order among themselves; the layers are separated as well as dispersed throughout the polymer matrix (Figure lc). 4

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Our recent interest is, however, on the in-situ intercalative polymerization of pyridine substituted acetylenic monomer (2-ethynylpyridine (2EPy)) within the galleries of Ca -montmorillonite ( C a - M M T ) (5). In this process, it is believed that monomer first intercalates into the galleries of montmorillonite, and then the interaction with clay surface initiates the polymerization (5). The X-ray 2+

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Figure 1: Schematic representation of layered structure of (a) clay (montmorillonite, MMT), (b) intercalated, and (c) exfoliated polymer nanocomposites (not to scale).

diffraction data shows a corresponding increase in the d-spacing with adsorption yield, and a trans-transoidal conformational model of the pyridine ring inside the clay lamella is proposed (J). The nature of polymer-clay interaction so far has not been very clear. Recent work using multi nuclei solid-state NMR techniques provides information regarding the structure and dynamics of the polymer chains and the nature of interaction of clay surface with the polymer chains (6,7). To the best of our knowledge, a limited literature deals with the structure of the intercalated substituted imine polyacetylene polymer and interaction of the clay surface with the polymer chains in the in-situ intercalative polymerization (J). The goal of this preliminary work is to investigate the nanocomposites formed as a result of in-situ intercalative polymerization of 2-ethynylpyridine by solid-state C CP/MAS NMR and X-ray diffraction techniques. I3

Experimental 2+

Materials: Ca -Montmorillonite (Ca-MMT) was purchased from Clay Minerals Repository with cation exchange capacity of 1.20 mequiv/g. 2-ethynylpyridine (2EPy) was purchased from Farchan Laboratory and used as received. Synthesis of Poly (2-ethynylpyridine)/Clay nanocomposites: 0.395 g of 2EPy was added to a suspension of 1.0 g of driedCa -MMT in 25 ml of benzene and 2+


297 then mixture was stirred at 65±3 °C for 24 hrs. The dark brown precipitate was separated from suspension by eentrifugation and washed several times with benzene to exclude the monomer trapped between the aggregates of clay particles and finally dried at 110 °C for 24 hrs. The adsorption yield of the monomer per unit weight of dried Ca -MMT was obtained by measuring the concentration in the benzene solution by UV/Vis absorption spectroscopy. The chemical synthesis of bulk-P2EPy, poly (2-ethnylpyridinium hydrochloride) was described elsewhere (8,9) Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 12, 2017 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch022

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N M R Spectroscopy: Solid-state C NMR experiments were carried out using a dedicated Bruker D M X 300 MHz NMR instrument. Zirconium oxide (Zr0 ) 4mm rotors were used with Kel-F caps for all the measurements. Crosspolarization with magic angle spinning (CP/MAS) and high power dipolar decoupling techniques were used to study these materials. All spectra were recorded using a rotor spinning speed of 10 kHz. The typical parameters for C CP/MAS NMR experiments were as follows: spin lock and decoupling field of 70 kHz, a 3 s recycle time, a contact time of 1 ms and a sweep width of 31 kHz. The total number of free induction decays (FIDs) co-added per spectrum ranged from -10,000 for bulk polymer to 160,000 for intercalated polymer-clay complexes. All the FIDs were processed by an exponential apodization function and with a line broadening of 20-50 Hz. The C CP/MAS NMR spectra were externally referenced to glycine by assigning the carbonyl signal at 176.03 ppm with respect to tetramethylsilane (TMS). 2

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Figure 2. Graph showing an increase in the basal d-spacing (XRD) of in-situ polymerized nanocomposites NnCl, NnC2, and NnC3, containing 8,15.3 and 21.3% of polymers, respectively.


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Results and Discussion 2+

The in-situ polymerization of 2EPy in the lamellar galleries of Ca -MMT resulted an increase in the d-spacing of Ca -MMT, as measured by X-ray diffraction at different adsorption yield (τ) (Figure 2). The increased d-spacing of Ca -MMT indicates the successful intercalation of 2EPy into the lamellar galleries. The d-spacing increased from 1.49 nm at low adsorption yield (8.0%) to 1.64 nm at an adsorption yield of 21.3%. XRD data suggests that polymers are formed in the gallery space between the aluminosilicate layers maintaining a well-defined spatial relationship to each other. 2+


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The solid-state C CP/MAS NMR spectrum of a nanocomposite, Poly (2Epy) / Ca -MMT with an adsorption yield of 21% is shown in Figure 3 together with the chemical shift values. The solution-state °C NMR spectrum of the monomer (2EPy) is shown in Figure 4. Comparing the NMR spectra of the monomer and Poly (2Epy)/Ca -MMT nanocomposites, it is clear that the peaks of the acetylenic carbons (82.9 and 77.7 ppm resonances in Figure 4) are not present in the solid-state NMR spectra shown in Figure 3. As there are no contributions from montmorillonite, the C NMR spectrum of the nanocomposite is a characteristic of the adsorbed, intercalated (based on XRD data), and polymerized species within the aluminosilicate matrix. The disappearance of the resonances for the acetylenic carbons and the appearance of broad distribution of resonances in the aromatic/olefînic region suggest the spontaneous in-situ intercalative polymerization of the 2EPy. The structural information obtained from solid-state NMR and an increased d-spacing (from Xray diffraction) suggests a spontaneous in-situ intercalative polymerization of 2EPy between lamella without any pre-treatment of MMT and without use of external initiator. Significantly the MMT not only acts as a matrix for the preparation of nanocomposites, but acts also as a solid acid catalyst for the polymerization. Resonances due to the ethylenic carbons in the Poly (2Epy) backbone are overlapped with the aromatic ring carbon of the pyridine ring, giving a broad resonance extendingfrom75 to 250 ppm (Figure 3). The tentative assignments of various resonances in Figure 3 are as follows: the most intense -125 ppm peak corresponds to the ethylenic-CH (C7) as well as the C2 (β) and C4 (β') carbons of the pyridine ring (Figure 5). The peak at 138.5 ppm is assigned to the ethylenic C (C6) and C3 (γ) carbons of the pyridine ring, and the most down field peak at 148 ppm is due to CI (a) and C5 (α'), both being bonded to the nitrogen atom. These assignments are consistent with the chemical shift values of the monomer spectrum (Figure 4) and agree with the work ofMaciel et al (10). The C CP/MAS NMR data were also collected (not shown) on nanocomposites containing varying amount of polymers (adsorption yield). As the adsorption yield increased the relative intensity of the 125.5 ppm peak increased compared to the 147.5 ppm peak. At the low adsorption yield of 8.0%, the solid-state C CP/MAS NMR spectra are broad and not well resolved. The strong peak in Figure 3 appears at 125.5 ppm, with a weak shoulder at 147.5 2+

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Figure 3. Solid-state C CP/MAS NMR spectrum of in-situ polymerized polymer-clay nanocomposite, Poly (2Epy)/Ca *-MMT, with an adsorption yield of 21.3% ofpolymer. 2

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Formation of Nanocomposites by In Situ Intercalative Polymerization of

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