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Confinement Enhances Dispersion in Nanoparticle–Polymer Blend Films

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Confinement Enhances Dispersion in Nanoparticle–Polymer Blend Films ARTICLE Received 14 Nov 2013 | Accepted 19 Mar 2014 | Published 8 May 2014 DOI: 10.1038/ncomms4697 Confinement enhances dispersion in nanoparticle–polymer blend films Sivasurender Chandran1, Nafisa Begam1, Venkat Padmanabhan2 & J.K. Basu1 Polymer nanocomposites constitute an important class of materials whose properties depend on the state of dispersion of the nanoparticles in the polymer matrix. Here we report the first observations of confinement-induced enhancement of dispersion in nanoparticle–polymer blend films. Systematic variation in the dispersion of nanoparticles with confinement for various compositions and matrix polymer chain dimensions has been observed. For fixed composition, strong reduction in glass transition temperature, Tg, is observed with decreasing blend-film thickness. The enhanced dispersion occurs without altering the polymer–particle interactions and seems to be driven by enhanced matrix-chain orientation propensity and a tendency to minimize the density gradients within the matrix. This implies the existence of two different mechanisms in polymer nanocomposites, which determines their state of dispersion and glass transition. 1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India. 2 Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, India. Correspondence and requests for materials should be addressed to J.B. (email: [email protected]). NATURE COMMUNICATIONS | 5:3697 | DOI: 10.1038/ncomms4697 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4697 lends of polymers and nanoparticles—polymer nanocom- do not get significantly altered with confinement. This clearly posites (PNC)—constitute a new class of materials exhibit- implies the existence of two different mechanisms in Bing tunable and novel physical properties1–4. A major nanoparticle–polymer blends, which determines their dispersion obstacle in achieving the desired properties of the blends is their and glass transition, allowing for their independent control. state of dispersion. Extensive research has, therefore, been devoted to understand the morphological and dynamical phase diagram of such blends and the consequent role it plays in Results determining the physical properties of these materials5–13. On the X-ray reflectivity. Table 1 summarizes the properties of the basis of these studies, various strategies are therefore being various polymer-grafted nanoparticles (PGNPs) used in these devised to enhance the dispersibilty of various functional experiments described in this manuscript. The particles have been nanoparticles in PNCs5–15. It is believed4,8,11,13 that a wetting prepared using a method described in the methods section. They nanoparticle–polymer interface is required for homogeneous have been characterized by using various methods described in nanoparticle dispersion in bulk PNCs. For bulk PNCs it has the Supplementary Information (Supplementary Fig. 1). Thin also been well established5,6,8,11 that for dewetting grafted chain– films of PGNPs mixed with linear polystyrene (PS) at various volume fractions, f , were prepared for thicknesses 65, 45 and matrix chain interface, the Tg of nanoparticle–polymer blends p decreases with increasing fraction of the nanoparticles, whereas 20 nm. Further details about the various samples have been specified in Table 2. Figure 1 shows the Fresnel-normalized XR Tg for wetting interface increases or remains unchanged. This profiles of 65, 45 and 20 nm of PNC films with PGNPs of various implies that Tg and dispersion of nanoparticles cannot be tuned independently in PNCs, at least for single-component graft fp and f as defined in the respective panels and Table 2. The molecules. Recent studies, however, have also shown that Fresnel-normalized reflectivity allows clear visualization of the presence of modulations in the XR profile, which is indicative of dispersion and thermomechanical properties could be 6,20,26 controlled using multicomponent grafted polymer chains16.On density variations along the film thickness . For a homo- the other hand, very little is known or understood about geneous thin film on a substrate, the Fresnel-normalized the nature of dispersion of nanoparticles in thin polymer reflectivity consists simply of film thickness oscillations, that is, films5–7,9,10,12. This is a bit surprising since, a vast amount of Kiessig fringes of constant or decaying amplitude depending on research has been devoted to understanding of structure and the magnitude of surface roughness. However, presence of density properties of thin polymer films, especially those related to the inhomogeneity or a layer of different density along the film perturbation of chain conformation17–21 and glass transition due thickness superimposes an additional modulation on the Kiessig to confinement5–8,11,22–25. Although there is significant progress fringes with the maximum at a wave vector qz, which is inversely in understanding the chain conformation with confinement, proportional to the thickness of the layer. The thickness of these B some conflicting reports still exist18–20,26,27. Similarly, in the layers is 6 nm, which corresponds to approximately one diffuse case of glass transition of thin polymer films, contradictory layer of PGNPs. As can be seen in all the XR data for various 65 nm films in Fig. 1, larger amplitude modulations are present reports on finite size effects on Tg in polymer thin films have emerged5–8,22–24. If polymer chain conformations do get indicative of presence of strong density inhomogeneity in these perturbed due to confinement, does it affect the state of films. In Fig. 1b we also compare the XR data for 45 and 20 nm dispersion and other physical properties in thin films of PNCs? films with fp ¼ 1.2% and f ¼ 0.033. Clear reduction in amplitude To address this fundamental question, we have probed the of the modulation with decreasing film thickness, indicative of structure and internal morphology of polymer and PNC films of confinement-induced density homogenization, is evident. This variable thickness using a combination of X-ray reflectivity (XR) and real-space imaging techniques as well as molecular dynamics (MD) simulations. We observe confinement-induced enhanced Table 1 | Properties of PGNP. dispersion of otherwise bulk phase-segregated blends of nanoparticles and polymers. To gain insight into the mecha- Sample Rc Mg R r (Chains Re À 1 2 nism responsible for enhanced dispersion of nanoparticles at a (nm) (g mol ) (nm) per nm ) (nm) microscopic level, we used coarse-grained MD simulations, PST3k-Au 2.1 3,000 1.2 1.98 3.3 which has been widely used for PNCs4,13,28–30. We simulated PST53k-Au 1.5 53,000 5.5 1.34 7.0 uniformly grafted nanoparticles in a homopolymer matrix PGNPs, polymer-grafted nanoparticles. confined between two smooth surfaces. We find that as we Rc is the radius of the core (gold nanoparticles), Mg is the molecular weight of the grafting agent, decrease the distance between the two surfaces, L , the nano- S is the thickness of the grafting layer, s is the grafting density and Re is the effective radius of z the particles, that is, Re ¼ Rc þ S. particles tend to disperse more readily and there is a significant drop in nanoparticle density near the surfaces, as observed in the experiments, and a corresponding increase in polymer segment density. In addition, calculated bond orientational parameter for matrix polymer chains indicate considerable chain orientation at Table 2 | Specification of PGNP-PS composite films. the interfaces, as observed earlier for linear polymers in 31–34 confinement with a finite value of this parameter being Sample Mg Mm /p (%) Wp (%) f observed to persist, even in the bulk, for the thinnest films (kg mol À 1) (kg mol À 1) simulated. S3 3 90 0.75 10.1 0.033 On the basis of our results, we show the effect of confinement S4 3 90 1.2 16.1 0.033 on dispersibility, D, of nanoparticles as a function of size S5 3 90 3.0 40.2 0.033 A 3 20 1.2 16.1 0.15 asymmetry f and fraction of inclusion fp, which would alter the reported bulk phase behaviour4,9,11,13,15. As a consequence of the B 53 90 1.2 16.1 0.59 C 53 20 1.2 16.1 2.65 enhanced dispersion, we observe a reduction in Tg of such blends as compared with that in the bulk at comparable nanoparticle PGNPs, polymer-grafted nanoparticles; PS, polystyrene. volume fractions. Interestingly, simulations indicate that the Mg and Mm are molecular weights of the grafting and matrix polymers, respectively; fp and Wp are the volume and weight fraction of PGNPs, respectively; size asymmetry, f ¼ Mg/Mm. nanoparticle–polymer interactions responsible for Tg reduction 2 NATURE COMMUNICATIONS | 5:3697 | DOI: 10.1038/ncomms4697 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4697 ARTICLE trend of density homogenization with confinement extends to electron density depth profile (EDP) from respective XR data. lower (Fig. 1a) and higher (Fig. 1b) fp and higher f (Fig. 1d). However, to quantify these observations accurately, we extracted However, as f increases further, the dispersion of the PGNPs in PS the respective EDP from all the XR data using detailed modelling films increases, leading to increased homogenization of density and analysis. The corresponding extracted EDPs for the XR data for such films as can be seen in Supplementary Fig. 2. These are also shown in the insets in Fig. 1. Also, refer Supplementary observations are independent of the model used to extract the Note 1 and Supplementary Figs 3–7 for the details of the models used, fitted profiles and the extracted EDPs. The obtained EDPs clearly bring out the features of the distribution of PGNPs in the a respective films. The EDP of 65 nm films shows clear indication S3-65 of presence of high-density layers of PGNPs at the film–substrate 0.6 ) S3-20 interface, which gives rise to the modulations observed in the XR –3 data.
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