Multiscale gold and silver plasmonic plastics by melt compounding

Abstract

This work presents a strategy to obtain plasmonic plastics by conventional large scale polymer processing methods using metal nanoparticles (Au and Ag) supported on sepiolite fibers acting as carriers. Two conventional polymers, polyethylene and polystyrene, have been used as matrices, and composites were prepared up to high inorganic contents. The resulting composites exhibited at all loadings the corresponding optical absorption plasmon bands ascribed to the metal nanoparticles. In addition the sepiolite fibers acting as carriers remarkably improved the thermal stability and produced mechanical reinforcement of the polymer matrices as well as they appear invisible due to the index matching with the matrix. Therefore, highly transparent and robust plasmonic plastics can be easily prepared by industrially scalable processing techniques.

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Introduction

Among the most interesting new phenomena arising from the nanosizing of materials are those derived from specific interactions between electromagnetic radiation and nanoparticles, as for instance optical properties. Selected metallic nanoparticles (typically Au, Ag and Cu) present Surface Plasmon Resonance (SPR),¹ which classically manifests itself as an absorption band in the visible spectrum, originated by large electronic movements due to a notable increase of the local electric field at the metal/matrix interface.

The main interest of this effect is that the local field enhancement allows the observation of several non-linear processes with conventional lighting sources. However, to take full advantage from SPR-derived physical processes it is in many cases mandatory to embed the nanoparticles into a homogeneous and transparent matrix which will enable the handling of the material. The embedding process presents two main handicaps, first how to integrate the particles in the matrix without aggregating them and second, how far do matrix-nanoparticle interactions modify the effective properties of the composite.^2,3 Matrices for embedding can be inorganic, like glasses⁴ or silicas,^5,6 though integrating the nanoparticles in polymeric matrices has many practical advantages and broadens the application fields.^7,8

Bulk metal-polymer nanocomposites have been mainly prepared by wet chemical strategies,^7,8 which can be divided into: (i) ex situ methods based on casting from dispersions of organically stabilized metal nanoparticles in polymer solutions or in monomer mixtures to be afterwards polymerized^9–12 or (ii) in situ methods in which metal nanoparticles are synthesized in the midst of the polymer matrix by the spontaneous or induced reduction of the metallic precursor.^13–15 Both approaches rival in being the most suitable to obtain good metal-polymer composites, but far from being solved, the final composite materials still present serious drawbacks and inconveniences, which compromise their industrial scalability and thus, their use in real applications. The most severe flaws regarding the nanoparticle dispersion and the performance of the composite: lack of thickness control and reproducibility, limited portfolio of available polymer matrices (arising from the polymer solution step in the former approach, and the metal reduction in the latter) or the unavoidable presence of surfactant and solvent traces in the composites. Therefore, both methods only enable the production of specific combinations of polymer/nanoparticle systems in very small amounts and mostly in the form of thin and inhomogeneous films, which will show dramatic flaws in fundamental properties. A more advantageous melt compounding processing, which is the processing method most commonly used at industrial scale (kg to tonnes), to obtain metal nanocomposites is found in a couple of examples but the resulting materials present particle aggregation and/or a dramatic loss of polymeric matrix properties.^16,17

An unexplored approach for the preparation of these systems is the use of carrier-supported metal nanoparticles, easier to handle and disperse in polymeric media and with a similar refractive index so that the plasmonic composite displays the same transparency as the matrix.¹⁸ In this work, we report the preparation of easy-to-produce, easy-to-handle plasmonic plastics by melt compounding of a polymer matrix and sepiolite-supported Au and Ag nanoparticles.^19,20Sepiolite is a needle-like phyllosilicate that can be finely dispersed in polymers by conventional techniques.²¹ The main highlights of this multiscale approach is that metal nanoparticles are immobilized on the carrier what precludes metal nanoparticle aggregation and avoids health concerns on nanosized materials. Following this procedure, transparent plasmonic materials with polymer-like mechanical and thermal properties are produced in large scale.

Experimental section

Materials

Sepiolite, provided by TOLSA, S.A (Spain), was treated following a previously reported method consisting basically on precipitation of metal salts on acid treated sepiolite and a subsequent thermal reduction.^19,20 The temperature treatment differs for both products, being 200 °C in the case of Au and 500 °C for Ag. The experimental conditions provide sepiolites with metal contents of 8 and 26 wt.% for Au and Ag, respectively. The polymers: low density polyethylene (LDPE, Alcudia® 003 from Repsol) and polystyrene (PS, 143E from BASF) were used as received.

Preparation of the composites

PS and LDPE sepiolite composites (Au and Ag-sepiolite contents varying from 2 to 40 wt.%) were prepared in a Haake MiniLab extruder. The appropriate quantities of polymer and sepiolite (total capacity is 5.5 g) were directly introduced into the machine. The processing temperature, shear rate and residence time for LDPE based materials were 140 °C, 80 rpm, and 20 min, respectively, whereas for PS composites these experimental parameters were 160 °C, 120 rpm and 20 min. In the case of PS composites, the compounding was subjected to a second 10 min-extrusion step at the same temperature and shear rate. The composites were afterwards compression moulded.

Characterization. Morphology and dispersion of the sepiolite fibers in the polymer matrices were evaluated by transmission electron microscopy (TEM, Phillips TECNAI 20).

The mechanical performance of the composites was tested by stress-strain measurements performed in an Instron 3366 with a 100-N load cell. The results given are the mean values of at least seven mechanical tests. Thermogravimetric Analysis (TGA) of the composites has been carried out under air atmosphere in a TA Q-500 applying a Hi-Res Dynamic method in which the initial ramp temperature, resolution and sensitivity in this method were set to 10 °C min⁻¹, 4, and 1, respectively. Finally, the absorption spectra were recorded on composite films by using a Varian Cary 3-Bio.

Results and discussion

This paper aims to show the feasibility of fabricating transparent (tinted) and mechanically stable polymer/metal nanocomposites following the multiscale approach graphically shown in Fig. 1.

Fig. 1 Preparation procedure to fabricate plasmonic plastics.

To exemplify this general strategy, two conventional thermoplastic polymer matrices were chosen, polystyrene (PS) and low-density polyethylene (LDPE), though the list of other candidates includes the very numerous family of polymers that can be melt compounded. The supported metallic nanoparticles selected have been Au and Ag–sepiolites, which have been used without ulterior organic surface modification.

TEM images of the Au and Ag–sepiolites are included in Fig. 2a and b, respectively. The macroscopic appearance featured by the intense and characteristic colors promoted by the metal nanoparticles is shown in the figures insets. Au–sepiolite retains the fiber structure of raw sepiolite (individual fibers can be longer than 1 μm). The size distribution of Au nanoparticles is remarkably narrow (8–10 nm) and these are evenly dispersed along the fiber. On the basis of previous works, the metal nanoparticles are mostly located inside the fiber structural channels and therefore protected by the inorganic coverage.²² In contrast, the Ag–sepiolite consists on small fiber aggregates and individual fibers with lower aspect ratio. The nanoparticle sizes are more heterogeneous (ranging from 2 to 20 nm) and some nanoparticles seem to be on the sepiolite surface and therefore, more exposed to the external medium. It must be noted that Ag–sepiolite underwent a severe thermal treatment (500 °C) while sepiolite crystalline structure starts to collapse above 350 °C leading to the fiber breaking and the formation of amorphous silica.²³ Thus, the different thermal treatments are responsible for the changes in aspect ratio and crystalline structure of sepiolite microparticles.^19,20

Fig. 2 TEM images of sepiolite fibers with supported Au (a) and Ag (b) nanoparticles. The insets show the optical appearance of these products.

These changes in the sepiolite structure result in significant differences in the dispersion ability of Au and Ag–sepiolites. Table 1 collects the metal-sepiolite composites prepared by extrusion with PS and LDPE matrices and their Au or Ag–sepiolite contents.

Table 1 Samples details: Au or Ag sepiolite content, TGA analysis: temperatures of 5% weight loss (T_5%) and 50% weight loss (T_50%), and mechanical properties: Young modulus (E) and elongation at break (ε_B)

Sample	Sepiolite wt.%	Thermal Stability		Mechanical Performance
		T 5%/°C	T 50%/°C	E/MPa	ε B/%
LDPE	—	302	358	242	507
LDPE–Ag–2	2	306	376	—	—
LDPE–Ag–5	5	333	387	303	331
LDPE–Ag–10	10	353	396	323	183
LDPE–Ag–20	20	370	402	422	44
LDPE–Ag–40	40	331	408	615	16
LDPE–Au–2	2	336	401	261	518
LDPE–Au–10	10	339	414	338	80
LDPE–Au–20	20	347	433	486	24
PS	—	293	315	2530	1.3
PS–Ag–2	2	288	314	—	—
PS–Ag–5	5	293	352	2900	1.3
PS–Ag–10	10	295	350	3070	0.8
PS–Ag–20	20	322	371	3296	0.6
PS–Au–2	2	290	322	—	—
PS–Au–10	10	307	348	3250	0.6
PS–Au–20	20	338	362	—	—

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Au–sepiolite disperses better in the polymer matrices, although both sepiolites gave rise to homogeneous composites even at high inorganic loadings. Fig. 3a collects two series of Au and Ag–sepiolite composites. All films look homogeneous and their color and transparency depend on the content and nature of the metallic sepiolite. For the same sepiolite load, the Au composites are more transparent mainly due to the lower metal content. Nevertheless, the light transmission in both series is surprisingly high considering the high inorganic loading. Regarding polymer matrices, slightly better dispersions are obtained with PS compared to LDPE, probably due to the higher shear rate applied to PS extrusion.

Fig. 3 Image of two series of 45 μm thick composites films (a) and TEM micrographs of representative composites (b and c).

In any case, sepiolite is fairly dispersed in all the mixtures, as depicted in the representative TEM images in Fig. 3b and c. Au–sepiolite fibers appear as larger and better dispersed than Ag–sepiolite fibers; though an additional fiber aspect ratio reduction by microtome cuts for TEM imaging cannot be ruled out.

Though sepiolite dispersion can probably be improved,²¹ that obtained with these unsophisticated experimental conditions is enough for our purpose. The thermal and mechanical properties in Table 1 show that even the most concentrated composites perform like thermoplastics. This is illustrated in Fig. 4a for 10 wt.% composites. The picture highlights the transparency and folding ability of the films, which would be inconceivable for composites exhibiting low quality dispersions or poor mechanical performance. Moreover, sepiolite addition produces an outstanding thermal stabilization (over 40 °C higher T_5%), as seen in Table 1 and Fig. 4b, which collects the TGA curves for a LDPE composites series. In general, the stabilizing effect is more pronounced in Au–composites, probably as a consequence of the fibers' larger aspect ratio and their better dispersion in the matrix, since both factors enable the formation of an efficient protective layer during the oxidation process.²⁴

Fig. 4 A photograph illustrating the mechanical performance of 45 μm thick films made from LDPE–Ag–10 and PS–Au–10 composites (a). TGA curves for the LDPE–Au composites series (b). Absorption spectra recorded on 45 μm thick films (except for PS–Ag–20 film which is 30 μm thick due to saturation at higher thickness) from 20 wt.% Au and Ag–sepiolite composites.

Regarding mechanical performance, the stress-strain Young Modulus increases continuously with the sepiolite content (Table 1), being maximum (a 2.5-fold increase) for the most concentrated LDPE composites. Reinforcement is higher in Au–composites mainly because of the fibers' larger aspect ratio and better dispersion. The lower metal content in Au–composites cannot be ruled out as an additional reason for better mechanical properties. Both composites series do not dramatically loose elastic properties as reflects the maximal elongation at break.

All these composites present intense colouring revealing the SPRs and high transparency. Because of refractive index matching between the silicate and the polymers, sepiolite is virtually invisible when embedded into the selected polymers,²¹ while Au and Ag nanoparticles display well developed SPR absorption bands in all the composites. This is presented in Fig. 4c which includes absorption spectra for 20 wt.% Au and Ag–sepiolite composites (45 μm thick films except for PS–Au–20 the spectrum of which were recorded on a 30-μm thick film since higher thickness led to the saturation of the SPR absorption band). Regardless of the sepiolite nature and content, and of the polymer matrix, similar spectra are recorded for the whole series of prepared materials. Neither the maxima location nor the SPR structure varies with optical path length; as composite films of varied thickness only exhibited differences in absorption intensity. The band intensity is higher for Ag–composites due to the higher metal content and the absorption maxima depends mainly on the metal nature (around 535 and 440 nm for Au and Ag containing composites, respectively) and only slightly on the polymer matrix and metal–sepiolite content.

Once assumed that the nanoparticle size and shape remain unaltered by the composite preparation procedure, the differences observed in the recorded spectra can be fully explained on the basis of aggregation state and the polymers' dielectric properties.^2,25–29

SPR red-shifts with increasing of the medium refractive index (n).^26,27 Thus, LDPE (n = 1.51) renders composites with SPR at lower wavelengths compared to those based on PS (n = 1.59). There is a stronger dependence of the polymer matrix on the plasmon location for Ag–composites in comparison with Au–composites. The same was detected in UV spectra recorded on suspensions of both metal sepiolites in different solvents. This means that sepiolite supported Ag–nanoparticles can reflect the surrounding media more conspicuously than Au–sepiolite. This fact must be related to the TEM observation suggesting Ag–nanoparticles partly to lie on the external surface of the sepiolite fibers, and being then in direct contact with the dispersing media.

Both decreasing the distance between nanoparticles and particle aggregation lead to an increase of low energy absorption.²⁵ This explains the smaller intensity and the SPR broadening at longer wavelengths in LDPE composites compared to PS ones, where the sepiolite fibers and consequently the metal nanoparticles are better dispersed.

The ultimate purpose of these Au and Ag plasmonic plastics is the development of flexible devices aimed at the fabrication of, for instance, magneto-optical sensors, where bulk plasmonic materials are a must. An approach to the development of materials with realistic application potential requires not only the existence of the surface plasmon, but also the production of a transparent, robust material with polymer-like mechanical properties and a sufficient thermal stability. The multiscale procedure here reported is a simple and industrially scalable solution. Being protected by sepiolite and the polymer matrix, the nanoparticles are less keen to aging, mechanical or chemical, and we have checked that the plasmon absorption is stable for long periods of time, without special storage conditions.

All these considerations indicate that these composites do really fulfil the basic requisites for developing certain magneto-optical based devices, as is, currently, being checked with appealing perspectives.

Conclusions

Transparent plasmonic multiscale nanocomposites based on common thermoplastic polymers can be obtained by conventional industrial polymer processing methods using supported metal nanoparticles (Au and Ag) on sepiolite fibers. Regardless of the metal sepiolite content, all the composites exhibit plasmon bands the maxima of which vary with the metal nature and the polymeric matrix. Sepiolite not only behaves as an excellent metal nanoparticle carrier, impeding their aggregation, but also acts as a transparent component in the final material. In addition, the resulting composites are mechanically reinforced and thermally stabilized compared to the neat polymer matrices as a consequence of the addition of the silicate. Therefore, this multiscale approach enables for the first time the large scale preparation of wide composition range bulk metal polymer composites which can be shaped by injection or compression molding, making plasmonic plastics eligible for unprecedented applications.

Acknowledgements

Financial support from the Spanish Ministry of Science and Innovation (MAT2008-06725-C03-01) is acknowledged. The authors are in debt to Jesús González (CAT-URJC) for their valuable work with the TEM imaging.

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