Langmuir 2008, 24, 5299-5305
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Cluster Formation and Rheology of Photoreactive Nanoparticle Dispersions Xiaofeng Yuan,‡,† Matthias Schnell,‡ Sandra Muth,‡ and Wolfgang Scha¨rtl*,‡ Institut fu¨r Physikalische Chemie, Johannes-Gutenberg-UniVersita¨t Mainz, Germany ReceiVed January 7, 2008. ReVised Manuscript ReceiVed February 21, 2008. In Final Form: February 21, 2008 We show how photocrosslinking of small nanoparticles within a very dilute colloidal dispersion leads to the formation of large fractal particle clusters, which have a strong influence on the viscosity of the dispersion although the overall solid content is well below 5 wt %. Furthermore, the solvent plays an important role because of its function as an optical filter, for example, in toluene only photocrosslinking but no photocleavage takes place. Therefore, a diffusion-controlled cluster growth mechanism, leading to clusters with low fractal dimension, is expected; on the other hand, in tetrahydrofuran the photoreaction is partially reversible. Therefore, the cluster growth in this case is reaction controlled, leading to more compact clusters with higher fractal dimension, which therefore only have a negligible effect on the rheological properties of the solvent. In this context, we will briefly discuss the possibility to use our nanoparticle system as opto-rheological switch.
1. Introduction The formation of aggregates in colloidal nanoparticle dispersions as a result of interparticle attraction, and the size and structure of the resulting clusters play a major role for technical applications, like the consistence of cement and concrete upon drying.1 On the other hand, controlling this aggregate formation by external stimuli may provide the means for rheological switches, for example optical control of the viscosity of photoreactive polymer solutions.2 Finally, understanding and manipulating the mechanism of cluster growth itself enables to one build well-defined supramolecular structures in terms of size, shape and porosity. This later possibility has to be considered with care because the details of cluster formation of nanoparticles in solution, starting from nonspherical particles and/or interplay of complex attractive and repulsive interparticle interactions, are not yet fully understood and the topic of ongoing research. Here, one has to note that in most cases, clusters formed are highly polydisperse in size and not very well defined in shape. Theoretical models, partially supported by computer simulations, have been developed to better understand the process of aggregation in particle dispersions; in case of perfectly irreversible aggregation, that is, the particles will stick to the growing cluster after contact, the process is controlled by diffusion alone. This so-called diffusion-limited cluster aggregation (DLCA) leads to aggregates with fractal dimension df < 2.0, compared to df ) 3.0 for compact three-dimensional objects. The clusters formed by DLCA in solution are rather flexible objects of low density, containing a large amount of solvent. Some simulations have given fractal dimensions as low as 1.25-1.5, values that have been confirmed by fitting the DLCA model to experimental results from hematite colloidal particles.3 On the other hand, if the aggregation process is partially reversible, particles loosely stuck to a growing cluster may rearrange their position until they reach * Corresponding author. E-mail:
[email protected]. † Johannes-Gutenberg-Universita¨t. ‡ Current address: Bayer Polyurethanes (Shanghai) Co., Ltd., Caojing, Shanghai, P.R. China. (1) Scherer, G. W. Cem. Concr. Res. 1999, 29, 1149–1157. (2) Pieroni, O.; Fissi, A.; Ciardelli, F. React. Funct. Polym. 1995, 26, 185– 199. (3) Gardner, K. H.; Theis, T. L.; Young, T. C. Coll. Surf., A 1998, 141, 237.
multiple contacts to neighboring particles. This so-called reactionlimited cluster aggregation mechanism (RLCA) consequently leads to a more compact aggregate with higher fractal dimension the order of df ) 2.3-2.5.4,5 It should be noted that in experimental practice, the structural difference between nanoparticle clusters formed by reversible or irreversible aggregation, respectively, can easily be detected by scattering methods, like X-ray scattering or static light scattering.5,6 Solving the RLCA as a kinetic model with different rate constants for the aggregation and disintegration process, Vorkapic and Matsoukas have been able to determine a scaling law predicting the final average cluster size in dependence of particle mass concentration and fractal dimension of the clusters.7 The authors confirmed their theoretical approach with the experimental example of titania nanocolloids. In general, clusters obtained via the RLCA mechanism seem to be more defined in terms of size and shape compared to DLCA clusters, which have also been found in computer simulations of sticky spheres under shear in which case the shear forces destabilize the growing clusters and therefore introduce the partially reversible disintegration component.8 An interesting experimental example for controlling the partially reversible aggregation process by external stimuli are spherical nanoparticles whose surface has been modified with DNA. Upon addition of the complementary RNA in aqueous solution, these particles will aggregate, whereas an increase in temperature leads to disintegration of the interparticle bonds.9 Alternatively, aggregation and disintegration could be switched by light, a stimulus that is easier to handle both in respect to time- and length-scale compared to temperature. As an example, Pieroni and co-workers have developed a system of photoreactive polymer chains that show a photoinduced change in structure, ranging from a coil-helix-transition to reversible aggregation.2 As a consequence of these structural changes, they found changes in solubility as well as in solution viscosity, thereby establishing (4) Terao, T.; Nakayama, T. Phys. ReV. E 1998, 58, 3490. (5) Terao, T.; Nakayama, T. J. Phys. Cond. Mat. 1999, 11, 7071. (6) Carpineti, M.; Giglio, M. Phys. ReV. Lett. 1993, 70, 3828LP–3831. (7) Vorkapic, D.; Matsoukas, T. J. Colloid Interface Sci. 1999, 214, 283–291. (8) Chen, D. H.; Doi, M. J. Colloid Interface Sci. 1999, 212, 286. (9) Ihara, T.; Kurohara, K.; Jyo, A. Chem. Lett. 1999, 10, 1041.
10.1021/la800043j CCC: $40.75 2008 American Chemical Society Published on Web 04/24/2008
5300 Langmuir, Vol. 24, No. 10, 2008 Scheme 1. Sketch of the Photocrosslinking of Photoreactive Nanoparticles
Scheme 2. Reversible 2 + 2 Photoaddition of Nitrocinnamatea
a Note that on average about 150 of these photoreactive dye molecules are chemically attached to a single polyorganosiloxane nanoparticle via an ester bond as indicated.
an optomechanical switch. Whereas this example is based on a cis-trans-transition of the chromophore that causes a change in solubility of the individual polymer chains, Chen and co-workers have developed a different approach: they employed a reversible 2 + 2 cycloaddition shown by certain chromophores, for example Coumarin or fullerene (C60), to photocrosslink polymer chains. Through the use of radiation of shorter wavelength, these crosslinks can be cleaved, leading to disintegration of the polymer network.10–12 In contrast to these studies, which are related to the photocrosslinking of polymer chains, to our knowledge we have been the first to investigate the optically controlled reversible cluster growth of spherical nanocolloids.13 As described in Schemes 1 and 2, our system of photoreactive spherical nanoparticles labeled with the chromophore nitrocinnamate undergo photochemical interparticle cross-linking if irradiated with light of wavelength >290 nm. Importantly, if the resulting cluster solution is irradiated with light of shorter wavelength (250 nm), the chemically crosslinked nanoparticle aggregates will disintegrate by photocleavage. The size of the growing clusters could be monitored by dynamic light scattering, by removing the sample, a 1 cm cylindrical light scattering cuvette of Suprasil glass containing a dilute solution of the photoreactive nanoparticles, from the irradiation setup after defined time periods. At first, the appropriate irradiation wavelengths for photocrosslinking or photocleavage, respectively, have been selected from the broad emission spectrum of our high pressure 200 W Hg/Xe lamp by suitable optical filters. This type of irradiation setup led to a dramatic loss in light intensity and consequently to cluster growth time-scales in the order of hours. Lately, we have found that the solvent of our photoreactive nanoparticle dispersion itself may play the role of an optical filter in a much better way. Through the use of the whole spectrum of our 200 W HgXe-lamp, that is, not employing any additional optical filters, to irradiate a toluene solution of the photoreactive nanoparticles, very large clusters >100 nm have been obtained after an irradiation time of a few minutes only. On the other hand, irradiating a tetrahydrofuran (THF) solution of the nanoparticles we found an even quicker cluster growth followed by a decrease in cluster size. This was interpreted according to the optical filter properties of the two different solvents, (10) Chen, Y.; Geh, J. L. Polymer 1996, 37, 4481–4486. (11) Chen, Y.; Jean, C. S. J. Appl. Polym. Sci. 1997, 64, 1759–1768. (12) Chen, Y.; Tsai, C. H. J. Appl. Polym. Sci. 1998, 70, 605–611. (13) Yuan, X.; Fischer, K.; Scha¨rtl, W. AdV. Funct. Mater. 2004, 14, 457.
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respectively. Toluene blocks all light of wavelengths
