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Dynamics in polymer brush-nanoparticle systems

Final Report Summary - POLYBRUSH (Dynamics in polymer brush-nanoparticle systems)

Polymers are very long molecules composed of repeated units. These molecular chains are normally entangled, but it is possible under certain conditions to graft them by one end to a surface or interface so that, if the density is high enough, their mutual repulsion induces the chains to stretch. These materials are called polymer brushes and, apart from their scientific interest, they are very interesting from the technological point of view, because they have a responsive behavior, making them suitable for the design of smart materials.

The chains in polymer brushes can swell or collapse as a function of the environment such as variations on the temperature, pH, ionic strength, solvent in which they are immersed, etc. Since these two states have distinctive properties such as thickness variations, stiffness, chain entanglement, refractive index, or hydrophobicity, they can be used in the myriad of applications that benefit from switching these properties on and off. For instance, in bioscience or medical research they can be used to switch cell attachment to a surface, or to capture drugs within the brush and then release them inside the body only under certain conditions. They can also used as chemical switches by attaching particles with catalytic properties to the chains, which are then only available to the reaction solution when the brush is in its swollen state. In nanotechnology they can be used as nanovalves, blocking certain paths in their swollen state but allowing free circulation when they are collapsed, and in general they can also be used as sensors, for instance changing their color or thickness when traces of certain contaminants are in the water.

To develop all these technological applications it is essential that interactions between the brush and other particles are well understood. However, due to the complexity of the polymer brush-nanoparticle systems and their intrinsic small volumes, experimental measurements of their dynamics have been elusive so far. Recently developed techniques, such as grazing incidence neutron spin echo (GINSE) spectroscopy or surface sensitive dynamic light scattering (DLS) could allow to tackle this problem. Here we have focused in resonant enhanced dynamic light scattering (REDLS).

The main goal of this project was to obtain a polymer brush-nanoparticle model system and optimize it so that its dynamics could be measured as a function of the various geometrical parameters (brush chain length and density, as well as particle size and concentration) in order to determine their effect in these kind of composite systems.

The polymer chosen for the model system was poly(2-(dimethylamino)ethyl methacrylate)) (PDMAEMA), because a range of monodisperse brush heights (10- 100 nm) can be obtained at different chain grafting densities (0.03-0.5 nm−2). Therefore, one single type of brush can be used to investigate all the parameters of interest, and the role of the parameters can be singled out more easily than if completely different systems were needed to explore different parameter ranges. The polymer chains were in turn grafted onto silicon substrates.

Gold nanoparticles were chosen as the spherical colloids for the model system because they can be synthesized in a wide range of nanometric sizes, and because they give an intense signal in REDLS, increasing the chances of successfully measuring their dynamics. They are also very versatile, and have uses in countless technological applications.

Several series of polymer brushes with various densities and chain lengths were successfully synthesized and characterized in their collapsed and swollen states using different techniques: ellipsometry was used to determine their thickness and refractive index, atomic force microscopy (AFM) was used to investigate their surface, and neutron and x-ray reflectometries (NR and XRR) were used to explore their density profile perpendicular to the surface.

A number of gold nanoparticle syntheses, mostly yielding citrate-capped nanoparticles, were also carried out and adapted, in order to obtain optimized nanoparticle batches that would be suitable for the demanding dynamics experiments. Citrate molecules, which are negatively charged, attach to the gold surface of the nanoparticles and stabilize them in the solution thanks to their repulsive interaction. Since PDMAEMA brushes are positively charged, using negatively charged particles for the polymer brush-gold nanoparticle composites enhances their incorporation into the brush. Additionally, citrate-capped gold nanoparticles are soluble in water, which is the solvent of choice to ensure compatibility with biotechnological, medical, and environmental applications. The particles were then characterized using ultraviolet-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), small angle x-ray scattering (SAXS) and dynamic light scattering (DLS) to determine their size distribution and possible aggregation.

Dialysis, filtration, and/or rinsing attempts on citrate-capped nanoparticle solutions proved to be quite unsuccessful, with changes in particle size, significant aggregation, and even second nucleations taking place. Therefore, brushes were immersed in the nanoparticle solution directly obtained from the synthesis to obtain the composites. The series of composites obtained in this manner were characterized using AFM, NR and XRR. These composites showed a high degree of complexity, even presenting domains at intermediate and low densities.

Preliminary dynamics experiments with the aforementioned brush and nanoparticle samples showed that the signal obtained with REDLS was too weak to carry out the measurements, and that the model system had to be optimized further. To improve surface homogeneity and, thus, its REDLS signal, a special device and a cell were designed to allow the substrates to sit horizontally facing downwards within the polymerization and nanoparticle solutions during all steps of sample preparation. This prevented gradients that appeared in the samples when they were kept submerged in a vertical position within the polymerization or nanoparticle solutions, as well as aggregates or small lumps that tended to form on the brush surface when the sample was kept horizontally within the solutions during preparation, but facing upwards.

Additionally, the nanoparticle solution had to be of a higher concentration, free of any kind of aggregates, and its size distribution as monodispersed as possible. However, citrate molecules proved to bind only weakly to the gold surface, which was resulting in quite unstable nanoparticles: whenever the free citrate was removed from the nanoparticle solution, citrate molecules protecting the nanoparticle surface redissolved into the solution leaving them unprotected. As a consequence, the integrity of these kind of particles is compromised, they cannot be reliably concentrated nor rinsed from the synthesis by-products, and display a propensity to aggregate.

To solve these problems, the citrate capping of the gold nanoparticles was substituted by an insulin capping, and then the nanoparticles were tested for stability. Since insulin has sulfur atoms that bind very strongly to gold, this capping is able to stabilize gold nanoparticles even in harsh conditions. This allows all kinds of nanoparticle manipulations: they can be completely rinsed from the synthesis by-products obtaining a pure solution of nanoparticles in water, and can also be concentrated to an extremely high degree without aggregation. Additionally, they are still soluble in water and even better suited for biomedical applications than citrate-capped nanoparticles. Such insulin-capped nanoparticles were characterized using UV-Vis, TEM, SAXS and DLS yielding extremely satisfactory results.

Composites were also successfully synthesized with insulin-capped gold nanoparticles and characterized using AFM, NR and XRR, yielding very different results than composites prepared with citrate-capped gold nanoparticles. Although some further improvement would be advisable before undertaking the systematic study of the dynamics of these samples with REDLS, they showed already to be very promising candidates on their preliminary experiments, with a significant increase on the intensity of their signal. Among other strategies, this further improvement could be achieved through the synthesis of nanoparticles with a narrower size distribution, or synthesizing the polymer brushes directly onto a gold substrate through an alternative procedure that initiates these surfaces for polymerization.

This work has allowed to successfully overcome many of the challenging obstacles that had prevented REDLS dynamics measurements of polymer brush-nanoparticle composites until now, and hence has opened the door to perform such measurements systematically as a function of geometrical parameters, which will help understand the interactions between nanoparticles and polymer brushes and optimize their design for all kinds of future applications.