A trithiocarbonate moiety was chosen because it can acts as chain transfer agent in a RAFT polymerization and therefore gives rise to block copolymers.14 On the other hand the thiol-group can be used for further reactions e.g. via thiol-ene chemistry. Its reduced reactivity towards nucleophiles might be a criterium for several other applications.165 From primary DLS and transmission electron microscopy (TEM) analysis, the successful anchoring of both TTC and thiol functional polyethylene onto gold and silver surfaces was confirmed. Since the synthesis of TTC terminated polyethylene was more efficient, the focus in the next chapter is on PETTC. Due to the comparable affinities of AuNPs and AgNPs towards sulfur containing moieties, both types of particles will be discussed in one chapter.
Dynamic Light Scattering
In order to characterize the solution behavior, dynamic light scattering analysis of the respective AuNP, PE−AuNP, AgNP and PE−AgNP was performed. Using this method, a determination of the hydrodynamic radius of the particles is gained depending on e.g. the solvent and
For the analysis, the capped and unfunctionalized nanoparticle dispersions were diluted with toluene and measured at a temperature of 90 °C to ensure a complete dissolution of the polymer-shell, respectively.
In addition, the obtained dispersions were cooled down to 25 °C and measured again.
The analysis of unfunctionalized AuNPs showed a narrow size distribution while the intensity maximum could be determined at ~10 nm (Figure 4-3). The DLS analysis of the PE−gold-nanoparticles resulted in two major differences. At a temperature of 90 °C the entire distribution is shifted to higher hydrodynamic diameters with a maximum at ~22 nm in combination with a slightly broadened shape of the curve. The shift to higher diameters can be attributed to the formation of a swollen polymer-shell around the AuNPs resulting in an increased size. Furthermore, the distribution of the unfunctionalized particles completely disappeared.
Consequently, from these data it can be extracted that a high surface loading of the respective nanoparticles with PE was achieved. In addition, it can be observed that the formed nanohybrids exhibit a strong stability even at high temperatures. This validated the assumption that sulfur containing polyethylene provides a strong interaction to the gold surface.
Figure 4-3. DLS measurement of AuNP at 90 °C/25 °C (black), PE−AuNP at 90 °C (red) and PE−AuNP at 25 °C (blue). All samples were measured in toluene and
normalized to the maximum.
10 100 1000
normalized number distribution
hydrodynamic diameter /nm Au-nanoparticles PE−Au-nanoparticles (90 °C) PE−Au-nanoparticles (25 °C)
Furthermore, at a temperature of 25 °C the PE−AuNPs showed an interesting behavior compared to unfunctionalized AuNPs. The distribution of the hydrodynamic diameter of the ungrafted gold nanoparticles was independent on the applied measuring temperature.
In contrast to this, after cooling to room temperature, the entire diameter-distribution of polyethylene capped AuNPs shifted to high values of about 1000 nm. This observation is assigned to aggregation of PE−AuNPs and indicates that the solution behavior of capped nanoparticles is entirely determined by the polyethylene-shell. Below the critical solution temperature, the PE-shell becomes insoluble resulting in a segregation of PE−AuNPs from the solvent and the formation of their agglomerates including non-bound polymer. Remarkably, after reheating the sample a decrease of the diameter is observed back to the primary distribution at maximum of ~22 nm. The reversibility of the aggregation/disaggregation process confirmed the presence of a stable PE-shell during the entire procedure and excluded the formation of unfunctionalized AuNP-aggregates.
The measurement of unfunctionalized and PE-grafted silver nanoparticles showed comparable results. The pure AgNPs exhibit a narrow hydrodynamic diameter distribution around 11 nm. The PE-AgNPs showed a strong increase of the measured diameter with a maximum at ~28 nm and a broadened shape of the curve that might be attributed to different grafting densities of the polyethylene shell. The results validated strong sulfur−silver-interaction and consequently a strong attachment of the polyethylene-shell even at harsh temperature conditions.
Figure 4-4. DLS measurement of AgNP (black) and PE−AgNP (red). All samples were measured in toluene at 90 °C.
From the DLS measurements of the respective unfunctionalized and PE-modified nanoparticles it could be extracted that the formation of polyethylene grafted gold and silver nanoparticles was successful. In addition, it was proven that an appropriate grafting density of the polymer-shell, a strong attachment to the surface and a high stability was achieved. Due to the observation it was validated that the dispersion behavior of PE−AgNP and PE−AuNP is entirely corresponding to the adhered polyethylene-shell.
Transmission Electron Microscopy
In order to validate the data obtained from DLS analysis and to visualize the behavior of the particles in the dried, solid state, transmission electron microscopy (TEM) was performed. Via an electron beam transmitted through a sample, a direct visualization of the particle’s structure in vacuum is possible. From the TE micrographs the size distribution of the respective nanoparticles can be extracted (Figure 4-5). The average
0 10 20 30 40 50
normalized number distribution
hydrodynamic diameter / nm
Ag-nanoparticles PE−Ag-nanoparticles
diameter was determined to ~7 nm for AuNPs and to ~9 nm for AgNPs.
In addition, it was observed that the silver nanoparticles exhibited a much narrower size-distribution compared to AuNPs. The data obtained from TEM analysis were in a good agreement with the size distribution obtained by DLS analysis (chapter 4.3.1), whereas the slight discrepancies can be attributed to the differences in the setup and measurement of the respective method.
From the congruence of DLS and TEM analysis it was confirmed that the increase of the hydrodynamic diameter discussed in chapter 4.3.1 is clearly corresponding to the attached polyethylene-shell.
Figure 4-5. Size distribution of AuNPs (left) and AgNPs (right) obtained from TEM measurements.
Besides the determination of the respective diameter, further information within the TE micrographs could be extracted (Figure 4-6). Both unfunctionalized gold as well as silver nanoparticles showed a strong tendency to form aggregates. This aggregation arises from tending to reduce the surface area resulting in strong attractive particle− particle-interactions. In contrast to this, opposite observations were made for PE−AuNPs and PE−AgNPs. By comparing the TE micrograph of AuNPs with PE−AuNPs an increase of the inter-particles distance could be observed. In addition, a perfect separation without the formation of aggregates of the capped particles was monitored. Similar observations were made for AgNPs and PE−AgNPs. In case of modified silver nanoparticles a hexagonal arrangement was noticed in some areas as reported before for other nanohybrids.141 This well-structured
self-0 5 10 15 20
homogenous size distribution of AgNPs.
Furthermore, the monitored interparticle distance of the respective PE capped nanoparticles was remarkable high compared to the relatively low molecular weight of the applied polymer. This might be explainable due to the usage of high crystalline polyethylene (~90 %) resulting in a stretched polymer-shell, especially assuming a high grafting-density.
Another explanation could be found in the uncomplete end-group functionality (~60 %) resulting in free polyethylene that can swell the attached polyethylene-shell.
From the DLS and TEM analysis it can be concluded that to each distinct particle a polyethylene shell is attached. This validated the successful and effective synthesis of metal-core−PE-shell hybrids in the applied grafting-to approach.
Figure 4-6. Exemplary TE micrographs of unfunctionalized as well as PE-capped gold (on top) and silver (bottom) nanoparticles.