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Nanoparticle adsorption as a function of refractive index value of polymers
One of the best ways to validate our model is check the amount of absorbed NPs on polymer films having different index of refraction. Hence experiments were conducted on PMMA, polypropylene (PP) and PS films having a similar thickness of about 200 nm for which the index of refraction oa was increasing. [34], [35] (Figure 2-3) In addition, a silane hydrophobic silica layer SiO2-CH3 (water contact angle of 115°) made by silanization of the SiO2-Si wafer with the help of ODTS molecules (octadecyltriethoxysilane) was also used.
In this case, (i. e. for thick films of PMMA, PP, PS and SiO2-CH3 layer) an adsorption time of 2 with the same concentration was used. As shown in Figure 2-3, the adsorbed amount of NPs was found to change by almost a factor of two for films having an index of refraction, oa going from 1.45 to 1.59 clearly illustrating the sensitivity of this novel approach. The fact that PP and PMMA films roughly show the same adsorbed amount clearly signs that the van der Waals interaction rather than the surface energy dictates the process since PP and PMMA have different contact angles (102° and 71° respectively). As the refractive index of ceria nanoparticles is large (~1.8-2.0), their adsorption onto a PS surface for example is strong and follows a high affinity adsorption isotherm as shown in Figure 2-4. solution concentration monitored by optical reflectometry during 2 hours. b AFM topographic image 1×1 μm2 showing the surface of the PS film densely covered with CeO2 NP. affinity isotherm as anticipated from the high refractive indexes of both the PS film and the CeO2 NP. Here the pH of the dispersion is 1.5 with 0.1M of NaNO3 (k-1 ~1 nm) to minimize the mutual NP repulsion and to increase the surface coverage.
Density and NP adsorption of PS thin films
The Sellmeier model is used to find out the wavelength and thickness dependency of n. It can be observed that the refractive index decrease with the increasing thickness of PS thin films. For the thicker films, the n converge towards the value of 1.589 and is accordance with the previously reported value of bulk polystyrene at the same wavelength. [36] The refractive index of PS films obtained from the effective Hamaker of the system [37] and the one obtained with multiple angle spectroscopic ellipsometry [7] also reported such a trend of increasing refractive index with the decreasing film thickness. An estimation of the amount of ceria NPs adsorbed onto PS/H-Si films at different film thickness (h) was obtained from AFM topographic images as shown in Figure 2-6. Considering three images for instance for easiness (a, c and f), One can clearly observe that the 6.7 nm thick film (a) is densely covered by the ceria NPs whereas the coverage on the 143.6 nm thick PS film (f) is comparatively very low; the 18.8 nm film falling in between (c).These AFM images, along with the evolution of percentage area of adsorbed NPs with respect to the PS film thickness (Figure 2-7) clearly shows that the percentage area of adsorbed nanoparticles decreases with increasing PS film thickness.
Density and NP adsorption of PMMA thin films
Following the observations of PS thin films, it is interesting to check how the percentage adsorption of the nanoparticle can sense the refractive index for PMMA on silicon wafer (SiOx-Si) with an oxide layer of 1.7 nm. Figure 2-8 shows the adsorption percentage of ceria nanoparticles on PMMA/SiOx-Si at various thicknesses. Contrary to what observed with the PS/H-Si, an increased adsorption of nanoparticle with increasing film thickness can be observed through the AFM images. The trend can be clearly observed with the percentage area of adsorbed nanoparticles as a function of PMMA film thickness as given in the Figure 2-9. Hence from Figure 2-9, one should expect a decrease in the refractive index of the PMMA films with decreasing film thickness. This is exactly the case, as observed by the ellipsometric measurements as shown in Figure 2-10. Structuration and glass transition temperature of the adsorbed polymer layer : some insights in the property deviations of the ultra-thin polymer film Aparna Beena Unni, 2016 Refractive index is measured to be 1.5 at a thickness of 95.1 nm which comparable to the bulk value 1.49 of the PMMA. [36] So from Figure 2-8 and Figure 2-9, the decrease in the refractive index of PMMA/ SiOx-Si is detected by the decreased NP adsorption corresponding to the decreasing film thickness.
A study of PMMA thin films by neutron reflectivity also reported similar decreased density when the films become thinner. [42] To explain the difference in density between PS and PMMA, a key parameter could be the nature of the interactions between the polymer and SiOx layer. Unlike PS chains that do not form hydrogen bond with the hydrophobic H-Si substrate, PMMA chains are bonded to the substrate by acid-base interactions involving the carbonyl and the hydroxyl – OH. For a given chain of atactic PMMA, the number of carbonyl in capacity to form hydrogen bonds is limited [43] and reduces the number of conformations that a chain can have. These conformational restrictions may lead to situations where larger segments of the chains can no longer arrange themselves in the same dense way as in the bulk.
Table of contents :
Chapter 1. Materials and Methods
1.1 Materials used
1.1.1 Polymers
1.1.2 Ceria Nanoparticles
1.2 Sample preparation
1.2.1 Substrate preparation
1.2.2 Thin film formation
1.3 Characterization
1.3.1 Ellipsometry
1.3.2 Atomic force microscopy
1.3.3 X-ray Reflectivity
1.3.4 Contact angle goniometry
1.3.5 Solvent rinsing using dip coater
Chapter 2. Density variations of polymer thin films under confinement; a study based on Ceria nanoparticle adsorption
2.1 Introduction
2.2 Ceria NP adsorption for sensing the density variations
2.3 NP deposition on polymer thin films and data acquisition
2.4 Nanoparticle adsorption as a function of refractive index value of polymers
2.5 Density and NP adsorption of PS thin films
2.6 Density and NP adsorption of PMMA thin films
2.7 Influence of substrate on NP adsorption
2.8 Correlation of density with thin film thermal behavior
2.9 Conclusion
Chapter 3. Stable ultrathin polymer films (<7nm) made by solvent rinsing
3.1 Introduction
3.2 Ultrathin polymer films
3.2.1 Formation of ultrathin film (< 7 nm) by direct spin-coating
3.2.2 Formation of ultrathin films (≤ 7 nm) by solvent rinsing treatment
3.3 Correlation between the kinetics of dissolution and annealing
3.4 Morphology and verification of residual layer on substrates surface
3.5 Stability or instability of residual films of tunable thickness
3.5.1 The stability of residual films on H-Si wafers
3.5.2 The instability/metastability of residual films on SiOx-Si wafers
3.6 Conclusion
Chapter 4. Characteristic features of ultrathin polymer residual layer studied by rinsing with different solvents.
4.1 Introduction
4.2 The film prior to solvent rinsing
4.3 Thickness of polymer residual layer after rinsing with various solvents
4.4 The morphology of polymer residual layer after rinsing with various solvents
4.5 The stability/instability of polymer residual layer analyzed with Van der Waals intermolecular theory
4.6 The stability/instability of polymer residual layer analyzed with effective free energy of the system
4.7 Conclusion
Chapter 5. Stability and glass transition temperature of residual films on H-Si wafers
5.1 Introduction
5.2 Thickness and morphology of polymer residual layer after rinsing with various solvents
5.3 The stability/instability of polymer residual layer analyzed with Van der Waals intermolecular theory
5.4 Glass transition properties of polymer residual layer
5.5 Conclusion
Conclusion and Perspectives
