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The understanding of soft matter at the nanoscale is required to anticipate the upcoming challenges for the construction of nanodevices, and may help to deepen our understanding of fundamental phenomena like glass formation and crystallization in general. Investigations can start from a specific product design or have a more fundamental approach towards the understanding of materials at such small lengthscales. At the Laboratory of Acoustics and Thermal Physics (ATF) research is directed more on a fundamental approach rather than having a specific end product in mind. In our group we are especially interested in the behavior of soft matter at the nanoscale, with several projects focusing on the effect of confinement on polymers. At these small lengthscales the role of interfaces has intrigued many researchers and is therefore a hot topic to investigate. We want to determine the effect of interfaces on the crystallization behavior of ultrathin polymer films.^ This topic is excellent to evaluate the possible deviations from bulk behavior as the parameters that govern the crystallization are affected by confinement and interfaces. We therefore use broadband dielectric spectroscopy to monitor the relaxation dynamics, adsorption and crystallization. This allows us to determine how the system changes upon confinement (a reduction in thickness for example), but also the exact critical lengthscale it takes place on. We start by giving a broad introduction on dielectric spectroscopy and show how it is used to study materials at the nanoscale. This includes a background on the working principle, signal analysis, experimental setup and data interpretation. The technique itself is in essence capable to monitor the rotational mobility of dipoles in our polymer material for different frequencies and temperatures by placing the soft matter between two electrodes.^ This allows us to determine changes in our sample during experiments or compared to a reference bulk sample. In the second chapter we describe the various typical properties of polymers, ranging from glass transition to crystallization and adsorption. We link these topics directly to how they are investigated by dielectric spectroscopy and describe how we can determine them for our soft matter system. In the last introductory chapter we describe the different sample geometries with their advantages and applications.In the 1st experimental chapter we study the non-crystallizable polymer poly(ethyl methacrylate) (PEMA) in order to determine the influence of an interface without any possible crystallization. By using a heating and cooling run we were able to determine the effect of annealing on our samples. The differences observed before and after the high temperature annealing indicate changes in the packing of our polymer layer at the interface.^ This also means that our system is in a non-equilibrium state at the start of the heating run, where counterintuitive faster segmental dynamics are observed. The faster polymer dynamics before annealing are ascribed to packing frustration in the adsorbed layer at the interface. After annealing a recovery to bulk behavior is observed for films thicker than 10 nm. We continued our investigation by determining the kinetics adsorption at the interface using a washing experiment. This allowed us to introduce the dimensionless parameter t* as a Deborah number for deviations from bulk behavior, expressing the annealing time that is required to erase a reduction in Tg (that is to erase the deviations from bulk behavior). In the thinnest films however the frustrated packing remained even after rigorous annealing, indicating that the system is stuck in a new meta-stable state.^ The 2nd experimental chapter describes the kinetics of crystallization and adsorption of poly(ethylene terephthalate) (PET) in its most simple (symmetric) confinement geometry, an ultrathin capped film. We therefore followed the dielectric strength of our samples during isothermal annealing, providing us information on the amount of mobile dipoles in our thin film. By doing this for samples of different thickness we can disentangle interfacial from confinement effects and determine the exact lengthscale where they dominate the crystallization kinetics. In our experiments we observed an increase of the crystallization time, that is a slower kinetics, for thinner films. These findings allowed to determine the role of a constant nucleation density in a smaller volume as this size effect already plays an important role at the micrometer scale.^ In the thinnest polymer films a threshold thickness, h* ~ 20 nm, is identified where no crystallization can be observed since all chains in such a sample are adsorbed at the interface. This was verified by determining the adsorption kinetics and the final thickness of the adsorbed layer, which corresponds perfectly to h*. The adsorbed layer is in fact an opposing energetic barrier to prevent crystallization (or slow it down to such a long timescale that it is not visible in the experimental conditions). A determination of the critical thickness h* in soft matter for nanoscale devices would be crucial as crystallization can drastically change the properties of a material (strength, gas diffusion,).In the 3rd experimental chapter we apply our findings on the crystallization and adsorption kinetics to a different material, the more flexible poly(l-lactide) (PLLA).^ We additionally used the thickness dependent shape parameters of our dielectric spectra, an approach that was also used for PEMA, to confirm our findings. In the case of PLLA the nucleation dominated regime for thicker films is clearly observed. The critical thickness h* ~ 10nm, where an inhibition of crystallization is found, further confirms the findings for PET. In samples of intermediate thickness (between 10 and 25nm) however a different regime is found with slower crystallization kinetics, not imputable to nucleation effects. We explained this in terms of the high flexibility of the chain, allowing the adsorbed chains to crystallize to a certain extent, something that is notobserved for the more rigid PET chains. The hypothesis of this chain flexibility is confirmed using both the shape parameters of the relaxation spectra and the determination of the mobility profile based on the dielectric strength of the segmental relaxation in our films.^ The findings of this chapter clearly show that the chain rigidity of the adsorbed layer has to be taken into account when crystallization in a confined environment is studied.The 4th experimental chapter is based on the crystallization of poly(ethylene terephthalate) in a more complex (non-symmetric) confinement geometry: sandwiched between a hard aluminum and soft polystyrene (PS) layer. We then analyzed the crystallization kinetics in function of: (a) the thickness of PET (the crystallizable layer), and (b) the thickness of PS (the non-crystallizable layer). The effect of the PET sample thickness is similar to that of a single layer of PET, except that we also observe an intermediate behavior between the nucleation and adsorption dominated regimes. This effect can be ascribed to the presence of only one adsorbed layer at the hard aluminum interface. The thickness of PS clearly indicates that the screening layer is capable of altering the crystallization kinetics.^ This effect is ascribed to changes in the long-range Van der Waals forces that affect the thickness of the adsorbed layer and through this mechanism alter the crystallization rate. Since we can calculate these dispersion forces, we are able to predict (within certain limitations) the effect of a screening layer and tune the geometrical environment to obtain the desired crystallization kinetics. For example, if an inhibition of crystallization is required to maintain the strength of our nanoscale layer, a specific interfacial multilayer can be constructed. Our findings were validated using Monte Carlo simulations, where a lattice box is used to determine the crystallization kinetics of a polymer, without the requirement of specific atomic models. It can therefore be seen as a tool to analyze the general behavior of polymers without specifically focusing on the polymer type.^ The simulations were performed on a system where the interaction energy was altered in order to change the crystallization kinetics, similar to the changes in the dispersion forces of our dielectric measurements. The experimental method we introduced permits remote control of the crystallization kinetics of confined polymer chains without changing the size of the crystallizable layer or the interfacial chemistry. In the 5th and last experimental chapter we combine the approach of single and double layers from the previous chapters in a new confinement geometry with a free upper surface: the interdigitated electrode (IDE). We first determined that it was possible to monitor the crystallization of thick films. Further investigation on thinner films should allows us to determine the effect of a free surface on the crystallization kinetics and determine the role of a screening PS layer without the introduction of a capping aluminum layer (in theIDE geometry no capping layer is required).^ In the case of confinement however the increasing background noise of our signal limited our investigations to films of ~70nm. A further reduction in thickness would require to take reference measurements and devise a procedure to take into account all signals that are not coming from the studied material (something we could not do yet). In the outlook of this chapter we also describe the advantages of the IDE and propose several investigation techniques that can be combined with dielectric spectroscopy in order to gain more insight in soft matter at the nanoscale.We can thus conclude that there is a major role of the interface on the crystallization kinetics of ultrathin polymer films, especially considering the adso

538.9 <043> --- Academic collection --- Physics of condensed matter (in liquid state and solid state)--Dissertaties --- Theses --- 538.9 <043> Physics of condensed matter (in liquid state and solid state)--Dissertaties

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academic collection --- 543.42 <043> --- 538.9 <043> --- Spectrum analysis. Spectroscopy. Spectrography. Spectrometry. Spectrophotometry. Fluorescence analysis--Dissertaties --- Physics of condensed matter (in liquid state and solid state)--Dissertaties --- Theses --- 538.9 <043> Physics of condensed matter (in liquid state and solid state)--Dissertaties --- 543.42 <043> Spectrum analysis. Spectroscopy. Spectrography. Spectrometry. Spectrophotometry. Fluorescence analysis--Dissertaties