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Surface imprinting involves the synthesis of thin polymer layer in the presence of a template species to make artificial receptors. The implementation of such synthetic receptors into various biomimetic sensors is promising and allows for fast, specific, and low-cost target detection. However, their fundamental detection mechanisms are not fully understood yet. In this study, cell recognition mechanisms of surface imprinted polymers (SIP) were analyzed. Specifically, the molecular dynamics of the sensing surface was evaluated in order to analyze cell residue signatures that might be present and have a role in the binding mechanism. Secondly, the thermal and dielectric properties of the polymer used for making SIP were studied to optimize its synthesis. Finally, we studied the effects of humidity and temperature on phospholipid bilayer dynamics, kinetics, and structure. For this purpose, we proposed the application of a novel compact device that combines a quartz crystal microbalance (QCM) and dielectric spectroscopy and simultaneously provides valuable information regarding water absorption or desorption in a thin organic film together with its molecular dynamics.

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The goal of the experimental investigations in the past two years was to understand the role of interfacial interactions on the structural relaxation process of polymers. To achieve our objectives, we measured and analyzed by means of broadband relaxation spectroscopy the dynamics of bulk and ultrathin films (thickness < 200 nm) of several amorphous and semicrystalline polymers [poly(vinyl acetate), PVAc; atactic polypropylene, a-PP; poly(ethylene terephthalate), PET; poly(3-hydroxybutytate), PHB; polystyrene, a-PS, atactic poly (2-vinyl pyridine), a-P2VP]. Owing to the wide frequency range of the principal technique (1 mHz - 10 MHz) the complex relaxation scenario above and below the glass transition temperature in those systems could be investigated in great detail. In this dissertation, we started with an introduction to the phenomenology of the structural relaxation process (alpha-relaxation) via the concepts of cooperative motion and dynamical heterogeneity (CHAPTER 1). The discussion was exemplified by the analysis of the relaxation behavior of a polar model polymer, PVAc. In CHAPTER 2 we described a recently introduced methodology for the investigation of non polar systems by means of dielectric probes. The experimental data reported filled the gap in the characterization of a-PP, being considered a model system for the study of the dynamics in amorphous polymers in the liquid state. The relatively low value of the intrinsic dipole moment of this polymer does not permit an easy analysis of its dielectric relaxation. Notwithstanding this, it was possible to characterize the glass transition dynamics of a-PP by broadband dielectric spectroscopy (DS) using small amounts of DBANS, an organic molecule with a high dipole moment. After an introduction to the phenomenology of deviations from bulk behavior in ultrathin polymers films (via six different key themes in CHAPTER 3), we reviewed the contribution of dielectric spectroscopy to the debate on confinement effects in the last ten years. In this experimental work, the dynamics of ultrathin polymer films was explored at different time and length scales. We extended the conventional dynamic range of dielectric relaxation spectroscopy by introducing real-time crystallization experiments in a confined geometry (motions of the whole polymer chain up to 100000 s). To understand the impact of interfacial interaction on the structural relaxation we characterized the changes in the alpha-process of PET down to 13 nm (CHAPTER 4). Ultrathin films were capped between aluminum electrodes, often discussed as the geometry of model nanocomposites, and the formation of stable chemical bonds between the polymer and the metal ensured the presence of a layer with slower dynamics at the interface. Deviations from bulk behavior, appearing as an increase in the relaxation time at a fixed temperature, a simultaneous reduction of the dielectric strength and a broadening of the structural peak, are observed for films of a thickness below 35 nm. The slowing-down acts as a shift factor slightly decreasing with the temperature, and does not affect the fragility. We introduced the concept of reduced mobility layer, RML, and showed how the dielectric response of ultrathin films is altered by the presence of layers with a different mobility. A simple explanation for the dual character (static – dynamical) of the confinement effects was proposed. As regarding crystallization phenomena, we proved the feasibility of DS as a tool to monitor the crystallization kinetics also in ultrathin polymer films (CHAPTER 5). We combined the real time monitoring of the conversion of the amorphous phase into crystalline structures with an analysis of the thickness dependence of the structural relaxation of PHB. We concluded that for this polymer the tremendous slowing down of the crystallization kinetics (the mean crystallization time increased by more than one order of magnitude) is not due to a change in the chain mobility on the time and lengthscale of the dynamic glass transition. The increase of the crystallization time was in fact accompanied by a constant value of the glass transition temperature. Arguing with similar examples from the recent literature we concluded that in ultrathin polymer films the slow-down of crystallization kinetics is not simply induced by a size-dependent increase in the glass transition temperature but by a reduction of the chain mobility at the very interface with an attractive substrate. Moreover, we considered the reduction of active nuclei density as a possible element contributing to the increase of the crystallization time in ultrathin films by comparing the effects of temperature and thickness on the diffusion limited crystallization kinetics (CHAPTER 6). We proved that in the regime of cold crystallization, the dynamics of thinner films corresponds to the one in bulk but at lower temperatures. To rationalize these results, we investigated the interplay between crystallization phenomena and structural relaxation arriving to a correlation between the crystallization time and the structural relaxation time (CHAPTER 7). We proposed a model to characterize the deviation from bulk behavior in ultrathin polymer films. As a main result the changes in the crystallization (structural relaxation) time depends on the temperature of the experiments and vanish at sufficiently high temperatures. We arrived to the same conclusions studying the relaxation behavior of extremely thin films (thickness < 6 nm) of a-PS (CHAPTER 8). By characterizing the response of these samples after different thermal histories we showed the possibility to obtain films with different dynamics by simple changing the annealing temperature. We observed an anomalous increase with the temperature of the electric capacitance and attributed it to the defreezing of the dead layer, i.e. the polymer chains absorbed by the substrate. Such defreezing was found to act as a precursor for the dynamic glass transition of the reduced mobility layer, and provides the direct link between interfacial interactions and molecular relaxations. In terms of a simple scaling expression we found an alternative path to show that the deviations from bulk behavior depend on the time scale of the experiment and thus on the temperature. Finally, we experimentally verified in ultrathin films of a-P2VP that the deviations from bulk behavior are temperature dependent (CHAPTER 9). We treated the influence of an absorbing substrate as a small perturbation in the activation energy of the a-modes related to the dynamic glass transition. We proposed that the deviations from bulk behavior derive from the balance of such a perturbation and the energy of the thermal bath. In conclusions we proposed a correlation between the structural time tau_a and the crystallization time, tau_cry, valid for bulk systems, at temperatures just above Tg. In ultrathin polymer films, the coupling between segmental mobility and crystallization rate breaks down because of interfacial interactions. We modeled the properties of the layers at the very interface with an attractive substrate in terms of a reduction of molecular mobility on the time and the length scale of the dynamic glass transition. Further we experimentally verified that the structural relaxation time, tau_a, increases in proximity of the walls and that the relative variations of tau_a vanish at sufficiently high temperatures. We thus support the idea that deviations from bulk behavior originate from changes of chains’ conformation at the interfaces, considering the latter as a particular case of surface ordering effects. We thus think to have (partially) replied to the question “What are the effects of interfacial interactions on the structural relaxation dynamics of ultrathin polymer films?”. Still, we do not know how to reply to questions like: Is the dynamics of polymer chains at a free surface faster than in bulk? If so, why? Can free surfaces be treated as liquid like layers even below bulk Tg? Is the size of cooperative motion tremendously affected by the presence of an interface? What is the penetration depth of this perturbation? What is the profile of the gradients of molecular mobility? Could those be described just in terms of conformational changes and distance from the wall? Are there different profiles of molecular mobility, or the deviations in the different correlated quantities (structural time, crystallization time, physical aging, diffusion ...) are only due to averaging issues? Are the changes in the cold crystallization kinetics and the diffusion dynamics completely governed by the a-modes, or should other molecular processes acting at intermediate length scales be taken into account? Why does an attractive substrate induce changes propagating up to 10 times the radius of gyration? We hope it will be possible to give an answer to those points in the near future. This dissertation aims to understand how and why interfaces may influence the structural relaxation of polymers. To study the interfacial properties of these materials we characterized the dielectric response of ultrathin polymer films (thickness < 200 nm) and monitored the changes in the material performance upon thickness reduction down to 4 nm. Considering the tremendous fundamental and technological relevance of polymer/substrate systems leading to an increase of the glass transition temperature, Tg, we concentrated our attention on the investigation of those interactions causing a partial or total immobilization of the polymer chains. We proposed a correlation between the structural relaxation (segmental) time tau_a and the crystallization time, tau_cry, valid in bulk systems, at temperature just above Tg. We found that in ultrathin polymer films such a coupling breaks down because of interfacial interactions. We thus modeled the properties of the layers at the very interface with an attractive substrate in terms of reduction of molecular mobility on the time and the length scale of dynamic glass transition. We experimentally verified that tau_a increases in proximity of the walls and that the relative variations of tau_a vanish at sufficiently high temperature.

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Ons streven in dit doctoraat is het ontwikkelen van kennis over 2 verschillende onderwerpen revelant voor polyamides. Ten eerste, om polyamide-producten te produceren is er een hoge temperatuur nodig door de hoge smelttemperatuur van de polyamides. Het zou van grote interesse zijn om te kunnen werken bij lagere temperaturen. Hiermee zou de verwerkingstijd mogelijk versneld kunnen worden; energie bespaard en kunnen er additieven met een lagere thermische stabiliteit gebruikt worden. Ten tweede, een probleem,specifiek voor polyamides, is de wateropname, wat leidt tot ongewenste fenomenen zoals dimensie-instabiliteit, krimp enz. In dit doctoraat is geprobeerd om het negatieve aspect van wateropname om te vormen in een positief aspect. Met de gevoeligheid van polyamides voor water in gedachten, hebben we een concept gebruikt ontwikkeld op DSM Research namelijk polyamides volledig op te lossen in water, en verder ook in andere solventen, zoals methanol en ethanol. Dit oplossen zorgt ervoor dat de temperaturen van kristallisatie en vervolgens weer oplossen drastisch dalen. Hierdoor komt de verwerking van polyamides bij extreem lage temperaturen in zicht. De wetenschappelijke doelen van dit doctoraat zijn een grondige beschrijving en begrip van het oplossen van polyamides in water en in andere solventen als functie van temperatuur, tijd en concentratie; vervolgens kristalliseren, oplossen en morfolgie-ontwikkeling; het effect van kristallisatie in oplossing op smelt- en kristallisatiegedrag na het verwijderen van het solvent. De methodes die hiervoor gebruikt zijn DSC, SALLS, SAXS/WAXD, Raman spectroscopie en solid state-NMR. Deze zijn met succes toegepast op PA6 /PA4.6 en dit met verschillende solventen zoals water, methanol, ethanol. Er werd duidelijk aangetoond dat PA6 oplosbaar is in water over het gehele concentratiebereik en dat water zich gedraagt als een kristallisatie- en smeltpuntsverlager. Uit WAXD-metingen en Platon berekeningen bleek dat de morfologie van de PA6 alfa-kristallen behouden blijft in het gehele concentratiebereik, en dat water de kristallen niet binnendringt. De opmerkelijke verlaging van de smelt- en kristallisatietemperaturen van PA6 in water kan niet enkel verkregen worden met water maar ook met andere solventen, zoals methanol en ethanol. De opmerkelijke verlaging van de smelt- en kristallisatietemperaturen kan ook verkregen worden met andere polyamiden zoals PA6.6 en PA4.6, en met andere solventen, zoals ethanol. PA4.6 heeft een gelijkaardig gedrag als PA6 en is oplosbaar in verschillende oplosmiddelen (ethanol en water), terwijl de overgangstemperatuur depressies groter zijn. Wanneer de PA4.6-water en de PA6-water systemen vergeleken worden, hebben de PA4.6-water systemen de grootste temperatuur depressies.

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The experimental work presented in this thesis can be divided into two main parts: one part concerning phase transitions in pure liquid crysalline systems and an other part regarding liquid crystalline mixtures with nonmesogenic sollutes. All experimental results were obtained by Adiabatic Scanning Calorimetry. First two ferroelectric liquid crystals were studied: S-(-)-4-(2’-methylbutyloxy)phenyl 4-n-octyloxybenzoate and S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4’-carboxylate. The experiments showed that no latent heat was present for the smectic A to smectic C* (AC*) transition to within the experimental resolution. Therefore the AC* transition in both samples could be classified as a continuous one. By fitting the specific heat data we showed that this phase transition could be described by an extended mean field model augmented with Gaussian fluctuation terms in the specific heat capacity expression. The second pure system investigated in this thesis was the liquid crystal 4-butyloxyphenyl-4’-decyloxybenzoate with the emphasis on the nematic to smectic A (NA) and the nematic to isotropic (NI) phase transitions. Because of the high resolution ASC technique used in this investigation we were able to accurately determine the latent heat of the first order NI transition. The NA transition was found to be a continuous one within the experimental resolution and the value of the effective specific heat capacity critical exponent, a, was found to be 0.23 ± 0.01, thus between the 3D XY and tricritical value. In a last part the effect of nonmesogenic impurities on the order of the nematic to smectic A phase transition was studied in a liquid crystal. Three different nonmesogenic impurities being biphenyl, cyclohexane and water were added to a liquid crystal, octylcyanobiphenyl (8CB), resulting in three different conclusions. The NA transition in pure 8CB is continuous and for all concentrations of biphenyl studied this transition remained continuous. The effective specific heat critical exponent decreased from 0.31±0.03 for pure 8CB down to 0.08±0.03 for the highest mole fraction of biphenyl studied, x=0.1415. For mixtures of 8CB and cyclohexane, however, crossover from continuous to first order is observed at a tricritical point of the mole fraction, x, of cyclohexane around 0.0460. After investigating mixtures of 8CB and water, the effective critical exponent increased up to 0.36±0.03 for mixtures with a mole fraction of water x=0.1105 and it remained constant for all higher concentrations studied (up to x=0.3509). By means of visual inspection of the samples we were able to conclude that for mixtures with a mole fraction of water higher than x=0.10, phase separation occured. The difference between these three systems and the crossover from continuous to first order for the system 8CB and cyclohexane was explained in terms of a mean field free energy density expression including coupling terms of the mole fraction, x, with the nematic and smectic A order parameters. Vloeibare kristallen vormen sinds hun ontdekking in 1880 een interessante onderzoekstopic zowel op experimenteel als theoretisch niveau. Het intrigerende aan vloeibare kristallen is dat ze tussen de vaste en vloeibare fase, fasen van gedeeltelijke ordening vertonen. Dankzij deze “mesofasen” kunnen vloeibare kristallen aangewend worden voor vele praktische toepassingen waaronder het LCD-scherm de meest gekende is. Het doel van dit thesisonderzoek was om faseovergangen tussen deze mesofasen te karakteriseren. In het algemeen kan een faseovergang van de eerste orde of continu zijn. Een eerste-orde faseovergang wordt gekenmerkt door een sprong in de enthalpie als functie van de temperatuur. Al naargelang de grootte van deze sprong kan een eerste-orde faseovergang kwalitatief opgedeeld worden in sterk of zwak eerste orde. Voor een continue faseovergang zal de enthalpie als functie van de temperatuur een continu verloop vertonen. Continue faseovergangen kunnen opgedeeld worden in verschillende universaliteitsklassen. Met iedere universaliteitsklasse worden kritische exponenten geassocieerd en de waarden van deze kritische exponenten bepalen het gedrag van een systeem in de buurt van een faseovergang. Overgangen die tot dezelfde universaliteitsklasse behoren zullen bijgevolg ook hetzelfde kritische gedrag vertonen. De meettechniek die gebruikt wordt in dit doctoraal onderzoek is adiabatische scanning calorimetrie (ASC). Aan de hand van deze hoge-resolutie techniek kunnen we zowel de enthalpie als de soortelijke warmtecapaciteit in functie van de temperatuur van een meetmonster opmeten, wat toelaat om de relevante kritische exponent te bepalen. De experimentele resultaten kunnen opgedeeld worden in twee delen. Een eerste deel behandelt faseovergangen in zuivere vloeibare kristallen. De smectische A-smectische C* overgang in twee ferroëlektrische vloeibare kristallen (D1, D2) werd bestudeerd. De resultaten toonden aan dat deze faseovergang continu was (binnen de experimentele resolutie) en dat deze overgang het best beschreven kon worden aan de hand van een gemiddelde-velduitdrukking die termen tot de zesde graad in de ordeparameter bevat. Verder bestudeerden we de nematische-isotrope (NI) en nematische-smectische A (NA) faseovergang in het vloeibare kristal 10O4. De NI-overgang kon geclassificeerd worden als zwak eerste orde. De NA-overgang was continu met een kritische exponent die een waarde had tussen deze van het 3D XY model en de trikritische waarde. Dit kon verklaard worden door de koppeling tussen de nematische en smectische A ordeparameters. In een tweede deel wordt de invloed van niet-mesogene onzuiverheden op de NA-overgang in een vloeibare kristal (8CB) onderzocht. Theoretisch werd er reeds gesteld dat het toevoegen van niet-mesogene onzuiverheden een invloed zou kunnen hebben op de orde van de faseovergang, dit werd echter nog niet experimenteel bevestigd. De NA-overgang in het zuivere systeem 8CB werd reeds uitvoerig bestudeerd in het verleden: ook hier werd er experimenteel een waarde voor de kritische exponent gevonden tussen de 3D XY en trikritische waarde. Er werden binaire mengsels van het vloeibare kristal 8CB met drie verschillende niet-mesogene onzuiverheden bestudeerd: bifenyl, cyclohexaan en water. In het geval van 8CB en bifenyl bleef de overgang continu en naarmate de concentratie van bifenyl steeg nam de waarde van de kritische exponent af. Voor 8CB en cyclohexaan vond er een overgang van continu naar eerste orde plaats: naarmate de concentratie van cyclohexaan steeg naderde de kritische exponent de trikritische waarde van 0.5 en vervolgens werd de overgang (zwak) eerste orde. Voor 8CB en water bleef de overgang ook continu. Er werd een kleine stijging van de kritische exponent voor een molaire fractie van water van x=0.1105 waargenomen en voor hogere concentraties bleef de kritische exponent constant. Door middel van een visualisatie-experiment van een reeks nieuwe mengsels konden we aantonen dat er voor mengsels met een molaire fractie van water groter dan x=0.10 fasescheiding optrad. Het verschil tussen deze drie systemen werd uitgelegd aan de hand van een gemiddelde-velduitdrukking voor de vrije energie dichtheid die koppelingstermen bevatte tussen de molaire fractie en de nematische en smectische A ordeparameters.