No description available.

Yes / A new approach is described herein, where neutron reflectivity measurements that probe changes in the density profile of thin films as they absorb material from the gas phase have been combined with a Love wave based gravimetric assay that measures the mass of absorbed material. This combination of techniques not only determines the spatial distribution of absorbed molecules, but also reveals the amount of void space within the thin film (a quantity that can be difficult to assess using neutron reflectivity measurements alone). The uptake of organic solvent vapours into spun cast films of polystyrene has been used as a model system with a view to this method having the potential for extension to the study of other systems. These could include, for example, humidity sensors, hydrogel swelling, biomolecule adsorption or transformations of electroactive and chemically reactive thin films. This is the first ever demonstration of combined neutron reflectivity and Love wave-based gravimetry and the experimental caveats, limitations and scope of the method are explored and discussed in detail.

No description available.

No description available.

Protein adsorption at the solid/liquid interface is of wide ranging importance in many different areas of science such as biomaterial design, the fate of nanoparticles and in the food industry. As a result, many studies have been undertaken with varying foci but there still remains a lack of agreement between many working in this field and fundamental questions regarding the adsorption of proteins at the solid/liquid interface. Neutron reflectivity is a powerful technique for probing the properties of adsorbed layers at interfaces due to its high structural resolution and the possibility of using isotopic substitution to distinguish between components of a mixture. In this work, neutron reflectivity has been used as the primary technique for the investigation of proteins adsorbed sequentially or from a binary mixture. Initially, the adsorption of four proteins (carbonic anhydrase II, lysozyme, human serum albumin and maltose binding protein) onto a clean silica surface was investigated which revealed the importance of electrostatic interactions and entropic contributions to the driving forces for adsorption. Most of the adsorbed layers were described by a 2-layer model with a thinner, denser layer adjacent to the surface and a thick, diffuse layer extending into the bulk solution. The presence of impurities is also shown to have a significant impact on the adsorption of HSA. A study of the HSA/myristic acid system shows that the presence of small amphiphiles can inhibit HSA adsorption and also remove a pre-adsorbed layer. A comparison was made between the protonated and deuterated forms of two proteins, HSA & MBP, showing the deuterated proteins to have a higher affinity for the surface with adsorption occurring in a 3:1 ratio when from a 1:1 mixture. Likewise, d-MBP displaced h-MBP more readily than vice versa in an investigation into the effect of incubation time on the properties of the protein layer. The extent of desorption into protein free buffer is not affected by incubation time but the extent to which d-MBP was displaced by h-MBP showed a clear trend of decreased exchange with increasing incubation time indicating an active exchange process was occurring. This was also observed to a lesser extent for the sequential adsorption of binary protein systems, HSA & LYS and HSA & MBP. When investigating binary protein mixtures the higher propensity for deuterated proteins to adsorb is observed. LYS dominates when adsorbed from a mixture with h-HSA but from a d-HSA & LYS mix both proteins were adsorbed. The marked difference between the adsorption characteristics of perdeuterated proteins and their protonated counterparts provides a good case study for testing the neutron reflectivity technique when investigating systems with more than one component. This thesis assesses the limitations of the methodology of contrast variation for investigating mixtures as well as using different solvent contrasts. A comparison of neutron reflectivity and dual polarisation interferometry (DPI) shows that the two techniques are similar and any small differences can be attributed to the small change in surface chemistry. This comparison also highlights the advantages of DPI; high throughput of samples and detailed information but the restriction to using a 1-layer model limits its use.

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No description available.

Στην παρούσα διδακτορική διατριβή έγινε μελέτη πολυμερικών βουρτσών που διαμορφώνονται από συμπολυμερή πολυστυρενίου-πολυβουταδιενίου που φέρουν στην άκρη τους ομάδες ικανές να προσροφηθούν (PS-PB-Zw) Η μελέτη έγινε κυρίως με την τεχνικής της μέτρησης δυνάμεων (SFA) και της ανάκλασης νετρονίων. Ιδιαίτερο ενδιαφέρον παρουσιάζουν οι γραμμικές πολυμερικές αλυσίδες όπου ένα μεγάλου μοριακού βάρους πολυμερές φέρει στην άκρη του μια μόνο ακραία ομάδα. Τα γραμμικά αυτά συμπολυμερή είχαν μελετηθεί και στο παρελθόν. Το ζητούμενο όμως ήταν να μεταβάλλουμε την αρχιτεκτονική αυτή και να παρατηρήσουμε τις πιθανές μεταβολές στα ποιοτικά χαρακτηριστικά των προσροφημένων πολυμερικών στρωμάτων που διαμορφώνουν. Ως μέτρο σύγκρισης μελετήσαμε αρχικά τις ήδη μελετημένες γραμμικές αλυσίδες και στη συνέχεια προχωρήσαμε στη μελέτη των νέων δειγμάτων. Προσθέσαμε δύο και τρεις ακραίες ομάδες στο άκρο της πολυμερικής αλυσίδας μεγάλου μοριακού βάρους και αναμέναμε σύμφωνα με θεωρητικές προβλέψεις αύξηση στο προσροφημένο ποσό. Η αύξηση αυτή θα ήταν εμφανής μέσω της αύξησης του πάχους του πολυμερικού στρώματος που εύκολα μπορεί να μετρηθεί με την τεχνική της μέτρησης των δυνάμεων αλληλεπίδρασης. Επίσης μετρήσαμε άμεσα το προσροφημένο ποσό και με την τεχνική της ανάκλασης νετρονίων για πολυμερή τριών διαφορετικών μοριακών βαρών (25000, 70000 και 150000). Το σημαντικότερο αποτέλεσμά μας είναι ότι δεν παρατηρήσαμε αύξηση στο προσροφημένο ποσό σε σχέση με τις γραμμικές αλυσίδες και σε χρόνους ενός τυπικού πειράματος. Υπήρχε ωστόσο η υποψία ότι σε βάθος χρόνου μπορεί να αυξάνεται το προσροφημένο ποσό. Για το λόγο αυτό διεξαγάγαμε μια σειρά από μετρήσεις «κινητικής» με την τεχνική της ανάκλασης νετρονίων, αλλά και με συμπληρωματικές τεχνικές (διέγερση επιφανειακών πλασμονίων). Καμία μεταβολή δεν παρατηρήσαμε στο προσροφημένο ποσό, ούτε σε βάθος χρόνου αλλά ούτε και με την προσθήκη επιπλέον ακραίων ομάδων ικανών να προσροφώνται σε επιφάνειες. Στη συνέχεια επεκτείναμε τη μελέτη μας στην «αντίστροφη» αρχιτεκτονική. Διατηρήσαμε μια ακραία ομάδα και προσθέσαμε δύο και τρεις πολυμερικές αλυσίδες μεγάλου μοριακού βάρους (Μr=70000). Η αρχιτεκτονική που διαμορφώσαμε με τον τρόπο αυτό ήταν ουσιαστικά αρχιτεκτονική αστεροειδών πολυμερών με μικρό αριθμό πλοκαμιών. Στη περίπτωση αυτή παρατηρήσαμε μια σταδιακή μείωση του προσροφημένου ποσού με την προσθήκη επιπλέον βραχιόνων στο αστέρι. Η μείωση αύτη έχει παρατηρηθεί τόσο από μετρήσεις δυνάμεων, όσο και από μετρήσεις με ανάκλαση νετρονίων. Στα προφίλ δυνάμεων παρατηρούμε διαφοροποιήσεις σε σχέση με την γενική εικόνα που εμφανίζουν οι πολυμερικές βούρτσες. Αυτές οι διαφοροποιήσεις μπορούν να ερμηνευτούν με τη βοήθεια θεωρητικών προβλέψεων για τη συμπεριφορά των αστεροειδών πολυμερών. Η μείωση του προσροφημένου ποσού με αύξηση του αριθμού των πλοκαμιών μπορεί να αποδοθεί είτε σε λόγους κινητικής, είτε σε λόγους αλληλεπίδρασης μεταξύ των πολυμερικών αλυσίδων και μεταξύ των πλοκαμιών ενός μακρομορίου. Επίσης στη παρούσα διδακτορική διατριβή μελετήσαμε τη συμπεριφορά προσροφημένων ασθενών πολυ-ηλεκτρολυτών. Οι πολυ-ηλεκτρολύτες συγκεντρώνουν σταδιακά την προσοχή όλο και περισσότερων ερευνητών καθώς εμφανίζουν αρκετές σημαντικές εφαρμογές και είναι συμβατοί με βιολογικά συστήματα. Μέχρι πριν λίγο καιρό οι ασθενείς πολυ-ηλεκτρολύτες, όπως είναι το πολυακρυλικό οξύ που μελετούμε στη παρούσα εργασία, δεν είχαν μελετηθεί εκτενώς. Η μελέτη τους παρουσιάζει δυσκολίες, λόγω της πολυπλοκότητας στη συμπεριφορά τους. Αυτή εξαρτάται τόσο από τη συγκέντρωση άλατος του διαλύματος, όσο και από το pH. Μεταβάλλαμε τόσο τη συγκέντρωση άλατος του διαλύματος, όσο και το pH του διαλύματος εξετάζοντας πως αυτά επηρεάζουν το πάχος του προσροφημένου στρώματος και τα ποιοτικά χαρακτηριστικά των πολυ-ηλεκτρολυτικών προσροφημένων στρωμάτων. Παρατηρήσαμε λοιπόν μείωση του πάχους της πολυ-ηλεκτρολυτικής βούρτσας με αύξηση της συγκέντρωσης άλατος. Το αποτέλεσμα αυτό είναι πλήρως συμβατό με θεωρητικές προβλέψεις. Στη συνέχεια εξετάσαμε τη συμπεριφορά των ασθενών πολυ-ηλεκτρολυτών μεταβάλλοντας το pH του διαλύματος. Παρατηρήσαμε ότι με αύξηση του pH οι πολυμερικές αλυσίδες εκτείνονταν αυξάνοντας το πάχος της πολυμερικής βούρτσας. Αυτή η συμπεριφορά είναι άμεσο αποτέλεσμα της αύξησης του βαθμού διάστασης των πολυμερικών αλυσίδων. Αυξάνοντας το pH περισσότερα φορτία αποσπώνται από την πολυμερική αλυσίδα φορτίζοντάς την έτσι ισχυρότερα. Οι απωστικές ηλεκτροστατικές αλληλεπιδράσεις μεταξύ των φορτίων εξαναγκάζουν την αλυσίδα σε έκταση επιμηκύνοντας την πολυμερική βούρτσα. Τα αποτελέσματα αυτά είναι επίσης σε πλήρη συμφωνία με θεωρητικές προβλέψεις. / Flexible polymer chains end-tethered to a surface in good solvent tend to extend away from the surface due to excluded volume interactions. At sufficiently high grafting densities the chains become elongated normal to the surface, this extension being opposed by an elastic restoring force of entropic origin to form a layer of stretched chains referred to as a “polymer brush”. These systems have been studied extensively in recent years by numerous experimental techniques and theoretical methods. In the present investigation we have studied asymmetric star-shaped polymers whereby the different arms are either non-adsorbing PS chains or short PB chains terminating in a zwitterionic end-group known to adsorb strongly on surfaces such as mica or quartz. In this manner, it is possible on the one hand to form brushes with a single PS chain, but multiple zwitterionic stickers, and on the other to study the reverse case of multiple non-adsorbing arms attached to a surface via a single zwitterions. We have used the surface force balance technique to determine the interaction between such brush-layers formed on mica and neutron scattering to determine the absorbed amount and interanchor distance. Interactions between polymer brushes formed by highly asymmetric star-like polymers with a long PS arm and one, two or three short PB arms each terminating in a zwitterionic end-group were studied in order to explore the effect of the sticking energy on the brush structure. Polystyrene stars with two and three tails bearing a single end group were also studied to investigate how the height of the adsorbed layer and the grafting density are affected. Our measurements show no significant differences between PS with 1, 2 and 3 end groups. This may be due to kinetic reasons since additional polymer chains are hindered from attaching to a brush-bearing surface. The behavior of two PS chains with one end-group ((PS)2-PB-X) seems not to differ appreciably from that of PS-PB-X copolymer. On the other hand three PS chains with one end-group ((PS)3-PB-X) appeared to form layers with smaller brush height and greater interanchoring distance, relative to PS-PB-X layers. We can attribute this to the extra stretching that the three-chain architecture imposes on the adsorbed brush. In the present investigation we have also studied weak polyelectrolytes. Polyelectrolytes (charged polymer chains) remain among the least understood materials despite their importance in biology (proteins, DNA) and materials science. Their behavior and characteristics are not yet fully understood because of complicated correlations due to their charged nature that gives rise to long-range interactions. The counterplay of their properties as polymers and electrolytes with counterions around polyelectrolyte chains imposes additional difficulties on explaining their behavior. The association of counterions around polyelectrolytes and the pH of the solution are two parameters than can affect the properties of such systems. Especially weak polyelectrolytes (like poly-acrylic acid) are even sensitive to pH changes because of the alterable degree of dissociation. We measured forces between two charged polymer layers of Poly (isoprene-acrylic acid) diblock copolymers adsorbed on mica surfaces. Poly (isoprene-acrylic acid) diblock copolymers can be adsorbed from one end (poly-isoprene) which is sorter than the poly-(acrylic acid) part. The properties of the polymer layer at various salt concentrations and different pH of the solutions were measured by Surface Forces Apparatus. Information about the height of the polymer layer, and the adsorbed amount were extrapolated. The extracted results were compared with theoretical predictions showing well fit.

Magnetic recording media refers to the disc shaped thin film magnetic medium present inside the hard disk drive of a computer. Magnetic recording is an important function of the hard disk drive by which information such as text, pictures, audio and videos are stored. Information is broken down to a simple binary format and is stored as magnetised bits along the tracks of the disk forming the hard drive. Over the years advancements in research on the type of magnetic materials used has allowed increased data storage capacities by reducing magnetic bit sizes. It is with this advancement in magnetic data storage, that we have today's hard disk drive technology, which uses a perpendicular magnetic medium to store data. A perpendicular magnetic medium is a multi-layered magnetic thin film structure with the topmost layer comprising nanoscale magnetic grains of high perpendicular anisotropy. The topmost recording layer (RL) is mapped into individual bits of 80-100 nm² area that consist of 5-10 nm diameter CoCrPt grains, embedded in an oxide matrix. A bit area is defined to ensure a significant number of stable grains allowing data to be stored in each bit as a ‘0' or a ‘1' depending on its switched magnetic state. The magnetic grains if sputtered below a threshold grain size tend to suffer from thermal fluctuation and instability due to super-paramagnetic effects, hence bringing limitations to grain size. As a result of this, research in recent years has been directed at introducing a softer magnetic exchange coupled composite (ECC) layer above the recording layer. This layer facilitates the delicate balance of switching smaller grains with strong magneto-crystalline anisotropy at lower magnetic fields, by exchange coupling with the CoCrPt grains in the recording layer. However this technique of increasing the efficiency in the perpendicular magnetic medium by introducing ‘facilitating' layers is an area that is still being widely researched and understood. Although numerous surface and bulk analysis techniques exist to study magnetic and surface properties of these materials, there is limited information on the structural and magnetic properties of these materials at the nanoscale level. The reported work investigates the structural and magnetic properties of the magnetic grains and multi-layers in the perpendicular magnetic medium using polarised neutron scattering and reflectivity techniques. The work investigates the structural and magnetic properties of the CoCrPt grains, apart from understanding the CoCrPt magnetic grain switching. The work also investigates the magnetisation in the layers of the thin film perpendicular media structure using polarised neutron reflectivity (PNR). Using polarised small angle neutron scattering (PolSANS), it has been shown that ferromagnetic ordered core region of the CoCrPt grain in the recording layer is smaller than the physical CoCrPt granular structure. The magnetic switching behaviour of the CoCrPt grain at different magnetic fields is also analysed and the experimental PolSANS data is fitted with non-interacting size-dependent analytical grain switching models. This result provides significant evidence that the magnetic anisotropy increases with grain size, with larger magnetic grains having larger magnetic anisotropy. Polarised neutron scattering experiments are carried out with the magnetically softer exchange coupled composite (ECC) layer included in the thin film magnetic structure. The first experiments investigate if the ECC layer contributes to the nuclear and magnetic interference scattering term in the experimenting scattering data. The experiments clearly show that there is no contribution from the ECC layer in the nuclear and magnetic scattering interference term. The role of the ECC layer in the magnetic switching process is then investigated at different magnetic fields. The ECC layer was found to influence the size-dependent magnetic grain switching of the CoCrPt grains in the recording layer and a detailed investigation is presented in the reported work. Polarised neutron reflectivity (PNR) experiments have also been carried out with the ECC layer on the perpendicular magnetic media samples. These experiments investigate the composition and thickness of the thin film structure, while also providing information on the magnetic state of the thin films under the influence of an in-plane magnetic field. The in-plane magnetisation in the recording and ECC layer is determined at different in-plane magnetic fields. The magnetisation values determined for the ECC layer and the recording layer (RL) at different in-plane magnetic fields help better understand the differences in their magnetic properties.

Interfacial features of polymers are a complex, fascinating topic, and industrially very important. There is clearly a need to understand interactions between polymer layers as they can be used for controlling surface properties, colloidal stability and lubrication. The aim of my Ph.D study was to investigate fundamental phenomena of polymers at interfaces, covering adsorption, interactions between polymer layers and surfactants, surface forces and friction between adsorbed layers. A branched brush layer with high water content was formed on silica surfaces by a diblock copolymer, (METAC)m-b-(PEO45MEMA)n, via physisorption. The adsorption properties were determined using several complementary methods. Interactions between pre-adsorbed branched brush layers and the anionic surfactant SDS were investigated as well. Surface forces and friction between polymer layers in aqueous media were investigated by employing the Atomic Force Microscopy (AFM) colloidal probe technique. Friction forces between the surfaces coated by (METAC)m-b-(PEO45MEMA)n in water are characterized by a low friction coefficient. Further, the layers remain intact under high load and shear, and no destruction of the layer was noted even under the highest pressure employed, about 50 MPa. Interactions between polymer layers formed by a temperature responsive diblock copolymer, PIPOZ60-b-PAMPTMA17 (phase transition temperature of 46.1 °C), was investigated in the temperature interval 25-50 °C by using the AFM colloidal probe technique. Friction between the layers increases with increasing temperature (25-45 °C), while at 50 °C friction was found to be slightly lower than that at 45 °C. We suggest that this is due to decreased energy dissipation caused by PIPOZ chains crystallizing in water above the phase transition temperature. The structure of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers was determined by X-ray reflectometry. Surface forces and friction between DPPC bilayer-coated silica surfaces were measured utilizing the AFM colloidal probe technique. Our study showed that DPPC bilayers are able to provide low friction forces both in the gel (below ≈ 41°C) and in the liquid crystalline state (above ≈ 41°C). However, the load bearing capacity is lower in the gel state. This is attributed to a higher rigidity and lower self-healing capacity of the DPPC bilayer in the gel state. Friction forces in single asperity contact acting between a micro-patterned silicon surface and an AFM tip was measured in air. We found that both nanoscale surface heterogeneities and the µm-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless, Amontons’ first rule described average friction forces reasonably well. Amontons’ third rule and Euler’s rule were found to be less applicable to our system. / <p>QC 20141209</p>