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There is a chemical hydrophobic main chain and hydrophilic side chain of poly (N-isopropyl acrylamide)(PNIPAM). The molecular chain conformation of PNIPAM is sensitive to temperature due to its special chemical structure that responses reversibly with temperature about physical conformation. Polyhedral oligomeric silsesquioxanes (POSS) is a cubic silica core surrounded by eight corner functional group which can be incorporated into polymer material. PNIPAM was prepared by atom transfer radical polymerization (ATRP) carried out at 0 oC and resulted the narrow polydisperity of homopolymer. We produced star-like organic/inorganic hybrid POSS-PNIPAM via click and hydrosilylation reactions. The synthesis of POSS-PNIPAM copolymers was characterized by GPC, 1H NMR and FTIR analyses. DLS, UV-Vis and PL spectrometer were use to measure the particle size, cloud point and photoluminescence of PNIPAM solution in this study.

The pursuit of a greater understanding of the biological membrane has led to the development of a number of mimetics and probing techniques. This thesis contributes to both of these efforts. Towards the development of mimetics, poly(N-isopropylacrylamide) (pNIPAM) microgels were explored as membrane supports. PNIPAM microgels are “smart” materials that experience a volume phase transition (VPT) at ~32°C, where they undergo a severe loss in volume. The core-shell microgels were synthesized with a low crosslinked pNIPAM core covered by a highly crosslinked pNIPAM shell that was functionalized with exploitable carboxylates. It was shown that a lipid bilayer could be coated on these microgels using either liposomes or bicelles. Specifically, lipid bilayer enclosed microgels made with liposomes (“Lipogels”) were created by using hydrophobically modified microgels, which possessed the ability to sequester liposomes that could ultimately be fused into a continuous bilayer. It was also found that above the VPT temperature, surface decoupled lipid protrude into highly curved structures. Hence, the VPT property could be used to control the curvature of the Lipogel bilayer. These particles could be useful platforms for conducting biophysical membrane studies as well as drug delivery vehicles. Bicelles were also explored as lipid sources for microgel coating, resulting in the creation of “Bicellogels”. Electrostatic attraction between cationic bicelles and unmodified anionic core-shell pNIPAM microgels resulted in the coating of the latter. Astonishingly, the resulting bilayer was made up of only the long chain bicellar lipid. Due to the simplicity of this method, it could be extended to easily coat all types of soft material. The last development on the pNIPAM front involved the coupling of intact liposomes to microgels to create “VESCOgels”. These complexes offer two distinct cargo holds through which temporally distinct release can be achieved. Hence, they could be very useful for applications in tandem release. Lastly, the 31P CODEX NMR technique was adapted to study the lateral diffusion of phospholipids in large liposomes. This technique allows for the measurement of lateral diffusion coefficients of multiple phospholipids simultaneously. This could prove useful for the study of such biologically relevant phenomena as domain formations and drug-lipid interactions.

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Le but de cette thèse est de faire évoluer les connaissances et la compréhension des systèmes lubrifiants en milieux aqueux, synthétiques comme biologiques. Pour cela, nous élaborons des systèmes de monocouches auto-assemblées de microgels thermosensibles de pNIPAM cationiques afin d’en étudier les propriétés mécaniques et nanotribologiques. Nous mettons au point plusieurs synthèses de microgels afin d’étudier l’effet de l’élasticité sur le comportement tribologique. Nous regardons également l’effet de la nature du greffage des microgels en sur-face, en élaborant une méthode de couplage chimique novatrice, pour comparer les propriétés de monocouches physi- et chimisorbées. Nous étudions les propriétés mécaniques en mi-lieux aqueux des couches des différents microgels en fonction de la température, de la nature du greffage et du sel en présence, à l’aide d’une Microbalance à Cristal de Quartz avec mesure de Dissipation (QCM-D). Le coeur de notre étude est réalisé à l’aide d’un Appareil de Forces de Surface (SFA) modifié pour permettre des mesures tribologiques, dont les résultats seront traités en deux parties. La première consiste à caractériser les forces normales de surface lors-que l’on comprime deux surfaces décorées de microgels. La seconde est constituée de l’ana-lyse de ces surfaces sous compression et cisaillement. Nous explorons les propriétés lubrifiantes du système et observons l’apparition une force de normale dépendant de la vitesse de cisaillement, et dont nous cherchons l’origine. Nous avons ainsi découvert un mécanisme propre au substrat souple, décoré de particules discrètes avec un contact répulsif sans friction à longue portée. / The aim of this project is to advance the knowledge and understanding of lubricating systems, whether synthetic or biological, in aqueous media. For this purpose, we develop self-assem-bled monolayer 2D-arrays of cationic pNIPAM thermosensitive microgels in order to study their mechanical and nanotribological properties. We establish several synthetic routes to modulate the microgel rigidity and study its effect on the tribological behaviour. We also look at the effect of the grafting nature of microgels on the substrate, by developing an innovative chemical coupling method, to compare the properties of physisorbed and chemisorbed mon-olayers. We probe the mechanical properties of the microgel layers in aqueous environment while varying the temperature, the nature of the grafting and the salts added to the system, primarily by using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). The core of our study is performed using a modified Surface Forces Apparatus (SFA) which allows for tribological measurements, the results of which will be treated in two parts. First, we char-acterise the normal surface forces when compressing two surfaces decorated with the micro-gel layers. Second, we study the behaviour of these surfaces under compression and shear. We explore their lubricant properties and observe the appearance of a shear-induced velocity-dependent lift force, whose origin we seek to determine. We thus discovered a mechanism specific to a compliant substrate, decorated with discrete particles presenting a repulsive con-tact without friction at long range.

Phospholipid liposome binding to hydrogel beads based on poly(N-isopropylacrylamide) (PNIPAM) is accomplished employing either avidin/biotin conjugation or hydrophobic modification of the microgels, and the ability to form single supported lipid bilayers is explored. The co-monomer acrylic acid (AA), evenly distributed or localized to the shell of the microgel, is included to facilitate post-polymerization chemical modification of the hydrogel beads. The degree of chemical modification of the microgels as well as the thermal behavior is monitored via 1H and 13C nuclear magnetic resonance (NMR). Liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phoshocholine (POPC) and a small amount of commercially available biotinylated-lipid are shown to bind as intact entities while sequestering internal contents to biotinylated hydrogel beads, utilizing avidin as the coupling agent. Under fusogenic conditions, these bound liposomes remain as individual vesicles. Alternatively, POPC liposomes are shown to bind to microgels modified to display single chain alkyl groups or cholesteryl moieties, and remain as intact vesicles. It is demonstrated that these liposomes become permeable at high hydrophobe content. Bound liposomes will fuse into larger structures under high hydrophobe content conditions, but remain permeable. The volume phase transition (VPT) characteristic of PNIPAM microgels is shown to influence the permeability of hydrophobically bound (low hydrophobe content), but not avidin/biotin conjugated, liposomes. The degree of liposome binding, as well as their resulting structures and permeability are investigated utilizing 31P NMR, fluorescence spectroscopy and microscopy. The microgel-bound liposome and microgel-supported lipid bilayer hybrid systems would be ideally suited to drug delivery and tissue engineering applications. The microgel-supported single lipid bilayer system would, in addition, potentially act as a cell model system for membrane dynamics and embedded amphiphile NMR studies.

Phospholipid liposome binding to hydrogel beads based on poly(N-isopropylacrylamide) (PNIPAM) is accomplished employing either avidin/biotin conjugation or hydrophobic modification of the microgels, and the ability to form single supported lipid bilayers is explored. The co-monomer acrylic acid (AA), evenly distributed or localized to the shell of the microgel, is included to facilitate post-polymerization chemical modification of the hydrogel beads. The degree of chemical modification of the microgels as well as the thermal behavior is monitored via 1H and 13C nuclear magnetic resonance (NMR). Liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phoshocholine (POPC) and a small amount of commercially available biotinylated-lipid are shown to bind as intact entities while sequestering internal contents to biotinylated hydrogel beads, utilizing avidin as the coupling agent. Under fusogenic conditions, these bound liposomes remain as individual vesicles. Alternatively, POPC liposomes are shown to bind to microgels modified to display single chain alkyl groups or cholesteryl moieties, and remain as intact vesicles. It is demonstrated that these liposomes become permeable at high hydrophobe content. Bound liposomes will fuse into larger structures under high hydrophobe content conditions, but remain permeable. The volume phase transition (VPT) characteristic of PNIPAM microgels is shown to influence the permeability of hydrophobically bound (low hydrophobe content), but not avidin/biotin conjugated, liposomes. The degree of liposome binding, as well as their resulting structures and permeability are investigated utilizing 31P NMR, fluorescence spectroscopy and microscopy. The microgel-bound liposome and microgel-supported lipid bilayer hybrid systems would be ideally suited to drug delivery and tissue engineering applications. The microgel-supported single lipid bilayer system would, in addition, potentially act as a cell model system for membrane dynamics and embedded amphiphile NMR studies.

Poly(N-isopropylacrylamide) (PNIPAM) is a thermo-responsive hydrogel; that is, it is a macromolecule which exists in a hydrated state beneath its lower critical solution temperature (LCST). Polymers such as PNIPAM undergo a phase transition in response to changes in temperature, pressure, pH, salt concentration, and the addition of co-solvents. Previously, visible-light microscopic measurements of the pressure-induced phase transition have been hindered by the lack of a pressurization apparatus with the short working distance and optical transmission properties necessary for high resolution microscopy. We employ a high pressure setup which uses a fused silica micro-capillary to contain the sample. Our experiment reveals differences in the spatial evolution of the phase change across the temperature and pressure thresholds, and Raman measurements allude to conformational differences in the evolution of the phase transitions. The Raman peaks positions are in agreement with previous FTIR measurements, and due to a difference in selection rules additional vibrational bands are observed in the Raman spectra.

Poly(N-isopropylacrylamide) (PNIPAM) belongs to a class of stimuli-responsive materials known as “smart” polymers. When cast in the form of a hydrogel, PNIPAM’s lower critical solution temperature (LCST) of 32°C serves as a threshold for volumetric change. For solution temperatures below LCST, PNIPAM hydrogels exist as swollen, hydrophilic networks of polymer and water, spontaneously expelling the bound water molecules to shrink (and become increasingly hydrophobic) as temperature increases beyond LCST. This thesis centers on PNIPAM hydrogel layers grafted along the inner diameter of glass capillaries in order to form a temperature-responsive gating mechanism that spontaneously seals for solution temperatures below LCST. Surprisingly, very thin layers (10-20µm) of PNIPAM have dramatic effects on bulk fluid flow through the capillary due to complex interactions at the swelling interface. Specifically, for the case of capillary pressure driven flow, the swelling PNIPAM interface gives rise to "stick-and-slip" motion for bulk flow. Experiments explore the extent of this phenomenon, while a theoretical framework is proposed to model how the evolving gel interface pins the contact line. Additionally, an exploratory segment of this work examines the ways in which PNIPAM hydrogel nanoarrays can be synthesized via scalable template methods. Nanostructured PNIPAM films exhibit dramatic changes in surface properties with temperature, characterized by very low contact angles (~10°) below LCST, and very high ones (~160°) above LCST. Results for several methods are presented with lessons learned to guide future development of surfaces with temperature-responsive wetting properties.

Two families of amphiphilic polymers were characterized with respect to their ability to respond to changing solution conditions using steady-state and time-resolved fluorescence. Firstly, a series of 5 sequential amphiphilic polypeptides consisting of hydrophilic and ionizable aspartic acid (Asp) and hydrophobic phenylalanine (Phe) in varying sequence (AspxPhey)n were studied. The effect of pH on the collapse and aggregation behaviour of the samples was investigated using light scattering which determined that the samples became insoluble when the fraction of ionized amino acids decreased below 0.2. Pyrene fluorescence measurments demonstrated that hydrophobic aggregate formation was the cause of this behaviour. The fluorescence and light scattering experiments yielded a detailed description of how pH affects the collapse and aggregation behaviour of amphiphilic polypeptides. Secondly, a series of poly(N-isopropylacrylamide) chains in aqueous solution, varying in length and end-labelled with pyrene (Py2-PNIPAM), were studied with respect to their temperature dependent solubility using turbidimetry, light scattering, and fluorescence. The cloud point temperature (Tc) of the Py2-PNIPAM samples was found to increase with increasing chain length. Steady-state fluorescence spectra yielded the ratio of excimer intensity to that of monomer intensity, or (IE/IM)SS, which reached a maximum at Tc. Time-resolved fluorescence decays were analyzed using Model-Free Analysis (MFA). The MFA yielded the average rate constant of excimer formation <k>, and the parameters fagg, fdiff and ffree, which reflect the molar fractions of pyrenes that form excimer from pre-formed aggregates, form excimer by diffusion, and do not form excimer, respectively. Increasing the temperature above Tc caused a strong decrease in <k> and ffree, and a sharp rise in fdiff, which is consistent with the formation of mesoglobules. Increasing the temperature above the dehydration temperature of unlabelled PNIPAM (Tm = 34 oC) caused the distribution of pyrene to stabilize. The third study focused on aqueous mixtures of Py2-PNIPAM and unlabelled PNIPAM, and their ability to mix when the temperature was raised above Tm. Using turbidimetry, separate transitions for Py2-PNIPAM and unlabelled PNIPAM were identified. Steady-state fluorescence of the labelled chains demonstrated separate (IE/IM)SS transitions for the labelled and unlabelled chains as well. The MFA of the time-resolved fluorescence decays yielded no transition in <k> at Tm, while fagg, fdiff and ffree showed transitions consistent with unlabelled PNIPAM entering the Py2-PNIPAM mesoglobules. This led to the conclusion that the two polymers are able to mix at and above Tm, and that the mesoglobules are not frozen and vitreous. Fourthly, a variety of pyrene-labelled polymers in organic solution were studied using steady-state and time-resolved fluorescence, where the results of Birks’ Scheme and the Fluorescence Blob Model (FBM) were compared to the results of the MFA. The MFA was found to be able to faithfully reproduce the results of the other, more established, models. In addition, the MFA allowed the calculation of (IE/IM)SPC, the ratio of excimer to monomer intensity as determined by time-resolved fluorescence, which scaled linearly with both (IE/IM)SS and <k>. From this it was concluded that the MFA is able to fit the decays of pyrene-labelled polymers with any architecture studied thus far, but also provides an absolute measure of IE/IM which can be reproduced in any lab.