Multilayer Based Nanogels and Bio-lubricants

• Main Author: Liu, Chao
• Format: Doctoral Thesis
• Language: English
• Published: KTH, Yt- och korrosionsvetenskap 2014
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-140688; http://nbn-resolving.de/urn:isbn:978-91-7501-993-2

Surface chemistry plays an important role in numerous technological innovations, and gives the ability to modify and control surface and interface properties. Layer-by-Layer (LbL) self-assembly is a simple concept that can provide a route to versatile combination of materials as well as fine control of film thickness, hydrophobicity, thermal, optical and electrical properties. This methodology has thus received attention from both academic and industrial experts. A large variety of polymers, proteins and nanoparticles can be utilized in the LbL process. In my PhD-thesis work I made use of the LbL technique to build surface grafted nanogels and bio-lubricant films. Various surface sensitive techniques have been applied in this PhD thesis work. The three main methods were quartz crystal microbalance with dissipation (QCM-D), total internal reflection Raman (TIR-Raman) spectroscopy, and atomic force microscopy (AFM). In lieu of conventional methods such as reflectometry or ellipsometry, we have combined data obtained from QCM-D and TIR-Raman to gain information on wet and dry LbL films as well as their water content. The relatively new AFM imaging mode known as PeakForce QNM was used to investigate topographical and nano-mechanical properties of LbL films. The colloidal probe technique was implemented with AFM for normal and lateral force measurements. It is becoming increasingly clear that biopolymers are important for a sustainable society since they are renewable, have useful properties and often are environmentally benign. One main part in this thesis work was to fabricate thin chitosan (CHI) nanogels covalently attached to solid surfaces. This was achieved by first assembling a chitosan/poly(acrylic acid) multilayer using silane chemistry and the LbL method. Next, the chitosan molecules were selectively cross-linked in the film, and finally poly(acrylic acid) was (partly) rinsed out of the nanogel. The final composition and the responsiveness of the nanogel to pH and ionic strength changes were found to depend on the cross-linking density. Statistical analysis, known as target factor analysis, was used to analyze TIR Raman spectra and draw conclusions about e.g. the composition of multilayers during the build-up process, and the kinetics of cross-linking of chitosan. The other main part in this thesis work also utilized the LbL methodology, but here the main goal was to gain understanding on the unprecedented lubrication of synovial joints. It is in general terms due to a sophisticated hierarchical structure of cartilage combined with synergistic actions of surface-active components present in the synovial fluid, but many aspects of this fascinating biotribological system remain poorly understood. I focused on the lubricating ability of synovial fluid components, and in particular on the association of two components of the synovial fluid, hyaluronan and dipalmitoyl phosphatidyl choline (DPPC), in bulk solution and at interfaces. We found that hyaluronan associated with DPPC vesicles in bulk and adsorbed to supported DPPC bilayers, and that the LbL method could be utilized for forming composite layers of these two components. These composite layers had very favorable lubrication properties, with a low friction coefficient as low as 0.01, and they were also sufficiently stable to shear and load up to the pressure that broke healthy cartilage.     === <p>QC 20140130</p>