Studies of water-induced surface reorganization of biocompatible amphiphilic copolymers by X-ray photoelectron spectroscopy using cryogenic sample handling techniques
Amphiphilic copolymers containing hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) and hydrophobic, low-surface-energy poly(dimethylsiloxane) (PDMS) are expected to have potential applications as biomaterials since both components possess many valuable properties, such as: the biocompatibility of both PHEMA and PDMS, the hydrophilicity and high mechanical strength of PHEMA, and the excellent oxygen permeability of PDMS. The surface composition of multicomponent copolymers or blends is different from the bulk composition due to the preferential segregation of one component over another toward the surface or interface under the thermodynamic driving force for minimizing the total free energy of the system. In air, the low-surface-energy hydrophobic segment can preferentially migrate to the surface; upon exposure to water, a surface reorganization can occur, resulting in a surface composed of more high-surface-energy hydrophilic phase. The extent and in-depth distribution of surface segregation in copolymers or blends are affected by a number of factors, such as polymer structure and morphology, processing conditions, and in hydrated copolymers or blends, hydration kinetics and water chemistry. A series of novel amphiphilic graft copolymers having a hydrophilic PHEMA backbone and hydrophobic PDMS side chains were synthesized to develop a model system to examine surface properties under dry and wet conditions. The surface composition of these graft copolymers was quantified by angle-dependent X-ray Photoelectron Spectroscopy (XPS). The effects of molecular weight distribution (MWD, M w / M n = 2.85-3.55 for copolymers with broad MWD, M w / M n = 1.09-1.16 for copolymers with narrow MWD), PDMS graft length and PDMS bulk content on the surface composition of the copolymers at dry state were studied and described in Chapter 1. For poly(HEMA- g -DMS), understanding of its surface composition and morphology in the water medium is important since this will help to understand its surface structures and interactions with aqueous phases and thus help to improve its applications. To use XPS to analyze hydrated samples that may undergo a reorganization in surface chemistry based on the environment, a special sample handling technique is needed since hydrated samples must be frozen to a very low temperature to stabilize the hydrated surface. To analyzed hydrated poly(HEMA- g -DMS), a cryogenic sample handling technique was developed and described in Chapter 2. Using the cryogenic sample handling techniques developed in Chapter 2, the surface composition of the hydrated poly(HEMA- g -DMS) was quantified by angle-dependent XPS and described in Chapter 3. In addition, the surface reorganization upon hydration and the surface reversibility upon dehydration, annealing and heptane treatments for poly(HEMA- g -DMS) were investigated as a function of PDMS graft length, hydration time and XPS sampling depth.