Materials Science and Technology of Polymers at the University of Twente

New Approaches in the Engineering and Characterization of Macromolecular Interfaces Across the Length Scales: Applications to Hydrophobic and Stimulus Responsive Polymers

The thesis discribes how to enhance characterization and surface engineering approaches to test and control physico-chemical changes on modified hydrophobic (LDPE and PDMS) and stimulus-responsive (PFS) polymers across different length scales. [Here LDPE denotes low density polyethylene, PDMS stands for poly(dimethylsiloxane), and PFS for poly(ferrocenyldimethylsilanes)]. Efforts have been made to design and engineer desired surface properties of selected polymers and to characterize the chemical composition, electrokinetic and mechanical properties by various experimental techniques from the nano to the meso scale. Furthermore, a comparison of these techniques has been carried out in order to understand the aforementioned issues.
In chapter 2 details are provided regarding essential issues for the scientific and technological background necessary to introduce the scope and the state of the art for this Thesis. Recent advances of modern approaches to surface analysis, as well as the state-of-the-art of corresponding instrumentation are reviewed. A high priority has been given to the investigation of adhesion and adherence across the length scales from micro- to nanometer contact areas. The physical principles and theories of polymer adhesion, and the various experimental approaches used in adhesion studies are introduced. Finally, some new developments of adhesion applications in polymer science are reviewed. Flame-treatment has been developed to modify the surface properties of LDPE in order to yield valuable finished products. The relationship between surface chemistry and morphology of flame-treated LDPE by various characterization techniques across different length scales has been described in chapter 3. The surface concentration of hydroxyl, carbonyl and carboxyl groups, as well as surface wettability as a function of the number of flame-treatment passes (which is proportional to the treatment time) was obtained. In addition, surface topography of LDPE (before and after flame-treatment) was examined by atomic force microscopy (AFM). Carboxyl functional groups were specifically identified by fluorescent labeling and the results were compared with time-of-flight secondary ion mass spectrometry (ToF-SIMS) data. The effective surface pKa values of carboxylic acid (-COOH) were revealed by chemical force titration curves and the effective surface pKa values were found to be around 6.
Chapter 4 demonstrates the effects of the flame-treated LDPE on the work of adhesion (W) and energy release rate (G) as assessed by a custom-built adhesion testing device (ATD). ATD measurements were performed between extracted PDMS lenses prior to, and following UV/ozone treatment (PDMS and PDMSOX) and LDPE films with different treatment level. Work of adhesion and pull-off force were studied as functions of treatment numbers. The dependence of the measured values of the energy release rate on the rate of separation, dwelling time (time in contact), flame-treatment effects, were also considered. The difference between the energy release rate and the thermodynamic work of adhesion, which is defined as the adhesion hysteresis increased systematically with increasing treatment numbers. The source of adhesion hysteresis observed was attributed to specific bonding of surface functional groups and viscoelastic effect. UV/ozone irradiation has been adopted for controlling the wettability of PDMS for a wide variety of applications. Studies of the surface chemistry and ionization state of cross-linked PDMS exposed to UV/ozone as a function of treatment time using various complementary and independent experimental techniques are presented in chapter 5. It was found that the top 1-2 nm surface layer was dominated by silanol groups (–SiOH) for which the concentration increased with increasing treatment dose as unveiled by quantitative studies using ToF-SIMS. The lateral distributions of the silanol groups were analyzed on the nanometer length scale by means of AFM with chemically functionalized tip probes in aqueous buffer solutions at varying pHs. The protolytic properties of oxidized PDMS surfaces in the presence of sodium chloride and phosphate electrolytes were further analyzed by streaming potential measurements as a function of the UV/ozone treatment time. The electrokinetic data were quantitatively interpreted on the basis of a classical double layer model that combined the Gouy-Stern picture and the site dissociation theory. A characteristic pKa constant of silanol groups was found to be a function of treatment time. We concluded that this shift was primarily due to the hydrophobic environment, which has a stabilizing effects on the surface ionization. The broadening of the pKa range that was also observed with increasing treatment dose was interpreted as a result of a mixed contribution of increasing probability of intermolecular hydrogen bonding and specific ion adsorption.
Chapter 6 presents the investigation of the dependence of the Young’s modulus of UV/ozone treated PDMS across different length scales. AFM has been used to probe PDMS surface mechanical properties at the nanometer length scale. The values of the Young’s modulus of each sample was calculated by comparing different continuum contact mechanics theories (Hertz and JKR) as well as different tip shape models (spherical and hyperboloid). Moreover, a custom-built ATD has been used to study the modulus at the micrometer length scale with PDMS using a Si3N4 lens. The modulus of PDMS increased with increasing treatment time as unveiled by both AFM and ATD techniques. Moreover, in comparison with PDMS bulk modulus values which were obtained by tensile tests, we showed that the modulus decreased with the increase the characteristic length scales of the different approaches. These surface mechanical properties studies of UV/ozone treated PDMS provide new insights into the contact performance used in surface engineering and tribological applications.
Chapter 7 describes quantitative friction and adherence (adhesion) measurements between Si3N4 tips and reversibly oxidized and reduced poly(ferrocenyldimethylsilane) (PFS) layers. Corrections for fluid refraction have been introduced in the calibration protocol of cantilever spring constants to obtain quantitative friction data in aqueous electrolytes. Measurements of interfacial friction as a function of applied load on the nanoscale revealed a significant change in friction between the oxidized and reduced PFS layers. The average friction coefficient (0.47) and adherence (0.61 nN) between oxidized PFS and Si3N4 tip was found to be larger than that of neutral PFS (0.3, and 0.03 nN, respectively). Issues related to the interpretation of observed friction and adherence changes were discussed. In particular, electrostatic force, conformational changes of polymer chains and ionic interactions in the presence of ClO4- anions were concerned. As shown in this Thesis, "surface engineering" can provided polymer surfaces with specific properties to suit wide applications without affecting the bulk properties. In-depth understanding of the chemical and physical properties of polymer surfaces is, however, essential to further improve those in surface modification techniques. New characterization approaches across the length scales from sub millimeter to nanometer can contribute to a significant enhancement of our understanding of surface properties, and optimizing treatment procedure. Specifically, with the advent of nanoscale surface analysis technique, such as AFM, the gap between atomic/molecular scale properties and bulk (ensemble average) performance has been significantly narrowed.