Microrheology and microstructure of poly(vinyl alcohol)-based physical gels
We study the microrheology and microstructure of physically cross-linked poly(vinyl alcohol) (PVA) gels using atomic force microscopy, dynamic light scattering and particle tracking. We compare the microscopic rheological properties with the bulk properties measured using conventional shear rheometry, and correlate the rheological properties to the structure of the materials. We develop a new technique for investigating the viscoelastic properties of soft materials using the atomic force microscope. The electronic feedback of an atomic force microscope is modified by adding a small oscillatory voltage to the deflection signal, and the amplitude and phase of the motion of the sample stage is monitored by a lock-in amplifier to determine the viscous and elastic moduli of the sample. We apply this method to PVA hydrogels and suspended PVA nanofibers. We find the moduli of both the fibers and the hydrogels to show a significant frequency dependence. We perform rheological and dynamic light scattering measurements on PVA/poly(ethylene glycol) (PEG) blends during aging. The properties of the blends change much faster with age than those of the pure PVA solution, and the blends undergo phase separation and gel over time. From dynamic light scattering experiments, we observe changes of the relaxation times in the blends as they age. We determine the gel point on the microscopic scale from the depression of the intensity autocorrelation function of the scattered light. We find that gelation of the PVA/PEG blends is induced by the growth of aggregates in the blends. By comparing the gel point determined by light scattering to that from the rheometry, we find that the macroscopic gel point is earlier than the microscopic one, indicating that the gel transition is length-scale dependent. Particle tracking microrheology is used to investigate the microrheology and microstructure of these PVA/PEG blends as a function of PEG concentration and aging time. Dynamic light scattering probes the ensemble--averaged motion of all tracer particles in the scattering volume, while particle tracking tracks the motion of many individual tracers. The local viscoelastic moduli are determined from the measurements using the generalized Stokes-Einstein relation. We find that addition of PEG to the PVA solutions influences the micro--environment significantly, and that phase separation happens before gelation as the blends age. We again find that the microscopic gel point occurs later than the macroscopic one, confirming the above results. The experimental results are consistent with a model in which the PVA/PEG blends consist of PVA-poor pores within a continuous PVA-rich domain which gels.
Soft materials, physical gels, viscoelasticity, microrheology
Networked Digital Library of Theses and Dissertations