| Summary: | 博士 === 國立中正大學 === 化學工程研究所 === 105 === Complex fluids such as polymer solutions, gels and colloidal suspensions often exhibit complicated quiescent/flow properties due to the intermolecular interactions among the dissolved or dispersed components in fluids. These complicated interactions often lead to structures and dynamics spanning a wide range of time/length scales, and therefore multiscale experimental schemes are necessary to unveil the underlying molecular mechanisms behind various macroscopic phenomena. In this dissertation, a combination of scattering techniques (i.e., small angle light scattering (SALS), static/dynamic light scattering (SLS/DLS), small angle X-ray scattering (SAXS)), rheometer and microscopy techniques (i.e., optical microscopy (OM), scanning electron microscopy (SEM) and cryogenic transmission electron microscopy (cryo-TEM)) was employed to explore the kinetic and structural features of conjugated polymer gel during a sol-gel transition, and the mesoscale aggregation properties of C60 solutions.
The structural evolution of an amorphous conjugated polymer, poly(2-methoxy-5-(2’-ehylhexyloxy)-1,4-phenylenevinylene (MEH-PPV), in a hybrid-solvent medium (nonane/chlorobenzene) during gelation process (i.e., aging for 24 h at 30 °C) was explored using multiscale experimental schemes. We found that the micrometer-sized colloidal aggregates served as gel precursor in solution, and later bridged each other mutually to form gel through a one-dimensional (1-D) to three-dimensional (3-D) kinetic pathway. Furthermore, our results indicated a concomitant structural reorganization within the colloidal aggregates, where polymer chains packed spontaneously to form local fiber-like structures that are elastic by nature. On the basis of the near coincidence of such microscopic and macroscopic phase alterations, we contend that fiber-like structures can be used for the bridging agent to fabricate colloidal strands upon gelation. The present findings elucidate complicated gelation phenomena of MEH-PPV, with the prospect of utilizing specific polymer-solvent interactions to incubate desirable colloidal aggregates and gels in room-temperature processing of conjugated polymers.
The scattering and microscopy techniques and analysis schemes were employed to resolve the nanoscale to microscale aggregation properties of C60 in two distinct aromatic solvents (i.e., toluene and chlorobenzene) and a range of concentrations (c=1-2 and c=1-5 mg/mL, respectively). Combined depolarized dynamic light scattering (DDLS) and cryo-TEM imaging identified that while C60 aggregate clusters are notably anisotropic in shape in chlorobenzene, they are basically isotropic in toluene. SLS and SAXS analyses revealed that these distinct clusters are formed by different, solvent-induced, nanoscale aggregate units, which apparently organized themselves into self-similar, mesoscale aggregate fractals. The SEM images and XRD (X-ray diffraction) patterns further confirmed that the morphologies of drop-casting thin films and the crystal structures of C60 are dependent on the prior solution state. In particular, previously unnoticed DLS features revealing peculiar relaxation (i.e., q-independent relaxation) behavior in both C60 solvent systems were suggested to arise from the confinement effect according to our number density analysis of aggregates in conjunction with known static structural features. Overall, a better knowledge of the fundamentals of solvent-tunable, self-organized fullerene aggregates may improve our ability to control a hierarchy of aggregate structures that would best meet the requirements for a wide range of nanotechnological applications.
The ongoing developments of rheo-SALS and microrheology are introduced in the final section. Rheo-SALS is useful in gaining insight into the locally varying structural features brought in by flow fields, as well as the correlation between structural changes and rheological responses. Microrheology can not only help assess the viscoelastic features in the high-frequency region (i.e., 100<w(rad/s)<100000), which is inaccessible in a conventional rheometer, but also provide structural information at a microscopic level. In summary, the use of multiscale (length and time) experimental protocols makes it possible to systematically explore hierarchical soft matter systems.
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