acase@tulane.edu / 1 / Giovanni M. Kelly II

A model of non-Markovian random block copolymers is developed and used to study the effects of blockiness and compositional polydispersity on the phase behaviour of random block copolymers. The model approximates a random copolymer as a series of segments with equal lengths, while each segment is composed of sequences of different monomers drawn randomly from a distribution. The phase behaviour of the model random copolymers is first examined using the random phase approximation (RPA) to study the effects of blockiness and polydispersity on the order-disorder transition. It is observed that the critical point is inversely proportional to the blockiness. Compositional polydispersity is found to facilitate phase separation, and could induce macrophase separation. Next, the model is implemented into self-consistent field theory (SCFT) in order to elucidate the full phase behaviour of symmetric (A/B)-A random copolymers. Finally, the model is applied to the particular case of poly(styrenesulfonate-b-methylbutylene) (PSS-PMB) to study the effects of blockiness on the phase behaviour. In particular, the stability and structure of the `swollen gyroid' morphology predicted by previous Monte Carlo simulations is examined. / Thesis / Master of Science (MSc)

The dynamics of thin polymer films display many differences from the bulk dynamics. Different modes of motions in polymers are affected by confinement in different ways. The enhancement in the dynamics of some modes of motion can cause anomalies in the glass transition temperature (Tg) of thin films, while other modes of motion such as diffusion can be substantially slowed down due to the confinement effects. In this thesis, different modes of dynamics are probed using different techniques. The interface healing of two identical polymer surfaces is used as a probe of segmental motion in the direction normal to the plane of the films and it is shown that this mode of motion is slowed down at temperatures above bulk glass transition, while the glass transition itself is decreased indicating that the type of motion responsible for the glass transition is enhanced. The glass transition measurements at different cooling rates indicate that this enhancement only happens at temperatures close to or below bulk glass transition temperature and it is not expected to be detected at higher temperatures where the system is in the melt state. It is shown that the sample preparation technique is not a factor in observing this enhanced dynamics, while the existence of the free surface can be important in observed reductions in the glass transition temperature. The dynamics near the free surface is further studied using a novel nano-deformation technique, and it is shown that the dynamics near the free surface is in fact enhanced compared to the bulk dynamics and this enhancement is increased as the temperature is decreased further below Tg. It is also shown that this mode of relaxation is much different from the bulk modes of relaxations, and a direct relationship between this enhanced motion and Tg reduction in thin films can be established. The results presented in this thesis can lead to a possib le universal picture that can resolve the behavior of different modes of motions in thin polymer films.

The dynamics of thin polymer films display many differences from the bulk dynamics. Different modes of motions in polymers are affected by confinement in different ways. The enhancement in the dynamics of some modes of motion can cause anomalies in the glass transition temperature (Tg) of thin films, while other modes of motion such as diffusion can be substantially slowed down due to the confinement effects. In this thesis, different modes of dynamics are probed using different techniques. The interface healing of two identical polymer surfaces is used as a probe of segmental motion in the direction normal to the plane of the films and it is shown that this mode of motion is slowed down at temperatures above bulk glass transition, while the glass transition itself is decreased indicating that the type of motion responsible for the glass transition is enhanced. The glass transition measurements at different cooling rates indicate that this enhancement only happens at temperatures close to or below bulk glass transition temperature and it is not expected to be detected at higher temperatures where the system is in the melt state. It is shown that the sample preparation technique is not a factor in observing this enhanced dynamics, while the existence of the free surface can be important in observed reductions in the glass transition temperature. The dynamics near the free surface is further studied using a novel nano-deformation technique, and it is shown that the dynamics near the free surface is in fact enhanced compared to the bulk dynamics and this enhancement is increased as the temperature is decreased further below Tg. It is also shown that this mode of relaxation is much different from the bulk modes of relaxations, and a direct relationship between this enhanced motion and Tg reduction in thin films can be established. The results presented in this thesis can lead to a possib le universal picture that can resolve the behavior of different modes of motions in thin polymer films.

Thesis (Ph. D.)--Cornell University, 2007. / (UMI)AAI3276765. Source: Dissertation Abstracts International, Volume: 68-08, Section: B, page: 5311. Adviser: Sol Gruner.

<p> Although the glass transition temperature (<i>T_g</i>) and dynamics of polymers confined to the nanoscale have been studied for twenty years, a physical understanding is still lacking. The reason for a polymer species dependent <i>T_g</i>-confinement effect and the role of neighboring polymer domains in perturbing the <i>T_g</i> of a confined species are areas with a need for greater study as they will inform many of the decisions regarding the use of polymers in nanomaterials. </p><p> In this work, fluorescence spectroscopy is used as the primary tool to characterize <i>T_g</i> in a number of systems. First, micelle core <i>T_g</i> and critical micelle temperatures can be determined via pyrenyl label fluorescence for block copolymers in organic solvent at polymer contents which cannot be reliably characterized by other standard methods. Next, measurements were extended to miscible polymer-polymer blend systems where two component <i>T_gs</i> can be determined via a single pyrene-labeled component. Fluorescence can characterize systems with small component <i>T_g</i> differences and near-infinitely dilute blend components unlike scanning calorimetry. </p><p> Studies of the near-infinitely dilute blend components reveal that a 0.1 wt% polystyrene component can have its <i>T_g</i> tuned over a 150 &deg;C range depending on the blend partner. Analogous tunability of <i>T_g</i> is also reported in multilayer film systems with an ultrathin PS layer surrounding by bulk neighboring domains. The same limiting <i>T_g</i> is reported by PS for a given neighbor indicating a common physical origin of perturbations in both systems. The perturbations are correlated with fragility which also tracks with the magnitude of <i>T_g</i>-confinement effects in single layer polymer films. Thus, fragility provides a unifying explanation of confinement effects in multilayer films, blends, and single layer films (in the absence of attractive interactions). </p><p> Surface wave dynamics are also examined in ultrathin polystyrene layers on various substrates. It is demonstrated that surface dynamics become much slower than anticipated by capillary wave theory as the film thickness decreases. Additionally, surface wave dynamics become orders of magnitude faster as the modulus of the supporting substrate decrease.</p>

<p> Presented here is a detailed study of electron transport in highly disordered insulating materials (HDIM). Since HDIMs do not lend themselves to a lattice construct, the question arises: How can we describe their electron transport behavior in a consistent theoretical framework? In this work, a large group of experiments, theories, and physical models are coalesced into a single formalism to better address this difficult question. We find that a simple set of macroscopic transport equations--cast in a new formalism--provides an excellent framework in which to consider a wide array of experimentally observed behaviors. It is shown that carrier transport in HDIMs is governed by the transport equations that relate the density of localized states (DOS) within the band gap and the occupation of these states through thermal and quantum interactions. The discussion is facilitated by considering a small set of simple DOS models. This microscopic picture gives rise to a clear understanding of the macroscopic carrier transport in HDIMs. We conclude with a discussion of the application of this theoretical formalism to four specific types of experimental measurements employed by the Utah State University space environments effects Materials Physics Group.</p>

This research focuses on the modeling of surfaces decorated by grafted polymers in order to understand their structure, energetics, and phase behavior. The systems studied include flat and curved surfaces, grafted homopolymers and random copolymers, and in the presence of solvent conditions, homopolymer melt conditions, and diblock copolymer melt conditions. We use self-consistent field theory to study these systems, thereby furthering the development of new tools especially applicable in describing curved particle systems and systems with chemical polydispersity. We study a polymer-grafted spherical particle interacting with a bare particle in a good solvent as a model system for a polymer-grafted drug interacting with a blood protein in vivo. We calculate the energy of interaction between the two particles as a function of grafting density, particle sizes, and particle curvature by solving the self-consistent field equations in bispherical coordinates. Also, we compare our results to those predicted by the Derjaguin approximation. We extend the previous study to describe the case of two grafted particles interacting in a polymer melt composed of chains that are chemically the same as the grafts, especially in the regime where the particle curvature is significant. This is expected to have ramifications for the dispersion of particles in a polymer nanocomposite. We quantify the interfacial width between the grafted and free polymers and explore its correlation to the interactions between the particles, and use simple scaling theories to justify our results. In collaboration with experimentalists, we study the behavior of the glass transition of polystyrene (PS) films on grafted PS substrates. Using the self consistent field theory methods described above as well as a percolation model, we rationalize the behavior of the glass transition as a function of film thickness, chain lengths, and grafting density. Grafting chemically heterogeneous polymers to surfaces in melt and thin film conditions is also relevant for both particle dispersion and semiconductor applications. To study such systems, we model a random copolymer brush in a melt of homopolymer that is chemically identical to one of the blocks. We modify the self-consistent field theory to take into account the chemical polydispersity of random copolymer systems and use it to calculate interfacial widths and energies as well as to make predictions about the window in which perpendicular morphologies of diblock copolymer are likely to form. We also explore the effect of the rearrangement of the chain ends on the surface energy and use this concept to create a simple modified strong stretching theory that qualitatively agrees with our numerical self-consistent field theory results. We explicitly study the system that is most relevant to semiconductor applications - that of a diblock copolymer melt on top of a substrate modified by a random copolymer brush. We explore the morphologies formed as a function of film thickness, grafting density, chain length, and chain blockiness, and make predictions about the effect of these on the neutral window, that is, the range of brush volume fractions over which perpendicular lamellae are expected to occur. / text

Hydrogel materials have become a heavily studied as materials for interfacing with biology both for laboratory investigations and the development of devices for biomedical applications. These polymers are water swellable and can be made responsive to many different stimuli by choice of monomers, co-monomers, and cross-linkers or functionalization with pendent ligands, substrates, or charged groups. The high water content, low moduli and potential responsively of these polymers make good candidates for biomaterials. A specific type of hydrogel called a microgel or a hydrogel micro/nanoparticle has similar properties to bulk hydrogel materials. Many of the interesting results and utility of the microgels in bioapplications are due to their inherent softness of the material. Here, the softness, flexibility, and conformability of these water swollen particles is used to create an interesting sensor platform, studied in the context of a microgel passing through a pore, and used as an emulsifier to create a drug delivery platform. The unifying theme of this dissertation is the softness of microgels which is critical for all of these experiments. However, the study of individual microgel softness is challenging and complex, since the softness is composed of two different components. The first is that the microgel is a swollen polymer which can be deswollen by an external stimuli or force. The second is that the microgel is a volume conserving elastic colloid which can deform without deswelling under the certain conditions. Throughout, this dissertation will discuss the ramifications of the complex softness of microgels in each experimental result and potential application.

<p> Self-replication exists everywhere in nature from bacteria to human beings. Several generations of scientists have worked on self-replication in nature. However, a more challenging breakthrough is to self-replicate through lifeless matter, such as colloids. To accomplish this paradigm shift, technically, we need to investigate thermodynamics, kinetics, multi-functionality, mobility, and the formation of specific covalent bonds of DNA-coated colloids. These are the essential studies for realizing colloidal self-replication. </p><p> We present and experimentally test a mean field thermodynamic model for DNA-functionalized colloidal aggregation and find excellent agreement when accounting for the binding configurations between a pair of particles and adding an additional entropic term due to restricted configurations for DNA bound to both surfaces. We study the kinetics of aggregation as a function of DNA coverage and salt concentration over the range: 4 minutes - 79 hours. The fundamental factor is an intrinsic hybridization time for a pair of complementary DNA in solution retarded by Coulomb repulsion, and the entropic search for inter-particle binding configurations. We investigate the flexibility of the DNA colloid system for colloidal architecture by evaluating theoretically and experimentally the number of specific associations each of our colloids can have with its neighbors. In theory, we find that our particles can recognize up to 76 different particles due to intrinsic properties of DNA hybridization and sequence combination while in experiment we confine that up to 40 different particles can be bound. A practical limit is &sim;100. To demonstrate the utility of our "polygamous particles," we synthesize a dual-phase material, which by control process forms either gels or liquids at the same temperature. </p><p> "Sticky" particles typically have low mobility. We demonstrate a novel solution to this problem by combining depletion and DNA interactions, and we successfully synthesize crystals and designed hexagon clusters. Finally, we use cinnamate-modified DNA to control formation of specific covalent bonds and develop a new DNA photolithography. We functionalize a patterned area on a gold surface by a controlled UV light pattern.</p>