Electrospinning Fibers via Complex Coacervation

Electrospun fibers are high-surface-area materials widely used in applications ranging from batteries to wound dressings. Typically, an electrospinning precursor solution is prepared by dissolving a high-molecular-weight polymer in an organic solvent to form a sufficiently entangled solution. Our approach bypasses the requirement for entanglements and completely avoids toxic chemicals by focusing on using an aqueous complex coacervates solution. Coacervates are a dense, polymer-rich liquid phase resulting from the associative electrostatic complexation of oppositely charged macroions. We were the first to demonstrate that liquid complex coacervates could be successfully electrospun into polyelectrolyte complex (PEC) fibers. A canonical coacervate system was formed with poly(4-styrene sulfonic acid, sodium salt) and poly(diallyldimethylammonium chloride). Characterization of the binodal phase behavior demonstrated the thermodynamic linkage of the polymer and salt concentrations (CP and CS): greater CP indirectly controlled by decreasing CS. Our results showed that electrospun fibers had smaller and more uniform diameters with increasing applied voltage, separation distance, and CP. The resulting fibers were ultra-stable to heat and organic solvents because of the strong electrostatic attraction between polymers. Coacervates have the potentials to be developed into an environmentally benign electrospinning precursor platform. Having demonstrated coacervates electrospinnability, we hypothesized that the associative interactions that drive coacervation can also enable electrospinning. Therefore, we synthesized a set of backbone-matched methacroloyl polymers of different chain lengths and formed coacervates. Amazingly, all the coacervates were successfully electrospun into continuous fibers, including an oligomeric Nanion/Ncation 6/9 coacervate system where no physical chain entanglements were possible. After correlating the spinnability of coacervates with their rheological behavior, we found out that spinnable coacervates had prolonged relaxation behaviors due to interpolymeric electrostatic interactions. The ability to electrospin oligomeric coacervates has significant impacts on decoupling polymer chain length from electrospinnability thus, enabling fibers formation from chemical species that were previously considered non-spinnable. Knowing the long history of applications where complex coacervates were used for encapsulation, we also investigated the ability to electrospin cargo-loaded PEC fibers via coacervation. We used a family of six fluorescent dyes with systematic structural differences as model drugs. All dyes preferentially partitioned into the coacervates phase, allowing the subsequent electrospinning of highly-loaded fibers. Dyes that were electrostatically attracted to PSS to undergo π-π interactions partitioned more favorably in the coacervate phase, slowed the release from within PEC fibers when exposed to aqueous solutions, as well as exhibited enhanced uptake by fibers. These findings have the potentials to use the PEC fibers in applications related to biomedicine, energy, and separations, where controlling the uptake and release of cargo into sponge-like mats is needed. In summary, this dissertation demonstrated the first electrospinning of aqueous complex coacervates solution into PEC fiber mats, identified that the spinning mechanism was electrostatic interactions as an alternative of entanglements, and studied the associated dye encapsulation and release properties of the mats to enable their use across a range of applications

Experimental Investigation of Thermal Cracking and Permeability Evolution of Granite with Varying Initial Damage under High Temperature and Triaxial Compression

Thermal cracking and permeability evolution of granite under high temperature and triaxial compression are the key to designing high-level waste disposal sites. In this paper, uniaxial compression tests of granite specimens with different axial compression are designed, and then a solid-head-designed coupling triaxial testing system is applied to study thermal cracking and permeability evolution of granite specimen with different damage at different inlet gas pressures (1, 2, 4, and 6 MPa) and temperatures (ranging from 100 to 650°C). The test results show that granite, nearly impermeable rocks, can show a striking increase of permeability by heating beyond the critical temperature. When the initial axial pressure is 60% or 70% of the uniaxial compressive strength, the growth of granite permeability exhibits three stages during 100∼650°C heating process. Permeability increases by two orders of magnitude, but it does not reach the maximum value (i.e., a network of interconnected cracks is not fully formed in the specimen). With increasing initial damage, permeability shows a sharp increase. Permeability increases by three orders of magnitude, it is in equilibrium state, and a network of interconnected cracks is fully formed in the specimen. Permeability of granite has a critical temperature at which permeability increases sharply. When the temperature is lower than the critical temperature, the magnitude of permeability is 10−18 m2 with a slight increase. When temperature is higher than the critical temperature, the magnitude of permeability is 10−15 m2 with a sharp increase. The critical temperature is related to the initial damage of specimen, and the critical temperature is smaller with the initial damage going larger. Therefore, studying thermal cracking and permeability evolution of granite with different initial damage under high temperature and triaxial compression is expected to provide necessary and valuable insight into the design and construction of high-level waste disposal structures

Understanding The Electrospinability Of Complex Coacervates

Complex coacervation is an associative, liquid-liquid phase separation that is driven by the electrostatic and entropic interactions between oppositely-charged polymers in water. For many coacervating systems it is possible to transition from the liquid coacervate state to a solid material by removing salt. This ‘saloplasticity’ allows for the processing of materials via methods such as spin coating, extrusion, etc. using the coacervate phase as a liquid precursor. In particular, we have developed an approach that uses complex coacervation as an environmentally friendly method for fabricating ultra-stable electrospun fibers directly from aqueous solutions. We have used this method to electrospin complexes of various synthetic polymers as well as natural biopolymers. These efforts have required the simultaneous exploration of the phase behavior of coacervate formation, as well as the rheology of the liquid coacervates

Electrospinning complex coacervates

As polymer-based materials become ever more integrated into our daily lives, there is an increasing need to develop both materials that are safe for the consumer, and manufacturing strategies that have a minimal impact on the environment. However, the vast majority of polymers require either organic solvents for dissolution, or the use of potentially cytotoxic cross-linking agents to prevent material dissolution. Additionally, many of the chemistries and solution conditions necessary for processing can damage cargo molecules and create biocompatibility issues for subsequent use. Complex coacervation is an associative, liquid-liquid phase separation that has the potential to circumvent many of the challenges associated with processing traditional polymers and encapsulating actives. Complex coacervation is driven by the electrostatic and entropic interactions between oppositely-charged polymers in water. For many coacervating systems, the solid or liquid nature of the complex can be tuned via the concentration of salt present. Additionally, the strength of the electrostatic interactions within the complex are such that in the absence of salt, solid complexes are highly resistant to thermal melting and/or solvent dissolution. Furthermore, complex coacervation has a strong history of use for the encapsulation of a range of cargo. We have taken advantage of this salt-driven plasticity to enable fabrication of ultra-stable electrospun fibers directly from aqueous solutions. These efforts have required the simultaneous characterization of coacervation, as well as the effect of cargo molecules on the phase behavior and rheology of the resulting coacervates/precursor solutions. Furthermore, these materials show tremendous promise for the use of electrospun coacervate-based nanofiber meshes across a range of applications

Vortex-Induced Vibration of a Marine Riser: Numerical Simulation and Mechanism Understanding

Marine riser is a key equipment connecting a floating platform and a seabed wellhead. Vortex-induced vibration (VIV) is the main cause of the fatigue damage of the riser. The prediction of marine riser VIV is very difficult because of its strong non-linearity, instability and uncertainty. In recent years, many numerical models of VIV of marine riser have been developed to explore the mechanism of marine riser VIV, providing scientific theoretical basis and practical engineering methods for vibration control and engineering design of marine riser. Combined with the authors’ own recent research, this chapter discusses the research progress on marine riser VIV in the ocean engineering, including phenomenon mechanism analysis and different numerical research methods

Multiplexed molecular imaging in the second near infrared window

The implement and multiplexing of the second near-infrared window (NIR-II) imaging was demonstrated. Multiplexing is a novel idea in medical imaging, which describes the paradigm of integrating various diagnostic and therapeutic methods around the information acquired by imaging for better medical practices. NIR-II imaging is the in vivo imaging method with the light in the wavelength range between 1050 nm and 1700 nm. The NIR-II imaging is superior to conventional imaging due to the lower scattering, deeper penetration, and negligible autofluorescence. In this thesis, the imaging system and fluorescent materials for NIR-II imaging, the implement of phantom and in vivo NIR-II imaging, as well as multiplexed NIR-II imaging with radiotherapy have been studied, and the NIR-II Cherenkov imaging based on Cu-64 has been explored. Two NIR-II imaging systems have been constructed to achieve high quality facile imaging, with the special optical design to boost the quantitative imaging ability. Three quantum dots with different emission peaks across the NIR-II region have been obtained, and synthetic protocols were provided. An online-monitoring system of the NIR-II fluorescence has been designed and constructed, to monitor the process of quantum dots synthesis real-time. NIR-II imaging have been conducted with the imaging system and the fluorescent quantum dots. The imaging results obtained with the commonly used Intralipid® liquid phantoms were analyzed with a quantitative pipeline, to evaluate the effect of various parameters in NIR-II imaging. Monte Carlo simulations were conducted to verify the findings. The NIR-II imaging was also applied on a tissue clearing technology called CLARITY, to obtain ex vivo images and further proved the imaging quality. Based on the previous results, two types of in vivo NIR-II fluorescence imaging studies have been implemented. The dynamic in situ perfusion imaging tracks the fluorophores injected to the blood vessels of the mouse at a high frame rate, and reveals the diffusion of the fluorescent nanoparticles in the blood stream. Tumor angiogenesis has been evaluated with this method. The real-time intestine imaging tracks the movement of the fluorophores inside the intestines of the mice and visualizes the intestine structure. The imaging experiments verified the superiority of the NIR-II imaging. To further exploit the value of the NIR-II imaging, it has been multiplexed with radiotherapy. Cherenkov emission is generated during the megavolt X-ray beams therapy. The NIR-II components of the Cherenkov emission is sufficient for in vivo imaging to reveal the radiation field. This Cherenkov emission could also excite the NIR-II-emitting quantum dots, and this multiplexed imaging method could be used to validate the delivery of the radiation dose. With the lead shield apparatus, the high-quality Cherenkov imaging and Cherenkov excited luminescence imaging results could be obtained with megavolt X-ray beams. An attempt was made to multiplex NIR-II imaging with positron emission tomography. A solid target electroplating technology was developed to enable the manufacture of Cu-64. With the nanoparticle probe labeled with Cu-64, effective tumor imaging was obtained. However, the NIR-II Cherenkov luminescence imaging and Cherenkov radiation energy transfer were not successful. This thesis has established the technical platform for NIR-II imaging, and conducted the multiplexed NIR-II imaging. There is a great significance in multiplexed NIR-II imaging which is worthy of exploration.Ph.D