Exploring the influence of magnetic fields and silica nanoparticles on the behavior of thermoresponsive polymer solutions

Abstract

Stimuli-responsive materials have recently sparked significant interest driven by growing demand for flexible electronics and wearable sensors. Among these, thermoresponsive polymer solutions stand out as a crucial class, comprising of a flexible polymer chain dissolved in a relatively low viscosity solvent. These materials can rapidly change their morphology and macroscopic properties in response to temperature fluctuations. Poly(N-isopropyl-acrylamide) (PNIPAM) and poloxamers are notable examples exhibiting thermoresponsive behavior in water, undergoing reversible dissolution-dehydration phase transitions. Both are amphiphilic polymers, containing hydrophobic and hydrophilic portions, leading to the formation of physical hydrogels at biologically-relevant temperatures. Despite previous studies on altering the phase transition behavior with silica nanoparticle (NP) additives and magnetic (B) fields in diamagnetic polymer solutions, the understanding of how these factors influence hydrogen bonding in polymer solutions remains incomplete. Therefore, developing methods to tune and characterize polymer-solvent interactions with NPs and B fields will significantly enhance the applicability and processability of these stimuli-responsive materials. The first goal of this thesis is to build an instrument capable of studying optical phase transitions of solutions under various concentrations of silica NPs and strengths of in-situ B fields. To do so, a turbidimeter will be designed, programmed and developed which can apply diverse and controlled temperature changes to solutions while simultaneously collecting light transmittance values. Turbidimetry studies the optical clarity of solutions; in polymer solutions, a 1- to 2-phase transition is often accompanied by a clear-to-cloudy transition, indicative of polymer dehydration. The modified turbidimeter (`magneto-turbidimeter') allows examinations into these polymer dehydration and aggregation mechanisms under different NP concentrations and B field strengths, revealing invaluable molecular-level insights about changes in interactions across these polymer phase transitions. The instrument was validated using aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions and compared to results collected manually by visually examining transmittance of vials of PNIPAM solution which were heated on a heating block. The second goal of this thesis is to study how B fields and silica NP additives impact the molecular level interactions in polymer solutions. Using a combination of calorimetric, spectroscopic and turbidimetric techniques--including the magneto-turbidimeter--the synergistic or antagonistic effects of silica NPs and B fields on the phase transition behavior of thermoresponsive polymer solutions are examined. Of particular interest is understanding how either NPs or B fields alter hydration shells or direct hydrogen bonds with PNIPAM side groups. While NP addition minorly impacts the PNIPAM thermodynamic and optical transitions, rheological transitions are dramatically altered and dependent upon NP quantity and shape. While NPs and B fields both reduce the phase separation energy barrier and lower optical transition temperatures by altering hydrogen bonding (H-bonding), infrared spectra demonstrate that the mechanism by which these changes occur is distinct. Magnetic fields primarily alter solvent polarization while NPs provide PNIPAM–NP H-bonding sites. Combining NP addition with field application uniquely alters the solution environment and results in field-dependent rheological behavior that is unseen in polymer-only solutions. Leveraging the insights of changes hydration with B fields, the third goal of this thesis is to study how the changes in polymer-solvent molecular interactions from B fields alter the rheological phase transition of PNIPAM solutions. Through the use of a magneto-rheological device, linearly-deforming oscillatory shear probed the formation of physical hydrogels from PNIPAM solutoins under various strengths of B fields. Here, we show that B fields weaken these physical hydrogels by limiting interactions between nearby PNIPAM-rich mesoglobules comprising the physical hydrogel network and by decreasing the size and water content of PNIPAM mesoglobules. Magnetic effects on hydrogel strength are also shown to depend upon the magnetization time and strength, as longer time and higher fields maximize B effects. Examinations of the two-step yielding behavior of PNIPAM hydrogels under various B field strengths suggest that B fields weaken mesoglobule-mesoglobule interactions without dramatic changes in the interaction length-scale. Conversely, B fields decrease the length-scale of longer-range mesoglobule correlations, likely due to a decrease in network connectivity from an increase in average inter-particle distance. Exploring the effects of B fields on the hydrogelation of PNIPAM solutions provides fundamental insight into the ability of B fields to interact with diamagnetic polymer solutions. The final goal of this thesis is to investigate the impact of B fields on the segmental dynamics of individual components in poloxamer solutions. Here, poloxamers are chosen as the material of interest over PNIPAM due to the highly uniform, ordered structures of the former, which minimize complications in QENS data. Through quasi-elastic neutron scattering (QENS) experiments conducted on aqueous poloxamer solutions, B fields increase the segmental dynamics of poly(ethylene oxide) (PEO) within the corona of poloxamer micelles. Curiously, diffusion of water molecules in the poloxamer corona was slightly slowed from magnetization, perhaps due to alterations in interactions between water-water H-bonding clusters due to magnetic polarization. Importantly, these changes in polymer and solvent dynamics are demonstrated to be independent of microstructural rearrangement, affirming that the application of a B field alters the interactions between polymer and solvent. The results of this thesis demonstrate that both B fields and hydrophilic silica nanoparticles are viable methods to alter the optical, thermodynamic, and rheological phase transition behavior of thermoresponsive polymer solutions. Exploring the manipulation of polymer-solvent interactions using these techniques expands their range of potential uses and reveals innovative pathways for crafting PNIPAM or poloxamer physical hydrogels. Specifically, understanding how nanoparticles or B fields alter polymer-solve hydrogen bonds offers novel avenues for tuning the directed assembly of soft matter systems.