Complex Phase Behaviors in Interacting Polymeric Mixtures : Thermodynamic Analyses and Molecular Dynamics Simulations

다국어 초록 (Multilingual Abstract)

Interacting polymeric mixtures have emerged as both a focal point and a versatile platform for designing stimuli-responsive materials, owing to the diversity and tunability of their intermolecular interactions. However, their phase behaviors are inherently complex, governed by the competition and coupling among multiscale interactions, which present significant challenges for theoretical interpretation and experimental prediction. To address this, this study investigates three representative systems—polymer/ionic liquid (IL) mixtures, polyelectrolyte solutions, and associating block copolymers—to elucidate the mechanisms by which multiscale interactions regulate phase behaviors and drive microstructural evolution, thereby providing a systematic framework for the design of smart materials. First, motivated by the remarkable thermal responsiveness of polymer/IL mixtures, experimental characterization and molecular dynamics (MD) simulations were combined to investigate the phase behavior of Polyvinylpyrrolidone (PVP) in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). During heating, the films exhibited a transparent–opaque–transparent transition, analyzed by differential scanning calorimetry (DSC) and light scattering to examine the effects of temperature and polymer concentration. The system exhibited an unusual loop-type phase behavior, featuring the coexistence of both a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST), surpassing conventional single thermal responsiveness in adaptability. To elucidate the underlying molecular mechanism, MD simulations using the OPLS5 all-atom force field were performed. Structural and cluster analyses revealed thermally induced conformational transitions of PVP chains from gradual collapse to re-extension, while free volume analysis further corroborated the loop-type phase behavior. Furthermore, by varying alkyl chain lengths of imidazolium-based ILs, the simulations revealed that PVP chains exhibit similar temperature-dependent conformational behaviors, indicating that this molecular feature effectively governs the loop-type phase behavior in PVP/ILs mixtures. These findings offer new insights into the molecular basis for designing thermoresponsive smart materials. Second, in polyelectrolyte solutions, the interplay between electrostatic interactions and chain conformations results in rich and complex phase behaviors. To provide a more comprehensive description of oppositely charged polyelectrolyte blends, we constructed a Landau free energy based on the Random Phase Approximation (RPA) theory combined with a compressible equation of state for charged systems (Cho–Sanchez EOS). This framework incorporates excluded volume effects, chain connectivity, dispersion interactions, and long-range Coulomb forces. Within this model, we investigated both conventional order-disorder transitions and more complex ordering transitions, spanning from fully charged to weakly charged systems. The analysis revealed the roles of chain-size asymmetry, charge effects, disparity in dispersion interactions, and pressure effects in shaping the phase behavior. Furthermore, we identified both barotropic and baroplastic responses of the ordering transitions, highlighting the potential of designing pressure-responsive polyelectrolyte materials. These results provide a mechanistic understanding for theoretical predictions and establish general principles for the rational design of polyelectrolyte systems. Finally, the theoretical framework was extended to associating AB diblock copolymers. Based on the RPA theory of combined with the Cho–Sanchez EOS for associating systems, we constructed a Landau free energy functional that accounts for excluded volume, dispersion interactions, and the effective Flory interaction parameter (𝜒), defined as the sum of enthalpic (𝜒𝐻) and entropic (𝜒𝑆) contributions. The RPA analysis identifies the critical conditions and quantifies the effective interaction parameters. Representative self-consistent field theory (SCFT) density profiles illustrate the disordered and lamellar morphologies predicted by the framework. The resulting analysis elucidates the temperature- and pressure-dependent ordering of different diblock copolymers. It was found that the competition between enthalpic and entropic contributions gives rise to both cooling-induced ordering and the less conventional heating-induced ordering. Skewed phase boundaries in the phase diagrams underscore the role of dispersion interaction asymmetry in driving ordered morphologies. Furthermore, barotropic and baroplastic pressure responses were identified, and the effects of multibody interactions on phase boundaries were further explored, broadening our understanding of the self-assembly mechanisms in associating block copolymers. These results provide theoretical guidelines for the rational design of nanostructured materials with tailored thermoresponsive and pressure-sensitive properties.

목차 (Table of Contents)

• Ⅰ. Introduction 1
• 1.1 Background 1
• 1.2 Theoretical methods 5
• 1.3 Molecular dynamics simulations 7
• Ⅱ. Loop-type Phase Behavior in Polyvinylpyrrolidone/Ionic Liquid Mixtures 8
• 2.1 Introduction 8
• 2.2 Methods 9
• 2.2.1 Experimental details and characterization 9
• 2.2.2 Molecular dynamics simulations 13
• 2.3 Results and discussion 18
• 2.3.1 Loop-type phase behavior of PVP/BMIMBF4 18
• 2.3.2 Molecular dynamics simulation analysis of PVP chains conformations ·· 26
• 2.3.3 Molecular dynamics simulation analysis of free volume 34
• 2.4 Summary 37
• Ⅲ. Complex Phase Behaviors in Polyelectrolyte Solutions 38
• 3.1 Introduction 38
• 3.2 Theoretical methods 39
• 3.3 Results and discussion 42
• 3.3.1 Fully charged system 42
• 3.3.2 Weakly charged system 47
• 3.4 Summary 55
• Ⅳ. Phase Behaviors of Associating A-b-B Diblock Copolymers 56
• 4.1 Introduction 56
• 4.2 Theoretical methods 58
• 4.3 Results and discussion 62
• 4.3.1 Spinodals from RPA and density profiles from SCFT 62
• 4.3.2 Temperature dependence of various phase transitions 67
• 4.3.3 Pressure dependence of various phase transitions 72
• 4.3.4 Effect of multibody interactions on phase boundaries 75
• 4.4 Summary 79
• Ⅴ. Conclusion 80
• References 86
• 국문초록 107