Interfacial Assembly of Amphiphilic Materials Synthesized by Living Radical Polymerization

Abstract

In fields of soft matter, there have been growing interests in utilizing amphiphilic materials due to their intriguing properties, such as surface activity as well as self-assembly. Amphiphilic material refers to a surfactant having a property of hydrophilic and hydrophobic so-called amphiphilic in a molecular structure. The structural toughness of these self-assemblies is mainly determined by according to the mechanical properties of the surfactants forming them. Therefore, in recent years, many polymer surfactants have been widely used because of their high mechanical properties. In this study, we developed a structurally stable self-assembly system using polymer-based surfactants. Subsequently, the research on the assembly technology with stable and high mechanical properties according to various external environments was conducted, and the applicability was expanded by confirming the rheological behavior of the assembly.

We first introduces a highliy stable nanoemulsion system using a novel triblock copolymer surfactants. For this, we synthesize a series of poly (2-(methacryloyloxy) ethyl phosphorylcholine)-b-poly (e-caprolactone)-b-poly (2-(methacryloyloxy) ethyl phosphorylcholine) (PMPC-b-PCL-b-PMPC) triblock copolymers by using atom transfer radical polymerization (ATRP). We have a particular interest in using PMPC as a hydrophilic block, since it can have both electrostatic repulsion and steric repulsion in complex fluid systems. Assembling them at the oil-water interface by using the phase inversion method enables production of highly stable nanoemulsions. From the analyses of the crystallography and self-assembly behavior, we have found that the triblock copolymers assemble to form a flexible but tough molecular thin film at the interface, which is essential for the remarkable improvement in the emulsion stability.

We also introduces a temperature-responsive attractive nanoemulsion (ANE) system, which is characterized by the polymer chain conformation-driven dipolar interaction across different oil droplets in an aqueous medium. To achieve this, highly stable ANEs were produced by co-assembly of amphiphilic triblock copolymers (ATCs), poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(ethylene oxide) (PEO-b-PCL-b-PEO), with lecithin at the oil-water interface. The dipolar attraction of the methoxy terminated-PEO (mPEO) of ATCs on one drop surface with the lecithin head located on the other drop surface led to the drop-to-drop association. We showed that the efficiency of this interdrop association was dominantly influenced by the chain conformation of mPEO blocks. From dense suspension rheology studies, it was demonstrated that the ANEs formed a gel-like phase below the lower critical solution temperature (LCST) of the mPEO, but transformed to a liquid-like phase above the LCST, which occurred reversibly, thus enabling the development of temperature-responsive emulsion fluids

Third, this study reports a robust and straightforward approach to fabricate polymer microcapsules with a silica reinforced polyelectrolyte thin shell layer. We showed that the layer-by-layer (LbL) deposition of silica nanoparticles with polyelectrolytes remarkably reinforced the shell layer, which was experimentally demonstrated by focused ion beam analysis. Moreover, we demonstrated that the molecular degradation of a model antioxidant encapsulated in our capsule system was effectively hindered during long-term storage. This indicates that the presence of the silica nanoparticles-reinforced polyelectrolyte shell layer displayed enhanced cargo retention against leakage of the antioxidant as well as oxygen attack from the surroundings.mperature-responsive emulsion fluids.

Finally, we introduce a new type of amphiphilic nanoplatelet (ANPL)-armored Pickering emulsion system that shows an unprecedented gelation behavior. The ANPLs, synthesized by simultaneous grafting of hydrophilic brushes and hydrophobic brushes on each surface of the platelet via surface-induced ATRP, exhibit a high level of energy barrier of ∆(〖∆E〗_min)≈3.9×〖10〗^5 k_B T, thus allowing them to adsorb strongly to the oil-water interface. Consequently, the ANPL-armored Pickering emulsions show surprisingly improved emulsion stability even under harsh storage conditions such as repeated freeze-thaw cycling and a wide range of medium pH and salinity. More interestingly, they show an unusual emulsion gel phase regardless of whether it is an oil-in-water (O/W) or a water-in-oil (W/O) emulsion from a very low volume fraction of dispersion phase, ~0.1. We suggest that it is presumably driven by either platelet surface-mediated hydrogen bonding or hydrophobic interaction between suspending droplets. Finally, the results obtained in this study highlight that the ANPLs can be used as a promising colloid surfactant that can regulate the drop-to-drop interaction.

|계면활성제는 한 분자구조에 친수성 및 소수성 소위 양친성이라고 하는 성질을 가지고 있는 물질을 말한다. 이러한 계면활성제는 용매상에서 스스로 자기회합이라는 물리적 거동을 통해 회합체 구조를 이룰 수 있다. 이러한 자기회합체들은 그들을 이루고 있는 계면활성제의 물성에 따라 구조의 강직성이 결정된다. 따라서 최근에는 기계적 물성이 증대된 고분자 계면활성제를 많이 사용하고 있는 추세이다. 본 연구에서는 상기 고분자를 기반으로 한 계면활성제를 이용하여 구조적으로 안정한 여러 회합체 기술을 개발했다. 이어서 다양한 외부 환경에 따른 안정적이면서도, 높은 기계적 물성을 지닌 회합체 기술에 관한 연구를 진행 및 회합체의 유변 거동을 확인함으로써 응용성을 확대하였다.

첫 번째 연구에서는 리빙 라디칼 중합을 이용하여 고분자 기반 쯔비터 이온 및 양이온성 삼중 블록 고분자 계면활성제를 합성하고, 이들을 유화 시스템에 적용하여 절대 안정한 나노에멀젼 시스템을 연구하였다. 이를 위해 피부 세포막을 이루는 인지질과 유사한 구조를 지닌 쯔비터 이온성 PMPC-b-PCL-b-PMPC와 양이온성 PAMA-b-PCL-b-PAMA를 합성하였다. 상전이 방법을 사용하여 유-수 계면에서 이들을 자기회합하면 절대 안정한 나노 에멀젼을 제조할 수 있다. 자기회합 거동 분석과 전자현미경 분석으로부터, 삼중 블록 공중 합체가 계면에서 유연하지만 강직한 계면 박막을 형성한다는 것을 발견하였으며, 이는 나노에멀젼이 절대 안정성을 나타내는 주요한 근거가 된다. 또한 이러한 나노에멀이 고농도의 염 조건에 대해서도 높은 장기 저장 안정성을 발현한다는 것을 증명하였다.

두번째 연구에서는 온도 감응형 고농축 회합형 나노에멀젼 시스템을 연구하였다. 이 나노에멀젼 시스템은 수상에서 서로 에멀젼 액적들의 고분자 사슬과 레시틴 사이의 유도 쌍극자 상호 작용을 특징으로 온도에 따라 제형의 유변 특성을 제어하는 시스템을 소개하였다. 이를 위해 양친성 삼중블록 고분자 계면활성제인 PEO-b-PCL-b-PEO를 축합반응을 통해 합성하고, 이를 이용하여 유-수 계면에서 레시틴과 함께 고에너지 유화법을 통해 나노에멀젼화를 한 뒤, 고농축 현탁과정을 거쳐 이에 대한 유변학적 특성을 조사하였다. PEO 말단에 있는 메톡시기와 레시친의 암모늄기가 유도 쌍극자 인력을 통해 액적간의 클러스터 구조를 형성할 수 있다는 것을 실험적으로 증명하였다. 그리고 이것은 친수성 PEO사슬의 분자량에 따라서 온도에 대한 유변제형 특성이 제어된다는 것을 검증하였다. 결과적으로 온도에 감응하여 PEO사슬이 가지고 있는 LCST 특성에 따라 나노에멀젼의 유변특징이 겔상 또는 솔상으로 가역적으로 변화될 수 있음을 설명하였다.

세 번째 연구에서는 리빙 라디칼 중합을 이용하여 고분자 기반 삼중블록 고분자 계면활성제를 합성하고 이를 자기회합하여 고분자 마이크로 입자를 제조하였다. 그리고 실리카 나노 입자와 고분자 전해질을 적층하여 고분자 마이크로 캡슐의 껍질의 기계적 물성을 강화하고 항산화 약물을 더욱 안정하고 효율적으로 담지 할 수 있는 회합체 기반 캡슐화 기술을 소개하였다. 이것은 실리카 나노 입자와 고분자 전해질 껍질 층의 존재가 항산화 물질의 누출을 효과적으로 막고, 강화된 껍질층을 통해 산소로 인한 항산화제의 분해를 현저하게 감소시킬 뿐만 아니라 항산화제의 누출에 대해 효과적으로 담지 할 수 있다는 것을 증명하였다.

마지막 연구는 이상적인 계면흡착력을 발현할 수 있는 양친성 판형 나노입자를 이용하여 분산상의 낮은 분율과 적은 양의 계면 활성제에서도 강한 인력으로 유도되는 점탄성 피커링 에멀전 젤 시스템을 연구하였다. 이를 위해, 표면 개시-라디칼 중합을 이용하여 에멀젼 상에서 판형입자의 각 표면에 친수성 고분자 및 소수성 고분자를 그라프팅하여, 양친성 판형입자 계면활성제를 합성하였다. 이 양친성 판형입자는 Δ (Δ𝐸𝑚𝑖𝑛) ≈3.9 × 105𝑘𝐵𝑇의 매우 높은 수준의 에너지 장벽을 가지며 유수 계면에서 흡착될 때 강한 흡착력을 발현하여 모노레이어 장벽을 지닌 피커링 에멀젼을 형성하였다. 결과적으로, 양친성 판형입자로 제조된 피커링 에멀젼은 동결-해동 사이클링과 같은 열악한 조건에서뿐만 아니라 광범위한 pH 및 염분 조건 하에서도 매우 개선된 에멀젼 안정성을 나타내었다. 또한, 피커링 에멀젼은 액적들 사이의 강한 수소 결합 및 소수성 상호 작용에 의해 낮은 분산상의 분율 뿐만 아니라 낮은 입자 농도에서 겔상과 같은 유변학적 거동을 나타냄을 확인하였다. 이러한 시스템은 농축 과정, 고농도의 계면활성제등의 조건이 아니더라도 높은 수준의 겔 특성을 지닌 피커링 에멀젼을 제조할 수 있음을 증명하였다.; In fields of soft matter, there have been growing interests in utilizing amphiphilic materials due to their intriguing properties, such as surface activity as well as self-assembly. Amphiphilic material refers to a surfactant having a property of hydrophilic and hydrophobic so-called amphiphilic in a molecular structure. The structural toughness of these self-assemblies is mainly determined by according to the mechanical properties of the surfactants forming them. Therefore, in recent years, many polymer surfactants have been widely used because of their high mechanical properties. In this study, we developed a structurally stable self-assembly system using polymer-based surfactants. Subsequently, the research on the assembly technology with stable and high mechanical properties according to various external environments was conducted, and the applicability was expanded by confirming the rheological behavior of the assembly.

We first introduces a highliy stable nanoemulsion system using a novel triblock copolymer surfactants. For this, we synthesize a series of poly (2-(methacryloyloxy) ethyl phosphorylcholine)-b-poly (e-caprolactone)-b-poly (2-(methacryloyloxy) ethyl phosphorylcholine) (PMPC-b-PCL-b-PMPC) triblock copolymers by using atom transfer radical polymerization (ATRP). We have a particular interest in using PMPC as a hydrophilic block, since it can have both electrostatic repulsion and steric repulsion in complex fluid systems. Assembling them at the oil-water interface by using the phase inversion method enables production of highly stable nanoemulsions. From the analyses of the crystallography and self-assembly behavior, we have found that the triblock copolymers assemble to form a flexible but tough molecular thin film at the interface, which is essential for the remarkable improvement in the emulsion stability.

We also introduces a temperature-responsive attractive nanoemulsion (ANE) system, which is characterized by the polymer chain conformation-driven dipolar interaction across different oil droplets in an aqueous medium. To achieve this, highly stable ANEs were produced by co-assembly of amphiphilic triblock copolymers (ATCs), poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(ethylene oxide) (PEO-b-PCL-b-PEO), with lecithin at the oil-water interface. The dipolar attraction of the methoxy terminated-PEO (mPEO) of ATCs on one drop surface with the lecithin head located on the other drop surface led to the drop-to-drop association. We showed that the efficiency of this interdrop association was dominantly influenced by the chain conformation of mPEO blocks. From dense suspension rheology studies, it was demonstrated that the ANEs formed a gel-like phase below the lower critical solution temperature (LCST) of the mPEO, but transformed to a liquid-like phase above the LCST, which occurred reversibly, thus enabling the development of temperature-responsive emulsion fluids

Third, this study reports a robust and straightforward approach to fabricate polymer microcapsules with a silica reinforced polyelectrolyte thin shell layer. We showed that the layer-by-layer (LbL) deposition of silica nanoparticles with polyelectrolytes remarkably reinforced the shell layer, which was experimentally demonstrated by focused ion beam analysis. Moreover, we demonstrated that the molecular degradation of a model antioxidant encapsulated in our capsule system was effectively hindered during long-term storage. This indicates that the presence of the silica nanoparticles-reinforced polyelectrolyte shell layer displayed enhanced cargo retention against leakage of the antioxidant as well as oxygen attack from the surroundings.mperature-responsive emulsion fluids.

Finally, we introduce a new type of amphiphilic nanoplatelet (ANPL)-armored Pickering emulsion system that shows an unprecedented gelation behavior. The ANPLs, synthesized by simultaneous grafting of hydrophilic brushes and hydrophobic brushes on each surface of the platelet via surface-induced ATRP, exhibit a high level of energy barrier of ∆(〖∆E〗_min)≈3.9×〖10〗^5 k_B T, thus allowing them to adsorb strongly to the oil-water interface. Consequently, the ANPL-armored Pickering emulsions show surprisingly improved emulsion stability even under harsh storage conditions such as repeated freeze-thaw cycling and a wide range of medium pH and salinity. More interestingly, they show an unusual emulsion gel phase regardless of whether it is an oil-in-water (O/W) or a water-in-oil (W/O) emulsion from a very low volume fraction of dispersion phase, ~0.1. We suggest that it is presumably driven by either platelet surface-mediated hydrogen bonding or hydrophobic interaction between suspending droplets. Finally, the results obtained in this study highlight that the ANPLs can be used as a promising colloid surfactant that can regulate the drop-to-drop interaction.