UV-induced in-situ transition of polydimethylsiloxane elastomer to functional adhesive : A Fluoride-Driven Route to On-Demand Silicone Adhesion = 사용 현장에서 자외선 자극에 의해 접착제로 전환되는 폴리 디메틸실록산 엘라스토머

국문 초록 (Abstract)

본 연구는 연성 로봇(soft robotics) 분야에서 요구되는 가역적이고 외부 자극으로 제어 가능한 부착 메커니즘을 구현하기 위해, 경화된 PDMS 엘라스토머가 UV 조사에 의해 접착성 물질로 전환될 수 있는지를 탐구하였다. 이를 위해 Ecoflex 00-30, 불소이온 발생 광개시제(DPI-HFP), 그리고 UV 조사 및 열 활성화 이후에만 반응하도록 설계된 차단(blocked) 폴리우레탄 프리폴리머를 조합하였다. 문헌 연구를 통해 UV로 인한 불소이온 생성, PDMS 주사슬 절단, 이소시아네이트 디블로킹(deblocking), 그리고 우레탄 결합 형성을 포함하는 4단계 반응 메커니즘을 설정하였으며, 이를 통해 열화된 매트릭스를 재결합시키고 접착 특성을 부여하는 것을 목표로 하였다.
기계적 평가 결과, (PPG 1:8 TDI) 1:20 Ecoflex 조성이 가장 유망한 물성(낮은 히스테리시스와 적절한 연성)을 보여 최적 후보로 선정되었다. 화학적 분석을 통해 이소시아네이트의 성공적인 차단이 확인되었고, FT-IR 분석을 통해 UV–열 활성화 과정에서 PDMS 분해와 우레탄 결합 형성이 실제로 일어남을 검증하였다. 그러나 전단 접착(lap-shear) 강도는 기대치(≈200 kPa)에 크게 못 미치는 20 kPa 이하의 매우 낮은 값을 보였으며, DPI-HFP 농도나 열 처리 시간의 영향도 초기 가설과 정반대의 양상을 나타냈다. 높은 DPI-HFP 함량 및 장시간 열처리는 오히려 기계적 취약성과 취성 증가를 초래하였다.
이러한 결과는 두 가지 주요 요인에 의해 설명될 수 있다. 첫째, 프리폴리머 합성 과정에서 발생한 광범위한 NCO 자기반응(self-reaction)이 반응성 이소시아네이트를 소모하고 hard/soft 도메인의 균형을 깨뜨렸다. 둘째, 장시간 열처리로 인해 PDMS 매트릭스가 과도하게 분해되면서 응집력이 감소하고 재결합 능력이 크게 저하되었다. 비록 목표했던 접착 성능에는 도달하지 못했으나, 본 연구는 UV 기반 PDMS 기능화의 화학적·기계적 기반을 체계적으로 확립하고 주요 실패 메커니즘을 규명했으며, MDI 적용, 고분자량 PPG 사용, 합성·활성화 공정 최적화와 같은 개선 방향을 제시함으로써 향후 연성 로봇용 자극응답형 접착 소재 개발을 위한 중요한 연구 기반을 마련하였다.

다국어 초록 (Multilingual Abstract)

This thesis explores the feasibility of creating a UV-triggered adhesive material from a cured PDMS elastomer, motivated by the need for controllable anchoring mechanisms in soft robotics. The proposed system combines Ecoflex 00-30, the fluoride-generating photoinitiator DPI-HFP, and a blocked polyurethane prepolymer designed to react only after UV exposure and heat activation. Literature-informed analysis led to a four-step mechanism involving UV-induced fluoride release, PDMS backbone cleavage, isocyanate deblocking, and subsequent urethane formation intended to strengthen the degraded matrix and generate adhesive behavior.
Mechanical screening identified (PPG 1:8 TDI) 1:20 Ecoflex as the most promising formulation, showing an advantageous balance of softness and low hysteresis. Chemical validation confirmed successful blocking of isocyanates, and FT-IR measurements verified that the UV–heat cascade produced both PDMS degradation and polyurethane bonds. However, lap-shear adhesion remained significantly lower than expected (< 20 kPa versus the targeted ≈200 kPa), and the influence of DPI-HFP concentration and heating duration contradicted the initial hypothesis. Higher photoinitiator content and longer activation times frequently resulted in weaker, more brittle materials.
Failure analysis revealed two primary causes: extensive NCO self-reactions during prepolymer synthesis, which depleted reactive isocyanates and disrupted the hard/soft domain balance, and over-degradation of the PDMS matrix during prolonged heating, which reduced cohesion and prevented effective repolymerization. Although the material did not achieve functional adhesion, this work establishes a solid chemical and mechanical framework, identifies the main failure pathways, and outlines improvement strategies —such as using MDI, higher-molecular-weight PPG, and optimized synthesis timing—offering a clear direction for advancing UV- triggered adhesive materials in soft robotic applications.

목차 (Table of Contents)

• Abstract i
• Table of Contents⁠ ii
• List of Figures ⁠ vi
• Chapter 1. Introduction ⁠ 1
• 1.1 Research Motivation ⁠ 1
• 1.2 Purpose of Research 3
• Chapter 2. Literature Review 5
• 2.1 Polydimethylsiloxane: Structure, Properties, and Inherent Limitations ⁠ 5
• 2.2 Soft Robotics: Principles, Applications, and Material-Driven Challenges 6
• 2.3 Fundamentals of Adhesion and UV-triggered Adhesives⁠ 8
• 2.3.1 Thermodynamic and Molecular Origins of Adhesion 8
• 2.3.2 Mechanical Interlocking and Chain Mobility ⁠ 9
• 2.3.3 Chemical Bonding and Intermolecular Interaction ⁠ 9
• 2.3.4 Characterizing Adhesion Performance ⁠ 9
• 2.3.5 UV-triggered Adhesives : Principles and Limitations ⁠ 10
• 2.4 PDMS Degradation 10
• 2.4.1 Thermal Degradation Mechanisms ⁠ 10
• 2.4.2 Catalytic Depolymerization⁠ 11
• 2.5 Fluoride-induced Si-O-Si Cleavage and UV Activation ⁠ 11
• 2.5.1 Mechanism of Fluoride Attack on the Siloxane Backbone ⁠ 12
• 2.5.2 UV-triggered Fluoride Generation Inside PDMS Elastomers 12
• 2.5.3 Self Amplifying Fluoride Degradation Cascades ⁠ 13
• 2.5.4 Fluoride-catalyzed Dynamic Siloxane Exchange and Implications for Network Design ⁠ 14
• 2.6 Polyurethane Chemistry, Adhesion and Blocking/Unblocking Mechanisms ⁠ 14
• 2.7 Research Gap : Synthesis of Literature Insights⁠ 16
• Chapter 3. Theoretical PDMS Functionalization Mechanism⁠ 18
• 3.1 Overview of PDMS Re-polymerization Processus ⁠ 18
• 3.1.1 UV-triggered fluoride Ion Generation by DPI-HFP ⁠ 19
• 3.1.2 Fluoride-Induced Silicone Decomposition ⁠ 20
• 3.1.3 Fluoride-Induced Diisocyanate Activation ⁠ 22
• 3.1.4 In-situ UV-triggered Repolymerization 23
• Chapter 4. Material Synthesis and Validation ⁠ 25
• 4.1 Chemicals & Materials ⁠ 25
• 4.1.1 Ecoflex 00-30 as the Silicone Elastomer Matrix ⁠ 25
• 4.1.2 DPI-HFP as the Photochemical Fluoride Source ⁠ 25
• 4.1.3 Di-isocyanates and Their Role in Urethane Formation ⁠ 26
• 4.1.4 Selection of PPG 1000 as the Polyol Soft Segment ⁠ 27
• 4.1.5 Catalysis and Blocking Strategy ⁠ 28
• 4.2 Prepolymer Synthesis ⁠ 28
• 4.3 Synthesis Validation of Blocked Prepolymers ⁠ 29
• 4.4 PDMS Composite Preparation ⁠ 29
• 4.5 Activation Protocol ⁠ 30
• 4.5.1 UV Irradiation ⁠ 30
• 4.5.2 Thermal Activation ⁠ 31
• 4.6 Heat Degradation Results & FT-IR analysis ⁠ 31
• 4.6.1 FT-IR analysis of degraded samples ⁠ 32
• Chapter 5. Mechanical Optimization of PDMS Composite ⁠ 35
• 5.1 Mechanical Characterization Methods ⁠ 35
• 5.2 Data Analysis Process ⁠ 35
• 5.3 Mechanical Property Windows for Soft Robotic Applications ⁠ 37
• 5.3.1 Young’s Modulus (Stiffness) ⁠ 37
• 5.3.2 Elongation at Break (Stretchability) ⁠ 38
• 5.3.3 Hysteresis and Energy Dissipation ⁠ 39
• 5.3.4 Cyclic Durability and Fatigue Resistance ⁠ 39
• 5.3.5 Implications for Material Selection in This Study ⁠ 40
• 5.4 Hard/Soft Segment Microstructure in PU–PDMSComposites ⁠ 40
• 5.5 Optimization of PPG : Diisocyanate Ratio ⁠ 42
• 5.5.1 Expected Influence of Prepolymer Composition on Mechanical Properties ⁠ 42
• 5.5.2 Cyclic Test Comparison Between PPG:Diisocyanate Ratios ⁠ 43
• 5.6 Influence of Prepolymer : Ecoflex Ratio on Mechanical Properties ⁠ 44
• 5.6.1 Expected Influence of Prepolymer : Ecoflex ratio on Mechanical Properties ⁠ 44
• 5.6.2 Cyclic Test Comparison Between Prepolymer : Ecoflex ratios ⁠ 45
• 5.7 Identification of the Optimal Composite Formulation ⁠ 46
• 5.7.1 Expected Influence of Diisocyanate Species on Mechanical Properties ⁠ 46
• 5.7.2 Cyclic Test Comparison Between Diisocyanate Species ⁠ 47
• 5.7.3 Identification of the Optimal Mechanical Formulation Within the Soft Robotics Framework ⁠ 49
• Chapter 6. Lap-Shear Adhesion & Failure Analysis ⁠ 51
• 6.1 Lap-Shear Adhesion Tests ⁠ 51
• 6.1.1 DPI-HFP Weight% and Heat Degradation Time Optimization Processus ⁠ 51
• 6.1.2 Expected Lap-shear Tests results ⁠ 53
• 6.1.3 Experimental Lap-shear Test Results ⁠ 54
• 6.2 Cohesion Failure Interpretation ⁠ 55
• 6.2.1 -NCO Self Reaction ⁠ 55
• 6.2.2 Cohesive Failure Hypothesis ⁠ 56
• Chapter 7. Future Work ⁠ 59
• 7.1 Routes of improvement ⁠ 59
• 7.1.1 Introduction of MDI as a New Di-isocyanate Component ⁠ 59
• 7.1.2 Using Higher-Molecular-Weight Polyol (PPG 2000) ⁠ 59
• 7.1.3 Improving the Synthesis and Activation Protocol ⁠ 60
• 7.2 New Synthesis Processus ⁠ 61
• Chapter 8. Conclusion ⁠ 63
• Glossary ⁠ 65
• Bibliography ⁠ 66
• Abstract in Korean ⁠ 70
• Annex ⁠ 71
• A. Elongation at Break Graphics ⁠ 71
• B. Optimized Synthesis Additionnal Content ⁠ 72
• B.1 Table of New Reactive Trials amount ⁠ 72
• B.2 Heat Degradation Tests With Optimized Synthesis Process ⁠ 72
• C. Automated Data Analyse code ⁠ 74