Solvent Operated Polydimethylsiloxane (PDMS) Sponge Coatings for Tunable Transmittance Smart Windows

Technical Summary: Solvent Operated Polydimethylsiloxane (PDMS) Sponge Coatings for Tunable Transmittance Smart Windows

Problem Statement
Heating and cooling systems account for approximately 30% of global energy consumption, with windows serving as primary sites for heat exchange. Existing smart window technologies, such as electrochromic, thermochromic, and liquid crystal-dispersed systems, face significant barriers to widespread adoption. These include high production costs, complex fabrication methods, toxicity concerns, sensitivity to environmental conditions, and limited durability. Furthermore, while radiative cooling technologies offer promise, they often rely on expensive nanoporous materials. There is a critical need for a sustainable, scalable, and cost-effective alternative that can dynamically regulate light and heat transfer without complex external control systems.

Methodology
The authors developed a tunable smart window coating based on Polydimethylsiloxane (PDMS) sponges. The fabrication process utilizes a simple, environmentally friendly "sugar-templating" method:

1. Material Synthesis: A mixture of PDMS prepolymer, a curing agent, and granulated sugar is prepared. The sugar acts as a removable template to create a porous structure.
2. Optimization: Key parameters were systematically optimized to balance mechanical strength, swelling behavior, and optical performance. The optimal formulation was determined to be a PDMS:curing agent:sugar weight ratio of 10:1:4, using sugar granules sized 500–200 µm.
3. Coating Application: The mixture was cast onto glass substrates, cured at 80°C for 3 hours, and the sugar template was removed via ultrasonic water washing, leaving a porous PDMS sponge layer (approx. 0.57 mm thick) adhered to the glass.
4. Tunability Mechanism: The optical properties are controlled by the infusion and release of a solvent. In the dry state, the air-filled pores create a refractive index mismatch ($\Delta n \approx 0.42$) between the PDMS ($n \approx 1.42$) and air ($n \approx 1.0$), causing light scattering and an opaque appearance. Upon infusion with silicone oil ($n \approx 1.40$), the refractive index difference is minimized ($\Delta n \approx 0.02$), rendering the coating transparent.

Key Results

• Optical Performance: The PDMS sponge-coated glass exhibits reversible switching between opaque and transparent states.
  • Opaque State (Dry): Blocks 84.6% of solar light.
  • Transparent State (Silicone Oil Infused): Allows 84.8% light transmission (blocking only 15.2%), behaving similarly to uncoated glass.
  • Solvent Selection: While hexanol provided slightly higher transmittance (~100%), silicone oil was selected as the optimal solvent due to its safety profile, moderate swelling ratio (27%), and high transmittance (86%).
• Thermal Regulation: Under IR irradiation (simulating solar heat), the opaque state demonstrated a significant temperature difference ($\Delta T$) of 4.40 ± 1.05 °C between the front and back surfaces, effectively blocking heat. In contrast, the transparent state showed a $\Delta T$ of only 0.83 ± 0.15 °C, allowing heat transmission similar to uncoated glass.
• Durability and Stability:
  • Mechanical: The coatings withstood 200 cycles of mechanical abrasion with a 20 g load on 600-grit sandpaper without delamination or significant deformation.
  • Chemical: The coatings remained stable after immersion in strong acidic (pH 1) and alkaline (pH 13) solutions.
  • Long-term: Samples maintained structural integrity and performance after 6 months of exposure to silicone oil.
• Versatility: The system was successfully applied to glass substrates of various shapes and geometries, demonstrating scalability.

Significance and Claims
The paper claims to present the first detailed optimization of a sponge-based system applied to glass surfaces for smart window applications. The authors highlight several advantages over state-of-the-art systems:

1. Simplicity and Cost: The fabrication relies on readily available, low-cost materials (PDMS, sugar, silicone oil) and avoids hazardous chemicals or complex synthesis.
2. Environmental Independence: Unlike thermochromic materials (e.g., VO₂ or pNIPAm) that require specific temperature thresholds to switch, this system operates via solvent infusion, offering consistent performance regardless of ambient temperature fluctuations.
3. Dual Functionality: The coating provides dynamic control over both solar light transmission and heat blocking, switching between a heat-blocking opaque state and a heat-transmitting transparent state.
4. Robustness: The system demonstrates high mechanical and chemical durability, addressing common limitations of existing smart window technologies.

The authors acknowledge limitations, specifically that the opaque state limits visibility and that the system currently requires an external mechanism (such as pumps) to inject and withdraw the solvent. However, they posit that this platform offers a viable, low-cost solution for building energy efficiency in applications where full-time transparency is not strictly required.