Impact of Cross-Linker Structure on the Properties of Durable and Selectively Degradable Silyl-Containing Polyurethane Networks

Polyurethane (PU) networks with covalent cross-links provide unique strength and durability, which has led to their use in numerous modern-day materials. However, these networks are difficult to degrade, which results in landfilling once their service life has ceased. To date, there are no examples of PU networks that offer the properties of traditional cross-linked PUs with the ability to selectively, rapidly, and completely degrade at room temperature under static conditions. Herein, we report on silyl-containing polyurethane (silyl-PU) networks that exhibit up to 301.7 °C thermal stability, up to 290% elongation, and up to 16.4 MPa strength when silyl diol cross-linkers with different appendage lengths and composition are employed. Upon immersion in a solution of fluoride ions at room temperature and under static conditions, these silyl-PUs, as unpigmented (clear) networks, demonstrated complete degradation within 15–120 min via a mechanism of cascading bond cleavages. When pigmented with titanium dioxide (TiO2), a white-colored silyl-PU displayed similar thermal stability and mechanical properties as a commercial white-colored PU network that is based on traditional PU chemistry (e.g., no silyl bonds) and used on military and commercial aircraft. However, when exposed in an accelerated weathering chamber to artificial sunlight and humidity at elevated temperatures, the white-colored silyl-PU demonstrated better photo-oxidative and hydrolytic stability than the white-colored nonsilyl-PU as evidenced by no observable bond changes for the silyl-PU via infrared analysis, a smaller decrease in surface yellowness (12.7% for silyl-PU vs 71.7% for nonsilyl-PU), and a smaller decrease in cross-link density (35.6% for silyl-PU vs 75.8% for nonsilyl-PU). Furthermore, the white-colored silyl-PU could be selectively degraded with a static solution of fluoride ions at room temperature and completely removed from a strongly adhered epoxy network within 20 min, whereas the aerospace-grade nonsilyl-PU showed no degradation or removal after 360 min in the same solution. These silyl-PU networks have potential applications as high-performance and selectively degradable materials (e.g., coatings, composites, adhesives) and may reduce environmental waste compared to PUs based on traditional, yet nondegradable, chemistries.