Clear Viscoelastic Film Based on Low-Hard-Segment Silicone Polyurea Elastomers for Flexible Display Applications

A clear viscoelastic film (CVF) plays a critical role in bonding different components in a display module. The recent flexible display application calls for a CVF with balanced viscoelasticity by integrating many combined material properties, such as high optical clarity, high adhesion, low glass transition temperature, low elastic modulus, and high recovery performance. In particular, a low glass transition temperature (Tg) is essential to achieve flexibility over a wide range of temperatures (i.e., −20 to 80 °C). Silicone polymers are an important class of elastomers with Tg below −100 °C and are expected to exhibit unique properties as CVF candidates in flexible display applications. In this report, silicone polyurea (SPU) elastomers with low hard segments were first synthesized using α,ω-bis(aminopropyl)poly(dimethylsiloxanes) and different diisocyanates. Interestingly, these silicone polyurea elastomers exhibited good elasticity despite a small quantity of bidentate hydrogen bonds. SPU CVF was then prepared by blending tackifier that hydrolytic condensation products of monofunctional silane (M) and tetrafunctional silane (Q) (MQ resin) at different ratios to adjust the viscoelastic properties. Specifically, blending an equal weight of methyl MQ resin could simultaneously achieve an optimum balance of high clarity (transmittance above 94%, haze lower than 1%), low elastic modulus (30 kPa at room temperature), low working temperature limit (Tg approximately −20 °C), decent peel adhesion (6.9 N/25 mm), good stress relaxation (71.43%), and high recovery (80%) at large strain (500 and 1000%). Moreover, the optimal sample could withstand 200 000 successive folding–unfolding tests without any cracking or delamination. The results revealed that the SPU-based viscoelastomers could function as CVFs for flexible display applications.