Synthesis of Polyetheramine-Grafted Epoxidized Natural Rubber and Its Role in Humidity Adhesive Sensors

The development of hydrophilic adhesives is critical for multifunctional applications ranging from wound healing patches to humidity sensors. With the advent of the internet of things, developing a sensor material that demonstrates flexibility in terms of its form factor is becoming a ubiquitous choice. Especially in the field of the humidity sensor, ceramic materials are leveraged. This causes impediments in terms of the form factor, while reports related to the development of flexible hydrophilic adhesive sensors from sustainable resources for such activity are less. Herein, we report the synthesis and characterization of a functional additive from a sustainable source that can be blended effectively in an adhesive formulation to impart hydrophilicity to the entire composition. Epoxidized natural rubber was selected as the main polymer backbone, while polyether amine was selected as the modifier. Various spectroscopic techniques probed the fidelity of modification, which indicated 8% grafting. A detailed rheological study was performed to decipher the structure of the functional additive at the molecular level in terms of various parameters like the plateau modulus, entanglement molecular weight, relaxation time, and diffusion coefficient of the functional additive. The functional additive was blended with a standard adhesive formulation and was tested for its hydrophilicity and adhesiveness. The functional additive showed a 284% increase in water absorption after 7 days and a 78% increase in water vapor uptake when blended with the adhesive formulation. The final composition was tested for its ability to act as a humidity sensor. The hydrophilic adhesive sensor was capable of reciprocating with variation in humidity. Such a finding can be readily extended to develop smart adhesive patches with end applications in bioelectronics.