A Simple Solvent-Free Strategy of Polybenzoxazine Film Elaboration with Controllable Performance via Molecular Weight Regulation between Cross-Linking Knots

Abstract

Polybenzoxazines (PBzs) belong to the next generation of highly cross-linked thermostable resins that go far beyond a traditional high-temperature polymer network due to their remarkable physical–chemical properties. However, some of their disadvantages, namely, a high brittleness (i.e., impossibility to form a free-standing film), limit the PBz applications in real-world devices. In this work, the influence of the high molecular amide-functionalized polypropylene glycol/polyethylene glycol blocks with different lengths between 3,4-dihydro-2H-1,3-benzoxazine fragments on the physical–chemical and gas permeation properties of PBz-based free-standing flexible films was studied for the first time. Such films were prepared by a simple solvent-free strategy by curing liquid amide-functionalized benzoxazine main-chain prepolymers. Due to such linkers, the elaboration of dimensionally stable, free-standing PBz films without loss of any properties is possible. In addition, the spacer length between cross-linking knots was correlated with physical and mechanical characteristics and the thermal stability of PBz-based materials. The thermal stability of materials (T 10% up to 360 °C) is found to increase with the rising of the molecular weight of the polypropylene glycol/polyethylene glycol blocks between cross-links. The performed gas permeation measurements also underline the influence of the molecular weight of the polypropylene glycol/polyethylene glycol blocks on the material properties. Moreover, the regulation of their molecular weight provides a sufficient degree of freedom for the polymer subchain movement and, consequently, for the formation of organized structures with tunable material properties (e.g., mechanical and transport), thus making the synthesized polymer resins attractive for many industrial applications.