Molecule control and thermal degradation study of phosphazene cyclomatrix nanocrystals: For simultaneous enhancement of flame retardant and mechanical properties

•Phosphazene cyclomatrix with controllable molecules were designed and studied.•Chemical structure, molecule size and thermal properties were correlated.•The macromolecules self-assemble into a unique cubic crystalline structure.•Thermal degradation mechanism of macromolecules shows self-intumescent feature.•Excellent flame retardant performance and mechanical reinforcement of the macromolecules was achieved. Cyclotriphosphazene which possesses hexafunctional framework is a versatile building block to construct functional materials and has been used in many areas such as catalysis, biomedicine, sensing and imaging, and flame retardancy. However, the proper control of the molecules and their condense state structures, in particular via the reaction of phosphonitrilic chloride trimer (PCT) with other multifunctional co-monomers is a challenge. Herein, we developed phosphazene cyclomatrix molecules using pentarythritol (PER) as the co-monomer. Through controlling the PCT/PER molar ratio and the feeding manner, the obtained product is either monomeric/oligomeric small molecules or macromolecules. Interestingly, the macromolecules self-assembled into three-dimensional structures, forming cubic nanocrystals. The thermal study shows that all molecules possess a low temperature degradation domain (LTD, 150–400 °C) and high temperature degradation domain (HTD, 500–750 °C) due to the same chemical bonds, while the macromolecules have the highest carbon forming among all. TG-IR and TG-MS results confirm the following thermal degradation features: (1) inert gases (NH3, CO2) and H2O release after 200 °C, (2) the carbonaceous phase forms at 300–450 °C when the PER segment content in the molecule is 50 % and higher, and (3) extra phosphate/phosphite agents release after 450 °C, which renders carbonizing capability. The addition of the crystalline macromolecules into polypropylene (PP) showed that under a low loading of only 18 and 22 wt%, UL94 V2 and V0 rate was achieved for the obtained composites. This indicates the high flame retardant efficacy of the macromolecules as a standalone additive, ascribing to the cooperative effect of the above thermal features in a single molecule. The synthesized macromolecule is also able to simultaneously reinforce PP in terms of tensile modulus and flexural modulus without sacrificing the impact strength and flexural strength, leading to dual functionality, i.e., enhancement flame-retardant (FR) performance and mechanical reinforcemen