Bifunctional linear polyphosphazene decorated by allyl groups: Synthesis and application as efficient flame-retardant and toughening agent of bismaleimide

As known, the poor toughness and low fire safety of bismaleimide resin (BMI) has become a problem which restricts its further application in advanced high-performance field. Therefore, a novel allyl-functionalized linear polyphosphazene (PMAP) was designed and synthesized. With inclusion of 3wt% PMAP, the peak heat release rate (PHRR) and total smoke production (TSP) of BMI/PMAP-3 are reduced by 51.3% and 17.8%, respectively. And the residual char of BMI/PMAP increases significantly as well. Furthermore, the flame-retardant mechanism of BMI/PMAP is proposed. In condensed phase, PMAP can participate in the formation of residual char of BMI/PMAP and the char layer is with an excellent physical barrier effect by the existence of phosphorus oxygen and phosphorus nitrogen cross-linking substances. In gas phase, phosphorous oxygen free radical is also generated from PMAP, which can capture gas-phase chain free radicals and inhibit gas-phase combustion. Moreover, the impact strength of BMI/PMAP-3 increases by 85.3%, which indicates that the toughness of BMI/PMAP is effectively enhanced. The toughening mechanism of PMAP on BMI can be assigned to elastomer toughening. Therefore, with modification of PMAP, BMI/PMAP is indeed of better comprehensive performance, which is in line with expectation and provides inspiration for the simultaneous flame-retardant and toughening modification of BMI.