In this work, hydroxyl-terminated oxalamide compounds N 1 , N 2 -bis(2-hydroxyethyl)oxalamide (OXA1) and N 1 , N 1′ -(ethane-1,2-diyl)bis( N 2 -(2-hydroxyethyl)oxalamide (OXA2) were synthesized to initiate the ring-opening polymerization of L-lactide for preparation of oxalamide-hybridized poly(L-lactide) (PLA OXA ), i.e. , PLA OXA1 and PLA OXA2 . The crystallization properties of PLA were improved by the self-assembly of the oxalamide segments in PLA OXA which served as the initial heterogeneous nuclei. The crystal growth kinetics was studied by Hoffman-Lauritzen theory and it revealed that the nucleation energy barrier of PLA OXA1 and PLA OXA2 was lower than that of PLA. Consequently, PLA OXA could crystallize much faster than PLA, accompanied with a decrease in spherulite size and half-life crystallization time by 74.8% and 86.5% (T = 125 °C), respectively. In addition, the final crystallinity of PLA OXA1 and PLA OXA2 was 6 and 8 times higher, respectively, in comparison with that of neat PLA under a controlled cooling rate of 10 °C/min. The results demonstrate that the hybridization of oxalamide segments in PLA backbone will serve as the self-heteronucleation for promoting the crystallization rate. The higher the content of oxalamide segments (PLA OXA2 compared with PLA OXA1 ) is, the stronger the promotion effect will be. Therefore, this study may provide a universal approach by hybridizing macromolecular structure to facilitate the crystallization of semi-crystalline polymer materials.
Endowing intumescent flame retardant (IFR) with free-radical quenching capability is a promising method to improve its efficiency. Herein, a novel N -alkoxyamine containing macromolecular intumescent flame retardant (RQMIFR) was synthesized via nucleophilic substitution reaction and combined with ammonium polyphosphate (APP) to improve the thermostability and fire resistance of polypropylene (PP). The results showed that there was a good synergism between RQMIFR and APP, which effectively enhanced the thermostability and flame retardancy of PP. When the content of RQMIFR was 8.33 wt% and APP was 16.67 wt%, the limiting oxygen index of PP increased from 19.0% to 39.5% and achieved a UL-94 V-0 rating. Besides, the peak heat release rate, total heat release, carbon monoxide production, and carbon dioxide production were remarkably reduced. It was revealed that during combustion, RQMIFR generated nitroxyl free-radicals and quenched the active free radicals produced by PP both in gas and condensed phase. Simultaneously, RQMIFR and APP worked synergistically to form a compact intumescent char layer through the esterification, cross-linking, cyclization, and foaming reactions, thus effectively improved the thermostability and flame retardancy of PP.
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