Expanding the application range of flame-retardant polymer biocomposites remains a huge challenge for a sustainable society. Despite largely enhanced flame retardancy, until now the resultant poly(lactic acid) (PLA) composites still suffer reduced tensile strength and impact toughness due to improper material design strategies. We, herein, demonstrate the design of a green flame retardant additive (ammonium polyphosphate (APP)@cellulose nanofiber (CNF)) via using the cellulose nanofibers (CNFs) as the green multifunctional additives hybridized with ammonium polyphosphate (APP). The results show that PLA composite with 5 wt % loading of APP@CNF can pass the UL-94 V-0 rating, besides a high limited oxygen index of 27.5%, indicative of a significantly enhanced flame retardancy. Moreover, the 5 wt % of APP@CNF enables the impact strength (σ i ) of the PVA matrix to significantly improve from 7.63 to 11.8 kJ/m 2 (increase by 54%), in addition to a high tensile strength of 50.3 MPa for the resultant flame-retardant PLA composite. The enhanced flame retardancy and mechanical strength performances are attributed to the improved dispersion of APP@CNF and its smaller phase size within the PLA matrix along with their synergistic effect between APP and CNF. This work opens up a facile innovative methodology for the design of high-performance ecofriendly flame retardants and their advanced polymeric composites.
Chiral aldehyde catalysis is uniquely suitable for the direct asymmetric α-functionalization of N-unprotected amino acids, because aldehydes can reversibly form imines. However, there have been few successful reports of these transformations. In fact, only chiral aldehyde catalyzed aldol reactions of amino acids and alkylation of 2-amino malonates have been reported with good chiral induction. Here, we report a novel type of chiral aldehyde catalyst based on face control of the enolate intermediates. The resulting chiral aldehyde is the first efficient nonpyridoxal-dependent catalyst that can promote the direct asymmetric α-functionalization of N-unprotected glycine esters. Possible transition states and the proton transfer process were investigated by density functional theory calculations.
The self-assembly behavior of (B1AB2)5 star copolymers, composed of five asymmetric BAB-triblock arms joined at the end of B2-blocks, has been investigated using the self-consistent field theory. The special architecture enables a few different sophisticated mechanisms such as the conformational asymmetry from the star topology, the effect of combinatorial entropy from the multiple arms that enhances the formation of bridging configurations for the core B2-blocks, the local segregation between the two different B-blocks, and the solubilization effect of the short B2-block in the majority A-domain, each of which has been individually demonstrated to play an important role in impacting the self-assembly behavior of block copolymers before. As a result, the combination of these mechanisms leads to many unusual phase behaviors of (B1AB2)5 with tunable asymmetry τ = f B1 /(f B1 + f B2 ) between the two B-blocks, where f B1 and f B2 are the volume fractions of B1- and B2-blocks, respectively. For example, reentrant phase transitions between the BCC and FCC spherical phases are observed with minority A-domains, whereas the width of the overall spherical phase region at the opposite side of the phase diagram exhibits two maxima as τ increases. The expansion of the spherical phase region at the first maximum is induced by the reduced effective volume fraction due to the solubilization effect and thus is solely occupied by the BCC phase. While the expansion at the second maximum originates from the formation of enlarged “core–shell” domains due to the effect of local segregation, leading to the formation of complex Frank–Kasper spherical phases. In addition, no stable gyroid phase composed of A-network is observed in the phase diagram of τ = 4/5, while the gyroid phase region in the opposite side of the phase diagram is expanded significantly. The absence of the gyroid phase is a very rare phenomenon for block copolymers and here may result from the combined effect of different sophisticated mechanisms.
A palladium-catalyzed, cascade 5-endo-dig cyclization-alkenylation synthesis of isoxazoles has been developed. The addition of 1 equiv of n-Bu(4)NBr significantly increases the yield of the desired 4-alkenyl-3,4,5-trisubstituted isoxazoles. A variety of trisubstituted isoxazoles are prepared in moderate to excellent yields. One example of the synthesis of a naphthoisoxazole is reported by a cascade cyclization-alkenylation-Heck reaction.
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