A comparative study on interfacial crystallization of isotactic polypropylene (iPP) surrounding macroscopic carbon nanotube and graphene fibers has been carried out in single fiber polymer composites by means of in situ polarized optical microscope, scanning electron microscope and X-ray diffraction. Ordered interfacial microstructures of iPP nucleate on both nanocarbon fibers in the form of a transcrystalline interphase. Nanotube fibers tend to promote negative birefringence transcrystals whereas graphene fibers induce positive birefringence transcrystals. The microstructures of transcrystals are strongly dependent on the thermal history and the double-layered transcrystals consisting of a negative inner layer and a positive outer layer occur under certain conditions. Transcrystallization kinetics has been studied and the Lauritzen-Hoffman theory of heterogeneous nucleation used to analyze the dynamic crystallization process. While the fold surface energy of iPP transcrystals surrounding both nanocarbon fibers shows little difference, the nanotube fiber promotes shorter induction time than the graphene fiber. Thermal resistance test demonstrates that the ordered interfacial microstructures possess higher melting temperature in the nanotube fiber composites than those in the graphene fiber composites. Under appropriate conditions, the-form transcrystals of iPP are observed. The amount of the-form iPP surrounding the nanotube fiber is much higher than that surrounding the graphene fiber. A theoretical model is proposed to interpret the difference between the nanotube and graphene fiber composites and the mechanisms behind its influence on interfacial crystallization.
The preparation of ring‐banded spherulites in poly(3‐butylthiophene) via controlled solvent evaporation of solution‐cast films is reported. The spherulites display unusual concentric ring‐banded structures under both polarized and unpolarized lights. The size of the ring‐banded spherulites is 300 ± 100 µm in diameter and the periodicity of the bands is 15 ± 2 µm. The periodic bands of the spherulite consist of alternating ridge and valley surface patterns and the crystalline lamellae in the bands are more or less parallel to the radial growth direction of the spherulites. Local lamellar bending and branching are observed analogous to that of classical non‐conjugated polymers. A possible diffusion‐induced rhythmic growth mechanism is proposed to interpret the formation of periodic banding of the spherulite.
Increasing polymer usage has demanded functional additives that decrease fire hazards for end users. While traditional flame‐retardant (FR) additives, such as halogenated, phosphorus, and metal hydroxides, greatly reduce flammability and associated fire hazards, research has continually exposed a litany of health and environmental safety concerns. This perspective aims to identify the key components of a successful FR additive and address material, environmental, and health concerns of existing additives. Legislation surrounding FRs and persistent organic pollutants is also discussed to highlight political perception that has resulted in the increased chemical regulations and subsequent banning of FR additives. Finally, future directions of this field regarding nonreactive additives, focusing on the use of bioinspired materials and transition metal chemistries to produce alternatives for polymers with efficacies surpassing traditional additives are presented.
The directed assembly of conjugated polymers into macroscopic organization with controlled orientation and placement is pivotal in improving device performance. Here, the supramolecular assembly of oriented spherulitic crystals of poly(3‐butylthiophene) surrounding a single carbon nanotube fiber under controlled solvent evaporation of solution‐cast films is reported. Oriented lamellar structures nucleate on the surface of the nanotube fiber in the form of a transcrystalline interphase. The factors influencing the formation of transcrystals are investigated in terms of chemical structure, crystallization temperature, and time. Dynamic process measurements exhibit the linear growth of transcrystals with time. Microstructural analysis of transcrystals reveals individual lamellar organization and crystal polymorphism. The form II modification occurs at low temperatures, while both form I and form II modifications coexist at high temperatures. A possible model is presented to interpret transcrystallization and polymorphism.
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