βα recrystallization refers to a process of melting of the β crystallites followed by a recrystallization into α phase in isotactic polypropylene (iPP). It is normally observed only after cooling ...down the β-nucleated iPP (β-iPP) from the crystallization temperature to below 100 °C before heating. In this work, several β-iPP with different molecular weights were employed to investigate the βα recrystallization in the heating process starting at the crystallization temperature (Tc). It turned out that the non-cooled β-iPP with a molar mass of about 12 kg/mol (β-iPP12K) showed a βα recrystallization during slowing heating. The extent of the βα recrystallization gradually reduced with the increase of the Tc of the β-iPP12K. This peculiar behavior of β-iPP12K can be assigned to the existence of less stable β-crystallites which are susceptible to βα recrystallization. The β-iPP12K preferred to crystallize into thin β-crystallites with low stability which could be melted at low temperatures to form “amorphous self-nuclei” during slow heating. These local ordered amorphous phase would serve as nucleation-sites for α growth in a favorable temperature range. With respect to the β-iPP with high molar mass, although the “amorphous self-nuclei” could be induced at a certain temperature, this temperature was already beyond the appropriate temperature for α growth.
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•It is the first time to observe βα recrystallization in β-iPP samples never cooled below 100 C.•The local ordered amorphous phase or “amorphous self-nuclei” originates from melting the less stable β crystallites.•The less stable β crystallites are responsible for inducing βα recrystallization with further heating.•Higher Tc would inhibit the βα recrystallization during direct heating, due to the decrease of less stable β crystallites.
Some zinc dicarboxylic acids can induce numerous β-crystals formation in isotactic polypropylene (iPP), thus improving the crystallization behavior and mechanical properties of iPP. However, the ...corresponding nucleation mechanism of β-crystals remains inadequate, leading to the absence of a theoretical basis for the design and preparation of new and high-efficiency β-nucleating agents (β-NAs). In this work, we demonstrate a new mechanism that β-crystals can be formed when the interfacial interaction between β-NAs and iPP is comparable to that between β-PP and iPP. The interfacial interaction occurs when iPP epitaxially crystallizes on the NA surface, which will affect the folding and arrangement of iPP chains, thereby leading to formation of different crystalline forms of iPP. The interaction energy of iPP chain with α-PP and β-PP is used as a judgment criterion, which is resolved by molecular dynamics (MD) simulation. We surprisingly found that when the interaction energy of NA/iPP was close to that of β-PP/iPP, the NA can be acted as β-NA; β-crystals were inclined to form when the interaction energy of NA/iPP was close to that of β-PP/iPP. Furthermore, the simulation results are well verified by the crystallization behavior of iPP in the presence of aromatic zinc dicarboxylic acids of zinc terephthalate (ZnCC1) and zinc 4-carboxybenzoate (ZnCC2), lending strong support to the proposed nucleation mechanism and method. This work proposes a new insight into the formation of β-crystals of iPP based on the interaction energy of NA and iPP, and demonstrates a method to rapidly predict and evaluate the β-nucleation effect of NAs, thus may serve as a theoretical guidance for the design of NAs that can induce iPP with specific crystalline forms.
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•The single crystals of zinc dicarboxylic acids were systematically cultured and single crystal structures were obtained.•The effects of single crystals on the isothermal crystallization behavior of iPP were observed.•A new simulation method is proposed to predict and evaluate the effect of NA on the crystallization behavior of iPP.
Compared with traditional injection foaming, co-injection foaming serves as a method to prepare products with high-quality surfaces and enhanced mechanical properties, demonstrating excellent ...technical advantages and economic benefits. The effects of high-density polyethylene (HDPE) content, foaming agent content, melting temperature of core material, and nano-organic montmorillonite (OMMT) content on the cell structure, mechanical properties, and crystallization behavior of isotactic polypropylene (iPP)/HDPE composite foam prepared by co-injection foaming were systematically studied. At a foaming agent content of 2 wt.%, HDPE content of 30 wt.%, OMMT content of 3 wt.%, and the melting temperature of core material of 190 °C, the composite foam exhibited a cell size measuring 85 μm, a cell density of 2.5 × 105 cells/cm3, a tensile strength of 25.8 MPa, and an impact strength of 12.5 kJ/m2. The addition of HDPE reduced the crystallinity of iPP matrix, while enhancing the solubility of carbon dioxide within the matrix. The non-miscible interface between HDPE and iPP helped reduce the energy barrier for cell nucleation. Furthermore, the introduction of OMMT as a nucleating agent in iPP/HDPE composite material dramatically decreased the energy barrier for cell nucleation. These synergistic effects result in foam with the smaller cell size and the higher cell density, exhibiting excellent comprehensive mechanical properties. This work provides a feasible method for preparing composite foam with excellent comprehensive performance.
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•The addition of HDPE reduced the crystallinity of iPP matrix, while enhancing the solubility of CO2 within the matrix.•The non-miscible interface between HDPE and iPP helped reduce the energy barrier of cell nucleation.•The introduction of OMMT in iPP/HDPE composite material dramatically reduced the energy barrier of cell nucleation.
Owing to the unique cross-hatched structure of α crystal, the deformation behavior of isotactic polypropylene (iPP) differs considerably with the traditional polymers. The influence of crystalline ...structures on the cavitation and shearing effects of α-iPP under tensile loading is still not well established. In this work, various iPP samples were prepared via crystallizing from 0 °C to 130 °C, after which the crystalline structures were characterized via differential scanning calorimetry, polarized optical microscopy and scanning electron microscopy. The lamellar thickness and crystallinity of α-iPP precursor films change slightly, while the content of tangential lamellae in α-spherulite and the spherulite size increase significantly with the decreasing supercooling. On the other hand, the morphological evolution of α-iPP during stretching was tracked by in-situ two-dimensional small angle X-ray scattering. It is found that cavitation effect become dominant instead of shearing effect when the size of α-spherulite exceeds 25 μm. Furthermore, we disclose four typical deformation behaviors of α-iPP resulting from the competition between intra-spherulitic deformation and inter-spherulitic deformation, namely the shearing without cavitation, shearing then localized cavitation, homogeneous cavitation with concomitant shearing, and cavitation with absence of shearing. Accordingly, we construct a route for the structural evolution of α-iPP with different crystalline structures during stretching, which provides an effective guidance to produce diverse iPP films with desired structures and tunable functionalities.
Diagram of interaction and competition between shearing and cavitation of α-iPP with different crystalline structures, the tensile direction is vertical. Display omitted
•The yield strength of α-iPP does not change linearly with spherulite size.•The tensile behavior of α-iPP depends on the spherulite size.•There is a critical spherulite size that affects cavitation and shearing.
It is of great significance to study the deformation mechanism of isotactic polypropylene (iPP) which differs considerably from that of traditional polymers in consideration of its unique ...cross-hatched structure of α crystal. In this work, the yielding mechanism of three iPP precursor films prepared via crystallizing in different supercooling during stretching under elevating temperature had been investigated systematically. The results show that three mechanisms of deformation had been aroused in the change of yielding stress with stretching temperature named as inter-spherulitic, intra-spherulitic and mix deformation, respectively. Moreover, the transition temperature (60 °C) is in close correlation with the α2 relaxation of iPP, where the chain diffusion within the crystal blocks is activated. The competition between inter-spherulitic deformation and intra-spherulitic deformation is not only determined by the spherulite structures, but also the stretching temperatures. Further studies indicate that, larger spherulitic size, more daughter lamellae and lower stretching temperature are dominant issues to induce inter-spherulitic deformation. This work is expected to provide a guideline for the preparation of iPP films with desired structures and functionalities.
Diagram of yielding mechanism of α-iPP during stretching at elevated temperatures. The tensile direction is vertical. Display omitted
•The yield mechanism changes with spherulite size and tensile temperature.•Larger spherulites bring greater spherulite rigidity.•α-relaxation is activated to change the yield mechanism.•There is a critical tensile temperature that affects yield mechanism.
The major challenge in membrane distillation for seawater desalination and/or wastewater purification is the development of high-efficiency membrane distillation (MD) membranes with high porosity, ...hydrophobicity, and adequate mechanical strength for long-term operation. Herein, a superhydrophobic two-tier interlocked composite membrane based on a hierarchically structured isotactic polypropylene (iPP) coating and an electrospun poly(vinylidene fluoride) (PVDF) nanofibrous support was engineering constructed. The obtained coating layer possessed a deformed micro/nanostructured microsphere surface with robust superhydrophobicity and further glorious anti-fouling property, which was owed to the synergistic effect of the low surface free energy material and hierarchical roughness. Specially, the middle transitional interlocking zone between the crystalline iPP microsphere coating and PVDF nanofibers endowed the resultant composite membrane with excellent structural integrity including the remarkable enhancement in mechanical performance compared with PVDF flat sheet or nanofibrous membranes, and also resulted in the iPP/PVDF composite membranes with robust durability against ultrasonication in isopropanol and strong acid/base attacks. Moreover, for the simulated high salinity sunset yellow (SY) wastewater, the optimized superhydrophobic composite membrane exhibited a competitive permeate flux of 53.9 kg/(m2·h) and complete rejection over 50 h operation (ΔT = 40 °C) because of its highly porous nanofibrous support and firmly sustainable liquid repellency.
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•A robust superhydrophobic two-tier interlocked composite membrane was demonstrated.•Hierarchically structured iPP layer was constructed onto nanofiber support.•The interlocked structure endowed the composite with excellent structural integrity and strength.•The resultant composite exhibited excellent superhydrophobicity and antifouling property.•The membrane showed outstanding DCMD performance for high salty dye wastewater.
Despite the maturity of the technology, processing of fiber-reinforced thermoplastic materials remains challenging, and difficulties in processability often result in material formulations with high ...modulus and strength, yet rather poor ductility compared to the pure polymer matrix. To gain fundamental insight into the deformation mechanisms present in such materials, the complexity of the system is step-wise increased; first, the effect of the most commonly applied adhesion enhancement, the addition of MAH-g-PP compatibilizer, on the bulk properties is assessed. The small-strain tensile properties, i.e., modulus and yield stress, appear to be only marginally affected by the addition of such compatibilization agent, however, the strain-at-break is strongly reduced, even before the addition of the fiber reinforcement. Subsequently, using in-situ X-ray characterization methods upon tensile deformation, the time evolution of crystal structure and lamellar morphology is determined, and at first glance the compatibilizer addition appears to better preserve the crystalline structure. The onset of local failure (cavitation) is quantified at the interface of a single glass fiber. By increasing the adhesive interaction between fiber and matrix the stress concentration at the interface is increased, leading to an acceleration in void formation followed by unstable growth, which in turn strongly embrittles the composite. By the addition of various selective nucleating agents, it is demonstrated that the role of local phase composition and morphology on the deformation kinetics and subsequent failure mechanisms is much more pronounced than the increased adhesion between fiber and matrix by compatibilization or sizing effects. These findings may specify a new route towards tougher fiber-reinforced composites with reduced complexity in the material formulation.
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•The mechanical performance of a composite grade iPP is systematically assessed.•With the addition of MAH-g-PP compatibilizer, the cavitation is more severe.•By the addition of a single fiber, strain is localized at the interface region.•Increased adhesion by sizing leads to accelerated cavitation and brittle fracture.•In β-phase polypropylene, despite extreme voiding the brittle fracture is suppressed.
Dewetting experiments on molten spherulites, previously grown at Tc = 120 °C in thin films of isotactic polypropylene (iPP), revealed two kinds of instabilities of the dewetting rims, namely a ...fingering instability and a fracture instability for dewetting temperatures Ts above and below ca. 165 °C, respectively. For Ts ≥ 165 °C, the dynamics of growing dewetting holes on slippery substrates was governed by viscous dissipation, causing undulations leading to a fingering instability of the rims. However, for 160 °C < Ts < 165 °C, the dewetting rims exhibited a fracture instability, which was related to the release of elastic energy stored in the rim. From the dewetting dynamics of the molten polymers, we derived the non-equilibrium viscosity ηnon-equ(Ts) and equilibrated viscosity ηequ(Ts) as a function of Ts. The values of ηnon-equ(Ts) were significantly larger than ηequ(Ts), with differences decreasing for increasing Ts. Interestingly, extrapolation of ηnon-equ(Ts) to the cross-over point ηnon-equ = ηequ yielded Ts,cross ≈ 166 °C. We relate Ts,cross with the maximum melting temperature of crystalline domains which existed within iPP spherulites. Below Ts,cross, not all crystallites were molten and the melt contained crystalline seeds. Above Ts,cross, the melt was homogeneous and free of any seeds. Our approach opens up a new possibility for determining the maximum melting temperature of polymer crystals.
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•The fingering instability and fracture instability were respectively observed in homogeneous melt and heterogeneous melt.•The rim instability changed from fingering to fracture at 165 °C was revealed by dewetting of iPP films.•The non-equilibrium viscosity extrapolated to the equilibrium viscosity at a cross-over temperature Ts,cross ≈ 166 °C.•The Ts,cross was considered as the maximum melting temperature of crystalline domains within iPP spherulites.
Through the hydrogen bonding between polyamide 12 (PA12) and N, N′-dicyclohexylterephthalamide (DCHT) nucleator, the intentionally controlled self-assembly of DCHT and the crystallization behavior of ...isotactic polypropylene (iPP) matrix are investigated. The introduction of PA12 could severely postpone the condensation of DCHT vapor in iPP matrix due to the reduced vapor concentration. As a result, the nucleating efficiency of DCHT at lower concentration for example iPP/PA12/0.05DCHT even failed at a relatively higher final heating temperature Tf. Such a failure of nucleating efficiency could be recoverable after a step-cycle treatment at lower Tf, where the sublimation of DCHT can hardly occur. The changes from a cluster-like structure to a network structure driven by hydrogen bonding between DCHT and PA12 could be a reason of nucleation failure and recovery. This means that the nucleating efficiency of DCHT and crystallization of iPP could be tuned by controlling the absorption and release of DCHT vapor at the surface of PA12 particles. For iPP/PA12/0.5DCHT with a higher concentration of DCHT, the absorption of PA12 could only reduce the DCHT vapor concentration in the iPP melt and thus, the dendrite structure of DCHT shows more and thinner branches, which results in better nucleating efficiency of DCHT than that in iPP/0.5DCHT.
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•The presence of PA12 could absorb DCHT vapor in iPP matrix and postpone its condensation.•The nucleation failure and recovery of DCHT could be regulated by step-cycle treatment.•The nucleating efficiency of DCHT could be tuned by controlling the absorption and release from PA12.
In this article, the optical and structural properties of iPP/TiO2 nanocomposite fibres, considering three distinct extrusion speeds (25, 50 and 78 m/min) in addition to blank isotactic polypropylene ...samples were determined. Employing computed tomographic scans, localised optical defects in the nanocomposite fibres are unveiled, while refractive indices are examined by analysing transmitted intensity with incident light vibrating parallel and perpendicular to the fibre axis. The internal structure is further characterised through birefringence and density calculations. Mechanical properties, specifically stiffness, are probed by measuring elastic modulus values along the fibre. The investigation extends to the presence of TiO2 nanoparticles in the isotactic polypropylene matrix, inspecting their influence on the uniform morphology along and across the fibre. While the addition of TiO2 nanoparticles has many advantages, including enhanced properties, the study shows adverse effects on the morphological integrity of the fibres, particularly at higher extrusion rates. Micrographs are included to visually illustrate these findings, providing a comprehensive understanding of the complex interaction between extrusion rates, TiO2 nanoparticle incorporation, and the resulting optical and structural properties in iPP fibres.
