Heterogeneous Nucleation and Self-Nucleation of Isotactic Polypropylene Microdroplets in Immiscible Blends: From Nucleation to Growth-Dominated Crystallization

3D Morphology of Different Crystal Forms in β-Nucleated and Fiber-Sheared Polypropylene: α-Teardrops, α-Teeth, and β-Fans

Polymorphism of semicrystalline polymers has significant influence on their physical properties, with each form having its advantages and disadvantages. However, real-life polymer processing often results in different coexisting crystal polymorphs, and it remains a challenge to determine their shape, spatial distribution, and volume fraction. Here, i-polypropylene (i-PP) sheets containing both α- and β-forms were prepared either by adding β-nucleating agent or by fiber pulling-induced crystallization. By adding a compatible dye that is partially rejected from the growing crystalline aggregates (spherulites and cylindrites), we visualize the shape of these objects in 3D using two-photon fluorescence confocal microscopy. To distinguish between crystal forms, we take advantage of the difference in dye-retaining ability of the α- and β-aggregates. Even in 2D, fluorescence microscopy (FM) distinguishes the two crystal forms better than polarized microscopy. In 3D imaging, the volume fraction and spatial distribution of α- and β-forms in different morphological types could be determined quantitatively. Morphologies described as α-teeth, β-fans, and α-teardrops were visualized for the first time in 3D. Furthermore, internal and surface microcracks were seen to be associated predominantly with the β-form and around the fiber. Spatial distribution of α- and β-forms was also determined by scanning with a synchrotron X-ray beam. Good agreement was obtained with 3D microscopy, but XRD could not match the detail obtainable by the tomography. The work demonstrates the ability of the 3D imaging method to distinguish different crystal forms and their specific morphologies.

β-Nucleated Polypropylene: Preparation, Nucleating Efficiency, Composite, and Future Prospects

The β-crystals of polypropylene have a metastable crystal form. The formation of β-crystals can improve the toughness and heat resistance of a material. The introduction of a β-nucleating agent, over many other methods, is undoubtedly the most reliable method through which to obtain β-PP. Furthermore, in this study, certain newly developed β-nucleating agents and their compounds in recent years are listed in detail, including the less-mentioned polymer β-nucleating agents and their nucleation characteristics. In addition, the various influencing factors of β-nucleation efficiency, including the polymer matrix and processing conditions, are analyzed in detail and the corresponding improvement measures are summarized. Finally, the composites and synergistic toughening effects are discussed, and three potential future research directions are speculated upon based on previous research.

Non-Isothermal Crystallization Kinetics of Poly (ɛ-Caprolactone) (PCL) and MgO Incorporated PCL Nanofibers

The study delves into the kinetics of non-isothermal crystallization of Poly (ɛ-caprolactone) (PCL) and MgO-incorporated PCL nanofibers with varying cooling rates. Differential Scanning Calorimetry (DSC-3) was used to acquire crystallization information and investigate the kinetics behavior of the two types of nanofibers under different cooling rates ranging from 0.5-5 K/min. The results show that the crystallization rate decreases at higher crystallization temperatures. Furthermore, the parameters of non-isothermal crystallization kinetics were investigated via several mathematical models, including Jeziorny and Mo's models. Mo's approach was suitable to describe the nanofibers' overall non-isothermal crystallization process. In addition, the Kissinger and Friedman methods were used to calculate the activation energy of bulk-PCL, PCL, and MgO-PCL nanofibers. The result showed that the activation energy of bulk-PCL was comparatively lower than that of nanofibers. The investigation of the kinetics of crystallization plays a crucial role in optimizing manufacturing processes and enhancing the overall performance of nanofibers.

Covalent Bonding Enhanced Polypropylene Based T-ZIF-8 Masterbatch with Superior Photocatalytic and Antibacterial Performances

In order to solve the problem of poor compatibility between modified-ZIF-8 nanoparticles and mask matrix polypropylene (PP) and melt-blown materials, in this work, PP based modified-ZIF-8 antibacterial masterbatch was prepared employing surface modification and torque blending method. IR, SEM, XRD, XPS, DSC results confirm that the antibacterial masterbatch maintains the chemical and crystal structure of modified-ZIF-8 and the thermal stability of PP. Photocatalytic performance indicates that the antibacterial masterbatch basically maintains the photoresponse range of modified-ZIF-8, has narrower band gap and the superior photocatalytic performance than that of modified-ZIF-8. The photocatalytic antibacterial mechanism of ·O2- and h+ as antibacterial active species is revealed according to the energy band structure and free radical capture experiment. The photocatalytic antibacterial activity of the antibacterial masterbatch against Staphylococcus aureus and Escherichia coli under different dosage holds that the relationship between antibacterial rate and antibacterial agent concentration conforms to Beta distribution, demonstrating second-order kinetic behavior. The antibacterial properties reach the maximum when the loading of modified-ZIF-8 is 2% of the total weight of PP and melt-blown materials. S. aureus and E. coli could be completely killed when the simulated sunlight is irradiated for 30 min. These results indicate that PP based modified-ZIF-8 antibacterial masterbatch has potential application in photocatalytic antibacterial masks.

Surface Roughness Enhances Self-Nucleation of High-Density Polyethylene Droplets Dispersed within Immiscible Blends

Highly linear or high-density polyethylenes (HDPEs) have an intrinsically high nucleation density compared to other polyolefins. Enhancing their nucleation density by self-nucleation is therefore difficult, leading to a narrow self-nucleation Domain (i.e., the so-called DomainII or the temperature Domain where self-nuclei can be injected into the material without the occurrence of annealing). In this work, we report that when HDPE is blended (up to 50%) with immiscible matrices, such as atactic polystyrene (PS) or Nylon 6, its self-nucleation capacity can be greatly increased. In addition, temperatures higher than the equilibrium melting temperature of the HDPE phase are needed to erase the significantly enhanced crystalline memory in the blends. Morphological evidence gathered by Scanning and Transmission Electron Microscopies (SEM and TEM) indicates that these unexpected results can be explained by the modification of the interface between blend components. The filling of the solid HDPE surface asperities by the low viscosity polystyrene during heating to the self-nucleation temperature, or the crystallization of the matrix in the case of Nylon 6, enhances the interface roughness between the two polymers in the blends. Such rougher interfaces can remarkably increase the self-nucleation capacity of the HDPE phase via surface nucleation.

Competing crystallization of α- and β-phase induced by β-nucleating agents in microdroplets of isotactic polypropylene

Crystallization of heterogeneously nucleated isotactic polypropylene microdroplets in an immiscible polystyrene matrix allows the estimation of intrinsic nucleating efficiency of nucleating agents promoting the formation of different polymorphs.

The Application of Organic Phosphate Nucleating Agents in Polypropylene with Different Molecular Weights

Two kinds of organic phosphate nucleating agent (NA-11 and NA-21) were used in PP with different molecular weights through the melt extrusion method. The dispersibility of the nucleating agents in PP, and the effect of the nucleating agents on the molecular weight, rheological behavior and crystallization behavior of PP were investigated. SEM and TEM analysis showed that the average radius of the dispersed particles (nucleating agents) was larger in LPP than that in HPP. The good dispersion of NA-21 also created more nucleation embryos for the adsorption of polypropylene molecules than the agglomerated NA-11. The gel permeation chromatography (GPC) analysis showed that the average molecular weight of HPP and LPP both decreased with the addition of a nucleating agent. The rotational rheometer and capillary rheometer analysis showed that the effect of NA-21 on reducing intermolecular entanglement was more significant, whether in HPP or LPP. The addition of NA-21 had less elastic energy storage and better flow stability, and could be processed at a higher speed. Simultaneously, the relaxation time in the blends with LPP was shorter than that with HPP. It was found that the crystallinity and nucleation efficiency of HPP/nucleating agent blends increased remarkably, while there was a barely perceptible increase in LPP/nucleating agent blends.

Crystallization of a Self-Assembling Nucleator in Poly(l-lactide) Melt

In the present work, crystallization of a soluble nucleator N, N', N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328) in a poly(l-lactic acid) (PLLA) matrix has been studied at different temperatures. Based on the change in solubility with temperature, different levels of supersaturation of TMC-328 in a PLLA matrix can be obtained. This nucleator presents a fibrous structure produced via self-assembling and develops into an interconnected network when the temperature is lowered. The TMC-328 crystal nuclei density is quantified via optical microscopy, using the average distance of the adjacent fibrillar structure, which shows a steady decrease with the decrease in temperature. The crystallization rates of TMC-328 were assessed through rheological measurements of network formation. Both fibrils' density and crystallization kinetics display a power law dependence on supersaturation. For the first time, the solid-melt interfacial energy, the size of the critical nucleus, and the number of molecules making up the critical nucleus of the nucleator TMC-328 in the PLLA matrix have been determined by adopting the classical nucleation theory. The subsequent crystallization of PLLA induced by this nucleator was investigated as a function of the fibrils' spatial density. The crystallization rate of PLLA is enhanced with the increase in the TMC-328 fibrils' density because of the availability of a larger nucleating surface. The self-assembled fibril of TMC-328 can serve as shish to form a hybrid shish-kebab structure after the crystallization of PLLA, regardless of the number of nucleation sites.

Temperature and Thickness Dependence of the Time Scale of Crystallization of Polymers under 1D Confinement

Confined in nanodomains, polymers crystallize much slower than in bulk due to both finite size and interfacial effects. These two factors are successfully disentangled in our phenomenological framework, which provides a measurement of the time scale of crystallization via a product of probabilities involving nucleation and of chain diffusion. In this Letter, we demonstrate that our model allows determining the Gibbs free energy of the formation of a critical size nucleus indicated by the classical nucleation theory for bulk polymer melts. In addition to that, by means of segmental mobility data and one single set of isothermal crystallization measurements at different confinement degrees, our model predicts the right temperature and thickness dependence of the crystallization time.