Crystallization of Isotactic Polypropylene from Prequenched Mesomorphic Phase

Crystallization with Nodular Aggregation near the Glass Transition Temperature for Syndiotactic Polypropylene

The isothermal crystallization from the melt state of syndiotactic polypropylene (sPP) has been studied by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and optical microscopy. The WAXD and SAXS results show the crystallization mechanism near the glass transition temperature in which the crystalline and mesomorphic nodules cover the entire sample with the formation of aggregation regions. For the SAXS analysis, the scattering function for the three-component system has been suggested. Furthermore, to analyze the growth kinetics of the aggregation region for sPP, the time-dependent structure factor combined with the homogeneous and inhomogeneous nucleation-and-growth kinetics has been suggested. The analysis shows that the growth kinetics of the aggregation region for sPP is the homogeneous nucleation-and-growth. The growth velocity of the aggregation region is a natural extrapolation of that of spherulite to the high supercooling region. These results might indicate that the crystallization with the nodular aggregation is a fundamental crystallization process near the glass transition temperature for polymers.

Specific deformation behavior of isotactic polypropylene films under a multiaxial stress field

The specific deformation behavior of crystalline polymer films, namely unoriented crystallized isotactic polypropylene (it PP) films, was investigated under a multiaxial stress field. Changes in the aggregation structure of the films were investigated during the bulge deformation process using in situ small-angle X-ray scattering, wide-angle X-ray diffraction (WAXD) measurements, and polarized high-speed-camera observations. The films had a thickness of approximately 10 μm. The it PP films were fixed at the hole of a plate, then bulge deformation was applied using N₂ or He gas pressure, and stress-strain curves were then calculated from the applied pressure and bulge height. Yielding was observed in the stress-strain curves. Below the yield point, in situ WAXD measurements revealed that the crystal lattice expanded isotropically at the center, edge, and bottom of the bulge hole. Above the yield point, a craze started to form slightly near the center, and crazes formed in various directions with a further increase in strain, while the crystal lattice expanded uniaxially along the circumference at the edge and bottom. Crazes oriented in various directions merged and lost birefringence, indicating a change to the isotropic orientation. The different directions of the crazes indicated several directions of stress. In other words, even if multiaxial deformation is applied to a crystalline it PP film, the string-shaped crystalline polymer chain structure produces local anisotropic uniaxial stress.

Assessing Fast Structure Formation Processes in Isotactic Polypropylene with a Combination of Nanofocus X-ray Diffraction and In Situ Nanocalorimetry

A combination of in situ nanocalorimetry with simultaneous nanofocus 2D Wide-Angle X-ray Scattering (WAXS) was used to study polymorphic behaviour and structure formation in a single micro-drop of isotactic polypropylene (iPP) with defined thermal history. We were able to generate, detect, and characterize a number of different iPP morphologies using our custom-built ultrafast chip-based nanocalorimetry instrument designed for use with the European Synchrotron Radiation Facility (ESRF) high intensity nanofocus X-ray beamline facility. The detected iPP morphologies included monoclinic alpha-phase crystals, mesophase, and mixed morphologies with different mesophase/crystalline compositional ratios. Monoclinic crystals formed from the mesophase became unstable at heating rates above 40 K s-1 and showed melting temperatures as low as ~30 K below those measured for iPP crystals formed by slow cooling. We also studied the real-time melt crystallization of nanogram-sized iPP samples. Our analysis revealed a mesophase nucleation time of around 1 s and the co-existence of mesophase and growing disordered crystals at high supercooling ≤328 K. The further increase of the iPP crystallization temperature to 338 K changed nucleation from homogeneous to heterogeneous. No mesophase was detected above 348 K. Low supercooling (≥378 K) led to the continuous growth of the alpha-phase crystals. In conclusion, we have, for the first time, measured the mesophase nucleation time of supercooled iPP melted under isothermal crystallization conditions using a dedicated experimental setup designed to allow simultaneous ultrafast chip-based nanocalorimetry and nanofocus X-ray diffraction analyses. We also provided experimental evidence that upon heating, the mesophase converts directly into thermodynamically stable monoclinic alpha-phase crystals via perfection and reorganization and not via partial melting. The complex phase behaviour of iPP and its dependence on both crystallization temperature and time is presented here using a time-temperature-transformation (TTT) diagram.

Structure and Mechanical Properties of Multi-Walled Carbon Nanotubes-Filled Isotactic Polypropylene Composites Treated by Pressurization at Different Rates

Isotactic polypropylene filled with 1 wt.% multi-walled carbon nanotubes (iPP/MWCNTs) were prepared, and their crystallization behavior induced by pressurizing to 2.0 GPa with adjustable rates from 2.5 to 1.3 × 10⁴ MPa/s was studied. The obtained samples were characterized by combining wide angle X-ray diffraction, small angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atomic force microscopy techniques. It was found that pressurization is a simple way to prepare iPP/MWCNTs composites in mesophase, γ-phase, or their blends. Two threshold pressurization rates marked as R₁ and R₂ were identified, while R₁ corresponds to the onset of mesomorphic iPP formation. When the pressurization rate is lower than R₁ only γ-phase generates, with its increasing mesophase begins to generate and coexist with γ-phase, and if it exceeds R₂ only mesophase can generate. When iPP/MWCNTs crystallized in γ-phase, compared with the neat iPP, the existence of MWCNTs can promote the nucleation of γ-phase, leading to the formation of γ-crystal with thicker lamellae. If iPP/MWCNTs solidified in mesophase, MWCNTs can decrease the growth rate of the nodular structure, leading to the formation of mesophase with smaller nodular domains (about 9.4 nm). Mechanical tests reveal that, γ-iPP/MWCNTs composites prepared by slow pressurization display high Young’s modulus, high yield strength and high elongation at break, and meso-iPP/MWCNTs samples have excellent deformability because of the existence of nodular morphology. In this sense, the pressurization method is proved to be an efficient approach to regulate the crystalline structure and the properties of iPP/MWCNTs composites.

Lamellae evolution of poly(butylene succinate-co-terephthalate) copolymer induced by uniaxial stretching and subsequent heating

The structural evolution of biodegradable poly(butylene succinate-co-terephthalate) copolymer was investigated during the uniaxial stretching and following heating processes via in situ small-angle X-ray scattering.

Fourier transform infrared (FT-IR) imaging coupled with principal component analysis (PCA) for the study of photooxidation of polypropylene

Fourier transform infrared (FT-IR) imaging coupled with principal component analysis (PCA) is used to characterize the photooxidation of polypropylene (PP) and identify the photooxidative products at different oxidation times. PP slices were exposed to ultraviolet (UV) irradiation for times up to 60 hours and spatially resolved spectra were acquired with a transmission FT-IR imaging system in order to view the steric inhomogeneity of the photooxidation process of PP. The evolution of the oxidized products with irradiation time is shown through the application of PCA. Carboxylic acid is the major oxidized product in the initial period from 0 h to 8 h while ketone becomes the major product with the increase of irradiation time. Carboxylic anhydride is identified for the first time to our knowledge in oxidized PP after 16 h irradiation. Carboxylate ester is also observed in the oxidized PP after 32 h irradiation. Possible mechanisms forming these products have been discussed.