Hongfei Li

Shear effects on crystallization behavior of poly(ethylene-co-octene) copolymers

Crystallization behavior of sheared polymer melts of a series of poly(ethylene-co-octene)s with different octene content was investigated by different scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and synchrotron small-angle X-ray scattering (SR-SAXS) techniques. The DSC results indicated that the hexyl branches content had dramatic effects on the thermal properties and crystallinity of polyethylene. It was also found that shear had no obvious effects on the size of either crystallite or lamellae. However, both crystallite and lamellae were oriented by shearing, especially for the lamellae. All obtained results indicate that the initial states of the polymer melt play an important role in affecting the crystallization behaviors. The difference of the shear-induced crystalline structure evolution and the orientation between crystallite and lamellae support the preordered mesomorphic phase of flexible polymer crystallization process proposed by Strobl.

Solvent vapor induced structural evolution of micelle clusters and square slices that form in PS-b-PEO solutions

Micelle clusters and square slices that formed in different polystyrene-b-poly(ethylene oxide) (PS-b-PEO) solutions were observed with AFM. This diblock copolymer assembled into small spherical micelles and then micelle clusters (diameters: 100–400xa0nm) after the PS-b-PEO/toluene solution had been aged for 24xa0h at room temperature. Square slices were obtained from solutions aged for two months. Solvent vapor induced morphological and structural transitions of the micelle clusters and the stability of the square slices on Si wafers were investigated. The results showed that the solvent vapor induced morphology of the sample aged for 24xa0h was more ordered than that aged for twoxa0months. Possible mechanisms for the formation of these structures in the solutions and the vapor-induced structural transition were proposed.

Mechanistic insights into the shear effects on isotactic polypropylene crystallization containing β-nucleating agent

The crystalline structures and crystallization behaviors of iPP containing β nucleation agent TMB-5 (iPP/TMB-5) were investigated by synchrotron radiation wide angel X-ray diffraction (SR-WAXD), differential scanning calorimeter (DSC) and polarized light microscope (PLM). It was found that α-crystallization lagged behind β-crystallization at normal temperatures, but the discrepancy reduced with increasing temperature. TMB-5 could not induce β-iPP when the nucleation agent is wrapped up with α-crystal that crystallized at high temperatures. The polymorphic composition of iPP/TMB-5 was susceptible to the introductory moment of shear. New crystallization process of β-nucleated iPP was proposed to understand the experimental phenomena which could not be explained by those reported in the literature. It was supposed that polymer crystallization initiated from mesophase, and the formations of iPP crystals involved the organization of helical conformation ordering within mesophase. It was proposed that the iPP melt contained mesophases with stereocomplex-type ordering of right-handed and left-handed helical chains which could be disturbed by shear or TMB-5, leading to different polymorphic structures.

Patterning Surfaces on Azo-Based Multilayer Films via Surface Wrinkling Combined with Visible Light Irradiation

Here, a simple combined strategy of surface wrinkling with visible light irradiation to fabricate well tunable hierarchical surface patterns on azo-containing multilayer films is reported. The key to tailor surface patterns is to introduce a photosensitive poly(disperse orange 3) intermediate layer into the film/substrate wrinkling system, in which the modulus decrease is induced by the reversible photoisomerization. The existence of a photoinert top layer prevents the photoisomerization-induced stress release in the intermediate layer to some extent. Consequently, the as-formed wrinkling patterns can be modulated over a large area by light irradiation. Interestingly, in the case of selective exposure, the wrinkle wavelength in the exposed region decreases, while the wrinkles in the unexposed region are evolved into highly oriented wrinkles with the orientation perpendicular to the exposed/unexposed boundary. Compared with traditional single layer-based film/substrate systems, the multilayer system consisting of the photosensitive intermediate layer offers unprecedented advantages in the patterning controllability/universality. As demonstrated here, this simple and versatile strategy can be conveniently extended to functional multilayer systems for the creation of prescribed hierarchical surface patterns with optically tailored microstructures.

Constructional details of polystyrene-block-poly(4-vinylpyridine) ordered thin film morphology

One of the most important issues for spin-coated diblock copolymer thin films is to distinguish the blocks by common techniques, especially for the diblock copolymers containing one block could form active domain. Aiming different estimations on the composition of surface microdomains in polystyrene-block-poly (4-vinylpyridine) (PS-b-P4VP) atomic force microscope (AFM) phase images, AFM, transmission electron microscopy, and droplet shape analyzer were used to identify the constructional details of the polymer thin film morphology. It was confirmed that PS block is harder than P4VP block in symmetric PS-b-P4VP films and corresponds to brighter regions in AFM phase image. It is helpful to distinguish the nanodomains that originate from the different blocks in PS-b-P4VP thin films. The structure evolutions of PS-b-P4VP film annealed with different selective solvents were studied and discussed. The results indicated that the core-corona inversion process is due to the cores of micelles swell and coalesce together rather than local reorganization of the chains.

Intra- and intermolecular-interaction-controlled reversible core–shell structures and photoluminescent properties of lanthanide ion-doped diblock copolymers

Lanthanide-based nanotechniques continue to attract considerable attention due to their current range of applications and broad potential in optical devices and biomedicine. Lanthanide ion-loaded block copolymers (BCPs) have gained interest, with questions remaining regarding their luminescence properties and structures. Solutions and films containing polystyrene-block-poly(2-vinylpyridine) (S2VP) and polystyrene-block-poly(4-vinylpyridine) (S4VP) loaded with trivalent lanthanide ions (Ln3+ = Eu3+, Tb3+) were prepared using a solvent displacement method and spin coating approach. Atomic force microscopy and transmission electron microscopy observations confirmed the Ln3+ distribution in the core of S2VP [Ln(x)] micelles and the corona of S4VP[Ln(x)] micelles. The coordination of Ln3+ and the S2VP core segment led to micelle shrinkage and film dewetting with increasing Ln3+ concentration, while the coordination of Ln3+ and the S4VP corona chain swelled the micelle. Photoluminescence investigations of the complex solutions showed typical reddish (Eu3+) and greenish (Tb3+) luminescence, and the emission intensity was enhanced with increasing Ln3+ concentration without typical fluorescence concentration quenching behavior. The luminescence intensity was distinctly enhanced when Ln3+ was distributed in the micellar core. Moreover, the emission spectra of mixed Eu–Tb ions complexes gave addition spectra of the two separated complexes. An orange color emission was observed because Eu3+ was more emissive. These results would improve understanding of ion coordination and lay the foundation for lanthanide ion/BCPs complex applications.

Deformation-induced structure evolution of poly(butylene terephthalate)/poly(carbonate) blends during uniaxial stretching

The crystalline structure evolution of poly(butylene terephthalate)/poly(carbonate) (PBT/PC) during stretching was investigated by in situ synchrotron wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) at 70 °C. The WAXS results demonstrated that the crystallinity and crystallite size (D100) decrease with deformation. The α–β transition was observed during the deformation of PBT/PC blends. The orientation degree of the PBT (100) diffraction increases with strain in the PBT/PC blends. The SAXS data showed that PC is located in the interlamellar region of PBT for the undeformed samples. A structure model was proposed to describe the stretching-induced structure evolution of the PBT/PC blends.

Effects of copolymer composition, film thickness, and solvent vapor annealing time on dewetting of ultrathin block copolymer films.

Effects of copolymer composition, film thickness, and solvent vapor annealing time on dewetting of spin-coated polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films (<20nm thick) were mainly investigated by atomic force microscopy. Surface chemical analysis of the ultrathin films annealed for different times were performed using X-ray photoelectron spectroscopy and contact angle measurement. With the annealing of acetone vapor, dewetting of the films with different thicknesses occur via the spinodal dewetting and the nucleation and growth mechanisms, respectively. The PS-b-PMMA films rupture into droplets which first coalesce into large ones to reduce the surface free energy. Then the large droplets rupture into small ones to increase the contact area between PMMA blocks and acetone molecules resulting from ultimate migration of PMMA blocks to droplet surface, which is a novel dewetting process observed in spin-coated films for the first time.

Influence of Crystallization on Molecular Dynamics of the Amorphous Phase in Poly(ε-caprolactone) and Poly(ε-caprolactone)/LiClO4 Complexes Investigated by Dielectric Relaxation Spectroscopy

The dielectric relaxation spectroscopy results indicate that the relaxation peak of PCL for pure PCL and PCL/LiClO4 complexes shifts to a lower frequency with increasing crystallization temperature, while the amorphous, condensed structure of PCL for pure PCL and PCL/LiClO4 complexes is slightly affected by crystallization temperature. These findings were obtained from small-angle X-ray scattering (SAXS) measurements. To further understand the effect of crystallization temperature on the relaxation behavior of PCL and PCL/LiClO4 complexes, the isothermal crystallization process of PCL and PCL/LiClO4 was probed in situ by dielectric spectra measurement. The analysis indicated that the relaxation dynamics for pure PCL and PCL/LiClO4 complexes decrease continuously during the isothermal crystallization process, even at the crystallization induction period. This paper presents a detailed discussion of the crystallization mechanisms of the PCL and PCL/LiClO4 complexes, and the probable explanation as to the effect of crystallization temperature on the relaxation dynamics of PCL and PCL/LiClO4 .