Banded Spherulites Of Poly Research Articles

Morphological analyses evidencing corrugate-grating lamellae assembly in banded spherulites of Poly(ethylene adipate)

The classical model of continuous helix-twist lamellae versus a novel corrugate-grating assembly in accounting for long-debated issues of periodic optical banding in poly(ethylene adipate) (PEA) polymer spherulites is critically assessed, by comparing and checking against a series of scientific evidence based on experimental data of scanning electron microscopy (SEM), ultrathin microbeam small-angle & wide-angle X-ray scattering (SAXS/WAXD), and optical microscopy. Analyses show that grating-structured model fits perfectly with all results. The novel proposition of grating-structured lamellae with a crossbar pitch fits perfectly with the experimental optical band spacing, and the interior morphology dissection is fully supported by the literature-reported microbeam X-ray data.

Organization of Twisting Lamellar Crystals in Birefringent Banded Polymer Spherulites: A Mini-Review

In this mini-review, we summarize the evidences of lamellar twisting in the birefringent banded polymer spherulites demonstrated by various characterization techniques, such as polarized optical microscopy, real-time atomic force microscopy, micro-focus wide angle X-ray diffraction, etc. The real-time observation of lamellar growth under atomic force microscopy unveiled the fine details of lamellar twisting and branching in the banded spherulites of poly(R-3-hydroxybutyrate-co-17 mol% R-3-hydroxyhexanoate). Organization of the twisting lamellar crystals in the banded spherulites was revealed as well. The lamellar crystals change the orientation via twisting rather than the macro screw dislocations. In fact, macro screw dislocation provides the mechanism of synchronous twisting of neighboring lamellar crystals. The driving force of lamellar twisting is attributed to the anisotropic and unbalanced surface stresses. Besides molecular chirality, variation of the growth axis and the chemical groups on lamellar surface can change the distribution of the surface stresses, and thus may invert the handedness of lamellar twisting. Thus, based on both experimental results and physical reasoning, the relation between crystal chirality and chemical molecular structures has been suggested, via the bridge of the distribution of surface stresses. The factors affecting band spacing are briefly discussed. Some remaining questions and the perspective of the topic are highlighted.

Novel approaches to study the crystal assembly in banded spherulites of poly(trimethylene terephthalate)

Poly(trimethylene terephthalate) (PTT), with large, well-defined, and strongly birefringent spherulites upon crystallization, has been chosen as a model to study the correlation between the interior lamellae assembly and the top surface banding structures. Approaches to investigate the bulk interior of the banded spherulites are needed to shed new light on the crystallization mechanism of polymers. Two etching methods (with two different etchants: methylamine and permanganate) were applied to two identical samples to enhance the contrast of crystal arrangements in three-dimensional perspectives. Methylamine, under mild conditions, was found to etch out lamellae bundles on ridges and leave discrete crevice-separated circumferential crystal layers. On the other hand, the oxidizing potassium permanganate was found to dissolve the circumferential crystal layers between the successive bands, exposing circular patterns. These two etching results mutually support the discontinuity of radial growth of PTT spherulites, providing valuable hints as to the actual crystal assembly of PTT.

Non‐Radial Growth of Helical Homopolymer Crystals: Breaking the Paradigm of the Polymer Spherulite Microstructure

Radial symmetry is essential for the conventional view of the polymer spherulite microstructure. Typically it is assumed that, in the course of the spherulite morphogenesis, the lamellar crystals grow radially. Using submicron X-ray diffraction, it is shown that in banded spherulites of poly(propylene adipate) the crystals have the shape of a helix with flat-on crystals winding around a virtual cylinder of about 6 µm in diameter. The helix angle of 30° implies that the crystal growth direction is tilted away from the spherulite radius by this angle. The implications of the helical crystal shape contradict the paradigm of the spherulitic microstructure. The radial growth rate of such spherulites does not correspond to the crystal growth rate, but to the propagation rate of the virtual cylinder the ribbons wind around.

Wavenumber Dependence of FT-IR Image of Molecular Orientation in Banded Spherulites of Poly(3-hydroxybutyrate) and Poly(l-lactic acid)

The chemical and the molecular chain orientation image of a banded spherulite of crystalline polymers, poly(l-lactic acid) (PLLA) and poly(3-hydroxybutyrate) (PHB), are visualized using FT-IR imaging with a newly proposed multipolarization calculation method. The vector representation of FT-IR image at different absorption spectral peaks visualizes the magnitude and direction of molecular chain orientation directly in comparison with the retardation and the slow-axis azimuthal images obtained by birefringence imaging. We found for the first time that the wavenumber dependence of in-plane orientation function in the band structure of a PHB spherulite, which is known as a double-band structure originated from a biaxial refractive index ellipsoid. The 15 absorption peaks in the IR spectrum of PHB can be classified into three groups by a different appearance of a band structure in view of the magnitude of molecular chain orientation. The azimuthal image of birefringence also shows the double-band structure; however, the FT-IR azimuthal images of orientation axis indicate the completely different distribution in which there is no band structure at any IR absorption peaks. By contrast, a good correspondence between the azimuthal image of the slow axis of birefringence and the orientation axis of FT-IR is observed in the spherulite of PLLA, which has a uniaxial refractive index ellipsoid. The advanced FT-IR imaging method clarifies the wavenumber-dependent two-types single-band structures of PHB, indicating the local molecular chain orientation along the crystallographic axes in a unit cell.

Local lamellar structures in banded spherulites analyzed by three‐dimensional electron tomography

AbstractThe lamellar morphology in banded spherulites of poly(ε‐caprolactone) blended with an amorphous polymer, poly(vinyl butyral), was investigated by three‐dimensional transmission electron tomography. It showed a local lamellar twist on a smaller scale than the band spacing by 2 orders of magnitude. It also indicated wavy lamellae and frequent variation in the direction of the lamellar plane. All these results indicated an S‐profiled lamellar structure; that is, the cross section perpendicular to the lamellar growth direction was S‐shaped. S‐profiled lamellae show these structures when they are sliced at a certain angle to the lamellar surface direction. Lamellar branching was also observed, but no screw dislocations that led to the formation of extinction rings were observed in this work. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1122–1125, 2007

Observation of banded spherulites and lamellar structures by atomic force microscopy

Lamellar structures of banded spherulites of poly(e-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends are observed using tapping mode atomic force microscopy (AFM). The surface of the PCL/PVC banded spherulites presents to be concentric periodic ups and downs. The period of the bands corresponds to the extinction rings under the polarized optical microscopy observation. The lamellae with edge-on orientation in the ridges and the flat-on lamellae in the valleys of the banded spherulites are observed clearly. The twisting between the edge-on and flat-on lamellae is also observed.