Formation Of Banded Spherulites Research Articles

Direct formation of banded spherulites in poly(l-lactide) from the glassy state: Unexpected synergistic role of chain structure and compressed CO2

Abstract Ring-banded spherulites have never been formed directly from the glassy state of poly( l -lactide) (PLLA), particularly in case of its copolymers containing some stereodefective D-lactic units, which kinetically hinder the crystallization. By utilizing compressed CO 2 , banded spherulites have been directly obtained by treating the glassy PLLA containing 4–6% d -isomer. Specifically, ring-banded spherulites with regular rings, consisting of periodically parallel-aligned lamellar crystals twist in the left-handed sense, were exclusively formed in CO 2 of high pressures (8−16 MPa) and low temperatures below the normal glass transition temperature ( T g ). Owing to the relatively high d -isomer content, PLLA possessed the adequate resistance to crystal nucleation during melt-quenching, which is inherently favorable for obtaining a fully glassy sample with negligible crystal nuclei and plays a significant role in promoting the formation of banded spherulites. The formation of banded spherulites in PLLA from the glassy state is plausibly attributed to a synergistic effect of the specific molecular structure and the suitable CO 2 atmosphere (i.e., low temperatures and high pressures). The unbalanced stress at opposite fold-surfaces responsible for the lamellar twisting was intensified by the d -isomer and released by the compressed CO 2 , which eventually resulted in cooperatively twisted lamellae for constituting the ring-banded spherulites.

Formation of banded spherulites and the temperature dependence of the band space in olefin block copolymer

The banded spherulites for olefin block copolymer result from continuous lamellar twisting with an intriguing temperature tendency of the band space.

Structures and growth mechanisms of poly-(3-hydroxybutyrate) (PHB) crystallized from solution and thin melt film

The spherulitic structures and morphologies of poly-(3-hydroxybutyrate) (PHB) crystallized from a solution and a thin melt film were investigated in this study. The formation mechanisms of banded spherulites under different crystallization conditions are proposed. It was found that the formation of banded spherulites was caused by the rhythmic crystal growth of the spherulites and lamellar twisting growth for the polymer crystallization from a thin melt film and a solution, respectively.

Morphological development of melt crystallized poly(propylene oxide) by in situ AFM: formation of banded spherulites

The morphology of poly(propylene oxide) (PPO) crystals grown from the melt was investigated. The spherulites of the optically pure S polymers displayed a regular pattern of concentric rings as observed by polarizing optical microscopy, while the stereocopolymer developed irregularly banded, or non-banded spherulites depending on the degree of undercooling. The organization of the lamellar crystals within the spherulites was examined by means of atomic force microscopy (AFM). For all cases, the lamellar structures appeared to adopt an alternating flat or edge-on orientation. Examination of the morphology of single crystals in the melt of the stereocopolymer revealed truncated-lozenge crystals, which were elongated in shape. Results from crystallization kinetics, obtained by in situ AFM observations, showed that the elongated habit is related to differences in the growth rates of the {2 0 0} and {1 1 0} facets. Interestingly, the melt-grown RS-PPO crystals developed a curved asymmetrical three-dimensional shape. Based on these observations it can be proposed that the chiral nature of the chain is transmitted to higher structural levels of ordering in the crystal aggregates.

Crystal Structure and Banded Spherulite of Poly(trimethylene terephthalate)

Developments of single crystal ED patterns in melt-crystallized poly(trimethylene terephthalate) (PTT) have been successfully achieved. Five different zonal electron diffraction patterns containing a total of 14 independent reflections were obtained. The PTT structure under strain-free conditions was identified as a triclinic structure with a = 4.5 Å, b = 6.3 Å, c = 18.2 Å, α = 97.5°, β = 91.4°, and γ = 111.7° with a calculated density of 1.44 g/cm3. On the basis of the corresponding reflections in ED patterns, each significant diffraction peak in wide-angle X-ray diffraction powder pattern was identified and indexed. A much more precise determination of the unit cell parameters has been thus achieved. They are a = 4.53 Å, b = 6.20 Å, c = 18.70 Å, α = 97.6°, β = 93.2°, and γ = 110.1°. Unlike poly(butylene terephthalate), structure deformation has not been found in PTT fibers after removal of the extension. This specific feature may explain the better performance in resilience recovery for PTT as compared to PET and PBT. Banded spherulite with negative birefringence has been observed in PTT under polarized light microscopy. The formation of banded spherulites is attributed to lamellar twisting. The twisting mechanism was evidenced by the observations of wavylike morphology from reflected light microscopy and transmission electron microscopy. The geometry of crystal lamellae has been identified according to lamellar morphology and its correlated single-crystal ED patterns. The PTT lamellae grow from the basal plane consisting of a and b axes. In connection with the observed morphology, we suggest that the lamellar twisting is attributed to the tilted chain stems which are nonorthogonal to fold surface. The nonorthogonal geometry results from the growth of PTT lamellae with triclinic structure where internal stress is gradually accumulated so as to drive the crystal twist along the radial direction of spherulite.