Coarse Spherulites Research Articles

Increased spherulitic growth rates of semicrystalline polymer due to enhanced limited local chain segmental mobility at the interface of double-layer thin films

In this work the nucleation and growth of spherulites for the below polylactide (PLA) layer in poly(ɛ-caprolactone)/polylactide (PCL/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PCL have been investigated by using polarized optical microscopy (POM). It is revealed that two types of spherulitic morphologies are observed in PCL/PLA double-layer films. One is the well defined highly birefringent spherulites, and the other one is the coarse spherulites. It is interesting to find that the spherulitic growth rate of the coarse spherulites is higher than that of the well defined spherulites. It is thought that the coarse spherulites nucleate and grow with the assistance of the interfaces between the PCL and PLA layers, and the well defined highly birefringent spherulites only nucleate and grow in the PLA layer.

Effect of poly(butylene succinate) on the morphology evolution of poly(vinylidene fluoride) in their blends

The effect of PBS on the morphological features of PVDF has been investigated by optical and atomic force microscopies under various conditions. It was found that neat PVDF forms large γ form spherulites with extraordinarily weak birefringence at 170°C. Adding 30% PBS makes PVDF exhibit intrigued flower-like spherulitic morphology. The growth mechanism was explained by the decrease of the supercooling and the materials dissipation. Increasing the PBS content to 70% favors the formation of ring banded spherulites. Temperature dependent experiments verify the γ→γ phase transition occurs from the junction sites of the α and γ crystals, while starts from the centers of α spherulites in the blends. Ring banded structures could be observed in neat PVDF, 70/30 blend and 30/70 blend when crystallized at 155°C, without γ crystals. The band period of PVDF α spherulites increases with crystallization temperature as well as the amount of PBS content. At 140°C, spherulites in neat PVDF lose their ring banded feature, while coarse spherulites consisting of evident lamellar bundles could be found in 30/70 blend.

Polypentadecalactone prepared by lipase catalysis: crystallization kinetics and morphology

AbstractBACKGROUND: This work addresses the need to better understand the crystallization kinetics and morphology of poly (ω‐pentadecalactone) (PPDL). This polyester has promising mechanical properties and a unique structure that resembles that of polyethylene. PPDL is a member of the poly(ω‐hydroxy fatty acid) family, which can be derived from biobased feedstocks.RESULTS: PPDL (Mn = 34 000 g mol−1 and dispersity D = Mw/Mn = 2.7) was synthesized using enzyme catalysis. Equilibrium melting enthalpy and equilibrium melting point were determined using extrapolation techniques, being 227 J g−1 and 101 °C, respectively. In addition, the equilibrium melting point ($T_{\rm {m}}^{0}$) was found to be 109.3 °C by the nonlinear Hoffman‐Weeks plot. For $T_{\rm {m}}^{0} = 101.0 ^{\circ}\hbox{C}$, the lateral surface free energy (σ), fold surface free energy (σe) and fold work (q) are 10.4 erg cm−2, 47.5 erg cm−2 and 2.6 kcal mol−1, respectively; while for $T_{\rm {m}}^{0} = 109.3 ^{\circ}\hbox{C}$, they are 25.1 erg cm−2, 46.6 erg cm−2 and 2.6 kcal mol−1, respectively. The results indicated the existence of a regime I to regime II transition during crystallization at about 80 °C. Polarized optical microscopy and AFM provided further evidence for the regime I–II transition. In regime I, coarse spherulites were formed through splaying out and occasional branching of lamellae, as well as stacking of lamellae through screw dislocation. In contrast, in regime II, banded spherulites were formed through crystal twisting.CONCLUSION: Morphological changes in PPDL at spherulitic and lamellar levels in regimes I and II were confirmed by differential scanning calorimetry, POM and AFM. Copyright © 2009 Society of Chemical Industry

Spherulitic crystallization of starch: influence of botanical origin and extent of thermal treatment

Numerous, well-developed spherulites exhibiting strong birefringence were formed in aqueous dispersions of mung bean and potato starch rapidly quenched (150 °C/min) from 180 °C. Ordinary maize starch yielded less numerous, less well-developed, coarse spherulites than did mung bean and potato starch. Acid-modified maize starch yielded large numbers of very round spherulites displaying moderately strong birefringence. Wheat, tapioca and arrowroot starches formed few or very poorly developed spherulites. No spherulitic morphology was observed in oat, rice, or sorghum starch dispersions. Equivocal results were obtained for amaranth and yellow pea starches. Potato and mung bean starches required heating to at least 170 °C before spherulite formation was observed. Acid-modified maize starch formed spherulites after heating to only 160 °C and possibly as low as 150 °C.

Morphology and mechanical behavior of polypropylene hot plate welds

AbstractPolypropylene tensile bars were hot plate welded and analyzed by microscopy and mechanical tests. The welding process caused significant modifications to the microstructure and shape of the bars at the weld zone. The weld morphology was affected by both the welding parameters and the thermal history of the components. Oriented textures, coarse spherulites, voids or sharp notches occurring at the beads are the key morphological factors that affect the weld quality. Weld performance is dependent upon the type of testing method used. A tensile impact test has been shown to be potentially useful for quality control, as the fracture behavior is affected by the overall morphoiogy.

Effect of nucleating agents and cooling rate on the microstructure and properties of a rotational moulding grade of polypropylene

Rotational moulding consists of coating the inside surface of a metal mould with a layer of plastic by rotating the mould, firstly in an oven and then in a cooling bay to induce solidification to the desired part shape. As the rotational speeds are slow (typically about 10 rev/min), the resulting hollow articles are practically stress free. The primary material used for rotationally moulded parts is polyethylene but there is an increasing interest in using polypropylene to provide stiffer, higher temperature products. Unfortunately the slow cooling combined with the slow crystallisation rate of polypropylene results in brittle mouldings with coarse spherulites. Since the inner surface of the plastic is in contact with air during moulding, degradation is also likely to occur. In order to improve the mechanical properties of the rotationally moulded polypropylene, α and β nucleating additives were added. The effect of using faster cooling rates was also studied. It was found that heterogeneous nucleation, both of β and α spherulites, did not improve the ductility of the samples. However, when fast cooling was used, the impact strength of the polypropylene improved markedly, independent of the presence of nucleating additives. In the rotationally moulded polypropylene parts, the fast cooling could only be applied to the outer surface of the mould, which led to asymmetric cooling. This resulted in severe warpage, and uneven morphology. This problem should be overcome by using fast cooling on both the inside and outside surfaces of the plastic.

On Spherulitic Growth and Cellulation in Polymers. A Unified Context

The new, unified, context for crystallization of linear macromolecules from the melt, established by recent work, is expounded. The morphology of spherulites shows that they form because of a short-range force operative at the branch points of dominant lamellae causing them to diverge at non-crystallographic angles ∼20°. Work on monodisperse n-alkanes has confirmed the identification of this short-range force with the pressure from (transient) cilia during growth. Accordingly, spherulites, like chainfolding, are a direct consequence of molecular length. It is suggested that the tendency to form coarse spherulites even for extended-chain growth at lower temperatures may result from the increasing difference between the lengths of nucleus and molecule. Crystallization on linear nuclei has been used to maximize the concentration of segregants at the growth front and to demonstrate cellulation in undoped polymers for the first time. The behavior of branched polyethylenes differs from the uniform growth of the linear polymer in coarsening and developing a corrugated growth front all the while slowing continuously towards an asymptotic steady state; differences of detail may be useful in distinguishing polymers of different catalytic origin and branch content. Spherulitic growth is also non-linear for these polymers but is always faster than for rows. When there is sufficient segregation spherulites themselves cellulate, increasingly so for higher branch content. Cellulation is thus an uncommon and secondary process which may be superposed on regular spherulitic growth beyond a certain distance. Cell dimensions do not scale with the diffusion length; in so doing the phenomenon displays new physics.

On the self-organization of linear macromolecules

The new unified context for crystallization of linear macromolecules from the melt, established by recent work, is expounded. The morphology of spherulites shows that they form because of a short-range force operative at the branch points of dominant lamellae that causes them to diverge at noncrystallographic angles of about 20°. Work on monodisperse n-alkanes has confirmed the identification of this short-range force with the pressure from dynamic cilia during growth. Accordingly, spherulites, like chain folding, are a direct consequence of molecular length. It is suggested that the tendency to form coarse spherulites even for extended-chain growth at lower temperatures may result from the increasing difference between the lengths of nucleus and molecule. Crystallization on linear nuclei has been used to maximize the concentration of segregants at the growth front and to demonstrate cellulation in undoped polymers for the first time. The behavior of branched polyethylenes differs from the uniform growth of the linear polymer in coarsening and developing protuberances at the growth front, all the while slowing continuously toward an asymptotic steady state; differences of detail may be useful in distinguishing polymers of different catalytic origin and branch content. Spherulitie growth is also nonlinear for these polymers, but is always faster than for rows. When there is sufficient segregation, spherulites themselves cellulate, increasingly so for higher branch content. Cellulation is thus an uncommon and secondary process may be superposed on regular spherulitie growth beyond a certain distance. Cell dimensions do not scale with the diffusion length; in so doing, the phenomenon displays new physics.

In situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope

The α- and β-spherulites in polypropylene (PP) were identified by direct observation in the scanning electron microscope after appropriate etching. The α-phase has a dark contrast while the β-phase is brighter. Results concerning the individual behaviour of α- and β-spherulites in polypropylene samples which have been subjected to tensile and shear loading are reported. Under tensile loading, the α-spherulites exhibit a brittle behaviour, while the β-phase deforms plastically up to high deformations. The brittle behaviour of the monoclinic structure is characterized by cavitation at an early stage of deformation at the spherulites boundaries or at their equatorial region perpendicular to the tensile axis. Under shear loading, the α-phase cavitation disappears and both phases are then capable of undergoing large strains. However, quantitative characterization of the local deformation in each spherulite species showed that the α-structure deforms less than the global deformation while the β-phase compensates for this lack of plastic deformation of the other phase. The interlocked structure of the α-spherulites is discussed as being the leading contributing factor towards their brittleness, since it makes the plastic glide of this phase very difficult. In contrast, the radial lamellae of the β-spherulites allow the initiation and propagation of plastic glide more easily. The presence of a β-phase in PP with coarse spherulites considerably improves the mechanical properties at room temperature.

Mode I interlaminar fracture of co-mingled yarn based glass/polypropylene composites

A 50:50% mixture of co-mingled glass/polypropylene fibers was selected to study the correlations between morphological details, mode I interlaminar fracture toughness and failure mechanisms. Unidirectional ‘prepreg’ sheets and laminates subjected to different thermal histories were manufactured in a steel mold in a hot press. Mode I interlaminar fracture tests were performed by using the DCB (double cantilever beam) test procedure. A simple model was established for identifying the energy contributions of various failure mechanisms to the interlaminar fracture toughness. Extensive fiber peeling and breakage events were observed for these co-mingled yarn based composites, which were consolidated at temperatures above the melting point of the polymer fibers, followed by different thermal cooling histories. When the composites were subjected to low cooling rates or an isothermal crystallization process, large voids and coarse spherulites existed in the resin-rich areas. These resulted in the presence of inter-spherulitic fracture paths between the debonded fiber-rich regions, so that the interlaminar fracture toughness was much lower than in the case where a rapid cooling condition was applied.

Thermal properties and morphology of hydrostatically extruded polypropylene

AbstractThe melting point, specific heat, and morphology and hydrostatically extruded polypropylene (PP) were studied to clarify the plastic deformation of three different parts of the extrudates (periphery, intermediate, and core) parallel to the direction of hydrostatic extrusion. Differential scanning calorimeter (DSC) measurements showed that, for all the parts studied, the peak and the end‐of‐transition, temperatures had a minimum value at a percentage reduction in area, R, of 50% Similar behavior was observed for the specific heat evaluated at several temperatures below the melting point. The morphological structure observed by use of a polarizing microscope indicated that the shape‐ of spherulites above R = 50% changed from spherulitic to elliptic. It is suggested that the pronounced changes taking place around R = 50% are closely related to the marked bend in the extrusion pressure versus extrusion ratio curve which takes place at the same R value. The molecular chains in the coarse spherulites appear to be deformed below R = 50% in such a way that finer spherulites are formed; but at R values above 50% the presence of elliptic spherulites implies that non‐uniform deformation is present under higher hydrostatic pressure of extrusion.

Morphological studies of poly(vinylidene fluoride) and its blends with poly(methyl methacrylate)

AbstractBoth pure poly(vinylidene fluoride) (PVF2) and its blends with poly(methyl methacrylate) (PMMA) develop a variety of morphologies when they are crystallized above the 420–424 K range. Two populations of spherulites as well as axialitelike growths are observed. Addition of the PMMA lowers the temperature where these new morphologies develop, makes the spherulites more open, causes the banding periodicity to decrease, and increases the number of small, coarse spherulites. These structures melt in three regimes. The highest‐melting‐point crystals arise only from a solid‐solid transformation of the lowest‐melting‐point ones. This solid‐state transition sometimes causes mixed spherulites to be formed in the blends. Electron and wide‐angle x‐ray diffraction show the lowest‐melting‐point species to be α crystals, while the other two are γ crystals. The highest‐melting‐point species, labeled γ′, and the α crystals seem to be more ordered than the other γ crystals.