Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Further study on double‐melting endotherms of isotactic polypropylene

The origins of the single‐ and double‐melting endotherms of isotactic polypropylene crystallized at different temperatures were studied carefully by differential scanning calorimetry, wide‐angle X‐ray diffraction, and small‐angle X‐ray scattering. The experimental data show that spontaneous crystallization occurs when the crystallization temperature is lower than 117°C; thus the lamellae formed are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae leads to double endotherms. On the other hand, when the crystallization temperature is higher than 136°C, two major kinds of lamellae with different thickness are developed during the isothermal process, which also results in the double‐melting endotherms. In the intermediate temperature range the lamellae formed are perfect, and there is only a single peak in the distribution of lamellar thickness. This explains the origin of the single‐melting endotherm. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 163–170, 2000     

Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper.Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C.We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature.Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60-105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.     

Solid-state reorganization,melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s⁻¹ suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.     

Crystallization studies of isotactic polystyrene

The kinetics of lamellar crystallization in thin films of isotactic polystyrene have been determined using transmission electron microscopy. The morphological changes accompanying crystallization have also been investigated as a function of solvent, supercooling and strain prior to crystallization. Crystallization temperatures have been attained by both cooling from the melt and warming from the glass. Similar growth rates were obtained in both cases. The nucleation density of spherulites is difficult to control when warming from the glass but does depend on the solvent used in preparing the thin film. The rate of lamellar growth follows a ‘bell’ shaped curve versus crystallization temperature and the kinetics were analysed using the secondary nucleation theory of Hoffman and Lauritzen. The end surface free energy, δ_e, of the lamellar crystals was determined using the variation of lamellar thickness with supercooling.     

On the triple melting behaviour of poly(ethylene succinate)

The morphology of melt-crystallized poly(ethylene succinate) (PES) was investigated by optical microscopy and scanning electron microscopy, and the melting behaviour of PES was studied by differential scanning calorimetry (DSC). At low crystallization temperature imperfect crystals were formed which could melt and recrystallize during the DSC scan. Triple melting peaks were observed, and the melting behaviour was strongly dependent on crystallization time and scan rate. It was observed that crystallization at high temperature perfected the crystals (dominant and subsidiary lamellae in the spherulitic structure). Increasing the scan rate reduced the chance for reorganization. However, at high crystallization temperature two melting peaks were observed. The material formed was much more perfect, so that the melting process was not dominated by recrystallization. Accordingly, the cause of dual melting is the existence of two kinds of crystal perfection.     

Effect of negative pressure on melting behavior of spherulites in thin films of several crystalline polymers

The melting behavior of spherulites in thin films of isotactic polypropylene, poly(ethylene oxide), poly(methylene oxide), and poly(ethylene adipate) crystallized isothermally at various temperatures has been studied by polarized light microscopy. The local increase of melting temperature in regions surrounding cavities and multiple boundary points, dependent on the crystallization temperature, was observed in all studied polymers. In pockets of occluded melt an arising negative pressure lowers an equilibrium melting temperature; hence, decreases an undercooling, which results in the increase of lamellae thickness and their melting temperature. The elevation of melting temperature and the negative pressure buildup depend on the polymer and the crystallization temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1380–1385, 1999     

Temperature-resolved SAXS studies of morphological changes in melt-crystallized poly(hexamethylene terephthalate) and its melting upon heating

Temperature-resolved small-angle X-ray scattering (SAXS) on poly(hexamethylene terephthalate) (PHT) samples crystallized from the melt yields direct information about the morphological changes in lamellar crystals and interlamellar amorphous layers upon melt-crystallization and subsequent heating to melting. Absolute intensities of these SAXS patterns were further analyzed via one-dimensional correlation and interface distribution functions. These analyses indicate that melt-crystallization at low temperature produces lamellar crystals having diverse thicknesses whereas crystallization at high temperature tends to favor growth of thick lamellar crystals with a nearly uniform distribution of thickness. When heating the PHT samples in the melting temperature region, the melting of the lamellar crystals was found to correlate well with the sequential-melting features. When these crystals are heated to higher temperatures, structural alterations from stacked lamellae to isolated lamellar crystals evolve with increasing extent of sequential melting, but, upon re-crystallization during extended annealing, the isolated lamellar crystals can pass through a reversible transition back to stacked lamellae.     

Isotactic polypropylene crystallized from the melt. I. Morphological study

The spherulitic structure of isotactic polypropylene (iPP) from the melt was studied by polarized light and scanning electron microscopy. From the crystallization morphology, it can be observed that crystallization of iPP from the melt below 132°C forms two types of spherulites, termed α- and β-spherulites. The structure of iPP isothermally crystallized above 132°C shows α-type only. The α-spherulites have a complex crosshatched array of radial and tangential lamellar structures, while β-spherulites have, to some extent, simpler lamellar morphology with lower crosshatching content compared with α-type. However, in α-spherulites the radial lamellar thickness is greater than that of tangential lamellae, but in β-spherulites the radial and tangential lamellae have approximately the same thickness. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1259–1265, 1998     

Characterization of the dual crystal population in an isothermally crystallized homogeneous ethylene-1-octene copolymer

The melting of a homogeneous ethylene-1-octene copolymer after isothermal crystallization is discussed based on DSC and time-resolved SALS, SAXS and WAXD data. Two melting peaks appear in DSC suggesting the presence of two crystal fractions. All crystals grow in a lamellar habit and there is no evidence for fringed micellar or isolated block-like crystals. The high melting fraction crystallizes while segregating comonomer-rich parts into separate regions where in a later stage the low melting fraction crystallizes. The data support the view of lamellae that grow via the secondary nucleation of crystalline blocks from a preexisting layer-like mesomorphic phase with preservation of the mesomorphic layer thickness. The stability of these blocks increases due to sintering, forming lamellae that melt slightly above the crystallization temperature. The high melting fraction is generated from those lamellae that are able to reduce the crystalline-amorphous interfacial tension.     

Crystallization and melting behavior of amorphous poly(iminosebacoyl iminodecamethylene)

Multiple melting behavior was observed in the differential scanning calorimetry (DSC) scans for the isothermally crystallized poly(iminosebacoyl iminodecamethylene) (PA1010) samples. Coexistence of crystal populations with different lamellar thickness in PA1010 was discussed by means of DSC, wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering techniques. During crystallization of the polymer, a major lamellar crystal population developed first, which possessed a higher melting temperature. However, a small fraction of the polymer formed minor crystal population with thinner lamellae, which was metastable and, upon post‐annealing, could grow into more stable and thicker lamellae through melting and recrystallization process. Lamellae insertion or stacks would develop during the post‐annealing at a lower temperature for the isothermally crystallized samples; thus, multiple crystal populations with different thickness could be produced. It is the multiple distribution of lamella thickness that gives rise to multiple melting behavior of crystalline polymers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 993–1002, 2000     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.     

AFM study of lamellar thickness distributions in high temperature melt-crystallization of β-polypropylene

Summary Atomic force microscopy (AFM) was used to study the lamellar thickness and its distribution in the β modification of isotactic polypropylene-(β-iPP) crystallized from the melt at high temperatures. Measurements were performed on lamellae oriented both flat-on and edge-on with respect to the examined surface. Average lamellar thickness was found to be dependent not only on the crystallization temperature, but also on factors such as nucleation density and isothermal lamellar thickening. The limitations and advantages of the AFM technique for evaluation of lamellar thickness are discussed. Received: 2 February 1998/Revised version: 10 July 1998/Accepted: 11 July 1998     

The effects of atacticity,comonomer content,and configurational defects on the equilibrium melting temperature of monoclinic isotactic polypropylene

A series of isotactic polypropylenes were investigated to account for total defect content using xylene fractionation and carbon‐13 NMR experimental methods. The defects of interest were percent atactic content, copolymer content, and configurational defects. Experimental equilibrium melting temperatures were obtained for each material using the Gibbs‐Thomson equation and extrapolation to infinite crystal thickness or the Hoffman‐Weeks analysis. The experimental equilibrium melting temperature was then compared with the theoretical equilibrium melting temperature predicted by Flory's melting point depression model. Flory's model was found to fit the experimental data using an equilibrium melting temperature of 186°C when configurational defects are ignored. However, to account for all defects, the equilibrium melting temperature for 100% isotactic polypropylene must be increased from 186 to 192°C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 229–236, 2001     

Atomic force microscopy study of the lamellar growth of isotactic polypropylene

The growth behaviors of cross-hatched and lath-like lamellae of α-form spherulites and flat-on lamellae of β-form pherulites of isotactic polypropylene were studied with a high-temperature atomic force microscopy in situ and in real time. The growth rates of crystal lamellae in types I, II and mixed α-form spherulites and in β-form spherulites, as well as the spatial frequency of tangential branching, were measured. The frequency of tangential branching increases with decreasing crystallization temperature, while the growth rates of leading radial and tangential lamellae are approximately the same at a given temperature. Observations of as-crystallized materials demonstrated that the spacing and length of transverse lamellae is sufficient to differentiate among spherulite types. Height measurements in the melt near the growth surface indicate roles of molecular transport in the crystallization process.     

Effect of lamellar thickness on the epitaxial crystallization of PE on oriented iPP films

The effects of lamellar thickness on the epitaxial crystallization of polyethylene on the oriented isotatic polypropylene have been studied by means of transmission electron microscopy. The results obtained from the bright field electron microscopy and electron diffraction show that the epitaxial orientation of the PE crystals on the iPP substrate depends not only on the thickness of the oriented iPP lamellae, but also on the lamellar thickness of PE crystals. No epitaxial orientation relationship between PE crystal and iPP substrate can be found, when the PE crystals are thicker than the lamellar thickness of iPP along the matching direction. This suggests, that the epitaxial nucleation of PE in the PE/iPP epitaxial system is controlled not only by the chain-row matching, but also by a secondary nucleation process. Received: 11 July 1996/Revised version: 30 September 1996/Accepted: 30 September 1996     

Effect of the structure at the micrometer and nanometer scales on the light transmission of isotactic polypropylene

The spherulitic superstructure, crystallinity, and structure and morphology of crystals of isotactic polypropylene were controlled by the conditions of melt crystallization and related to the transmittance of visible light. Spherulitic samples, which contained monoclinic lamellae, were prepared by slow cooling of the quiescent melt at rates lower than 10 K/s and by isothermal melt crystallization at temperatures between 373 and 413 K. Nonspherulitic specimens, which contained nonlamellar mesomorphic domains, in contrast, were obtained by rapid cooling of the melt with rates faster than 100 K/s. The crystallinity and the size of crystals were furthermore fine‐tuned by subsequent annealing at elevated temperatures. Analysis of such films of different structure by ultraviolet–visible spectroscopy revealed that the light transmission was independent of (1) the fraction, (2) the internal structure, and (3) the size of the crystals. In contrast, the light transmission increased with decreasing size of spherulites and finally exceeded 90% in films of 100 μm thickness when spherulites were completely absent. The crystallinity and the structure and size of the crystals of the films of isotactic polypropylene could be adjusted within wide limits without affecting the light transmission. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphology,crystallization, and thermal behavior of isotactic polypropylene/propylene‐g‐styrene blends

Optical microscopy, differential scanning calorimetry, and small angle X‐ray scattering techniques were used to study the influence of the crystallization conditions on morphology and thermal behavior of samples of binary blends constituted of isotactic polypropylene (iPP) and a novel graft copolymer of unsaturated propylene with styrene (uPP‐g‐PS) isothermally crystallized from melt, at relatively low undercooling, in a range of crystallization temperatures of the iPP phase. It was shown that, irrespective of composition, no fall in the crystallinity index of the iPP phase was observed. Notwithstanding, spherulitic texture and thermal behavior of the iPP phase in the iPP/uPP‐g‐PS materials were strongly modified by the presence of copolymer. Surprisingly, iPP spherulites crystallized from the blends showed size and regularity higher than that exhibited by plain iPP spherulites. Moreover, the amount of amorphous material located in the interspherulitic amorphous regions decreased with increasing crystallization temperature, and for a given crystallization temperature, with increasing uPP‐g‐PS content. Also, relevant thermodynamic parameters, related to the crystallization process of the iPP phase from iPP/uPP‐g‐PS melts, were found, composition dependent. The equilibrium melting temperature and the surface free energy of folding of the iPP lamellar crystals grown in the presence of uPP‐g‐PS content up to 5% (wt/wt) were, in fact, respectively slightly lower and higher than that found for the lamellar crystals of plain iPP. By further increase of the copolymer content, both the equilibrium melting temperature and surface free energy of folding values were, on the contrary, depressed dramatically. The obtained results were accounted for by assuming that the iPP crystallization process from iPP/uPP‐g‐PS melts could occur through molecular fractionation inducing a combination of morphological and thermodynamic effects. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2286–2298, 2001     

Effect of molding temperature on crystalline and phase morphologies of HDPE composites containing PP nano-fibers

A high-density polyethylene (HDPE)/isotactic polypropylene (PP) (75/25) blend containing 25 wt% of PP was fibrillated by roller drawing at 138 °C. The fibrillated blend was processed again at temperatures ranging from 155 to 200 °C by compression molding or extrusion. The effects of molding temperature on the morphology and mechanical properties of the blend were investigated. Wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) were used to study the morphology of the samples. The roller-drawn blend exhibited a fibrous structure with the chain direction aligned parallel to the drawing direction. After molding at 155 °C, the HDPE formed parallel-stacked lamellae retaining the parallel orientation after the melting of the PE crystals. As the molding temperature increased the parallel orientation gradually vanished and some of the parallel-stacked lamellae changed into twisted lamellae. The PP phase existed as fibrils in the PE matrix and the crystals stayed with their molecular chain aligned parallel to the fibrillation direction even when the molding temperature was far above the melting temperature of PP. Nevertheless, the orientation of the crystals did not change as the molding temperature increased from 155 to 165 °C. The internal structure of the PP fibrils changed from a needle structure to a parallel-stacked one. The PP fibrils induced the crystallization of the PE melt, leading to the formation of a trans-crystalline layer at their surface. As the molding temperature increased, more PE lamellae protruded into the PP fibrils and the interface between the PP fibrils and the PE matrix became diffuse.     

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman-Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.