Spherulitic morphology in polyethylene and isotactic polystyrene: Influence of diffusion of segregated species

Morphological consequences of a localized diffusion of segregated species at crystal growth fronts have been studied in two specific contexts: (1) variation of texture in spherulites grown in unfractionated polyethylene over a range of crystallization temperatures mostly in regime II, and (2) development of elongated lamellar habits in spherulites of a polymer (isotactic polystyrene) whose native crystal habit is regularly polygonal. In relation to (1) it is shown that, as crystallization temperature is varied, there is a correlation between mean thickness of stacks of lamellae and an averaged diffusion range of segregated molecules of lower molecular weight. It is noted that lamellar organization appears to be significantly different in polyethylene fractions. In relation to (2) it is shown that principal contributors to the evolution of spherulitic texture from hedritic precursors are fragmentation of lamellae by screw dislocations and radially biased growth under the influence of concentration gradients of segregated species.     

Nonintegral and integral folding crystal growth in low-molecular mass poly (ethylene oxide) fractions. III. Linear crystal growth rates and crystal morphology

Correlations of the linear crystal growth rates with the change in crystal morphology for poly (ethylene oxide) (PEO) (MW 3000) and α,ω--methoxy-poly (ethylene oxide) (MPEO) (MW 3000) fractions have been established over a wide range of supercooling (ΔT = 25 K). Two linear crystal growth rates were measured, namely, the linear crystal growth rate of spherulites or hedrites and the lateral (linear) crystal growth rate of single lamellar crystals along different crystalline planes (below ΔT = 11–12 K). At a low supercooling of 5 K, the crystal growth rate of the MPEO fraction passes through a minimum. Subsequently, the rate increases abnormally and reaches a maximum at even lower supercooling. This crystal growth retardation has been attributed to the competition between differing chain conformations during crystal growth. In this case, particularly, these conformations are the extended chain and the once-folded chain conformations. This retardation is not observed in the PEO fraction, since in this low supercooling region any once-folded chain conformations formed during crystal growth are of the double-layer lamella type. The change in morphology in this region supports this judgement. Above ΔT = 7.6 K, the crystal growth behavior in these two fractions can be described by the present nucleation theory on flat crystal growth faces. Below that ΔT, however, different crystal growth mechanisms can be clearly seen. This may be associated with the crystal growth on the highly serrated growth faces in these two fractions.     

Spherulitic growth in block copolymers and blends of miscible crystalline polymers

Spherulitic morphology and growth rate of block copolymers comprised of miscible crystalline constituents, namely poly(ethylene succinate) (PES) and poly(ethylene oxide) (PEO), were investigated. The results of the copolymers were compared with those of the blends with the same composition and molecular weight. Interpenetrating spherulites, where spherulites of one component grow in those of the other component, were observed in the copolymers as in the blends. Copolymerization, namely the connectivity of the PES and PEO blocks, reduced the spherulitic growth rate in the melt for both components. The growth inside the spherulites of the other component was discussed based on the lamellar and fibrillar (or lamella‐stack) structures, which are influenced by the interblock connectivity. Suppression of molecular mobility in the interlamellar regions resulted in the reduced nucleation and growth rate of the component growing in the spherulites of the other constituent. PES of the copolymer showed dendrites around 60 °C or above. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Effect of poly(butylene succinate) crystals on spherulitic growth of poly(ethylene oxide) in binary blends of the two substances

The effects of the lamellar growth direction, extinction rings, and spherulitic boundaries of poly(butylene succinate) (PBSU) on the spherulitic growth of poly(ethylene oxide) (PEO) were investigated in miscible blends of the two crystalline polymers. In the crystallization process from a homogeneous melt, PBSU first developed volume‐filling spherulites, and then PEO spherulites nucleated and grew inside the PBSU spherulites. The lamellar growth direction of PEO was identical with that of PBSU even when the PBSU content was about 5 wt %. PEO, which intrinsically does not exhibit banded spherulites, showed apparent extinction rings inside the banded spherulites of PBSU. The growth rate of a PEO spherulite, G_PEO, was influenced not only by the blend composition and the crystallization temperature of PEO, but also by the growth direction with respect to PBSU lamellae, the boundaries of PBSU spherulites, and the crystallization temperature of PBSU, T_PBSU. The value of G_PEO first increased with decreasing T_PBSU when a PEO spherulite grew inside a single PBSU spherulite. Then, G_PEO decreased when T_PBSU was further decreased and a PEO spherulite grew through many tiny PBSU spherulites. This behavior was discussed based on the aforementioned factors affecting G_PEO. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 539–547, 2009     

The nature of multiple melting transitions in cis-polyisoprene

The multiple melting transitions previously reported for cis-polyisoprene have been related to different morphological species observed in thin films using transmission electron microscopy. Two distinct types of spherulitic lamellar crystal have been identified which have characteristic growth rates, lamellar thicknesses, and fold planes. In addition to the α-lamellar crystals, which grow in prestrained films with the a axis perpendicular to the stretch direction, a second type of lamellar crystal was identified with the b axis perpendicular to the stretch direction: β-lamellae. From an analysis of the kinetics of growth and the variation of lamellar thicknesses with crystallization temperature, values of the side and surface free energies of the two types of lamellar crystal have been calculated.     

Nonintegral and integral folding crystal growth in low-molecular mass poly (ethylene oxide) fractions. II. End-group effect: α,ω-methoxy-poly (ethylene oxide)

Differential scanning calorimetry (DSC) and in situ small-angle x-ray scattering (SAXS) indicate that in an α ω-methoxy-poly(ethylene oxide) (MPEO) fraction (MW 3000) a transient nonintegral folding (NIF) crystal initially forms during crystallization throughout a wide range of crystallization temperatures. Subsequent transformations of the NIF to IF (integral folding) crystals at low temperatures occur mainly through isothermal thickening or thinning via perfection processes or, at higher temperatures, through primary crystal formation. The NIF crystal is thermodynamically the least stable state among the crystal forms, but its growth is the most rapid. The overall crystallization and crystal melting of this MPEO fraction reveal that the NIF crystal and the NIF → IF crystal transformations are common to low-molecular mass PEO fractions without regard to the end group. Nevertheless, diffusion coefficient and viscosity measurements provide clear evidence of an end-group effect in PEO and MPEO fractions. The difference in the overall crystallization and isothermal thickening and thinning kinetics of low-molecular mass PEO and MPEO fractions can lead to further understanding of end-group effects.     

Study of bundle formation during crystallization in polymer blends

The development of texture which exists in polymer spherulites grown from single phase melts containing an appreciable amount of noncrystallizable material was investigated. This texture generally consists of lamellar bundles separated by amorphous regions, both of which are typically 0.1–1 μm thick. A space–time finite element model previously developed by us was used to simulate the growth of a group of polymer lamellae. The model determines the impurity concentration field in the melt surrounding the growing lamellae and tracks the growth of each lamella. Important variables are the initial melt concentration of noncrystallizable material, the mass diffusion coefficient of noncrystallizable species, lamellar thickness, long period, and the rate of molecular attachment at the growth front. Under certain conditions, bundles did indeed develop during the simulations. These results were used to predict bundle thicknesses. The predictions of bundle texture were compared to actual textures observed in blends of syndiotactic and atactic polystyrene. It was found both experimentally and numerically that bundle thickness was a strong function of crystallization temperature and a relatively weak function of both the initial composition of noncrystallizable species and the degree of crystallinity of the lamellar stack. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 873–888, 1998     

Morphology and crystal structure of the poly(ethylene oxide)–hydroquinone molecular complex

The occurrence of a molecular complex between poly(ethylene oxide) (PEO) and p‐dihydroxybenzene (hydroquinone) has been determined using different experimental techniques such as differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared spectroscopy (FTIR). From DSC investigations, an ethylene oxide/hydroquinone molar ratio of 2/1 was deduced. During the heating, the molecular complex undergoes a peritectic reaction and spontaneously transforms into a liquid phase and crystalline hydroquinone (incongruent melting). A triclinic unit cell (a = 1.17 nm, b = 1.20 nm, c = 1.06 nm, α = 78°, β = 64°, γ = 115°), containing eight ethylene oxide (EO) monomers and four hydroquinone molecules, has been determined from the analysis of the X‐ray diffraction fiber patterns of stretched and spherulitic films. The PEO chains adopt a helical conformation with four monomers per turn, which is very similar to the 7₂ helix of the pure polymer. A crystal structure is proposed on the basis of molecular packing considerations and X‐ray diffraction intensities. It consists of a layered structure with an alternation of PEO and small molecules layers, both layers being stabilized by an array of hydrogen bonds. The morphology of PEO–HYD crystals was studied by small angle X‐ray scattering and DSC. As previously shown for the PEO–resorcinol complex, PEO–HYD samples crystallize with a lamellar thickness corresponding to fully extended or integral folded chains. The relative proportion of lamellae with different thicknesses depends on the crystallization temperature and time. Finally, the observed morphologies are discussed in terms of intermolecular interactions and chain mobility. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1197–1208, 1999     

Correlation between crystallization kinetics and microdomain morphology in block copolymer blends exhibiting confined crystallization

The crystallization kinetics of poly(ethylene oxide) (PEO) blocks in poly(ethylene oxide)‐block‐poly(1,4‐butadiene) (PEO‐b‐PB)/poly(1,4‐butadiene) (PB) blends were previously found to display a one‐to‐one correlation with the microdomain morphology. The distinct correlation was postulated to stem from the homogeneous nucleation‐controlled crystallization in the cylindrical and spherical PEO microdomains, where there existed a direct proportionality between the nucleation rate and the individual domain volume. This criterion was valid for confined crystallization in which the crystallization was spatially restricted within the individual domains. However, it was possibly not applicable to PEO‐b‐PB/PB, in that the melt mesophase was strongly perturbed upon crystallization. Therefore, it may be speculated that the crystal growth front developed in a given microdomain could intrude into the nearby noncrystalline domains, yielding the condition of cooperative crystallization. To establish an unambiguous model system for verifying the existence of microdomain‐tailored kinetics in confined crystallization, we crosslinked amorphous PB blocks in PEO‐b‐PB/PB with a photoinitiated crosslinking reaction to effectively suppress the cooperative crystallization. Small‐angle X‐ray scattering revealed that, in contrast to the noncrosslinked systems, the pre‐existing domain morphology in the melt was retained upon crystallization. The crystallization kinetics in the crosslinked system also exhibited a parallel transition with the morphological transformation, thereby verifying the existence of microdomain‐tailored kinetics in the confined crystallization of block copolymers. Homogeneous nucleation‐controlled crystallizations in cylindrical and spherical morphologies were demonstrated in an isothermal crystallization study in which the corresponding crystallinity developments followed a simple exponential rule not prescribed by conventional spherulitic crystallization. Despite the effective confinement imposed by the crosslinked PB phase, crystallization in the lamellar phase still proceeded through a mechanism analogous to the spherulitic crystallization of homopolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 519–529, 2002; DOI 10.1002/polb.10121     

Unusual phase behavior in mixtures of poly(ethylene oxide) and ethyl alcohol

Poly(ethylene oxide) (PEO), soluble in both aqueous and organic solvents, is one of the most intriguing polymers. PEO solution properties have been extensively studied for decades; however, many of the studies have focused on specific properties, such as clustering, of PEO in aqueous solutions, and the behavior of PEO in organic solvents has not been adequately explored. The results presented here demonstrate that PEO crystallizes into a lamellar structure in ethyl alcohol after the mixture is quenched to room temperature from a temperature above the crystal melting point. Above the melting temperature, PEO completely dissolves in ethyl alcohol, and the mixture exhibits regular polymer solution thermodynamic behavior with an upper critical solution temperature (UCST) phase diagram. Remarkably, the UCST phase boundary is significantly below the melting temperature, and this indicates that the system undergoes a crystallization process before the phase separation can occur upon cooling and, therefore, possesses an unusual phase transition. The phase transition from the crystalline state to the miscible solution state is reversible upon heating or cooling and can be induced by the addition of a small amount of water. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 557–564, 2006     

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 so- lution 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.     

Optical and SAXS characterization of poly(phenylene sulfide) (PPS) oligomers

Previously reported oligomeric PPSs prepared via the melt reaction of sulfur with excess p-diiodobenzene have been examined by optical microscopy and small-angle x-ray scattering (SAXS) techniques. A transition was seen from lamellar crystals for longer chains in the PPS samples of this work to a different type, which probably are extended chain crystals, occurring at about 20 DP. Spherulitic growth was observed optically for 38.8 DP and above. SAXS data established 31.8 DP as the point where a long period peak was first observed. The lamellar thickness of these bulk crystallized samples was established as about 50 Å which corresponds to a DP of 10. Maximum intensity of the SAXS peak increased with oligomer DP indicating increasing crystal perfection. Because of the random nature of the crystallization process, in PPS the average oligomer chain must be longer than three times this lamellar thickness to allow for folding and a spherulitic growth habit. © 1994 John Wiley & Sons, Inc.     

Probing the interplay between chain diffusion and polymer crystal growth under nanoscale confinement: a study by single molecule fluorescence microscopy

Motions of single poly(ε-caprolactone) (PCL) molecules during the formation of the dendrite crystals in ultrathin films are captured by single molecule fluorescence microscopy. The relationship of single molecule diffusion coefficient with the crystal growth rate, together with radius curvature, side-branch spacing of dendrite crystal and morphology are examined. The results support Mullins-Sekerka (MS) instability as the origin of lamellar branching induced by a diffusion field generated by a gradient of polymer segment density ahead of the crystal. Further analysis of the molecular trajectories has recognized different types of motions, depending on the distance to the crystal front: Fickian diffusion in regions far away from the crystal, sub-diffusion in regions adjacent to the crystal, and directed motion between these two regions. Anti-correlation of successive steps is discovered accompanying the sub-diffusion, providing a clear signature of macromolecule crowding at the crystal growth front. This anomalous diffusion process in polymer ultrathin films presents a new insight into the understanding of the retarded dynamics of interfacial mass transport towards the crystal front. It is considered to play a decisive role in controlling the crystal growth and evolution of crystal morphology.     

相态结构对聚氧化乙烯/丁二腈/高氯酸锂复合电解质室温电导率的影响

采用聚氧化乙烯(PEO)、丁二腈和高氯酸锂(LiClO4)的复合电解质体系, 制备了一系列不同配比的PEO/SN/LiClO4复合电解质, 对其室温电性能和相态结构进行了表征, 并探讨了相态结构对室温电导率的影响.     

Lamellar structure of poly(ethylene oxide) molecular complexes

The phase diagrams of some binary systems such as poly(ethy lene oxide)-p-dihalogenobenzene, poly(ethylene oxide)-resorcinol and poly(ethylene oxide)-p-nitrophenol show the existence of molecular complexes with a well definite stoichiometry. The crystal structure of these molecular complexes has been determined by wide-angle X-ray diffraction. The morphology of these molecular complexes crystallized from the melt is investigated by differential scanning calorimetry and small angle X-ray scattering. PEO-p-dichlorobenzene and PEO-resorcinol complexes crystallize from the melt as extended chains (EC) or integral folded chain (IFC) lamellar crystals. As observed for PEO oligomers, the fraction of EC crystals of PEO-resorcinol increases with the crystallization temperature. However EC crystals are present in a larger range of crystallization temperatures than for pure PEO. On the other hand, the PEO-p-nitrophenol complex crystallizes over all the studied crystallization temperature range as stable non integral folded chain (NIFC) crystals. Explanations related to the crystal structure of these complexes and to their mode of growth are invoked to explain these two deeply different lamellar morphologies.     

Crystallization of poly(tetramethyl-p-silphenylene)-siloxane (TMPS) polymers. Part II

The crystallization kinetics and morphology of poly(tetramethyl-p-silphenylene)siloxane spherulites have been investigated over a temperature range of 25–130°C. The effect of molecular weight on the spherulitic growth rates, ranging from the monomer to molecular weights about 10⁶, is discussed in terms of conventional rate theory. Surface free energies of crystal growth are computed on the basis of a spherulitic model in which the polymer chains are presumed to be incorporated within the lamellar crystallites which are comprised in the spherulites. Mention is made of the change in mechanical properties with molecular weight.     

Interaction via diffusion of polymer crystals in growing spherulites

Previous calculations of how diffusion ranges should be averaged when polymer molecules of various lengths are segregated at crystal growth fronts have been extended. Crystals of acicular or lamellar habit are addressed directly, necessarily using more approximate methods, and earlier conclusions regarding appropriate averaging are reaffirmed. Results also lead to a better understanding of why crystalline texture in polyethylene spherulites appears to scale with the averaged diffusion range at crystallization temperatures within regime II but not at higher temperatures within regime I.     

Crystal structure of poly(ethylene-oxide) supramolecular assemblies

The formation of poly(ethylene oxide) (PEO) supramolecular complexes is discussed in terms of intermolecular interactions and molecular packing. On the basis of the different known crystal structures, several mechanisms are proposed. First, the PEO complexes can be formed by an Intercalation or Inclusion process, guest molecules diffusing into the PEO unit cell. On the other hand, molecular complexes based on hydrogen bonding cannot be obtained by such a way, their formation requires the complete removal of the initial PEO structure either by melting or dissolution. Finally the relations between the crystal lamellar morphology, the host-guest interactions and the PEO chain mobility are discussed.     

MOLECULAR AND SUPRAMOLECULAR ORDERING IN CONFINED ENVIRONMENTS

Crystal and phase morphologies and structures determined by self-organization of crystalline-amorphous diblockcopolymers, crystallization of the crystallizable blocks, and vitrification of the amorphous blocks are reviewed through asystematic study on a series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers. On the base ofcompetitions among these three processes, molecular and supramolecular ordering in confined environments can beinvestigated. In a concentration-fluctuation-induced disordered (D_(CF)) diblock copolymer, the competition between crystalli-zation of the PEO blocks and vitrification of the PS blocks is momtored by time-resolved simultaneous small angle X-rayscattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques. In the case of T_c相似文献   

Nonintegral and integral folding crystal growth in low-molecular mass poly(ethylene oxide) fractions. I. Isothermal lamellar thickening and thinning

The overall crystallization and crystal melting of one low-molecular mass poly(ethylene oxide) (PEO) fraction (MW 3000) have been investigated by differential scanning calorimetry (DSC) and in situ small-angle x-ray scattering (SAXS). The salient new results indicate that initial transient crystals with nonintegral folding (NIF) chain lengths form over a wide range of crystallization temperatures. This NIF structure subsequently transforms into crystal forms with integral folding (IF). The PEO IF crystals consist of the extended chain (n = 0) crystal and the once-folded chain (n = 1) crystal, while the NIF has an intermediate fold length. The NIF → IF transformation occurs either by lamellar thickening or thinning. The NIF crystal is less stable than the IF(n = 1) crystal, but its growth is more rapid. Crystallization of the PEO (MW 3000) fraction is thus recognized as a compromise between the direction of the thermodynamic driving force and the kinetic pathway. Some potential consequences of these observations are also addressed.