Structure of ultrathin polyethylene layers in multilayer films

The crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high-density polyethylene and polystyrene. Transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and wide-angle x-ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row-nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row-nucleated morphology imparts highly anisotropic mechanical properties to the nanolayer polyethylene.     

Three-dimensional numerical simulations of lamellar structure via two-step surface-directed phase separation in polymer blend films

Lamellar structure via two-step surface-directed phase separation in polymer blend films is numerically investigated in three-dimensional (3D) space, which is more physically appropriate for the experimental situation than that in two-dimensional (2D) space [L.-T. Yan and X. M. Xie, J. Chem. Phys. 128, 034901 (2008)]. The 3D phase morphology and its evolution dynamics in both critical and off-critical conditions have been studied. The wetting layer formation mechanism during the second quench has been concerned. The effects of noise on the ordered phase structures have also been examined. The simulated results in 3D space give a more certain evidence that the lamellar structure can be induced by the surface or interface when the system is in the equilibration state with very shallow quench depth first and then imposed on a further quench depth in the unstable region of the phase diagram. It is found that the lamellar structure can also be induced in the polymer blends with off-critical condition. The simulated results demonstrate that the formation of the lamellar structure can present two basic processes and obey logarithmic growth law at the initial and metaphase stages. The results also show that a stronger thermal noise corresponds to a smaller region with the lamellar structure.     

Lateral structures of buried interfaces in ABA-type triblock copolymer films

The lateral structure of an ABA-type triblock copolymer polyparamethylstyrene- block-polystyrene- block-polyparamethylstyrene at the buried silicon substrate interface is studied as a function of different substrate surface treatments. With grazing incidence small-angle neutron scattering (GISANS), high interface sensitivity is reached. With GISANS, the orientation and degree of order of the morphology are probed. The powderlike oriented lamellar structure in the bulk orients along the surface normal in the vicinity of the substrate. A modification of the short-ranged interface potential of the substrate introduces a lateral stretching of this lamellar structure of up to 8% as compared to the bulk. The decay in stretching toward the volume structure is probed with depth profiling. It extends at least up to a distance of 51 nm from the solid surface.     

Practical methods to control morphology of heterogeneous polymer particles

This review focusses on processes in which emulsion polymerizations are carried out in stages so that previously formed particles are either overcoated in subsequent polymerization stages or engulf the second and later stage polymers. These products are often called “core-shell” particles. Basically, the most stable state of the final system is the one with the lowest net interfacial energy. In the case of a two-stage emulsion polymerization there can be three interfacial tensions to consider. Several mutually consistent, effective thermodynamic treatments have been published. At present, they serve primarily to predict when the morphology of multi-stage polymerization products may not be a simple reflection of the synthesis sequences. It is possible, and frequently desirable, however to produce particle structures that appear at first glance to be thermodynamically forbidden. This is achieved either by changing the surface characteristics of a polymer from those of the bulk material or by employing kinetic factors to anchor energetically unprofitable morphologies. This paper summarizes methods of both types that have been reported to control the texture of structured latex particles in order to produce designed morphologies.     

Interface-induced morphology transition in triblock copolymer films swollen with low-molecular-weight homopolymer

The morphology transition due to midblock swelling with low-molecular-weight homopolymer polystyrene of an ABA-type triblock copolymer polyparamethylstyrene-block-polystyrene-block-polyparamethylstyrene at the buried silicon substrate interface is studied as a function of different substrate surface treatments. With grazing incidence small-angle neutron scattering (GISANS), high interface sensitivity is reached. The powderlike oriented lamellar structure in the bulk becomes oriented along the surface normal in the vicinity of the substrate. A transition of the lamellar into a cylinder phase at the polymer-silicon interface is probed with GISANS. The transition is induced by the addition of the homopolymer, but the modification of the short-ranged interface potential of the substrate influences the amount of homopolymer that is necessary for this transition. Without and with 0.1 vol % added homopolymer, the lateral spacing is stretched at the interface as compared to the bulk whereas for a higher added amount of homopolymer no stretching occurs.     

On-line small-angle and wide-angle x-ray scattering studies on melt-spinning poly(vinylidene fluoride) tape using synchrotron radiation

On-line small-angle and wide-angle x-ray scattering experiments were performed during the melt spinning of polyvinylidene fluoride using the DESY synchrotron light source. In these studies, the melt-spinning apparatus consisting of a screw extruder, a metering pump, and a take-up motor system were assembled on two separate stepper-motor-driven platforms. To investigate the structure development during crystallization, the tape location at the desired distance from the die could be positioned at the beam level with synchronous vertical movement of extruder and take-up platforms. Small-angle and wide-angle x-ray patterns were taken simultaneously with a two-dimensional wire detector and one-dimensional wire detector. In a separate study, two-dimensional WAXS data were also taken under identical processing conditions to observe the off-equator diffraction behavior during the crystallization. The data obtained for a variety of take-up speeds generally indicate that SAXS d-spacings first appear large in the early stages of crystallization and gradually decrease along the spin-line (as the crystallization progresses). As the take-up speed increases, the crystallization onset position moves away from the die and d-spacings observed at the onset increase. In addition, the shape of the discrete scattering pattern starts as a meridional streak and converts to a teardrop shape with the tip of the pattern pointing toward the beam stop at the early stages of crystallization for high take-up speeds. This does not occur at low take-up speeds and is attributed to the differences in crystallization behavior which is spherulitic or sheaflike to row nucleated crystallites. Our studies also showed that SAXS patterns appear earlier than the wide-angle crystalline diffraction peaks indicating the SAXS technique to be more sensitive to the structural changes at these stages. An idealized model is developed to explain the observed patterns at high take-up speeds. This consists of two regimes. In regime 1, the periodic fluctuations due to the alternating crystalline and amorphous regions form along the regions where eventually the “shish” structures develop. In the second stage, a volume filling crystallization takes place between the existing crystallites along the shish structure and simultaneously radial overgrowth of crystallites (i.e., the “kebabs”) takes place. This causes the observed reduction in the average d-spacing. © 1993 John Wiley & Sons, Inc.     

Formation processes of the integrated ordered mesostructure of silica at liquid-liquid interface using synchrotron radiation X-rays

The formation processes of the integrated ordered mesostructure of silica at the liquid-liquid interface in laurylamine/tetraethoxysilane system were elucidated by measuring the small-angle X-ray scattering pattern every second during the formation processes using strong synchrotron radiation X-rays. Cylinders of silica were formed in random configurations in the very early stage of reactions at the liquid-liquid interface. Since these cylinders were restricted in a two-dimensional liquid-liquid interface, they aligned easily with each other and made an ordered structure composed of aligned cylinders at the interface. These ordered structures then accumulated with each other, making an integrated ordered structure, such as a hexagonal array.     

Crystallization of silver carboxylates from sodium carboxylate mixtures

Silver carboxylates can be made by the reaction of silver nitrate and the corresponding sodium carboxylates. The length of the alkyl chain has a significant impact on the product behavior. In this study, 18, 20, and 22 carbon chains (stearate, arachidate, and behenate, respectively) have been selected. All three sodium carboxylates are very insoluble in water at room temperature. Solutions are obtained above the Krafft temperature, which precipitates lamellar crystals if cooled at the proper cooling rate. Depending on the chain length, metastable morphologies, such as vesicles and tiny fibers, can be seen consecutively before hexagonal plates form. The carboxylate with the shorter chain length reaches equilibrium more quickly. All three silver carboxylates also take on a lamellar structure. Small-angle X-ray scattering (SAXS) shows that the d spacing of the crystals increases as the chain length increases. Cryo-TEM illustrates that the crystallites are the result of micelle nucleation and micelle aggregation. In addition, the crystallization process in the presence of silver bromide nanocrystals has been investigated. In the initial stage, an epitaxial interface is formed between the silver carboxylate crystallites and the cubic silver bromide grains. Budlike and strandlike structures grow because of it. The consequent strand enclosure restrains the crystal growth, which reduces the size and changes the morphology of the crystals.     

Comparative study on the lamellar structure of polyethylene foams

A comparative study on the lamellar morphology of a collection of polyethylene foam (LDPE) has been performed in order to obtain a better understanding of the morphology of the crystalline phase of these materials. The lamellar structure was measured by small-angle X-ray scattering (SAXS), differential scanning calorimetric (DSC) and Raman spectroscopy. The results have shown that the lamellar structure of the foams is different to that of a LDPE solid sheet. Moreover, the different sensitivity of the three experimental techniques to the lamellar structure has also been analyzed.     

Ordered structure of block polymer/homopolymer mixtures, 4. Vesicle formation and macrophase separation

Self-assembled structures in solvent-cast films of binary mixtures of poly(styrene-block-isoprene) (SI) and homo-polystyrenes (HS) were studied as a function of r_s, the ratio of molecular weights of HS and polystyrene block (PS) in SI and of w_HS, the weight fraction of HS. For r_S<1 (a criterion for “wet brush”), HS tends to be solubilized into the microdomain of PS and to swell the PS chains, causing a change in microdomain morphology with long-range order. Spherical microdomains composed of polyisoprene block (PI) dispersed randomly in the matrix of HS and PS (“spherical micelles”) are formed at the limit of high w_HS (w_HS→1). For r_S≳1 (a criterion for “dry brush”), HS tends either to be segregated from SI, forming a macrophase-separated morphology (at r_S>>1), or to be solubilized into the PS microdomains without swelling PS or without a significant change of a degree of swelling with w_HS, if any, (at r_S≌1). In the latter case, HS does not significantly affect the lamellar domains of PI for the particular SI studied here, which by itself gives alternating lamellar microdomains of PS and PI, but increases the mean distance between them. AS w_HS→1, the long-range spatial order of the PI lamellae is lost, while keeping their thicknesses constant and uniform, resulting in morphologies of lamellar, cylindrical, and spherical vesicles. The vesicular morphology was analyzed by small-angle X-ray scattering and transmission electron microscopy. At r_S>>1 (still in the criterion of dry brush), SI and HS undergo macrophase separation, forming SI domains composed of alternating lamellae in the matrix of HS.     

SAXS experiments on gel-spun polyethylene fibers

The properties of gel-spun polyethylene fibers hot-drawn to the maximum draw ratio depend on the spinning conditions. Different spinning conditions result in two types of structure in the paraffin oil containing fibers: an isotropic lamellar structure or a shish-kebab structure. Meridional SAXS experiments can identify the structure present. After extraction, these structures are still present but can be detected only in a more indirect way by SAXS experiments because of an excessive contribution of void scattering. During hot-drawing both structures are transformed into a more fibrillar structure. The shish-kebab structures can be drawn only to relatively low hot-draw ratios with an incomplete transformation of the lamellar overgrowth into the fibrils, as demonstrated by the presence of a meridional SAXS maximum/shoulder. This leads to relatively weak fibers. Lamellar structures can be drawn to high draw ratios by chain unfolding. A nearly complete transformation of the lamellae into fibrils is obtained and the fibers have excellent properties. The information about the morphology obtained by SAXS, DSC, WAXS, and SEM can be used to establish a relation between morphology and properties.     

Effect of thermal transport on spatiotemporal emergence of lamellar branching morphology during polymer spherulitic growth

Spatiotemporal emergence of lamellar branching morphology of polymer spherulite has been investigated theoretically in the framework of a phase field model by coupling a crystal solidification potential pertaining to a nonconserved crystal order parameter with a temperature field generated by latent heat of crystallization. A local free-energy density having an asymmetric double well has been utilized to account for a first-order phase transition such as crystallization. To account for the polymorphous nature of polymer crystallization, the phase field order parameter of crystal at the solidification potential of the double-well local free-energy density is modified to be supercooling dependent. The heat conduction equation, incorporating liberation of latent heat along the nonuniform solid-liquid interface, has led to directional growth of various hierarchical structures including lamella, sheaflike structure, and spherulite. Two-dimensional calculations have been carried out based on experimentally accessible material parameters and experimental conditions for the growth of syndiotactic polypropylene spherulite. The simulations illustrate that, under self-generated thermal field, the initial nucleus is anisotropic having lamellar stacks that transforms to a sheaflike structure and eventually to a lamellar branching morphology with a dual-eye-pocket texture at the core. It appears that the released latent heat is responsible for the lamellar side branching and splaying from the main lamellae. On the same token, the heat build-up seemingly prevents the interface boundaries of neighboring spherulites from over running on each other during impingement, thereby forming the grain boundary.     

Characterization of selectively degraded poly(ethylene terephthalate)

To investigate the morphology of unoriented poly(ethylene terephthalate) (PET) films and the selective character of the aminolysis of PET, 67% crystalline polymer samples were degraded with 40% aqueous methylamine at room temperature. The aminolyzed PET samples were subjected to gel permeation chromatography (GPC), viscometry, electron microscopy, and small-angle x-ray diffraction (SAXD). Weight loss and density crystallinity measurements were also made. After 24 hr of aminolysis, the amorphous regions and chain folds were completely removed. The long molecular chains in the semi-crystalline polymer were reduced to monodisperse rods having a molecular weight of 1,800. The corresponding lamellar thickness was calculated to be 101 Å, consistent with the x-ray diffraction and electron microscope (EM) measurements. The EM photographs of “stripped” crystals show the lamellar structure previously found for other selectively degraded polymeric materials. The weight of crystalline debris remaining was consistent with the initial crystallinity. After degradation the crystallinity as determined by density was 96%.     

Folded chain crystallization of random terpolymer liquid crystal polymers from the nematic state

The morphology of ca. equimolar random terpolymer liquid crystal polymers of an aliphatic segment of 4–7 carbon atoms, oxybenzoate, and dioxyphenyl crystallized from the nematic state in the form of thin films on glycerine by slow cooling and quenching has been characterized by electron microscopy (TEM) and diffraction (ED). In all cases a folded chain, lamellar structure is found. The ED studies suggest adjacent reentry, and despite a large ΔH, indicate no change in lateral molecular packing at the crystal-“liquid crystal” transition with a transformation to the nematic state at a higher temperature “liquid crystal”-liquid crystal transition. The results are interpreted as suggesting similar folded chain, lamellar morphology in the nematic state. © 1992 John Wiley & Sons, Inc.     

A novel hierarchical crystalline structure of injection-molded bars of linear polymer: co-existence of bending and normal shish–kebab structure

The hierarchy structures and orientation behavior of high-density polyethylene (HDPE) molded by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined by using scanning electronic microscopy (SEM) and 2D wide-angle X-ray diffraction (2D-WAXD). Results show that the spatial variation of crystals across the thickness of sample molded by CIM was characterized by a typical skin–core structure as a result of general shear-induced crystallization. Unusually, the crystalline morphologies of the parts prepared by GAIM, primarily due to the penetration of secondary high-compressed gas that was exerted on the polymer melt during gas injection, featured a richer and fascinating supermolecular structure. Besides, the oriented lamellar structure, general shish–kebab structure, and common spherulites existed in the skin, sub-skin, and gas channel region, respectively; a novel morphology of shish–kebab structure was seen in the sub-skin layer of the GAIM parts of HDPE. This special shish–kebab structure (recognized as “bending shish–kebab”) was neither parallel nor perpendicular to the flow direction but at an angle. Furthermore, there was a clear interface between the bending and the normal shish–kebab structures, which may be very significant for our understanding of the melt flow or polymer rheology under the coupling effect of multi-fluid flow and complex temperature profiles in the GAIM process. Based on experimental observations, a schematic illustration was proposed to interpret the formation mechanism of the bending shish–kebab structure during GAIM process.     

High-pressure phases in polymers. III. The nature of the high-pressure phase in cis-polyisoprene

The morphology and growth of the disordered hexagonal phase which crystallizes in films of cis-polyisoprene at pressures in excess of 3 kbar is discussed. A two-stage growth process is proposed consisting of nucleation and crystallization followed by a pressure-enhanced thickening process. The phase is metastable and is replaced by normal spherulitic growth at long times. It transforms into lamellar sheafs with higher than usual lamellar thicknesses when pressure is lowered at constant temperature.     

Facile synthesis of the structural hierarchy in chrysanthemum–snowball-like self-organized polyaniline

A facile synthesis of the “chrysanthemum–snowball”-shaped polyaniline (PAni) has been prepared by using self-assembly polymerization of the host–guest monomeric inclusion complex of β-cyclodextrin (β-CD) with aniline. The amount of the monomer complex plays a role as a structural regulator during fabrication of the inclusion polymer as chrysanthemum–snowball structure/nanorods via intermolecular interactions such as: hydrogen bonding between β-CD and PAni, π–π interactions and cooperative interaction between PAni with FeCl₃ in an aqueous medium. The microstructure and morphology of the resulting materials were investigated by using various analytical techniques such as Fourier transform infrared, wide-angle X-ray diffraction, small-angle X-ray scattering, field emission scanning electron microscopy and transmission electron microscopy. After observing the growth process, a tentative mechanism is proposed to elucidate the formation of the PAni hierarchical structures.     

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. II. Morphology and crystallization kinetics by time resolved X‐ray scattering

The development of the morphology in poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) (PVDF/PHB) blends upon isothermal and anisothermal crystallization is investigated by time‐resolved small‐ and wide‐angle X‐ray scattering. The components are completely miscible in the melt but crystallize separately; they crystallize stepwise at different temperatures or sequentially with isothermal or anisothermal conditions, respectively. The PVDF crystallizes undisturbed whereas PHB crystallizes in a confined space that is determined by the existing supermolecular structure of the PVDF. The investigations reveal that composition inhomogeneities may initially develop in the remaining melt or in the amorphous phases of the PVDF upon crystallization of that component. The subsequent crystallization of the PHB depends on these heterogeneities and the supermolecular structure of PVDF (dendritically or globularly spherulitic). PHB may form separate spherulites that start to grow from the melt, or it may develop “interlocking spherulites” that start to grow from inside a PVDF spherulite. Occasionally, a large number of PVDF spherulites may be incorporated into PHB interlocking spherulites. The separate PHB spherulites may intrude into the PVDF spherulites upon further growth, which results in “interpenetrating spherulites.” Interlocking and interpenetrating are realized by the growth of separate lamellar stacks (“fibrils”) of the blend components. There is no interlamellar growth. The growth direction of the PHB fibrils follows that of the existing PVDF fibrils. Depending on the distribution of the PHB molecules on the interlamellar and interfibrillar PVDF regions, the lamellar arrangement of the PVDF may contract or expand upon PHB crystallization and the adjacent fibrils of the two components are linked or clearly separated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 974–985, 2004     

Structure of a microemulsion in the critical region: Neutron small-angle scattering results

Neutron small-angle scattering has been used to study the structure of an oil in water microemulsion in the critical region. The structure, one of small “swollen” micelles, is independent of the critical fluctuations. This together with other evidence strongly suggests that the latter are clusters of swollen micelles.     

Effect of thermal oxidation on the spherulitic morphology of high-density polyethylene

The supermolecular structure of high-density polyethylene (HDPE) at various stages of oxidation was studied using polarized optical microscopy and small-angle x-ray scattering (SAXS). Samples of HDPE crystallized isothermally at 123°C show a pronounced change in their spherulitic structure with progressive thermal oxidation. Polarized optical micrographs and SAXS data indicate that the average lamellar thickness decreases concomitantly with thermal treatment. Solid-state oxidative scission occurs preferentially at the chain folds where the polymer molecules are strained. This process increases the level of crystallinity of the polymer due to the more efficient packing of crystallites formed by the shorter cleaved chains. The morphological changes are related to the polymer melt flow index, molecular weight distribution, crystallinity and peak melting temperature. A model is proposed to account for the changes in the spherulitic morphology.