Confined crystallization of polyethylene oxide in nanolayer assemblies

Interfacial Diffusion and Bonding in Multilayer Polymer Films: A Molecular Dynamics Simulation

As a stacked form of ultrathin polymer films, multilayer nanostructures are of great interest in various applications. Coarse-grained molecular dynamics simulations were carried out to understand the behaviors of interfacial diffusion and bonding of multilayer polymer films. We found two obvious stages for the interfacial diffusion of polymers in the multilayer film, and it is 3 times faster in the first stage than in the second one due to the evolution of molecular conformations. The polymers near the interfaces have an in-plane mobility much higher than the out-of-plane one. The strength of interfacial bonding has been characterized by the fast tensile stress-strain curve along the normal direction. It shows multiple yielding points for the multilayer polymer films, which is distinct from the tensile behavior of the bulk. The ultimate tensile stress (UTS) and corresponding separating strain, surprisingly, do not necessarily increase with diffusion time. Because of the dramatic molecular rotation and extension during the first stage of interfacial diffusion, the interlayer interpenetration is nonuniformly distributed in the plane of the interface. Such a nonuniform distribution may be one of the reasons for the decrease of the UTS and separating strain.

Interfacial phenomena at a surface of individual and complex fumed nanooxides

Investigations of interfacial and temperature behaviors of nonpolar and polar adsorbates interacting with individual and complex fumed metal or metalloid oxides (FMO), initial and subjected to various treatments or chemical functionalization and compared to such porous adsorbents as silica gels, precipitated silica, mesoporous ordered silicas, filled polymeric composites, were analyzed. Complex nanooxides include core-shell nanoparticles, CSNP (50-200nm in size) with titania or alumina cores and silica or alumina shells in contrast to simple and smaller nanoparticles of individual FMO. CSNP could be destroyed under high-pressure cryogelation (HPCG) or mechanochemical activation (MCA). These treatments affect the structure of aggregates of nanoparticles and agglomerates of aggregates, resulting in their becoming more compacted. The analysis shows that complex FMO could be more sensitive to external actions than simple nanooxides such as fumed silica. Any treatment of 'soft' FMO affects the interfacial and temperature behaviors of polar and nonpolar adsorbates. Rearrangement of secondary particles and surface functionalization affects the freezing-melting point depression of adsorbates. For some adsorbates, open hysteresis loops became readily apparent in adsorption-desorption isotherms. Clustering of adsorbates bound in textural pores in aggregates of nanoparticles (i.e., voids between nanoparticles in secondary structures) causes reduced changes in enthalpy during phase transitions (freezing, fusion, evaporation). Freezing point depression and melting point elevation cause significant hysteresis freezing-melting effects for adsorbates bound to FMO in the textural pores. Relaxation phenomena for both low- and high-molecular weight adsorbates or filled polymeric composites are affected by the morphology of primary particles, structural organization of secondary particles of differently treated or functionalized FMO, content of adsorbates, co-adsorption order, and temperature.

Processing and surface modification of polymer nanofibers for biological scaffolds: a review

Polymeric fibrous constructs possess high surface area-to-volume ratios when compared with solid substrates and are quite commonly used as tissue engineering and cell growth scaffolds. An overview of important design and material considerations for fibrous scaffolds as well as an outline of both established and emerging solution- and melt-based fabrication techniques is provided. Innovative post-process surface modification avenues using "click" chemistry with both single and dual active cues as well as gradient cues, which maintain the fibrous structure are described. By combining process parameters with post-process surface modification, researchers have been able to selectively tune cellular response after seeding and culturing on fibrous constructs.

Proton Conductive Nanosheets Formed by Alignment of Metallo-Supramolecular Polymers

Linear Fe(II)-based metallo-supramolecular polymer chains were precisely aligned by the simple replacement of the counteranion with an N,N'-bis(4-benzosulfonic acid)perylene-3,4,9,10-tetracarboxylbisimide (PSA) dianion, which linked the polymer chains strongly. A parallel alignment of the polymer chains promoted by the PSA dianions yielded nanosheets formation. The nanosheets' structure was analyzed with FESEM, HRTEM, UV-vis, and XRD in detail. The nanosheets showed more than 5 times higher proton conductivity than the original polymer due to the smooth ionic conduction through the aligned polymer chains. The complex impedance plot with two semicircles also suggested the presence of grain boundaries in the polymer nanosheets.

Crystal Morphology of Poly(3-hydroxybutyrate) on Amorphous Poly(vinylphenol) Substrate

The crystalline morphology and orientation of poly(3-hydroxybutyrate) (PHB) thin film on the poly(vinylphenol) (PVPh) sublayer with different thickness was studied by atomic force microscopy, X-ray diffraction, and infrared spectroscopy. PVPh sublayer influences the morphology of PHB greatly. Although edge-on lamellae form on both Si and PVPh surfaces at relatively lower crystallization temperature, the morphology of them is quite different. It appears as sheaflike edge-on lamellar morphology on PVPh sublayer. In addition, the edge-on lamellae prefer to form on the PVPh sublayers at much higher crystallization temperature compared with that on Si wafer. The PVPh layer thickness also influences the crystalline morphology of PHB. On a 30 nm thick PVPh layer, sheaflike edge-on lamellae form in a wide range of crystallization temperatures. When the PVPh thickness increases to 65 nm, fingerlike morphology is observed when the crystallization temperature is lower than 95 °C. The fingerlike morphology is caused by a diffusion-limited aggregation process, and it requires an optimum condition. Thickness ratio between PHB and PVPh sublayer and temperature are two key factors for the formation of fingerlike morphology.

Interfacial Interaction in Anodic Aluminum Oxide Templates Modifies Morphology, Surface Area, and Crystallization of Polyamide-6 Nanofibers

Here, we demonstrated that, when the precipitation process of polyamide-6 (PA6) solution happens in cylindrical channels of an anodized aluminum oxide membrane (AAO), interface interactions between a solid surface, solvent, non-solvent, and PA6 will influence the obtained polymer nanostructures, resulting in complex morphologies, increased surface area, and crystallization changes. With the enhancing interaction of PA6 and the AAO surface, the morphology of PA6 nanostructures changes from solid nanofibers, mesoporous, to bamboo-like, while at the same time, metastable γ-phase domains increase in these PA6 nanostructures. Brunauer-Emmett-Teller (BET) surface areas of solid, bamboo-like, and mesoporous PA6 nanofibers rise from 16, 20.9, to 25 m(2)/g. This study shows that interfacial interaction in AAO template fabrication can be used in manipulating the morphology and crystallization of one-dimensional polymer nanostructures. It also provides us a simple and novel method to create porous PA6 nanofibers with a large surface area.

Mechanics of an Asymmetric Hard-Soft Lamellar Nanomaterial

Nanolayered lamellae are common structures in nanoscience and nanotechnology, but most are nearly symmetric in layer thickness. Here, we report on the structure and mechanics of highly asymmetric and thermodynamically stable soft-hard lamellar structures self-assembled from optimally designed PS1-(PI-b-PS2)3 miktoarm star block copolymers. The remarkable mechanical properties of these strong and ductile PS (polystyrene)-based nanomaterials can be tuned over a broad range by varying the hard layer thickness while maintaining the soft layer thickness constant at 13 nm. Upon deformation, thin PS lamellae (<100 nm) exhibited kinks and predamaged/damaged grains, as well as cavitation in the soft layers. In contrast, deformation of thick lamellae (>100 nm) manifests cavitation in both soft and hard nanolayers. In situ tensile-SAXS experiments revealed the evolution of cavities during deformation and confirmed that the damage in such systems reflects both plastic deformation by shear and residual cavities. The aspects of the mechanics should point to universal deformation behavior in broader classes of asymmetric hard-soft lamellar materials, whose properties are just being revealed for versatile applications.

Puncture characterization of multilayered polypropylene homopolymer/ethylene 1-octene copolymer sheets

Both the puncture strength and energy of the multilayered sheets were significantly higher than those of the conventional blend.

Innovative enhancement of gas barrier properties of biodegradable poly(butylene succinate) nanocomposite films by introducing confined crystals

In this work, we creatively obtain high gas barrier poly(butylene succinate) (PBS)/clay nanocomposite films by introducing confined crystals taking advantage of the spatial confinement effect which commonly exists in polymer/nanofiller systems.

A grazing incident XRD study on the structure of poly(3-hydroxybutyrate) ultrathin films sandwiched between Si wafers and amorphous polymers

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology.

Co-Extrusion Layer Multiplication of Rheologically Mismatched Polymers: A Novel Processing Route

Co-extruded films with up to 65 layers of two rheologically mismatched polymer systems – polystyrene/poly(methylmethacrylate) (PS/PMMA) and hard/soft thermoplastic polyurethanes (TPUs) – were successfully produced using a combination of a 9-layer feedblock, low-pressure drop multiplier dies, and external lubricants. Formation of viscoelastic instabilities was studied using a custom visualization and by finite element method (FEM) simulations of a standard multiplier. The results showed that the flow inside the standard multiplier die is highly non-uniform, with severe gradients in shear and normal stresses and viscous encapsulation occurring mainly in the initial multiplication stages where there is enough material available in the low-viscosity layers to proceed with the encapsulation. To mitigate layer degradation the standard 2- or 3-layer feedblock was replaced with a 9-layer one, thereby decreasing the thickness of each layer at the end of the feedblock. Also, subsequent layering was performed using a low flow resistance die. This new multiplier die yields a more uniform flow profile and imparts a more homogeneous thermo-mechanical history on the melt which results in an improved layer stability. Simulations showed that in the standard die the second normal-stress difference (N2) responsible for elastic instabilities at the edges of the die are very high. These can be reduced by inducing slip at the wall resulting in be much improved layer uniformity and stability. This was accomplished experimentally via the use of external lubricants, and the resulting layered structure was indeed much better than was possible to achieve with the conventional multiplier dies.

Strong and electrically conductive graphene-based composite fibers and laminates

Graphene is an ideal candidate for lightweight, high-strength composite materials given its superior mechanical properties (specific strength of 130 GPa and stiffness of 1 TPa). To date, easily scalable graphene-like materials in a form of separated flakes (exfoliated graphene, graphene oxide, and reduced graphene oxide) have been investigated as candidates for large-scale applications such as material reinforcement. These graphene-like materials do not fully exhibit all the capabilities of graphene in composite materials. In the current study, we show that macro (2 inch × 2 inch) graphene laminates and fibers can be produced using large continuous sheets of single-layer graphene grown by chemical vapor deposition. The resulting composite structures have potential to outperform the current state-of-the-art composite materials in both mechanical properties and electrical conductivities (>8 S/cm with only 0.13% volumetric graphene loading and 5 × 10(3) S/cm for pure graphene fibers) with estimated graphene contributions of >10 GPa in strength and 1 TPa in stiffness.

Confine Clay in an Alternating Multilayered Structure through Injection Molding: A Simple and Efficient Route to Improve Barrier Performance of Polymeric Materials

Various methods have been devoted to trigger the formation of multilayered structure for wide range of applications. These methods are often complicated with low production efficiency or require complex equipment. Herein, we demonstrate a simple and efficient method for the fabrication of polymeric sheets containing multilayered structure with enhanced barrier property through high speed thin-wall injection molding (HSIM). To achieve this, montmorillonite (MMT) is added into PE first, then blended with PP to fabricate PE-MMT/PP ternary composites. It is demonstrated that alternating multilayer structure could be obtained in the ternary composites because of low interfacial tension and good viscosity match between different polymer components. MMT is selectively dispersed in PE phase with partial exfoliated/partial intercalated microstructure. 2D-WAXD analysis indicates that the clay tactoids in PE-MMT/PP exhibits an uniplanar-axial orientation with their surface parallel to the molded part surface, while the tactoids in binary PE-MMT composites with the same overall MMT contents illustrate less orientation. The enhanced orientation of nanoclay in PE-MMT/PP could be attributed to the confinement of alternating multilayer structure, which prohibits the tumbling and rotation of nanoplatelets. Therefore, the oxygen barrier property of PE-MMT/PP is superior to that of PE-MMT because of increased gas permeation pathway. Comparing with the results obtained for PE based composites in literature, outstanding barrier property performance (45.7% and 58.2% improvement with 1.5 and 2.5 wt % MMT content, respectively) is achieved in current study. Two issues are considered responsible for such improvement: enhanced MMT orientation caused by the confinement in layered structure, and higher local density of MMT in layered structure induced denser assembly. Finally, enhancement in barrier property by confining impermeable filler into alternating multilayer structure through such simple and efficient method could provide a novel route toward high-performance packaging materials and other functional materials require layered structure.

Crystallization-Induced Redox-Active Nanoribbons of Organometallic Polymers

Polymer/inorganic functional nanostructures are essential for the fabrication of high-performance nanodevices in the future. The synthesis of hybrid nanostructures is hindered by complicated synthetic protocols or harsh conditions. Herein, we report a facile and scalable method for the synthesis of organometallic polymer nanoribbons through crystallization of polymers capped with a ferrate complex. Nanoribbons consisted of a single crystalline polymer lamella coated with a redox-active ferrate complex on both sides. The nanoribbons had a width of approximately 70 nm and a thickness of 10 nm. With the merit of highly ordered crystalline structures of polymers and functional coating layers, as well as a highly anisotropic nature, the nanoribbons are useful in nanodevices and biosensors.

Effect of Layer and Film Thickness and Temperature on the Mechanical Property of Micro- and Nano-Layered PC/PMMA Films Subjected to Thermal Aging

Multilayered polymer films with biomimicking, layered structures have unique microstructures and many potential applications. However, a major limitation of polymer films is the deterioration of mechanical properties in working environments. To facilitate the design and development of multilayered polymer films, the impact of thermal aging on the mechanical behavior of micro- and nano-layered polymer films has been investigated experimentally. The composition of the polymer films that have been studied is 50 vol% polycarbonate (PC) and 50 vol% poly(methyl methacrylate) (PMMA). The current study focuses on the effect of film and layer thickness and temperature on the mechanical properties of the materials subjected to thermal aging. To study the effect of film and layer thickness, films with the same thickness, but various layer thicknesses, and films with the same layer thickness, but various film thicknesses, were thermally aged at 100 °C in a constant temperature oven for up to six weeks. The results show that as the layer thickness decreases to 31 nm, the film has a higher stiffness and strength, and the trend of the mechanical properties is relatively stable over aging. The ductility of all of the films decreases with aging time. To study the effect of temperature, the films with 4,096 layers (31 nm thick for each layer) were aged at 100 °C, 115 °C and 125 °C for up to four weeks. While the 100 °C aging results in a slight increase of the stiffness and strength of the films, the higher aging temperature caused a decrease of the stiffness and strength of the films. The ductility decreases with the aging time for all of the temperatures. The films become more brittle for higher aging temperatures.

Orientation of PVDF α and γ crystals in nanolayered films

Wide-angle X-ray scattering in conjunction with pole figure technique was used to study the texture of poly(vinylidene fluoride) (PVDF) α and γ phase crystals in nanolayered polysulfone/poly(vinylidene fluoride) films (PSF/PVDF) produced by layer-multiplying coextrusion. In all as-extruded PSF/PVDF films, the PVDF nanolayers crystallized into the α phase crystals. A large fraction of those crystals was oriented with macromolecular chains perpendicular to the PSF/PVDF interface as evidenced from the (021) pole figures. Further refinement of the texture occurs during isothermal recrystallization at 170 °C in conjunction with transformation of α to γ crystals. The γ crystals orientation was probed with the (004) pole figures showing the c-axis of PVDF γ crystals perpendicular to the PSF/PVDF interface. The thinner the PVDF layers the stronger the orientation of γ crystals. It was proven that the X-ray reflections from the (021) planes of α crystals and from the (004) planes of γ crystals are not overlapped with other reflections and can be effectively used for the texture determination of PVDF nanolayers in multilayered PSF/PVDF films.

Structural tailoring of hydrogen-bonded poly(acrylic acid)/poly(ethylene oxide) multilayer thin films for reduced gas permeability

Hydrogen bonded poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) layer-by-layer assemblies are highly elastomeric, but more permeable than ionically bonded thin films. In order to expand the use of hydrogen-bonded assemblies to applications that require a better gas barrier, the effect of assembling pH on the oxygen permeability of PAA/PEO multilayer thin films was investigated. Altering the assembling pH leads to significant changes in phase morphology and bonding. The amount of intermolecular hydrogen bonding between PAA and PEO is found to increase with increasing pH due to reduction of COOH dimers between PAA chains. This improved bonding leads to smaller PEO domains and lower gas permeability. Further increasing the pH beyond 2.75 results in higher oxygen permeability due to partial deprotonation of PAA. By setting the assembling pH at 2.75, the negative impacts of COOH dimer formation and PAA ionization on intermolecular hydrogen bonding can be minimized, leading to a 50% reduction in the oxygen permeability of the PAA/PEO thin film. A 20 bilayer coating reduces the oxygen transmission rate of a 1.58 mm natural rubber substrate by 20 ×. These unique nanocoatings provide the opportunity to impart a gas barrier to elastomeric substrates without altering their mechanical behavior.

Preparation of alternating multilayered polyethylene oxide/poly(ε-caprolactone) and the confined crystallization of the composites

Due to the effects of the interfacial interaction between PEO and PCL layers and the spatial confinement. The crystallization behaviors of PEO and PCL in multilayered structure varied distinguishably as the layer number became higher.