Morphologies of miscible PCL/PVC blends confined in ultrathin films

Morphologies of ultrathin films (10–60 nm) of miscible poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends have been investigated under isothermal crystallization conditions by real time atomic force microscopy, and electron diffraction techniques. It was found that the morphology and growth rate of PCL/PVC blends strongly depend on the blend composition, crystallization temperature and film thickness. At a film thickness of 30 nm, the truncated lozenge-shape morphology of pure PCL crystals, found when the growth rate is slow, bent with increasing PVC content to form S-shaped or inverted S-shaped crystals, the curvature increasing by lowering the crystallization temperature. Electron diffraction patterns reveal that these crystals are flat-on single crystals with the PCL molecular chains (c axis) in the blends slightly tilted with respect to the lamella normal, while the b direction of the crystal lattice, corresponding to the fast growing direction of the growth front, follows a S line. Upon decreasing the film thickness (<30 nm), the S-shaped or inverted S-shaped crystals transform into four-branch dendritic lamellae.     

Crystallization of ultrathin poly(ε-caprolactone) films in the presence of residual solvent, an in situ atomic force microscopy study

The influence of minute amounts of residual tetrahydrofuran on the crystallization of ultrathin poly(ε-caprolactone) (PCL) films has been investigated in real-time by atomic force microscopy. Crystallizations were performed at 30 °C, after melting at 64 °C, for different periods of time; both the morphology and growth rate of the crystals were found to depend on the thermal history of the sample, i.e. on the heating time, due to the evaporation of the residual solvent trapped in the spin-coated polymer. The residual solvent, acting as a plasticizer, facilitates the diffusion of the polymer chains to the crystal growth front; its elimination slows down the growth rates and leads to a decrease of the width of the dendrites. Growth rates were measured for the edge-on and for the flat-on lamellae, which are nucleated from them. Edge-on lamellae were found to crystallize 15 times faster than flat-on lamellae. Finally, spinodal dewetting was observed in the melt left between dendrites grown at 30 °C, after evaporation of the residual solvent, on a substrate of lower wettability. These observations are discussed in terms of the mobility of polymer chains in the melt.     

Compatibilization and morphology development of immiscible ternary polymer blends

We report a novel and effective strategy that compatibilizes three immiscible polymers, polyolefins, styrene polymers, and engineering plastics, achieved by using a polyolefin-based multi-phase compatibilizer. Compatibilizing effect and morphology development are investigated in a model ternary immiscible polymer blends consisting of polypropylene (PP)/polystyrene(PS)/polyamide(PA6) and a multi-phase compatibilizer (PP-g-(MAH-co-St) as prepared by maleic anhydride (MAH) and styrene (St) dual monomers melt grafting PP. Scanning electron microscopy (SEM) results indicate that, as a multi-phase compatibilizer, PP-g-(MAH-co-St) shows effective compatibilization in the PP/PS/PA6 blends. The particle size of both PS and PA6 is greatly decreased due to the addition of multi-phase compatibilizer, while the interfacial adhesion in immiscible pairs is increased. This good compatibilizing effect is promising for developing a new, technologically attractive method for achieving compatibilization of immiscible multi-component polymer blends as well as for recycling and reusing of such blends. For phase morphology development, the morphology of PP/PS/PA6 (70/15/15) uncompatibilized blend reveals that the blend is constituted from PP matrix in which are dispersed composite droplets of PA6 core encapsulated by PS phase. Whereas, the compatibilized blend shows the three components strongly interact with each other, i.e. multi-phase compatibilizer has good compatibilization between the various immiscible pairs. For the 40/30/30 blend, the morphology changed from a three-phase co-continuous morphology (uncompatibilized) to the dispersed droplets of PA6 and PS in the PP matrix (compatibilized).     

Dendritic crystallization in thin films of PEO/PMMA blends: A comparison to crystallization in small molecule liquids

Dendritic crystallization of poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) thin films is reported. The film thickness is kept constant while the PMMA molar mass and blend composition are varied. Some basic features of dendritic growth, such as the diffusion length and tip curvature are discussed. The diffusion coefficient is tuned by varying the molar mass of the non-crystallizable PMMA and the blend composition. The observed dendrite tip radius is on the order of 50 nm and the shape of the growth envelope varies from square to needle-like as the PMMA molar mass or PMMA content is increased. The sidebranch spacing increases with the distance from the dendrite trunk with a power-law relationship that is also dependent on the PMMA molar mass and PMMA content. This coarsening process is similar to that reported for other classes of materials. These similarities (the curved dendrite tip, power-law relationship of the sidebranches, and the sidebranch coarsening processes) indicate that the large scale crystallization morphologies of the polymeric materials we study are similar to those found in crystallization of small molecules and metals.     

Elongational creep experiments - A new method for investigations of morphology development in polymer blends

This work is focused on the changes of phase structure in polystyrene/polyethylene blends with up to 15 wt.% of dispersed phase during elongational experiments in creep. In the first part, features of the experiments at constant stress with a special attention to morphology development in polymer blends are discussed. In the second part of the paper the deformation behavior of the dispersed droplets in dependence on applied stress and total strain is studied. It was found that with increasing the initial particle size the formation of homogeneously deformed long fibrils is preferred during the elongation. A maximum deformability of the droplets was observed, which cannot be increased by applying higher stresses, although the affine deformation of the droplets was not reached.     

Crystallization, melting and morphology of PEO in PEO/MWNT-g-PMMA blends

The dispersion of multi-walled carbon nanotubes (MWNTs) in crystalline poly(ethylene oxide) (PEO) is significantly improved by grafting with poly(methyl methacrylate) (PMMA) on surface of MWNTs via emulsion reactions. The synthesized MWNTs-g-PMMA is soluble in solvents that can dissolve PMMA and is well dispersed in PEO. The effects of the MWNTs-g-PMMA on PEO crystallization and its use as a reinforcement for PEO are investigated using DMA, DSC, POM, and SAXS. DMA data show that the PEO/MWNTs-g-PMMA blends containing up to 30 wt% MWNTs-g-PMMA are compatible. DSC data show the crystallization of PEO is enhanced by the MWNTs-g-PMMA, accompanying with a decreased thickness of crystal layers and an increased thickness of amorphous layers of the PEO lamellar stacks, in combination with SAXS data.     

Crystallization of double crystalline block copolymer/crystalline homopolymer blends: 1. Crystalline morphology

The crystalline morphology formed in binary blends of poly(ε-caprolactone)- block-polyethylene (PCL-b-PE) copolymers and PCL homopolymers has been examined using synchrotron small-angle X-ray scattering (SR-SAXS) and differential scanning calorimetry (DSC) as a function of the homopolymer fraction in the blend. The PE block crystallized first on quenching from a lamellar microdomain structure to set a hard lamellar morphology (PE lamellar morphology) in the blend, followed by the crystallization of PCL chains (i.e., PCL homopolymers + PCL blocks). Two binary blends were studied by considering the miscible state of PCL homopolymers in the microdomain structure: when the PCL homopolymers were uniformly mixed with PCL blocks, they formed a mixed crystal. When the PCL homopolymers were localized between PCL blocks in the microdomain structure, DSC results suggested the possible formation of separate PCL crystals in the PE lamellar morphology. The effect of the advance crystallization of PE blocks on the subsequent crystallization of PCL chains was discussed as compared with the crystalline morphology formed in PCL-block-polybutadiene copolymer/PCL homopolymer blends, where the crystallization of PCL chains started directly from a microdomain structure without forming the hard lamellar morphology.     

Labyrinthine pattern of polymer crystals from supercooled ultrathin films

We report the observation of a labyrinthine crystal pattern with a periodic structure in the crystal width direction in ultrathin films of poly(ethylene oxide) fractions with molecular weight ranging from 25,000 to 932,000 g/mol. The polymer thin films are crystallized at temperatures well below the bulk melting temperature, thus the system is characterized by limited diffusion of the polymer chains and rapid growth of the crystal fronts. The competition of these two competing factors leads to the formation of the labyrinthine pattern. This mechanism is supported by a scaling relation between the long period and molecular weight, , indicating the importance of chain diffusion. Furthermore, a linear relationship between and is observed, implying that the crystal growth is dominated by a secondary nucleation.     

Interface morphology snapshots of vertically segregated thin films of semiconducting polymer/polystyrene blends

We present atomic force microscopic images of the interphase morphology of vertically segregated thin films spin coated from two-component mixtures of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and polystyrene (PS). We investigate the mechanism leading to the formation of wetting layers and lateral structures during spin coating using different PS molecular weights, solvents and blend compositions. Spinodal decomposition competes with the formation of surface enrichment layers. The spinodal wavelength as a function of PS molecular weight follows a power-law similar to bulk-like spinodal decomposition. Our experimental results indicate that length scales of interface topographical features can be adjusted from the nanometer to micrometer range. The importance of controlled arrangement of semiconducting polymers in thin film geometries for organic optoelectronic device applications is discussed.     

The dynamics of montmorillonite clay dispersion and morphology development in immiscible ethylene-propylene rubber/polypropylene blends

The evolution of morphology during the melt compounding of polypropylene (PP), maleated ethylene-propylene rubber (EPR-g-MAn) and onium-ion exchanged montmorillonite clay (NR₄⁺-MM) is described. Irrespective of the ratio of components, clay partitions into the EPR-g-MAn phase exclusively, with significant amounts of mineral exfoliation occurring in the very early stages of compounding. These changes in filler distribution and dispersion are accompanied by reductions in the size of the dispersed PP phase, as the rate of droplet coalescence falls in response to an elevated EPR-g-MAn matrix viscosity. However, when NR₄⁺-MM is localized in a dispersed EPR-g-MAn phase, coalescence increases as a result of hindered particle break-up.     

Crystallization of hydroxybutyrate oligomers: 1. morphology and folding

Exact length hydroxybutyrate oligomers containing 24 and 32 repeat units have been prepared and their crystallization behaviour and crystal morphology observed.The oligomers form crystals from dilute solution and from the melt that have similar overall morphologies to crystals of the polymer, poly(hydroxybutyrate). Electron diffraction indicates that the chains are closely perpendicular to the basal planes of the crystals.Wide angle X-ray diffraction suggests that all crystals, no matter what their thickness, have a high degree of crystallinity and the same crystal structure.The crystals have a range of thicknesses, as measured by small angle X-ray diffraction. The most common thicknesses correspond to the extended stem length and to one half or, surprisingly, two thirds of that value. A model is proposed for the non-integer fraction crystals in which half of the chain ends are incorporated in the crystals themselves.     

Morphology development in polymer blends produced by chaotic mixing at various compositions

Whereas blending devices commonly entail complex flow fields and internal geometries, chaotic mixing can be instilled by simple periodic motion of bounding surfaces in simple devices. Breakup and coalescence of spatially expansive structures in components can give blends with a wide variety of morphologies. In this study, polystyrene and low density polyethylene were used as model components to study the effect of composition and processing time on gradual morphology development in immiscible binary blends. Inspections of samples disclosed attainable morphologies and also how transitions between morphologies occurred. Novel findings included blends with encapsulated fibers, abundant platelets, and two distinct morphologies having single phase continuity. Additionally, interpenetrating blends formed over a broad compositional range. Results suggest that chaotic mixing is a useful tool for studying relationships among processing conditions, morphology development, and blend properties and may serve as a means to more deliberately obtain target morphologies.     

The effect of end groups of PEG on the crystallization behaviors of binary crystalline polymer blends PEG/PLLA

The effect of end groups (2OH, 1OH, 1CH₃ and 2CH₃) of poly(ethylene glycol) (PEG) on the miscibility and crystallization behaviors of binary crystalline blends of PEG/poly(l-lactic acid) (PLLA) were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). A single glass-transition temperature was observed in the DSC scanning trace of the blend with a weight ratio of 10/90. Besides, the equilibrium melting point of PLLA decreased with the increasing PEG. A negative Flory interaction parameter, χ₁₂, indicated that the PEG/PLLA blends were thermodynamically miscible. The spherulitic growth rate and isothermal crystallization rate of PEG or PLLA were influenced when the other component was added. This could cause by the change of glass transition temperature, T_g and equilibrium melting point, T⁰_m. The end groups of PEG influenced the miscibility and crystallization behaviors of PEG/PLLA blends. PLLA blended with PEG whose two end groups were CH₃ exhibited the greatest melting point depression, the most negative Flory interaction parameter, the least fold surface free energy, the lowest isothermal crystallization rate and spherulitic growth rate, which meant better miscibility. On the other hand, PLLA blended with PEG whose two end groups were OH exhibited the least melting point depression, the least negative Flory interaction parameter, the greatest fold surface free energy, the greatest isothermal crystallization rate and spherulitic growth rate.     

Structural relaxation and dynamic fragility of freely standing polymer films

In addition to a tremendous reduction in the glass transition temperature, dielectric spectra of freely standing films reveal two other intriguing features: a temperature dependent asymmetric broadening of the structural relaxation peak towards lower temperatures and a reduction of the dynamic fragility down to the monomer limit. We verified that this experimental evidence is a manifestation of a gradient of glass transition temperatures across the film thickness induced by an enhanced molecular mobility at the two free surfaces of the membrane. As a direct implication of the peculiar features just described, the properties of freely suspended membranes neither correspond to those in bulk nor to a simplified scenario where the structural relaxation peak is merely shifted towards lower temperatures.     

Study of the morphology of poly(butylene succinate)/poly(ethylene oxide) blends using hot-stage atomic force microscopy

Blends of poly(butylene succinate) (PBS) and poly(ethylene oxide) (PEO) were cast into films, melted, and crystallized. A number of PBS/PEO blend compositions, ranging from 85/15 to 20/80 were used. The PBS, with a higher melting point, always crystallizes first, providing a scaffold on which the PEO would crystallize. AFM phase and height images were made at room temperature and at higher temperatures, as the PEO melted, allowing one to determine the morphology and location of the PEO. It was found that at low PEO concentrations (below 15 w/o) the PEO resides preferentially between PBS lamellae. This interlamellar PEO does not crystallize, except under extreme undercooling. At higher concentrations, larger amorphous domains exist within the PBS crystalline scaffold and PEO can crystallize in these domains. Two unexpected phenomena are observed: (1) the reversible exuding of PEO from interlamellar spaces to the surface for crystallization and (2) an unusual orientation of PEO lamellae within amorphous domains in the PBS scaffold.     

Morphology and finite strain rheologyof NBR/TPU blends

Due to orientational and dispersional interactions between molecules in polar polymer blends, such as NBR/TPU, secondary molecular networks are formed, which, despite their weakness, essentially affect rheological properties of the blends. Being highly susceptible to strain and temperature changes, the secondary networks under such conditions undergo breakdown, causing changes in blends' rheological characteristics. Particularly suitable method for tracking such network breakdown is by measuring the blends' dynamic mechanical functions at different strains and temperatures. For elucidation of these measurements a model is chosen, analysing a secondary network breakdown by statistical mechanics, whose final result is strain, temperature and compositional dependence of the blends' dynamic mechanical functions. Along with the measurements', the chosen model, originally devised to study rheological properties of carbon black filled rubbers, also provides means for quantitative characterization of secondary networks in polar polymeric systems.     

High performance LDPE/thermoplastic starch blends: a sustainable alternative to pure polyethylene

Thermoplastic starch (TPS), as opposed to dry starch, is capable of flow and hence when mixed with other synthetic polymers can behave in a manner similar to conventional polymer-polymer blends. This paper presents an approach to preparing polyethylene/thermoplastic starch blends with unique properties. A one-step combined twin-screw/single screw extrusion setup is used to carry out the melt-melt mixing of the components. Glycerol is used as the starch plasticizer and its content in the TPS is varied from 29 to 40%.Under the particular one-step processing conditions used it is possible to develop continuous TPS (highly interconnected) and co-continuous polymer/TPS blend extruded ribbon which possess a high elongation at break, modulus and strength in the machine direction. The PE/TPS (55:45) blend prepared with TPS containing 36% glycerol maintains 94% of the elongation at break and 76% of the modulus of polyethylene. At a composition level of 71:29 PE/TPS for the same glycerol content, the blend retains 96% of the elongation at break and 100% of the modulus of polyethylene. These excellent properties are achieved in the absence of any interfacial modifier and despite the high levels of immiscibility in the polar-nonpolar TPS-PE system. The 55:45 blend possesses a 100% continuous or fully interconnected TPS morphology, as measured by hydrolytic extraction. This highly continuous TPS configuration within the blend should enhance its potential for environmental biodegradation. The elongation at break in the cross direction of these materials, although lower than the machine direction properties, also demonstrates ductility at high TPS concentrations. At a glycerol content of 36% in the TPS, the blends demonstrate only very low levels of sensitivity to moisture. A high degree of transparency is maintained over the entire concentration range due to the similar refractive indices of PE and TPS and the virtual absence of interfacial microvoiding.Effective control of the glycerol content, TPS concentration and processing conditions can result in a wide variety of morphological structures including spherical, fiber-like, highly continuous and co-continuous morphologies. These various blend morphologies are shown to be the determining parameters with respect to the observed mechanical properties.This material has the added benefit of containing large quantities of a renewable resource and hence represents a more sustainable alternative to pure synthetic polymers.     

Crystallization of polycarbonate induced by spinodal decomposition in polymer blends

We found that amorphous polycarbonate (PC) can be crystallized in several minutes by blending poly(ethylene oxide) (PEO). When the blends were annealed in the two-phase region below the upper critical solution temperature, highly interconnected two-phase structure characteristic of the spinodal decomposition was developed and then the crystallization occurred in the PC-rich phase during the spinodal decomposition. As the molecular weight of PEO decreased, the crystallization rate decreased and the crystallizable temperature became narrower in spite of the acceleration of the polymeric segmental motion. These results suggest that the crystallization of the PC is not induced by the acceleration of the polymeric segmental motion, but by the up-hill diffusion of the liquid-liquid phase separation via spinodal decomposition. Owing to the competitive progress of the crystallization and the spinodal decomposition, the melting peak of the PC crystallites shifted to lower temperature with increasing annealing temperature.     

Evolution of TiOx-SiOx nano-composite during annealing of ultrathin titanium oxide films on Si substrate

This paper discusses the formation of the TiO_x-SiO_x nano-composite phase during annealing of ultrathin titanium oxide films (~27 nm). The amorphous titanium oxide films are deposited on silicon substrates by sputtering. These films are important for high-k dielectrics and sensing applications. Annealing of these films at 750 °C in the O₂ environment (for 15–60 min) resulted in the polycrystalline rutile phase. The films exhibit Raman peaks at 150 cm⁻¹ (B_1g), 435 cm⁻¹ (E_g), and 615 cm⁻¹ (A_1g) confirming the rutile phase. The signature TO (1078 cm⁻¹) and LO (1259 cm⁻¹) infrared active vibrational modes of Si–O–Si bond confirms the presence of silicon-oxide. The X-ray photoelectron spectra of the TiO_x films show multiple peaks corresponding to Ti metal (453.8 eV); Ti⁴⁺ state (458.3 eV (Ti 2p_3/2) and 464 eV (Ti 2p_1/2)); and Ti³⁺ state (456.4 eV (Ti 2p_3/2) and 460.8 eV (Ti 2p_1/2)). The O1s XPS spectra peaks at 530–533 eV can be attributed to Ti–O and Si–O bonds of the TiO_x-SiO_x nano-composite phase in the annealed films. The depth profiling XPS study shows that the top surface of the annealed film is mainly TiO_x and the amount of SiO_x increases with the depth.