Spherulitic Structure Research Articles

Role of nanofiller in friction of polymers based on polycaproamide: Indirect effect

The effect of nanofillers (multiwalled carbon nanotubes) on the tribological behavior of polycaproamide is studied when introduced into the polymer by polymerization filling. The introduction of insignificant quantities (∼0.1 wt %) of nanotubes provides their homogeneous distribution in the polymer and contributes to reducing the wear and friction coefficient. This is mainly due to the role of nanotubes as nucleators of the fine spherulitic polycaproamide structure. Such an indirect effect of the nanofiller on the polymer’s tribological behavior is seen when nanotubes are added to the melt in the course of granulation of polycaproamide compositions reinforced by carbon fibers, the effect of nanotubes being reduced with increasing amount of the reinforcing filler.

Synthesis and characterization of polyurethane-based side-chain cholesteric liquid crystal polymers

Polyurethane-based side-chain cholesteric liquid crystalline polymers (ChLCPs) with variable clearing temperatures were synthesized in a two-step reaction. The chemical structures of ChLCPs were confirmed by FT-IR and 1H-NMR spectroscopy. The mesogenic properties and phase transition behavior were investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction measurements. The DSC studies show that the melting temperature and isotropic transition temperature of the ChLCPs increased with the weight percentage of cholesterol in the polymer. POM shows that the ChLCPs had a distinct spherulite structure that melted at about 140 °C, and these results are consistent with those of the DSC studies. The thermogravimetric studies show that the ChLCPs were stable up to 200 °C, though there was a reduction in the thermal stability as the weight proportion of cholesterol and glycerol in the polymer increased.

Morphology of polypropylene/poly(ethylene-co-propylene) in-reactor alloys prepared by multi-stage sequential polymerization and two-stage polymerization

Abstract The morphology of two polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys prepared by multi-stage sequential polymerization (MSSP) and two-stage polymerization (TSP) processes, respectively, was investigated. It is observed that the alloy prepared by MSSP (sample 1) exhibits lower phase separation temperature than the alloy prepared by TSP (sample 2), probably due to the higher content of PP segments in the blocky copolymer fractions of sample 1. Two thermal treatments were applied to the samples: (1) The samples were directly quenched from 230 °C to 132 °C for isothermal crystallization; (2) The samples were firstly held at 160 °C for 60 min for phase separation and then cooled to 132 °C for crystallization. It is found that both microstructure and thermal treatment affect the morphology of the alloys, and the differences in morphology are interpreted in terms of phase diagram. For sample 1 and for the samples subjected to phase separation prior to crystallization, the EPR-rich phase contains more PP and thus is more viscous, which leads to more inclusion of the EPR-rich phase into the spherulites. A coarse spherulitic structure is formed due to crystallization of PP in the included EPR-rich phase. More included EPR-rich phase and its stronger crystallizability can further lead to the narrower boundaries and formation of connections between the adjacent spherulites.

Effect of Crystallinity and Spherulite Structure on the Mechanical Properties of Poly(1-butene)

The effect of crystallinity and spherulite structure on the properties of Poly(1-butene) (PB-1), including tensile strength, yield strength and flexural modulus, was studied. The results show that isotacticity plays an important role in the spherulitic morphology and mechanical properties. With the increasing in isotacticity, the crystallinity increased and the size of the PB-1 spherulitic crystals decreased, therefore the mechanical properties were enhanced.

Additional Supra-Self-Assembly of Human Serum Albumin under Amyloid-Like-Forming Solution Conditions

Protein aggregation has a multitude of consequences ranging from affecting protein expression to its implication in different diseases. Of recent interest is the specific form of aggregation leading to the formation of amyloid fibrils, structures associated with diseases such as Alzheimer's disease. These fibrils can further associate in other more complex structures such as fibrillar gels, plaques, or spherulitic structures. In the present work, we describe the physical and structural properties of additional supraself-assembled structures of human serum albumin under solution conditions in which amyloid-like fibrils are formed. We have detected the formation of ordered aggregates of amyloid fibrils, i.e., spherulites which possess a radial arrangement of the fibrils around a disorganized protein core and sizes of several micrometers by means of polarized optical microscopy, laser confocal microscopy, and transmission electron microscopy. These spherulites are detected both in solution and embedded in an isotropic matrix of fibrillar gels. In this regard, we have also noted the formation of protein gels when the protein concentration and/or ionic strength exceds a threshold value (the gelation point) as observed by rheometry. Fibrillar gels are formed through intermolecular nonspecific association of amyloid fibrils at a pH far away from the isolectric point of the protein where protein molecules seem to display a "solid-like" behavior due to the existence of non-DLVO (Derjaguin-Landau-Verwey-Overbeck) intermolecular repulsive forces. As the solution ionic strength increases, a coarsening of this type of gel is observed by environmental scanning microscopy. In contrast, at pH close to the protein isoelectric point, particulate gels are formed due to a faster aggregation process, which does not allow substantial structural reorganization to enable the formation of ordered structures. This behavior also additionally corroborates that the existence of particulates might also be a generic property of all polypeptide chains as amyloid fibril formation under suitable conditions.

Isothermal Crystallization Behavior of Poly(ϵ-Caprolactone) Diol/Functionalized-Multiwalled Carbon Nanotube Composites

Isothermal crystallization behavior of poly (ϵ-caprolactone) diol (PCL)/functionalized-multiwalled carbon nanotube (f-MWNTs) composites at three different temperatures (30°, 35°, 40°C), with varying molecular weights (MW) of PCL (2000, 4000, 8000 g/mol) and loadings of f-MWNTs (0, 0.1, 1.0, 5.0 wt.%), was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction methods. DSC isotherm showed that introduction of f-MWNTs into PCL induced a heterogeneous nucleation in the crystallization growth process, thereby lowering the activation energy for the crystallization process appreciably. POM showed the spherulite structure of neat PCL, whereas the composites showed the spherulite structure containing f-MWNTs at the center, indicating nucleating action of f-MWNTs in the crystallization process. The spherulite size decreased but its number increased with increasing weight proportion of f-MWNTs and MW of PCL.

Structural evolution of tensile deformed high-density polyethylene at elevated temperatures: Scanning synchrotron small- and wide-angle X-ray scattering studies

The structural evolution of an ice-quenched high-density polyethylene (HDPE) subjected to uniaxial tensile deformation at elevated temperatures was examined as a function of the imposed strains by means of combined synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The data show that when stretching an isotropic sample with the spherulitic structure, intralamellar slipping of crystalline blocks was activated at small deformations, followed by a stress-induced fragmentation and recrystallization process yielding lamellar crystallites with their normal parallel to the stretching direction. Stretching of an isothermally crystallized HDPE sample at 120 °C exhibited changes of the SAXS diagram with strain similar to that observed for quenched HDPE elongated at room temperature, implying that the thermal stability of the crystal blocks composing the lamellae is only dependent on the crystallization temperature. The strain at a characteristic transition point associated with the first indication for the occurrence of a fibrillar structure remains essentially constant in spite of the large changes in drawing temperature and crystalline thickness. In addition, WAXS experiments were used to probe the texture changes accompanying the uniaxial elongation and yield the relationship between the orientational order parameters associated with the crystallites and the amorphous chain segments, and the imposed strain. The results support the existence of intralamellar slip processes from the very beginning of tensile deformation.

Structure and properties of MDO stretched polypropylene

Abstract Two polypropylene cast films of different crystalline structures (one with coexisting small rows of lamellae and spherulites and the other with only a spherulitic structure) were prepared by extrusion. The produced cast films were uniaxially hot drawn at T = 120 °C using a machine direction orientation (MDO) unit and the changes in structure and morphology were examined and related to barrier as well as tear and puncture properties. Structural changes in terms of the degree of crystallinity and crystal size distribution, orientation of the amorphous and crystalline phases, and the deformation behavior at the crystal lattice and lamellae scales were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), respectively. A significant effect of the original crystal morphology on the alignment of the amorphous and crystalline phases was observed from FTIR and WAXD. The results also revealed that the deformation behavior of the crystal structure was dependent on the draw ratio (DR). Our findings showed that by increasing DR the crystal lamellae first broke up and oriented along the drawing direction and then, at large DR, they were deformed and created a fibrillar structure. Morphological pictograms illustrating the effects of original morphology and draw ratio on the stretched film microstructure are proposed. The tear resistance along the machine direction (MD) decreased significantly with increasing DR whereas the puncture resistance increased drastically. Finally, the oxygen transmission rate (OTR) of the MDO stretched films could be correlated with the orientation parameters as well as the β-relaxation peak magnitude of the amorphous tie chains.

Influence of structural organization on tensile properties in mesomorphic isotactic polypropylene

Abstract The effects of annealing on the structure and mechanical properties of mesomorphic isotactic polypropylene have been investigated using wide-angle and small-angle X-ray scattering and rheo-optics in addition to tensile tests. Young's modulus of mesomorphic phase was estimated to be 5 GPa using Takayanagi model. The α-crystallitic iPP prepared by annealing the quenched mesomorphic iPP was transparent because of the absence of spherulitic structure. It was found that the mechanical yielding of α-crystallitic iPP is dominated by the plastic flow of crystalline structural units whereas the yield process of α-spherulitic iPP quenched at 80 °C is caused by the fracture or fragmentation of crystalline structural units.

Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl carbonate diluent via thermally induced phase separation

AbstractDiphenyl carbonate was used as a diluent to prepare microporous polyvinylidene fluoride (PVDF) membrane via thermally induced phase‐separation method. The liquid–liquid phase‐separation phenomenon was found in PVDF/diphenyl carbonate system during the thermally induced phase‐separation process. The monotectic point of this system appeared at a high polymer concentration approximately 56 wt %. This suggested that PVDF membrane with a bicontinuous or cellular structure could be obtained when the PVDF concentration was less than this point value. The effects of polymer concentration and quenching temperature on the membrane structure, porosity, and tensile strength were also investigated. The results showed that low polymer concentration and high quenching temperature led to a large pore size membrane. The tensile strength of the membrane increased with increase in the polymer concentration, but the porosity of the membrane decreased. On condition that the PVDF concentration was higher than 60 wt %, only polymer crystallization occurred and a typical compact spherulitic structure was obtained. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Investigating the inner structure of irregular $ \beta$ -lactoglobulin spherulites

When beta-lactoglobulin in low p H aqueous solutions is exposed to high temperature for extended time, spherulites composed of amyloid fibrils of the beta-lactoglobulin protein form. Many of these spherulites have fibrils that radiate out from a centre and, under crossed polarisers, exhibit a symmetric Maltese Cross structure. However, a significant fraction (50 of the 101 observed spherulites) of beta-lactoglobulin spherulites formed under these conditions demonstrate various forms of irregularity in apparent structure. The irregularities of spherulites structures were qualitatively investigated by comparing optical microscopy images observed under crossed polarisers to computationally produced images of various internal structures. In this way, inner spherulite structures are inferred from microscopy images. Modelled structures that were found to produce computed images similar to some of the experimentally viewed images include fibrils curving as they radiate from a single nucleation point; multiple spherulites nucleating in close proximity to one another; and fibrils curving in opposite directions above and below a single nucleation point.

Experimental and numerical study of creep and creep rupture behavior of PA6

Polyamide 6 (PA6) is a semi-crystalline polymer utilized in many structural components working under steady load. At room temperature, PA6 exhibits time dependent (viscoplastic) deformation. The aim of this work is to investigate the mechanical response and the crack growth of PA6 under creep conditions. The experimental database consists of tests carried out on smooth and notched round bars. Load versus displacement curves were recorded for monotonic tests tensile at various crosshead speeds. Then, creep tests at constant load were performed allowing the record of the creep strain history according to the applied load. Microscopic observations highlight the initial spherulitic structure. Smooth and notched specimens were utilized in order to identify and to validate material coefficients dedicated for analytical modeling. The non linear fracture mechanics for creeping solids was applied to results on “pre-cracked” specimens. For this kind of loading, use of the load parameter C ∗ is recommended. By plotting C ∗ values versus time to failure, a unique correlation was obtained. The knowledge of this master curve allows lifetime assessment of PA6 cracked bodies.

Phase behaviour of oleanolic acid/stearyl stearate binary mixtures in bulk and at the air–water interface

The solid–liquid phase behaviour of oleanolic acid (OLA)/stearyl stearate (SS) was investigated by differential scanning calorimetry and polarizing optical microscopy. A eutectic type diagram, with the eutectic composition close to pure SS was obtained. Complementary studies by NMR, X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy were performed. A mutual influence was detected in mixtures: the low melting form of SS is favoured at low OLA molar fractions ( X OLA) and spherulitic structures appear at high X OLA and high temperature. Additionally, H-bonding between OLA carbonyl groups increases in the presence of SS. The study of OLA/SS by the Langmuir method and Brewster angle microscopy revealed the organization at the air–water interface: OLA interacts with water in the first layer, while SS is completely segregated to the upper layer for X OLA > 0.3, and it distributes in the first and upper layers for X OLA < 0.3.

Attachment of Human Primary Osteoblast Cells to Modified Polyethylene Surfaces

Ultra-high-molecular-weight polyethylene (UHMWPE) has a long history of use in medical devices, primarily for articulating surfaces due to its inherent low surface energy which limits tissue integration. To widen the applications of UHMWPE, the surface energy can be increased. The increase in surface energy would improve the adsorption of proteins and attachment of cells to allow tissue integration, thereby allowing UHMWPE to potentially be used for a wider range of implants. The attachment and function of human primary osteoblast-like (HOB) cells to surfaces of UHMWPE with various levels of incorporated surface oxygen have been investigated. The surface modification of the UHMWPE was produced by exposure to a UV/ozone treatment. The resulting surface chemistry was studied using X-ray photoelectron spectroscopy (XPS), and the topography and surface structure were probed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), which showed an increase in surface oxygen from 11 to 26 atom % with no significant change to the surface topography. The absolute root mean square roughness of both untreated and UV/ozone-treated surfaces was within 350-450 nm, and the water contact angles decreased with increasing oxygen incorporation, i.e., showing an increase in surface hydrophilicity. Cell attachment and functionality were assessed over a 21 day period for each cell-surface combination studied; these were performed using SEM and the alamarBlue assay to study cell attachment and proliferation and energy-dispersive X-ray (EDX) analysis to confirm extracellular mineral deposits, and total protein assay to examine the intra- and extracellular protein expressed by the cells. HOB cells cultured for 21 days on the modified UHMWPE surfaces with 19 and 26 atom % oxygen incorporated showed significantly higher cell densities compared to cells cultured on tissue culture polystyrene (TCPS) from day 3 onward. This indicated that the cells attached and proliferated more readily on the UV/ozone-treated UHMWPE surfaces than on untreated UHMWPE and TCPS surfaces. Contact guidance of the cells was observed on the UHMWPE surfaces by both SEM and AFM. Scanning electron micrographs showed that the cells were confluent on the modified UHMWPE surfaces by day 10, which led to visible layering of the cells by day 21, an indicator of nodule formation. In vitro mineralization of the extracellular matrix expressed by the HOB cells on the modified UHMWPE surfaces was confirmed by SEM and EDX analysis; spherulite structures were observed near cell protrusions by day 21. EDX analysis confirmed the spherulites to contain calcium and phosphorus, the major constituents in calcium phosphate apatite, the mineral phase of bone. Overall cell attachment, functionality, and mineralization were found to be enhanced on the UV/ozone-modified UHMWPE surfaces, demonstrating the importance of optimizing the surface chemistry for primary HOB cells.

Structure and properties of polypropylene/organic rectorite nanocomposites

AbstractMelt blending using a twin-screw extruder was used to prepare composites of polypropylene (PP)/organic rectorite (PR). The organic rectorite (OREC) was modified with dodecyl benzyl dimethyl ammonium bromide (1227). Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy were used to investigate the dispersion of OREC in the composites. The d spacings of OREC in PR composites was greater than in OREC itself. The dispersion of OREC particles in the PP polymer matrix was fine and uniform when the clay content was small (2 wt.%). The rheology was characterized using a capillary rheometer. The processing behaviour of the PR system improved as the amount of OREC added increased. Non-isothermal crystallization kinetics were analysed using differential scanning calorimetry. It was shown that the addition of OREC had a heterogeneous nucleation effect on PP, and can accelerate the crystallization. However, only when fine dispersion was achieved, and at lower rates of temperature decrease, was the crystallinity greater. Wide-angle X-ray diffraction and polarized light microscopy were used to observe the crystalline form and crystallite size. The PP in the PR composites exhibited an a-monoclinic crystal form, as in pure PP, and in both cases a spherulite structure was observed. However, the smaller spherulite size in the PR systems indicated that addition of OREC can reduce the crystal size significantly, which might improve the ‘toughness’ of the PP. The mechanical properties (tensile and impact strength) improved when the amount of OREC added was appropriate. Dynamic mechanical analysis showed that the storage modulus (E′) and loss modulus (E″) of the nanocomposites were somewhat greater than those of pure PP when an appropriate amount of OREC was added. Finally, thermogravimetric analysis showed that the PR systems exhibited a greater thermal stability than was seen with pure PP.

Self‐Organization, Optical, and Electrical Properties of α‐Quinquethiophene–Dinucleotide Conjugates

The synthesis and properties of (5')TA(3')-t5 (8a) and (5')CG(3')-t5 (8b) conjugates, in which the self-complementary dinucleotides TA and CG are covalently bound to the central ring of alpha-quinquethiophene (t5), are described. According to molecular mechanics calculations, the preferred conformation of both 8a and 8b is that with the dinucleotide folded over the planar t5 backbone, with the nucleobases facing t5 at stacking distance. The calculations show that the aggregation process of 8a and 8b is driven by a mix of nucleobase-thiophene interactions, hydrogen bonding between nucleobases (non Watson-Crick (W&C) in TA, and W&C in CG), van der Waals, and electrostatic interactions. While 8b is scarcely soluble in any solvents, 8a is soluble in water, indicating that the aggregates of the former are more stable than those of the latter. Microfluidic-induced self-assembly studies of 8a showed the formation of lamellar, spherulitic, and dendritic supramolecular structures, depending on the concentration and solvent evaporation time. The self-assembled structures displayed micrometer dimensions in the xy plane of the substrate and nanometer dimensions in the z direction. Spatially resolved confocal microscopy and spectroscopy showed that the aggregates were characterized by intense fluorescence emission. Cast films of 8a from water solutions showed chirality transfer from the dinucleotide to t5. The hole mobility of the cast films of 8a was estimated using a two-electrode device under high vacuum and found to be up to two orders of magnitude greater than those previously measured for dinucleotide-quarterthiophene conjugates under the same experimental conditions.

Novel polypropylene-based microporous membranes via spherulitic deformation

A novel method for creating a microporous membrane via spherulitic deformation is illustrated. The microporous structure was generated by the combination of intra-spherulitic and inter-spherulitic deformations. Polypropylene was selected due to its unique cross-hatched lamellar morphology facilitating inter-spherulitic deformation. A precursor film with a spherulitic structure was made under low-stress melt processing condition. A tangential lamellae-rich spherulite was created and identified with a positive birefringence sign. A sequential annealing process improved the crystalline structure, and in particular the thickness of the tangential lamellae. The annealing process proved to be critical for initiating the inter-spherulitic deformation. A WAXS examination provides a quick characterization method for the inter-spherulitic deformation. A highly interconnected solvent-resistant porous polypropylene membrane having a pore size in the range of 50–100 nm and a porosity of about 0.18 was thereby developed in this study. This concept can be further expanded by using an α-nucleating agent to reduce spherulite sizes. A highly methanol permeable membrane with an estimated porosity of 0.29 was produced with the nucleated polypropylene samples.

Banded spherulitic structures of poly(ethylene adipate), poly(butylene succinate) and in their blends

Banded spherulites of PEA, PBS and their PBS-PEA blends were studied by optical and atomic force microscopies. It was found that PEA forms a regular banded structure within the temperature range 23 degrees C to 36 degrees C, otherwise only Maltese-cross spherulites are observed. The banded PEA spherulites exhibit a double ring feature with band spacing between two equivalent birefringent rings increasing with temperature. The ring extinction in PEA banded spherulites is caused by a lamellar twist. There exist two equivalent positions in one twisting period, which show no birefringence. The PBS can also grow in ring-banded spherulites in a wide temperature window with the high temperature threshold for forming banded structures that are not sharply delineated, as in the case of PEA. Also the double ring feature is not so pronounced for the spherulites of pure PBS. Blending PBS with PEA is found to favor the formation of PBS banded structures. During the crystallization process, the early growing PBS spherulites at 75 degrees C show weak birefringence with an evident ring-banded structure. The crystallization of PEA at 0 degrees C makes a great contribution to the birefringence increment of the initially-birefringent rings. This is caused by the analogic crystal orientation of PBS and PEA based on interlamellar phase separation as revealed by AFM observation. The band spacing of PBS spherulites is found to be increase with both increasing temperature and increasing PEA content. This stems from an increment in chain mobility of PBS both with temperature and addition of the PEA component.

Competition between crystallization and dewetting fronts in thin polymer films

We investigate the competition between dewetting and crystallization fronts in thin polymer films where the velocities of these fronts are modulated by varying the temperature from a low temperature regime, where crystallization dominates, to a high temperature regime, where dewetting is more rapid and dominates the film structure. Specifically, thin films of poly(caprolactone) (PCL) on n-octyldimethylchlorosilane (ODS)-treated substrates are investigated as a model system exhibiting this general phenomenon. Structure formation at low temperatures is dominated by polycrystalline spherulitic structures that nucleate at random positions within the plane of the film, but at intermediate temperatures, where the crystallization and dewetting are strongly coupled, dewetting occurs preferentially at the grain boundaries between the spherulites. We term this phenomenon crystallization-induced dewetting (CID) and provide evidence that it arises from the high stresses at the spherulite grain boundaries that accompany local volume changes of the polymer material upon crystallization. CID is prevalent in a temperature range in which the angles between the grains are larger than a critical angle and this effect allows for considerable control over the resulting film morphology.

Morphological and crystalline studies of isotactic polypropylene plastically deformed and evaluated by small-angle X-ray scattering, scanning electron microscopy and X-ray diffraction

Morphological and structural properties of isotactic polypropylene (i-PP) submitted to uniaxial plane strain deformation at ambient temperature with compression pressures of 3, 10 and 20 MPa, were investigated using wide angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Two kinds of samples were studied, namely: sample A: Mw = 117,400 g/mol Mn = 17,300, Mw/Mn = 6.8 and sample B: Mw = 271,000 g/mol Mn = 43,700 Mw/Mn = 6.2, both with isotactic content = 95%. Crystalline α- and β-phases are clearly identified by WAXD. The lamellar long period, as well as, the crystalline and amorphous lamellar thickness for the non-deformed samples measured by SAXS indicates the presence of a more symmetric spherulitic structure for sample A, while the sample B displays anisotropic scattering. The WAXD study of the apparent relative crystallinity and the orientation of crystallites, revealed that plastic deformation of i-PP by plane-strain compression, leads to preferred orientation of main axis of crystallites at relatively early stage of the deformation process induced a monocrystal texture and an excellent molecular alignment along the FD, in both samples. The SEM evaluation shows that a gradual change occurs in the spherulitic structure and seems to transform gradually and disappear almost for the 37% deformations. The sample with highest deformation shows thin shear bands oriented along the FD-view which originate an appearance of a layered structure. Concomitantly the crystalline lamellae were detected by TEM technique.