Thick Lamellar Crystals Research Articles

Molecular and Thermal Characterization of a Nearly Perfect Isotactic Poly(propylene)

The structure formation upon cooling of the quiescent melt of a nearly defect‐free isotactic poly(propylene) (iPP) homopolymer is investigated in this study. This iPP, having a molecular weight of 1 × 106 g mol−1 and a narrow molecular weight distribution, has been produced with a C2 symmetric hafnocene complex. Information about the crystallization kinetics is collected in a broad range of cooling rates. Standard differential scanning calorimetry is used for cooling rates in the range 3–100 °C min−1 and FSC is employed to analyze the solidification process at cooling rates larger than 100 °C s−1. The near absence of regio‐ and stereo‐defects allows this iPP to crystallize at significantly high temperatures and to form crystals at cooling rates higher than commonly observed for standard Ziegler–Natta iPP. The data reveal that the iPP homopolymer with close‐to‐perfect stereo‐regularity form crystals at high cooling rates as, e.g., α‐nucleated iPP. The defect‐free polymer chain allows the formation of relatively thick lamellar crystals which resulted in a relatively high tensile modulus of 2088 ± 8 MPa. image

Influence of Titanium Dioxide Content on the Crystallization Behavior and Solar Reflectance of Polyethylene/Titanium Dioxide Composites

Titanium dioxide (TiO2) particles were introduced to improve the solar reflectance of high-density polyethylene (HDPE). The organic-inorganic hybrids were fabricated by melt blending. A series of characterizations were taken to study the crystallization behavior, morphology, solar reflectance, and real cooling property. TiO2 particles acted as nucleation agents in the HDPE matrix and made the HDPE form thick lamellar crystals. TiO2 particles could disperse well into the HDPE matrix under 2.5 wt.% loading but agglomerated with 3 wt.%. Solar reflectance was related to the reflective index of TiO2 and the microstructure of HDPE. The real cooling property depended on the solar reflectance and the dispersion of the TiO2 particles in the HDPE matrix.

ChemInform Abstract: Ferroelectricity of Polymers Based on Vinylidene Fluoride

Characteristics of the ferroelectricity in polyvinylidene fluoride (PVDF) and copolymers based on vinylidene fluoride (VDF) with different previous thermomechanical histories are analysed. Switching of polarisation and the fluctuation mechanism of formation of new nuclei upon the appearance of conformation defects such as kink bonds are considered. The role of the space charge, which stabilises the new polarisation position, is noted. It is shown that the Curie transition can be either a first- or second-order phase transition depending on the copolymer composition. The influence of the morphology of PVDF on some ferroelectric characteristics is found. In particular, the morphology of thick lamellar crystals is favourable for enhanced spontaneous polarisation. It is shown that a ferroelectric structure approaching the texture of an ideal single crystal can be created in Langmuir–Blodgett films. As the number of unimolecular layers decreases, the concentration of conformational defects in these films increases and the crystallographic order in the lattice is violated. This intensifies the low-temperature Curie transition. The bibliography includes 247 references.

Thermal and mechanical characteristics of polylactide filaments drawn at different temperatures

Polylactide (PLA) filaments produced by melt spin-drawing in different conditions (draw ratio, temperature of the draw roll) have been studied to demonstrate the influence of the process conditions on the tensile properties of the PLA filaments. Draw ratio varies from 2 to 3.5 and temperature of the draw roll from 80°C to 120°C. The thermal characteristics have been investigated using modulated differential scanning calorimetry (MDSC) and crystallinity using X-Ray Diffraction (XRD). Shrinkage of PLA multifilaments depending on the processing conditions is also investigated. It is demonstrated that the temperature of the draw roll is essential to obtain PLA filaments with optimum properties. At 80°C, the size of the lamellar crystals, measured using DRX, is not the highest. It is necessary to set the temperature of the draw roll at 110°C to obtain thick lamellar crystals. So, in those conditions, the PLA filaments have their optimum mechanical properties.

Temperature-resolved SAXS studies of morphological changes in melt-crystallized poly(hexamethylene terephthalate) and its melting upon heating

Temperature-resolved small-angle X-ray scattering (SAXS) on poly(hexamethylene terephthalate) (PHT) samples crystallized from the melt yields direct information about the morphological changes in lamellar crystals and interlamellar amorphous layers upon melt-crystallization and subsequent heating to melting. Absolute intensities of these SAXS patterns were further analyzed via one-dimensional correlation and interface distribution functions. These analyses indicate that melt-crystallization at low temperature produces lamellar crystals having diverse thicknesses whereas crystallization at high temperature tends to favor growth of thick lamellar crystals with a nearly uniform distribution of thickness. When heating the PHT samples in the melting temperature region, the melting of the lamellar crystals was found to correlate well with the sequential-melting features. When these crystals are heated to higher temperatures, structural alterations from stacked lamellae to isolated lamellar crystals evolve with increasing extent of sequential melting, but, upon re-crystallization during extended annealing, the isolated lamellar crystals can pass through a reversible transition back to stacked lamellae.

Effect of different modified clays on the thermal and physical properties of polypropylene-montmorillonite nanocomposites

This paper studies the isothermal and nonisothermal crystallization of maleic anhydride grafted polypropylene-montmorillonite nanocomposites, compared to that of the polypropylene matrix (PP), by differential scanning calorimetry (DSC). The nanocomposites were prepared by melt-intercalation. The DSC results show that the presence of modified montmorillonite (mMMT) clay results in a shift of the onset and peak temperatures towards higher values. This suggests that thicker lamellar crystals appear. Thermomechanical analysis (TMA) was used to monitor the changes in volume expansion of PP in the presence of mMMT sheets.

Crystal morphologies and enzymatic degradation of melt-crystallized thin films of random copolyesters of ( R)-3-hydroxybutyric acid with ( R)-3-hydroxyalkanoic acids

Spherulitic and lamellar morphologies of melt-crystallized thin films for poly[( R)-3-hydroxybutyric acid- co-16 mol%-( R)-3-hydroxypentanoic acid] and poly[( R)-3-hydroxybutyric acid- co-8 mol%-( R)-3-hydroxyhexanoic acid] were characterized by optical, transmission electron and atomic force microscopies. Two-dimensional spherulites were formed throughout the thin films of bacterial copolymers after isothermal crystallization at 110 °C. The second monomer units in the random copolymer chains were excluded from the crystalline regions of poly[( R)-3-hydroxybutyric acid] (P[( R)-3HB]) lamellar crystals during isothermal crystallization process. Formation of the thin crystals with 5–7 nm thickness was found on the surface of original lamellar crystals with 10–13 nm thickness in the course of storage at room temperature after isothermal crystallization at 110 °C. Both the thermal analysis and morphological observation of copolymer thin films suggest that a certain sequential length of ( R)-3HB units in copolymer is needed for the formation of the original lamellar crystals at a crystallization temperature. The enzymatic degradation of copolymer thin films was performed in an aqueous solution of PHB depolymerase from Ralstonia pickettii. At the initial stage of enzymatic degradation, only the thin crystals formed at room temperature were eroded by the enzyme. By the continuous degradation reaction, the original thick lamellar crystals showed a comb-like texture and grooves along the crystal growth direction, suggesting that disordered chain-packing regions are accumulated in the original lamellar crystals during crystal growth process.

THE RELATIONSHIP BETWEEN PROCESSING HISTORY AND THE MORPHOLOGY OF INJECTION MOLDED TOUGHENED POLYOLEFINS

The solid-state morphologies of three polyolefins, namely isotactic polypropylene (iPP), isotactic poly(propylene-co-ethylene) (iPcE), and high-density polyethylene (PE), toughened with an elastomeric poly(ethylene-co-1-octene) (EcO) have been investigated. Morphologies ranged from dispersed droplets with mean diameters in the range 0.2 to 0.6 μm, for iPP, to a fine interpenetrating morphology comprising 17 nm thick lamella crystals and elastomer-rich regions of length-scale 170 nm when the major component was linear PE. In the iPcE formulations, 10 nm thick lamellar crystals of the matrix polymer were observed in the elastomer domains. Dynamic mechanical analysis and microscopy of quenched and annealed samples showed that each system was phase separated in the solid state and in the melt under quiescent conditions, respectively. The differing solid state morphologies were inferred to result from the mixing under melt flow of the partially miscible polymers during processing and by the subsequent liquid–liquid and solid–liquid phase separation on cooling.

Time-Resolved SAXS, WAXS, and DSC Study of the Annealing of Poly(aryl ether ether ketone) (PEEK) from the Glassy State

The annealing of poly(aryl ether ether ketone) (PEEK) from the glassy state was investigated by time-resolved simultaneous small-angle (SAXS) and wide-angle (WAXS) X-ray scattering using synchrotron radiation and by differential scanning calorimetry (DSC). Each experiment consisted of a heating ramp from 100 °C to the annealing temperature (280, 300, 310, 315, 340, and 350 °C) at 10 °C/min, followed by a 30 min isotherm at this temperature and a cooling ramp down to 100 °C at −10 °C/min. The long spacing L and the invariant Q were estimated from the Lorentz-corrected SAXS intensity curves. The crystalline (Lc) and amorphous (La) thicknesses and the linear degree of crystallinity (vclin) within the stacks of lamellar crystals were obtained from the correlation function γ(r). The annealing temperature range can be divided into two regions based on the final morphology of the PEEK films. Samples annealed at temperatures up to 315 °C contain lamellar crystals that have crystallized during the isotherm. For annealing temperatures above 315 °C, thick lamellar crystals grow during the isotherm. However, a large fraction of the material remains molten at the end of the isotherm. The magnitude of this molten fraction increases with the annealing temperature. This molten fraction nonisothermally crystallizes during cooling as revealed by calorimetry and X-ray diffraction. The linear degree of crystallinity of PEEK crystallized from the glassy state at temperatures between 280 and 340 °C is always below 0.5, implying that the crystalline thickness of the lamellae corresponds to the smallest value of the two partial thicknesses obtained from the correlation function. For the last experiment, the polymer is fully molten at 350 °C and nonisothermally crystallizes during the cooling as revealed by calorimetry. In contrast, the analysis of the corrected invariant and the integrated WAXS intensity for this last sample indicates that the latter consists of lamellar stacks with a linear degree of crystallinity of up to 0.75.

Crystallization, field-induced phase transformation, thermally induced phase transition, and piezoelectric activity in P(vinylidene fluoride-TrFE) copolymers with high molar content of vinylidene fluoride

Field-induced phase transformation and thermally induced phase transition in vinylidene fluoride (VDF) and trifluoroethylene copolymers with VDF content higher than 82 mol% and their ferroelectric and piezoelectric properties have been studied by means of x-ray diffraction, DSC, D-E hysteresis, and piezoelectric resonance. It is found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of α, β, and possibly γ phases. Application of a strong ac electric field transforms these mixed phase crystals into β-phase crystals completely. The β-phase crystals are ferroelectric and exhibit strong piezoelectric activity stable up to the melting temperature (160–180 °C depending on VDF content); the Curie point coincides with the melting point. Unpoled, mixed phase film has a paraelectric or rotational phase below the melting temperature, in which thick lamellar single crystals are grown extensively, whereas β-phase film prepared by mechanical drawing has no paraelectric phase below the melting point. Definite ferroelectric-to-paraelectric phase transition in P(VDF-TrFE) is inferred to occur only in thick lamellar crystals grown in the paraelectric phase. A phase diagram of P(VDF-TrFE) is described.