Recrystallization of Quenched β‑Form Isotactic Polypropylene Lamellar Crystals in Thin Films

The melting–recrystallization characterizations of quenched β-form isotactic polypropylene (β-iPP) lamellar crystals in ultrathin films were investigated via the self-seeding method. We found lath-like shaped α-iPP crystals generated within the region occupied by the initial β-iPP lamellae and calculated their average number density (Nα). Intriguingly, two slopes were derived from the semi-logarithmic plot of Nα versus partial melting temperature (Ts), which reflects that Nα is determined by βα-recrystallization and αα-recrystallization as Ts is below and above 157 °C, respectively. Furthermore, our experimental observations clearly demonstrate that two phase selection pathways, that is, ββ- and βα-recrystallization, are possible upon melting of the quenched β-iPP lamellae. In particular, we found that ββ-recrystallization emerges as the preferred route for the case of an initial β phase with sufficiently high thermal stability, which is probably thermodynamically controlled rather than kinetically controlled. Otherwise, βα-recrystallization will be the dominant process due to the growth kinetics

Crystalline Structure of Injection Molded β‑Isotactic Polypropylene: Analysis of the Oriented Shear Zone

Although both shear flow and β-nucleating agent (β-NA) could separately induce β-crystal in isotactic polypropylene (iPP), their combination, particularly in the typical industrial processes, in fact has received comparatively little attention. In the current study, two-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering (2D-WAXD/SAXS) measurements were performed to investigate the effect of β-NA on the crystalline structure of the oriented shear zone in injection molded iPP. It is observed that, regardless of the β-NA concentration, parent–daughter structure of α-crystal can be formed in iPP. Furthermore, the fraction of daughter lamellae elevates with the increasing concentration of β-NA. Interestingly, unexpected scattering patterns of (300) reflection for β-crystal, which is similar with that for parent–daughter lamellar branching of α-crystal, is exclusively found in iPP with higher concentration of β-NA (1.0 wt %). The most possible explanation is that the addition of high content of β-NA lowers the free energy barrier. Additionally, the same change tendency of long period, crystal lamellar thickness and lateral dimension, <i>d</i>-spacing and crystallite size is found, viz., they first increase and then decrease with the increasing β-NA content. The results demonstrate that the concentrations of β-NA have a significant effect on the crystal grain structure under the practical molding process

Study on Crystallization Kinetics of Partially Melting Polyethylene Aiming To Improve Mechanical Properties

Partially melting high-density polyethylene (HDPE) is investigated by differential scanning calorimetry and in situ Fourier transform infrared spectroscopy (FTIR) measurements. The results show that some incompletely molten crystals can be preserved in the partially melting HDPE, thus accelerating subsequent crystallization. Moreover, quantitative calculation of such incompletely molten crystals has been done based on the FTIR results. Interestingly, the kinetically enhanced crystallization results from the improved nucleation rate because of the incompletely molten crystals. Furthermore, such interesting self-nucleation originating from the partially melting HDPE has been deliberately applied to practical polymer processing, i.e., extrusion, aiming to investigate the influence of partially melting HDPE on the microstructural development and the resultant mechanical properties of the extruded products. Unexpectedly, an increment in tensile strength has been achieved in the sheets extruded from partially melting HDPE compared with that extruded from completely melted HDPE. Wide-angle X-ray diffraction and small-angle X-ray scattering results show that the crystallinity, long period, and lamellar thickness increase for the sheets extruded from the partially melting HDPE. These increases are considered to be responsible for the mechanical increment. This work opens a new gateway for applying the self-nucleating effect of partially melting polymer melt into practical processing with the purpose of preparing high-performance polymer products

Carbon Nanotubes-Adsorbed Electrospun PA66 Nanofiber Bundles with Improved Conductivity and Robust Flexibility

Electrospun polyamide (PA) 66 nanofiber bundles with high conductivity, improved strength, and robust flexibility were successfully manufactured through simply adsorbing multiwall carbon nanotubes (MWNTs) on the surface of electrospun PA66 nanofibers. The highest electrical conductivity (0.2 S/cm) and tensile strength (103.3 MPa) were achieved for the bundles immersed in the suspension with 0.05 wt % MWNTs, indicating the formation of conductive network from adsorbed MWNTs on the surface of PA66 nanofibers. The decrease of porosity for the bundles immersed in the MWNT dispersion and the formation of hydrogen bond between PA66 nanofibers and MWNTs suggest a superb interfacial interaction, which is responsible for the excellent mechanical properties of the nanocomposite bundles. Furthermore, the resistance fluctuation under bending is less than 3.6%, indicating a high flexibility of the nanocomposite bundles. The resistance of the nanocomposite bundle had a better linear dependence on the temperature applied between 30 and 150 °C. More importantly, such highest working temperature of 150 °C far exceeded that of other polymer-based temperature sensors previously reported. This suggests that such prepared MWNTs-adsorbed electrospun PA66 nanofiber bundles have great potentials in high temperature detectors

Baseline Characteristics and Measures of Endothelial Function, Metabolic Parameters, Inflammation, Bacterial Translocation, and Oxidative Stress in the Overall Study Cohort (N = 23) and in the Efavirenz (N = 12) and Protease Inhibitor (N = 11) Subgroups.

<p>NOTES: Data presented as medians (quartile 1, quartile 3) unless otherwise specified; all p-values are for within-individual changes from baseline (only those ≤0.10 are shown).</p><p>ABBREVIATIONS: EFV, efavirenz; PI, protease inhibitor; FMD, flow-mediated dilation; NTGMD; nitroglycerin-mediated dilation; LDL, low-density lipoprotein; HDL, high-density lipoprotein; HOMA-IR, homeostatic model assessment-insulin resistance; hsCRP, high sensitivity C-reactive protein; IL-6, interleukin-6; MCP-1, monocyte chemoattractant protein-1; sTNFR2, soluble tumor necrosis factor receptor-2; IP-10, interferon gamma inducible protein-10; sVCAM-1, soluble vascular cell adhesion molecule-1; sCD14, soluble cluster of differentiation-14; PTH, parathyroid hormone; FGF-23, fibroblast growth factor-23.</p

Pearson Correlation Coefficients of Markers of Microbial Translocation with Brachial Artery Flow- Mediated Dilation.

*<p>Includes all subject data from baseline A5152s evaluations and all subjects in the Indiana cross-sectional study.</p><p>ART – antiretroviral therapy. sCD14 – soluble CD14. LPS – lipopolysaccharide. Significant correlations are shown in bold.</p

Morphological Changes of Isotactic Polypropylene Crystals Grown in Thin Films

Morphological variations of lamellae of isotactic polypropylene (iPP) grown in thin films have been examined experimentally by optical microscopy (OM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A flower-shaped morphology of iPP crystals, composed of several petal-like lamellae radiating from a nucleus, was typically found. At crystallization temperatures (<i>T</i>_c) below 135 °C, initially petal-like lamellae with a flat α-iPP backbone and many regular branches were formed, which were able to induce epitaxial nucleation of γ-iPP, resulting in features similar to a dendrite growing in the plane of the slow growth direction (i.e., <i>b</i>-axis of α-iPP). With increasing <i>T</i>_c, these dendritic structures disappeared gradually, and the lamellae exhibited a faceted lath-like shape for <i>T</i>_c > 150 °C. Interestingly, periodic lateral splitting (the crystal splayed into a pair of branches) at the fast growth plane was observed at a critical width (<i>W</i>_max) which increased with <i>T</i>_c. In particular, the measured temperature dependence of the products of <i>W</i>_max²<i>G</i> (<i>G</i> represents the growth rate along the <i>a</i>*-axis) was found to be constant. We discuss the role of the diffusion field at the growth front and epitaxial crystallization with respect to morphological changes of iPP lamellae in thin films

Median % flow mediated dilation by tertile of plasma lipopolysaccharide levels among ART treated subjects in the IU study.

<p>Median % flow mediated dilation by tertile of plasma lipopolysaccharide levels among ART treated subjects in the IU study.</p

Systematic Control of Self-Seeding Crystallization Patterns of Poly(ethylene oxide) in Thin Films

Using optical microscopy and atomic force microscopy, we studied systematically crystallization patterns in thin films of a low molecular weight poly­(ethylene oxide) (PEO) resulting from a kinetically controlled self-seeding approach. In particular, the influence of seeding temperature (<i>T</i>_s) and heating rate (<i>V</i>_h) on the various resulting crystallization patterns was investigated. Crystallization at 49 °C resulted in dendritic PEO crystals consisting of almost exclusively twice-folded chains. Upon heating these crystals, we observed crystal thickening due to a reduction in the average number of chain folds. On the basis of the detected morphology, we deduced that the density of seeded PEO crystals decreased when increasing <i>T</i>_s from 54 to 57 °C. At the highest <i>V</i>_h (i.e., 100 °C/min), only a few well-separated faceted single crystals of PEO were grown from individual seeds. In contrast to such random distribution of crystals, because of a faster reduction of chain folds at the edges of PEO lamellae, an almost continuous sequence of seeded crystals was formed at the periphery of the original crystals at significantly lower <i>V</i>_h (i.e., 10 °C/min). Interestingly, reflecting the different metastable states within the initial crystal resulting from seeding at <i>T</i>_s = 54 °C, the seeding probability for crystals at the diagonals was higher than for the major side branches. In addition, we estimated activation energies (213–376 kJ/mol) for thickening of PEO lamellar crystal from an Arrhenius-type behavior of the lateral spreading rates as a function of <i>V</i>_h. Our findings suggest that the interplay between thickening and melting of metastable states within the initial crystals is considered as responsible for the resulting nucleation density and crystal morphology induced by self-seeding

Subject Characteristics.

<p>Values are means (±standard deviation) except where noted. ART – antiretroviral therapy. sCD14 – soluble CD14. LPS – lipopolysaccharide. FMD - Flow mediated dilation.</p>*<p>p = 0.02 for the difference between baseline and week 24 in ACTG 5152s subjects.</p