Understanding isothermal crystallization and subsequent melting behavior of syndiotactic polypropylene

Various issues related to isothermal quiescent crystallization and subsequent melting behavior of syndiotactic polypropylene (sPP) were investigated in this dissertation. On the study of isothermal melt- and cold-crystallization kinetics and subsequent melting behavior of sPP, the overall crystallization rate parameters for melt-crystallization process, when plotted as a function of crystallization temperature, exhibited an unmistakable double bell-shaped curve; whereas, those for cold-crystallization process showed the typical bell-shaped curve. Comparison of the overall crystallization rate parameters obtained for both melt- and cold-crystallization processes indicate that crystallization from the glassy state proceeds at a much greater rate than from the melt state. The multiple-melting behavior observed in subsequent melting endotherms is attributed to the contributions from: 1) melting of the secondary crystallites and their re-crystallization, 2) partial melting of the less stable fraction of the primary crystallites and their re-crystallization, 3) melting of the primary crystallites, and lastly 4) re-melting of the re-crystallized crystallites formed during the heating scan. Analysis of the linear growth rate data of sPP#1 and other data sets taken from the literature in the context of the Lauritzen-Hoffman secondary nucleation theory suggested an unmistakable regime II-III transition at the crystallization temperature of 110°C. Regardless of the crystal structure, if the growth is assumed to occur on the bc plane, the lateral surface free energy σ = 11.3 erg-cm-2 and the fold surface free energy σe = 63.7 ± 7.1 erg-cm-2 were found. On the other hand, if the growth is assumed to occur on the ac plane, the fold surface free energy is found to be σe= 82.4 ±9.1 erg-cm-2, while the lateral surface free energy is the same as previously noted. The sensitivity of the crystal growth parameters on changes in the values of the input parameters was also investigated. Isothermal crystallization behavior after partial or complete melting of syndiotactic polypropylene was also investigated. On subsequent crystallization after partial melting, the total concentration of predetermined nuclei was found to decrease with increasing fusion temperature and increasing time period the sample spent at a specific fusion temperature. On subsequent crystallization after complete melting, the total concentration of predetermined nuclei was found to approach a constant value, which is the concentration of infusible heterogeneous nuclei (e.g., impurities, catalyst residues, etc.) present originally in the sample. At a specific fusion temperature, the concentration of predetermined athermal nuclei was found to decrease exponentially with the time period spent in the melt. Applicability of four macrokinetic models; namely the Avrami, Tobin, Malkin, and simultaneous Avrami models; in describing the time-dependent relative crystallinity data, using sPP as the model system, was tested using a non-linear multivariable regression program. Based on the quality of the fit, only the Avrami, Malkin, and the simultaneous Avrami models were found to describe the experimental data well, resulting in the rejection of the Tobin model in describing isothermal crystallization data of sPP. Comparison of the Avrami kinetics parameters obtained from the program with those obtained from the traditional analytical procedure suggested that use of non linear multi-variable regression program in data analysis is satisfactorily reliable. Lastly, a technique of using differential scanning calorimeter (DSC) to study crystallization behavior and the kinetics of the process at high crystallization temperatures or low degrees of undercooling was proposed. The technique was carried out based on the observations of, and the measurements of the enthalpy of fusion from, the subsequent melting endotherms after isothermal crystallization for various time intervals. Comparison of the overall crystallization data obtained from this proposed technique with those obtained from the traditional technique evidently indicated that the proposed technique of using information acquired from subsequent melting endotherms in studying crystallization kinetics is at least reliable and applicable to describe isothermal crystallization of sPP at the conditions studied. Apparent advantages and disadvantages of the proposed technique were also given

Material Characterisation Thermal, crystallization, and rheological characteristics of poly(trimethylene terephthalate)/poly(butylene terephthalate) blends

Abstract Blends of poly(trimethylene terephthalate) (PTT) and poly(butylene terephthalate) (PBT) were miscible in all of the blend compositions studied, as evidenced by an observed single and composition-dependent glass transition temperature for each blend composition. The variation of the glass transition temperature with the blend composition was well predicted by the Gordon-Taylor equation, with fitting parameter being ca. 6.9. The cold crystallization (peak) temperature was found to increase, while the melt crystallization (peak) temperature was found to decrease, with increasing PTT content. The subsequent melting behavior for these blends (after cold crystallization) showed the melting point depression behavior, in that the melting (peak) temperature for each component was lowered with increasing content of the other component. During crystallization, the pure components crystallized simultaneously to form their own crystals. The blend having 60 percent by weight of PTT showed the lowest apparent degree of crystallinity. The steady shear viscosities for the pure components and the blends showed slight decrease with increasing shear rate (within the shear rate range of 0.25-25 s Ϫ1 ), with those of the blends lying in between those of the pure components

Preparation of Hydrolyzed Electrospun Polyacrylonitrile Fiber Mats as Chelating Substrates: A Case Study on Copper(II) Ions

Ultrafine polyacrylonitrile (PAN) fiber mats were prepared by electrospinning and subsequently hydrolytically treated with a sodium hydroxide ethanolic/aqueous solution to impart the ability to chelate metal ions. This was achieved through the conversion of the nitrile functional groups on the surface of the PAN fibers into imine conjugated sequences, which was confirmed by Fourier-transform infrared spectroscopy. The chelating property of the hydrolyzed electrospun PAN fiber mats (i.e., H-ePAN fiber mats) was evaluated against Cu(II) ions. The amounts of the Cu(II) ions adsorbed onto the H-ePAN fiber mats were influenced by the initial pH and the initial concentration of the metal ion solutions. At the optimal pH of 5.0, the amounts of the Cu(II) ions adsorbed onto the H-ePAN fiber mats increased with an initial increase in the time the materials were in contact with the metal ion solution to finally reach the maximal, plateau values after about 5 h of immersion. The maximal adsorption capacity of the H-ePAN fiber mats for the Cu(II) ions was determined to be 31.3 mg g^–1, and the fully adsorbed materials could be regenerated upon submersion in 0.1 M hydrochloric acid aqueous solution for 30 min. Finally, X-ray photoelectron spectroscopy indicated specific interactions between the adsorbed Cu(II) ions and the N- and/or the O-atoms associated with the imine groups on the surface of the H-ePAN fibers

Performance of Electropun Polyacrylonitrile Nanofibrous Phases, Shown for the Separation of Water-Soluble Food Dyes via UTLC-Vis-ESI-MS

Research in the miniaturization of planar chromatography led to various approaches in manufacturing ultrathin-layer chromatography (UTLC) layers of reduced thickness (<50 µm) along with smaller instrumentation, as targeted in Office Chromatography. This novel concept merges 3D print & media technologies with miniaturized planar chromatography to realize an all-in-one instrument, in which all steps of UTLC are automated and integrated in the same tiny device. In this context, the development of electrospun polyacrylonitrile (PAN) nanofiber phases was investigated as well as its performance. A nanofibrous stationary phase with fiber diameters of 150–225 nm and a thickness of ca. 25 µm was manufactured. Mixtures of water-soluble food dyes were printed on it using a modified office printer, and successfully separated to illustrate the capabilities of such UTLC media. The separation took 8 min for 30 mm and was faster (up to a factor of 2) than on particulate layers. The mean hRF values ranging from 25 to 90 for the five food dyes were well spread over the migration distance, with an overall reproducibility of 7% (mean %RSD over 5 different plates for 5 dyes). The individual mean plate numbers over 5 plates ranged between 8286 and 22,885 (mean of 11,722 over all 5 dyes). The single mean resolutions RS were between 1.7 and 6.5 (for the 5 food dyes over 5 plates), with highly satisfying reproducibilities (0.3 as mean deviation of RS). Using videodensitometry, different amounts separated in parallel led to reliable linear calibrations for each dye (sdv of 3.1–9.1% for peak heights and 2.4–9.3% for peak areas). Coupling to mass spectrometry via an elution head-based interface was successfully demonstrated for such ultrathin layers, showing several advantages such as a reduced cleaning process and a minimum zone distance. All these results underline the potential of electrospun nanofibrous phases to succeed as affordable stationary phase for quantitative UTLC

Biologically inspired hierarchical design of nanocomposites based on poly(ethylene oxide) and cellulose nanofibers

Attempts to create hierarchically structured, uniaxially oriented nanocomposites comprising cellulose nanowhiskers (CNWs), which promise anisotropic mechanical properties, are exceedingly rare. We report here the fabrication of uniaxially-oriented arrays of microfibers based on poly(ethylene oxide) (PEO) and CNWs by electrospinning. Compared with the neat PEO fibers, the incorporation of CNWs within the fibers increased the storage modulus (E′) of arrays along the fiber axis of the PEO/CNW nanocomposite fibers. Successful incorporation of the CNWs within each of the as-spun PEO/CNW nanocomposite fibers in the direction parallel to the fiber axis was verified by both scanning and transmission electron microscopy

Improvement of dual-leached polycaprolactone porous scaffolds by incorporating with hydroxyapatite for bone tissue regeneration

<div><p>Polycaprolactone (PCL)/hydroxyapatite (HA) composite scaffolds were prepared by combining solvent casting and salt particulate leaching with a polymer leaching technique. The hydrophilicity of the dual-leached scaffold was improved by alkaline (NaOH) treatment. Well-defined interconnected pores were detected by scanning electron microscopy. The water absorption capacity of the NaOH-treated PCL/HA dual-leached scaffold increased greatly, confirming that the hydrophilicity of the scaffold was improved by NaOH treatment. The compressive modulus of the PCL/HA dual-leached scaffold was greatly increased by the addition of HA particles. An indirect evaluation of the cytotoxicity of all PCL dual-leached scaffolds with mouse fibroblastic cells (L929) and mouse calvaria-derived pre-osteoblastic cells (MC3T3-E1) indicated that the PCL dual-leached scaffolds are non-toxic to cells. The ability of the scaffolds to support mouse calvaria-derived pre-osteoblastic cell (MC3T3-E1) attachment, proliferation, differentiation, and mineralization was also evaluated. Although the viability of cells was lower on the PCL/HA dual-leached scaffold than on the tissue-culture polystyrene plates (TCPS) and on the other substrates at early time points, both the PCL and NaOH-treated PCL/HA dual-leached scaffolds supported the attachment of MC3T3-E1 at significantly higher levels than TCPS. During the proliferation period (days 1–3), all of the PCL dual-leached scaffolds were able to support the proliferation of MC3T3-E1 at higher levels than the TCPS; in addition, the cells grown on NaOH-treated PCL/HA dual-leached scaffolds proliferated more rapidly. The cells cultured on the surfaces of NaOH-treated PCL/HA dual-leached scaffolds had the highest rate of mineral deposition.</p></div

Silver nanoparticle-embedded poly(vinyl pyrrolidone) hydrogel dressing: gamma-ray synthesis and biological evaluation

<div><p>Silver nanoparticle (nAg)-embedded poly(vinyl pyrrolidone) (PVP) hydrogels, to be used as antibacterial wound dressings, were prepared by γ-irradiation at various doses: 25, 35, and 45 kGy. The formation and characteristics of the silver nanoparticles were investigated with a UV–vis spectrophotometer, transmission electron microscopy, and scanning electron microscopy with energy-dispersive X-ray. The hydrogels were characterized for physical and biological properties. Based on the antibacterial determination, the 1 and 5 mM nAg–embedded PVP hydrogels were effective, with 99.99% bactericidal activity at 12 and 6 h, respectively. The indirect cytotoxicity evaluation based on 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay indicated that both the neat and the nAg-embedded PVP hydrogels were non-toxic to mouse fibroblasts (L929). The 5 mM nAg-embedded PVP hydrogels not only provided a clean, moist environment for wound healing, but also effectively prevented bacterial infection and enhanced wound recovery.</p></div