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

A kinetic study of the formation of smectic phases in novel liquid crystal ionogens

A multi-rate non-isothermal kinetic analysis of the isotropic-melt to liquid crystalline phase transition of novel liquid crystalline ionogenic copolymers, LCIs, the 10-(4-methoxyazobenzene-4′-oxy)decyl methacrylate]-co-2-(acrylamido-2-methyl-1-propanesulfonic acid)s, 10-MeOAzB/AMPS, copolymers, has been performed by means of calorimetric experiments. An analytical methodology which includes the study of the phase transition rate parameter, the determination of the activation energies by using Kissinger and Flynn-Wall-Ozawa models, and the study of the phase transition kinetics by the use of the Avrami theory, has been applied. The formation of the mesophases from the isotropic state occurred close to thermodynamic equilibrium. The results evidence the presence of several individual processes in the formation of liquid crystalline phases from the melt and a strong dependence of phase transition rates and activation energies with acid contents. A decrease in the phase transition rate, related to a decrease in the overall change of the transition entropy, has been observed. The final inhibition of the liquid crystal (LC) behaviour is ascribed to an exponential increase in the activation energy of the phase transition, promoted by strong acid aggregation. An optimum composition of the 10-MeOAzB/AMPS copolymers to achieve the dual characteristics of LCIs (ionogenic and liquid crystalline behaviour) requires acid concentrations capable of promoting structure-forming effects on the LC phases and the evolution of phase separated morphologies

Mechanical reinforcement of electrospun poly(vinyl alcohol) by α‐FeOOH nanowires

The authors kindly acknowledge the financial support of the Estonian Research Council for the post-doctoral research grants of personal research funding in projects PUT1096 and PUTJD578 as well as Institutional Research Funding Projects, IUT20-17, and IUT23-7.We report the mechanical performance of α‐FeOOH nanowire reinforced poly(vinyl alcohol) (PVA) composite nanofiber mat, fabricated using straightforward aqueous processing methods. Goethite (α‐FeOOH) nanocrystals have a high elastic modulus and –OH rich surface, ensuring strong interactions with hydrophilic polymers and effective reinforcement. Needle‐less electrospinning resulted in alignment of the nanowires along fibre axis, as confirmed by transmittance electron microscopy studies. Produced composite PVA nanofibers containing 10 wt% goethite nanoparticles exhibited an outstanding fivefold increase in Young's modulus and 2.5‐fold improvement of tensile strength compared to mats of neat PVA. The addition of α‐FeOOH had a significant influence on glass transition temperature indicating formation of interphase regions around nanowire inclusions. Observed properties are explained by nanowire grafting in the precursor solution, extensive interactions between the adsorbed PVA chains and the matrix and percolation of interphase regions at 10 wt% α‐FeOOH.Estonian Research Council PUT1096 and PUTJD578; Institutional Research Funding Projects, IUT20-17, and IUT23-7; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Investigation on viscosity and non-isothermal crystallization behavior of P-bearing steelmaking slags with varying TiO2 content

The viscous flow and crystallization behavior of CaO-SiO2-MgO-Al2O3-FetO-P2O5-TiO2 steelmaking slags have been investigated over a wide range of temperatures under Ar (High purity, >99.999 pct) atmosphere, and the relationship between viscosity and structure was determined. The results indicated that the viscosity of the slags slightly decreased with increasing TiO2 content. The constructed nonisothermal continuous cooling transformation (CCT) diagrams revealed that the addition of TiO2 lowered the crystallization temperature. This can mainly be ascribed to that addition of TiO2 promotes the formation of [TiO6]-octahedra units and, consequently, the formation of MgFe2O4-Mg2TiO4 solid solution. Moreover, the decreasing viscosity has a significant effect on enhancing the diffusion of ion units, such as Ca2+ and [TiO4]-tetrahedra, from bulk melts to the crystal–melt interface. The crystallization of CaTiO3 and CaSiTiO5 was consequently accelerated, which can improve the phosphorus content in P-enriched phase (n2CaO·SiO2-3CaO·P2O5). Finally, the nonisothermal crystallization kinetics was characterized and the activation energy for the primary crystal growth was derived such that the activation energy increases from −265.93 to −185.41 KJ·mol−1 with the addition of TiO2 content, suggesting that TiO2 lowered the tendency for the slags to crystallize

Controlled three-dimensional polystyrene micro- and nano-structures fabricated by three- dimensional electrospinning

The combination of electrospinning with extrusion based 3D printing technology opens new pathways for micro- and nanofabrication, which can be applied in a wide range of applications. This simple and inexpensive method has been proven to fabricate 3D fibrous polystyrene structures with controlled morphology and micro- to nano-scale fibers diameter. The controllable movement of the nozzle allows precise positioning of the deposition area of the fibers during electrospinning. A programmed circular nozzle pattern results in the formation of controllable 3D polystyrene designed shapes with fiber diameters down to 550 nm. The assembly of the fibrous structures starts instantaneously, and a 4 cm tall and 6 cm wide sample can be produced within a 10 minutes electrospinning process. The product exhibits high stability at ambient conditions. The shape, size, and thickness of fibrous polystyrene structures can be easily controlled by tuning the process parameters. It is assumed that the build-up of 3D fibrous polystyrene structures strongly depends on charge induction and polarization of the electrospun fibers