1,721,017 research outputs found
Effect of self-seed crystal structure on growth of polymorphs in poly(butylene 2,6-naphthalate): A cross-nucleation study
No full textCross-nucleation between different crystal polymorphs is a particular, self-seed assisted type of heterogeneous nucleation, where a fast-growing polymorph nucleates at a pre-existing crystal surface of another polymorph. Here, we present a study on cross-nucleation between different crystalline phases of poly(butylene 2,6-naphthalate) (PBN), employing hotstage polarized-light optical microscopy and temperature-resolved wide-angle X-ray scattering as analysis tools. PBN forms α-crystals at relatively low temperature and β′-crystals at rather high temperature, with cross-nucleation experiments designed such to first obtain few α- or β′-seed crystals (mother phase) which then are transferred to higher or lower temperature, respectively, to monitor the continuation of the crystallization process and possible growth of the other polymorph. In case of cooling β′-crystals to lower temperature where typically α-crystals form in the non-seeded isotropic melt, β′-crystals nucleate growth of α-crystals, following many examples of cross-nucleation in the literature. In contrast, if low-temperature-generated α-crystals are heated to a temperature where β′-crystals form in a non-seeded melt, the cross-nucleation efficacy is reduced as, beside growth of cross-nucleated β′-crystals, also growth of the mother phase is observed. This unexpected result demonstrates the importance of the structure of the nucleating substrate and the interplay between kinetic and thermodynamic aspects of crystal growth
Melt Crystallization of Poly(butylene 2,6-naphthalate)
No full textPoly(butylene 2,6-naphthalate) (PBN) is a crystallizable linear polyester containing a rigid naphthalene unit and flexible methylene spacer in the chemical repeat unit. Polymeric materials made of PBN exhibit excellent anti-abrasion and low friction properties, superior chemical resistance, and outstanding gas barrier characteristics. Many of the properties rely on the presence of crystals and the formation of a semicrystalline morphology. To develop specific crystal structures and morphologies during cooling the melt, precise information about the melt-crystallization process is required. This review article summarizes the current knowledge about the temperature-controlled crystal polymorphism of PBN. At rather low supercooling of the melt, with decreasing crystallization temperature, β’- and α-crystals grow directly from the melt and organize in largely different spherulitic superstructures. Formation of α-crystals at high supercooling may also proceed via intermediate formation of a transient monotropic liquid crystalline structure, then yielding a non-spherulitic semicrystalline morphology. Crystallization of PBN is rather fast since its suppression requires cooling the melt at a rate higher than 6000 K·s−1. For this reason, investigation of the two-step crystallization process at low temperatures requires application of sophisticated experimental tools. These include temperature-resolved X-ray scattering techniques using fast detectors and synchrotron-based X-rays and fast scanning chip calorimetry. Fast scanning chip calorimetry allows freezing the transient liquid-crystalline structure before its conversion into α-crystals, by fast cooling to below its glass transition temperature. Subsequent analysis using polarized-light optical microscopy reveals its texture and X-ray scattering confirms the smectic arrangement of the mesogens. The combination of a large variety of experimental techniques allows obtaining a complete picture about crystallization of PBN in the entire range of melt-supercoolings down to the glass transition, including quantitative data about the crystallization kinetics, semicrystalline morphologies at the micrometer length scale, as well as nanoscale X-ray structure information
Stability of crystal nuclei of poly (butylene isophthalate) formed near the glass transition temperature
Full text linkTammann's two-stage crystal-nuclei-development method is applied for analysis of the thermal stability of homogenously formed crystal nuclei of poly(butylene isophthalate) (PBI) as well as their possible reorganization on transferring them to the growth temperature, using fast scanning chip calorimetry. Crystal nuclei were formed at 50 °C, that is, at a temperature only slightly higher than the glass transition temperature, and developed to crystals within a pre-defined time at the growth temperature of 85 °C. The number of nuclei, overcritical at the growth temperature, was detected as a function of the transfer-conditions (maximum temperature, heating rate) by evaluation of the developed crystal fraction. For different size-distributions of crystal nuclei, as controlled by the nucleation time, there is detected distinct reduction of the nuclei number on heating to maximum temperatures higher than about 90 to 110 °C, with the latter value holding for longer nucleation time. Longer nucleation allows for both increasing the absolute nuclei number and generation of an increased fraction of larger nuclei. Heating at 1000 K/s to 140-150 °C causes "melting" of even the most stable nuclei. While direct transfer of crystal nuclei from the nucleation temperature (50 °C) to the growth temperature (85 °C) reveals negligible effect of the transfer-heating rate, in-between heating to higher temperatures is connected with distinct nuclei-reorganization above 85 °C on heating slower than 1000-10.000 K/s. The performed study not only provides specific valuable information about the thermal characteristics of crystal nuclei of PBI but also highlights the importance of proper design of Tammann's nuclei development experiment for analysis of nuclei numbers. With the evaluation of critical rates of temperature-change for suppression of non-isothermal formation of both nuclei and crystals, the kinetics of crystallization of the slow crystallizing PBI is further quantified
Mesophase formation in poly(propylene-ran-1-butene) by rapid cooling
No full textThe effect of random insertion of low amount of 1-butene of less than about 11 mol% into the isotactic polypropylene chain on structure formation at non-isothermal crystallization at different rate of cooling
was investigated by X-ray scattering, density measurements, and atomic force and polarizing optical microscopy. Emphasis is put on the evaluation of the condition of crystallization for replacement of lamellar crystals by mesomorphic nodules on increasing the cooling rate/supercooling. In the polypropylene homopolymer, mesophase formation occurs on cooling at rates larger about 150–200 K s 1, while in case of poly(propylene-ran-1-butene) mesophase formation is observed on cooling at a lower
rate of about 100 K s 1. It is suggested that the lowering of the critical rate of cooling for mesophase formation in poly(propylene-ran-1-butene) is due to a reduction of the maximum rate of formation of
monoclinic/orthorhombic crystals at low supercooling, compared to the homopolymer. The data of the present study allowed the establishment of a non-equilibrium phase diagram which shows ranges of existence of phases as a function of the cooling rate on solidification the quiescent liquid and the concentration on 1-butene co-units
Enthalpy relaxation, crystal nucleation and crystal growth of biobased poly(butylene isophthalate)
Full text linkThe crystallization behavior of fully biobased poly(butylene isophthalate) (PBI) has been investigated using calorimetric and microscopic techniques. PBI is an extremely slow crystallizing polymer that leads, after melt-crystallization, to the formation of lamellar crystals and rather large spherulites, due to the low nuclei density. Based upon quantitative analysis of the crystal-nucleation behavior at low temperatures near the glass transition, using Tammann's two-stage nuclei development method, a nucleation pathway for an acceleration of the crystallization process and for tailoring the semicrystalline morphology is provided. Low-temperature annealing close to the glass transition temperature (Tg) leads to the formation of crystal nuclei, which grow to crystals at higher temperatures, and yield a much finer spherulitic superstructure, as obtained after direct melt-crystallization. Similarly to other slowly crystallizing polymers like poly(ethylene terephthalate) or poly(l-lactic acid), low-temperature crystal-nuclei formation at a timescale of hours/days is still too slow to allow non-spherulitic crystallization. The interplay between glass relaxation and crystal nucleation at temperatures slightly below Tg is discussed
Structure formation of random isotactic copolymers of propylene with 1-hexene and 1-octene at rapid cooling
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
In situ X-ray analysis of mesophase formation in random copolymers of propylene and 1-butene
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