Poly (butylene succinate): low-temperature nucleation and crystallization, complex morphology and absence of lamellar thickening

Melt-crystallization of poly (butylene succinate) (PBS) at largely different melt-supercooling was analyzed by X-ray scattering techniques, direct imaging using microscopy, and by fast scanning chip calorimetry, with the latter employed to achieve high melt-supercooling. Crystallization at 100 and 20 °C, representing structure formation at low and high melt-supercooling, respectively, yields lamellae with thicknesses of around 8 and 4 nm. In both cases, lamellar thickening is excluded as main mechanism of isothermal secondary crystallization, suggesting that absence of intracrystalline chain-sliding diffusion is not dependent on the absolute value of the thickness of lamellae. The wide-angle X-ray pattern of PBS crystallized at low temperature shows less and broader peaks than in case of PBS crystallized at high temperature, pointing to presence of crystal defects, and causing an enlargement of the unit cell. Secondary crystallization leads to an increase of the melting temperature, being distinctly larger (per unit annealing time) for low-temperature crystallized PBS. Since lamellar thickening is absent, and imperfection of the bulk crystal structure persists even on long-term annealing, merging of crystalline blocks and lateral crystal growth at the crystallization temperature, for explaining the observed increases of the crystallinity and melting temperature, are suggested. Though there is detected a large rigid amorphous fraction in PBS crystallized at high supercooling, vitrification only occurs continuously on cooling, after the isothermal crystallization process

Pressure- and Temperature-Dependent Crystallization Kinetics of Isotactic Polypropylene under Process Relevant Conditions

In this study, a non-nucleated homopolymer (HP) and random copolymer (RACO), as well as a nucleated HP and heterophasic copolymer (HECO) were investigated regarding their crystallization kinetics. Using pvT-measurements and fast scanning chip calorimetry (FSC), the crystallization behavior was analyzed as a function of pressure, cooling rate and temperature. It is shown that pressure and cooling rate have an opposite influence on the crystallization temperature of the materials. Furthermore, the addition of nucleating agents to the material has a significant effect on the maximum cooling rate at which the formation of α-crystals is still possible. The non-nucleated HP and RACO materials show significant differences that can be related to the sterically hindering effect of the comonomer units of RACO on crystallization, while the nucleated materials HP and HECO show similar crystallization kinetics despite their different structures. The pressure-dependent shift factor of the crystallization temperature is independent of the material. The results contribute to the description of the relationship between the crystallization kinetics of the material and the process parameters influencing the injection-molding induced morphology. This is required to realize process control in injection molding in order to produce pre-defined morphologies and to design material properties

Process induced skin-core morphology in injection molded polyamide 66

Polyamide 66 (PA 66) was injection-molded to obtain samples with a structure gradient between skin and core, as it was revealed by analysis of the semi-crystalline morphology using polarized-light optical microscopy (POM). Wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) were employed to characterize thin sections with a thickness in the order of magnitude of 50 µm, allowing detection of crystals of different perfection, as a function of the distance from the surface. It was found that the transparent and non-spherulitic skin layer contains rather imperfect α-crystals while the perfection of α-crystals continuously increases with extending distance from the surface. Since variation of the molding conditions allows tailoring the skin-core morphology, the present study was performed to suggest a reliable route to map the presence of specific semi-crystalline morphologies in such sample

Cold-crystallization of poly(butylene 2,6-naphthalate) following Ostwald's rule of stages

Melt-crystallization of poly (butylene 2,6-naphthalate) (PBN) at temperatures lower than about 160 \ub0C follows Ostwald's rule of stages, leading first to formation of a transient smectic liquid crystalline phase (LC) which then may convert in a second step into crystals, controlled by kinetics. In the present work, the PBN melt was cooled at different rates in a fast scanning chip calorimeter to below the glass transition temperature, to obtain different structural states before analysis of the cold-crystallization behavior on heating. It was found that heating of fully amorphous PBN at 1000 K/s leads to a similar two-step crystallization process as on cooling the quiescent melt, with LC-formation occurring slightly above Tg and their transformation into crystals at their stability limit close to 200 \ub0C. In-situ polarized-light optical microscopy provided information that the transition of the LC-phase into crystals on slow heating is not connected with a change of the micrometer-scale superstructure, as the recently found Schlieren texture remains unchanged

Enthalpy relaxation, crystal nucleation and crystal growth of biobased poly(butylene isophthalate)

The 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

The effect of the skin-core structure of injection-molded isotactic polypropylene on the stress distribution in bending tests

We examine the effect of the skin-core structure of isotactic polypropylene (iPP) in bending tests. The depth-dependent material properties are determined in tensile tests and mapped to a finite element model. This enables the examination of internal stresses during bending numerically. In a bending test, one usually expects a monotonic stress distribution across the thickness, provided that the material is homogeneous and does not strain-soften. We found that the structural gradient of injection-molded iPP easily overcompensates the monotonic stress dependence, such that the maximal equivalent von Mises stress lies well below the surface in the so called shear layer. The latter is a result of the injection molding process

Homogeneous crystal nucleation in polymers

© 2017 IOP Publishing Ltd. The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 10 6 K s -1 , allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation in polymers is discussed in the framework of the classical nucleation theory. The majority of our observations are consistent with the theory. The discrepancies may guide further research, particularly experiments to progress theoretical development. Progress in the understanding of homogeneous nucleation is much needed, since most of the modelling approaches dealing with polymer crystallization exclusively consider homogeneous nucleation. This is also the basis for advancing theoretical approaches to the much more complex phenomena governing heterogeneous nucleation

Structure, Properties, and Release Kinetics of the Polymer/Insect Repellent System Poly (l-Lactic Acid)/Ethyl Butylacetylaminopropionate (PLLA/IR3535)

The insect repellent ethyl butylacetylaminopropionate (IR3535) was used as a functional additive for poly (l-lactic acid) (PLLA) to modify its structure and mechanical properties and achieve insect repellency. PLLA/IR3535 mixtures at various compositions were prepared via melt extrusion. In the analyzed composition range of 0 to 23 m% IR3535, PLLA and IR3535 were miscible at the length scale represented by the glass transition temperature. Addition of IR3535 resulted in a significant decrease in the glass transition temperature of PLLA, as well as in the elastic modulus, indicating its efficiency as a plasticizer. All mixtures were amorphous after extrusion, though PLLA/IR3535 extrudates with an IR3535 content between 18 and 23 m% crystallized during long-term storage at ambient temperature, due to their low glass transition temperature. Quantification of the release of IR3535 into the environment by thermogravimetric analysis at different temperatures between 50 and 100 °C allowed the estimation of the evaporation rate at lower temperatures, suggesting an extremely low release rate with a time constant of the order of magnitude of 1–2 years at body temperature

Measuring National Innovation Performance: The Case of Austria

In diesem Kapitel wird der Rahmen skizziert, den der Rat für Forschung und Technologieentwicklung - das zentrale Beratungsgremium der österreichischen Bundesregierung für bildungs-, wissenschafts-, forschungs- und innovationspolitische Angelegenheiten - zur Messung und Bewertung der Leistungsfähigkeit des nationalen Innovationssystems Österreichs im internationalen Vergleich anwendet. Ziel des Kapitels ist es, einen Überblick über die Entwicklung des Rahmens in Zusammenarbeit mit dem Österreichischen Institut für Wirtschaftsforschung und in Abstimmung mit den zuständigen Ministerien zu geben. Weiters soll die Anwendung des Rahmens im Rahmen der jährlichen Gutachten des Rates zur wissenschaftlich-technologischen Leistungsfähigkeit Österreichs beschrieben werden