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

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

Effect of molar mass on enthalpy relaxation and crystal nucleation of poly (L-lactic acid)

© 2017 Elsevier Ltd The effect of the chain length/molar mass of poly (L-lactic acid) (PLLA) on the formation of homogeneous crystal nuclei below and above the glass transition temperature (T g ) has been analyzed by fast scanning chip calorimetry, employing Tammann's nuclei development method. It has been found that the kinetics of crystal nucleation below T g is independent on the chain length within the investigated range of molar masses from about 60 to 600 kDa, similar as the kinetics of enthalpy relaxation. In contrast, at temperatures well above T g there is detected that crystal nucleation is slower in samples of higher molar mass. This observation is confirmed by non-isothermal nucleation experiments which showed that suppression of nucleation occurs on cooling at rates of 200, 100, and 30 K/s in samples with molar masses of around 60, 100, and 600 kDa, respectively. The different effect of molar mass on crystal nucleation in the glass and well above T g is discussed in terms of the length-scale of required chain-mobility and of the critical size of nuclei

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

© 2018 Elsevier B.V. Melt-crystallization of poly (butylene 2,6-naphthalate) (PBN) at temperatures lower than about 160 °C 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 °C. 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

Flame retarding polyamide 11 with exfoliated vermiculite nanoflakes

Polyamide 11‐based bionanocomposites were prepared by melt compounding with 10 wt% clays of different chemistry and morphology. This included vermiculite nanoflakes obtained by consecutive thermal and ultrasonic exfoliation in both neat and organo‐modified form. The mechanical reinforcement‐ and flame‐retardant performance of the vermiculite clays were compared to organo‐modified montmorillonite (Cloisite 30B) and needle‐shaped sepiolite (Pangel S9). Electron microscope investigations revealed different structures and dispersion levels of the clay nanoparticles in the polymer matrix. Tensile tests showed that the addition of clays led to considerable improvements in Young's modulus without compromising the elongation at break. Compared to the neat polymer, all clays reduced the peak heat release rate and the smoke production rate in cone calorimeter testing. Surprisingly, the needle‐shaped sepiolite clay and the two vermiculites outperformed the montmorillonite organoclay in the fire testing even though it featured the highest degree of exfoliation in the polymer matrix.The Deutsche Forschungsgemeinschaft (DFG) through Grant AN 212/18-1 and from the National Research Foundation (NRF) via the South African/Algeria research partnership program under Grant 87453.https://onlinelibrary.wiley.com/journal/154826342019-10-01hj2019Chemical Engineerin

Enthalpy of formation and disordering temperature of transient monotropic liquid crystals of poly(butylene 2,6-naphthalate)

© 2018 Elsevier Ltd Melt-crystallization of poly(butylene 2,6-naphthalate) (PBN) at temperatures lower than about 160 °C follows Ostwald's rule of stages via intermediate formation of a smectic liquid crystalline phase (LC-phase). The transient LC-phase has been isolated by interruption of the isothermal crystallization process at 140 °C at sub-second timescale, and then its disordering was analyzed on heating at a rate of 2000 K/s, which suppresses the transition into α-crystals. The disordering temperature of the LC-mesophase is slightly lower than 200 °C, and as such 20–30 K lower than the melting temperature of α-crystals formed from the LC-phase at 140 °C. Analysis of the bulk enthalpy of formation of the LC-phase revealed that it covers only 20–25% of the total bulk enthalpy of crystallization, which is considered further proof of its smectic nature

Nanometer scale thermal response of polymers to fast thermal perturbations

© 2018 Author(s). Nanometer scale thermal response of polymers to fast thermal perturbations is described by linear integro-differential equations with dynamic heat capacity. The exact analytical solution for the non-equilibrium thermal response of polymers in plane and spherical geometry is obtained in the absence of numerical (finite element) calculations. The solution is different from the iterative method presented in a previous publication. The solution provides analytical relationships for fast thermal response of polymers even at the limit t → 0, when the application of the iterative process is very problematic. However, both methods give the same result. It was found that even fast (ca. 1 ns) components of dynamic heat capacity greatly enhance the thermal response to local thermal perturbations. Non-equilibrium and non-linear thermal response of typical polymers under pulse heating with relaxation parameters corresponding to polystyrene and poly(methyl methacrylate) is determined. The obtained results can be used to analyze the heat transfer process at the early stages of crystallization with fast formation of nanometer scale crystals

Nucleation and crystallization in bio-based immiscible polyester blends

Bio-based thermoplastic polyesters are highly promising materials as they combine interesting thermal and physical properties and in many cases biodegradability. However, sometimes the best property balance can only be achieved by blending in order to improve barrier properties, biodegradability or mechanical properties. Nucleation, crystallization and morphology are key factors that can dominate all these properties in crystallizable biobased polyesters. Therefore, their understanding, prediction and tailoring is essential. In this work, after a brief introduction about immiscible polymer blends, we summarize the crystallization behavior of the most important bio-based (and immiscible) polyester blends, considering examples of double-crystalline components. Even though in some specific blends (e.g., polylactide/polycaprolactone) many efforts have been made to understand the influence of blending on the nucleation, crystallization and morphology of the parent components, there are still many points that have yet to be understood. In the case of other immiscible polyester blends systems, the literature is scarce, opening up opportunities in this environmentally important research topic.The authors would like to acknowledge funding by the BIODEST project ((RISE) H2020-MSCA-RISE-2017-778092