Detection and characterization of folded-chain clusters in the structured melt of isotactic polypropylene

Despite ceaseless efforts in past decades, the memory effect of semi-crystalline polymers has not been elucidated completely yet. An important reason why is that residual lamellar crystals in the structured melt are difficult to characterize. Recently, we developed a new small-angle X-ray scattering (SAXS) theory [Li et al. (2019). IUCrJ, 6, 968–983] and Fourier transform method [Li et al. (2020). CrystEngComm, 22, 3042–3058] for lamellar crystals that could derive structural information from SAXS readily. In this study, we tried to employ the new theory and method to characterize residual lamellar crystals in the structured melt. It was found that although scattering peaks cannot be observed in raw scattering profiles, they actually exist. Subtracting free-melt scattering and multiplying by q 4 benefit the observation of these weak scattering peaks. With the new Fourier transform method, it was found that indeed as proposed previously, thicker lamellar crystals exist in the structured melt. To determine the lateral size of residual lamellar crystal especially, a new method was developed under the guidance of the new theory. With the new method, it was found that although the crystallinity is very low (∼1% at 174°C), the lateral sizes in the structured melts are still large, e.g. 45.3 nm at 174°C, much greater than the critical nucleation size. This implies that these residual lamellar crystals can act as athermal nuclei after quenching to a lower temperature, as proposed by Ziabicki & Alfonso [(1994). Colloid Polym. Sci. 272, 1027–1042; (2002). Macromol. Symp. 185, 211–231] more than 20 years ago. The methodologies proposed here could also be applied to other polymer lamellar systems.

Comment on “Crystallization of Supercooled Liquids: Self-Consistency Correction of the Steady-State Nucleation Rate” by Abyzov et al., Entropy 2020, 22, 558

It is shown that in the growth region (above the critical nucleation size) the transient distributions obtained numerically from the Becker-Döring equation (BDE) by Abyzov et al., Entropy 2020, 22, 558, are in accurate correspondence with the matched asymptotic (singular perturbation) solution by Shneidman, Sov. Phys. Tech. Phys. 1988, 33, 1338. The solution is unmodified by “self-consistency” corrections which affect only the steady state rate. Sensitivity of the results to selection of a specific form of the BDE (the “nucleation model”) also is briefly discussed.

Swi5–Sfr1 stimulates Rad51 recombinase filament assembly by modulating Rad51 dissociation

Eukaryotic Rad51 protein is essential for homologous-recombination repair of DNA double-strand breaks. Rad51 recombinases first assemble onto single-stranded DNA to form a nucleoprotein filament, required for function in homology pairing and strand exchange. This filament assembly is the first regulation step in homologous recombination. Rad51 nucleation is kinetically slow, and several accessory factors have been identified to regulate this step. Swi5–Sfr1 (S5S1) stimulates Rad51-mediated homologous recombination by stabilizing Rad51 nucleoprotein filaments, but the mechanism of stabilization is unclear. We used single-molecule tethered particle motion experiments to show that mouse S5S1 (mS5S1) efficiently stimulates mouse RAD51 (mRAD51) nucleus formation and inhibits mRAD51 dissociation from filaments. We also used single-molecule fluorescence resonance energy transfer experiments to show that mS5S1 promotes stable nucleus formation by specifically preventing mRAD51 dissociation. This leads to a reduction of nucleation size from three mRAD51 to two mRAD51 molecules in the presence of mS5S1. Compared with mRAD51, fission yeast Rad51 (SpRad51) exhibits fast nucleation but quickly dissociates from the filament. SpS5S1 specifically reduces SpRad51 disassembly to maintain a stable filament. These results clearly demonstrate the conserved function of S5S1 by primarily stabilizing Rad51 on DNA, allowing both the formation of the stable nucleus and the maintenance of filament length.

Off-Lattice KMC Simulation of the Growth Process of Ti-Si-N Film

In order to investigate the progress of the composite films growth, and the influence of the process parameters to the film‘s structure, this paper uses the off-lattice KMC method to research the Ti-Si-N nanocomposite film, and calculate the influence of deposition temperatures, deposition rate and Si content to the film’s surface roughness, nucleation size and density of initial stage. The results show that with deposition temperature and deposition rate ascend, it can increase transition event, decrease the film’s surface roughness. Si content works on nucleation in initial stage, and the more Si contents, the larger of nucleation rate and the smaller of the grain crystal.

Aerosol particle number size distributions and particulate light absorption at the ZOTTO tall tower (Siberia), 2006–2009

This paper covers measurements of the number-size distribution of aerosol particles, particulate absorption at 570 nm wavelength and CO as tracer gas from 2006 through 2009 at 50 and 300 m on the ZOTTO tower, Siberia at 60.8° N; 89.35° E. Average number, surface and volume concentrations are similar to results given for continental and boreal background locations. When fitted with lognormal functions, the probability distribution function of modal diameters shows three main maxima in the Aitken and accumulation size range and a possible secondary maximum in the nucleation size range below 25 nm. The seasonal distributions of the different particle parameters differ substantially. Particulate absorption has a clear single maximum in high winter and minimum values in mid-summer. The 90%-percentile, however, indicates a possible secondary maximum in July/August that may be related to forest fires. The strongly combustion derived CO shows a single winter maximum and a late summer minimum, albeit with a considerable smaller seasonal swing than the particle data due to its longer atmospheric lifetime. Total volume and even more so total number show a more complex seasonal variation with maxima in winter, spring, and summer. A cluster analysis of back trajectories complemented by local stability information yielded ten clusters with three levels of particle concentration: Low concentrations for the northernmost (Arctic) clusters mid-level concentrations for clusters reaching rapidly west between 55° and 65° latitude, and high concentrations for the cluster reaching southwest via Kazakhstan to the central Russian industrial region.

Asymmetry of the latent heat signature in b-axis oriented single crystal Gd5Si2Ge2

ABSTRACTA 100 micron fragment of a b-axis oriented single crystal Gd5Si2Ge2 has been studied using microcalorimetry, enabling the separate measurement of the heat capacity and the latent heat. The sample was taken from the same crystal previously studied with Hall probe imaging, which showed that the phase transition is seeded by a second phase of Gd5Si1.5Ge1.5 nanoplatelets on the increasing field sweep direction only. The multiple transition features observed in the latent heat signature suggests a nucleation size of approximately 20 μm, consistent with the lengthscale suggested by Hall imaging. The difference in nucleation and growth process with field sweep direction is clearly identified in the latent heat. We show that the latent heat contribution to the entropy change is of the order of 50% of the total entropy change and unlike other systems studied, the transition does not broaden (and the latent heat contribution does not diminish significantly) as magnetic field and temperature are increased within the parameter range explored in these experiments.