Correlation between Grafting Density and Confined Crystallization Behavior of Poly(ethylene glycol) Grafted to Silica

The interfacial interactions of polymer-nanoparticles have dramatical effects on the crystallization behavior of grafted polymers. In this study, methoxy polyethylene glycol (MPEG) (molecular weights 750, 2000 and 4000 g mol−1) was grafted onto amino-modified nanosized silica (SiO2-NH2) by the “grafting to” method. The effects of the grafting density and molecular weight on the confined crystallization of grafted MPEG (MPEG-g-SiO2) were systematically investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and wide-angle X-ray scattering (WAXS). It was found that confinement effects are stronger when lower molecular weights of grafted MPEG are employed. These grafted MPEG chains are more difficult to stretch out on SiO2-NH2 surfaces than when they are free in the bulk polymer. Both crystallization temperature (Tc) and crystallinity of grafted MPEG chains decrease with reductions of grafting density. Additionally, covalent bonding effects and interfacial interaction confinement effects are strengthened by the decrease in grafting density, leading to an increase in decomposition temperature and to the disappearance of the self-nucleation Domain (i.e., Domain II), when self-nucleation experiments are performed by DSC. Overall isothermal crystallization kinetics was studied by DSC and the results were analyzed with the Avrami equation. An Avrami index of n≈3 was obtained for neat MPEG (indicating that instantaneous spherulites are formed). However, in the case of MPEG-g-SiO2 with the lowest grafting density, the Avrami index of (n) was less than 1 (first order kinetics or lower), indicating that nucleation is the determining factor of the overall crystallization kinetics, a signature for confined crystallization. At the same time, the crystallization from the melt for this MPEG-g-SiO2 with the lowest grafting density occurs at Tc ≈-30 ºC, a temperature close to the glass transition temperature (Tg) of MPEG, indicating that this confined MPEG crystallizes from homogeneous nuclei.This project was supported by the National Natural Science Foundation of China (21574141) and the Ministry of Science and Technology of China (2017YFE0117800). The authors gratefully acknowledge the funding of project BIODEST, Research and Innovation Staff Exchange (RISE) H2020-MSCA-RISE-2017-778092. The authors thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time and helps during experiments

Isothermal Crystallization Kinetics of Poly(ethylene oxide)/Poly(ethylene glycol)-<i>g</i>-silica Nanocomposites

In this work, the crystallization kinetics of poly(ethylene oxide) (PEO) matrix included with poly(ethylene glycol) (PEG) grafted silica (PEG-g-SiO2) nanoparticles and bare SiO2 were systematically investigated by differential scanning calorimetry (DSC) and polarized light optical microscopy (PLOM) method. PEG-g-SiO2 can significantly increase the crystallinity and crystallization temperature of PEO matrix under the non-isothermal crystallization process. Pronounced effects of PEG-g-SiO2 on the crystalline morphology and crystallization rate of PEO were further characterized by employing spherulitic morphological observation and isothermal crystallization kinetics analysis. In contrast to the bare SiO2, PEG-g-SiO2 can be well dispersed in PEO matrix at low P/N (P: Molecular weight of matrix chains, N: Molecular weight of grafted chains), which is a key factor to enhance the primary nucleation rate. In particular, we found that the addition of PEG-g-SiO2 slows the spherulitic growth fronts compared to the neat PEO. It is speculated that the interfacial structure of the grafted PEG plays a key role in the formation of nuclei sites, thus ultimately determines the crystallization behavior of PEO PNCs and enhances the overall crystallization rate of the PEO nanocomposites

An effective Pd-promoted gold catalyst supported on mesoporous silica particles for the oxidation of benzyl alcohol

A bimetallic Pd-promoted gold catalyst with mesoporous silica nanoparticles (MSNs) as support, PdAu/MSN, was prepared by an impregnation-hydrogen reduction method, and its catalysis for the base-free oxidation of benzyl alcohol was investigated. It was found that adding a small amount of Pd, with a Pd/Au atomic ratio as low as 0.05/1, can significantly decrease the size of the gold particles and thereby remarkably enhance the catalyst's activity for aerobatic oxidation. At the optimal Pd/Au atomic ratio of 0.2/1, the catalyst Pd0.2Au/MSN showed 8 times and 3 times higher activity than the monometallic catalysts Au/MSN and Pd/MSN, respectively. The prepared catalysts were comprehensively characterized by XRD, DRUV-vis, TEM, XPS, and H-2-TPR to correlate the enhanced activity with the promotional effect induced by adding Pd. (C) 2013 Elsevier B.V. All rights reserved

Catanionic Surfactant-Assisted Mineralization and Structural Properties of Single-Crystal-like Vaterite Hexagonal Bifrustums

Crystalline vaterite is the most thermodynamically unstable polymorph of anhydrous calcium carbonate (CaCO₃), and various morphologies can be controlled in the presence of organic additives. Constructing vaterite with minimal defects, determining its distinctive properties, and understanding the formation mechanism behind a biomimetic process are the main challenges in this field. In this paper, a unique single-crystal-like vaterite hexagonal bifrustum with two hexagonal and 12 trapezoidal faces has been fabricated through a catanionic surfactant-assisted mineralization approach for the first time. Compared with the polycrystalline vaterite aggregates, these bifrustums clearly present a doublet for Raman <i>v</i>₁ symmetric stretching mode, a low depolarizaiton ratio for carbonate molecular symmetry, and a high structural stability. These indicate a dominant position of hexagonal phase in each crystallite and confirm the Raman <i>v</i>₁ doublet characteristics of synthetic and biomineral-based vaterites. Our finding may provide evidence to distinguish vaterite with different structures and shed light on a possible formation mechanism of vaterite single crystals

Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State

The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an <i>n-</i>octadecane C₁₈/SiO₂-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state ¹³C nuclear magnetic resonance (VT solid-state ¹³C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C₁₈ molecules involve two phase transitions unknown from the bulk C₁₈. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interface-frozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles’ surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the first-order phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C₁₈

Confined Crystallization of <i>n</i>‑Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study

Crystallization and phase transition behaviors of <i>n</i>-hexadecane (<i>n</i>-C₁₆H₃₄, abbreviated as C₁₆) confined in microcapsules and <i>n</i>-alkane/SiO₂ nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C₁₆ is enhanced in both the microcapsules and SiO₂ nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid–solid phase transition is observed only in the microencapsulated C₁₆, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C₁₆/SiO₂ composite, however, no novel rotator phase emerges during the cooling process, and C₁₆ crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C₁₆/SiO₂ composite, leading to phase transition behaviors similar to those of bulk C₁₆

Direct Relationship between Dispersion and Crystallization Behavior in Poly(ethylene oxide)/Poly(ethylene glycol)-g-Silica Nanocomposites

Unformatted post-print version of the accepted articleThe inclusion of polymer-grafted nanoparticles (PGNPs) can impart various functional properties to polymer nanocomposites (PNCs). For semicrystalline polymers, we can control the spatial dispersion of PGNPs and presumably use it to modulate the nucleation rate of the polymer. In this work, the correlation between the dispersion quality of poly(ethylene glycol) (PEG) grafted silica (PEG-g-SiO2) nanoparticles and the crystallization ability of poly(ethylene oxide) (PEO) nanocomposites is systematically investigated by varying the grafting density (σ, chains/nm2) and the value of P/N (P: molecular weight of matrix chains, N: molecular weight of grafted chains). The variation of PEG-g-SiO2 dispersion state was studied by morphological characterization and small-angle X-ray scattering (SAXS). It was found that, in contrast to the unmodified SiO2 and poly(methyl methacrylate) grafted silica (PMMA-g-SiO2), PEG-g-SiO2 (high σ and low P/N) can increase the nucleation rate of PEO even under conditions where they are well dispersed in the PEO matrix. Evidently, the nature of the graft, i.e., amorphous PMMA vs. crystallizable PEO, has profound consequences in this context, a novel result that has not been anticipated based on previous work. NP aggregation occurs at higher P/N values and limits the effectiveness of the grafted PEG on the crystallization ability of PEO nanocomposites. Based on differential scanning calorimetry (DSC) and polarized light optical microscopy (PLOM) characterization, we deduced that the increased nucleation density at high σ and low P/N has a strong impact on accelerating the overall crystallization of PEO nanocomposites.This project is supported by the Ministry of Science and Technology of China (2017YFE0117800) and the National Natural Science Foundation of China (21574141). We would like to thank the financial support provided by the BIODEST project; this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778092. The authors thank Prof. Yongfen Men (CIAC) for providing the SAXS beam time and helps during experiments. The discussion of TEM data with Andrew M. Jimenez (Columbia University, New York, NY) is gratefully acknowledged. We also thank B. Guan, J. L. Yue and K. A. Liu for help in cryo-TEM experiments