Confined Crystallization of Polymers within Nanopores

Unformatted post-print version of the accepted articleCrystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called “fractionated crystallization”, i.e., multiple crystallization peaks during cooling. As the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material. Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the “chain perpendicular to” to “chain parallel to” AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth, nucleation and crystal growth rate. An orientation diagram has been established for PEO/AAO system, considering the cooling condition and pore size. The interfacial polymer layer has different physical properties as compared to the bulk. In poly(L-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.This work was supported by the National Key R&D Program of China (Grant No. 2017YFE0117800) and the National Natural Science Foundation of China (Grant Nos. 21873109, 51820105005, and 21922308). We also acknowledge the financial support from the BIODEST project; this project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 778092. A.J.M. acknowledges funding from MINECO, Grant No. MAT2017-83014-C2-1-P, and from the Basque Government through Grant No. IT1309-19. G.L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. Y201908)

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

Recent applications of the Successive Self-nucleation and Annealing thermal fractionation technique

Successive Self-nucleation and Annealing (SSA) is a thermal fractionation technique that is performed by Differential Scanning Calorimetry (DSC). The combination of non-isothermal and isothermal steps applied during SSA achieves efficient molecular segregation during polymer crystallization. Such molecular segregation magnifies the effect of defects in polymer chain crystallization, thereby providing information on chain structure. The technique was created and implemented by Müller and co-workers in 1997, becoming a powerful resource for studying ethylene/α-olefin copolymers. The different variables to design the SSA protocol: fractionation window, fractionation time, scanning rate, sample mass, and the first self-nucleation temperature to be applied (Ts, ideal), have been previously reviewed, together with the different applications of SSA. SSA versatility, simplicity (when properly applied), and short times to produce results have allowed its use to study novel and more complex polymeric systems. This review article explores the most recent applications of SSA of the past decade. First, the principles of the technique are briefly explained, covering all the relevant variables. Next, we have selected different cases that show how SSA is employed in various novel fields, such as studying intermolecular interactions and topological effects in homopolymers; supernucleation and antinucleation effects in nanocomposites, including the pre-freezing phenomenon; crystallization modes in random copolymers; solid-solid transitions; miscibility, co-crystallization and composition in blends; evaluation of polymer synthesis variables; and the novel information that could be gained by using fast scanning chip-based calorimetry. Finally, we offer a perspective on SSA, a technique that has become a powerful method for studying the distribution of defects affecting crystallization in semi-crystalline polymers.This work has received funding from the Basque Government through grant IT1503-22 and from MICINN (PID2020-113045GB-C21). We would also like to acknowledge the financial support from the BIODEST and the REPOL projects; these projects have received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreements No. 778092 and No. 860221. It has also been supported by the National Natural Science Foundation of China (51820105005, 52050410327)

Uniaxial and Mixed Orientations of Poly(ethylene oxide) in Nanoporous Alumina Studied by X-ray Pole Figure Analysis

The orientation of polymers under confinement is a basic, yet not fully understood phenomenon. In this work, the texture of poly(ethylene oxide) (PEO) infiltrated in nanoporous anodic alumina oxide (AAO) templates was investigated by X-ray pole figures. The influence of geometry and crystallization conditions, such as pore diameter, aspect ratio, and cooling rates, was systematically examined. All the samples exhibited a single, volume-dependent crystallization temperature (Tc) at temperatures much lower than that exhibited by bulk PEO, indicating “clean” microdomains without detectable heterogeneous nucleation. An “orientation diagram” was established to account for the experimental observations. Under very high cooling rates (quenching), crystallization of PEO within AAO was nucleation-controlled, adopting a random distribution of crystallites. Under low cooling rates, growth kinetics played a decisive role on the crystal orientation. A relatively faster cooling rate (10 °C/min) and/or smaller pores lead to the * ║ pore axis (n⃗) mode (uniaxial orientation). When the cooling rate was lower (1 °C/min), and/or the pores were larger, a mixed orientation, with a coexistence of * ║ n⃗ and * ║ n⃗ , was observed. The results favor the kinetic model where the fastest growth direction tends to align parallel to the pore axis.This work is supported by the National Natural Science Foundation of China (NSFC, 21873109, 51820105005, 21274156). G. L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015026). G. L., D. W., and A. J. M. also acknowledge European funding by the RISE BIODEST project (H2020-MSCA-RISE-2017-778092). The authors thank Dr. Zhongkai Yang for assistance with pole figure measurement

Temperature modulated DSC for composition analysis of recycled polyolefin blends

Post-consumer plastic waste contains blends of numerous types of polyethylene (PE) and isotactic polypropylene (i-PP), whose recycling is challenging due to the complexity of this waste stream. A comprehensive knowledge of the composition of these recyclates is essential to understand the structure-property relationship of these systems and therefore upcycle them for high-value applications. To this aim, we used Temperature Modulated DSC (TMDSC) to develop a quantitative method to evaluate PE and Low-density PE (LDPE) content in recycled polyolefin blends. TM-DSC was carried out on 29 virgin PE materials, spanning densities between 960 and 862 kg/m3, characterizing a wide range of PE microstructures. Moreover, several PE/i-PP model blends were prepared by selecting LDPE, High-density PE (HDPE) and Linear Low-density PE (LLDPE) materials to blend them with i-PP of three types: homopolymer (PP-H), block copolymer (PP-B) and random copolymer (PP-R), mimicking the composition of real recyclates. Results from the TM-DSC analysis of these blends allowed us to establish methods for quantifying the amount of overall PE content and also the LDPE fraction within recyclates. The developed methods were applied to real post-consumer recycled grades, and results were compared with the ones obtained from Cross-Fractionation Chromatography (CFC) analysis and Nuclear Magnetic Resonance (NMR) spectroscopy, displaying good agreement between the latter and the TM-DSC method.We acknowledge the financial support from the REPOL project; this project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860221. The authors wish to thank Tamara Carmeli and all the Thermal Analysis group for the help with the TM-DSC measurements, Karin Kemper for the help with procuring the materials, Andreas Albrecht for the CFC measurements, and Gerhard Hubner for the NMR measurements

Peculiar self-nucleation behavior of a polybutene-1/ethylene random copolymer

Unformatted post-print version of the accepted articleThe self-nucleation behavior of a polybutene-1/ethylene random copolymer, P(B1-ran-E), which undergoes a complex crystal-crystal transition behavior, has been studied in detail. Similar to PE random copolymers, this material shows a strong melt memory effect even above equilibrium melting point of PB-1 homopolymer. Different polymorphic forms can be obtained when P(B1-ran-E) is cooled from different self-nucleation Domains. The trigonal form I' could only be nucleated in the presence of remaining form I crystals via self-seeding, while the melt memory in Domain IIa could only act as self-nuclei for kinetically favored form II. Furthermore, observations from optical microscopy illustrated that melt memory is able to enhance nucleation density but it does not affect the spherulitic growth rate.Financial supports from the National Science Foundation of China (Grant No. U1510207) and the Key Program for Coal-based Science and Technology of Shanxi Province (MH-2014-08) are gratefully acknowledged. We would like to acknowledge the financial support from 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. AJM would also like to acknowledge funding from the Basque Government through grant IT1309-19

Competition between Chain Extension and Crosslinking in Polyamide 1012 during High Temperature Thermal Treatments as Revealed by SSA Fractionation

Unformatted post-print version of the accepted articleSelf-nucleation and annealing (SSA) is an efficient way to thermally fractionate semi-crystalline polymers. The thermal fractions produced by SSA have distinct melting points that correspond to different average lamellar thickness. In this research, SSA was adopted to investigate the in-situ evolution of lamellar thickness of polyamide 1012 (PA1012), which was affected by high temperature thermal treatments. SSA successfully fractionated PA1012 into 4 thermal fractions with different average lamellar thicknesses. The integrated area of the first or second SSA fraction against the total endothermic integrated area was plotted as a function of thermal treatment time to study the kinetics of lamellar thickness changes. Two opposing structural effects, chain growth and crosslinking, occurred during the applied thermal treatment (which consisted in thermally treating the material by holding it isothermally at temperatures in the range of 140-250 ºC) and they were detected as a function of time by SSA, rheology and dissolution behavior. The structural changes increased the viscosity and Tg and decreased the overall crystallization rate. Based on the construction of a master curve of “time-temperature superposition” at a reference temperature (T0) of 190 oC, the mechanism for lamellar thickness evolution was divided into three stages: (a) Stage I: Initially, PA1012 end groups reacted rapidly with active sites to generate chemically crosslinked structures. (b) Stage II: As the number of end groups rapidly increased, amidation reactions between carboxylic end groups and amine end groups resulted in linear chain growth. Linear chain growth and crosslinking occurred simultaneously, and there was no change in lamellar thickness or its distribution. (c) Stage III: Eventually, an increasing number of end groups was formed in the system, most of which led to linear chain growth via chain end-group reactions. These structural changes during the applied thermal treatments enhanced the mechanical properties and the heat resistance of PA1012. This work provides specific guidance for improving the toughness, strength and heat resistance of polyamide materials.We acknowledge generous financial support from the following grants: National Key R&D Program of China (2017YFB0307600) and STS project of Chinese Academy of Sciences (KFJ-STS-QYZX-113). A.J.M. acknowledges funding from the Basque Government through grant IT1309-19. 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 Sklodowska-Curie grant agreement no. 778092

Characterization and Modification of Red Mud and Ferrosilicomanganese Fines and Their Application in the Synthesis of Hybrid Hydrogels

In this work, hybrid hydrogels were synthesized with the inclusion of two types of clay materials that are considered industrial waste: red mud (RM) and ferrosilicomanganese fines (FeSiMn). These solid waste materials were characterized by studying their particle size and chemical composition, which are two key variables for their application in the synthesis of hybrid hydrogels. The morphology imaged by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), showed, in the case of RM, heterogeneous size and shape particles, with 73% of the particles having lengths of less than 5 μm. On the other hand, FeSiMn had particles with a circular morphology of nanometric sizes. Regarding the synthesis of the hybrid hydrogels, it was determined that the incorporation of small percentages (0.1%) of the inorganic phases improved the capacity of the materials to absorb water (swelling indices of 1678% and 1597% for the RM and FeSiMn hydrogels, respectively) compared to the conventional polyacrylamide hydrogel (1119%). An improvement in Vickers microhardness and storage modulus (G′) was also observed: the hybrid with 10% RM presented a G′, 50 times higher than conventional hydrogel. The results show the merit of RM and FeSiMn in improving the properties of hydrogels.A.J.M. gratefully acknowledges funding from MICINN (PID2020-113045GB-C21) and the Basque Government (IT1503-22)

Synthesis, Structure and Crystallization Behavior of Amphiphilic Hetero-arm Molecular Brushes with Crystallizable Poly(ethylene oxide) and N-Alkyl Side Chains

Unformatted post-print version of the accepted articleA series of hetero-arm amphiphilic molecular brushes (AMBs) with poly(ethylene glycol) (PEG) and long chain n-alkyl side chains were synthesized via conventional free radical polymerization (FRP) of mainly 4-vinyl benzyl-PEG methyl ether and N-alkylmaleimide macromonomers. By varying PEG side chain degree of polymerization (D.P. = 12, 16 and 20) and n-alkyl chain lengths (C16 and C20), AMBs with varying combinations of side chain lengths were produced. This enabled the elucidation of the effect of side chain length on AMB phase behavior, semicrystalline morphologies and crystallization kinetics, via differential scanning calorimetry, polarized light optical microscopy and x-ray diffraction experiments. Calculations of segregation strength together with SAXS measurements indicate that all materials are probably phase segregated structure in the melt. Most of the AMB materials prepared were double crystalline, i.e., contained crystals from alkyl and PEG chains. AMB crystallization was constrained by AMB architecture, the frustration being most evident in AMBs with combinations of either low D.P.PEG, or short alkyl chain lengths. Large, well-developed spherulites, implying break-out crystallization from a weakly segregated melt, were only observed for the AMBs with the combination of the longest PEG chain (D.P. = 20) and longest alkyl chain length (C20). A peculiar behavior was found when spherulitic growth rates and overall crystallization rates of the PEG chains, within this particular AMB sample, were determined as a function of crystallization temperature. In both cases, a distinct minimum with decreasing temperature was observed, probably caused by the challenges encountered in crystal packing of the PEG side chains, tethered to an amorphous backbone, which also contained already crystallized C20 chains. This minimum is analogous to that observed in the crystallization of long chain n-alkanes, or high molar mass polyethylenes with bromine pendant groups that has been attributed to a self-poisoning effect; this is the first observation of this phenomenon in AMBs.This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology (DST) and National Research Foundation (NRF) of South Africa (Grant No 46855). J.M. acknowledges support from the Provincial Council of Gipuzkoa under the program Fellow Gipuzkoa and “Fomento San Sebastián” in the framework program “Retorno del Talento Local” Donostia up! 2016. This work has received funding from the European Union´s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 778092, from MINECO, project: MAT2017-83014-C2-1-P and from the Basque Government through grant IT1309-19. We also thank ALBA Synchrotron facility for providing funding and beam time (proposal number: 2018082953)

Surface Roughness Enhances Self-Nucleation of High-Density Polyethylene Droplets Dispersed within Immiscible Blends

[EN] Highly linear or high-density polyethylenes (HDPEs) have an intrinsically high nucleation density compared to other polyolefins. Enhancing their nucleation density by self-nucleation is therefore difficult, leading to a narrow self-nucleation Domain (i.e., the so-called Domain II or the temperature Domain where self-nuclei can be injected into the material without the occurrence of annealing). In this work, we report that when HDPE is blended (up to 50%) with immiscible matrices, such as atactic polystyrene (PS) or Nylon 6, its self-nucleation capacity can be greatly increased. In addition, temperatures higher than the equilibrium melting temperature of the HDPE phase are needed to erase the significantly enhanced crystalline memory in the blends. Morphological evidence gathered by Scanning and Transmission Electron Microscopies (SEM and TEM) indicates that these unexpected results can be explained by the modification of the interface between blend components. The filling of the solid HDPE surface asperities by the low viscosity polystyrene during heating to the self-nucleation temperature, or the crystallization of the matrix in the case of Nylon 6, enhances the interface roughness between the two polymers in the blends. Such rougher interfaces can remarkably increase the self-nucleation capacity of the HDPE phase via surface nucleation.The authors acknowledge technical and human support provided by SGIker (UPV/EHU/ERDF, EU). This work has also received funding from the Basque Government through grant IT1309-19