Prediction of environmental stress cracking in polycarbonate by molecular modeling

Environmental stress cracking (ESC) is a premature failure of a polymer exposed to a fluid, under stress which is much less than its yield stress. Many experimental works were done before in an effort to predict experimentally the ESC potential of a fluid in different polymers. None of the previous works applied molecular modeling techniques to predict this potential so this work is a pioneering work. This study's goal was to apply atomistic molecular modeling techniques to gain a better understanding of the ESC mechanism and to predict the ESC potential of different fluids in polymers. Our model experimental system was amorphous polycarbonate (PC) with water as an ESC fluid. The computational study was expanded to include a high level ESC fluid for PC–toluene and a non ESC fluid–BD, together with the moderate ESC fluid–water. A clear distinction between ESC fluids and non ESC fluids for PC was achieved by means of molecular modeling. The experimental work approved that water is an ESC fluid for PC as predicted in the computational part. Copyright © 2014 John Wiley & Sons, Ltd.     

Stabilization of vinylidene chloride barrier resins

Vinylidene chloride copolymers have a number of superior properties, most notably a high barrier to the transport of oxygen and other small molecules. As a consequence, these materials have assumed a position of prominence in the packaging industry. At processing temperatures these copolymers tend to undergo degradative dehydrochlorination. Unsaturation generated via interaction of the polymer with a variety of agents commonly encountered during polymerization or processing introduces an allylic dichloromethylene unit which may function as a major defect (labile) site for the initiation of degradation. Three approaches to the potential stabilization of these materials have been examined. The first involved the addition of agents, e.g. metal formates, capable of converting labile dichlormethylene units into non-reactive groups which would interrupt propagation of the degradative dehydrochlorination. The second involved the incorporation into the polymer of a commoner capable of scavenging free chlorine atoms. The third involved the preparation of copolymers which contains units capable of reaction with (consumption of) a mole of hydrogen chloride to expose a good free radical stabilizer to scavenge chlorine atoms.     

Full notch creep test (FNCT) of PE-HD – Characterization and differentiation of brittle and ductile fracture behavior during environmental stress cracking (ESC)

The damage mechanisms slow crack growth (SCG) and environmental stress cracking (ESC), relevant for PE-HD materials are characterized based on improved full notch creep testing (FNCT) of two selected typical PE-HD materials for container applications. In this context, a distinction of the failure mechanisms as well as a categorization of involved media is suggested. Employing a novel FNCT device, elongation data were obtained in addition to conventional time-to-failure results of stress-dependent as well as temperature-dependent measurements. Changes in failure behavior, as determined by fracture surface analysis based on light microscopy (LM) and laser scanning microscopy (LSM), are correlated with FNCT results and used to introduce an additional possibility for the identification of brittle/ductile fracture behavior.     

Effect of thermal history on the deformation and failure of polyimides

Thermal‐processing structure‐property relationships for polyetherimide (PEI), poly(4,4′‐oxydiphenylene pyromellitimide) (POPPI), and phenylethynyl‐terminated imide (PETI‐5) composite matrices are reported from a fundamental perspective. For thermoplastic PEI, deformation and failure depend primarily on free volume as evidenced by moisture‐absorption, mechanical‐property, and mass‐density changes as a function of annealing. The deformation of POPPI can be divided into the following three regimes as a function of annealing temperature: (1) physical aging‐induced glassy state free‐volume decreases, (2) thermally activated microvoid collapse, and (3) chemical degradation. In the case of PETI‐5, macroscopic defects, free volume, and polymer morphology control deformation. The effects of residual crystallinity on deformation are reported, and it is shown that mechanical toughness can be significantly decreased upon annealing below the glass‐transition temperature. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2947–2959, 2001     

QuanPol: A full spectrum and seamless QM/MM program

The quantum chemistry polarizable force field program (QuanPol) is implemented to perform combined quantum mechanical and molecular mechanical (QM/MM) calculations with induced dipole polarizable force fields and induced surface charge continuum solvation models. The QM methods include Hartree–Fock method, density functional theory method (DFT), generalized valence bond theory method, multiconfiguration self‐consistent field method, Møller–Plesset perturbation theory method, and time‐dependent DFT method. The induced dipoles of the MM atoms and the induced surface charges of the continuum solvation model are self‐consistently and variationally determined together with the QM wavefunction. The MM force field methods can be user specified, or a standard force field such as MMFF94, Chemistry at Harvard Molecular Mechanics (CHARMM), Assisted Model Building with Energy Refinement (AMBER), and Optimized Potentials for Liquid Simulations‐All Atom (OPLS‐AA). Analytic gradients for all of these methods are implemented so geometry optimization and molecular dynamics (MD) simulation can be performed. MD free energy perturbation and umbrella sampling methods are also implemented. © 2013 Wiley Periodicals, Inc.     

Polymerization of free secondary amine bearing monomers by RAFT polymerization and other controlled radical techniques

This work describes the polymerization of the free secondary amine bearing monomer 2,2,6,6‐tetramethylpiperidin‐4‐yl methacrylate (TMPMA) by means of different controlled radical polymerization techniques (ATRP, RAFT, NMP). In particular, reversible addition‐fragmentation chain transfer (RAFT) polymerization enabled a good control at high conversions and a polydispersity index below 1.3, thereby enabling the preparation of well‐defined polymers. Remarkably, the polymerization of the secondary amine bearing methacrylate monomer was not hindered by the presence of the free amine that commonly induces degradation of the RAFT reagent. Subsequent oxidation of the polymer yielded the polyradical poly(2,2,6,6‐tetramethylpiperidinyloxy‐4‐yl methacrylate), which represents a valuable material used in catalysis as well as for modern batteries. The obtained polymers having a molar mass (M_n) of 10,000–20,000 g/mol were used to fabricate well‐defined, radical‐bearing polymer films by inkjet‐ printing. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Polymer Nanoassembly as Delivery Systems and Anti‐Bacterial Toolbox: From PGMAs to MSN@PGMAs

Researches on cargo delivery systems have received burgeoning attention and advanced rapidly. For synthetic nanodevices, polymer nanoassemblies and their inorganic‐organic hybrid materials, especially smart mesoporous silica nanoparticle (MSN)‐polymer hybrids (e. g., MSN@PGMAs), have attracted increasing attention in recent years. Their superior characteristics and unique features such as dynamic transition of morphology endow them the ability to efficiently entrap cargo molecules and undergo smart cargo delivery and release in response to various external stimuli. In this Personal Account, we present our recent research progress in the construction of cargo delivery systems based on polymers, poly(glycidyl methacrylate) (PGMA) and its derivatives in particular, ranging from polymer nanoparticles, reverse micelles, to vesicles and reverse vesicles, and their performance in the delivery and controlled release of model molecules and therapeutic agents. Significantly, MSN‐PGMA hybrid nanoassemblies (MSN@PGMAs), constructed with the aid of atom transfer radical polymerization, host‐guest interactions, or layer‐by‐layer self‐assembly techniques, and their potential bio‐related applications and anti‐bacterial applications as new nanocarriers are reviewed. Finally, the prospects and challenges of such nanoplatforms are also discussed.     

Converging ligand‐binding free energies obtained with free‐energy perturbations at the quantum mechanical level

In this article, the convergence of quantum mechanical (QM) free‐energy simulations based on molecular dynamics simulations at the molecular mechanics (MM) level has been investigated. We have estimated relative free energies for the binding of nine cyclic carboxylate ligands to the octa‐acid deep‐cavity host, including the host, the ligand, and all water molecules within 4.5 Å of the ligand in the QM calculations (158–224 atoms). We use single‐step exponential averaging (ssEA) and the non‐Boltzmann Bennett acceptance ratio (NBB) methods to estimate QM/MM free energy with the semi‐empirical PM6‐DH2X method, both based on interaction energies. We show that ssEA with cumulant expansion gives a better convergence and uses half as many QM calculations as NBB, although the two methods give consistent results. With 720,000 QM calculations per transformation, QM/MM free‐energy estimates with a precision of 1 kJ/mol can be obtained for all eight relative energies with ssEA, showing that this approach can be used to calculate converged QM/MM binding free energies for realistic systems and large QM partitions. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.     

The CHARMM–TURBOMOLE interface for efficient and accurate QM/MM molecular dynamics,free energies,and excited state properties

The quantum mechanical (QM)/molecular mechanical (MM) interface between Chemistry at HARvard Molecular Mechanics (CHARMM) and TURBOMOLE is described. CHARMM provides an extensive set of simulation algorithms, like molecular dynamics (MD) and free energy perturbation, and support for mature nonpolarizable and Drude polarizable force fields. TURBOMOLE provides fast QM calculations using density functional theory or wave function methods and excited state properties. CHARMM–TURBOMOLE is well‐suited for extended QM/MM MD simulations using first principles methods with large (triple‐ζ) basis sets. We demonstrate these capabilities with a QM/MM simulation of Mg²⁺(aq), where the MM outer sphere water molecules are represented using the SWM4‐NDP Drude polarizable force field and the ion and inner coordination sphere are represented using QM PBE, PBE0, and MP2 methods. The relative solvation free energies of Mg²⁺ and Zn²⁺ were calculated using thermodynamic integration. We also demonstrate the features for excited state properties. We calculate the time‐averaged solution absorption spectrum of indole, the emission spectrum of the indole excited state, and the electronic circular dichroism spectrum of an oxacepham. © 2014 Wiley Periodicals, Inc.     

The influence of interlayer cations on the photo‐oxidative degradation of polyethylene/montmorillonite composites

The photo‐oxidative degradation of polyethylene/montmorillonite (PE/MMT) nanocomposite and microcomposite has been investigated. It has been found that the rate of photo‐oxidative degradation of PE/MMT nanocomposite and PE/Mⁿ⁺MMT (where Mⁿ⁺ stands for multivalent transition metal cation) microcomposites is much faster than that of pure PE. For the PE/MMT nanocomposite, the acceleration of photo‐oxidative degradation is due to the influence of MMT and ammonium ion, in which the influence of ammonium is primary. The decomposition of ammonium ion may create acidic sites on layered silicates; meanwhile, the complex crystallographic structure and habit of clay minerals could also result in some active sites. The reversible photo‐redox reaction of transition metal cations has a catalytic effect in the degradation of the polymer matrix. All these catalytic active sites can accept single electrons from donor molecules of polymer matrix and induce the formation of free radical upon UV irradiation. The generation of free radical leads to the oxidization and break of molecular chain. Thus, the materials suffer degradation and their mechanical strength decreases. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3006–3012, 2004     

Coarse‐grained molecular dynamics simulations of polymerization with forward and backward reactions

We develop novel parallel algorithms that allow molecular dynamics simulations in which byproduct molecules are created and removed because of the chemical reactions during the molecular dynamics simulation. To prevent large increases in the potential energy, we introduce the byproduct molecules smoothly by changing the non‐bonded interactions gradually. To simulate complete equilibrium reactions, we allow the byproduct molecules attack and destroy created bonds. Modeling of such reactions are, for instance, important to study the pore formation due to the presence of e.g. water molecules or development of polymer morphology during the process of splitting off byproduct molecules. Another concept that could be studied is the degradation of polymeric materials, a very important topic in a recycling of polymer waste. We illustrate the method by simulating the polymerization of polyethylene terephthalate (PET) at the coarse‐grained level as an example of a polycondensation reaction with water as a byproduct. The algorithms are implemented in a publicly available software package and are easily accessible using a domain‐specific language that describes chemical reactions in an input configuration file. © 2018 Wiley Periodicals, Inc.     

Comparison of extraction methods for sampling of low molecular compounds in polymers degraded during recycling

The demand for mechanical recycling of plastic waste results in an increasing amount of recycled polymeric materials available for development of new products. In order for recycled materials to find their way into the material market, high quality is demanded. Thereby, a complete and closed loop of polymeric materials can be achieved successfully. The concept of high quality for recycled plastics imply that besides a pure fraction of e.g. polyethylene (PE) or polypropylene (PP), containing only minor trace amount of foreign plastics, knowledge is required about the type and amount of low molecular weight (LMW) compounds. During long-term use (service-life), products made of polymeric materials will undergo an often very slow degradation where a series of degradation products are formed, in parallel, additives incorporated in the matrix may also degrade. These compounds migrate at various rates to the surrounding environment. The release rate of LMW products from plastics depends on the initiation time of degradation and the degradation mechanisms. For polymers the formation of degradation products may be initiated already during processing, and subsequent use will add products coming from the surrounding environment, e.g. fragrance and aroma compounds from packaging. During recycling of plastics, emissions which contain a series of different LMW compounds may reach the environment leading to unwanted exposure to additives and their degradation residues as well as degradation products of polymers.Several extraction techniques are available for sampling of LMW compounds in polymers before chromatographic analysis. This paper reviews and compares polymer dissolution, accelerated solvent extraction (ASE), microwave assisted extraction (MAE), ultrasound assisted extraction (UAE), super critical fluid extraction (SFE), soxhlet extraction, head-space extraction (HS), head-space solid phase micro extraction (HS-SPME), and head-space stir bar sorptive extraction (HSSE) as appropriate sampling methods for LMW compounds in recycled polymers. Appropriate internal standards useful for these kinds of matrices were selected, which improved the possibility for later quantification. Based on the review of extraction methods, the most promising techniques were tested with industrially recycled samples of HDPE and PP and virgin HDPE and PP for method comparison.     

Magnetic orientation of diamagnetic amorphous polymers

Molecular order in an amorphous polymer with anisotropic magnetic susceptibility is altered by applying external magnetic fields. Disks of atactic polystyrene (a‐PS) are prepared by solvent casting outside or inside a magnet. The effect of the magnetic field on the polymer samples is investigated by magnetic levitation and solid state NMR spectroscopy. Magnetic levitation of the a‐PS disks shows that the orientation of disk symmetry axis with respect to the magnetic field gradient depends on the magnitude and direction of the applied field during casting. Similarly, carbon‐13 two‐dimensional cross‐polarization/magic angle spinning rotor‐synchronized NMR measurements in these samples show modulation patterns of the spinning side bands only on disks prepared in the presence of a magnetic field. These findings suggest that macromolecular order could be induced in a fluid or fluid–solid phase transition with cooperative segmental motions reorienting the diamagnetic susceptibility tensor to minimize the magnetic contribution to free energy of the sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1009–1015, 2010     

Superhydrophilic Phytic‐Acid‐Doped Conductive Hydrogels as Metal‐Free and Binder‐Free Electrocatalysts for Efficient Water Oxidation

Recently, metal‐free, heteroatom‐doped carbon nanomaterials have emerged as promising electrocatalysts for the oxygen evolution reaction (OER), but their synthesis is a tedious process involving energy‐wasting calcination. Molecular electrocatalysts offer attractive catalysts for the OER. Here, phytic acid (PA) was selected to investigate the OER activity of carbons in organic molecules by DFT calculations and experiments. Positively charged carbons on PA were very active towards the OER. The PA molecules were fixed into a porous, conductive hydrogel with a superhydrophilic surface. This outperformed most metal‐free electrocatalysts. Besides the active sites on PA, the high OER activity was also related to the porous and conductive networks on the hydrogel, which allowed fast charge and mass transport during the OER. Therefore, this work provides a metal‐free, organic‐molecule‐based electrocatalyst to replace carbon nanomaterials for efficient OER.     

RECENT ADVANCES IN THE PREPARATION OF MOLECULARLY IMPRINTED POLYMERS VIA CONTROLLED RADICAL POLYMERIZATION TECHNIQUES

Molecular imprinting technique is a simple and efficient method for the preparation of polymer materials （i. e., molecularly imprinted polymers, MIPs） with tailor-made recognition sites for certain target molecules. The resulting MIPs have proven to be versatile synthetic receptors due to their high specific recognition ability, favorable mechanical, thermal and chemical stability, and ease of preparation. Recent years have witnessed significant progress in the synthesis and applications of MIPs. This review focus on the recent developments and advances in the preparation of MIPs via various controlled radical polymerization techniques.     

Ermod: Fast and versatile computation software for solvation free energy with approximate theory of solutions

ERmod is a software package to efficiently and approximately compute the solvation free energy using the method of energy representation. Molecular simulation is to be conducted at two condensed‐phase systems of the solution of interest and the reference solvent with test‐particle insertion of the solute. The subprogram ermod in ERmod then provides a set of energy distribution functions from the simulation trajectories, and another subprogram slvfe determines the solvation free energy from the distribution functions through an approximate functional. This article describes the design and implementation of ERmod, and illustrates its performance in solvent water for two organic solutes and two protein solutes. Actually, the free‐energy computation with ERmod is not restricted to the solvation in homogeneous medium such as fluid and polymer and can treat the binding into weakly ordered system with nano‐inhomogeneity such as micelle and lipid membrane. ERmod is available on web at http://sourceforge.net/projects/ermod . © 2014 Wiley Periodicals, Inc.     

Nuclear proteome analysis of undifferentiated mouse embryonic stem and germ cells

Embryonic stem cells (ESCs) and embryonic germ cells (EGCs) provide exciting models for understanding the underlying mechanisms that make a cell pluripotent. Indeed, such understanding would enable dedifferentiation and reprogrammation of any cell type from a patient needing a cell therapy treatment. Proteome analysis has emerged as an important technology for deciphering these biological processes and thereby ESC and EGC proteomes are increasingly studied. Nevertheless, their nuclear proteomes have only been poorly investigated up to now. In order to investigate signaling pathways potentially involved in pluripotency, proteomic analyses have been performed on mouse ESC and EGC nuclear proteins. Nuclei from ESCs and EGCs at undifferentiated stage were purified by subcellular fractionation. After 2‐D separation, a subtractive strategy (subtracting culture environment contaminating spots) was applied and a comparison of ESC, (8.5 day post coïtum (dpc))‐EGC and (11.5 dpc)‐EGC specific nuclear proteomes was performed. A total of 33 ESC, 53 (8.5 dpc)‐EGC, and 36 (11.5 dpc)‐EGC spots were identified by MALDI‐TOF‐MS and/or nano‐LC‐MS/MS. This approach led to the identification of two isoforms (with and without N‐terminal acetylation) of a known pluripotency marker, namely developmental pluripotency associated 5 (DPPA5), which has never been identified before in 2‐D gel‐MS studies of ESCs and EGCs. Furthermore, we demonstrated the efficiency of our subtracting strategy, in association with a nuclear subfractionation by the identification of a new protein (protein arginine N‐methyltransferase 7; PRMT7) behaving as proteins involved in pluripotency.     

Free energy perturbation and MM/PBSA studies on inclusion complexes of some structurally related compounds with β-cyclodextrin

Molecular dynamics (MD) simulations have been conducted to explore time-resolved guest–host interactions involving inclusion complex formation between β-cyclodextrin and organic molecules bearing two peripheral benzene rings in aqueous solution. Moreover, free energy perturbation (FEP) and thermodynamic integration (TI) methods at different simulation times have been employed to estimate the relative free energy of complexation. Also, the less computer-time demanding molecular mechanics/Poisson–Boltzmann surface area (MM/PBSA) method was used to estimate the free energy of complexation based on only 1-ns MD simulation. Results showed that both FEP and TI methods were able to reasonably reproduce the experimental thermodynamic quantities. However, long simulation times (e.g. 15 ns) were needed for benzoin mutating to benzanilide (BAN), while moderately shorter times were sufficient for BAN mutating to phenyl benzoate and for benzilic acid mutating to diphenylacetic acid. The results have been discussed in the light of the differences in the chemical structural and conformational features of the guest molecules. In general, it was apparent that the TI method requires less time for convergence of results than the FEP method. However, the less expensive MM/PBSA method proved capable of producing results that are in agreement with those of the more expensive TI and FEP methods.     

Conjugated Polymer with Intrinsic Alkyne Units for Synergistically Enhanced Raman Imaging in Living Cells

Development of Raman‐active materials with enhanced and distinctive Raman vibrations in the Raman‐silent region (1800–2800 cm⁻¹) is highly required for specific molecular imaging of living cells with high spatial resolution. Herein, water‐soluble cationic conjugated polymers (CCPs), poly(phenylene ethynylene) (PPE) derivatives, are explored for use as alkyne‐state‐dependent Raman probes for living cell imaging due to synergetic enhancement effect of alkyne vibrations in Raman‐silent region compared to alkyne‐containing small molecules. The enhanced alkyne signals result from the integration of alkyne groups into the rigid backbone and the delocalized π‐conjugated structure. PPE‐based conjugated polymer nanoparticles (CPNs) were also prepared as Raman‐responsive nanomaterials for distinct imaging application. This work opens a new way into the development of conjugated polymer materials for enhanced Raman imaging.