Telechelic Polybutadienes or Polyisoprenes Precursors for Recyclable Elastomeric Networks

(Bis)furan‐telechelic, low‐molar‐mass polybutadienes and polyisoprenes are synthesized by controlled degradation of high molar mass polymers and chain‐end modifications yielding difunctional, trifunctional, or tetrafunctional polymers. Addition of a bismaleimide to the liquid‐modified polymer leads to the formation of a thermoreversible elastomeric network based on the Diels–Alder chemistry for the trifunctional or tetrafunctional polymers, whereas only chain extension occurs for the bifunctional one. Dynamic mechanical analyses or tensile tests are performed on the networks and reveal a similar behavior for polyisoprene and polybutadiene with nevertheless quite different Young modulus or strain at break. The retro Diels–Alder reaction occurs upon heating, allowing the remolding of the used elastomer. The remolded network exhibits the same mechanical properties as the initial network, showing an efficient material recyclability.     

Correlated fluctuations in polymer networks

Polymer micronetworks with Cayley tree and random topologies are compared. The fluctuations of junctions in random networks exhibit significant departures from the mean value corresponding to those in Cayley tree topology. Correlations among the fluctuations are examined for the two types of networks. Correlations are stronger, and cover longer distances along the chain contours in random networks, compared to those observed in Cayley trees. This is a consequence of the non-uniform spatial distribution of connected vertices in a random network, as opposed to Cayley trees in which chains extend between junctions located in successive tiers only. The relaxation spectra of the two types of networks are compared. The cumulative distribution of frequencies obeys a power law dependence on frequency, with exponents of 1.4 and 1.1 for trifunctional Cayley tree and random networks, respectively. These exponents are significantly smaller than the Debye value of 3 for regular three dimensional crystals.     

Molecular-dynamics simulations of the transport properties of a single polymer chain in two dimensions

Molecular-dynamics simulations are conducted to elucidate the critical factors affecting the transport properties of isolated polymer chains in strictly two dimensions. The relevance of surface inhomogeneity is critically examined. We unequivocally find that surface inhomogeneity is critical in obtaining transport behavior consistent with the recent measurements of surface diffusion for polymers adsorbed at the solid-liquid interface. For a systematic investigation of this point, heterogeneity was introduced by decorating the surface with impenetrable elements and we find that chain diffusivity crossed over from Rouse-type behavior to reptationlike with increasing surface coverage of obstacles. This transition in behavior occurred when the mean distance between obstacles is approximately equal to the end-to-end distance, Re, of the two-dimensional chain. Our results underscore the importance of surface disorder (not only literal obstacles but by reasonable extension also to other types of disorder) in determining the transport behavior of chains adsorbed to solids.     

Two-dimensional folded chain crystals of a synthetic polymer in a Langmuir-Blodgett film

Isotactic poly(methyl methacrylate) monolayers deposited from a water surface onto mica at different surface pressures were studied by atomic force microscopy, and their structure formation from single chains to two-dimensional folded chain crystals was clearly observed. Furthermore, gentle crystallization of the monolayer by slow compression on the water surface enabled the observation of crystals at a molecular level, thus visualizing the chain foldings and tie-chains for the first time. The resulting molecular level information will provide an important clue toward the understanding of polymer crystals not only in two dimensions but also in three dimensions.     

Randomly branched polycyanurates - modification of the network structure

Polycyanurates (poly(2,4,6-trisaroxy-1,3,5-triazines)) are built up through a step-growth polycyclotrimerization of difunctional aromatic cyanic acid esters and form an ideally trifunctional randomly branched network, where all initial cyanato groups are linked with the triazine branching units at full conversion. Results on modifications of the high network density of pure polycyanurates are reported for three types of comonomers: monofunctional cyanic acid esters, difunctional phenols and difunctional aromatic glycidyl ethers. Using the coreaction of mono- and difunctional aromatic cyanic acid esters, additional chain segments, dangling ends and low molecular weight compounds are built, the fractions of which are dependent on the initial admixture of the monofunctional compound and its relative reactivity. Therefore, the critical conversion at the gel point is shifted to higher values and the network density at full conversion decreases. The abstraction of phenols from intermediately formed iminocarbonic ester structures is utilized for the modification of the polycyanurate network through the coreaction of difunctional aromatic cyanic acid esters with phenols. It was found that the critical conversion and the network structure are dependent on all the initial amount of phenols, their reactivity and functionality. The modification with the help of glycidyl ethers is characterized by an insertion of the oxirane ring into the cyanurate and the succeeding reactions into isocyanurates and oxazolidinones. Depending on the initial mixture and the reaction conditions, a large spectrum of chemical and topological network structures can be obtained, what is demonstrated by both experimental and theoretical investigations.     

TMA characterization of urethane elastomers,with the structure varying from chain to network

TMA has been used to investigate the thermomechanical behaviour of six series of elastomers in connection with their chemical constitution and physical structure. The elastomers, were synthesized from an NCO-terminal pre-polymer, based on oligo (ethylene adipate), 1,4-butanediol and 2,4-toluylene diisocyanate, by curing with systems of two agents: a bifunctional one (1,4-butanediol, bistethylene glycol)terephthalate, or monoethanolamine), and a trifunctional one (1,1,1-trimethylolpropane or diethanolamine). The TMA results are presented as due to the superposition of the chemical cross-linking and the physical network, formed through microphase segregation. The TMA suggests that diethanolamine unexpectedly acts as a chain extender, rather than a cross-linking agent.     

Model polyurethane networks prepared from poly(ethylene oxide) and poly(tetramethylene oxide)

Polyurethane elastomers of known degrees of cross-linking were prepared from hydroxylterminated poly(ethylene oxide) (PEO) and poly(tetramethylene oxide) (PTMO) chains having numberaverage molecular weights in the range 880–6820 g mol^?1. The chains were end-linked into “model” trifunctional networks using a specially prepared aromatic triisocyanate. The networks thus obtained were studied with regard to their stress-strain isotherms in both the unswollen and swollen states, in elongation at 25°, and with regard to their equilibrium swelling in benzene at 57.9°. Values of the modulus in the limit at high deformation were in good agreement with corresponding results previously obtained on trifunctional networks of poly(dimethylsiloxane) (PDMS). Since PEO has a much higher value of the plateau modulus in the uncross-linked state, this agreement indicates that inter-chain entanglements do not contribute significantly to the equilibrium modulus of an elastomeric network. These values of the high deformation modulus are also in good agreement with recent molecular theories as applied to the non-affine deformation of a “phantom” network. The swelling equilibrium results were in very good agreement with the new theory of network swelling developed by Flory.     

Dynamics and scaling of polymers in a dilute solution: analytical treatment in two and higher dimensions

We consider the dynamical scaling of a single polymer chain in good solvent. In the case of two-dimensional systems, Shannon and Choy [Phys. Rev. Lett. 79, 1455 (1997)] have suggested that the dynamical scaling for a dilute polymer solution breaks down. Using scaling arguments and analytical calculations based on the Zimm model, we show that the dynamical scaling of a dilute two-dimensional polymer system holds when the relevant dynamical quantities are properly extracted from finite systems. Most important, the polymer diffusion coefficient in two dimensions scales logarithmically with system size, in excellent agreement with our extensive computer simulations. This scaling is the reason for the failure of the previous attempts to resolve the dynamical scaling of dilute two-dimensional polymer systems. In three and higher dimensions our analytic calculations are in agreement with previous results in the literature.     

甘氨酸与间硝基苯甲酸加合物的合成及晶体结构

The title 1:1 adduct, has been prepared and a large single crystal with dimensions of 5 mm×50 mm×20 mm was obtained by slow-cooling method. It produces the green radiation at 532 nm under the irradiation of Nd~3+: YAG laser beam at 1064nm. The crystal structure of this potential non-linear optical material was determined by X-ray diffraction method. The crystal is orthorbombic, space group Pca2_1, with a=2.2701(5) nm, b=0.5852(2) nm, c=0.7815(2) nm, Z=4; final R is 0.054 for 702 observed reflections. The intermolecular hydrogen bonds are formed between the amino and carboxyl groups of glycines, which connect the glycine molecules to form two-dimensional network parallel to the (100) plane, while the intermolecular hydrogen bonds between the carboxyl group of glycine and the carboxyl group of m-nitrobenzoic acid let the latter link to above mentioned two-dimensional network.     

A molecular dynamics simulation study on polymer networks of end-linked flexible or rigid chains

The differences in formation and structural properties of polymer networks consisting of end-linked flexible or rigid chains were studied by molecular dynamics simulation. Networks were formed from monodisperse, linear, short, flexible or rigid chains with functional end groups and a stoichiometric ratio of trifunctional cross-linkers. The rigid chains had a rodlike shape defined by an angle potential, while the flexible chains had no angle potential. In order to understand the influence of chain rigidity, all parameters of precursor chains (length, reactivity, bond potential, nonbonding potential) were the same, with the exception of the angle potential. The system density rho, corresponding to the concentration of monomer in solvent, was varied from 0.01 to 0.11. Different network structures resulting from the different processes of network formation were observed. Simulations showed that the flexible chains created an inhomogeneous network on a large scale via microgel cluster formation, in agreement with experimental observations, whereas the rigid chains rapidly created a homogeneous network in the entire system volume without first generating microgel clusters, with the additional difference that they gave rise to mutually interpenetrating networks at the local scale.     

Synthesis of well‐defined three‐armed polystyrene having thiourethane–isocyanurate as the core structure derived from trifunctional five‐membered cyclic dithiocarbonate

The synthesis of a three‐armed polymer with an isocyanurate–thiourethane core structure is described. Monofunctional reversible addition–fragmentation chain transfer (RAFT) agent 2 and trifunctional RAFT agent 5 were prepared from mercapto‐thiourethane and tris(mercapto‐thiourethane), which were obtained from the aminolysis of mono‐ and trifunctional five‐membered cyclic dithiocarbonates, respectively. The radical polymerization of styrene in the presence of 2,2′‐azobis(isobutyronitrile) and RAFT agent 2 in bulk at 60 °C proceeded in a controlled fashion to afford the corresponding polystyrene with desired molecular weights (number‐average molecular weight = 3000–10,100) and narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight < 1.13). On the basis of the successful results with the monofunctional RAFT agents, three‐armed polystyrene with thiourethane–isocyanurate as the core structure could be obtained with trifunctional RAFT agent 5 in a similar manner. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5498–5505, 2005     

Characterization of PDMS model junctions and networks by solution and solid-state silicon-29 NMR spectroscopy

The application of ²⁹Si solution and solid-state cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) techniques to the study of structural features in polydimethylsiloxane (PDMS) model endlinked elastomeric networks is explored. The relationship between the topological (network) functionality of a structural moiety, which determines network mechanical properties, and its chemical (spectral) functionality, which is reported by the NMR, is discussed. The second-order spectral shifts corresponding to topographical functionality variation within a chemical functionality class are usually sufficiently well resolved in these networks to allow positive identification of a variety of structural features. The basic PDMS repeat unit, ? OSi(CH₃)₂? , is found to possess an axially symmetric chemical shift tensor with σ_∥ = ?56.8 ppm downfield from TMS, and σ_⊥ = ?4.4 ppm. This axial symmetry does not result from rapid reorientation about the chain axis. The NMR spectrum reveals defects in model endlinked networks. In the case of vinyl-endlinked systems, the defects are ascribed to the formation of elastically ineffective loops. Hydroxyl-endlinked systems contain either loops or else trifunctional junctions (hydrolyzed before chain coupling could take place) and dangling chain ends. The CP/MAS technique provides an order-of-magnitude reduction over standard solution techniques in the time to acquire a spectrum from a network not containing paramagnetic doping. ¹³C spectra of PDMS systems are not as informative as ²⁹Si spectra.     

Synthesis of molecular objects: Two-dimensional polymers

Throughout this century polymer science has studied the linear chain and its architectural derivatives which include familiar forms such as the branched chain and the three dimensional network. Other derivatives with unique properties have been investigated more recently and include macromolecular rings, dendrimers, stars, combs and ladders. The objective of this work is to depart from the focus on linear chains and explore the “bulk” synthesis and properties of polymer molecules that can be considered molecular objects. Ideal molecular objects should be macromolecules with well defined shapes that persist as systems transform reversibly from solids to melts or solutions. The limited access to conformational space which is required in order to define and maintain shape in liquid and solid states of the system is an unusual molecular characteristic in common polymers. Our ability to create such objects through bulk reactions of reasonable scale will undoubtedly extend the current boundaries of polymer science and technology. Shapes that are particularly interesting are those not common in the conformational space of linear chains, for example, two-dimensional polymers shaped as plates and macromolecular bundles shaped as cylinders or parallelepipeds. The molecular object to be discussed in this lecture is a rigid and anisotropic two-dimensional polymer with planar dimensions greater than its thickness and a shape-granting skeleton built by covalent bonds. We have so far developed three different strategies for their synthesis, all involving systems in which reactive oligomers organize spontaneously into the necessary planar assemblies to form the object. In one strategy molecular recognition driven by homochiral interactions plays a key role in the formation of two-dimensional polymers.¹ A different methodology relies on entropy-driven nanophase separation in rodcoil block molecules in which a rigid segment is covalently bonded to a flexible one sharing the same backbone. The third strategy involves the folding of oligomers into hairpin structures which self assemble into two-dimensional liquids. The lecture will also describe examples of unique properties that could be achieved in materials containing these rigid two-dimensional objects. These examples will include bulk materials with self-organized surfaces and also remarkably stable nonlinear optical properties.     

Reappraisal of four different approaches for finding the mean reaction time in the multi-trap variant of the Adam-Delbrück problem

Adam and Delbrück argued that the dimensionality of the diffusion space determines the average lifetime of a diffusing particle confined to a region with a central trap. Doubts have often been aired as to whether their calculation is relevant to real biological systems, where the number of traps is usually much larger than unity; or whether the rate enhancement is merely a manifestation of an increase in the concentration of the traps; or whether the diverse multi-trap versions of their expression for the mean lifetime in two dimensions are trustworthy. These issues are addressed, and the long-standing problem of finding the low-density limit of trapping time in two dimensions solved, by examining previous treatments of the problem, and by carrying out simulations of two-dimensional systems in which the particles undergo a Pearsonian random walk, and the traps are distributed randomly or on a square lattice. The mean lifetimes are found to be different in the two situations, and it is concluded that the neglect of this aspect lies at the root of the conflict between some of the existing expressions for the mean lifetime. Relations expressing the mean lifetime as a function of the concentration of the traps are presented together with a discussion of their applicability.     

Monte Carlo simulation studies of ring polymers at athermal and theta conditions

By use of an intramolecular criterion, i.e., the direct proportionality between mean square dimension and chain length, theta conditions for linear chains and ring shaped polymers are evaluated for several types of cubic lattice chains (simple cubic, body centered cubic, and face centered cubic). The properties of the rings are evaluated for the same thermodynamic conditions under which they are prepared thus allowing for a natural amount of knots which have been identified by use of Alexander polynomials. For the limit of infinite chain lengths the same theta parameter is found for linear chains and rings. On the contrary, a significant theta point depression occurs due to an additional excluded volume effect if unknots are exclusively regarded. Parameters characteristic of the shape of rings and chains under theta conditions extrapolated to infinite chain length fairly well coincide with respective data for random walks. Mean square dimensions (characteristic of the size) of theta systems are slightly in excess as compared to nonreversal random walks due to the necessity of avoiding overlaps on a local scale. Furthermore athermal systems are studied as well for comparison; mean square dimensions are described by use of scaling relations with proper short chain corrections, shape parameters are given in the limit of infinite chain length.     

MOF-Mediated Fabrication of a Porous 3D Superstructure of Carbon Nanosheets Decorated with Ultrafine Cobalt Phosphide Nanoparticles for Efficient Electrocatalysis and Zinc–Air Batteries

Superstructures have attracted great interest owing to their potential applications. Herein, we report the first scalable preparation of a porous nickel-foam-templated superstructure of carbon nanosheets decorated with ultrafine cobalt phosphide nanoparticles. Uniform two-dimensional (2D) Co-metal organic framework (MOF) nanosheets (Co-MNS) grow on nickel foam, followed by a MOF-mediated tandem (carbonization/phosphidation) pyrolysis. The resulting superstructure has a porous 3D interconnected network with well-arranged 2D carbon nanosheets on it, in which ultrafine cobalt phosphide nanoparticles are tightly immobilized. A single piece of this superstructure can be directly used as a self-supported electrode for electrocatalysis without any binders. This “one-piece” porous superstructure with excellent mass transport and electron transport properties, and catalytically active cobalt phosphide nanoparticles with ultrasmall size (3–4 nm), shows excellent trifunctional electrocatalytic activities for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR), achieving great performances in water splitting and Zn–air batteries.     

Thermomechanical properties and phase structure of epoxy/silica nano‐hybrid materials constructed from a linear silicone oligomer

Epoxy‐grafted silicone oligomer (ESO), which has a linear silicone chain in the backbone moiety, was synthesized from a trifunctional alkoxysilane via a sol–gel reaction. Characterization of ESO was performed with ¹H and ²⁹Si NMR, Fourier transform infrared, and gel permeation chromatography. The number‐average molecular weight of ESO was 3300. By adding the silicone oligomer as the inorganic source in the curing process of the epoxy resin, novel epoxy/silica hybrid materials were prepared. It was observed by transmission electron microscope that fine silica‐rich domains of about 5‐nm diameter were uniformly dispersed in the cured epoxy matrix. Thermomechanical properties of the hybrid materials were also investigated. The storage modulus in the rubbery region and the peak area of the tan δ curve at the glass‐transition region increased and decreased, respectively, with the hybridization of the silica network. The mobility of the epoxy network chains should be considerably suppressed by the hybridization with the silica network. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1631–1639, 2005     

Cholic acid-based fluorescent probes for enantioselective recognition of trifunctional amino acids

The ditopic fluorescent photoinduced electron transfer (PET) amino acid sensory probes and were designed and synthesized from cholic acid. To confer the probes with specific binding ability, an amidothiourea moiety and a cyclic diamino-chiral receptive site were introduced on the C17 side chain and the C7 and C12 hydroxyl pendants, respectively. In acetonitrile, the probes demonstrated differential binding toward trifunctional amino acids like serine, lysine, threonine and tyrosine against other simple amino acids. Enantioselectivities (KD/KL) of up to 8.9 and sensitivities in the micromolar range with the probes were observed for trifunctional amino acids.     

Internal motion in organosilicon polymers. II. Dimethylpolysilazanes crosslinked through silicon

An investigation of the internal motion in organosilicon polymers by wideline nuclear magnetic resonance has been extended to a pair of dimethylpolysilazanes crosslinked through trifunctional silicon. The data suggest that there is considerable internal motion in all silazanes at 77°K. Evidence is presented for the presence of C₃ rotation of the methyl groups, as well as rotation or torsional oscillation of the Si(CH₃)₂ groups about the polymer backbone. Upon warming the NMR line is seen to narrow, and this is associated with the onset of additional motion, including chain translation and chain flexing or bending. Crosslinking through silicon increases the barrier to chain translation while decreasing the barrier to chain flexing or bending.     

Prediction of three-dimensional fractal dimensions using the two-dimensional properties of fractal aggregates

Fractal dimension analysis using an optical imaging analysis technique is a powerful tool in obtaining morphological information of particulate aggregates formed in coagulation processes. However, as image analysis uses two-dimensional projected images of the aggregates, it is only applicable to one and two-dimensional fractal analyses. In this study, three-dimensional fractal dimensions are estimated from image analysis by characterizing relationships between three-dimensional fractal dimensions (D(3)) and one (D(1)) and two-dimensional fractal dimensions (D(2) and D(pf)). The characterization of these fractal dimensions were achieved by creating populations of aggregates based on the pre-defined radius of gyration while varying the number of primary particles in an aggregate and three-dimensional fractal dimensions. Approximately 2000 simulated aggregates were grouped into 33 populations based on the radius of gyration of each aggregate class. Each population included from 15 to 115 aggregates and the number of primary particles in an aggregate varied from 10 to 1000. Characterization of the fractal dimensions demonstrated that the one-dimensional fractal dimensions could not be used to estimate two- and three-dimensional fractal dimensions. However, two-dimensional fractal dimensions obtained statistically, well-characterized relationships with aggregates of a three-dimensional fractal characterization. Three-dimensional fractal dimensions obtained in this study were compared with previously published experimental values where both two-dimensional fractal and three-dimensional fractal data were given. In the case of inorganic aggregates, when experimentally obtained three-dimensional fractal dimensions were 1.75, 1.86, 1.83+/-0.07, 2.24+/-0.22, and 1.72+/-0.13, computed three-dimensional fractal dimensions using two-dimensional fractal dimensions were 1.75, 1.76, 1.77+/-0.04, 2.11+/-0.09, and 1.76+/-0.03, respectively. However, when primary particles were biological colloids, experimentally obtained three-dimensional fractal dimensions were 1.99+/-0.08 and 2.14+/-0.04, and computed values were both 1.79+/-0.08. Analysis of the three-dimensional fractal dimensions with the imaging analysis technique was comparable to the conventional methods of both light scattering and electrical sensing when primary particles are inorganic colloids.