Summary: Using molecular dynamics simulations, we determine the linear and nonlinear viscoelastic properties of a model polymer melt in the unentangled regime. Several approaches are compared for the computation of linear moduli and viscosity, including Green‐Kubo and nonequilibrium molecular dynamics (NEMD). An alternative approach, based on the use of the Rouse modes, is also discussed. This approach could be used to assess local viscoelastic properties in inhomogeneous systems. We also focus on the contributions of different interactions to the viscoelastic moduli and explain the microscopic mechanisms involved in the mechanical response of the melt to external solicitation.
Atom transfer radical polymerization (ATRP) is a robust method for the preparation of well‐defined (co)polymers. This process has also enabled the preparation of a wide range of polymer brushes where (co)polymers are covalently attached to either curved or flat surfaces. In this review, the general methodology for the synthesis of polymer brushes from flat surfaces, polymers and colloids is summarized focusing on reports using ATRP. Additionally, the morphology of ultrathin films from polymer brushes is discussed using atomic force microscopy (AFM) and other techniques to confirm the formation of nanoscale structure and organization.
A dextran‐based dual‐sensitive polymer is employed to endow gold nanoparticles with stability and pH‐ and temperature‐sensitivity. The dual‐sensitive polymer is prepared by RAFT polymerization of N‐isopropylacrylamide from trithiocarbonate groups linked to dextran and succinoylation of dextran after polymerization. The functionalized nanoparticles show excellent stability under various conditions and can be stored in powder‐form. UV and DLS measurements confirm that the temperature‐induced optical changes and aggregation behaviors of the particles are strongly dependent on pH.
Summary: New polymer gelators consisting of poly(propylene glycol) or poly(ethylene glycol) and L ‐lysine‐based low‐molecular‐weight gelators have been developed. These polymer gelators were synthesized according to a simple procedure with high reaction yield, and formed organogels in many organic solvents. The organogelation mechanism was proposed from the transmission electron microscopy and FTIR spectroscopy studies.
Structures of the polymer gelators synthesized here. 相似文献
Summary: A new strategy was developed to prepare disorderly exfoliated nanocomposites, in which a soft siloxane surfactant with a weight‐average molecular weight ( ) of 1 900 was adopted to modify the clay. The modified clay slurry was then mixed with silicone rubber by hand, and exfoliation was achieved. The proposed mechanism thereof was verified by TEM and XRD. The physical entanglement of the soft siloxane surfactant plays a vital role in the diffusion and intercalation of the matrix molecules during the compounding of the slurry‐polymer mixture. This simple method is applicable to other silicone‐based materials reinforced by clay.
TEM micrograph of silicone rubber/clay‐sil nanocomposite. 相似文献
Summary: Magnetic nanoparticles have been prepared by a co‐precipitation method and modified with methacryloxypropyltrimethoxysilane. Magnetic molecularly imprinted polymer particles have been prepared by suspension polymerization in silicone oil. The particles possess a high affinity to the template molecules and are rapidly separated under a magnetic field.
Amphiphilic polymer brushes grafted onto gold nanoparticles impart distinct solvent‐responsive behavior via the change to particle size and surface chemistry and, therefore, wide application prospects can be expected. Coarse‐grained simulations are performed for block and/or mixed polystyrene (PS)/poly(ethylene oxide) (PEO)‐modified amphiphilic gold nanoparticles (AuNP) to investigate their responsive behavior in five different solvents by analyzing their morphology, distribution density profiles, and gyration radii. Typical core–shell, Janus‐type, buckle‐like, ring‐like, jellyfish‐like, and octopus‐like morphologies are formed. Influence of block sequence, mixing mode, and several other effects are discussed. Responsive particle size and surface hydrophilicity can be successfully reproduced by altering solvents.
A simple approach to improve the structural ordering in block copolymer/nanoparticle nanocomposites is presented. It is to blend a small molecular weight homopolymer with the composites, which can uniformly swell the preferred domain where the nanoparticles locate and increase the conformational entropy of the domain. Consequently, the interfaces between the block copolymer domains become smooth that improves the long range order in the nanocomposites. Furthermore, the uniform swelling of the preferred domain by the homopolymer will allow higher loading of nanoparticles without adversely affecting the long range order.
Summary: A polymer‐iron oxide nanocomposite film has been successfully synthesized by using a microporous regenerated cellulose film as template and aqueous ferrous chloride as precursor. The nanocomposite film was investigated with XRD, XPS, SEM, TEM, and VSM. The nanoparticles synthesized in situ were disk‐shaped with a mean diameter of ≈24 and thicknesses of 2.5–3.5 nm. For the first time, the nanodisks were well aligned in the cellulose matrix to form an ordered multibilayer structure by the shrinkage of the hybrid film while drying. The nanocomposite film displayed anisotropic magnetic properties as a result of the alignment of magnetic nanodisks. This work provides a novel and facile method for template synthesis of nanoparticles and aligned nanocomposites.
TEM image of the air‐dried nanocomposite film; slice perpendicular to the film plane. 相似文献
The identification and control of a critical stage of polyaniline “nanotube” self‐assembly is presented, namely the granular agglomeration or growth onto nanorod templates. When the synthesis pH is held above 2.5, smooth insulating nanorods exhibiting hydrogen bonding and containing phenazine structures are produced, while below pH 2.5, small 15–30 nm granular polyaniline nanoparticles appear to agglomerate onto the available nanorod surface, apparently improving conductivity of the resulting structures by three orders of magnitude. This finding affects both fundamental theories of polyaniline nanostructure self‐assembly and their practical applications.
Catechin was oxidatively polymerized by laccase in a mixture of a polar organic solvent and buffer to give a new class of flavonoid polymers. Myceliophthora laccase showed high catalytic activity for the polymerization. Using a mixed solvent of acetone and pH 5 acetate buffer efficiently produced the polymer. Under the selected conditions, DMF‐soluble polymers were obtained in good yields. Effects of reaction parameters on the yield, solubility, and molecular weight of the polymer have been systematically investigated. A radical species was detected in the polymer by ESR spectroscopy. The polymers showed greatly amplified superoxide scavenging activity and xanthine oxidase inhibitory activity compared with monomeric catechin.
Superoxide scavenging activity of poly(catechin)s, n = 3. ○: catechin, □: poly(catechin). 相似文献
Results of different NMR investigations of elastomers are reviewed with respect to their significance for statistical models of rubber elasticity. In contrast to earlier work based on lineshape analysis and relaxometry, results of recent multiple‐quantum experiments indicate that the NMR‐detected dynamic chain order parameter, which reflects the conformational space of individual monomer units at which the signal is detected locally, is a rather narrowly distributed quantity. Constraints to the dynamics and the conformations of a network chain thus act uniformly and appear as a dynamic average over chains of different length and with different end‐to‐end separations. All our findings are in good agreement with large‐scale computer simulations. Anomalies on swelling such as chain desinterspersion at the early stages and the appearance of heterogeneities, are also discussed.
Polymer‐encapsulated silver nanoparticles were synthesized and sterically stabilized by a new core‐shell type system consisting of poly(S‐alt‐MA)‐graft‐PMMA copolymer that acts as a scaffold for the synthesis of size confined nanoparticles. The graft copolymer is synthesized via ambient temperature ATRP using the CuBr/PMDETA catalytic system at ambient temperature. The graft copolymer is hypothesized to function as a scaffold with the anhydride part interacting strongly with the silver ions, while the PMMA graft functions as a polymer brush that stabilizes the dispersion and prevents the particle aggregation due to a ‘polymer brush effect’. UV absorption and TEM studies confirm that the synthesized silver composite particles have a core‐shell structure.
One‐dimensional methyl orange fibrils can be easily prepared. They are stable in acidic aqueous solutions and soluble in neutral water. When used to synthesize conducting polymer microtubules, the fibrils act as “hard templates” formally but as “soft templates” effectively. Microtubular structures of polypyrrole, polyaniline, and poly(3,4‐ethylenedioxythiophene) have been achieved successfully via such water‐soluble versatile templates.
Summary: From kinetic theory we have rigorously derived singularity‐free Brownian dynamics analyses of nanoparticle rotational dynamics. The rigid non‐spherical nanoparticles incorporate all three rotational degrees of freedom. This was achieved by using the components of Cartesian rotation vectors as the generalized coordinates describing angular orientation. The new results constitute an important advance compared to the situation when Eulerian angles specify angular orientation. Our finding eliminates one of the main longstanding obstacles to detailed studies of nanoparticle rotational dynamics in the diffusion time domain. The described formalism is applicable to a wide range of nanoparticle systems including liquid crystals, biopolymers, and colloids.
The effects of temperature and solvent on the β‐phase formation and energy transfer in an Ir(III) complex‐containing polyfluorene were investigated. Efficient energy transfer from polyfluorenes host to Ir complexes guest can be realized at low temperature. The formation of β‐phase was observed both in THF solution at low temperature and as suspended nano‐particles at room temperature. In addition, phosphorescent polymer nanoparticles were prepared successfully and exhibited efficient phosphorescent emission.
Nanoparticles in a flexible polymer melt film often segregate to the substrate due to attractive depletion interactions between the nanoparticles and the substrate. Here, molecular dynamics simulations are performed to study the effect of chain stiffness on this segregation. The nanoparticles are modeled as spheres and the polymers as semi‐flexible bead‐spring chains. Both purely repulsive and attractive forces are considered, while assuming non‐selective interactions among all species. The nanoparticles are found to be well‐dispersed in the system having repulsive forces only and aggregate into clusters in the completely attractive system. For the repulsive system, adding chain stiffness substantially decreases the nanoparticles' segregation, and hence their concentration, at the substrate.
A solution processible polymer—poly(3,3‴‐didodecylquaterthiophene) (PQT‐12) is investigated at the liquid/solid interface using the scanning tunneling microscopy (STM). Two‐dimensional ordered films made up of self‐assembled domains, with dimensions of 100 nm × 50 nm adsorbed on highly oriented pyrolytic graphite (HOPG) were formed. These domains consist of parallel lamellar polymer chains, with the alkyl chains forming interdigitated structures, along with U‐shaped and closed ring segments of the polymer chains. A polymer chain packing model is proposed herein, which attempts to propose a correlation between the packing of long chains and charge mobilities. These STM results could help in understanding the relationship between the extended conjugation and molecular organization of the PQT‐12 chains.
