This study presents a molecular model for the amplitude‐dependent dynamic moduli of polymer melts reinforced with nanoparticles. This study shows that intense strain‐thinning reported in experimental studies of polymer nanocomposites can be attributed to disentanglement of bulk polymer chains from those strongly adsorbed to the surface of nanoparticles. This flow‐induced relaxation is what is frequently termed as convective constraint release and is similar to the cohesive slip of polymer melt at solid interfaces.
Living ethylene/1‐olefin copolymerization with multiple comonomer feeding stages allows the production of living block copolymers (LBCs) with well‐controlled microstructures. A dynamic Monte Carlo model is developed to simulate the production of LBCs in a semibatch reactor, and it is used to study how the polymer microstructure evolves during the polymerization. The model also describes how chain transfer reactions affect the microstructure of LBC blocks. These model predictions provide useful guidelines for producing LBCs with precisely designed microstructures.
Dissipative particle dynamics simulation is employed to study the chain exchange kinetics between micelles of diblock copolymer in aqueous solution via in silico hybridization method. One focus is placed on the effect of chain flexibility on the dynamic behavior by varying the spring constant in the bead‐spring model. The length ratio of hydrophilic to hydrophobic block is also varied. It is found that chain expulsion/insertion is the dominant mechanism in the chain exchange process. The most interesting finding is the multimodal relaxation behavior for the chain exchange and expulsion when the spring constant is small or the length ratio of hydrophilic to hydrophobic block is large. This phenomenon is due to an increase in size polydispersity of micelles with rising population of small aggregates/micelles, for which the exchange kinetics is faster. Micelles with larger aggregation numbers (>10) are found to follow single exponential relaxation kinetics.
The structural determination and manipulation of bottle‐brush polymers, a class of polymers with serially grafted side‐chains, is challenging due to the interplay of side‐chain and backbone interactions over various length scales. The present work performs a detailed analysis, using molecular dynamics simulation techniques, to unravel these interactions by probing the distinct rod to a flexible real‐chain with self‐avoiding walk (SAW) type crossover in the backbone static structure factor. This analysis elucidates the deviation from flexible chain behavior, while also providing a quantitative measure of persistence length, . Significantly, the results identify a trend in which is consistent with the debated theoretical prediction of , where Ns is the number of monomers in each side‐chain of the bottle‐brush polymer.
The molecular systems, modeling composite materials, are formed by the epoxy‐amine oligomers with covalently bound chromophore‐containing dendritic fragments ( EAD ) as a host and additional azo‐chromophores as guests. The relationship between the structure and quadratic nonlinear optical characteristics of the model systems is established by atomistic modeling and quantum chemistry. The effect of the chain length of the model systems and the choice of the dendron structure, in particular, the length of the groups tethering chromophores to the branching center, on the values of first hyperpolarizability is studied. Molecular dynamics, performed for the model systems at various temperatures, reveals chromophores local mobility at temperature close to 130 °C; the calculations being fulfilled with the force field MMFF94s modified in terms of the ESP partial charges estimate at B3LYP/6‐31G(d) level.
A modeling pathway and software tool for linking entangled linear polymer molecular properties to linear viscoelasticity and melt index (MI) values is presented. A reptation model links molecular properties to the flow curve, and then, an ANSYS Polyflow model calculates MI values based on the flow curve predicted. The method is thoroughly tested and validated for uni‐ and bimodal, low‐ and high‐density polyethylene grades. An overall accuracy level in the range of 90% on average is exhibited, considering both model prediction steps: (i) molecular weight distribution to flow curve and (ii) flow curve to MI.
A unique fabrication process of low molar mass, crystalline polypeptoid fibers is described. Thermoresponsive fiber mats are prepared by electrospinning a homogeneous blend of semicrystalline poly(N‐(n‐propyl) glycine) (PPGly; 4.1 kDa) with high molar mass poly(ethylene oxide) (PEO). Annealing of these fibers at ≈100 °C selectively removes the PEO and produces stable crystalline fiber mats of pure PPGly, which are insoluble in aqueous solution but can be redissolved in methanol or ethanol. The formation of water‐stable polypeptoid fiber mats is an important step toward their utilization in biomedical applications such as tissue engineering or wound dressing.
By anchoring alkynylplatinum(II) terpyridine molecular tweezer/pyrene recognition motif on the chain‐ends of telechelic polycaprolactone, high‐molecular‐weight supramolecular polymers have been successfully constructed via noncovalent chain extension, which demonstrate fascinating rheological and thermal properties. Moreover, the resulting assemblies exhibit interesting temperature‐ and solvent‐responsive behaviors, which are promising for the development of adaptive functional materials.
Polydisperse linear polymers are studied in startup of steady shear flow simulations using dissipative particle dynamics. The results show that with an increase in polydispersity the stress overshoot declines while the steady‐state stress increases. Various physical characteristics of the systems are studied including frequency of nonbonded interactions, gyration radius data, flow alignment angles, and average bond lengths. The patterns in the data suggest higher forces are necessary to orient and stretch long chain fractions in the flow direction. Relaxation modulus data prove the broad range of relaxation mechanisms in polydisperse systems. Linear viscoelasticity theory is used to quantify the relaxation spectrum. The results indicate an increase in the longest relaxation time in systems with higher polydispersity. The steady‐state shear viscosity results show higher viscosities with an increase in polydispersity at all shear‐rates. The good agreement of the characteristic behaviors of modeled polydisperse polymers with experiments is encouraging for future work.
A statistical–mechanical theory of thermoreversible gelation which is formed by monodisperse telechelic associating polymers with junction multiplicity of three is developed. In the present theory, the effect of loop formation is considered. Using the theory, the properties of the system are obtained, such as the state of association of polymers, the sol–gel transition line, and the number concentration of elastically effective chains. In addition, a Monte Carlo simulation of a bead‐spring model of monodisperse telechelic associating polymers is performed. The simulation results are in good agreement with the present theoretical results. Furthermore, the shear modulus is calculated by an application of phantom network theory and compared with the experimental data. The theoretical results agree well with the experimental results. It is shown that the loop formation occurs especially in dilute regime and causes the decrease of the modulus in the regime.
The self‐assembly of a binary blend of nanoparticles in a homopolymer matrix using molecular dynamics (MD) simulations is studied here. The systems consist of polymer matrix, “bare” ungrafted spherical nanoparticles and polymer‐grafted nanoparticles, where the particle cores are identical and grafted chains are similar to matrix polymer. It is observed that addition of grafted nanoparticles to a blend of polymer and bare particles can result in the formation of anisotropic structures. By carefully selecting the graft density and molecular weight of the grafted chains, the clusters go from spherical to cylindrical to branched cylinders. This study suggests that it is indeed possible to control the morphology of bare nanoparticles in polymer without directly modifying their surface properties. It is believed that this phenomenon might be of high importance, especially in cases such as polymer‐based solar cells, where it is not feasible to graft the nanoparticles with polymer chains to achieve a greater level of control over the morphology.
In order to control the branching behavior of polymers, the comparison of experimental and simulated data is important. The utilization of a nonlattice, self‐avoiding necklace‐bead random walk simulator is reported, which allows for the calculation of radii of gyration rg of polymer molecules with branched structures. The focus is on sensitivity toward short‐chain branches, long‐chain branches (LCBs), and the copolymer composition. Using only two parameters—the size of monomer beads and the minimum angle between three subsequent beads—a fast and reliable parameter fit procedure based on experimental data is described. The procedure is exemplarily shown for copolymers of vinylidene fluoride and hexafluoropropene (HFP) with HFP contents in the copolymer of at most 0.3 and is easily transferable to other polymers that may be analyzed by size‐exclusion chromatography/multiangle laser light scattering close to θ conditions. Applying the Zimm–Stockmayer equation to simulated rg data allows for comparing the “effective” number of LCBs with the number of LCBs given by kinetic simulations. A tool for better estimation of rate coefficients associated with the formation of short‐ and long‐chain branches is provided.
Cationic polyelectrolytes showing an upper critical solution temperature (UCST) are synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization in water at a temperature well above the UCST. The polymerization is well controlled by the RAFT process, with excellent pseudo‐first‐order kinetics. The cloud point is highly dependent on the polyelectrolyte concentration, molecular weight, and presence of added electrolyte. Alkylation of a neutral polymer is also conducted to obtain polyelectrolytes with different hydrophobic groups, which are shown to increase the cloud point.
The morphological features of carbon nanotube (CNT) polymer composites and their influence on the effective modulus are evaluated. The considered features include bundle formation from the helical sub‐bundles made of individual CNTs. The formation of bundles is considered as a result of agglomeration of individual nanotubes above and below onset of percolation and is related to electrical conductivity. The proposed geometrical model yields a bundle diameter that agrees closely with that of the experimentally measured by voltage‐contrast method and scanning electron microscopy analysis of polyimide nanocomposites. The proposed micromechanical analytical model includes the helical structure of a bundle and provides close agreement of the effective Young's modulus of nanocomposite over a wide range of CNT content. It is shown that considering the helical structure of CNT bundles and its effect on bundle modulus is vital for predicting the effective modulus of CNT‐polymer nanocomposite.
Similar to the traditional self‐assembly strategy, polymerization induced self‐assembly and reorganization (PISR) can produce a myriad of polymeric morphologies through morphology transitions. Besides the chain length ratio (R) of the hydrophobic to the hydrophilic blocks, the chain mobility in the intermediate nano‐objects, which is a requisite for morphology transition, is a determining factor in the formation of the final morphology. Although various morphologies have been fabricated, hexagonally packed hollow hoops (HHHs) with highly ordered internal structure have not, to the best of our knowledge, been prepared by PISR. In this article, the fabrication of HHHs through morphology transition from large compound vesicles to HHHs is reported. HHHs with highly regular internal structure may have significance in theoretical research and practical applications of nanomaterials.
This paper reports on the synthesis of well‐defined polyacrylamide‐based nanogels via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization, highlighting a templateless route for the efficient synthesis of nanogels based on water‐soluble polymers. RAFT dispersion polymerization of acrylamide in co‐nonsolvents of water–tert‐butanol mixtures by chain extension from poly(dimethylacrylamide) shows well‐controlled polymerization process, uniform nanogel size, and excellent colloidal stability. The versatility of this approach is further demonstrated by introducing a hydrophobic co‐monomer (butyl acrylate) without disturbing the dispersion polymerization process.
In this work, the combined iterative Boltzmann inversion/conditional reversible work scheme is extended with a little modifications to derive the systematically coarse‐grained (CG) potentials for simulating two typical atactic polymer blends composed of poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC) or polystyrene (PS). Molecular dynamics simulations are extensively performed on the two blends with a wide formulation range. It is revealed by these simulations that, throughout the entire composition range, the PMMA/PVC blend is homogeneous whereas the PMMA/PS blend undergoes phase separation, which agrees well with the experimental observation that the former exhibits strong interactions that are absent in the latter. Depending upon the formulation, the immiscible PMMA/PS blend presents one single‐ or double‐continuous phase. It is further confirmed that intermolecular interactions play the key roles in forming the phase morphologies, which in turn can be inferred from only the three nonbonded CG potentials of one unlike pair and two like pairs.
An advanced Monte Carlo (MC) method is developed, using weight‐based selection of polymer chains, to predict the molecular weight distribution (MWD) and branching level for arborescent polyisobutylene (arbPIB) at the end of a batch reaction. This new weight‐based MC method uses differential equations and random numbers to determine the detailed structure of arbPIB molecules. Results agree with those from an advanced number‐based MC method. The proposed weight‐based algorithm requires approximately twice the computation time of the number‐based method, but produces more accurate results in the high‐molecular‐weight portion of the MWD when the same number of polymer chains is assembled.
