The planar elongational melt rheology and structural properties of dendrimers and hyperbranched polymers of different molecular weights (generations 1-4) and their linear counterparts have been studied using nonequilibrium molecular dynamics simulation techniques in the isothermal-isobaric ensemble. The extensional viscosity showed three distinctive regions against strain-rate, including an initial Newtonian region at low strain-rates, followed by a thickening behavior at medium strain-rates and terminated with a thinning region at very high strain-rates, in agreement with the Sarkar and Gupta model [J. Rein. Plast. Comp. 20, 1473-1484]. In addition, a structural analysis was performed to study the size, shape, and spatial distributions within globular dendrimers and hyperbranched polymer molecules under planar elongational flow (PEF). Ratios of the eigenvalues of the gyration tensor showed that contrary to shear flow, under PEF even at low strain rates, dendrimers and hyperbranched molecules have ellipsoidal conformations and change to a much more flattened prolate shape at higher strain rates. In combination with the eigenvalue ratios, the distribution of monomers from the central core of the molecules showed that the thickening region occurs due to branches being stretched, and terminal thinning behavior stems primarily from flow-induced alignment and finite extensibility effects. V

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Shape memory elastomers (SMEs) are a class of intelligent materials characterized by their ability to deform and recover shapes under applied force and external stimuli. Heat and ultraviolet radiation are examples of the most common external stimuli. With the emerging prevalence of internet of things devices and the ensuing need for smart materials and structures, SMEs provide significant opportunities to support the development of novel applications in robotics, remotely actuated systems, and packages, including those promised for the space industry. To harness the immense potential in the emerging applications of these materials, one approach is the systematic multi‐scale modeling coupled with artificial intelligence‐assisted design leading to the development of next‐generation intelligent systems. This review covers several aspects of the synthesis/materials chemistry and applications of SMEs with a view towards enabling such an approach. The synthesis procedures emphasizing dynamic covalent bond reactions are reviewed. Then, liquid crystalline elastomers are introduced as a specific elastomeric material class that exhibits excellent shape memory characteristics and distinctive transition temperatures. The utilization of advanced manufacturing methods such as additive manufacturing, three‐dimensional printing, and the emerging four‐dimensional printing technologies assisted by machine learning are detailed in producing and predicting SMEs. Finally, the current trends in the use of SMEs are summarized in areas of industrial and space engineering and biomedical applications.

Molecular dynamic (MD) simulation techniques are increasingly being adopted as efficient computational tools to design novel and exotic classes of materials for which traditional methods of synthesis and prototyping are either too costly, unsafe, and time-consuming in laboratory settings. Of such class of materials are liquid crystalline elastomers (LCEs) with favorable shape memory characteristics. These materials exhibit some distinct properties, including stimuli responsiveness to heat or UV and appropriate molecular structure for shape memory behaviors. In this work, the MD simulations were employed to compare and assess the leading force fields currently available for modeling the behavior of a typical LCE system. Three force fields, including Dreiding, PCFF, and SciPCFF, were separately assigned to model the LCE system, and their suitability was validated through experimental results. Among these selected force fields, the SciPCFF produced the best agreement with the experimentally measured thermal and viscoelastic properties compared to those of the simulated steady-state density, transition temperature, and viscoelastic characteristics. Next, shape fixity (R f ) and shape recovery (R r ) of LCEs were estimated using this force field. A fourstep simulated shape memory procedure proceeded under a tensile mode. The changes in molecular conformations were calculated for R f and R r after the unloading step and reheating step. The results revealed that the model LCE system exhibit characteristic behaviors of Rf and Rr over the thermomechanical shape memory process, confirming the suitability of selected force field for use in the design and prediction of properties of typical LCE class of polymers. I. INTRODUCTIONShape memory elastomers (SMEs) are a significant member of intelligent polymers which can be deformed to another temporary shape and subsequently recovered to their permanent shape again. 1,2 This process is typically achieved through an applied load and forms of trigger by an external stimulus. 3 Currently, a variety of external stimuli are utilized to activate shape memory effects, including heat 4 , pH 5 , water 6 , UV light 7 , or electric and magnetic fields 8,9 . Among these, heat is the most commonly used stimulus. Furthermore, molecular architecture of these materials, containing net-points (hard phase) and switch units (soft phase), is the crucial determining factor which promotes the shape memory behavior. 10 The net-points responsible for maintaining the permanent shape are generated by covalent

We present nonequilibrium molecular dynamics (NEMD) simulation results for the miscibility, structural properties, and melt rheological behavior of polymeric blends under shear flow. The polymeric blends consist of chemically identical linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1-4. The number fraction x of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. The miscibility of blend species is measured, using the pair distribution functions gDL, gLL, and gDD. All the studied systems form miscible blend melts under the conditions investigated. We also study the effect of shear rate γ̇ and dendrimer generation on inter-penetration between blend species for different blend systems. The results reveal that shear flow increases the interpenetration of linear chains toward the core of the dendrimers. We also calculate the shear-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the shear-induced deformation of the molecules in the blend. Melt rheological properties including the shear viscosity and first and second normal stress coefficients obtained from NEMD simulations at constant pressure are found to fall into the range between those of pure dendrimer and pure linear polymer melts.

This work presents a framework governing the development of an efficient, accurate, and transferable coarse-grained (CG) model of a polyether material. The framework combines bottom-up and top-down approaches of coarse-grained model parameters by integrating machine learning (ML) with optimization algorithms. In the bottom-up approach, bonded interactions of the CG model are optimized using deep neural networks (DNN), where atomistic bonded distributions are matched. In the top-down approach, optimization of nonbonded parameters is accomplished by reproducing the temperature-dependent experimental density. We demonstrate that developed framework addresses the thermodynamic consistency and transferability issues associated with the classical coarse-graining approaches. The efficiency and transferability of the CG model is demonstrated through accurate predictions of chain statistics, the limiting behavior of the glass transition temperature, diffusion, and stress relaxation, where none were included in the parametrization process. The accuracy of the predicted properties are evaluated in context of molecular theories and available experimental data.

We apply our recent continuum theory for stress-gradient-induced migration of polymers in solution (G. Zhu et al., J. Rheol., 2016, 60, 327-343) to rotational shearing flow in the gap between concentric cylinders (the so-called Taylor-Couette flow), where we have also accounted for the effect of polymer depletion from the solid boundaries on migration patterns. The steady-state distribution of dilute solutions of polymer dumbbells, obtained both using a systematic perturbation analysis in terms of the Weissenberg number (Wi) and by solving numerically the transport problem coupled to the upper-convected Maxwell equation, shows accumulation of polymers near the inner cylinder. This accumulation becomes so strong that most polymers are driven near the inner cylinder once Wi > 4. We also show that there is no first-order contribution to the polymer migration in Taylor-Couette flow due to the absence of a velocity component in the r-direction. Brownian dynamics (BD) simulations for a Hookean dumbbell give a concentration distribution in good agreement with the theoretical predictions of our theory, confirming the accuracy of the theory when the dumbbell radius of gyration is around an order of magnitude or much smaller than the gap. The demonstration of the accuracy of our continuum theory by direct molecular simulation opens the door to application of the theory to journal bearing and other lubrication flows containing polymers that may migrate due to stress gradients.

The structure and rheology of model polymer blends under planar elongational flow have been investigated through nonequilibrium molecular dynamics simulations. The polymeric blends consist of linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1 - 4. The number fraction, x, of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. We study the effect of extension rate, dendrimer generation, and dendrimer number fraction on pair distribution functions for different blend systems. We also calculate the extension-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the elongation-induced deformation of the molecules in the blend. Melt rheological properties including the first and second extensional viscosities are found to fall into the range between those of pure dendrimer and pure linear polymer melts, which are correlated with the mass fraction and generation of the dendrimers in the blend.