We compare predictions of two of the most advanced versions of the tube model, namely the "Hierarchical model" by Wang et al. [J. Rheol. 2010, 54, 223] and the BoB (branch-on-branch) model by Das et al. [J. Rheol. 2006, 50, 207], against linear viscoelastic G′ and G″ data of binary blends of nearly monodisperse 1,4-polybutadiene 4-arm star polymer of arm molar mass 24 000 g/mol with a monodisperse linear 1,4-polybutadiene of molar mass 58 000 g/mol. The star was carefully synthesized and characterized by temperature gradient interaction chromatography and by linear rheology over a wide frequency region through time−temperature superposition. We found large failures of both the Hierarchical and BoB models to predict the terminal relaxation behavior of the star/linear blends, despite their success in predicting the rheology of the pure star and pure linear polymers. This failure occurred regardless of the choices made concerning constraint release, such as assuming arm retraction in "fat" or "skinny" tubes. Allowing for "disentanglement relaxation" to cut off the constraint release Rouse process at long times does lead to improved predictions for our blends, but leads to much worse predictions for other star/linear blends described in the literature, especially those of Shivokhin et al. [Macromolecules 2014[Macromolecules , 47, 2451. In addition, our blends and those of Shivokhin et al. were also tested against a coarse-grained slip-link model, the "clustered fixed slip-link model (CFSM)" of Schieber and co-workers [J. Rheol. 2014, 58, 723], in which several Kuhn steps are clustered together for computational efficiency. The CFSM with only two molecular-weight-and chain-architecture-independent parameters was able to give very good agreement with all experimental data for both of these sets of blends. In light of its success, the CFSM slip-link model may be used to address the constraint release issue more rigorously and potentially help develop improved tube models.

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During a study of hereditary motor and sensory neuropathy-Lom in Bulgaria, a previously unrecognized neurological disorder was encountered, mainly in Wallachian Gypsies, who represent a relatively recent genetic isolate. The disorder has been termed the congenital cataracts facial dysmorphism neuropathy (CCFDN) syndrome to emphasize its salient features. Fifty individuals from 19 extended pedigrees were identified and examined clinically and electrophysiologically. At least 1 patient from each family was admitted to the hospital in Sofia for full investigation. Pedigree analysis indicates autosomal recessive inheritance. The disorder is recognized in infancy by the presence of congenital cataracts and microcorneas. A predominantly motor neuropathy beginning in the lower limbs and later affecting the upper limbs develops during childhood and leads to severe disability by the third decade. Associated neurological features are a moderate nonprogressive cognitive deficit in most affected individuals together with pyramidal signs and mild chorea in some. Accompanying nonneurological features include short stature, characteristic facial dysmorphism, and hypogonadotrophic hypogonadism. Nerve conduction studies suggest a hypomyelinating/demyelinating neuropathy, confirmed by nerve biopsy. The CCFDN syndrome is thus a pleomorphic autosomal recessive disorder displaying a combination of neurological and nonneurological features.

The discrete slip-link theory is a hierarchy of strongly connected models that have great success predicting the linear and nonlinear rheology of high-molecular-weight polymers. Three of the four parameters of the most-detailed model, which can be extracted from primitive-path analysis, give quantitative agreement with experimental data for all examined chemistries (polystyrene, polyisoprene, polybutadiene, and polyethylene). Here, we attempt to extract the remaining friction parameter from atomistic simulations. In particular, an available quantum chemistry-based force field for polyethylene oxide (PEO) was used to perform molecular-dynamics simulations of a 12 kDa melt. The Kuhn friction is obtained from the mean-squared displacement of the center-of-mass of the chains (MSD of COM) in the melt. The result is also corroborated using the relaxation modulus calculated through the Green–Kubo formula. Once the four parameters are determined for any chemistry, all parameters for all members of the slip-link hierarchy are determined. Then, using a coarser member of the hierarchy, the dynamic modulus of a 256 kDa PEO melt was predicted. The predictions are compared to experimental measurements performed at the same temperature. Unfortunately, the extracted friction is about 30% larger than the one observed in the experiment. However, two fundamentally different methods, one utilizing the MSD of COM and the other the relaxation modulus, gave consistent results for the extracted Kuhn friction. Therefore, the discrepancy presumably arises from insufficient accuracy in the force field. Nonetheless, the work demonstrates that theory predictions without adjustable parameters should be possible.

The mechanical properties of a cross-linked polydimethylsiloxane (PDMS) network swollen with nonreactive entangled PDMS solvent was previously studied experimentally. In this article, we use the discrete slip-link model to predict its linear and nonlinear rheology. Model parameters are obtained from the dynamic modulus data of pure solvent. Network rheology predictions also require an estimate of the fraction and architecture of dangling or inactive strands in the network, which is not directly measurable. The active strand fraction is estimated from dynamic modulus measurements, and the molecular weight is adjusted to fit the dynamic modulus data. Then, the nonlinear rheology can be predicted without adjustments. These successful predictions strongly suggest that the observed rheological modification in the swollen blend arises from the constraint dynamics between the network chains and the dangling ends. V C 2014 American Institute of Chemical Engineers AIChE J, 60: 1372AIChE J, 60: -1380AIChE J, 60: , 2014 We dedicate this article to Prof. R. Byron Bird on the occasion of his 90th birthday. We attempt to follow the twin exhortations of Bob to be both mathematically rigorous and practically relevant.