Dynamics of asymmetric star polymers under coarse grain simulations

The asymmetric star polymers are studied by coarse grain simulations. Each polymer chain is represented by number of consecutive soft blobs and additional uncrossability constraints are added to prevent chain crossings. In this work two types of asymmetric star polymers with different backbone lengths are structured. Their dynamical properties are discussed by comparisons with corresponding linear chains, the one covers chain length along with the asymmetric arm through the branch point to one of the symmetric arm, or the backbone chain between two symmetric arm ends, or the largest linear possesses the same molecular weight of the entire star. To reveal the influence of the asymmetric arm length on their relaxation decay times, the autocorrelation function of the vectors from each branching point to corresponding asymmetric arm end are calculated, results are compared with the symmetric star having the same backbone chain.

Influence of Strain on the Microstructure and Transverse Relaxation Characteristics of HTPB Coating Using Low-Field 1H NMR Spectroscopy

During the storage process, the HTPB (hydroxyl-terminated polybutadiene) coating is continuously affected by the strain, and the microstructure and mechanical properties will be degraded, which will seriously affect the performance of composite solid propellant and solid rocket motor and cause great harm. In order to analyze the microstructure and transverse relaxation characteristics of HTPB coating under different strains, low-field 1H NMR tests was carried out under 0%, 5%, 10% and 15% strain conditions, and the crosslinking density and transverse relaxation parameters of HTPB coating were analyzed. The results show that, the transverse relaxation decay can be divided into two segmental mobilities corresponding to two distinct transverse relaxation times. With the increase of strain, the crosslinking density shows a decline tendency, the transverse relaxation decay amplitude slows down, and the inversion curve has a tendency to move to the right. The ratio of the fast transverse relaxation time and the peak area are much larger than the slow transverse relaxation time, and the proportion of the fast relaxation time and the peak area enlarge with the increase of the strain, while the proportion of slow transverse relaxation time is reduced. With the increase of strain, there is a transition from slow transverse relaxation to fast transverse relaxation, and there is an inverse linear relationship between crosslinking density and transverse relaxation time.

Self-trapping relaxation decay investigated by time-resolved photoelectron spectroscopy

The present work combines time-resolved photoelectron spectroscopy on isolated species with high-level data processing to address an issue which usually pertains to materials science: the electronic relaxation dynamics towards the formation of a self-trapped exciton (STE).

Parameters characterizing the charge state of dielectrics

This paper presents three physical sources of the electric field in dielectrics: excess free volume charges with the distribution qv(x,y,z), free surface charges with the distribution qs(x,y,z) and frozen polarization state in the dielectric. They have a deciding influence on the parameters of the electret, in particular they determine the total lifetime of the electret and technical components made of it. The indeterminacy related to the mutual proportions of the spatial and surface charges was discussed: one can find an infinite number of distributions of surface qse(x,y,z) and spatial qve(x,y,z) charges leading to the same distribution of the electric field E(x,y,z). A general case of electret was considered, where a coexistence of relaxation decay of frozen polarization and Maxwellian relaxation dependent on volume conductivity of the dielectric is assumed. An attempt to interpret the charge lifetime in real electrets was made.

Assessing the Effect of Matrix Metalloproteinase-9 on the Growth of Mice Teeth

Dentin and bone are formed when odontoblasts and osteoblasts synthesize and secrete collagen type I-rich extracellular matrix that mineralizes in a highly controlled manner. A wide spectrum of mouse and human disorders affecting tooth and bone biomineralization shows that dentin and bone formation are under strict genetic control. Although the controlling mechanisms of dentinogenesis and osteogenesis require further study, a large body of evidence points to the importance of the matrix metalloproteinases (MMPs) participate in a wide variety of extracellular matrix degradation. Detailed knowledge of MMPs may be important for understanding the pathogenesis of tooth development. Some researchers have pointed MMP-9 is an extracelluar proteinase that is highly expressed in osteoclasts and has been postulated to play an important role in their resorptive activity. Although MMP-9 has been reported to play a role in bone resorption, the association of this enzyme during deciduous tooth resorption has not yet been clarified. Based on accumulating evidence, we hypothesized that MMP-9 should play a role in teeth attrition. In this study, we have applied NMR relaxation technique to assess age-related MMP-9 KO tooth quality in vitro by quantifying changes in dentin and pulp simultaneously. The major hypothesis in this paper was that whether noninvasive NMR relaxation time measurements could be used to characterize MMP-9 KO changes in dentin and pulp, and to predict tooth quality. Specifically, we tested that age-related MMP-9 KO tooth changes result in an alteration of the NMR spin-spin (T2) relaxation time signal due to the structural changes in the tooth matrix. This signal can be further processed to produce a T2 relaxation distribution spectrum related to dentin and pulp, and their derived parameters can be used as descriptors of age-related MMP-9 KO tooth changes. In this study, the proton liquid-like NMR spin-spin (T2) relaxation decay signal was obtained from the Carr-Purcell-Meiboom-Gill (CPMG) NMR spin echo train method [1,2], then the relaxation decay signal was converted to T2 relaxation distribution spectra describing the size domain of dentin and pulp. Therefore, we can calibrate the intensities in NMR inversion T2 relaxation distribution spectra corresponding to the amount of dentin and pulp related to the structural changes. Here, we propose an NMR calibration method “NMR standard estimation” — the ratio of the amount of pulp to the amount of dentin obtained from NMR T2 distribution spectra that can be used to measure the age-related MMP-9 KO structural changes in teeth [3]. We are cognizant of the biological and physiological variability manifest in teeth size variations, but feel that this kind of NMR standard estimation — the ratio of amount of dentin to amount of pulp from the NMR T2 inversion spectrum can be used to determine age-related MMP-9 KO structural changes in teeth and eliminate any variations in size of teeth.

Molecular Mobility of Unfilled and Carbon-Black-Filled Isoprene Rubber: Proton NMR Transverse Relaxation and Diffusion

In an effort to understand the effect of ultrasound on the devulcanization of gum and filled isoprene rubber vulcanizates, solid state NMR 1H transverse relaxation (T2) was employed to analyze rubber molecular mobility. The T2 relaxation decay of the unfilled and the black filled IR was successfully described by a two-component model. The short T2 component arose from the chemically crosslinked (gel) and physically entangled (heavy sol) network. The long T2 decay came from the unentangled sol and dangling network chain ends. Vulcanization decreased the molecular mobility; however, ultrasound devulcanization partially reversed this effect. Addition of processing oil in the filled IR significantly altered the dependence of T2 on the sol fraction. T2 and pulsed-gradient diffusion experiments were carried out on IR melt specimens after sonication with or without subsequent vulcanization. The lowered and broadened M-distribution produced results quantitatively related to earlier work in natural rubber.