Unraveling the Glass-like Dynamic Heterogeneity in Ring Polymer Melts: From Semiflexible to Stiff Chain.

Ring polymers are an intriguing class of polymers with unique physical properties, and understanding their behavior is important for developing accurate theoretical models. In this study, we investigate the effect of chain stiffness and monomer density on the static and dynamic behaviors of ring polymer melts using molecular dynamics simulations. Our first focus is on the non-Gaussian parameter of center-of-mass displacement as a measure of dynamic heterogeneity, which is commonly observed in glass-forming liquids. We find that the non-Gaussianity in the displacement distribution increases with the monomer density and stiffness of the polymer chains, suggesting that excluded volume interactions between centers of mass have a strong effect on the dynamics of ring polymers. We then analyze the relationship between the radius of gyration and monomer density for semiflexible and stiff ring polymers. Our results indicate that the relationship between the two varies with chain stiffness, which can be attributed to the competition between repulsive forces inside the ring and from adjacent rings. Finally, we study the dynamics of bond-breakage virtually connected between the centers of mass of rings to analyze the exchanges of intermolecular networks of bonds. Our results demonstrate that the dynamic heterogeneity of bond-breakage is coupled with the non-Gaussianity in ring polymer melts, highlighting the importance of the bond-breaking method in determining the intermolecular dynamics of ring polymer melts. Overall, our study sheds light on the factors that govern the dynamic behaviors of ring polymers.

Effects of chain length on Rouse modes and non-Gaussianity in linear and ring polymer melts.

The dynamics of ring polymer melts are studied via molecular dynamics simulations of the Kremer-Grest bead-spring model. Rouse mode analysis is performed in comparison with linear polymers by changing the chain length. Rouse-like behavior is observed in ring polymers by quantifying the chain length dependence of the Rouse relaxation time, whereas a crossover from Rouse to reptation behavior is observed in linear polymers. Furthermore, the non-Gaussian parameters of the monomer bead displacement and chain center-of-mass displacement are analyzed. It is found that the non-Gaussianity of ring polymers is remarkably suppressed with slight growth for the center-of-mass dynamics at long chain length, which is in contrast to the growth in linear polymers for both the monomer bead and center-of-mass dynamics.

Understanding the scaling of boson peak through insensitivity of elastic heterogeneity to bending rigidity in polymer glasses.

Amorphous materials exhibit peculiar mechanical and vibrational properties, including non-affine elastic responses and excess vibrational states, i.e., the so-called boson peak (BP). For polymer glasses, these properties are considered to be affected by the bending rigidity of the constituent polymer chains. In our recent work [Tomoshige,et al2019,Sci. Rep.919514], we have revealed simple relationships between the variations of vibrational properties and the global elastic properties: the response of the BP scales only with that of the global shear modulus. This observation suggests that the spatial heterogeneity of the local shear modulus distribution is insensitive to changes in the bending rigidity. Here, we demonstrate the insensitivity of elastic heterogeneity by directly measuring the local shear modulus distribution. We also study transverse sound wave propagation, which is also shown to scale only with the global shear modulus. Through these analyses, we conclude that the bending rigidity does not alter the spatial heterogeneity of the local shear modulus distribution, which yields vibrational and acoustic properties that are controlled solely by the global shear modulus of a polymer glass.

Effect of dietary calcium deficiency and altered diet hardness on the jawbone growth: A micro-CT and bone histomorphometric study in rats.

OBJECTIVES: We examined the effects of a low-calcium diet and altered diet hardness on bone architecture and metabolism in the maxilla and mandible. MATERIALS AND METHODS: Male rats (n=48, 3 weeks old) were divided into six groups. In total, 24 rats were given a normal-calcium diet and the others were given a low-calcium diet. Each group was then divided into three subgroups, which were fed a 'hard̕ diet for 8 weeks, a 'soft̕ die for 8 weeks, or switched from the soft diet after 4 weeks to the hard diet for 4 weeks. The bone architecture was analyzed using cephalometry and micro-computed tomography, in addition, the bone metabolism was analyzed using serum bone markers and bone histomorphometry in the maxilla and mandible. Moreover, the bone formation patterns were evaluated using histopathologically in the midpalatal suture. RESULTS: The low-calcium diet affected bone architecture by increasing bone turnover and the soft diet affected bone architecture mainly by increasing bone resorption. The soft diet changed the chondrocyte cell layers into fibrous connective tissues in the midpalatal suture. At 4 weeks after the return to a hard diet from a soft diet, recovery of the deterioration in bone architectures was seen in the maxilla and mandible. CONCLUSIONS: We demonstrated that mastication with a hard diet is effective for recovering the collapsed equilibrium of jaw bone turnover and the deteriorating jaw bone architectures due to the poor masticatory function during the growing period.

Influence of differences in the hardness and calcium content of diets on the growth of craniofacial bone in rats.

OBJECTIVE: To examine the effects of a soft diet and a low-calcium diet on the craniofacial growth and bone architectures of the maxilla and mandible. MATERIALS AND METHODS: Male rats (n  =  20, 3 weeks old) were divided into four groups. Ten rats were given a normal-calcium diet, and the other rats were given a low-calcium diet. Each group was then divided into two subgroups, which were fed a hard or a soft diet. After 4 weeks, craniofacial growth and architecture in maxillary and mandibular bone were analyzed using cephalometry, micro-computed tomography, and histopathology. RESULTS: The low-calcium diet had no effect on serum calcium levels. The low-calcium diet had the greatest effect on craniofacial bone growth, while the soft diet affected the growth of several bone sites that are attached to the masseter muscle. A low-calcium diet resulted in the deterioration of the connectivity of the trabeculae in the furcation region of the maxillary and mandibular first molar, while a soft diet resulted in the diffuse disappearance of trabeculae in the central part of the furcation regions. In the midpalatal suture, a low-calcium diet resulted in inhibition of cartilaginous ossification, although the midpalatal suture had a normal cartilaginous structure. A soft diet resulted in narrower cartilage cell layers in the midpalatal suture. CONCLUSIONS: We demonstrated that a low-calcium diet and a soft diet resulted in a deterioration of bone structures in both the maxilla and in the mandible; however, the mechanisms underlying these effects differed between diets.

Microstructural analysis of three-dimensional canal network in the rabbit lumbar vertebral endplate.

Insufficient nutrient supply through vertebral canal structures to the intervertebral disc (IVD) has been considered as an important contributor for disc degeneration. Despite previous canal structure characterization studies using histology, scanning electron microscopy, and angiography, among others, their three-dimensional (3D) topology inside the vertebral endplate remains poorly understood. This study aims to characterize the 3D canal structure in the rabbit lumbar vertebral endplate using micro computed tomography (µCT). Vertebral endplates were imaged using high-resolution µCT with 1.4 × 1.4 × 1.8 µm voxel size. Diameter, length, orientation, and depth starting from the vertebral endplate surface were analyzed for each canal using individual 3D canal models from the vertebral endplate scans. In the layer underneath the vertebral endplate, at a mean depth of 76.2 µm, longitudinally-oriented relatively short-length (57.6 µm) and small diameter (45.7 µm) canals were dominant. Large-scale canals with a mean diameter of 152.1 µm running parallel to the endplate surface were isolated at the depth of 224.1 µm. These canals were connected to both IVD and bone marrow spaces through vertically oriented canals.