Low-intensity mixing process of high molecular weight polymer chains leads to elastomers of long network strands and high fatigue threshold.

Many polymer networks are prepared by crosslinking polymer chains. The polymer chains and crosslinkers are commonly mixed in internal mixers or roll mills. These intense processes break the polymer chains, lower viscosity, and ease mixing. The resulting polymer networks have short chains and a fatigue threshold of ∼100 J m-2. Here, we show that a low-intensity process, a combination of kneading and annealing, preserves long chains, leading to a network of polybutadiene to achieve a fatigue threshold of 440 J m-2. In a network, each chain has multiple crosslinks, which divides the chain into multiple strands. At the ends of the chain are two dangling strands that do not bear the load. The larger the number of crosslinks per chain, the lower the fraction of dangling strands. High fatigue threshold requires long strands, as well as a low fraction of dangling strands. Once intense mixing cuts chains short, each short chain can only have a few crosslinks; the strands are short and the fraction of dangling strands is high-both lower the fatigue threshold. By contrast, a low-intensity mixing process preserves long chains, which can have many crosslinks; the strands are long and the fraction of dangling strands is low-both increase the fatigue threshold. It is hoped that this work will aid the development of fatigue-resistant elastomers.

Effects of vegetation type on the microbial characteristics of the fissure soil-plant systems in karst rocky desertification regions of SW China.

In karst regions, shallow karst fissure (SKF) soil has proven to be an important plant habitat and soil resource. However, how plants affect the microbial abundance and community composition of SKF soil remains poorly studied. We explored the soil microbial community structure differences in fractured soil-plant systems by determining phospholipid fatty acid (PLFA) profiles under three vegetation types (herbs, shrubs and trees) in SKF and used a bare SKF without vegetation as the control in a karst rocky desertification area. The total microbial biomass and microbial community composition differed between surface soil and SKF soil. The total microbial biomass in surface soil was higher than that in SKF soil. In addition, in contrast to surface soil, the microbial communities in SKF soil were more vulnerable to the effects of environmental variables. Furthermore, plants had a significant positive effect on the accumulation of microbial biomass in surface and SKF soil: shrubs had the strongest effect, followed by trees. Vegetation types significantly affected the ratios of saturated PLFAs to monounsaturated PLFAs (SAT/MONO ratio) and cyclopropyl PLFAs to precursors (cy/pre ratio). In contrast to the SKF without vegetation, the SAT/MONO ratio and cy/pre ratio under grasslands, shrublands and trees were low. Herbs and shrubs had the greatest capacity to enhance the ability of soil to respond to environmental stress compared to trees. Our results suggest that, as an important plant habitat in karst regions, the condition of SKF soil should be urgently improved. The stereoscopic collocation of shrub-grass vegetation may be the preferred measure for vegetation restoration. Deep-rooted shrubs and grasses are best at improving soil and plant growth. Our study can be useful for developing strategies for vegetation rehabilitation in karst regions.

[Analysis on vegetations spectral characteristics along the altitudinal gradients in south-facing slope of Dangxiong Valley].

The present study focused on variation of vegetation types and canopy spectra along the altitudinal gradients in south-facing slope of Dangxiong valley in Tibet. Spectral extraction methods including red edge analysis and vegetation indices were used for vegetation spectral characteristics analysis. Through the hierarchical clustering analysis based on the vegetation spectral features, the feasibility of remote sensing classification of vegetation types along the elevation gradients in the experimental area was evaluated. The experimental results showed that: there were significant differences in spectral features including water index (WI), red edge POSITION (REP), and normalized difference vegetation index (NDVI) in different plots along elevation gradients in the study area, and there were strong correlations between WI and leaf water content, REP and dry biomass, NDVI and vegetation coverage. The hierarchical clustering analysis result of 12 vegetation samples along the altitudinal gradients is consistent with the ground survey, which shows that the selected vegetation spectral features can characterize the vertical distribution of vegetation types in the experimental area. The vegetation spectral analysis in this study can provide the priori knowledge support of spectral characteristics for the vegetation vertical distribution information extraction in the Tibet Plateau.

[Observation of CT three-dimensional reconstruction in the treatment of standard manipulation for lumbar intervertebral disc herniation].

OBJECTIVE: To observe the morphological change of prominence through CT three-dimensional reconstruction before and after manipulative treatment and in order to investigate biomechanical effect of manipulation in treating lumbar intervertebral disc herniation (LIDH). METHODS: From December 2009 to May 2010, 24 patients with LIDH (32 herniated discs) with the unilateral typing,which were treated with manipulation (on alternate day one time and every time about 20 min, 3 weeks as a course of treatment). There were 10 males and 14 females, ranging in age from 25 to 54 years with an average of 36.2 years, in course of disease from 2 days to 10 years with an average of 6.9 years. Protrusible 12 discs were in L4,5 and 20 discs were in L5S1. According to typing of distance between prominence and zygapophysial joint or vertebral plate (ligamentum flavum), 5 cases were type I, 13 cases were type II and 6 cases were type III. After a course of treatment,the morphological changes of prominences were analyzed in the same level of CT three-dimensional reconstruction, including contour map of nerve root sheath side distance (TD), the distance between prominence and zygapophysial joint or vertebral plate (ligamentum flavum), the deviated angle of prominence (AN value) and the sagittal index (SI value). RESULTS: From the contour map of TD, 19 patients (79.2% of the total) can be identified morphological changes after the treatment; from the distance between prominence and zygapophysial joint or vertebral plate (ligamentum flavum), 7 cases with type II turned into type I and 2 cases with type III turned into type II after treatment; AN value increased after treatment (P<0.05),it showed prominence occurred morphological change toward deviated direction of intervertebral foramina; there was no significant difference in SI value between before and after treatment (P>0.05). CONCLUSION: Standard manipulation can make prominence change, the prominence and nerve roots release, and mutual position improve,which can provide imaging evidence for the study in biomechanical effects.