STRUCTURAL CHARACTERIZATION OF POLYETHYLENES BY DSC ANALYSIS AFTER CRYSTALLIZATION SEGREGATION

The molecular structure of polyethylene (PE) samples with various comonomers including propylene, I-butane and 1-hexene was investigated by DSC and C-13-NMR techniques. The density of the samples varies from 0.948 g/cm(3) to 0.917 g/cm(3), and the molecular weight determined by the GPC method is in the range of 1 similar to2 x 10(5). The branch paint content of the samples was determined by C-13-NMR measurements and was found to be less than 20 per 1000 C atoms along the main chain. Crystallization segregation DSC technique (CSDSC) was used to characterize the branch point distribution or the segment length distribution of PEs. The crystallization segregation was performed in a successive annealing process at decreasing temperatures. The interval of two successive annealing temperatures was 6 K, and the time length of each annealing step was 2.5 h. The CSDSC results clearly indicate that all the PE samples used, including some metallocene PEs, more or less exhibit their non-uniformity in segment length distribution, and bimodal or multimodal CSDSC curves were usually observed. For quantitative characterization of the CSDSC curves and the segment length distribution two parameters, the average melting point, T-mAV, and the root-mean-square deviation of melting temperature, (DeltaT(m)(AV)(2))(1/2), were proposed. TmAV is corresponding to the average segment length due to branching and (DeltaT(m)(AV)(2))(1/2) gives information about the: width of the segment length distribution. Experimental results show that both the degree of average melting temperature depression and the width of the distribution seem to increase with increasing the branching content and are dependent on the type of comonomers. Very good reproducibility and additivity of the CSDSC method were evidenced experimentally. It was concluded that the CSDSC technique is a sensitive and convenient method for characterizing the segment length distribution of branched polyethylenes and will be of great interest in structure-property relationship studies of crystalline polymers.

CRYSTALLIZATION SEGREGATION DSC STUDIES OF PP/PE COPOLYMERS

The structural analysis of two PP/PE copolymer samples,1 and 2,was conducted by using^(13)C-NMR,GPC and crystallization segregation DSC(CSDSC)techniques.A pure polypropylene sample was also used for comparison.It was found that the two copolymer samples are very close in composition(the ethylene mol content is 4.3%and 4.5%,respectively),stereoregularity(96%and 97%)and molecular weight(M_(w)=7.0 x 10^(4)and 7.3 x 10^(4);M-(w)/M_(n)=5.0 and 6.1,respectively).While the CSDSC thermograms of the two samples are quite different from each other.Sample 1 shows a higher average melting temperature and a broader distribution of its thermogram These phenomena were explained as an indication of a less uniform distribution of ethylene units along the PP chains for sample 1.It was noted that CSDSC is a very sensitive and convenient technique for structural studies of copolymers.

Preparation and tensile properties of DD5 single crystal castings

The preparation procedure of DD5 single crystal castings was optimized. The microstructure characteristics of DD5 single crystal superalloy were investigated by microstructure observation and segregation behavior examination. The results show that the grain orientation is optimized by constraining the spiral crystallizer in [001] orientation and spatial scale. Also, the γ＇ phase of inter-dendrites is larger and more irregular than that in dendrite arms. High temperature tensile tests of DD5 single crystal castings exhibit that the peak stress increases with increasing temperature, while the area reduction shows an opposite trend, when the temperature is below 800℃; meanwhile, when the temperature is between 800℃ and 1000℃, the fracture stress of the alloy is the same as the peak stress. The fracture mode changes from shear to ductile with increasing temperature from 600℃ and 1000℃.

Study on Intermediate Crack in Continuous Casting Slab of Medium Carbon Steel

According to the analysis of the macro and micro characteristics of the center of continuous casting blooms by acid wash, SEM, metallographic and scanning-type electron microscope, there was a serious segregation and some shrinkage cavities in the slab; Cracks occurred at the end of the columnar crystal and spread along the boundary of the primary dendrite. At the crack enriched a large number of impurity elements, carbon, phosphorus, sulfur, whose crystal boundary segregation is the internal factor of the intermediate cracks. Determining the originated location of intermediate cracks and calculating the total strain in this area during solidification process of slab, it is found that the load is the ex- ternal factor of cracks’ generation and expansion.