Citation: . STRUCTURAL CHARACTERIZATION OF POLYETHYLENES BY DSC ANALYSIS AFTER CRYSTALLIZATION SEGREGATION*[J]. Chinese Journal of Polymer Science, ;2000, 18(6): 557-564. shu

STRUCTURAL CHARACTERIZATION OF POLYETHYLENES BY DSC ANALYSIS AFTER CRYSTALLIZATION SEGREGATION*

Received Date: 22 March 2000

The molecular structure of polyethylene (PE) samples with various comonomers including propylene, 1-buteneand 1-hexene was investigated by DSC and 13C-NMR techniques. The density of the samples varies from 0.948 g/cm3 to0.917 g/cm3, and the molecular weight determined by the GPC method is in the range of 1～2×105. The branch point contentof the samples was determined by 13C-NMR measurements and was found to be less than 20 per 1000 C atoms along themain 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 2.5h.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,TmAV,and the root-mean-square deviation of melting temperature,(△Tm2 AV)1/2 were proposed.TmAV is corresponding to the average segment length due to branching and(△Tm2 AV)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 of 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.
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