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1 Materials Science Poland

1 2005

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Segmented block copolymers of natural rubber and 1,4-butanediol-toluene diisocyanate oligomers

Gopakumar S., Paul C. J., Gopinathan Nair M. R.

Materials Science Poland

2005

Vol. 23, No. 1

227--245

Segmented block copolymers were synthesized from hydroxyl terminated liquid natural rubber (HTNR) and polyurethane oligomers (PU), formed from 1,4-butanediol (1,4-BDO) and toluene diisocyanate (TDI). The samples were synthesized in solution in two series by the one-step and two-step methods. They were characterized by spectral analysis, thermal analysis, etching studies, microscopy and mechanical testing. IR and NMR spectral data, along with etching studies, support the notion that a chemical reaction leads to block copolymerization. Differential scanning calorimetric (DSC) analysis showed a soft segment glass transition temperature between -62 °C and -63 °C and a hard segment glass transition temperature between 87 °C and 100 °C for different samples. This observation and two-stage thermal decomposition of the samples in thermogravimetric analysis (TGA) clearly indicate that the block copolymers are completely phase-segregated systems. The amorphous heterophase morphology of the samples is indicated by SEM, which shows well-defined beads of hard phase dispersed in a matrix. SEM results, along with the etching studies, revealed that the samples are systems consisting of block copolymers and some quantity of uncoupled polyurethane homopolymers in the form of beads. The homopolymer beads suggest that the efficiency of the chain extension in the present method of synthesis is slightly lower than expected. Tensile properties improved with the hard segment content in the samples. Low hard segment content leads to a flexible elastomer, while at the high one rigid plastics are formed. Intermediate compositions yielded rigid elastomers. Two-step samples showed slightly better properties compared to the one-step samples. This may be due to the systematic way by which the reaction progresses in two-step synthesis. The overall properties are found to be lower than that of the conventional polyurethane elastomers. This is attributed to the absence of phase mixing and the inability of the soft segments to crystallise under strain due to their short segments.