Effect of polymer glycols on micro-aggregation structure and mechanical properties of Spherulite size graded polyurethane elastomers

Spherulite size graded polyurethane elastomers (PUEs) were prepared using a mold with a temperature gradient from different polymer glycols, 4,40-diphenylmethane diisocyanate (MDI) and a mixture of 1,4-butanediol (BD), and 1,1,1-trimethylol propane (TMP) as a curing agent by a prepolymer method. The used polymer glyocols were poly(oxytetramethylene)glycol (PTMG, M _n = 2000), poly(ethylene adipate)glycol (PEA, M_n = 2000), and poly(hexamethylene adipate)glycol (PHA, M_n = 2000). The effect of polyether and polyester glycols and the temperature gradient on the micro-aggregation structure and mechanical properties were studied by polarized optical microscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy, micro-hardness test, pulsed nuclear magnetic resonance spectroscopy, and tensile test. The prepared PUEs had spherulite size graded super-structure. Spherulite sizes of the PUEs depended significantly on the temperature gradient of the mold. Spherulite of the PUEs in contact with the mold at low temperature was significantly smaller and more dense than that contacting the mold at high temperature. The spherulite diameter of PTMG-, PEA-, and PHA-PUEs changed from 12.6 to 16.3 m̈m, 8.4 to 15.6 m̈m, and 10.4 to 16.0 m̈m, respectively from the lower temperature side (LTS) toward the higher temperature side. In contrast, the number of spherulites became sparser toward the higher one. The glass transition temperature (T_g) of PTMG-, PEA-, and PHA-based PUEs elevated from -55.3°C, -19.0°C and -33.8°C at the LTS to -48.6°C, -17.8°C and -32.8°C at the HTS, respectively. T_g of all PUEs exhibited elevation from the LTS toward the higher one. This result associated with the strong microphase separation of the LTS.