High cis polybutadienes (cis-PB) with different molecular weights

molecular weight distributions (MWDs) and microstructures have been prepared via coordination polymerization of butadiene with rare earth catalyst system.The commercial cis-PB products (A9712

C9520 and C9634) were also selected for comparison.Isothermal crystallization kinetics and crystal morphology of cis-PB samples were investigated at various temperatures ranging from -18℃ to -27℃ by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) equipped with in situ heating and cooling device

respectively.The Avrami exponent (n) for various cis-PB samples was determined to be in the range of 2.0~3.0

which indicated that the isothermal crystallization of all the cis-PB samples proceeded via three-dimensional spherulite growth.The spherulites with different sizes and shapes could be observed for the cis-PB samples with different molecular weights

MWDs and cis-1

4 contents.The size of spherulites increased with increasing cis-1

4 contents of cis-PB samples under the same crystallization conditions.The half-crystallization time (t1/2) decreased and the crystallization rate increased with an increase in cis-1

4 content of cis-PB samples by keeping the other conditions almost the same when the cis-1

4 content of cis-PB samples was lower than 98.4%.However

the crystallization rate decreased with further increasing the cis-1

4 content of cis-PB samples from 98.4% to around 99.0%.The crystallization could be greatly accelerated with simultaneous increases in molecular weight and cis-1

4 content of cis-PB samples by keeping the polydispersity almost the same.The crystallization could also be accelerated with narrowing molecular weight distribution of cis-PB samples by keeping the molecular weight and cis-1

4 content almost the same for the cis-PB samples.It was found that the degree of undercooling (ΔT)

a driving force for crystallization

increased with increasing the cis-1

4 content of cis-PB samples.The crystallization activation energy (ΔE) was determined to be in the range of -115~-140 kJ/mol.The negative values suggest that the crystallization could be accelerated with decreasing temperature.The ΔE values decreased from -115 kJ/mol to -130 kJ/mol with increasing the cis-1

4 content of cis-PB samples from 95.2% to 98.2%

indicating that the crystallization rate could be increased more greatly with decreasing temperature for the cis-PB samples with higher cis-1

4 contents when cis-1

4 contents were lower than 98.2%.Moreover

the ΔE values kept at around -137 kJ/mol when the cis-1

4 content of cis-PB samples was further increased from 98.2% to 99.0%

indicating that the dependence of crystallization rate on temperature was similar for the cis-PB samples with more than 98.2% of cis-1

4 contents.