稀土高顺式聚丁二烯等温结晶动力学及结晶形态的研究

Isothermal Crystallization Kinetics and Morphological Features of High Cis Polybutadiene Produced with Rare Earth Catalyst

通过稀土催化丁二烯配位聚合制备了一系列不同分子量、分子量分布及顺-1,4结构含量的稀土高顺式聚丁二烯(cis-PB),采用扫描示差量热仪法(DSC)研究了cis-PB在-18^-27℃范围内等温结晶过程,采用配有在线冷热台的偏光显微镜(POM)观测结晶形态.结果表明,在cis-PB等温结晶过程中,Avrami指数(n)在2.0~3.0之间,呈现三维球晶的生长方式,与微观结构参数及结晶温度无关,但其球晶尺寸与微观结构有关,并随着顺-1,4结构含量升高而增大;当顺-1,4结构含量小于98.4%时,半结晶期(t1/2)随其含量增加而缩短,结晶速率加快,但若其含量继续增加至99.0%左右时,结晶速率反而有所降低;若同时提高聚合物分子量及顺-1,4结构含量或者在分子量及顺-1,4结构含量相近的前提下使分子量分布变窄,均有利于提高结晶速率;随顺-1,4结构含量增加,过冷度(ΔT)增加,结晶驱动力增大;结晶活化能(ΔE)为负值,降低结晶温度有利于提高结晶速率,当顺-1,4结构含量大于98.2%时,ΔE值接近(^-137 k J/mol),结晶速率对温度的依赖程度相近.

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-1,4contents 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 than98. 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 k J/mol. The negative values suggest that the crystallization could be accelerated with decreasing temperature. The ΔE values decreased from-115 k J/mol to-130 k J/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 k J/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-1,4contents.