The mechanism of nucleating agents(NAs)accelerating the crystallization of semi-crystlline polymers has received continuous attention due to the extreme importance in academic research and industry application.In this work,the nucleation effect and probable mechanism of 1,3.2,4 bis(3,4-dimethylbenzylidene)sorbitol(DMDBS)on promoting the crystallization of syndiotactic polypropylene(sPP)was systematically investigated.Our results showed that DMDBS could significantly accelerate the crystallization process and did not change the crystalline form of sPP.The in situ infrred spectra recorded in the crystallization process showed that in pristine sPP the tttt conformers decreased and the tgg conformers increased subsequently.In sPP/DMDBS system,DMDBS could promote the increase of tgg conformers rather than the decrease of tttt conformers.The further analysis by 2D-IR spectra revealed that ttgg conformers increased prior to the decrease of tttt conformers in the sPP/DMDBS system comparing with pristine sPP.Considering that ttgg conformers were basic elements of helical conformation of Form I crystal for sPP,we proposed a probable nucleation mechanism of DMDBS for sPP:DMDBS could stabilize the ttgg conformers which induced these ttgg conformers to pre-orientate and aggregate into helical conformation sequences as initial nuclei quickly and early to promote the sPP crystallization.Our work provides some new insights into the nucleation mechanism of NAs for sPP.
Xin-Xin WangJian-Jun YiLi WangYuan YuanJia Chun Feng
The effect of clay on the nucleating behavior of 1,3：2,4-bis（3,4-dimethylbenzylidene） sorbitol （DMDBS） in cryatallization of isotactic polypropylene （iPP） was investigated by means of differential scanning calorimetry （DSC）, dynamic rheology and polarized light microscopy （PLM）. It is interesting to note that the incorporation of layered clay nanoparticles into DMDBS-nucleated iPP may induce a synergetic nucleation effect while the DMDBS content is below 0.1 wt%, otherwise it restricts the crystallization rate prominently as the DMDBS content increases up to 0.3 wt%, which has exceeded the content threshold to yield a nucleating agent （NA） network. As shown by dynamic rheological investigations, the clay nanoparticles demonstrate an obstructive effect of disturbing the consistency of DMDBS fibrils network. Moreover, to further demonstrate the importance of NA network formation in the crystallization of iPP, we used another NA named HPN-20e, which can not form network structure at all over the concentration studied, for comparison. In this case, the nucleated-crystallization rate is independent on the addition of clay nanoparticles, as the nucleating mechanism is an individual nuclei manner without NA network forming.
