ANNEALING INDUCED VARIATION OF CRYSTALLINE STRUCTURE AND MECHANICAL PROPERTIES OF SYNDIOTACTIC POLYPROPYLENE FIBERS

Syndiotactic polypropylene （sPP） as-spun fiber （sPP1） and drawn fiber （sPP2） were prepared by melt-spinning and melt-spinning/hot-drawing, respectively. The structure transition of the two fibers induced by annealing at different temperatures and the corresponding mechanical properties were subsequently investigated by the combination of Fourier transform infrared spectroscopy （FTIR）, wide-angle X-ray diffraction （WAXD） and tensile testing. The results indicate that the chain conformation and crystal forms of the two sPP fibers are not obviously changed at low annealing temperature （40℃）. With increasing the annealing temperature, the trans-planar conformation and mesophase in sPP1 and sPP2 fibers can be completely transformed to helical conformation and crystal form I under tension. Upon removing the tension, a small amount of mesophase and trans-planar conformation will be regained. The mechanical properties of the annealed fibers are manifestly dependent on their initial structure and the annealing temperature.

Deformation Temperature and Lamellar Thickness Dependency of FormⅠto Form Ⅲ Phase Transition in Syndiotactic Polypropylene during Tensile Stretching

Phase transition from form Ⅰ to form Ⅲ in syndiotactic polypropylene crystallized at different conditions during tensile deformation at different temperatures was investigated by using in situ synchrotron wide angle X-ray diffraction technique. In all cases, the occurrence of this phase transition was observed. The onset strain of this transition was found to be crystalline thickness decided by crystallization temperature and drawing temperature dependent. The effect of drawing temperature on this phase transition is understood by the changes in mechanical properties with temperature. Moreover, crystalline thickness dependency of the phase transition reveals that this form Ⅰ to from Ⅲ phase transition occurs first in those lamellae with their normal along the stretching direction which have not experienced stress induced melting and recrystallization.

Polymorphic Behavior of Syndiotactic Polypropylene Induced by Heat Treatment

The polymorphic behavior of syndiotactic polypropylene (s-PP) crystallized from the melt under different conditions was investigated by means of WAXD and DSC. The isothermal melt crystallization of s-PP proceeded in form I at tc≥ 110℃ and in form Ⅱ at tc≤110 ℃ s-PP crystallized nonisothermally from the melt was the mixture of forms and . With increasing the cooling rate (a), the content of form increased, and the content of form decreased. On heating, at 10 K/min, of quenched s-PP, it crystallized in form at t≤70 , continuing heating resulted in form form transition, which was an endothermal process.

Investigating the Nucleation Effect of DMDBS on Syndiotactic Polypropylene from the Perspective of Chain Conformation

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.