低温冷拉对具有排核片晶结构的PVDF微孔膜成孔的影响

Effect of Low-temperature Cold Draw on Porosity of PVDF Microporous Membrane with Lamellar Crystalline Structure

采用聚偏氟乙烯(PVDF)材料制备锂电池用微孔膜,使用熔体挤出拉伸法制备了预制膜片,通过牵伸比的调节成功获得了拥有排核片晶结构的预制膜片。结果表明,该预制膜具有沿着流动方向的高取向度。退火后,在拉伸预制膜成孔的过程中,由于PVDF松弛时间较短,即使预制膜具有排核片晶结构,在常温下的成孔效果也较差;根据时温等效原理,降低冷拉时的拉伸温度,可以达到与提高松弛时间同样的效果;对比室温下冷拉和-20℃低温下冷拉膜片的扫描电子显微镜照片,低温冷拉后的膜片具有更多的孔结构和更好的孔分布情况,并初步探索了低温拉伸条件对于成孔促进作用的成孔机理。

As a polymer with excellent mechanical strength,heat resistance ability and chemical stability,poly（vinylidene fluoride）（PVDF）was used to prepare microporous membrane for lithium battery through melt extrusion and post draw.A row-nucleated lamellar crystalline structure was achieved by adjusting the draw ratio during the process.The investigation results indicated that the casting film was highly orientated along the flowing direction.It is hard for cast film to gain a proper porous structure at room temperature stretching due to short relaxation time of PVDF,even if the cast film achieved the row-nucleated lamellar structure.Based on the timetemperature equivalence principle,the reduction of cold-drawing temperature can lead to the same effect by increasing relaxation time.More plastic deformation was generated in the amorphous region when the cast film was drawn at a low temperature below room temperature.By comparing the SEM images of the film drawn at different temperature,it was observed that the film colddrawn at low temperatures exhibited much better porous structure and pore-size distribution.The formation mechanisms of microporous structure at low-temperature cold draw were also discussed.