Abstract
The deformation-induced crystallization of an isotactic polypropylene (iPP) sheet containing a β-nucleating agent was evaluated. The phase transformation of the β-modifications was investigated and the crystal morphology was observed at room temperature after stretching at different temperatures. The results showed that the crystallinity increased after solid-state stretching. When the stretching temperature was below the initial crystallization temperature, stretching deformation promoted the orientation of amorphous molecular chains. When the deformation temperature exceeded the crystallization temperature, part of the β-modifications underwent a phase transformation process and was stretched into a shish-kebab structure. However, once the stretching temperature was close to the melting point, the β-modifications melted and recrystallized, and the shish-kebab structure underwent stress relaxation due to poor thermal stability, transforming into α-modifications. It was revealed that the crystal phase transformation mechanism of the β-modifications was based on the orientation of the molecular chains between the adjacent lamellae. In addition, the shish-kebab cylindrite structure played an important role in modifying the tensile and impact properties of the iPP sheet. The tensile and impact strengths increased by as much as 34% and 126%, respectively.
Highlights
When the stretching temperature was below the crystallization temperature (i.e., 124 ◦ C), the external force did not overcome the internal friction of the molecular chain movement of the lamellae
This study investigated the deformation-induced crystal process of an isotactic polypropylene (iPP) sheet containing the TMB-5 nucleating agent
The crystal orientation and the transformation thea shish-kebab iPP sheet were cylindrite structure was crystallized when the stretching temperature was above the crystallization evaluated during a solid-state stretching process. β-modification lamellae were broken and a shishtemperature
Summary
Many researchers have studied the crystallization behavior of iPP containing a β-nucleating agent, and reported that the growth rate of the β-modification was 70% higher than that of the α-modification [10,11,12,13]. Varga and Menyhárd [14,15] proved that the nucleating agent could affect the molecular structure of iPP, and revealed that multiple β-modification morphologies existed in iPP crystalline modifications. Lezak et al [17] studied the morphology and deformation behavior of iPP homopolymer containing γ-modifications obtained from isothermal crystallization at a high pressure of 200 MPa. Mostly, γ-modification iPP exists under high pressure—the higher the pressure, the larger the fraction of γ-modifications [18].
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