| Polypropylene (PP), as a kind of semi-crystalline polymer, exhibits different crystal forms including α-form PP (named as a-PP), β-form PP (named as β-PP) and γ-form (named as γ-PP), et al. Among them, α-form and β-PP have been widely researched in recent years, while γ-PP has been rarely researched due to its harsh preparation conditions. Generally, high pressure is needed to prepare the PP articles with high content of γ-PP. Therefore, it is very significant to research how to prepare PP articles with high content of γ-form in a very efficient way.In this paper, the high efficient nucleation agent, i.e. a nucleating agent (DMDBS), or β nucleating agent (TMB-5), was introduced into PP. The effects of nucleation agents on crystallization behavior of PP under high pressure were investigated systematically. Since the reduction of the degree of supercooling could possibly improve the formation of γ-PP, it was expected to joint the effects of pressure and nucleating agent to alleviate the critical condition of obtaining γ-PP; At the same time, the competition growth of PP in different crystal forms (β-PP and γ-PP) during the crystallization process was also investigated; Besides, the synergistic effect and/or competition between the nucleating agent and pressure on the crystallization of PP were investigated by changing the content of nucleating agent and pressure condition; At last, a simple comparison was made to understand the degree of the dependence of γ-PP formation on the PP molecular chains. Here, three different PP including isotactic polypropylene (iPP), polypropylene block copolymer (PPB), and polypropylene random copolymer (PPR) were investigated. The main conclusions obtained in this work were listed as follows:(1) At atmosphere pressure, the pure iPP and PPB could not form into β-PP. But with the increase of the pressure, the amount of the α-PP reduced gradually while γ-PP appears and increases. So, enhancing pressure is an important method to obtain high content of γ-PP.(2) The simultaneous addition of DMDBS and pressure can promote the formation of the γ-PP efficiently whether in iPP or in PPB samples. This can be attributed to the largely decreased degree of supercooling, which is realized by the nucleation effect of the DMDBS, providing more available condition for the formation of the γ-PP. At the same time, the existence of nucleators changes the crystallization morphology of γ-PP. A feather-like γ-PP rather than the common spherulitic γ-PP was observed in this work.(3) The effects of TMB-5and pressure on the formation of γ-PP were proved to be dependent upon the value of the pressure. At relatively low pressure, TMB-5and pressure exhibited the synergistic effect on the formation of γ-PP; However at relatively high pressure, TMB-5and pressure exhibited a competitive effect. This was mainly because that at high pressure, TMB-5still induced the information of a few amount of β-PP, consequently, the content of γ-PP was smaller than that of iPP with crystallized under the same pressure condition.0.2wt%TMB-5could form into the rod-like structure through self-assembly, and then induced the iPP to crystallize into a novel γ-PP with novel crystalline morphology. Based on this, the new model that iPP nucleated and grew on the rod-like TMB-5crystal was established, and the novel theory of y phase’s growth was also presented. In addition, at the certain pressure (193MPa), the unconventional material with coexistence of β/γ-PP was obtained. This provided the theoretical foundation for the development of this kind of material.(4) DMDBS could induce the formation of γ-PP in PPB samples more efficiently, but there existed a critical content (about0.2wt%-0.3wt%). When the content of DMDBS in PPB was smaller than the critical content, DMDBS and pressure could synergistically induce the formation of γ-PP in PPB samples, and the content of γ-PP increased with the increase of the DMDBS; When the content of DMDBS in PPB was bigger than the critical one, the competition between DMDBS and pressure appeared, this moment, the content of γ phase decreased with the increase of the DMDBS, namely, the promotion effect of DMDBS on γ-PP decreased. At relatively low pressure, DMDBS exhibited the dominant effect in controlling the formation of α-PP in PPB samples; however at relatively high pressure, pressure was proved to be the dominant factor, inducing the formation of γ-PP in PPB samples. For the sample with the critical content of0.2wt%(PPB-0.2DM). only the pressure of154MPa was needed to prepare the PPB material with100%content of γ-PP.(5) The content of γ-PP in iPP exhibited the strong dependence on the pressure. The degree of the dependence of γ-PP content on pressure in different PP materials was in the order of iPP>EPS30R>PPR3260. PP who exhibited excellent crystallization ability and high regularity in molecular chains could not form into γ phase at atmosphere pressure, but the crystallization behavior of such PP was sensitive to the variation of pressure. Enhancing the pressure induced the greatly increase of the γ-PP content during the crystallization process. But, PP who exhibited poor regularity in molecular chains and had high content of ethylene could form into the γ-PP at atmosphere pressure. The effect of the pressure on the formation of γ-PP was inconspicuous. Addition of the other additives, such as nucleators, possibly facilitated the formation of γ-PP at moderate pressure condition. |
PDF Full Text Request