Crystallization of isotactic polypropylene from mesomorphic phase: a constant heating rate study

Abstract

Abstract.We have studied crystallization behaviour of isotactic polypropylene (iPP) from mesomorphic phase in structural point of view. Time-resolved wide-angle X-ray diffraction (WAXD) measurements during a heating process have been performed using a synchrotron radiation (SR) X-ray beam line at SPring-8, Japan. The heating process was so programmed to reproduce a thermal trace of differential scanning calorimetry (DSC) with a constant heating rate (10 °C/min) in order to compare the structural change with thermal behaviour. SR-WAXD sensitively detected the crystallization behaviour and we have obtained fractions of alphacrystal, mesomorphic phase and amorphous phase as a function of temperature by analysing the data. The results showed that the crystallization from mesomorphic phase proceeds in between 60 and 120 °C (meso-alpha transition). During this process, the crystallization from amorphous hardly takes place. The crystalline fraction shows almost constant in between 120 and 140 °C; meanwhile, the mesomorphic fraction still decreases above 120 °C. The crystalline fraction starts to decrease above 140 °C and the most extensively decreases at around 165 °C (melting point). We have also determined the energy level of the mesomorphic phase (metastable state) relative to that of alpha-crystal (stable state), considering the balance among the fractions of alpha-crystal, mesomorphic phase and amorphous. IntroductionFrom a standpoint of thermal process, crystallization of bulk polymer is classified into mainly two categories; one is cooling from molten state and another is heating from "pre-quenched state". In both processes usually growth of spherulites in micrometer scale are observed, though the latter process yields more or less higher nucleus density than the former one. In those polymers "pre-quenched state" is mostly equivalent to "glassy state". However, this is not the case for isotactic polypropylene (iPP). In the heating process of pre-quenched iPP, the growth of spherulites is never observed and the macroscopic appearance is almost transparent up to the melting temperature T m . The absence of the spherulites and the transparency not necessarily imply that the pre-quenched iPP does not crystallize. The pre-quenched iPP indeed crystallizes in the heating process [1], but the higher order structure is quite different from one that is crystallized from molten state [2]. The specific behaviour of the crystallization of iPP from the pre-quenched state is considered to be due to the presence of mesomorphic phase. IPP has three well-established polymorphisms; such as alpha (monoclinic), beta

Phase Transitions in Prequenched Mesomorphic Isotactic Polypropylene during Heating and Annealing Processes As Revealed by Simultaneous Synchrotron SAXS and WAXD Technique

“…However, in our case, crystallization hardly occurs from the amorphous region, but occurs mainly from the mesomorphic phase. A reasonable explanation can be found in the most recent work of Asakawa et al, 30 in which it is verified that compared with the transitions of mesomorphic to amorphous phases and amorphous to crystalline phases, the energy level for the transition of mesomorphic to crystalline phases is much lower. For the iPP samples prequenched at the rates of 1000 and 686 °C/s, the temperatures at which the α-monoclinic crystals are completely melted are 162.5 and 158.3 °C, respectively.…”

In situ observations of the mesophase formation of isotactic polypropylene—A fast time-resolved X-ray diffraction study

“…8,[10][11][12][13][14][15] Additionally, the crystallization of iPP via the mesophase has been extensively studied in the literature. 7,[16][17][18][19][20][21] However, the ongoing process of mesophase formation has been a black box for many years, and in situ observation of the formation process, which occurs in a very short period of time, has been challenging.…”

“…7,24 Thus, the formation of the mesophase of iPP is one of the solidification processes, which accompanies an exotherm (latent heat) sufficient to stabilize the mesophase at room temperature (above the T g ). 21 The understanding of the formation process of the mesophase iPP has progressed significantly as a result of thermal analysis.…”

Charge trapped mechanism for semi‐crystalline polymer electrets: quasi‐dipole model

“…2 a is the Wide‐Angle X‐ray Diffraction (WXRD) spectra of the MBPP fabrics annealed at 60°C. It is shown that there are two strong wide peaks, wherein 2 θ values are 15.0° and 21.3°, respectively, which are the characteristic peaks of mesomorphic crystallite [44]. No obvious change of WXRD spectra is observed when the samples are annealed for more time, although the sketch of other peaks appears due to a glass transition‐like behaviour resulting from the chain reorganisation of the interfacial region of the mesophase [45].…”