Detection and characterization of folded-chain clusters in the structured melt of isotactic polypropylene

Abstract

Despite ceaseless efforts in past decades, the memory effect of semi-crystalline polymers has not been elucidated completely yet. An important reason why is that residual lamellar crystals in the structured melt are difficult to characterize. Recently, we developed a new small-angle X-ray scattering (SAXS) theory [Li et al. (2019). IUCrJ, 6, 968–983] and Fourier transform method [Li et al. (2020). CrystEngComm, 22, 3042–3058] for lamellar crystals that could derive structural information from SAXS readily. In this study, we tried to employ the new theory and method to characterize residual lamellar crystals in the structured melt. It was found that although scattering peaks cannot be observed in raw scattering profiles, they actually exist. Subtracting free-melt scattering and multiplying by q 4 benefit the observation of these weak scattering peaks. With the new Fourier transform method, it was found that indeed as proposed previously, thicker lamellar crystals exist in the structured melt. To determine the lateral size of residual lamellar crystal especially, a new method was developed under the guidance of the new theory. With the new method, it was found that although the crystallinity is very low (∼1% at 174°C), the lateral sizes in the structured melts are still large, e.g. 45.3 nm at 174°C, much greater than the critical nucleation size. This implies that these residual lamellar crystals can act as athermal nuclei after quenching to a lower temperature, as proposed by Ziabicki & Alfonso [(1994). Colloid Polym. Sci. 272, 1027–1042; (2002). Macromol. Symp. 185, 211–231] more than 20 years ago. The methodologies proposed here could also be applied to other polymer lamellar systems.

Polymer Chains Fold Prior to Crystallization

“…In the past two decades, several important findings have been reported in both experimental − and computational works. − For example, chip DSC (cDSC) provided formations of nuclei for poly­( l -lactic acid) (PLLA), isotactic -poly­(1-butene) ( i PB1), poly­(ε-caprolactone), etc. , even below the glass transition temperature, T g .…”

Microstructural Change of Poly(1-butene) during Crystallization, Phase Transition, and Melting Revealed by Synchrotron Small-Angle X-ray Scattering

“…As seen in Figure b, I ev b concentrates mainly in the small q range, while I ev i concentrates in the high q range. Multiplying q 4 can suppress I ev b effectively, which has been shown in the Supporting Information of ref . The second small question has been addressed also in reality; nevertheless, it was addressed by the first method.…”

“…If these scatterings were not removed or reduced, the determinations of lateral size, lamellar thickness, and long period could be influenced seriously. Over the past few years, we developed a full set of methodologies to obtain lateral size, lamellar thickness, and long period under the guidance of a new SAXS theory. , To help more researchers to understand and employ these methods, we would like to introduce these methods in this study systematically, with form I and form II of poly­(1-butene) as examples. Simultaneously, some methods may need slight revision when dealing with some new polymers or phases.…”

The influence of melt status and beta-nucleation agent distribution on the crystallization of isotactic polypropylene

“…[57][58][59] No ordered structure would exist in the melt. 60 For safety, we employed synchrotron wide-angle X-ray diffraction and smallangle X-ray scattering to check whether αcrystals existed above 174 °C in the β-nucleated iPP sample. Fig.…”

“…To observe the structural change from SAXS more clearly, SAXS was corrected with a new method proposed by us recently. [60][61][62][63] SAXS was first subtracted by free-melt scattering and then multiplied by q 4 . The aim of subtracting free-melt scattering is to remove other minor scatterings such as air scattering, scattering induced directly by incident X-ray.…”