Structure and morphology development in syndiotactic polypropylene during isothermal crystallization and subsequent melting

Abstract

Structure and morphology development during the isothermal crystallization and subsequent melting of syndiotactic polypropylene (sPP) was studied with differential scanning calorimetry (DSC), time-resolved simultaneous small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) methods with synchrotron radiation. The morphology of sPP isothermally crystallized at 100°C for 3 h was also characterized with transmission electron microscopy (TEM). Time-and temperature-dependent parameters such as the long period (L), crystal lamellar thickness (l c ), amorphous layer thickness (l a ), scattering invariant (Q), crystallinity (X c ), lateral crystal sizes (L 200 and L 010 ), and unit cell dimensions (a and b) were extracted from the SAXS and WAXD data. Results indicate that the decreases in L and l c with time are probably due to the formation of thinner crystal lamellae, and the decreases in a and b are due to crystal perfection. The changes in the morphological parameters (Q, X c , L, and l c ) during subsequent melting exhibited a twostage process that was consistent with the multiple melting peaks observed in DSC. The two high-temperature peaks can be attributed to the melting of primary lamellae (at lower temperatures) and recrystallized lamellae (at higher temperatures). An additional minor peak, located at the lowest temperature, was also visible and was related to the melting of thin and defective secondary lamellae. TEM results are consistent with the SAXS data, which supports the assignment of the larger value (l 1 ) from the correlation function analysis as l c . WAXD showed that the thermal expansion was greater along the b axis than the a axis during melting.

Crystallization Kinetics and Melting Behaviour of the Novel Biodegradable Polyesters Poly(propylene azelate) and Poly(propylene sebacate)

“…Thus, multiple-melting endotherms in differential scanning calorimeter (DSC) traces of semicrystalline polymers are often observed. [17][18][19][20][21][22] In fact, interpretation of these phenomena is quite difficult. The most common concepts concerning multiple-melting behaviours of polymers are the melting of crystals of different stability (dual morphology mechanism) [23] and the melting, recrystallization, and remelting process (reorganization mechanism).…”

Time‐Resolved Synchrotron SAXS Observations on Sheared Syndiotactic Poly(propylene) Crystallization Process

“…Since 1980s highly stereoregular syndiotactic poly(propylene) (sPP) was obtained by metallocene catalysts, [18] sPP has attracted much attention and led to more market in many fields. [19][20][21][22][23] As is known to us, sPP has three types of crystalline forms. The form which has a T 2 G 2 chain conformation for setting up a helix with two syndiotactic units per turn is considered to be the most stable one and it is always obtained by meltcrystallization.…”

Excess X-ray Scattering Observed at Low Angles during Melting of Crystalline-Amorphous Diblock Copolymers

“…[29][30][31] Wang et al, for example, investigated the melting behavior of syn-polypropylene (sPP, M w $ 174;000) using time-resolved SR-SAXS, 31 and found a significant upturn scattering in the melting temperature range of sPP. They concluded that this excess scattering originated from the transient heterogeneous structure appearing during the melting of sPP crystals.…”

“…The excess upturn X-ray scattering at low angles was sometimes reported during the melting process of crystalline homopolymers, [29][30][31] which has been believed to arise from the heterogeneous distribution of electron density when the crystallized lamellar morphology was melting during heating. However, the substantial origin of this excess scattering is not clarified mainly because of the transient process observed only with the time-resolved SR-SAXS technique.…”