Epitaxially Dominated Crystalline Morphologies of the γ-Phase in Isotactic Polypropylene

• Published in: Macromolecules Vol. 42; no. 13; pp. 4758 - 4768
• Main Authors: Cao, Yan, Van Horn, Ryan M, Tsai, Chi-Chun, Graham, Matthew J, Jeong, Kwang-Un, Wang, Bojie, Auriemma, Finizia, De Rosa, Claudio, Lotz, Bernard, Cheng, Stephen Z. D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 14-07-2009
• Subjects: Applied sciences; Condensed Matter; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Physics; Properties and characterization; Soft Condensed Matter; Structure, morphology and analysis; Gamma form; Melt crystallization; Epitaxy; Isotactic polymer; Propylene polymer; Epitaxial film; Experimental study; Crystal morphology; Formation mechanism; Isothermal crystallization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma900475h

In the past, the crystallographic relationship between the γ-phase and the α-phase in isotactic polypropylene (i-PP) was extensively studied and established via i-PP oligomers of which the γ-phase can be formed at atmospheric pressure. We attempt to investigate how the epitaxial domination of the crystal morphologies takes place in the γ-phase of chain-folded crystals using high molecular weight i-PP samples with a controlled number of stereodefects. These specifically synthesized samples favor the isothermal growth of the γ-phase crystals from a thin film melt at 100−140 °C at atmospheric pressure. It is known that the γ-phase unit cell has the very unique characteristic of an orthorhombic lattice with alternating stem orientations in every two stem layers along the c-axis. Due to the specific epitaxial growth of the γ-phase on the elongated α-phase single crystals, two different morphologies were identified via transmission electron and atomic force microscopies (TEM and AFM). The first γ-phase crystalline morphology is needle-like. The selected area electron diffraction (SAED) results showed that either the [1̅10] or [110] zone axis was parallel to the thin film normal, and the needle direction was along the c-axis of the γ-phase. The epitaxial growth of this type of γ-phase crystal was generated from the stem direction in the initial α-phase single crystal being parallel to the thin film (and thus, the lamellar) normal. In this case, the stem length of the α-phase single crystal provided a limit for the growth of the γ-phase to develop toward the thin film normal. On the other hand, the stem length oriented at ±80° away from the film normal in the γ-phase crystal was limited by the folded chain crystal growth kinetics, which is proportional to the reciprocal supercooling. These two factors thus resulted in the formation of these peculiar needle-like crystals. The second γ-phase crystalline morphology was “flat” lamellae. The SAED results indicated that the chain orientations in the “flat” lamellae were tilted at ± 40° from the thin film normal within the ab-plane of the γ-phase. Macroscopically, growth of the “flat” lamellae was thermodynamically more stable compared with the needle-like crystals due to their larger crystal size. On the basis of the tilted SAED and dark field results from TEM, the microscopic formation mechanism of this morphology revealed that the initial α-phase single crystal had to have a stem orientation tilted ± 25° away from the thin film normal within the ac-plane around the b-axis. Therefore, the epitaxial growth of the γ-phase on the ac-plane of the α-phase did not possess a chain orientation of ± 40° from the thin film normal. The final “flat” lamellar γ-phase crystals might have resulted from a continuous twist of the chain orientation from +25°/−55° or −25°/+55° to ±40° from the thin film normal.