Fold Surfaces of Polymer Crystals Are Nucleation Sites: The Link between Polymer Decoration, Secondary Crystallization, and the Rigid Amorphous Fraction (RAF)

Abstract

The fold surfaces of polymer crystals may become suitable nucleating sites for uncrystallized material left over during the primary crystallization process. This “nucleation by the folds surface” (NFS) ability is highlighted in the so-called polymer decoration technique in which vapors of polyethylene condense and crystallize on a cold fold surface substrate to form edge-on lamellae that reveal the folds and loops orientations in different growth sectors. In bulk materials, the primary crystallization leaves confined and uncrystallized material between lamellae. NFS, based on epitaxy-type interactions, is expected to take place in these confined and supercooled melt fractions before “true” homogeneous nucleation sets in. The insights provided help elucidate the structural features of the interlamellar layers of crystalline polymers, which overcomes the limits of global, indirect investigation techniques (e.g., differential scanning calorimetry (DSC)) used in attempts to analyze their structure in situ. The interlamellar layer is known to be divided into a rigid amorphous fraction (RAF) located in contact with the fold surfaces and a mobile amorphous fraction (MAF) at its center. The nature of the RAFamorphous but rigid above its T g, or crystallineis still debated. The NFS provides a structural identity for the RAF that associates conformation disorder with stem orientation: the RAF is actually a “condis oriented” fraction. On heating, the small endotherm observed in DSC curves corresponds to its isotropization. The RAF is also a protonucleus for secondary crystallization and induces the growth of edge-on lamellae on top and in the interlamellar spaces of the primary crystals. As such, the RAF has also a layered lamellar structure. Polymer decoration, the “condis oriented” RAF, and the specific edge-on lamellar secondary crystallization are manifestations of the nucleation by the fold surfaces.

Proliferation and Arrangement of Poly(vinylidene fluoride) Oval γ Single Crystals Grown in Its Poly(1,4-butylene adipate) Blends

“…It may thus imply the existence of some ordered structure on the fold surface. As suggested by Chan and Li, loose folds and/or cilia with some orientation order imposed by the basal lamella can organize into a new nucleus. , Similarly, in a recent article, Lotz has discussed the nucleation by the fold surfaces . He suggests that the rigid amorphous fraction (RAF) directly in contact with the lamellar crystals at fold surfaces can act as nucleation sites to develop secondary crystallization.…”

“…65,66 Similarly, in a recent article, Lotz has discussed the nucleation by the fold surfaces. 67 He suggests that the rigid amorphous fraction (RAF) directly in contact with the lamellar crystals at fold surfaces can act as nucleation sites to develop secondary crystallization.…”

Molecular Mechanism of Surface Melting of Polyamide12 as Elucidated by Synchrotron Radiation X-ray Scattering and 1H Spin Diffusion NMR

“…The origin of the RAF is recently revealed by Lotz to be a "codisoriented" fraction, whose molecular conformation is disordered but stem orientation exists. 74 Such a RAF can serve as the secondary nucleation site. Our results further clarify the dynamic features of RAF at different temperatures.…”

“…Due to its close connection with the rigid crystalline domain, such molecular structure within the interphase may exhibit rapid but constrained motion. The origin of the RAF is recently revealed by Lotz to be a “codisoriented” fraction, whose molecular conformation is disordered but stem orientation exists . Such a RAF can serve as the secondary nucleation site.…”

The Isothermal Melting Kinetics of Ultra‐High Molecular Weight Polyethylene Crystals

“…Melting of semicrystalline polymer strongly depends on the crystallization history [ 1–4 ] and displays complex behaviors, such as a broad melting temperature range, [ 5,6 ] parallel occurrence of melting, and recrystallization, [ 7,8 ] as well as superheating‐dependent melting kinetics. [ 9,10 ] For long‐chain polymers, crystallization from highly entangled melts generally leads to a semicrystalline state with alternating layer‐like lamellar crystals and entangled amorphous layers.…”