Strain Rate Dependence of Amorphous Phase Instability in Semicrystalline Polymers: Insights from the Scale of Lamellar Stacks

Given the structural hierarchy in semicrystalline polymers, there is a compelling need to elucidate the mechanisms behind the instability of the interlamellar amorphous phase at the scale of lamellar stacks, which constitute fundamental building units with a biphasic nature. We specifically chose a hard-elastic isotactic polypropylene film composed of highly oriented lamellar stacks as a model sample. By utilizing synchrotron-based in situ wide-, small-, and ultrasmall-angle X-ray scattering techniques (WAXS/SAXS/USAXS), along with postmortem scanning electron microscopy (SEM) analysis, we studied the structural instabilities of lamellar stacks across a wide range of strain rates (from 0.001 to 0.5 s–1). Owing to the inherently dynamical asymmetry of the amorphous phase, we propose an insight into its instability characterized by stress-induced microphase separation based on the stress–concentration coupling model, where the extreme outcome aligns with the classical viewpoint, the formation of a fibrillar bridge/void system. With an increase in the Weissenberg number, a greater number of stress transmitters within the amorphous phase tend to be retained, thereby impeding the advancement of stress-induced microphase separation but promoting the crystalline phase instability. Furthermore, during the transition from a slow to a rapid stretching process, the amorphous phase instability undergoes a shift from a growth-dominated to a nucleation-dominated mode. This kinetic transition results in a more uniform dispersion of lamellar clusters that encompass unstable amorphous layers

Mixing Assisted Direct Formation of Isotactic Poly(1-butene) Form I′ Crystals from Blend Melt of Isotactic Poly(1-butene)/Polypropylene

The influence of mixing of iPB-1/iPP blend on the polymorphism of iPB-1 under processing-relevant conditions is studied with emphasis on the competition between the thermodynamically stable form I′ crystal and the kinetically favored form II. <i>In situ</i> optical microscopy measurements reveal that the upper critical solution temperature (UCST) of iPB-1/iPP blend locates in the range of 180–200 °C. Unexpectedly, by quenching mixed iPB-1/iPP melt down to temperatures below UCST and melting point, form I/I′ can be produced directly which is further identified as form I′ by temperature-dependent WAXS and DSC. The formation of form I′ is promoted by increasing the annealing time above UCST, while is suppresses by raising the quenching temperature. In addition, the crystallization of iPP also displays a similar trend as iPB-1 does. The correlated crystallization of each constituent with dependence on the initial mixing degree suggests that the crystallization behavior of the binary blends is determined by the interplay between simultaneous processes concomitant with the liquid–solid transition. The experimental results reveal the possibility to modify the crystallization pathway of iPB-1 in iPB-1/iPP blend through the mixing degree which is initially controlled by annealing but is subject to evolve during the subsequent thermal treatment. Possible mechanisms are discussed including the roles of phase separation and concentration fluctuation in crystallization

Kinetic Process of Shish Formation: From Stretched Network to Stabilized Nuclei

On the basis of the duality of the shish-kebab superstructure, coil–stretch transition (CST) is well recognized as the molecular mechanism for shish-kebab formation in polymer melts, which, however, is challenged by recent results in flow-induced crystallization (FIC). In this work, we perform a real time investigation on FIC of polyethylene bimodal blends by combing a unique homemade extensional rheometer and synchrotron radiation small-angle X-ray scattering. The results show that the critical strain for shish formation decreases with increasing long chain concentration, which contradicts the role of CST but agrees well with stretched network model (SNM). Quantitative analyses indicate that the formation of shish is determined by the degree of network deformation rather than solely by strain or long chain concentration at a specific temperature. In addition, three types of shish with different stability are observed sequentially by increasing strain. On the basis of our results, strong support is given to the idea that shish formation is a kinetic process. When stretched to a critical deformation degree, the aligned segments couple with each other to form fibrillar-like type I shish, which further transform into type II shish embedded with sporadic lamellae and type III shish embedded with well-defined periodic lamellae sequentially by increasing flow intensity. Our results and the resulting conceptual model not only demonstrates that shish formation is derived from SNM but unveils its kinetic process from initial chain configuration to final stable nuclei

Extension-Induced Crystallization of Poly(ethylene oxide) Bidisperse Blends: An Entanglement Network Perspective

The role of long chains in extension flow-induced crystallization was studied with a combination of extension rheological and <i>in situ</i> small-angle X-ray scattering (SAXS) measurements at 52 °C. To elucidate the effects of long chains, bidisperse blends of poly­(ethylene oxide) (PEO) with the long-chain concentration above the overlap concentration were prepared, constructing long-chain entanglement network in short-chain matrix. Rheological data of step extension on PEO melt are divided into two regions with fracture strain of pure short-chain sample as a boundary. Distinctly different features of crystallization kinetics and crystal morphologies are observed in these two regions, exactly corresponding to rheological behavior. A new mechanism based on entanglement network perspective is proposed, in which the second entanglement network constructed by long chains has three effects: (i) helping flow to change the free energy of polymer melt more effectively; (ii) ensuring the specific work can impose on the system; (iii) favoring the formation of precursors. This mechanism captures both rheological observation and crystallization behavior successfully and offers a new viewpoint for FIC study

A New Three-Dimensional (3D) Multilayer Organic Material: Synthesis, Swelling, Exfoliation, and Application

A novel fully rigid, rod-shaped oligo­(<i>p</i>-benzamide) (OPBA-6) molecule was designed and synthesized, which can be recrystallized into a three-dimensional (3D) multilayer material via an antiparallel molecular packing model. Intermolecular hydrogen bonding and π–π interaction are brought to ensure a strong intralayer interaction, while decoration of layer surface with sulfonic groups promotes water to enter interlayer space and facilitates the swelling and exfoliation of sample. With a simple dispersion in water, the obtained multilayer material can be easily swollen by water without destruction of in-plane morphology and subsequently delaminated into 2D nanosheets with thickness of about 5.38 nm. This achievement may be the first attempt to exfoliate layered organic materials and thus provide a new strategy to prepare 2D organic nanosheets without using any substrates or templates as required by conventional and widely used self-assembly routes. Based on exfoliated nanosheets, poly­(vinyl alcohol) nanocomposites were prepared using a simple water solution processing method. A 64% increase in tensile stress and a 63% improvement in Young’s modulus were achieved by addition of 7 wt % OPBA-6 loading