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Abstract The relationship between the macroscopic non-linear mechanics and the microscopic crystal structural evolution of pre-oriented high-density polyethylene (HDPE) is investigated by in situ synchrotron radiation wide-angle X-ray diffraction (WAXD) measurement over a wide temperature range from −10 to 130 °C. With the concept of stress-induced disordering of crystal, the ratio ( φ a / b ) of lattice parameters a to b is defined as a new structural variable, which can reflect the lattice distortion and then the microscopic stress state of orthorhombic crystal (O-crystal). According to the temperature-dependent non-linear variation of φ a / b with strain, the contributions of O-crystal and monoclinic crystal (M-crystal) to the macroscopic mechanics including linear elasticity, yielding, stress softening and strain hardening are clarified. It is found that M-crystal bears the main extensional stress once formed, although it survives within a limited strain window relying on temperature. By further combining the extensional phase diagram constructed in strain-temperature space, the HDPE deformation is recognized to undergo successively one-dimensional (1D) chain segments rotation of crystal, two-dimensional (2D) crystal plan shearing or slipping and three-dimensional (3D) recrystallization along with increasing strain or stress, demonstrating a multiscale structural transition and energy dissipation mechanism. Graphical abstract Image 1 Highlights • A new structural parameter is defined to characterize the lattice distortion and the microscopic stress state of crystal. • Multiscale structural evolutions of crystal are connected to the macroscopic non-linear mechanics of HDPE under deformation. • The research method can be generalized to other semicrystalline polymers to understand the structure-mechanics relationship. [ABSTRACT FROM AUTHOR]

The influence of strain rate on the deformation induced crystal transition from forms II to I of Polybutene-1 is investigated by in situ Wide Angle X-ray Diffraction at different temperatures. The structural evolution during the subsequent stress relaxation is further monitored. As strain rate increases, the phase transition rate decreases apparently, while the dependence on stress and strain also becomes weak. The phase transition during deformation with three strain rates shows consistent dependence on external work. The transition rate in terms of form I content drops instantly when stretching stops. Further analysis on the evolution of crystallinity and orientations of both forms II and I shows that a small amount of recrystallization occurs in form II during stress relaxation at 80 °C, accompanied by a continuous increase in the content of form I, which further confirms occurrence of deformation induced melting and recrystallization. Structural evolution of Polybutene-1 during the deformation induced phase transition from forms II to I and the following stress relaxation process is investigated by in situ Synchrotron Radiation Wide Angle X-ray Diffraction (SR-WAXD). During the tensile with different strain rate, the phase transition rates show consistent dependence on external work. After deformation at 80 oC, crystallinity of unstable form II increase inversely, while the phase transition from forms II to I continues at a slower rate. The full cycle consideration provides a more comprehensive basis for the use of materials. [Display omitted] • The structural evolution of Polybutene-1 during stress relaxation is first considered. • The phase transition rates during deformation with three strain rates show consistent dependence on external work. • Recrystallization of unstable form II is found during stress relaxation after deformation at high temperature. [ABSTRACT FROM AUTHOR]

In this study, Fourier transform infrared spectroscopy, synchrotron small-angle and wide-angle X-ray scattering were employed to reveal the underlying mechanism leading to conformational reorganization during mesomorphic-monoclinic phase transition. Results indicated that conformation reorganization occurred from 50 to 130 °C, evidenced by conformational infrared bands at 800 and 841 cm −1 . The conformation reorganization led to an immediate increase in the density fluctuation but not induced phase transition immediately. Only when the long period started to increase quickly from 80 °C, can phase transition occur. This indicated that intrachain conformational ordering occurred prior to crystalline ordering. Moreover, temperature dependence of long period changed before and after phase transition, implying that chain sliding was the main driven force for the phase transition rather than rewinding and translation of single stems. With the new information obtained, a two-step model for phase transition was proposed. Within a single-chain bundle, two adjacent helices with opposite handedness can interlock into double helices with higher density, and urge each other extending to entire stem during moving along opposite directions with the help of chain slipping, resulting in ordered arrangement of helical stems. Afterwards, single-chain clusters coalesce, leading to interchain crystalline ordering. The new model is helpful to understand the phenomena related to mesomorphic iPP. [ABSTRACT FROM AUTHOR]

In many industrial processes, polymer melt undergoes complex flow-cooling environments before being solidified into the final products. Investigating polymer crystallization under processing conditions is a necessary step to predict and customize the structures and properties of semicrystalline polymers. Herein, the crystallization behavior of high-density polyethylene (HDPE) under combined melt stretching and fast cooling fields was studied through a homemade extensional rheometer. A series of HDPE samples were prepared by cooling the stretched melt at different cooling rates (0.02–220 °C/s) immediately after flow, or after waiting for different times of isothermal crystallization at 128 °C. Microstructural characterizations confirm four different structural states related to the evolution of shish-kebabs after melt stretching, namely generation of shish precursors, shish crystal growth, kebab lamellar growth, and perfection of shish-kebabs, which diversify greatly the structures and morphologies of the final solidified samples. By prolonging the isothermal crystallization time at high temperature, the fast cooling-induced lamellae keep decreasing in thickness. This supports the occurrence of molecular segregation caused by shish-kebabs growth at high temperature, which reduces the molecular weight of the remaining un-crystallized melt. In addition, quantitative analysis of crystalline orientation manifests that the lamellar growth during isothermal process takes strictly the shish as a template and adopts a high orientation, while the fast cooling-induced lamellae are only related to the chains conformation of un-crystallized melt prior to cooling. This work would be essential to advance the in-depth understanding on the actual industrial processing of semicrystalline polymeric materials. [Display omitted] • Melt crystallization of HDPE under combined stretching and fast cooling fields was studied. • Four different structural states related to the evolution of shish-kebabs after flow were identified. • Molecular segregation caused by shish-kebabs growth at high temperature was suggested. • Isothermal and non-isothermal crystallization under flow presented different lamellar growth modes. [ABSTRACT FROM AUTHOR]

Abstract: In this work extension induced crystallization of end-linked polyethylene oxide (PEO) network was investigated with in-situ small angle X-ray scattering (SAXS) and rheological measurement. The coupling between crystallinity and stress of crosslinked network makes the morphology time dependent. With a large strain the structure evolution in the stretched network is found to follow a two-stage process: i) crystals with high numerical density and small dimension form first, which laterally correlate with adjacent ones; ii) continuous melting and fusing of adjacent crystal happen after the first nucleation stage, leading to increase of crystal dimension and stability. Decreasing strain a gradual transformation to normal lamellae formation is found. Strain tuned confinement and crystallization induced confinement release is supposed to induce the structural change. [Copyright &y& Elsevier]

Abstract: With three polymers (poly(1-butene), isotactic polypropylene and syndiotactic polypropylene) as model systems, the effect of extensional strain on the nucleation rate was studied using a home-made miniature extensional rheometer and in-situ X-ray scattering. A non-monotonic correlation between nucleation rate and strain is observed around rheological yield point. Increasing strain leads to an increase in nucleation rate as expected when strain is smaller than yield strain, however, it results in a decrease in nucleation rate when strain slightly exceeds yield. We proposed that disentanglement occurs at yield point and thus decelerates flow-induced nucleation. Disentanglement provides freedom for chains to retract and leads to a dissipation of the energy initially imposed by extensional flow, which consequently weakens the effect of flow field on the enhancement of nucleation. [Copyright &y& Elsevier]