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The flow-induced crystallization (FIC) is commonly encountered in the polymer industry as more than 70% of commercial polymers are crystalline, which needs to be processed before the final application. The complicated external flow field, i.e., shear or extensional flow, results in a great challenge in understanding the FIC phenomenon from both general thermodynamics and detailed molecular level aspects. The current tutorial first describes the general phenomenon of FIC from the aspect of different morphologies and enhanced kinetics induced by the flow. Second, characterization methods for monitoring FIC are introduced. Here, the in situ synchrotron x-ray scattering and non-equilibrium molecular dynamics simulation are selected as typical examples. Then, the theoretical descriptions of FIC are summarized from the aspects of molecular origin and thermodynamics. The coil–stretch transition theory and later developed stretched network theory are highlighted, where the former mainly accounts for polymer dilute solution and the latter for highly entangled cases. Also, multi-step features for the formation of various intermediate states during flow-induced nucleation are depicted. Despite non-equilibrium nature, the FIC can still be treated by thermodynamics, especially under weak flow conditions. The classic entropy reduction model is introduced together with later modifications. In all, understanding the fundamental mechanism of FIC is crucial for optimizing external processing parameters and internal molecular characteristics, and useful to guide current or further applied techniques. [ABSTRACT FROM AUTHOR]

The structural evolution during the linear low‐density polyethylene (LLDPE) film blowing has been studied with a combination of home‐made blown film apparatus and in situ WAXS and SAXS measurements. Analyzing the evolution of orientation parameters and crystallinity of the bubble shows that the blown film process can be divided into four regions. Distinctly different features of LLDPE bubble are observed in first three regions: (a) lower orientation parameters during the blown film process, (b) higher crystallinity is required to form a deformable crystalline crosslinked network, and (c) the weaker stretching effect and the difficulty of reaching equilibrium when the crystal network deforms. These results should provide a basis for understanding the poor blown film stability of LLDPE. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1404–1412 In situ WAXS and SAXS technique studies have been used successfully to investigate the structural evolution during the linear low‐density polyethylene (LLDPE) film blowing. It is found that the LLDPE blown film process can be divided into four regions. The higher crystallinity in the end of Region II and the weaker change of orientation in Region III are expected to explain the LLDPE blown film poor stability. [ABSTRACT FROM AUTHOR]

Aiming to establish the relationship between structures of precursor film and its capability of pore formation, high density polyethylene (HDPE) precursor films are prepared under varied draw ratios and die temperatures, which are further subjected to stretching to generate microporous membranes. Structures like long period, lateral size of lamellae, orientation parameter, and fibril content of the precursor films are obtained by small/wide angle X-ray scattering (SAXS/WAXS), and morphologies of the microporous membranes are examined with scanning electron microscope. It is found that the content of fibrils and orientation parameter of lamellar stacks in the precursor films play counter effects on the capability of micropore formation. A critical window to balance the conflict is revealed with an optimal fibril content of about 6.4%, at which maximal porosity and most homogeneous porous structures are obtained. A schematic model is proposed to account for the role of the fibrils and the oriented lamellar stacks during pore formation, which may guide the production of high-performance HDPE microporous membranes. [ABSTRACT FROM AUTHOR]

Recent efforts attempting to in-situ track the polymer film formation during film blowing are summarized. The specially designed film blowing apparatus is firstly introduced, where the air ring is aligned along with the die, and it's capable of installation in the synchrotron beamline (SR-beamline). Later on, detailed structural evolution of different polymeric systems during film blowing, including polyethylene (PE) and poly(butylene adipate-co- butylene terephthalate) (PBAT), are obtained with the assistance of simultaneous acquired Small- and Wide Angle X-ray Scattering (SAXS/WAXS). The transient structure evolution information from oriented polymer melt to final crystalline tubular film clarifies the contribution of the crystallite-based network to final macroscopic performances. Also, the experimental results have shown the influence of intrinsic molecular characteristics and external processing parameters on the formation of stable tubular bubble. Additionally, the post-modification of the blown film, including stretching and surface treatment is also discussed. Current experimental efforts are expected to provide more detailed parameters for mathematical modeling of film blowing, which could help us modify or develop new numerical models. The micro-structural evolution of polymer during film blowing as in-situ characterized by synchrotron radiation X-ray scattering. Image 1 • Feature article focuses on in-situ tracking of structural evolution during Film Blowing. • A new film blowing machine which can be installed in the SR-beamline is designed. • Formation of crystal-based network is only influenced by molecular parameters. • Flow- and temperature-induced crystallization coexist during film blowing. [ABSTRACT FROM AUTHOR]

The copolymerization of ethylene and different α-olefins could result in polyethylene (PE) with different structural topologies, and lead to polyethylene products with different macroscopic performances. Herein, three different polyethylene samples, namely low-density polyethylene (l -PE), metallocene catalyzed ethylene-hexene copolymer (h -PE) and ethylene-octene copolymer (o -PE), were selected as representatives to construct the structure-process-property relationship during film blowing. The detailed crystal-based network evolution during film blowing was first characterized by in-situ synchrotron radiation X-ray scattering. The crystallization process of l -PE film is determined by the coupling effects of temperature and flow, while those of h -PE and o -PE films are dominated by the temperature. Furthermore, the hierarchical crystal structure from the molecular scale to micrometers of final films and segmental dynamics were systematically characterized by multiple ex-situ characterization techniques, i.e. Solid-State NMR, FTIR, SEM. l -PE film shows the crystalline morphology of the row-nucleated structure, whereas h -PE and o -PE show spherulite-like superstructure with better mechanical properties. The current study tentatively constructs the relation of primary chemical structure, microstructural evolution and macroscopic performances of different polyethylene copolymers during film blowing. • Three ethylene/α-olefin copolymers were selected to construct the structure-process-property relationship on the film blowing. • The structural evolution during film blowing was captured by a combination of in-situ SR-SAXS/WAXS and a custom-built film blowing device. • The multi-scale microstructure and dynamics of final films were characterized by ex-situ SAXS/WAXS, FTIR, SEM, and SS NMR. • The mechanical and optical properties of films are well correlated with their structural evolution and microstructure. [ABSTRACT FROM AUTHOR]

Structural evolutions of cellulose triacetate (CTA) film during uniaxial stretching process at different temperatures were studied by in-situ synchrotron radiation wide-angle X-ray scattering (WAXS) combined with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The results show that four temperature regions can be divided for the CTA film stretching, in which different structural evolutions and deformation mechanisms will be observed. The structural evolutions of CTA film at temperatures between 60 and 125 °C (region I) are seriously influenced by the absorbed moisture, resulting in a large variation of the crystallinity ( χ c ). At temperatures around T g (195 °C), stretch-induced orientations for crystal can be achieved more easily although χ c and crystal size have no large change compared with the flexible-chain semi-crystalline polymers like polyethylene. Interestingly, the CTA film presents various deformation mechanisms including the elongation of amorphous and slipping, formation, destruction and recrystallization of crystals during stretching at different temperatures, which are often observed for the flexible-chain semi-crystalline polymers. These results can provide valuable guidance for the hot-stretching processing and application of CTA retardation film. Image 1 • The structural evolutions of CTA film during stretching were studied by in-situ WAXS. • Four temperature regions with different structural evolutions were observed. • The structural evolution is affected by absorbed moisture at low temperature. • Stretch-induced orientation can be achieved more easily at temperatures around T g. [ABSTRACT FROM AUTHOR]

Abstract The structural evolution of poly (ethylene terephthalate) (PET) with distinct water absorption during uniaxial deformation slightly above glass transition temperature (T g) were investigated by in-situ synchrotron radiation wide angle X-ray scattering (WAXS). Results show that the locally ordered structures are not conducive to stretch-induced crystallization (SIC) although the corresponding entropy is more close to crystals compared to random chains, which is due to the limited chain mobility therein. The "frozen orders" should be destroyed to achieve the critical chain mobility required by SIC even with the presence of water as plasticizer. Unexpectedly, absorption of water does not enhance but suppresses SIC though it does promote the chain mobility, originating from lower stress level with higher water contents. Stress and chain mobility directly correlate with chain stretch and orientation in positive and negative manners, respectively, demonstrating that chain stretch and orientation also play important role in SIC around T g. Graphical abstract Image 10457 Highlights • The structural evolutions of PET samples with different contents of water are investigated with in-situ WAXS. • It is necessary to accumulate sufficient chain mobility for SIC by destroying the frozen orders. • The SIC in PET can be inhibited by water above glass transition temperature. [ABSTRACT FROM AUTHOR]

Lamellar stacks, arranged alternatively with hard crystal and soft amorphous nano-layers, are the basic building units to determine the intrinsic mechanical properties of semi-crystalline polymeric materials. The mechanical instability of hard-elastic isotactic polypropylene (iPP) films with highly parallel lamellar stacks is studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during cyclic tensile deformation. Unexpectedly, the micro-strain, deduced from the relative variation of lamellar periodicity, shows an accelerated increase at the onset of instability and reaches values larger than the corresponding macro-strain after yield, during which no irreversible plastic destruction of crystal is observed. Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXS measurements, we propose that stress-induced microphase separation of interlamellar amorphous phase is responsible for the yield behavior and the hyperelasticity of hard-elastic iPP films, which stems from the heterogeneous distribution of tie chain/trapped entanglement in interlamellar amorphous nano-layers. This reversible stress-induced non-equilibrium phase transition of interlamellar amorphous phase is different from current plastic deformation models with crystal destruction in semi-crystalline polymers but in line with the nearly 100% elastic recovery ratio of hard-elastic films. [ABSTRACT FROM AUTHOR]

Structure evolution of slightly oriented high density polyethylene (HDPE)-plasticizer film is recorded with small- and wide- angle X-ray scattering (SAXS/WAXS) during stretching. The HDPE has number average molecular weight ( M n ) of 300 kg/mol and polydispersity index ( M w / M n ) of 20, respectively. The plasticizer is mixture of aliphatic hydrocarbon with chain length of 11–13 carbon atoms. Experiments are carried out at industrially relevant conditions, with film thickness of 650 μm, plasticizer content of 56%, strain rate of 0.5 s −1 and stretch temperature from 80 °C to 100 °C, respectively. An interesting evolution of SAXS patterns from anisotropic ring, four-point, six-point, and two-lobule is observed. From WAXS data, a monotonous increasing after the first decline of crystallinity is obtained, by which melting-recrystallization is validated to occur during stretching. With peak shift of WAXS pattern at different azimuthal angle ( Φ ), triaxial local stresses exerted on lamellae oriented along different directions are acquired. As consequence, stress exerted along and that perpendicular to chain direction are revealed to induce melting of lamellae at azimuthal angle from 0° to 60° and that from 60° to 90°, respectively. Due to the heterogeneous stress distribution of the system, the appearance of four-point SAXS pattern stems from the sequential melting of lamellae at different azimuthal angles. Through thermodynamic analysis, the physical essence of stress induced melting of lamellae is validated to be elastic energy driven crystal to amorphous phase transition. [ABSTRACT FROM AUTHOR]

Aiming to unveil the effect of cold stretching conditions including strain and temperature on the morphology and performance of membrane product, high density polyethylene (HDPE) microporous membranes were prepared by uniaxial stretching. The evolutions of lamellar stacks and micropore formation were tracked by combining in-situ and ex-situ X-ray scattering and scanning electron microscopy methods. It was found that cold stretching strain mainly affected separation degree of lamellar stacks under strain lower than 60%, while slip of crystal at higher strain. As for cold stretching temperature, stretching below 60 °C can promote separation of lamellar stacks, while stretching over 80 °C is beneficial to pull out molecular chains from lamellae, resulting in fibrils. A linear relation between separation degree of lamellar stack and air permeability of final membrane was established, which can provide guidance for producing HDPE microporous membranes with high performance. [ABSTRACT FROM AUTHOR]

Aiming to unveil the key structures and processing parameters of precursor film for the micropore formation, the structure and mechanical behavior of isotactic polypropylene (iPP) casting films prepared at different processing conditions are systematically studied with small and wide angle X-ray scattering measurements as well as tensile analysis. Draw ratio and cast roll temperature are found to be the key processing parameters, which determine orientation of lamellar stack and thickness of lamellar as well as amorphous layers, respectively. Furthermore, these two important structural parameters, the orientation of lamellar stack and the thickness of amorphous layer, show positive correlations with the formation of micropores. Considering coupling effects of tight folds and lamellar-fibrillar or lamellar-amorphous transformation, a semi-quantitative deformation model for the micropore formation is proposed to correlate surface porosity and elastic recovery rate with structural parameters of precursor film. [ABSTRACT FROM AUTHOR]

The evolution of poly (vinyl alcohol) PVA structure and dichromatic substance induced by uniaxial stretching PVA in the mixed solution with iodine and boric acid (BA) is systematically studied by the combination of in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXD). The experiment is carried out under an orthogonal experimental design for the iodine concentrations of 0.01, 0.2, 1 M, and the BA concentrations of 0.3, 1, and 3 wt%, respectively. We divide the stretch process into three regions, whose boundary is the yield point and strain harden point. The PVA-I 3 - co-crystal generating at high and moderate iodine concentrations is less stable as compared with the PVA crystal, which leads to the earlier decrease of crystallinity at the end of the first region. In the second region, the broken crystal, namely the regular amorphous, is the precursor of the PVA-polyiodide complex. Further increasing the strain, the PVA-I 3 - co-crystals are generated from the one dimension polyiodide surrounded by the PVA chains through a melt-reconstruction process. The adequate BA can make it easy to form the polyiodide at the initial state. Still, the increasing rate of polyiodide is suppressed by further added BA due to the limitation of chain movement by the crosslinking agent. In the final region, the emergence of nanofibril can account for the strain hardening of the film. The evolution of polyiodide and PVA-I 3 - co-crystal revealed by this work gives a better understanding of the polarizer production process, which may guide the resolution of blue leakage of polarizer via an enhancement of the I 3 − in the crystal form during stretching. Image 1 • The structural evolution of dichromatic substance with poly (vinyl alcohol) at different concentrations of boric acid during stretch is investigated by in-situ WAXD and SAXS. • The metastable PVA-I 3 - cocrystal will be easily broken during stretching, which leads to the distinct yield point. • Polyiodide first emerges in the amorphous originated from the broken crystal and then in the highly oriented amorphous during stretching. • The one-dimension polyiodide along with the PVA chain is the precursor of PVA-I 3 - cocrystal. • Dichromatic substance emerges earlier with the increasing of boric acid concentration; however, their final content is less due to the limitation of chain mobility by the crosslinking agent. [ABSTRACT FROM AUTHOR]

Hard-elastic polyethylene (HEPE) cast films are the key intermediate product for producing PE microporous membranes (dry process) used as separators in Lithium battery. The effects of temperature on the deformation mechanisms of HEPE cast films are systematically studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during stretching in a wide temperature range from 25 to 135 °C. The structural evolutions and mechanical behaviors show three distinct features, which contribute to the rough divisions of the temperature space into three regions (I/II/III) with α-I relaxation temperature (T αI) and the onset melting temperature (T onset) as the boundaries. On the basis of the evolutions of the structural parameters like long period (L m), micro-strain (ε m), orientation parameter (f 200), crystallinity (X c), and etc., lamellar separation is the main deformation mode in the linear elastic strain zone. And microphase separation of interlamellar amorphous, lamellar crystal slipping and melt-recrystallization are proposed to determinate the later non-linear mechanical behaviors in the three temperature regions, respectively. The full view of deformation mechanisms in the 2D temperature-strain space aids to deepen the understanding of the nonequilibrium structural evolutions in hard-elastic polyethylene films and guide the manufacture of high-performance microporous membranes with dry process. Image 1 • The structural roadmap of hard-elastic polyethylene films in temperature-strain space was constructed. • The deformation modes depend on the coupled effects of external field and chain mobility. • The microphase separation of interlamellar amorphous dominates in region I. • Plastic deformation like crystal slipping and melt-recrystallization is important in regions II and III. [ABSTRACT FROM AUTHOR]

Mechanical and structural evolution of poly (vinyl alcohol) (PVA) induced by uniaxial deformation at different concentrations of boric acid is systematically studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXS). The PVA films were stretched uniaxially in the aqueous solution at room temperature with concentrations of boric acid at 0.3 wt%, 1 wt% and 3 wt%, respectively. The stretching process can be divided into three stages through WAXS and SAXS results, whose boundary is the end of the crystallinity platform and the onset of the formation of nanofibrils. In the first stage, the films show elastic stretch with the crystallinity keeping constant and the d -spacing of (101) plane (d 101) decreasing. The unstable crystal breaks up and transfers into amorphous during the second stage so that the force is gradually removed from the unit cell. The connection between the crystal network breaks up in the third stage when the content of boric acid is not sufficient enough. Moreover, increasing the concentration of boric acid leads to the earlier emergence of nanofibrils due to the lower energy barrier induced by entropy decrease, which also results in a reduction of nanofibrils content by limiting the mobility of molecular chain. The results have been applied to establish relationships between the microstructure and the physical characteristics of PVA to help us tune the processing parameters during the practical production. •The structural evolution of poly (vinyl alcohol) at different concentrations of boric acid during stretch were investigated with in-suit WAXS and SAXS. •The addition of adequate boric acid can maintain the connection between the crystal. •Increasing the concentration of boric acid leads to the earlier emergence of nanofibrils. •The boric acid works as the crosslinking agent which limits the mobility of the molecular chain to form more nanofibrils. [ABSTRACT FROM AUTHOR]