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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]

Poly(butylene adipate-co-butylene terephthalate) (PBAT) films under uniaxial stretching at room temperature were characterized by in situ synchrotron radiation wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). Based on the macroscopic stress–strain curve and the microscopic structural information obtained by SAXS/WAXS, three regions are defined. In region I, the molecular chain of the amorphous region undergoes elastic deformation, namely affine deformation. After the yielding point (region II), the oriented amorphous chain starts to crystallize into the metastable β-crystal. In region III, where the strain-hardening happens, the transition from α-crystal to β-crystal occurs through a solid–solid transition mechanism. Therefore, the β-crystal is found to be formed through two different pathways during stretching. Meanwhile, it can be observed that the change in transmittance and phase transition are almost synchronous. This is attributed to the similarity of the refractive index of both the amorphous and crystalline regions after stretching. The relationship between the PBAT phase transition and its optical properties is revealed on the microscopic scale and could provide theoretical guidance for the preparation of agricultural films with higher transmittance. [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 reveal the stabilization mechanism of micropore embryos formed during cold stretching in high-density polyethylene films, samples are subsequently subjected to temperature elevation and strain holding at 25 °C, respectively. The corresponding structure evolution is tracked. It is found that after strain holding at 25 °C and subsequent strain recovery, inhomogeneously distributed cavities are produced, most of which can be healed as temperature is elevated to 110 °C. Consequently, only a small number of nonevenly distributed micropores are formed during the subsequent hot stretching. While for thermal pathway, micropores and fibrils can be formed as temperature is elevated. The hot stretching membrane exhibits uniformly distributed micropores and the micropores are well interconnected, indicating that micropores stabilized via temperature elevation are permanent and homogeneous. The results reveal different stabilization mechanisms of micropores via the thermal and mechanical pathways with regard to the distribution as well as the amount of permanent micropores. [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]

Polymer films based on polyethylene (PE) and ionomer ethylene/methacrylic acid (EMAA) copolymer blend were prepared by film blowing, whose surface properties were tuned by varying processing parameters, i.e., take up ratio (TUR). Blends of PE/EMAA copolymer were firstly prepared by the melt-mixing method, before being further blown to films. The wettability of the film was investigated by measuring the contact angle/water-film encounter time, and optical properties, i.e., the haze and transmittance. The wettability was found to be enhanced with the increase of TUR. So too was the haze, while the transmittance was found to be almost independent of TUR. The XPS and AFM results directly show the increasing polar functional groups (–COO−) on the surface and roughness with increasing TUR. Further analysis of the 2D SAXS and WAXS unveiled the origin of the invariant transmittance, which resulted from the minor change of the crystallinity and the monotonic increase of the haze, with TUR resulting from the evolution of crystal orientation. In addition to other post-modification methods, the current study provides an alternative route to prepare large-scale PE films as the template for the advanced potential applications, i.e., covering in the layer of roof, the privacy of protective windows, and multitudes of packaging. [ABSTRACT FROM AUTHOR]

One prerequisite for the large-scale application of biodegradable polymers is the manipulation of macroscopic performances of commercially available biopolymers during processing according to different real service requirements. Herein, the microstructural evolution of poly(butylene adipate- co -butylene terephthalate) (PBAT) modified by chain extender during film blowing was investigated by in situ synchrotron radiation X-ray scattering to unveil the origin of different performances. The chain dynamics difference induced by the chain extender was first characterized by the rheological measurement and 1 H Multiple Quantum (MQ) NMR. It shows that the terminal relaxation is significantly slowed down, while the locally segmental dynamics is not apparently changed. With the assistance of the custom-built film blowing apparatus, the microstructure right above the die exit ( D = 13-165 mm) was in situ, simultaneously captured by small- and wide-angle scattering (SAXS/WAXS), where four distinct regimes can be defined. Only the PBAT melt signals are found in regime I, whereas the formation of the mesomorphic domains as shown by the SAXS streaks appearing in regime II. The crystal shows up in regime III, where the WAXS signal appears. A dramatic increment of the crystallinity is found in regime III, which contributes to the continuous increasing bubble modulus with the formation of the crystal-based network. Such a crystal-based network is filled with crystals in regime IV, where the diameter of the PBAT bubble remains constant. The addition of the chain extender is found to significantly influence the structural evolution within different regimes. These dynamics and structure information could supply general guidance for bubble stability improvement and modification of macroscopic performances of biodegradable polymer products.

Deformation-driven alignment of macromolecules or nanofibers leading to anisotropy is a challenge in functional soft materials. Here, tough cellulose hydrogels that exhibited deformation-induced anisotropy are fabricated by reacting cellulose with a small amount of epichlorohydrin (EPI) in LiOH/urea solution and subsequent treating with dilute acid. The loosely cross-linked network that was obtained via chemical cross-linking of cellulose with EPI as a large framework maintained the elasticity of hydrogels, whereas nanofibers produced by the acid treatment formed physical cross-linked networks through hydrogen bonds which could efficiently dissipated mechanical energy. Meanwhile, the nanofibers could further aggregate to form submicrobundles and participate in the formation of frameworks during the acid treatment. Under deformation, the nanofibers and submicrobundles in the physical networks synchronize easily to align with the large framework, generating the rapidly responsive birefringence behaviors with highly stable colors. Thus, the cellulose hydrogels possessing sensitively mechano-responsive behavior could be utilized as a dynamic light switch and a soft sensor to accurately detect small external force, respectively. This work opens a novel pathway to construct tough and mechanoresponsive hydrogels via a green conversion of natural polysaccharide.