From Molecular Entanglement Network to Crystal-Cross-Linked Network and Crystal Scaffold during Film Blowing of Polyethylene: An in Situ Synchrotron Radiation Small- and Wide-Angle X-ray Scattering Study

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Circular Economy Assessment in Recycling of LLDPE Bags According to European Resolution, Thermal and Structural Characterization

According to the Circular Economy Package promoted by the European directive, plastic bags companies must use in their formulations a percentage of polyethylene waste (industrial and/or domestic) greater than 70%. Following that regulation requires an understanding of its consequences in the final product from an industrial point of view. This manuscript analyzes the thermal and morphological changes related to the tear resistance of linear-low density polyethylene (LLDPE) samples from industrial waste generated by the company Sphere Spain subjected to the degradation produced by the recycling cycles. The process is analogue to the industrial, starts from samples in pellets then a film by blow extrusion is obtained (odd steps) and posteriorly this film is recycled to pellets again (even steps). The results obtained show that the LLDPE samples develop two crystalline structures (CS1 and CS2) which evolve differently through the recycling cycles with a tendency to decrease in crystallinity due to degradation that is not the same for the process of obtaining film or recycling to pellet. The molecules with a more linear structure and a longer chain break and branch. The more branched structure increases and tends to crosslinking. This leads to a decrease in tear strength in the longitudinal direction, which is not so evident in the transversal direction. The samples could admit four recycling cycles with and acceptable tear resistance. The longitudinal tear strength value decreases by 40% for each film and 20% in the case of tearing in the transverse direction. The results obtained in this research work show that the regulations included in the cited circular economy package can be applied in the manufacture of consumer bags, helping also to reduce the dependence of manufacturers on fluctuations in delivery by collapses in shipping.

SGTools: a suite of tools for processing and analyzing large data sets from in situ X-ray scattering experiments

In situ synchrotron small-angle X-ray scattering (SAXS) is a powerful tool for studying dynamic processes during material preparation and application. The processing and analysis of large data sets generated from in situ X-ray scattering experiments are often tedious and time consuming. However, data processing software for in situ experiments is relatively rare, especially for grazing-incidence small-angle X-ray scattering (GISAXS). This article presents an open-source software suite (SGTools) to perform data processing and analysis for SAXS and GISAXS experiments. The processing modules in this software include (i) raw data calibration and background correction; (ii) data reduction by multiple methods; (iii) animation generation and intensity mapping for in situ X-ray scattering experiments; and (iv) further data analysis for the sample with an order degree and interface correlation. This article provides the main features and framework of SGTools. The workflow of the software is also elucidated to allow users to develop new features. Three examples are demonstrated to illustrate the use of SGTools for dealing with SAXS and GISAXS data. Finally, the limitations and future features of the software are also discussed.

Manipulation of Chain Entanglement and Crystal Networks of Biodegradable Poly(butylene adipate-co-butylene terephthalate) During Film Blowing through the Addition of a Chain Extender: An In Situ Synchrotron Radiation X-ray Scattering Study

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 ¹H 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.

Preparation of Polyethylene and Ethylene/Methacrylic Acid Copolymer Blend Films with Tunable Surface Properties through Manipulating Processing Parameters during Film Blowing

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.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

Structural evolution and phase transition of uniaxially stretched poly(butylene adipate-co-butylene terephthalate) films as revealed by in situ synchrotron radiation small and wide angle X-ray scattering

Structural evolution and phase transition of poly(butylene adipate-co-butylene terephthalate) during uniaxial stretching.

Tailoring the degradation and mechanical properties of poly(ε-caprolactone) incorporating functional ε-caprolactone-based copolymers

Functional block copolymers (COPs) were synthesized through the ring-opening polymerization, and the effects of COPs on the hydrolytic & oxidative degradation and mechanical properties of PCL/COP composites were studied.