Deformation of confined poly(ethylene oxide) in multilayer films

Effects of network junctions and defects on the crystallization of model poly(ethylene glycol) networks

Polymer crystallization drastically changes the physical properties of polymeric materials. However, the crystallization in polymer networks has been little explored. This study investigated the crystallization behavior of a series of poly(ethylene glycol) (PEG) networks consisting of well-defined branched precursors. The PEG networks were prepared by drying gels synthesized at various conditions. The PEG networks showed slower crystallization with lower final crystallinity than uncrosslinked PEGs with amine end groups. Surprisingly, the effect of network formation was not as significant as that of the relatively bulky end-groups introduced in the uncrosslinked polymer. The molecular weight of the precursor PEG, or equivalently the chain length between neighboring junctions, was the primary parameter that affected the crystallization of the PEG networks. Shorter network chains led to lower crystallization rates and final crystallinity. This effect became less significant as the network chain length increased. On the other hand, the spatial and topological defects formed in the gel synthesis process did not affect the crystallization in the polymer networks at all. The crystallization in the polymer networks seems insensitive to these mesoscopic defects and can be solely controlled by the chain length between junctions.

Multilayered Nanocomposites Prepared through Quadruple-Layering Approach towards Enhanced Mechanical Performance

Multilayered materials are widely studied due to their special structures and great properties, such as their mechanical ones. In this paper a novel and effective technique, a quadruple-layering approach, was used to fabricate multilayered materials. This approach increases the number of layers rapidly via simple operations. Materials with 4, 16, and 64 layers with alternating layers of polypropylene and nanocomposites were fabricated using this approach, and their film morphology and mechanical properties were studied. The influence of the number of layers on the mechanical properties of the materials and the relationship between the mechanical properties of each material were investigated. The results illustrated that the tensile modulus and strength were enhanced and elongation at the break increased when the layer numbers of the multilayered materials increased. However, this approach has a defect in that as the layer number increases, the layer thickness was not uniform, thus restricting the improvement of properties. This may need to be further studied in future work.

Interfacial Phenomena in Multi-Micro-/Nanolayered Polymer Coextrusion: A Review of Fundamental and Engineering Aspects

The multilayer coextrusion process is known to be a reliable technique for the continuous fabrication of high-performance micro-/nanolayered polymeric products. Using laminar flow conditions to combine polymer pairs, one can produce multilayer films and composites with a large number of interfaces at the polymer-polymer boundary. Interfacial phenomena, including interlayer diffusion, interlayer reaction, interfacial instabilities, and interfacial geometrical confinement, are always present during multilayer coextrusion depending on the processed polymers. They are critical in defining the microstructural development and resulting macroscopic properties of multilayered products. This paper, therefore, presents a comprehensive review of these interfacial phenomena and illustrates systematically how these phenomena develop and influence the resulting physicochemical properties. This review will promote the understanding of interfacial evolution in the micro-/nanolayer coextrusion process while enabling the better control of the microstructure and end use properties.

Controlled 3D Shape Transformation Activated by Room Temperature Stretching and Release of a Flat Polymer Sheet

Shape transformation of polymeric materials, including hydrogels, liquid crystalline, and semicrystalline polymers, can be realized by exposing the shape-changing materials to the effect of a variety of stimuli such as temperature, light, pH, and magnetic and electric fields. Herein, we demonstrate a novel and different approach that allows a flat sheet or strip of a polymer to transform into a predesigned 3D shape or structure by simply stretching the polymer at room temperature and then releasing it from the external stress, that is, a 2D-to-3D shape change is activated by mechanical deformation under ambient conditions. This particular type of stimuli-controlled shape-changing polymers is based on suppressing plastic deformation in selected regions of the flat polymer sheet prior to stretching and release. We validated the design principle by using a polymer blend composed of poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA), and tannic acid (TA) whose plastic deformation can be locally inhibited by surface treatment using an aqueous solution of copper sulfate pentahydrate (Cu²⁺ ink) that cross-links PAA chains through a Cu²⁺-carboxylate coordination and, consequently, increases the material's Young's modulus and yield strength. After room temperature stretching and release, elastic deformation in the Cu²⁺ ink-treated regions leads to 3D shape transformation that is controlled by the patterned surface treatment. This facile and effective "stretch-and-release" approach widens the scope of preparation and application for shape-changing polymers.

Multiple grafting to enzymatically synthesized polyesters

Enzymatic polymerization is an environmentally benign process for the synthesis of biodegradable and biocompatible polymers. The regioselectivity of lipase B from Candida Antarctica (CAL-B) produces linear functional polyesters without protection-deprotection steps. In this work, two different methods for the enzymatic synthesis of functional polyesters based on renewable resources, as, e.g., glycerol, using CAL-B are outlined. Poly(glycerol adipate) was synthesized by enzymatic transesterification between glycerol and divinyl adipate or dimethyl adipate. Methods are also reported to graft poly(glycerol adipate) with different amounts of hydrophobic side chains (lauric, stearic, behenic, and oleic acids) and hydrophilic poly(ethylene glycol) side chains, respectively. The hydrophilicity or lipophilicity of grafted polyesters is well controlled by changing the degree of grafting of hydrophilic and hydrophobic side chains. The multiple grafted polyesters are characterized by NMR spectroscopy, differential scanning calorimetry, gel permeation chromatography, and X-ray diffraction. Furthermore, the self-assembly of the graft copolymers in water and their use as steric stabilizers for cubosomes are discussed. For this purpose mainly dynamic light scattering and small angle X-ray scattering have been employed.

Nanolayer coextrusion: An efficient and environmentally friendly micro/nanofiber fabrication technique

Researchers have developed many types of nanoscale materials with different properties. Among them, nanofibers have recently attracted increasing interest and attention due to their functional versatility and potential applications in diverse industries, including tapes, filtration, energy generation, and biomedical technologies. Nanolayer coextrusion, a novel polymer melt fiber processing technology, has gradually received attention due to its environmental friendliness, efficiency, simplicity and ability to be mass-produced. Compared with conventional techniques, nanolayer coextruded non-woven nanofibrous mats offer advantages such as a tunable fiber diameter, high porosity, high surface area to volume ratio, and the potential to manufacture composite nanofibers with different components to achieve desired structures and properties. Dozens of thermoplastic polymers have been coextruded for various applications, and the variety of polymers has gradually continued to increase. This review presents an overview of the nanolayer coextrusion technique and its promising advantages and potential applications. We discuss nanolayer coextrusion theory and the parameters (polymer and processing) that significantly affect the fiber morphology and properties. We focus on varied applications of nanolayer coextruded fibers in different fields and conclude by describing the future potential of this novel technology.

Critical Role of Interfacial Diffusion and Diffuse Interphases Formed in Multi-Micro-/Nanolayered Polymer Films Based on Poly(vinylidene fluoride) and Poly(methyl methacrylate)

It is known that the macroscopic properties of multilayer polymer films are largely dominated by the diffuse interphase formed via interfacial diffusion between neighboring layers. However, not much is known about the origin of this effect. In this work, we reveal the role of interfacial diffusion and the diffuse interphase development in multilayer polymer films, based on a compatible poly(vinylidene fluoride)/poly(methyl methacrylate) system fabricated by forced-assembly micro-/nanolayer coextrusion. Interestingly, the layer morphology is found to prevail in all investigated multilayer films, even for the nanolayered system where the interdiffusion is substantial. It is also demonstrated that, in the presence of macromolecular and geometrical confinements, interfacial diffusion significantly alters the crystalline morphology and microstructure of the resulting micro-/nanolayered films, which leads to quantitatively different dielectric and rheological properties. More importantly, the combination of dielectric relaxation spectroscopy and energy-dispersive X-ray analysis further reveals that multiple diffuse interphases with various length scales exist in the multilayer structures. The presence of these multiple interphases is explained in terms of a proposed physical picture for the interdiffusion of fast-mode mechanism occurring in the coextrusion process, and their length scales (i.e., interphase thicknesses) are further mapped quantitatively. These findings provide new insights into the effects of interfacial diffusion and diffuse interphases toward tailoring interfaces/interphases in micro-/nanolayered polymer structures and for their advanced applications.

Chain Tilt and Crystallization of Ethylene Oxide Oligomers with Midchain Defects

Many text books and publications do not focus on the necessity of chain tilting in crystalline lamellae of oligomers and polymers, a fundamental aspect of their crystallization already discussed by Flory. Herein we investigate the chain tilt of ethylene oxide oligomers (EOs) containing various midchain defects by WAXS, SAXS and solid state ¹³C MAS NMR spectroscopy. At low temperatures, one out of the two EO chains of EO₉-meta-EO₉ and EO₁₁-TR-EO₁₁ containing a 1,3-disubstituted benzene or a 1,4-disubstituted 1,2,3-triazole defect in central position of the oligomer chain forms crystals and the other EO chain as well as the defect remain in the amorphous phase. The aromatic midchain defect of these two oligomers can be incorporated into the crystalline lamella upon heating below T_m. Then, the adjoining amorphous EO chain crosses from the lamellae to the amorphous regions at an angle ξ, which is preordained by the substitution pattern of the aromatic defect, revealing that the chain tilt angle ranges between 36° ≤ ϕ ≤ 60°.

Structure and Barrier Properties of Multinanolayered Biodegradable PLA/PBSA Films: Confinement Effect via Forced Assembly Coextrusion

Multilayer coextrusion processing was applied to produce 2049-layer film of poly(butylene succinate-co-butylene adipate) (PBSA) confined against poly(lactic acid) (PLA) using forced assembly, where the PBSA layer thickness was about 60 nm. This unique technology allowed to process semicrystalline PBSA as confined polymer and amorphous PLA as confining polymer in a continuous manner. The continuity of PBSA layers within the 80/20 wt % PLA/PBSA layered films was clearly evidenced by atomic force microscopy (AFM). Similar thermal events to the reference films were revealed by thermal studies; indicating no diffusion of polymers during the melt-processing. Mechanical properties were measured for the multilayer film and the obtained results were those expected considering the fraction of each polymer, revealing the absence of delamination in the PLA/PBSA multinanolayer film. The confinement effect induced by PLA led to a slight orientation of the crystals, an increase of the rigid amorphous fraction (RAF) in PBSA with a densification of this fraction without changing film crystallinity. These structural changes allowed to strongly improve the water vapor and gas barrier properties of the PBSA layer into the multilayer film up to two decades in the case of CO₂ gas. By confining the PBSA structure in very thin and continuous layers, it was then possible to improve the barrier performances of a biodegradable system and the resulting barrier properties were successfully correlated to the effect of confinement on the microstructure and the chain segment mobility of the amorphous phase. Such investigation on these multinanolayers of PLA/PBSA with the aim of evidencing relationships between microstructure implying RAF and barrier performances has never been performed yet. Besides, gas and water permeation results have shown that the barrier improvement obtained from the multilayer was mainly due to the reduction of solubility linked to the reduction of the free volume while the tortuosity effect, as usually expected, was not really observed. This work brings new insights in the field of physicochemical behaviors of new multilayer films made of biodegradable polyesters but also in interfacial processes due to the confinement effect induced in these multinanolayer structures obtained by the forced assembly coextrusion. This original coextrusion process was a very advantageous technique to produce eco-friendly materials with functional properties without the help of tie layer, additives, solvents, surface treatments, or inorganic fillers.

Preparation of alternating multilayered polyethylene oxide/poly(ε-caprolactone) and the confined crystallization of the composites

Due to the effects of the interfacial interaction between PEO and PCL layers and the spatial confinement. The crystallization behaviors of PEO and PCL in multilayered structure varied distinguishably as the layer number became higher.

Speeding of spherulitic growth rate at the late stage of isothermal crystallization due to interfacial diffusion for double-layer semicrystalline polymer films

In this study a unique phenomenon has been found for isothermal crystallization of double-layer semicrystalline polymer films. It is surprisingly found that there exists a speeding of poly(l-lactic acid) (PLA) spherulitic growth rate for poly(ethylene oxide)/poly(l-lactic acid) (PEO/PLA) double-layer films at the late stage of isothermal crystallization, which does not exist for PLA/PEO blend films and neat PLA films. The mutual diffusion between PEO and PLA layers plays the key factor to bring out the observed speeding of spherulitic growth rate. This type of study provides an avenue for understanding the interplay between polymer crystallization and interfacial diffusion in multilayer polymer films, which is not available when employing the polymer blend films.

Structural evolution during mechanical deformation in high-barrier PVDF-TFE/PET multilayer films using in situ X-ray techniques

Poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF-TFE) is confined between alternating layers of poly(ethylene terephthalate) (PET) utilizing a unique multilayer processing technology, in which PVDF-TFE and PET are melt-processed in a continuous fashion. Postprocessing techniques including biaxial orientation and melt recrystallization were used to tune the crystal orientation of the PVDF-TFE layers, as well as achieve crystallinity in the PET layers through strain-induced crystallization and thermal annealing during the melt recrystallization step. A volume additive model was used to extract the effect of crystal orientation within the PVDF-TFE layers and revealed a significant enhancement in the modulus from 730 MPa in the as-extruded state (isotropic) to 840 MPa in the biaxially oriented state (on-edge) to 2230 MPa in the melt-recrystallized state (in-plane). Subsequently, in situ wide-angle X-ray scattering was used to observe the crystal structure evolution during uniaxial deformation in both the as-extruded and melt-recrystallized states. It is observed that the low-temperature ferroelectric PVDF-TFE crystal phase in the as-extruded state exhibits equatorial sharpening of the 110 and 200 crystal peaks during deformation, quantified using the Hermans orientation function, while in the melt-recrystallized state, an overall increase in the crystallinity occurs during deformation. Thus, we correlated the mechanical response (strain hardening) of the films to these respective evolved crystal structures and highlighted the ability to tailor mechanical response. With a better understanding of the structural evolution during deformation, it is possible to more fully characterize the structural response to handling during use of the high-barrier PVDF-TFE/PET multilayer films as commercial dielectrics and packaging materials.