Morphological, interfacial and rheological properties in multilayer polymers: A review

Correlations between adhesion and molecular interactions at buried interfaces of model polymer systems and in commercial multilayer barrier films

Sum frequency generation vibrational spectroscopy (SFG) was applied to characterize the interfacial adhesion chemistry at several buried polymer interfaces in both model systems and blown multilayer films. Anhydride/acid modified polyolefins are used as tie layers to bond dissimilar polymers in multilayer barrier structures. In these films, the interfacial reactions between the barrier polymers, such as ethylene vinyl alcohol (EVOH) or nylon, and the grafted anhydrides/acids provide covalent linkages that enhance adhesion. However, the bonding strengths vary for different polymer-tie layer combinations. Here, using SFG, we aim to provide a systematic study on four common polymer-tie interfaces, including EVOH/polypropylene-tie, EVOH/polyethylene-tie, nylon/polypropylene-tie, and nylon/polyethylene-tie, to understand how the adhesion chemistry varies and its impact on the measured adhesion. Our SFG studies suggest that adhesion enhancement is driven by a combination of reaction kinetics and the interfacial enrichment of the anhydride/acid, resulting in stronger adhesion in the case of nylon. This observation matches well with the higher adhesion observed in the nylon/tie systems in both lap shear and peel test measurements. In addition, in the polypropylene-tie systems, grafted oligomers due to chain scission may migrate to the interface, affecting the adhesion. These by-products can react or interfere with the barrier-tie chemistry, resulting in reduced adhesion strength in the polypropylene-tie system.

Confinement Effect in Multilayer Films Made from Semicrystalline and Bio-Based Polyamide and Polylactic Acid

Bio-based multilayer films were prepared by using the innovative nanolayer coextrusion process to produce films with a number of alternating layers varying from 3 to 2049. For the first time, a semicrystalline polymer was confined by another semicrystalline polymer by nanolayering in order to develop high barrier polyamide (PA11)/polylactic acid (PLA) films without compromising thermal stability and mechanical behavior. This process allows the preparation of nanostratified films with thin layers (down to nanometric thicknesses) in which a confinement effect can be induced. The stratified structure has been investigated, and the layer thicknesses have been measured. Barrier properties were successfully correlated to the microstructure, as well as the thermal behavior, and mechanical properties. The layer continuity was fully achieved for most of the films, but some layer breakups have been observed on the film with the thinnest PLA layer (2049-layers film). Coextruding PLA with PA11 has induced an increase in PLA crystallinity (from 4 to 16%) along with an increase in thermal stability of the multilayer films without impacting PA11 properties. Gas barrier properties were driven by the PLA confined layers due to the microstructural rearrangement by increasing crystallinity, whereas water barrier properties were governed by the PA11 confining layers due to its lower water affinity. As a consequence, a decrease of water permeability (up to 11 times less permeable for the 6M film) but an increase of gas barrier properties (barrier improvement factor (BIF) of 66% for the 0M film for N2 and BIF of 36% for the 6M film for CO2 for instance) were evidenced as the layer number was increased. This study paves the way for the development of ecofriendly materials with outstanding barrier performances and highlights the importance of nonmiscible polymers adhesion at melt state and additives presence.

Nanoporous air filtering systems made from renewable sources: benefits and challenges

There is a crucial need for air purification systems due to increasing air contamination, while conventional air-filtering materials face challenges in eliminating gaseous and particulate pollutants. This review examines the development and characteristics of nanoporous polymeric materials developed from renewable resources, which have rapidly advanced in recent years. These materials offer more sustainable alternatives for nanoporous structures made out of conventional polymers and significantly impact the properties of porous polymers. The review explores nanoporous materials' production from renewable sources, filtering mechanisms, physicochemical makeup, and sensing capabilities. The recent advancements in this field aim to enhance production techniques, lower pressure drop, and improve adsorption efficiency. Currently, supporting approaches include using adsorbent layers and binders to immobilize nanoporous materials. Furthermore, the prospects and challenges of nanoporous materials obtained from renewable sources used for air purification are discussed.

Non-Stick Length of Polymer-Polymer Interfaces under Small-Amplitude Oscillatory Shear Measurement

Interfaces in soft materials often exhibit deviation from non-slip/stick response and play a determining role in the rheological response of the overall system. We discuss detection techniques for the excess interface rheology using small-amplitude oscillatory shear (SAOS) measurements. A stacked bilayer of different polymers is sheared parallel to the interface and the dynamic shear response is measured. Deviation of the bilayer shear modulus from the superposition of the shear moduli of the component layers is analysed. Furthermore, we introduce a frequency-dependent non-stick length based on the bilayer SAOS response to characterize the excess interface rheology. We observe an approximate stick response in the interface in bilayers composed of the chemically same monomer as well as an apparent slip in the interface between immiscible polymers. The results suggest that the proposed non-stick length in SAOS is capable of detecting the apparent interfacial slip. The non-stick length in SAOS is readily applicable to other complex interfaces of different soft materials and offers a convenient tool to characterize the excess interface rheology.

Improved Optimization of a Coextrusion Die with a Complex Geometry Using the Coupling Inverse Design Method

The main challenge in a polymer coextrusion process is to have a good die design prior to the process, which can minimize the geometric errors that are caused by extrusion swell and interface motion. For this purpose, a coupling method of optimization and inverse design for a coextrusion die was studied for a medical striped catheter. In the study, the main material was thermoplastic polyurethane (TPU), and the auxiliary material was TPU filled with 30 wt% barium sulfate. An overall optimization design method was used to optimize the geometry of the extrusion die channel for the striped catheter, which had a complex geometry. In the global optimization process, the local inverse design method was used to design the inlet of the auxiliary material. The non-linear programming by quadratic Lagrangian (NLPQL) algorithm was used to obtain the optimal geometric solution of the coextrusion die runner. The experimental verification results showed that the coupling method for coextrusion die design improved the design efficiency of the coextrusion die remarkably. The value of the objective function, which was used to measure the geometric error of the product, was reduced by 72.3% compared with the initial die design.

Overview of the Cast Polyolefin Film Extrusion Technology for Multi-Layer Packaging Applications

The review article presents the technology of producing polyolefin-based films by extrusion casting. Due to the wide use of this type of film as packaging for food and other goods, obtaining films with favorable properties is still a challenge for many groups of producers in the plastics market. The feedblock process and multimanifold process are the main methods of producing multi-layer film. In the case of food films, appropriate barrier properties are required, as well as durability and puncture resistance also at low temperatures. On the other hand, in order to properly pack and present products, an appropriate degree of transparency must be maintained. Therefore, processing aids such as anti-slip, anti-block and release agents are commonly used. Other popular modifiers, such as waxes, fatty acid amides and mineral fillers-silica, talc or calcium carbonate-and their use in film extrusion are discussed. The article also presents common production problems and their prevention.

Optimization of Processing Conditions and Mechanical Properties for PEEK/PEI Multilayered Blends

The goal of producing polyetheretherketone/polyetherimide (PEEK/PEI) blends is to combine the outstanding properties that both polymers present separately. Despite being miscible polymers, it is possible to achieve PEEK/PEI multilayered blends in which PEEK crystallinity is not significantly inhibited, as opposed to conventional extruding processes that lead to homogeneous mixtures with total polymer chain interpenetration. This study investigated a 50/50 (volume fraction) PEEK/PEI multilayered polymer blend in which manufacturing parameters were tailored to simultaneously achieve PEEK-PEI adhesion while keeping PEEK crystallinity in order to optimize the mechanical properties of this heterogeneous polymer blend. The interface adhesion was characterized with the use of three-point bending tests, which proved that a processing temperature below the melting point of PEEK produced weak PEEK-PEI interfaces. Results from differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and X-ray diffraction analysis (XRD) showed that under a 350 °C consolidation temperature, there is a partial mixing of PEEK and PEI layers in the interface that provides good adhesion. The thickness of the mixed homogeneous region at this temperature exhibits reduced sensitivity to processing time, which ensures that both polymers essentially remain separate phases. This also entails that multilayered blends with good mechanical properties can be reliably produced with short manufacturing cycles. The combination of mechanical performance and potential joining capability supports their use in a wide range of applications in the automotive, marine, and aerospace industries.

Interfacial structure and properties of isotactic polybutene-1/polyethylene blends

Polymer blending is one of the most economical and effective techniques for achieving products with high comprehensive performances. However, the immiscibility between polymers results in a weak interface, which is typically the position where material failure starts when an external force is applied. Therefore, understanding and controlling the interfacial structure are important for controlling the failure behavior of polymer blends and achieving advanced materials. In this study, the related work was performed on a crystal/crystal blend of isotactic polybutene-1 and polyethylene (iPB-1/PE). The results indicated that iPB-1 and PE were partially miscible in a wide temperature window (140–220°C), and the phase separation of iPB-1/PE blends was retarded at 180°C, resulting in an increase in the interfacial thickness and interfacial adhesive strength when iPB-1/PE crystallized at a low temperature. In addition, the iPB-1/high-density PE (HDPE) samples exhibited higher interfacial adhesive strength than the iPB-1/linear low-density PE, which was attributed to the relative streamline chain structure and the wide molecular weight distribution of HDPE and improved the interpenetration, crystallization, and miscibility of iPB-1 and HDPE at the interface. During storage at room temperature, the interfacial adhesive strength of iPB-1/PE decreased because of the spontaneous crystal transition of iPB-1.

Upcycling of semicrystalline polymers by compatibilization: mechanism and location of compatibilizers

With the continuous increase of global plastics production, there is a demand to develop energy efficient processes to transform mixed plastic wastes into new products with enhanced utility - a concept that is often referred to as upcycling. Compatibilization is one of the most promising strategies to upcycle communal waste plastics. In this work, poly(ethylene terephthalate) (PET) and high-density polyethylene (HDPE), both widely used semicrystalline packaging polymers, are used as the target polymer blend. We systematically evaluate and compare three commercial ethylene copolymer based compatibilizers, ELVALOY™ AC 2016 Acrylate Copolymer (EAA), ELVALOY™ PTW Copolymer (PTW), and SURLYN™ 1802 Ionomer (Surlyn). They represent different compatibilization mechanisms. Furthermore, this work tackles a challenging question: where the compatibilizers are located in the blend. We discover that the location of the compatibilizer molecules can be predicted by comparing the crystallinity change of PET and HDPE in binary and ternary systems. Gaining this knowledge will facilitate root cause analysis of an ineffective compatibilizer and guide the design strategy to upcycle commingled waste plastics.

High-pressure treatment of water-filled co-extruded polylactide films: Effect on microstructure, barrier, thermal, and rheological properties

The impact of high-pressure treatments (450 and 600 MPa) on the morphological, thermal, structural, and barrier properties of commercial coextruded polylactide (PLA) packaging films has been explored to evaluate their applicability in food processing. Pouches filled with water as a food simulant were subjected to high-pressure treatment for 15 min at ambient temperature. Results indicated no significant changes in the visual appearance, color, integrity, or water barrier properties of the post-process pouches. However, high-pressure treatment affected mechanical property results. Thermal analysis of the film showed endothermic double melting peaks (165.12 and 170.55°C), which did not change with the pressurization; however, the exothermic crystallization peak (118.08°C) varied significantly. Both SEM and AFM micrographs demonstrated that the surface morphology and roughness parameters (arithmetic mean [S_a ] and root mean square height [S_q ]) of the films were significantly affected by the HP treatment, which is further complemented by the FTIR spectra and XRD diffractogram. Melt rheology (175-205°C) of the pressure-treated films showed a significant drop (20-30%) in mechanical rigidity (G') when compared to the untreated sample. Changes in the microstructure/crystallinity in the PLA films were indicated by van Gurp and Palmen plot. PRACTICAL APPLICATION: The results reported here can help to improve the design of the coextruded packaging materials so that it can be successfully implemented to high-pressure processing and high pressure-assisted thermal processing of food.

Influence of the Size of the Fiber Filler of Corn Stalks in the Polylactide Matrix Composite on the Mechanical and Thermomechanical Properties

This paper presents results of a study on the effect of filler size in the form of 15 wt% corn stalk (CS) fibers on the mechanical and thermomechanical properties of polylactide (PLA) matrix composites. In the test, polylactidic acid (PLA) is filled with four types of length of corn stalk fibers with a diameter of 1 mm, 1.6 mm, 2 mm and 4 mm. The composites were composed by single screw extrusion and then samples were prepared by injection molding. The mechanical properties of the composites were determined by static tensile test, static bending test and Charpy impact test while the thermo-mechanical properties were determined by dynamic mechanical thermal analysis (DMTA). The composite structures were also observed using X-ray microcomputed tomography and scanning electron microscopy. In the PLA/CS composites, as the filler fiber diameter increased, the degradation of mechanical properties relative to the matrix was observed including tensile strength (decrease 22.9-51.1%), bending strength (decrease 18.9-36.6%) and impact energy absorption (decrease 58.8-69.8%). On the basis of 3D images of the composite structures for the filler particles larger than 2 mm a weak dispersion with the filler was observed, which is reflected in a significant deterioration of the mechanical and thermomechanical properties of the composite. The best mechanical and thermomechanical properties were found in the composite with filler fiber of 1 mm diameter. Processing resulted in a more than 6-fold decrease in filler fiber length from 719 ± 190 µm, 893 ± 291 µm, 1073 ± 219 µm, and 1698 ± 636 µm for CS1, CS1.6, CS2, and CS4 fractions, respectively, to 104 ± 43 µm, 123 ± 60 µm, 173 ± 60 µm, and 227 ± 89 µm. The fabricated green composites with 1 to 2 mm corn stalk fiber filler are an alternative to traditional plastic based materials in some applications.

Enhanced Anticorrosion Properties through Structured Particle Design of Waterborne Epoxy-Styrene-Acrylate Composite Emulsion

In order to develop a waterborne epoxy-styrene–acrylate composite latex with a better stability and anticorrosion resistance, a novel synthetic approach has been proposed. First, modified by methyl acrylic, epoxy resin containing terminal C=C double bonds was successfully synthesized, where epoxide groups were partially retained. Then, by structural design and multi-stage seed emulsion copolymerization, a stable waterborne epoxy-styrene-acrylate composite latex composed of a modified epoxy resin acrylate polymer as the core, inert polystyrene ester as the intermediate layer, and carboxyl acrylate polymer as the shell was successfully fabricated. The structure of the obtained latex was characterized by fourier transform infrared (FTIR) and transmission electron microscopy (TEM). The stability of the composite latex was tested based on the wet gel weight, Zeta potential, and storage stability, and the corrosion resistance of the composite latex films was analyzed by electrochemical measurements and salt spray tests. The thickness of each layer of the composite latex was calculated by the temperature random multi-frequency modulation DSC (TOPEM-DSC) technique. In addition to the successful emulsion copolymerization that occurred between the modified epoxy resin and acrylate monomer, the presence of carboxyl groups in the obtained latex was evidenced, while the epoxide groups were partially retained. The anticorrosion resistance and stability of the multilayer composite latex with the intermediate layer are better than that of the conventional core-shell latex. The outstanding stability and corrosion resistance is attributed to the multilayer core-shell structure. The TOPEM-DSC approach can accurately determine the thickness of the intermediate layer in the multilayer core-shell particles and is a new strategy for characterizing the core-shell structure of polymer particles with a similar monomer composition.

Common Nettle (Urtica dioica L.) as an Active Filler of Natural Rubber Biocomposites

Common nettle (Urtíca Dióica L.), as a natural fibrous filler, may be part of the global trend of producing biocomposites with the addition of substances of plant origin. The aim of the work was to investigate and explain the effectiveness of common nettle as a source of active functional compounds for the modification of elastomer composites based on natural rubber. The conducted studies constitute a scientific novelty in the field of polymer technology, as there is no research on the physico-chemical characteristics of nettle bio-components and vulcanizates filled with them. Separation and mechanical modification of seeds, leaves, branches and roots of dried nettle were carried out. Characterization of the ground plant particles was performed using goniometric measurements (contact angle), Fourier transmission infrared spectroscopy (FTIR), themogravimetric analysis (TGA) and scanning electron microscopy (SEM). The obtained natural rubber composites with different bio-filler content were also tested in terms of rheological, static and dynamic mechanical properties, cross-linking density, color change and resistance to simulated aging processes. Composites with the addition of a filler obtained from nettle roots and stems showed the highest mechanical strength. For the sample containing leaves and branches, an increase in resistance to simulated ultraviolet and thermo-oxidative aging processes was observed. This phenomenon can be attributed to the activity of ingredients with high antioxidant potential contained in the plant.