Dripping-onto-substrate (DoS) rheometry is a well-established method for measuring the extensional rheology of low-viscosity liquids. However, clear guidelines on the capabilities and limitations of the technique are lacking. In the present work, we define operational limits for measuring a transient extensional viscosity directly from observations of the rate of filament thinning, as well as model-based bounds on calculating a viscosity η and extensional relaxation time τE of a liquid using DoS. Dilute solutions of polyethylene oxide and polyacrylamide are used to probe the lower limit of measurable τE, demonstrating that values as low as 0.1 ms can be resolved, provided (a) the intrinsic Deborah number (based on the ratio of the relaxation time and the Rayleigh breakup time scale) is De≥O(0.1) and (b) an instrumental constraint related to spatial and temporal resolution is satisfied. This instrumental constraint is quantified through a new metric we define as the filament capture rate, a “figure of merit” (expressed in s−1) that can be used to quantify the number of data points within the elastocapillary regime that are available for extraction of τE. Finally, model-agnostic bounds for reliable measurements of the transient extensional viscosity ηE+ using DoS rheometry are derived. These bounds prescribe experimental limitations due to inertia, gravity, and measurement resolution (spatial and temporal) that can assist rheologists in the design and interpretation of extensional viscosity measurements using DoS. Collectively, these results provide a quantitative roadmap for reliable DoS rheometry and affirm its use for measuring the transient extensional viscosity of liquid-like materials.

We derive an operating limit line for the non-ideal artifacts caused by machine stiffness (instrument compliance) which causes measured apparent viscoelastic moduli to be systematically lower than the true values. The limit is represented as a maximum measurable apparent shear modulus G_max , or tensile modulus E_max , which can be shown explicitly on plots of viscoelastic moduli independent of the applied displacement, load, or frequency. Uncorrected data should be much lower than these limits. Corrected data can be above these limits and credible. These interpretations are supported by studying how correction equations can be re-written in terms of G_max or E_max and how error propagates in the corrections. We also show how the dynamic compliance representation leads to simpler corrections and how machine stiffness can be calibrated from apparent dynamic compliance measurements of a single sample at two different geometry conditions. Equations are provided for rotational rheometers as well as linear displacement dynamic mechanical analyzers. Used as an operational limit line, G_max or E_max , the method can assess the credibility of data from others—even without access to their primary data of displacement, force, torque, or amount of correction, which are rarely reported. The method can also anticipate future issues before data are taken, e.g., to understand operational limits when selecting instruments and test geometries.

This work demonstrates a greater than expected enhancement of oxygen barrier properties in linear low-density polyethylene (LLDPE)-based materials by blending LLDPE with high-density polyethylene (HDPE). The films made by melt pressing the LLDPE/HDPE blends had a greater reduction in oxygen permeability coefficients (PO2) than predicted using common permeability reduction models, i.e., the harmonic average model and zero-permeability nanofiller model. The reduction of PO2 was attributed to the presence of spherulite crystal structures, as revealed by atomic force microscopy combined with infrared spectroscopy (AFM-IR). The LLDPE matrix exhibited significant spherulite formation even at a relatively low addition of HDPEs, which likely formed tortuous pathways for diffusing oxygen molecules. Transport results from melt-pressed films contrast with the results from films with similar compositions prepared by film blowing, which did not show barrier enhancement beyond expectation. AFM-IR revealed that the blown films lacked spherulite crystals likely due to stretching in the machine direction followed by rapid cooling. These findings demonstrate the role of processing in controlling microstructures and thus the oxygen barrier performance. This work offers the possibility of achieving easily recyclable LLDPE-based packaging materials by simple blending of polyethylenes with different crystalline content.

This review assesses the advantages and limitations of extensional rheometry using flow through an abrupt contraction. Analyses used to estimate extensional viscosity from entrance pressure drop and flow rate measurements are reviewed. These are compared with each other and to results from other extensional rheometers. A great advantage of entrance flow extensional rheometry is that it is perhaps the easiest method to obtain extensional data, especially with complex and industrial fluids such as hot melt adhesives, dough, emulsions and polymer melts, since it is readily adapted to standard capillary rheometers. Disadvantages are that the flow is not homogeneous and shear is always present. Nonetheless, pressure drop through an abrupt contraction can be especially useful in quickly ranking extensional effects between different material formulations.

Properly addressing the global issue of unsustainable plastic waste generation and accumulation will require a confluence of technological breakthroughs on various fronts. Mechanical recycling of plastic waste into polymer blends is one method expected to contribute to a solution. Due to phase separation of individual components, mechanical recycling of mixed polymer waste streams generally results in an unsuitable material with substantially reduced performance. However, when an appropriately designed compatibilizer is used, the recycled blend can have competitive properties to virgin materials. In its current state, polymer blend compatibilization is usually not cost-effective compared to traditional waste management, but further technical development and optimization will be essential for driving future cost competitiveness. Historically, effective compatibilizers have been diblock copolymers or in situ generated graft copolymers, but recent progress shows there is great potential for multiblock copolymer compatibilizers. In this perspective, we lay out recent advances in synthesis and understanding for two types of multiblock copolymers currently being developed as blend compatibilizers: linear and graft. Importantly, studies of appropriately designed copolymers have shown them to efficiently compatibilize model binary blends at concentrations as low as similar to 0.2 wt %. These investigations pave the way for studies on more complex (ternary or higher) mixed waste streams that will require novel compatibilizer architectures. Given the progress outlined here, we believe that multiblock copolymers offer a practical and promising solution to help close the loop on plastic waste. While a complete discussion of the implementation of this technology would entail infrastructural, policy, and social developments, they are outside the scope of this perspective which instead focuses on material design considerations and the technical advancements of block copolymer compatibilizers.

Due to its low cost, stiffness, and recyclability, isotactic polypropylene (iPP) is an excellent candidate for packaging applications. However, iPP is notoriously difficult to thermoform due to its low melt strength. The addition of just 10 thin layers of high-molecular-weight, linear low-density polyethylene (LLDPE) into iPP sheets by coextrusion significantly increased extensional viscosity and reduced sag. Both LLDPE and iPP were metallocene-catalyzed with excellent adhesion as measured in our previous work. We performed a series of hot tensile tests and sheet sag measurements to determine the properties of the iPP sheet and the multilayer sheet between 130 and 180 °C. To evaluate the thermoformability of these multilayer sheets, truncated conical cups were positive vacuum formed at different temperatures and heating times, and the crush strength was measured. Cups that released easily from the mold with good shape retention and a crush strength within 80% of the maximum value were used to define a temperature-time thermoformability window. We estimated the maximum stress that occurred during the thermoforming process to be 5 MPa. Layer thicknesses before and after thermoforming were used to estimate an average strain of 0.78. The thin LLDPE layers decreased the yield stress below 5 MPa. This enabled thermoforming at sheet temperatures as low as 150 °C. The immiscible LLDPE interfaces increased extensional viscosity, which decreased sag in the multilayer sheets compared to iPP. This broadened the thermoforming range to temperatures as high as 180 °C and allowed longer heating times. These highly thermoformable, layered sheets may be recycled as iPP since they contain only 8% of LLDPE.

In this work, we study the effect of strong electric fields on a polymer/graphene composite and the resulting morphology upon its dielectric breakdown. Our model system was produced by compounding up to 0.25 wt % graphene nanoplatelets (GNP) into poly(ethylene-co-vinyl acetate) (EVA), which is a soft polymer with low melt viscosity. A strong electric field of up to 400 Vrms/mm was applied to the EVA/GNP composite in the melt. The sample’s resistance over the electric field application was simultaneously measured. Despite the low GNP loading, which was below the theoretical percolation threshold, the electric conductivity of the composite during electric field application dramatically increased to >10−6 S/cm over 5 min of electric field application before reaching the current limit of the experimental apparatus. Conductivity growth follows the same scaling relationship of the theoretical model that predicts the rotation and translation time of GNPs in a polymer melt as a function of electric field strength. Since no significant GNP alignment in the composite was observed under transmission electron microscopy (TEM), we hypothesized that the increase in electrical conductivity was due to local electrical treeing of the polymer matrix, which eventually leads to dielectric breakdown of the composite. Electrical treeing is likely initiated by local GNP agglomerates and propagated through conductive channels formed during progressive dielectric breakdown.

Blending with ethylene-based flexible polymers such as polyethylene (PE) is one of the strategies to toughen poly(lactic acid) (PLA), an inherently brittle biodegradable plastic enjoying growing demands worldwide. Interfacial tension plays a crucial role in blend formulation. Yet several literature reports on the PE/PLA interfacial tension contradict each other, giving similar to 5 mN/m and similar to 11 mN/m. In this work, we demonstrate that the PE/PLA interfacial tension is at least 9 mN/m. We use a cocontinuous PE/polystyrene (PS)/PLA ternary blend. Scanning electron microscopy (SEM) revealed complete wetting morphology with PS phase separating PE and PLA phases in the ternary blend. In addition, the complete wetting behavior was maintained at a PS volume fraction as low as 3%. This morphology together with the Harkins equation, indicate that the PE/PLA interfacial tension is higher than 10.5 +/- 1.4 mN/m at 180 degrees C.

Fire safety in polymers is critically important with products such as textiles and consumer goods, as well as materials used in construction, aerospace, transportation, and furniture. Additives used as anti-dripping and flame-retardant agents impose acute human health issues and have negative environmental impacts. With a focus on developing additive-free solutions, we used multilayer coextrusion to fabricate layered polyethylene/polypropylene (PE/PP) films and investigated the effect of a layered morphology on dripping and burning rates. The experimental results for multilayered PE/PP samples were compared to control groups of melt blended PE/PP samples. The multilayer configuration provides two significant advantages: 154% delay in the time to the first drip and up to 87% reduction in the number of drips compared to the value for the melt blended samples with no additives. PE/PP blends with up to 2 wt % polytetrafluoroethylene, a commonly used anti-dripping agent in the industry, showed only 85% delay in the time to the first drip and about 60% reduction in the number of drips. We discovered a linear relationship between film thickness and burning rate, which allows for the use of the volumetric burning rate to normalize the effect of film thickness. No benefit in the burning rate was observed when comparing multilayered structures and conventional melt blended architectures or blends with PTFE. Annealing studies and transmission electron microscopy at areas close to the combustion zones confirmed that higher extensional viscosities induced by the layered structures and morphological changes due to the layer breakup play an important role in reducing dripping. By combining the first drip time and overall drip numbers, a parameter, "dripping index", was defined to establish a fire safety map to simplify material comparisons. This study demonstrates that a multilayer film geometry can provide an additive-free solution to yield control over fire safety parameters in polymers.

Conductive polymer composites enjoy specialized applications such as electrostatic discharge protection. In this work, we create interfacially localized graphene nanoplatelets (GNPs) in a cocontinuous polymer blend of polylactide (PLA) and poly(ethylene-co-vinyl acetate) (EVA). Based on the wetting coefficient analysis, GNPs favor localization in the EVA phase. A two-step compounding sequence is designed such that a PLA/GNP masterbatch is first prepared via solution blending, and then melt compounded with the EVA. In the second step, GNPs transfer from the PLA phase to the EVA phase but become kinetically trapped at the interface, as confirmed by electron microscopy. We achieve an ultralow percolation threshold of 0.048 wt. % GNPs and obtain blends with electrical conductivities of similar to 10(-5) S/cm at 0.5 wt. % GNP concentration. We systematically study the shear and extensional rheology of the ternary composite system. Cocontinuous blends with interfacial GNPs exhibit higher shear and extensional viscosities compared to samples with GNPs localized entirely within the EVA phase. Rheology, in situ dielectric measurements, and transmission electron microscopic imaging after nonlinear deformations all show the interfacial GNP network undergoes structure recovery and largely remains at the PLA/EVA interface. Moreover, high electrical conductivity is maintained during 2-10 min melt compounding and conductivity recovers with annealing after nonlinear deformations. These results suggest that these robust GNP networks preserve their bulk electrical conductivity during subsequent melt processing. (c) 2021 The Society of Rheology

Molecular dynamics simulations are used to obtain mode I and mode II fracture energies and cohesive laws for bulk epoxy and interfaces formed between epoxy and single-layer graphene (SLG), multilayer graphene (MLG), and multilayer graphene oxide (MLGO). The elastic moduli and ultimate tensile and shear strengths of epoxy–graphene interfaces are calculated from uniaxial tension and simple shear loadings. The results show that Young’s modulus and the ultimate tensile strength increase relative to bulk epoxy, whereas the shear modulus and ultimate shear strength are reduced. Failure of epoxy–graphene interfaces in tension occurs due to the formation of voids in the epoxy. Failure in shear is due to tangential slipping at the interface. Under mixed-mode conditions, the shear modulus and shear strength decrease with increasing tensile load. The critical energy release rate $$G_{\text{c}}$$ for the studied epoxy–SLG/MLG/MLGO systems is obtained using a continuum fracture mechanics approach and is found to be significantly lower than for bulk epoxy. All of the results are combined to define mode I and II cohesive laws for bulk epoxy and epoxy–SLG/MLG/MLGO interfaces that can be used in theoretical models and numerical methods, such as finite elements, that employ cohesive zones.

We have demonstrated the concept of imprinting graphene on a polymer substrate via coextrusion of polypropylene/graphene (PP/G) composite and target polymers. First, we predicted by wetting coefficients that carbon nanotubes (CNTs) or graphene would prefer to localize in polyethylene (PE), polycarbonate (PC), or polystyrene (PS) over PP. On the basis of this hypothesis, we laminated target polymer substrates with PP/CNT or PP/G to verify the concept of imprinting prior to the coextrusion experiments. Fillers migrated from PP to target polymers exhibiting electrical conductivity at their surfaces. Migration was promoted by elevated temperature, long interfacial contact time, or applied pressure. We speculate that migration is driven by wetting of the fillers and their translational Brownian motion. The coextrusion process was mimicked by a dual-bore capillary rheometer with a miniature coextrusion die. Capillary coextrusion successfully transferred graphene from PP to the surface of PE, PC, or PS, in <10 s contact time, resulting in surface resistivity as low as similar to 10(5) ohm/sq. In the process, the graphene-loaded PP is stripped off and can be recycled. Lower resistance should be possible in an industrial process which can run at higher temperature. Imprinting via coextrusion could be a universal and recyclable process for fabricating functional substrates with surfaces coated with nanosized fillers of any kind.

Two trimellitic anhydride-functionalized, thermally reduced graphenes with different aspect ratios, A f, and the same C/O ratio (8:1) were prepared and melt-mixed into poly(ethylene terephthalate) (PET), and the mechanical properties of the resulting nanocomposites were studied with a focus on plastic deformation behavior. A slight increase in the G' of the melt was observed for the surface-modified low-A f graphene composites (A f = 20) below the percolation threshold, whereas a significant enhancement in G' was observed for higher-A f graphene composites (A f = 80) at all graphene loadings, both below and above the percolation concentration. Furthermore, the use of modified low-A f graphene caused an improvement both in Young's modulus and elongation at break of the resulting PET nanocomposites because of enhancement of interfacial adhesion between filler and matrix which resulted in the formation of a coupled network via covalent bonding and the suppression both of strain-induced orientation and strain-induced crystallization. By contrast, the use of modified higher-A f surface graphene in nanocomposites caused a drastic improvement in Young's modulus but lower elongation-at-break than with the unmodified counterpart; the former effect is due to the formation of denser coupled networks and stronger interfacial adhesion as a result of graphene surface modification and the latter is due to the added geometrical restriction in unentangling chains from the PET matrix in the presence of higher-A f graphene. The preceding observations demonstrate the potential impacts of tuning both surface chemistry and aspect ratio of graphene in the fabrication of PET/graphene composites.

This paper reports the thermomechanical properties of a thermosetting polymer formed by curing a DGEBA resin with a Jeffamine D230 agent predicted by molecular dynamics (MD) simulations. A multistep crosslinking approach is used to form the crosslinked network of the thermosetting polymer. The radial distribution function and X-ray diffraction pattern of the MD predicted crosslinked structure are calculated and compared with experimental results to validate the epoxy network system. Thermomechanical properties such as mass density, gel point, glass transition temperature (Tg), elastic moduli (Young’s modulus and shear modulus), and yield strength in shear and tension are calculated at different temperatures and crosslinking conversions by employing the DREIDING and AMBER force fields. The MD predicted results are in good agreement with theoretical studies and existing experimental data. We find a significant increase in Tg and yield strength with crosslinking conversion. The elastic modulus is less sensitive to the strain rate, but the yield strength is significantly strain-rate dependent. The high-quality digital epoxy configurations developed in this work are available in LAMMPS data format from the journal website.

Graphene of two different aspect ratios, Af, was melt mixed with poly(ethylene terephthalate) (PET) to form amorphous PET/graphene composites with less than 5% crystallinity. The higher-order structure and mechanical properties of poly(ethylene terephthalate) (PET) in these composites were investigated using techniques such as differential scanning calorimetry and dynamic mechanical analysis, whereas transmission electron microscopy, melt rheology, and electrical conductivity were used to study the graphene dispersion. A decrease in heat capacity changes, ΔCp, of PET in nanocomposites at the glass transition temperature, Tg, without Tg change suggests that a rigid amorphous fraction (RAF) of PET was formed at the PET/graphene interface. The stiffening effect of graphene below 1 wt % loading is quite small in the glassy state region and independent of the Af of graphene. Above 2 wt %, graphene forms a mechanical percolation network with the RAF of PET and the PET chains are geometrically restricted by the incorporation of graphene with a high Af, resulting in an unexpectedly higher modulus of nanocomposites both below and above Tg.

Many tougheners have been developed for thermosetting resins. Numerous studies indicate that the mode-I fracture toughness of a thermosetting resin ( G IC_Resin ) can be effectively enhanced by rubber tougheners such as liquid rubber and core–shell rubber, and inorganic rigid particles such as silica, clay, carbon nanotubes, or graphene. Can these additives also toughen fiber-reinforced polymers (FRPs)? In particular, can they improve the mode-I interlaminar fracture toughness of FRPs ( G IC_Comp )? To answer how much toughness improvement is transferred from resin to FRPs, we reviewed data from more than 50 publications related to interlaminar toughening. The performance of various types of tougheners in the resin and/or FRPs is summarized, and toughening mechanisms are also discussed. We found a wide range of improvement in fracture toughness in FRPs with the addition of nanoparticles, from negative improvement in some silica and carbon particle studies to an improvement ratio equal to that achieved in the resin. Overall, rubber tougheners are the most effective tougheners, but on average, only about 30% of the relative improvement in resin toughness translated to G IC_Comp increase. The enhancement in G IC_Comp after incorporating rigid particles tougheners is even less, but rigid particles do not decrease the strength and modulus of the final FRPs. Other toughening strategies, such as using multiple types of tougheners, and coating or depositing nanoparticle onto the fiber reinforcements are also discussed, and we suggest some strategies to design FRPs with optimal delamination resistance.

Graphene‐based nanomaterial tougheners can effectively toughen thermosetting resins at an extremely low loading level with minimal change in Tg or modulus. However, previous research has indicated that this toughening effect shows only small responses to changing graphene oxide (GO) surface modifications, resin dispersion processing methods, and GO loading levels. It is hypothesized that the physical dimensions of GO particles affect the toughening behavior of graphene‐based additives. In this study, the sizes of primary and aggregated surface‐modified GO (mGO) particles were measured directly using scanning electron microscopy. The effects of the differences in size on fracture toughness (KIC), bending modulus, and flexural strength of an unsaturated polyester resin were evaluated. The sizes of primary mGO particles were estimated from the sizes of GO particles prior to their surface modification and the sizes of mGO aggregates after dispersion in the resin. This study shows that as little as 0.005 wt% mGO can increase KIC by nearly 20%. This toughening effect is independent of primary particle size. Reducing mGO aggregate sizes by chemical modification or sonication shifts the optimal loading level of mGO‐based tougheners to lower concentrations, but the maximal achievable toughness cannot be changed by improving dispersion or particle–matrix interactions. POLYM. COMPOS., 40:3886–3894, 2019. © 2019 Society of Plastics Engineers