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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.

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 situgenerated 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 ∼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.

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 ∼105ohm/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.

Poly(ethylene terephthalate) (PET) and polyethylene (PE) are two of the most commonly used polymers in the packaging industry, and they make up a large portion of the recycling stream. Adhesion between these two immiscible polymers is poor without compatibilization. Maleic anhydride (MAH) grafted PE is a popular compatibilizer for PET/PE but is limited by the slow rate of reaction. In this study, we propose tin additives for improving adhesion and explore the mechanism of that improvement. We demonstrate that either dibutyltin oxide (DBTO) or di-n-octyltin oxide (DOTO) can dramatically enhance adhesion with only 60 s of contact time during lamination. A range of MAH and DOTO concentrations are examined, yielding adhesion up to 1300 J/m2. Possible reactions at the interface are investigated with small molecule model compounds, namely 1-hexadecanol, octylsuccinic anhydride (OSA), and ethylene glycol dibenzoate. DOTO does not improve the reaction rate in the hexadecanol–OSA system. This suggests that another reaction is responsible for the adhesion improvement. We provide evidence that tin oxides can open the ring of alkylsuccinic anhydrides and form complexes that can interesterify PET. This overall reaction appears to be third order with an activation energy of 100 kJ/mol. Extrapolating these kinetic results to 270 °C, the lamination temperature, we estimate that interfacial interesterification is very fast. Thus, adhesion development is limited by diffusion of the tin additives.

Conductive polymer composites (CPCs) enjoy broad industrial applications such as electrostatic discharge (ESD) protection. Herein, we constructed CPCs by first solution blending graphene nanoplatelets (GNPs) into a miscible blend of poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) and then inducing PMMA/SAN spinodal decomposition by annealing well above the lower critical solution temperature. The resulting composite showed spatially regular, co-continuous polymer domains, in which GNPs preferentially localized within the SAN-rich phase and formed a conductive network. We found that GNPs induced local nucleation of SAN into surface layers of ∼4Rgin thickness. Small PMMA domains formed next to these SAN layers and were stable against long annealing times. As a result, GNPs created local blend morphologies that were different from the bulk morphology arising from spinodal decomposition alone. During annealing, GNPs suppressed domain coarsening and preserved the co-continuous morphology, while their connectivity in the polymer matrix was improved. Additionally, inducing PMMA/SAN phase separation significantly increased the ternary blend’s electrical conductivity by over 5 orders of magnitude. Compared to the conventional approach of CPC manufacture of compounding carbon black into a homopolymer matrix, our approach achieved bulk electrical conductivity of ∼10–8S/cm at 1 wt % GNP loading, rendering this system suitable for ESD protection.

Interfaces between immiscible polymers are widely known to be the origin of premature failure in multilayers and blends. Developing a small-scale, continuous method to fabricate interfaces from polymer melts with short contact times to simulate industrial coextrusion is critical for assessing new materials, polymeric systems, and processing conditions. Here we develop a miniature coextrusion die that can be fitted to a twin bore capillary rheometer for precise temperature and contact time control from 4.4 to 440 s. The design of the feedblock, stacker, manifold, and land zones was optimized via flow simulation software to allow bilayer coextrusion with minimal (<10%) edge distortions. Bilayer films of isotactic polypropylene (iPP) and polyethylene (PE) were produced using 90% less material than a lab-scale continuous multilayer coextrusion device, and the interfacial adhesion was in excellent agreement with iPP/PE bilayers produced using a continuous coextrusion technique for a range of PE molecular architectures. Finally, poly(ethylene terephthalate) (PET) and maleic anhydride grafted PE were coextruded. With a melt interfacial contact time of 22 s, interfacial adhesion between functionalized PE (both 0.8 and 1.5 wt %) and PET increased at least 30-fold over nonfunctionalized PE and PET. Using a moderate anhydride grafting density (0.8 wt %), interfacial adhesion also increased by a factor of 10 when interfacial contact time was increased from 4.4 to 22 s. The die designed here represents a convenient and accessible method to screen interfacial adhesion using less than 50 g of material, ideal for assessing new polymers synthesized at small scale.

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10–3–10 s) to the formation of a dry thin film (a “capillary coating”; 102–103s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings.

Melt-blown fibers (dav∼1 μm) were produced from blends of poly(butylene terephthalate) (PBT) and a partially fluorinated random multiblock copolyester (PFCE) leading to enhanced hydrophobicity and even superhydrophobicity (static water contact angle = 157 ± 3°) of the associated fiber mats. XPS measurements demonstrated quantitatively that the surface fluorine content increased systematically with the bulk loading of PFCE, rising to nearly 20 atom %, which corresponds to 41 wt % PFCE at a bulk loading of 10 wt %. The PBT/PFCE fibers exhibit greater fluorine surface segregation than either melt-blown PBT/poly(ethylene-co-chlorotrifluoroethylene) (PBT/PECTFE) fibers or electrospun fibers obtained from blends of poly(styrene) and fluoroalkyl end-capped polystyrene (PS/PSCF). Dynamic contact angle measurements further demonstrated decreased surface adhesion energy of the melt-blown PBT/PFCE fiber mats due to the blooming of PFCE to the surface.

We have investigated particle size, interior structure, drug release kinetics, and anticancer efficacy of PEG-b-PLGA-based nanoparticles loaded with a series of paclitaxel (PTX)-silicate prodrugs [PTX-Si(OR)3]. Silicate derivatization enabled us to adjust the hydrophobicity and hydrolytic lability of the prodrugs by the choice of the alkyl group (R) in the silicate derivatives. The greater hydrophobicity of these prodrugs allows for the preparation of nanoparticles that are stable in aqueous dispersion even when loaded with up to ca. 75 wt % of the prodrug. The hydrolytic lability of silicates allows for facile conversion of prodrugs back to the parent drug, PTX. A suite of eight PTX-silicate prodrugs was investigated; nanoparticles were made by flash nanoprecipitation (FNP) using a confined impingement jet mixer with a dilution step (CIJ-D). The resulting nanoparticles were 80–150 nm in size with a loading level of 47–74 wt % (wt %) of a PTX-silicate, which corresponds to 36–59 effective wt % of free PTX. Cryogenic transmission electron microscopy images show that particles are typically spherical with a core–shell structure. Prodrug/drug release profiles were measured. Release tended to be slower for prodrugs having greater hydrophobicity and slower hydrolysis rate. Nanoparticles loaded with PTX-silicate prodrugs that hydrolyze most rapidly showed in vitrocytotoxicity similar to that of the parent PTX. Nanoparticles loaded with more labile silicates also tended to show greater in vivoefficacy.

The wetting properties of poly(butylene terephthalate) (PBT) melt blown fibers were tuned by alkaline hydrolysis and subsequent fluorination. Fiber mats were exposed to a NaOH methanol solution for controlled periods of time at several temperatures, resulting in surface hydrolysis (h-PBT). Subsequent simple solution chemistry was applied to the h-PBT fibers, leading to fluorination of the surface (f-PBT) and the transformation of the wetting properties of the material. Electron microscopy revealed that hydrolysis leads to a textured surface that is retained in the fluorinated product. Sessile drop wetting measurements demonstrated superhydrophilicity for the h-PBT fiber mats and sticky superhydrophobicity with the f-PBT fiber mat.

Flash nanoprecipitation (FNP) is a process that, through rapid mixing, stabilizes an insoluble low molecular weight compound in a nanosized, polymer-stabilized delivery vehicle. The polymeric components are typically amphiphilic diblock copolymers (BCPs). In order to fully exploit the potential of FNP, factors affecting particle structure, size, and stability must be understood. Here we show that polymer type, hydrophobicity and crystallinity of the small molecule, and small molecule loading levels all affect particle size and stability. Of the four block copolymers (BCP) that we have studied here, poly(ethylene glycol)-b-poly(lactic-co-glycolic acid) (PEG-b-PLGA) was most suitable for potential drug delivery applications due to its ability to give rise to stable nanoparticles, its biocompatibility, and its degradability. We found little difference in particle size when using PLGA block sizes over the range of 5 to 15 kDa. The choice of hydrophobic small molecule was important, as molecules with a calculated water–octanol partition coefficient (clogP) below 6 gave rise to particles that were unstable and underwent rapid Ostwald ripening. Studies probing the internal structure of nanoparticles were also performed. Analysis of differential scanning calorimetry (DSC), cryogenic transmission electron microscopy (cryo-TEM), and 1H NMR experiments support a three-layer core–shell–corona nanoparticle structure.

The largest use of polyurethane (PU) is as closed cell rigid foams for thermal insulation. One problem is loss of blowing gases, which leads to slow increase in thermal conductivity. PU composites with plate-like nanofillers create a diffusion barrier, reducing gas transport and slowing insulation aging. In this research, a new in situ intercalative polymerization is described to disperse vermiculite (VMT) in PU. When VMT was modified by cation exchange with long-chain quaternary ammonium, the dispersion in methylene diphenyl diisocyanate (MDI) was significantly improved. Dispersion of clay in MDI was further improved by combining high intensity dispersive mixing with a polyol-clay preblend (master-batch). The VMT dispersibility was characterized using rheology, microscopy, and X-ray scattering/diffraction. With the method of polyol-assisted VMT dispersion, electron microscopy revealed extensive intercalation and exfoliation of clay particles. In contrast, simple mixing of organoclay in MDI resulted in macroscopic localization and poor distribution of clay particles in PU. The final nanocomposites prepared by the master-batch method showed enhancement of mechanical properties (85% increase in elastic modulus) and reduction in permeability to CO2, as much as 40%, at a low clay concentration of 3.3 wt %.

Lightweight graphene structures have promising applications, for example as separation media, as microreactor environments, for use in energy storage or in microelectronics. The assembly of graphene or its derivatives into such frameworks typically requires rather elaborate and expensive methods which are moreover difficult to scale up. In the present work we propose a simpler route which is based on arresting the temperature induced phase separation in a water–lutidine mixture containing graphene oxide particles. The 2D nature of graphene oxide sheets, combined with an adequate surface chemistry simply controlled by their carbon to oxygen ratio, causes them to collect and assemble at the interfaces and create highly elastic layers which arrest the phase separation process. Soft solid graphene oxide frameworks are created which have sufficient mechanical stability to allow removal of the solvents by freeze drying. The advantage of the method is that the structure and mesh size of the frameworks can be easily controlled, providing direct access to the graphene-oxide surfaces while creating a mechanically robust and percolating network of graphene oxide tiles. This could be used, for example, as electrodes for supercapacitors or fuel cells.

To understand the effect of processing and co-monomer content on interfacial adhesion, we quantified adhesion levels of bilayers of a polyethylene (PE) with various polypropylenes (PPs) prepared using bilayer co-extrusion and lamination processes. We tested adhesion between a medium-density PE (MDPE) with different types of PPs, including impact-modified PP (with various amount of ethylene), isotactic PP and ethylene–propylene random copolymers. Increasing the concentration of ethylene or ethylene–propylene rubber gave rise to increased adhesion. The impact-modified PP with 20?wt% ethylene content exhibited adhesion with MDPE almost two orders of magnitude higher compared with other PPs. Although lamination and co-extrusion processes showed good agreement in these trends with ethylene content, the operation parameters are critical for adhesion control. For lamination, ice-water cooling generated a stronger adhesion than that with air cooling. Faster cooling rates in co-extrusion also gave rise to stronger adhesion. Increasing draw down ratio and varying flow rate to put the interface near the wall resulted in stronger adhesion. Fast quenching rate and increased crystallinity induced by drawing down are believed to be the causes. Both atomic force microscopy and transmission electron microscopic images exhibited roughened interfaces for samples with strong adhesion.

Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas aggregated graphene sheets exhibited the lowest hemolytic activity. Coating graphene oxide with chitosan nearly eliminated hemolytic activity. Together, these results demonstrate that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, the cytotoxicity of graphene oxide and graphene sheets was investigated by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. However, appropriate alternate assessments, using the water-soluble tetrazolium salt (WST-8), trypan blue exclusion, and reactive oxygen species assay reveal that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e., whether or not aggregation occurs) and mode of interaction with cells (i.e., suspension versus adherent cell types).

We report a new, simple, hydrazine-free, high-yield method for producing single-layer graphene sheets. Graphene sheets were formed from graphite oxide by reduction with simple deionized water at 95 °C under atmospheric pressure. Over 65% of the sheets are single graphene layers; the average sheet diameter is 300 nm. We speculate that dehydration of graphene oxide is the main mechanism for oxygen reduction and transformation of C−C bonds from sp3to sp2. The reduction appears to occur in large uniform interconnected oxygen-free patches so that despite the presence of residual oxygen the sp2carbon bonds formed on the sheets are sufficient to provide electronic properties comparable to reduced graphene sheets obtained using other methods.

Nanodispersions of clays in polyurethane components have been prepared. Nanoclays (both natural and organically modified) of various aspect ratios are used. The fillers are dispersed separately in polyurethane components, viz., polyol and polyisocyanate. The nanodispersions are characterized by the combined use of solution rheology, X-ray scattering, cryo-electron microscopy, and IR spectroscopy. Reactive foaming of these nanodispersions is carried out to make polyurethane nanocomposite foams. The status of the dispersion of fillers in components and in foams has been compared to investigate the effect of the foaming process in exfoliation. Interpretation of the results from different characterization techniques describes the state of the dispersion of fillers in components and in foam. The rheological and physicochemical behaviors of nanodispersions are shown to have a significant influence on the properties of nanocomposite foams.

Carboxylic acid terminated polycaprolactones PCLCOOHs with narrow polydispersity were synthesized and coupled with polyethylene glycol HO–PEG–OH to afford PCLPEG–OH copolymers. The hydroxyl groups in the PCLPEG–OHs were then converted to maleimide groups to afford maleimide terminated PCLPEGMALs that contained 70–90 maleimide functionality. Nanoparticles with maleimide functionality on their surfaces were prepared by impingement mixing. Particle sizes and size distributions were determined by dynamic light scattering. Conjugation of reduced glutathione with model maleimides and two MALfunctional nanoparticles was also demonstrated. The amount of accessible maleimide on the particle surface was measured using Ellmans reagent to range between 51–67.

Discrete nanopores develop in the interfacial regions of polyolefin blends and layered films under tension. They originate from crazelike structures in the early development stage, which later undergo disruption processes with an increase in strain. The disruption processes appear to be triggered by the existence of adjacent ductile polyolefins and their micromechanical deformations. The degree of nanopore development significantly varies among blends and layered systems, depending on several material parameters, such as the interfacial strength. Discrete nanopores are observed more often in metallocene systems and blends than in Ziegler–Natta systems and layered films. Macroscopically, with the development of discrete nanopores, metallocene systems show higher strain to break and significant shear yielding under tension. The disruption mechanism can be used to prepare nanoporous materials or to improve the mechanical performances of polyolefins. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 708–718, 2005

Porous polymeric materials can be prepared by using micromechanical deformations. The development of porous structures in the tension of polyolefin blends are studied with the goal of developing a novel technique to make porous films. It is found in polypropylene/polyethylene copolymer blends of near 50/50 weight ratio that a metallocene polymer pair can produce stable nanopores throughout a wide range of strain, ~50–700%. Its strain-to-break was relatively high, although its continuous phase is a brittle polypropylene. The typical size of nanopores is 10–300 nm; they start to develop at approximately 50% strain. The disruption of craze-like structures into discrete nanopores seems to be the key mechanism in stabilizing pore development. Varying stretching direction and speed controls the pore morphology. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3642–3650, 2004

Compatibilized blends of thermoplastic polyurethane (TPU) and polypropylene (PP) were developed using amine (primary or secondary) functionalized PP's (PP‐g‐NH2or PP‐g‐NHR). The strategy of reactive compatibilization is based on fast reactions between amine functional groups and urethane linkages or traces of free isocyanates released by thermal degradation of TPU. Excellent compatibilization between TPU and PP was confirmed by rheological, morphological, and mechanical properties. Much finer domain size, higher interfacial adhesion, and more stable morphologies were clearly observed by scanning electron microscopy. Significant improvements in the overall mechanical properties (tensile, tear, abrasion) imply significantly more reaction between TPU and PP phases in the two TPU/PP blends containing PP‐g‐NH2or PP‐g‐NHR than a TPU/PP blend using PP‐g‐MA as a compatibilizing agent.

Two model urethane compounds, dibutyl 4,4'-methylenebis(phenyl carbamate) (BMB) and dioctyl 4,4'-methylenebis(phenyl carbamate) (OMO) were prepared by capping 4,4'-methylenebis(phenyl isocyanate) with n-butanol and n-octanol, respectively. The reactions of the two model urethane compounds with several small monofunctional compounds as well as two model poly(ethylene glycols) were carried out with neat mixtures at elevated temperatures. The ranking of reactivity of the functional groups with the urethanes was determined as follows—primary amine > secondary amine ≫ hydroxyl ~ acid ~ anhydride ≫ epoxide. Nuclear magnetic resonance spectroscopy (NMR) was used for the quantitative analysis. Fourier transform infrared spectroscopy was used to complement the NMR analysis. Conversions of carbamate in each reaction were monitored over time at constant temperature (200 °C). The reactions between OMO and primary amine were conducted at 170, 180, 190, and 200 °C and best described with a second-order bimolecular reaction model. The rate constant was estimated to be 1.8 × 10⁻³ L · mol⁻¹ · s⁻¹ and activation energy 115 kJ · mol⁻¹. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2310–2328, 2002