Postconsumer recycled plastics typically exhibit low environmental stress cracking resistance (ESCR), which limits their use in the rigid packaging industry. A styrenic triblock copolymer increased the ECSR of virgin and recycled high-density polyethylene (HDPE) to 130%. Scanning electron microscopy revealed well-dispersed styrene-b-ethylene-b-ethylene/propylene-b-styrene copolymer domains within HDPE spherulites. It is postulated that the rigid polystyrene domains, interconnected by flexible polyolefin midblocks, act as anchors that delay disentanglement and pullout of the tie molecules. Strain hardening modulus and in situ Fourier-transform infrared-tensile tests validated this reinforcing effect, indicating a promising approach to improving the recyclability of HDPE.

Here we report on the influence of an aliphatic oxalamide-based nucleating agent (OXA3,6) on the viscoelastic and mechanical properties of isotactic polypropylene (iPP) blended with ultrahigh molecular weight iPP (UHMWiPP). The linear viscoelastic properties are investigated by using a plate-plate rheometer; the presence of only 0.5 wt % of OXA3,6 in the iPP-UHMWiPP blends results in a reduction of the complex viscosity in the entire frequency domain examined. This observation holds irrespective of the UHMWiPP weight fraction, up to 25 wt %. The viscosity suppression is attributed to an acceleration of the reorientation times of the chains, especially the longer ones, due to partial alignment of their molecular segments. Furthermore, SAXS measurements on cooled, injection molded samples hint that OXA3,6-containing samples display enhanced alignment under processing conditions (injection molding). Compared to samples without OXA, an increase in the SAXS intensity along the equator is observed in the OXA3,6 samples, implying an enhanced ability of shish-kebab formation. Such a morphological development results in higher yield stress while maintaining the elastic behavior.

Manufacturing of three-dimensional scaffolds with multiple levels of porosity are an advantage in tissue regeneration approaches to influence cell behavior. Three-dimensional scaffolds with surface roughness and intra-filament open porosity were successfully fabricated by additive manufacturing combined with chemical foaming and porogen leaching without the need of toxic solvents. The decomposition of sodium citrate, a chemical blowing agent, generated pores within the scaffold filaments, which were interconnected and opened to the external environment by leaching of a water-soluble sacrificial phase, as confirmed by micro-CT and buoyancy measurements. The additional porosity did not result in lower elastic modulus, but in higher strain at maximum load, i.e. scaffold ductility. Human mesenchymal stromal cells cultured for 24 h adhered in greater numbers on these scaffolds when compared to plain additive-manufactured ones, irrespectively of the scaffold pre-treatment method. Additionally, they showed a more spread and random morphology, which is known to influence cell fate. Cells cultured for a longer period exhibited enhanced metabolic activity while secreting higher osteogenic markers after 7 days in culture. STATEMENT OF SIGNIFICANCE: Inspired by the function of hierarchical cellular structures in natural materials, this work elucidates the development of scaffolds with multiscale porosity by combining in-situ foaming and additive manufacturing, and successive porogen leaching. The resulting scaffolds displayed enhanced mechanical toughness and multiscale pore network interconnectivity, combined with early differentiation of adult mesenchymal stromal cells into the osteogenic lineage.

Stereo-diblock copolymers of high molecular weight polylactide (PLA) were synthetized by the one pot-sequential addition method assisted by a heteroscorpionate catalyst without the need of a co-initiator. The alkyl zinc organometallic heteroscorpionate derivative (Zn(Et)(κ3-bpzteH)] (bpzteH = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide) proved to assist in the mechanism of reaction following a coordination-insertion process. Kinetic studies along with the linear correlation between monomer and number average molecular weight (Mn) conversion, and the narrow polydispersities supported the truly living polymerization character of the initiator, whereas matrix-assisted laser desorption/Ionization-time of flight (MALDI-TOF) studies showed a very low order of transesterification. The high stereo-control attained for the afforded high molecular weight derivatives was revealed by homonuclear decoupled 1H NMR spectra and polarimetry measurements. The nanostructure of the PLA derivatives was studied by both wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) and the stereocomplex phase of the PLA stereo-diblock copolymers was successfully identified.

The processing temperature of thermotropic liquid crystalline polymers (LCPs) can be reduced by increasing the flexibility of the backbone, simultaneously decreasing the relaxation time and resulting in a decrease in the mechanical properties in the processed polymer. Here, we report on the development of thermotropic LCPs with low processing temperatures, an increased relaxation time, and excellent mechanical performance via a versatile two-step synthesis. These LCPs can be processed at temperatures below 200 degrees C into injection-molded parts which have a mechanical performance comparable to that of commercially available grades with Young's modulus and a tensile strength over 12.5 GPa and 200 MPa, respectively. These LCPs are synthesized using carboxylic acid-terminated thermotropic prepolymers that are produced via a standard acidolysis polycondensation at 220 degrees C, a temperature that allows incorporation of monomers having lower thermal stability. Subsequently, the prepolymers are subjected to a chain-extension reaction with bis(2-oxazoline)s in a reactive extrusion step. The facile two-step method provides excellent control over the chemical composition, molecular weight, and chain architecture of the resulting polymer. As the molecular weight of the LCPs increases, they display interesting and uncharacteristic viscoelastic behavior. The viscoelastic response of the LCPs changes from that of an unentangled melt, typically observed in rigid LCPs, toward a physically constrained melt indicated by the appearance of a crossover point with an increase in molar mass. Such a response is normally not observed in commercial thermotropic LCPs even with an increase in molar mass. We attribute the change in the viscoelastic response to the chain-extension reaction. This fundamental change in the state of the nematic melt greatly increases the relaxation timescales of both the interchain orientation and the nematic texture, providing further evidence toward their interrelation. The change in the relaxation mechanism of the nematic phase, with increasing molar mass, is followed by time-resolved X-ray diffraction and optical microscopy.

We demonstrate a novel way of creating three-dimensional microfluidic channels capable of following complex topographies. To this end, substrates with open channels and different geometries were 3D-printed, and the open channels were consecutively closed with a thermoplastic using a low-resolution vacuum-forming approach. This process allows the sealing of channels that are located on the surface of complex multiplanar topographies, as the thermoplastic aligns with the surface-shape (the macrostructure) of the substrate, while the microchannels remain mostly free of thermoplastic as their small channel size resists thermoplastic inflow. This new process was analyzed for its capability to consistently close different substrate geometries, which showed reliable sealing of angles >90°. Furthermore, the thermoplastic intrusion into channels of different widths was quantified, showing a linear effect of channel width and percentage of thermoplastic intrusion; ranging from 43.76% for large channels with 2 mm width to only 5.33% for channels with 500 µm channel width. The challenging sealing of substrate ‘valleys’, which are created when two large protrusions are adjacent to each other, was investigated and the correlation between protrusion distance and height is shown. Lastly, we present three application examples: a serpentine mixer with channels spun around a cuboid, increasing the usable surface area; a cuvette-inspired flow cell for a 2-MXP biosensor based on molecular imprinted polymers, fitting inside a standard UV/Vis-Spectrophotometer; and an adapter system that can be manufactured by one-sided injection molding and is self-sealed before usage. These examples demonstrate how this novel technology can be used to easily adapt microfluidic circuits for application in biosensor platforms.

Thermoplastic composites consisting of a liquid crystalline polymer (LCP) and poly(lactide) (PLA) have the potential to combine good mechanical performance with recyclability and are therefore interesting as strong and sustainable composite materials. The viscoelastic behavior of both the LCP and the PLA is of great importance for the performance of these composites, as they determine the LCP morphology in the composite and play a crucial role in preventing the loss of mechanical performance upon recycling. Though the effect of the matrix viscosity is well-documented in literature, well-controlled systems where the LCP viscosity is tailored are not reported. Therefore, four LCPs, with the same chemical backbone but different molecular weights, are used to produce reinforced LCP-PLA composites. The differences in viscosity of the LCPs and viscosity ratio between the dispersed phase and the matrix of the blends are evident in the resultant composite morphology: in all cases fibrils are formed; however, the diameter increases considerably as the viscosity ratio increases for the higher molar mass LCPs. The fibril diameter ranges from several hundred nanometer to a few micrometer. A typical layered structure in the injection molded composites is observed, where the layer-thickness is influenced by the LCP viscosity. The LCPs are found to effectively reinforce the PLLA matrix, increasing the Young's modulus by 60% and the maximum stress by 40% for the composite containing 30 wt % of the most viscous LCP. Remarkably, this did not result in an increase in brittleness, effectively increasing the toughness of the composite compared to pure PLLA. The feasible reprocessability of this composite is confirmed, by subjecting it to three reprocessing cycles. The relaxation of the LCPs orientation upon heating is measured via in situ WAXD. We compare the relaxation in an amorphous PLA matrix and in a semicrystalline PLLA matrix with that of the pure LCPs. The matrix viscosity is found to strongly influence the relaxation. For example, in a low viscous amorphous matrix relaxation of the LCP fibrils into droplets dominates the process, whereas a semicrystalline matrix helps in maintaining the fibril morphology and intermolecular orientation of the LCP. In the latter case, the LCPs relax via contraction and coalescence of the polydomain texture and maintains a significant degree of orientation until the PLLA crystals melt and the matrix viscosity decreases. The insights gained in this study on the role of the LCP viscosity on the morphology and performance of thermoplastic composites, as well as the relaxation of LCPs in a matrix, will aid progression toward sustainable and reprocessable LCP reinforced thermoplastic composites.

In this work, a series of oxalamide based organic compounds (OBOC) are synthesized and their capability to enhance the nucleation of polylactide is reported. The OBOCs are soluble in the polylactide melt and crystallize upon cooling, providing surface for heterogeneous nucleation to the polylactide matrix. One interesting observation is that the nucleation efficiency of the OBOCs increases at high cooling rates, making the use of OBOCs as a nucleating agent attractive for industrial processing conditions. The paper addresses the mechanism involved in the cooling rate dependence on the nucleation efficiency of the OBOC: The nucleation efficiency of polylactide is significantly enhanced in the presence of OBOC crystals, as a transcrystalline PLA morphology grows from the OBOC crystal surface (at relatively low supercooling from the equilibrium melting temperature of PLA). However, such crystallization of PLA occurs only when the OBOC crystals are formed at or below 145 degrees C while cooling at 10 degrees C/min. In contrast, when the OBOC crystals are formed above 145 degrees C (i.e. from a lower supersaturated state), the OBOC crystals display a significantly reduced capability for PLA nucleation. These findings override the possibility of both epitaxy and soft-epitaxy as a plausible nucleation mechanism. Supported by polarized optical microscopy, differential scanning calorimetry, plate-plate rheology and molecular modelling we evaluate the possibility of nucleation resulting from local stresses imposed on the polylactide melt invoking stress-enhanced nucleation. The imposed local shear rates, facilitated by the rapid growth of the OBOC crystals, are found high enough to facilitate contour orientation of the high molecular weight PLA chains next to the growing OBOC crystals, confirming the possibility for stress-enhanced nucleation. In addition, we identify surface roughness of OBOC crystals as a second parameter that influences the PLA nucleation process; the OBOC crystallization at high supersaturation is expected to yield smaller/defected OBOC crystals, which are presumed to provide high surface area and surface roughness. In contrast, when the OBOC crystals are grown at lowered supersaturation (>150 degrees C), they are bound to anneal during crystallization, providing a smoother surface and lower surface area - retrospectively exhibiting a decreased capability to promote the PLA nucleation, irrespective of the PLA supercooling. Interestingly, both the surface roughness of OBOC crystals and the local stresses they impose on the PLA melt increase when the OBOC crystal growth proceeds from a highly supersaturated state, providing an explanation to the cause of the favored crystallization of PLA at the high cooling rates in the presence of the chosen OBOCs.

We report on the role of temperature and shear on the melt behavior of iPP in the presence of the organic compound N1,N1'-(propane-1,3-diyl)bis(N2-hexyloxalamide) (OXA3,6). It is demonstrated that OXA3,6 facilitates a viscosity suppression when it resides in the molten state. The viscosity suppression is attributed to the interaction of iPP chains/subchains with molten OXA3,6 nanoclusters. The exact molecular mechanism has not been identified; nevertheless, a tentative explanation is proposed. The observed viscosity suppression appears similar to that encountered in polymer melts filled with solid nanoparticles, with the difference that the OXA3,6 compound reported in this study facilitates the viscosity suppression in the molten state. Upon cooling, as crystal growth of OXA3,6 progresses, the decrease in viscosity is suppressed. Retrospectively, segmental absorption of iPP chains on the surface of micrometer-sized OXA3,6 crystallites favors the formation of dangling arms, yielding OXA3,6 crystallites decorated with partially absorbed iPP chains. In other words, the resulting OXA3,6 particle morphology resembles that of a hairy particle or a starlike polymer chain. Such hairy particles effectively facilitate a viscosity enhancement, similar to branched polymer chains. This hypothesis and its implications for the shear behavior of iPP are discussed and supported using plate-plate rheometry and slit-flow experiments combined with small-angle X-ray scattering analysis.

In this work, we report on a novel hydrogen-bonding compound, N,N'-bis(2-hydroxyethyl)-terephthalamide (BHET), and its potential as additive in poly-L-lactide (PLA). Although the hydroxyl groups of BHET can participate in transesterification with the PLA matrix, we demonstrate through gel permeation chromatography that extrusion at 200 degrees C does not result in a drastic decrease in molecular weight. When dissolved in the PLA matrix, BHET facilitates a plasticizing effect, indicated by a suppression in both melt viscosity and glass-transition temperature. Additionally, BHET can crystallize from the PLA melt during cooling, where the generated BHET crystals facilitate heterogeneous nucleation of the PLA matrix. In general, BHET crystallization is favored at high undercooling/superstaturation, as this yields BHET crystals with a high surface area-to-volume ratio. However, rapid cooling to temperatures below the glass-transition temperature prevents the crystallization of the BHET, effectively yielding samples where only the plasticizing effect of BHET is evident. A suppression in yield point is observed during mechanical analysis of samples with increasing BHET concentration, a characteristic feature for samples with decreasing glass-transition temperatures. In contrast, when allowed to crystallize during processing, BHET can be used to generate PLA crystals oriented along the flow direction, effectively enhancing the tensile modulus. Overall, the combined plasticizing and nucleating effect of BHET makes it a verstaile additive for controlled processing and performance of PLA.

In this study, we report on the synthesis and characterization of thermosets based on the renewable N,N'-octamethylene-bis(pyrrolidone-4-carboxylic acid) (BP-C-8) and 1,3-bis(4,5-dihydro-2-oxazolyl)benzene (IAox). The BP-C-8, having two pyrrolidone rings, is obtained via a bulk aza-Michael addition reaction of 1,8-diaminooctane with 2 equiv of itaconic acid using water as catalyst. Thermosets are prepared by mixing the desired ratio of BP-C-8 with IAox, followed by thermal curing at 180 degrees C. The cross-link density of the thermosets is controlled by the usage of an excess of bis(2-oxazoline), and achievement of full conversion is confirmed by Fourier transform infrared spectroscopy. As anticipated, the glass transition temperature of the thermosets increases with increasing cross-linking density to values up to 100 degrees C. All thermosets are found to absorb water upon storage, resulting in a strong plasticizing effect and thus a suppression of the glass transition temperature. To assess the role of water on the thermo-mechanical properties, all thermosets are exposed to various relative humidities and their mechanical performance is evaluated. In general, we observe that the suppression in T-g under the influence of water facilitates a brittle-to-ductile transition as the T-g of the thermosets approaches room temperature. Additionally, in this study we demonstrate that hydrolysis of the ester bonds in the thermosets can be enforced at elevated temperatures in the presence of water, resulting in the liberation and migration of BP-C-8 monomer. Interestingly, BP-C, can readily be isolated by filtration as it crystallizes from water during cooling, thereby providing a promising route for its chemical recycling. Overall, these findings indicate that water plays a pivotal role in these poly(ester-amide)s as it functions as a catalyst in the BP-C-8 synthesis, governs the thermal and mechanical properties of the thermosets, and opens up routes for chemical recycling of BP-C-8 after hydrolysis as it acts as a recrystallization (co)solvent.

A bio-derived monomer called 2,3:4,5-di-O-isopropylidene-galactarate acid/ester (GalXMe) has great potential in polymer production. The unique properties of this molecule, such as its rigidity and bulkiness, contribute to the good thermal properties and appealing transparency of the material. The main problem, however, is that like other biobased materials, the polymers derived thereof are very brittle. In this study, we report on the melt blending of GalXMe polyamides (PAs) with different commercial PA grades using extrusion as well as blend characterization. Biobased PA blends showed limited to no miscibility with other polyamides. However, their incorporation resulted in strong materials with high Young moduli. The increase in modulus of the prepared GalXMe blends with commercial PAs ranged from up to 75% for blends with aliphatic polyamide composed of 1,6-diaminohexane and 1,12-dodecanedioic acid PA(6,12) to up to 82% for blends with cycloaliphatic polyamide composed of 4,4′-methylenebis(cyclohexylamine) and 1,12-dodecanedioic acid PA(PACM,12). Investigation into the mechanism of blending revealed that for some polyamides a transamidation reaction improved the blend compatibility. The thermal stability of the biobased PAs depended on which diamine was used. Polymers with aliphatic/aromatic or alicyclic diamines showed no degradation, whereas with fully aromatic diamines such as p-phenylenediamine, some degradation processes were observed under extrusion conditions (260/270 °C).

In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the slow coalescence of the polydomain texture. The timescale of the orientation parameter’s relaxation matched with that of the slow coalescence process. All processes were found to scale with shear rate in the tested regime. These observations can have far reaching implications for the processing of liquid crystal polymers as they indicate that increased shear rates during processing can correspond to an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropy and material properties after cooling.

In this work, we report on the small scale polycondensation and consecutive analysis of novel polyesters based on the potentially renewable 1,3-cyclopentanediol (CPdiol).

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

In this study, the performance of three aliphatic oxalamide based compounds as nucleating agent (NA) for isotactic polypropylene (iPP) has been evaluated. The NAs evaluated in this study contain two oxalamide motifs that are connected through linking spacers with 2, 3, or 4 methylene units in between the oxalamide groups, while containing flexible hexyl end-groups on the outside. It is demonstrated that the onset of crystallization in the presence of these NAs under quiescent conditions, generally observed around 120-125 degrees C, is strongly dependent on both the crystallization conditions and NA concentration. Although these oxalamide compounds are only moderately effective under quiescent conditions, they show promise for use under industrially relevant processing conditions that require high cooling and shear rates: When the NA is forced to undergo shear-induced self-assembly and shear alignment during cooling, the onset of crystallization of the iPP phase is observed in SAXS at temperatures up to 140 degrees C.