Elastic behavior and platelet retraction in low- and high-density fibrin gels.

Fibrin is a biopolymer that gives thrombi the mechanical strength to withstand the forces imparted on them by blood flow. Importantly, fibrin is highly extensible, but strain hardens at low deformation rates. The density of fibrin in clots, especially arterial clots, is higher than that in gels made at plasma concentrations of fibrinogen (3-10 mg/mL), where most rheology studies have been conducted. Our objective in this study was to measure and characterize the elastic regimes of low (3-10 mg/mL) and high (30-100 mg/mL) density fibrin gels using shear and extensional rheology. Confocal microscopy of the gels shows that fiber density increases with fibrinogen concentration. At low strains, fibrin gels act as thermal networks independent of fibrinogen concentration. Within the low-strain regime, one can predict the mesh size of fibrin gels by the elastic modulus using semiflexible polymer theory. Significantly, this provides a link between gel mechanics and interstitial fluid flow. At moderate strains, we find that low-density fibrin gels act as nonaffine mechanical networks and transition to affine mechanical networks with increasing strains within the moderate regime, whereas high-density fibrin gels only act as affine mechanical networks. At high strains, the backbone of individual fibrin fibers stretches for all fibrin gels. Platelets can retract low-density gels by >80% of their initial volumes, but retraction is attenuated in high-density fibrin gels and with decreasing platelet density. Taken together, these results show that the nature of fibrin deformation is a strong function of fibrin fiber density, which has ramifications for the growth, embolization, and lysis of thrombi.

Effects of polydispersity on confined homopolymer melts: a Monte Carlo study.

New insight into the molecular scale details of polymer melts under confined conditions is obtained from the first dynamic Monte Carlo study incorporating polydispersity. While confinement effects on polymers have been widely explored, little work exists on the effects of polydispersity. This is surprising given the near universal presence of polydispersity in physical systems. To address this shortcoming, a new variation of on-lattice dynamic Monte Carlo simulation is used to provide an understanding of how polydispersity alters confinement effects on polymer melts. Polymer melts of varying polydispersity are simulated between two hard walls (surface interaction parameter, χ(s) = 0) of variable spacing. As plate spacing decreases, polymer chains adopt conformations in which the end-to-end vector is parallel to the hard walls. However, polydisperse melts with the same length average molecular weight, N(w) (which is analogous to the weight average molecular weight, M(w)) show reduced orientation effects. Polydispersity provides greater degrees of freedom; that is, there are more configurations for the system to adopt to accommodate confinement without ordering. At plate spacings of four radii of gyration and only modest polydispersity index values (polydispersity index, PDI = 1.42), the order parameters are reduced by 15% compared to the monodisperse case. The same PDI value corresponds to a 10% reduction in the perturbations of the end-to-end vector and Rouse time. Interestingly, length-based migration effects are observed. Longer chains reside away from the walls and the shorter chains are found nearer the walls; at equilibrium there is a molecular weight based fractionation across the gap. Confinement also leads to a "speeding up" of the polymer dynamics. Altered dynamic phenomena include a reduction of the Rouse time for the same average molecular weight and an altered scaling behavior with plate spacing. Reptation times are also reduced and polydispersity smoothes out the transitions between different scaling regimes. The overall picture that emerges is not unexpected­polydispersity profoundly affects the behavior of confined homopolymers.

Molecular scale simulation of homopolymer wall slip.

The first molecular scale simulation of highly entangled polydisperse homopolymers that is capable of capturing all three regions--no slip, weak slip, and strong slip--of the hydrodynamic boundary condition is presented. An on-lattice dynamic Monte Carlo technique capable of correctly capturing both unentangled and entangled polymer dynamics is used to study the molecular details of wall slip phenomena for homopolymers and energetically neutral walls. For unentangled chains (those exhibiting Rouse dynamics) weak slip is not present but evidence of strong slip is manifest at very high shear rates. For entangled chains (of sufficient length to exhibit reptation dynamics), both weak and strong slip are observed. Consistent with numerous experimental studies, disentanglement and cohesive failure occur at high shear rates. Disentanglement is clearly evidenced in a nonlinear velocity profile that exhibits shear banding, in an excess of chain ends at the slip plane, and perhaps most importantly in a nonmonotonic stress versus shear rate response. The chain end density exhibits a pretransitional periodicity prior to disentanglement. Unentangled Rouse chains do not show this pretransitional response or a bifurcation in their stress versus shear rate response. Finally, it is shown that when polydispersity is introduced, slip phenomena are severely reduced and the inherent constitutive bifurcation is limited to a small region. Predictions are in post facto agreement with many experiments, are distinct from existing results obtained using molecular dynamics simulation techniques, and shed light on fundamental mechanisms of polymer wall slip.

Supra-molecular ecobionanocomposites based on polylactide and cellulosic nanowhiskers: synthesis and properties.

Successful filler dispersion and establishment of good interfacial contact with the surrounding matrix are essential for optimized reinforcement in polymeric nanocomposites. In particular, in renewable-based composites this can be challenging, where hydrophilic attractions between nanofillers facilitate aggregation. Here an innovative approach to prepare cellulosic nanowhisker (CNW) reinforced polylactide (PLA) is presented. The lactide ring-opening polymerization is initiated from CNW surface hydroxyl groups after partial acetylation to control the grafting density. Grafting of PLA chains is verified by Fourier transform infrared spectroscopy. The resulting nanocomposites display exceptional properties; a heat distortion temperature of 120 °C is achieved at 10 wt % CNW loading and can be further enhanced to reach 150 °C at 15 wt % CNW. The formation of a percolating network is verified by comparison of modulus data with an established theoretical model. Additionally, nucleation by CNWs reduces the crystallization half-time to 15 s compared with 90 s for PLA. Melt-pressed films retain transparency indicating good filler dispersion.

Hybrid Chemomechanical Plastics Recycling: Solvent-free, High-Speed Reactive Extrusion of Low-Density Polyethylene.

Low-density polyethylene (LDPE) is ubiquitous in the packaging industry owing to its flexibility, toughness, and low cost. However, it is typically contaminated with other materials, seriously limiting options for mechanical recycling. Interest in chemical recycling techniques such as pyrolysis and hydrothermal liquefaction is growing, but most of these processes face technoeconomic challenges that have limited commercial deployment. This study concerns a hybrid chemomechanical approach using reactive twin-screw extrusion (TSE) for tailoring the molecular weight and chain structure of reclaimed LDPE. Two types of zeolite catalysts at several loading levels are evaluated over a range of processing conditions. Structural, thermal, and rheological properties of the extruded samples are investigated and compared to virgin LDPE and LDPE extruded without the catalyst. NMR spectroscopy is used to investigate changes in the structure of the polymer. LDPE extruded with microporous Y zeolite shows lower degradation temperature and increased short chain branching. Mesoporous MCM-41 also induces increased branching but has no effect on the degradation temperature. The theoretical mechanical energy input for the chemical modification is calculated by using process modeling. The demonstrated hybrid reactive extrusion process provides a potential low-cost, simple approach for repurposing LDPE-based flexible packaging as coatings and adhesives.

Single-step method for the isolation and surface functionalization of cellulosic nanowhiskers.

Surface modification of cellulosic nanowhiskers (CNW) is of great interest, especially to facilitate their use as polymer reinforcements. Generally, alteration of the surface chemistry is performed using multiple reaction steps. In contrast, this study demonstrates that the needed hydrolysis of amorphous cellulose chains can be performed simultaneously with the esterification of accessible hydroxyl groups to produce surface functionalized CNW in a single step. The reaction is carried out in an acid mixture composed of hydrochloric and an organic acid (acetic and butyric are both demonstrated). Resulting CNW are of similar dimensions compared to those obtained by hydrochloric acid hydrolysis alone; sizes are verified by multiangle laser-light scattering and transmission electron microscopy. However, narrower diameter polydispersity indices indicate that surface groups aid the individualization of the nanowhiskers (Px = 2.5 and 2.1 for acetic and butyric acid, Px = 3.0 for hydrochloric acid). More than half of the hydroxyl groups located on the CNW surface are substituted under the employed reaction conditions as determined by quantitative Fourier-transform infrared-spectroscopy. The resulting surface modified CNW are dispersible in ethyl acetate and toluene indicating increased hydrophobicity and thus are presumably more compatible with hydrophobic polymers when used as a reinforcing phase.

Cellulosic nanowhiskers. Theory and application of light scattering from polydisperse spheroids in the Rayleigh-Gans-Debye regime.

Mathematical treatment of light scattering within the Rayleigh-Gans-Debye limit for spheroids with polydispersity in both length and diameter is developed and experimentally tested using cellulosic nanowhiskers (CNW). Polydispersity indices are obtained by fitting the theoretical formfactor to experimental data. Good agreement is achieved using a polydispersity of 2.3 for the length, independent of the type of acid used. Diameter polydispersities are 2.1 and 3.0 for sulfuric and hydrochloric acids, respectively. These polydispersities allow the determination of average dimensions from the z-average mean-square radius (z) and the weight-average molecular weight (M w) easily obtained from Berry plots. For cotton linter hydrolyzed by hydrochloric acid, the average length and diameter are 244 and 22 nm. This compares to average length and diameter of 272 and 13 nm for sulfuric acid. This study establishes a new light-scattering methodology as a quick and robust tool for size characterization of polydisperse spheroidal nanoparticles.

Propionic acid production from corn stover hydrolysate by Propionibacterium acidipropionici.

BACKGROUND: The production of value-added chemicals alongside biofuels from lignocellulosic hydrolysates is critical for developing economically viable biorefineries. Here, the production of propionic acid (PA), a potential building block for C3-based chemicals, from corn stover hydrolysate is investigated using the native PA-producing bacterium Propionibacterium acidipropionici. RESULTS: A wide range of culture conditions and process parameters were examined and experimentally optimized to maximize titer, rate, and yield of PA. The effect of gas sparging during fermentation was first examined, and N2 was found to exhibit improved performance over CO2. Subsequently, the effects of different hydrolysate concentrations, nitrogen sources, and neutralization agents were investigated. One of the best combinations found during batch experiments used yeast extract (YE) as the primary nitrogen source and NH4OH for pH control. This combination enabled PA titers of 30.8 g/L with a productivity of 0.40 g/L h from 76.8 g/L biomass sugars, while successfully minimizing lactic acid production. Due to the economic significance of downstream separations, increasing titers using fed-batch fermentation was examined by changing both feeding media and strategy. Continuous feeding of hydrolysate was found to be superior to pulsed feeding and combined with high YE concentrations increased PA titers to 62.7 g/L and improved the simultaneous utilization of different biomass sugars. Additionally, applying high YE supplementation maintains the lactic acid concentration below 4 g/L for the duration of the fermentation. Finally, with the aim of increasing productivity, high cell density fed-batch fermentations were conducted. PA titers increased to 64.7 g/L with a productivity of 2.35 g/L h for the batch stage and 0.77 g/L h for the overall process. CONCLUSION: These results highlight the importance of media and fermentation strategy to improve PA production. Overall, this work demonstrates the feasibility of producing PA from corn stover hydrolysate.

Molecular-scale simulation of cross-flow migration in polymer melts.

The first ever molecular-scale simulation of cross-flow migration effects in dense polymer melts is presented; simulations for both unentangled and entangled chains are presented. At quiescence a small depletion next to the wall for the segmental densities of longer chains is present, a corresponding excess exists about one-half a radii of gyration away from the wall, and uniform values are observed further from the wall. In shear flow the melts exhibit similar behavior as the quiescent case; a constant shear rate across the gap does not induce chain length based migration. In contradistinction, parabolic flow (where gradients in shear rate are present) causes profound migration for both unentangled and entangled melts. Mapping onto polyethylene and calculating stress shows the system is far below the stress required to break chains. Accordingly, our findings are consistent with flow induced migration mechanisms predominating over competing chain degradation mechanisms thus resolving a 40 year old controversy.