Symmetric Blends Research Articles

Enhancing the Stereocomplex Crystallization of PLLA/PDLA Symmetric Blend by Controlling the Degree of Molecular Chain Entanglement

In order to enhance the content of stereocomplex crystals (SC) in poly L-lactic acid (PLLA)/poly D-lactic acid (PDLA) symmetric blend, thereby improving the defects of low crystallinity and poor heat resistance of polylactic acid (PLA)-based materials, the samples with different entanglement degrees were prepared by freezing extration method. The critical overlap concentration of the blend system was quantitatively obtained using an Ubbelohde viscometer. Based on this concentration, samples with a series of concentration gradient were prepared to control the different degrees of molecular chain entanglement inside. The results indicated that the chain disentanglement could greatly increase the content of SC in the blend, whether in the precipitation crystallization or the melt crystallization process. Especially after the isothermal crystallization for 30min at 130oC, the L/D-0.1 sample with the highest degree of disentanglement almost only formed SC, accounting for 95.2% of the total crystallinity. Samples with lower entanglement degree had more crystal nuclei and faster growth rate of formed spherulites. For completely entangled sample L/D-50, the crystal nucleus of homo-crystalls (HC) could be considered as SC. Besides, the Vicat softening temperature (VST) of L/D-0.1 sample increased from 79.9oC of L/D-50 sample to 103.8oC with an obvious increase of 24.9oC. All these results provide new ideas for improving the performance of PLA-based materials and show promising development prospects of green and sustainable polymers.

Modeling the competition between phase separation and polymerization under explicit polydispersity.

The dynamics of phase separation for polymer blends is important in determining the final morphology and properties of polymer materials; in practical applications, this phase separation can be controlled by coupling to polymerization reaction kinetics via a process called 'polymerization-induced phase separation'. We develop a phase-field model for a polymer melt blend using a polymerizing Cahn-Hilliard (pCH) formalism to understand the fundamental processes underlying phase separation behavior of a mixture of two species independently undergoing linear step-growth polymerization. In our method, we explicitly model polydispersity in these systems to consider different molecular-weight components that will diffuse at different rates. We first show that this pCH model predicts results consistent with the Carothers predictions for step-growth polymerization kinetics, the Flory-Huggins theory of polymer mixing, and the classical predictions of spinodal decomposition in symmetric polymer blends. The model is then used to characterize (i) the competition between phase separation dynamics and polymerization kinetics, and (ii) the effect of unequal reaction rates between species. For large incompatibility between the species (i.e. high χ), our pCH model demonstrates that the strength for phase separation directly corresponds to the kinetics of phase separation. We find that increasing the reaction rate k̃, first induces faster phase separation but this trend reverses as we further increase k̃ due to the competition between molecular diffusion and polymerization. In this case, phase separation is delayed for faster polymerization rates due to the rapid accumulation of slow-moving, high molecular weight components.

Miscibility and exchange chemical potential of ring polymers in symmetric ring-ring blends.

Generally, differences of polymer topologies may affect polymer miscibility even with the same repeated units. In this study, the topological effect of ring polymers on miscibility was investigated by comparing symmetric ring-ring and linear-linear polymer blends. To elucidate the topological effect of ring polymers on mixing free energy, the exchange chemical potential of binary blends was numerically evaluated as a function of composition ϕ by performing semi-grand canonical Monte Carlo and molecular dynamics simulations of a bead-spring model. For ring-ring blends, an effective miscibility parameter was evaluated by comparing the exchange chemical potential with that of the Flory-Huggins model for linear-linear polymer blends. It was confirmed that in the mixed states satisfying χN > 0, ring-ring blends are more miscible and stable than the linear-linear blends with the same molecular weight. Furthermore, we investigated finite molecular weight dependence on the miscibility parameter, which reflected the statistical probability of interchain interactions in the blends. The simulation results revealed that the molecular weight dependence on the miscibility parameter was smaller in ring-ring blends. The effect of the ring polymers on miscibility was verified to be consistent with the change in the interchain radial distribution function. In ring-ring blends, it was indicated that the topology affected miscibility by reducing the effect of the direct interaction between the components of the blends.

Critical Percolation Threshold for Solvation-Site Connectivity in Polymer Electrolyte Mixtures

Poly(ethylene oxide) (PEO)-based polymer electrolytes are often mixed with rigid, nonconductive polymers to improve mechanical strength. The suppressed conductivity of the mixture typically arises from a reduced segmental mobility and a diminished connectivity between conductive PEO sites. To decouple these two mechanisms, we compare transport in symmetric miscible blends and disordered block copolymers (BCP) of PEO and poly(methyl methacrylate) (PMMA). Because the two systems have identical physicochemical properties, differences in their conductivity directly reflect the underlying PEO network connectivity. We find that, at short distances (<5 Å), the Li+ solvation networks are identical for the two systems; however, a distinct variation in the network connectivity arises at length scales between 5 and 10 Å. Specifically, the BCP exhibits a lower connectivity, and therefore a lower conductivity than the blend. A quantitative model is proposed that associates long-range Li+ transport with local miscibility; the concept of network connectivity discussed here could be useful for designing polymer electrolyte systems.

Composite entanglement topology and extensional rheology of symmetric ring-linear polymer blends

Extensive molecular simulations are applied to characterize the equilibrium dynamics, entanglement topology, and nonlinear extensional rheology of symmetric ring-linear polymer blends with systematically varied ring fraction ϕR. Chains with degree of entanglement Z≈14 are mixed to produce 10 well-entangled systems with ϕR varying from neat linear to neat ring melts. Z is large enough that except for very large ϕR, the rings are threaded by multiple linear chains in equilibrium. Primitive path analysis is used to visualize and quantify the structure of the composite ring-linear entanglement network. We measure the quantity of ring-linear threading and linear-linear entanglement as a function of ϕR and identify with simple arguments a ring fraction ϕR≈0.4 where the topological constraints of the composite entanglement network are maximized. These topological analyses are used to rationalize the ϕR-dependence of ring and linear chain dynamics, conformations, and rheology. Simulations of startup uniaxial elongation flows demonstrate the extensional stress overshoot observed in recent filament stretching experiments and characterize how it depends on the blend composition and entanglement topology. The overshoot is driven by an overstretching and recoil of ring polymers due to the convective unthreading of rings from linear chains.

Stereocomplex and individual crystallization behavior of symmetric or enantiomeric substituted Poly(lactic acid)s random copolymers with high crystallizabilities

Stereocomplex (SC) crystallization is reported for the first time for the enantiomeric blends of random copolymers of l-configured poly(l-2-hydroxybutanoic acid-co-l-2-hydroxy-3-methylbutanoic acid) [P(L-2HB-co-L-2H3MB)] (49/51) and d-configured poly(d-2-hydroxybutanoic acid-co-d-2-hydroxy-3-methylbutanoic acid) [P(D-2HB-co-D-2H3MB)] (47/53) with high individual crystallizabilities. Unblended l-configured P(L-2HB-co-L-2H3MB) and d-configured P(D-2HB-co-D-2H3MB) formed individual crystallites through purification, solvent evaporation, and melt crystallization. Three types of individual crystallites (provisionally named as α-, β-, and γ-forms) were observed in the unblended copolymers depending on the crystallization methods. SC crystallites were formed in the blends through solvent evaporation and melt crystallization method. All types of monomer units cocrystallized in the individual and SC crystallites. The melting and equilibrium-melting temperatures of SC crystallites in the blends were 192.5–203.7 °C and 205.2 °C, respectively, which were significantly higher than the melting temperatures of individual crystallites in the unblended copolymers (61.6–133.1 °C). The maximum radial growth rate of SC crystallites (13.5 μm min−1) was 10 and 27 times higher than those of individual crystallites of the unblended copolymers (1.4 and 0.5 μm min−1), indicating that SC crystallization increased the crystallization rate. SC crystallization proceeded via regimes II and III. The copolymer crystallizability, monomer cocrystallizability, and crystallization rate of P(L-2HB-co-L-2H3MB) (49/51) and P(D-2HB-co-D-2H3MB) (47/53) are compared with those reported for the series of symmetric or enantiomeric polymer blends of poly(l-lactic acid-co-l-2-hydroxybutanoic acid) (56/44) and poly(d-lactic acid-co-d-2-hydroxybutanoic acid) (52/48) with lowindividualcrystallizabilities and poly(l-lactic acid-co-l-2-hydroxy-3-methylbutanoic acid) (47/53) and poly(d-lactic acid-co-d-2-hydroxy-3-methylbutanoic acid) (47/53) with noindividualcrystallizability.

Fascinating morphology and crystallization behavior of melt miscible binary blends of crystalline homopolymers depicting nearly simultaneous melting transitions

We report on the interesting crystallization behavior of crystalline/crystalline polymer blends, composed of poly(butylene adipate) (PBA) and poly(ethylene oxide) (PEO), with nearly simultaneous melting and crystallization transitions. The miscibility of the two components ensured that the crystallization, on cooling, occurred from the homogenous phase in the melt. The crystalline morphology and structure was investigated using a combination of DSC, PLOM, WAXD and SAXS measurements. The crystalline structure and morphology of the PEO/PBA blends were found to be governed by a strong competition between the faster crystallization kinetics of PEO and relatively much higher nucleation density of PBA. In the PEO-rich blend, the faster crystallization kinetics of the PEO dominated the crystallization morphology development such that the minority component i.e. PBA was found to get entrapped in the interlamellar and interfibrillar regions of PEO spherulites. Interestingly, the resulting crystallization of PBA, within the confined intraspherultic regions of PEO, favored the formation of β phase crystals even at very high isothermal crystallization temperatures. In the symmetric and PBA-rich blends, the higher nucleation density of PBA had a dominant influence on the crystalline morphology development, especially at high degree of supercooling. This resulted in the formation of a variety of interesting morphologies which included interpenetrating spherulites, PEO templated PBA and PBA templated PEO spherulites. Furthermore, the α crystal phase of PBA was found to be more stable in a large supercooling window plausibly due to the dilution effect of PEO. The present study provide novel insight into the competitive role of crystallization rate and nucleation density on the crystallization behavior of crystalline/crystalline blends, with similar melting points, which has been scarcely studied in the past.

Dissipative Particle Dynamics Study on Interfacial Properties of Symmetric Ternary Polymeric Blends.

We investigated the interfacial properties of symmetric ternary An/AmBm/Bn and An/Am/2BmAm/2/Bn polymeric blends by means of dissipative particle dynamics (DPD) simulations. We systematically analyzed the effects of composition, chain length, and concentration of the copolymers on the interfacial tensions, interfacial widths, and the structures of each polymer component in the blends. Our simulations show that: (i) the efficiency of the copolymers in reducing the interfacial tension is highly dependent on their compositions. The triblock copolymers are more effective in reducing the interfacial tension compared to that of the diblock copolymers at the same chain length and concentration; (ii) the interfacial tension of the blends increases with increases in the triblock copolymer chain length, which indicates that the triblock copolymers with a shorter chain length exhibit a better performance as the compatibilizers compared to that of their counterparts with longer chain lengths; and (iii) elevating the triblock copolymer concentration can promote copolymer enrichment at the center of the interface, which enlarges the width of the phase interfaces and reduces the interfacial tension. These findings illustrate the correlations between the efficiency of copolymer compatibilizers and their detailed molecular parameters.

Directional percolating pathways in demixing blends on a wetting substrate

Understanding the dynamical evolution of a percolating network during liquid–liquid demixing is crucial for many technological applications, including plastic electronics, such as organic photovoltaics, whose performance depends on the efficiency to transport the positive and negative charge carriers to the corresponding electrodes. The transport and collection of the charge carriers require sufficient asymmetry between the donor and acceptor phases by attaining a minimum concentration of the majority fluids in it, called the percolation threshold. We investigate demixing in symmetric binary blends on a substrate preferentially wet by one of the fluids from the perspective of such a percolation threshold to achieve directed and connectivity percolation. We also study the influence of the strength of the substrate interaction and the property of the blend right before the quench with respect to the critical point. It is commonly assumed that the bicontinuous morphology of a symmetric blend guarantees percolation, where the average concentration of the blend distinguishes the two phases. However, if the percolation threshold is larger than the average concentration, we find that percolating pathways grow monotonically and a percolating cluster forms only after a time lag. Furthermore, we find that this time lag is characterized by two universal kinetic regimes that can explain all our observations. The first regime is associated with the percolation threshold itself, which grows exponentially. The second regime displays an algebraic growth with an exponent of 1/3 and we argue that it must be associated with the directional connectivity of the wetting phase to the substrate.

Direct Transesterification for Biodiesel Production and Testing the Engine for Performance and Emissions Run on Biodiesel-Diesel-Nano Blends.

In the current research, the biodiesel was prepared from feedstocks of Neem oil and Karanja oil employing a single step direct transesterification method using acid-base catalysts simultaneously. The fuel properties of both Neem and Karanja biodiesel along with different biodiesel-diesel blends were studied and compared. Biodiesel produced from Neem oil was found better in terms of kinematic viscosity, calorific value and cloud point for all its blends with diesel compared to Karanja biodiesel-diesel blends. Experiments were conducted to study the effects of addition of graphene nano particles on fuel properties of biodiesel-diesel blends. The B20 biodiesel-diesel blend was selected, which was blended with graphene nano particles in different proportions (35, 70, 105 ppm) to get different stable and symmetric B20-nano blends. The fuel properties except kinematic viscosity were further improved with higher dosages of nano particles with the biodiesel-diesel blend. The performance and emissions tests were conducted on 4-stroke variable compression ratio diesel engine. Higher concentrated B20-nano blends of Neem (NOME20GO105) and Karanja (KOME20GO105) resulted in 31 and 30.9% of brake thermal efficiency, respectively, compared with diesel of 32.5%. The brake-specific fuel consumption (BSFC) was reduced by 10 and 11% for NOME20GO105 and KOME20GO105, respectively, compared to their respective B20 blends. Similarly, carbon monoxide (CO) was reduced significantly by 27 and 29% for NOME20GO105 and KOME20GO105, respectively.

Coexistence of Polymeric Microemulsion with Homopolymer-Rich Phases

Recent field-theoretic simulations of symmetric ternary blends of A- and B-type homopolymers with AB diblock copolymer [Vorselaars, B.; Phys. Rev. Lett., 2020, 125, 117801]predicted three-phase coe...

Instability of the Microemulsion Channel in Block Copolymer-Homopolymer Blends.

Field theoretic simulations are used to predict the equilibrium phase diagram of symmetric blends of AB diblock copolymer with A- and B-type homopolymers. Experiments generally observe a channel of bicontinuous microemulsion (BμE) separating the ordered lamellar (LAM) phase from coexisting homopolymer-rich (A+B) phases. However, our simulations find that the channel is unstable with respect to macrophase separation, in particular, A+B+BμE coexistence at high T and A+B+LAM coexistence at low T. The preference for three-phase coexistence is attributed to a weak attractive interaction between diblock monolayers.

Dynamical self-consistent field theory captures multi-scale physics during spinodal decomposition in a symmetric binary homopolymer blend.

We study the spinodal decomposition in a symmetric, binary homopolymer blend using our recently developed dynamical self-consistent field theory. By taking the extremal solution of a dynamical functional integral, the theory reduces the interacting, multi-chain dynamics to a Smoluchowski equation describing the statistical dynamics of a single, unentangled chain in a self-consistent, time-dependent, mean force-field. We numerically solve this equation by evaluating averages over a large ensemble of replica chains, each one of which obeys single-chain Langevin dynamics, subject to the mean field. Following a quench from the disordered state, an early time spinodal instability in the blend composition develops, before even one Rouse time elapses. The dominant, unstable, growing wavelength is on the order of the coil size. The blend then enters a late-time, t, scaling regime with a growing domain size that follows the expected Lifshitz-Slyozov-Wagner t1/3 power law, a characteristic of a diffusion-driven coarsening process. These results provide a satisfying test of this new method, which correctly captures both the early and late time physics in the blend. Our simulation spans five orders-of-magnitude in time as the domains coarsen to 20 times the coil size, while remaining faithful to the dynamics of the microscopic chain model.

Stress relaxation in symmetric ring-linear polymer blends at low ring fractions.

We combine linear viscoelastic measurements and modelling in order to explore the dynamics of blends of the same-molecular-weight ring and linear polymers in the regime of the low volume fraction (0.3 or lower) of the ring component. The stress relaxation modulus is affected by the constraint release (CR) of both rings and linear components due to the motion of linear chains. We develop a CR-based model of ring-linear blends that predicts the stress relaxation function in the low fraction regime of ring component in excellent agreement with experiments. Rings trapped by their entanglements with linear chains can only relax by linear-chain-induced constraint release, resulting in much slower relaxation of rings than of linear chains. The relative viscosity of the blend with respect to the linear melt viscosity η L at ring overlap volume fraction increases proportionally to the square root of ring molecular weight . Our experimental results clearly demonstrate that it is possible to enhance the viscosity and simultaneously the structural relaxation time of linear polymer melts by adding a small fraction of ring polymers. These results not only provide fundamental insights into the physics of the CR process but also suggest ways to fine-tune the flow properties of linear polymers by means of adding rings.

Microphase Separation in Polyelectrolyte Blends: Weak Segregation Theory and Relation to Nuclear “Pasta”

We develop the weak segregation theory of microphase separation in stoichiometric blends of oppositely and weakly charged polyelectrolytes, which would be immiscible in the absence of charged units. Short-range repulsions between polycation and polyanion monomers induce the formation of oppositely charged domains, the size of which is controlled by their excess Coulomb energy. The diagram of blend morphologies is constructed in the framework of Leibler’s mean-field approach and to account for fluctuations within the Brazovskii–Fredrickson–Helfand approximation. Phase behavior of the polyelectrolyte blends is fully analogous to that of neutral diblock copolymers. The mean-field analysis predicts that the order–disorder transition (ODT) is second order for the symmetric blend. In asymmetric mixtures, an increasing incompatibility between polyelectrolytes triggers the usual cascade of first-order phase transitions, dis → bcc → hex → lam. Fluctuations alter the phase diagram topology and allow the direct transitions from the disordered state (dis) to hex and lam microphases; for the symmetric blend, ODT becomes weakly first order. We also discuss that both microphase separation in polyelectrolyte blends and the formation of nuclear “pasta” phases within neutron stars are governed by similar physical principles, despite the 6 orders of magnitude difference in the periods of these structures.

Calibration of a lattice model for high-molecular-weight block copolymer melts.

The Morse calibration is applied to a lattice model designed for efficient simulations of two-component polymer melts of high molecular weight. The model allows multiple occupancy per site, which results in high invariant polymerization indices, and interactions are limited to monomers within the same site, which enhances the computational speed. The calibration maps the interaction parameter of the lattice model, α, onto the Flory-Huggins χ parameter of the standard Gaussian-chain model, by matching the disordered-state structure function, S(k), of symmetric diblock copolymers to renormalized one-loop predictions. The quantitative accuracy of the calibration is tested by comparing the order-disorder transition of symmetric diblock copolymer melts to the universal prediction obtained from previous simulations. The model is then used to confirm the universality of fluctuation corrections to the critical point of symmetric binary homopolymer blends.

Nanostructure stability and swelling of ternary block copolymer/homopolymer blends: A direct comparison between dissipative particle dynamics and experiment

ABSTRACTTernary block copolymer (BCP)‐homopolymer (HP) blends offer a simple method for tuning nanostructure sizes to meet application‐specific demands. Comprehensive dissipative particle dynamic (DPD) simulations were performed to study the impact of polymer interactions, molecular weight, and HP volume fraction (φHP) on symmetric ternary blend morphological stability and domain spacing. DPD reproduces key features of the experimental phase diagram, including lamellar domain swelling with increasing φHP, the formation of an asymmetric bicontinuous microemulsion at a critical HP concentration , and macrophase separation with further HP addition. Simulation results matched experimental values for and lamellar swelling as a function of HP to BCP chain length ratio, α = NHP/NBCP. Structural analysis of blends with fixed φHP but varying α confirmed that ternary blends follow the wet/dry brush model of domain swelling with the miscibility of HPs and BCPs depending on α. Longer HPs concentrate in the center of domains, boosting their swelling efficiencies compared to shorter chains. These results advance our understanding of BCP‐HP blend phase behavior and demonstrate the value of DPD for studying polymeric blends. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 794–803

Monte Carlo Simulation of Strain-Enhanced Stereocomplex Polymer Crystallization.

We performed dynamic Monte Carlo simulations to investigate strain-induced polymer crystallization under separate enhancements of the driving forces for homocomponent and stereocomplex crystallization in the half-half symmetric racemic polymer blends. The results showed that the polymer strain significantly enhances the stereocomplex crystallization, in comparison to the parallel cases of template-induced crystal growth without any strain in the previous simulations. We attributed the results to the strain-induced polymer crystallization favoring intermolecular crystal nucleation at high temperatures, which benefits the stereocomplex crystallization. Our observations provided a molecular-level interpretation to the strain- or shear-enhanced stereocomplex crystallization in the racemic polylactide blends.

Colloid silica nanoparticles trapped morphology of polymer blends during solvent evaporation

During the solvent evaporation induced phase separation of PS and PMMA blends with silica nanoparticles, the dispersed and bicontinuous morphologies need to be stabilized by the absorption of silica nanoparticles at the interface. In order to accomplish this, the silica nanoparticles need to be chemically neutral to PS and PMMA homopolymers. For this purpose, their surfaces were modified by attaching mono-hydroxyl group terminated polystyrene poly(methyl methacrylate) random (PS-r-PMMA-OH) copolymer via the condensation reaction between the hydroxyl groups of silica particles and copolymer. The resulting morphology was found to be affected by the rate of solvent evaporation, blend compositions, particle loading and size. In the symmetric blend system, the dispersed phase was observed at low evaporation rate and bicontinuous morphology was obtained at high evaporation rate. The silica nanoparticles were found to be densely packed at the interface of PS and PMMA. For asymmetric blend system, only dispersed phase was observed at both low and high evaporation rates. The size of phase separated domains decreases with increasing solvent evaporation rate and particle loading. The silica nanoparticles with smaller size resulted in relatively smaller domain size due to their higher surface area to volume ratio. Porous membranes with partially exposed nanoparticles were fabricated by etching PMMA phase in acetic acid.

A simulation method for the phase diagram of complex fluid mixtures.

The phase behavior of complex fluid mixtures is of continuing interest, but obtaining the phase diagram from computer simulations can be challenging. In the Gibbs ensemble method, for example, each of the coexisting phases is simulated in a different cell, and ensuring the equality of chemical potentials of all components requires the transfer of molecules from one cell to the other. For complex fluids such as polymers, successful insertions are rare. An alternative method is to simulate both coexisting phases in a single simulation cell, with an interface between them. The challenge here is that the interface position moves during the simulation, making it difficult to determine the concentration profile and coexisting concentrations. In this work, we propose a new method for single cell simulations that uses a spatial concentration autocorrelation function to (spatially) align instantaneous concentration profiles from different snapshots. This allows one to obtain average concentration profiles and hence the coexisting concentrations. We test the method by calculating the phase diagrams of two systems: the Widom-Rowlinson model and the symmetric blends of freely jointed polymer molecules for which phase diagrams from conventional methods are available. Excellent agreement is found, except in the neighborhood of the critical point where the interface is broad and finite size effects are important. The method is easy to implement and readily applied to any mixture of complex fluids.