Computer Simulations of Local Concentration Variations in Miscible Polymer Blends

The influence of elongation-induced concentration fluctuations on segmental friction in polymer blends

Recent experimental studies have revealed a lack of universality in the extensional behavior of linear polymers, which is not envisioned by classical molecular theories. These surprising findings, particularly the sharp contrast between polymer melts and solutions, have catalyzed the development of new theoretical ideas, including the concept of friction reduction in highly stretched polymer melts. By presenting evidence from rheology and small-angle neutron scattering, this work shows that deformation-induced demixing, which is due to the viscoelastic asymmetry in binary mixtures, contributes to the observed nonuniversality. In the case of polystyrene/oligostyrene blends, demixing increases the effective glass transition temperature of the long chain, leading to an apparent friction enhancement. On the other hand, the opposite case is found for the polystyrene/poly(α-methylstyrene) blend. These results highlight the important influence of deformation-induced concentration fluctuations on polymer segmental friction.

Experimental evidence of co-existence of equilibrium and nonequilibrium in two-glass-transition miscible mixtures

Two glass-transitions have been observed in some miscible molecular mixtures with notable differences in geometry or chemistry of constituents. The explanation of the phenomena has been puzzling with diverse structural models. Here, we present detailed studies on two glass-transition mixtures composed of tripropyl phosphate (TPP) and polystyrene (PS) by using calorimetric and dielectric measurements. We found that ageing between the two transitions always generates endothermic peaks at temperatures ∼4 K higher than the ageing temperatures and, subsequent thermal cycles around the peaks can remove the ageing effect and restore the systems, confirming the co-existence of nonequilibrium and equilibrium states in the regions. We also found that the broad glass transition thermogram is associated with highly stretched relaxation dynamics. The results allow us to draw a conclusion of continuous mobility gradient spanning the two TPP-PS glass-transitions, rather than complete phase separation.

Spatio-temporal heterogeneities in nanosegregated single-molecule polymeric nanoparticles

The study of the coupling between structural and dynamical heterogeneities in nanostructured systems is essential for the design of hybrid materials with the desired properties. Here, we use atomistic molecular dynamics simulations to closely examine the dynamical heterogeneities in nanostructured single-molecule nanoparticles consisting of mikto-arm star copolymers with poly(ethylene oxide), PEO, and polystyrene, PS, arms. The particles exhibit an internally nanostructured morphology, resembling either "Janus-like" or "patchy-like" morphology when the functionality of the stars varies. The differences in the local environment result in strong intramolecular dynamical heterogeneities. In the proximity of the star core, geometric constraints promote unfavorable PEO:PS contacts that lead to a behavior similar to dynamically asymmetric miscible polymer blends or disordered copolymers. In contrast, further away from the core, the nanosegregation induces segmental dynamics very similar to the one found in the homopolymer star analogues.

Exploring Disordered Morphologies of Blends and Block Copolymers for Light-Emitting Diodes with Mesoscopic Simulations

Recently, disordered blends of semiconducting and insulating polymers have been used to prepare light-emitting diodes with increased luminous efficiency. Because the thermodynamic stability of the disordered phase in blends is limited, equivalent diblock copolymers (BCPs) could be an alternative. However, the choice between disordered blends and BCPs requires understanding structural differences and their effect on charge carrier transport. Using a hybrid mesoscopic model, we simulate blends and equivalent BCPs of two representative semiconducting and insulating polymers: poly(p-phenylene vinylene) (PPV) and polyacrylate. The immiscibility is varied to mimic annealing at different temperatures. We find stable or metastable disordered morphologies until we reach the mean-field (MF) spinodal. Disordered morphologies are heterogeneous because of thermal fluctuations and local segregation. Near the MF spinodal, segregation is stronger in BCPs than in the blends, even though the immiscibility, normalized by the MF spinodal, is the same. We link the spatial distribution of PPV with electric conductance. We predict that the immiscibility (temperature at which the layer is annealed) affects electrical percolation much stronger in BCPs than in blends. Differences in the local structure and percolation between blends and BCPs are enhanced at a high insulator content.

Confinement and localization effects revealed for thin films of the miscible blend poly(vinyl methyl ether)/polystyrene with a composition of 25/75 wt%⋆

Thin films (200-7nm) of the asymmetric polymer blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) (25/75wt%) were investigated by broadband dielectric spectroscopy (BDS). Thicker samples ([Formula: see text]37 nm) were measured by crossed electrode capacitors (CEC), where the film is capped between Al-electrodes. For thinner films ([Formula: see text]37 nm) nanostructured capacitors (NSC) were employed, allowing one free surface in the film. The dielectric spectra of the thick films showed three relaxation processes ( [Formula: see text] -, [Formula: see text] - and [Formula: see text] -relaxation), like the bulk, related to PVME fluctuations in local spatial regions with different PS concentrations. The thickness dependence of the [Formula: see text] -process for films measured by CECs proved a spatially heterogeneous structure across the film with a PS-adsorption at the Al-electrodes. On the contrary, for the films measured by NSCs a PVME segregation at the free surface was found, resulting in faster dynamics, compared to the CECs. Moreover, for the thinnest films ([Formula: see text]26 nm) an additional relaxation process was detected. It was assigned to restricted fluctuations of PVME segments within the loosely bounded part of the adsorbed layer, proving that for NSCs a PVME enrichment takes place also at the polymer/substrate interface.

Rheology of Concentrated Polymer/Ionic Liquid Solutions: An Anomalous Plasticizing Effect and a Universality in Nonlinear Shear Rheology

An anomalous plasticizing effect was observed in polymer/ionic liquid (IL) solutions by applying broad range of rheological techniques. Poly(ethylene oxide)(PEO)/IL solutions exhibit stronger dynamic temperature dependence than pure PEO, which is in conflict with the knowledge that lower-Tg solvent increases the fractional free volume. For poly(methy methacrylate)(PMMA)/IL solutions, the subtle anomaly was detected from the fact that the effective glass transition temperature Tg,eff of PMMA in IL is higher than the prediction of the self-concentration model, while in conventional polymer solutions, Tg,eff follows the original Fox equation. Observations in both solutions reveal retarded segmental dynamics, consistent with a recent simulation result (Macromolecules, 2018, 51, 5336) that polymer chains wrap the IL cations by hydrogen bonding interactions and the segmental unwrapping delays their relaxation. Start-up shear and nonlinear stress relaxation tests of polymer/IL solutions follow a universal nonlinear rheological behavior as polymer melts and solutions, indicating that the segment-cation interaction is not strong enough to influence the nonlinear chain orientation and stretch. The present work may arouse the further theoretical, experimental, and simulation interests in interpreting the effect of complex polymer-IL interaction on the dynamics of polymer/IL solutions.

Nanoscale Concentration Quantification of Pharmaceutical Actives in Amorphous Polymer Matrices by Electron Energy-Loss Spectroscopy

We demonstrated the use of electron energy-loss spectroscopy (EELS) to evaluate the composition of phenytoin:hydroxypropyl methylcellulose acetate succinate (HPMCAS) spin-coated solid dispersions (SDs). To overcome the inability of bright-field and high-angle annular dark-field TEM imaging to distinguish between glassy drug and polymer, we used the π-π* transition peak in the EELS spectrum to detect phenytoin within the HPMCAS matrix of the SD. The concentration of phenytoin within SDs of 10, 25, and 50 wt % drug loading was quantified by a multiple least-squares analysis. Evaluating the concentration of 50 different regions in each SD, we determined that phenytoin and HPMCAS are intimately mixed at a length scale of 200 nm, even for drug loadings up to 50 wt %. At length scales below 100 nm, the variance of the measured phenytoin concentration increases; we speculate that this increase is due to statistical fluctuations in local concentration and chemical changes induced by electron irradiation. We also performed EELS analysis of an annealed 25 wt % phenytoin SD and showed that the technique can resolve concentration differences between regions that are less than 50 nm apart. Our findings indicate that EELS is a useful tool for quantifying, with high accuracy and sub-100 nm spatial resolution, the composition of many pharmaceutical and soft matter systems.

Dynamic heterogeneity in fully miscible blends of polystyrene with oligostyrene

Binary blends of polystyrene with oligostyrene are perfectly miscible (χ=0) yet dynamically heterogeneous. This is evidenced by independent probing of the dipole relaxation perpendicular to the backbone by dielectric spectroscopy and molecular dynamics. The self-concentration model with a single intramolecular length scale qualitatively describes the slower segmental dynamics. A quantitative comparison based on MD, however, requires a composition-dependent length scale. The pertinent dynamic length scale that best describes the slow segmental dynamics in miscible blends relates to both intra- and intermolecular contributions.

Role of distributions of intramolecular concentrations on the dynamics of miscible polymer blends probed by molecular dynamics simulation

Using molecular dynamics simulations we show that a given monomer in a miscible polymer blend experiences broad distributions of both connectivity driven self-concentrations and thermodynamically controlled intermolecular concentration fluctuations. While these distributions should play a significant role in determining the constituent's dynamics across the whole concentration range, the distribution of self-concentrations is particularly important in the dilute limit, where intermolecular concentration fluctuations should be absent. These conclusions allow us to rationalize the recent literature results that report the apparent self-concentration determined in the dilute limit surprisingly depended on the blend partner.

Segmental dynamics in miscible polymer blends: recent results and open questions

In this short review we summarize the outcome of the large amount of effort made during the past decade from both the experimental and the theoretical point of view in order to understand the effect of blending on the segmental dynamics in polymers. Each of the two families of models proposed-one based on thermally activated concentration fluctuations, the other on chain connectivity effects-account for each of the two main experimental observations: the broadening of the component response with respect to that of the homopolymer and the dynamic heterogeneity, respectively. The complementarity of these approaches, their main achievements and failures, are critically revised. We also include recent results on blends of components with very different mobilities. In the neighbourhood of the glass-transition of the slow polymer, the dynamics of the other component seem to be confined within the frozen chains. We suggest possible ingredients and new routes to be considered in order to elaborate more predictive theoretical frameworks for all these phenomena.

Dynamics of a poly(ethylene oxide) tracer in a poly(methyl methacrylate) matrix: Remarkable decoupling of local and global motions

The tracer diffusion coefficient of unentangled poly(ethylene oxide) (PEO, M=1000 gmol) in a matrix of poly(methyl methacrylate) (PMMA, M=10 000 gmol) has been measured over a temperature range from 125 to 220 degrees C with forced Rayleigh scattering. The dynamic viscosities of blends of two different high molecular weight PEO tracers (M=440 000 and 900 000 gmol) in the same PMMA matrix were also measured at temperatures ranging from 160 to 220 degrees C; failure of time-temperature superposition was observed for these systems. The monomeric friction factors for the PEO tracers were extracted from the diffusion coefficients and the rheological relaxation times using the Rouse model. The friction factors determined by diffusion and rheology were in good agreement, even though the molecular weights of the tracers differed by about three orders of magnitude. The PEO monomeric friction factors were compared with literature data for PEO segmental relaxation times measured directly with NMR. The monomeric friction factors of the PEO tracer in the PMMA matrix were found to be from two to six orders of magnitude greater than anticipated based on direct measurements of segmental dynamics. Additionally, the PEO tracer terminal dynamics are a much stronger function of temperature than the corresponding PEO segmental dynamics. These results indicate that the fastest PEO Rouse mode, inferred from diffusion and rheology, is completely separated from the bond reorientation of PEO detected by NMR. This result is unlike other blend systems in which global and local motions have been compared.

Self-confined polymer dynamics in miscible binary blends

The segmental dynamics of PVME within the single-phase state of poly(styrene)/poly(vinyl methyl ether) blends (PS/PVME) was examined by dielectric spectroscopy. A particular attention has been given to the high PS concentration regime. In this latter, rather localized, weakly cooperative motions of the PVME segments are detected at low temperatures, in addition of the secondary relaxation processes. This feature is attributed to confinement effects induced by the PS chains on the PVME.

Out of equilibrium dynamics of poly(vinyl methyl ether) segments in miscible poly(styrene)-poly(vinyl methyl ether) blends

The local dynamics of the low-T(g) component in a polymer blend, dynamically asymmetric poly(styrene)-poly(vinyl methyl ether) (PS-PVME), is studied below the glass transition, via dielectric relaxation spectroscopy. A particular attention has been paid to blends with a high PS content (PS weight fraction higher than 50%). A relaxation process, slower than the localized motions inducing the PVME secondary relaxations, is detected. Even though these blends fall out of equilibrium in this temperature regime, the structural recovery process is not efficient on the time scale of this PVME motional process. This relaxation is attributed to rather localized, weakly cooperative PVME motions resulting from the topological constraints imposed by the frozen PS chains.