What Length Scales Control the Dynamics of 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.

Dynamics of aqueous peptide solutions in folded and disordered states examined by dynamic light scattering and dielectric spectroscopy

Characterizing the segmental dynamics of proteins, and intrinsically disordered proteins in particular, is a challenge in biophysics. In this study, by combining data from broadband dielectric spectroscopy (BDS) and both depolarized (DDLS) and polarized (PDLS) dynamic light scattering, we were able to determine the dynamics of a small peptide [ε-poly(lysine)] in water solutions in two different conformations (pure β-sheet at pH = 10 and a more disordered conformation at pH = 7). We found that the segmental (α-) relaxation, as probed by DDLS, is faster in the disordered state than in the folded conformation. The water dynamics, as detected by BDS, is also faster in the disordered state. In addition, the combination of BDS and DDLS results allows us to confirm the molecular origin of water-related processes observed by BDS. Finally, we discuss the origin of two slow processes (A and B processes) detected by DDLS and PDLS in both conformations and usually observed in other types of water solutions. For fully homogeneous ε-PLL solutions at pH = 10, the A-DLS process is assigned to the diffusion of individual β-sheets. The combination of both techniques opens a route for understanding the dynamics of peptides and other biological solutions.

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.

Is kinetic polymer arrest very specific to multiwalled carbon nanotubes?

In this study we have assessed, using dielectric relaxation spectroscopy (DRS), the confinement effects of the more mobile chain in partially miscible polymeric blends of PS/PVME (polystyrene/poly(vinyl methyl ether)) in the presence of anisotropically shaped MWCNTs (multiwalled carbon nanotubes). To understand if this confinement effect is very specific to MWCNTs, the characteristic dimensions of which are often close to the radius of gyration of the polymeric chains, a few other particles like spherical silver, stacked clay tactoids and platy graphene sheets at similar weight fractions were also incorporated and systematically studied. The DRS studies reveal that the more mobile chain (here PVME) experiences possibly two different environments in the presence of frozen PS and more importantly in the presence of MWCNTs at temperatures close to and not so far from the blend T_g. The presence of bimodal relaxations with a weak temperature independent faster relaxation in the blends is composition dependent (PS rich blends). Assuming that there are no chemical interactions of PVME with the particles, these confinement effects seem to be very specific to MWCNTs as the bimodal relaxations were completely absent in the case of other nanoparticles. In the case of polymer blends, when two different chains are brought together, a loss in the deformational entropy is expected due to the excluded volume interaction and chain connectivity effects. In the presence of nanoparticles, especially MWCNTs, the polymer coils are subjected to perturbation leading to entropic loss in the system, which determine the miscibility in the blends. The configurational entropy near glass transition was assessed to understand the improved miscibility due to MWCNTs in this particular blend. The length of cooperativity suggests a cooperative motion of PS and PVME over shorter length scales in the case of MWCNTs as compared to other particles. This also hints at perturbed PVME motion in the network of MWCNTs. Taken together, our study reveals that the kinetic PVME arrest results in two different environments and is dependent on the effective concentration of MWCNTs in the blends.

Revealing the dynamic heterogeneity of PMMA/PVDF blends: from microscopic dynamics to macroscopic properties

An effort was made to demonstrate the dynamic heterogeneity of poly(methyl methacrylate) (PMMA)/poly(vinylidene fluoride) (PVDF) blends, where its composition dependence and the role of interphase were probed. Firstly, the composition dependence of thermorheological complexity of PMMA/PVDF blends in the melt was revealed. The molecular entanglement state involving intra- and interchain entanglements was found to govern the scenario of thermorheological complexity. Intriguingly, local heterogeneity was further demonstrated to exist in the melt-state blends with intermediate compositions, and its origin was depicted to be the interphase. The interphase, coupled with unfavourable interchain entanglements in those blends, could explain the reduced viscosity and speed-up relaxations, contributing to the overall thermorheological complexity. Besides, two experimental glass transition temperatures of blends were resolved in view of segment motions in the miscible phase and the crystal-amorphous interphase, and further assessed via the "self-concentration" concept. The presence of a crystal-amorphous interphase, likely leading to three distinct dynamics of segments in blends, was supposed to contribute to the dynamic heterogeneity in segment relaxations for PMMA/PVDF blends in the solid state. Lastly, effects of dynamic heterogeneity on dynamic mechanical properties were also evaluated.

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.

Route to calculate the length scale for the glass transition in polymers

The occurrence of glass transition is believed to be associated to cooperative motion with a growing length scale with decreasing temperature. We provide a route to calculate the size of cooperatively rearranging regions (CRR) of glass-forming polymers combining the Adam-Gibbs theory of the glass transition with the self-concentration concept. To do so we explore the dynamics of glass-forming polymers in different environments. The material specific parameter alpha connecting the size of the CRR to the configurational entropy is obtained in this way. Thereby, the size of CRR can be precisely quantified in absolute values. This size results to be in the range 1-3nm at the glass transition temperature depending on the glass-forming polymer.

"Self-concentration" effects on the dynamics of a polychlorinated biphenyl diluted in 1,4-polybutadiene

The mobility of isolated polychlorinated biphenyl (PCB54) in 1,4-polybutadiene (PB) has been investigated by means of broadband dielectric spectroscopy. The aim was to provide new insights about the effect of the environment on the dynamics of PCB54. The authors' results indicate that PCB54 structural dynamics is neither independent of the PB matrix nor slaved to the matrix itself. The authors interpret these results as a consequence of the limited size of cooperatively rearranging regions (CRRs) involved in PCB54 structural relaxation possessing an effective concentration different from the macroscopic one. This implies a non-negligible influence of "self-concentration," already proven for the component segmental dynamics in polymer blends, also in the relaxation of binary mixtures involving low molecular weight glass formers. This allowed the evaluation of the size of CRR, which was about 1 nm for PCB54 in PB. This means that the cooperativity extends over the first shell around PCB54 molecules.

Glass transition of miscible binary polymer-polymer thin films

The average glass transition temperatures, Tg, of thin homopolymer films exhibit a thickness dependence, Tg(h), associated with a confinement effect and with polymer-segment-interface interactions. The Tg's of completely miscible thin film blends of tetramethyl bisphenol-A polycarbonate (TMPC) and deuterated polystyrene (dPS), supported by SiO(x)/Si, decrease with decreasing h for PS weight fractions phi >0.1. This dependence is similar to that of PS and opposite to that of TMPC thin films. Based on an assessment of Tg(h, phi), we suggest that the Tg(h, phi) of miscible blends should be rationalized, additionally, in terms of the notion of a self-concentration and associated heterogeneous component dynamics.

The single chain limit of structural relaxation in a polyolefin blend

The influence of composition on component dynamics and relevant static properties in a miscible polymer blend is investigated using molecular dynamics simulation. Emphasis is placed on dynamics in the single chain dilution limit, as this limit isolates the role of inherent component mobility in the polymer's dynamic behavior when placed in a blend. For our systems, a biased local concentration affecting dynamics must arise primarily from chain connectivity, which is quantified by the self-concentration, because concentration fluctuations are minimized due to restraints on chain lengths arising from simulation considerations. The polyolefins simulated [poly(ethylene-propylene) (PEP) and poly(ethylene-butene) (PEB)] have similar structures and glass transition temperatures, and all interactions are dispersive in nature. We find that the dependence of dynamics upon composition differs between the two materials. Specifically, PEB (slower component) is more influenced by the environment than PEP. This is linked to a smaller self-concentration for PEB than PEP. We examine the accuracy of the Lodge-McLeish model (which is based on chain connectivity acting over the Kuhn segment length) in predicting simulation results for effective concentration. The model predicts the simulation results with high accuracy when the model's single parameter, the self-concentration, is calculated from simulation data. However, when utilizing the theoretical prediction of the self-concentration the model is not quantitatively accurate. The ability of the model to link the simulated self-concentration with biased local compositions at the Kuhn segment length provides strong support for the claim that chain connectivity is the leading cause of distinct mobility in polymer blends. Additionally, the direct link between the willingness of a polymer to be influenced by the environment and the value of the self-concentration emphasizes the importance of the chain connectivity. Furthermore, these findings are evidence that the Kuhn segment length is the relevant length scale controlling segmental dynamics.

Is there a higher-order mode coupling transition in polymer blends?

We present simulations on a binary blend of bead-spring polymer chains. The introduction of monomer size disparity yields very different relaxation times for each component of the blend. Competition between two different arrest mechanisms, namely, bulklike dynamics and confinement, leads to an anomalous relaxation scenario for the fast component, characterized by sublinear time dependence for mean squared displacements, or logarithmic decay and convex-to-concave crossover for density-density correlators. These anomalous dynamic features, which are observed over time intervals extending up to 4 decades, strongly resemble predictions of mode coupling theory for nearby higher-order transitions. Chain connectivity extends anomalous relaxation over a wide range of blend compositions.

Local and chain dynamics in miscible polymer blends: A Monte Carlo simulation study

Local chain structure and local environment play an important role in the dynamics of polymer chains in miscible blends. In general, the friction coefficients that describe the segmental dynamics of the two components in a blend differ from each other and from those of the pure melts. In this work, we investigate polymer blend dynamics with Monte Carlo simulations of a generalized bond fluctuation model, where differences in the interaction energies between nonbonded nearest neighbors distinguish the two components of a blend. Simulations employing only local moves and respecting a no bond crossing condition were carried out for blends with a range of compositions, densities, and chain lengths. The blends investigated here have long time dynamics in the crossover region between Rouse and entangled behavior. In order to investigate the scaling of the self-diffusion coefficients, characteristic chain lengths N(c) are calculated from the packing length of the chains. These are combined with a local mobility mu determined from the acceptance rate and the effective bond length to yield characteristic self-diffusion coefficients D(c)=muN(c). We find that the data for both melts and blends collapse onto a common line in a graph of reduced diffusion coefficients DD(c) as a function of reduced chain length NN(c). The composition dependence of dynamic properties is investigated in detail for melts and blends with chains of length N=20 at three different densities. For these blends, we calculate friction coefficients from the local mobilities and consider their composition and pressure dependence. The friction coefficients determined in this way show many of the characteristics observed in experiments on miscible blends.

Describing the component dynamics in miscible polymer blends: Towards a fully predictive model

We have recently proposed [D. Cangialosi et al., J. Chem. Phys. 123, 144908 (2005)] an extension of the Adam-Gibbs [J. Chem. Phys. 43, 139 (1965)] theory, combined with the concept of self-concentration, to describe the temperature dependence of the relaxation time for the component segmental dynamics in miscible polymer blends. Thus, we were able to obtain the dynamics of each component in the blend starting from the knowledge of the dynamic and thermodynamic data of the pure polymers, with a single fitting parameter (alpha) which had to be obtained from the fitting of the experimental data. In the present work we demonstrate that this model is also suitable to describe the polymer segmental dynamics in concentrated polymer solutions. From this result we have developed a new route for determining the value of the alpha parameter associated with any given polymer. Once this value is known for the two components of a possible polymer blend, our model for polymer blends dynamics becomes fully predictive.