The memorizing capacity of polymers

Advancements in Novel Mechano-Rheological Probes for Studying Glassy Dynamics in Nanoconfined Thin Polymer Films

The nanoconfinement effects of glassy polymer thin films on their thermal and mechanical properties have been investigated thoroughly, especially with an emphasis on its altered glass transition behavior compared to bulk polymer, which has been known for almost three decades. While research in this direction is still evolving, reaching new heights to unravel the underlying physics of phenomena observed in confined thin polymer films, we have a much clearer picture now. This, in turn, has promoted their application in miniaturized and functional applications. To extract the full potential of such confined films, starting from their fabrication, function, and various applications, we must realize the necessity to have an understanding and availability of robust characterization protocols that specifically target thin film thermo-mechanical stability. Being nanometer-sized in thickness, often atop a solid substrate, direct mechanical testing on such films becomes extremely challenging and often encounters serious complexity from the dominating effect of the substrate. In this review, we have compiled together a few important novel and promising techniques for mechano-rheological characterization of glassy polymer thin films. The conceptual background involved in each technique, constitutive equations, methodology, and current status of research are touched upon following a pedagogical tutorial approach. Further, we discussed each technique's success and limitations, carefully covering the puzzling or contradicting observations reported within the broad nexus of glass transition temperature-viscosity-modulus-molecular mobility (including diffusion and relaxation).

An entropy generation approach to the molecular recoiling stress relaxation in thin nonequilibrated polymer films

In polymers, the equilibrium state is achieved when the chains have access to the maximum number of conformational states, which allows them to explore a larger conformational space, leading to an increase in the entropy of the system. Preparation of thin polymer films using the spin-coating technique results in polymer chains being locked in a nonequilibrium state with lower entropy due to possible stretching of chains during the process. Allowing enough time for recovery results in the relaxation of the spin-coating-induced molecular recoiling stress. Annealing such a film generates entropy due to its inherent irreversibility. We employed the dewetting technique to determine the molecular recoiling stress relaxation time in poly-(tertbutyl styrene) thin films. Furthermore, we qualitatively differentiated the metastable states achieved by the polymer film using entropy generation in a relaxing polymer film as an effect of thermal entropy and associated it with the conformational entropy of polymer chains utilizing the molecular recoiling stress relaxation time. This enabled us to explain molecular recoiling stress relaxation using a rather simplistic approach involving segmental level molecular rearrangements in polymer chains by attaining transient metastable states through an entropically activated process driving toward equilibrium.

The memory of thin polymer films generated by spin coating

We present results from isothermal and temperature-sweep creep experiments adapted to filaments which were derived from spin coated and subsequently crumpled thin polystyrene films. Due to the existence of residual stresses induced by preparation, the filaments showed significant shrinkage which we followed as a function of time at various temperatures. In addition, the influence of preparation conditions and subsequent annealing of supported thin polymer films on shrinkage and relaxation behavior was investigated. The temporal evolution of shrinkage revealed a sequence of relaxation regimes. We explored the temperature dependence of this relaxation and compared our observations with published results on drawn melt-spun fibers. This comparison revealed intriguing similarities between both systems prepared along different pathways. For instance, the magnitudes of shrinkage of melt-spun fibers and of filaments from crumpled spin coated polymer films are similar. Thus, our results suggest the existence of generic mechanisms of "forgetting", i.e., how non-equilibrated polymers lose their memory of past processing events.

Fast equilibration mechanisms in disordered materials mediated by slow liquid dynamics

The rate at which a nonequilibrium system decreases its free energy is commonly ascribed to molecular relaxation processes, arising from spontaneous rearrangements at the microscopic scale. While equilibration of liquids usually requires density fluctuations at time scales quickly diverging upon cooling, growing experimental evidence indicates the presence of a different, alternative pathway of weaker temperature dependence. Such equilibration processes exhibit a temperature-invariant activation energy, on the order of 100 kJ mol^-1. Here, we identify the underlying molecular process responsible for this class of Arrhenius equilibration mechanisms with a slow mode (SAP), universally observed in the liquid dynamics of thin films. The SAP, which we show is intimately connected to high-temperature flow, can efficiently drive melts and glasses toward more stable, less energetic states. Our results show that measurements of liquid dynamics can be used to predict the equilibration rate in the glassy state.

Nascent structure memory erased in polymer stretching

Stretching of semicrystalline polymer materials is fundamentally important in their mechanical performance and industrial processing. By means of dynamic Monte Carlo simulations, we compared the parallel stretching processes between the initially bulk amorphous and semicrystalline polymers at various temperatures. In the early stage of stretching, semicrystalline polymers perform local and global melting-recrystallization behaviors at low and high temperatures, while the memory effects occur upon global melting-recrystallization at middle temperatures. However, the final crystallinities, crystalline bond orientations, chain-folding probabilities, residual stresses, and crystallite morphologies at high enough strains appear as the same at each temperature, irrelevant to the initially amorphous and semicrystalline polymers, indicating that the common post-growth melting-reorganization processes determine the final products. In addition, both final products harvest the highest crystallinities in the middle temperature region because the postgrowth stage yields the vast nuclei followed with less extent of crystal growth in the low temperature region and few nuclei followed with large extent of crystal growth in the high temperature region. Our observations imply that a large enough strain can effectively remove the thermal history of polymers, similar to the thermal treatment at a high enough temperature; therefore, the fracture strength of semicrystalline polymers depends upon their final structures in stretching, not related to their nascent semicrystalline structures.

Nonequilibrium free energy during polymer chain growth

During fast diffusion-influenced polymerization, nonequilibrium behavior of the polymer chains and the surrounding reactive monomers has been reported recently. Based on the laws of thermodynamics, the emerging nonequilibrium structures should be characterizable by some “extra free energy” (excess over the equilibrium Helmholtz free energy). Here, we study the nonequilibrium thermodynamics of chain-growth polymerization of ideal chains in a dispersion of free reactive monomers, using off-lattice, reactive Brownian dynamics computer simulations in conjunction with approximative statistical mechanics and relative entropy (Gibbs–Shannon and Kullback–Leibler) concepts. In the case of fast growing polymers, we indeed report increased nonequilibrium free energies Δ Fneq of several kB T compared to equilibrium and near-equilibrium, slowly growing chains. Interestingly, Δ Fneq is a non-monotonic function of the degree of polymerization and thus also of time. Our decomposition of the thermodynamic contributions shows that the initial dominant extra free energy is stored in the nonequilibrium inhomogeneous density profiles of the free monomer gas (showing density depletion and wakes) in the vicinity of the active center at the propagating polymer end. At later stages of the polymerization process, we report significant extra contributions stored in the nonequilibrium polymer conformations. Finally, our study implies a nontrivial relaxation kinetics and “restoring” of the extra free energy during the equilibration process after polymerization.

Fingerprint of semi-crystalline structure memory in the thermal and ionic conduction properties of amorphous ureasil-polyether hybrid solid electrolytes

Correlations among the structure, thermal properties, and ionic conductivity of solid polymer electrolytes (SPEs) were studied using a ureasil-polyethylene oxide (U-PEO) organic-inorganic hybrid prepared according to a simple sol-gel route, employing a low molecular weight PEO macromer (M w = 1900 g mol-1). The behavior of an amorphous sample loaded with lithium triflate (LiTFSI) at an optimum ratio between ether oxygen and lithium (EO/Li+ = 15) was compared with that of a semicrystalline sample prepared without salt loading. The temperature range investigated by differential scanning calorimetry (DSC), Raman spectroscopy, small angle X-ray scattering (SAXS), and complex impedance spectroscopy covered both the glass transition and the melting temperature of the U-PEO. The gauche to trans conformational transformation of the (O-C-C-O)Li+ sequence showed similarity between the temperature evolution of the semi-crystalline U-PEO and amorphous U-PEO:Li+ samples, providing an indication of the local structural memory of crystalline state in the amorphous SPE. The linear thermal expansion of the average correlation distance between the siloxane crosslink nodes and the long-distance period of the lamellar semi-crystalline edifice were determined by SAXS. Comparison of the expansion curves suggested that although the siloxane nodes were excluded from the PEO crystalline edifice, the sharp expansion of the amorphous region between the lamellae during melting permitted modulation of the free volume of the hybrid network. In addition, the temperature-induced Li+-EO decomplexation observed by Raman spectroscopy explained the change of the average activation energy of the conduction process revealed by the different Arrhenius regimes. These results evidence the key role of the ionic conductivity decoupling from the segmental motion of chain pair channels on the improvement of ion mobility through the free volume between chains. This concept may inspire materials chemistry researchers to design optimized structures of polymer electrolytes with minimized structural memory of crystaline building blocks and improved ionic conductivity.

Controlling solvent quality by time: Self-avoiding sprints in nonequilibrium polymerization

A fundamental paradigm in polymer physics is that macromolecular conformations in equilibrium can be described by universal scaling laws, being key for structure, dynamics, and function of soft (biological) matter and in the materials sciences. Here we reveal that during diffusion-influenced, nonequilibrium chain-growth polymerization, scaling laws change qualitatively, in particular, the growing polymers exhibit a surprising self-avoiding walk behavior in poor and θ solvents. Our analysis, based on monomer-resolved, off-lattice reaction-diffusion computer simulations, demonstrates that this phenomenon is a result of (i) nonequilibrium monomer density depletion correlations around the active polymerization site, leading to a locally directed and self-avoiding growth, in conjunction with (ii) chain (Rouse) relaxation times larger than the competing polymerization reaction time. These intrinsic nonequilibrium mechanisms are facilitated by fast and persistent reaction-driven diffusion ("sprints") of the active site, with analogies to pseudochemotactic active Brownian particles. Our findings have implications for time-controlled structure formation in polymer processing, as in, e.g., reactive self-assembly, photocrosslinking, and three-dimensional printing.

Effects of Glass Transition and Structural Relaxation on Crystal Nucleation: Theoretical Description and Model Analysis

In the application of classical nucleation theory (CNT) and all other theoretical models of crystallization of liquids and glasses it is always assumed that nucleation proceeds only after the supercooled liquid or the glass have completed structural relaxation processes towards the metastable equilibrium state. Only employing such an assumption, the thermodynamic driving force of crystallization and the surface tension can be determined in the way it is commonly performed. The present paper is devoted to the theoretical treatment of a different situation, when nucleation proceeds concomitantly with structural relaxation. To treat the nucleation kinetics theoretically for such cases, we need adequate expressions for the thermodynamic driving force and the surface tension accounting for the contributions caused by the deviation of the supercooled liquid from metastable equilibrium. In the present paper, such relations are derived. They are expressed via deviations of structural order parameters from their equilibrium values. Relaxation processes result in changes of the structural order parameters with time. As a consequence, the thermodynamic driving force and surface tension, and basic characteristics of crystal nucleation, such as the work of critical cluster formation and the steady-state nucleation rate, also become time-dependent. We show that this scenario may be realized in the vicinity and below the glass transition temperature, and it may occur only if diffusion (controlling nucleation) and viscosity (controlling the alpha-relaxation process) in the liquid decouple. Analytical estimates are illustrated and confirmed by numerical computations for a model system. The theory is successfully applied to the interpretation of experimental data. Several further consequences of this newly developed theoretical treatment are discussed in detail. In line with our previous investigations, we reconfirm that only when the characteristic times of structural relaxation are of similar order of magnitude or longer than the characteristic times of crystal nucleation, elastic stresses evolving in nucleation may significantly affect this process. Advancing the methods of theoretical analysis of elastic stress effects on nucleation, for the first time expressions are derived for the dependence of the surface tension of critical crystallites on elastic stresses. As the result, a comprehensive theoretical description of crystal nucleation accounting appropriately for the effects of deviations of the liquid from the metastable states and of relaxation on crystal nucleation of glass-forming liquids, including the effect of simultaneous stress evolution and stress relaxation on nucleation, is now available. As one of its applications, this theoretical treatment provides a new tool for the explanation of the low-temperature anomaly in nucleation in silicate and polymer glasses (the so-called "breakdown" of CNT at temperatures below the temperature of the maximum steady-state nucleation rate). We show that this anomaly results from much more complex features of crystal nucleation in glasses caused by deviations from metastable equilibrium (resulting in changes of the thermodynamic driving force, the surface tension, and the work of critical cluster formation, in the necessity to account of structural relaxation and stress effects) than assumed so far. If these effects are properly accounted for, then CNT appropriately describes both the initial, the intermediate, and the final states of crystal nucleation.

Irreversible adsorption of polymer melts and nanoconfinement effects

For almost a decade, growing experimental evidence has revealed a strong correlation between the properties of nanoconfined polymers and the number of chains irreversibly adsorbed onto nonrepulsive interfaces, e.g. the supporting substrate of thin polymer coatings, or nanofillers dispersed in polymer melts. Based on such a correlation, it has already been possible to tailor structural and dynamics properties - such as the glass transition temperature, the crystallization rate, the thermal expansion coefficients, the viscosity and the wettability - of nanomaterials by controlling the adsorption kinetics. This evidence indicates that irreversible adsorption affects nanoconfinement effects. More recently, also the opposite phenomenon was experimentally observed: nanoconfinement alters interfacial interactions and, consequently, also the number of chains adsorbed in equilibrium conditions. In this review we discuss this intriguing interplay between irreversible adsorption and nanoconfinement effects in ultrathin polymer films. After introducing the methods currently used to prepare adsorbed layers and to measure the number of irreversibly adsorbed chains, we analyze the models employed to describe the kinetics of adsorption in polymer melts. We then discuss the structure of adsorbed polymer layers, focusing on the complex macromolecular architecture of interfacial chains and on their thermal expansion; we examine the way in which the structure of the adsorbed layer affects the thermal glass transition temperature, vitrification, and crystallization. By analyzing segmental dynamics of 1D confined systems, we describe experiments to track the changes in density during adsorption. We conclude this review with an analysis of the impact of nanoconfinement on adsorption, and a perspective on future work where we also address the key ideas of irreversibility, equilibration and long-range interactions.