Chain Retraction in Highly Entangled Stretched Polymer Melts

Stable polydisperse free-standing porous films made by mechanical deformation

Using molecular dynamics simulations, we show that the methodology of making thin stable nanoporous monodisperse films by biaxial mechanical expansion and subsequent cooling into the glassy state, also works for polydisperse films. To test this, a bidisperse polymer system of an equal number of very long (≈72 entanglements) and short (≤4 entanglements) chains with a polydispersity index of 1.80 is considered. The void formation and the development of the local morphology upon expansion, relaxation, and cooling are investigated. As for the monodisperse case, long chains in thin porous polydisperse films extend over several pores, stabilizing the whole morphology. The short chains do not fill up the pores but tend to aggregate inside the polymer matrix and to avoid surface areas and reduce conformational constraints imposed by the surrounding, a scenario very similar to strain-induced segregation between the strained long and relaxed short chains.

Entanglement-Stabilized Nanoporous Polymer Films Made by Mechanical Deformation

We present a new simulation-guided process to create nanoporous materials, which does not require specific chemical treatment and solely relies on mechanical deformation of pure highly entangled homopolymer films. Starting from fully equilibrated freestanding thick polymer melt films, we apply a simple "biaxial expansion" deformation. Upon expansion holes form, which are prevented from growing and coalescing beyond a characteristic size due to the entanglement structure of the melt. We investigate the local morphology, the void formation upon expansion, and their stabilization. The dependence of the average void (pore) size and void fraction (porosity) on the total strain and subsequent relaxation is investigated. Furthermore, the stabilization of the porous structure of the thin expanded films through cooling below the glass transition temperature Tg is discussed.

Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation

A new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters is presented. The method is based on fast mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench far below the glass transition temperature T_g . The process is first designed generally by simulation and then verified for the example of polystyrene films. The methodology does not need any chemical processing, supporting substrate, or self assembly process and is solely based on polymer inherent entanglement effects. Pore diameters are of the order of ten polymer reptation tube diameters. The resulting membranes are stable over months at ambient conditions and display remarkable elastic properties.

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.

Clustering of Entanglement Points in Highly Strained Polymer Melts

Polymer melts undergoing large deformation by elongation are studied by molecular dynamics simulations of bead-spring chains in melts. By applying a primitive path analysis to strongly deformed polymer melts, the role of topological constraints in highly entangled polymer melts is investigated and quantified. We show that the overall, large scale conformations of the primitive paths (PPs) of stretched chains follow affine deformation while the number and the distribution of entanglement points along the PPs do not. Right after deformation, PPs of chains retract in both directions parallel and perpendicular to the elongation. Upon further relaxation we observe a long-lived clustering of entanglement points. Together with the delayed relaxation time this leads to a metastable inhomogeneous distribution of topological constraints in the melts.

Optical Tweezers Microrheology Maps the Dynamics of Strain-Induced Local Inhomogeneities in Entangled Polymers

Optical tweezers microrheology (OTM) offers a powerful approach to probe the nonlinear response of complex soft matter systems, such as networks of entangled polymers, over wide-ranging spatiotemporal scales. OTM can also uniquely characterize the microstructural dynamics that lead to the intriguing nonlinear rheological properties that these systems exhibit. However, the strain in OTM measurements, applied by optically forcing a microprobe through the material, induces network inhomogeneities in and around the strain path, and the resultant flow field complicates the measured response of the system. Through a robust set of custom-designed OTM protocols, coupled with modeling and analytical calculations, we characterize the time-varying inhomogeneity fields induced by OTM measurements. We show that homogenization following strain does not interfere with the intrinsic stress relaxation dynamics of the system, rather it manifests as an independent component in the stress decay, even in highly nonlinear regimes such as with the microrheological large-amplitude-oscillatory-shear (MLAOS) protocols we introduce. Our specific results show that Rouse-like elastic retraction, rather than disentanglement and disengagement, dominates the nonlinear stress relaxation of entangled polymers at micro- and mesoscales. Thus, our study opens up possibilities of performing precision nonlinear microrheological measurements, such as MLAOS, on a wide range of complex macromolecular systems.

Hierarchical modelling of polystyrene melts: from soft blobs to atomistic resolution

We demonstrate that hierarchical backmapping strategies incorporating generic blob-based models can equilibrate melts of high-molecular-weight polymers, described with chemically specific, atomistic models. The central idea is first to represent polymers by chains of large soft blobs (spheres) and efficiently equilibrate the melt on large scales. Then, the degrees of freedom of more detailed models are reinserted step by step. The procedure terminates when the atomistic description is reached. Reinsertions are feasible computationally because the fine-grained melt must be re-equilibrated only locally. We consider polystyrene (PS) which is sufficiently complex to serve method development because of stereo-chemistry and bulky side groups. Our backmapping strategy bridges mesoscopic and atomistic scales by incorporating a blob-based, a moderately coarse-grained (CG), and a united-atom model of PS. We demonstrate that the generic blob-based model can be parameterised to reproduce the mesoscale properties of a specific polymer - here PS. The moderately CG model captures stereo-chemistry. To perform backmapping we improve and adjust several fine-graining techniques. We prove equilibration of backmapped PS melts by comparing their structural and conformational properties with reference data from smaller systems, equilibrated with less efficient methods.