Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session M15: Structural Relaxation in Permanent Crosslinked Polymer NetworksFocus
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Sponsoring Units: DPOLY Chair: Ralm Ricarte, FAMU-FSU College of Engineering Room: Room 207 |
Wednesday, March 8, 2023 8:00AM - 8:12AM |
M15.00001: Nonlinear Network Deformation and Loop Effect Yuan Tian, Michael S Jacobs, Andrey V Dobrynin The characterization of network structure remains one of the unresolved problems of polymer science. To investigate the effect of the network topology and establish contributions from different network structural elements, we developed a forensic-like framework utilizing network response to nonlinear deformations. By applying this approach to coarse-grained networks of phantom strands studied by molecular dynamics simulations, we obtained the degree of polymerization of network strands between crosslinks, quantified the contributions of loops and dangling ends, and established the actual network functionality. In particular, we determined the loop coefficient for phantom networks Cloop = 0.87±0.04, which describes the reduction of the network structural modulus by contributions from different types of loops. Furthermore, our analysis showed that the dangling ends not only influence the density of the stress-supporting strands but also decrease the effective crosslink functionality. |
Wednesday, March 8, 2023 8:12AM - 8:24AM |
M15.00002: Effect of Penetrant Volume on Diffusion in Dense Crosslinked Networks Tsai-Wei Lin, Charles E Sing Passive membranes have been used for separations because they are less energy intensive compared to thermal separations. There remains a challenge to separate molecules with similar molecular weight and intramolecular interactions. Tight flexible polymer networks are proposed to improve separation efficiency because the sub-nm mesh sizes can discriminate differences in geometries of two molecules. Nevertheless, the mechanisms for how tight networks regulate transport for different molecules are not fully understood. We seek to provide a molecular-level understanding of tight network-based selectivity to design materials for separation applications with Molecular Dynamics simulation with the standard bead-spring model for semi-flexible polymers. The model is parametrized for poly-n-butyl-acrylate (PnBA) networks synthesized by experimental collaborators to model dilute spherical penetrant diffusion in the networks. We studied the effect of crosslink on the alpha relaxation time and glass transition temperatures (Tg) for networks, and the results agree with experiments and theory. Diffusion coefficients for spherical penetrants with various diameters were calculated and we observed a stronger exponential dependence of diffusion coefficient and the ratio of system temperature to Tg which is also found in experiment and theory. The results show the importance of penetrant volume in determining its diffusivity and the success of our simple model to capture key physics of molecule transport in dense crosslinked networks at temperatures close to Tg. We also extend the model to study penetrant diffusion in vitrimers, polymer networks with associative dynamic covalent bonds. We aim to understand how the bond exchange mechanism impacts molecular transport. |
Wednesday, March 8, 2023 8:24AM - 8:36AM |
M15.00003: Effect of Crosslink Homogeneity on the Mechanochemical Activation of Elastic Polymer Networks Victoria Kong, Jennifer E Laaser We use force-responsive mechanophores to investigate how network homogeneity impacts the molecular-scale force distributions in elastic polymer networks with different network topologies. Mechanochromic regularly-crosslinked networks are first synthesized by coupling tetrafunctional n-butyl acrylate star polymers with a difunctional naphthopyran crosslinker. Randomly crosslinked networks with comparable crosslink densities are then synthesized by free-radical polymerization of n-butyl acrylate with the same naphthopyran crosslinker, and all samples are characterized via simultaneous tensile tests and optical absorption measurements. We find that networks with randomly and regularly crosslinked topologies exhibit different onsets of mechanochemical activation, even when the bulk moduli of the materials are the same. These results suggest that network uniformity is critical for distributing stress evenly throughout polymer networks, and provides insight into how network structure can be manipulated to prepare materials with well-defined force-responsive properties. |
Wednesday, March 8, 2023 8:36AM - 8:48AM |
M15.00004: Bond Compression upon Chain Extension Liel Sapir, James Brock, Danyang Chen, Qi Liao, Sergey Panyukov, Michael Rubinstein Extending polymer chains entails a strictly positive chain tension. On the level of individual bonds, tension can be either negative or positive, and depends both on chain tension and bulk pressure. In specific polymeric systems this dependence may not be intuitive, whereby chain tension increases while bond tension decreases, or in other terms: the entire chain is extended, while individual bonds are compressed. For example, increasing grafting density of a polymer brush results in chain extension along the direction perpendicular to the grafting surface, while the individual bonds are compressed. A similar phenomenon is also predicted in deformed polymer networks. Upon compression of polymer networks, the extension of chains oriented in the “free” direction increases, while their bonds are getting more compressed. We demonstrate this phenomenon in molecular dynamics simulations and explain it by the fact that the pressure contribution to bond tension is dominant over a wide range of network deformations and polymer brush grafting densities, while chain conformations are still almost Gaussian. |
Wednesday, March 8, 2023 8:48AM - 9:00AM |
M15.00005: Competition Between Phase Separation and Photopolymerization Kinetics During Polymeric Network Development Lauren A Zakrzewski, Yeongsik Kim, Younghan Song, Chang Y Ryu, Chulsung Bae The competition between phase separation and photopolymerization kinetics during photopolymerization-induced phase separation (photo-PIPS), can easily be studied using a multiscale method to visualize the extent of phase separation and its overall effect on the cured material. Our main characterization method for phase separation detection is a UV transmittance experiment performed using a custom-built UV light transmission apparatus. Real time-Fourier transform infrared spectroscopy and scanning electron microscopy are also used as further characterization methods. In the UV transmittance experiment, rapid reductions in transmittance indicate the onset of phase separation due to light scattering. Ultimately, the transmittance reaches a minimum point, at which phase separation is at a maximum, and then recovers. This recovery could be due to photoinitiator consumption; however, the recovery is too large to rely solely on this theory. Our phase separating resin is irradiated with varying durations of UV light, at which point the light is turned off and only turned back on intermittently to probe the system and obtain a transmittance value. This enables the system to be frozen in various states of phase separation and network development. By probing the system, it is seen if the transmittance recovery still occurs without further photoinitiator consumption. This study aims to verify whether photoinitiator consumption is the sole reasoning behind the transmittance recovery. |
Wednesday, March 8, 2023 9:00AM - 9:12AM |
M15.00006: Network Structure Influences Bulk Modulus of Nearly Incompressible Rubbery Polymers Christopher W Barney, Olivia G Long, Matthew E Helgeson, Megan T Valentine Soft materials represent a unique class of matter where the bulk modulus (~1 GPa) exceeds Young's modulus ( |
Wednesday, March 8, 2023 9:12AM - 9:48AM |
M15.00007: Theory of structural relaxation and vitrification in permanent and associating polymer networks Invited Speaker: Ken S Schweizer
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Wednesday, March 8, 2023 9:48AM - 10:00AM |
M15.00008: Effective Medium Theory for Mechanical Phase Transition of Fiber Networks Sihan Chen, Tomer Markovich, Frederick C MacKintosh Disordered fiber networks, such like extracellular matrix of collagen, are responsible for the mechanical properties for cells and tissues. With an average connectivity between 3 to 4 in 3 dimensions, fiber networks do not meet the Maxwell threshold for mechanically stable networks. Such networks are floppy under small deformation, with linear elasticity governed by the bending energy. However, these sub-isostatic networks can become rigid under large strain via a critical phase transition. In this talk we present an effective medium theory (EMT) for this strain-controlled critical phase transition. Our theory describes the non-affine deformation of the disordered networks and predicts the mechanical phase transition using a Landau-type elastic energy. Our theory qualitatively captures the critical behavior with predictions of mean-field critical exponents. |
Wednesday, March 8, 2023 10:00AM - 10:12AM |
M15.00009: Role of terminal groups of cis-1,4-polyisoprene in the formation of physical cross-linking in NR --- All-atom simulation study --- Takashi Taniguchi, Mayank Dixit Hevea natural rubber (NR) consists of 99% cis- polyisoprene with dimethyl allyl-(trans-1,4-isoprene) (ω) and α terminal groups. These terminal groups provide excellent mechanical and physical properties to NR. Oochi et al[1] have elucidated the structures of six types of α terminals of NR by using a solid -state NMR study. We examine four types of cis-1,4- polyisoprene (PI) melt systems with different combinations of dimethyl allyl-(trans-1,4-isoprene)(ω) and hydroxylated isoprene (α6) terminal groups, i.e., pure PI (no terminal) (PII), ω-PI-ω (PIII), ω-PI-α6 (PIIII), and α6-PI-α6 (PIIV). From the analysis of the end-to-end vector autocorrelation function C(t), average relaxation time τ, and self-diffusion coefficients of polymer chains, we found that the presence of a hydrogen bond between α6 residues makes the dynamics of polymer chains slower in PIIII and PIIV melt systems. From the analyses of RDFs and the potentials of mean force (W(r)), the association between α6 terminals in ω-PI-α6 and α6-PI-α6 is significantly stronger than the isoprene−isoprene association in PII and the ω−ω association in PIII. By the analysis based on the obtained potential of the mean field, we calculated the cluster-formation fraction of terminal groups associated in clusters of a given size in the four systems. We found [1] that in the PII and PIII systems no firm cluster formation is observed, but in the ω-PI-α6 and α6-PI-α6 systems, on the other hand, stable clusters of α6 terminals with a size s (2 ≤ s ≤ 5) are observed, which supports the formation of multiple branching points in between polyisoprene chains through their α6 terminals. We also found that the cluster-formation fraction in the α6-PI-α6 system is significantly enhanced compared to that in the ω-PI-α6 system. |
Wednesday, March 8, 2023 10:12AM - 10:24AM |
M15.00010: Discrete polymer network models for stimuli-responsive, biological, and curing networks via free energy minimization over rotations Matthew J Grasinger Polymer network models allow one to model the constitutive relationships of a broader polymer network from the behavior of a single polymer chain (e.g. viscoelastic response to applied forces, applied electromagnetic fields, etc.). These network models have been used to characterize multiscale phenomena in a variety of contexts such as rubber elasticity, soft multifunctional materials, biological materials, and even the curing of polymers. For decades, a myriad of polymer network models have been developed with different representative volume elements (RVEs). Different RVEs consist of collections of differing numbers of chains that are crosslinked in many different ways. To complicate matters further, there are also competing assumptions for how macroscopic variables (e.g. deformation) are related to individual chains within the RVE. |
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