Mechanical Properties of Tetrapolyethylene and Tetrapoly(ethylene oxide) Diamond Networks via Molecular Dynamics Simulations

Abstract

The tensile response to uniaxial deformation of polyethylene-based (Tetra-PE) and polyethylene glycol-based (Tetra-PEG) networks of various strand lengths with idealized diamond connectivity have been studied via atomistic molecular dynamics simulations. Tetra-PE and Tetra-PEG diamond networks with the same strand length show comparable maximum extensibility but the Young's moduli and tensile strength of the former are significantly lower than those of the latter, consistent with stronger intersegmental attractions in the amorphous Tetra-PEG networks. The stress-strain curves show that the stress in short-stranded networks increased rapidly and monotonically with strain while for long-stranded networks it increased very little at small strain, in a non-monotonic fashion at intermediate strains, and then very sharply as the limit of extensibility was approached. Spontaneous partial crystallization of a long-stranded Tetra-PE diamond network under supercooling was demonstrated, and the resulting system was used to: (1) Estimate its melting point as the temperature where any crystalline material disappeared abruptly, and (2) show that the presence of crystalline material in the undeformed state leads to higher stress responses upon deformation compared to amorphous samples, a result consistent with experimental observations. The spontaneous crystallization of Tetra-PEG networks at large supercooling was unsuccessful due to the slow motions of the network beads and the prohibitively long crystal nucleation times entailed.iii BIOGRAPHICAL SKETCHThe author, Endian Wang, born in Fujian, China, attended his middle school and high school in He studied and enjoyed sketching and oil painting during middle school and high school, and would like to continue painting in the near future.iv ACKNOWLEDGEMENTS

Swelling and Tensile Properties of Tetra‐Polyethylene glycol via Coarse‐Grained Molecular Models

“…To explore the effect of swelling on mechanical properties of gel networks, the systems were expanded or compressed to the desired swelling degree φ m and equilibrated in either NPT or NVT ensemble simulations at 328 and 343 K for the 10k and 20k tetra‐PEG systems. The resulting gels were then uniaxially deformed to obtain the Young's moduli using a strain rate of fs −1 . The uniaxial deformation simulations can be performed either under constant volume or constant lateral pressure conditions.…”

“…Two CG tetra‐PEG ideal diamond network systems were constructed with the degree of polymerization n = 112 and 224, corresponding to tetra‐functional prepolymers with M w = 10 and 20 kg mol −1 , respectively, to compare with “realistically” cross‐linked gels of the same strand molecular weight. The topological structure of an ideal diamond network has been discussed previously . The network has the mean topological locations of its cross‐links corresponding to the sites of a diamond lattice, and each network strand corresponding to the “bond” between neighbor lattice sites.…”

“…Despite the recent improvements in computer power, fully atomistic simulations of many soft matter systems are still severely limited in terms of accessible time and length scales . Previously, we used a tetra‐PEG ideal diamond network model using a realistic united‐atom force field via molecular dynamics simulations, and studied the dependence of mechanical properties of a dried amorphous tetra‐PEG network on degree of polymerization of the network strand . However, similar atomistic modeling of a tetra‐PEG diamond network in a swollen state would be beyond our current computational resources as these would require the explicit incorporation of a large number of water molecules.…”

“…[36] Previously, we used a tetra-PEG ideal diamond network model using a realistic unitedatom force field [37] via molecular dynamics simulations, and studied the dependence of mechanical properties of a dried amorphous tetra-PEG network on degree of polymerization of the network strand. [38] However, similar atomistic modeling of a tetra-PEG diamond network in a swollen state would be beyond our current computational resources as these would require the explicit incorporation of a large number of water molecules. To overcome these limitations, CG molecular chain models with implicit water interactions calibrated from atomistic, water explicit simulations, reduce substantially the number of molecular degrees of freedom to allow the efficient computation of relevant thermophysical properties of tetra-PEG gels.…”

Strength of Alkane–Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms

“…Alkanes are modeled with two different united-atom (UA) force fields: PYS [14][15][16] and OPLS [17,18]. These force fields have been widely used to investigate the structure, interfacial properties and phase transitions of alkanes [4,12,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Water is modeled with the monatomic water model, mW [38], which has been extensively used to study the structure, thermodynamics, interfacial properties, and phase transitions of water .…”