The Effect of Intrachain Cross-Linking on the Thermomechanical Behavior of Bulk Polymers Assembled Solely from Single Chain Polymer Nanoparticles (original) (raw)
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Cross-linked polymers in strain: Structure and anisotropic stress
arXiv (Cornell University), 2011
Molecular dynamic simulation enables one to correlate the evolution of the microstructure with anisotropic stress when a material is subject to strain. The anisotropic stress due to a constant strain-rate load in a cross-linked polymer is primarily dependent on the mean-square bond length and mean-square bond angle. Excluded volume interactions due to chain stacking and spatial distribution also has a bearing on the stress response. The bond length distribution along the chain is not uniform. Rather, the bond lengths at the end of the chains are larger and uniformly decrease towards the middle of the chain from both ends. The effect is due to the presence of cross-linkers. As with linear polymers, at high density values, changes in mean-square bond length dominates over changes in radius of gyration and end-to-end length. That is, bond deformations dominate over changes in size and shape. A large change in the mean-square bond length reflects in a jump in the stress response. Short-chain polymers more or less behave like rigid molecules. Temperature has a peculiar effect on the response in the sense that even though bond lengths increase with temperature, stress response decreases with increasing temperature. This is due to the dominance of excluded volume effects which result in lower stresses at higher temperatures. At low strain rates, some relaxation in the bond stretch is observed from = 0.2 to = 0.5. At high strain rates, internal deformation of the chains dominate over their uncoiling leading to a rise in the stress levels.
Polymer, 2018
To establish relationships between the molecular structure of polyolefines and their physical characteristics which determine possible commercial applications, structural changes and tensile deformation response up to deformations beyond the natural draw ratio were investigated using a variety of experimental approaches. True stress-strain curves were measured at different temperatures so as to estimate the available effective network density, which will eventually define the failure mode of the material under investigation. Analysis of the deformation by means of tensile strain hardening, assuming the Haward-Thackray spring dashpot decoupling assumption by means of Edward-Vilgis' non-Gaussian rubber-elastic slip-link model, reveals the role of transient and fixed network nodes. It was established by differential scanning calorimetry and X-ray diffraction analysis that the transformation from lamellar to fibrillar morphology passes through the several pronounced stages: deformation of initial lamellae ( < 1.5); destruction of lamellar structure through the tilt; slippage of molecules in the crystallites; simultaneous formation of fibrils with structural characteristics depending on the molecular structure and on deformation conditions; deformation of the formed fibrillar structure; tiltingformation of chevrons for high molecular weight low density polyethylene or slippage of fibrils and void formation. Distinction between fixed and transient slip link network contributions reveals neatly that although there is a slight drop in the fixed link network density with increasing temperature, this contribution remains Manuscript Click here to view linked References
Tensile Stress Generation on Crystallization of Polymer Networks
ACS Applied Polymer Materials, 2019
When crystallized under constant strain, conventional semicrystalline networks exhibit a loss in tension due to directionally preferred crystallization of chains along the strain axis. This study shows that a dual-cured, semicrystalline polymer network with programmed chain bias can generate ~400 kPa of tensile stress upon crystallization. Constant strain experiments were conducted to monitor stress in configurationally biased networks as they were cooled and crystallized. Time resolved wide angle X-ray scattering was used to study crystallization kinetics and the evolution of crystallite orientation. In configurationally biased networks without external load, initial crystallization along the internal chain bias direction is rapidly followed by crystal growth in the orthogonal direction. Under external loads, crystallization first occurs along the external load direction followed by the configurational chain bias direction. Calorimetry measurements indicate that two distinct crystal populations form, upon cooling, with different crystallization temperatures. In summary, embedding chain bias can act as a source of tension. The magnitude of tensile stress can be controlled though different crystallization
Macromolecules, 2013
Stress−strain relations and strain-induced crystallization (SIC) of unvulcanized and vulcanized states of natural rubber (NR) and synthetic polyisoprene (IR) were studied using synchrotron X-ray at various temperatures from −50 to +75°C. Unvulcanized IR is a polymer melt that shows a viscous response with yield stress that is related to entanglement and no SIC at 25°C. However, unvulcanized IR shows SIC at 0, −25, and −50°C. Entanglements in unvulcanized IR become pivots to align chains and induce crystals at low temperatures. On the other hand, unvulcanized NR shows SIC and stress upturns in stress−strain relations at 25°C. Since a permanent set is observed after large extension and retraction, unvulcanized NR has a pseudo end-linked network. The pseudo end-linked networks make entanglements as permanent entanglements and show stress upturn and SIC. Vulcanization makes IR to a rubber which shows a stress upturn and SIC by chemical bond network. The stress of vulcanized NR and IR appear almost the same at strains less than 3.0, however the stress of vulcanized NR is much higher than vulcanized IR beyond strain 3.0. The onset strain of SIC of vulcanized NR is much smaller than vulcanized IR. This different behavior is caused by the pseudo end-linked network. The stress in stress−strain relation at higher temperatures is significantly lower than the stress at lower temperatures. This tendency does not seem to follow the theory of rubber elasticity. The onset of SIC delays the upturn of stress as a shoulder or plateau in the stress−strain relation. SIC contributes to the stress, even though the stress smoothly increases with strain. At higher strain, SIC become big network points to bind many chains and reduce the limit of extensibility. The effect of SIC and the limited extensibility to the stress is not distinguishable.
Polymer, 2015
Crosslinked polymer formation commonly occurs when two or more multi-functional precursors react to form a three-dimensional network. The resulting networks may contain a significant number of topological imperfections such as loops or dangling ends when formed using crosslinkers with high functionality or when crosslinking at high temperatures. We employ molecular dynamics simulations to analyze these topological imperfections in coarse-grained networks generated from precursors consisting of 'chain extenders' composed of two beads (dimers) and a crosslinker of functionality f = 3 or 6 for a wide range of crosslinking temperatures and final conversions. It is shown that these imperfections result in networks in which the number of elastically active chains, the cycle rank and the number of elastically active junctions is smaller than predicted by the Miller-Macosko theory. Such defects must adversely affect the mechanical properties, resistance to solvent swelling and, possibly, the long-term protective properties of polymer networks.
Probing Rubber Cross-Linking Generation of Industrial Polymer Networks at Nanometer Scale
The journal of physical chemistry. B, 2016
We present improved analyses of rheometric torque measurements as well as (1)H double-quantum (DQ) nuclear magnetic resonance (NMR) buildup data on polymer networks of industrial compounds. This latter DQ NMR analysis allows finding the distribution of an orientation order parameter (Dres) resulting from the noncomplete averaging of proton dipole-dipole couplings within the cross-linked polymer chains. We investigate the influence of the formulation (filler and vulcanization systems) as well as the process (curing temperature) ending to the final polymer network. We show that DQ NMR follows the generation of the polymer network during the vulcanization process from a heterogeneous network to a very homogeneous one. The time variations of microscopic Dres and macroscopic rheometric torques present power-law behaviors above a threshold time scale with characteristic exponents of the percolation theory. We observe also a very good linear correlation between the kinetics of Dres and rhe...
The influence of cross-link coordination on the mechanical properties of polymers
2020
Reversible cross-linking is a powerful strategy employed by nature to specifically tailor the mechanical performance of load-bearing polymeric structures. These cross-links are found in a variety of biological systems such as bone, silk or mussel threads. They provide an efficient toughening mechanism due to hidden length unraveling and repeated rupture and reformation of cross-links during the course of deformation. In this dissertation, the main objective is to investigate the influence of cross-link coordination on the mechanical properties of polymeric structures. Three different structures are investigated: a single linear chain, fiber bundles and a stiff random network. The aim is to contrast the deformation behavior of polymers cross-linked with two-fold coordinated cross-links only (the "classical" system) with the behavior of system where three-fold coordination of cross-links is energetically most favorable. The inspiration for the current study stems from variou...
Journal of Materials Research, 2010
Mechanical and thermodynamical properties of bulk polyethylene have been scrutinized using coarse-grained (CG) molecular dynamics simulations. Entangled but cross-link-free polymer clusters are generated by the semicrystalline lattice method for a wide range chain length of alkane modeled by CG beads, and tested under compressive and tensile stress with various temperature and strain rates. It has been found that the specific volume and volumetric thermal expansion coefficient decrease with the increase of chain length, where the specific volume is a linear function of the bond number to all bead number ratios, while the thermal expansion coefficient is a linear rational function of the ratio. Glass-transition temperature, however, does not seem to be sensitive to chain length. Yield stress under tension and compression increases with the increase of the bond number to all bead number ratio and strain rate as well as with decreasing temperature. The correlation found between chain l...