Molecular Dynamics Studies of Entropic Elasticity of Condensed Lattice Networks Connected with Uniform Functionality f = 4

To study the linear region of entropic elasticity, we considered the simplest physical model possible and extracted the linear entropic regime by using the least squares fit and the minimum...

Crosslinked Elastomers: Structure–Property Relationships and Stress-Optical Law

We present a combination of independent techniques in order to characterize crosslinked elastomers. We combine well-established macroscopic methods, such as rheological and mechanical experiments and equilibrium swelling measurements, a more advanced technique such as proton multiple-quantum NMR, and a new method to measure stress-induced segmental orientation by in situ tensile X-ray scattering. All of these techniques give access to the response of the elastomer network in relation to the crosslinking of the systems. Based on entropic elasticity theory, all these quantities are related to segmental orientation effects through the so-called stress-optical law. By means of the combination of these techniques, we investigate a set of unfilled sulfur-vulcanized styrene butadiene rubber elastomers with different levels of crosslinking. We validate that the results of all methods correlate very well. The relevance of this approach is that it can be applied in any elastomer materials, including materials representative of various industrial application, without prerequisite as regards, e.g., optical transparency or simplified formulation. Moreover, the approach may be used to study reinforcement effects in filled elastomers with nanoparticles.

Polymer Networks for Enrichment of Calcium Ions

In this study, solvogels containing (2-((2-(ethoxycarbonyl)prop-2-en-1-yl)oxy)-ethyl) phosphonic acid (ECPA) and N,N′-diethyl-1,3-bis-(acrylamido)propane (BNEAA) as the crosslinker are synthesized by UV induced crosslinking photopolymerization in various solvents. The polymerization of the ECPA monomer is monitored by the conversion of double bonds with in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The morphology of the networks is characterized by in situ photorheology, solid state NMR spectroscopy, and scanning electron microscopy (SEM) of the dried gels. It is demonstrated that the storage modulus is not only determined by the crosslinker content in the gel, but also by the solvent used for preparation. The networks turn out to be porous structures with G′ being governed by a rigid, phase-separated polymer phase rather than by entropic elasticity. The external and internal pKa values of the poly(ECPA-co-BNEAA) gels were determined by titration with a specially designed method and compared to the calculated values. The polymer-immobilized phosphonic acid groups in the hydrogels induce buffering behavior into the system without using a dissolved buffer. The calcium accumulation in the gels is studied by means of a double diffusion cell filled with calcium ion-containing solutions. The successful accumulation of hydroxyapatite within the gels is shown by a combination of SEM, energy-dispersive X-ray spectroscopy (EDX) and wide-angle X-ray scattering (WAXS).

Rheological and Rheo-optical Behaviors of Nanocellulose Suspensions Containing Unfibrillated Fibers

Cellulose nanofibers (CNFs) produced by mechanical processing have a more uneven fiber shape, diameter, and length than those produced by chemical processing. Depending on the manufacturing conditions, CNFs containing insufficient fibrillated fibers may be produced. In order to find practical applications for CNFs containing unfibrillated fibers, it is important to understand how to control the rheological behavior of these systems. In this study, we investigated the relationship between the nanosized volume fraction and the rheological behaviors of CNF suspensions containing unfibrillated fibers prepared by a wet refining system (Water Jet System). The macroscopic structural changes in those suspensions under shear flow were also discussed based on rheo-optic measurements. According to the frequency sweeps of the CNF suspensions, it was found that they were elastic-dominated gels, and the elasticity was attributed to the nanofibers. The elastic moduli increased with the volume fraction of the nanofibers, suggesting that the entanglement of the nanofibers was enhanced. The pseudo-plateau modulus Gp' is proportional to the nanofiber volume fraction, with the constant α = 1.5, suggesting that the entropic elasticity is dominant. The viscosity curves of the CNF suspensions showed a shear thinning behavior, in which the viscosity linearly decreased with the increasing shear rate. From the Rheo-SALS measured at the same time, we found that the aggregates of the nanofibers elongated in the flow direction and deformed into an elliptical shape with the applied shearing. The shape change of the aggregates comprised of the nanofibers became more pronounced with the increased nanofiber volume fraction. However, the effect of the shape change of the aggregates was hardly observed on the viscosity curve. We speculate that this is due to the fact that the unnanosized fibers, which exhibit a Newtonian flow, play a significant role in the flow behavior of the CNF suspensions.

ORIGIN OF ENERGETIC ELASTICITY AND ENTROPIC ELASTICITY OF NATURAL RUBBER WITH NANODIAMOND NANOMATRIX STRUCTURE

ABSTRACT The origin of energetic elasticity in conjunction with the entropic elasticity for natural rubber with a nanodiamond nanomatrix structure was investigated in terms of bound rubber formed between nanodiamonds, based on the interaction between natural rubber and nanodiamonds inside the nanomatrix. The natural rubber with a nanodiamond nanomatrix structure was prepared by reacting nanodiamonds with deproteinized natural rubber in the presence of tert-butylhydroperoxide/tetraethylenepentamine at 30 °C in the latex stage followed by drying. Morphology of the products was observed by two-dimensional and three-dimensional transmission electron microscopies. The effect of bound rubber on the mechanical properties of the products was investigated by measurements of the dynamic mechanical properties and differential scanning calorimetry. The contribution of bound rubber was estimated by combining the Takayanagi equation and modified Guth–Gold equation. A significant increase in complex modulus was attributed to the effect of the bound rubber.

Mechanically strong and resilient shape memory polyurethane with hexamethylene diisocyanate as mixing segment

Mechanical robustness and flexibility of shape memory polyurethane (SMPU) make them a prominent candidate in various field. However, the shape memory characteristics are hampered due to the lower breaking stress and strain originating from the slippage of hard segments during deformation and entropic elasticity of the segments. Herein, SMPU is synthesised by modification of Polycaprolactone diol (PCL) based soft segment by introducing a linear chain diisocyante, that is hexamethylene diisocyanate (HDI) as the mixing segment and rigid MDI (4,4′-methylene bis-phenyl diisocyanate) as the hard segment. The HDI based soft segment is expected to improve the chain flexibility, and MDI will retain the strength factor. The SMPU is characterised by chemical, structural and thermal analysis. The stress relaxation behaviour of the film was analysed w.r.t time and correlated with recovery studies using the Maxwell model. The thermomechanical conditions are optimised to attain higher shape fixity (SF) and shape recovery (SR) and the SMPU shows maximum SF (60.8%) and SR (97%) at 70°C temperature and 50% strain condition. Also, SMPU shows the tensile strength of 23.4 MPa with elongation at break of nearly 1270%. Thus, the combination of both diisocyanate and soft segments imparts strength and ductility to the SMPU.

Tensile and torsional elastomer fiber artificial muscle by entropic elasticity with thermo-piezoresistive sensing of strain and rotation by a single electric signal

Artificial muscles are developed by using twisted natural rubber fiber coated with buckled carbon nanotube sheet, which show tensile and torsional actuations and sensing function via the resistance change by a single electric signal.