Bimodal Chain Length Distribution Research Articles

Highly Stretchable, Soft, and Clear Viscoelastic Film with Good Recoverability for Flexible Display.

The advancement of flexible electronic devices has prompted new material development for the display application. For flexible display, a suitable clear viscoelastic film (CVF) is essential to bond different layers in a display stack in order to improve the visualization and durability during the repeated folding process. However, it is challenging to integrate different properties in the CVF by overcoming many contradictory requirements, such as low modulus/glass transition temperature (Tg) and high adhesion or high recoverability and good stress-relaxation. In this work, a CVF was prepared using an interpenetrating polymer network (IPN) with bimodal chain length distribution, and it exhibited several favorable properties. The bimodal elastomer was composed of short-chain polyurethane (PU) and long-chain polyacrylate. The long-chain polyacrylate network provided a large amount of entanglement that conferred stretchability, adhesion, and stress-relaxation, whereas the short PU chain network acted as an entropy spring and contributed mostly to the recoverability. The experimental data suggested the presence of a hydrogen-bonding interaction and interlocked polymer chains between the two networks. When the components of the IPN are adjusted, the CVF can simultaneously achieve good stress-relaxation, high strain recovery at large strain (1000%), high toughness, clarity, and adhesion. Moreover, the CVF displayed low glass transition temperature (-57 °C) and low storage modulus (20 to 30 kPa at room temperature). To the best of our knowledge, this is the first report using the IPN concept to prepare a CVF with well-balanced properties.

All-Around Universal and Photoelastic Self-Healing Elastomer with High Toughness and Resilience.

Ultimately soft electronics seek affordable and high mechanical performance universal self‐healing materials that can autonomously heal in harsh environments within short times scales. As of now, such features are not found in a single material. Herein, interpenetrated elastomer network with bimodal chain length distribution showing rapid autonomous healing in universal conditions (<7200 s) with high efficiency (up to 97.6 ± 4.8%) is reported. The bimodal elastomer displays strain‐induced photoelastic effect and reinforcement which is responsible for its remarkable mechanical robustness (≈5.5 MPa stress at break and toughness ≈30 MJ m−3). The entropy‐driven elasticity allows an unprecedented shape recovery efficiency (100%) even after fracturing and 100% resiliency up to its stretching limit (≈2000% strain). The elastomers can be mechanically conditioned leading to a state where they recover their shape extremely quickly after removal of stress (nearly order of magnitude faster than pristine elastomers). As a proof of concept, universal self‐healing mechanochromic strain sensor is developed capable of operating in various environmental conditions and of changing its photonic band gap under mechanical stress.

1H multiple-quantum nuclear magnetic resonance investigations of molecular order in polymer networks. II. Intensity decay and restricted slow dynamics.

We present an approach towards the analysis of the intensity decay in proton multiple-quantum experiments on polymeric networks in terms of slow fluctuations of the residual dipole-dipole coupling tensor. Solutions for individual spin pairs as well as the three-spin system of methyl groups are derived, and the influence of the cycle time of the multiple-quantum pulse sequence is evaluated. The multiple-quantum strategy discussed herein features the advantage that the magnitude of the fluctuating part of the residual dipole-dipole coupling constant and the correlation time of the slow process can be determined independently of the integral residual coupling constant as well as its distribution. The theory is applied to experiments on end-linked poly(dimethylsiloxane) model networks with mono- and bimodal chain length distributions, where it is found that, for all samples, correlation times of the slow processes average to about 1 ms, and that the magnitude of the fluctuating part of the dipole-dipole coupling is significantly smaller than the average dipole-dipole coupling constant. This observation is interpreted in terms of considerably restricted reorientations of topological constraints.

A simulation study of lyotropic isotropic–nematic phase transitions in polydisperse chain systems

The isotropic–nematic (I–N) phase behavior of athermal chain-length polydisperse systems was investigated by molecular simulation. The approach adopted involved the formulation of a novel semigrand Gibbs-type of ensemble for polydisperse chain systems, the use of extrapolation methods (like histogram reweighting) to target polydispersities of interest, and the implementation of both intermutation moves among species and expanded-ensemble moves (for the transfer of the shortest chains) to attain chemical potential equilibration. This approach was first used to illustrate how the results of a system with bimodal polydispersity distribution can be used to get coexistence data for a bidisperse system of short and long chains, thereby circumventing the need to insert/delete the longest molecules. Simulations were then used to examine the effect of osmotic pressure (concentration) and the shape of the chain-length polydispersity function on the I–N phase transitions. In agreement with the predictions of Flory theory, the phase behavior of a unimodal (Poisson) parent distribution showed significant partitioning of the components between the coexisting phases, wherein longer chains concentrate in the nematic phase. Within the biphasic region, the order parameter in the nematic phase increased with pressure despite the growing proportion of shorter chains. Very short chains, like dimers and trimers, were found to be disordered in the nematic phase. Partitioning effects are more pronounced when the parent system had a flatter or a bimodal chain-length distribution. Flory theory gives a good description of the partition effects, but significantly overestimates the coexisting concentrations and the nematic ordering.

Evaluation of the Chain Length Distribution in Free-Radical Polymerization, 2. Emulsion Polymerization

Emulsion polymerization is characterized by the segregation of the active chains in the polymer particles, which can strongly affect the kinetics of the process and in particular the chain length distribution (CLD). To correctly take into account these effects, a model has been developed which is constituted by a set of population balance equations based on the well-known concept of distinguished particle distribution. The model has been solved with the discretization technique developed in the first part of the work, which allows the directly evaluation of the CLD. The obtained model has been used to calculate the CLD resulting from complex polymerization kinetic mechanisms, such as, for example, those of living free-radical polymerization. In all cases rather accurate results have been obtained, including situations characterized by bimodal CLDs, where indirect methods, typically based on the method of moments, fail.

Synthesis and investigation of polyphenyldimethylsiloxane block copolymers with bimodal chain length distribution of oligodimethylsiloxane blocks

Siloxane block copolymers containing linear dimethylsiloxane (DMS) and cyclolinear phenylsilsesquioxane blocks were synthesized. A peculiarity of the copolymers is a bimodal distribution of linear DMS blocks in the polymer chain. The results of X-ray diffraction. thermomechanical, and DSC studies of bimodal block copolymers indicate a higher degree of microphase separation of the blocks as compared to unimodal block copolymers.

Toughness of polymeric networks: The limited importance of crosslink density

AbstractFour series of polymeric model networks were prepared with bimodal chain length distribution between crosslink points and two types of dangling chains as network defects. In the last series the crosslink density was changed without a large change in the chemical composition. The fracture toughness of those networks were compared with that of the defect–free networks.The fracture toughness of the various networks is surprisingly little influenced by the introduction of defects. Neither bimodality, nor dangling chains, nor a high sol fraction alters the toughness of the network. A good correlation between KIc and the weight fraction of polyether is observed. A much smaller dependence of KIc on the strand density can be deduced. The yield stress is high and approximately invariant for all systems studied.It is concluded that the toughness of a polymeric network does not seem to be influenced by its perfection and only to a small extent by its degree of crosslinking.

Elastomeric Networks with Bimodal Chain-Length Distributions

Elastomeric Networks with Bimodal Chain-Length Distributions

Polysiloxane elastomers with bimodal chain-length distributions

Most elastomers are prepared by an adventitious, random cross-linking process and thus have a broad, unimodal distribution of network chain lengths. It is possible, however, to obtain materials of controlled chain-length distribution by restricting the reactivity of the chains to their ends, and then end linking these chains with a multi-functional reactant. The networks of this type that have proved to be of greatest interest consist of short chains end linked with long chains to yield abimodal distribution of network chain lengths. These bimodal networks have unusually high extensibility for their values of the modulus and ultimate strength, and thus considerable toughness. Most such elastometers have been prepared from chains of poly(dimethylsiloxane), by carrying out either a condensation reaction between hydroxyl-terminated chains and tetraethoxysilane, or an addition reaction between vinyl-terminated chains and a poly(methylhydrogen siloxane) oligomer. The present review discusses the preparation of such materials, the characterization of some of their properties, and the interpretation of these properties in terms of the molecular theories of rubberlike elasticity.

Torsion Studies of Thermoelasticity and Stress–Strain Isotherms of Unimodal, Bimodal, and Filled Networks of Poly(dimethylsiloxane)

End-linking reactions were used to convert chains of poly(dimethylsiloxane) (PDMS) into (i) unfilled networks having either unimodal or bimodal chain-length distributions, or (ii) filled networks in which either a separately-prepared zeolite had been blended into the polymer prior to end linking, or silica was precipitated into the networks afterwards. The resulting elastomers were studied in torsion, the aspect of particular interest in the case of the unfilled networks being the fraction Me/M of the total couple which is due to changes in conformational energy. The values of Me/M were found to be independent of crosslink density, extent of torsional strain, and extent of any elongation imposed during the torsion. The values for the bimodal networks, however, were much smaller than those for the unimodal ones, presumably because of the non-Gaussian nature of the short chains. None of the unimodal or bimodal unfilled networks showed any upturn in modulus at high torsional deformations similar to the upturns shown by bimodal networks in elongation, biaxial extension, and shear. Neither did the filled networks, but they did show significant increases in modulus in general, thus demonstrating reinforcement at least in this regard.

Effect of bimodal chain length distribution on the thermal characteristics of poly(ethylene oxide) end-linked networks

Poly(ethylene oxide) (PEO) end-linked networks have been prepared with bimodal distributions of molar masses between crosslinks ( M c). The thermal characteristics of the PEO networks have been investigated in the unstretched state over the temperature range 173–373 K using differential scanning calorimetry. The elastomer with the lowest M c (205 g mol −1) showed a much higher glass transition temperature ( T g) than the infinite molar mass value T g(∞) for PEO. Although it is known that molecular topology affects T g, this system involves incorporating bulky junction points, thus also causing a significant chemical change upon curing. The mechanical and swelling properties of the bimodal PEO networks have suggested that strain-induced crystallization may be facilitated by the relatively stiff short network chains acting as nucleation sites for crystallization in the stretched state. In the present study, however, it is seen that the extent of crystallization of the PEO networks decreases with an increase in short chain content in the unstretched state.

Properties of stretched trans(CH) x systems: Analysis of polarized resonant Raman scattering

Polarized Resonant Raman Scattering (RRS) spectra of trans (CH) x taken at different excitation wavelengths ( λ L=647 nm and 458 mm) have been recorded for oriented samples obtained by using a new catalyst for the synthesis. The experimental results display a similar behaviour to that of RRS spectra recently analyzed. In particular, the ‖ ‖ spectra at λ L=458 nm show two bands with a double peaked sufficiently well resolved structure, due to the contribution of long and short conjugation length segments. On the contrary, the |; ‖, ‖ |; , |; |; polarized spectra only display two structureless bands peaked at higher frequencies with respect to the previous ones. These results confirm the theoretical predictions. Oriented trans (CH) x doped with iodine have also been studied in polarized RRS. A detailed analysis of the band shapes and of the depolarization ratios shows differences with respect to the undoped trans (CH) x case in agreement with a modification upon doping of the bimodal chain length distribution.

Model Networks of End-Linked Polydimethylsiloxane Chains. X. Bimodal Networks Prepared in Two-Stage Reactions Designed to Give High Spatial Heterogeneity

Abstract End-linking mixtures of very short and relatively long polydimethylsiloxane chains with a multi-functional alkoxy silane yields model elastomeric networks having a bimodal chain length distributions, and earlier studies have shown that this compositional heterogeneity increases the ultimate properties of the networks. The present investigation focuses on possible additional effects of the spatial heterogeneity introduced by the prereaction of some of the very short chains into tightly crosslinked domains. Increasing the extent of prereaction did not give any significant increases in modulus or ultimate strength. Thus, it is indicated that the unusual ultimate properties of bimodal networks are due to limited chain extensibilities, rather than to reinforcing effects.