Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties

Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties

Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties

Gels of Semiconducting Polymers in Benign Solvents

Gels of semiconducting polymers have many potential applications, including biomedical devices and sensors. Here, we report a self-assembled gel system consisting of isoindigo-based semiconducting polymers with galactose side chains in benign, alcohol-based solvents. Because of the carbohydrate side chains, the modified isoindigo polymers are soluble in alcohols. We obtained thermoreversible gels in 1-propanol using these polymers and di-Fmoc-l-lysine, a molecular gelator. The polymers and molecular gelators have been selected in such a way that they do not have significant physical interactions. The molecular gelator self-assembled to form a fibrous structure that confines the polymer chains in the interstitial spaces of the fibers. The polymer chains formed local aggregations and increased the shear moduli of the gels significantly. Bulky galactose side chains and the less planar nature of the polymer backbone hindered the formation of long-range assembled structures of the polymers. However, the dispersion of polymers throughout the gel samples resulted in a percolated structure in the dried gel films. The bulk electrical conductivity of dried gels confirmed the presence of such percolated structures. Our results demonstrated that carbohydrate-containing conjugated polymers can be combined with molecular gelators to obtain gels in eco-friendly solvents

Multicolor Nitrogen-Doped Carbon Quantum Dots for Environment-Dependent Emission Tuning

Carbon quantum dots (CQDs) have potential applications in many fields such as light-emitting devices, photocatalysis, and bioimaging due to their unique photoluminescence (PL) properties and environmental benignness. Here, we report the synthesis of nitrogen-doped carbon quantum dots (NCQDs) from citric acid and m-phenylenediamine using a one-pot hydrothermal approach. The environment-dependent emission changes of NCQDs were extensively investigated in various solvents, in the solid state, and in physically assembled PMMA–PnBA–PMMA copolymer gels in 2-ethyl-hexanol. NCQDs display bright emissions in various solvents as well as in the solid state. These NCQDs exhibit multicolor PL emission across the visible region upon changing the environment (solutions and polymer matrices). NCQDs also exhibit excitation-dependent PL and solvatochromism, which have not been frequently investigated in CQDs. Most CQDs are nonemissive in the aggregated or solid state due to the aggregation-caused quenching (ACQ) effect, limiting their solid-state applications. However, NCQDs synthesized here display a strong solid-state emission centered at 568 nm attributed to the presence of surface functional groups that restrict the π–π interaction between the NCQDs and assist in overcoming the ACQ effect in the solid state. NCQD-containing gels display significant fluorescence enhancement in comparison to the NCQDs in 2-ethyl hexanol, likely because of the interaction between the polar PMMA blocks and NCQDs. The application of NCQDs-Gel as a solid/gel state fluorescent display has been presented. This research facilitates the development of large-scale, low-cost multicolor phosphor for the fabrication of optoelectronic devices, sensing, and bioimaging applications

Effects of Poly(3-hexylthiophene) Molecular Weight and the Aging of Spinning Solution on the Electrospun Fiber Properties

The electrospinning technique is an attractive route for processing conjugated polymers in a significant quantity for large-scale applications. However, the processing–structure–property relationship of the electrospinning process for conjugated polymers is not well understood. Here, we report the electrospinning of poly(3-hexylthiophene) (P3HT) for three different molecular weights of P3HT: 31, 58, and 83 kDa. Chloroform was used as a solvent, and a high molecular weight poly(ethylene oxide) (PEO) was utilized to facilitate the processing of P3HT. Electrospinning was performed on the freshly prepared and 24 h aged spinning solutions. The aging of the spinning solution led to the self-assembly of P3HT chains, particularly with dominant H-aggregation for 83 kDa P3HT. The structure development and properties of the fibers were investigated, including the single-fiber electrical conductivity measured using a custom-built setup. Electrical conductivity has been found to increase with increasing molecular weight, and as high as a fivefold enhancement in single-fiber electrical conductivity was obtained for the fibers from the aged solution compared to the fiber from the freshly prepared solution. Despite a 25% PEO concentration in the fibers, the maximum electrical conductivity of a single fiber was found to be ≈2.7 × 10–5 S/cm, similar to the pristine P3HT thin films. Our study provides an additional understanding of P3HT structure development in electrospun fibers as a function of polymer molecular weight and processing steps and relates that to fiber properties