Polymer Relaxation Research Articles

The roles of polymer relaxation phenomena, aqueous dye solubility and the physical properties of water in the mechanism of adsorption of a disperse dye on poly(ethylene terephthalate) fibres: Part 1 polymer relaxation phenomena

AbstractTo investigate the contributions of polymer relaxation phenomena to the mechanism of disperse dye adsorption on poly(ethylene terephthalate) fibres, a commercial grade dye was applied to poly(ethylene terephthalate) fabric at temperatures between 30°C and 130°C. Three regions of temperature‐dependent dyeing behaviour were identified, in which the promotional effect imparted by increasing dyeing temperature varied, depending on whether dyeing had been carried out at temperatures that were below, within or above the polymer's glass transition temperature, Tg, namely < 65°C, between 65°C and 110°C and between 110°C and 130°C, respectively. When experimentally determined colour strength data points (log 1/fk) were fitted to plots of polymer structural relaxation (log aT) calculated using the Williams, Landel and Ferry equation as a function of (T − Tg), three different levels of correspondence were achieved which paralleled the observed three regions of temperature‐dependent dye uptake. The adsorption of the commercial disperse dye on the poly(ethylene terephthalate) fibre therefore concurs with the free volume model of dye diffusion insofar as the diffusional behaviour of the dye is related to the relaxation time of the molecular motions occurring within the poly(ethylene terephthalate) polymer. The finding that the poly(ethylene terephthalate) substrate's glass transition extends over a broad range of temperature upto ~110°C explains why elevated dyeing temperatures in the region of 130/140°C must be used in High Temperature dyeing processes, and also, why ~75% of uptake of the commercial disperse dye on the poly(ethylene terephthalate) fabric occurs over the very narrow 20‐30°C critical temperature range between 110 and 130°C/140°C.

Disentangling water, ion and polymer dynamics in an anion exchange membrane.

Semipermeable polymeric anion exchange membranes are essential for separation, filtration and energy conversion technologies including reverse electrodialysis systems that produce energy from salinity gradients, fuel cells to generate electrical power from the electrochemical reaction between hydrogen and oxygen, and water electrolyser systems that provide H2 fuel. Anion exchange membrane fuel cells and anion exchange membrane water electrolysers rely on the membrane to transport OH- ions between the cathode and anode in a process that involves cooperative interactions with H2O molecules and polymer dynamics. Understanding and controlling the interactions between the relaxation and diffusional processes pose a main scientific and critical membrane design challenge. Here quasi-elastic neutron scattering is applied over a wide range of timescales (100-103 ps) to disentangle the water, polymer relaxation and OH- diffusional dynamics in commercially available anion exchange membranes (Fumatech FAD-55) designed for selective anion transport across different technology platforms, using the concept of serial decoupling of relaxation and diffusional processes to analyse the data. Preliminary data are also reported for a laboratory-prepared anion exchange membrane especially designed for fuel cell applications.

Viscoelastic necking dynamics between attractive microgels

HypothesisMicrogels can deform and interpenetrate and display colloid/polymer duality. The effective interaction of microgels in the collapsed state is governed by the interplay of polymer–solvent interfacial tension and bulk elasticity. A connecting neck is shown to mediate microgel interaction, but its temporal evolution has not been addressed. We hypothesize that the necking dynamics of attractive microgels exhibits liquid-like or solid-like behavior over different time and length scales. ExperimentsWe simulate the merging and pinching of attractive microgels with different crosslinking densities in explicit solvent using dissipative particle dynamics. The temporal coalescence dynamics of microgels is investigated and compared with simple liquid and polymeric droplets. We model the neck growth on long time scales using Maxwell model of polymer relaxation and compare the theoretical prediction with simulation data. The mechanical strength of the neck is characterized systematically via simulated pinch-off of microgels by steered molecular dynamics. FindingsWe evidence a crossover in the coalescence dynamics reflecting the viscoelastic signature of microgels. In contrast to the common knowledge that viscoelastic materials respond elastically on short time scales, the early expansion of the microgel neck exhibits a linear behavior, similar to the viscous coalescence of liquid droplets. However, the late regime with arrested dynamics resembles sintering of solid particles. Through an analytical model relating microgel dynamics to neck growth, we show that the long-term behavior is governed by stress relaxation of the polymers in the neck region and predict an exponential decay in the rate of growth, which agrees favorably with the simulation. Different from coalescence, the thread thinning in microgel breakup primarily highlights its polymeric characteristics.

Influence of polymers on flow and heat transfer due to peristaltic waves: a molecular approach

This work aims to have an insight into the effects of polymers on flow and heat transfer due to peristaltic waves. Oldroyd-B model, based on kinetic theory, is considered to study the relation between the molecular structure of the polymers and the rheological properties. The main advantage of using this model is the inclusion of information based on molecular structure. Such inclusion in stress tensor incorporates the effects of stretching and orientation of the molecules. Some significant applications polymer-peristalsis can be found in tissue engineering, intake of food, and medicines. Approximations of long-wavelength and low Reynolds number are used to investigate the main features of the problem. Pressure gradient, velocity, temperature, trapped bolus, drag coefficient, and heat flux at the wall are studied numerically. The analysis is based on the change in important parameters like polymer concentration, relaxation time, Weissenberg number, and Brinkman number. Results show that by increasing polymer concentration parameter, Weissenberg number, and relaxation time parameter, the negative pressure gradient in the flow geometry increases. By increasing the polymer concentration parameter and time relaxation parameter, the velocity and temperature profiles increase. An increase in polymer concentration increases the size of the trapped bolus. Drag parameter at the wall decreases while heat flux increases when polymer concentration and Weissenberg number are increased.

DESIGN, 23 FACTORIAL OPTIMIZATION AND IN VITRO–IN VIVO PHARMACOKINETIC EVALUATION OF ROSUVASTATIN CALCIUM LOADED POLYMERIC NANOPARTICLES

Objective: The objective is to incorporate low bioavailable Rosuvastatin Calcium (20%) into polymeric nanoparticles (PNs) to improve its biopharmaceutical properties of Rosuvastatin Calcium. Methods: The PNs were prepared by solvent evaporation method by applying 23 factorial designs. The formulated PN are investigated for particle size (PS) and shape, zeta potential (ZP), polydispersity index (PI) and entrapment efficiency (EE), in vivo pharmacokinetic. Results: Among 8 formulations, PN7 shows least PS of 159.9±16.1 nm, which enhance the dissolution, surface area and permeability; ZP of-33.5±1.54 mV, which shows good stability; PI of 0.587±0.16 shows monodisperse distribution pattern; high EE of about 94.20±2.46 %; percentage yield of 96.80±2.08 %; maximum in vitro drug release of about 96.54±2.02 % at 24 h with controlled and predetermined release pattern. PN7 drug release obeys zero-order release kinetics, non-fickian diffusion mechanism with r2 value>0.96 and release exponent ‘n’ value falls between 0.5-0.8 for peppas kinetic model i.e., the mechanism of drug diffusion is based on polymer relaxation. In vivo pharmacokinetic studies illustrate enhance in AUCo-α in mg/ml, which proves a significant enhancement of bioavailability of Rosuvastatin Calcium by PNs. Conclusion: This PN shows a significant enhancement of bioavailability by minimizing the dose-dependent adverse side effects of rosuvastatin calcium.

In Situ Liquid Phase TEM of Nanoparticle Formation and Diffusion in a Phase-Separated Medium.

Colloidal nanoparticles are synthesized in a complex reaction mixture that has an inhomogeneous chemical environment induced by local phase separation of the medium. Nanoparticle syntheses based on micelles, emulsions, flow of different fluids, injection of ionic precursors in organic solvents, and mixing the metal organic phase of precursors with an aqueous phase of reducing agents are well established. However, the formation mechanism of nanoparticles in the phase-separated medium is not well understood because of the complexity originating from the presence of phase boundaries as well as nonuniform chemical species, concentrations, and viscosity in different phases. Herein, we investigate the formation mechanism and diffusion of silver nanoparticles in a phase-separated medium by using liquid phase transmission electron microscopy and many-body dissipative particle dynamics simulations. A quantitative analysis of the individual growth trajectories reveals that a large portion of silver nanoparticles nucleate and grow rapidly at the phase boundaries, where metal ion precursors and reducing agents from the two separated phases react to form monomers. The results suggest that the motion of the silver nanoparticles at the interfaces is highly affected by the interaction with polymers and exhibits superdiffusive dynamics because of the polymer relaxation.

Influence of molecular weight on the distribution of segmental relaxation in polymer grafted nanoparticles

The segmental dynamics of one-component nanocomposites (OCNC) is significantly influenced by the molecular weight $({M}_{w})$ of the grafted polymer. Neutron backscattering shows that compared to the neat polymer the OCNCs exhibit a shift from slower segmental dynamics at low ${M}_{w}$ to faster dynamics at high ${M}_{w}$. We model the local relaxation as distribution of exponential diffusers. This approach reveals the presence of fast and slow segments in both OCNCs. However, their fractional contribution varies with ${M}_{w}$ leading to different average relaxation times. Our results present important insights into the origin of segmental mobility in OCNC and address the inconsistencies in different literature reports.

Shape Permanence in Diarylethene-Functionalized Liquid-Crystal Elastomers Facilitated by Thiol-Anhydride Dynamic Chemistry.

Diarylethene-functionalized liquid-crystalline elastomers (DAE-LCEs) containing thiol-anhydride bonds were prepared and shown to undergo reversible, reprogrammable photoinduced actuation. Upon exposure to UV light, a monodomain DAE-LCE generated 5.5 % strain. This photogenerated strain was demonstrated to be optically reversible over five cycles of alternating UV/Visible light exposure with minimal photochrome fatigue. The incorporation of thiol-anhydride dynamic bonds allowed for retention of actuated states. Further, re-programming of the nematic director was achieved by heating above the temperature for bond exchange to occur (70 °C) yet below the nematic-to-isotropic transition temperature (100 °C) such that order was maintained between mesogens. The observed thermal stability of each of the diarylethene isomers of over 72 h allowed for decoupling of photo-induced processes and polymer network effects, showing that both polymer relaxation and back-isomerization of the diarylethene contributed to LCE relaxation over a period of 12 hours after actuation unless bond exchange occurred.

Shape Permanence in Diarylethene‐Functionalized Liquid‐Crystal Elastomers Facilitated by Thiol‐Anhydride Dynamic Chemistry

AbstractDiarylethene‐functionalized liquid‐crystalline elastomers (DAE‐LCEs) containing thiol‐anhydride bonds were prepared and shown to undergo reversible, reprogrammable photoinduced actuation. Upon exposure to UV light, a monodomain DAE‐LCE generated 5.5 % strain. This photogenerated strain was demonstrated to be optically reversible over five cycles of alternating UV/Visible light exposure with minimal photochrome fatigue. The incorporation of thiol‐anhydride dynamic bonds allowed for retention of actuated states. Further, re‐programming of the nematic director was achieved by heating above the temperature for bond exchange to occur (70 °C) yet below the nematic‐to‐isotropic transition temperature (100 °C) such that order was maintained between mesogens. The observed thermal stability of each of the diarylethene isomers of over 72 h allowed for decoupling of photo‐induced processes and polymer network effects, showing that both polymer relaxation and back‐isomerization of the diarylethene contributed to LCE relaxation over a period of 12 hours after actuation unless bond exchange occurred.

Cyclodextrin mediated controlled release of edaravone from pH-responsive sodium alginate and chitosan based nanocomposites

The objective of the study is to enhance the aqueous solubility and stability of edaravone, a free radical scavenger drug. Inclusion complexes of edaravone with β-cyclodextrin were prepared by microwave irradiation and physical mixture method and confirmation of inclusion complexes were investigated by different analytical techniques such as FT-IR, ROESY, DSC, and 1H NMR. pH-sensitive nanocomposites based on chitosan (CH), sodium alginate (ALG), and bentonite (BN) were synthesized. To get the maximum percentage swelling different reaction parameters that are responsible for the synthesis of the nanocomposite were optimized and characterized by various techniques such as FESEM, EDS, XRD, and FT-IR. To regulate the drug delivery, inclusion complexes were directly loaded into the CH/ALG hydrogel, and CH/ALG/BN nanocomposite and release studies were evaluated at different pH environments. The solubility of edaravone was investigated by phase solubility and the graph results in a typical AL type behavior, suggesting the formation of a 1:1 stoichiometry inclusion complex. The comparative evaluation of drug release was explored by kinetic models. Controlled release of drug was achieved from CH/ALG/BN nanocomposite in comparison to CH/ALG hydrogel. The exploratory kinetic investigation revealed that β-CD plays a critical role in the drug release process by influencing polymer relaxation, resulting in slow release.

Management and control of intraocular pressure applying macitentan hydrogel film formulation: improved effect of surfactant and cosurfactant system.

Macitentan blocks endothelin receptors in order to control the pulmonary arterial hypertension (PAH). Oral administration of macitentan is associated with painful urination and troubled breathing. Formulated macitentan hydrogel film was used for examining the control of intraocular pressure, and the effect of surfactant and cosurfactant was studied. Macitentan ocular film formulation has been prepared in hydroxypropyl methylcellulose (HPMC) matrix system using different surfactant/co-surfactant system, and intraocular pressure was monitored on normotensive rabbit eyes after application in the cul-de-sac. The solid state characterization of the film indicated amorphisation of macitentan and no issues regarding major incompatibility was observed. Combination of surfactant, co-surfactant and hydrophilic co-solvent systems in the said films markedly improved the drug release and mucosal tissue permeation. Presence of PEG and Transcutol significantly improved ex vivo corneal permeation of MP and MT respectively compared to other films. Transcutol (MT) exhibited greatest difference among the formulations by improving the vesicular bilayer fluidity and reducing the mucosal tissue barrier facilitating the transcorneal diffusion. A combination of diffusion and erosion control behavior was observed in drug release and corneal permeation of the films due to the balanced liquid penetration and polymeric chain relaxation rate. MP and MT films were used for further in vivo studies to achieve possible effective and prolonged control of intraocular pressure. In vivo study has revealed the reduction in intraocular pressure upto about 23 % when tested on normotensive rabbit model. The films has managed to lower the IOP upto 3 h. Developed macitentan hydrogel film containing Transcutol (MT) could have a high potential for the control and management of ocular hypertension after topical application.

Transmission of Sodium Chloride in Pdms Membrane During Pervaporation Based on Polymer Relaxation

Transmission of Sodium Chloride in Pdms Membrane During Pervaporation Based on Polymer Relaxation

An experimental and theoretical study of the magnetic relaxation in heterometallic coordination polymers based on 6-methyl-2-oxonicotinate and lanthanide(III) ions with square antriprismatic environment.

Two new isostructural compounds based on 6-methyl-2-oxonicotinate (6m2onic) ligand and sodium and lanthanide(III) ions are reported. The structural and chemical characterization reveals the following chemical formula: {[Ln(6m2onic)2(μ-6m2onic)2Na(H2O)3]·8H2O}n [where Ln(III) = Dy (1Dy) and Er (2Er)]. These compounds crystallize in the form of one-dimensional arrays held together into a hydrogen-bonded structure, in which 6m2onic ligands establish four O,O' chelating rings with the lanthanide to render a distorted square antiprism (SAPR) geometry. Magnetic dc and ac susceptibility measurements confirm that 1Dy and 2Er behave as SIMs. Magnetic dilutions using Y(III) matrices have been made to achieve a Dy(III) counterpart (1Y/Dy) that presents slow magnetic relaxation under zero dc field. Under an optimized Hdc field (of 1000 Oe and 1500 Oe for 1Y/Dy and 2Y/Er, respectively), 1Y/Dy reveals the occurrence of two well-separated maxima, attributed to SR (Ueff = 65.2 K (45.3 cm-1) and τ0 = 2.76 × 10-9 s) and FR processes (Ueff = 23.2 K (16.1 cm-1) and τ0 = 1.40 × 10-8 s), whereas 2Y/Er shows a multiple relaxation pathway that considers quantum tunnelling of the magnetization (QTM), Orbach, Raman and direct mechanisms. Ab initio calculations have been carried out to support the experimental evidence and to explain the lanthanide ion-dependent behaviour deepen the understanding of the magneto-structural relationship of the SAPR environment.

Experimental investigation of anomalous molecular probe diffusion in entangled polymer melts.

Investigations on the diffusion of small molecules or particles in polymeric materials are important to numerous technologies and can also be used to gain insight on polymer chain dynamics. Systems where the probe size is comparable to (or smaller than) a characteristic length of the polymer chain, the tube diameter for example, are of particular interest because the diffusion coefficient of the probe can be orders of magnitude larger than the value predicted by the Stokes-Einstein relation. In the present study, we employ the optical technique known as forced Rayleigh scattering to study the diffusion of a molecular probe (dye) in several entangled polymer melts over a wide range of length and time scales. The probe size is much smaller than the tube diameter for the systems studied. We find the diffusion coefficient is larger by four to five orders of magnitude than the Stokes-Einstein prediction. More interestingly, we observe anomalous, non-Fickian, diffusion where the value of the measured diffusion coefficient can abruptly change by as much as 50%. We suggest that this unexpected behavior occurs when the time scale for diffusion is larger than the relaxation time associated with the constraint release mechanism for polymer chain dynamics.

Anionically modified N-(alkyl)acrylamide-based semi-IPN hybrid gels reinforced with SiO2 for enhanced on-off switching and responsive properties.

Semi-interpenetrated (semi-IPN) poly(N-isopropylacrylamide-co-methacrylic acid)/polyacrylamide P(NIPA-MA)/PAAm hybrid gels containing linear poly(acrylamide) PAAm chains were designed by incorporation of different amounts of silica particles (SiP). Formation of temperature-sensitive semi-IPN hybrid gels was evaluated by simultaneous radical polymerization under different polymerization temperatures, and the effect of polymer/particle interfaces on the swelling and elasticity was explained. Nanoparticle-mediated enhancements were studied to understand the effect of addition of SiP to anionically modified semi-IPNs. Hybrid network formation was confirmed by FTIR, with an increase in SiP resulting in an increased Si-O-Si absorption peak in hybrid samples. P(NIPA-MA)/PAAm/SiP gels showed a reduction in the degree of swelling with the addition of SiP. The Flory-Huggins interaction parameter of the semi-IPN hybrid-solvent was estimated using the extended equation. The compression test results showed an improvement in the stiffness and modulus attributed to stress transfer from the hybrid network to nanoparticles. Swelling processes of semi-IPN hybrids prepared by cold polymerization have anomalous diffusion owing to polymer relaxation, while Fickian behavior was observed for the hybrids obtained by warm polymerization. During oscillation shrinking-swelling of semi-IPN hybrids upon ionic-strength switching in NaCl solutions, the gels retained their shape and integrity for 10 cycles of testing. To evaluate the adsorption characteristics of semi-IPN hybrids, methyl violet (MV) was chosen as a model cationic dye. The effects of contact time, silica content and initial dye concentration were studied and the time-dependent adsorption data were fitted with six kinetic models. The MV uptake capacity of semi-IPN hybrids increased with an increase in the initial MV concentration as well as with silica content. The adsorption process followed pseudo-second order type adsorption kinetics and the mechanism of process was better described by intraparticle diffusion. These results will contribute to the understanding of roles of anionic comonomer and linear polymer doping in nanoparticle-mediated synthesis of hybrid gels and to the development of next-generation materials for pharmaceutical and environmental-based applications.

Preparation and characterization of starch-cellulose interpenetrating network hydrogels based on sequential Diels-Alder click reaction and photopolymerization

Herein, starch-cellulose interpenetrating network (IPN) hydrogels were fabricated by sequential Diels-Alder click reaction and photopolymerization in water. Moreover, β-cyclodextrin, a commonly used host molecule in supramolecular chemistry, was also introduced to improve the performance of the IPN hydrogel. Firstly, the starch-based dienes were synthesized by modifying starch with N-maleoyl-β-alanine, and the cellulose-based dienophiles were obtained by the reaction of cellulose and furfurylamide succinate; Secondly, the as-synthesized starch-based dienes, cellulose-based dienophiles, polymerizable β-cyclodextrin, crosslinker, and acrylamide were dissolved in water and obtained a transparent solution. The solution was maintained in a water bath of 50 °C for 3 h, forming the first network via catalyst-free click Diels-Alder reaction, subsequently, the second network was formed by photopolymerization. Their preparation conditions were optimized via one-factor experiments and their properties and structures were characterized. Finally, 5- fluorouracil (5-Fu) was used as a model drug to study the sustained release behavior of the drug-loaded hydrogels. Release profile was found to fit in Ritger-Peppas kinetic model and polymer relaxation and drug diffusion made a valuable contribution to drug release. Taking into account the virtues of easily controllable photopolymerization and catalyst-free Diels-Alder reaction, the strategy described here has a potential application in the preparation of IPN hydrogels.

Compressive deformations of ring polymers in a confining channel

We used Langevin Dynamics Simulations to study compression and relaxation processes of confined ring polymers in a nanochannel by a sliding piston. Our findings show that ring polymers undergo a series of complex deformations in compression, dependent on the degree of confinement and the topological constrains: the weakly confined flexible ring is compressed into the condensed blobs, while the strongly confined semiflexible ring undergoes deformations from double helix to collapsed states, in order to reduce the bending energy. In the relaxation process, with the piston drawing back, the weakly confined ring relaxs to the initial states, while the strongly confined ring extends from the compacted states to the randomly folded states, due to the interpenetrated and self-entangled structures formed in the compression process. These findings are important to understand the complex deformations and relaxation of ring polymers in mechanical compression or driven transport arising from the geometric confinement.

Ionic and Enzymatic Multiple-Crosslinked Nanogels for Drug Delivery.

Both ionic and enzymatic crosslink are efficient strategies for constructing network materials of high biocompatibility. Here chitosan was modified firstly and then crosslinked by these two methods for complementary advantages. The preparation methods and ionic crosslinkers can regulate the size and uniformity of the multiple-crosslinked nanogels. The multiple-crosslinked nanogels with the smallest size and the best uniformity was selected for the drug delivery. The drug-loading content and encapsulation efficiency were up to 35.01 and 66.82%, respectively. Their release behaviours are correlated with the pH value and the drug dosage. In general, the lower pH value and the lower drug dosage promoted the drug release. With the assistance of several kinetic models, it is found that drug diffusion plays a preponderant role in drug release, while polymer relaxation has a subtle effect. The multiple-crosslink resulting from ionic compounds and enzymes may provide a new perspective on developing novel biocompatible materials.

Nonlinear evolutions of streaky structures in viscoelastic pipe flows

The dynamics of streaky structures in a viscoelastic pipe flow of FENE-P fluids is studied numerically. The values of Weissenberg number ( W i , polymer relaxation time over flow turn-over time) and viscosity ratios are varied to evaluate the effects of viscoelasticity in the pipe. For Reynolds numbers 3000 ∼ 3500 (based on pipe radius and maximum laminar velocity) near the transition boundary, we select finite-amplitude two-dimensional rolls as initial conditions and add infinitesimal three-dimensional perturbations to trigger the transition. The 2D streamwise rolls are highly unstable to small 3D disturbances, which helps to better understand the evolution of large scale structures. The stronger temporal intermittency in viscoelastic flows makes it more difficult to identify a successful transition; we found that quantities directly related to viscoelasticity can serve as better indicators of transition to turbulence. The role of polymer stress is studied through the kinetic energy budget and it is found that polymer torque opposes the vorticity and become more dominant for higher W i , consistent with previous studies. The results show that the secondary instability of the streaky structures can be enhanced by viscoelasticity at low W i , while at high W i viscoelasticity delays the transition to turbulence, sometimes leading to relaminarization. • Direct numerical simulations of viscoelastic pipe flow of FENE-P fluids are conducted. • Viscoelasticity changes the evolution of streaks and causes strong intermittency. • Polymer stress is studied via turbulent kinetic energy budget in streak breakdown. • Non-monotonic influence of Weissenberg number on threshold boundary of transitions.

Remarkable Suppression of Vibrational Relaxation in Organic Semiconducting Polymers by Introducing a Weak Electron Donor for Improved NIR‐II Phototheranostics

AbstractExploration of high‐efficiency agents for near‐infrared‐II fluorescence imaging (NIR‐II FI) promotes the development of NIR‐II FI in life science. Despite the extensive use of organic semiconducting nanomaterials for NIR‐II FI, the fluorescence efficiency is barely satisfying, and the molecular guideline to improve the imaging quality has not been clarified yet. This contribution designs self‐brightened organic semiconducting polymers (OSPs) for improved NIR‐II phototheranostics of cancer. The amplification of NIR‐II brightness is realized by incorporating a weak electron‐donating unit (5,5′‐dibromo‐4,4′‐didodecyl‐2,2′‐bithiophene, DDB) into the semiconducting backbone with strong electron donor–acceptor alternated structure, which exhibits 6.3‐fold and 25‐fold fluorescence enhancement compared with the counterpart OSP at the same optical concentration and mass concentration, respectively. The broadband femtosecond transient absorption spectra experimentally elucidate the DDB doping‐induced suppression of vibrational relaxation as the underlying reason for the NIR‐II fluorescence amplification. Biocompatible nanoparticles fabricated from the optimal OSP12 exhibit excellent NIR‐II phototheranostic performance both in vitro and in vivo. Our research not only reveals the mechanistic insights for fluorescence enhancement of the designed OSPs from the essential view but also highlights an effective molecular methodology to guide the rational design of imaging agents with enhanced NIR‐II brightness for improved phototheranostics in living subjects.