Initial Monomer Concentration Research Articles

Effects of network structures on the fracture of hydrogel

The polymer network structures and topologies in hydrogels can significantly affect their mechanical properties. The network structures can vary dramatically with different synthetic conditions of hydrogels. Herein, we study the effects of the polymer network structures on mechanical properties of poly(acrylamide) (PAAm) hydrogels synthesized through free radical polymerizations. We focus on investigating how the initial monomer concentration ϕ0 during the synthesis influences the network structures and thus their mechanical properties. Specifically, we measure the swelling capability of the polymer network, the elasticity and fracture of swollen hydrogels with a fixed volume fraction of polymer. Our results have revealed that for PAAm hydrogels with the same volume fraction of polymer, their elasticity, stretchability, work of rupture and fracture toughness can vary significantly if ϕ0 during the synthesis is different. Our study may provide important guideline for synthesizing hydrogels with tailorable mechanical properties without changing the chemistry and synthesis procedure.

Synthesis and Heavy-Metal Sorption Studies of N,N-Dimethylacrylamide-Based Hydrogels.

In this work, a hydrogel system was produced via radical polymerization of N,N-dimethylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid in the presence of N,N-methylene-bis-acrylamide as a crosslinker and ammonium persulfate as an initiator. Parameters that impact the conversion of copolymerization (such as initial concentration of monomers, temperature, initiator dose, and time) were studied. The swelling degree of the hydrogel was investigated with the addition of a crosslinker and initiator at different pH levels. A hydrogel with high conversion and high swelling degree was selected to investigate their ability for adsorption of Pb(II) ions from solutions. Adsorption behavior of Pb(II) ions in a hydrogel was examined as a function of reaction time and concentration of lead ions from a solution of Pb(II) ions.

Highly thermostable high molecular-weight low k PIM polymers based on 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-Tetramethylspirobisindane, decafluorobiphenyl, and bisphenols

High-molecular-weight (number average molecular weight Mn = 11–81 kDa) polymers with intrinsic microporosity (PIM) were prepared by the solution polycondensation of 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane, decafluorobiphenyl, and bisphenols in an aprotic polar solvent at an initial monomer concentration of 0.1 mol/L at 100°C-140 °C for 10–144 h. Transparent (minimum cutoff wavelength of 310 nm), highly thermostable (glass transition temperature of 161°C-376 °C by dynamic mechanical analysis, and 5% weight loss temperature in N2 of 410°C-514 °C) polymer films were fabricated using a solvent-cast method. The copolymer films showed good mechanical properties (tensile strengths of 45–67 MPa, breaking elongation of 2.2%–23.1%, and tensile elastic moduli of 1.5–3.3 GPa) and insulating properties (relative permittivity of 2.27–2.59, and dissipation factor of 0.00191–0.00466 at 20 GHz frequency). The prepared new PIM copolymers are candidates for next-generation low k dielectric materials.

Diverse Aggregation Kinetics Predicted by a Coarse-Grained Peptide Model.

Protein and peptide aggregation is a ubiquitous phenomenon with implications in medicine, pharmaceutical industry, and materials science. An important issue in peptide aggregation is the molecular mechanism of aggregate nucleation and growth. In many experimental studies, sigmoidal kinetics curves show a clear lag phase ascribed to nucleation; however, experimental studies also show downhill kinetics curves, where the monomers decay continuously and no lag phase can be seen. In this work, we study peptide aggregation kinetics using a coarse-grained implicit solvent model introduced in our previous work. Our simulations explore the hypothesis that the interplay between interchain attraction and intrachain bending stiffness controls the aggregation kinetics and transient aggregate morphologies. Indeed, our model reproduces the aggregation modes seen in experiment: no observed aggregation, nucleated aggregation, and rapid downhill aggregation. We find that the interaction strength is the primary parameter determining the aggregation mode, whereas the stiffness is a secondary parameter modulating the transient morphologies and aggregation rates: more attractive and stiff chains aggregate more rapidly and the transient morphologies are more ordered. We also explore the effects of the initial monomer concentration and the chain length. As the concentration decreases, the aggregation mode shifts from downhill to nucleated and no-aggregation. This concentration effect is in line with an experimental observation that the transition between downhill and nucleated kinetics is concentration-dependent. We find that longer peptides can aggregate at conditions where short peptides do not aggregate at all. It supports an experimental observation that the elongation of a homopeptide, e.g., polyglutamine, can increase the aggregation propensity.

Self-Initiated Butyl Acrylate Polymerizations in Bulk and in Solution Monitored By In-Line Techniques.

High-temperature acrylate polymerizations are technically relevant, but yet not fully understood. In particular the mechanism and the kinetics of the thermal self-initiation is a topic of current research. To obtain more detailed information the conversion dependence of the polymerization rate, rbr, is determined via in-line DSC and FT-NIR spectroscopy for reactions in bulk and in solution at temperatures ranging from 80 to 160 °C. Solution polymerizations revealed that dioxane is associated with the highest rbr, while aromatic solvents result in the lowest values of rbr. Interestingly, rbr for polymerizations in solution with dioxane depends on the actual monomer concentration at a given time in the system, but is not depending on the initial monomer concentration. The overall rate of polymerization in bulk and in solution is well represented by an equation with three or four parameters, respectively, being estimated by multiple linear regression and the temperature as additional parameter.

An In Situ Film-to-Film Transformation Approach toward Highly Crystalline Covalent Organic Framework Films

An In Situ Film-to-Film Transformation Approach toward Highly Crystalline Covalent Organic Framework Films

Modeling Amyloid Aggregation Kinetics: A Case Study with Sup35NM

Understanding the aggregation mechanism of amyloid proteins, such as Sup35NM, is essential to understanding amyloid diseases. Significant recent work has focused on using the fluorescence of thioflavin T (ThT), which undergoes a red shift when bound to amyloid aggregates, to monitor amyloid fibril formation. In the present study, the progression of the total mass of aggregates during fibril formation is monitored for initial monomer concentrations in order to infer the relevant aggregation mechanisms. This workflow was implemented using the amyloid-forming fragment Sup35NM under different agitation conditions and for initial monomer concentrations spanning 2 orders of magnitude. The analysis suggests that primary nucleation, monomeric elongation, secondary nucleation, and fragmentation might all be relevant, but their relative importance could not be determined unambiguously, despite the large set of high-quality data. Discriminating between the fibril-generating processes is shown to require additional information, such as a fibril length distribution. Using Sup35NM as a case study, a framework for fitting the parameters of arbitrary amyloid aggregation kinetics is developed based on a population balance model (PBM), which resolves not only the total aggregate mass (monitored experimentally via ThT fluorescence) but the entire fibril length distribution over time. In addition to the rich new set of ThT fluorescence data, we have reanalyzed a previously published aggregate size distribution using this method. With the size distribution, it was determined that in the reanalyzed in vitro experiment, secondary nucleation generated significantly fewer new Sup35NM fibrils than fragmentation. The proposed strategy of applying the same PBM to a combination of kinetic data from fluorescence monitoring and experimental fibril length distributions will allow the inference of aggregation mechanisms with far greater confidence than fluorescence studies alone.

A novel method for the in vitro assembly of virus-like particles and multimeric proteins.

To develop a method for the efficient assembly of viral or multimeric proteins into virus-like particles (VLP) or other macro structures. Protein monomers were assembled by eliminating calcium ions through precipitation. The model protein, rotavirus VP6, assembled into stable, long nanotubes with better quality than the assemblies obtained directly from cell culture. Nanotube length was directly proportional to the initial concentration of VP6 monomers, in accordance with the classic nucleation theory of capsid assembly. The quality of the obtained assemblies was confirmed when the nanotubes were functionalized with metals, yielding unique nanobiomaterials. Assembly efficiency was improved in comparison with other previously proposed methods. The novel method presented here is simpler and faster than other reported methods for the assembly and disassembly of viral proteins, a step needed for most applications.

New Grafted Copolymers Carrying Betaine Units Based on Gellan and N-Vinylimidazole as Precursors for Design of Drug Delivery Systems.

New grafted copolymers possessing structural units of 1-vinyl-3-(1-carboxymethyl) imidazolium betaine were obtained by graft copolymerization of N-vinylimidazole onto gellan gum followed by the polymer-analogous reactions on grafted polymer with the highest grafting percentage using sodium chloroacetate as the betainization agent. The grafted copolymers were prepared using ammonium persulfate/N,N,N′,N′ tetramethylethylenediamine in a nitrogen atmosphere. The grafting reaction conditions were optimized by changing one of the following reaction parameters: initiator concentration, monomer concentration, polymer concentration, reaction time or temperature, while the other parameters remained constant. The highest grafting yield was obtained under the following reaction conditions: ci = 0.08 mol/L, cm = 0.8 mol/L, cp = 8 g/L, tr = 4 h and T = 50 °C. The kinetics of the graft copolymerization of N-vinylimidazole onto gellan was discussed and a suitable reaction mechanism was proposed. The evidence of the grafting reaction was confirmed through FTIR spectroscopy, X-ray diffraction, 1H-NMR spectroscopy and scanning electron microscopy. The grafted copolymer with betaine structure was obtained by a nucleophilic substitution reaction where the betainization agent was sodium chloroacetate. Preliminary results prove the ability of the grafted copolymers to bind amphoteric drugs (cefotaxime) and, therefore, the possibility of developing the new sustained drug release systems.

Kinetics and Modeling of Aqueous Phase Radical Homopolymerization of 3-(Methacryloylaminopropyl)trimethylammonium Chloride and its Copolymerization with Acrylic Acid

The radical homopolymerization kinetics of 3-(methacryloylaminopropyl) trimethylammonium chloride (MAPTAC) and its batch copolymerization with nonionized acrylic acid (AA) in aqueous solution are investigated and modeled. The drift in monomer composition is measured during copolymerization by in situ NMR over a range of initial AA molar fractions and monomer weight fractions up to 0.35 at 50 °C. The copolymer becomes enriched in MAPTAC for monomer mixtures containing up to 60 mol% MAPTAC, but is enriched in AA for MAPTAC-rich mixtures; this azeotropic behavior is dependent on initial monomer content, as electrostatic interactions from the cationic charges influence the system reactivity ratios. Models for MAPTAC homopolymerization and AA-MAPTAC copolymerization are developed to represent the rates of monomer conversion and comonomer composition drifts over the complete range of experimental conditions.

Synthesis and study of oligobenzotriazolylimides

Three-dimensional heterocyclic polymers can be obtained by the migration copolymerization of bis-imides of unsaturated dicarboxylic acids and such nucleophiles as diamines, bisphenols and bismercaptans. As-obtained nitrogen-containing heat-resistant polymers are of practical interest due to their high performance and the availability of the feedstock. In this work, we propose to obtain heat-resistant polymeric materials through the synthesis of oligobenzotriazolylimides by interaction of bis-maleimides with benzotriazoles. One-stage melt synthesis producing no by-products and requiring no organic solvents seems to be the most suitable method from the technological and environmental standpoint. Optimal conditions for the synthesis of polymers were determined using the method of compositional orthogonal planning. Reduced viscosity was chosen as the optimization parameter; the initial concentration of monomers, as well the duration and temperature of synthesis, were the variable factors affecting the optimization parameter. The structure, properties and mechanical characteristics of the obtained polymers were studied. The structure of oligomers and polymers was confirmed by IR and NMR spectroscopy data. A series of experiments was carried out to investigate the potential of extrusion-rolling casting technology for obtaining composite materials based on oligobenzotriazolylimides. This technology maintains initial substances in a fluid state for an extended period of time. The slower curing rate of oligomers facilitates the dissipation of the heat released during structuring, thus causing no local overheating of the material and resulting mechanical stresses. This improves the technological and physical-mechanical characteristics of the composites based on these oligomers. Optimal conditions for the production of polymer composites were identified, and their physical and mechanical properties were studied. A technology was proposed for obtaining materials resistant to aggressive media and temperature drops.

A Thermodynamic Roadmap for the Grafting-through Polymerization of PDMS11MA.

Grafting-through atom transfer radical polymerization (ATRP) was used to polymerize a sterically hindered poly(dimethylsiloxane) methacrylate (PDMS11MA, Mn = 1000) macromonomer to high conversion as a function of temperature, solvent, initial monomer concentration, and pressure. Higher polymerization yields were obtained when polymerizations were conducted at (i) lower temperature (T), (ii) in a poor solvent for the side chain, (iii) higher initial monomer concentration ([M]0), and (iv) higher pressure by mitigating the contribution of the equilibrium monomer concentration ([M]eq). The enthalpy of polymerization (ΔHp) and entropy of polymerization (ΔSp) were more negative in poor solvents. Polymerizations at ambient pressure required higher [M]0, use of a poor solvent, and lower temperatures to reach higher conversion with good control, whereas high pressure ATRP (HP-ATRP) displayed better control under dilute conditions. Grafting-through polymerization at high P and higher [M]0 was less controlled, plausibly due to limited solubility and mobility of the copper catalyst in the highly viscous medium.

Potentiometric Sensors Based on Nafion Membranes Modified by PEDOT for Determining Procaine, Lidocaine, and Bupivacaine in Aqueous Solutions and Pharmaceuticals

Hybrid materials based on perfluorinated sulfonic acid Nafion-type membranes and poly-3,4-ethylenedioxythiophene (PEDOT) with a gradient distribution of the latter along the film length were synthesized by in situ oxidative polymerization. The initial monomer concentration (0.01 and 0.002 M) and the concentration ratio of the monomer to the oxidant (1/1.25 and 1/2.5) were varied. We studied the effect of the equilibrium and transport properties of the obtained materials on the characteristics of cross-sensitive DP-sensors (analytical signal is the Donnan potential) in aqueous solutions of procaine, lidocaine, and bupivacaine hydrochlorides, including those containing sodium chloride, in a concentration range from 1.0 × 10–4 to 1.0 × 10–2 M and pH from 2 to 6. The relative error in determining the active substance in the Novokain preparation using a DP-sensor based on the Nafion/PEDOT membrane (0.002 M, 1/2.5) was 0.4%. An array of DP-sensors based on Nafion and Nafion/PEDOT (0.002 M, 1/1.25) membranes was used to determine bupivacaine hydrochloride and sodium chloride in the Markain® Spinal preparation with an error of 11 and 6%, respectively.

Size-Controlled Synthesis of Iron and Iron Oxide Nanoparticles by the Rapid Inductive Heating Method.

Inductive heating synthesis is an emerging technique with the potential to displace the hot-injection synthesis method to prepare colloidal particles very rapidly with a narrow size distribution, controlled size, and high crystallinity. In this work, the inductive heating synthesis is applied to produce a short-temperature jump to mimic conditions like the hot-injection method to prepare traditional iron and iron oxide nanoparticles (IONPs) in the 3–11 nm size range within various solvents, precursors, and reaction time conditions. Moreover, this inductive heating technique can be used under unique experimental conditions not available for hot-injection reactions. These conditions include the use of very high initial monomer concentrations. Considering benefits over conventional methods, the inductive heating technique has the potential to provide an industrial level scale-up synthesis. The magnetization of these particles is consistent with the magnetization of the particles from the literature.

Analysis of the Competition between Cyclization and Linear Chain Growth in Kinetically Controlled A2 + B2 Step‐Growth Polymerizations Using Modeling Tools

AbstractA mathematical model for the kinetics and chain length development in A2 + B2 step‐growth polymerization with competition between cyclization and linear chain growth is presented. The model requires two kinetic rate constants, one for the several reactions producing linear polymer molecules (k), and another one for chain length dependent cyclization (kc). The model describes well the qualitative behavior of the system, including the effects of initial monomer concentration and ratio of limiting to excess functional groups on degree of cyclization. The model is validated using two polymerization systems, a conventional A2 + B2 polymerization of hexamethylene diisocyanate (HDI) and poly(ethylene glycol) (PEG200), as well as the superacid catalyzed polyhydroxyalkylation of modified isatin and biphenyl. Four different kinetic rate constants for the four reactions producing linear polymer molecules are required in the second case.

Poor Solvents Improve Yield of Grafting-Through Radical Polymerization of OEO19MA.

Radical polymerization of poly(ethylene glycol) methyl ether methacrylate (OEO19MA, Mn ∼ 950) at an initial monomer concentration of 150 mM was investigated as a function of solvent composition. Conventional and controlled radical polymerizations in anisole at 60 °C converged at approximately the same equilibrium monomer concentration ([M]eq) of ∼38 mM, suggesting that livingness or diminished termination did not affect the thermodynamic parameters of polymerization. Conventional radical polymerizations (RPs) in anisole, dimethylformamide (DMF), toluene, and 1×PBS buffered water were taken to approximately 98% thermal initiator decomposition to determine [M]eq at reaction completion within a broad temperature range. The enthalpy (ΔHp) and entropy (ΔSp°) of polymerization were solvent-dependent. Polymerizations in 1×PBS were the most thermodynamically favorable, followed by those in DMF, toluene, and anisole. -ΔHp and -ΔSp increased with the square of the difference in the Hansen solubility parameters of poly(ethylene glycol) and the solvent. It is proposed that poor solvents favor polymer-polymer interactions over polymer-solvent interactions, which improves the thermodynamic polymerizability.

Inclusion of isolated α-amino acids along the polylactide chain through organocatalytic ring-opening copolymerization

Degradable polymers based on α-hydroxy acids and α-amino acids constitutes a potent class of biomaterials, combining high hydrolyzability with structural features that mimics peptides. Driven by the design criteria to construct isolated α-amino acid units along a main polylactide chain, a copolymer system was developed based on two monomers with distinctly different equilibrium behaviors. This was uncovered by detailed understanding on the kinetic and thermodynamic polymerizability of 3S,6S-dimethylmorpholine-2,5-dione (DMMD) and L-lactide (LLA) at low reaction temperatures. Under Brønsted base-promoted ring-opening copolymerization (ROCOP) conditions, the equilibrium nature of the copolymerization was shown susceptible to changes in the system, such as catalyst basicity, solvent polarity and initial monomer concentrations. Subsequently, high equilibrium conversions of both monomers with control over molecular weight and dispersity could be achieved within short reaction times by modulation of these factors. Thermodynamic elucidations of the copolymerization system revealed that DMMD behaved as an unstrained monomer with a large propagation barrier, favored by an increase in polymerization temperature. Ultimately, the high propagation barrier of DMMD in the system resulted in a kinetically controlled mechanism with the formation of completely isolated units of DMMD along the polylactide backbone. These results extend current ROCOP strategies of morpholine-2,5-diones and cyclic esters to a mild and selective copolymerization platform for the construction of sequence-controlled α-amino acid decorated polyesters for medical applications.

Shape Morphable Hydrogel/Elastomer Bilayer for Implanted Retinal Electronics.

Direct fabrication of a three-dimensional (3D) structure using soft materials has been challenging. The hybrid bilayer is a promising approach to address this challenge because of its programable shape-transformation ability when responding to various stimuli. The goals of this study are to experimentally and theoretically establish a rational design principle of a hydrogel/elastomer bilayer system and further optimize the programed 3D structures that can serve as substrates for multi-electrode arrays. The hydrogel/elastomer bilayer consists of a hygroscopic polyacrylamide (PAAm) layer cofacially laminated with a water-insensitive polydimethylsiloxane (PDMS) layer. The asymmetric volume change in the PAAm hydrogel can bend the bilayer into a curvature. We manipulate the initial monomer concentrations of the pre-gel solutions of PAAm to experimentally and theoretically investigate the effect of intrinsic mechanical properties of the hydrogel on the resulting curvature. By using the obtained results as a design guideline, we demonstrated stimuli-responsive transformation of a PAAm/PDMS flower-shaped bilayer from a flat bilayer film to a curved 3D structure that can serve as a substrate for a wide-field retinal electrode array.

Synthesis of Poly(2-aminothiazole)-Coated Polystyrene Particles and Their Excellent Hg(II) Adsorption Properties.

Synthesis of conjugated polymer-coated latex particles is an effective method to improve the poor processability of conjugated polyheterocycles. The key to success is to control the overlayer thickness so it is less than the size of the solvated layer of polymeric stabilizer. This paper presents a protocol to coat polymer latex particles with poly(2-aminothiazole) (PAT), which is a relatively new heterocyclic conjugated polymer. The protocol is based on chemical oxidative polymerizations of 2-aminothiazole using copper chloride as the oxidant at a fixed oxidant/monomer molar ratio of 0.5 in aqueous media in the presence of poly(N-vinyl-2-pyrrolidone)-functionalized polystyrene (PS) latex. The effects of monomer concentration, PS concentration, and polymerization temperature on the morphology of the PAT-coated PS composite particles were investigated by SEM and TEM, and the resulting composite particles characterized by FTIR and XPS. Optimization of the initial monomer concentration allowed colloidally stable PAT-coated PS composite particles to be formed at ambient temperature, and the PAT loading was easily adjusted by varying the initial PS concentration. The Hg(II) adsorption properties of selected PAT-coated PS composite particles were assessed preliminarily. The maximum adsorption capacity at 25 °C reached 440.25 mg/g, which is much higher than many other adsorbents.

Tracking Oligomerization of Alpha-Synuclein Demonstrates Pivotal Role of Mitochondria in Seeding

Aggregation of α-synuclein, which normally exists as an unfolded monomer, in human brain contributes to the major pathogenesis of the synucleinopathies, Parkinson's disease and Lewybody dementia. However, the initial triggers to form early aggregates are unknown. Of note, visualizing the dynamic process of aggregation in neurons has been limited due to the inability to detect the early soluble toxic intermediates, oligomers, inside cells. Here we report a Foster resonance energy transfer (FRET) based method that enables the kinetics of intracellular oligomerization to be tracked in live cells. We first visualized that neurons uptake externally applied monomers and intracellularly form early and late oligomers, dependent on the initial monomer concentration. Then we utilized mutant α-synuclein carrying A53T point mutation that causes early onset progressive Parkinson's disease to investigate triggers of aggregation. We demonstrated that the A53T mutant monomer progressively oligomerizes due to the fast intracellular uptake than WT and other α-synuclein mutants. In addition, correlative light and electron microscopy (CLEM) allowed us to identify cellular regions in human neurons in which oligomerization is detected, such as mitochondria and lysosome. Importantly, A53T monomer produces excess reactive oxygen species (ROS) and opens mitochondria permeabilization pore (PTP) prior to cell death unlike the WT monomer. Importantly prevention of mitochondrial ROS inhibits the oligomerization of the A53T monomer and maintains the protein in its monomeric state, and this subsequently prevents cell death. In summary, our study provides evidence that mitochondria seed the aggregation of α-synuclein in neurons and mitochondrial stress in the form of enhanced ROS production is a major trigger for oligomerization.