Content Of Meso Research Articles

Phase-Separated Nanodroplets Formed below the Cloud Point for the Aqueous Solution of Stereo-Controlled Poly(N-isopropylacrylamide).

The relationship between dehydration of polymer chains and the nanodroplet formation through the macroscopic liquid-liquid phase separation (LLPS) has been investigated for the aqueous solution of stereocontrolled poly(N-isopropylacrylamide) (PNiPAm) and poly(N-diethylacrylamide) (PNdEAm). The fluorescent probe method reveals that the temperature range of dehydration for PNiPAm chains is much narrower than that for PNdEAm. The sharp dehydration of polymer chains may give rise to the characteristic thermoresponsive behavior of PNiPAm in water. For meso-rich PNiPAms, the dehydration point (Tdh), which is defined as the temperature where the single chains start assembling in the solution, locates far from the cloud point (Tc). That is, the dehydration of the chain occurs antecedently before the system undergoes a macroscopic LLPS. For PNdEAm, however, the dissociation between Tdh and Tc is not found. For the aqueous solution of PNiPAm with 52% of the meso content, the fluorescence correlation spectroscopy has revealed that nano-order droplets (ca. 45 nm of the hydration radius) are stabilized in the intermediate state between Tdh and Tc. The sharp dehydration of PNiPAm chains may enable an acute condensation of polymers in droplets, causing a viscoelastic hindrance in the coalescence of droplets.

Preparation of Polyacrylamide with Improved Tacticity and Low Molecular Weight Distribution

Polyacrylamide with improved tacticity and low molecular weight distribution was obtained via stereospecific atom transfer radical polymerization (ATRP) using the mixture of Lewis acids Y(OTf)3 and AlCl3 in a certain ratio as stereospecific catalyst and chloroacetic acid/ Cu2O / N,N,N’,N’-tetramethylethylenediamine( TMEDA) as initiating system. The initiating system afforded persistently controlled ATRP of acrylamide with lower polydispersity index ranging from 1.12 to 1.35 as well as a moderate polymerization process. The participation of the mixture of Lewis acids Y(OTf)3 and AlCl3 as stereospecific catalyst in the stereospecific ATRP of acrylamide contributed optimal stereospecific PAM with the meso content 80%~83%. Polymerization kinetics displayed a living/controlled nature of the present polymerizations.

Dynamic Evolution of Nanoscale Pores of Different Rank Coals Under Solvent Extraction

During the coalification process, coalbed methane (CBM) is formed and mainly adsorbed in the pores of coal. Pore structure evolution is critical to CBM adsorption/desorption and extraction. This paper puts forward two parameters, namely the variety degree x and variety gene σ, for characterizing pore structure through mercury injection tests. Then, under extraction with different solvents, the dynamic evolution characteristics of nanoscale pores are addressed and quantified by taking four different rank coals (lignite, medium-volatile bituminous coal, low-rank anthracite and mediumrank anthracite) from different coal mines of China as the study object. The results indicate that the content of meso- and macropores after solvent extraction is much larger, but that there is no obvious law with the content of transition pores and micropores in the size range of 50-7.2 nm, according to the basic data sets of specific surface area (SSA) and pore volume (PV) of all coal samples. This phenomenon can be explained by the pore increase and expansion effects in nanoscale pores during solvent extraction. Generally, with the increasing of the solvent extraction degree, the difference in variety degree x with respect to the total PV and total SSA of different coals shows a significant decreasing trend, which expresses a homogeneous development in the change in pore structure. In regard to different solvents, benzene mainly causes pore expansion in meso- and macropores, and CS₂ has a great effect on micropores. Whereas acetone plays an important role in mesopores and transition pores with pore expansion, THF has various effects on different size pores. Further study with higher variety gene σ values shows that the total PV mainly depends on the change in the absolute content of meso- and macropores. While the change in the absolute content of transition pores and micropores (less than 50 nm) has a great influence on the total SSA. As the extraction degree increases, the influence of the transition pores and micropores on the total PV is increased, and then, the content of meso- and macropores also plays an important role on the total SSA. However, this effect is highly different for raw coals of different ranks.

Synthesis of activated carbon from high-carbon coal fly ash and its hydrogen storage application

Activated carbons (ACs) have desirable characteristics that make them attractive for many industrial applications. In order to reduce their production cost, there is always a need to find alternative low-cost feedstock precursors. Nowadays zero-waste technologies play an important role in sustainable development, therefore using of coal by-product as source of AC is strongly recommended. In this study, coal fly ash (CFA) sample with a high unburned carbon content was used to synthesize ACs. The effect of acid pre-treatment of the CFA sample using HF and HCl prior to thermochemical activation using KOH was also investigated. The acid washing was found to be effective since it removed most of the inorganic components found in the CFA, as was confirmed by EDS and XRF. The resulting carbon-rich feedstock had relatively high content of meso-/macropores as well as with relative increase in specific surface area (46.19 m2/g - 81.20 m2/g). The obtained AC sample was found to exhibit high specific surface (946.77 m2/g) that was dominated by high microporosity and was tested for hydrogen storage. The H2 uptake (1 bar, 77 K) was found to be 1.35 wt% and with a predictable potential for even higher capacity when measurements are conducted at high pressures.

Rapid single-step synthesis of porous carbon from an agricultural waste for energy storage application

Single-step synthesis of porous carbon (PC) from biomass is a challenge via microwave heating, because biomass rarely absorbs the microwave energy. Herein, wheat-straw-derived char, as a good microwave absorber, was used to achieve rapidly single-step synthesis of PC from an agricultural waste (wheat straw). KOH was used to generate abundant micropores in the PCs. High heating rate caused by microwave heating combined with the pyrolysis gases resulted in the formation of meso-/macropores. A series of post-oxidation reactions between active sites in the PCs and oxygen in the air led to the doping of oxygen-containing chemical groups. Consequently, the obtained PC possessed a high specific surface area of 1905 m2 g−1, a balanced pore distribution with abundant micropores (0.62 cm3 g−1), considerable content of meso-/macropores (0.53 cm3 g−1), and an oxygen-enriched structure (oxygen content up to 21.6%). These characteristics not only contributed to the achievement of a high specific capacitance of 268.5 F g−1 at 0.5 A g−1 for the resultant supercapacitor, but also resulted in an excellent rate capability with a high capacitance retention of 81.2% at 10 A g−1 in a gel electrolyte (polyvinyl alcohol/LiCl). This supercapacitor can extract a high energy density of 21.5 W h kg−1 at 0.5 A g−1 and a high power density of 7.2 kW kg−1 at 10 A g−1.

Relationship between the phase diagram and hysteresis in demixing and remixing for atactic and meso-rich Poly(N-isopropylacrylamide)s in water

Effects of the stereoregularity on the hysteresis in demixing and remixing of the aqueous solution of poly(N-isopropylacrylamide) (PNiPAm) observed as the dissociation of transmittance curves in heating and cooling have been studied. For the liquid-liquid phase separation of the monomer model (N-isopropyl propionamide)/water binary mixture, no hysteresis occurs in demixing and remixing when the scanning rate (vT) is small enough. On the other hand, the aqueous solution of an atactic PNiPAm with 46% of the meso content (m) shows the hysteresis even under a very dilute condition (0.01 wt%) at a slow vT of 0.01 °C min−1. The difference between demixing and remixing temperatures at vT → 0 systematically increases as m increases. The effect of stereoregularity is more significant for the cooling (remixing) process of the aqueous PNiPAm solution compared with that for the heating (demixing) process. These results strongly suggest that hysteresis arises from the structure of phase diagrams of the aqueous PNiPAm solution in which the liquid-liquid phase separation and sol-gel transition coexist [F. Tanaka et al. React. Funct. Polym. 2013, 73 (7)]. Above the demixing temperature (Tdmx), a dilute and a concentrated PNiPAm solutions appear, and the concentrated one of which solution undergoes physical gelation. The physical gel formed at the polymer-rich region returns to a sol by cooling at the remixing temperature Trmx, which lies possibly at lower than Tdmx. Thus, the hysteresis in demixing and remixing is observed.

(Invited) Relationship Between the Phase Diagram and Hysteresis in Demixing and Remixing for Thermo-Responsive Polymers in Water

The effects of the stereoregularity on the hysteresis in demixing and remixing of the aqueous solution of poly(N-isopropylacrylamide) (PNiPAm), which is observed as the dissociation of transmittance curves in heating and cooling, has been studied in detail. For the liquid-liquid phase separation of the monomer model (N-isopropyl propionamide) / water binary liquid, there is no hysteresis in demixing and remixing when the scanning rate (v T) is slow enough. On the other hand, the aqueous solution of an atactic PNiPAm with 46% of the meso content (m) shows the hysteresis even at a very dilute condition (0.01 wt%) with a slow v T of 0.01 °C min−1. The difference between demixing and remixing temperatures (Dhys T) at v T ® 0 systematically increases as m increases. The effect of stereoregularity is more significant for the cooling (remixing) process of the aqueous PNiPAm solution compared with that for the heating (demixing) process. These results can be interpreted from the phase diagram of the aqueous PNiPAm solution, in which the liquid-liquid phase separation and sol-gel transition coexist [F. Tanaka et al. React. Funct. Polym. 2013, 73 (7)]. Above the cloud point temperature (T cp), a dilute and a concentrated PNiPAm solutions coexist, and the concentrated phase get into the physical-gel phase. The concentrated PNiPAm solution undergoes the gel-to-sol transition in cooling at a temperature below T cp, causing the hysteresis in demixing and remixing.

Study of theThermo-/pH-Sensitivity of Stereo-Controlled Poly(N-isopropylacrylamide-co-IAM) Copolymers via RAFT Polymerization.

In this work, a smart copolymer, Poly(nipam-co-IAM) was synthesized by copolymerization of N-isopropylacrylamide (nipam) and itaconamic acid (IAM) through reversible addition-fragmentation chain-transfer (RAFT) polymerization. Poly(nipam-co-IAM) has been studied previously synthesized via radical polymerization without stereo-control, and this work used cumyl dithiobenzoate and Ytterbium(III) trifluoromethanesulfonate as RAFT and stereo-control agents, respectively. The stereo-control result in this work shows that tacticity affects the lower critical solution temperature (LCST) and/or the profile of phase separation of Poly(nipam-co-IAM). In the pH 7 and pH 10 buffer solutions, the P(nipam-co-IAM) copolymer solutions showed soluble–insoluble–soluble transitions, i.e., both LCST and upper critical solution temperature (UCST) transitions, which had not been found previously, and the insoluble to soluble transition (redissolved behavior) occurred at a relatively low temperature. The insoluble to soluble transition of P(nipam-co-IAM) in alkaline solution occurred at a temperature of less than 45 °C. However, the redissolved behavior of P(nipam-co-IAM) was found only in the pH 7 and pH 10 buffer solutions and this redissolved behavior was more prominent for the atactic copolymers than in the isotactic-rich ones. In addition, the LCST results under our experimental range of meso content did not show a significant difference between the isotactic-rich and the atactic P(nipam-co-IAM). Further study on the soluble-insoluble-soluble (S-I-S) transition and the application thereof for P(nipam-co-IAM) copolymers will be conducted.

Stereospecific preparation of polyacrylamide with low polydispersity by ATRP in the presence of Lewis acid

The stereospecific atom transfer radical polymerizations of acrylamide were achieved in the presence of the Lewis acid Y(OTf)3 or AlCl3 as stereospecific catalyst using chloroacetic acid/CuCl/N,N,N′,N′-tetramethyl-ethylenediamine (TMEDA) as initiating system. The addition of Lewis acid Y(OTf)3 in the ATRP of acrylamide led to an increased polymerization rate and an improved tacticity of polyacrylamide (m∼71%) at the expense of controllability of the molecular weight distribution. In the case of AlCl3, the polymerizations were committed to afford the resultant polyacrylamide with lower polydispersity index ranging from 1.03 to 1.42 and well-controlled tacticity with meso content ranging from 57 to 76% depending on the different reaction conditions used. Lower temperature or higher concentration of the feeding Lewis acid helped to obtain the polyacrylamide with increased tacticity which revealed a decreased glass transition temperature (Tg).

Crystallization and Microstructure of Poly(l-lactide-co-meso-lactide) Copolymers

Small-angle X-ray scattering (SAXS), optical microscopy and differential scanning calorimetry were used to establish relationships between crystallization, microstructure, and equilibrium melting temperature (Tm0) for random copolymers of poly(l-lactide-co-meso-lactide). Tm0s derived using the Gibbs−Thomson and data-fitting23 approaches were found to be in excellent agreement and decreased significantly with increasing meso-lactide concentration. Analysis of these equilibrium melting temperatures using models for copolymer crystallization leads to the conclusion that the meso defects are rejected from the crystalline regions. From analysis of the SAXS data, it was concluded that the copolymers contained significant interfibrillar regions whose concentration increased with higher comonomer content. Spherulitic growth rates were strongly dependent on meso content and were analyzed using the Lauritzen−Hoffman kinetic theory of crystal growth.

Intramolecular chelation as a factor in the stereochemistry of anionic vinyl polymerization

AbstractThe methylation (CH3I) of 1,3‐bis(2‐pyridyl)butyllithium in THF at −78°C is highly meso‐selective (>98%) but the selectivity decreases with increasing cation‐size or ‐coordination. The reaction of 1,3‐bis(2‐pyridyl)butyllithium with other electrophiles such as i‐C3H7Br, PhCH2Cl, Me3SiCl (CD3)2CO and 4‐vinylpyridine is also stereoselective under these conditions giving meso‐like products. On the other hand, addition of 2‐vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2‐pyridyl group and this is supported by equilibria involving proton abstraction of 1,3‐bis(2‐pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (−1.4 and −1.3 kcal respectively) whereas the AH for K ion is very small (−0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (‐2.60 and −0.40 e.u. respectively). The lack of stereoregular polymerization of 2‐VP in the presence of Li ion is most likely due to the requirement that the Li ion of the newly formed 2‐pyridyl anion is coordinated with the 2‐pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2‐pyridine does not necessarily lead to isotactic‐polymerization.

Determination of the Tacticity of Poly(methyl methacrylate) by Hydrolysis Rate Measurements

The rate of hydrolysis of poly(methyl methacrylate) increases with the content of meso(isotactic) diads in the polymer. The increase in the rate of hydrolysis of isotactic polymers is due to an intramolecular nucleophilic attack by the adjacent carboxylic ion on the carbonyl carbon atom in the ester group. An analysis of the content of sequences of variously arranged ester and carboxylic groups shows that the hydrolysis of s-poly(methyl methacrylate) is a random process, while i-poly(methyl methacrylate) is hydrolyzed gradually (by zip mechanism), starting from the carboxylate group formed by an external attack by hydroxylic ions. The kinetic data thus obtained cannot be interpreted in terms of a simple kinetic model. The tacticity of poly(methyl methacrylate) may be estimated from the dependence of the rate of hydrolysis on the content of meso diads determinated by NMR spectrometry. The rate of hydrolysis depends not only on the tacticity of the polymer, but probably also on its configurational sequence statistics.

Cationic Polymerization of Exo-2,3-epoxynorbornane

Abstract The homopolymerization of exo-2,3-epoxynorbornane and its copolymerization with tetrahydrofuran were carried out at different temperatures in dichloroethane with triphenylmethyl hexafluoroarsenate as the initiator. The reaction products were characterized by infrared and nuclear magnetic resonance (NMR) spectroscopy as well as by gel-permeation chromatography (GPC), viscometry, and differential scanning calorimetry. The products showed low viscosities and exhibited low glass transition temperatures. The GPC molecular weight distributions of the products prepared at different temperatures were multi-nodal, the proportions of the low molecular weight component being higher in reactions carried out at low temperatures. The NMR analyses of the products obtained at various temperatures suggest that the meso content in the polymer decreases as the temperature of reaction is increased.