A new method is presented that provides experimental information which is qualitatively and quantitatively sensitive to assumptions made as to the mechanisms of free radical entry and of latex particle formation in emulsion polymerization systems. The method consists of (1) obtaining (by electron microscopy) the full particle-size distributions (PSDs) at several different times soon after the cessation of latex particle nucleation, (2) using these PSDs to determine the volume dependences of the various rate coefficients governing particle growth by fitting the data to the appropriate evolution equations, and (3) employing these empirical rate coefficients to find that time dependence of the nucleation rate which fits the early-time PSD (again using the evolution equations). This method is quite sensitive to mechanistic assumptions: for example, one is able to determine whether or not the nucleation rate is an increasing or decreasing function of time. The technique is applied to a styrene nucleation system employing sodium dodecyl sulfate as surfactant at well above the critical micelle conventration. The data cannot be fitted even qualitatively by a simple one-step nucleation mechanis, whether it involes micellar entry or homogeneous nucleation. It is found, on the other hand, that the results can be accurately fitted by assuming that coagulation events between primary colloidal particles, perhaps formed by homogeneous nucleation, dominate both the nucleation process and the entry of free radicals into mature latex particles. In addition, the data indicate that the rate of free radical entry into the latex particles decreases with increasing particle size, at least for particles of unswollen radius less than ca. 40 nm. 相似文献
A detailed dynamic mathematical model that describes the evolution of particle size distributions (PSDs) during emulsion copolymerization reactions in a continuous loop reactor was developed and compared with experimental data. The model is based on the assumption that two distinct particle populations exist: precursor particles and stable latex particles. Precursor particles are colloidally unstable and therefore may undergo coagulation with other precursors and be absorbed by stable latex particles. It is shown that the kinetic model is able to reproduce the rather complex dynamic behavior of the vinyl acetate/Veova10 emulsion copolymerization in a continuous loop reactor, including the development of oscillatory responses of PSDs during reaction start‐up. It is also shown that, for the studied polymerization system, oscillatory responses are obtained only when both particle populations are assumed to exist and when both coagulative and micellar particle nucleations are simultaneously considered. 相似文献
Summary: The number (N) and size distribution of particles (PSD) of a styrene emulsion polymerization above the CMC were studied by means of asymmetric flow‐field flow fractionation (AF4). Bimodal PSDs were obtained, suggesting that coagulation of the primary particles is not as extensive as would be expected, according to the coagulative mechanism. AF4 allowed it to be demonstrated that N is constant during interval II, and that the resolution limit of other particle sizing techniques can lead to erroneous mechanistic inferences, from the evolution of N.
Particle size distribution measured at low conversion for the emulsion polymerization of styrene, obtained by AF4 and DLS. The initial surfactant (S0), initiator (I0) and monomer (M0) concentrations are indicated in the figure. 相似文献
The uptake kinetics of two major RDX (hexahydro-1,3,5-trinitro-1,3,5-triazacyclohexane) metabolites, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX), into passive sampling devices (PSDs), and the ability of PSDs to serve as surrogates for evaluating bioavailability of MNX and TNX were investigated in laboratory sand and two soil types. The results indicate that MNX and TNX absorption into PSDs was best fitted with a polynomial curve model: y = ax2 + bx + c (y: amount of MNX or TNX absorbed into PSD; x: incubation time of PSDs in soil), with an excellent correlation coefficient (>0.95) for each type of soil amended with 10 mg/kg MNX or TNX. TNX was more readily absorbed by PSDs than MNX. Soil conditions, especially organic matter content, affected MNX and TNX uptake into PSDs. A relatively good correlation between MNX and TNX uptake into PSDs and uptake into earthworms was obtained in two types of natural soils (a silt loam soil from Nebraska and a sandy loam soil from Texas) and laboratory sand. A linear relationship between PSD uptake and earthworm uptake was observed. The correlation coefficients (r2) were > or = 0.82 for all test soils spiked with MNX or TNX. Organic matter content is one soil factor that affected the ratio of MNX or TNX uptake into earthworms versus uptake into PSDs. These data indicate that C18 PSDs may be used as a surrogate for soil organisms such as earthworms and provide a simple and easy chemical test for assessing the bioavailability of contaminants in soils. 相似文献
Using the thin film polymerization approach, we have studied the texture evolution when synthesizing a novel liquid crystalline polymer (LCP) system based on Sumikasuper® LCPs. The main monomers used were p-acetoxybenzoic acid (ABA), 4,4′-biphenol (BP), isophthalic acid (IA), phthalic acid (PA) and terephthalic acid (TA). Polarizing optical microscopy (POM), FTIR, X-ray diffraction (XRD) and atomic force microscopy (AFM) were employed to study the thin film polymerization process and characterize the products. The generation and evolution of liquid crystal phases were monitored; the results revealed that there exists a composition range for the monomers to react and form liquid crystal materials. The critical temperature for LC formation in ABA/BP/IA system decreased with increasing ABA content. FTIR results confirmed the formation of polymers. AFM investigation suggested a similar process of morphological change to that observed using POM. An increasing surface roughness of the thin films with the progress of polymerization was also obtained from AFM analysis. A nematic LC texture of the polymer system was suggested by XRD examination. Results obtained by replacing BP with acetylated BP, and by conducting polymerization using two-monomer systems, suggest that BP units are included in polymers obtained by the thin film polymerization method. 相似文献
Radical polymerization of several alkyl allyl oxalates, including methyl allyl oxalate (MAO), ethyl allyl oxalate, propyl allyl oxalate, butyl allyl oxalate, and octyl allyl oxalate, was conducted in the evolution of carbon dioxide at elevated temperatures, and was compared with the anomalous polymerization behavior of diallyl oxalate (DAO) discussed in our earlier article
1 A. Matsumoto, I. Tamura, M. Yamawaki, and M. Oiwa, J. Polym. Sci. Polym. Chem. Ed., 17 , 1419 (1979).
. The kinetic equations for the polymerization of alkyl allyl oxalate were derived following the kinetic treatment of the DAO polymerization by further consideration of the absence of cyclization of the growing polymer radical and the effective reinitiation by alkyl radical, and were then satisfactorily applied to the polymerization of MAO, as a representative alkyl allyl oxalate. The evolution of carbon dioxide in the polymerization of alkyl allyl oxalates was enhanced with the increase of bulkiness of the alkyl substituent, as a result of steric suppression of the propagation of the growing polymer radical. 相似文献
Kinetics of the in situ bulk polymerization of methyl methacrylate in the presence of organomodified montmorillonite (MMT)
was investigated using differential scanning calorimetry (DSC) and gravimetrically. Different amount and types of MMT under
the trade names Cloisite were employed. Using DSC, the amount of heat released versus time, under isothermal conditions, was
recorded, and eventually, the time evolution of polymerization rate and monomer conversion was calculated. Results on the
variation of monomer conversion with reaction time were in good agreement to corresponding from the gravimetric measurements.
The nanocomposites prepared were characterized with WAXD, TEM and FTIR, and their glass transition temperature, Tg, was measured with DSC. Depending on the added amount of nano-MMT, either exfoliated or intercalated structures were obtained.
An enhancement of the polymerization rate with the presence of the nanoparticles was observed especially in the gel effect
region. This was accompanied by a higher Tg and average molecular weight, as measured by GPC, of all nanocomposites compared to neat PMMA. 相似文献
Time-resolved gel-permeation chromatography (GPC) profiles were measured over a long period of time (t=0–768 h) for the HNO3-catalyzed reaction mixture of a well-defined polymeric silane coupling agent, triethoxysilyl-terminated polystyrene. The results showed that HNO3 catalyzes the rate of polymerization, but only to a small extent. Two-dimensional (2-D) GPC correlation spectra were calculated from time-resolved GPC profiles and were used to examine the mechanism of polymerization. The resolution enhancement obtained by 2-D correlation provided ample evidence for existence of four monomeric components. It has been demonstrated that dynamic variation in the population of these components occurs over a long reaction time, reflecting the mechanism of a slow polymerization process. 相似文献
Molecular dynamics of diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with an epoxidized poly(styrene-b-butadiene) (SepB) linear block copolymer has been monitored during cure and microphase separation process by dielectric relaxation spectroscopy (DRS) for wide frequency and temperature ranges. Different primary and secondary relaxation processes have been analyzed for neat components and ternary mixture. Relaxational behaviour has been modelled with Havriliak-Negami, Vogel-Fulcher-Tammann and Arrhenius equations and fitting parameters and their evolution have been obtained. The retention of the epoxidized poly(butadiene) (PepB) block in the epoxy-rich phase during all the polymerization process, previously detected by our group with atomic force and transmission electron microscopies, has been confirmed by dielectric relaxation spectroscopy. The evolution of molecular dynamics during the polymerization process of the epoxy resin in the ternary system indicates a change in the trend of the main relaxation at times that agree with phase separation detected by rheology. 相似文献
Summary: A “series” hybrid model based on material balances and artificial neural networks to predict the evolution of weight average molecular weight, , in semicontinuous emulsion polymerization with long chain branching kinetics is presented. The core of the model is composed by two artificial neural networks (ANNs) that calculate polymerization rate, Rp, and instantaneous weight‐average molecular weight, from reactor process variables. The subsequent integration of the material balances allowed to obtain the time evolution of conversion and , along the polymerization process. The accuracy of the proposed model under a wide range of conditions was assessed. The low computer‐time load makes the hybrid model suitable for optimization strategies.
Abstract Aging is one of several processes that are known to affect exposure of chemicals to organisms by decreasing the available fraction of chemical contaminants in soil. This phenomenon has important implications in the assessment of the hazards of chemicals and regulations for soil cleanup. Passive sampling devices (PSDs) are potentially direct chemical indicators for assessing bioavailability of pesticides (and other chemicals). PSDs consist of lipophilic material within a semi-permeable membrane, similar to biological systems. In this study, a pesticide mixture was aged in soil for up to eight months. Earthworms and PSDs were placed in soil and chemical uptake into both was determined over time. Uptake rates into PSDs and maximum concentrations were observed to positively correlate with uptake rates and maximum concentrations in earthworms for both of the soil types studied (sandy loam, silt loam). These results indicate that PSDs may be used as a surrogate for earthworms and provide a chemical technique for assessing the availability of aged chemical residues in soil. 相似文献
An approach for polymerization to produce polymethylmethacrylate (PMMA) was developed, in which the reaction was initiated
by the glow discharge electrolysis (GDE) rather than chemical initiators. The highest number-average molecular weight (Mn) and the lowest polydispersity index (PDI) of the resulting polymer were 1.12 × 106 g/mol and 1.21, respectively. The following parameters such as the applied voltage, discharge time, the content of methylmethacrylate
(MMA), the amount of a suspension stabilizer (polyvinyl alcohol), polymerization temperature and time were examined in detail,
which could affect the conversion, molecular weight and polydispersity index. The Mn and PDI of polymer can be monitored by changing the discharge parameters and polymerization conditions. PMMA was characterized
by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), cold field emission scanning electron
microscopy (FESEM), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). Results indicate that using the
GDE technique to initiate the polymerization reaction is successful, because the product obtained has the same properties
with one obtained by chemical method, for example, in chemical structure, tacticity and thermal stability. Moreover, the polymer
particles for the former are smaller than the latter. The kinetic observation was that the polymerization of MMA initiated
by the GDE plasma obeys the first order of reaction with an obvious induction period. 相似文献
A new solution for chain‐length distributions of “dead” polymer and their moments (obtained by solving the coupled system of non‐linear differential equations in full generality) is presented and compared to existing theoretical and numerical approaches. The new solution for chain‐length distributions (cld) (at least numerically) coincides with our earlier results (analytical solution as well as data computed by use of numerical models) which were obtained by starting with strict coupling of time and degree of polymerization followed by an a posteriori Poisson broadening. (As a consequence the new solution also coincides with a numerical treatment based on fluctuating propagation probabilities by introducing an a priori Poisson broadening.) In turn, for termination exclusively by disproportionation this earlier solution is fully equivalent to Aleksandrov's approach (after some modification of the latter expression). This holds true also for termination exclusively by combination if one corrects for errors in Aleksandrov's derivation. The new solution of the moments of the cld agrees with our former results, while Aleksandrov's expressions are shown to be worthless under the usual experimental conditions. 相似文献
The polymerization of methylated β‐cyclodextrin (m‐β‐CD) 1 : 1 host‐guest compounds of methyl methacrylate (MMA) ( 1 ) or styrene ( 2 ) is described. The polymerization of complexes 1 a and 2 a was carried out in water with potassium peroxodisulfate (K2S2O8)/sodium hydrogensulfite (NaHSO3) as radical redox initiator at 60°C. Unthreading of m‐β‐CD during the polymerization led to water‐insoluble poly(methyl methacrylate) (PMMA) ( 3 ) and polystyrene ( 4 ). By comparison, analogously prepared polymers from uncomplexed monomers 1 and 2 in ethanol as organic solvent with 2,2′‐azoisobutyronitrile (AIBN) as radical initiator showed significantly lower molecular weights and were obtained in lower yields in all cases. Polymerization of m‐β‐CD complexed MMA in water, initiated with 2,2′‐azobis(N,N ′‐dimethyleneisobutyroamidine) dihydrochloride, occurred much faster than the polymerization of uncomplexed MMA in methanol under similar conditions. Furthermore, it was shown, that the precipitation polymerization of complexed MMA from homogeneous aqueous solution can be described by equations (Pn–1 ∝ lsqb;Irsqb;0.5) similar to those for classical polymerization in solution. 相似文献