Novel Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers with pH/Temperature Response and Self-Assembly Behavior
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A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by 1H NMR and 19F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA30-b-PDEA10 diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA10-b-PDMA30 diblock copolymers can form flowerlike micelles in basic aqueous solution.Keywords:
Gel permeation chromatography
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Thermodynamics of micellization
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Thermodynamics of micellization
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Thermodynamics of micellization
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Thermodynamics of micellization
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The micelle formation and micellar size (diameter) of various surfactants were studied by light scattering. Each critical micelle concentration (cmc) determined from change in the intensity of scattered light with concentration was slightly higher than that based on change in surface tension with concentration. The concentration at which micelles could first be detected in solution by dynamic light scattering was considerably higher than the cmc determined as above. This difference may be due to the ability of light scattering to detect change in monomer or micelle concentration. Micellar size as determined by dynamic light scattering decreased with increasing concentration of surfactant due to intermicellar (micelle-micelle) interactions.
Thermodynamics of micellization
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Static light scattering
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The effect of dimethyl sulphoxide(DMSO)on micelle conformation of hexadecyltrimethylammonium bromide(CTAB)was studied by means of surface tension and negative staining TEM measurements. The results show that CTAB solution has two critical micelle concentration(CMC)values due to the effect of DMSO. The appearance of the second CMC of the CTAB/DMSO/H 2O system may be attributed to a transition process from sphere like micelle to rod like micelle.
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We present a theory of spherical micelle formation from cationic amphiphiles in the absence and in the presence of DNA. The distribution of micelle sizes as well as the critical micelle and aggregation concentrations (cmc and cac) are calculated. Micelle formation is favored by the hydrophobic tails but disfavored by the entropic cost associated with counterion condensation. Counterion release drives the complexation between DNA and amphiphiles and causes micellation at a much smaller concentration than in the absence of DNA. The stiffness of double-stranded DNA favors the formation of large micelles leading to a bimodal distribution of micelle sizes.
Cationic polymerization
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The fluorescence intensity rations (F2.F1) of excimer (F2) to monomer (F1) of pyrene were measured as a function of the concentration of sodium dodecyl sulfate (SDS). It was found that there were not gross changes in size and shape of sphere-shape micelles in the first micelle concentration, while at concentrations above the second critical micelle concentration (CMC) the micelles grew in size with increasing concentration. Fluorescence intensities of 8-anilinonaphthalene 1-sulfonate (ANS) were also monitored as a micellar probe with varying concentrations of SDS. Results suggested that a phase transition from sphere-shaped micelles to hemicapped rod-like micelles occurred at the second CMC (17). A general formula for the axial ratio of ellip-soil-shaped micelle in the first micelle concentration was suggested. According to this general formula, the axial ratio of SDS, sodium lauryl ether sulfate and sodium laurate were 1:1, 5:2, and 5:3, respectively. The electrolyte-induced phase transition from spherical to hemicapped rod-like micelles occurred and the size of hemicapped rod-like micelles grew with increasing electrolyte concentrations. The maximum concentrations of solubilzed benzene in sphere-shaped micelles and hemicapped rod-like micelles were measured by differential spectrohpotometry. The hemicapped rod-like micelles in the presence of electrolytes grew in size with increasing amount of benzene solubilized.
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The formation process of micelle by the ionic surface-active agent CTAB in water solution is studied by microcalorimetric method. The power-time curve of the micelle formation is determined. By setting the result, the authors get the critical micelle concentration (CMC) and thermodynamic parameter.
Thermodynamics of micellization
Isothermal microcalorimetry
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