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  • 1
    Digitale Medien
    Digitale Medien
    Stretching and overstretching of DNA in pulsed field gel electrophoresis. I. A quantitative study from the steady state birefringence decay (1993)
    New York : Wiley-Blackwell
    Biopolymers 33 (1993), S. 1347-1357 
    Details
    ISSN: 0006-3525
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Using a sensitive birefringence instrument, the birefringence arising from the orientation of the DNA chain during electrophoretic transport has been recorded. This birefringence is shown to proceed both from the alignment (stretching) of the molecule in the direction of the electric field and from the extension of the length of its primitive path (overstretching). The contribution of these two processes can be separated in the decay of the birefringence after the end of the application of the electric field. The fast relaxation of the overstretching occurs first and is demonstrated to be the main contribution to the birefringence. The orientation factor of the remaining stretched state and its decay can be quantitatively understood using the biased reptation model. It provides, in addition, a high value for the tube diameter or gel pore size a (4500 ± 450 Å for a 0.7% agarose gel with a c-0.6g dependence in the agarose concentration cg) and a low value for the effective charge per base pair (0.2e as compared to 0.5e using the condensation hypothesis). The contribution of overstretching to the birefringence is also quantitatively interpreted in term of the change in the mean length l of DNA inside a pore size a. The dynamics of decay of this overstretching is well represented by a stretched exponential with a stretching exponent α = 0.44. The mean decay time decreases slightly with increasing fields and scales with the overall DNA length close to N20. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 14 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bip.360330905
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  • 2
    Digitale Medien
    Digitale Medien
    Stretching and overstretching of DNA in pulsed field gel electrophoresis. II. Coupling of orientation and transport in initial response to the field (1993)
    New York : Wiley-Blackwell
    Biopolymers 33 (1993), S. 1359-1363 
    Details
    ISSN: 0006-3525
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The rise of the birefringence following the application of an electric field to DNA in an agarose gel has been quantitatively analyzed to demonstrate the major role played by the chain overstretching. By analyzing the field free decay after short times, we demonstrate that the overshoot is completely due to overstretching. Its time of appearance τov varies with the field and DNA length in fair agreement with the E-lN0ln N0 prediction of Lim et al. [ (1990), Journal of Chemical Physics, Vol. 92, pp. 709-721]. The τov is also the time corresponding to one tube renewal after relaxation of the overstretching. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bip.360330906
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  • 3
    Digitale Medien
    Digitale Medien
    Synthesis and initiation/transfer properties of carbamoyl disulfides (1993)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 194 (1993), S. 2897-2906 
    Details
    ISSN: 0025-116X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Three model tetraalkylcarbamoyl disulfides were prepared and their initiation/transfer capabilities tested. The compounds studied were: bis(1-piperidinecarbonyl) disulfide (1), bis(4-(2-hydroxyethyl)piperazine-1-carbonyl) disulfide (2), bis[N-ethyl-N-(2-(trimethylsiloxyethyl)-carbamoyl)] disulfide (3). It was found that, in contrast to tetraalkylthiruam disulfides, the carbamoyl disulfides were not able to initiate free radical polymerization of methyl methacrylate (MMA) and styrene (ST), even at the highest temperatures applicable to these monomers. Apparently, the central S - S bond in carbamoyl disulfides is much stronger than the corresponding S - S bond in thiuram disulfides. Moreover, the carbamoyl disulfides are rather poor transfer agents. The transfer constants were estimated to be 2,5 10-3, 18.10-3 and 2,8. 10-3 for 1,2 and 3 in thermal polymerization of MMA at 110, 110 and 100°C, respectively, and 2,1. 10-3 for 3 in polymerization of MMA initiated with AIBN at 70°C. Chemistry aspects of synthesis and stability of hydroxy-functionalized carbamoyl disulfides are discussed.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/macp.1993.021941022
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  • 4
    Digitale Medien
    Digitale Medien
    Triblock copolymers via thermal “macroiniferters”: Block copolymers of ethyl acrylate with methyl methacrylate or styrene (1991)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 192 (1991), S. 579-590 
    Details
    ISSN: 0025-116X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The concept of macro iniferter (MI) was applied to realize the synthesis of triblock copolymers of ethyl acrylate (EA) with styrene (St) or methyl methacrylate (MMA), with EA forming the end blocks. The syntheses involved 3 steps. Secondary amine terminated poly(ethyl acrylate) was synthesisted by polymerization of EA in presence of butyl-(2-mercaptoethyl)ammonium chloride as a functional chain-transfer agent. The macroamine so obtained was converted to the macrothiuram disulfide by reaction with CS2 and I2. Thermal polymerization of MMA or St in presence of this macroiniferter led to the respective triblock copolymers. The kinetics of polymerization of MMA and styrene using MI and two different chain lengths was done in limited concentration ranges. The kinetic parameters indicated that the iniferter action was not affected by the incorporation of the thiuram disulfide groups in the polymer backbone. Triblock copolymers of differing block lengths of hard and soft segments could be prepared by varying the chain length of the macroamine and the concentration of the iniferter in the reaction system. The copolymers were characterized by GPC analysis and spectral or elemental analyses. In the case of St, analysis showed formation of perfect triblocks, whereas for MMA, tendencies to form diblocks were observed on increasing the chain length of the iniferter-forming block. DSC analyses showed demixing of the soft EA block and the hard central blocks.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/macp.1991.021920310
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  • 5
    Digitale Medien
    Digitale Medien
    Arylpropenoates as photoactive units in photocontrollable polymers (1992)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 193 (1992), S. 1499-1508 
    Details
    ISSN: 0025-116X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The synthesis of photochemical properties of a range of arylpropenoates are described and their suitability as photoactive units for the reversible manipulation of polymer properties assessed. The Z and E isomers of the 1-naphthyl-, 1-(4-methoxynaphthyl)- and 9-phenanthryl- derivatives have sufficiently different absorption characteristics to allow their selective exciation to give photostationary states having high concentrations of each isomer. Incorporation of the photoactive units into methacrylate based polymers changes the photostationary state concentrations of the Z and E isomers but the effect on the 1-(4-methoxynaphthyl)-derivative is small and here the photointerconversion is not accompanied by fatigue. The 9-anthryl derivative undergoes facile (4π + 4π) photodimerization and this may have potential as a reversible crosslinking mechanism.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/macp.1992.021930626
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  • 6
    Digitale Medien
    Digitale Medien
    The synthesis of siloxane-based polymers containing photoactive cinnamate moieties (1991)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 192 (1991), S. 2801-2810 
    Details
    ISSN: 0025-116X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Three ways of synthesizing cinnamate-based siloxane polymers suitable for photoisomerization purposes have been employed. The route involving direct hydrosilation of the photoactive unit and a non-photoactive alkene with the homopolymer poly(methylsiloxane) has the severe disadvantages that the polymer is coloured, crosslinking during work-up or storage commonly occurs and also some of the cinnamate CC double bonds are reduced during the hydrosilation. This loss of the chromophore can be decreased but not eliminated by its hydrosilation with the copolymer poly(dimethylsiloxane-co-methylsiloxane). All of the available Si—H sites are now reacted with the cinnamate ester but a low concentration of active units on the polymer is still maintained. The most successful method of preventing loss of the chromophore is best achieved by functionalising the polymer via hydrosilation with allyl acetate, followed by hydrolysis of the acetate to the alcohol and finally esterification with the acid chloride of the chromophore. This procedure in contrast to the other methods, yields the photoactive polymer as an almost colourless, mobile liquid which is stable on storage and in which no reduction of the chromophore has occurred.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/macp.1991.021921124
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  • 7
    Digitale Medien
    Digitale Medien
    The effect of chain transfer agent on the entry of free radicals in emulsion polymerization (1992)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 193 (1992), S. 303-313 
    Details
    ISSN: 0025-116X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The model of Maxwell et al. (Macromolecules 24, 1629 (1991)) for initiator efficiencies in emulsion polymerization has been extended to take into account the effect of added chain transfer agent (CTA). The model supposes that the rate-determining step for radical entry into latex particles is aqueous-phase propagation of the primary free radicals to a critical chain length z required for entry (forming species such as .MzSO-4, where M is a monomer entity and peroxodisulfate S2O2-8 is the initiator). The effect of CTA on the entry rate is assumed to occur by facilitating the production of aqueous-phase free-radical species (CTA.) by transfer between species such as .MzSO-4 (where n 〈 z) and CTA in the aqueous phase. The CTA. will be formed at a reasonable rate provided the CTA is not too water-insoluble (e.g. C12H25SH); it should also be capable of entering the latex particles rapidly because of its relative insolubility. If the monomer-derived .MnSO-4 tend to suffer aqueous-phase termination rather than entry, the overall rate of entry (and hence initiator efficiency) will be increased. This can explain the accelerating effect of intermediate molecular weight CTA's on emulsion polymerization of monomers such as butadiene, where z is large and hence initiator efficiency is very low in the absence of CTA, because most .MnSO-4 undergo termination rather than entry into the latex particles.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/macp.1992.021930201
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  • 8
    Digitale Medien
    Digitale Medien
    Liquid crystal elastomers: Synthesis and characterization (1990)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 28 (1990), S. 1455-1472 
    Details
    ISSN: 0887-624X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The synthesis is described of some acrylate based polymers containing both a mesogenic group (4-cyanophenyl benzoate) connected to the polymer backbone by a flexible spacer, and varying degrees of crosslinking, introduced by copolymerization. Optical and calorimetric techniques are used to determine the phase behavior, which is found to depend strongly on the crosslink density (and the flexible spacer length); though (nominally) a relatively high degree of crosslinking is possible without completely destroying the liquid crystal phase. Mechanical measurements in conjunction with supplementary analytical data, where available, are used to determine the efficiency of copolymerization at introducing intermolecular crosslinking. Some possible explanations for the unexpectedly low efficiency of this process are discussed. The properties of materials prepared in this fashion are compared briefly with some examples where crosslinking is introduced after polymerization; this latter process is seen to be rather more efficient.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pola.1990.080280613
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  • 9
    Digitale Medien
    Digitale Medien
    Consistent values of rate parameters in free radical polymerization systems. II. Outstanding dilemmas and recommendations (1992)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 30 (1992), S. 851-863 
    Details
    ISSN: 0887-624X
    Schlagwort(e): polymerization ; kinetics ; free radical ; transfer ; photolysis ; propagation ; rate coefficients ; termination ; EPR ; copolymerization ; initiation ; Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The problems of determining reliable, well-characterized values of kinetic parameters in free-radical polymerizations are discussed. The origins of the fact that experimental determinations of rate coefficients of ostensibly identical systems often result in quite different values being reported can be ascribed to subtle mechanistic assumptions made in data interpretation, which are considered in detail. A series of recommendations to assist in overcoming these problems, and to highlight their origins, are presented, with emphasis placed on new techniques including those employing laser photolysis and EPR.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pola.1992.080300516
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  • 10
    Digitale Medien
    Digitale Medien
    Cinnamate ester containing liquid crystalline side chain polymers (1991)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 29 (1991), S. 251-259 
    Details
    ISSN: 0887-624X
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The synthesis of methacrylate esters of 4-cyanophenyl-(4-(ω-hydroxyalkyloxy)) cinnamates, with spacer lengths of 2 and 6 methylene units and the synthesis of the corresponding acrylate ester with a spacer of 2 methylene units are described. The methacrylate monomers were polymerized by free radical polymerization, both as homopolymers and as copolymers with the analogous benzoate monomer of spacer length 6. The acrylate ester could not be polymerized successfully under the same reaction conditions. Polymers were characterized by NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermo-optic observations. Of the monomers prepared, only the cinnamate with a hexamethylene spacer shows a mesophase, seen on supercooling of the melt. All of the polymers prepared were liquid crystalline, with smectic behavior predominating in the polymethacrylates with the longer spacer group. A narrow nematic region is seen just below the clearing temperature with a range of 3-9°C, nematic character is increased in the copolymer series with the degree of incorporation of the cinnamate monomer with the spacer group of length 2.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pola.1991.080290213
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