Crosslinked polymers, nuclear

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Although crosslinked polymers and polymer gels are not soluble, the spectra of swollen, low crosslink density networks exhibit reasonably narrow C-13 NMR line widths, sufficiently resolved to reveal details of microstructure 13S). Thus, recording the spectra under scalar low power decoupling yields characterization information and some dynamic measurements, concerning T, T2 (line widths) and nuclear Overhauser enhancement (NOE) for lightly crosslinked polymers. 

Schaefer J, High-resolution pulsed carbon-13 nuclear magnetic resonance analysis of some crosslinked polymers, Macromolecules, 4 110-112, 1971. 

Therefore, a preliminary NMR study of the Nuclear Overhauser Effect on linear pol)rmers and crosslinked polymer was made. The NMR spectra were taken on resin slurries in deuterochloroform. 

If we were to have an isolated polymer chain with a single nuclear spin attached to each segment (the marked chain) crosslinked into an unmarked network, the second moment of the NMR line of that spin species would carry information relating to the separation of chain segments, and to their relative orientation with respect to the field direction. If the network were to be subjected to a bulk deformation, these geometrical parameters would be altered, and hence we would expect a corresponding change in the value of the experimentally measured... 

A brief investigation of the potential thermal and photo-lytical chemistry of resins 3 and 5 was undertaken. It was prompted by a report of the observation of site-site isolation on irradiation of 18% cross linked 3 (22). This is in contrast to soluble CpCo(C0)2 which forms di- and trinuc.lear clusters under the same conditions ( 33, 34). Similar apparent prevention of the formation of higher nuclear species was noted in the decarbonyla-tions of CH2Fe(C0)2H (22) and CH2Cgi(C0) 3H (M = Cr, Mo, W) (35), and in the catalytic activity of CfoCpTiCp (25). However, these species are all supported by a relativelyTiLghly cross linked polymer, and there was ample indication that lesser crosslinking enables "bimolecular" reactions of active centers (36,37,38,39). 

It can be concluded that it is very difficult to predict the result from a polymer macrostructure, but it is relatively easy to measure the secondary species generated on irradiation by using known analytical techniques, such as measuring swelling, tensile tests, analysis using nuclear magnetic resonance (NMR), etc. The yield is then expressed by the G value, which represents the number of cross-links, scissions, double bonds, etc., produced for every 100 eV (1.6 X 10 J) dissipated in the material. For example, G (cross-links), abbreviated G(X), = 3.5 means that 3.5 cross-links are formed in the polymer per 100 eV under certain irradiation conditions. Similarly, the number of scissions formed is denoted by G(S). In order to determine the number of crosslinks or G(X), the number of scissions or G(S), etc., it is necessary to know the dose or dose rate and the time of exposure for these irradiation conditions. From the product yields it is possible to estimate what ratio of monomer units in a polymer is affected by irradiation. ... 

One of the most characteristic properties of crosslinked rubbers is the ability to swell in appropriate solvents to a constant volume. Not only is this property exploited for estimation of parameters such as crosslink densities and polymer-solvent interaction parameters, but the resultant change in nuclear magnetic resonance (NMR) parameters allows a large number of new and interesting NMR experiments. It is the aim of this chapter to introduce some simple concepts of polymer swelling and to examine the information obtainable for the range of NMR experiments possible on swollen gels. 

Manatt SL, Horowitz D, Horowitz R, Pimnell RP, Solvent swelling for enhancement of carbon-13 nuclear magnetic resonance spectral information from insoluble polymers chloromethylation levels in crosslinked, Anal. Chem., 52 1529-1532, 1980. 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

The direct detection of radiation induced crosslinks in polyethylene has been a major goal of radiation chemists for many years. It was recognized as early as 1967 that solution 13c nuclear magnetic resonance (NMR) spectroscopy could be used to detect structures produced in polymers from ionizing radiation. Fischer and Langbein(l) reported the first direct detection of radiation induced crosslinks (H-links) in polyoxymethylene using 13c NMR. Bennett et al.(2) used 13c NMR to detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the molten state. Bovey et al.(3) used this technique to identify both radiation induced H-links and long chain branches (Y-links) in n-alkanes... 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

Characterization of the chemical structure of highly cross-linked polymers, and of the chemical changes that accompany degradation processes, relies on spectroscopic methods. Solid-state nuclear magnetic resonance techniques have the potential to allow a more detailed characterization than before possible of the chemical environment and structure of chemical crosslinks in elastomers and thermoset epoxies. Degradation processes in cross-linked systems have been studied by using infrared spectroscopy, solid-state NMR, and electron spin resonance. 

Disuccinimidyl (DS)-PEG, a crosslinker of hydrogel, has been synthesised by the reaction between N,N -disuccinimidyl carbonate (DSC) and the hydroxyl groups at both ends of PEG-diol. Firstly, DSC and triethylamine were added to anhydrous acetonitrile containing PEG-diol. The synthesised polymer was precipitated, filtered and then lyophilised in benzene. The molecular weight of the obtained polymers was then determined using gel permeation chromatography and H-nuclear magnetic resonance [83]. 

Koizumi, H. Taguchi, M. Kobayashi, Y, and T. Ichikawa. 2003. Crosslinking of polymers in heavy ion tracks. Nuclear Instruments and Methods in Physics Research B 208 161-165. 

CDs can form inclusion complexes with iodine, which makes them candidates for iodine-sorption from nuclear waste gases. In model experiments, it was shown that the aqueous solutions containing CDs and crosslinked CD polymers were selective and effective iodine absorbers [86]. Especially the cr-CD derivatives (methylated and crosslinked) have high sorption capacity. On the basis of the results, the binding of elemental and organic iodine emitted into the air by chemical and nuclear power plants can be made effectively by immobilizing iodine vapor in aqueous CD solutions or in CD polymer gel beds. Such new sorbents can be employed in the air filtration systems (Table 8.4). 

The relaxation dynamics of junctions in polymer networks have not been well known until the advent of solid-state P NMR spin-lattice relaxation measurements in a series of poly(tetrahydrofuran) networks with tris(4-isocyanatophenyl)-thiophosphate junctions (Shi et al., 1993). The junction relaxation properties were studied in networks with molecular weights between crosslinks, Me, ranging from 250 to 2900. The dominant mechanism for P nuclear spin lattice relaxation times measured over a wide range of temperatures were fit satisfactorily by spectral density functions, 7([Pg.225]

In attempts to reconstitute nucleosomes on SV40 circular DNA, Cap-lan et al. (37) observed that if the histones were ADP-rihosylated prior to reconstitution assembly of nucleosomes was inhibited by as much as 80%. If nucleosomes were first allowed to assemble, then modified by ADP-ribosylation, no eflPect on stability was observed. These data led the authors to speculate that ADP-ribosylation of histones prior to nucleosome assembly might be of physiological importance. Perella and Lea (168, 169) have shown that in rat liver nuclei, polyamines cause an increase in histone Hi ADP-ribosylation and histone HI dimer synthesis, which is accompanied by a decrease in core histone ADP-ribosylation. Data such as these have led to speculation that Hi dimer formation may function in the condensation or stabilization of chromatin fibers (35, 119). The data of Lorimer et al. (124) indicate that dimer synthesis is inversely related to the nuclear activity of the poly(ADP-ribose) glycohydrolase. Thus, these authors (124) inferred that the Hl-Hl-polymer complex formation is of a transient nature. As pointed out by Purnell et al. (171), if this crosslink were to function in the stabilization of chromatin, the modified histone Hi would have... 

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