Concentrations crosslink

Aldehydes (glutaric, succinic [53-56]) are used to crosslink PVA, the network density being controlled by the ratio of aldehyde to OH-groups. The swelling degree of these hydrogels does not exceed 35-40 ml g-1, and there are almost quantitative correlations of the network density with the crosslinker concentration... 

To determine the crosslinking density from the equilibrium elastic modulus, Eq. (3.5) or some of its modifications are used. For example, this analysis has been performed for the PA Am-based hydrogels, both neutral [18] and polyelectrolyte [19,22,42,120,121]. For gels obtained by free-radical copolymerization, the network densities determined experimentally have been correlated with values calculated from the initial concentration of crosslinker. Figure 1 shows that the experimental molecular weight between crosslinks considerably exceeds the expected value in a wide range of monomer and crosslinker concentrations. These results as well as other data [19, 22, 42] point to various imperfections of the PAAm network structure. 

The Effect of Crosslinker Concentration on the Rate of Polymerization. Ethylene glycol dimethacrylate is used most frequently as the crosslinker for HEMA formulations useful in contact lens manufacturing. To demonstrate the effect of crosslinker concentration on the curing rate, formulations derived from HEMA/Glycerine/BME at 85/15/0.17, while varying EGDMA (from 0.34 to 0.68), the peak times were about the same (3.73 and 3.61 minutes respectively). This is reasonable due to the similarity in molecular structure of the crosslinker and the monomer, and the low amount of crosslinker used. The possible presence of other crosslinker, such as the dimerization product of HEMA, is even less a factor to be considered in polymerization kinetics, due to low concentration (normally much less than 0.1 %, in-house information). 

From N2 absorption by BET method b X = crosslinker concentration, PV90 organic phase/aqueous phase = 10/90 (90% phase volume) PE = petroleum ether T = toluene. 

Table III. Calculations of Radical, Scission, and Crosslink Concentrations in Irradiated Polystyrene...

The results of the friction measurements are shown in Fig. 8. The friction coefficient of the poly(acrylamide) gel decreases with an increase of the crosslinker concentration in the major part of the concentration range studied, except that it sharply decreases at a mole fraction of 0.2% of the cross-linker concentration. The measurements cannot be performed below this concentration because the gel becomes too soft to be used in the present experimental setup. It suggests that the system is dose to the gelation threshold. The sharp decrease of the friction observed in the gel with a mole fraction of 0.2% may be due to the effect of the sol-gel transition. It is also found from visual inspection that the gels becomes opaque above a cross-linker concentration of 3 mole-% fraction. 

The crosslink concentration is a very important structural parameter because it is directly related to the elastic modulus of the network in the... 

Let us consider an A2 + A4 chainwise polymerization e.g., a vinyl - divinyl system with af = 0.01 (a very small concentration of the crosslinker in order to keep ideal conditions), and q = 0.999. Figure 3.22 shows the fraction of tetrafunctional crosslinks, nc 4/[A4o], as a function of conversion, for two limiting values of ,. Termination by combination (E, = 1) increases the crosslink concentration with respect to termination by chain transfer or disproportionation (2, = 0). At full conversion, termination by combination leads... 

By plotting (Tg-Tg,)/veT gi versus F, one must obtain a straight line of slope Kdm. This approach was tested on about 100 systems of Tg values ranging from 270 to 520 K, and crosslink densities ranging from 0 to about 10 mol kg-1. We obtained excellent results with KDM = 2.91, when the crosslink density was expressed in terms of crosslink concentration (for trifunctional crosslinks). This leads to KDM 2, when the crosslink density is expressed in chain concentration. 

Separations of complex steroid mixtures were achieved recently by Que et al. [76] using both isocratic and gradient elution. Mass spectrometric detection gave femto-mole detection limits while laser-induced fluorescence of dansylated ketosteroids ranged in attomole levels (Fig. 10.16). Monolithic column packings were used with a 35 cm (25 cm packed bed) x 100 pm i.d. capillary packed with a polymer prepared from 5% T (total monomer concentration), 60% C (total crosslinker concentration), 3% polyethylene glycol, 10% vinylsulfonic acid and 15% lauryl acrylate. Details of the monolithic column preparation can be found in refs. 36,76, and 193. Similar monolithic columns can be used for the separation of bile acids [194],... 

All the reactants are individual compounds of high chemical purity with epoxy equivalent close to theoretical values. It has been shown 6,7,16,17) that, at Tcure iS 110-120 °C and especially with diglycidyl ether of resorcinol (DGER) or Bis-phenol-A (DGEBA) and w-phenylenediamine (w-PhDA) as reactants, Eq. (I) proceeds practically without side reactions and yields a polymer with a chemical structure very close to that shown in above. Using different initial ratios of reactants (P = [NH]0/[EP]0 molar ratio), one can prepare a family of polymers with rather broad variations of crosslink concentration per unit volume or number of unreacted groups of different type and properties of the final products. 

Since the DGEBA/DDS networks are tetrafunctional and of stoichiometric composition, the theoretical value of z is 2. Furthermore, the crosslink concentration, c, is simply the DDS molecule concentration. Performing the necessary calculations yields the theoretical M, listed in Table 4. Compared to the experimental M, the theoretical values are very consistent. If it is assumed that the DGEBA/DDS networks are not phantom-like (i.e., A = 1), then the ratio of the theoretical and experimental values may serve as an estimate of the dilation factor, These ratios are listed in Table 4, and show that is approximately unity for all the networks. If the experimental M had been calculated using the actual network densities (instead of q = 1 g/cm), the ratios would be even closer to unity, being reduced by approximately 20 percent. 

The networks were prepared using a different molecular weight of hydroxyl terminated polydimethylsiloxane (PDMS-OH) and in different crosslinker concentrations. The swelling results are summarized in Table 1. From Table 1 we can conclude that both the molecular weight of the hydroxylterminated polydimethylsiloxane and the crosslinker concentration have an important influence on the emission of the probe. The self reaction of the crosslinker and the flexibility of the network are important quantities to describe the emission behavior of the probes applied in this work. 

Table 1. Summary of the swelling results in dependence on the crosslinker concentration and the molecular weight of the siloxane used, swelling solvent tetrahydrofurane THF. ...

Fig. 3. Fluorescence spectra of dimethylaminobenzoic acid esters in swollen polydimethylsiloxane networks as a function of the crosslinker concentration corresponding [-Si-0H] [-Si-0-C2H5 ratios are drawn in the figure, the intensity is plotted in a.u.

Fig. 3 Scission and crosslinking concentrations produced in polypropylene after 16 weeks and 34 weeks of UV exposure in the laboratory. Samples taken at different depths from the exposed surface. Each sample was 0.1 mm deep the corresponding result is plotted at the midpoint of the layer. (More details in Shyichuk, A.V. Turton, T.J. White, J.R. Syrotynska, I.D. Different degradability of two similar polypropylenes as revealed by macromolecule scission and cross-linking. Polym. Degrad. Stab. 2004, 86, 377-383.)...

Particle Shape. The delineation of two families of materials on the basis of particle shape is very clear from the electron microscope evidence. That the families are xx, lx and xl, 11 points directly to the presence or absence of crosslinking in the PBMA as a source of particle shape differentiation. Overall concentration of the PBMA precursors is not a strong factor, since the irregular particles were observed in all three of the xx compositions, but an examination of the effect of crosslinker concentration alone was not carried out. In related work on an epoxy-butyl acrylate system in which component polymerization rates and the simultaneity of the reactions were matched, it was reported (3) that, prior to gelation of the matrix, irregular particles of crosslinked acrylate were formed but spherical particles were found in the absence of crosslinker. Together with the observation of an apparent bimodal size distribution, our results are similar, even though our system and conditions are markedly different from those in the earlier study. 

Fig. 20 Phase-contrast micrographs of BFAECs on various gels at the initial stage (6 h, column 1) and at a prolonged culture time (120h, column IF). Crosslinker concentration of gels PVA 6, PAA 2, PMAA 1, PNaSS 10, and PNaAMPS 6mol%. Original magnification lOx scale bar. 100 pm. (Reproduced, with permission, from [95])...

Laboratory and Numerical Simulation Studies Both 2D and 3D physical models were built to study the effectiveness of the profile control. In the 2D model, the incremental oil recovery factor was 8.19% over aquifer drive. In the 3D model, the incremental oil recovery factor was 6.2% (Li et al., 2005c). In the 3D model, 0.08 PV of 3000 mg/L polymer was injected. When crosslinked polymer was injected, high permeability channels were immediately blocked, the injection pressure rose, and the water cut fell. However, because of strong edge water, water bypassed the blocked zone, the injection pressure fell, and the water cut quickly rose again. A numerical simulation was carried out to study the feasibility of polymer injection and optimize the program (Yao et al., 2005). The optimum concentrations from the laboratory results were 0.3 to 0.5% polymer, 0.2% crosslinker concentration, pH 5,... 

Table 1. Spin-coated CPB thin film (<20 nm) deposition condition and pentacene-based TFT device performance (mobility and on/off ratio) - polymer/crosslinker concentration ratio (mg/ml mg/ml), solvent, film thickness (D, nm), RMS roughness (p, nm), leakage current density at an electric field of 2 MV/cm (J, A/cm2), mobility (ji, cm2/Vs), and Current On/off Ratio (/<, /< ) ...

The crosslink concentration Is much higher at the surface than Is found In the unexposed coatings and decreases with depth. 

Hint 1. One can plot the ratio versus conversion or crosslinker concentration or mole fraction of a comonomer or weight fraction of polymer, and observe how it behaves with respect to unity. 

The determination of the concentration of crosslinks in a crosslinked polymer sample ib possible by several procedures [30]. The most frequent are equilibrium swelling, stress-strain measurements and sol-gel anafysis. These methods applied to identical crosslinked samples yield, however, differoit values of crosslink concentration [31,32]. 

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