SANS experiments

The size and shape of polymer chains joined in a crosslinked matrix can be measured in a small angle neutron scattering (SANS) experiment. This is a-chieved by labelling a small fraction of the prepolymer with deuterium to contrast strongly with the ordinary hydrogenous substance. The deformation of the polymer chains upon swelling or stretching of the network can also be determined and the results compared with predictions from the theory of rubber elasticity. 

The SANS experiment is applicable to polymeric networks containing some deuterium labeled chains. The chain geometry can be probed not only in the unperturbed network, but changes in chain shape and size can be measured as a function of strain or swelling. This enhances the applicability of SANS experiments for elastomeric systems. 

Table I contains calculations of changes in molecular dimensions which would be obtained from a SANS experiment if the models of the network proposed in the previous section apply.

The SANS experiments of Clough et al. (21) on radiation crosslinked polystyrene are presented in Figure 9, and appear to fit the phantom network model well. However, these networks were prepared by random crosslinking, and the calculations given are for end-linked networks, which are not truly applicable. 

The first SANS experiments on end-linked elastomers with a well-defined functionality were carried out by Hinkley et al, (22). Hydroxy-terminated polybutadiene was crosslinked by a trifunctional isocyanate, and the resultant polymer was uniaxially stretched. 

Figure 1 Aspect of the sample recorded simultaneously to the SANS experiments. For this synthesis, the precipitation time equals 23 minutes.

The SANS experiments [51] were performed with solutions of G8 PAMAM dendrimer in D20, methyl-d4, ethyl-d6, and n-butyl-d10 alcohol at a temperature of T = 20.0 °C. PAMAM dendrimers do not dissolve in acetone, but they readily dissolve in methyl alcohol/acetone mixtures over a wide range of composition. Solvents of different composition, were prepared and added to a weighed amount of dried G5 or G8 dendrimer. In a separate set of experiments, the NIST NG7 30 m instrument was used to measure the effects of charging on the dendrimer size. PAMAM G8 dendrimers in D20 were charged by addition of HC1 in the presence of various amounts of NaCl to the charged dendrimers to screen the electrostatic interactions. 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

By the definition of T (f), is the volume of the cavity in which the particular solvent is replaced by the solute. Therefore Vp depends on the particular solvent chosen for the SANS-experiment and also on concentration. For the dilute regime under consideration here the latter dependence can safely be dismissed. 

The discussion in Sect. 3.3 has suggested that the intensity measured in SANS-experiments contains an appreciable contribution not related to the structure of the dissolved dendrimers. Only the part Isiq) (see Eq. 4) is connected to the average spatial structure as expressed through the shape function T(r). Figure 6... 

Agreement of model (3) with experimental data is shown on Figure 11 for ESR experiments in zone A and has been confirmed by SANS experiments (H). 

Fig. 2.11 Schematic representation of the transition from hexagonally packed cylinders to disorder following cessation of large-amplitude shear, deduced from SANS experiments on an asymmetric PEP-PEE diblock (Bates et al. 1994). Shear was used to stabilize a hex phase above the equilibrium ODT, and the relaxation back to the equilibrium disordered phase was followed after the shear was stopped. Close to the ODT, the transition r, —> u was postulated, while at a higher temperature, (ri) > rr ) r, was indicated.

Cohen et al. (1990) studied a poly(styrene)-poly(ethylene) (PS-PE) diblock that was solvent cast from toluene. Crystallization within microphase-separated PE spheres occurred when solvent-casting was done above the PE block melting temperature, Tm (see Fig. 5.2). When solvent was removed below Tm crystallization did not occur within spherical microdomains, instead TEM and SANS experiments suggested an irregular structure. Nojima et al. (1994) suggest that crystallization from the melt in this sample occurred within the microphase-separated block in the former case due to the high molecular weight of the... 

Whitmore and Noolandi (1985b) developed the self-consistent field theory to examine micellization in AB diblocks in a blend of AB diblock and A homopolymer solvent . The model was applied specifically to the case of PS-PB diblocks in PB homopolymer for comparison with the results of small-angle neutron scattering (SANS) experiment by Selb et al. (1983). This model is discussed in more detail in Section 3.4.2. 

Controversial discussions have surrounded the three-dimensional structure of flexible dendrimers in solution during the past two decades. Now, on the basis of numerous SANS experiments using the contrast variation technique, the idea that isolated flexible dendrimers in good solvents do not take on the originally predicted dense shell structure but instead assume a dense core structure [47] appears to be gaining general acceptance. This means that in such dendrimers the segment density reaches a maximum at the centre of the molecule and decreases towards the periphery (cf. Fig. 7.6). 

From SANS-experiments it is possible to determine the ionic strength inside the polyelectrolyte shell, cs nt. Its dependence on the added salt concentration is shown in Fig. 7 [49]. 

SANS experiments were undertaken to confirm the DLS observations. Results obtained with phenol solutions are shown in Figures 7 and 8. In the conditions of contrast matching employed here, where the scattering length density of the water was adjusted to be equal to that of the polymer, clustering... 

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