Neutron reflectivity deuterium labeling

In neutron reflectivity, neutrons strike the surface of a specimen at small angles and the percentage of neutrons reflected at the corresponding angle are measured. The an jular dependence of the reflectivity is related to the variation in concentration of a labeled component as a function of distance from the surface. Typically the component of interest is labeled with deuterium to provide mass contrast against hydrogen. Use of polarized neutrons permits the determination of the variation in the magnetic moment as a function of depth. In all cases the optical transform of the concentration profiles is obtained experimentally. 

Polymer-polymer interfaces are an important area of study since the interfacial behaviour is fundamental to the bulk properties of the system. This is particularly true when two or more polymers are mixed to form a blend, but the interface also plays a dominant role in areas such as adhesion, welding, surface wetting and mechanical strength. To understand fully polymer behaviour in such applications, the interface must be characterised at a microscopic level. Through deuterium labelling the interface between otherwise indistinguishable polymers can be studied, and neutron reflectivity provides unprecedented detail on interfacial width and shape. In addition to the inherent interdiffusion between polymers at a polymer-polymer interface, the interface is further broadened by thermally driven capillary waves. Capillary waves... 

For all these specialty polymers, deuterium can be used as a label on one or the other monomer. Deuterium labeling allows the use of techniques based on ion detection such as forward recoil spectrometry (FRES), nuclear reaction analysis (NRA) or secondary ion mass spectrometry (SIMS). If a high-resolution depth profile of the interfacial region is needed, neutron reflectivity can also be used. The main drawback of that approach is the cost of the deuterated polymers while deuterated styrene and methyl methacrylate are expensive but commercially available, other monomers need to be synthesized and the cost can be quite prohibitive. 

Much of the simple theory of reflection is presented in optics textbooks a classic and clear presentation is to be found in Bom and Wolf (1975). The general theory of the reflection of waves, both electromagnetic and particle, is dealt with in more detail in the book by Lekner (1987). Reviews of x-ray and neutron reflectivity, including their application to polymers, are available (Russell 1990, Thomas 1995). In this section we concentrate on neutron reflectivity, primarily because of the advantages offered by deuterium labelling, though as x-ray reflectivity can also provide useful information, contrasts between reflection of neutrons and x-rays will be pointed out where necessary. 

Figure 6.20. Segment distributions of a styrene-(methyl methacrylate) block copolymer (relative molecular masses of each block were in the range 48 000-65 000) at an interface between polystyrene (relative molecular mass in the range 110000-127 000) and poly(methyl methacrylate) (relative molecular mass in the range 107000-146 000), revealed by a series of neutron reflection experiments in which various parts of the copolymer and/or one of the homopolymers was labelled with deuterium. The bold lines are the segment density profiles for all styrene and methyl methacrylate segments, summed over both the homopolymer and the copolymer the solid lines are the homopolymers, and the dotted lines are the styrene and methyl methacrylate blocks of the copolymer. After Russell et al. (1991).

Neutron reflectivity was used to investigate the segment density distribution of symmetric diblock copolymers of PS and PMMA [39] (molecular weights of about 100,000) at the interface between PS and PMMA homopolymers (molecular weights of about 100,000). Selective deuterium labeling of either a block of the PS-Z -PMMA or of the PS or PMMA homopolymers provided the contrast necessary to isolate the distribution of the segments of the individual components at the interface. Results from a series of experiments were used simultaneously to yield the density profiles of the PS and PMMA segments of the homopolymers, and of the copolymer blocks at the interface (Fig. 22). 

Fig. 22 Volume fraction profiles of the PS and PMMA homopolymers thin solid lines), the PS and PMMA blocks of the PS- -PMMA copolymer dashed lines), and the total PS and PMMA segments summed over the homopolymer and the respective copolymer blocks thick solid lines). The results were obtained by simultaneous analysis of neutron reflectivity experiments with different deuterium labeling of copolymer and homopolymer segments [39]...

From Eq. 1, the evaluation of the characteristic ratio requires measurement of the unperturbed dimensions of the polymer chain. Polymers exhibit their characteristic unperturbed dimensions in the bulk amorphous state, i.e., chain dimensions under these conditions reflect the influence of short range, rotational isomeric state, effects only. Prior to the availability of small-angle neutron scattering (SANS) in the mid-1970s [32-34], no method was available that allowed chain dimensions in polymer melts to be measured directly. SANS utilizes the fact that different isotopes result in different scattering amplitudes for neutrons. Thus, selective deuterium labelling of some chains, followed by dispersing these chains in a solvent of otherwise identical but non-deuterated chains, allows the conformational properties of individual chains to be probed in the melt. 

In the case of lamellar-forming BCPs, where the morphology consists of alternating layers of each block, NR profiles similar to that shown in Figure 3 are obtained where strong interferences are evident as a result of constructive and destructive interferences of the reflected neutron beam as a consequence of the layered morphology. By analysis of the NR profile quantitative information on Lg, the thickness of each layer, and the width of the interfaces between the lamellar microdomains can be obtained and quantitatively compared to theoretical arguments. In addition, since selective parts of the BCP can be labeled with deuterium, for example, the junction points between the blocks or a chain end, exceptional detail on... 

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