Two high‐resolution, general‐purpose, small‐angle neutron scattering instruments have been constructed at the National Institute of Standards and Technology's Center for Neutron Research. The instruments are 30 m long and utilize mechanical velocity selectors, pinhole collimation and high‐data‐rate two‐dimensional position‐sensitive neutron detectors. The incident wavelength, wavelength resolution and effective length of the instruments are independently variable, under computer control, and provide considerable flexibility in optimizing beam intensity and resolution. The measurement range of the instruments extends from 0.0015 to 0.6 Å−1 in scattering wavevector, corresponding to structure in materials from 10 Å to nearly 4000 Å. The design and characteristics of the instruments, and their modes of operation, are described, and data are presented which demonstrate their performance.

Small‐angle scattering (SAS) curves are characterized by two main features: the Guinier region and the Porod region. Standard linear plots are available to fit SAS data and obtain a radius of gyration and a Porod exponent. A new Guinier–Porod empirical model is introduced to fit SAS data from spherical as well as nonspherical objects such as rods or platelets. It also applies to shapes intermediate between spheres and rods or between rods and platelets. The new model is used to fit SAS data from a Pluronic solution that sequentially forms unimers, then spherical micelles, then cylindrical micelles, then lamellar micelles upon heating. This single model can fit structures associated with all four phases as well as the intermediate structures.

The clustering phenomenon has been observed in many macromolecular systems. Poly-(ethylene oxide) solutions are characterized by a clustering effect that has been extensively discussed in the literature. Its origin has remained elusive. Using small-angle neutron scattering from PEO solutions in various deuterated solvents, the possible causes of clustering that have been given in the literature are analyzed here. These include impurities in water, possible PEO crystallization, a subtle phase transition whereby a concentrated phase coexists with free polymer coils, hydrogen-bond physical crosslinking, and finally chain ends effect. We have shown that under the experimental conditions considered here (4% PEO weight fraction) the mostly forgotten chain ends effect is at the origin of clustering in poly(ethylene oxide) solutions.

Bottlebrush polymers are highly branched macromolecules with potential applications in antifouling coatings, rheological modifiers, and drug delivery systems. However, the solution conformation of bottlebrush polymers has been studied in only a limited set of materials made primarily by grafting-from polymerization. Here we present small-angle neutron scattering (SANS) measurements on a series of polystyrene bottlebrush polymers with varying side-chain and backbone lengths in d 8-toluene to analyze their size, shape, and conformation. Bottlebrush polymers with 2–7 kg mol–1 polystyrene side chains (degree of polymerization DP = 14–54) and poly(oxanorbornene) backbones (DP = 10–264) were synthesized using reversible addition–fragmentation chain transfer (RAFT) followed by a ring-opening metathesis polymerization (ROMP) grafting-through synthesis scheme. Analysis by Guinier–Porod, rigid cylinder, and flexible cylinder models provided estimates of the bottlebrush polymer length, radius, and stiffness. The bottlebrush polymer cross-sectional area depends primarily on side-chain DP, and the radius of gyration R g exhibits a power-law dependence with side-chain DP. We also observe a sphere-to-cylinder transition with increasing backbone DP, with the transition occurring at a backbone DP of approximately 120 for the polystyrene bottlebrush polymers studied. The maximum molecular dimension for the series studied varies from 25 to 350 nm.