Small-angle neutron scattering (SANS) can be used to investigate Polymer sizes, form, orientation, anisotropy, molecular interactions and defects on the polymer chains. Phase equilibria of Polymer gels and Volume Phase Transition can be analysed.
The basis of neutron scattering is that when a neutron beam of known wavelength impinges upon a sample, scattering occurs by interaction with atomic nuclei or by magnetic dipole interaction with atoms having unpaired electron spins. The scattered neutrons can change both their energy and direction (termed inelastic scattering) or the scattering can be elastic, without change in energy. The scattered neutrons are detected and analysis of the scattering pattern reveals the relative positions of corresponding atoms in the sample. Typically, scattering intensity I(q) is plotted as a function of the scattering vector (q). Energy transfer effects, during inelastic scattering, enable dynamic observations of translational, rotational and vibrational motions of atoms and molecules. Neutron sources can be steady-state nuclear fission reactors or spallation sources, where high energy protons collide with heavy atoms to produce a pulsed neutron beam, typically with a 25 Hz or 50 Hz frequency.
Small-angle neutron scattering (SANS) is based on using very small scattering angles (5 to 10 degrees), relative to the incident beam. The neutrons have wavelengths of the order of 5-20 Å, which is comparable to interatomic spacing. This enables molecular structures to be investigated. The energy of the incident neutrons are in a broad range of 5 to 100 meV. SANS has been found useful for the study of polymers in a variety of conditions ranging from dilute solutions to polymer gels and melts. SANS provides a variety of information covering size, form, orientation of molecular chains, anisotropy, molecular interactions and defects on the polymer chains. Structural evolution with time can also be studied. An important aspect of the SANS technique involves deuterium labelling, where Hydrogen in the polymer molecule is exchanged with Deuterium. This is chemically inconsequential, but increases scattering contrast, enabling single molecules or their parts to be studied.
Polymeric gels are semisolid three-dimensional polymer networks with wide-ranging industrial and biomedical applications. SANS is very useful in the study of Polymer gels as scattering contrast can be contrived by producing the gel in a deuterated solvent or immersion of the gel in a deuterated solvent. Further, special sample measurements of parameters such as shear deformation, stretching under high pressures or controlled temperature and humidity can be done using robust metal, quartz or sapphire windows. Neutron beams can penetrate the thick windows.
Volume Phase Transition (VPT) is a special kind of phase equilibrium pertaining to polymer gels. This is relevant to production of crosslinked polymeric gel structures of controlled pore size, of which ion-exchange resins are an example. VPT relates to the identification of the stage when the swollen crosslinked polymer phase has reached equilibrium with the surrounding liquid and monomer. This case study describes how SANS was used to study VPT of Poly (N-isopropylacrylamide) (PNIPA) hydrogels. The VPT was triggered by creating an electrically charged gel, upon adding a small amount of ionizable co-monomer such as acrylic acid (AAc). Figures 1(a) and 1(b) show the phase diagram of the neutral gel and the weakly charged PNIPA/AAc gel undergoing VPT, respectively. The corresponding optical micrographs of the two situations are also shown. Figure 1(a) shows that the swollen gel gradually shrinks along the isobar line when heated from 200C to 360C and the shrinkage is visible in the photographs. However, if the heating process is abrupt (‘T jump’), preventing temperature equilibrium, then phase separation occurs along a constant volume (isochoric) line. Figure 1(b) for the charged gel shows the existence of a bi-phase region extending over a large volume change, before reaching the same end state. Figure 2 shows SANS intensity curves of PNIPA/AAc hydrogels observed at various temperatures (left) and an optical photograph showing a hydrogel undergoing collapsed (or swelling) transition on a slight increase (or decrease) in temperature. The SANS curves in the bi-phase region show a distinct scattering peak indicating large concentration fluctuations in the gel, with polymer-rich and poor domains. The existence of a scattering peak is an indication of microphase separation in gels.
Figure 1: (a) Phase diagram and photos of neutral PNIPA gel and (b) charged PNIPA+AAc gel showing bi-phasic region during volume phase transition (VPT).
Figure 2: SANS curves of N-isopropylacrylamide/ acrylic acid (NIPA/AAc) weakly charged gel as a function of temperature. The associated photograph shows a swollen and collapsed bi-phase state.
Mitsuhiro Shibayama, Small-angle neutron scattering on polymer gels: phase behavior, inhomogeneities and deformation mechanism, Polymer Journal (2011) 43, 18–34.