Small-angle x-ray scattering study of kinetics of spinodal decomposition inN-isopropylacrylamide gels

Abstract

We present synchrotron-based time-resolved small-angle x-ray scattering (SAXS) measurements of spinodal decomposition in a covalently cross-linked N-isopropylacrylamide gel. The range of wave numbers examined is well beyond the position of the maximum in the structure factor S(q,t). The equilibrium structure factor is described by the sum of a Lorentzian and a Gaussian. Following a temperature jump into the two phase region, the scattered intensity increases with time and eventually saturates. For early times the linear Cahn-Hilliard-Cook (CHC) theory can be used to describe the time evolution of the scattered intensity. From this analysis we found that the growth rate R(q) is linearly dependent on q(2), in agreement with mean-field theoretical predictions. However the Onsager transport coefficient Lambda(q) approximately q(-4), which is stronger than the q dependence predicted by the mean-field theory. We found that the growth rate R(q)>0, even though the wave numbers q probed by SAXS are greater than sqrt[2]q(m) where q(m) is the position of the peak of S(q,t), also in agreement with the mean-field predictions for a deep quench. We have also examined the range of validity of the linear CHC theory, and found that its breakdown occurs earlier at higher wave numbers. At later times, a pinning of the structure was observed. The relaxation to a final, microphase-separated morphology is faster and occurs earlier at the highest wave numbers, which probe length scales comparable to the average distance between crosslinks.

SAXS Investigation of the Effect of Temperature on the Multiscale Structure of a Macroporous Poly(N-isopropylacrylamide) Gel

“…[15], R G increases slightly between 18 and 32°C, i.e., above T c . It can be noticed that these values, ranging between 2.5 and 5.4 nm are much smaller than that reported for gels [15,16]. This feature could be related to the particular method of polymerization (cryopolymerisation) used for cryogels.…”

“…The amount of cross-linker, N,N′methylenebisacrylamide (MBAA), was 6.6 wt% of the total amount NIPA, corresponding to 1 M for 15 M of NIPA yielding a degree of cross-linking larger than for the pNIPA cryogel we have investigated previously (1/40 in ref. [25]) and for the gel investigated by Shibayama et al [16].…”

Micellar Solutions of a Symmetrical Amphiphilic ABA Triblock Copolymer with a Temperature‐Responsive Shell

“…Its LCST at 32 °C is attributed to alterations in the hydrogen‐bonding interactions of the amide group 8–10. So far, macroscopic gels,11–13 microgels,14–17 and core‐shell lattices with a cross‐linked hydrophobic core and a cross‐linked responsive shell18–21 have been studied.…”

Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

“…The interesting behavior of PNIPAM particles derives from the dramatic swelling/shrinking volume transition of the polymer network in response to a small external stimulus, such as temperature, pH, or electrolyte concentration 3, 20, 21. This volume transition is directly related to the liquid–liquid phase separation of uncross‐linked PNIPAM polymer in water, which exhibits a lower critical solution temperature (LCST) at about 33 °C.…”