Page 35 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
32-33
Figure 2
Time dependence of correlation length (red triangles) and radius of gyration of the mesoglobules (blue circles) obtained from fits to the SANS data.
The pre-release SANS curve, as well as the initial kinetics (regime I), are characteristic of semidilute solutions undergoing concentration fluctuations (figure 1b). After 1.4 s, an additional contribution with rapidly increasing intensity appears at low momentum transfers (q < 0.1 nm-1), reflecting the formation and growth of aggregates from collapsed chains called mesoglobules (regime II). After ~30 s, their growth slows down significantly (regime III).
Fitting of structural models yields characteristic lengths (figure 2). In regime I, the correlation length of the concentration increases, but the intensity only increases weakly. These findings indicate that small clusters form by a nucleation and growth mechanism (figure 3).
In regime II, mesoglobules are formed, and their radius of gyration increases with time according to a power law Rg(t) ∝ t1/3, i.e., they grow by diffusion-limited coalescence [2]. Moreover, a dense shell emerges that traps water inside the mesoglobules. In regime III, the growth follows Rg(t) ∝ log(t), suggesting the appearance of an energy barrier that hinders coalescence. A large energy barrier (several kBT) implies that aggregation
is greatly slowed down in regime III [3]. This may be
attributed to the viscoelastic effect (entanglement force) hindering the coalescence of two mesoglobules because of the low mobility of the polymers [4].
Thus, in situ, real-time SANS in combination with a fast pressure jump initiating a phase transition captures a comprehensive view of the structure-forming processes over time scales ranging from tens of milliseconds to thousands of seconds. Detailed structural information could be gained on length scales of ~1–100 nm. In particular, our study reveals that, in a semidilute PNIPAM solution, the growth of the mesoglobules features several regimes. First, loose clusters form from the homogeneous solution by a nucleation process. Second, the clusters evolve into compact aggregates having a dense shell and grow rapidly by diffusion-limited coalescence. Finally, their growth slows down substantially because the low polymer mobility in the aggregates hinders
their coalescence. These findings could only be made possible by combining a milli-second pressure jump and fast SANS. The method opens up new opportunities
for investigating the pathways of structural changes in complex systems with unprecedented time resolution.
Figure 3
Schematic representation of the formation and structural evolution of the mesoglobules during the pressure jump.
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