No abstract
The local dynamics of 1,4 polybutadiene below and above the merging of the ␣ and  relaxations have been investigated by combining neutron spin echo ͑NSE͒ and dielectric spectroscopy. The study of the dynamic structure factor measured by NSE over a wide momentum transfer range allows us to characterize the ␣ relaxation as an interchain process while the  relaxation originates from mainly intrachain motions. At temperatures below the merging, the dynamic structure factor can be described by a superposition of elemental processes for the  relaxation as obtained from dielectric spectroscopy. The elemental motions behind this process can be related to rotational jumps of the chain building blocks around their center of mass. Furthermore, we have been able to consistently describe the dynamic structure factor above the merging of the ␣ and  relaxations by assuming that both processes are statistically independent. In the framework of this scenario a procedure for analyzing the dielectric response in the ␣- merging region has been developed. Its application to the dielectric data allows us to describe the dielectric response in this region on the basis of the low temperature behavior of the ␣ and  processes and without considering any particular change in the relaxation mechanism of these processes. The temperature dependence found for the relaxation time of the ␣ process follows now the viscosity, a masked feature in the experimental data due to the merging process. In this way, we have been able to consistently describe the relaxation of both, the polarization and the density fluctuations, by using the same scenario, i.e., independent ␣ and  processes, and considering the same functional forms and temperature dependences of the characteristic times of the two processes. ͓S1063-651X͑96͒07209-1͔
We investigate, by means of computer simulations, the formation of soft nanoparticles by irreversible intramolecular cross-linking of homofunctional polymer precursors in good solvent. Simulations reveal that the early and intermediate stages of the cross-linking process are dominated by bonding at short contour distances. Because of the initial selfavoiding character of the precursor, bonding at long contour distances, which is the efficient mechanism for global compactation, is a rare event that essentially occurs in the late stage of cross-linking. Thus, irreversible cross-linking of precursors with identical molecular weight and linker fraction produces both compact and sparse objects. This is confirmed by a detailed analysis of the size and shape distribution of the fully cross-linked nanoparticles. We also investigate intramolecular cross-linking of heterofunctional polymers with two species of orthogonal linkers, bonding between distinct species being forbidden. It is found that simultaneous cross-linking of both species and sequential cross-linking (activation of one species after full cross-linking of the other) lead to the same structural properties for the resulting nanoparticles. The heterofunctional nanoparticles are on average smaller and more spherical than the homofunctional counterparts, though still a significant fraction of sparse objects is found. The simulation results are compared with results from SEC/MALLS and SAXS experiments in real polymeric nanoparticles.
Single-chain nanoparticles (SCNPs) are unimolecular soft nano-objects, consisting of individual polymer chains collapsed to a certain degree by means of intramolecular bonding. Many of the potential applications of SCNPs rely on their particular molecular architecture. Even if the ultimate goal is to produce globular protein-like soft nanoparticles, recent SANS and SAXS results -supported by computer simulations-indicate that SCNPs in solution actually adopt sparse configurations. Herein we compile size data from the literature for a large number of SCNPs in solution, covering from covalent to non-covalent bonded SCNPs, and provide a comparison with the corresponding data for compact or partially swollen globules of the same nature and molar mass. This comparison gives a clear idea of how far from the compact globule limit are current SCNPs. A quantification of the departure from the globular state is provided in terms of size scaling laws. This procedure facilitates a comparison with the size scaling laws observed for folded proteins with globular conformation as well as intrinsically disordered proteins which, on average, exhibit a certain local compaction when compared to chemically denatured proteins. Lastly, the underlying physical mechanism for the noncompact morphology of SCNPs in solution is put forward and guidelines for the potential synthesis of true SCNP globules in solution are suggested.
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