Influence of network defects on the conformational structure of nanogel particles: From “closed compact” to “open fractal” nanogel particles

Abstract

We propose an approach to generate a wide range of randomly branched polymeric structures to gain general insights into how polymer topology encodes a configurational structure in solution. Nanogel particles can take forms ranging from relatively symmetric sponge-like compact structures to relatively anisotropic open fractal structures observed in some nanogel clusters and in some self-associating polymers in solutions, such as aggrecan solutions under physiologically relevant conditions. We hypothesize that this broad “spectrum” of branched polymer structures derives from the degree of regularity of bonding in the network defining these structures. Accordingly, we systematically introduce bonding defects in an initially perfect network having a lattice structure in three and two topological dimensions corresponding to “sponge” and “sheet” structures, respectively. The introduction of bonding defects causes these “closed” and relatively compact nanogel particles to transform near a well-defined bond percolation threshold into “open” fractal objects with the inherent anisotropy of randomly branched polymers. Moreover, with increasing network decimation, the network structure of these polymers acquires other configurational properties similar to those of randomly branched polymers. In particular, the mass scaling of the radius of gyration and its eigenvalues, as well as hydrodynamic radius, intrinsic viscosity, and form factor for scattering, all undergo abrupt changes that accompany these topological transitions. Our findings support the idea that randomly branched polymers can be considered to be equivalent to perforated sheets from a “universality class” standpoint. We utilize our model to gain insight into scattering measurements made on aggrecan solutions.

Molecular dynamics study of the swelling and osmotic properties of compact nanogel particles

“…This same situation arises in linear polymer chains, but where the good solvent and θ -solvent scaling exponents are different from branched polymers. Future work will describe the size of compact 20 and open 21 nanogel particles that are more suitable when considering an extensive range of polymer molecular mass. We next briefly show that a simple extension of the renormalization (RG) group theory to describe the swelling of our nanogel particles appears to be promising for describing this type of crossover above the θ -point.…”

“…The mass scaling of the ‘open gels’ was found to conform rather well with randomly branched polymers (‘lattice animals’), but the situation was found to be more complicated for the perfect nanogel particles whose size did not seem to conform with the expectations of randomly branched polymers. 21 In particular, the apparent mass scaling exponents describing the average size of these nanogel particles depended on the mesh spacing in the network. The radius of gyration mass scaling exponent when the mesh size was small was found to be consistent with that for the swollen linear chains composing the network in isolation ( ν ≈ 0.588) with increasing M while the apparent exponent was found to be near ( ν ≈ 1/3) with increasing branched points in each direction.…”

“…Our previous study of polymers having a network topology indicated that the apparent exponents describing the mass scaling of these polymers under good solvent conditions generally depend on the density of branch points in the network and the fraction of bonds cut in the network. 20,21 This led us to define two distinct general classes of networks. First, the 'ideal' or 'compact nanogel particles' have a network topology defined by a lattice-like structure, resulting in a more symmetric particle-like geometry.…”

“…However, a cross-over to a wavevector scaling is consistent with a higher effective fractal dimension at larger length scales. 20,21 It is apparent from these observations that the structures simulated cannot be described as being 'fractal' objects, but they are rather hybrid structures having properties in common to fractals, epitomized by linear polymer chains, and symmetric particles having uniform density. It is difficult from the limited size simulations to deduce the asymptotic mass scaling in the limit of large polymer mass from the study of individual network polymer chains, but additional insights into these mass scaling characteristics can be obtained from the study of the swelling of these structures as solvent quality is varied and from the concentration dependence of the osmotic pressure under variable solvent conditions.…”

The role of charge content in the two-step deswelling of Poly(N-isopropylacrylamide)-based microgels

“…Since we became aware of a different method used in the literature to estimate the hydrodynamic radius of macromolecules, also recently applied to nanogels, 56 that is based on the ZENO algorithm, 57 we also use this approach to evaluate R H for all investigated microgels. It is important to note that we find an overall consistency between the two methods, so that both appear to be valid approaches to estimate R H from implicit solvent simulations.…”