Network confinement and heterogeneity slows nanoparticle diffusion in polymer gels

Abstract

Nanoparticle (NP) diffusion was measured in polyacrylamide gels (PAGs) with a mesh size comparable to the NP size, 21 nm. The confinement ratio (CR), NP diameter/mesh size, increased from 0.4 to 3.8 by increasing crosslinker density and from 0.4 to 2.1 by adding acetone, which collapsed the PAGs. In all gels, NPs either became localized, moving less than 200 nm, diffused microns, or exhibited a combination of these behaviors, as measured by single particle tracking. Mean squared displacements (MSDs) of mobile NPs decreased as CR increased. In collapsed gels, the localized NP population increased and MSD of mobile NPs decreased compared to crosslinked PAGs. For all CRs, van Hove distributions exhibited non-Gaussian displacements, consistent with intermittent localization of NPs. The non-Gaussian parameter increased from a maximum of 1.5 for crosslinked PAG to 5 for collapsed PAG, consistent with greater network heterogeneity in these gels. Diffusion coefficients decreased exponentially as CR increased for crosslinked gels; however, in collapsed gels, the diffusion coefficients decreased more strongly, which was attributed to network heterogeneity. Collapsing the gel resulted in an increasingly tortuous pathway for NPs, slowing diffusion at a given CR. Understanding how gel structure affects NP mobility will allow the design and enhanced performance of gels that separate and release molecules in membranes and drug delivery platforms.

Controlled release of entrapped nanoparticles from thermoresponsive hydrogels with tunable network characteristics

“…At the initial ballistic regime (τ < 0.1 µs), the MSD is independent of the polymer concentration, because the polymers do not interact with the NPs at short times, thus having little impact on the motion of NPs. The results are in close qualitative agreement with available experimental results [37,38]. Correspondingly, we present the VACF and diffusion coefficient of NPs in Figure 3(a).…”

“…To quantify the confinement of polymer network on NPs, we introduce the van Hove displacement distributions [37,38,58,59], which characterize the probability distribution of the distance a particle moving along x, y or z direction and can be calculated based on [59]:…”

Tracer Diffusion in Tightly-Meshed Homogeneous Polymer Networks: A Brownian Dynamics Simulation Study

“…Accordingly, previous studies on tracer diffusion in polymer networks have focused on understanding the effect of different network mesh sizes [7][8][9]19,[24][25][26][27][28][29][30][31]. For instance, recent experiments on tracer diffusion in hydrogels investigated the effect of changes in ξ by varying the degree of crosslinking and by volume phase transition [7][8][9]19,26,31]. ξ decreases with an increasing degree of crosslinking and with a decrease in swelling ratio, and it was shown that the diffusion of a tracer particle in hydrogels, with variable crosslinking or swelling, is largely determined by ξ relative to the size of a tracer particle σ tr , i.e., ξ/σ tr .…”

“…ξ decreases with an increasing degree of crosslinking and with a decrease in swelling ratio, and it was shown that the diffusion of a tracer particle in hydrogels, with variable crosslinking or swelling, is largely determined by ξ relative to the size of a tracer particle σ tr , i.e., ξ/σ tr . However, the systematic and quantitative investigation on the relation between tracer diffusion and the ratio of ξ/σ tr has been complicated due to the heterogeneity inherent in the polymer networks during the course of crosslinking or volume phase transition [19,31,32]. Computer simulations of polymer models, on the other hand, allow for the preparation of regularly crosslinked, homogeneous polymer networks and the investigation of tracer diffusion in the absence of network heterogeneity.…”

“…Average mesh size, ξ, is calculated for each polymer network and the dynamics of a tracer particle is compared as a function of ξ/σ tr . Although several factors influence the dynamics of a tracer particle in polymer networks, such as interactions between a tracer particle and polymer segments [8,19,33], aggregation of polymer segments of the network [30], and network heterogeneity [19,31,32], we solely focus on the size-dependent obstructive effect of small meshes by investigating the diffusion of an inert, nonsticky tracer particle in homogeneous polymer networks. Similar with those in polymer solutions and melts [24,34,35], the dynamics of a tracer particle in polymer networks can be distinguished by the relative mesh size to that of a tracer particle, ξ/σ tr [36].…”