Reports: ND754028-ND7: Nanosphere and Nanorod Diffusion in Polymer Melts
printer friendlyDiffusion of Polymer-Grafted Nanospheres
We investigated poly(methyl methacrylate)-grafted iron oxide NP diffusion in poly(methyl methacrylate), PMMA, using Rutherford backscattering spectrometry. The brush grafting density, brush molecular weight, and matrix molecular weight were systematically varied to determine each factor’s effect on diffusion. NPs (d = 5 nm) were prepared and named as follows: IO21L (N = 21 kg/mol, σ = 0.17 chains/nm2), IO21M (N = 21 kg/mol, σ = 0.33 chains/nm2) and IO16H (N = 16 kg/mol, σ = 0.55 chains/nm2). All NPs were determined by TEM to be well dispersed within PMMA and the diffusion coefficients of the NPs were independent of annealing time, suggesting that NPs do not form aggregates while diffusing. As matrix molecular weight increased, the diffusion coefficients of all three NPs decreased monotonically. When matrix chains wet the brush for samples IO21L and IO21M, NP diffusion was slowed down by 70% relative to Stokes Einstein (SE) diffusion, suggesting that matrix chains interpenetrate the brush causing a significant drag. For IO16H, however, diffusion increased rapidly due to less matrix interpenetration in the brush, eventually becoming faster than SE. Using self-consistent field theory, brush and matrix density profiles were calculated and showed that grafting density most strongly effects interpenetration of the brush and matrix and effective brush size. This study underlines the importance of polymer brush structure on NP center of mass diffusion in polymer melts; the stretching and collapse of polymer brushes can slow down or accelerate diffusion relative to the classical SE predictions.
Diffusion of Quantum Dot Nanoparticles in Swollen Polymer Gels
The incorporation of nanoparticles (NPs) to form hybrid polymer gels is integral to maintaining the desirable qualities of gels, such as high swelling ratios, flexibility, and light weight, while adding functionality. To determine systematic relationships between NP mobility and polymer gel properties, a model system of polyacrylamide gels (PAGs) were chosen due to the ability to modify the network by changing crosslinking concentration and swelling ratio. The mesh size of PAGs can be tuned by the addition of bis-acrylamide, allowing average mesh sizes from 5 to 50 nm. The mesh sizes were determined using rubber elastic theory, the polymer volume fraction determined from thermogravimetric analysis, and the zero frequency shear modulus determined from rheometry. Using a 20 nm quantum dot NP, average confinement ratios (hydrodynamic radius of the NP/mesh size) from 0.4 to 3.8 were achieved with these mesh sizes, ranging from larger to smaller than the NP diameter. Additionally, PAGs undergo a continuous volume phase transition in acetone/water solutions, changing the effective mesh size from 50 to 10 nm, confinement ratios from 0.4 to 2.
To characterize NP mobility within hydrated polymer systems, single particle tracking (SPT) of quantum dot NPs was used. The spatial coverage, mean squared displacements, and van Hove displacement distributions of the NPs within each gel system were analyzed. In the PAGs, NP diffusion could be separated into three categories: localized, diffusive, and intermittent diffusion based on NP trajectories and van Hove distributions. Confinement ratios greater than one resulted in an order of magnitude decrease in diffusion coefficient, slowing to 0.1µm2/s. Increasing acetone percentage in the swelling solution decreased the bulk volume swelling ratio, from 2.6 to 0.3, and decreased diffusion more strongly than expected based on confinement alone. Non-Gaussian dynamics were observed for both confinement increased by crosslinking and by decreased swelling. The non-Gaussian parameter, Ng, of the NPs distribution was shown to be in the range of 1.5 to 7 for samples with decreased swelling ratios, while water swollen samples have a maximum Ng of 1.5. This increase in heterogeneity corresponds to more tortuous diffusion pathways than in water, resulting in decreased diffusion for a given confinement ratio. Thus, both average confinement and changes in the network arrangement due to the VPT, which impact the dynamic heterogeneity of the NPs, contribute to NP mobility in these gels. From this gained insight, gels can be better designed for improved separation and delivery capability.
Diffusion of Quantum Dot Nanoparticles in the Cell Cytoplasm
The impact of cytoskeletal organization and connectivity on non-specific NP mobility was probed by injecting NPs with 5k PEG brushes (10 nm hydrodynamic diameter) into the cytoplasm of healthy, fibroblasts, and cancerous, fibrosarcoma, cells, as well as within these cells exposed to cytoskeletal destabilizers, cytoclasin D and nocodazole. NPs injected into fibrosarcomas, which had more disorganized actin than fibroblasts, showed faster diffusion, 0.1µm2/s versus 0.04µm2/s, and an increase in the amount of space explored by the NPs. When actin was disrupted with cytoclasin D, diffusion was increased in the fibroblast cells, while diffusion was minimally impacted in fibrosarcomas. Nocodazole did not greatly impact the diffusion behavior of NPs within either cell line. By characterizing the distributions of NP displacements, an average mesh size could be characterized. The displacement distributions also indicated that NPs within fibrosarcoma cells and fibroblasts exposed to cytoclasin D moved between meshes more easily than control fibroblasts. This research demonstrated the sensitivity of intracellular QD diffusion to actin cytoskeletal modifications. The organization and extent of polymerization of the actin within each cell controls the confinement the NPs experience and their ability to diffuse.
