Macromolecular Diffusion in a Crowded Polymer Nanocomposite

Abstract

Macromolecular motion is reduced in crowded polymer nanocomposites. Tracer diffusion is measured for deuterated polystyrene (dPS) into a polystyrene (PS):silica nanoparticle (NP) matrix using elastic recoil detection. This nanocomposite is ideal for studying diffusion in a crowded system because the interparticle distance (ID) that defines confinement can be varied from much greater than to much less than the size of the dPS chain, which is described by 2R g, the radius of gyration, and varies from 10 to 40 nm in this study. Diffusion is observed to be significantly slower than that predicted by the Maxwell model. The tracer diffusion coefficient of dPS in the nanocomposite relative to the pure PS matrix (D/D 0) plotted against the NP separation relative to probe size (i.e., ID/2R g) falls on a master curve, indicating that crowding is a property of both the dPS size and confinement in the nanocomposite. Moreover, the normalized diffusion coefficient decreases more rapidly when ID/2R g is less than ∼1, suggesting strong confinement conditions. The scaling of the diffusion coefficient with chain length is in excellent agreement with the entropic barrier model that accounts for the slowing down associated with the loss of chain entropy due to constrictive bottlenecks.

Effects of carbon nanotubes on polymer physics

“…The simplest theory, for example, a Maxwell theory, 32 which considers only the steric barrier imposed by the filler, suggests a decrease in diffusion constant and all models that consider only steric barriers predict a decrease. Studies with silica nanoparticles 33 found a monotonic decrease in diffusion constant with increasing filler concentration, another study with nanoclays found no change in diffusion coefficient at 5% clay content in polystyrene with a factor of three decrease in polymethyl methacrylate. 34 The results for carbon nanotubes are significantly more complicated.…”

Polymer and spherical nanoparticle diffusion in nanocomposites

“…However, in the dilute noparticle regime there is discrepancy to the data. In the experimental nanocomposite, tracer diffusion 24 collapses onto a master curve when the neat homopolymer diffusion (D/D 0 ) is plotted versus a confinement parameter defined as the interparticle distance relative to the polymer radius of gyration (ID/2R g ) 23,24,92 where the interparticle distance ID is given as:…”

“…While, most models for polymer nanocomposites, predict a decrease in polymer diffusivity due to tortuosity, only recent models [19][20][21][22] are based on the interaction of polymer chains with the nanoparticle surface. Polymer tracer diffusivity decreases in nanocomposites of large silica nanoparticles dispersed in polystyrene (PS) 23 or (PMMA) 24 . The tracer diffusivity decreases across a wide range of polymer molecular weight, nanoparticle size 23,24 , and unexpectedly decreases more strongly at higher temperatures 25 .…”

“…Polymer tracer diffusivity decreases in nanocomposites of large silica nanoparticles dispersed in polystyrene (PS) 23 or (PMMA) 24 . The tracer diffusivity decreases across a wide range of polymer molecular weight, nanoparticle size 23,24 , and unexpectedly decreases more strongly at higher temperatures 25 . In addition, Richter and co-workers explored the dynamics of entangled 26,27 and unentangled poly(ethylene-altpropylene) (PEP) polymers 28 in nanocomposites by neutron spin echo (NSE) experiments.…”

Modeling of Intermediate Structures and Chain Conformation in Silica–Latex Nanocomposites Observed by SANS During Annealing

“…In some sense, a zone of restricted flow may develop across the sample. Alternatively, a 'bottleneck' explanation has been put forward 65 , where the three-dimensional silica structure is thought to constrain possible motion of the polymer latex molecules. This may apply to the a priori percolated silica structure at 20%.…”