Molecular Weight Dependence of the Glass Transition Temperature (T_g)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer T_g and Matrix T_g

Abstract

Nanocomposites (NCs) of poly(2-vinyl pyridine) (P2VP) and nanosilica were prepared by spin-coating and characterized by fluorescence to assess the alterations in matrix glass transition temperature (T g,matrix ) and interfacial layer T g (T g,inter ) as functions of P2VP molecular weight (MW) and silica content. Relative to neat P2VP T g (T g,neat ), major increases in T g,matrix were observed with increasing silica content in low-MW, 2.1 kg/mol P2VP NCs; T g,matrix − T g,neat = 15 and 35 °C at 1.0 and 10 vol % silica, respectively. High-MW, 100 kg/mol P2VP NCs exhibited much smaller increases in T g,matrix . To understand better the T g,matrix enhancements, trace levels of dye were labeled to silica to characterize the nearinterface (≤4 nm from the surface) T g (T g,inter ). Profound increases in T g,inter were observed in low-MW P2VP NCs: T g,inter − T g,neat = 25 and 40 °C at 1.0 and 10 vol % silica, respectively. In contrast, in high-MW P2VP NCs, T g,inter − T g,neat = 8 and 16 °C at 1.0 and 10 vol % silica, respectively. Because interfacial T g perturbations propagate tens of nanometers into a matrix, the larger increases in T g,inter in low-MW P2VP and at increasing silica content lead to larger increases in T g,matrix . At a given silica content, the greater alteration in T g,inter in low-MW P2VP NCs is explained by differences in interfacial conformations of low-and high-MW P2VPs and greater interfacial hydrogen bonding in low-MW P2VP. The stronger attractive interfacial interactions in low-MW P2VP NCs lead to greater alterations in T g,inter , which result in greater changes in T g,matrix . In contrast, the increase in T g,inter with increasing filler is due to T g,inter perturbations propagating into the matrix. At sufficient silica loading, perturbations to T g from one interface modify T g s at other interfaces. At a constant silica content in the range of 3.0−10 vol %, spatial T g gradients in the NCs and near-interface gradients have little MW dependence. At 0.5 and 1.0 vol % silica, our results suggest that near-interface gradients are larger in low-MW P2VP NCs.

Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets

“…They described the results of both NMR and DSC measurements in terms of a surface induced gradient of local T g ; the local T g , was found to be the maximum at the nanoparticle surface and converges to the bulk value after few nanometers. Wei and Torkelson [ 4 ] inserted dye molecules at different locations in the nanocomposites of poly(2-vinyl pyridine) and silica and used fluorescence technique to study T g at those locations; increases of T g at the silica interface were reported. Choi et al [ 5 ] used neutron reflectivity to measure the diffusion coefficients of PMMA and polystyrene melts confined between graphene oxide sheets; for highly confined PMMA films (film thickness less than ≈3 bulk gyration radius), large decreases of diffusion coefficient were reported.…”

Direct Visualization and Characterization of Interfacially Adsorbed Polymer atop Nanoparticles and within Nanocomposites

“…Such variations may account for disparate reports of T g perturbations in seemingly similar systems. For example, the introduction of silica nanoparticles has been observed to cause polystyrene (PS) T g to increase, decrease, , and remain essentially unchanged. ,, Analogous discrepancies have been reported for other pairings, such as silica/poly­(methyl methacrylate) (PMMA) (increased, , decreased, ,, and essentially unchanged T g ) and alumina/poly­(2-vinylpyridine) (strongly or very weakly increased T g ).…”

“…Adsorbed layer formation is strongly dependent upon processing conditions. Due to governing chain kinetics during a typical experimental timescale, adsorption occurs more readily at elevated temperatures, coinciding for practical purposes with standard processing parameters. − Moreover, longer annealing times elicit adsorbed layers of greater thickness. ,, The processing-dependent nature of irreversibly adsorbed layers has important implications in polymer nanocomposites, whose preparation typically involves a sustained high-temperature annealing step. − Taken together with the large specific surface area of polymer nanocomposites, the temperature dependence of adsorption suggests that adsorbed layers may significantly contribute to modifying nanocomposite properties. However, direct investigations of irreversible adsorptionits evolution and its consequenceshave not yet extended beyond polymer thin films , to consider polymer nanocomposites.…”

“…More sophisticated techniques such as modulated DSC, dielectric spectroscopy, and neutron scattering enable somewhat more precise measurements and have supplied evidence of regions of locally perturbed properties in polymer nanocomposites. − However, such measurements remain indirect, leaving parameters such as layer thickness to be inferred based on estimates of local density. One of the more useful strategies for elucidating local polymer properties inaccessible by conventional techniques is fluorescence spectroscopy, ,,− wherein fluorescent labels at specific locations within a polymeric material can directly convey information about local properties such as T g at nanoscale resolution. This approach has been applied to study polymer thin films − and multilayer films composed of fluorescently labeled and unlabeled components, ,, facilitating valuable insights into local T g gradients under confinement.…”

The ascension of nanosponges as a drug delivery carrier: preparation, characterization, and applications

“…Nanomaterials are defined as physical substances that are having at least one proportion in 1–100 nm range [ 3 ]. Nanoparticles are fragments between 1 and 1000 nm in size with a neighboring interfacial layer [ 4 ]. These nanoparticles are accessible in various configurations like polymeric nanoparticles, hard- phospholipid nanoparticles, Nano emulsions, nanosponges (NSs), carbon nanotubes, micellar systems, dendrimers etc.…”