When conventional materials are fabricated on the nano-scale, many physical properties of the materials are changed by the length scale at 1 to 20 nm. If at least one of the dimensions is smaller than the length scale of some property, that property is confined and behaves differently from the larger dimensional states. Polymer nano-particles offer an ideal system for the study of the confinement effects on the glass transition (T_g) because the T_g of polymers describes a cooperative motion at the length scale of 10 - 20 nm. The glass transition is one of the most intriguing puzzles in the physics of condensed matter, the study of which will ultimately influence different fields, in particular biophysics and biochemistry....[ Read more ]

When conventional materials are fabricated on the nano-scale, many physical properties of the materials are changed by the length scale at 1 to 20 nm. If at least one of the dimensions is smaller than the length scale of some property, that property is confined and behaves differently from the larger dimensional states. Polymer nano-particles offer an ideal system for the study of the confinement effects on the glass transition (T_g) because the T_g of polymers describes a cooperative motion at the length scale of 10 - 20 nm. The glass transition is one of the most intriguing puzzles in the physics of condensed matter, the study of which will ultimately influence different fields, in particular biophysics and biochemistry.

Single molecule chain polymers occupy nano-space. The understanding of the effects of this confinement is limited at present. For example, although there are many studies of the glass transition behaviour of polymers in confined environments, e.g. in ultra-thin films, in pores and near surfaces, the experimental results are controversial even for thin films - the simplest confined geometry. In this study spherical monomolecular polymer particles were prepared by a novel application of spray-drying. The spray-drying process is a classic example of the use of heat and mass transfer fundamentals.

The focus of this study was the characterization of the particles produced by spray-drying the water soluble polymers poly(N-vinyl pyridine) (PVP) and polyacrylamide (PAL). The morphologies and glass transition behaviour of these fine particles were investigated by thermal analysis (DSC) and atomic and electron microscopies (AFM, SEM). The diameters of the particles were measured as 50 - 80 nm for PAL, (M̄_w = 1.8 x 10[to the power of seven] kg kmol^-1) and 14 - 16 nm for PVP (M̄_w = 1.3 x l0[to the power of six] kg kmol^-1). Calculations show the diameters of the sprayed particles are similar to the dimensions of single chain molecules for both polymers. The AFM images of the PVP show a honeycomb-like structure with a central particle surrounded by six or seven other particles.

The glass transition temperature of the PVP single chain nano-particles is ~3[degress celsius] higher than that of the bulk polymer, as shown by normal DSC. An exothermic peak immediately after the glass transition was observed and explained as being due to the energy associated with the interaction of the mesophase structure with its immediate matrix.

The high pressure DSC results indicate a lack of plasticisation even though there is a high CO₂ sorption. An explanation using a combination of peak area and fictive temperature data is presented. The PVP nano-particles, with their high surface area to volume ratio, are sufficiently compact that the CO₂ is adsorbed, but there is little or no permeation of the gas molecules into the monomolecular spheres. This is in contrast to what happens in the bulk film sample.