Effect of Nanopore Geometry in the Conformation and Vibrational Dynamics of a Highly Confined Molecular Glass

Abstract

The effect of nanoporous confinement on the glass transition temperature (T g) strongly depends on the type of porous media. Here, we study the molecular origins of this effect in a molecular glass, N,N′-bis­(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), highly confined in concave and convex geometries. When confined in controlled pore glass (CPG) with convex pores, TPD’s vibrational spectra remained unchanged and two T g’s were observed, consistent with previous studies. In contrast, when confined in silica nanoparticle packings with concave pores, the vibrational peaks were shifted due to more planar conformations and T g increased, as the pore size was decreased. The strong T g increases in concave pores indicate significantly slower relaxation dynamics compared to CPG. Given TPD’s weak interaction with silica, these effects are entropic in nature and are due to conformational changes at molecular level. The results highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.

Glass Transition Dynamics of Poly(phenylmethylsiloxane) Confined within Alumina Nanopores with Different Atomic Layer Deposition (ALD) Coatings

“…This points out that some other phenomena might probably occur close to the surface, not considered so far. One possible scenario is related to density variation close to the substrate, as suggested by Wang and co-workers . In this view, in confined material, we can distinguish high- and low-density regions with different T g s. In the N , N ′-bis­(3-methyl­phenyl)- N , N ′-diphenyl­benzidine (TPD) case, studied by Wang et al, the pore curvature promotes itself various molecular packing.…”

Effect of the Surface Polarity, Through Employing Nonpolar Spacer Groups, on the Glass-Transition Dynamics of Poly(phenyl methylsiloxane) Confined in Alumina Nanopores

“…The modification of the surface conditions aims to affect the specific interactions of the polymer with the confining walls causing T g2 to disappear. However, the presence of two T g ’s has also been seen even in the absence of strong interactions for other systems . One of the possibilities, noted by Wang et al, is to link high and low T g ’s seen in nanopore confinement with a variation in the density, which for the regions close to the smooth pores induce denser packing, while for the regions away from the pores results in consistently lower density areas .…”

“…However, the presence of two T g 's has also been seen even in the absence of strong interactions for other systems. 93 One of the possibilities, noted by Wang et al, is to link high and low T g 's seen in nanopore confinement with a variation in the density, which for the regions close to the smooth pores induce denser packing, while for the regions away from the pores results in consistently lower density areas. 93 Such an explanation would explain the disappearance of two T g scenarios for rough pore walls.…”

“…93 One of the possibilities, noted by Wang et al, is to link high and low T g 's seen in nanopore confinement with a variation in the density, which for the regions close to the smooth pores induce denser packing, while for the regions away from the pores results in consistently lower density areas. 93 Such an explanation would explain the disappearance of two T g scenarios for rough pore walls. 94 What the current study shows is that the particular surface chemistry can either promote or hinder the equilibration phenomena that bring back the nanoconfined system to the time scale of the bulk material.…”

Effect of Nanoscale Confinement on Polymer-Infiltrated Scaffold Metal Composites

“…We now transition from examining the kinetics of polymer infiltration in NPG to the thermal properties of the resulting composites. Specifically, polymer chains in confined geometries, such as nanosized pores, are expected to have different segmental dynamics than unconfined chains. ,− The effect of confinement on segmental relaxation dynamics can be captured by measuring the T g of the polymer using in situ SE to monitor changes in the refractive index as a function of temperature. This contrasts with measurements of the infiltration kinetics of the polymer chains (cf.…”