Enhanced mobility and increased gas sorption capacity in thin film and nanoconduit confined polymers

Poly(tert butyl acrylate) (PTBA) is found to exhibit enhanced mobility when spun cast into thin films or impregnated into cylindrical anodic aluminum oxide (AAO) nanoscale pores. In a thin film configuration, the glass transition temperature of 20 nm thick PTBA is found to decrease almost 20 °C compared to the bulk. Consistent with this mobility increase, an increased volume fraction of interphase polymer leads to at least a 2.4 times viscosity reduction when PTBA is impregnated in 100 nm pores versus 200 nm pores. Such increases in mobility result in a 15‐fold increase in CO₂ permeability for an AAO confined geometry compared to a bulk film. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 434–441, 2010     

Insight into reverse selectivity and relaxation behavior of poly[1-(trimethylsilyl)-1-propyne] by flux-lateral force and intrinsic friction microscopy

Novel scanning force microscopy techniques (SFM) were employed to investigate poly[l-(trimethylsilyl)-1-propyne] (PTMSP), a high-free volume, highly permeable reverse-selective membrane material. This study reports, for the first time, reverse selectivity in relation to the interfacial gas adsorption capacity of the PTMSP membrane with the gas permeants CO₂ and helium. With flux-lateral force microscopy (F-LFM), mechanical property changes caused by permeant gas infiltration were recorded within the polymer interfacial downstream region. In conjunction with bulk permeation measurements and varying sequential exposure to the two permeants, CO₂ is found to saturate the membrane faster, i.e. at a lower differential pressure by about 0.3 bar, in comparison to helium. It is also identified as modifying agent for PTSMP causing a significant change in the mechanical properties of the polymer matrix, which consequentially leads to a considerable helium transport reduction, and thus, an increase in reverse selectivity from 1.2 to 4.7. Also in this study, thermally available activation modes of 6–8 kcal/mol were revealed by intrinsic friction analysis (IFA) that were attributed to backbone methyl-group rotations in accordance with conformational calculations. Bulk thermally activated modes were found to be modestly affected by interfacial constraints on the sub-100 nanometer scale, which is an important finding for interpreting interfacial constraints in PTMSP nanocomposites involving silicon-oxides.     

Glass and Structural Transitions Measured at Polymer Surfaces on the Nanoscale

This paper reviews our recent progress in determining the surface glass transition temperature, T_g, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of T_g. The T_g of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to T_g will be discussed.This revised version was published online in November 2005 with corrections to the Cover Date.     

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.     

Comparative study of single and multi domain CVD graphene using large‐area Raman mapping and electrical transport characterization

We systematically investigate the impact of granularity in CVD graphene films by performing Raman mapping and electrical characterization of single (SD) and multi domain (MD) graphene. In order to elucidate the quality of the graphene film, we study its regional variations using large‐area Raman mapping and compare the G and 2D peak positions of as‐transferred chemical vapor deposited (CVD) graphene on SiO₂ substrate. We find a similar upshift in wavenumber in both SD and MD graphene in comparison to freshly exfoliated graphene. In our case, doping could play the dominant role behind the observation of such upshifts rather than the influence due to strain. Interestingly, the impact of the polymer‐assisted wet transfer process is the same in both the CVD graphene types. The electrical characterization shows that SD graphene exhibits a substantially higher (a factor 5) field‐effect mobility when compared to MD graphene. We attribute the low sheet resistance and mobility enhancement to a decrease in charge carrier scattering thanks to a reduction of the number of grain boundaries and defects in SD graphene.