Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid-like Behavior

Abstract

We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm M w arm , thin films supported by silicon oxide, SiO x , is strongly dependent on M w arm and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, T g , decreases with decreasing film thickness (ΔT g < 0). However, for sufficiently large f and small M w arm , T g becomes independent of film thickness (ΔT g ≈ 0). In this region, where ΔT g ≈ 0, the star-shaped macromolecules self-assemble into ordered, periodic structures, similar to that of soft spheres or colloids, as revealed by simulations and experiments. This is identified as the soft-colloidal region. The transition from the linear-chain-like to the soft-colloidal-like region occurs over an intermediate range of functionalities and arm lengths; throughout this transition range ΔT g > 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to competing entropic interactions associated with changes in f and M w arm . The vitrification behavior of thin star-shaped PS films on SiO x is summarized in terms of a "diagram of states".

Free Surface Relaxations of Star-Shaped Polymer Films

“…6,19,20 However $T_{\mathrm{g}}^{\text{surf}}$ became gradually lower than $T_{\mathrm{g}}^{\text{bulk}}$ as M w arm became much larger than M e, the molecular weight between entanglements; this trend is consistent with the behavior of linear chain polymers. 6,19,20 Molecular dynamics simulations revealed that this behavior– $T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$-might be due to positional correlations of the star-shaped molecular at the free surface of the film. 20 In this letter we demonstrate using XPCS that the in contrast to the linear chain polymers, the free surface layer relaxations are slow compared to the bulk and this persists to temperatures T > T g + 50 K. Molecular dynamics simulations of supported thin polymer films indicate that the slow dynamics, and $T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$, would be due to a preferential localization of these molecules at the free surface.…”

“…$T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$. 6,19,20 However $T_{\mathrm{g}}^{\text{surf}}$ became gradually lower than $T_{\mathrm{g}}^{\text{bulk}}$ as M w arm became much larger than M e, the molecular weight between entanglements; this trend is consistent with the behavior of linear chain polymers. 6,19,20 Molecular dynamics simulations revealed that this behavior– $T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$-might be due to positional correlations of the star-shaped molecular at the free surface of the film.…”

“…Simulations of thin films of star polymers having M = 10 segments per arm were performed; details of the simulations are described elsewere. 20,28 Based on previous studies, 20,28 we know that there is a stronger positional correlation (concentration profile) of the core particles at the free interface for films having 8-arm star polymers than the linear chain analogues; this implies that star polymers are preferentially located at the free surface compared to the interior of the film, as illustrated in Fig. 5a.…”

“…6,19,20 Molecular dynamics simulations revealed that this behavior– $T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$-might be due to positional correlations of the star-shaped molecular at the free surface of the film. 20 In this letter we demonstrate using XPCS that the in contrast to the linear chain polymers, the free surface layer relaxations are slow compared to the bulk and this persists to temperatures T > T g + 50 K. Molecular dynamics simulations of supported thin polymer films indicate that the slow dynamics, and $T_{\mathrm{g}}^{\text{surf}}>T_{\mathrm{g}}^{\text{bulk}}$, would be due to a preferential localization of these molecules at the free surface. The XPCS measurements indicate that the dynamics are restricted spatially, or “caged,” at temperatures tens of degrees above the glass transition temperature, due to the persistence of structural ordering.…”

pH responsive highly branched poly(N-isopropylacrylamide) with trihistidine or acid chain ends

“…Study of polymer materials in recent years have focused on the development of advanced polymer architectures with tuneable materials properties [1][2][3][4][5] . Branched polymers are interesting because their solution 6,7 , rheological [8][9][10] and surface properties 10,11 are different to their linear analogues.…”