Dynamics of poly(ethylene oxide) in a blend with poly(methyl methacrylate): A quasielastic neutron scattering and molecular dynamics simulations study

Abstract

In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, Tg, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond-10 nanoseconds scale at temperatures close and above the Tg of the blend. The main focus was on the PEO component, i.e., that of the lowest Tg, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the alpha-methyl rotation; an additional process detected in the experimental window 65 K above the blend-Tg can be identified as the merged alphabeta process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of approximately 1 nm. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure.

Self- and Collective Dynamics of Syndiotactic Poly(methyl methacrylate). A Combined Study by Quasielastic Neutron Scattering and Atomistic Molecular Dynamics Simulations

“…Samples. Two PMMA samples (Polymer Source, Inc.) with different deuteration levels were investigated: fully deuterated (PMMA-d 8 ) and a protonated material including deuterated ester methyl groups (PMMA-d 3 ). The molecular weights and polydispersities were M w ) 26 800 and 25 300 g/mol and M w /M n ) 1.04 and 1.09, respectively.…”

“…The homopolymer investigated as reference showed extremely strong deviations from the Gaussian approximation for the average self-correlation function of all PMMA hydrogens that were experimentally accessed. The second work 8 did not report on the homopolymer dynamics. Finally, in the neutron spin-echo study on collective motion, 20 the temperature range was restricted to only two temperatures very close to T g for pure PMMA.…”

“…[2][3][4][5][6] (ii) Because of stronger hindrance, the rotation of the R-methyl group occurs only at higher temperatures. 7,8 Furthermore, other subglass and glass relaxation processes in PMMA have been studied for a long time mainly by dynamic mechanical analysis, dielectric spectroscopy, and advanced NMR spectroscopy. [9][10][11][12][13][14][15][16][17] Especially in the temperature range close to and above the glass-transition temperature (T g ≈ 400 K), PMMA exhibits a complicated dynamics.…”

Dynamics of PEO in Blends with PMMA: Study of the Effects of Blend Composition via Quasi-Elastic Neutron Scattering

“…We note that it has been reported that QENS data for this system indicate confinement below the T g of PMMA, where the system may be regarded as mobile PEO segments "confined" by the immobile PMMA. 12 It may be that the character of the observed process changes under these conditions (PEO confined between immobile PMMA segments) and is no longer represented by models designed to describe the R-relaxation. The R-relaxation of PEO has been reported to disappear entirely when confined in other systems, which is consistent with all the above evidence.…”

“…1,2 The difference in glass transition temperatures [T g 's] between PEO and PMMA is significant (∼180 K), with PEO having the low T g and thus being the more mobile component. Two dynamic responses are observed through a variety of experimental techniques, [3][4][5][6][7][8][9][10][11][12] and two glass transitions have recently been reported. 58 If characteristic relaxation times are compared at a common temperature, the mobility of PMMA increases with blending while that of PEO decreases.…”