Effect of Tacticity on the Phase Behavior and Demixing of PαMSAN/dPMMA Blends Investigated by SANS

Abstract

We investigate the effect of polymer tacticity on the phase behaviour and phase separation of polymer mixtures by small angle neutron scattering (SANS). Poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) and deuterated poly(methyl methacrylate) (dPMMA) with two degrees of syndiotacticity, were selected as a model partially miscible blend, as one of the most highly-interacting systems known (defined by the temperature dependence of blend's interaction parameter). One-phase (equilibrium) and time-resolved, spinodal demixing experiments, were analysed by the de Gennes' Random Phase Approximation (RPA) and Cahn-Hilliard-Cook (CHC) theory, respectively. The second derivative of the Gibbs free energy of mixing with respect to composition (G ≡ ∂ 2 ∆G m /∂φ 2) and corresponding χ parameter were obtained from both RPA and CHC analysis, and found to correlate well across the phase boundary. We find that blends with higher PMMA syndiotacticity exhibit greater miscibility, and a steeper G temperature-dependence by ∼40%. The segment length of dPMMA with higher syndiotacticity was found to be a = 7.4Å, slightly larger than 6.9Å reported for lower syndiotacticity dP-MMA. Consideration of thermal fluctuations is required for the self-consistent analysis of the non-trivial evolution of the spinodal peak position q * over time, corroborated by CHC model calculations. The temperature dependence of the mobility parameter, M , can be described by a 'fast mode' average of the diffusion coefficients of the blend constituents, except for quenches originating near the glass transition. A minimum de mixing lengthsc ale of Λ ≈ 40 nm is obtained, in agreement with theory for deeper quenches, but deviates at shallower quenches, whose origin we discuss. CHC correctly describes demixing length and timescales, except for quenches into the vicinity of the spinodal boundary. Our data demonstrate the significant effect of relatively minor polymer microstructure variations on polymer blend behaviour across both sides of the phase boundary.

Thermodynamics of Model PαMSAN/dPMMA Blend: A Combined Study by SANS, Ellipsometry, and Locally Correlated Lattice (LCL) Theory

“…The structure factor of a polymer blend S(q) in the one phase region can be obtained from the SANS scattering intensity I(q) following where K is a contrast factor SANS data from polymer blends are frequently analysed using the Random Phase Approximation (RPA) and we have previously reported 18,19 an extensive analysis of these blends employing this approach. In the current work, we explore the thermodynamic behavior of the blend and are thus principally interested in determining G .…”

“…We then provide a detailed thermodynamic analysis of the effect of PMMA tacticity on the polymer blend miscibility, and summarize our findings. have been previously reported by Aoki et al 18,19 Experimental and Modeling Methods…”

“…18 This blend presents the interesting case of a very modest change in tacticity of the PMMA component giving rise to a sizeable shift of 16 K in the phase boundary. The monomer structures, phase diagram, calorimetry data, previously reported SANS measurements, 18,19 and G (T ) analysis are summarized in Figures 1 (a 18,19 to determine the main thermodynamic quantity of interest, G , defined by Eq. 1.…”

“…In this paper, for the first time, we combine analysis of SANS and ellipsometry data with the LCL theory to understand the data recently published for the blend of poly(α-methyl styrene-co-acrylonitrile), or PαMSAN, with deuterated poly(methyl methacrylate), or dPMMA. 18 This blend presents the interesting case of a very modest change in tacticity of the PMMA component, giving rise to a sizeable shift of 16 K in the phase boundary. The monomer structures, phase diagram, calorimetry data, previously reported SANS measurements, 18,19 and G″(T) analysis are summarized in Figure 1a−h of the current paper.…”

“…18 This blend presents the interesting case of a very modest change in tacticity of the PMMA component, giving rise to a sizeable shift of 16 K in the phase boundary. The monomer structures, phase diagram, calorimetry data, previously reported SANS measurements, 18,19 and G″(T) analysis are summarized in Figure 1a−h of the current paper. This paper is organized as follows: In the Experimental and Modeling Methods section, we describe the experimental sample preparation, including the SANS and ellipsometry procedures, and present the background on the locally correlated lattice (LCL) model, which will be used to analyze the results.…”