Small Angle Neutron Scattering Study of the Thermodynamics of Highly Interacting PαMSAN/dPMMA Blends

Abstract

Poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) form partially miscible blends with lower critical solution temperature (LCST) behaviour. We revisit this system using small angle neutron scattering (SANS), examining the effect of molecular weight (M w) of deuterated PMMA (dPMMA), blend composition (φ) and temperature (T) in the homogeneous region. All data are well described by the Random Phase Approximation (RPA) theory, enabling us to determine thermodynamic and structural parameters, including the correlation length ξ, G (the second derivative of the free energy of mixing with respect to composition), and the statistical segment length a of each component. Phase boundaries are computed by extrapolation of G with temperature, to yield the spinodal, and inspection of Kratky plots to yield the binodal. For PαMSAN, a is determined to be 10.1±0.4Å. Unsurprisingly, this system deviates strongly from Flory-Huggins expectations, exhibiting a minimal M w dependence of the phase boundaries and φ-dependence of effective interaction parameter (χ). Comparison of G with values for other blend systems places PαMSAN/dPMMA in a class of highly interacting blends, expected from Cahn-Hilliard theory to yield small initial phase sizes upon spinodal demixing. This is confirmed experimentally, with an illustrative temperature jump resulting in an initial phase size of 30 nm.

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 .…”

“…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.…”

“…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.…”

“…Figure 10:Comparison of phase diagrams for the PαMSAN/dPMMA40k and PαMSAN/dPMMA99k blends 19. LCL model spinodal boundary (curves) and corresponding experimental spinodal data (points).…”