Intra- vs Intermolecular Cross-Links in Poly(methyl methacrylate) Networks Containing Enamine Bonds

Abstract

The molecular dynamics of a copolymer composed of methyl methacrylate (MMA) and (2-acetoacetoxy)ethyl methacrylate (AEMA) monomers and the influence on it of intra- to intermolecular cross-links of AEMA units with ethylenediamine (EDA) was studied by combining dielectric relaxation experiments and thermal investigations. The dielectric spectra of the non-cross-linked copolymer show three dynamical processes: a slow relaxation (α) and a faster (β), both dominated by the MMA dynamics, and an even faster secondary relaxation (γ) reflecting the AEMA dynamics. Already for low cross-linking densities, the γ process is very much affected and eventually disappears, increasing the cross-linking density. The secondary β relaxation however was nearly unaffected by cross-linking. The effect of cross-linking on the α relaxation was very pronounced with an important increasing of the glass transition temperature T _g. There was also an increase of the dynamic heterogeneity and the relaxation intensity when increasing the cross-linking density (up to the maximum explored, 9 mol % EDA). The quality of the average time scale and T _g value have similarities in behavior for intra- and intermolecular cross-linking, but clear differences in the dynamic heterogeneities where observed. These differences can be interpreted in connection with the sparse internal structure of the collapsed single chains obtained by intramolecular cross-linking.

Scheme 1. (a) Chemical Structure of the Cross-Linking between AEMA Units by Ethylenediamine (EDA) via Enamine Bond Formation (see ref (25) and the SI); (b) Preparation of THF Solutions above and below the Overlap Concentration for Inter- and Intra-Chain Cross-Linking, Respectively; (c) Procedure of Film Preparation from the above Solutions

Figure 1

DSC heat flow curves of the samples investigated in this work as measured during cooling at 3 °C/min: neat poly(MMA_0.7-ran-AEMA_0.3) (denoted as NEAT, dotted-dashed line), networks based on intramolecular cross-linked SCNPs (denoted as INTRAx, solid lines), and networks based on intermolecular cross-linked poly(MMA_0.7-ran-AEMA_0.3) chains (denoted as INTERx, dashed lines) prepared with different EDA/AEMA molar ratios, from x = 9% to 50%.

Figure 2

(a) Dielectric permittivity losses (ε″) as function of frequency (f) for the neat poly(MMA_0.7-ran-AEMA_0.3) copolymer at different temperatures. (b) Experimental −dε′(f)/d ln f as function of log₁₀f at three representative temperatures in the merging region and corresponding fitting lines (see text for details).

Figure 3

Relaxation map of the poly(MMA_0.7-ran-AEMA_0.3) copolymer. The solid lines are the fitting using eq 6 for α-relaxation and the Eyring equation for the β- and γ-relaxations (see text for details).

Figure 4

Spectra of −dε′(f)/d ln f as function of log f for (a) INTRA9, (b) INTRA15, and (c) INTRA50. Continuous lines are fits by the model function (see text for details).

Figure 5

(a) Relaxation map of INTRA9 (solid square points), INTRA15 (solid star points), INTRA50 (solid up-triangle points), INTER15 (open star points), and INTER50 (open up-triangle points). (b) σ_cross parameter as a function of temperature for different cross-linking density (same symbols as in (a)).

Figure 6

Spectra of −dε′(f)/d ln f as a function of log f for (a) INTER15 and (b) INTER50 at different temperatures. Continuous lines are fits by merging model and conductivity (σ) contributions.

Figure 7

(a) T_g and (b) T₀ of networks based on intramolecular cross-linked SCNPs (solid circles) and networks based on intermolecular cross-linked poly(MMA_0.7-ran-AEMA_0.3) chains (open circles) as a function of the cross-linking degree (ϕ) calculated as the relative number of repeating units involved in the reactions. The values are normalized by the parameters of the neat poly(MMA_0.7-ran-AEMA_0.3) copolymer. (c and d) Corresponding behavior of the fragility index m and the additional dynamic heterogeneity σ_cross.