Influence of Particle Size and Polymer−Filler Coupling on Viscoelastic Glass Transition of Particle-Reinforced Polymers

Abstract

The viscoelastic glass-to-rubber softening transition is analyzed for various cross-linked polymers reinforced with filler particles. We find that the loss modulus peak corresponding to the segmental relaxation process (glass transition) is not significantly affected by the particle surface area in carbon black-filled polybutadiene or by silane chemical coupling of poly(styrene-co-butadiene) to silica. Large differences in shape and magnitude of the peak in the loss tangent (tan δ) vs temperature are noted for these materials; however, this is due to variations in the storage modulus at small strains in the rubbery state, which is influenced by the nature of the jammed filler network. The use of a simple relaxation model demonstrates this feature of the viscoelastic glass transition in filled rubber. It is not necessary to invoke concepts involving a mobility-restricted polymer layer near the filler surfaces to explain the viscoelastic results. Atomic force microscopy conducted with an ultrasharp tungsten tip indicates that there may be some stiffening of the elastomer in the proximity of filler particles, but this does not translate into an appreciable effect on the segmental dynamics in these materials. IntroductionDespite significant research activity on the effect of nanoscale confinement on the glass transition temperature (T g ) of polymers, many controversial issues remain unresolved, as recently reviewed by Alcoutlabi and McKenna.1 Of particular relevance to the field of elastomers is the influence of reinforcing particles on the polymer T g . It is reasonable to expect that physical adsorption or chemical attachment of polymer chains to rigid particles can slow down the polymer dynamics, which might increase the glass transition of the polymer chains near particle surfaces. However, while some published studies show increases in T g upon the addition of carbon black, silica, or other fillers, others report no change in T g or even T g decreases.2-26 The nature of the interfacial interactions between the polymer and particles may account for some of the disparate results concerning the effect of fillers on T g .

Flocculation, Reinforcement, and Glass Transition Effects in Silica-Filled Styrene-Butadiene Rubber

“…To characterize the segmental relaxation process, the loss modulus (G ) versus the temperature peak is used rather than the tanδ peak because the latter occurs at higher temperatures and has shape and magnitude which are unduly affected by the rubbery modulus. 21 The dynamic mechanical results presented in Figure 11 corroborate the lack of filler-induced T g change noted by DSC. A review article in this area concludes that there are many such examples for the materials described in Table III.…”

“…[16][17][18] There are observations that the segmental relaxation (α-relaxation) and glass transition temperature (T g ) are not significantly affected by the presence of filler, despite significant levels of "bound" polymer from chemically modified polymer-filler interfaces and from well dispersed particles with high surface area. [19][20][21][22][23][24] On the other hand, there are other reports which show that the filler can have a significant influence on the glass transition dynamics. [25][26][27][28][29][30][31] In particular, the research of Tsagaropoulos and Eisenberg 25,26 is often cited as support for the existence of severely retarded segmental motion of the polymer near the surfaces of small particles (glassy polymer shell).…”

“…However, it was previously suggested 17,21,32 and later conclusively demonstrated 33 that the high temperature viscoelastic response was a flow-related phenomenon rather than the glass transition of a mobility-restricted polymer shell. Numerous studies have employed nuclear magnetic resonance (NMR) relaxation experiments to evaluate the effect of filler particles on polymer dynamics, and the results of this research are summarized elsewhere.…”

Glass Transition and Interfacial Segmental Dynamics in Polymer-Particle Composites

“…Dynamic mechanical testing is a common technique to study the effect of particles on the T g of polymers, [11][12][13][14][15][16][17][18][19][20][21][22][23] and a summary of these studies is given in Table I. Many investigators draw conclusions based on the temperature of the maximum in the isochronal loss tangent, tanδ = G"/G' (abbreviations are defined in the appendix).…”

“…Many investigators draw conclusions based on the temperature of the maximum in the isochronal loss tangent, tanδ = G"/G' (abbreviations are defined in the appendix). This can be problematic because tanδ in the glass-to-rubber softening region is influenced not only by the local segmental dynamics, as reflected in the magnitude of the loss modulus toward lower T, but also by filler-induced changes in both G' and G" at higher T. As shown recently by Robertson et al for BR reinforced with carbon black or SBR reinforced with silica, 23 the shape and position of the loss modulus peak are unaffected by particle surface area and the intensity of the polymer-filler interaction, despite substantial changes in the loss tangent peaks (see Figure 1). Vieweg et al 21 also reported negligible modification of the segmental relaxation behavior in filled SBR, as illustrated in Figure 2.…”