Microscopic Relaxation Processes in Branched-Linear Polymer Blends by Rheo-SANS

Abstract

The relaxation time spectrum in blends of architecturally different polymers with strongly disperse time scales has been investigated by their time-dependent small angle neutron scattering signal after a fast uniaxial step strain. Modelhyperbranched dendrimeric polymers of second generation, dilutely dispersed within linear homopolymer matrices, acted like sensitive probes for structurally, though not firmly, established features of the tube model for bidisperse melts. We showed that the equilibration time of the linear matrix determines the size of the fluctuations that the outer and inner arms experience. Within a random phase approximation (RPA) treatment, which accounts for the different degrees of freedom inherent to the broad time scales, the observed loss of anisotropy with time was described in terms of two parameters only, namely the tube diameter and the fraction of relaxed arms of the minority component. The scattering data reveal details of mechanisms, which cannot be extracted from but determine the macroscopic flow properties. At intermediate times, a tube relaxation process was detected. At long times, the dynamic dilution model is confirmed. ■ INTRODUCTIONIn the last few decades, the flow behavior of branched polymers has attracted an increasing attention and the description of their linear rheology by means of the tube model has achieved remarkable progress. 1−16 Of particular interest is that the dynamics of branched polymers is, on one hand, much more complex and demanding as compared to that of simple linear chains but, on the other hand, allows to distinguish different processes according to the hierarchy of times in branched polymers. Therefore, the coupling and competition of diffusive reptation and contour length fluctuations, which have dominate until today the understanding of the dynamics of linear chains in detail, does not occur in the same way. 17−20 Also, interesting new properties due to this hierarchical approach emerge in the nonlinear rheological response. This bears consequences especially on processing. Blends of short and long linear chains 21,22 as well as those consisting of linear and long-chainbranched polymers have well-known additional advantages. Here, we investigated branched polymers immersed in a linear matrix as the major constituent. This allows controlling the time constants of the matrix in relation to the characteristic times of different sections of the branched chains. 9,11,19,23,24 Vice versa, the branched polymer may be regarded as a probe for mechanisms of topological constraints and constraint release processes in linear systems, comparable to probe rheology, which was recently reintroduced by Bailly et al. 25 Although probe rheology is a very sensitive and interesting technique for systems with sufficiently different time scales, it is based on the subtraction of large modulus contributions to isolate the response function of the probe without its environment. A rigorous underlying theory like the Random phase Approximation (RPA) for scattering is, unfor...

Effect of the salt-induced micellar microstructure on the nonlinear shear flow behavior of ionic cetylpyridinium chloride surfactant solutions

“…This finding is in line with many examples of complex systems, where such additional stress relaxation mechanisms cannot be detected through simple rheology. Examples include blends of architecturally different polymers with strongly disperse times [50][51][52][53]. In these systems, depending on the concentration of the various structures, the hierarchical relaxation mechanism of the different characters could be hidden in a rheological measurement, while showing up clearly in scattering experiments.…”

Stress growth and relaxation of dendritically branched macromolecules in shear and uniaxial extension

“…Concerning the present data, given the small range of times we refrain from considering power laws. Unfortunately, we do not have independent information of the flow-induced stretching in different generations and respective relaxation upon flow cessation (in fact, very limited data exist in general [7,12,60]) since this requires deuterated samples which are not available currently.…”

“…Indeed, substantial progress has been made in decoding the linear viscoelasticity of model branched polymers (such as combs or pom-pom-like macromolecules), and more recently their nonlinear response in shear and extension. The rheological properties of model polymers provide profound implications in finessing molecular models and improving material performance in technological applications [5][6][7][8][9][10][11][12][13][14][15]. Focusing on the nonlinear rheology of comb polymers, one important consequence of the recent progress in experimental extensional and shear rheology is the quantitative assessment of the role of dynamic dilution in transient strain hardening and stress overshoot, respectively.…”

KWS-1 high-resolution small-angle neutron scattering instrument at JCNS: current state

“…Complex magnetic nanoparticles such as dimers (Schladt et al, 2012), antiferromagnets (Feygenson et al, 2010) and steels for reactor components (Heintze et al, 2012) might profit from the small-angle neutron scattering of polarized neutrons as well. Besides that, soft matter materials like polymers (Maccarrone et al, 2007;Ruocco et al, 2013), proteins (Zhang et al, 2012), carbon nanotubes (Doe et al, 2010), hydrogen storage materials (Sartori et al, 2010) etc. can be studied using conventional SANS as before, but with the additional possibilities of using large sample areas to increase the intensity on the sample with the help of lenses and the use of the chopper for time-resolved measurements.…”