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Identifier 000384231
Title Dynamics and rheology of model branched polymers
Alternative Title Δυναμική και ρεολογία πρότυπων διακλαδωμένων πολυμερών
Author Λεντζάκη, Ελένη
Thesis advisor Βλασόπουλος, Δημήτριος
Reviewer Benoit, Loppinet
Jonas, Ulrich
Read, Daniel
Ruymbeke, Evelyne
Πετεκίδης, Γεώργιος
Briels, Wim
Abstract There is an intimate relationship between the molecular structure of industrial polymers, their rheological properties and their final processing and mechanical properties. The tube model of Doi, Edwards and de Gennes (Doi and Edwards, 1986; de Gennes, 1971) enables a molecular understanding of this relationship. The linear rheology data of linear and star polymers can be quantitatively predicted by adaptations of the original tube model. All parameters can be determined self-consistently from the chemistry except for the dilution exponent α and friction parameter p2 whose exact values are still being debated (van Ruymbeke et al., 2012). However, the inherent problems of the tube model theory arise with predictions of complex branched structures and with predictions of these polymers in complex non-linear shear and extensional flows. One of the reasons for this is due to the uncertainty related to relaxation mechanisms such as constraint release. Moreover, it is also a problem of obtaining well-defined mondisperse branched polymers, accurate characterization of the branching structure and developing reliable non-linear flow experiments where experimental artificats are avoided. This is the exact goal of this work, to combine well-defined anionic synthesized polymers, state-of-the-art characterization tools such as TGIC and systematic rheological studies in both the linear and non-linear regime in order to validate and improve existing current tube model theories. More specifically, our study focuses on the determination of the physical origin of chain stretch in complex branched polymers. We use the Sentmanat Extensional Rheometer fixed to a strain controlled rheometer to perform uniaxial extensional rheology. Uniaxial extensional rheology is difficult to measure in experimental set-ups but is a crucial experiment for introducing chain stretch. We investigate three types of architecture from order of branching complexity: linear, H, comb polymers. The uniaxial extensional rheology of linear polymers is highly rate dependent and the onset rate of experimental strain hardening (the macroscopic consequence of chain stretch) is equivalent to the theoretical prediction of the inverse Rouse time. Moreover, even linear polymers will stretch considerably under high deformations until they reach finite extensibility. The molecular dynamic picture becomes more complicated when introducing two or more branch points and two or more relaxation 7 times. We study H polymers, which have been anionically synthesized by (Roovers, 1984) and characterized recently by state-of-the-art TGIC. We discover that due to the hierarchical relaxation scheme of the H polymer, there is a greater degree of chain stretch and an earlier onset rate. We determine that as the experimental extensional rate is increased, the chain stretch is increased until it is no longer entropically favourable to do so, and branch point withdrawal occurs. Contrary to linear polymers, the maximum stretch is independent of stretch rate and only depends on architecture such that λ=q where q is the number of arms on each branch point. This can be explained by the simple rationale that the backbone segment is not free to relax until the branches have full retracted (McLeish and Larson, 1998). When increasing the number of entanglements of the arms and backbone of the H polymer, the effect of chain stretch is magnified. Moreover, we study well-defined comb polymers (Roovers, 1979) with long molar mass of backbone Mb and rather short arm ends. When doubling the number of entanglements of the arms systematically while keeping the Mb constant, the onset of chain stretch occurs at earlier rates. This can be rationalized by accounting for the effect of dynamic tube dilution and extra drag from the arms that results in an effectively slower stretch relaxation time. We modify the original differential pom-pom model of (McLeish and Larson, 1998) with the recently added modification to include drag strain coupling (Blackwell et al., 2000) by specifying the coupling of stretch between adjacent backbone segments. The model is validated successfully by comparison with a wide variety of combs (with different molecular features) and a wide range of extensional rates. At high rates, the maximum stretch condition is reached and branch point withdrawal occurs, when arms are first oriented and then withdrawn into the stretched backbone tube segments, first from the free ends and then gradually progressing towards the center. By studying the internal dynamics of the backbone segments, we discover that at this maximum stretch condition, the central backbone segment has a stretch factor equal to λ=ns/2 and that the stretch factor decreases by a value of 1 at each adjacent backbone segment. At these high rates, the addition of drag strain coupling smoothens the transition to maximum stretch and allows for better predictions. Moreover, our study focuses on the effects of the environment on the reptation and fluctuations of model H and comb polymers. By systematically varying the length of the linear chains, we study the acceleration factor related to the arm and backbone relaxation times. The acceleration factor has a strong dependence on the length of the linear chains. The shorter the chains, or the larger the difference between the relaxation times of the linear 8 matrix and the branched polymer, the more enhanced is the acceleration factor. For the study of bidisperse linear blends, there is the Struglinksy-Graessley parameter which is often invoked to explain the transition from static dilution of the short linear chains in a dilated tube to reptation in a skinny tube. There is no similar interpretation for blends of branched polymers and linear chains. We model the SAOS data using the Time Marching Algorithm by estimating a priori whether the linear polymer would be taken as a theta solvent or whether the reptation occurs in a skinny tube. The criterion for this estimation is based on the relaxation timescale separation between the linear matrix and the H or comb. In addition, the BOB model is used to model the SAOS data of the mixtures and is shown to match the data moderately well. The second diluted plateau modulus is overpredicted, indicating that the full dilution is not taken into account.
Language English
Issue date 2014-06-28
Collection   Faculty/Department--Faculty of Sciences and Engineering--Department of Materials Science and Technology--Doctoral theses
  Type of Work--Doctoral theses
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