Park, Gun Woo
(2017)
Modeling the NonLinear Rheology of Linear Polymers and Associating Telechelic Polymers.
[Tesi di dottorato]
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Item Type: 
Tesi di dottorato

Resource language: 
English 
Title: 
Modeling the NonLinear Rheology of Linear Polymers and Associating Telechelic Polymers 
Creators: 
Creators  Email 

Park, Gun Woo  gunwoo.park@unina.it 

Date: 
10 April 2017 
Number of Pages: 
249 
Institution: 
Università degli Studi di Napoli Federico II 
Department: 
Ingegneria Chimica, dei Materiali e della Produzione Industriale 
Dottorato: 
Ingegneria dei prodotti e dei processi industriali 
Ciclo di dottorato: 
29 
Coordinatore del Corso di dottorato: 
nome  email 

Mensitieri, Giuseppe  giuseppe.mensitieri@unina.it 

Tutor: 
nome  email 

Ianniruberto, Giovanni  UNSPECIFIED 

Date: 
10 April 2017 
Number of Pages: 
249 
Keywords: 
telechelic associating polymer, transient network, CoxMerz rule, shear thickening, monomeric friction reduction, nonlinear rheology, nematic interaction 
Settori scientificodisciplinari del MIUR: 
Area 09  Ingegneria industriale e dell'informazione > INGIND/24  Principi di ingegneria chimica 
Date Deposited: 
25 Apr 2017 17:33 
Last Modified: 
08 Mar 2018 14:17 
URI: 
http://www.fedoa.unina.it/id/eprint/11762 
DOI: 
10.6093/UNINA/FEDOA/11762 
Collection description
The nonlinear rheology of ordinary linear polymers and linear polymers with associable endgroup has been examined by means of modeling.
This work is motivated by discrepancies between experiments and existing theoretical expectations in the nonlinear regime. In the case of associating polymers, we aim at understanding the breakdown of CoxMerz rule, shear thickening, and strain hardening of shear startup, while for conventional linear polymers we focus on the discrepancies typically encountered in the fast uniaxial extensional flows. It is appropriate to mention that most efforts are related to developing a stochastic simulation of associating telechelic polymers (part 1) while the studies of linear polymers in the fast flows (part 2) is rather limited in the persepectives of nematic interactions of oligomertype solvent.
For reference sample of associating telechelic polymers, the hydrophobically modified ethoxylated urethane (HEUR) is selected because of plenty of experimental data have been reported in the past. This information is collected into chapter 1 considering both morphology and dynamical aspects. HEUR is made up by poly(ethylene oxide) (PEO) endcapped with short hydrophobic groups. Above the socalled critical micelle concentration, HEUR in aqueous solutions forms flowerlike micelles where the core is composed of aggregated hydrophobic endgroups. Since the aggregation is physically reversible, chain ends can detach from the core, and attach to neighboring micelles (thus forming bridges). The probability of bridge formation increases with increasing HEUR concentration, and a transient network eventually builds up. The linear viscoelastic behavior of HEUR systems is somehow simple since they exhibit a singlemode Maxwelllike response with a dominant relaxation time (related to the association/dissociation dynamics), exhibiting a powerlaw dependence on HEUR concentration and molar mass. On the contrary, HEUR solutions exhibit a complex nonlinear rheological behavior. The CoxMerz rule is often violated since the steady shear viscosity can reveal shear thickening while the dynamic viscosity only shows shear thinning. In the shear rate range of the viscosity thickening, the first normal stress coefficient remains at its LVE value. As regards the shear startup response at high shear rates, strain hardening is often observed both for the viscosity and for the first normal stress coefficient. Remarkably, the overshoot of stress growth function is well beyond linear viscoelastic envelope. Motivated by these experimental observations, new stochastic simulation is proposed where its coarsegraining level is the consequence of trade off between computation time (within few days for nonequilibrium simulation) and availability to capture detailed mechanism behind rheological observations, especially for number of elastically active chains.
This newly developed stochastic simulation is based on Langevin dynamics coupled with an additional stochastic step for topological renewal. Parameters of the simulations are size of micelles and chains, stiffness of micelle structure, micelle aggregation number, length being related to micelle core, and time ratio between micelle diffusion time and loopdissociation time. After detailing the algorithm in chapter 2, chapter 3 explores the effect of various parameters on static and dynamical observables. Selected samples are examined in chapter 4 both under equilibrium and nonequilibrium conditions. Results show scaling exponents consistent with experimental data, understanding strain hardening of shear startup in the way of finite extensibility of chains, confirm breakdown of CoxMerz rule due to persistence of bridges, and capture shearthickening. Details of simulations are reported in the appendix together with the theoretical background and strategy of code development.
In part 2, we examined entangled linear polymers in the extensional flows at flow rates higher than the reciprocal Rouse time. In the classical molecular models, the steadystate extensional viscosity is characterized by four regimes: (i) the linear regime with Trouton ratio equal to 3, (ii) viscosity thinning with exponent 1, (iii) upturn due to chain stretch, and (iv) approach to an asymptotic value due to finite extensibility. The advent of data from extensional rheology, however, reveals that the theoretical expectation is not strictly true, and the tendency depends on the details of chemistry. To be specific, polystyrene (PS) melt shows the spontaneous decrease even beyond the reciprocal Rouse time with an exponent of 1/2, while polyisoprene (PI) and poly(nbutyl acrylate) (PnBA) shows the upturn around reciprocal Rouse time. These differences are believed to be due to the sensitivity of the monomeric friction coefficient to alignment in the statistical segments of polymer chain when the flow rate is larger than reciprocal Rouse time. This is confirmed by measuring components for friction tensor and order parameter of oligomertype molecular simulations in the simple shear where shear rate is higher than reciprocal selfdiffusion time (chapter 6). In this context, we also analyze PS solutions in its oligomeric solvents, all having the same linearviscoelasticity (chapter 7). The suggested model uses the frictional change due to the change of order parameter that accounts for the nematic interactions. The results quantitatively predict the experimental data from extensional flow.
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