Nonlinear Polymer Rheology
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SHI-QING WANG, PhD, is Kumho Professor of Polymer Science at the University of Akron. He has been teaching at the university level for more than 28 years and has over 150 peer reviewed publications. Dr. Wang is a reviewer for many journals and a Fellow of both the American Physical Society (APS) and American Association for the Advancement of Science (AAAS).
Content
Preface xv
Acknowledgments xix
Introduction xxi
About the Companion Website xxxi
Part I Linear Viscoelasticity and Experimental Methods 1
1 Phenomenological Description of Linear Viscoelasticity 3
1.1 Basic Modes of Deformation 3
1.1.1 Startup shear 4
1.1.2 Step Strain and Shear Cessation from Steady State 5
1.1.3 Dynamic or Oscillatory Shear 5
1.2 Linear Responses 5
1.2.1 Elastic Hookean Solids 6
1.2.2 Viscous Newtonian Liquids 6
1.2.3 Viscoelastic Responses 7
1.2.3.1 Boltzmann Superposition Principle for Linear Response 7
1.2.3.2 General Material Functions in Oscillatory Shear 8
1.2.3.3 Stress Relaxation from Step Strain or Steady-State Shear 8
1.2.4 Maxwell Model for Viscoelastic Liquids 8
1.2.4.1 Stress Relaxation from Step Strain 9
1.2.4.2 Startup Deformation 10
1.2.4.3 Oscillatory (Dynamic) Shear 11
1.2.5 General Features of Viscoelastic Liquids 12
1.2.5.1 Generalized Maxwell Model 12
1.2.5.2 Lack of Linear Response in Small Step Strain: A Dilemma 13
1.2.6 Kelvin-Voigt Model for Viscoelastic Solids 14
1.2.6.1 Creep Experiment 15
1.2.6.2 Strain Recovery in Stress-Free State 15
1.2.7 Weissenberg Number and Yielding during Linear Response 16
1.3 Classical Rubber Elasticity Theory 17
1.3.1 Chain Conformational Entropy and Elastic Force 17
1.3.2 Network Elasticity and Stress-Strain Relation 18
1.3.3 Alternative Expression in terms of Retraction Force and Areal Strand Density 20
References 21
2 Molecular Characterization in Linear Viscoelastic Regime 23
2.1 Dilute Limit 23
2.1.1 Viscosity of Einstein Suspensions 23
2.1.2 Kirkwood-Riseman Model 24
2.1.3 Zimm Model 24
2.1.4 Rouse Bead-Spring Model 25
2.1.4.1 Stokes Law of Frictional Force of a Solid Sphere (Bead) 26
2.1.4.2 Brownian Motion and Stokes-Einstein Formula for Solid Particles 26
2.1.4.3 Equations of Motion and Rouse Relaxation Time tR27
2.1.4.4 Rouse Dynamics for Unentangled Melts 28
2.1.5 Relationship between Diffusion and Relaxation Time 29
2.2 Entangled State 30
2.2.1 Phenomenological Evidence of chain Entanglement 30
2.2.1.1 Elastic Recovery Phenomenon 30
2.2.1.2 Rubbery Plateau in Creep Compliance 31
2.2.1.3 Stress Relaxation 32
2.2.1.4 Elastic Plateau in Storage Modulus G' 32
2.2.2 Transient Network Models 34
2.2.3 Models Depicting Onset of Chain Entanglement 35
2.2.3.1 Packing Model 35
2.2.3.2 Percolation Model 38
2.3 Molecular-Level Descriptions of Entanglement Dynamics 39
2.3.1 Reptation Idea of de Gennes 39
2.3.2 Tube Model of Doi and Edwards 41
2.3.3 Polymer-Mode-Coupling Theory of Schweizer 43
2.3.4 Self-diffusion Constant versus Zero-shear Viscosity 44
2.3.5 Entangled Solutions 46
2.4 Temperature Dependence 47
2.4.1 Time-Temperature Equivalence 47
2.4.2 Thermo-rheological Complexity 48
2.4.3 Segmental Friction and Terminal Relaxation Dynamics 49
References 50
3 Experimental Methods 55
3.1 Shear Rheometry 55
3.1.1 Shear by Linear Displacement 55
3.1.2 Shear in Rotational Device 56
3.1.2.1 Cone-Plate Assembly 56
3.1.2.2 Parallel Disks 57
3.1.2.3 Circular Couette Apparatus 58
3.1.3 Pressure-Driven Apparatus 59
3.1.3.1 Capillary Die 60
3.1.3.2 Channel Slit 61
3.2 Extensional Rheometry 63
3.2.1 Basic Definitions of Strain and Stress 63
3.2.2 Three Types of Devices 64
3.2.2.1 Instron Stretcher 64
3.2.2.2 Meissner-Like Sentmanat Extensional Rheometer 65
3.2.2.3 Filament Stretching Rheometer 65
3.3 In Situ Rheostructural Methods 66
3.3.1 Flow Birefringence 66
3.3.1.1 Stress Optical Rule 67
3.3.1.2 Breakdown of Stress-Optical Rule 68
3.3.2 Scattering (X-Ray, Light, Neutron) 69
3.3.3 Spectroscopy (NMR, Fluorescence, IR, Raman, Dielectric) 69
3.3.4 Microrheology and Microscopic Force Probes 69
3.4 Advanced Rheometric Methods 69
3.4.1 Superposition of Small-Amplitude Oscillatory Shear and Small Step Strain during Steady Continuous Shear 69
3.4.2 Rate or Stress Switching Multistep Platform 70
3.5 Conclusion 70
References 71
4 Characterization of Deformation Field Using Different Methods 75
4.1 Basic Features in Simple Shear 75
4.1.1 Working Principle for Strain-Controlled Rheometry: Homogeneous Shear 75
4.1.2 Stress-Controlled Shear 76
4.2 Yield Stress in Bingham-Type (Yield-Stress) Fluids 77
4.3 Cases of Homogeneous Shear 79
4.4 Particle-Tracking Velocimetry (PTV) 79
4.4.1 Simple Shear 80
4.4.1.1 Velocities in XZ-Plane 80
4.4.1.2 Deformation Field in XY Plane 80
4.4.2 Channel Flow 82
4.4.3 Other Geometries 83
4.5 Single-Molecule Imaging Velocimetry 83
4.6 Other Visualization Methods 83
References 84
5 Improved and Other Rheometric Apparatuses 87
5.1 Linearly Displaced Cocylinder Sliding for Simple Shear 88
5.2 Cone-Partitioned Plate (CPP) for Rotational Shear 88
5.3 Other Forms of Large Deformation 91
5.3.1 Deformation at Converging Die Entry 91
5.3.2 One-Dimensional Squeezing 92
5.3.3 Planar Extension 95
5.4 Conclusion 96
References 97
Part II Yielding - Primary Nonlinear Responses to Ongoing Deformation 99
6 Wall Slip - Interfacial Chain Disentanglement 103
6.1 Basic Notions of Wall Slip in Steady Shear 104
6.1.1 Slip Velocity Vs and Navier-de Gennes Extrapolation Length b 104
6.1.2 Correction of Shear Field due to Wall Slip 105
6.1.3 Complete Slip and Maximum Value for b 106
6.2 Stick-Slip Transition in Controlled-Stress Mode 108
6.2.1 Stick-Slip Transition in Capillary Extrusion 108
6.2.1.1 Analytical Description 108
6.2.1.2 Experimental Data 109
6.2.2 Stick-Slip Transition in Simple Shear 111
6.2.3 Limiting Slip Velocity V*s for Different Polymer Melts 113
6.2.4 Characteristics of Interfacial Slip Layer 116
6.3 Wall Slip during Startup Shear - Interfacial Yielding 116
6.3.1 Theoretical Discussions 117
6.3.2 Experimental Data 118
6.4 Relationship between Slip and Bulk Shear Deformation 120
6.4.1 Transition from Wall Slip to Bulk Nonlinear Response: Theoretical Analysis 120
6.4.2 Experimental Evidence of Stress Plateau Associated with Wall Slip 122
6.4.2.1 A Case Based on Entangled DNA Solutions 122
6.4.2.2 Entangled Polybutadiene Solutions in Small Gap Distance H~50 µm 123
6.4.2.3 Verification of Theoretical Relation by Experiment 126
6.4.3 Influence of Shear Thinning on Slip 127
6.4.4 Gap Dependence and Independence 128
6.5 Molecular Evidence of Disentanglement during Wall Slip 131
6.6 Uncertainties in Boundary Condition 134
6.6.1 Oscillations between Entanglement and Disentanglement Under Constant Speed 134
6.6.2 Oscillations between Stick and Slip under Constant Pressure 134
6.7 Conclusion 134
References 135
7 Yielding during Startup Deformation: From Elastic Deformation to Flow 139
7.1 Yielding at Wi1 140
7.1.1 Elastic Deformation and Yielding for Wi<1 140
7.1.2 Steady Shear Rheology: Shear Thinning 141
7.2 Stress Overshoot in Fast Startup Shear 143
7.2.1 Scaling Characteristics of Shear Stress Overshoot 144
7.2.1.1 Viscoelastic Regime (WiR <1) 145
7.2.1.2 Elastic Deformation (Scaling) Regime (WiR >1) 145
7.2.1.3 Contrast between Two Different Regimes 148
7.2.2 Elastic Recoil from Startup Shear: Evidence of Yielding 149
7.2.2.1 Elastic Recoil for WiR >1 149
7.2.2.2 Irrecoverable Shear at WiR <1 149
7.2.3 More Evidence of Yielding at Overshoot Based on Rate-Switching Tests 153
7.3 Nature of Steady Shear 154
7.3.1 Superposition of Small-Amplitude Oscillatory Shear onto Steady-State Shear 155
7.3.2 Two Other Methods to Probe Steady Shear 157
7.4 From Terminal Flow to Fast Flow under Creep: Entanglement-Disentanglement Transition 159
7.5 Yielding in Startup Uniaxial Extension 163
7.5.1 Myth with Considère Criterion 163
7.5.2 Tensile Force (Engineering Stress) versus True Stress 164
7.5.3 Tensile Force Maximum: A Signature of Yielding in Extension 165
7.5.3.1 Terminal Flow (Wi<1) 166
7.5.3.2 Yielding Evidenced by Decline in sengr 167
7.5.3.3 Maxwell-Like Response and Scaling for WiR >1 170
7.5.3.4 Elastic Recoil 173
7.6 Conclusion 175
7.A Experimental Estimates of Rouse Relaxation Time 175
7.A.1 From Self-Diffusion 175
7.A.2 From Zero-Shear Viscosity 176
7.A.3 From Reptation (Terminal Relaxation) Time td 176
7.A.4 From Second Crossover Frequency~1/te 176
References 176
8 Strain Hardening in Extension 181
8.1 Conceptual Pictures 181
8.2 Origin of "Strain Hardening" 184
8.2.1 Simple Illustration of Geometric Condensation Effect 184
8.2.2 "Strain Hardening" of Polymer Melts with Long-Chain Branching and Solutions 185
8.2.2.1 Melts with LCB 185
8.2.2.2 Entangled Solutions of Linear Chains 187
8.3 True Strain Hardening in Uniaxial Extension: Non-Gaussian Stretching from Finite Extensibility 188
8.4 Different Responses of Entanglement to Startup Extension and Shear 190
8.5 Conclusion 190
8.A Conceptual and Mathematical Accounts of Geometric Condensation 191
References 192
9 Shear Banding in Startup and Oscillatory Shear: Particle-Tracking Velocimetry 195
9.1 Shear Banding After Overshoot in Startup Shear 197
9.1.1 Brief Historical Background 197
9.1.2 Relevant Factors 198
9.1.2.1 Sample Requirements: Well Entangled, with Long Reptation Time and Low Polydispersity 198
9.1.2.2 Controlling Slip Velocity 199
9.1.2.3 Edge Effects 199
9.1.2.4 Absence of Shear Banding for b/H«1 201
9.1.2.5 Disappearance of Shear Banding at High Shear Rates 202
9.1.2.6 Avoiding Shear Banding with Rate Ramp-Up 202
9.1.3 Shear Banding in Conventional Rheometric Devices 203
9.1.3.1 Shear Banding in Entangled DNA Solutions 203
9.1.3.2 Transient and Steady Shear Banding of Entangled 1,4-Polybutadiene Solutions 204
9.1.4 From Wall Slip to Shear Banding in Small Gap Distance 208
9.2 Overcoming Wall Slip during Startup Shear 209
9.2.1 Strategy Based on Choice of Solvent Viscosity 209
9.2.2 Negligible Slip Correction at High Wiapp 213
9.2.3 Summary on Shear Banding 213
9.3 Nonlinearity and Shear Banding in Large-Amplitude Oscillatory Shear 214
9.3.1 Strain Softening 214
9.3.2 Wave Distortion 215
9.3.3 Shear Banding 215
References 217
10 Strain Localization in Extrusion, Squeezing Planar Extension: PTV Observations 221
10.1 Capillary Rheometry in Rate-Controlled Mode 221
10.1.1 Steady-State Characteristics 221
10.1.2 Transient Behavior 223
10.1.2.1 Pressure Oscillation and Hysteresis 223
10.1.2.2 Input vs. Throughput, Entry Pressure Loss and Yielding 224
10.2 Instabilities at Die Entry 226
10.2.1 Vortex Formation vs. Shear Banding 226
10.2.2 Stagnation at Corners and Internal Slip 227
10.3 Squeezing Deformation 230
10.4 Planar Extension 233
References 233
11 Strain Localization and Failure during Startup Uniaxial Extension 235
11.1 Tensile-Like Failure (Decohesion) at Low Rates 237
11.2 Shear Yielding and Necking-Like Strain Localization at High Rates 239
11.2.1 Shear Yielding 239
11.2.2 Constant Normalized Engineering Stress at the Onset of Strain Localization 243
11.3 Rupture-Like Breakup: Where Are Yielding and Disentanglement? 245
11.4 Strain Localization Versus Steady Flow: Sentmanat Extensional Rheometry Versus Filament-Stretching Rheometry 247
11.5 Role of Long-Chain Branching 250
11.A Analogy between Capillary Rheometry and Filament-Stretching Rheometry 250
References 251
Part III Decohesion and Elastic Yielding After Large Deformation 255
12 Nonquiescent Stress Relaxation and Elastic Yielding in Stepwise Shear 257
12.1 Strain Softening After Large Step Strain 258
12.1.1 Phenomenology 258
12.1.2 Tube Model Interpretation 261
12.1.2.1 Normal Doi-Edwards Behavior 261
12.1.2.2 Type C Ultra-strain-softening 262
12.2 Particle Tracking Velocimetry Revelation of Localized Elastic Yielding 265
12.2.1 Nonquiescent Relaxation in Polymer Solutions 266
12.2.1.1 Elastic Yielding in Polybutadiene Solutions 266
12.2.1.2 Suppression of Breakup by Reduction in Extrapolation Length b 269
12.2.1.3 Nonquiescent Relaxation in Polystyrene Solutions 269
12.2.1.4 Strain Localization in the Absence of Edge Instability 270
12.2.2 Nonquiescent Relaxation in Styrene-Butadiene Rubbers 272
12.2.2.1 Induction Time and Molecular Weight Dependence 273
12.2.2.2 Severe Shear Banding before Shear Cessation and Immediate Breakup 275
12.2.2.3 Rate Dependence of Elastic Breakup 275
12.2.2.4 Unconventional "Step Strain" Produced at WiR <1 278
12.3 Quiescent and Uniform Elastic Yielding 279
12.3.1 General Comments 279
12.3.2 Condition for Uniform Yielding and Quiescent Stress Relaxation 280
12.3.3 Homogeneous Elastic Yielding Probed by Sequential Shearing 281
12.4 Arrested Wall Slip: Elastic Yielding at Interfaces 283
12.4.1 Entangled Solutions 283
12.4.2 Entangled Melts 283
12.5 Conclusion 286
References 287
13 Elastic Breakup in Stepwise Uniaxial Extension 291
13.1 Rupture-Like Failure during Relaxation at Small Magnitude or Low Extension Rate (WiR <1) 292
13.1.1 Small Magnitude (e ~ 1) 292
13.1.2 Low Rates Satisfying WiR <1 292
13.2 Shear-Yielding-Induced Failure upon Fast Large Step Extension (WiR >1) 293
13.3 Nature of Elastic Breakup Probed by Infrared Thermal-Imaging Measurements 297
13.4 Primitive Phenomenological Explanations 298
13.5 Step Squeeze and Planar Extension 299
References 299
14 Finite Cohesion and Role of Chain Architecture 301
14.1 Cohesive Strength of an Entanglement Network 302
14.2 Enhancing the Cohesion Barrier: Long-Chain Branching Hinders Structural Breakup 306
References 308
Part IV Emerging Conceptual Framework and Beyond 311
15 Homogeneous Entanglement 313
15.1 What Is Chain Entanglement? 313
15.2 When, How, and Why Disentanglement Occurs? 315
15.3 Criterion for Homogeneous Shear 316
15.4 Constitutive Nonmonotonicity 318
15.5 Metastable Nature of Shear Banding 319
References 322
16 Molecular Networks as the Conceptual Foundation 325
16.1 Introduction: The Tube Model and its Predictions 326
16.1.1 Basic Starting Points of the Tube Model 327
16.1.2 Rouse Chain Retraction 328
16.1.3 Nonmonotonicity due to Rouse Chain Retraction 328
16.1.3.1 Absence of Linear Response to Step Strain 328
16.1.3.2 Stress Overshoot upon Startup Shear 329
16.1.3.3 Strain Softening: Damping Function for Stress Relaxation 330
16.1.3.4 Excessive Shear Thinning: The Symptom of Shear Stress Maximum 331
16.1.3.5 Anticipation of Necking Based on Considère Criterion 332
16.1.4 How to Test the Tube Model 332
16.2 Essential Ingredients for a New Molecular Model 333
16.2.1 Intrachain Elastic Retraction Force 334
16.2.2 Intermolecular Grip Force (IGF) 335
16.2.3 Entanglement (Cohesion) Force Arising from Entropic Barrier: Finite Cohesion 336
16.2.3.1 Scaling Analysis 337
16.2.3.2 Threshold for decohesion 338
16.3 Overcoming Finite Cohesion after Step Deformation: Quiescent or Not 339
16.3.1 Nonquiescence from Severe Elastic Yielding 339
16.3.1.1 With WiR >1 339
16.3.1.2 With WiR«1 340
16.3.2 Homogeneous Elastic Yielding: Quiescent Relaxation 340
16.4 Forced Microscopic Yielding during Startup Deformation: Stress Overshoot 341
16.4.1 Chain Disentanglement for WiR <1 341
16.4.2 Molecular Force Imbalance and Scaling for WiR >1 342
16.4.3 Yielding is a Universal Response: Maximum Engineering Stress 346
16.5 Interfacial Yielding via Disentanglement 346
16.6 Effect of Long-Chain Branching 347
16.7 Decohesion in Startup Creep: Entanglement-Disentanglement Transition 349
16.8 Emerging Microscopic Theory of Sussman and Schweizer 350
16.9 Further Tests to Reveal the Nature of Responses to Large Deformation 351
16.9.1 Molecular Dynamics Simulations 352
16.9.2 Small Angle Neutron Scattering Measurements 353
16.9.2.1 Melt Extension at WiR«1 353
16.9.2.2 Step Melt Extension With WiR >1 354
16.10 Conclusion 354
References 355
17 "Anomalous" Phenomena 361
17.1 Essence of Rheometric Measurements: Isothermal Condition 361
17.1.1 Heat Transfer in Simple Shear 362
17.1.2 Heat Transfer in Uniaxial Extension 364
17.2 Internal Energy Buildup with and without Non-Gaussian Extension 366
17.3 Breakdown of Time-Temperature Superposition (TTS) during Transient Response 368
17.3.1 Time-Temperature Superposition in Polystyrene Solutions and Styrene-Butadiene Rubbers: Linear Response 368
17.3.2 Failure of Time-Temperature Superposition: Solutions and Melts 369
17.3.2.1 Entangled Polymer Solutions Undergoing Startup Shear 369
17.3.2.2 Entangled Polymer Melts during Startup Extension 370
17.4 Strain Hardening in Simple Shear of Some Polymer Solutions 372
17.5 Lack of Universal Nonlinear Responses: Solutions versus Melts 374
17.6 Emergence of Transient Glassy Responses 378
References 380
18 Difficulties with Orthodox Paradigms 385
18.1 Tube Model Does Not Predict Key Experimental Features 385
18.1.1 Unexpected Failure at WiR«1 387
18.1.2 Elastic Yielding Can Lead to Nonquiescent Relaxation 387
18.1.3 Meaning of Maximum in Tensile Force (Engineering Stress) 388
18.1.4 Other Examples of Causality Reversal 389
18.1.5 Entanglement-Disentanglement Transition 390
18.1.6 Anomalies Are the Norm 390
18.2 Confusion About Local and Global Deformations 391
18.2.1 Lack of Steady Flow in Startup Melt Extension 391
18.2.2 Peculiar Protocol to Observe Stress Relaxation from Step Extension 392
18.3 Molecular Network Paradigm 392
18.3.1 Startup Deformation 392
18.3.2 Stepwise Deformation 393
References 394
19 Strain Localization and Fluid Mechanics of Entangled Polymers 397
19.1 Relationship between Wall Slip and Banding: A Rheological-State Diagram 398
19.2 Modeling of Entangled Polymeric Liquids by Continuum Fluid Mechanics 399
19.3 Challenges in Polymer Processing 400
19.3.1 Extrudate Distortions 401
19.3.1.1 Sharkskin Melt Fracture (Due to Exit Boundary Discontinuity) 401
19.3.1.2 Gross (Melt Fracture) Extrudate Distortions Due to Entry Instability 403
19.3.1.3 Another Example Showing Pressure Oscillation and Stick-Slip Transition 403
19.3.2 Optimal Extrusion Conditions 404
19.3.3 Melt Strength 405
References 406
20 Conclusion 409
20.1 Theoretical Challenges 410
20.2 Experimental Difficulties 413
References 415
Symbols and Acronyms 417
Subject Index 421
Preface
Nonlinear Polymer Rheology explores the rich phenomenology of the mechanical behavior of polymer melts and concentrated polymer solutions. My main purpose is to expose the reader to the latest knowledge and understanding of the subject, developed in the past decade. This book explores and establishes a microscopic foundation that provides a coherent molecular-level interpretation for various nonlinear rheological behaviors. In absence of such a foundation, the book would not and could not have been written.
Covering nearly every aspect of the nonlinear rheological responses of entangled polymers, this book may be used as a textbook to introduce essential phenomenological information. The reader does not need to be an experienced researcher in the field of rheology. The book presents the subject in a self-contained manner, although familiarity with the literature on nonlinear polymer rheology would allow the reader to contrast different standpoints.
My approach to nonlinear polymer rheology places a great emphasis on understanding transient viscoelastic responses. While steady-flow behavior is also of interest, Nonlinear Polymer Rheology differs from other works in that it treats nonlinear responses as primary and linear responses as secondary. We aim to collect sufficient first-hand phenomenology before proposing theoretical concepts, although key concepts including yielding and disentanglement are utilized in as early as Chapter 6 and 7 without elaborative discussion. Since fresh viewpoints are required, the reader will recognize sharp contrasts with conventional knowledge and methodology.
Many excellent books have treated the subject of polymer rheology in a traditional way. The most classical literature is cited and discussed in books including those by Ferry,[1] Doi and Edwards,[2] Bird et al.,[3] Dealy and Larson,[4] Graessley,[5] and Phillies.[6] In general, these books do not have a sufficient discussion of nonlinear rheology that is based on a coherent gathering of key phenomenology. Limited by the available space, the present book omits discussion of the older literature before 2000 and only includes a few pertinent references since 2000.
Scientific inquiries develop in three stages: A. "Empirical," where we find out what happens; B. "Phenomenological," where we learn how it happens; C. "Theoretical," where we explain why it happens. For a complicated subject such as polymer rheology, it is not feasible to formulate a theory without first having sufficient and coherent phenomenological knowledge. Nonlinear Polymer Rheology acknowledges this logical sequence and strives to collect and establish the phenomenology before developing any theoretical treatment and formulation. However, these three types of research can and do often proceed interactively and interchangeably. For example, a particular theoretical idea or picture can prompt one to organize phenomenological information in a more coherent manner and to design additional experiments using hypothetical and unproven concepts.
The responses of polymeric liquids to large and rapid external deformations are challenging to depict and understand in molecular terms. The task is difficult because molecular behavior on nanometer scales dictates rheological properties characterized on macroscopic (millimeter) scales: There exists a gap of six orders of magnitude in length scales. Thus, we should begin by "listening to" what the experiments tell us, and first build a sound phenomenological base. Until we have a sense of the full picture, which could stem from an adequate analysis of the available phenomenology, it is challenging and risky to make theoretical simplifications, for example, in modeling such complex behavior as the response of polymer entanglement to sudden, fast, large external deformations.
Our goal is to arrive at a realistic physical picture for nonlinear polymer rheology. Having collected the essential phenomenology, the process of rational thinking must take us beyond empirical knowledge. To illustrate the merit of reason-guided thinking, let us consider, for example, the phenomenon of viscoelasticity. By definition, all viscoelastic materials are mechanically solid-like (elastic) when probed on short time scales. Viscoelastic liquids become completely viscous only on long time (relative to the material relaxation time) scales. Where does the "elasticity" come from in such a liquid? What is the structure of the material that produces the elasticity? How should we think about the microscopic origin of viscoelasticity?
Upon external deformation, it is clear from the established phenomenology that an initial elastic response must end, and a transition to flow must begin. In other words, all viscoelastic liquids undergo yielding when subjected to fast deformation. The transition to flow suggests that there exists some kind of a potential barrier on short time scales. The next question is what creates such a potential barrier. As soon as we ask questions like this, we can make progress toward an instructive understanding of viscoelasticity, instead of stopping at the level of phenomenological models including the Maxwell model and the Oldroyd[7] model.
This idea to associate the "elasticity" with a potential barrier for any viscoelastic materials is useful even for an external deformation rate R that is lower than the reciprocal of the dominant relaxation time t, that is, when the Weissenberg number Wi < 1. In other words, a transition from elastic deformation to flow must occur even when the product Wi = R t is below unity. For Wi » 1, the initial elastic deformation can be rather remarkable, persisting up to many strain units in the case of entangled polymers. The termination of the elastic-dominant response apparently stems from a breakdown of some microscopic structure. It is an essential task of polymer rheology to identify and delineate the nature of potential barriers, in terms of intermolecular interactions, and show how the structural breakdown takes place.
Figuring out the nature of intermolecular interactions in entangled polymeric liquids under large deformation is a daunting task. Historically, the task has challenged the brightest minds in polymer science. An entangled polymer can be regarded as a physical network of Gaussian chains. Its stress response to startup shear reveals a finite cohesive strength of the network junctions. Maxwell was right[8]: Entangled polymer melts yield, just like ductile polymeric solids (e.g., glasses) do, under continuous external deformation. The rate dependence of the yielding response indicates that chain entanglements are dynamic and have finite lifetimes.
In 1979, Maxwell and Nguyen[9] described the stress overshoot of polystyrene melts upon startup shear by stating "the yielding behavior indicates that, as straining progresses, the structure of the melt is broken down, thereby permitting flow." In the same year, Doi and Edwards published Paper 4, completing their treatment of nonlinear response aspects in the tube model,[10-13] building on the appealing idea of reptation from de Gennes.[14] The tube model had a very different molecular interpretation of shear stress overshoot. Perhaps the tube model made the Maxwell and Nguyen's idea of yielding unnecessary and obsolete. Ever since 1979, theory, experiment, and interpretation of polymer rheology have developed on the presumption that the Doi-Edwards tube model encompasses the right physical picture and tells us how to understand the nonlinear rheological behavior of entangled polymers. The tube model paradigm provides a huge backdrop, against which this book discusses the same subject, polymer rheology, especially nonlinear rheology of entangled polymers.
It seems that modern scientific inquiries rarely follow the preaching of Karl Popper (1902-1994) concerning the objective of doing science, that is, (i) to develop falsifiable theories and (ii) to falsify existing theories with experiment. On the contrary, we prefer to work within an existing paradigm as described by Kuhn (1922-1996).[15] There is a tendency for one to do anything and everything to validate and defend a given theory instead of performing experiments aimed at falsifying it. The notion of scientific truth is often not established by objective criteria and logical rationalization in the Popperian sense but instead by the consensus of a scientific community. Consequently, for two reasons it may be difficult to carry out unconventional research: (i) We are predisposed to accept textbooks and literature results. (ii) Unconventional ideas can be inharmonious. Transformative knowledge find it hard to gain acceptance by the community, especially by people who are accustomed to the standard knowledge and approach. Fortunately, science is ultimately not an affair of democracy, dictated by popular vote. The state of our knowledge is not defined by the status quo.
Doing science often amounts to sorting out relationships between causes and effects. Depending on the level of description, causality can be confused or even reversed. Something taken as the cause at a coarser level may actually be an effect at a finer, deeper, or higher level. For polymer rheology, it is unnecessary to go to the quantum-mechanical level; but it is unacceptable to stay at the continuum mechanical level as the Maxwell model does when it is feasible to probe molecular origins. A short presentation has been posted at www.youtube.com/watch?v=2HDD51Mxu8U to discuss this matter of causality in nonlinear polymer rheology.
Our objective as well...
