1.2 Polymers in the Eyes of Physicists 3 1.2 Polymers in the Eyes of Physicists In 1990.the Nobel Physics Prize laureate p-G.de gennes delivered his Nobel lectur titled with"Soft Matter"(de Ger 19).He used poly as one of oles of soft matter.Another commonly used term for soft matter is"comple The hardness,or softness,of matter is normally characterized by their cohesive energy density,i.e.the interaction energy of particles in each unit of volume, Eela,where e is the interaction energy between two particles and a is the inter-particle distance. The conventional hard matter includes metals,glasses and ceramics.The atoms are connected by strong chemical bonds with the interaction energy of the order of 10-1s J.The bond lengths or atomic at the leve el of angstr m.The of hard natter is estimated 2N/m sive energy de which is about the Young's modulus of diamon In contrast,the soft matter includes polymers.liquid crystals,colloids,nano particles,self-assembled or hybrid materials,foams,foods (Fortunately our teeth are some sort of hard matter!).and even the life systems(Our body is unfortunately not as hard as the superman in science fictions!).The building blocks of soft matter typically interact via the sub-valence bonds,with the interaction energy at the order of magnitude 10-20 J that is much lower than the chemical bonds,and their spacing ranges from nanometers to micrometers,a=10-810-m.Therefore,the cohe of soft matter as low as 10-2-10+N/m2. The tio between much er tha ab break response to thermal fluctuations near room temperatures or to weak mechanical disturbances.Therefore,the soft matter that we daily encounter can undergo a gigantic structural change around the ambient conditions,and some of its phase transformations are even driven by the entropic changes.From this perspective,soft matter can be described as materials "comprising all physicochemical systems which have large response functions."(de Gennes'words(de Gennes 2005)). Current classification of chemical substances is also limited in reflecting the ctural cha racters of polym omp nds Che 1 substa n be divid ces and thei rthe divided ents an as a typical se matter,can change their molecular shapes (conformations)to a large extent,and may contain multiple chemical components in each macromolecule to behave like a mixture Accordingly,as pure substances,they are more complicated than normal small molecular compounds.In 1990,Wunderlich proposed to divide all chemical compounds into three classes(Wunderlich 1990): Class I includes conventional small molecules.They stay in all three states of ing the inte of oy grity of che ical nds Ex mples of thi the molecules tively.There are are nitroge and me hane,respec ds of sma molecules
1.2 Polymers in the Eyes of Physicists In 1990, the Nobel Physics Prize laureate P.-G. de Gennes delivered his Nobel lecture titled with “Soft Matter” (de Gennes 1992). He used polymers as one of examples of soft matter. Another commonly used term for soft matter is “complex fluids”. The hardness, or softness, of matter is normally characterized by their cohesive energy density, i.e. the interaction energy of particles in each unit of volume, E e/a3 , where e is the interaction energy between two particles and a is the inter-particle distance. The conventional hard matter includes metals, glasses and ceramics. The atoms are connected by strong chemical bonds with the interaction energy of the order of 1018 J. The bond lengths or atomic spacing are at the level of angstrom, a ¼ 1010 m. Therefore, the cohesive energy density of hard matter is estimated as 1012 N/m2 , which is about the Young’s modulus of diamond. In contrast, the soft matter includes polymers, liquid crystals, colloids, nanoparticles, self-assembled or hybrid materials, foams, foods (Fortunately our teeth are some sort of hard matter!), and even the life systems (Our body is unfortunately not as hard as the superman in science fictions!). The building blocks of soft matter typically interact via the sub-valence bonds, with the interaction energy at the order of magnitude 1020 J that is much lower than the chemical bonds, and their spacing ranges from nanometers to micrometers, a ¼ 108 ~ 106 m. Therefore, the cohesive energy density of soft matter is as low as 102 ~ 104 N/m2 , much lower than that of hard matter! The connection between particles is liable to break as a response to thermal fluctuations near room temperatures or to weak mechanical disturbances. Therefore, the soft matter that we daily encounter can undergo a gigantic structural change around the ambient conditions, and some of its phase transformations are even driven by the entropic changes. From this perspective, soft matter can be described as materials “comprising all physicochemical systems which have large response functions.” (de Gennes’ words (de Gennes 2005)). Current classification of chemical substances is also limited in reflecting the structural characters of polymer compounds. Chemical substances can be divided into pure substances and their mixtures. The pure substances can be further divided into elements and compounds. Polymer compounds, as a typical soft matter, can change their molecular shapes (conformations) to a large extent, and may contain multiple chemical components in each macromolecule to behave like a mixture. Accordingly, as pure substances, they are more complicated than normal small molecular compounds. In 1990, Wunderlich proposed to divide all chemical compounds into three classes (Wunderlich 1990): Class I includes conventional small molecules. They stay in all three states of gas, liquid and solid, reserving the integrity of chemical bonds. Examples of this class are the molecules of oxygen, hydrogen, nitrogen and methane, respectively. There are currently more than 107 kinds of small molecules. 1.2 Polymers in the Eyes of Physicists 3
I Introduction and solid,in orde Evaporation o such macromolecules requires so high level of therm I energy that the chemical bonds are actually broken before reaching that level.The molecular flexibility in the liquid mainly comes from the internal rotation of the main-chain C-C bonds This class includes structural materials of synthetic polymers such as Nylon PVC.PET,and PC.adhesives such as PVA.epoxy resins and Glue 502. elastomers such as natural rubber.polvurethane.SBs and EPDM (rubbe could be regarded as the cross-linked liquid polymers.),biomaterials s ch a cellulo arch,silks and wools,and en b cules s such as DNA RNA and prote .The above exible macron ules corresponds to the sof matter def Class III includes rigid macromolecules.They stay only in the solid states for reserving the integrity of chemical bonds.Examples of this class include metals oxides,salts,ceramics,silicon glasses,diamond,graphite,and some conductive polymers without any solvent or melting point.The class of rigid macromolecules com onds to the hard matter defined above In this classification,chain-like structur s,as the major reason for polymer onging to In fact,chai -like structures are mainly responsible for those unique physical behaviors of polymers in our study.This kind of structures exhibits anisotropic properties,i.e.strong covalent bonds along the backbone of the chain,and much weaker sub-valence interactions on the normal directions of the chain.In thermal fluctuations and Brownian motions of condensed matter macromolecules.the strong correlation along the chain dominates physical properties of polymers. especially in their am prepa ration of chain-like or usin as building mplicated ma rom ecules.In ng them construc st.polymer phys those physical behaviors brought by chain-likesctre although as building blocks the latter may construct the more complicated topological architectures o macromolecules Conventionally.we categorize the chain structures of polymers according to their spatial length scales.The primary structures,also called the short-range structures on the polymer chain,mainly characterize the chemical microstructures or the chemical configurations (note that this configuration is different from that defined in the physics of classical mechanics.where the configuraion space meansll the ossible tial rdinates and m es c only be mo ified by chemical reactic s for making specific sequenc units and their connections along the chain.The secondary structures,also called the long-range structures on the polymer chain,mainly reflect the chain conformations such as the conventional random coils of polymers,the alpha-helices and the beta-sheets of proteins,etc.The secondary structures are changed with thermal fluctuations or phase transitions.The tertiary structures mainly describe the steric assembly of secondary structures in the single protein molecules.The spontaneous
Class II includes flexible macromolecules. They stay only in the states of liquid and solid, in order to reserve the integrity of chemical bonds. Evaporation of such macromolecules requires so high level of thermal energy that the chemical bonds are actually broken before reaching that level. The molecular flexibility in the liquid mainly comes from the internal rotation of the main-chain C-C bonds. This class includes structural materials of synthetic polymers such as Nylon, PVC, PET, and PC, adhesives such as PVA, epoxy resins and Glue 502, elastomers such as natural rubber, polyurethane, SBS and EPDM (rubber could be regarded as the cross-linked liquid polymers.), biomaterials such as celluloses, starch, silks and wools, and even bio-macromolecules such as DNA, RNA and proteins. The class of flexible macromolecules corresponds to the soft matter defined above. Class III includes rigid macromolecules. They stay only in the solid states for reserving the integrity of chemical bonds. Examples of this class include metals, oxides, salts, ceramics, silicon glasses, diamond, graphite, and some conductive polymers without any solvent or melting point. The class of rigid macromolecules corresponds to the hard matter defined above. In this classification, chain-like structures, as the major reason for polymers belonging to the soft matter, have received a highlight. In fact, chain-like structures are mainly responsible for those unique physical behaviors of polymers in our study. This kind of structures exhibits anisotropic properties, i.e. strong covalent bonds along the backbone of the chain, and much weaker sub-valence interactions on the normal directions of the chain. In thermal fluctuations and Brownian motions of condensed matter macromolecules, the strong correlation along the chain dominates physical properties of polymers, especially in their amorphous states. Polymer chemistry mainly concerns the preparation of chain-like structures, or using them as building blocks to construct more complicated macromolecules. In contrast, polymer physics mainly concerns those physical behaviors brought by chain-like structures, although as building blocks the latter may construct the more complicated topological architectures of macromolecules. Conventionally, we categorize the chain structures of polymers according to their spatial length scales. The primary structures, also called the short-range structures on the polymer chain, mainly characterize the chemical microstructures or the chemical configurations (note that this configuration is different from that defined in the physics of classical mechanics, where the configuration space means all the possible combinations of spatial coordinates and momentums.). The primary structures can only be modified by chemical reactions for making specific sequences of structural units and their connections along the chain. The secondary structures, also called the long-range structures on the polymer chain, mainly reflect the chain conformations, such as the conventional random coils of polymers, the alpha-helices and the beta-sheets of proteins, etc. The secondary structures are changed with thermal fluctuations or phase transitions. The tertiary structures mainly describe the steric assembly of secondary structures in the single protein molecules. The spontaneous 4 1 Introduction
1.3 Role of Polymer Physics 5 assembly of macromolecules to form a (either intramolecular or intermolecular) multi-level hierarchical structure via strong sub-valence interactions is often called the molecular self-assembly process (Lehn 1995). 1.3 Role of Polymer Physics Polymer physics is a multi-disciplinary subject derived mainly from polymer chemistry and condensed matter physics,and pushed forward by the high demands of materials,engineering and life sciences.It studies physical states and processes as well as their intrinsi correlations to the OY structur s and molec of the cules A omprehens ndi ng of the nciples governing the polymeric behaviors constitutes the ma in obje 0 polymer physics evolved in the last century directly from organic chemistry that was the subject of organic substances,while inorganic chemistry was the earliest subject of inorganic substances evolved since the epoch of alchemy.Such an evolution sequence of chemical subjects coincides with the creation of corresponding substances in nature,following the common trend of evolutions from simple to complex. such as nucleic acids,carbohydrates and prote genetic inb ins acting rately as the basic substar nd hie fur tioning in the living body ha e exhibit ed admirabl xity and accuracy by the e of their strong yet flexible chain- Inspir y nature.ou knowledge of polymer chemistry has expanded extremely fast on making.measur ing and modeling of polymeric materials.The field of traditional condensed matter physics also faces the new challenge of soft matter(sometimes called complex fluids).Polymers are a typical kind of soft matter,featured with metastable states and nonlinear viscoelasticity.Many basic theoretical tools of condensed matter physics,such as the mean-field theory.the scaling analysis,the self-consistent-field the density fur ctional th olecular mics s ulation 、and mo y: dyn commonly app the beh polymers.In ou ife polyme mater als have b materials common as met and ceral The early strategy to investigate poryme materials was mainly based on the trial-and-error experiments,i.e.synthesizing a series of polymer compounds with varing chemical structures and compositions,to identify a proper range of useful properties.Nowadays,the molecular design of the properties has given impetus to the development of new polymer materials.Such an oach demands for our deep understanding of the relationships between the olecular-level str and nolvr ties.Many n olve s pr ma engine g o olvn ood proce ing,oil recovery and ig-d iences also d of physics The rapid of life and microscopic mechanisms of living processes.As the vitally important substances macromolecules are often involved into the microscopic living processes
assembly of macromolecules to form a (either intramolecular or intermolecular) multi-level hierarchical structure via strong sub-valence interactions is often called the molecular self-assembly process (Lehn 1995). 1.3 Role of Polymer Physics Polymer physics is a multi-disciplinary subject derived mainly from polymer chemistry and condensed matter physics, and pushed forward by the high demands of materials, engineering and life sciences. It studies physical states and processes, as well as their intrinsic correlations to the microscopic structures and molecular motions of the macromolecules. A comprehensive understanding of the basic principles governing the polymeric behaviors constitutes the main objective of polymer physics. As a subject of macromolecular substances, polymer chemistry evolved in the last century directly from organic chemistry that was the subject of organic substances, while inorganic chemistry was the earliest subject of inorganic substances evolved since the epoch of alchemy. Such an evolution sequence of chemical subjects coincides with the creation of corresponding substances in nature, following the common trend of evolutions from simple to complex. Macromolecules, such as nucleic acids, carbohydrates and proteins acting separately as the basic substances in genetic inheritance, energy storage and hierarchical functioning in the living body, have exhibited admirable complexity and accuracy by the use of their strong yet flexible chain-like backbones. Inspired by nature, our knowledge of polymer chemistry has expanded extremely fast on making, measuring and modeling of polymeric materials. The field of traditional condensed matter physics also faces the new challenge of soft matter (sometimes called complex fluids). Polymers are a typical kind of soft matter, featured with metastable states and nonlinear viscoelasticity. Many basic theoretical tools of condensed matter physics, such as the mean-field theory, the scaling analysis, the self-consistent-field theory, the density functional theory, molecular dynamics simulations and Monte Carlo simulations, have been commonly applied to investigate the behaviors of polymers. In our daily life, polymer materials have become basic materials as common as metals and ceramics. The early strategy to investigate polymer materials was mainly based on the trial-and-error experiments, i.e. synthesizing a series of polymer compounds with varing chemical structures and compositions, to identify a proper range of useful properties. Nowadays, the molecular design of the properties has given impetus to the development of new polymer materials. Such an approach demands for our deep understanding of the relationships between the molecular-level structures and polymer properties. Many engineering processes involve macromolecules, such as the chemical engineering of polymer materials, food processing, oil recovery and long-distance piping. The rapid progress of life sciences also demands our approaches of physics and chemistry to elucidate the microscopic mechanisms of living processes. As the vitally important substances, macromolecules are often involved into the microscopic living processes. 1.3 Role of Polymer Physics 5
1 Introduction Polymer science Polymer chemistry->Polymer physics Research Synthesis→Structure→ &Monomer synthesis-→Polymerization-→Molding-Characterzation→Tesl Development Polymer chemical engineeringPolymer materials science Polymer engineering Fig.1.1 Diag ship between polymer science I polymer engineering Integrated with the demands of the multiple disciplines above,polymer physics has attracted a great deal of attention with the approaches often concerted with theory. simulation and experiment. The role of polymer physics can be elucidated,for a typical example,in the interdisciplinary field of polymer science and polymer engineering.To make it more explicit,let us look at the conventional p ocedure for the reparation of ials.The route starts fron nd th hen pe e tes centra ontal of Fig.1.1 Above the horiz ntal is the fundamenta research:polymer synthesis mainly covers the early stage from monomer synthe to polymerization;polymer structure covers the middle stage from polymerization to characterization:and polymer property covers the final stage from molding to performance test.Above this level,polymer chemistry concerns the early stage from synthesis to structure,while polymer physics concerns the later stage from structure to property.These two constitute polymer science at the top.Below the central horizontal is the industrial develop ent:polymer chemical gineering s the e e covers the s to moldi whi olyme ter stage from mol g to perto】 e tes ese two con: tute polymer engin almost the second half of this diagram. The common polymers for plastics,rubbers and fibers have been produced at a large industrial scale.It appears difficult to modify them from the early stage of the preparation route.Currently,most of modifications either via physical methods or via chemical treatments are based on their structure-property relationships.The specific functional polymers for coatings.adhesives.adsc n resins and filtration mbrane upy a relatively small market and thei odifications often start nomer syn Bom asa multi-disciplinary subject of chemistry and physics.polymer physics i also a bridge connecting materials sciences and life sciences.At the early history of polymer science,many fundamental concepts of polymer physics were actually
Integrated with the demands of the multiple disciplines above, polymer physics has attracted a great deal of attention with the approaches often concerted with theory, simulation and experiment. The role of polymer physics can be elucidated, for a typical example, in the interdisciplinary field of polymer science and polymer engineering. To make it more explicit, let us look at the conventional procedure for the preparation of synthetic polymer materials. The route starts from monomer synthesis, to polymerization, molding, characterization, and then performance tests, as shown on the central horizontal of Fig. 1.1. Above the central horizontal is the fundamental research: polymer synthesis mainly covers the early stage from monomer synthesis to polymerization; polymer structure covers the middle stage from polymerization to characterization; and polymer property covers the final stage from molding to performance test. Above this level, polymer chemistry concerns the early stage from synthesis to structure, while polymer physics concerns the later stage from structure to property. These two constitute polymer science at the top. Below the central horizontal is the industrial development: polymer chemical engineering covers the early stage from monomer synthesis to molding, while polymer materials science covers the later stage from molding to performance test. These two constitute polymer engineering at the bottom. One can see that polymer physics occupies almost the second half of this diagram. The common polymers for plastics, rubbers and fibers have been produced at a large industrial scale. It appears difficult to modify them from the early stage of the preparation route. Currently, most of modifications either via physical methods or via chemical treatments are based on their structure–property relationships. The specific functional polymers for coatings, adhesives, adsorption resins and filtration membranes occupy a relatively small market, and their modifications often start from monomer synthesis. Born as a multi-disciplinary subject of chemistry and physics, polymer physics is also a bridge connecting materials sciences and life sciences. At the early history of polymer science, many fundamental concepts of polymer physics were actually Fig. 1.1 Diagram of the subjects along the conventional preparation route of synthetic polymers to demonstrate the relationship between polymer science and polymer engineering 6 1 Introduction
1.4 Focusing of this Book invented in the study of natural polymers such as celluloses and natural rubbers The fast development of global science and technology in the middle of twentieth ntury resulted in a b application of the d t concepts in the research and me of tic poly twent -firs century developins ta Weoinueoe materials sciences ha titative life sciences ar cutting-edge knowledge,by following the calls for advanced materials,new energies and green environments of our society.It remains nourishing.and being nourished by,the flourishing of life sciences.As already pointed out by Staudinger in 1953(Staudinger 1953),"In the light of this new knowledge of macromolecular chemistry,the wonder of Life in its chemical aspect is revealed in the astounding abundance and masterly macromolecular architecture of living matter." 1.4 Focusing of this Book Polymer physics covers a wide landscape of polymer structures and their physical properties.The description of the relationships between structures and properties evolves from the early-stage trial-and-error empirical equations to the currently well-established statistical thermodynamic and kinetic theories nas Kuhn ha、 ed out in hi s well-known book"The Structure of Scientific Revolut (Kuhn the scientific of each subject xperienc four pha the pre-paradigm phase.the normal science.the anomaly and crisis.and the revolutionary science."Normal science means research firmly based upon one or more past scientific achievements,achievements that some particular scientific com- munity acknowledges for a time as supplying the foundation for its further practice."(P10)"These are the community's paradigms,revealed in its textbooks, lectures.and laboratory exercises.By studving them and by practicing with them.the members of the comre ponding community learn their trade."(P43) Ther tatistical thermody s that hasic the etical para in polyme tment of physics s.The first the Itre rmations used to calculate ational entropy.This theory allows us to apply the scaling analysis (as well as the self-consistent-field theory)to treat more realistic single-chain confor mation and to describe chain dynamics based on Brownian motions.The first theory and its extension cover the first half content of this book.The second theory is the Flory-Huggins lattice statistical treatment of multi-chain conformations.used to calculate the mixing entropy.This theory allows us to apply the mean-field treat ment to estimate the int actions and then to understand the the c proces sses of chain ssemb uch as li uid-li uid phase 10 h stallizat ory and cover the f content of this boo Both these th res are on the assu nption tha chain conformations can be modeled by the trajectories of random walks.In othe words,both theories rest on the framework of Brownian motion,which is the basic dynamic feature for all types of soft matter particles
invented in the study of natural polymers such as celluloses and natural rubbers. The fast development of global science and technology in the middle of twentieth century resulted in a broad application of these concepts in the research and development of synthetic polymer materials. Entering twenty-first century, materials sciences have been well established, while quantitative life sciences are still developing fast. We speculate that polymer physics will continue to expand its cutting-edge knowledge, by following the calls for advanced materials, new energies and green environments of our society. It remains nourishing, and being nourished by, the flourishing of life sciences. As already pointed out by Staudinger in 1953 (Staudinger 1953), “In the light of this new knowledge of macromolecular chemistry, the wonder of Life in its chemical aspect is revealed in the astounding abundance and masterly macromolecular architecture of living matter.” 1.4 Focusing of this Book Polymer physics covers a wide landscape of polymer structures and their physical properties. The description of the relationships between structures and properties evolves from the early-stage trial-and-error empirical equations to the currently well-established statistical thermodynamic and kinetic theories. Thomas Kuhn has pointed out in his well-known book “The Structure of Scientific Revolutions” (Kuhn 1996) that, the scientific progress of each subject experiences four phases: the pre-paradigm phase, the normal science, the anomaly and crisis, and the revolutionary science. “Normal science means research firmly based upon one or more past scientific achievements, achievements that some particular scientific community acknowledges for a time as supplying the foundation for its further practice.”(P10) “These are the community’s paradigms, revealed in its textbooks, lectures, and laboratory exercises. By studying them and by practicing with them, the members of the corresponding community learn their trade.”(P43) There are two statistical thermodynamic theories that can be regarded as the basic theoretical paradigms in polymer physics. The first theory is the Gaussian statistical treatment of ideal single-chain conformations, used to calculate the conformational entropy. This theory allows us to apply the scaling analysis (as well as the self-consistent-field theory) to treat more realistic single-chain conformation and to describe chain dynamics based on Brownian motions. The first theory and its extension cover the first half content of this book. The second theory is the Flory-Huggins lattice statistical treatment of multi-chain conformations, used to calculate the mixing entropy. This theory allows us to apply the mean-field treatment to estimate the inter-chain attractions and then to understand the thermodynamic processes of chain assembly, such as liquid-liquid phase separation and polymer crystallization. The second theory and its extension cover the second half content of this book. Both these theories are based on the assumption that chain conformations can be modeled by the trajectories of random walks. In other words, both theories rest on the framework of Brownian motion, which is the basic dynamic feature for all types of soft matter particles. 1.4 Focusing of this Book 7