Polymer Science and Engineering The Shifting Research Frontiers (1994) / Chapter Skim
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4. Enabling Science
4. Enabling Science
Pages 116-168
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From page 116... ...
POLYMER SYNTHESIS Synthesis provides the underpinnings for all advances in the science and engineering of polymeric materials. The past decade has produced many notable synthetic advances: the extension of living polymerization methods to new classes of reactive intermediates and new classes of monomers, the appearance of hyperbranched polymers, the controlled preparation of inorganic polymers and organic-inorganic hybrids, the development of efficient biological polymerization strategies, and many others.
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From page 117... ...
A striking recent advance has been the preparation of hyperbranched—or dendritic macromolecules, in which iterative branching steps lead to structures in which segment density grows rapidly as one proceeds radially from the molecular "core" (Figure 4.11. Dendritic polymers of narrow molecular weight distribution have been prepared by laborious stepwise synthesis, while "one-pot" methods have been developed for polydisperse samples.
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From page 118... ...
118 1: ~ ~ _ 1 /\ _~( in ~ ,\: — C)
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From page 119... ...
All of these methods are capable of controlling not only the average chain length but also the molecular weight distribution. However, chain length control is still statistical.
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From page 120... ...
. POLYMER SCIENCE AND ENGINEERING 0 catalyst "Living" Mn up to 250,000 MW / Mn < 1 ~ OSiMe3 PMMA ' OMe study to delineate the factors affecting chain termination and chain transfer, events that adversely affect chain length control, is also needed if one is to solve completely the problem of molecular weight control.
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From page 121... ...
This material is used in a variety of applications and displays a glass transition temperature of about-90°C. Coordination catalysis can generate rather high isomeric purity in all of these systems.
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From page 122... ...
Free radical polymerization produces mostly atactic structures, with the temperature being one important parameter, which can modestly control placement. In general, ionic polymerization, and especially coordination catalysis, is required to produce either highly isotactic or highly syndiotactic materials.
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From page 123... ...
Advances in catalysis that could produce better understanding of polymerization mechanisms are needed to further refine the microstructure and to produce improved materials. Synthesis of Polymers of Controlled End-Group Structure An increasingly important class of polymers are telechelic polymers, which contain reactive end groups that can be used to further increase the molecular weight of a polymer during processing or to generate block copolymers.
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From page 124... ...
24 POLYMER SCIENCE AND ENGINEERING These nominally difunctional materials are used in a wide variety of applications in which they are reacted with isocyanates to prepare segmented polyurethanes and polyureas (Figure 4.61. Polyols can be prepared by either ring-opening or step-growth polymerization.
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From page 125... ...
In cases in which the reaction is not living and cannot be controlled to be living, chain transfer with a functionalized agent is the only solution for the preparation of telechelic polymers. Some success has been realized using this technique for radical polymerizations, and recent advances have been made in the preparation of telechelic polymers using difunctional acyclic olefins as chain transfer agents in ring-opening metathesis polymerization.
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From page 126... ...
X—CH2 - CH-Li / \ ~ ~ H2C CH2 BLOCK COPOLYMER POLYMER SCIENCE AND ENGINEERING INITIATION PROPAGATION ROH 1,3-butadiene ~ ~tw~rCH2 - CH2OH FUNCTIONALIZATION TERMINATION FIGURE 4.7 Introduction of reactive end groups by use of a functional terminator.
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From page 127... ...
The substituent approach results in defects in the polymer chains and diminishes interchain interactions that are important to some desired properties. Given the real and potential advances in the use of conjugated polymers in device applications, a vigorous synthetic attack on such problems is badly needed.
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From page 128... ...
· Thermally Stable Heterocyclic/ Heteroatom Units Imides, Benzoxazoles, Phenyl Phosphine Oxides, Fluorinated Materials · Conformational Non-Planarity Biphenyls CF3 CF3 Phenyl Phosphine Oxide "3F" Systems CF~ _ CF3 ~P~ ,D~ ~CF~ . Chain Length/End Group Control Non-Reactive Thermoplastics; Reactive, Uniform Network Thermosets FIGURE 4.8 Concepts for molecular design of processible macromolecules with exceptional thermal and thermo-oxidative stability.
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From page 129... ...
. Modification of Polymer Surfaces Chemical surface modification of solid organic polymers is generally carried out to change chemical or physical surface properties of preformed objects (e.g., film, fibers, and bottles)
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From page 130... ...
The structure of the solution-solid interface in a modification reaction is extremely system dependent, and a large variety of situations can be envisioned. The mobility of polymer chains in contact with the solution depends on the polymer morphology (degree of crystallinity)
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From page 131... ...
This mobility can effect desired surface properties and likely plays an important role in further chemistry of these surfaces. Certainly significant advances have been made in this field in the past decade, but with respect to the highly evolved disciplines of organic polymer synthesis and synthetic organic chemistry, the field is in its infancy in terms of elegance and versatility.
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From page 132... ...
This approach offers significant advantages, in that it leads to uniform chain populations of controlled chain length, sequence, and stereochemistry, all the important structural components in polymer synthesis. In addition, the factors that control the secondary structure of proteins can be used to impose three-dimensional structure on the synthetic polymers made by this technique.
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From page 133... ...
New developments in catalysis and in organometallic chemistry will be critical to such discoveries and should be accorded high priority in exploratory research programs in polymer synthesis. Exploring the Periodic Table: Inorganic Polymers Although most current polymeric materials are based on the chemistry of carbon, the remainder of the periodic table is accessible through the synthesis of inorganic polymers and networks or hybrid organic-inorganic systems often composed of interpenetrating organic and inorganic polymers.
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From page 134... ...
- Na+, which in the rate-determining step reacts with dichlorosilane, adding one silicon unit and producing a chlorine-ended chain. Under ultrasound irradiation, at low temperature, high molecular weights and monomodal molecular weight distributions are obtained.
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From page 135... ...
There are of course many examples of less common inorganic polymers. Polymeric sulfur and selenium are synthesized by free radical polymerization of cyclic molecules.
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From page 136... ...
Opportunities and Challenges With most of the elements of the periodic table available, the opportunities for chemists to synthesize new inorganic polymers and networks with unique properties are clearly unlimited. However, the greater chemical and structural diversity represented by this class of materials compared to traditional organic polymers provides daunting synthetic challenges.
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From page 137... ...
Over the last decade, there has emerged a broader appreciation of the role of noncovalent interactions in the synthesis of polymeric materials, and polymerizations in monolayers, bilayers, crystals, and liquid crystals have become a popular area of investigation. This work has been motivated by interest in (1)
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From page 138... ...
Although they are significantly weaker than covalent bonds, hydrogen bonds exhibit directional character and are therefore useful in controlling both the size and the shape of molecular aggregates of dimensions comparable to those of polymer chains. Penetrating studies of molecular recognition processes in organic chemistry are providing new strategies for the assembly of large-scale structures, and it is clear that a broad view of polymer synthesis, a view that embraces both covalent and non-covalent bond-forming steps, should be encouraged.
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From page 139... ...
There are five main areas of application: . chains; Molecular characterization of the architecture of isolated polymer · Characterization of solutions, melts, and elastomers, especially the dynamics of polymer chains; · Characterization of polymer solid-state structure and properties, wherein new microscopies can offer major advances; · Characterization of polymer surfaces and interfaces, especially with new depth profiling techniques; and · Characterization of biopolymers to provide information for development of biotechnology.
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From page 140... ...
Solutions, Melts, and Elastomers · Many new techniques for measuring diffusion · Simultaneous theological and optical/X-ray/neutron measurements · New nuclear magnetic resonance and optical techniques for measuring local polymer dynamics and tertiary structure of biopolymers · Neutron spin-echo techniques for measuring intermediate-scale polymer motion · Neutron-scattering methods for determining the thermodynamics of polymer blends 3. Solid-State Structure and Properties · Environmental scanning electron microscopy · Near-field optical microscopy · Simultaneous X-ray and calorimetry measurements · Solid-state nuclear magnetic resonance techniques · Molecular imaging with transmission electron microscopy · Confocal optical microscopy · Transmission electron microscope image-processing techniques · Techniques using synchrotron radiation sources for solution of the phase problem in solving the structure of large biomolecules 4.
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From page 141... ...
Surfaces and Interfaces · Analysis techniques that can characterize curved interfaces · Interface analysis techniques with both good depth resolution and good lateral resolution ~ Ways to use atomic force microscopy to characterize local mechanical properties 5. Biopolymers · Scanning tunneling microscopy to read out biopolymer sequences · Rapid sequencing methods · Computer algorithms to predict biomolecular structures from sequences · Higher-resolution electrophoretic methods for separating large biopolymers · Time-resolved Laue diffraction methods for X-ray crystal structure determination · Hydrogen exchange nuclear magnetic resonance for local motions and internal structure of biopolymers The number and average molecular weight of soluble polymers can be obtained by numerous methods, including colligative property measurements, scattering, and ultracentrifugation.
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From page 142... ...
and for use at high temperatures. Methods for obtaining molecular weight distributions of insoluble polymers are, however, in their infancy; one example involves mass spectroscopy, which has recently been used for molecules with molecular weights as high as 100,000.
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From page 143... ...
We are all familiar with the wonders that three-dimensional NMR imaging has wrought for medical diagnosis; such imaging has great potential for polymer science as well. At the 1- to 0.1-millimeter (mm)
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From page 144... ...
Neutron scattering has become an indispensable tool in the characterization of polymer melts and elastomeric networks. Neutrons of wavelengths from 0.2 to 2.5 nm available from modern neutron sources permit studies of the dimensions of single polymer molecules as well as the chain conformation over much shorter length scales.
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From page 145... ...
Currently, available neutron fluxes are too low to permit dynamic studies, so that the prospect of a high-flux advanced neutron source is a welcome possibility. The case for an advanced neutron source is reinforced by the emergence of neutron reflectometry as a primary tool for characterizing polymer surfaces and interfaces (see below)
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From page 146... ...
, which can operate in a partial pressure of water vapor, can eliminate the necessity for coating samples with metal to prevent charging artifacts, thus achieving superior resolution. Solvent as well as water effects on the mechanical properties can be stud.
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From page 147... ...
Tunable soft X-ray sources are available at synchrotron radiation facilities, and the necessary focusing X-ray optics are now becoming available; already, spatial resolutions of approximately 50 nm have been demonstrated. For most of the applications of solid polymers, mechanical properties are of primary importance.
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From page 148... ...
Experimental techniques to probe the microscopic nature of the deformation and fracture of such systems, preferably in a time-resolved fashion at typical impact strain rates, are needed. Characterization of Polymer Surfaces and Interfaces Surfaces and interfaces present challenges that are distinctly different from those presented by bulk, three-dimensional polymeric materials.
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From page 149... ...
The work of adhesion between a wide variety of surfaces can thus be measured with little more than an optical microscope and an analytical balance. More conventional ways of measuring surface properties include the determination of contact angles of fluids on polymer surfaces and the measurement of the shapes of polymer melt and solution droplets, under conditions where these shapes are modified by the action of gravity or centrifugal force.
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From page 150... ...
has already had an impact on the measurement of the surface topology of polymers. It seems likely, however, that the AFM can be suitably modified to allow measurement of very local surface properties as well as mechanical properties of polymers.
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From page 151... ...
The main methods for determining biopolymer structures are X-ray crystallography, NMR spectroscopic methods, electron microscopy, and scanning tunneling microscopy. The highest-resolution structures have been obtained by X-ray crystallography.
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From page 152... ...
· Improved characterization methods for structures between the monomeric and macroscopic levels are needed, for solutions, surfaces, solid-state polymers, and insoluble polymeric materials. Methods and instrumentation need to be developed.
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From page 153... ...
This is a central problem in the design of new materials. Modern polymeric materials often involve mixtures of several different types of polymers, of different molecular weights, and in complex solvents at different temperatures and pressures.
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From page 154... ...
The strong interdependence of experiment and theory in polymer science is well illustrated by the Nobel Prizes in polymer theory awarded in 1974 to Paul J Flory (in chemistry)
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From page 155... ...
States of Matter Polymer Solutions Because many theories of polymers in concentrated solutions and bulk rely on single-chain concepts developed and tested in dilute solutions, the understanding of dilute polymer solutions has repercussions throughout polymer science. Hence, dilute solutions of flexible polymers have received much attention in the past 50 years, and as a result many aspects of the average conformation of an isolated polymer molecule are now well understood.
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From page 156... ...
, and in the solid state. The spontaneous ordering of these polymers provides a unique means of aligning polymer chains to obtain materials having exceptional strength, rigidity, and toughness.
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From page 157... ...
Because many of these materials must be processed in the liquid state, it is important to understand those features governing the phase diagrams of polymer liquid mixtures, that is, those factors determining whether the system is homogeneous or phase separated at a given temperature, pressure, composition, and so on. Traditionally, theories of this phase behavior have been based on simple models, but such models have not been fully satisfactory in relating the thermodynamic behavior of liquid polymer mixtures to the detailed chemical structures and interactions of their constituents; this is necessary for the molecular design of novel composite materials.
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From page 158... ...
There has been much progress in understanding properties of polymer interfaces, but we do not yet have a satisfactory understanding, at the molecular level, of the factors involved in strength and failure. In addition, the viability of certain- polymeric materials is affected by the segregation of their components at the interfaces, so the properties of these interfaces are likewise of interest, especially in systems containing block copolymers.
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From page 159... ...
Nor is much known about the interactions between polymer surfaces and other materials such as liquid crystals, for example, in flat panel displays. Experimental methods to study the structures and properties of polymers on the 10- to 100-\ thickness scales are limited.
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From page 160... ...
Advances are likewise desirable in treating thermodynamics and phase behavior of polymers under the strong flows typical of processing conditions used to fabricate commercial materials. The molecular rheology of stiff chains, with its relationship to the nature of liquid crystalline order and their domain boundaries or disclinations, is another area that requires much more effort.
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From page 161... ...
The use of multiphase polymers, particularly block copolymers and polymer blends, has increased substantially in the last 10 years, but the deformation and toughening mechanisms of these systems are little understood. In general, a molecular-level description for mechanical properties of glassy polymers, such as toughness and fatigue, is still at a primitive stage, although computer simulation studies have recently shown promising results in understanding mechanical modulus and certain plastic flow processes.
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From page 162... ...
Despite the impressive amount of experimental work so far, advances in device applications are hampered by the lack of predictive capabilities for the optical absorption, linear optical, and nonlinear optical characteristics of chromophore structures, and chromophore-polymer interactions. Semiempirical computational methods, with the parameters optimized by using available experimental data, do provide reasonable trends within a given class of materials, but they are not reliably transferable to different types of chromophores.
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From page 163... ...
It is based on the interatomic forces, appropriately parameterized from either experiments on simple systems or ab initio quantum mechanical methods. Molecular dynamics (MD)
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From page 164... ...
164 in o Cal in UJ o _ 1 Cal lo C`3 o _ a)
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From page 165... ...
Exhaustive simulations are the most complete in this regard, but they suffer either a power-law or exponential dependence of computer time on chain length and are
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From page 166... ...
Among the most exciting new developments are the Gibbs ensemble Monte Carlo and configurational bias Monte Carlo techniques. They permit computation of phase diagrams from force-field simulations, and chemical potentials for Insertion or polymer chains, even into h~gh-density media.
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From page 167... ...
Conclusions · Theoretical and computational methods play a fundamental role in developing new properties and polymeric materials. Explosive developments in computer technology have fueled the growth of computational experiments and simulations.
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From page 168... ...
· Practical challenges include applying theory and computation to polymer behaviors in complex medi~polymer blends, liquid crystalline polymers, semicrystalline materials, composites, block copolymers, interfaces, the rheology of mixtures, branched molecules, soft matter, and so on. REFERENCES Butera, R., L.J.
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