Pergamon Chemical Engineerin 0 Science, Vol. 50, No. 24, pp. 4123-4141, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/95 $9.50 + 0.00 0009-2509(95)00273-1

CHEMICAL ENGINEERING OF POLYMERS: PRODUCTION OF FLEXIBLE, FUNCTIONAL MATERIALS

MATTHEW TIRRELL Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A. (Received: 1 July 1995) Abstract--Polymers, with their large spatial extent and chemical variety, afford materials scientists the opportunity to be architects at the molecular level. Once the architecture is decided, however, the task of construction moves to the chemical engineer to build the material in a faithful, stable and efficient manlaer. This involves assembling not only the molecular structure but also the larger scale internal micro- and meso-structure of the material. New processes are continually becoming available to the chemical engineer to accomplish these ends. New catalysts, new macromoleeular building blocks, reactive processing, self-assembly, manipulation of phase behavior, applications of strong orienting fields and genetic engineer- ing of materials are among the processing of polymers is the ability to measure product characteristics. Scattering and imaging methods have been the most powerful and the fastest growing techniques giving insight into polymer structure. Chemical engineering is as central as it always has been to leadership in the development and processing of polymers and other soft materials.

INTRODUCTION provided little opportunity to tailor, is still a very Polymeric materials are architecturally designed at lively arena for innovations in chemistry and pro- the molecular level. Since macromolecules have large cessing. spatial extent, and have a multiplicity of subunits of With these thoughts in mind, this article will review subunits and components built into them, moleeular recent accomplishments and future prospects for pro- design is an important part of the conception and use gress relevant to chemical engineering research. As we of polymers. Chemical engineers are the building con- embark on this, we should be conscious of the fact struction analogy breaks down rather quickly in the that a similar review was prepared several years ago production of polymers. End-use properties, which (Tirrell, 1991). As we proceed, there are opportunities are the ultimate measure of the efficacy of a polymer to compare the perspective advanced five years ago to manufacturing operation are generally affected as this one. They are not identical and some of the much by the chemical and physical processing as by reasons for that are interesting and will be discussed. the molecular design. Polymer production is not sim- We begin this review with a discussion of reaction ply a matter of executing the molecular blueprint. chemistry applied to polymers. Physical phenomena, There is a level of interaction between the process and such as rheology, dynamics and inteffacial phe- the product with polymers that is much richer, more nomena are dealt with next. Developments in experi- complex and more intricate than for most chemical mental techniques and instrumentation, which are products. Polymer properties are not intrinsic, as are greatly advancing our understanding of relationships the properties of simple chemicals, but rather they can between structure and properties of polymers, are be manipulated widely by polymerization and pro- discussed in this connection. We conclude with a sec- cessing conditions (see Fig. 1). tion on processing of polymers. Throughout, the aim For this reason, chemical engineers producing is several-fold: to inform the reader about the role of polymers must be conversant in molecular design and chemical engineering in this area; to make clear the molecular designers must be aware of processing con- special engineering challenges (common to much of siderations. (Wimberger-Friedl, 1995). Even the most chemical processing of materials) when the product is basic polymer, polyethylene, exhibits new aspects in a material the quality of which is judged by end-use this regard. A recent workshop on industrial perspect- properties; and most importantly, to illuminate signif- ives in polymer manufacturing (Layman, 1994) gener- icant new fields of research for chemical engineers. ated a remarkable statement to the effect that the highest priority need of the industry is the ability to tailor the polyethylene molecule more accurately. This POLYMER REACTIONS chemically simple polymer of methylene units with Catalysis of polymerization occasional branches, which accounted for more than It is commonplace assertion (Wirth, 1994; National 20 of the 70 billion pounds of plastics produced in the Research Council, 1994) that new developments in U.S. in 1993, and which one might have thought synthesis and construction of new polymeric materials 4123 4124 M. TIRRELL

Polymer ~ Polymerization ~-- Process

Product

Performance

Fig. 1. Schematic interplay among the "five p's" representing: molecular design, polymerization, process- ing, product applications and end-use properties. Adapted from Wimberger-Friedl (1995).

Linear low-density polyethylene 8% Polystyrene 11% Low-density All other: A,~ [yethylene 15%

Polypropylene 18% Thermoplastic polyester 4%

[igh-density Polyvinyl chloride Acrylonitrile- polyethylene 18% & copolymers 19% butadiene-styrene 3% Top

Furniture & furnishings 5% Other tckaging 32% Electrical equipment 6%

Exports 12% ouuumg & Consumer & Transportation construction institutional equipment 4% 14% products 13% Bottom Fig. 2. (Top) U.S. production of thermoplastics by type, 1990. Adapted from National Research Council (1994). (Bottom) Categories of uses for thermoplastics in the U.S., 1990. Adapted from National Research Council (1994). will come more in the specialty polymer areas, such as wide production of plastics; see Fig. 2. This produc- high-performance engineering plastics and polymeric tion volume naturally provides considerable motiva- additives for other products, than in the main com- tion for attention and, combined with elasticity in modity polymer areas. However, as the remarks in the price and a widening array of applications with more Introduction let on, the scope for innovative advances finely tailored properties, gives rise to conditions in polymerization of these commodity polymers is far favoring significant developments. from exhausted and some recent activity in catalyst Polyolefin production is dominated by products of development has run counter to this conventional organo-metallic, Ziegler-Natta catalysis, including view. Five standard commodity thermoplastics: poly- high-density polyethylene (HDPE), linear, low- ethylene (typically classified into high- and low-den- density polyethylene (LLDPE) and polypropylene sity categories), polypropylene, polyvinyl chloride, (PP). Several factors drive catalysis research in this polystyrene and thermoplastic polyester, constitute field. These include: increases in catalytic activity (of more than half of the 250 billion pounds of world- order 10ag polymer/metal h, at constant monomer Chemical engineering of polymers 4125 concentration, in the most active examples) the group 4b transition metallocenes, typically tita- (Kaminsky et al., 1992; Alt et al., 1994); control of nium, zirconium or hafnium, with bulky, substituted molecular architecture, molecular architecture, mo- cyclopentadienyl ligands. In 1980, Kaminsky and lecular weight distribution and resulting improve- coworkers (Sinn and Kaminsky, 1980) discovered that ments in properties (Coates and Waymouth, 1995; the presence of methyl alumoxane (MAO) with these Reisch, 1995; Spaleck et al., 1992; Kaminsky and metallocenes produced an extremely active catalyst Steiger, 1988); and mechanistic insight into the pol- for ethylene polymerization. Chien et al. (1991) went ymerization (Huang and Rempel, 1995). For poly- on to develop one of the most promising systems for ethylene, the principal goal is the control over the high activity, stereoregular propylene polymerization, number, placement and length of branches in the a bis-indenyl zircocene or titanocene with a bridging repeating methylene unit structure; for polypropylene, unit between the two ligands (Wild et al., 1985) to lock the main goal is control of the level of crystallizable, the ligands into a symmetric or asymmetric configura- isotactic placement of the propylene monomers in the tion to produce the desired stereochemistry, as chain. sketched in Fig. 3. In general, products of these cata- Chemical engineering has been particularly impor- lysts are different from heterogeneous Ziegler-Natta tant to the development of commercial Ziegler-Natta polymerization in that they have higher degree of polymerization, and to enhancement of mechanistic stereoregularity, narrow molecular weight distribu- insight, over the last decade. An issue of prime signifi- tion and more uniform sequence distribution in cance for heterogeneously catalyzed Ziegler-Natta copolymers of ethylene and higher ct-olefins. polymerization has been the especially broad molecu- Huang and Rempel (1995) describe some of the lar weight distributions that evolve, even when poly- applications that the capabilities of these new cata- mers with excellent stereochemical control are being lysts open up. Among them are: 100% atactic produced (Schmeal and Street, 1971, 1972). Various polypropylene for blending with elastomers, homo- origins of these broad distributions have been pro- geneous copolymers for LLDEPE, poycycloalkenes posed: diffusion-limited access to catalytic sites, ex- for stable high strength, high-temperature applica- acerbated by the heterogeneous nature of the catalyst tions, and elastomeric polypropylene, consisting of and the high production of insoluble polymer that blocky sequences of isotactic and atactic polypropy- quickly envelops the catalyst particles; multiplicity of lene. This last application has been advanced nicely types of active sites within one catalyst system; and recently by Coates and Waymouth (1995), who have temporal deactivation of catalysts during polymeriz- developed an "oscillating" catalyst, depicted in Fig. 3, ation. Ray and coworkers (Nagel et al., 1980; Floyd where the bulkiness of the substituted indene ligand et al., 1986a, b, 1987; Hutchinson et al., 1992) led the generates a rotation frequency for switching between way in the analysis of diffusion limitations in hetero- isotactic-producing and atactic-producing positions geneous catalysis, developing a realistic and versatile, that is comparable to the growing chain lifetime. The multigrain model, which showed how the structure of configurational switches are made during the growth growing polymer particles could produce molecular of each chain and stereoblockiness, leading to elas- weight distributions approaching the large observed tomeric behavior, is built into the macromolecules. breadths. Deactivation as the main source of molecu- Metallocene catalysts offer many possibilities to lar weight polydispersity has been ruled out. Experi- synthesize novel polyolefins (Horton, 1994; Reisch, mental support for the role of multiple sites is found, 1995). Variants of this class could conceivably begin for example, in the early results (Dotson et al., 1995) to displace other commodity plastics as well as create which showed great variations in polydispersity be- new engineering plastics. There are research chal- tween different kinds of catalysts, as well as more lenges to meet before this potential is fully, realized. recent results with metallocene catalysts (Kaminsky Molecular weights of these polymers are often lower and Steiger, 1988) which have only one site and give than desirable for the application. MAO, at the levels quite narrow distributions. While the weight of evid- currently employed, is expensive and should be re- ence suggests that diffusion limitations are important, duced in amount or replaced. There may be a need to over time it has become more likely that multiple develop heterogeneous versions of these catalysts, for catalytic sites do exist and that they must be invoked various reasons, including the control of polymer par- in modeling the performance of traditional Zieg- ticle morphology (see next section), without sacrificing ler-Natta catalysts to represent properly the broad their marvellous advantages. Heterogeneous catalysts experimental molecular weight distributions seen in development would bring analysis of diffusion con- practice (Gaivhn and Tirrell, 1986a, b; McAuley et al., trolled reaction back to the fore. 1990; de Carvalho et al., 1989, 1990). Fundamental analysis of diffusion-controlled catalytic reactions has M acromolecular building blocks been the key to these insights. Increasingly, polymeric materials are being, and Until recently, homogeneous Ziegler-Natta cata- will continue to be, synthesized, not directly from lysts have been less important commercially then het- monomers, but from preformed, functionalized, mac- erogeneous versions, but this is rapidly changing romolecular subunits. This is particularly true for (Horton, 1994; Van Santen, 1995). The main compon- multicomponent macromolecules. These building ent of homogeneous Ziegler-Natta catalyst systems is blocks are, in the simplest case, end-functionalized 4126 M. TIRRELL

Atactic

m m r m r r r m r m

Isotactic

m m m m m m m m m m

Isotactic- atactic stcmoblock

Atactic polymer ~ P r polymer

Isotactic ~ ~ Atac~ block Kpi Kpa~ block

Fig. 3. Various stereochemistdes for polypropylenes can be produced by metallocene catalysts. Adapted from Coates and Waymouth (1995). oligomers or polymers (sometimes called telechelics) search on macromolecular building blocks (Fr~chet, that are linked into larger structures by coupling 1994). The term "functional polymer" is often used to reactions. More complex examples of subunits are describe polymers that carry reactive functional highly structured molecular objects and particles that groups that can participate in chemical processes fuse into the desired materials. without degradation of the original polymeric unit. This modular construction of polymeric materials Functional polymers are abundant in nature. Molecu- raises some particular challenges for chemical engin- lar architecture has significant effects on the charac- eers. One generic challenge is in the production of the teristics of functional polymers. Most polymers subunits. In order to achieve many of the desired consist of largely linear chains that are randomly properties of materials put together in this manner, coiled and entangled with their neighbors. Introduc- the subunits have to be made with a level of precision tion of substantial amounts of branching is always unprecedented at the production scale. Near perfect accompanied by substantial changes in flow proper- uniformity in molecular architecture and in degree of ties. Highly branched and star-like polymers have functionalization is often desirable. A second generic significantly lower melt viscosities than linear poly- challenge is the creation of the reaction environments mers of the same molecular weight (Graessley, 1974). for assembly of the modules into materials. Ways of From the point of view of functionalization, a major meeting these challenges are advanced in subsequent effect of branching is the multiplication of reactive sections. This section deals with the motivation for the chain ends. examples of new developments in macromolecular Dendrimers are highly branched, three-dimen- building blocks. sional macromolecules with branch points at every An intriguing development in the last few years, monomer, as illustrated in Fig. 4 from Fr~chet (1994), unanticipated in a review five years ago (Tirrell, 1991), leading to a structure that has essentially as many is the burst of activity in the synthesis and applica- end-groups as it has monomer units. When syn- tions of dendrimer polymers (Voit, 1995). In several thesized by controlled, convergent growth, these ways, activity in this area typifies this subset of re- dense regular structures adopt compact, well-defined Chemical engineering of polymers 4127

q

) I

O

¢ II

R= i-io¢ ~-,~

Fig. 4. (Left) Dendritic polyester. (Right) Amphiphilic dendrimer. Adapted from Fr~chet (1994).

spherical shapes and, in initial explorations, have be- and designable solvophilicity at the surfaces (even gun to exhibit physical properties not seen even in with controlled, local variation over the surface of the other more traditional forms of highly branched poly- same molecule, see Fig. 4). mers. For example, the intrinsic viscosity of de- Dendrimers are species on the low side of the ndrimeric polymers (Mourley et al., 1992) goes borderline of size and mass between molecules and through a maximum with increasing molecular particles, occupying a position somewhat analogous weight, in striking contrast to the monotonic increase to fullerenes, though their synthesis is much more with molecular weight in linear polymers. This is due versatile and controllable. There is exciting activity in to the fact that intrinsic viscosity measures the mo- several quarters aimed at extending the desirable fea- lecular volume pervaded per unit molecular mass; for tures exhibited by these structures up to larger sizes higher generation dendrimers, the mass increases (molecular masses of 108-109 ) and different control- faster than the pervaded volume. Some other highly lable shapes. Stupp has termed these target species ramified products, known as hypcrbranched poly- "molecular objects" (Stupp et al., 1993). Large, two- mers, also exhibit some of these physical phenomena, dimensional, flat sheet polymers are examples of such but lack the structural precision and its potential objects. Like dendrimers, they possess persistent advantages (Kim and Webster, 1990). shapes under changing thermal and phase state condi- Tomalia and Dvornic (1994) have succinctly sum- tions and have well-defined, functionalizable surfaces. marized some of the exciting new applications for In general, though, molecular objects are being con- dendrimers being explored in industry and universi- structed with forms of monomer interconnection dif- ties for these molecules. These include uses as poly- ferent from the highly branched route to dendrimers, meric catalyst particles and nanoscale reactors (Turro two-dimensional cross-linking, molecular recogni- et al., 1991), molecular mimics of miceUes (Jansen tion, noncovalent bonding and nematogenic or self- et al., 1994), delivery vehicles for magnetic resonance assembly tendencies are among the routes being used imaging agents, immune-diagnostics and gene ther- to form large molecular objects. One can readily im- apy (Haensler and Zoka, 1993), and building blocks agine applications for stratified two-dimensional for more elfiborate supermolecular structures polymers, where functionalized top and bottom strata (Tomalia, 1994). The molecular features enabling would link an active middle stratum into a larger these applications are persistent and controllable structure. Several alternative concepts are illustrated nanoscale dimensions in the range from 1 to 100 nm, in Fig. 5 (from Stupp, see Acknowledgements). Treat- some control over shape via molecular design of the ing surfaces with two-dimensional molecular objects, dendrimer core (Fr~chet, 1994), precise masses that where desired surface functionality has been built into can approach 100,000 (with polydispersities of the molecule, bears a relationship to alternative sur- Mw/Mn ~ 1.0005), chemically reactive surface func- face treatments by adsorption of self-assembly some- tionality, interiors that can be specifically tailored to thing like the difference between wallpaper and paint- hydrolytically or thermally demanding environments, ing. In the former, the desired pattern is preformed, 4128 M. TIRRELL Concepts in Advanced Materials with 2D Polymers

Electronic

semiconducting nanolayer

Photonic

nonlinear optical nanolayer

Biological cell adhesive nanolayer

substrate bonding nanolaye

Adhesive sticky nanolayer

Transport semi- permeable nanolayers

Structural inorganic hard layer organic soft layer inorganic hard layer

Fig. 5. Possibilities envisioned by Stupp (see Acknowledgments) for different structures and applications of two-dimensional polymers. then applied; in the latter, pattern is created during special molecular geometry are discussed by Eisen- the application process. Both have their uses. Other bach et al. (1995). sorts of molecular objects under study are cylindrical In the same spirit, the next step up the size scale (Imrie, 1995) and virus shapes (Kwon et al., 1994) and from the nano- to the micro-level, and equally impor- molecular ribbons and tapes (Whitesides et al., 1991; tant to the chemical processing of advanced polymeric Lehn et al., 1992). Additional polymer systems of materials, is the production of polymer particles of Chemical engineering of polymers 4129 well-defined size. Emulsion and dispersion polymeriz- terials are naturally very important in themselves and ation are the common routes for the production of provide stimulating impetus for the development of these materials. Surface functionalization, in the form new, soft composite materials. of core-shell morphologies and other routes, gives Several areas of current activity illustrate the re- particles similar roles to those of smaller molecular search problems and potential for interesting develop- objects in materials development. Galli (1994) de- ments in this area. Work on multilayer, thin films scribes the key role of particle size and morphology typifies some of the possibilities. Many of the applica- control in polyolefin manufacturing and, how it is tions are related to those illustrated in Fig. 5. The possible, directly from the reactor, a broad range of modification of the surface of solids with polymers has polyolefin products in spherical form, with a soft, become a major challenge in both basic research and porous morphology. Spherical-form polymer par- applied materials science. Tailoring of surface proper- ticles are utilizable without any pelletization opera- ties enables control of properties such as chemical tion, which is expensive and risks degrading product binding for sensor applications, biocompatibility, properties. The importance of particle size and shape conductivity for anti-static coatings, adhesion and control in the solids flow and handling properties, friction. Controlled surface treatments permit the con- vital in polymer processing, is being increasingly struction of a materials surface property while retain- understood and is a point of contact between impor- ing desirable bulk materials properties. tant issues in polymer materials and particle techno- Decher and coworkers (Decher and Schmitt, 1992; logy (Zukoski, 1995). Decher et al., 1994; Schmitt et al., 1993) have pion- To the extent that well-defined functionalized poly- eered a new technique of constructing multilayer poly- mers, molecular objects and particles are important mer assemblies by consecutively alternating adsorp- for the future of polymeric materials, chemical engin- tion of anionic and cationic polyelectrolytes and/or eers will have to learn to produce them in large scales, bipolar amphiphiles. This synthesis process is driven with unprecedented levels of precision and control. by the electrostatic attraction between opposite This has implications, in turn, for necessary develop- charges as illustrated schematically in Fig. 6. In con- ments in instrumentation and process control. trast to chemisorption methods (Gun and Sagiv, 1986; Whitesides et al., 1991) that require a reaction yield of Noncovalent synthesis 100%, or lateral cross-linking, in order to stabilize Intermolecular coupling by routes other than a certain surface functional density after each reaction covalent bonds can be very important in producing or step, no covalent bonds need to be formed. Addition- interlinking the building blocks of the previous sec- ally, an advantage over the classic Langmuir- tion. Noncovalent bonding is also of great importance Blodgett method (LB) (Roberts, 1990) is that adsorp- in linking polymeric materials with their surround- tion processes are relatively independent of substrate ings, as in adhesive or composite materials and ap- size and geometry, whereas LB deposition works only plications. In a larger sense, noncovalent bonding is on large, smooth substrates. the foundation of supramolecular materials synthesis. The principle of self-assembly of multilayers in this Within the last few years, "supramolecular science" fashion is sketched in Fig. 6. A solid substrate with has become the cumulative title to describe the rap- a positively charged surface is immersed in a solution idly emerging achievements at the contact lines of the anionic polyelectrolyte and a layer of polyanion among chemistry, physics and biology (Ringsdorf, is adsorbed (step A). The surface can adsorb polyan- 1994). It applies the principles of self-organization, ion until all of the surface charges are compensated by regulation, replication, communication and adsorbed anions. If the adsorbate concentration in cooperativity, generally life-science-derived principles, solution is high, however, the adsorbed polymers will to the development of new materials. Further at- retain many unbound anions exposed to the interface tributes of biological systems, such as morphogenesis, with the solution. In this way, the charge of the surface pain (i.e. intrinsic capacity for defect detection) and is effectively reversed. After rinsing in pure water, the healing (defect repair), may eventually also be instilled substrate is immersed in a solution of cationic poly- in materials. electrolyte. Polycations adsorb on the anionic surface Most current systems based on noncovalent syn- (step B) and, again with sufficient reservoir in solution, thesis are considerably less sophisticated than bio- the surface charge can be overcompensated and rever- logy, though they may be based on the same types of sed back to positive. Repetition of steps A and B in intermolecular interactions, such as hydrogen bond- a cyclic fashion leads to an alternating multilayer ing, electrostatic interactions, hydrophobic interac- assembly. The minimum charge functionality per mol- tions or n-n stacking. Many biological materials are ecule necessary to accomplish this is two and the first composed of hierarchical scales of structures. For examples of this method of assembly were accomp- exampl% in connective tissue, long molecules lished with bipolar (i.e. double-ionic-headed, some- (~ 100A) form helices (~ 100nm) which organize times called bolaform) amphiphiles. Mao and into fibers on the micron scale, which, in turn, organ- coworkers (Mao et al., 1993, 1994, 1995) have produc- ize into a fibrous network structure at the largest ed such assemblies with cationic bolaform am- scale. Noncovalent forces are often the binding mech- phiphiles and anionic polyelectrolytes and have meas- anism between different structural scales. These ma- ured their mechanical properties in compression. The 4130 M. TIRRELL quirement for the construction of more complex film architectures, such as (ABCB), or (ABCDEF),, is that a single molecular layer be deposited in each cycle that effectively overcompensates the previous surface functionality. Such structures will have useful proper- ties in their own right and can also serve as scaffolds and templates for insertion of other structural and functional elements (Maoz et al., 1995). This line of materials production by noncovalent synthesis is by no means confined to surface recon- struction. In bulk media, polyelectrolyte complexes A made of two oppositely charged polyelectrolytes also spontaneously form complexes, sometimes termed simplexes, which exhibit interesting material proper- ties (Kabanov and Zezin, 1984). Stable porous and selective membrane structures have been made by this route. Similarly, complexes of polyeletrolytes and sur- factants of opposite change have been shown (Anto- nietti and Conrad, 1994; Antonietti et al., 1994) to assemble spontaneously into materials possessing in- ternal, layered or cylindrical microstructures and in- teresting, elastomeric properties. In these ways, these noncovalently bonded materials resemble the struc- tures formed by ampliphilic molecules and polymers B (Bates and Fredrickson, 1990). Conceptually closely related to these polymer-sur- factant complexes are synthetic, nanophase, organic- inorganic hybrid materials (Dangani, 1992). In these materials, polymers are bound by adsorption within an inorganic host, for example, a layered silicate, which generally imposes some ordered microstructure ® on the assembly. Polymers are inserted into these microstructures by several routes: intercalation of At By ,.o a monomer followed by polymerization (Cao and Mallouk, 1991), polymer intercalation from solution ® (Messersmith and Stupp, 1992), or from the melt (Vaia et al., 1993). These nanocomposites combine the high strength and thermal stability of ceramics with the processibility and crack-deflecting properties of poly- Fig. 6. Schematic diagram depicting bnild-up of multilayer assemblies by sequential adsorption of anionic and cationic mers. The dielectric properties of these materials are polyelectrolytes. Counterions omitted for simplicity. essentially the same as the bulk polymer making them Adapted from Decher et al. (1994). very attractive materials for insulating layers in microelectronics applications. The converse of these organic-inorganic hybrids, where the inorganic synthetic options are widely varied. The ability to phase largely determines the morphology of the com- continue the sequential adsorption reproducibly posite, is the interesting work going on in several through a large number of adsorption cycles has been chemical engineering groups, using the self-organizing demonstrated often and for many different molecular properties of organic molecules, such as amphiphilic combinations (Mao et al., 1993, 1994, 1995; Decher polymers and surfactants, to make templates for inor- and Schmitt, 1992; Decher et al., 1994; Schmitt et al, ganics, such as zeolites or microstructured ceramics, 1993). where the organic may eventually by removed by This inversion and reconstruction of surface prop- high-temperature processing (Archibald and Mann, erties can be viewed as a simple form of template- 1993); Fig. 7 illustrates some possibilities. Stucky, controlled growth regulated by molecular recogni- Chmelka and coworkers (Monnier et al., 1993; tion. The solid-liquid interface acts as a template for Firouzi et al., 1995) have demonstrated a strong surface the deposition of the subsequent layers. The next templating effect in the synthesis of these materials. layer, if adsorbed properly, will refunctionalize the Research efforts by chemical engineers in this gen- surface, providing a template for the following layer. eral field of noncovalent synthesis is needed in many Sequential adsorption of two compounds produces an directions. Relative to covalent bonds with energies alternating sequence (AB),, but there is no require- ranging from i00 to 300 kcal/mol, bonding in non- ment that this be a two-cycle process. The only re- covalent assemblies is very weak with bond energies Chemical engineering of polymers 4131

Tetrapropylammonium Lamellar phase Hexagonal phase Bicontinuous cubic phase O

ZSM-5 Layered organic- MCM-41 Cubic organic- silica phase silica phase

Fig. 7. Somearchitectures that can be obtained by organic templating of inorganic materials:(top) organic molecules and aggregates; (bottom) the zeolite or microporous oxide. Adapted from Davis (1992).

(hydrogen, electrostatic, dipolar, van der Waals) in the exquisite tailoring and uniformity of macromolar neighborhood of 0.1-5 kcal/mol. Effective processing structure have never been greater. This, coupled with of these materials will require development of under- the potential to eliminate reliance on petroleum feed- standing of the kinetics of these physicochemical pro- stocks, is driving strong efforts in the synthesis of cesses, analogous to chemical reaction kinetics. Such macromolecular materials by microorganisms. Mac- syntheses require the development of multiple sets of romolecular synthesis of polymers by biological interactions coordinated in time and space. This re- routes is a large subset of the emerging field of mark, in fact, applies to all self-assembly processes biomolecular materials (Tirrell et aL, 1994). With in- where overwhelming attention has been given to the creasing frequency, new materials or processing strat- structures formed and insufficient investment has egies are emerging, inspired by biological examples or been made in kinetics, mechanisms and ultimate developed directly from biological systems. Re- properties of the resultant materials. Most of the searchers are engineering bacteria or other organisms examples cited here consist of materials built up from to synthesize monomers for polymer production. electrostatic interactions with other molecules and They are synthesizing and expressing artificial genes surfaces. Weak bond energies implies either weak to produce protein-like materials with mechanical structures or that multiple bonds have to be formed to properties of silk, or other materials containing elastic stabilize noncovalent structures. In general, non- fibers (Bailey, 1995). The aim is to harness the efficien- covalent structures are soft (meaning exhibiting signif- cy of biosynthesis. Organisms carry out astonishingly icant responses to weak stimuli, such as ambient ther- elaborate sequences of organic reactions that convert mal energy), but can be strengthened as much as simple building blocks into complex natural products. desired by additional bonding. Polymeric nemato- The comparable degree of efficiency in an industrial gens, such as thermotropic, main-chain liquid crystal- chemical process would be the equivalent of convert- line polymers, owing mainly to their high cost have ing a single chemical feedstock into end products not made much commercial success as hard, stiff ma- without having to isolate or purify any intermediate. terials. However, for applications where higher cost Examples of commercially successful monomer or can be borne, such optoelectronic properties, poly- polymer precursor (Mobley, 1994) production via bio- mers capable of organizing themselves through synthesis include: the ICI invention of a selective noncovalent nematic interactions are increasingly im- bacterial oxidation of benzene to produce 5,6-cis. portant (Imrie, 1995). dihydroxycyclohexa-l,3-diene (DHCD) which is con- ventionally chemically acylated and free radically Biological synthesis of polymeric materials polymerized to polyphenylene (which becomes an or- A theme underlying several phases of the foregoing ganic semiconductor on doping) and; the General discussion is the growing need for the precise Electric fungal, regiospecific, hydroxylation of bi- synthesis of modular building blocks for polymeric phenyl to the 4,4'-dihydrnxy derivative, which is used materials. Whether for surface modification or for to prepare GEs engineering plastic Ultem II. noncovalent synthesis by molecular recognition or for A number of bacteria produce biodegradable precise control of material properties, the needs for polyhydroxyalkanoate (PHA) polyesters. The most 4132 M. TIRRELL common PHA, poly(3-hydroxybutyrate (PHB), an energy and carbon reserve material in many bacteria F'Nx r"~ f-~ (Anderson and Dawes, 1990), is accumulated as in- tracellular granules under nutrient-limiting condi- tions other than carbon limitation. A wide variety of other PHA are also synthesized in response to nutri- ent deficiencies. PHB is quite brittle and melts at 180 K. PHA copolymers are considerably more flex- ix-helix or ible and are stable at temperatures high enough to withstand conventional plastics processing methods. /',,-)~y ",._~y ',,...),y y A PHA copolymer developed by ICI is in commercial Reverse turn use in biodegradable films, coatings, molded materials Fig. 8. Polypeptide chains that could fold back and forth at such as bottles, and controlled drug release applica- regular intervals would produce synthetic lamellar protein tions. Engineering PHA-producing organisms are to crystals of well-defined size and with predetermined func- assimilate and utilize effectively for synthesis of a var- tional groups positioned on fold surfaces. Compare with Fig. iety of unnatural, but inexpensive and mechanically 5. Adapted from Tirrell et al. (1994). beneficial, PHA copolyesters (Jackson and Srienc, 1994). Engineering protein and polypeptide-based mater- ial is attracting increasing attention and has enor- r-sheet), the size and thickness of the crystals and the mous possibilities for new materials from structural chemical functionality of the two surfaces. A major proteins, like silk, collagen, and elastin that have long challenge that is being approached in several ways is intrigued materials scientists (Urry et al., 1992), to how to incorporate entities beyond the naturally oc- materials to optimized for other properties such as curring amino acids. Successes have been achieved in cell adhesion or chemical and biological sensors. Gen- incorporating elements such as selenium (via sele- etically engineered bacterial systems offer superior nomethionine) and fluorine (via p-fluorophenyl- control over the architecture of polymer chains. New alanine) (Tirrell et al., 1994). routes to materials synthesis will increasingly require precise molecular units. The basic strategy involves, as Environmentally benign synthesis of polymers a first step, the solid-state synthesis of double- The foregoing sections deal with chemical engineering stranded DNA segments that code for the desired developments in polymer production that are driven amino acid repeat units. More than one of these by the creation of new polymer structures and proper- segments may be stitched together with ligation en- ties. There are other drivers of new polymerization zymes to produce multiple coding units arranged in methods. Among the most challenging technically is tandem. This synthetic DNA sequence (an artificial the pursuit of more environmentally friendly commer- gene) is then inserted into a plasmid and the recom- cial reactions for monomer and polymer production. binant molecule introduced into a strain of Es- An example of an alternate process with an environ- cherichia coli that can express the protein. However, mental rationale is a route to polycarbonate, an im- use of these genetic engineering techniques to produce pact-resistant engineering thermoplastic developed by unnatural proteins and polypeptides presents several General Electric, which does not involve the use of problems, outlined by Tirrell et al. (1994). First, repeti- phosgene. Condensation polymerization of phosgene tive DNA sequences are highly susceptible to re- with bisphenol-A is the traditional route. The newer arrangement and deletion. Second,the messenger technology (Wirth, 1994) replaces phosgene with RNA needed to synthesize a "foreign" protein may be diphenylcarbonate (which is transesterified with bi- subject to rapid degradation. Third, because most sphenol-A to make polycarbonate) which, in turn, is amino acids are specified by more than one codon, it produced from methanol and carbon monoxide. The is possible to select a sequence in the artificial gene key to this process is a detailed understanding of the that is inefficiently processed during protein synthesis. process variables and careful process control. Finally, foreign proteins may be toxic to the host cell A second example in the synthesis area driven by and kill it before useful amounts of the protein is environmental rationale pertains to the production of synthesized. fluorocarbon polymers, such as Teflon and related Despite all these adversities, progress is being made materials, which are used extensively for their low (Creel et al., 1991; McGrath et al., 1992; Dougherty energy surface properties, as lubricants in computer et al., 1993; Parkhe et al., 1993). De novo design of disk drives, protective coatings and sealants (Scheirs protein-based materials opens great possibilities for et al., 1995). Fluoropolymers are generally insoluble in imaginative molecular architecture. A materials de- most organic solvents, but are readily soluble in the sign target that has received considerable attention is chemically similar chlorofluorocarbon solvents the synthesis of polypeptides that could fold over and (CFCs). However, scientific evidence and govern- back at regular intervals to produce a lamellar protein mental regulation now dictate diminishing use of crystal, as illustrated in Fig. 8. Construction details CFCs owing to environmental concerns, posing tech- that have to be attended to include the secondary nical problems for the polymerization and processing structure of the lamellar-spanning chains (~t-helical or of fluoropolymers (Haggan, 1995). DeSimone and Chemical engineering of polymers 4133 coworkers (DeSimone et al., 1992, 1994; Shaffer and both from a fundamental point of view and for their DeSimone, 1995) have demonstrated that supercriti- significance in controlling material structure and cal CO2 is an excellent alternative to CFCs for con- properties. Classes of materials exhibiting this behav- ducting homogeneous solution homo- and ior include thermotropic and lyotropic liquid crystals copolymerizations of fluorinated monomers. They and block copolymers. Block copolymers can be have shown that it is possible to synthesize high viewed as displaying both lyotropic and thermotropic molecular weight fluoropolymers at reasonable rates behavior since phase transitions can be induced by in supercritical CO2. The more widespread use of changing the composition, f, and temperature, T, re- supercritical CO2 as a polymerization and processing spectively (F6rster et al., 1994). Symmetrical (f = 0.5, solvent is being explored, giving further impetus to the that is, both blocks of equal chain length) diblock field of supercritical solvents in general (McHugh and copolymers are conventionally represented, and ex- Krukonis, 1986; Savage et al., 1995). perimentally found, to have a transition between a disordered phase and an ordered lamellar phase at zN-~ 10 (Leiber, 1980). Typically, two limiting re- POLYMER PHYSICAL PHENOMENA gimes in the "phase diagram" are distinguishable for Recognition of additional fundamental parameters the ordered regime. (We note that since ~N and fare governing phase behavior and entanglement not intensive variables, this is not a proper phase Organization of information about the physical diagram in strict thermodynamic terms.) In the strong phenomenology of polymers in a form that displays segregation limit (SSL), zN >> 10, enthalpic effects some universal character is particularly useful for dominate, leading to a sharp interfacial composition engineering purposes. Key to this is the identification profile that approaches a step function. In the weak of all the fundamental governing parameters for the segregation limit, zN ~ 10, entropic contributions be- behavior of interest. Our long experience with scaling come more important, leading to a broad interface and dimensional analysis teaches us this. For the with a composition profile that tends toward purposes of the first part of this discussion, we are a sinusoidal form. The weak segregation limit is going to confine our attention to rheological, diffu- a more complex and delicate area of the phase dia- sion and phase behavior of polymers. Chemical engin- gram, very relevant for processing of these materials, eering insight into thermodynamics, phase equilibria since most processing involves transient passage and transport phenomena has long been a mainstay through this state. Moving away from f= 0.5, to of progress in polymer physical science (Bird et al., asymmetric diblock copolymers, produces an array of 1987). Much of the spectrum of bulk behavior in these ordered phases with different symmetry, most com- areas for long-chain polymers has been organized in monly hexagonally arrayed cylinders and BCC- terms of just five fundamental parameters: X, the inter- ordered spheres, as well as bicontinuous phases, for action energy (in units of kT) that must be paid to certain values offi Some of the structures that have move a segment into an environment where it inter- been observed are illustrated in Fig. 9. acts with a segment of different chemical nature; N, the number of segments per chain; b, the effective (often called the "statistical") segment size (Flory, 1969); ~, the friction coefficient per segment; and M,, Im3m HEX la3d the molecular weight between entanglements. These are the parameters in which the theories of polymer physics are written; the data are plotted in terms of these or their combinations. Phase behavior of polymers is influenced by the first three parameters and, in fact, the first two act in combination, with :tN being the important parameter. This combination signifies that chain length, N, be- Spheres Cylinders Bicontinuous comes an effective thermodynamic parameter in the sense that a small intersegmental effect, Z, can be multiplied into an arbitrarily large intermolecular ef- HPL HML LAM fect by increasing the molecular weight. Mixtures of chemically different polymers of equal N have a criti- cal point for demixing at xN = 2 and • (volume fraction) = 0.5. Multicomponent polymers in addi- tion to blends, such as block copolymers, also exhibit phase behavior governed largely by xN. Phase transitions involving ordered states on a me- Perforated Modulated soscopic scale occur in a wide variety of materials of Layers Lamellae Lamellae interest from solid-state physics to structural biology Fig. 9. Ordered microstructures observed in A-B diblock to applied materials science. These transitions present copolymer melts near the order~iisorder transition. a rich field of current interest for chemical engineers Adapted from Bates et al. (1994b). J

CES 50-24-S 4134 M. TIRRELL Recently, several groups have seen that the phase gyration per unit chain length. Thus, compositional diagram for block copolymers is asymmetric about and conformational effects are driving interfacial cur- f = 0.5, which would not be expected if composition vature in different directions. Phase symmetry is not and zN were the only relevant parameters. This asym- dictated by copolymer composition alone. metry is particularly pronounced in the weak segrega- Conformational asymmetry effects have also re- tion limit (WSL) near xN = 10. The reason for the cently been shown to be important in determining appearance of these new phases, and their asymmetric surface segregation and phase organization in thin placement with respect to changes in fin the WSL, is films of block copolymers (Sikka et al., 1994). There is the ability of the additional factors to influence the every reason to believe that further consideration and behavior. In the strong segregation limit (SSL), or- exploration of these trade-offs between conformation dered-phase organization is dictated by the enthalpic and space-filling packing, and their interactions with drive to demix unlike segments, tempered only by the classical parameters, will lead to a better under- classical translational entropy. Mean-field analyses standing of phase diagrams of amphiphilic molecules, are satisfactory in the SSL. In the WSL, chain stretch- in general, and the connection between low molecular ing, chain packing and fluctuation effects come into weight surfactants and block copolymers, in particu- play in addition to the classical factors. These effects lar. Chemical engineers can gain practical information require additional parameters for their description. on the utilization and processing of these materials for (Bates et al., 1994b). The magnitude of fluctuation a wide range of applications in pursuing such re- corrections to mean-field theory is controlled by search. (Fredrickson and Helfand, 1988) Interestingly, the conformational parameter fl (Hel- fand and Sapse, 1975) has recently appeared in a dif- N = a6N/v 2 (1) ferent important context, that is, in the connection where, for the time being, we are assuming that seg- between polymer molecular weight, density, chain di- ments of both are of the same length a and same mensions, and melt viscoelastic properties (Fetters volume v. Note that v # 4.3ha, 3. v is a real packing et al., 1994). One of the principal goals of polymer volume, the occupied volume of the segment, whereas science has been to relate the structure of macro- the effective segment length, a, is a chain configura- molecular chains to their macroscopic properties. In tional property relating the radius of gyration of the the context of polymer melt rheology, dynamics and coil to N. The importance of fluctuations decreases processing, the objective has taken the form of predic- with increasing N. A second important nonclassical tion of the degree to which polymer chains entangle parameter and manifest viscoelastic rheology. In a foregoing paragraph, we mentioned the basic entanglement e = fl2a/fl~ (2) parameter, Me, which is a well-tabulated quantity (Ferry, 1980) for a wide range of polymers. It is experi- gauges the differences in space-filling characteristics of mentally well-defined from the plateau modulus ex- the two blocks A and B. fl, referred to as the confor- hibited by entangled polymer melts, and given a value mational asymmetry parameter, is defined from the for Me, or a related quantity, the reptation theories ratio of the radius of gyration, Na=/6, to the volume of stemming from the pioneering work of de Gennes the chain, Nv: (1971) and Doi and Edwards (1978) can predict a wide f12 = a=/6v (3) range of rheological behavior (Macosko, 1994). Rep- tation as a physical mechanism has been vividly ob- and provides a measure of the eonformational vs served in striking experiments by Chu and co-workers volume-filling behavior of a polymer. (Perkins et al., 1994), However, there has never been Fluctuation effects, parametrized by N, dictate the a reliable a priori predictor of Me. It now appears that appearance of new phases for lower molecular weight Fetters et al. (1994) have discovered one. polymers in the WSL. As fluctuation effects increase, Their model is predicated on the idea that there is the ordered system will be driven toward increasingly a relation between the sizes of the polymer coils and isotropic states, such as the cubic bicontinuous states the degree to which they are entangled with one that have recently been identified. This relieves some another. The essence of their model is that, the larger of the packing frustration (Seddon, 1990) experienced the dimensions of a chain, the greater the volume it by low molecular weight polymers and amphiphiles sweeps out, and the greater the number of other in forming an ordered phase. Conformational asym- chains it will encounter and with which it might en- metry effects, parametrized by e, play off against the tangle. This requires knowledge of the volume a chain compositional asymmetry of fin dictating interfacial occupies (given by M/p, where p is the polymer den- curvature and therefore the morphology of micro- sity) and also the volume pervaded or spanned, I/sp, phase separation. A composition off> 0.5 tending by the random walk chain estimated as being propor- toward a spontaneous curvature that might lead to- tional to the three-halves power of the radius of gyra- ward a cylindrical morphology can be neutralized by tion of the chains. From these quantities, Fetters et al. a balancing conformational asymmetry, e > 1, which (1994) estimate Me by determining the molecular means that the compositional majority component of weight at which the pervaded volume of a chain is just the copolymer also adopts a more compact radius of large enough to permit there to be, on average, within Chemical engineering of polymers 4135 the unoccupied space within the pervaded volume, polymers (Kannan et al., 1993) or block copolymers just two full chains (it takes two to entangle). This (Koppi et al., 1992, 1993), the opportunities for and model involves the same conformational and packing susceptibility to processing via external fields goes up. parameters that were discussed in connection with The growing activity in a variety of chemical engineer- block copolymers, and when these ideas are properly ing groups on the shearing of ordered block assembled, the resulting prediction is that copolymers provides good examples of the research needs and opportunities in this area. Me = Kpp 3 (4) Work in the area of shearing diblock copolymers where K is a numerical constant that can be esti- started (Koppi et al., 1992, 1993; Albalak and mated, and p = fl-2, which gives the conformational Thomas, 1993) with the objectives of preparing sam- parameter the dimensions of length, sometimes ples where the small-scale domains could be induced termed a packing length (Witten et al., 1989). Since to form a uniform large-scale, sample-spanning many models of rheological properties are cast in orientation of the "crystallographic" planes of the terms of Me, this leads immediately to property pre- microstructure, and examining structure-property re- dictions such as the plateau modulus: lationships in these materials. Single domain samples can be used more readily for structure determination G ° ~ p/Me ~ p-3. (5) and for studying the anisotropy of properties, such as diffusion (Balsara et al., 1991). We use the language of Figure 10 shows how well this works for about two crystallography here since these efforts parallel devel- dozen different polymers. opments of several decades ago in metallurgy in look- It appears that these relationships apply to a very ing at multigrain materials vs single crystal materials broad range of linear polymers, from rubbers to and deducing the effects of grain boundaries on prop- engineering thermoplastics. They should greatly erties. We view these efforts in part as an opening of assist the a priori prediction of the properties of new the important, and largely untouched, field of under- polymers by polymer modeling software packages. standing the defect structure, and its implications, in Accurate prediction of chain conformations leads im- microstructured polymers. mediately to viscoelastic property predictions. Fur- The ordered microstructures of block copolymer thermore, this packing length parameter is already melts, and various other types of soft condensed appearing in other contexts, such as the packing matter lead to a variety of special properties. In gen- length parameter is already appearing in other con- eral, the long-time relaxation behavior of ordered texts, such as the packing of interfacial chains tethered block copolymers follows neither a liquid-like ter- to an interface (Witten et al., 1989), where packing minal response (i.e. G' ~ to2 and G" ~ to) nor a truly interactions result in chain stretching in the direction solid-like behavior (i.e. G' independent of to) (Bates, normal to the interface and give rise to a range of 1984; Rosedale and Bates, 1990). Even when the interesting properties (Halperin et al., 1992). rheological properties of the polymer chains compris- ing each block are similar, the presence of composi- Effects of external fields on microstructured polymers tion gradients at the microdomain interfaces can lead Control of molecular orientation has always been to complex dynamics, including highly non-Newto- a key component in successful polymer processing nian and nonlinear rheological properties. Defects operations. Particular examples include fibers and within the ordered structure of copolymer melts can films where strong uniaxial and biaxial extensional also affect the relaxation behavior, and the polycrys- flows are essential to the development of good proper- talline nature of quench-ordered materials further ties during processing. When the material contains complicated the dynamic response of these materials. larger scale, and possibly more complex and delicate, Techniques for producing monodomain, "single internal microstructures, such as liquid crystalline crystal" samples have been known for over 20 years (Keller et al., 1970; Hadziioannou et al., 1979) but a surprising array of new features and fruitful lines of

2.5 research have been uncovered in the last three years. First, it has been found that lamellar-forming micro- 2.0 y structures is simple shear can be formed into single crystal samples with very few defects (Koppi et al., 1992, 1993; Winey et al., 1993); however, the orienta- o~ 1.0 tion of the lamellae can be either parallel or perpen- O.5 dicular to the planes of shear, depending on the material and on the temperature and deformation 0.0 0.05 0.1 0.15 0.2 parameters (strain and strain rate). A variety of inter- esting ideas and methods are being pursued currently to understand the orientation selection mechanism in Fig. 10. Linear plot of plateau modulus vs inverse cube of both lamellar and cylindrical samples (Morrison et al., the packing length, testing eq. (5). Adapted from Fetters et al. 1993; Kannan and Kornfield, 1994; Tepe et al., 1995). (1994). The theory for these phenomena is only developed to 4136 M. TIRRELL a very basic level (Fredrickson, 1994). There are paral- view of adsorbed amount and chain configurations on lels between these observation and phenomenon oc- surfaces (Amiel et al., 1995; Watanabe and Tirrell, curring in sheared suspensions of particles (Chen 1993). et al., 1992). This is not the place for a thorough review of all the A second new feature, that may have even more possible research opportunities in polymer surfaces fundamental and practical significance, is that the and interfaces. However, some general commentary is application of shear has been shown to affect the in order since the potential for significant engineering order-disorder transition temperatures rather research in this area is enormous. What is most strongly (Koppi et al., 1993; Bates et al., 1994a). The needed is a shift in emphasis from the recent concen- basic reason for this appears to be the effect of shear tration on structure toward a focus on the properties on the fluctuation spectrum in the material (Cates and of polymer surfaces. Engineers need to take up the Milner, 1989); fluctuations tend to depress the order- direction of the development of surface structure- ing transition temperature, since fluctuations screen property relationships in polymers. interactions among dissimilar segments (Fredrickson An excellent example of this line of work is in the and Helfand, 1988). Shearing suppresses fluctuations study of the adhesiveness of polymer interfaces. There and their role in depressing the transition; thus, the is currently no fundamental theory from which one ordering transition temperature rises. can reliably predict the adhesion at a polymer inter- More generally, shearing of these materials is face. There is no parallel to out understanding of bulk a prime example of the effect and utility of a external structure-property relationships. This endeavor is field on the properties and processing of polymers. challenging because adhesion is a complex property Magnetic and electric field poling of liquid crystalline, combining rheology, fracture mechanics and surface optoelectronic and nonlinear optical polymers is an science. Brown (1991, 1993, 1994) has summarized equally rich and important field of chemical engineer- many of the important issues. Some current attention ing research (Knoll, 1993). Another general lesson is being focused on the adhesion at the interface be- from examining the opportunities in this field is the tween a cross-linked elastomer and a solid surface need for continuing work on the rheology and related with linear chains end-anchored to the solid. The aim dynamic properties of polymers with new molecular is to pose well-defined questions on model systems architectures. No engineering discipline is better pre- where the structure is both simple enough to model pared to master the delicate interplay of effects neces- and readily determinable in sufficient detail experi- sary to control the processing of organic materials mentally. The basic goal in this research is to under- than chemical engineering. stand how the number of length of polymer chains attached to the surface enhance the adhesion by pen- Polymer surfaces and interfaces etration into the network. Simple theory predicts lin- Surfaces are the regions through which materials earity in adhesion enhancement with both variables connect and interact with their surroundings. Trans- (RaphaEl and de Gennes, 1992). Experiments show mission of stress, adhesion, friction, abrasion, per- (Deruelle et al., 1995) that adhesion is indeed en- meability to gases and liquids, compatibility with hanced by polymer grafting but there is a maximum biological or harsh corrosive environments are all with increasing density. This problem is currently properties of polymeric materials that are dominated being dissected by detailed study of the degree of by the structure and characteristics of that portion of interpenetration between the layer and the elastomer. the material that finds itself within tens of nanometers Other important problems in this surface struc- of the surface. All multicomponent polymers have an ture-property vein include: studies of shearing and important internal structure comprising interfaces other dynamic phenomena in this polymer films (Van that determine their properties, such as the rheology Alsten and Granick, 1990) and near surfaces (Chak- discussed in the preceding section. Furthermore, some raborty and Adriani, 1992), leading to enhanced basic components of multicomponent polymers may segre- understanding of friction; microscopic probes of ad- gate preferentially to the external surfaces of the ma- hesion, aiming to break down adhesion phenomena terial and therefore exert an influence on surface prop- into their rheological and surface chemistry compo- erties disproportionate to their bulk composition nents (Mangipudi et al., 1994, 1995); wall slip, and (Parsonage and Tirrell, 1993). Much of modern poly- related flow instabilities in exit flows of polymer fluids mer engineering research is dominated by surface and (Denn, 1980); biocompatibility, and learning to make interfacial issues. polymers that are desionedfor, rather than adapted to, Tremendous advances have been made in the last contact with tissue. decade in the instrumental methods to examine poly- mer surfaces and interfaces (Parsonage and Tirrell, Instrumentation to advance polymer science and 1993; Stamm, 1992). It is now possible to determine engineering compositions and molecular ordering of polymers Significant advances in characterization methods near surfaces with depth and lateral resolutions of the have always paralleled the development of new poly- order of nanometers in the most favourable cases. meric materials. This is a very germane point for Adsorption of polymers at interfaces between solids chemical engineers, related to our discussion in the and polymer fluids can be followed from the point of Introduction. One can only control a process when Chemical engineering of polymers 4137 the desired outputs are well-measured or estimated. than proportional amount of unprecedented informa- Generally speaking, with polymeric products, these tion. can often be difficult-to-measure structural features or New imaging methods are likely to be the dominant end-use properties. Instrumentation must advance as innovations in both solid-state and surface character- new applications are moved forward. The Stein Com- ization. Among these will be scanning probe micro- mittee (National Research Council, 1994) categorizes scopies, of which scanning tunneling microscopy and the recent and potential breakthroughs in instrumen- atomic force microscopy are the first examples; op- tal methods of polymer characterization into five tical, magnetic, electric field and ionic versions are main target areas: molecular characterization of the in various stages of development and realization. architecture of single polymer chains; characterization Micromechanical analysis, combining deformation of the architecture of single polymer chains; character- with in situ microscopy (Michler, 1995), will be in- ization of solutions, melts and elastomers, especially creasingly important tools to probe complex molecu- in their dynamic properties; solid-state structure and lar architectures. properties; surfaces and interfaces; and characteriza- tion of biopolymers. Only a few highlights of needs and opportunities for chemical engineers will be men- PROCESSING POLYMERS FOR HIGH PERFORMANCE tioned here. The growth of polymers, both in volume and in Related to the molecular characterization'area, the number of uses, is in part related to their ease in Polymer Reactions section of this paper discussed processing. Means of processing are many and varied, many ways in which seemingly subtle manipulations tailored to their applications. The Stein Committee of feature such as placement of chain branching, se- Report (National Research Council, 1994) has quence distribution and stereoregularity can be of pointed to two broad areas as most significant for major technological importance for polymer prod- future advanced technology applications of polymers: ucts. Better means, and faster means (for process con- health, medicine and biotechnology; and information trol), to measure these quantities and couple the and communications. These applications in turn sug- measurements with production are very much needed. gest the importance of certain types of processing: NMR and other high-speed, high-resolution spectro- surface modification and the development of soft, scopies seem most promising. Molecular weight deter- composite materials for biomedical applications; pre- mination, even for relatively high molecular weight cise control of molecular orientation for optoelec- polymers, is on the verge of becoming a nearly exact tronic polymers, of growing importance in informa- methodology with the continuously increasing capa- tion and communications. bility of matrix-assisted laser desorption ionization There are other drivers of polymer processing tech- time of flight mass spectrometry (MALDI-TOF). Pre- nology beyond applications. Effective, new processing cise molecular weights up to 100,000 have been deter- methods are required for each of the new molecular mined along with more subtle, important features architectures discussed in previous sections if they are such as simultaneous end-group analysis (Belu et al., to become of real practical significance. New architec- 1994). tures require new means of construction; the processes Molecular rheology is a field with a rich tradition, will remain inextricably part of the product. Multi- and an important future, in chemical engineering. The component polymer blends are a rich example of this relation between molecular level structure in solu- close relationship between processing and product. tions, melts and elastomers, and their deformation Polymer blends constitute over 30% of all plastics and flow characteristics is essential to control pro- sold and this fraction continues to increase. The ma- perly polymer processing operations such as injection terials produced include high impact resistant mater- molding, fiber spinning or extrusion. Perhaps most ials, thermoplastic elastomers, polymers modified for worthy of developmental time and energy expenditure ease of processing and blends created for the purposes are methods that couple flow with on-line optical, of plastics recycling (Hamid and Atiqullah, 1995). scattering or imaging methodologies that operate in Nearly all of these blends are immiscible, yet the real time. Rheo-optics [e.g. Kannan and Kornfield resulting two-phase morphologies, if stable, can pro- (1994)1, on-line neutron [e.g. Koppi et al. (1992, 1993)] duce desirable properties (Utracki, 1989). The most or X-ray scattering, flow ellipsometry, and other re- common method for creating new polymer blends is lated methods yet to be conceived, are all worthy of to disperse one thermoplastic melt into another via development to the maximum extent possible, which a twin screw extruder. In these blends, the minor will not be adequate until we can see all essential fine phase must be reduced in size to critical scale, typi- molecular details of structure and orientation, on-line cally microns, and stabilized against coalescence, that in real processing operations. There is great potential is, the blend must be compatibilized. for interaction here between basic and applied re- Reactive processing is increasingly becoming the search. Some of this capacity will come from instru- optimum method for these purposes. In the broadest mental development; some will come from peripheral sense, reactive processing includes both the synthesis (e.g. fast, multidimensional detectors) and software of high polymers from monomers and the chemical development. Scattering methods with increasingly modification of polymers. To produce compatible intense sources will pay back their costs with a more polymer blends, reactive processing is a particularly 4138 M. TIRRELL effective alternative (Kumpf et al., 1995). Interfacial ance; rather, the current research landscape in poly- agents, typically block copolymers, are necessary for mers has physical phenomena and advanced synthesis several purposes in stabilizing polymer blends: reduc- inextricably connected. Self-assembly, noncovalent tion of interfacial tension, enabling smaller scale dis- synthesis and reactive processing are all, in a sense, persion of droplets; prevention of subsequent coales- physical methods of polymer synthesis. cence by steric stabilization; and mechanical intercon- However, it is not just a rearrangement of priorities nection and augmentation of adhesion between that have marked the last few years. A few qualitat- phases. One of the advantages of reactive processing ively new features have emerged. Precision construc- revolves around the means by which the agent is tion of supermolecular building blocks, the need for delivered to the phase boundary. Mixing in a third macromolecules with very uniform architecture and component, the block copolymer surfactant, to the molecular weight, processing under orienting fields, blend requires additional mixing operations and, on-line monitoring of physical and molecular prop- more importantly has been shown (Nakayama et al., erty development and the continual drive toward new 1993) to be less effective than making the block applications that goes hand-in-hand with new mater- copolymer in situ by having a portion of the blend ials and improved processing are but a few of the components have reactive end groups so that inter- unique aspects of the field of polymer engineering that facial encounters lead to reactions that chemically will engage chemical engineers in intellectual and link the two components into one well-suited surfac- technological leadership for the foreseeable future. tant molecule. Twin screw extruders have gained wide acceptance Acknowledgements--The preparation of this review for conducting reactive processing operations because benefited enormously from wide-ranging discussions of their flexibility in design, effective mixing, pumping with many friends and colleagues. Singular among and heat transfer. They are good current examples of these have been very stimulating discussions with Sam continuous reactors for materials processing, parti- Stupp whose conception of the potential of two-di- cularly for new polymer products, where the volume mensional polymers is embodied in Fig. 5. of product produced may not b e appropriate for other continuous reactor designs. Control of these polymer REFERENCES reactors must develop simultaneously. New analytical Alt, H. G., Milius, W. and Paclackal, S. 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