preface, introduction - American Chemical Society

Downloaded by UNIV OF NEBRASKA - LINCOLN on April 10, 2015 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.pr001

PREFACE Since early in this century, water in polymers has been of interest to scientists concerned with living matter and natural polymers. However, the significance and role of water in synthetic polymers and commercial resins have become recognized only recently, with significant initiation of research in this area and in allied fields. The symposium was timely, with objectives (a) to bring together scientists from different parts of the field to exchange and discuss information, (b) to teach those reseachers who are presently active on the boundary of the field, and (c) to bring before the entire group some of the latest thinking in connection with the structure and behavior of water itself. Several divisional cochairmen collected and arranged the papers in the symposium. I should like to thank them: John A. Rupley and Irwin D. Kuntz, Jr. of Division of Biological Chemistry; Daniel F. Caulfield of Cellulose, Paper and Textile Division; Curt Thies of Division of Colloid and Surface Chemistry; Cornells A. J. Hoeve of Division of Organic Coatings and Plastics Chemistry; and P. Anne Hiltner of Division of Polymer Chemistry. My special thanks go to Martin Karplus and John Rupley for consultation and advice and to Frank Stillinger for obligingly and capably filling the void when an ill Henry S. Frank had to cancel his opening paper for the symposium. I thank the Petroleum Research Fund of the ACS for financial assistance granted to this symposium. The expected professional expertise of the participants in quality of research and presentations was appreciated gratefully. The same extends to the preparation of manuscripts for this monograph. The symposium attendees found the papers and program to be highly informative. I hope that the reader, too, will find the monograph instructive, informative, and useful for perspective to methodology, results, and active researchers. The chapters in the monograph are grouped into eight sections to suggest to the reader certain close relationships among the chapters within each section. However, there are other relationships across sections, some of which are brought out in the Introduction. Southern Regional Research Center AR, SEA, USDA New Orleans, Louisiana 70179

STANLEY P. ROWLAND

January 3, 1980 ix

In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.



INTRODUCTION STANLEY P. ROWLAND Southern Regional Research Center, P.O. Box 19687, New Orleans, LA 70179


IRWIN D. KUNTZ, JR. Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94143

For many reasons there is considerable concern for the nature of interactions between water and polymers. However, the two major reasons are: the water-polymer interaction is essential to biological processes, and the water-polymer interaction is often beneficial or detrimental to performance of commercial polymers. These primary concerns are evident throughout the chapters of this monograph, but superimposed atop these is a search for an understanding of the way(s), the driving force(s), and the consequence(s) of the interactions of water with dissolved, swollen or rigid polymeric surfaces. The initial section of the monograph deals with The Structure of Water as evidenced by its character in the supercooled state [1] (numbers in brackets refer to chapters listed in the Table of Contents), as affected by a dipeptide [2], and as affected by a variety of solutes and polymers [3]. One chapter is regrettably missing; Henry S. Frank, a pioneer of stature in studies of the structure of water, became ill before the symposium and was unable to attend or to complete his manuscript. The second section, Perspective: Macro- and Microinteractions of Water and Polymers, consists of three chapters. It covers a brief historical introduction leading to an insight into selected aspects of current thought on interactions of water with model solutes and proteins [4], consideration of water-polymer and ice-polymer interfacial regions [5], and examination of stages in the process of protein hydration assessed by a variety of types of measurements [6], The chapters in sections 1 and 2 provide perspective pertinent to chapters in subsequent sections. Section 3. Proteins: The Mobile Water Phase 4 Chapters Section 4. Proteins: Ordered Water 4 Chapters Section 5. Polysaccharide Interactions with 3 Chapters Water Section 6. Permeation, Transport, and 7 Chapters Ion Selectivity

0-8412-0559-0/ 80/47-127-001 $05.00/0 © 1980 American Chemical Society


2

WATER IN POLYMERS

Section 7. Section 8.

Synthetic Polymers: Water Interactions Synthetic Polymers: Water and Polymer Performance

5 Chapters 5 Chapters


An Overview of Results The Structure of Water. Pertinent to the bound, nonfreezing, or interface water in polymers, a subject of subsequent sections, are the studies of Angell et al. (see réf. in [1]). He shows that pure water at normal pressures cannot be supercooled below -40°C and that virtually all physical properties of water point to a "lambda anomaly" at -45°C [1], The fact that surfaces disrupt the natural order of a bulk phase serves as basis for Stillinger's attention to supercooling of water in small droplets or clusters [1]. His statistical mechanical approach to the structure of ice Ih and water as hydrogen-bonded in ordered and disordered polygonal structures, respectively, results in a qualitative estimate of the depression of temperature of maximum density. His approach also explains the behavior of supercooled water in terms of structural fluctuations in the bonded bicyclic octameric water network that represents ice Ih. By molecular dynamics simulation of a dilute aqueous solution of an alanine dipeptide, Karplus [2] estimates the influence of the solute on the dynamic properties of water to be limited to the first solvation layer. The different structures of water in the vicinity of polar and nonpolar groups were resolved to bulklike hydrogen bonding involving a reduced number of neighbors in the former case and the absence of hydrogen bonding together with decreased water mobility in the latter case. Overtone infrared spectroscopy described by Luck [3] is an effective means for determining quantitatively the concentrations of water in nonbonded and hydrogen bonded OH groups. Interesting results have been obtained for a variety of situations, including salt solutions, water-organic solvent mixtures, interface effects, organic molecule hydration, and diffusion in polymeric substrates. From such studies, Luck classifies water structure as (a) first shell water hydrate, (b) second shell, disturbed liquid-like water, and (c) liquid-like water. For salt transport in membranes, for diffusion of dyes in fibers, and for life in plant and animal cells, water of types b and c are essential. Protein-Water Interactions. There appears a general agreement that all thermodynamic measurements, such as heat capacity results of Rupley et al. [6] and Hoeve [7] or the NMR data on frozen materials of Bryant [8], indicate that 0.3-0.4 g HÔ/g protein form an unfrozen boundary layer at subzero temperature. The water is primarily associated with polar groups. It seems



INTRODUCTION

Rowland

and Kuntz

3

appropriate to call this layer "bound" water because its properties are demonstrably different from either supercooled water or ice at the same temperature. Is this layer a useful concept in dilute solution? The thermodynamic measurements do not provide a direct answer because one cannot use a nearby phase boundary to make the sharp distinctions possible below the freezing point. One source of information, hydrodynamic measurements, has recently been reviewed by Squire and Himmel Q J . Their analysis suggests large amounts of water associated with globular proteins, but even their restricted data set shows large variations between the amount of hydration calculated from sedimentation or diffusion results. Because the cube of the friction factor enters into the calculation, highly accurate data are required to resolve this matter. The issue of hydrophobic hydration is obscure. Karplus' calculation [2] and much early work do suggest that water near hydrophobic surfaces should have modified properties. To date, the thermodynamic experiments at low temperature and the structural studies (see below) give l i t t l e direct indication of how many such water molecules are present and whether their properties are different from either "bulk" or "bound" water molecules. Several papers deal with the spatial arrangements of water molecules near proteins. The neutron-scattering experiments of Schoenborn [12] set a high standard for careful crystallographic analysis. Refined x-ray studies are also in progress. Ordered water molecules are found hydrogen-bonded to polar sites or to each other in very short chains. Some dozens of such water molecules are observed at high occupancy. The number of water molecules placed by these methods is becoming smaller as the structures become more refined. Hermans [11] suggested that part of the difficulty is the high salt concentration in many crystals. We expect this question to be answered as resolution improves and neutron scattering comes into wider use. On the theoretical side, work discussed by Karplus [2] and by Hermans [11] and previous studies of Hagler and Moult (2j and Stillinger and Rahman (3) make it clear that a similar structural ordering is found with standard potential functions. Although no simulation of a protein solution with internal degrees of freedom for the protein and a large number of water molecules has been conducted, we may expect that similar structural sites will be maintained in solution. Two issues deserve comment. First, there is as yet no evidence for clatherate water structures surrounding globular proteins. Second, there is no sharp "boundary" on structural features. The diffraction experiments encourage us to think of a spectrum of structural sites characterized by an order parameter ranging from highly ordered to completely random distribution over two or three layers of water molecules.



4

WATER IN POLYMERS

It has been difficult to unravel the microscopic energies of interaction between water molecules and a protein surface by direct experiment. Thermodynamic methods on two component sys tems never permit a rigorous molecular dissection of this kind. Infrared and Raman spectroscopy, as described by Luck [3], have some promise. The theoretical attack represented by Stilunger [1], Karplus [2], and Hermans [11] has become increasingly productive. In brief, water-water energies are relatively large for nonbonded interactions and water-protein interactions are roughly comparable in strength, making water-protein surfaces of the "soft-soft" kind in Adamson's nomenclature [5]. A clear hierarch is emerging, with interaction strengths decreasing in the progression: ion-ion > water-ion > water-polar = polar-polar = water-water > water-hydrophobic. The major issues are (a) how good are the current potential functions for quantitative studies, and (b) how should one treat entropy terms that are surely important, i f not crucial, to our final understanding of these systems. Several chapters deal with the dynamics of water molecules near protein surfaces. Recent dielectric experiments by Grigera and Berendsen (4) and Hoeve [7] suggest that low (1 MHz) and intermediate (100 MHz) dielectric dispersions result largely from protein-water interface effects, from protein rotation, and from sidechain reorientation. NMR experiments, represented here by Koenig [9] and Bryant [8], offer clear evidence that water molecule motion reflects both rigid rotation of the protein in solution and some other motion in the nanosecond region (-30°C to +30°C). Lillford's theory [10] of water-proton relaxation con siders compartmentalization and relates complex relaxation to heterogeneous mass distribution down to ^10μ dimensions. The hydrodynamics of coupling the macromolecular motions to the water relaxation processes are currently obscure but are under study. Hundreds, i f not thousands, of water molecules per protein are implicated in the nanosecond motion. The details of the NMR theory are incomplete but pose a serious interpretive issue when taken with the neutron scattering results which report a much smaller number of water molecules strongly affected by protein surface. Of course, there is no fundamental reason that the dyna mics (governed by activation barriers) and the static structure (controlled by the width and depth of the potential wells) must exactly correspond. Extension of molecular dynamics calculations to and beyond the nanosecond region may provide insight into these questions. Polysaccharide-Water Interactions. These polymers exhibit a range of hydrophilicity comparable to that of proteins and in teract with water in many ways similar to those already noted for


INTRODUCTION

Rowland

and Kuntz

5

proteins. The subject is covered here by a diversity of substrates, representing different compositions, solubilities, and crystallinities. Bound water, primarily observed as a nonfreezing component, is reported in two chapters; but by comparison to the value 0.04-0.10 g H 0/g cotton cellulose (5), the unfrozen boundary layer found by Deodhar and Luner [16] for wood pulp is 0.4-0.6 g HÔ/g and that measured by Ikada et al. [17] for mucopolysaccharides is 0.4-0.7 g/g. The former authors associate nonfreezing water in wood pulps with pore size of the cellulose pulp, estimating 40 A as the largest pore that can carry 100% nonfreezing water. Actual pore measurements coupled with various measurements of water-polymer interactions, such as those covered by Rupley et al. [6] would be revealing and are in order on wood pulps and cotton celluloses. The effect of salts in structuring and destructuring water in the pores of wood pulps [16] (see Luck [3]) seems complicated by the necessity for the salt to penetrate small pores to be effective. The salt-free crystalline polysaccharides reviewed by Bluhm et al. [15] are stabilized in characteristic crystalline unit cells by specific amounts of water. Two kinds of locations have been proposed for the water molecules; one is unique, i . e . , the water lies clustered in an existing interstitial cavity between double helices of B-starch. The other has water bound at specific sites within each unit cell. Additional water in this second type expands one or more unit cell dimension. This almost continuous expansion of the unit cell with increasing content of water may represent a more ordered aspect of the same interaction that occurs between water and accessible, disordered surfaces of celluloses crystallites (and other imperfectly crystalline polysaccharides). At an opposite extreme from crystalline water-containing polysaccharides are the mucopolysaccharides described by Ikada et al. [17]. In addition to the nonfreezing water that characterizes these polymers in the solid state, one to three thermal transitions measured on solutions suggest complex interactions with water.


2

Permeation, Transport, and Ion Selectivity. Under this heading, we bring together those compositions that have in common a pore-like structure and that have been studied in this regard in these chapters. Tirrell et al. [21] measured nonfreezing water to the extent of 0.36-0.74 g rÔ/g copolyoxamide membrane, noted that these values are a direct function of surface area, and suggested that bound water exists in bulk polymer and over greater distances than a few molecular diameters on the polymer surfaces. The model developed by Bel fort and Sinai [19] for porous glass predicts that bulk water can form in pores above 27-38 A (see



6

WATER IN POLYMERS

Deodhar et al. [16]), and, therefore, desalting cannot be expected for pores larger than this range. Permeation measured by Kim et al. [20] for hydrophilic solutes in aqueous media through poly(hydroxyethyl methacrylate) (p-HEMA) is consistent with transport by bulk-like water. Permeation is reduced for crosslinked p-HEMA or HEMA copolymers as a result of non-bulk-like water. Southern and Thomas [22] showed that hydrophilic impurities in rubber provide loci for water molecules and account for diffusion of aqueous solutions through this hydrophobic substrate. Breuer et al. [18] developed a mechanism for the swelling of hair from consideration of the contribution that swelling makes to the overall free energy changes accompanying the water absorption. The dominant thermodynamic driving force for water absorption is site binding, i.e., interaction with discrete polar side chains and peptide bonds. But this force is supplemented with capillary condensation and entropie gains from the mixing of water with polymer chains. The mechanism is similar to that described for cellulosic fibers (5). Breuer's mechanism with minor modifications might be applied to other highly hydrophilic polymers in this section. From studies of the selectivity of porous synthetic ion exchange resins, Marinsky et al. [23] concluded that differences in excess free energy from ion-solvent interactions contribute most importantly to differences in affinity of ion pairs for the resin and, therefore, to ion selectivity. In studies of a completely different type of porous structure, Lipshitz and Etheredge [24] showed that articular cartilage is anisotropic in flow of interstitial fluid and that its properties are a function of the impedence to flow during and following compression. Water Interactions with Bulk Polymers and Resins. Starkweather [25] describes the water first absorbed in nylon 66 as most tightly bound and as nonfreezing. Bretz et al. [32] refer to tightly bound water at the level of 0.02-0.03 g/g nylon 66, but it may approach 0.08 g/g [25]. The water that is absorbed at low levels and that is uniformly distributed in such relatively hydrophobic polymers seems to fall in this category. This behavior appears to be one of the ways in which water is absorbed near polar groups in amorphous regions of partially crystalline polymers or throughout amorphous polymers. The spatial arrangement of absorbed water at low partial pressure is proposed to be in hydrogen bonds between two amide units in nylon 66 by Starkweather [25], at hydrogen bonding sites (ester or carboxyl groups) in amorphous acrylic polymers by Brown [26], at polar groups in epoxy resins by Moy and Karasz [30], and in the ionic cluster phases of perfluorosulfonic acid polymers by Pineri et al. [28,29]. Above a characteristic level of water for each polymer, the water appears in a second form, generally termed "clusters"; the terminology is used rather



INTRODUCTION

Rowland

and Kuntz

1

broadly [26]. Clusters may consist of an average of three water molecules bonded at polar sites [25] or a separate phase of water in a less polar polymer [27]. In both nylon 66, studied by Starkweather [25], and highperformance epoxy resin, described by Moy and Karasz [30], the water is believed to hydrogen bond between polar sites in the polymers, acting as crosslinks at low temperatures but plasticizing at elevated temperatures because of the greater thermal mobility of water compared to segments of the polymer chain. Water-induced plasticization of polymers by disruption of intermolecular hydrogen bonding between polymer chains that are generally considered to be hydrophobic is rather common. Moy and Karasz [30] show that the lowering of Tg for an epoxy-diamine resin is proportional to the amount of water in the system. Johnson et al. [27] report that polysulfone and polyvinyl acetate) show enhanced low-temperature β-loss transitions in proportion to the unclustered water. Clustered water in polyvinyl acetate) has no effect on T , although T shifts with increasing amount of unclustered water. Fuzek [3τ] found that water absorbed by synthetic fibers and silk at room temperature and 65% RH substan tially lowers T V s , the effect being reflected in several dif ferent fiber properties. Wet soaking has an additional effect. The action of water in poly(methyl methacrylate) is interpreted by Moore and Flick [33] as one of general plasticizing character. These effects are not limited to synthetic polymers and resins (note silk, above); Scandola and Pezzin [13] note the lowering of Tg of elastin by water and describe other aspects of the inter action that are closely related to the plasticization of poly vinyl chloride) and polycarbonate. The effect of water in polymers and resins is not simple and is not necessarily predictable on the basis of the foregoing comments. Some complexity is indicated by the necessity for a two-parameter sorption isotherm to correlate water sorption [26] and by the antiplasticizing effect of low levels of water at low temperature [25]. The epoxy composites of Π linger and Schneider [34] show large changes following water sorption, suggesting changes in structure of the substrate resulting from the sorption. A more subtle situation appears in the studies of Bretz et al. [32]; as water is imbibed by nylon 66, fatigue crack propagation decreases to about one-fifth and then increases about three fold. The level of water corresponding to maximum fracture energy is 1 H2O/4 amide units, approximately half the value at which sub stantial clustering begins [25]. Comments in the foregoing sections also suggest some of the complexities that might be anticipated in more detailed studies. g

Q

A Final Word In a symposium of this kind, which is oriented toward a phenomenon and whose ideal objective is a broad understanding of


8

WATER IN POLYMERS


the nature and limitations of the phenomenon, it seems that more questions have been raised than answered. However, despite the diversity of polymeric substrates under examination and the wide spectrum of experimental and theoretical approaches, there do appear to be strong threads of commonality in general results and conclusions among many, i f not a l l , of the chapters. We heartily recommend the reading of each and all of the following chapters with the thought that there is much to be learned from each and that one reader may, with additional research, put the phenomenon into perspective and quantitative array. Literature Cited 1.

Squire, P. G.; Himmel, M. E. Arch. Biochem. Biophys., 1979, 196, 165-177.

2.

Hagler, A. T.; Moult, J . Nature, 1978, 272, 222-226.

3.

Stillinger, F. H.; Rahman, A. J . Chem. Phys., 1978, 68, 666-670.

4.

Grigera, J . R.; Berendsen, H. J . Biopolymers, 1979, 18, 47-57.

5.

Rowland, S. P., in "Textile and Paper Chemistry and Technology"; J . C. Arthur, J r . , Ed., Symposium Series No. 49, American Chemical Society, Washington, D.C., 1977, pp. 20-45.

RECEIVED

January 4, 1980.


preface, introduction - American Chemical Society

Recommend Documents