2394
Langmuir 1991, 7, 2394-2400
Dynamics of Surface Property Change in Response to Changes in Environmental Conditions H. Yasuda,* E. J. Charlson, and E. M. Charlson University of Missouri-Columbia, Columbia, Missouri 65212
T. Yasuda, M. Miyama, and T. Okuno Mukogawa Women's University, Nishinomiya, Japan Received June 28,1990. In Final Form: March 27, 1991
It is commonly known that the characteristics of a material at an interface (interfacial characteristics) are considerably different from the corresponding properties of the same material in the bulk phase (bulk characteristics). There are practically no parameters in common use that describe the changes undergone by a surface that is brought into contact with a new surrounding medium. It is questionable whether the parameters that are used to describe the bulk characteristics of a material can be used to estimate or describe surface phenomena. Using a technique developed to measure surface configurationchanges which occur when polymer films are placed in water, it was possible to measure a transition temperature, T,,at which an abrupt change in the rate of change of surface characteristicswas observed in poly(ethy1ene terephthalate) [PET], Nylon 6, [Nylon] poly(methy1 methacrylate) [PMMA], and polyethylene [PE]. This method involves low-temperature plasma treatment of the polymer to incorporate fluorine as a labeling atom in the surface of the polymer. The rate of disagpearance of the fluorine from the surface was measured as a function of immersion time. While the bulk glass transition temperatures, Tg, of these polymers vary from -125 O C (PE) to 105 "C (PMMA), T,,the characteristic temperature for the surface change which occurs on immersion of polymer films, was consistently found to be around 15 OC, which is one of the Drost-Hansen temperatures for vicinal water. These results indicate that the primary driving force for the surface configurationchanges investigated here is the interaction between water and the fluorine atoms which were used as labels. Results also indicate that the changes which occur are due to the type of molecular motion referred to as j3 and/or y transitions, which require no large segmental motion. These changes could take place at temperatures significantly below Tv The meaning of T,is discussed based on the concenpt of the equilibration process between the surface states of water and of the polymer at the interface.
Introduction Surfaces of polymer solids are typically different from those of more rigid materials such as metals, glasses, and ceramics due to the mobility and flexibility of the constitutional units, which are organic macromolecules. Polymer molecules have much greater freedom to rearrange themselves at the surface in order to accommodate a change of chemical potential in the surrounding environment. Consequently, the surface characteristics of polymer solids may vary significantly from the bulk properties of the polymer. Therefore, the interfacial characteristics of a polymer should be treated as a function of the conditions of the environment in which the polymer exists. In our recent studies,*I2a CFI plasma surface labeling technique was used to investigate the kinetic aspects of the changes in surface configuration (details regarding the use of this term will appear later) characteristics when a hydrophobic polymer surface is immersed in water. The change of surface configuration as a result of such a process takes place over a relatively long time and can be expressed by a general equation which describes the phenomenon of diffusing species chemically reacting with a medium A, = (1) where A0 is the value of a parameter that describes a surface property, such as the contact angle of water or the ESCA FI, peak intensity of the surface before immersion. At is the corresponding value of A for a sample which was immersed in water for a time t . (1)Yasuda, T.;Okuno, T.;Yoehida, K.; Yasuda, H. J. Polym. Sci., Port B Polym. Phye. 1988,26,1781. (2) Yasuda, T.; Yoshida, K.; Okuno, T.; Yasuda, H. J . Polym. Sci., Port B Polym. Phye. 1988, 26,2061.
The rate constant, k, can be obtained either from the above relationship or from the initial slope of the plot of fluorine content vs immersion time. This constant can be interpreted as expressing the rate with which the variation of surface characteristics occurs under a specific set of conditions. Thus, the value of k can be used to judge the ease with which such a surface configuration changes in a given polymer. An examination of the k values obtained revealed that (1) the variation of the surface configuration took place at a temperature (25 "C) that is significantly below the glass transition temperature of the polymers (T,> 80 O C for the polymers investigated) and (2) the change is partially reversible, indicating that the change is not due to the loss of labeling moieties. These results suggest that any surface change which occurs in a polymer as a consequence of a change in the surrounding medium cannot necessarily be correlated to the bulk properties of the polymer and that further examination of the basic nature of such changes is needed to explain the phenomenon. In this paper, we wish to present data that show the influence of temperature on changes in dynamic surface characteristics when fluorinelabeled hydrophobic polymers are immersed in water.
Experimental Section In our recent studies,1@2 the CF, plasma surface labeling technique was used to investigate the kinetic aspect of the changes that occurred in surface characteristics when a hydrophobic polymer surface is immersed in water. The experimental procedure involved the following steps: (1)cleaning of sample by washing (storage under vacuum); (2) surface treatment of a polymer by CF, plasma; (3) immersion in water, for a predetermined period of time; (4) immersion in liquid nitrogen, for 1
0743-7463/91/2407-2394$02.50/0 0 1991 American Chemical Society
Langmuir, VoZ. 7, No. 10,1991 2395
Dynamics of Surface Property Change Table I. Samples and Treatments degree of thickcrystalli- ness, polymer film source orientation nity, 96 pm poly(ethy1ene Toray Co. biaxially 50.7 50 terephthalate) drawn (PET) Nylon 6 Toray Gosei undrawn 24.6 75 Film Co. poly(methyl Mihubishi cast film 50 methacrylate) Rayon Co. (undrawn)
Hydrogen
0
Carbon
0
Oxygen
Top View Side View
1
(PMMA) polyethylene (PE)
Nihon Gosei undrawn Film Co. (high density)
(90)
35
h; (5) freeze-drying a t -113 "C for over 1h; (6) conditioning of samples a t test temperature; (7)measurement of surface characteristics of freeze-dried samples. In this study, the identical experimental procedure was followed except the temperature of the immersion water was changed in order to investigate the apparent activation energy of the phenomenon. The water temperature was controlled within 0.3 "C at 0 "C, and within 0.2 "C a t temperatures above 10 "C. Water purified by ion-exchange columns plus reverse osmosis was used for the water immersion process. Materials Used. The polymer films that were studied are listed in Table I. None of the films had surface treatments of any kind. The rate constant k was obtained from the initial slope of the change of the ESCA F1,/CI, ratio as a function of the time of water immersion a t a given temperature. The contact angles of water on untreated and CFr plasma treated surfaces of PMMA and PE are so close that contact angle data cannot be used as a monitor of the surface chemistry. Thus, ESCA data were used in this study.
Theoretical Consideration and Background Information It is generally accepted that the surface of a material is different from the bulk of the same material. There are few parameters that are commonly used to describe differences between bulk and surface properties of a material. With many surface phenomena, such as adsorption and chemical reactions at the surface, it is often assumed that a polymer surface is a rigid and imperturbable plane on which many substances are adsorbed or on which chemical reactions take place. In recent years, it has been gradually recognized that the surfaces of polymers are highly perturbable and that the surface configuration of a polymer changes when the surface is brought into contact with a new surrounding medium, for instance, when a polymer surface is immersed in water.%ll The morphology or conformationalstate of macromoleculesat the interface changes in response to a change of contacting medium. It is important to recognize that this interfacial change may precede any changes that occur in the subsurface layer or in the bulk of the material. Furthermore, the change of surface characteristics is not necessarily a (3) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (4) Baszkin, A.; Nishino, M.; TerMinassian-Sarage, L. J . Colloid Interface Sci. 1976,54, 317.
( 5 )Ratner, B.; Weatherby, P.; Hoffman, A.; Kelly, M.; Scharpen, L. J. Appl. Polym. Sci. 1978,22, 643. (6) Thomas, R.; and Trifilet, R. Macromolecules 1979, 12, 1197. (7) Sung, C.; Hu, C. J. Biomed. Mater. Res. 1979, 13,45. (8) Yasuda, H.; Sharma,A. K.; Yasuda,T. J.Polym. Sci., Polym.Phys. Ed. 1981, 19, 1285. (9) Hanazawa E.; Ishimoto, R. Nippon Sechaku Kyokaishi 1982,18, 247. (10) Hanazawa, E.; Ishimoto, R. Nippon Sechaku Kyokaishi 1983,19, 95. (11) Harttig, H.; Huttinger, K. J. Colloid Interface Sci. 1983,48,520.
1
Bottom View
1
Figure 1. Schematic representation of molecular model of poly(methylene oxide) showing top and bottom views.
consequence of morphologicalor conformational changes of macromolecules. Langmuir, in 1938, made the following observations about surface configuration:12 (1)The wettability of a surface involves only short-range forces and depends primarily on the nature and arrangement of the atoms that form the actual surface and not on the arrangement of the underlying molecular layers. (2) Under special circumstancesa layer at the surface may undergo an almost instantaneous reversal of orientation. (3) Such a change can be prevented by anchoring the reactive groups by forming complexes. Surface Configuration of Polymeric Materials. The surface properties of a polymer, according to Langmuir's statements, are determined by the specific arrangement of atoms at the surface, surface configuration, rather than the configuration of a macromolecule as a whole. Surface configuration refers to the specific spatial arrangement of chemical moieties which are components of a macromoleculewhen they are exposed to the interface in a given environment. It should be distinguished from the bulk configuration of a macromolecule. The concept of surface configuration can be illustrated by the schematic representation of top and bottom views of a molecular model of poly(methy1eneoxide) placed on a plane in such a way that all CH2 groups face down. The chain extends in a line as shown in Figure 1. In the surface configuration of poly(methy1ene oxide) represented by the top view, all oxygen atoms are exposed to the surface, and in the surface configuration represented by the bottom view, all oxygen atoms are hidden below the level of the CH2. In these cases, both the configuration and the conformation of the macromolecule are identical. It is difficult to visualize the difference of surface configuration shown in Figure 1 in a two-dimensional chemical structure. With the identical molecular configuration and conformation, however, the surface configurations shown in the top and bottom views are obviously different. The surface configuration, in this context, is a function of the nature of the environment as well as of temperature. The arrangement of surface atoms under consideration changes ~~
(12) Langmuir, I. Science 1938,87, 493.
Yasuda et al.
2396 Langmuir, Vol. 7, No. 10,1991
by virtue of the change of conformation of macromolecules, but the configuration of macromolecules remains unchanged. The driving force behind the change of surface configuration is the free energy difference a t the interface due to the change of the surrounding medium. Therefore, the rearrangement of atoms at the surface occurs immediately when the surrounding medium is changed. Upon immersion of a polymer film in water, initiation of the diffusion of water and the reconfiguration of surface atoms occur simultaneously. This means that the surface configuration change is not a consequence of the diffusion of water and cannot be correlated to the bulk characteristics of the polymer. While the glass transition temperature is related to the mobility of macromolecules at a given temperature, it cannot be used to predict the onset of a change of surface configuration, since the glass transition is related to large scale segmental motions of macromolecules, whereas the change of surface configuration can be accomplished by motions of much shorter range, such as rotation along the axis of a macromolecule. Therefore, the surface change can occur at a temperature considerably lower than the glass transition temperature of a polymer if the driving force is great enough. The transition associated with the change of surface configuration might be related to transitions that are known to occur below the glass transition temperature. There are many modes of molecular motion that occur at and below the glass transition temperature of a polymer and the transitions associated with those shorter range motions of macromolecules are known by the terms cy, 8, and y transitions, etc., starting from the glass transition temperature, in order of decreasing temperature. It is very likely that the change of surface configuration occurs as a consequence of the molecular motions which result in those sub-glass-temperature transitions. Probably the most dramatic demonstration of the role of surface configuration may be seen in the hydrophilicity of a water-swollen hydrogel. A hydrogel, which contains a large amount of water in a swollen polymer network, often shows very hydrophobic surface characteristics at the air-hydrogel interface in spite of the fact that the polymer had to be highly hydrophilic in order to make such a hydrogel. Holley and Refojo3 explained this transition from the hydrophilicity that exists a t the water-hydrogel interface to the hydrophobicity a t the air-hydrogel interface, which occurs within a fraction of a second when a hydrogel surface is brought into contact with air, as being possible because of the high mobility of macromolecules in a hydrogel. Such a reversal of characteristics is not observed for Agar-Agar hydrogeL8 The surface of Agar-Agar hydrogel in air is always wettable by water because the chemical structure of the polysaccharide consists of hydrophilic functional groups located symmetrically along the axis of the polysaccharide chain. In such a chemical structure, minimal surface configuration change can occur. Thus, the molecular structure of macromolecules (molecular configuration) has an important impact on surface configuration. Stable surface properties can be obtained with macromolecules having chemical structures which permit minimum change in surface configuration. Results and Discussion In order to demonstrate that the change of surface characteristics due to water immersion is not due to the loss of chemical moieties, simple experiments were carried
Table 11. Effect of Water Immersion and Heat Treatment PTFE PVdF sample CAa FICb CA FIC untreated 100.4 1.84 69.8 1.01 immersed HzO 120 min (25 "C) immersed heat treated 10 min (100 "C) a
100.3 100.4
1.82 1.82
64.8 69.3
0.71
0.90
*
Contact angle of water 25 "C. ESCA atomic ratio (Fl8/Cl,).
t
le5
I 0
20
40
60
l h m (-1
Figure 2. ESCA peak ratio FI./C1. versus immersion time for Nylon. The effect of temperature of immersion water on rate of change of surface characteristics is shown. out on polyttetrafluoroethylene) [PTFE] and polytvinylidene fluoride) [PVdF],which have built-in fluorine which functions as a labeling atom. PTFE is symmetric (in the context of surface configuration) along the axis of the polymer chain, while PVdF is asymmetric. T o be consistent with the description of surface configuration discussed above, water immersion should have no effect on PTFE but would show an effect on PVdF. The results shown in Table I1 confirm that the phenomena reported previously on CFr plasma treated samples are not limited to plasma-treated surfaces. The reversible nature of surface configuration change is clearly evident in the data from PVdF. While our intention is to investigate the dependence of k on the temperature of immersing water, the overall influence of water temperature can easily be seen by comparing curves taken a t different temperatures. For this purpose, smoothed curves for Nylon are compared in Figure 2 for the entire time scale of the measurement and in Figure 3 for the early time only. Changes that occur on a relatively long time scale appear to be more complex, and it is not possible to represent an entire curve with a simple mathematical equation. Nevertheless, the influence of water temperature is clearly seen in Figures 2 and 3. The selection of k using the initial slope or k using eq 1,which represents a longer time scale, results in different values of k. However, the same temperature dependence, with respect to the surface transition temperature T,is observed. In this study, the initial slopes (e.g., from Figure 3) are used to calculate the rate constant k and the change of k as a function of the temperature of water. A plot of k versus 1/T is shown in Figure 4. Figure 4 shows conspicuous breaks in the Arrhenius plots for all polymers investigated in this study. The activation energies of the change in surface characteristics for these polymers are tabulated in Table 111. The temperature at which the change of slope occurs is referred to as T,.
Langmuir, Vol. 7, No. 10, 1991 2397
Dynamics of Surface Property Change
1
1.4
1.o
1 : O'C 2 : 10%
3 : 20%
0.8
4 : 30% 5 :40.C
'
I
0
80
40
120
TLrW (-1 Figure 3. Initial time period of ESCA peak ratio FI,/CI, versus immersion time for Nylon.
1 : PET 2 : PMMA 3 : PE 4 : NYLON 6 I .1
3.0
3.2
3.4
I/T
3.6
3.8
x103 (I/K)
Figure 4. Plot of k versus l/Tfor PET, PMMA, PE, and Nylon. Table 111. Summary of the Effect of Temperature of Immersion
polymer
T., O C k (25"C) T,,"C E, kcal/mol E', kcal/mol
60-85 PET1' Nylon 617 40-52 PMMA 105 -125 PE
0.74 0.16 0.43 0.48
15.2 15.4 15.2 15.4
8.2 9.7 9.3 12.6
2.2 2.0 2.0 2.3
The results given in Table I11 indicate the following important aspects not known previously: (1)The surface characteristics change due to water immersion occurs rather quickly, even with glassy polymers. The major initial change seems to take place within a couple of minutes. (2) The activation energies observed at temperatures above T,are relatively small; i.e., 8-13 kcal/ mol, and below T,,a nearly constant activation energy of around 2.1 kcal/mol is observed. (3)The T, observed for all four polymers, whose glass transition temperatures vary from -125 to 105 "C,are nearly identical at around 15 "C. The migration of surface moieties, such as those containing fluorine which were used in this study, away from the top surface layer into the inner layer of the surface region or into the bulk phase of the material can be viewed in light of two fundamental factors. The first factor is the mobility of the molecular segments to which the labeling atoms are attached, and the second is the driving force that causes the migration. Activation energies for the permeation of small molecules such as oxygen and carbon dioxide through most polymers are on the order of 5-15 kcal/mol, and for dye molecules (molecularweight around 300 g/mol) activation
energies are about 30 kcal/mol. If surface changes occur due to large scale migration of polymeric segments, it is anticipated that much larger activation energies would be required than were actually observed. Since the change occurs rather rapidly, it is reasonable to conclude that the migration of moieties is achieved primarily by relatively simple motions, such as rotation along the axis of a polymeric molecule, since these types of motion do not require movement of large segments of a polymer. The rates a t which the migration of fluorine-containing moieties takes place are dependent on the mobilities of the polymer molecules to which they are attached. Therefore, the rate constants (k)as well as the activation energies (E)of the processes are dependent on the nature of the polymer. However, the fact that a kink is observed in the temperature dependence of the phenomenon does not necessarily depend on the mobility of the polymers. The kink could be observed if the change in the driving force occurred at a given temperature. Its position on the 1 / T axis could indicate the specific temperature a t which the change in driving force took place. If we assume that the driving force of the phenomenon is the specific interaction between fluorine atoms and water molecules, it can be explained by the change of interaction rate a t T,. There is ample evidence to support the contention that an abrupt change occurs in the interaction rate at 15 "C, as explained below. These results are best illustrated by observing that the surface states of two phases will establish an equilibrium at the interface between the phases. The change of surface characteristics occurs during the process of equilibration of the surface states. This concept is explained as follows. SurfaceStateof a Material. The surface of a material is different from the bulk phase of the material. In the bulk phase the force acting on an atom, a molecule, or a segment of a macromolecule is balanced by a threedimensional interaction with neighboring atoms or molecules. At a surface such a balance is broken by the absence of neighboring elements beyond the plane of the surface. The influence of this imbalance of forces may extend into a portion of the bulk phase. It is this phase which is referred to as the surface state of a material. In this context, a "surface" is a two-dimensional plane, and a "surface state" is a three-dimensional phase. Interfacial phenomena are interpreted by examining the interaction of two surface states. The word "state" is used to describe a state of matter. It is often quoted (Crook's statement) that plasma is the fourth state of matter (according to increasing order of kinetic energy). The term "state" is used in this paper in precisely the same context. It is proposed, however, that plasma is considered to be the zeroth state of matter (by reversing the order of kinetic energy) and that surface states occur between different states of matter, which would make surface states the fourth state of matter. This situation is schematically depicted in Figure 5. At an interface between two states, e.g., liquid and solid, there exist two surface states, one in the solid and another in the liquid. These two surface states are in equilibrium. It is of interest to determine the thickness of the surface region, however there is probably no clear boundary between the surface region and the bulk material. The influence of the surface region on some characteristics of a material may penetrate deeper than that on some other characteristics. Our recent study of contact electrification of polymer sheets coated with ultrathin layers of plasma polymers indicates that the contact electrification phenomenon becomes independent of the substrate material
Yasuda et al.
2398 Langmuir, Vol. 7, No. 10,1991 B
U
\ \ r J A
B
4. Surface State
Figure 5. The states of matter.
at a thickness on the order of approximately 150 nm. In other words, the thickness of the surface state region observed by contact electrification measurements is roughly 150 nm. Equilibration of Surface States at an Interface. The changes in the surface state of a polymer that occur when the surrounding medium is abruptly changed might be described as the process by which surface state equilibration is reached at an interface. When contact is established between two different materials, the surface states of the contacting materials begin to equilibrate. In the case of a polymer immersed in water, the surface state of the polymer and the surface state of the water approach equilibrium. The concept of equilibration of surface states at an interface may be illustrated by the case in which the two contacting phases are solids. In such a case, the energy levels of the surface state electrons can be used to explain the surface state equilibration which occurs on contact. The energy levels of the surface state of a solid can be illustrated by using the model for a semiconductor. Although there is some question as to whether free electrons exist in a polymer solid, by considering a polymer as a semiconductor with a large band gap and also by assigning quantized energy levels within the bangap to "surface state electrons", equilibration can be represented as a process involving electron transfer between two solid surfaces. Figure 6 depicts the surface state electrons of two surfaces and the transfer of electrons when a contact is established between the two surfaces. When two surfaces are separated, each surface retains the equilibrium surface electrons which it acquired as a result of the contact. Consequently, the surface that donates electrons becomes positively charged, and the surface that receives electrons becomes negatively charged. This is the principle by which static charges are built up on surfaces due to contact and subsequent separation. Transfer of electrons can be measured by completing a circuit and measuring the current that flows on contact and separation. A typical result from our recent study on contact electrification is shown in Figure 7. The time constants associated with the current peaks can be adjusted by inserting a series resistance in the measurement circuit. In real time, the equilibration of surface state electrons occurs instantaneously. The plasma polymer of tetrafluoroethylene was deposited on two different substrates, Nylon film and polished brass, and the contact and separation currents were measured with a brass probe (reference surface). The time scale of the plots has been shifted so that the contact and separation currents are paired for each sample and the two different substrates can be distinguished. The thickness of the plasma polymer
A
J \ r
B
+ -1 .# +
Figure 6. Surface state electron transfer when contact is established between two materials. CURRENT (AMPERES) 1.2E.11 I SEPARATION CURRENT
0
-1E.11
I 0
m 20
LO
CONTACTCURRENT
BO
80
100
120
TIME (ARB)
Figure 7. Currenta resulting from contact and subsequent separation of a brass probe and Nylon 6 treated with plasma polymerized TFE and braas coatedwith plasma polymerized TFE.
deposition is roughly 140nm. The data presented in Figure 7 clearly show that the surface state equilibration illustrated in Figure 6 occurs on metal-polymer contact. It is also important to note that, in this range of thickness, the plasma polymer surface shows similar behavior regardless of the substrate material on which it is deposited. This implies that the thickness of the surface state of a polymer is less than the thickness of the plasma polymer applied in this case. The overall equilibration of the surface state takes a much longer time, since there are a number of phenomena
Dynamics of Surface Property Change Table IV. Changes of Nylon 6 Surface Observed by Contact Current and ESCA FI.Peak Intensity. sample FI,/CI, contact current, pA 0 289 untreated CF4 plasma treated 2.04 -470 CF4 plasma treated, water 1.21 -397 immersed CF4 plasma treated, water 1.9 -490 immersed, heat treated a Water immersion at 25 O C for 1h. Heat treatment at 100 "C for 10 min. Contact current measured against a platinum reference, in air, at room temperature.
that contribute to the total process. For instance, the interdiffusion of atoms or molecules occurs on a much longer time scale. The change of surface configuration is another of the phenomena which occurs as a part of the equilibration of the two surface states. Nevertheless, the concept of surface state equilibration on contact is clearly demonstrated by the transfer of electrons shown in Figure 6. It is interesting to note that the change of surface characteristics due to the water immersion and subsequent heat treatment described in Table I can also be observed by measuring the surface contact electrification current as is shown in the following example. Nylon is one of the most positive materials in the triboelectric series. In other words, the surface of Nylon, when it establishes contact with another material, will charge positively. Fluorine is one of the most negative and electrophilic atoms. Surface treatment by the CF4 plasma employed in this study will alter the energy levels of the surface states of the treated films. The treated Nylon film becomes more electrophilic and a shift of ita position in the triboelectric series is observed. The results of ESCA F1,peak counts and the contact current measured against a platinum reference plate are summarized in Table IV. These results clearly indicate the same trend found in Table 11. Change of the Surface State of Water at 15 "C. The concept of a surface state, usually understood to be applicable to solids, can be extended to the liquid phase. In most liquids, in which the mobility of molecules is high and the tendency to form any kind of association is minimal, the surface state might be reduced to an atomic layer. However, in the case of water, which has an exceptionally high cohesive energy density, the surface state may extend to such a significent distance from the interface with a solid that the surface state of water could be considered in the same manner as the surface state of a solid. It is important to note that there is much evidence to support the concept of the existence of surface states in water and their importance in interfacial phenomena, particularly in biological phenomena.13-16 It has been recognized that the properties of water adjacent to most solid interfaces differ from the corresponding bulk properties. Hence it has been interpreted that the structure of the interfacial water (water in the surface state in contact with a solid surface) differs from the structure of bulk water. Such "surface modified" water is referred to as uicinal water by Etzler and Drost(13) Pethig, R. Dielectric and Electronic Properties of Biological Materials; John Wiley and Sons, Ltd.: New York, 1979. (14) Droet-Hansen, Walter The Effectson Biologic Systems of HighOrder Phase Tramitions in Water;New York Academy of ScienceAnnals; New York Academy of Science: New York, 1965; Vol. 125, pp 471-501. (15) Etzler, F. M.;Conners, J. J. Langmuir 1990, 6, 1250. (16) Drost-Hansen, W.; Etzler, F. M.Langmuir 1989,5, 1439.
Langmuir, Vol. 7, No. 10, 1991 2399
Hansen.l7 The structure of "vicinal water" has been investigated in recent years. It was recognized that many physical properties such as density, viscosity, heat capacity, and ion selectivity differ significantly from those of bulk water. It appears that the vicinal water structure decays in an approximately exponential manner and that significant vicinal structuring extends 3-6 nm from the surface.18 An important transition temperature of vicinal water is 15 "C, and 15 OC is referred to as the DrostHansen temperature by some investigators. I t is important to point out that these observations and statements refer to vicinal water a t the interface with biological cells, while our attention is focused on the surface state of polymers in contact with water. The results of our study indicate that it is logical to consider that the surface state of water as well as the surface states of polymers are different from the corresponding bulk states. Changes in the surface state of water at temperatures below 15 "C observed in biological systems are very likely to be observed at the interface with other solid surfaces, as evidenced by the data presented in this paper. According to the concept of surface configuration described earlier, the main driving force that causes the migration of fluorine containing moieties out of the interface is believed to be the interaction between water molecules and fluorine atoms. If the magnitude of such an interaction were reduced due to a structural change in the vicinal water a t acertain temperature, it could certainly result in a reduction in the driving force for the migration. Because this change is due to changes in the water but not in the polymer, the kink observed in the log k - l / T plots is independent of the nature of the polymers. Conclusion (1)The change of surface configuration due to a change of surrounding medium is the main reason why surface characteristics change upon water immersion. (2) The nature and extent of the change of surface characteristics that occur in a polymer due to a change of surrounding medium (an interfacial phenomenon) cannot be explained or predicted by parameters that describe bulk characteristics such as the glass transition temperature of a polymer. (3) The migration of moieties, which is responsible for the surface change, requires a relatively small amount of energy. It is therefore very likely that the rotational migration of moieties, rather than large polymer segments, is the major mode of migration. (4) The apparent transition temperature T,a t which an abrupt change in the rate of change of surface configuration occurs, as a consequence of molecular motion, is below the glass transition temperature of many glassy polymers. (5) The results obtained for PET, Nylon, PMMA, and PE, all of which were plasma treated with CF4 in this study, strongly suggest that the major factor which influences surface changes is the interaction of water and the fluorine atoms used as labeling atoms on the surface. The repulsive force between water molecules and fluorine atoms, in the case of the fluorine-labeled samples used in this study, can be considered as the major driving force of the surface changes observed. (17) Etzler,FrankM.;Droet-Hansen, W. Ahlefor WaterinBiological Rate Processes. In Cell-Associated Water; Droet-Hansen, W., Clegg, James S., Eds.; Academic Press: New York, 1979. (18)Etzler, F. M.Langmuir 1988, 4,878.
2400 Langmuir, Vol. 7, No.10, 1991
(6) The fact that nearly identical *surface transition temperatures" were observed for four different polymers and that the value of T,observed coincided with the DrostHansen temperature of vicinal water strongly indicates that a change in the rate of interaction between water and the fluorine atoms attached to polymer molecules due to a structural change of the water is responsible for the kink observed in the temperature dependence. (7) The behavior of the surface region of a polymer is a function of the surrounding medium. Changes in the surface properties of a polymer are seen as occurring as the surface state approaches equilibrium, following a change of surrounding medium. (8)Surface properties and interfacial properties should be distinguished.
Yasuda et al. Surface properties are measures of the surface state of a polymer at the polymer/air (vacuum) interface, which is different from the surface state of the polymer at the polymer/water interface. Most surface properties are evaluated at either the polymer/air or polymer/vacuum inteface, but polymers are often used in environments that are significantly different from air or vacuum. This study indicates that interface properties could be significantly different from the surface properties evaluated by most conventional methods. Registry No. PET, 25038-59-9;PMMA, 9011-14-7;PE, 900288-4; HzO, 7732-18-5;Nylon 6,25038-54-4.
