Langmuir 1992,8, 1425-1430
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Dynamics of the Surface Configuration Change of Polymers in Response to Changes in Environmental Conditions. 2. Comparison of Changes in Air and in Liquid Water T. Yasuda and M. Miyama Mukogawa Women's University, Nishinomiya 663, Japan
H. Yasuda* Center for Surface Science & Plasma Technology, University of Missouri-Columbia, Columbia, Missouri 65212 Received October 17,1991. I n Final Form: February 19, 1992 The change of electron spectroscopy for chemical analysis (ESCA) F1$C1, peak area ratios of plasmafluorinated (by CF4 plasma treatment) poly(ethy1ene terephthalate) (PET), Nylon 6, and poly(methy1 methacrylate) (PMMA)samples was observed as a function of time and temperature of dry heat treatment. The results of dry heat treatment are comparedwith the similar results obtained when samples are immersed in water. The decrease of the ratio is observed in both cases which indicates that F atoms disappear from the top surface region of samples as a consequenceof these treatments. The surface configuration changes in response to the changes in the contacting medium are interpreted on the basis of the concept of the surface state of the polymer which tends to equilibrate with a new surrounding medium. Arrhenius plots of the rate constants of such decays showed conspicuous breaks at characteristic surface transition temperature T,. In the case of dry heat treatment, T,represents the change of mobility of a polymer and corresponds to T of the polymer. In the case of water immersion, T,represents the change of driving force due to the change in basic properties of water in contact with a surface (vicinal water) at the major Drost-Hansen temperature at 15 OC and is independent of Tgof the polymer. The activation energies for the surface configuration change in the dry heat treatment are found to be greater than those for the water immersion case; however, the differences are relatively small. Results indicate that the surface configuration changes do not require large segmental motions or migration of macromolecules. Motions responsible for surface configuration changescould be well perceived as relatively small rotational motions such as the ones seen in sub-8 transition temperatures. Therefore, depending on the nature of the driving force, a significantextent of the surface configuration change can take place at temperatures substantially below Tgof the polymer.
Introduction It was first pointed out by Langmuir that the surface properties of a solid are determined by the surface configuration rather than the configuration of molecules which constitute the surface.' In other words, the factor which determines whether a solid surface is hydrophilic or hydrophobic is not the presence or absence of hydrophilic moieties in the molecules which constitute the surface, but whether hydrophilic moieties are actually located at the surface or not. The original work of Langmuir was concerned with the orientation of soap molecules, which have a distinctively hydrophilic head with a rather long hydrophobic tail, when they were placed on a solid surface such as a glass plate. The principle also applies to polymer surfaces which consist of macromolecules with a variety of moieties within a chain that contribute to the overall characteristics of s ~ r f a c e s . ~ - l ~ (1) Langmuir, I. Science 1938,87, 493. (2) Ray, B.R.; Anderson, J. R.; Scholz, J. J. J.Phys. Chem. 1958,62, 1220. (3) Cuthrell, R. E. J. Appl. Polym. Sci. 1967, 11, 1495. (4) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. ~~~~
(6)Baezkin, A.; Nishino, M.; Ter Minaesian-Saraga, L. J. Colloid Interface Sci. 1976,54, 317. (6) Yasuda, H.; Sharma,A. K.; Yasuda,T. J.Polym. Sci.,Polym.Phys. Ed. 1981,19,1285. (7)Gagnon, D. R.; McCarthy, T. J. J.Appl. Polym. Sci. 1984,29,4335. (8)Lavielle, L.; Schultz, J. J. Colloid Interface Sci. 1985, 106, 438. (9) Ruckenstein, E.: Gourisankar. S. V. J.Colloid Interface Sci. 1985. 107,488. (10) Ikada, Y .;Mataunage,T.; Suzuki, M. Nippon Kagaku Kaishi 1985, 1079. (11) Tam, Y.; Takaoka, K. Kobunshi Ronbunshu 1986, 43,361.
In the case of a polymeric solid, the surface characteristics will be determined by how macromolecules orient themselves a t the surface, and consequently the overall surface properties cannot be simply predicted by the configuration of the macromolecule alone. A good example may be seen in the surface hydrophilicity of a synthetic hydrogel" or of a gelatin gel6 at the gel/air interface. The contact angle of water on the surface of a gelatin gel which contains more than 90% water is often more than 90°, indicating that the surface is very hydrophobic. In such a case, most hydrophilic moieties are oriented toward the bulk phase of agel, which contains a large amount of water, and the gel/air interface is populated with rather hydrophobic moieties of gelatin molecules. The motions of moieties seen in the example of a gelatin gel may be described as rotational motions along the axis of a polymeric chain. In our recent studies,'"'7 the CFI plasma surface labeling technique was used to investigate the kinetic aspect of the changes in surface properties which occurred when a CF4 plasma treated (hydrophobic) polymer surface is immersed in water. The fluorine atoms introduced by the plasma treatment were used to fogow (12) Holmes-Farley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G. M. Langmuir 1987,3, 799. (13) Takahara, A.; Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1988,21, 2443. (14) Cross, E. M.; McCarthy, T. J. Macromolecules 1990, 23, 3916. (15) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. J. Polym. Sci., Part B: Polym. Phys. 1988,26, 1781. (16) Yasuda, T.; Yoshida, K.; Okuno, T.; Yasuda, H. J. Polym. Sci., Part B Polym. Phys. 1988,26, 2061. (17) Yasuda,H.;Charlson,E. J.;Charlson,E.M.;Yasuda,T.;Miyama, M.; Okuno, T. Langmuir 1991, 7, 2394.
0743-7463/92/2408-1425$03.0Q~Q 0 1992 American Chemical Society
Yasuda et al.
1426 Langmuir, Vol. 8,No. 5, 1992 the disappearance of fluorine atoms from the surface on water immersion by electron spectroscopy for chemical analysis (ESCA) of surfaces. The experimental procedure involved the following steps: (1)cleaning of the sample by washing (storage in vacuum), (2)surface treatment of a polymer by CFq plasma, (3)immersion in water, for a predetermined period of time, (4)immersion in liquid nitrogen, for 1 h, to freeze the motion of moieties near the surface, ( 5 ) freeze drying at -113 O C for over 1h, and (6)ESCA analysis of freeze-dried samples. In a recent study (part l),I7 an attempt was made to investigate the effect of temperature on such rotational motions. Several noteworthy observations were reported in the paper. The following seem to be of particular importance. (1)High degrees ofmobilities of surface molecules, which allow the surface configuration change to occur in response to the change in the surrounding medium, were observed at temperatures substantially below the glass transition temperature of polymers. (2)Plots of rate constants for disappearance of ESCA F1$C1, peak area ratios against 1 / T showed conspicuous breaks, indicating the presence of characteristic transition temperatures for such surface configuration changes. (3) These transition temperatures were found to be around 15OC regardless of the types and Tgof the polymers investigated. (4)The transition temperature for the surface configuration change, T,, (observed for all polymers at around 15 "C) coincides with the major Drost-Hansen temperature for the change of structural characteristics of water in contact with a surface (vicinal water). It has been known for years that many physical properties of water in contact with a surface, which is termed as vicinal water, change at several transition temperatures known as Drost-Hansen temperatures.ls A major Drost-Hansen temperature exists at around 15 "C. The results were explained by the concept of the equilibration of surface states. According to this concept, when a polymer surface, of which the surface state was previously in equilibrium with a surrounding medium (often air), is brought into a contact with a new surrounding medium, the surface-state equilibration process occurs in order to establish a new equilibrium between the surface state of the polymer with (the surface state of) the new surrounding medium. When a new surrounding medium is a polymer, the contact electrification (static charge creation) occurs as a consequence of the equilibrium process to establish an equilibrium energy level of surface-state electrons. In such a case, minimal or no surface configuration change is seen. When the new surrounding medium is liquid water, the equilibration process between the surface state of the polymer and the surface state of water which has been recognized as 'vicinal water" occurs. In this case, the change in surface configuration of the polymer occurs in order to minimize the interfacial tension. Such a change is driven by the interaction between water molecules and soxfie functional moieties on the polymer surface. It is postulated that the surface transition temperature observed in water immersion experiments reflects the change in the interaction between a polymer surface and water at the temperature. In other words, the surface transition temperature of the polymer in water immersion (18)Etzler, F. M.; Drost-Hansen, W. A Role for Water in Biological Rate Processes. In Cell-Associated Water;Drost-Hansen, W., Clegg, J. S.,Eds.;Academic Press: New York, 1979.
Table I. Description of Polymer Films degree of crystallinity, thickness, film source orientation % pm PET Toray Co. biaxially 50.7 50 drawn 24.6 75 Nylon 6 Toray Gosei undrawn Film Co. PMMA Mitsubishi cast film 50 Rayon Co. (undrawn)
experiments is observed as the consequence of the change in the driving force of the phenomena rather than the change in the mobility of macromolecules. Thus, such a surface transition temperature is independent of the mobilities of polymers. In order to gain more insight into the role of the mobilities of macromolecules at the surface and of the driving force in the surface configuration changes, the effect of temperature in the absence of water was investigated in this study by employing the similar procedure. Dealing with the change of surface properties with time, four major cases can be considered, i.e., (1)decay of the hydrophilic property in (dry)air or a hydrophobic medium, (2) change of the hydrophilic property in water, (3) decay of the hydrophobic property in water, and (4)decay of the hydrophobic property in (dry) air or a hydrophobic medium. Most attention has been focused in case 1. The increased rate of change was recognized at a higher temperature, and the higher rate has been attributed to a higher mobility of polymer segments. The effect of the glass transition temperature, TB, of polymers has been recognized, but only in a qualitative manner. We present an overall picture of the dynamics of surface configuration changes in response to changes in environmental conditions by comparing the correspondingchanges in cases 3 and 4 mentioned above.
Experimental Section In this study, the similar technique and procedure developed in the previous study1"17 were followed except the water immersion and freeze-drying steps were replaced by a dry heat treatment. The logics of measurements and details of procedures were given in the references, and only a brief summary of experimental procedures are presented here. Polymer Samples. The polymer films used in this study are listed in Table I. None of the films had surface treatments of any kind. The surfacesof the filmswere cleaned by a conventional cleaning procedure described previously. The surface cleaned samples were stored in a vacuum desiccator with silica gel, after air drying in a clean air room for 24 h. Plasma Treatment. Low-temperature ashing equipment (International Plasma, Inc.) was used for CF, plasma treatment of films. The reactor has a chamber (2035 mL) with a pair of external electrodes which is operated with a 13.56-MHz rf generator to create plasma by capacitive external electrode glow discharge. The treatment was done by fixed conditions: CF4flow rate pressure discharge power treatment time
30 sccm 0.7 Torr 20 w 2 min
Heat Treatment. Immediately after the CF, plasma treatment, films were heat treated at a predetermined temperature (40-160 "C)and for a predetermined time (0.25-5 min). Films were clamped (at the top part of a film) to an aluminum sample holder and treated under a tensionless condition. In the case of Nylon 6, a sample dried in a desiccator with silica gel at 21 OC is used as a sample heat treated at the temperature. ESCA Analysis. The surface analysis by ESCA was performed immediately after the heat treatment of a film. A Shi-
Surface Configuration Change of Polymers
polymer PET Nylon PMMA
21 0.043
Langmuir, Vol. 8, No. 5,1992 1427
Table 11. Values of Decay Rate Constant, k,in Dry Heat Treatment k at various temperatures, “C 40 60 80 100 120 0.025 0.039 0.067 0.142 0.294 0.043 0.056 0.110 0.190 0.354 0.032 0.065 0.122 0.217 0.456
madzu ESCA 750and ESPAC 200 (data processing system) were used for the measurements of Cl., Fll, and 01, peaks, using an aluminum foil-filtered Mg Ka X-ray (8kV, 300 mA). Background Information on Surface Configuration Changes and t h e Surface State of a Polymer Solid. The concepts of surface configuration and of surface state are presented in detail in part 1 of this series of studies;” however, because these concepts are vitally important in interpreting data presented in this paper, a brief digest is presented below. In dealing with many surface-related phenomena such as adsorption and chemical reactions at the surface, it is often assumed that a polymer surface is a rigid and imperturbable plane which can be represented by a line in a two-dimensional illustration. In recent years, however, it has been gradually recognized617 that the surfacesof polymersare highly perturbable and that the surface characteristics of a polymer change when the surface is brought into contact with a new surrounding medium, for instance, when a polymer surface is immersed in water. The morphology or conformational state of macromolecules at the interface changes in response to a change of contacting medium. 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 determine the surface characteristics. Therefore, the term surface configuration should be distinguished from configuration of macromolecules. It is important to recognize that surface configuration changes could be accomplished with little change in the conformation of surface molecules and with no change in the configuration of macromolecules. In dealing with surface-related phenomena, it seems to be advantageous to recognize the surface region of a material as a state of matter, i.e., surface state. The concept of surface state is based on the recognition of the fact that the surface of amaterial is significantly 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 more or less balanced by a threedimensional interaction with neighboring atoms or molecules. At a surface such a balance is broken by the abeence 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. The word “state”is used to describe a state of matter. With adaptation of the surface state, the surface properties of a solid could be dealt with the corresponding surface state, which represents a three-dimensional phase, rather than with the conventional term “surfacem,which represents only a twodimensional plane. The influence of the surface imbalance of forces on some characteristics of a material may penetrate deeper than that on some other characteristics. Consequently,the depth or thickness of the surface state we can perceive is different depending upon what kinds of properties or phenomena are being considered. For instance, the triboelectric property of a plasma-deposited polymer becomes independent of the substrate materials when the thickness of the plasma polymer reaches roughly 100-140 nm. Thus, the thickness of the surface state corresponding to the contact electrification (surface electron transfer) is roughly on the order of 120 nm; however, it does not mean that the thickness of the surface state of the plasma polymer is around 120 nm. Surface configuration changes dealt with in this paper, the depth of the surface state, which is responsible for the surface configurational change, might be significantly different from the value obtained by the contact electrification phenomenon. The ESCA results presented in this study (and also in part 1)represent
140 0.594 0.591 0.978
160 1.068 0.930 1.896
the surface state with a depth of 2-5 nm on the basis of the escaping depth of photoelectrons (take off angle 90”). The changes in surface configuration, which occur when a polymer surface is brought in contact with a new surrounding medium, e.g., contact with another polymer, metal, liquid, or gas, can be viewed as the process to establish a new equilibrium between the surface state of the polymer with the surface state of the new surrounding medium. Such an equilibration process is obviously a function of thermodynamic parameters of the two phases under consideration. In order to investigate the influence of temperature, we must expose a surface to the surrounding medium at different temperatures. Consequently, the heat transfer from the surrounding medium to the surface or vice versa takes place at the interface. This process alone is rather complex depending upon the heat transfer coefficient and the heat capacities of the two phases in consideration. Nevertheless, it is important to recognize that we are dealing with a transient phenomena which is in the direction to establish a new equilibrium between two surface states, which were previously in equilibrium with totally different media and at different temperatures. It is important to reckon that we are not dealing with the equilibrium surface states in contact with air at different temperatures, but with the initial rate of transition from one temperature to another. The influence of temperature on surface configuration cannot be ignored in any circumstances whenever a surface is exposed to a new medium at a different temperature. The major issue is whether the temperature has a dominant effect on surface configuration changes or some other factors might have predominantly greater influence than the temperature. In the case of water immersion of plasma-fluorinated polymers, the effect of interaction between water and fluorine atoms appears to be predominantly great to obscure the thermal effect according to our recent study. The experiments carried out in the absence of liquid water in this study will eliminate the influence of waterfluorine interaction, and consequently the process to establish a new equilibrium of the surface state of a polymer is totally different. By comparing these two different experiments, Le., with or without liquid water, we will be able to distinguish the thermal factor and the interaction factor, both of which influence surface configuration changes in response to changes in the surrounding medium. It should be reiterated here that the surfacefluorination by plasma was used as a surface-labeling tool, and the main objective of this study is an investigation of motions of molecules in the surface state of polymers.
Results and Discussion Upon heat treatment in air, peak area ratios of ESCA F1$C1, for CF4 plasma treated films decrease, and the decrease is greater at higher temperature and at longer treatment time, although the extent of decrease is much smaller than those observed in water immersion. In order to see the general trends in the effects of temperature and time of heat treatment, smoothed curves are compared in Figures 1-3. Profiles of ESCA peaks for samples heat treated for 5 min are shown in Figures 4-6. The data for the water immersion case showed that the decrease of the F1,/C1, ratio and the immersion time can be correlated by a linear equation between log ( F I J C 3 and log t. This relationship was used to estimate the initial slope of the decay rate to express the decay rate constant k a t a given temperature. Thus obtained rate constant k’s are shown in Table 11. The similar ft values for the water
1428 Langmuir, Vol. 8,No. 5, 1992
Yasuda et al. Fls
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7
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540
530
290
260
Binding Energy (eV)
1. Change of ESCA FIJC1, peak ratio of CF4 plasma
treated PET films with heat treatment time. 1.3
--690
Time (min)
Figure 4. Influence of heat treatment temperature on profiles
1
of ESCA peaks for CF4 plasma treated PET films (bmin heat treatment). Temperature ("C): (1) 160,(2)140,(3)120,(4)100, (5)80,(6)60,(7) 40,(8)no heat treatment, (9) untreated film.
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2
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Figure 2. Change of ESCA FI&, peak ratio of CF4 plasma
treated Nylon 6 films with heat treatment time.
---600
540
530
400
290
260
Blnding Energy (eV)
Figure 5. Influence of heat treatment temperature on profiles
of ESCA peaks for CF4 plasma treated Nylon 6 films (Bmin heat treatment). Temperature (OC): (1)160,(2)140,(3)120,(4)100, (5)80,(6)60,(7) 40,(8)no heat treatment, (9) untreated film.
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Figure 3. Change of ESCA FIJCI, peak ratio of CF4 plasma treated PMMA films with heat treatment time. immersion case are shown in Table 111. Comparison of Tables I1 and I11 clearly shows that the decay rate in air is much smaller than that in water immersion at a given temperature, implyingthat the factor due to water-fluorine interaction is much greater than the factor due to the thermal motion of polymer molecules. The details of the temperature effect can be seen in Arrhenius plots depicted in Figures 7-9. Conspicuous breaks in Arrhenius plob are observed for all polymers.
The break in an Arrhenius plot is taken as the transition temperature of the surface configuration change, T, (surface transition temperature). The values of T,and the activation energy for above and below T,are tabulated in Table IV. The corresponding values for the water immersion case are shown in Table V. It is interesting and also important to point out that the value of T,observed in dry heat treatment coincides with TBof the corresponding polymer, whereas the value of TB in the water immersion case coincides with the DrostHansen temperature of vicinal water. This difference of T,observed with or without liquid water is an important indication of the nature of the surface configuration change which occurs in response to the change in the contacting medium. The change of surface properties can be best understood ifwe adopt the concept of surface state and treat the change as the process to establish a new equilibrium between the surface state of the polymer and the surface state of the new contacting medium. The surface state of the new contacting medium could be a monolayer of adsorbed gas or (non-structure-forming) liquid, or the surface state of water recognized as the vicinal water in the case of highly structure-forming liquid water.
Surface Configuration Change of Polymers Fl8
Langmuir, Vol. 8, No. 5, 1992 1429
Cl8
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in Water Immersion k at various temperatures, O C 0 10 20 30 0.292 0.339 0.458 0.738 0.056 0.063 0.090 0.156 0.162 0.184 0.253 0.433
Heat Treatment activation energy, kcal/mol
PET Nylon6 PMMA
60-85 40-52 105
I
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Figure 8. Temperature dependence of k for heat treatment in air and k for water immersion for Nylon 6 films. The surface state of a material is a function of thermodynamic parameters such as temperature and pressure of a system. Therefore, it is tempting to correlate the surface configurational change to the glass transition temperature of the polymer. However, it is necessary to reiterate that we are dealing with change of the surface configuration rather than the equilibrium surface configuration at different temperatures. The rate by which such surface configuration changes is influenced by the mobility of macromolecules and also by the driving force that causes such configurational changes. This situation is analogous to the flux (configurational change) related to the diffusion coefficient (mobility of macromolecule) and the concentration gradient (driving force gradient). The discrepancy in T,observed in water
74.5 53.8 104.4
10.6 8.1 12.2
4.6 7.3
Table V. Summary of Surface Transition in Water Immersion
4.0
Figure 7. Temperature dependence of k for heat treatment in air and k for water immersion for PET films.
40 1.126 0.260 0.698
Table IV. Summary of Surface Transition in Dry
I/T x103 ( 1 1 ~ )
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Table 111. Values of Decay Rate Constant, k,
280
Figure 6. Influence of heat treatment temperature on profiles of ESCA peaks for CF*plasma treated PMMA films (5-minheat treatment). Temperature(OC): (1) 160, (2) 140, (3) 120, (4) 100, (5) 80, (6) 60, (7) 40, (8) no heat treatment (9) untreated film.
j
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Binding Energy (eV)
0.0
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Figure 9. Temperature dependence of k for heat treatment in air and k for water immersion for PMMA films.
--600
'
polymer PET
Nylon 6 PMMA
T., O C 15.2 15.4 15.2
activation energy, kcal/mol above T. below T. 8.2 2.2 9.7 2.0 9.3 2.0
immersion and in dry heat treatment clearly shows that (1) the mobility of macromolecules responsible for the surface configuration change in these two cases is different and (2) the driving force which causes the surface configuration change in these two cases is also different. In the case of the water immersion of plasma-fluorinated polymer surfaces, the interaction of water and fluorine (driving force) is the dominant factor, and the change of the interaction is seen as the surface transition temperature. By this reason, T,observed corresponds to the Drost-Hansen temperature of the vincinal water. In the case of dry heat treatment of the same surfaces, the mobility of macromolecules in response to the thermal gradient is the dominant factor. Therefore, in this case, T,corresponds to the glass transition temperaure, Tg, of a polymer. Comparison of activation energies shown in Tables IV and V suggests that the activation energy for the surface configuration change due to the thermal motion is greater than that for water-driven migration of fluorine atoms, although the difference is relatively small. The basic nature of the surface configuration change may be visualized as relatively simple rotational motion without involving large segments or migration of whole macromolecules. It is often intuitively assumed that the surface configuration of a glassy polymer is unperturbable at temperatures below its glass transition. On the basis of such an
1430 Langmuir, Vol. 8,No.5, 1992
assumption, it is often further assumed that water has little effect on the surface properties of a glassy hydrophobic polymer, such as poly(methy1 methacrylate) and poly(ethy1ene terephthalate), at around room temperature or at body temperature. Results shown here indicate that such intuitive assumptions could be far from reality. In the dry heat treatment of PET, there is practically no change at 40 "C, whereas in the water immersion case the change in the surface configuration is much greater than that which occurs at 160 OC in the dry heat treatment of the polymer.
Conclusion Surface properties of a polymer are determined by the surface configuration of a surface,which is a specificspatial arrangement of moieties within a polymer molecule under a particular environment. The change of surface configuration can occur without involving large segmental motions, conformational changea, or morphological changes of a polymer surface. The surface configuration change of a polymer can be well represented by the concept that such changes occur as the process to establish a new equilibrium between the surface state of the polymer and that of the new surrounding medium. The vicinal water corresponds to the surface state of water in contact with a surface. The overall surface configuration change is a function of the mobilities of macromolecules and the driving force which causes the surface configuration change in an
Yasuda et al.
analogous manner that the flux is related to the diffusion coefficientand the concentration gradient. Changeseither in the mobility or in the driving force at a certain temperature could cause a break in an Arrhenius plot for k and T,which can be recognized as a surface transition temperature T,. In dry heat treatments of a polymer surface,the thermal mobility of macromolecules plays a predominant role in the surface configuration change, and accordingly TBcorresponds to the glass transition temperature of the polymer. In the case of the water immersion of a polymer surface, the interaction between water and hydrophobic moieties (the driving force of the surface configuration change) is the dominant factor, and in this case T,corresponds to the Drost-Hanaen temperature of vicinal water at 15 "C and becomes independent of the nature of the polymers. The surface of a polymer solid is much more mobile and purturbable than it has been often assumed intuitively that polymer surfaces a t temperature below TBare rigid and unperturbable. Consequently, it seems to be important to emphasize that important properties of a polymer surface are interfacial properties rather than so-called surface properties which are often represented by the interfacial properties with air or vacuum. Regietry No. PET (SRU), 25038-59-9; PMMA (homopolymer), 9011-14-7; CFI, 75-73-0; H20,7732-18-5; Nylon 6, 2503854-4.
