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Roles of Molecular Architecture and End-Group Functionality on the Surface Properties of Branched Polymers Joshua A. Orlicki, Neil O. L. Viernes, and Jeffrey S. Moore* Department of Chemistry, The Beckman Institute for Advanced Science and Engineering, University of Illinois, Urbana, Illinois 61801
Ibrahim Sendijarevic and Anthony J. McHugh* Department of Chemical Engineering, University of Illinois, Urbana, Illinois 61801 Received July 22, 2002. In Final Form: October 8, 2002 The effects of molecular architecture on the surface properties of polyetherimide polymer films were investigated. For polymers ranging in structure from hyperbranched to completely linear, it was found that the presence of branches limits the mobility of polymeric segments, which inhibited the migration of interior end-groups to the film surfaces. As a result, the surface energy of randomly branched polymers scales with the number of terminal segments which are located at the periphery of molecules. For a series of branched polymers with compositionally identical end-groups and repeat units, the surface properties were independent of molecular architecture. The results of this study also indicate that the surface properties of hyperbranched polymers strongly depend on the functionality of end-groups.
Introduction Understanding the surface and interfacial properties of polymeric materials is vital for predicting adequate performance in various applications. For example, the surface energies of polymeric materials determine their adhesion properties to substrates, which in turn can affect the mechanical properties and performance of composite materials.1 In packaging and biomedical applications, the wetting behavior and barrier properties of films are of crucial importance.2 Considerable efforts have been expended in understanding the roles of structural variables on the surface properties of polymeric materials. Studies conducted in the past decade indicate that, in addition to chemical composition of the polymeric backbone, end-group functionality can also have a significant impact on the surface properties of polymers. Although arguments have been made that, in athermal systems, end-groups migrate to surfaces to minimize the loss of configurational entropy,3-5 it has been shown theoretically and experimentally that the differences in surface energies between the end-groups and repeat units dominate the concentration profiles of chain ends. Jalbert et al.1 utilized χ, the dimensionless difference between surface energies of ends and repeat units, in a lattice self-consistent model and Monte Carlo simulation to predict the location of end-groups. This approach successfully predicted the concentration profiles of end-groups in polymers with surface energies of ends lower, the same, or greater than that of the backbone repeat units. Similarly, in polyurethane and poly(ethylene glycol) (PEG) systems, Chen et al.6 showed that hydro(1) Jalbert, C.; Koberstein, J. T.; Hariharan, A.; Kumar, S. K. Macromolecules 1997, 30, 4481. (2) Sigurdsson, S.; Shishoo, R. J. Appl. Polym. Sci. 1997, 66, 1591. (3) Kumar, S. K.; Vacatello, M.; Yoon, D. Y. Macromolecules 1990, 23, 2189. (4) Hariharan, A.; Kumar, S. K.; Russell, T. P. Macromolecules 1990, 23, 3584. (5) Yethiraj, A.; Hall, C. K. Macromolecules 1990, 23, 1865.
phobic end-groups preferentially concentrate at the air interface while the hydrophilic end-groups remain in the bulk. Affrossman et al.7 presented results for styrene polymers, having one or both ends capped with perfluoro groups, that show preferential surface migration of the low energy end-group that scales with their bulk concentration. Although these studies show that in energetically favorable cases end-groups preferentially concentrate at surfaces, for linear polymers their effectiveness on surface energy decreases with molecular weight. However, in branched polymer systems, the number of end-groups scales with the branching density, consequently enhancing the role of end-groups on surface properties. Of particular interest for modification of polymeric surfaces, via modification of end-groups, are highly branched polymers, such as hyperbranched polymers (HBPs). HBPs are usually synthesized by reaction of ABx functional monomers, with x g 2, resulting in large number of end-groups that scale with the degree of polymerization.8 As a result, the surface properties of HBPs are expected to strongly depend on the functionality of end-groups for all molecular weights. Interestingly, the literature is nearly devoid of studies addressing the role of end-groups on the surface properties of HBPs. In a recent publication, Mackay et al.9 were the first to show that the melt surface tension of hydroxyl- terminated HBPs is high and approaches that of water. In addition, the surface (6) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y.; Somrjai, G. A. J. Am. Chem. Soc. 2000, 122, 10615. (7) Affrossman, S.; Bertrand, P.; Hartshorne, M.; Kiff, T.; Leonard, D.; Pethrick, T. A.; Richards, R. W. Macromolecules 1996, 29, 5432. (8) For comprehensive reviews of HBPs see: (a) Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2505. (b) Hult, A.; Johansson, M.; Malmstro¨m, E. Adv. Polym. Sci. 1999, 143, 1. (c) Hawker, C. J. Adv. Polym. Sci. 1999, 147, 113. (d) Kim, Y. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685. (e) Malstro¨m, E.; Hult, A. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1997, C37, 555. (f) Fre´chet, J. M. J.; Hawker, C. J.; Gitsov, I.; Leon, J. W. J. Macromol. Sci., Pure Appl. Chem. 1996, A33 (10), 1399. (9) Mackay, M. E.; Carmezini, G.; Sauer, B. B.; Kampert, W. Langmuir 2001, 17, 1708.
10.1021/la020663p CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002
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Figure 1. Reaction scheme for AB/AB2 PEI copolymers with xAB ) 0-1. (1) 1 + 2, DMAc, 150 °C, cat. CsF, 5 min. For synthesis of AB/AB2/A′ PEI copolymers add the following step: (2) 3, DMAc, 150 °C, 10 min. Molecular architecture varies with starting molar fraction of AB monomers, xAB, from hyperbranched (xAB ) 0) to linear (xAB ) 1).
tension of HBPs terminated with alkyl end-groups approached the surface tension of linear polyethylene. These data, therefore, suggest that HBPs are efficient at presenting end-groups at polymeric surfaces. The effects of molecular architecture and end-group functionality on the propensity of chain ends to express at a surface are still mostly unexplored. In addition to an increase in the number of ends with increased branching density, the configurational entropy of the polymer backbone is expected to decrease, in turn decreasing the mobility of chain segments. The mobility of end-groups is expected to decrease as well. Therefore, in enthalpicly favorable cases, in which the end-groups have lower surface energies that those of the repeat units, expression of chain ends at surfaces is expected to depend on the branching density of the polymer backbone. The objective of the present work has been to systematically examine the effects of molecular architecture in model branched systems on the ability of end-groups to segregate at the surfaces and affect surface properties. For this task we used randomly branched polyether imide (PEI) polymers of various degrees of branching (DB) from 0.66 to 0.00, corresponding to HBPs and linear molecules, respectively.10-12 The molecular architecture of these PEIs is systematically controlled by varying the ratio of starting linear (AB) and branched (AB2) monomers. The resulting model branched systems have nearly constant weight average molecular weights, Mw, and mole fractions of starting AB monomer, xAB, systematically varied from 0 to 1. For one series of AB/AB2 PEI copolymers end-groups were capped with tert-butyldimethylsilyl groups (TBS), which have lower surface energy than that of the interior repeat units.10,11 A second series of AB/AB2 PEI copolymers12 were prepared with end-groups that were chemically identical to the repeat units. Therefore, for this series the chemical compositions and surface energies of ends and repeat units were identical for the entire range of xAB.12 (10) Markoski, L. J.; Thompson, J. L.; Moore, J. S. Macromolecules 2000, 33, 5315. (11) Markoski, L. J.; Moore, J. S.; Sendijarevic, I.; McHugh, A. J. Macromolecules 2001, 34, 2695. (12) Sendijarevic, I.; McHugh, A. J.; Markoski, L. J.; Moore, J. S. Macromolecules 2001, 34, 8811.
In addition, to investigate the effect of end-group functionality on surface energies of HBPs, PEI HBPs with various concentrations and functionalities of endgroups were evaluated. Materials Polyetherimides with Various Branching Densities. Previously we reported on the syntheses and properties of randomly branched polyetherimide (PEI) AB/AB2 copolymers with various degrees of branching.10-12 By controlling the ratio of AB (linear) and AB2 (branched) monomers, PEI AB/AB2 copolymers (4a-n) of nearly constant weight average molecular weight, Mw, ranging in molecular structure from fully hyperbranched to linear were synthesized (Figure 1). By increasing the mole fraction of AB monomers, xAB, the number of unreacted tert-butyldimethylsilyl (TBS) end-groups that affect the elemental composition decreases as well.10,11 It is therefore difficult to decouple the effects of branching and elemental composition on material properties. To isolate the effects of branching on properties, we prepared a second series of AB/AB2 copolymers that were compositionally invariant over the same range of molecular architecture as that for the previous series. A terminating segment (3) of A′ functionality was prepared, and the ((aAB2 + bAB) + aA′) copolymers were synthesized as shown in Figure 1. The copolymers (5a-n), with xAB ranging from 0 to 1, maintain constant elemental composition and nearly constant Mw. The material properties characterization and synthetic procedures involved in preparation of these materials are described in more detail elsewhere.12 Polyetherimide Hyperbranched Polymers with Various End-Groups. The PEI HBPs were synthesized using AB2 monomers containing a pair of tert-butyldimethylsilyl (TBS)protected phenols and an activated aryl fluoride, together with a catalytic amount of CsF. The resultant HBPs contain a large number of unreacted TBS end-groups, proportional to the degree of polymerization (Figure 2, structure 16). The unreacted TBS end-groups were replaced with A1terminating segments, thereby affecting end-group dependent material properties such as solubility, thermal properties, and surface energy. A1-terminating segments are listed in Table 1. In a previous publication13 we described the synthesis of alkylterminated PEI HBPs (11a-c, 12a-c). The composition of the terminating units varied from methyl (CH3), to octyl (C8H17), to octadecyl (C18H37). The concentration of alkyl end-chains was varied by using monofunctional (6a-c) or difunctional (7a-c) terminating segments. (13) Orlicki, J. A.; Thompson, J. L.; Markoski, L. J.; Sill, K. N.; Moore, J. S. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 936.
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Figure 2. Reaction scheme for PEI HBPs with indicated capping groups.
Results
Table 1. Capping Groups for PEI HBPs Described in Figure 2 A1 segment
HBP
end-group composition
6a 6b 6c 7a 7b 7c 8a 8b 8c 9 10
11a 11b 11c 12a 12b 12c 13a 13b 13c 14 15
X, Y ) H; Z ) OCH3 X, Y ) H; Z ) OC8H17 X, Y ) H; Z ) OC18H37 Y ) H; X, Z ) OCH3 Y ) H; X, Z ) OC8H17 Y ) H; X, Z ) OC18H37 X, Z ) H; Y ) OCH2(CF2)2CF3 X, Z ) H; Y ) OCH2(CF2)7CF3 X, Z ) H; Y ) O(CH2CH2O)3CH3 X, Y, Z ) H Y ) H; X, Z ) CF3
PEI HBPs, capped with perfluoro (13a-b, 15), phenyl (14), hydroxyl (17), and ethylene glycol (13c) end-groups, were evaluated as well. The synthetic procedures and characterization of these PEI HBPs are described in detail elsewhere.13
Experiments HBP FilmssSpin-Coated Method. Spin-coated films were prepared from 3.0 wt % solutions of polymer in appropriate solvents. N-Methyl pyrollidinone (NMP) was used as solvent for the preparation of branched copolymer films (4a-n, 5a-n). HBPs with modified end-groups were dissolved in chloroform, with the exception of the long-chain perfluoro-terminated HBP (13b), which was dissolved in tetrahydrofuran, and the hydroxyterminated HBP (17), which was dissolved in NMP. All films were spin-coated onto glass cover slips at a rate of 4000 rpm for 60 s. To ensure complete solvent evaporation, films (4a-n, 5a-n) were annealed at 80 °C for 24 h in a vacuum oven. No difference in surface properties was observed with films annealed at higher temperatures. End-group-modified HBPs were annealed at 200 °C under nitrogen atmosphere. Contact Angle Measurement. Contact angle measurements were performed on the surfaces of films with deionized water (γL ) 72 mN/m, γLD ) 22 mN/m) and methylene iodide (γL ) γLD ) 27 mN/m) using a Rame-Hart goniometer model 100-00. Advancing and receding contact angle values were obtained. Surface energies were obtained using the method outlined by Owens et al.14 X-ray Photoelectron Spectroscopy (XPS). Samples were spin-coated by depositing 3.0 wt % solutions of 13a-b and 15 onto gold-covered glass slides. To ensure complete solvent evaporation, films were annealed at 150 °C for 24 h in a vacuum oven. XPS spectra were taken on a Physical Electronics PHI 5400 X-ray photoelectron spectrometer with an Al KR X-ray source at a take off angle of 45°. The instrument was operated at a power of 400 W, a source-operating voltage of 15 kV, and pass energies of 35.75 eV. (14) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
Effect of Molecular Architecture. Contact angle data and surface energies of the AB/AB2 and AB/AB2/A′ series of PEI copolymers are listed in Table 2. The advancing contact angle data were reproducible within the experimental error of (2°, while the receding contact angle values were less reliable. For better illustration, advancing contact angle values for both series of PEI copolymers are plotted as a function of starting molar fraction of AB monomers, xAB, in Figure 3. The data for AB/AB2 PEI copolymers (capped with TBS end-groups) show decreasing contact angle values with xAB. The observed trend reflects a change in an overall elemental composition, caused by a decrease in the number of TBS end-groups with xAB. The TBS end-groups have lower surface energy than that of the repeat units and are expected to preferentially migrate to the air interface to minimize the interfacial energy. Consequently, the contact angle data and surface energy are expected to scale roughly with the bulk concentration of TBS end-groups, which decreases linearly with xAB. A deviation from this trend suggests that a fraction of TBS end-groups are trapped in the interior of the molecule, limiting their effect on the surface properties. These surface property results can be rationalized with the aid of predictions made via the polymerization kinetics model of Frey et al.15,16 They developed the basic definitions to describe the topological changes of AB/AB2 copolymers, on the basis of the assumption of equal reactivity of all groups. The probabilities of forming dendritic (D), linear (L), and terminal (T) segments from AB2 and L or T segments for AB monomers, see Figure 1, are given by eqs 1 and 2, respectively.
AB2 units: r+1 D(AB2) ) p3A (r + 2)2 1 r+1 L(AB2) ) 2p2A 1 - pA r+2 r+2
(
)
1 r+1 T(AB2) ) pA 1 - pA r+1 r+2
(
)
2
(1)
(15) Ho¨lter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 30. (16) Frey, H.; Ho¨lter, D. Acta Polym. 1999, 50, 67.
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Figure 3. Advancing contact angle data with H2O and CH2I2 for AB/AB2 and AB/AB2/A′ PEI copolymers versus the starting molar composition of AB monomers, xAB. Table 2. Contact Angle Data and Surface Energies (γs) for AB/AB2 Copolymers (4a-n) and AB/AB2/A′ Copolymers (5a-n) H2O 4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n
H2O
CH2I2
xAB
adv
rec
adv
rec
γs (mN/m)
0.00 0.25 0.50 0.75 0.78 0.80 0.83 0.85 0.88 0.90 0.93 0.95 0.98 1.00
99 98 95 91 90 90 89 89 87 87 85 83 82 84
90 81 78 76 76 74 75 75 74 74 73 72 72 69
56 57 53 45 44 42 39 38 35 33 29 29 27 27
43 43 36 25 25 26 25 23 20
31 31 33 38 39 40 41 42 43 44 46 46 47 47
19 17 13
5a 5b 5c 5d 5e 5f 5g 5h 5i 5j 5k 5l 5m 5n
CH2I2
xAB
adv
rec
adv
0 0.4 0.67 0.86 0.89 0.9 0.92 0.93 0.95 0.96 0.97 0.98 0.99 1.00
84 84 83 86 85 85 85 83 84 83 84 85 85 84
75 71 70 71 69 70 72 72 72 71 71 71 71 69
24 24 23 23 25 22 22 23 21 22 23 22 23 23
rec
γs (mN/m) 50 51 51 49 48 49 49 49 49 49 48 48 48 48
where r is defined as xAB/(1 - xAB) and pA is the mole
AB units: r L(AB) ) p2A r+2 r r+1 T(AB) ) pA 1 - pA r+1 r+2
(
)
(2)
fraction of reacted A groups. Due to the high degree of polymerization, we assume complete conversion of A groups, pA ) 1. It is important to note that the experimental results from a previously published model compound study are in reasonable quantitative agreement with Frey’s predictions.17 As shown in Figure 1, TBS end-groups are present on T(AB2), T(AB), and L(AB2) segments of branched AB/AB2 PEI copolymers. Since the mobility of chain segments is restricted by the presence of branched segments, TBS endgroups from L(AB2) are unlikely to express at the surfaces. In addition, surface expression of TBS end-groups from L(AB2) segments would result in unfavorable conformations of molecules near the surfaces, resulting in loss of configurational entropy. Therefore, it is unlikely that TBS end-groups from L(AB2) segments reside at the surfaces. The number of terminal segments, T(AB) and T(AB2), and their sum, T(AB) + T(AB2), are plotted versus xAB in (17) Markoski, L. J.; Thompson, J. L.; Moore, J. S. Macromolecules 2002, 35 (5), 1599.
Figure 4. Number of moles of terminal AB, T(AB), terminal AB2, T(AB2), and their sum, T(AB) + T(AB2), versus the starting molar composition of AB monomers, xAB. DP ) 100.17
Figure 4. These results indicate that for highly branched polymers (i.e., xAB < 0.3) the majority of terminal ends are T(AB2) segments. On the other hand, for lightly branched polymers (xAB > 0.80) T(AB) segments compose the majority of the chain ends. We note that the changes in the sums of terminal segments with xAB (Figure 4) correspond to the trends observed in the contact angle data with xAB (Figure 3). Indeed, the plots of advancing contact angle data and surface energy show a linear
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Figure 5. (a) Advancing contact angle data and (b) surface energy of AB/AB2 PEI copolymers versus the total number of endgroups, T(AB2) + T(AB).
dependence with the sum of chain ends (see Figure 5a and b), suggesting that terminal segments of branched polymers reside at polymer surfaces. The fact that the surface energy data scale with the number of terminal segments and not the number of terminal TBS end-groups (T(AB2) has two and T(AB) has one TBS end-group) suggests that the surface energies of T(AB2) and T(AB) are nearly the same. The effects of the functionality of terminal segments on the HBP surface properties will be investigated in more detail in the next section. Interestingly, the first two points in Figure 5b, corresponding to the two least branched PEI copolymers (xAB ) 1.000 and xAB ) 0.975), appear to deviate from the linear dependence. A possible explanation is that, in these two samples, the negligible number of branched segments, D(AB2), allows for a significant mobility of chain ends compared to the case of the rest of the series, which has higher DB. As a result, low surface energy TBS end-groups are more mobile and therefore more likely to preferentially segregate to the surfaces, resulting in lower than expected surface energy. On the other hand, for xAB e 0.95, the presence of D(AB2) segments is significant, resulting in rigid and globular polymeric structures. In that case, due to the low mobility of chain segments, TBS end-groups that are terminated in the interior of a molecule would be prevented from migrating to the surfaces. In addition, it is expected for globular structures that a fraction of the terminal segments are trapped in the bulk (side of the molecules away from the surface). Therefore, the surface concentration of terminal segments should be lower than their bulk concentration. XPS and SIMS studies further addressing these issues are planned. The AB/AB2/A′ PEI copolymer series, with chemically identical end-groups and repeat units, exhibits a nearly constant surface energy with xAB (see Table 2 and Figure 3). Therefore, if the end-groups and repeat units are chemically identical, the elemental composition and surface energy of polymers are independent of the branching density. From these data, however, no conclusion can be drawn on the concentration profiles of endgroups. End-Group Functionality. Contact angle values and surface energies of PEI HBPs terminated with alkane end-groups are presented in Table 3. As expected, the surface energy decreases with the length of alkane endgroups (compare 11a-c and compare 12a-c). Within experimental error, however, methyl-terminated HBPs (11a) have the same surface properties as phenyl-capped HBPs (14). Therefore, it can be concluded that the surface energies of methyl end-groups and repeat units in the
Table 3. Advancing Contact Angle Data and Surface Energies for Alkane-Terminated PEI HBPs HBP (capping group)
H2O
CH2I2
γs
14 (H) 11a (mono-C) 11b (mono-C8) 11c (mono-C18) 12a (di-C) 12b (di-C8) 12c (di-C18) (PE)9
84 86 92 99 84 97 110 96
28 28 46 65 27 54 63 58
48 46 37 27 47 33 27 30
backbone are similar. Thus, it is not surprising that doubling the concentration of methyl end-groups (compare 11a and 12a) yields a negligible change in surface energies. Doubling the concentration of octyl (compare 11b and 12b) end-groups results in reduction of surface energy. This suggests a higher surface concentration of octyl groups with increased bulk concentration of octyl groups. However, these changes are negligible compared to the differences in surface energies arising from increases in the number of alkane repeat units. Doubling the concentration of octadecyl end-groups (compare 11c and 12c) results in no change in surface energies. It is possible that octadecyl end-groups are saturated at surfaces for 11c HBP. In that case, doubling the bulk concentration of endgroups would not necessarily yield higher end-group surface concentrations. Indeed, the surface energies of 11c and 12c are close to that of polyethylene (∼30-31 mN/m). Mackay et al.9 also showed that the surface tension of polyester HBPs with 90% of the end-groups capped with C20/22 alkane chains approaches that of polyethylene. Contact angle and surface energy data for the HBPs terminated with various end-groups are presented in bar graph form in parts a and and b, respectively, of Figure 6. As expected, hydroxyl (17)- and ethylene glycol (13c)terminated HBPs are hydrophilic, high surface energy materials. On the other extreme, HBP 13b, capped with perfluoro end-groups with a large number of repeat units, has the lowest surface energy (13 mN/m) in the series (lower than that of Teflon, 20 mN/m). As seen with alkane end-groups, the length of fluorinated end-groups (compare 13a, 13b, and 15) strongly affects the surface energy of the HBPs. The surface content of perfluoro-terminated HBPs was analyzed with XPS. Since fluorine atoms are unique to perfluoro end-groups, the ratio of fluorine to carbon was used as a measure of perfluoro end-group concentration at the surfaces. The experimental ratio of F/C is compared to the calculated bulk ratios in Figure 7. The data indicate that, with increased length of perfluoro end-groups, their surface concentration increases. For
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Figure 6. (a) Contact angle data with deionized water and (b) surface energy (γs) for PEI HBPs with indicated end-groups.
Conclusions
Figure 7. Surface ratio of F/C of PEI HBPs (11a, 13a, and 13b) determined by XPS versus the calculated bulk ratio.
HBPs 13a and 13b, which contain three and eight fluorocarbon repeat units, respectively, the concentration of fluorine at the air interface exceeds the bulk concentration. One explanation is that very low surface energy perfluoro groups are preferentially migrating to the surface to minimize the interfacial surface energy of the system, despite the restrictions in chain mobility due to the highly branched backbone structure of HBPs. On the other hand, end-groups for HBPs 13a and 13b are more flexible than, for example, TBS end-groups, which could result in an increased surface presence of perfluoro end-groups from the segments which are terminated in the interior of branched molecules.
The effects of molecular architecture and end-group functionality on the surface properties of branched polymers were investigated. It was found that the molecular architecture affects the mobility of chain segments and therefore expression of end-groups at polymer-air interfaces. In highly branched structures, where the mobility of end-groups was limited, a fraction of TBS endgroups was trapped in the interior of the molecule, limiting their effect on surface properties. Therefore, in branched PEIs, only chain ends from terminal segments reside at the periphery of the molecules and affect the surface properties. Although it is expected that this trend will hold for other highly branched structures, increased flexibility of the backbone may affect the mobility and surface expression of internal segments. As expected, in AB/AB2/A′ PEI copolymers, with chemically identical endgroups and repeat units, changes in molecular architecture had no effect on surface properties. By varying the functionality of end-groups, the surface properties of HBPs can be tuned over a wide range of surface energies. As expected, increasing the length of alkane end-groups resulted in decreased surface energy. High surface energy materials were obtained with hydroxy-terminated PEI HBPs. At the other extreme, PEI HBPs terminated with long perfluoro end-groups exhibit lower surface energy than that of Teflon. XPS data indicate that increasing the number of repeat units in perfluoro end-groups results in an enhanced surface concentration of the end-groups. Acknowledgment. This work has been supported under a grant from the U.S. Army Research Office under contract/grant number DAAG55-97-0126. The authors acknowledge L. J. Markoski (U of I) for helpful discussions and synthesis of the branched PEIs. We would also like to thank Professor Gupta and his group (U of I) for the use of their goniometer. LA020663P
