Controlled Pore Functionalization of Poly(ethylene terephthalate

Sep 1, 2007 - Surface grafting on poly(ethylene terephthalate) track-etched microporous membrane by activation with trifluorotriazine: Application to ...
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Langmuir 2007, 23, 10316-10322

Controlled Pore Functionalization of Poly(ethylene terephthalate) Track-Etched Membranes via Surface-Initiated Atom Transfer Radical Polymerization Alexander Friebe and Mathias Ulbricht* Lehrstuhl fu¨r Technische Chemie II, UniVersita¨t Duisburg-Essen, 45141 Essen, Germany ReceiVed June 8, 2007. In Final Form: July 24, 2007 A new method for surface-initiated atom transfer radical polymerization (ATRP) on the technical polymer poly(ethylene terephthalate) (PET) has been developed which allows controlling and estimating the layer thickness of the grafted polymer in the isocylindrical pores of track-etched membranes. After PET surface treatment by oxidative hydrolysis, the bromoalkyl initiator was immobilized on the PET surface in a two-step solid-phase reaction; the isoporous membrane structure was preserved, and the pore diameter was increased from 760 to 790 nm. Poly(Nisopropylacrylamide) (PNIPAAm) was grafted under ATRP conditions from a methanol/water mixture at room temperature. Both monomer concentration and reaction time could be used as parameters to adjust the degree of grafting. Effective grafted layer thickness and its response to temperature were estimated from pure water permeability. All data, especially the high polymer densities (0.37 g/cm3) in the swollen layers at 25 °C, indicate that grafted PNIPAAm with a “brush” structure has been achieved. For dry PNIPAAm layer thicknesses on the PET pore walls of up to 80 nm, a temperature-induced swelling/deswelling ratio of ∼3 had been observed. Reduction of the brush grafting density, via composition of the reaction mixture used in solid-phase synthesis for initiator immobilization, led to an increase of that swelling/deswelling ratio. Further, density and temperature response of the grafted PNIPAAm layers synthesized via ATRP were compared with those obtained in the same membranes by less controlled photografting, leading to lower grafting density and larger gradients in grafted layer density and, consequently, much higher swelling/ deswelling ratios (>15).

Introduction A large variety of methods has been developed and is used to functionalize material’s surfaces to change their properties according to the required application, and heterogeneous graft copolymerization (“grafting-from”) has become an important technology.1 In comparison to planar surfaces, the controlled functionalization of porous materials is more complicated but also most relevant for advanced functional materials such as membranes or adsorbers for mass separations.2 Important issues to be investigated are the accessibility of and the mass transfer in pores during grafting reactions and the consequences for grafted layer structure and functionality. In previous work, track-etched membranes made from poly(ethylene terephthalate) (PET) with uniform cylindrical pores at very narrow size distribution had been established as a versatile tool to investigate various grafting reactions in pores and the effects on a material’s function.3-6 Recently, it was demonstrated that, under appropriate experimental conditions, effective thicknesses (in the range of a few to several hundred nanometers) of grafted polymer layers on the pore walls could be precisely estimated from membrane permeability data.7 In that work, grafting-from with poly(Nisopropylacrylamide) (PNIPAAm) was performed by precoating the entire PET surface with the photoinitiator benzophenone and * Corresponding author: [email protected].

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(1) Kato, K; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y. Prog. Polym. Sci. 2003, 28, 209. (2) Ulbricht, M. Polymer 2006, 47, 2217. (3) Geismann, C.; Ulbricht, M. Macromol. Chem. Phys. 2005, 206, 268. (4) He, D. M.; Ulbricht, M. Macromol. Chem. Phys. 2007, 208, 1582. (5) Hicke, H. G.; Paulke, B. R.; Becker, M.; Ulbricht, M. J. Membr. Sci. 2006, 282, 413. (6) Becker, M.; Provart, N.; Lehmann, I.; Ulbricht, M.; Hicke, H. G. Biotechnol. Prog. 2002, 18, 964. (7) Geismann, C.; Yaroshchuk, A.; Ulbricht, M. Langmuir 2007, 23, 76.

subsequent UV irradiation in the presence of the monomer in the pores. PNIPAAm with a lower critical solution temperature (LCST) of ∼32 °C is a well-known example of a stimuliresponsive polymer.8,9 With the grafted PNIPAAm layers on the pore walls, the effective hydrodynamic pore diameter of the membrane could be switched reversibly by change of temperature. The photografting-from method used before is easy to use and very robust, but it also has disadvantages. First, it is not possible to make precise variations of grafting density because that depends on the surface density of the physically attached photoinitiator (which may vary as a function of the reaction conditions) and its reactivity with the base polymer (which will be different for different adsorption sites). Indeed, previous data indicate that only relatively low grafting densities can be achieved.3,7 Much higher grafting densities on the PET pore surface are accessible by chemically attaching a co-initiator (“synergist”) for the dissolved photoinitiator.4 However, the second disadvantage of conventional grafting-from is due to the uncontrolled mechanism of radical polymerization, leading to large polydispersity of grafted chain lengths. The aim of this study was to develop a method to graft functional polymer layers from the PET pore walls by using a fundamentally different and better controllable mechanism, and surface-initiated atom transfer radical polymerization (ATRP) had been selected. Since the discovery of this novel radical polymerization technique,10,11 many studies have been reported about ATRP in homogeneous solutions or heterogeneous systems.12,13 In contrast to other controlled methods such as (8) Shibayama, M.; Tanaka, T. AdV. Polym. Sci. 1993, 109, 1. (9) Okano, T. AdV. Polym. Sci. 1993, 110, 179. (10) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. (11) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614. (12) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921.

10.1021/la7016962 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/01/2007

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Figure 1. Schematic representation of PET pore functionalization by “grafting-from” of stimuli-responsive PNIPAAm via two different strategies and change of effective pore diameter by increasing/decreasing the temperature around the LCST (32 °C).

anionic, cationic, or nitroxide-mediated polymerization, the ATRP is very flexible with respect to the selection of monomers and it also permits the presence of water.12,13 Surface functionalizations using ATRP have mostly been studied using planar inorganic substrates, such as gold or quartz, and typically after substrate premodification with a self-assembled monolayer (SAM) comprising the initiator.14-16 Grafted PNIPAAm layers in the brush regime have been synthesized by surface-initiated ATRP on initiator SAM-coated gold14 or on polystyrene-based colloidal particles with a shell containing the immobilized initiator.17 Recently, temperature-gated porous membranes have been prepared by sputtering the outer surface of track-etched polycarbonate membranes having pore diameters of 80-200 nm with gold, initiator SAM coating, and finally ATRP grafting of PNIPAAm.18 Studies on the surface-initiated ATRP graftingfrom modification of technical polymers are very rare (e.g., refs 19 and 20), and to our knowledge, the functionalization of polyesters including PET via ATRP have not yet been reported. The ATRP for the functionalization of the inner surface of porous materials has also only occasionally been described.21,22 Recently, two groups have reported the first successful steps toward controlled intraporous functionalization of membranes using surface-initiated ATRP. Husson and co-workers used a multistep premodification/-coating of a PVDF microfiltration membrane to introduce the bromoisobutyrate initiator and grafted thereafter poly(2-vinylpyridine) under ATRP conditions.23 Bruening and co-workers functionalized membranes from aluminum oxide via initiator immobilization, ATRP, and subsequent polymeranalogous reaction to obtain a high-capacity protein adsorber.24 Here we report a newly developed approach for surface-initiated ATRP on the technical polymer PET which allows controlling (13) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14. (14) Plunkett, K. N.; Zhi, X.; Moore, J. S.; Leckband, D. E. Langmuir 2006, 22, 4259. (15) Kim, J. B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616. (16) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. (17) Kizhakkedathu, J. N.; Norris-Jones, R.; Brooks, D. E. Macromolecules 2004, 37, 734. (18) Lokuge, I.; Wang, X.; Bohn, P. W. Langmuir 2007, 23, 305. (19) Liu, D.; Chen, Y.; Zhang, N.; He, X. J. Appl. Polym. Sci. 2006, 101, 3704. (20) Liu, Y. L.; Luo, M. T.; Lai, J. Y. Macromol. Rapid Commun. 2007, 28, 329. (21) Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69, 4577. (22) Kruk, M.; Dufour, B.; Celer, E. B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. J. Phys. Chem. B 2005, 109, 9216. (23) Singh, N.; Husson, S. M.; Zdyrko, B.; Luzinov, I. J. Membr. Sci. 2005, 262, 81. (24) Sun, L.; Dai, J.; Baker, G. L.; Bruening, M. L. Chem. Mater. 2006, 18, 4033.

and estimating the layer thickness of the grafted polymer in the isocylindrical pores of track-etched membranes (Figure 1). Further, density and temperature response of the grafted PNIPAAM layers synthesized via ATRP are compared with those obtained by photografting in the same membranes.7 Experimental Section Materials. PET track-etched membranes Rotrac with a nominal pore diameter of 400 nm and a thickness of 23 µm were from Oxyphen GmbH, Dresden, Germany. Diisopropylcarbodiimide (99%), Nhydroxybenzotriazole hydrate (98%), R-bromoisobutyrylbromide (98%), ethanolamine (99%), propylamine (99%), triethylamine (pro analysis), and copper(II) bromide (99+%, extra pure) were purchased from Acros and used as received. Ethanolamine (>99%), thionin acetate (for microscopy), and N,N,N′,N′′,N′′-pentamethyl diethylentriamine (PMDETA) were from Fluka and also used as received. 4-(N′N′-Dimethylamino)pyridine (DMAP) was also from Fluka and recrystallized from toluene. N-Isopropylacrylamide (NIPAAm; 97%) and copper(I) bromide (99.999%) were obtained from Aldrich. NIPPAm was recrystallized twice from hexane to remove the inhibitor. Syntheses. All membrane samples (25 mm diameter) were pretreated (“carboxylated”) by “oxidative hydrolysis” as described before.3 Carboxyl group analysis on PET surfaces was performed by reversible binding of the cationic dye, thionin acetate, according to the method described earlier.3 The two-step immobilization of ATRP initiator according to Figure 2 was as follows: Immediately after carboxyl activation with diisopropylcarbodiimide and Nhydroxybenzotriazole according to ref 3, the membrane samples were put in a solution of 0.5 mol/L ethanolamine in DMF for 3 h. In the experiments toward “dilution” of immobilized initiator, a solution of 125 mmol/L ethanolamine and 375 mmol/L propylamine was used. Then, the samples were washed twice in DMF and twice in ethanol. Excessive solvent in the pores was removed in an oven at 50 °C for 2 h. Thereafter, the samples were put in 10 mL of a solution of 80 mmol/L R-bromoisobutyrylbromide, 100 mmol/L triethylamine, and 5 mmol/L 4-(N′N′-dimethylamino)pyridine in dry acetonitrile for 2 h. This solution was prepared in a dried flask with a narrow neck to protect it against moisture uptake, and the reaction was conducted in 50 mL glass vessels which were tightly sealed immediately after inserting the sample. After reaction, the samples were washed twice in acetonitrile and twice in methanol, and after drying overnight at 50 °C, they were weighed using the balance GENIUS (accuracy: (10 µg) from Sartorius (Germany). The ATRP grafting (cf. Figure 2) was carried out in a cascade of 50 mL two-neck flasks, each with one septum, under argon atmosphere. This cascade allowed the parallel functionalization of up to nine membrane samples (one per flask) using the same reaction mixture and, if intended, different reaction times. To prepare the reaction solution in a 100 mL flask, the required amounts of NIPAAm and PMDETA were dissolved in a methanol/water mixture 7:3 (v/v).

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Figure 2. Two-step immobilization of the ATRP initiator on carboxylated PET surface and subsequent surface-initiated ATRP of the functional monomer NIPAAm (NEt3 ) triethylamine, DMAP ) 4-(N′N′-dimethylamino)pyridine, AN ) acetonitrile, PMDETA ) N,N,N′,N′′,N′′pentamethyl diethylentriamine). After 15 min degassing with argon, the copper(I) bromide was added with strong stirring under a continuous argon stream. When a light yellow and clear solution had been formed, a volume of 8 mL was injected with help of a gastight syringe through the septum into the flask containing one membrane sample. To stop the polymerization, the samples were immersed in 10 mL of a solution of 50 mg of copper(II) bromide and 125 µL of PMDETA in methanol/water 1:1 (v/v). The washing procedure was as follows: The first solution, 10 mL for each sample, contained 125 µL of PMDETA in methanol/ water 1:1 (v/v) to remove copper traces. Subsequently, the samples were washed three times in methanol/water 1:1 (v/v). In the last solution, the samples remained for 2 h. After drying overnight at 50 °C, samples were weighed again to determine the degree of graft functionalization, which was calculated according to the following: DG )

mgr - mo Aspec

(1)

where mo is the membrane sample weight after initiator immobilization, mgr is the membrane weight after grafting, and Aspec is the specific surface area of the membrane sample used (in cm2, calculated from 1.0 m2/g, measured by nitrogen adsorption and BET isotherm analysis). Considering the accuracy of the balance, the smallest weight difference that can be measured ((10 µg; cf. above) corresponds to a DG difference of (0.1 µg/cm2 for the sample with a diameter of 25 mm (cf. above). The dry layer thickness, ldry, was calculated from DG assuming a polymer density of 1.1 g/cm3 and even coverage of the entire surface area. In addition, membranes were qualitatively and quantitatively characterized by transmission FTIR spectroscopy (see Supporting Information). Measurements. Pore size distributions of membranes in the dry state were measured by using liquid dewetting permporometry with a capillary flow porometer from PMI (New York, U.S.A.) as described in detail before.3,7 Measurements of pure water permeability in thermostated Amicon cells (Millipore, MA, U.S.A.) as well as calculations of membrane pore density (for unmodified membrane) and reduction of pore diameter (for modified membranes) were also done according to the earlier described procedures.7 Briefly, permeability resulted from dividing the flux by the transmembrane pressure, and the pore size of the membrane was calculated from the permeability data by using the equation of Hagen-Poiseuille: V π × ∆P × r4 ) ∆t 8×η×L

(2)

where V is the volume of the permeate relating to a single cylindrical membrane pore, ∆t is the time interval, ∆P is the transmembrane pressure, r is the pore radius, η is the viscosity of water, and L is the capillary length (i.e., the membrane thickness). The first result is the effective hydrodynamic pore diameter, dh ()2r). The thickness of the grafted layer on the pore wall, lh, is calculated as the difference

Figure 3. Pore size distributions from liquid dewetting permporometry for unmodified membrane (s), ATRP initiator-modified membrane (- - -), and a PNIPAAm-grafted membrane with a DG of 6.1 µg/cm2 (prepared using 0.75 mol/L NIPAAm at 40 min reaction time (‚ ‚ ‚); cf. Table 1). between the pore radius of the initiator-immobilized membrane and the pore radius of the grafted membrane.

Results and Discussion The reaction sequence for the surface functionalization, including immobilization of the ATRP initiator and graft copolymerization, is summarized in Figure 2. The pore diameter of the original membrane determined by permporometry was ∼760 nm (Figure 3). Such considerable discrepancy between nominal pore diameter according to this manufacturer and experimental data from two independent methods (permporometry and SEM) has been observed and discussed before.3,7 Presumably, the manufacturer had “over-etched” this batch of membrane (as also indicated by the relatively large porosity of 18.3%) and used only flux-based characterization methods. Initially, the surface of the PET base membranes was treated by oxidative hydrolysis to maximize the density of carboxyl groups without degradation of the pore structure.7 The pore diameter determined by permporometry after this step was 790 nm, and this slight increase was due to dissolution of fractions of degraded PET. The obtained concentration relative to the specific surface area was 155 pmol/ cm2 (i.e., 1.5 carboxyl groups per nm2).

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Figure 4. Degree of grafting, DG, of PET track-etched membranes as a function of monomer concentration at constant reaction time (40 min) and 25 °C.

Thereafter, the bromoisobutyrate initiator was immobilized in two steps via straightforward solid-phase reactions. Conditions have been developed so that high conversion is achieved on the one hand and the isoporous structure of the membrane is preserved on the other hand. Most important, the cylindrical pore morphology with very narrow size distribution of the PET membranes remained unchanged after initiator fixation, as indicated by SEM data (not shown) and permporometry (the pore diameter was only slightly increased from ∼760 to ∼790 nm, but this value was identical with the data measured after oxidative hydrolysis; cf. Figure 3). The carboxyl concentration after the first step was 70 pmol/cm2, corresponding to a carboxyl conversion of 55%. This conversion is somewhat lower than for the reaction with diethylamine in the previous work (85%).7 No quantitative data for the PET-bound bromo compound and, hence, for the conversion in the second step are available because this analysis on the surface of a technical polymeric material is rather challenging. However, due to the moderate hydroxyl density (cf. above) and the high reactivity of the R-bromoisobutyrylbromide, close to quantitative conversion of the surface hydroxyl groups may be expected (cf. ref 25). Therefore, the surface density of initiator with respect to the specific surface area (and hence pore surface) is not known exactly, but it should not be larger than ∼0.8 groups per nm2. Another batch of membranes with “diluted” ATRP initiator on the PET surface was prepared by using a 1:3 mixture of ethanolamine and propylamine in the step before the acylation reaction (cf. Figure 2 and Experimental Section). Finally, the PET membranes with surface-bound initiator were functionalized under conditions identified by other groups as well suited for ATRP grafting of NIPAAm on planar gold surfaces.14 Solvent was a mixture of methanol and water in a volume ratio of 7:3. A molar ratio NIPAAm/copper(I) of 100:1 and the ligand PMDETA were used. The ratio of PMDETA/ copper(I) of 3:1 permits that all copper(I) remains in solution during the polymerization. Under those conditions, we achieved with relatively low monomer concentration in relatively short reaction times at ambient temperature significant amounts of (25) Bao, Z.; Bruening, M. L.; Baker, G. L. Macromolecules 2006, 39, 5251.

Figure 5. Degree of grafting, DG, of PET track-etched membranes as a function of reaction time at constant monomer concentration (0.75 mol/L) and 25 °C.

grafted polymer which could be determined gravimetrically. Transmission IR analyses enabled the identification of the characteristic amide band for the grafted PNIPAAm on the PET membranes, and a good correlation between normalized IR intensities of this band and degree of grafting from gravimetry has been found (see Supporting Information). Figure 4 shows the degree of grafting relative to the specific surface area of the membrane as a function of monomer concentration at constant reaction time, and the results for a functionalization series with increasing reaction time at constant monomer concentration are presented in Figure 5. Further, the results obtained after identical grafting conditions for membranes with diluted ATRP initiator are compared with those for the membranes having the highest initiator density achieved under our reaction conditions (Table 1).

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Table 1. Overview of Membrane Preparations with Two Different Prefunctionalization Conditions (Maximum Hydroxyl Density and 1:3 Dilution)a Prefunctionalization Mixture ethanolamine/ propylamine 1:3 ethanolamine (mol/mol) DG [µg/cm2] 6.1 ( 0.3 dry layer thickness [nm] 55 ( 3 water permeability [L/m2‚bar‚min] @ 25 °C 8123 ( 1051 @ 45 °C 54201 ( 1599 hydrodynamic layer thickness [nm] @ 25 °C 159 ( 8 @ 45 ˚C 51 ( 2 number of independent samples 4

4.3 ( 0.4 39 ( 4 10695 ( 689 65789 ( 3223 142 ( 5 34 ( 5 5

a Identical subsequent functionalization steps (including immobilization of the ATRP initiator and ATRP grafting with 0.75 mol/L NIPAAm for 40 min (cf. Figure 2): dry layer thickness was estimated from degree of grafting (DG); hydrodynamic layer thickness at low and high temperature was deduced from water permeability.

The dry layer thickness estimated from gravimetry assuming an even coverage of the entire PET surface and ignoring the small additional effect of curvature of the concave pore surface (cf. Table 1) agreed quite well with the change of diameter of the cylindrical pores because a reduction from ∼790 to ∼700 nm had been measured (cf. Figure 3). The slight deviation can be explained by some minor irregularities in the shape of the track-etched pores discussed in an earlier paper,3 that is, some regions with wider diameters within the membrane which, however, do not extend through the entire thickness. Grafted polymer in those regions will not fully contribute to the pore diameter measured with the pore-dewetting method because according to this method’s principle only the smallest diameter of a cylindrical pore is detected. It is possible to control quite precisely the DG value by changing monomer concentration at moderate reaction time because the reproducibility is good (cf. Table 1) and the trend is almost linear (cf. Figure 4). This is what would be expected for a controlled polymerization. The slight decrease of slope at higher monomer concentration may be due to some termination reactions or depletion of monomer in the pores. It should be noted that the highest DG values of up to 18 µg/cm2 correspond to a degree of pore filling of ∼90% (based on the membrane porosity of 18.3% and a polymer density of 1.1 g/cm3, a degree pore filling of 100% is equivalent to a DG of ∼20 µg/cm2) and can only be obtained by diffusion of 5-6 times more monomer into the membrane pores than present there in the beginning of the reaction (note that synthesis is performed in methanol/water, i.e., with PNIPAAm not in fully swollen conformation). The trend for increasing reaction time is different: after a very steep DG increase for very short reaction time, the functionalization seems to reach a region with significantly slower but almost constant rate (between ∼10 and 120 min; cf. Figure 5). This result is similar to several other studies on surfaceinitiated ATRP, and it had been related to a high local concentration of active species in the boundary layer close to the surface leading to initiator deactivation and also to hindrance of chain growth due to increasing layer thickness.12-15 Plunkett et al.14 had used identical reaction conditions for ATRP grafting of NIPAAm from a planar initiator SAM gold surface: They had also observed a strong decay of grafting rate with time, and after 40 min reaction time, they had obtained dry layers with 140 nm thickness from 3.9 mol/L NIPAAm and 40 nm from 2.0 mol/L NIPAAm solution. Our data for the PET surface (including the pore walls) are 165 nm (3.9 mol/L) and 105 nm (2.0 mol/L).

Irrespective of the deviations in absolute values, it is remarkable that such similar results have been obtained for two completely different substrates (SAM14 vs polymer and planar14 vs porous) with different analysis methods (ellipsometry and AFM14 vs gravimetry). The dilution of surface-bound initiator yielded lower DG values under otherwise identical reaction conditions (cf. Table 1). This should primarily be related to a lower grafting density. However, because the values are higher than expected based on an assumed initiator density of only 25%, the termination of initiation and hindrance of chain growth evoked above seemed to be less pronounced than for the maximum initiator density. Qualitatively, this trend is in line with results of other groups.25 All functionalized membranes had significantly lower water permeability than unmodified ones, and permeabilities below and above the LCST of 32 °C were markedly different (cf. Table 1). The analysis of effective grafted PNIPAAm layer thickness based on water permeabilities of the membranes had been done as developed and discussed before,7 and the results are summarized in Figure 6. It is important to note that this analysis requires a well-defined and well-characterized membrane pore structure with cylindrical pores at very narrow size distribution and, hence, known specific surface area because all of these characteristics are used for the calculations from measured data. That this assumption is indeed valid is strongly supported by the permporometry data (cf. Figure 3). The same track-etched PET membrane type as in the previous study has been used (nevertheless, the batches differed slightly in initial pore diameter: 6927 vs 760 nm), and the results for the membranes from ATRP grafting are also compared with those synthesized using photografting.7 The first important observation is the very good linear correlation between the DG value (expressed as dry layer thickness) and ATRP-grafted PNIPAAm layer thickness at 25 °C. At 45 °C, the same is true for membranes up to a moderate DG of up to 9 µg/cm2, while a significant deviation to larger values is observed for membranes at higher DG. It should be noted that the latter samples could not be measured at 25 °C because the pores were blocked (cf. Figure 6). That the slope at 45 °C was close to 1 can be taken as indirect evidence that, similar to the case of photografting,7 the entire membrane surface including all pore walls had been rather evenly functionalized (the slight deviation to smaller values can again be related to the minor irregularities in pore shape; cf. discussion above). Consequently, an increase in DG leads to a proportional increase in grafted layer thickness in water. The slope at 25 °C is about 3-fold higher than that at 45 °C, and this means that the layer thickness in the swollen conformation (T < LCST) is 3-fold larger than that in the collapsed conformation (T > LCST). However, grafted PNIPAAm synthesized using the adsorbed photoinitiator and UV irradiation had approximately the same layer thickness in the collapsed state, but upon temperature decrease, the layers expanded by a factor of 15.6. Hence, the ATRP-grafted PNIPAAm layers had a much smaller temperature response. It is interesting to note that the membranes prepared using the diluted ATRP initiator fit very well to the correlation at high temperature, but a significant deviation to larger swelling is seen at low temperature (cf. Figure 6). With known polymer mass per surface area (i.e., DG) and layer thickness in water, the average density of the polymer in the grafted layer can be calculated; the results for PET membranes with grafted PNIPAAm prepared by two different methods are summarized in Table 2. For all PNIPAAm layers, the average polymer density at high temperature is around 1 g/cm3, indicating

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Figure 6. Effective hydrodynamic PNIPAAm layer thickness at 25 and 45 °C, estimated from water permeability data and pore diameter and pore density of the membrane before grafting, as function of the dry layer thickness, calculated from DG assuming even coverage of the specific surface area of the membrane with grafted polymer (∆, 0 ) synthesized via ATRP at varied monomer concentration, and measured at 25 and 45 °C, respectively; 2, 9 ) synthesized via ATRP at varied reaction time, and measured at 25 and 45 °C, respectively; O, ] ) synthesized via ATRP from membrane with diluted initiator (cf. Table 1) and measured at 25 and 45 °C, respectively; - - - ) linear fits for membranes from ATRP grafting, in order to enable a comparison between high and low temperature; the linear regressions at 25 and 45 °C have been done for the same set of samples (i.e., up to 90 nm dry thickness) - - - ) linear fits for membranes from photografting7). Table 2. Average PNIPAAm Density in the Grafted Layer in Water at Low and High Temperature for Two Different Preparation Methodsa Average Density of Grafted Polymer, Fp [g/cm3] preparation method

@ 25 °C

@ 45 °C

photografting ATRP grafting ATRP grafting, dilute initiator

0.06 0.37 0.31 ( 0.04

0.82 1.04 ∼1

a DG ranges: ATRP grafting 2.5-18.6 µg/cm2, photografting 0.4-5 µg/cm2; values calculated from linear fits in Figure 6 or (for ATRP grafting with dilute initiator) data in Table 1.

that the water content of the collapsed layer is low. In contrast, at low temperature, a very pronounced difference is seen as a function of preparation method. While the PNIPAAm density is very low after photografting, the value is markedly higher after ATRP grafting. In fact, water content for the ATRP-grafted PNIPAAm layers is much lower than data typically observed for free swelling of PNIPAAm in water (e.g., Fp < 0.03 g/cm3 for a very weakly cross-linked PNIPAAm hydrogel at 25 °C26). We do not claim that the absolute density values are very precise, but we note that the obtained data give most valuable information about the internal structure of the grafted layers in water. In fact, these data provide strong evidence that the ATRP-grafted PNIPAAm layers in their swollen state are in the “brush” conformation, while the photografted layers are in the “mushroom” regime (cf. Figure 1). This can be directly related to the grafting density, which is obviously much higher for the ATRP(26) Fa¨nger, C.; Wack, H.; Ulbricht, M. Macromol. Biosci. 2006, 6, 693.

grafted than for the photografted structures. Plunkett et al.14 had done (after dissolving the SAMs from the substrate) independent analyses of molar mass for grafted PNIPAAm for layers prepared under the same reaction conditions as in our study, and they had found that for molar masses between ∼50 and ∼300 kg/mol a density of as little as 0.05 chains per nm2 would still lead to a brush conformation. Grafted PNIPAAm with molar masses between ∼100 and 800 kg/mol at a density of 0.04 chains per nm2 on polystyrene particles was also found to be in the brush regime.17 The maximum initiator density in our study is 0.8 groups/nm2 (cf. above; but because selective graft copolymer cleavage is much more complicated in our case, we have no data for molar mass of grafted PNIPAAm). Therefore, even at a low initiation efficiency of ∼10%, as often found for surface-initiated ATRP from materials with maximum initiator density (see, for example, ref 25), brush layers in the PET membrane pores are obtained when the reaction conditions allow formation of high enough molar masses (note that our dry layer thicknesses are in the same range as those in the other studies;14 cf. above). Further, it is very interesting to note that, for the membranes where the initiator density had been reduced (cf. Table 1), a significantly larger swollen layer thickness (cf. Figure 6) and, hence, lower average polymer density have been found (cf. Table 2). This is in agreement with experimental and theoretical data indicating that PNIPAAm brushes of same average chain length show a maximum in the temperature-induced swelling/deswelling ratio at lower grafting densities.27 (27) Mendez, S.; Curro, J. G.; McCoy, J. D.; Lopez, G. P. Macromolecules 2005, 38, 174.

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It should be noted that our findings with respect to PNIPAAm swelling including its temperature response are in marked contrast to the data of Plunkett et al.14 who reported between 3 and 10 times larger layer thickness for collapsed PNIPAAm in water at 36 °C (from force-distance measurements) than in the dry state (from ellipsometry or AFM); the further increase in thickness in water after lowering the temperature to 26 °C (factor of 0-0.3, from force-distance measurements) had been much smaller than found in other work. For example, for PNIPAAm on colloidal particles at a grafting density of 0.043 chains per nm2, a swelling/ deswelling factor of 5-6 had been measured by light scattering,17 while in our present study, this factor was ∼3 for the maximum grafting density and ∼4.2 for the lower grafting density. The much larger temperature response of layer thickness for the photografted membranes may be qualitatively explained by the low grafting density along with a broad chain length distribution (due to the uncontrolled radical polymerization). At the same degree of grafting and in a capillary of the same diameter, such polymer layers will reduce the water flux much more than a densely packed brush layer consisting of chains with narrow length distribution (as expected for ATRP grafting). With increasing concentration of the monomer solution in ATRP grafting, we have observed deviations from the linear growth of the films (cf. Figure 4). The resulting membranes with higher DG values show also significant deviations toward larger layer thickness (in collapsed state) as compared to the membranes in the lower DG range (cf. Figure 6). The same effect of PNIPAAm chain length polydispersity as evoked above for the photografted samples may serve as an explanation for this behavior. Even for an ideal PNIPAAm brush layer, a density gradient from the (planar) surface has been predicted.27 Furthermore, the curvature of the substrate has an additional impact, as it has been demonstrated by the increasing density gradient for “spherical” PNIPAAm brushes on microparticles (diameter 509 nm).17 For our “concave” pore surfaces (diameter ∼790 nm), the opposite effect would be expected. However, one must keep in mind that the layer thickness is deduced from liquid flow through the pores. For the low-density, photografted PNIPAAm layers, significant effects of shear rate onto effective layer thickness have been observed before (e.g., an increase of flow rate by a factor of 2.5 led to a reduction of the swelling/deswelling ratio from 16 to 12).7 Preliminary data indicate that this shear sensitivity is indeed lower for the ATRP-grafted PNIPAAm samples. However, more

Friebe and Ulbricht

studies with a broader range of grafted membranes and flow conditions are necessary and already in progress. For an even better control over surface-initiated ATRP grafting, an optimization of the reaction conditions by fine-tuning the initiator reactivity, in particular, by using a copper(I)/copper(II) system in order to increase the “living” character of the ATRP12,13 is also underway.

Conclusions In this study, we have demonstrated that porous base materials from technical polymers can be functionalized in a controlled way by surface-initiated ATRP, and that important information about the functionalization mechanism as well as the structure and functionality of the resulting porous composite materials can be obtained by using the cylindrical pores of track-etched PET membranes as a model system. All data indicate that, for the maximum initiator density introduced to PET surfaces by solid-phase synthesis, grafted PNIPAAm layers with a brush structure can be synthesized, and both monomer concentration and reaction time can be used as parameters to adjust the degree of grafting. For dry PNIPAAm layer thicknesses on the PET pore walls of up to 80 nm, a temperature-induced swelling/ deswelling ratio of ∼3 has been observed. Reduction of the brush grafting density, via composition of the reaction mixture used in the solid-phase synthesis for initiator immobilization, led to an increase of that swelling/deswelling ratio. On the basis of our results, it is expected that the attractive features of controlled polymerization can be used for the functionalization of the pore space in the submicrometer dimension with many different functional macromolecules at varied densities and layer thicknesses, and including various grafted architectures, including block copolymers. Such complex stimuli-responsive materials could be developed into novel modules, such as for controlled release, which may find applications in microfluidic systems or other applications. Supporting Information Available: Transmission FTIR spectra of PET membranes with grafted PNIPAAm and correlation between relative IR intensity for amide band and gravimetrically determined degree of grafting. This material is available free of charge via the Internet at http://pubs.acs.org. LA7016962