stimulus - responsive 
 surfaces continue to attract considerable interest 
 since they enable a range of properties such as wettability , stiffness , 
 biocompatibility , frictional coefficients , or specific binding to 
 be regulated and controlled . in principle , such smart  surfaces have diverse potential 
 applications , such as drug delivery , aqueous lubrication , coatings for biomedical devices , and nanoparticle 
 transport . in particular , the combination of the so - called  grafting 
 from  process with living radical polymerization techniques 
 such as atom transfer radical polymerization ( atrp ) has enabled well - defined polymer brushes to be prepared with thicknesses 
 of up to several hundred nanometers from either planar or colloidal 
 substrates . depending on the chemical composition of such polymer brushes , 
 their conformation can be manipulated using external stimuli such as ph , temperature , solvent , and light or an 
 electrical stimulus . for example , 
 okano and co - workers reported that planar substrates coated with 
 thermo - responsive poly(n - isopropylacrylamide ) brushes 
 enable cell adhesion on the collapsed hydrophobic brush layer at 37 
 c with subsequent facile cell detachment on cooling below 32 
 c , since the brush becomes highly hydrophilic under these conditions . one of the most studied classes of stimulus - responsive 
 surfaces 
 is ph - responsive polymer brushes . ionization ( or protonation ) of functional 
 groups in the brush structure causes chain extension or collapse , 
 depending on the solution ph in which the brush layer is immersed . for example , both polyacids , such as poly(acrylic acid ) or poly(methacrylic acid ) , and polybases , such 
 as poly(2-(dimethylamino)ethyl methacrylate ) , poly(2-(diethylamino)ethyl methacrylate ) , or poly(2-(diisopropylamino)ethyl methacrylate ) , have been widely studied by various research groups . at low ph , weak polybase brushes swell when the counterion contribution 
 to the osmotic pressure exceeds the combinatorial excluded volume 
 contribution . moglianetti et al . reported that 
 pdmaema brushes became more swollen below ph 3 as a result of a greater 
 osmotic driving force , causing the brush chains to stretch away from 
 the interface . the fraction of protonated 
 monomer repeat units varied through the brush layer , with lower degrees 
 of protonation within the brush interior . jia et al . reported that the degree of swelling of pdmaema brushes 
 depended on their grafting density , with lower grafting densities 
 displaying higher degrees of swelling . one commercially available secondary 
 amine - based vinyl monomer is 2-(tert - butylamino)ethyl 
 methacrylate ( tbaema ) . well - defined ptbaema - based block copolymers 
 have been prepared using living anionic polymerization via sequential 
 monomer addition . morse and co - workers recently 
 prepared well - defined lightly cross - linked ptbaema latex particles 
 using aqueous emulsion polymerization . these ph - responsive latexes proved to be effective pickering emulsifiers 
 for a range of oils when emulsified at ph 10 , with demulsification 
 occurring on addition of acid since this caused a latex - to - microgel 
 transition . jerome and co - workers prepared 
 ptbaema - functionalized polyolefin fibers via two protocols . in one case , a polyolefin - ptbaema diblock copolymer was synthesized 
 by atrp and dispersed within low - density polyethylene , which led to 
 bactericidal activity toward e. coli . in the second 
 study , primary amine - functionalized ptbaema chains were prepared via 
 an azide - functional initiator , which led to chemical grafting of the 
 ptbaema to maleic anhydride - grafted polypropylene . dispersion of this 
 graft copolymer within native polypropylene led to long - lasting antibacterial 
 activity . however , to the best of our knowledge , there is very little 
 work on the synthesis and ph - responsive behavior of well - defined ptbaema 
 brushes in the literature . in the present study the ph - responsive behavior of these linear 
 brushes was characterized using ellipsometry and compared to that 
 of cross - linked brushes . for example , if stimulus - responsive polymer 
 brushes can be covalently cross - linked while achieving spatial confinement , 
 this is expected to produce more robust surface layers with minimal 
 detrimental effect on their stimulus - responsive character . cross - linking was achieved using a commercially available polymeric 
 diisocyanate , which reacts with the secondary amine groups in the 
 tbaema residues . spatial control was achieved by selecting the solvent 
 used for the cross - linking reaction . thus , using a good solvent ( e.g. , 
 thf ) for the ptbaema brush allows relatively uniform cross - linking 
 throughout the highly swollen brush layer , whereas using a poor solvent 
 ( e.g. , n - hexane ) leads to cross - linking being confined 
 to the upper surface of the collapsed brush ( see scheme 1 ) . in principle , such spatial confinement should significantly 
 affect the ph - responsive behavior of the brush layer : this hypothesis 
 is examined using various surface characterization techniques , including 
 in situ ellipsometry , atomic force microscopy ( afm ) , and x - ray photoelectron 
 spectroscopy ( xps ) . the former solvent results 
 in a uniformly cross - linked brush , whereas the latter results in a 
 surface - cross - linked brush . silicon wafers ( orientation , boron - doped , 
 0100  cm ) were purchased from compart technology ( peterborough , 
 u.k . ) . 3-aminopropyltriethoxysilane ( aptes ) ( > 98% ) , 2-bromoisobutyryl 
 bromide ( bibb ) ( 98% ) , and triethylamine ( 99% ) were purchased from 
 sigma - aldrich ( gillingham , u.k . ) . hydrogen peroxide ( 30% ) , sulfuric 
 acid ( 95% ) , ethanol ( 99.8% , hplc grade ) , ammonium hydroxide ( analar ) , 
 dichloromethane ( hplc grade ) , tetrahydrofuran ( thf ) ( 99.5% , hplc grade ) , n - hexane ( hplc grade ) , and basic alumina were obtained from 
 fisher scientific ( loughborough , u.k . ) . copper(i ) 
 bromide ( > 98% ) , copper(ii ) bromide ( > 99% ) , tolylene 2,4-diisocyanate - terminated 
 poly(propylene glycol ) ( ppg - tdi ) , tris(2-pyridylmethyl)amine ( tpma , 
 98% ) , and 2-(tert - butylamino)ethyl methacrylate ( tbaema , 
 97% ) were obtained from sigma - aldrich ( gillingham , u.k . ) . tbaema was treated with basic 
 alumina to remove inhibitor and stored at 4 c before use . solution for 2 h. ( caution : piranha solution comprises three 
 parts hydrogen peroxide to seven parts concentrated sulfuric acid ; 
 it is an extremely strong oxidizing agent that has been known to detonate 
 spontaneously upon contact with organic material ) . the treated 
 glassware and substrates were rinsed copiously with deionized water 
 and then sonicated for 10 min , followed by oven drying at 120 c 
 for 1 h. these clean silicon wafers were then immersed in a 1:1:5 
 solution of ammonium hydroxide , 30% hydrogen peroxide , and deionized 
 water . this reaction solution was heated to 85 c for 30 min 
 before being allowed to cool to 20 c . the treated wafers were 
 rinsed with deionized water , sonicated , and then oven - dried before 
 use . a 2.0% v / v ethanolic solution of 3-aminopropyltriethoxysilane 
 ( aptes ) was aged for 5 min at 20 c . silicon wafers were immersed 
 in this aptes solution for 30 min , rinsed with ethanol , dried using 
 a nitrogen gas stream , and annealed for 30 min at 120 c . the resulting amine - functionalized wafers were then immersed in 
 a solution of 2-bromoisobutyryl bromide ( bibb ) ( 0.37 ml , 3 mmol ) and 
 triethylamine ( 0.41 ml , 3 mmol ) in dichloromethane ( dcm ; 60 ml ) for 
 30 min at 20 c . subsequently , the initiator - functionalized wafers 
 were rinsed with ethanol and dcm and dried using a nitrogen gas stream 
 prior to use . a coherent innova 300c fred frequency - doubled argon 
 ion laser ( coherent 
 uk , ely , u.k . ) was used to irradiate samples at a uv wavelength of 
 244 nm . micropatterns 
 were obtained by irradiation of the 3-(2-bromoisobutyramido)propyl 
 triethoxysilane ( bibaptes ) monolayer on the silicon wafer using an 
 electron microscope copper grid ( agar , cambridge , u.k . ) as a convenient 
 mask . interferometric lithography ( il ) was conducted 
 using a two - beam interference system ( lloyd s mirror ) , as reported 
 elsewhere . micropatterning was achieved 
 by exposing bibbaptes - functionalized silicon wafers to 2.2 j cm laser radiation at 244 nm using a 2000 mesh electron 
 microscope grid as a mask . friction force microscopy 
 ( ffm ) was used to image the resulting surface patterns . nanoscale 
 patterning was carried out using il to achieve photolytic patterning 
 of the bibbaptes surface layer on the silicon wafer . a lloyd s 
 mirror dual - beam interferometer was utilized whereby a coherent uv 
 laser beam was directed toward a mirror and the sample was placed 
 at an angular separation of 2. one - half of the laser beam is 
 incident on the mirror , from where it is reflected onto the sample 
 to interfere with the other half of the beam , forming a sinusoidal 
 intensity pattern with a period of /(2  sin   ) , 
 where  is the laser wavelength of 244 nm . in this experiment , tbaema ( 5.0 g , 27 
 mmol ) was dissolved in isopropanol ( ipa ; 5.0 ml ) at 20 c , deoxygenated 
 for 30 min , and stored under n2 prior to use . tpma ( 58.8 
 mg , 0.203 mmol ) and cu(ii)br2 ( 15.0 mg , 0.068 mmol ) were 
 added to the solution and deoxygenated for 20 min , after which cu(i)br 
 ( 19.0 mg , 0.135 mmol ) was added and the monomer / catalyst mixture was 
 deoxygenated for a further 10 min . initiator - coated wafers were sealed 
 in schlenk tubes and deoxygenated via three vacuum / nitrogen cycles . the monomer / catalyst solution ( 5 ml ) was syringed into each tube under 
 a nitrogen atmosphere , and surface polymerization of tbaema was allowed 
 to proceed at 20 c for a given reaction time . each polymerization 
 was quenched after the desired reaction time by removing the wafer 
 from its schlenk tube , followed by washing with ipa and ethanol several 
 times . uniformly 
 cross - linked ptbaema brushes ( with dry brush thicknesses ranging from 
 4 to 28 nm , as judged by ellipsometry ) were prepared by immersing 
 the linear ptbaema brush - coated wafers prepared as described : 1 ml 
 of a 1 mg / l stock solution of 2,4-diisocyanate - terminated poly(propylene 
 glycol ) ( ppg - tdi ) in dry thf at 20 c . the grafting density 
 (  ) of the ptbaema brushes was calculated from the dry brush 
 thickness ( h ) using  = hna / mn , where 
  is the density of ptbaema ( 1.1 g cm),na is 
 avogadro s number , and mn is the 
 number - average molecular weight . while the thickness h is readily determined by ellipsometry , mn values are not available . however , following established literature 
 protocols , we also synthesized soluble 
 ( nongrafted ) ptbaema chains in the bulk solution using a soluble initiator 
 while simultaneously growing brushes from an initiator - modified planar 
 silicon wafer . if we assume that the rate of polymerization in solution 
 is the same as that on the planar surface , this allows the relationship between the mn of the soluble ptbaema chains ( determined by gpc ) and the 
 ellipsometric brush thickness ( h ) for the ptbaema 
 brush to be established . this approximate method has been used by 
 other groups , even though it is known that 
 the mn of solution - grown polymers is not 
 precisely the same as that of polymer brushes grown from planar surface . gpc analysis of free ptbaema chains grown in 
 solution indicates an mn of 12  700 
 g mol ( vs poly(methyl methacrylate ) standards ) . 
 a ptbaema brush of 10 nm dry thickness grown in the same reaction 
 solution is assumed to have the above mn value ( hence , the likely systematic error incurred by using inappropriate 
 gpc standards is ignored in this analysis ) . on this basis , the brush 
 grafting density was calculated to be 0.55 chains nm . for this 10 nm ptbaema brush grown from a 1 cm area 
 of silicon wafer ( 9.7  10 mol ) we used 1.0 
 ml of a 1.0 mg dm stock solution of 2,4-diisocyanate - terminated 
 poly(propylene glycol ) ( ppg - tdi ) ( 4.34  10 mol ) for the cross - linking reaction ; thus , we calculate that there 
 are approximately 22 secondary amine groups per isocyanate . given 
 that some fraction of the isocyanate groups is likely to be wasted 
 via intramolecular reaction with secondary amine groups on the same 
 brush chain ( as opposed to the desired intermolecular cross - linking 
 reaction ) , this is considered to be an upper limit value . this 
 protocol ensures that the brush layer is not overcross - linked , 
 so that it retains some degree of stimulus - responsive behavior . after 
 10 min , the wafer was rinsed copiously with thf to remove any unreacted 
 ppg - tdi cross - linker and dried under a stream of nitrogen gas . the 
 same protocol was utilized to prepare surface - cross - linked ptbaema 
 brushes , except that dry n - hexane was used instead 
 of thf . ellipsometric studies were 
 conducted using a alph - se ellipsometer ( j. a. woollam co. , lincoln , 
 ne ) equipped with a he  ne laser (  = 633 nm ) at an incident 
 angle (  ) of 70 from the normal . measurements were conducted 
 from 300 to 700 nm , and modeling was performed using wvase software 
 ( j. a. woollam co. , usa ) . fit quality was assessed using the root - mean - square 
 error ( rmse ) between the measured and the modeled ellipsometric constants  
 and  over all measured wavelengths . the dry films were modeled 
 as a single layer of variable thickness with refractive index given 
 by the cauchy parameters of an = 1.5 m and bn = 0.005 m ( found by fitting these values for a thick ptbaema film ) . the ellipsometric 
 brush thickness for each sample was determined in at least three different 
 places on the wafer and reported as the mean  standard error . in situ measurements of brush thickness in aqueous 
 solution the sample cell 
 was rinsed several times with deionized water between each measurement . ellipsometric data were fitted using a single - slab model with a refractive 
 index given by a linear effective medium approximation ( ema ) between 
 the ptbaema brush and water . again , three measurements 
 were recorded for each brush sample and reported as the mean  
 standard error . afm measurements were conducted using a digital 
 instruments nanoscope iv multimode atomic force microscope ( veeco , 
 santa barbara , ca ) with a silicon nitride nanoprobes ( digital instruments , cambridge , 
 u.k . ) with nominal spring constants of either 0.06 or 0.12 n m and tip radii of 2060 nm were used for contact 
 mode imaging . silicon probes with nominal spring constants ranging 
 from 20 to 80 n m were used for tapping mode imaging . all samples were allowed to stand in the liquid cell for at least 
 5 min prior to any measurements in order to attain equilibrium . mean 
 heights were determined for both dry micro- and nanopatterned brushes 
 in air and also for swollen brushes immersed in ph buffer solutions 
 ranging from ph 2 to 12 . x - ray photoelectron spectroscopy was 
 carried out using a kratos 
 axis ultra spectrometer ( kratos analytical , manchester , u.k . ) with 
 a monochromatic al k x - ray source operating at a power of 150 
 w with an emission current of 8 ma . electron energy analyzer pass energies of 20 and 160 ev were 
 used to acquire core - line spectra and survey scans , respectively . core - line spectra were peak - fitted 
 using casa xps software , and all binding energies were referenced 
 relative to the main hydrocarbon c1s signal calibrated at 285 ev . a malvern zetasizer nanozs model zen 3600 instrument equipped with 
 a malvern streaming potential cell attachment was used to measure 
 the surface zeta potential of these polymer brushes . surface zeta 
 potentials were measured at 25 c in the presence of 1 mm kcl 
 using a sterically stabilized polystyrene latex as noninteracting 
 tracer particles . the steric stabilizer was selected to be a highly 
 hydrophilic zwitterionic polymer , namely , poly(2-(methacryloyloxy)ethyl 
 phosphorylcholine ) ( pmpc ) . a pmpc - stabilized polystyrene latex was 
 synthesized as reported previously by armes and co - workers . the first stage in 
 brush synthesis was functionalization of a silica surface with 3-aminopropyltriethoxysilane 
 ( see scheme 2a ) . the surface amine groups were 
 subsequently converted into 2-bromoisobutyryl amide initiator sites 
 by reaction with excess 2-bromoisobutyryl bromide . the resulting films 
 were characterized by xps ( see figures s1a and s1b , supporting information ) . this living radical 
 polymerization technique has been widely used to grow polymer brushes . in particular , ding et al . reported the rather slow growth of ptbaema brushes from 
 a planar silicon substrate via living radical polymerization . an atrp 
 formulation involved using a cucl / cucl2/hmteta catalyst 
 at 90 c in the bulk , whereas a single electron transfer living 
 radical polymerization ( set - lrp ) formulation employed cu(0 ) in dmso 
 at 75 c . the former protocol produced 
 a relatively well - defined brush of around 20 nm dry thickness after 
 72 h , while the latter gave a less uniform brush of approximately 
 17 nm after 96 h. however , no detailed information regarding the kinetics 
 of ptbaema brush growth was reported for either formulation , and perhaps 
 surprisingly , the ph - responsive behavior of such brushes was not investigated . various formulations were examined 
 in the present work , including 
 different combinations of ligand , catalyst , solvent , and temperature . the objective was to identify a suitable formulation that would allow 
 well - controlled ptbaema brush growth to be achieved within a few hours 
 at 20 c. empirically , it was found that a suitable atrp 
 formulation comprised cu(i)br , cu(ii)br2 , and tpma ligand 
 in ipa , with a [ tbaema]:[cubr]:[cubr2]:[tpma ] relative 
 molar ratio of 200:1:0.5:1.5 . figure 1 shows 
 the ptbaema brush dry thickness as a function of polymerization time 
 using this optimized protocol . a linear growth regime was observed 
 from 15 to 125 min , with departure from linearity between 150 
 and 350 min as the rate of surface polymerization becomes retarded . evolution 
 of ellipsometric dry brush thickness versus polymerization 
 time for growth of linear ptbaema brushes prepared via surface atrp 
 in isopropyl alcohol at 20 c . brushes were cross - linked using tolylene 2,4-diisocyanate - terminated 
 poly(propylene glycol ) ( ppg - tdi ) dissolved in either thf or n - hexane . the molar ratio of secondary amine ( from ptbaema 
 brushes ) to isocyanate end - groups ( from ppg - tdi ) was approximately 
 22 . this ensures that the brush layer is not over - cross - linked , so 
 that it retains its stimulus - responsive behavior after cross - linking . xps is a surface - specific analysis technique with a sampling depth 
 of around 10 nm . the elemental compositions of the linear , uniformly 
 cross - linked , and surface - cross - linked ptbaema brushes are presented 
 in table 1 . the dry brush thickness was 18 
 nm in each case . for linear ptbaema brushes , the elemental compositions 
 ( c , n , o ) were similar to the calculated values ( c = 77.0% , n = 7.6% , 
 and o = 15.3% ) . after cross - linking , the o / n atomic ratio increased 
 because of the introduction of o atoms in the ppg - tdi cross - linker . the o / n atomic ratio increased for the surface - cross - linked ptbaema 
 brushes , suggesting that there is a higher degree of cross - linking 
 at the brush surface . these observations are consistent with the hypothesis 
 that ppg - tdi cross - linking conducted using a poor solvent ( n - hexane ) yields cross - linking that is predominantly confined 
 to the upper surface of the brush layer . there is a higher o / n atomic 
 ratio for the uniformly cross - linked brushes prepared in thf compared 
 to the linear brush . the c1s spectrum of an as - prepared ptbaema brush is 
 shown in figure 2a . the peaks corresponding 
 to hydrocarbon carbon 
 atoms at 285.0 ev ( c  c  c ) 
 and to the carbon atoms adjacent to nitrogen at 285.8 ev ( c  c  nh ) were separated by an energy difference 
 close to that of the resolution of the spectrometer ; hence they were 
 fitted using a single component . separate components were included 
 to account for the c  c  o 
 signal at 286.4 ev and the ester carbonyl component at 288.9 ev . the 
 respective relative peak areas after fitting were 5.7:2.41:1 , which 
 are in reasonable agreement with the calculated theoretical ratio 
 of 6:3:1 . x - ray photoelectron core - line spectra recorded for a series of 
 ptbaema brushes ( each of 18 nm dry thickness ) . ( a ) c1s spectrum obtained 
 for a linear ptbaema brush . ( b ) c1s spectrum obtained for the same 
 surface - cross - linked ptbaema brush prepared using ppg - tdi in n - hexane . ( c ) c1s spectrum obtained for the same uniformly 
 cross - linked ptbaema brush prepared using ppg - tdi in thf . the c1s spectrum of a ptbaema brush after surface 
 cross - linking 
 in n - hexane is presented in figure 2b . a substantial increase in intensity is observed for the 
 c  c  o component at 286.4 ev . this is attributed to the presence of additional c  o bonds 
 contributed by the ppg - tdi cross - linker ( which confers 34 polymerized 
 propylene glycol units per cross - link ) . the actual degree of cross - linking 
 can not be determined easily , since some unknown fraction of the ppg - tdi 
 reagent is likely to also react with two n h bonds on the same 
 ptbaema chain to form an intramolecular cycle , rather than forming 
 an intermolecular cross - link . cross - linking the ptbaema brush 
 in thf ( figure 2c ) also led to an increase 
 in the contribution from the c  c  o 
 component . however , this change was smaller 
 than that observed after cross - linking in n - hexane . 
 given that the xps sampling depth is ca . 10 nm , these data are consistent 
 with the hypothesis that using ppg - tdi in combination with a good 
 solvent ( thf ) yields more extensive uniform cross - linking within the 
 brush layer ( and hence a somewhat lower extent of surface cross - linking ) . thus the cross - links are concentrated within the xps sampling depth 
 for collapsed brushes cross - linked in n - hexane , but 
 are distributed more uniformly throughout the brush layer when cross - linking 
 is conducted in thf ; hence , only some fraction of the cross - links 
 are located within the xps sampling depth in the latter case . tapping - mode afm height images of 
 linear and cross - linked ptbaema brushes grown from planar silicon 
 wafers ( 15 nm dry brush thickness ) were acquired by afm ( figure 
 s2 , supporting information ) . a modest increase 
 in the root - mean - square roughness was observed after cross - linking , 
 but the extent of this change was not statistically significant . previous studies have demonstrated 
 that exposure to uv light causes debromination of the initiator , rendering 
 it inactive for subsequent atrp . in friction 
 images of the patterned initiator ( figure s3a , supporting information ) , high contrast was observed between 
 the masked regions ( dark contrast ) and exposed regions ( light contrast ) . the contrast difference arises from the more polar nature of the exposed 
 regions , leading to a stronger adhesive interaction with the afm probe 
 and hence a greater rate of energy dissipation and a larger friction 
 force . figure 3 shows a tapping - mode afm height 
 image recorded for a ptbaema brush film grown from a micropatterned 
 substrate prepared as described above . brighter regions correspond 
 to the brush layer , whereas darker regions correspond to the exposed 
 regions where no brush growth occurs because of photolysis of the 
 c  br bond in the surface - bound initiator sites . the mean brush 
 thickness can be determined by measuring the height difference between 
 the masked and the exposed regions in the image . tapping - mode afm studies 
 of the periodic brush height recorded 
 for a micropatterned linear ptbaema brush : topographical image ( left ) , 
 cross - section analysis ( right ) . ptbaema brushes were also obtained 
 by growing brushes from nanopatterned 
 atrp initiators fabricated by exposure to uv light in an interferometer . a typical friction force microscopy ( ffm ) image is presented in figure 
 s3b ( see supporting information ) . the bright 
 bands of 160  7 nm full width at half - maximum ( fwhm ) correspond 
 to areas of extensive surface modification , resulting from exposure 
 to maxima in the interferogram , whereas the dark bands of 140  
 6 nm ( fwhm ) correspond to regions of little or no surface modification 
 that were exposed to minima in the interferogram . the tapping - mode 
 afm topographical image confirmed that there was no height contrast , 
 indicating that no removal of material had occurred ( figure 4 ) . it was observed that brushes grown from such 
 nanopatterns were significantly thinner than brushes grown from micropatterns 
 under the same conditions . for example , the tapping - mode afm topographical 
 image of nanopatterned ptbaema brushes indicated a mean brush height 
 of 4.5  0.5 nm , whereas for micropatterned ptbaema brushes grown 
 under identical conditions the average brush height was about 14  
 3 nm . . also reported a reduction in brush height on 
 reducing the feature size of the pattern . these workers hypothesized that  at constant grafting density 
 a polymer brush in good solvent adopts a vertically less ordered and 
 laterally more extended conformation , induced by the lack of lateral 
 restraint the brush experiences at its boundaries , leading to less 
 chain crowding and thus less chain stretching .  polymer structures 
 grown from finer patterns are also likely to be more relaxed ( less 
 brush - like ) near the edges of the pattern , resulting in thinner polymer 
 brushes even in the dry state . tapping - mode afm studies of the periodic 
 brush height recorded 
 for a nanopatterned linear ptbaema brush : topographical image ( left ) , 
 cross - section analysis ( right ) . the secondary amine groups presented 
 in the weakly basic ptbaema brushes are readily protonated when immersed 
 in acidic solution . the resulting cationic 
 brushes become highly swollen and stretch away from the surface because 
 of the strong lateral electrostatic repulsive forces between adjacent 
 chains . according to the literature , swelling 
 of polymer brushes may be attributed to two factors : a combinatorial 
 excluded volume effect and a counterion - induced osmotic pressure . the former term is significantly reduced via chemical or physical 
 cross - linking , but the latter term remains relatively unaffected . for example , 
 moglianetti et al . studied the interaction between 
 pdmaema brushes and sodium dodecyl sulfate ( sds ) in both acidic and 
 alkaline media . a reduction in pdmaema brush swelling after physical 
 cross - linking using sds surfactant was attributed to annealing of 
 the excluded volume . the ph - responsive 
 behavior of ptbaema brushes grown from silicon wafers was investigated 
 by immersion in aqueous buffers ranging from ph 2 to 12 . in acidic 
 aqueous solution , the brushes become fully protonated and attain their 
 maximum swollen thickness . in alkaline media , this ph - modulated brush thickness for chemically cross - linked and 
 non - cross - linked ( linear ) ptbaema brushes was studied using both ellipsometry 
 ( see figure 5 ) and afm ( see figure 6 ) . in situ ellipsometric thickness of ptbaema brushes immersed 
 in 
 aqueous solution as a function of solution ph . (  ) linear ptbaema 
 brush ( original dry thickness = 16 nm ) . (  ) same ptbaema brush 
 uniformly cross - linked in thf using ppg - tdi at 20 c . (  ) 
 same ptbaema brush surface cross - linked in n - hexane 
 using ppg - tdi at 20 c . in situ brush height determined by tapping - mode afm as a function 
 of solution ph for micropatterned ptbaema brushes : (  ) linear 
 ptbaema brush ( original dry brush thickness = 15 nm ) ; (  ) uniformly 
 cross - linked ptbaema brush prepared using ppg - tdi in thf ; (  ) 
 surface - cross - linked ptbaema brush prepared using ppg - tdi in n - hexane . all brushes exhibited 
 similar thicknesses when collapsed in the 
 dry state and in alkaline media . however , under acidic conditions , 
 the degree of swelling varied with the nature of the normal distribution 
 of the covalent cross - links between chains . figure 5 shows that the mean thickness of a linear ptbaema brush increased 
 from ca . 16 ( in the dry state ) to 58 nm below ph 8 . above ph 8 , the 
 ptbaema chains became deprotonated and collapsed to give a brush layer 
 of ca . 18 nm thickness , which is close to the dimensions of the dry 
 brush . thus the linear swelling factor ( the ratio of the thickness 
 of the swollen brush to that of the collapsed brush ) is ca . the 
 mean thickness of a uniformly cross - linked ptbaema brush ( prepared 
 using ppg - tdi in thf ) increased from 17 nm in alkaline media to 43 
 nm in acidic solution , corresponding to a linear swelling factor of 
 ca . thus uniform cross - linking throughout the brush layer imposes 
 a significant constraint on its ph - responsive behavior , as expected . this is attributed to the multiple cross - links between the ptbaema 
 chains , which restrict the chain mobility . in contrast , for the surface - cross - linked 
 ptbaema brush prepared using ppg - tdi in n - hexane , 
 the brush thickness increased from 17 nm when immersed in alkaline 
 media to approximately 51 nm in acidic solution , which corresponds 
 to a linear swelling factor of 3.0 . thus the surface - cross - linked 
 ptbaema brushes exhibit similar ph - responsive behavior to the linear 
 ptbaema brushes . this is because the ppg - tdi cross - linker barely penetrates 
 the collapsed brush layer and hence can only react with the secondary 
 amine groups present in its outer surface . an extensive cross - link 
 network is not created within the brush layer under these conditions ; 
 hence this surface - confined cross - linking has rather less effect on 
 the swelling / collapse behavior of the brush compared to uniformly 
 cross - linked brushes . afm studies of the micropatterned linear , 
 uniformly cross - linked , 
 and surface - cross - linked ptbaema brushes were conducted ( see figure 6 and figure s4 , supporting information ) . as expected , the linear ptbaema brushes exhibited the strongest 
 ph - responsive behavior : the mean brush height difference increased 
 from 18 nm for the collapsed state ( above ph 8) to 64 nm for the fully 
 stretched brush ( below ph 8) . in contrast , the uniformly cross - linked ptbaema 
 brushes swelled from 15 ( above ph 8) to 36 nm ( below ph 7 ) . again , 
 the surface - cross - linked ptbaema brushes exhibited intermediate behavior , 
 with a swollen brush height of 51 nm and a collapsed brush height 
 of 18 nm , indicating a linear swelling ratio of 2.9 . these data are 
 in good agreement with the ellipsometric data discussed above , thus 
 confirming that the ph - responsive behavior of ptbaema brushes can 
 be modulated by conducting cross - linking in either a good or a bad 
 solvent . the resulting 3d ( or 2d ) cross - link networks dictate the 
 observed ph - responsive brush behavior , with greater cross - linking 
 restricting the ph response . the effect of spatially - confined 
 cross - linking was also studied 
 for nanopatterned brushes ( figure s5 , supporting 
 information ) . thus the same afm experiments were conducted 
 on nanopatterned linear , uniformly cross - linked , and surface - cross - linked 
 ptbaema brushes ( see figure 7 ) . very similar 
 ph - reponsive behavior was observed for the three brush types compared 
 to the micropatterned brushes discussed above , except that the ph 
 response was somewhat weaker for the nanopatterned brushes . this is 
 probably because , for such small features , the brush chains are more 
 likely to deform at the periphery of the patterned feature ; hence 
 brush - like chain stretching normal to the surface is likely to be 
 attenuated . it is perhaps worth emphasizing that the critical ph for 
 the brush collapse / swelling transition is always observed at around 
 ph 78 . further systematic studies within a relatively narrow 
 ph window ( see figure 7 ) suggest that the brush 
 swelling transition actually occurs between ph 7.6 and 7.8 , which 
 is a little lower than the pka of approximately 
 8.0 recently reported for linear ptbaema homopolymer in dilute aqueous 
 solution by morse and co - workers . covalent 
 cross - linking of the brush chains using ppg - tdi appears to have rather 
 little effect on the effective pka of 
 the ptbaema brushes described in this study ( see figure 8) . tapping - mode afm brush height 
 determined as a function 
 of solution ph for nanopatterned ptbaema brushes immersed in aqueous 
 solution : (  ) linear ptbaema brush ( original dry brush thickness 
 = 4.5  0.5 nm ) ; (  ) uniformly cross - linked ptbaema brush 
 prepared using ppg - tdi in thf ; (  ) surface - cross - linked ptbaema 
 brush prepared using ppg - tdi in n - hexane . in situ tapping - mode afm brush height determined over 
 a relatively 
 narrow ph window for micropatterned ptbaema brushes immersed in aqueous 
 solution : (  ) linear ptbaema brush ( original dry brush thickness 
 = 13 nm ) ; (  ) uniformly cross - linked ptbaema brush prepared 
 using ppg - tdi in thf ; (  ) surface - cross - linked ptbaema brush 
 prepared using ppg - tdi in n - hexane . the surface zeta potentials of linear and cross - linked 
 ptbaema 
 brushes were determined from ph 3 to 10 ( see figure s6 , supporting information ) . as expected , surface 
 zeta potentials for linear ptbaema brushes were + 45  5 mv at 
 ph  7 , with a gradual reduction in zeta potential being observed 
 at higher ph as the secondary amine groups become progressively more 
 deprotonated . as expected , the surface zeta potential is reduced after 
 cross - linking because of the reduction in the number of secondary 
 amine groups in the brush layer . moreover , the surface zeta potential 
 of the surface - cross - linked ptbaema brush is somewhat less cationic 
 than that of the uniformly cross - linked ptbaema brush . linear ptbaema brushes of up to 28 nm dry thickness have been prepared 
 via surface atrp at 20 c . ellipsometric studies confirm that 
 such brushes exhibit ph - responsive behavior , as expected . highly swollen 
 cationic protonated brushes are produced at low ph , with collapsed 
 neutral deprotonated brushes being obtained above a critical ph of 
 approximately ph 7.7 . ptbaema brushes can be readily cross - linked 
 via their secondary amine groups using a commercially available polymeric 
 diisocyanate ( ppg - tdi ) to produce robust urea bonds . xps studies confirmed 
 the presence of the reacted ppg - tdi cross - linker within the ptbaema 
 brush layers . moreover , the choice of solvent determines the spatial 
 location of this cross - linking reaction . using thf , which is a good 
 solvent for ptbaema , leads to relatively uniform cross - linking throughout 
 the swollen brush layer . on the other hand , conducting the same cross - linking 
 reaction in n - hexane , which is a poor solvent for 
 ptbaema , leads to surface cross - linking of the collapsed brush layer . this spatial confinement profoundly affects the subsequent ph - responsive 
 behavior of the latter brush layer , which becomes significantly more 
 swollen when immersed in acidic solution than the former uniformly 
 cross - linked brush layer , as judged by ellipsometry studies . however , 
 brush swelling occurs at approximately ph 7.7 regardless of the spatial 
 location of the cross - linking reaction , which is essentially the same 
 critical ph as that found for the linear brush . however , this critical 
 ph is slightly lower than the pka of around 
 8.0 reported for linear ptbaema chains . these findings are corroborated by brush heights determined by afm 
 measurements on either micro- or nanopatterned ptbaema brushes prepared 
 via uv laser irradiation or interferometric lithography . in summary , 
 this study provides some interesting new perspectives for the design 
 of ph - responsive polymer brushes .
surface - initiated 
 atom transfer radical polymerization ( atrp ) of 
 2-(tert - butylamino)ethyl methacrylate ( tbaema ) produced 
 ph - responsive secondary amine - functionalized polymer brushes with 
 dry thicknesses ranging from 4 to 28 nm , as determined by ellipsometry . 
 at low ph , linear ptbaema brushes became protonated and highly swollen ; 
 brush collapse occurred when the solution ph was increased to ca . 
 7.7 due to deprotonation . 
 ptbaema brushes were subsequently cross - linked 
 using tolylene-2,4-diisocyanate - terminated poly(propylene glycol ) 
 ( ppg - tgi ) in either thf ( a good solvent for ptbaema ) or n - hexane ( a poor solvent ) . 
 the intensity of the c  c  o component ( 286.5 ev ) in the c1s x - ray photoelectron 
 spectrum increased after reaction with ppg - tdi , suggesting that cross - linking 
 was successful in both solvents . 
 ellipsometry studies indicated that 
 the ph - responsive behavior of these cross - linked brushes is dictated 
 by the spatial location of the ppg - tdi cross - linker . 
 thus , uniformly 
 cross - linked brushes prepared in thf became appreciably less swollen 
 at a given ( low ) ph than surface - cross - linked brushes prepared in n - hexane . 
 micro- and nanopatterned ptbaema brushes were 
 prepared via uv irradiation and interference lithography , respectively , 
 and characterized by atomic force microscopy . 
 the change in brush 
 height was determined as a function of ph , and these afm observations 
 correlated closely with the ellipsometric studies .