Patent Application: US-201214233032-A

Abstract:
a method for producing a surface - functionalised object involves : providing negative mould with a surface topography complementary to that desired on the object ; applying a functional entity to the mould surface , in an exciting medium ; forming the object in or on the mould , the object being in direct contact , as it forms , with the functional entity ; and releasing the object from the mould , wherein during steps and / or , at least some of the functional entity is transferred from the mould to the object , whilst a proportion remains on the mould surface . the method can be used to functionalise the surface of an object as it is cast , and the mould can be re - used to form multiple replicate objects . the invention also provides an object produced using the method , and a surface - functionalised negative mould for use in the method .

Description:
the scheme shown in fig1 illustrates two alternative methods for producing an object having a functionalised surface of a desired topography . the method ( b ) depicted on the right is a cure - activated nanolayer transfer process in accordance with the invention . a surface 1 to be replicated ( in this case a natural surface such as a leaf ) is used as a template for the formation of a negative mould 2 . the mould is produced by forming a removable polymer layer , for example of a poly ( vinylsiloxane ), on the surface 1 . according to method ( a ), the negative mould is used to form ( for example to cast ) a replicate object 10 which replicates the surface topography of the template surface 1 . the surface of the replicate 10 is then chemically functionalised by the application of a functional surface layer 11 . for example , a hydrophobic polymer layer may be deposited onto the surface of the replicate 10 , for instance using a plasma deposition technique . method ( b ), in accordance with the present invention , involves application ( using an exciting medium ) of a functional surface layer 20 to the replica - facing surface of the negative mould 2 . again , the surface layer 20 may comprise a hydrophobic polymer and may be deposited onto the mould for instance by plasma deposition . subsequently , a replicate object 21 is formed in the functionalised mould , in contact with the functional surface layer 20 . when the object 21 is removed from the mould , it takes with it a thin layer 22 of functional material from the surface layer 20 . the result is an object which has the desired surface topography ( replicating that of the template 1 ) and an additional functional coating . such a process can be used to produce objects having superhydrophobic surfaces mimicking those found in nature . it is believed that on curing the material from which the object 21 is formed , against the functionalised negative mould , a thin layer ( typically a nanolayer ) of the functional material is transferred to the surface of the object as it forms . when the object is removed from the mould , it carries with it this functional surface coating , conforming exactly to the surface topography of the original template . at the same time , functional material is still left on the surface of the mould : this allows one or more further objects to be formed , and functionalised , within it in the same fashion . in this example , functionalised biomimetic surfaces were produced using a method in accordance with the invention . corydalis elata plant leaves and attacus atlas moth wings were selected as natural templates for this study , with the aim of replicating their natural superhydrophobicity on synthetic surfaces . the templates were rinsed with water to remove any surface debris and allowed to dry in air . negative moulds of the rinsed surfaces were prepared by application of a polyvinylsiloxane base and cure mixture ( president plus jet light body , coltene / whaledent ag ) to the substrate [ 25 , 29 ] and immediately pressing down using a glass slide for a cure period of 10 minutes . once a negative mould had hardened , it was carefully peeled away from the natural substrate surface , rinsed with water and left to dry . positive replicas were then prepared from the negative moulds using epoxy resin ( epoxy resin l and hardener s , r & amp ; g faserverbundwerkstoffe gmbh ). the epoxy resin was thoroughly mixed in a 5 : 2 ratio of resin to hardener , and then poured over the negative mould . any trapped air bubbles were removed by placing under vacuum , and then the mixture was left to cure overnight in a desiccator . finally , the negative moulds were gently peeled away to reveal the positive replica of the natural substrate . for the products prepared according to the present invention , by cure - activated nanolayer transfer , a functional coating was plasma deposited onto the negative mould prior to the application of epoxy resin to produce the positive replica . pulsed plasma deposition of the low surface energy precursor , 1h , 1h , 2h , 2h - perfluorooctyl acrylate (+ 95 %, fluorochem ltd , purified using several freeze - pump - thaw cycles ) was carried out in an electrode - less cylindrical glass reactor ( 5 cm diameter , 520 cm 3 volume , base pressure of 1 × 10 − 3 mbar , and with a leak rate better than 1 . 8 × 10 − 9 kg s − 1 ) enclosed in a faraday cage . the chamber was fitted with a gas inlet , a pirani pressure gauge , a 30 l min − 1 two - stage rotary pump attached to a liquid cold trap , and an externally wound copper coil ( 4 mm diameter , 9 turns , spanning 8 - 15 cm from the precursor inlet ). all joints were grease free . an l - c network was used to match the output impedance of a 13 . 56 mhz radio frequency ( rf ) power generator to the partially ionised gas load . the rf power supply was triggered by a signal generator and the pulse shape monitored with an oscilloscope . prior to each experiment , the reactor chamber was cleaned by scrubbing with detergent , rinsing in water and propan - 2 - ol , and then oven drying . the system was then reassembled and evacuated . further cleaning consisted of running an air plasma at 0 . 2 mbar pressure and 50 w power for 30 minutes . next , epoxy resin positive replicas , polyvinylsiloxane negative moulds , and control silicon ( 100 ) wafer ( memc materials inc ) and glass slides ( vwr international llc ) were inserted into the centre of the reactor , and the chamber pumped back down to base pressure . at this stage , 1h , 1h , 2h , 2h - perfluorooctyl acrylate monomer vapour was introduced at a pressure of 0 . 2 mbar for 5 minutes prior to ignition of the electrical discharge . the optimum conditions for functional group retention corresponded to a peak power of 40 w , a duty cycle on - time of 20 μs and an off - time of 20 ms . deposition was allowed to proceed for 5 minutes to yield 50 ± 5 nm thick layers . upon plasma extinction , the precursor vapour continued to pass through the system for a further 3 minutes , and then the chamber was evacuated back down to base pressure . leaf samples for scanning electron microscopy ( sem ) analysis were fixed overnight in 2 % gluteraldehyde in phosphate buffer solution ( ph 7 . 4 , sigma ). the leaves were then rinsed twice with buffer solution before undergoing dehydration through a graded series of ethanol solutions . the drying process was completed using a critical point dryer ( samdri 780 ). dried leaf , moth wing , and epoxy resin positive replica samples were mounted onto aluminium stubs using carbon discs and coated with a 15 nm gold layer ( polaron sem coating unit ). surface topography images were taken with a scanning electron microscope ( cambridge stereoscan 240 ). advancing and receding liquid contact angle measurements were made by increasing or decreasing the liquid drop volume at the surface whilst observing using a video capture system ( vca 2500xe ) [ 30 ]. the test liquid employed was high purity water ( iso 3696 grade 1 ). x - ray photoelectron spectroscopy ( xps ) surface characterisation was carried out using an electron spectrometer ( vg escalab mkii ), equipped with a non - monochromated mg kα 1 , 2 x - ray source ( 1253 . 6 ev ) and a concentric hemispherical analyser ( cae mode , pass energy = 20 ev ). elemental compositions were calculated using sensitivity ( multiplication ) factors derived from chemical standards : c ( 1s ): o ( 1s ): f ( 1s )= 1 . 00 : 0 . 45 : 0 . 34 . all binding energies were referenced to the c ( 1s ) hydrocarbon peak at 285 . 0 ev . a marquardt minimisation computer program was used to fit core level envelopes with fixed - width - at - half - maximum ( fwhm ) gaussian peak shapes [ 31 ]. film thickness measurements were made using a spectrophotometer ( nkd - 6000 , aquila instruments ltd ). transmittance - reflectance curves , over a wavelength range of 350 - 1000 nm , were fitted to a cauchy model for dielectric materials using a modified levenberg - maquardt method [ 32 ]. corydalis elata is a perennial plant with an alternate , 2 - 3 ternate leaf arrangement , as seen in fig2 ( a ) and ( b ). its leaves possess a hierarchical structure consisting of microscale papillae covered by nanoscale grooves ( fig2 ( c ) and ( d )). the adaxial leaf surface was found to display a high water contact angle and low hysteresis , indicative of superhydrophobic behavior ( see table 1 below ). attacus atlas moths are documented as being one of the largest moths in the world , with an average wingspan of 24 cm [ 33 ]. elongated scales ( measuring approximately 150 μm in height and 70 μm in width ) cover the wing surface and consist of several layers of chitinous material with a fine nanoscale structure , as seen by electron microscopy ( fig3 ( a )-( c )). a large water contact angle value and low hysteresis confirmed superhydrophobicity for this natural wing surface ( table 1 ). epoxy resin positive replicas of the plant leaf and moth wing surfaces were fabricated using a soft moulding process , as shown in fig1 . this entailed imprinting to produce a negative polyvinylsiloxane mould of the natural substrate , which itself was then moulded using epoxy resin to create a positive replica . in order to achieve individual scale replication , the negative mould was soaked in 50 % hcl solution prior to creating the positive replica , in order to dissolve any natural scales that were stuck in the mould . sem analysis of the corydalis elata leaf epoxy resin replicas confirmed successful duplication of the natural surface structural features ( fig2 ). however , although the surfaces displayed hydrophobicity , the contact angle hysteresis was relatively large when compared to that measured for the original leaf ( table 1 ). epoxy resin positive replicas of the attacus atlas moth also yielded high definition replication of individual scale features , with both the micro - and nanoscale features closely resembling those seen on the native wing surface ( fig3 ). water contact angle measurements were comparable to those of the hydrophobic corydalis elata leaf replica , but again the large hysteresis values pointed to the absence of superhydrophobicity ( table 1 ). 50 nm thick poly ( 1h , 1h , 2h , 2h - perfluorooctyl acrylate ) low surface energy films were plasma deposited onto the respective negative ( polyvinylsiloxane ) and positive ( epoxy resin ) replicas depicted in fig1 . in the case of the former , for cure - activated nanolayer transfer , the coated negative polyvinylsiloxane mould was used to fabricate the functionalised positive epoxy resin replica . for each type of functionalised positive replica , xps analysis confirmed the presence of the low surface energy perfluorocarbon functionalities , and these were found to be stable towards solvent washing in propan - 2 - ol , methanol , acetone , dichloromethane , tetrahydrofuran , dimethylformamide , toluene and cyclohexane . electron microscopy verified the retention of surface topography for both cases ( see fig2 and 3 ). water contact angle values increased significantly compared to those of the unfunctionalised replicas , with an accompanying drop in hysteresis values ( table 1 ). in fact , the hysteresis values for the cure - activated nanolayer transfer replicas were much closer to those measured for the parent natural species when compared to the plasma coated positive replicas . in the case of cure - activated nanolayer transfer , the same negative mould could be used several times to fabricate surfaces exhibiting comparable superhydrophobic properties . table 1 below shows the advancing and receding water contact angle measurements and hysteresis values for the natural surfaces and control surfaces used in example 1 , and for the functionalised surfaces generated in accordance with the invention . fig2 ( a ) and ( b ) are optical images of corydalis elata , showing , respectively , the plant and a single leaf . fig2 ( c ) to ( j ) are sem micrographs of the adaxial surface of corydalis elata at low and high magnifications , in which ( c ) and ( d ) show the native leaf ; ( e ) and ( f ) the epoxy resin replica of the leaf ; ( g ) and ( h ) the epoxy resin replica functionalised via cure - activated film transfer ; and ( i ) and ( j ) the epoxy resin replica functionalised via direct plasma deposition . fig3 shows sem micrographs of the attacus atlas moth wing surface at three different magnifications . figures ( a ) to ( c ) show the native wing ; ( d ) to ( f ) the epoxy resin positive replica ; and ( g ) to ( i ) the epoxy resin replica functionalised via cure - activated nano layer transfer . this example demonstrates the successful synthesis of biomimetic , superhydrophobic surfaces , using the method of the present invention . the inherent simplicity and nanoscale precision of this approach can make it highly attractive for a wide range of surface functionalisation and patterning applications . the replica surfaces fabricated in this study display an overall retention of the fine structure contained in the original natural template surface , which is consistent with the application of this replica moulding technique to other natural surfaces [ 25 , 29 ]. the key advantage of the present invention is that it can avoid the long processing times and / or high temperatures associated with alternative methods [ 17 , 28 , 34 , 35 , 36 , 37 , 38 ], where the consequent dehydration of the natural substrate tends to be an issue leading to shrinkage of the surface replica features compared to the parent natural substrate . furthermore , the replication of the individual scales of insect wings has not previously been achieved [ 35 ]. rather , there has been replication of insect wings via calcination , where a wing is coated with a very thin inorganic layer ( usually through atomic layer deposition [ 39 , 40 ] or chemical vapour deposition [ 41 ] technologies ) and subsequently fired at high temperatures to pyrolyse the natural substrate , culminating in shrinkage and deformation compared to the original structure . an aspect of the invention can therefore provide an object having a surface topography replicating that of an insect wing , in which individual scales are individually replicated , for instance as in example 1 above . such an object may be produced by casting in or on a negative mould as described above . its surface may be chemically functionalised , which may be achieved by producing the object according to the method of the first aspect of the invention . functionalisation of positive replica surfaces , in order to lower their surface energy to create superhydrophobicity , has previously been attempted using separate post - replica formation processes such as self - assembled monolayers [ 28 ] or dip coating [ 29 ]. the present cure - activated nano layer transfer approach can provide a way of imparting permanent surface functionality during the replica fabrication stage , without the need for further process steps . this in turn can reduce the risk of subsequent damage to the cast replica . one possible mechanism for the cure - activated nano layer transfer process demonstrated in example 1 is that the epoxy resin impregnates or reacts with the plasma deposited perfluorocarbon film present on the negative , low adhesion polyvinylsiloxane mould surface during the cure process [ 42 ]. the resultant positive replica epoxy resin surface then becomes enriched with these interpenetrating functionalities upon peeling away from the mould surface , due to adhesion between the outermost epoxy resin surface and the transferred functional film . thus in a method according to the invention , the ( typically polymeric ) material from which the object is cast may be chosen so as to have a degree of affinity with the functional entity on the mould surface . the observation that the same negative mould can be used multiple times , to impart superhydrophobicity on sequential positive epoxy resin replicas , indicates that ultrathin layers of the plasma deposited poly ( 1h , 1h , 2h , 2h - perfluorooctyl acrylate ) film are transferred during each subsequent curing process . in principle , the method of the invention can be used to introduce a range of other surface groups onto the surface of a cast object , including hydroxyl [ 43 ], carboxylic acid [ 44 ], anhydride [ 45 ], epoxide [ 46 ], furfuryl [ 47 ], amine [ 48 ], cyano [ 49 ], halide [ 50 ] and thiol [ 51 ] functionalities . also , the method should be easily adaptable so that the soft negative mould can be surface - loaded with the functional entity by other methods , such as chemical vapour deposition , inking or dip coating . finally , multifunctional surface patterning can be envisaged by preparing spatially - functionalised negative moulds using conventional lithographic techniques . autumn , k . ; liang , y . a . ; hsieh , s . t . ; zesch , w . ; chan , w . p . ; kenny , t . w . ; fearing , r . ; full , r . j . nature 2000 , 405 , 681 . arzt , e . ; gorb , s . ; spolenak , r . proc . natl . acad . sci . u . s . a . 2003 , 100 , 10603 . parker , a . r . ; lawrence , c . r . nature 2001 , 414 , 33 . wilson , s . j . ; hutley , m . c . j . mod . opt . 1982 , 29 , 993 . byun , d . ; hong , j . ; saputra , k . ; ko , j . h . ; lee , y . j . ; park , h . c . ; byun , b .- k . ; lukes , j . r . j . bionic eng . 2009 , 6 , 63 . fang , y . ; sun , g . ; wang , t . ; cong , q . ; ren , l . chin . sci . bull . 2007 , 52 , 711 . nosonovsky , m . ; bhushan , b . j . phys . condens . matt . 2008 , 20 , 395005 . feng , x .- q . ; gao , x . ; wu , z . ; jiang , l . ; zheng , q .- s . langmuir , 2007 , 23 , 4892 . wei , p . j . ; chen , s . c . ; lin , j . f . langmuir 2009 , 25 , 1526 . goodwyn , p . p . ; de souza , e . ; fujisaki , k . ; gorb , s . acta biomaterialia 2008 , 4 , 766 . epstein , a . k . ; pokroy , b ; seminara , a . ; aizenberg , j . proc . natl . acad . sci . u . s . a . 2011 , 108 , 995 . cassie , a . b . d . ; baxter , s . trans . faraday soc . 1944 , 40 , 546 . singh , r . a . ; yoon , e - 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