v w u JOURNÁL F SCIENCE Nat., Sci., & Tech., T.XXIII, NọỊ, 2007 PLASMA-INDUCED GRAFT POLYMERIZATION OF ACRYLIC ACID ONTO POLY(ETHYLENE TEREPHTHALATE) FILMS: HYDROPHILIC MODIFICATION Nguyen Kien Cuong D ep a rtm en t o fC h em istry, College o f Science, V N U ABSTRACT A complete and permanent hydrophilic modiĩication of poly (ethyleneterephthalate) (PET) ĩilms is achieved by argon-plasma irradiation, subsequently graíting acrylic acid (AA) in vapor phase onto their suríace Both Ar plasma irradiation alone and post graíting AA rendered a complete hydrophilicity to PET surtaces Hovvever, the hydrophilicity of the PET suríace, only treated with the Ar plasma, is not permanent In contrast, PET íilms, irradiated by the Ar plasma, exposed to air, and subsequently gratted with AA monomer, are permanently hydrophilic Degradation of polymer chains on the plasma-irradiated surtace is proportional to time of exposure Electron spectroscopy for chemical anaỉysis (ESCA) coníirmed the grafting of AA onto the íilm suríace, whỉch results ỉn a large amount of incorporated oxygen-containing íunctional groups like carboxylic (O ũ C' = O) and carbonyl (C' = O) The morphology of graíted suríaces, observed by scanning electron microscopy (SEM), displays some large area of microporosity compared to relative smooth morphology of the control one Gratted íunctional groups and suríace microporous structure are the main íactors to enhance hydrophilicity of the PET ĩilms Keywords: Plasma-induced graft polymerization, polymer degradation, oxygencontaining íunctional groups, hydrophilicity, microporosity and eiectron spectroscopy ỉorchemical analysis (ESCA) In tr o d u c tio n P olym eric m a te ria ls hold considerable in te re st in th e íĩeld of b io m aterials for s c ie n tis ts in re c e n t y ears T issu e en g in eerin g culture, m in im izin g p ro tein adso rp tio n to p re v e n t m em b ran e-fo u lin g for p ro tein u ltra filtra tio n , im m obilization of biologically activ e m olecules a n d liv in g cells, etc., are r a th e r closely re la te d to hydrophilic c h a c te rs of polym er su rfaces [1-3] S urface hydrophilicity of th e polym er can be achiev ed by th e in co rp o ratio n of oxygen-containing fu n ctio n al groups, such as — COOH a n d — OH, w hich a re u su a lly n o t coupled w ith m olecular ch ain s of th e polym er surface S u ría c e m o difications could en h an ce m echanical in terlocking, a n d cre a te functional groups, im p ro v in g w e ttin g an d /o r chem ical bonding of a polym er surface S ynthetic polym ers, th erefo re, o ften re q u ire selective m odiíĩcations to in tro d u ce speciíìc functional groups onto suríaces for proper purposes, ex binding of biomolecular, gas b a rrie r, etc T h e co n v en tio n al m eth o d s (wet chem istry) for th e hydrophilic m odiíìcation of polym er s u ría c e s h av e b een períorm ed by various chem ical tre a tm e n ts , usually accom panied by d am a g in g polym er bulk, hence affecting its p ro p erties In c o n tra st to 47 48 N guyen Kien C uong th e w et ch em istry , th e polym er 8urface, exposed to p la sm a , can be m odiíĩed to en h an ce its hydrophilicity, co m p atib ility an d bioĩunctionality M oreover, th e m odiíìed su rface is, in gen eral, coníìned to a top-surface lay er less th a n several h u n d r e d n a n o m eters th ro u g h polym er th ick n ess T hereíore, d esirab le p ro p e rtie s of b u lk lay ers a re u su a lly m ain tain ed H ow ever, on m ost polym er surfaces, th e g ain ed h y d ro p h ilicity is u su a lly not p e rm a n e n t, a n d d isa p p e a rs or d im in ish es s ig n iíĩc a n tly a fte r only p la sm a irra d ia tio n T he irra d ia te d surface g rad u ally re s to re s its h y d ro p h o b icity d u e to a g m e n te d low -polym er c h a in s on 8urface layers, te n d in g to re o rie n t into b u lk layers T his resu lte d in d ecrease in a n u m b er of fu n ctio n a l groups, th e re b y d e c re a sin g its hydrophilicity P o st-g ft copolyraerization can fìx d ic a ls by g ftin g a hydrophilic m onom er onto th e irra d ia te d su ríace, therefore, is in g th e lifetim e of su rface hydrophilicity In ad d itio n , th e g ítin g of a speciíìc m onom er m a k e s a su rface m odified w ith su ita b le chem ical fu n ctio n ality for b io m aterial a p p lic a tio n s [4-7] In previous p a p e r [8], hydrophilic im p ro v em en t of P E T ílb ers in m oisture ab so rp tio n an d d yeing p erío rm an ce h a s been re p o rte d A bsorption e n h a n c e m e n t are due to th e existence of carboxyl groups: — = o , in c o rp o rated on to P E T fĩber su rĩaces, íu rth e rm o re , th e ccn d itions of th e p la sm a irra d ia tio n aa \vell as graítpolyraerization h av e co n sid erab ly eíTects on th e h y d ro p h ilic d u b ility of P E T fibers c This paper describes PET íìlms, irrad iated w ith a m ix tu re of in e rt g a se s like heliu m /arg o n (He/Ar) a t p re ssu re of o n e-atm o sp h ere, th e n su b se q u e n tly graítpolym erized w ith acrylic acid in v apor to en h an ce th e irs h y d ro p h ilic d u b ility over tim e EíTects of irra d ia tio n tim e on a w eigh loss ratio a n d g ítin g d egree of P E T ’s films w ere in v estig ated O xygen-containing functional gro u p s, c h a c te riz e d by electron spectroscopy for chem ical a n aly sis (ESCA), w ere u se d to ro u g h ly e s tim a te hydrophilic cap ab ility of th e g fte d surface S urface m orphology of th e g rafted su rface was observed by sc an n in g electro n m icroscopy (SEM) In ílu e n c e of th e g rafted íunctional groups an d su ría ce m orphology upon surface h y d ro p h ilicity w ill be discussed E x p e rim e n ta l P ro c e d u re s — C —— - 2.1 Sam ple p rep aration Ị \ » \ N e N_, Rị — c = m olecular bonds w h en Ito c tro n i M* bond, sy ram etrically-bonded to an aro m atic ring, seem to be stable B esides, th e re a re — C H — C H bonds \vith low er b o n d in g energy H ence, th e d e g d a tio n of m olecular c h ain s on its surface m ig h t occur a t c — H a n d c — c r a d lc a ls c" © In P E T fílm s tru c tu re , two groups of R # lon* o l» c tro n lc e x c lte d p a r t id i uv-mdlatlon 1VH 1X1 * ® ỉ g c M* R* © Substrate e lo c tro d ỡ lúncdonal groups r f pow er th e Fig.1 Principle o f plasma reaction insỉde electrodes V N U J o u rn a ì o Ị S c ie n c e , N a i., S ci đc T e ch , TJOQỈỈ, N 0Ỉ , 20)7 Plasm a - Induced graft poíym crization o f acrylic acid onto 49 m o lecu lar ch ain abso rb s p lasm a-en erg y from a c tiv a te d species an d u ltra v io le t rays d u rin g th e p lasm a irra d ia tio n The principle of p lasm a reac tio n o ccurring betw een two electro d es is described in íìg u re Glow discharge p la sm a a t o n e-atm o sp h ere w as g e n e te d in a p la sm a re ac to r (m a n u ía ctu red by P e a rl Kogyo Co L td, O saka, J a p a n ) cou p led w ith p a lle l p la te electrodes, w hich w ere covered by dielectric b arrierceram ic, an d o p e tin g a t rad io írequency of 13.56 M Hz A P E T film sam p le of 0.2 mm in th ic k n e ss, provided by A sah i G lass F ibers Co Ltd (Ja p a n ), w as placed betw een two electro d es, an d th e n irra d ia te d w ith th e m ix tu re of H e/ A r in e rt gases, introduced by th e c o n s ta n t flow te of 850m l /150m l m in (STP), an d in tro d u ced into a plasm a ch a m b e r Irra d ia te d from 10 sec to m in, w ith p lasm a p o w er-d en sity of 1.75 w /cm 2, a t electro d e su ría c e te m p e tu re of ab o u t 70°c • 80°c, each sam p le w as rem oved from th e p la sm a ch am ber, th e n im m ed iately w eighted to e s tim a te d e g rad atio n State of surfacelay ers T he irra d ia te d sam p le vvas th e n grafted w ith acrylic acid (AA) of 99.5% conc in a glass tu b e e v ac u a te d to 133 Pa a t two level of c o n s ta n t te m p e tu re : ° c as w ell as 70°C; th e g ftin g process la ste d for h o u rs an d h o u r, respectively T ak en from th e g lass tu b e, th e sam p le w as e x tra c te d by hot m eth an o l in a S oxhlet e x tra c to r for h ours to rem ove u n re a c te d re m a in in g m onom er an d hom opolym ers 2.2 ESCA ch a cteriza tio n of m od iíìed surface ESCA m e a s u re m e n t w as períorm ed on a K rato s ESCA -3300 spectro m eter, em ploying M gK a (1253.6 eV) X-ray source T he electro n tak e-o ff an g le w as ad ju sted a ro u n d °c w ith resp ect to th e íìlm suríace T he p re ss u re in th e an a ly sis ch am b er was m a in ta in e d a t a b o u t P a du rin g th e d a ta acq u isitio n T he X -ray source w as ru n a t th e anode voltage of kV a n d c u rre n t of 30 mA 2.3 S urface m orp h ology ob served by SEM S u ría c e m orphology o f th e g rafted íílm s w as observed by a scan n in g electron microscope (SEM ), m odel J E O L JSM -5200 F or b e tte r electric conductivity, a sam p le’s su rface w as coated w ith th in gold lay er before th e ex am in atio n T he observation w as perío rm ed to d e te rm in e th e q u ality of polym er depositions, a n d especially to check w h e th e r m icropores a p p e a r on th e g rafted surface R e su lts a n d D iscu ssio n 3.1 D egrad ation o f plasm a-irradiated su ríace T h e d e g d a tio n of th e film su ríace irra d ia te d by p la sm a seem s to be p red o m in a n t effects of th e d isch arge in tera ctio n betvveen its su ríace an d activ ated species like ions, p a rtic le s, etc T his process led to an a lm o st coraplete breakdow n of c — H o r c — c bonds, p ro d u cin g carbon rad icals on irra d ia te đ su rfaces T he polym er d eg rad atio n can be describ ed a s follows: V N U J o u r n a l o f S cien c e N a t S ci.,& T e c h T X a i l N al , 0 50 Nguyen Kicn Cuoĩĩtg r ọ 9H Hl II / ^ r \ 11 I I o — c - \ ) —c — o — c — c - H Plasma irradiaíion 9 H H o - c - \/ ế0~ /\ - c!L—o - c -' c H - Radical ' H c W here: • is a d ic a l grow n by th e d e g d a tio n of a m olecular ch ain on th e P E T surface T h e p o ly m er d e g d a tio n , c h a c te riz e d by w eight-loss ratio , w as calculated in th e following ex p ressio n : WL(%) = 100 *(W,- w0)/w0 (1) W here: WL (%) is th e w eig h t-loss tio ; W0 a n d W] a re th e w eig h t of a sam p le beíore a n d a fte r th e G D P tre a tm e n t T h e m in u s m a r k Ĩ8 d enoted as th e w eight loss of th e m olecular c h a in s d u e to th e d e g rad atio n The d e g d a tio n of th e m olecular ch a in s on th e irra d ia te d su rface lay e rs v e rsu s th e tim e of exposure 18 in d ic a te d in th e Fig It 18 clearly th a t th e w e ig h t loss ratio , in d ic a tin g th e level of th e d eg rad atio n , w e n t u p w ith f u rth e r exposure tim e L a rg e d isp e rsio n of th e vveight loss is a sc rib e d to th e effect of th e d e n s ity of a c tiv a te d species, w hich collided w ith th e film 8urface as w ell a s th e cro ss-lin k in g of rad ica ls g e n e te d on th e P E T su rface d u rin g th e p la sm a irra d ia tio n T h e s im ila r re s u lts h av e Exposure tim e (sec) also b een found in th e re p o rt of Flg Degradation o f polym er surtace versus Y asu d a et aỉ.[9 ] E xposed to a ir, the tim e of irradiation th e se rad icals w ere re a c te d w ith oxygen in a ir to p ro d u ce p ero x id es a n d (— CO OH) h ydroperoxides T h ese peroxides, b eing in itia to rs for th e s u b s e q u e n t g ft p o ly m eriza tio n w ere form ed as following reactions: f oII o II t r r \ 1I ■ o -c -(0 H r o II oII H Hi I I I I H^ Exposuredto Air rz\ I I ) - c — o — c — c - - ■o—c (0/ c o c C-H HO O H- V N U J o u r n a l o ỊS c ie n c e N a t Sci A T e c h T x a i l N 01 20(7 51 Plasm a - Induccd graft polym erization o f acrylic acid onto 3.2 E ữ ects o f th e ex p o su re tlm e on g ftin g d eg ree O w ing to th e rm a lly -in d u c e d d e g d a tio n co in cid en t w ith th e p resen ce of th e AA m ono m er in vapor, c o • a n d • OH rad icals, decom posed from th e hydroperoxides, w ere th e n graft- polym erized in a g lass tu b e, ev acu ated to 133 P a a t te m p e tu re of °c for h rs a s well a s °c for h r G fte d w ith th e AA m onom er of 99.5% conc., th e se c o * d ic a ls, in itia lly serv in g a s a c tiv a te sites, re a c te d w ith th e m onom er to cre a te copolym ers w hile *OH d ic a ls, also re a c te d w ith th e sa m e m onom er, w ere changed in to hom opolym ers o o H H*1 ow oII H H, Thermally r II II 1 r —\ - induced rzr\ II 1 -■o—c-(0)—c —o—c —c- - - —►- o—c-(0)—c—o—c —c- - + 'OH ( HOO Ọ Ọ H II H- H II I - C - ^ ) - C —O - C — c 'O I HJ •o Acrylic acid — > Graýt o o H- H H II /CT\ II o -c -(Q )-c -o i c -c -t 1 o Copolymers CH COOH ồH AA — • H O -C H ^ C H -C O O H ĩ COOH Homopolymer T h e w e tta b ility of th e g rafted sam ple, reílected by g ftin g degree, w as calcu lated a s follows: (%) = 100 * (W2 ■ vv,) / Wl (2) W here: Wj a n d W a re th e sam p le’s w eig ht m e a su re d b eíore an d aíler th e graft polym erization, respectively i ọp c