STUDY ON THE GRAVIMETRIC MEASUREMENT OF THE SWELLING BEHAVIORS OF POLYMER FILMS

Transcription

1 452 Rev. Adv. Maer. Sci. 33 (2013) J. Liu, X.J. Zheng and K.Y. Tang STUDY ON THE GRAVIMETRIC MEASUREMENT OF THE SWELLING BEHAVIORS OF POLYMER FILMS J. Liu, X. J. Zheng and K. Y. Tang College of Maerials Science and Engineering, Zhengzhou Universiy, Zhengzhou , P. R. China Received: Ocober 17, 2011 Absrac. The presen heories and experimenal mehods o evaluae he swelling behaviors by gravimeric measuremen were experimenally esed for polyvinyl alcohol (PVA) films. I was found ha he swelling behaviors and swelling kineic parameers, such as diffusion exponen n, by gravimeric measuremen may be grealy influenced by experimenal procedures. In his paper, boh he coninuous (denoed as scheme A) and he separaed (scheme B) experimenal procedures of he gravimeric mehod o evaluae he swelling behaviors were presened and he difference beween he wo schemes were analyzed. The resuls sugges ha he coninuous diffusion of waer molecules in films during gravimeric deerminaion may be he key facor affecing he swelling behaviors. The advanages and disadvanages of he wo schemes were discussed. I was concluded ha an appropriae selecion of experimenal schemes is effecive o exend he applicabiliy range of gravimeric mehodology and o increase he accuracy of gravimeric measuremen. 1. INTRODUCTION Polymer films based on biomaerials or oher hydrophilic polymers are generally sensiive o waer and environmenal humidiy because hey are normally hygroscopic wih pleny of hydrophilic groups on he macromolecule chains [1]. The knowledge of swelling behaviors of polymer films is of significan imporance for various applicaions in bimolecular elecronics and sensors, drug delivery sysems, wound dressings, adsorpion of chemical maerials and conac lenses [2,3]. They have in- many heoreical models have been developed wih he developmen of maerials sciences [4,5]. Swelling curve (swelling raio vs. ime) is usually used o characerize he swelling behaviors of films or hydrogels [5]. Some imporan parameers can be obained from swelling curves, such as diffusion exponens and solven diffusion coefficiens in films. The swelling mechanisms can also be analyzed Corresponding auhor: K. Y. Tang, keyongangzzu@yahoo.com based on swelling curves. Therefore, accurae experimenal swelling daa are paricularly imporan for he developmen of synhesis and applicaions of films. To obain he swelling curve of films, he gravimeric measuremen has been exensively adoped because of is simpliciy and easy implemenaion [4-6]. Mos of he previous sudies abou he gravimeric measuremens were focused on he swelling behaviors for differen kinds of films [3-7]. The major experimenal mehod can be simply summarized as follows: he analysis of he dynamic swelling characerisics of films is performed by accuraely weighing he sample during he measuremen, he swollen films are removed from he immersing soluion a regular ime inervals, and he film sample is weighed afer removing he excess surface waer. The sample was pu ino he soluion again and he procedure was repeaed unil he swelling equilibrium was reached. Yin and coworkers measured

2 Sudy on he gravimeric measuremen of he swelling behaviors of polymer films 453 he swelling raios of sodium alginae-g-acrylic acid by he coninuous mehod, and he swollen gels were wihdrawn a regular ime inervals and weighed afer removal of surface waer [6]. The mos imporan feaure of his kind of measuremen is ha only one sample is needed and used during he whole process and he experimen is coninued afer weighing. Therefore, i is called coninuous mehod. On he oher hand, here is a differen procedure ha requires a series of samples, in which each sample is immersed in soluion for differen ime duraions. The swelling raios a differen ime can be obained wih separaed idenical samples, and he swelling mehod is usually used in he sudy of dimensional measuremen [8]. So i is correspondingly called he separaed mehod. Pornsak and coworkers sudied he swelling behaviors of calcium polysaccharide gels by boh gravimeric mehods, bu hey failed o compare he differences beween coninuous and perimenal procedures are always considered o be idenical in he sudy of swelling properies of films. In his work, he swelling behaviors of PVA films wih coninuous and separaed mehods were sudied and he differences of swelling curves and calculaed parameers by hese wo schemes were analyzed. To explain he differences beween he wo differen schemes, a possible mechanism abou he diffusion of solven and experimenal scheme was proposed. The influences of physical properies of films and experimenal design on he experimenal daa were analyzed. I may helpful for selecing suiable experimenal schemes a differen condiions. The purposes of his work were o improve he accuracy of he deerminaion of swelling daa, o exend he applicabiliy range of gravimeric mehodology, and o elucidae he relaionship beween experimenal design and swelling behaviors of films. 2. EXPERIMENTAL 2.1. Maerials and film producion The polymer films sudied in his work were prepared using a commercial poly (vinyl alcohol) wih a hydrolysis degree of 88% and an average polymerizaion degree of 1750 from Tianjin Kermel Co. Ld., Tianjin, China. Gluaraldehyde (GTA) was used as a cross-linking agen. All he reagens were used as received wihou furher purificaion. PVA was dissolved in ho waer (90 C) while being sirred for 3 hours o obain he film-forming soluion conaining 10 g PVA per 100 g of soluion. The film-forming soluion was hen cooled o room emperaure. PVA films were obained by casing waer evaporaion a room emperaure for 48 h. The hicknesses of he films were conrolled by carefully conrolling he dry maer / area of plae raio. Gluaraldehyde (GTA) cross-linked PVA films were prepared by he following in-siu cross-linking procedures: PVA films were immersed in a GTA (2% v/v) soluion a 60 o C for 10 min, and hen he crosslinked films were washed wih double-disilled waer and air dried a room emperaure. There were sulfuric acid and sodium sulfae (Na 2 SO 4 ) in he gluaraldehyde soluion, here he sulfuric acid was used o caalyze he acealizaion reacion, and sodium sulfae was used o inhibi he solubiliy of he PVA films during he in-siu cross-linking process Gravimeric deerminaion Analysis of he dynamic swelling behaviors of PVA films were performed in a PBS buffer soluion. PBS soluion was used as he soluion medium in he swelling process in order o provide a consan ph, mm), KCl (3 M), Na 2 HPO 4 (8.1 mm), and KH 2 PO 4 (1.5 mm). Afer humidiy regulaion in a consan humidiy (65% RH) and emperaure (25 o C) chamber, he circular shaped PVA films, 12 mm in diameer, was used in all swelling experimens. The so- o C for all he swelling experimens. The swelling experimens were as follows: Experimenal Scheme A (Coninuous mehod): film was weighed before swelling and he weigh was recorded as m 0. The sample was hen immersed in a 200 ml PBS soluion. A he same ime, a sop wach was iniiaed o record he swelling ime (he ime for he sample immersed in PBS soluion) of he sample. Afer a definie swelling ime 1, he sample was removed from he soluion and he surface waer was quickly removed wih a filer paper. The sample was weighed on an elecronic balance (0.1 mg accuracy) and he weigh was denoed as m 1 for he firs immersion sep. The sample was immersed in he soluion for he second ime, and he procedure was repeaed unil he equilibrium of swelling was reached. A ime, he weigh of he sample was denoed as m. The swelling raio a differen swelling ime can be calculaed by Eq. (1). Experimenal Scheme B (Separaed mehod): Differen from he experimenal scheme A, he swelling raio can be deermined wih a series of samples. In his mehod, he pre-weighed films numbered from 1 o n were immersed in he PBS soluion, removed

3 454 J. Liu, X.J. Zheng and K.Y. Tang Fig. 1. Schemaic diagram of experimenal scheme A (coninuous mehod). Fig. 2. Schemaic diagram of experimenal scheme B (separaed mehod). ou, and weighed a differen swelling ime 1, 3 n, respecively. For example, a ime 1, sample 1 was removed ou and weighed afer he absorbing he surface waer. The sample 1 was no used again. A ime 2, sample 2 was removed ou and weighed. The same procedure was repeaed unil he las sample n was weighed. Each sample has he same iniial weigh, m 0. Alhough someimes he same level may be kep, i is beer o accuraely record he weigh of he samples. Swelling behavior was described by he normalized swelling raio (M ) a ime. I can be obained in grams of waer per gram of dry sample using Eq. (1). M m m 0 (1) m 0 100%, where m 0 and m are weighs of samples before swelling and afer swelling ime, respecively. Swelling curves of films can be obained by he plo of M vs. ime. As menioned above, m 0 in Eq. (1) may be differen in separaed mehod, and only he corresponding iniial weigh of he sample for deermining he M can be used. 3. RESULTS AND DISCUSSION 3.1. Analysis of wo differen experimenal schemes The experimenal process for he wo differen schemes relaed o gravimeric measuremen was shown in Figs. 1 and 2. As has been known, here is a necessary sep for absorbing he surface soluion before weighing he samples. So he ime iner- val for feching ou of he films from he soluion is ineviable. In he sudy, a ime inerval of 30 seconds was se and i was called dry ime ( d ). I is he ime ha he samples do no say in soluion. As shown in Fig. 1, films are frequenly moved ou of soluion during he whole experimenal process. is used o denoe he immersion duraion of film in soluion beween wo weighing operaions. To simplify he analysis, we supposed ha he immersion duraion was he same during he whole measuremen, hough someimes migh be changed a lile according o he swelling properies a differen swelling sages. So here were a series of original weigh daa corresponding o he daa poins on swelling curve a 1 = = 2 n = n. However, he addiional dry ime ( d ) for weighing he samples was usually negleced. In order o fully undersand he influence of he dry ime on he resuls, he acual experimenal procedure is discussed. The iniial weigh (m 0 ) of he dry film is weighed a he ime of 0 = 0 as shown in Fig. 1. A he swelling ime of 1, he film is aken ou of he soluion and weighed, noed as m 1. The m 1 is he weigh afer he swelling duraion 1 ( 1 = ) on swelling curve, whereas he acual duraion is 1 + d or + d as shown in Fig. 1. Similarly, a swelling ime 2, we supposed 2 = 2 1 = 2 was he oal swelling ime. So he acual ime spen from he very beginning is no 2 because of he addiional ime 2 d. Therefore, acually d or d has passed o obain he weigh of film afer wice immersions. A swelling ime n, n + n d corresponds o m n, he final weigh of film. In Fig. 1, he black circles wih

4 Sudy on he gravimeric measuremen of he swelling behaviors of polymer films 455 arrows marked as m 1, m 2, and m n indicae he weighs of samples a swelling ime 1 and n. If we suppose he dry ime d is definie for all he repeaed procedures, a series of swelling raio M of swollen film a ime 1 n can be calculaed by: M m m 100% / m, 1 1d M m m 100% / m, 2 22 d m m md M m m 100% / m. (2) If he oal duraion of he measuremen is n +n d, he addiional ime n d is corresponding o he oal duraion weigh in air, which is he oal dry ime for he swelling measuremen. Acually, he second, hird and more parallel measuremens may be performed o obain more reliable swelling daa. However, he oal addiional dry ime n d may affec he swelling behaviors, compared o hose wihou dry ime. As has been menioned above, he oal swelling ime for experimenal scheme A o obain a swelling curve is n + n d. So he proporion of dry ime in scheme A (P A ) can be denoed as: P m d d A n n d d ( n 1,2,3...). (3) In scheme B, a series of samples were used. They were immersed in soluion unil he prese imes were reached. Each separaed sample corresponds o a single daa poin on he swelling curve. Fig. 2 shows ha every poins on he swelling curve was by an independen sample. If he iniial weigh m 0 of each dry film is he same, afer differen period of swelling ( = 1, 3, n ), he weighs of each swollen film are m 1, m 2, m 3 m n. However, he weigh deerminaion of each sample also requires a surface waer removing and weighing process. In oher words, an addiional dry ime d is needed o obain he weigh of he sample. Therefore, he swelling raio M of swollen film a ime = 1, 3, n can be calculaed as: M m m 100% / m, 1 1d M m m 100% / m, 2 22 d n n nd M m m 100% / m. (4) Each sample has only one chance o be swollen and weighed. Therefore, he proporion of dry Fig. 3. Swelling curves (M vs. ime) of PVA films measured by scheme A and B. ime in scheme B (P B ) is described by he following equaion: P d B (5) n, ( n 1,2,3...) Influence of experimenal schemes on he swelling behaviors Fig. 3 shows he swelling curves of PVA films, 0.22 mm in diameer, by experimenal scheme A and B, respecively. As has been menioned above, a series of differen ime inervals were used in he sudy, whereas he dry ime is consan (30 s). The swelling raios of he PVA films by schemes A and B are consisen a he iniial sage of he swelling. Fig. 4. Linear fiing of ln(m /M e )~ln curves of PVA films measured by scheme A and B.

5 456 J. Liu, X.J. Zheng and K.Y. Tang Fig. 5. Schemaic diagram of he diffusion of waer molecules in gravimeric measuremen (scheme A). Wih he increase of swelling ime, he differences beween he wo curves become more and more obvious in he firs few hours of experimen. When he swelling processes are approaching equilibrium, he swelling raios of he wo curves are approaching each oher. In order o clearly describe he differences of he wo curves, he imes needed for he swelling raio o reach 60% of he equilibrium swelling raio (denoed as 60% ) were deermined as shown in Fig. 3. The 60% were 130 s and 334 s for swelling curves by scheme A and B, respecively. For he same kind of samples and a he same swelling condiion (ype of soluion, emperaure, ec.), measuremen mehod should no affec he resuls of swelling experimen. In he sudy, however, he swelling curves obained by he wo mehods are significanly differen and he swelling raios by scheme A is much larger han hose by scheme B, especially a he inermediae swelling sage. I is very imporan o obain reliable swelling daa in he sudy of films, which may help us o develop new film and o use suiable films for differen kinds of purposes. In order o furher undersand he influence of experimenal schemes on he swelling behaviors, experimenal daa can be reaed [5-7] and he resuls are shown in Fig. 4. From he plos of ln(m /M e ) versus ln, he parameer n may be obained by he slopes of he linear simulaion, which can be used o describe he mechanism of diffusion process of films. The values of swelling parameer correspond o experimenal schemes A and B is 0.77 and 0.63, respecively. This difference can no be explained wih experimenal errors A possible explanaion for he experimenal resuls As discussed above, dry ime may be he key facor affecing he swelling resuls by he wo experimenal schemes. From a macroscopic poin of view, swelling is a peneraion process driven by pressure gradien or concenraion gradien beween he inerior of he film and he exernal soluion. Waer ransporaion in he film can be supposed as following assumpion: waer ransporaion occurs mainly under he moisure concenraion gradien beween he surface layer of he film and he inerior of he film [7]. Fig. 5 shows a schemaic diagram abou he influence of diffusion on he swelling behaviors. To simplify he problem, i is supposed ha film sample is symmerical in geomeric shape and he model wih only one surface conacing he soluion was sudied as shown in Fig. 5. Therefore, o explain he difference of swelling resuls beween he wo experimenal schemes, he experimenal procedure in scheme A may divided ino four sages: Firs, sample was immersed in waer and he waer molecules diffuse ino he inner par immediaely (Fig. 5a). Secondly, he sample was aken ou of he waer wih only swelling waer molecules exising in i (Fig. 5b). There are hree such pars in he swelling gel as compleely swollen gel, hydraing polymer layer, and dried polymer [7]. The moisure concenraion gradien beween differen layers drives waer molecules o diffuse from he surface pars of film o he inner dry pars unil swelling equilibrium is reached. When polymer film is kep in soluion, coninuous supplemen of waer molecules consrucs a successive process for he swelling of film. When he film is aken ou of he soluion during he dry ime ( d ), hough no free waer from soluion may fill up he surface vacancies wih he progress of diffusion, he moisure concenraion gradien sill drives he waer molecules in ouer par o penerae o he dryer inner par of film (Fig. 5c). Finally, afer a period of d, he film is pu back ino he soluion, in which case, due o he diffusion of waer during d, he moisure concenraion in he surface layer of film is reduced (Fig. 5d). Therefore, afer saying in air, addiional diffusion during d will

6 Sudy on he gravimeric measuremen of he swelling behaviors of polymer films 457 make he diffusion of waer molecules from soluion ino he film much easier. In scheme B, each samples are kep in soluion hroughou he swelling process unil he predeermined ime are reached, hough a he iniial sage of swelling, he influence of dry ime is almos he same as ha in scheme A. Wih he progress of he gravimeric measuremen, he swelling resuls will be slighly affeced by he addiional diffusion of waer molecules during dry ime. Therefore, scheme A always yields a bigger swelling raio, compared wih ha of scheme B. So he difference beween he swelling curves in Fig. 3 is explained Influence of film properies on he swelling resuls by differen schemes To verify he possible explanaion proposed above, he influence of film properies on he swelling resuls by differen experimenal schemes was invesigaed. In he presen sudy, uncross-linked and GTA cross-linked PVA films were used. Fig. 6 shows swelling curves of cross-linked and uncross-linked PVA films by scheme A and B. The cross-linked film swells slower han he uncross-linked one, and he equilibrium swelling raio becomes smaller afer cross-linking reamen. A he same swelling ime, he swelling raios of boh uncross-linked and crosslinked film by scheme A was significanly higher han hose by scheme B. Afer cross-linking, a he same swelling ime, he differences in swelling raios beween scheme A and B decrease obviously, which indicaes ha gel properies affec he swelling daa by differen schemes. In order o know he variaion of experimenal resuls, 60% was sudied and he resuls from Fig. 6 are 275 s and 370 s for crosslinked sample by experimenal schemes A and B, respecively. The difference of 60% before and afer cross-linking is 204 s and 95 s. The decrease of 60% afer cross-linking reamen indicaed ha he swelling resuls by he experimenal schemes A and B approached each oher. Uncross-linked film behaves rapidly and he cross-linked one behaves slowly in swelling process. In order o obain more daa on swelling curve o assure he accuracy of daa simulaion in he sudy of swelling kineics, he swelling duraion in scheme A for cross-linked film can be increased due o he longer ime of swelling before equilibrium is reached. Considering d is consan in all he swelling experimen, P A decreases wih he increase of in Eq. (3), which means ha he effec of he negleced swelling during d on he swelling behav- Fig. 6. Swelling curves of uncross-linked and GTA cross-linked PVA films by scheme A and B. iors of film is reduced. Then, wih he increase of waer-resisance of films, he difference of swelling daa by scheme A and B becomes unobvious. Therefore, i is concluded ha scheme B is more applicable for he films swelling very quickly in moisure, while for slowly swelling samples such as highly cross-linked polymer neworks, boh scheme A and scheme B can be used o accuraely deermine he swelling behaviors. Scheme A is usually preferable because only one sample is needed for he sudy. 4. CONCLUSIONS The difference beween wo experimenal schemes of gravimeric measuremen o sudying he swelling behaviors of PVA films was sudied. The effec of he addiional diffusion process a dry ime was invesigaed wih he resul ha he usually negleced diffusion is he key facor affecing he resuls of swelling behaviors. The accuracy of experimenal daa may be influenced by he properies of films. For he slowly swollen films, such as cross-linked PVA film, he difference of he swelling resuls by differen experimenal schemes is lile. The difference is significan for easily swollen samples. More exac swelling resuls may be obained by separaed mehod. For slowly swollen film, coninuous mehod is an effecive and sample-saving mehod, while separaed mehod is beer o obain more reliable swelling behaviors of rapidly swollen samples such as uncross-linked PVA film. ACKNOWLEDGEMENTS The financial suppors from he Naional Naural Science Foundaion Commission of China (Gran

7 458 J. Liu, X.J. Zheng and K.Y. Tang Commiee (No. 2009HASTIT015, 2009A430016) are graefully acknowledged. The cooperaion from he College of Maerials Science & Engineering, Zhengzhou Universiy is grealy appreciaed. REFERENCES [1] X. Kun, W.J. Hua, C. Qiang, Y. Yumei, Z. Wende and W. Pixin // J. Colloid Inerf. Sci. 321 (2008) 272. [2] S.H. Allan // Adv. Drug Deliver. Rev. 43 (2002) 3. [3] N.A. Peppas, K.B. Keys, M. Torres-Lugo and A.M. Lowman // J. Conrol Release 62 (1999) 81. [4] Aca Biomaer. 4 (2008) 733. [5] S. Pornsak and A.K. Ross // In. J. Pharm. 358 (2008) 205. [6] Y.Y. Hua, J.X. Min, D. Hui, Y. Yi and Z. Hua // Carbohydr. Polym. 71 (2008) 682. [7] B. Taiana and N. Parick // Macromolecules 31 (1998) [8] F. Gheorghe, C. Mariea and A. Paolo // Biomaerials 29 (2008) 2767.