Melting of lmellr phses in temperture sensitive colloi-polymer suspensions A. M. Alsye, Z. Dogic, n A. G. Yoh Deprtment of Physics n Astronomy, University of Pennsylvni, Philelphi, Pennsylvni 19104-6396 (Dte: Ferury 4, 2004) We investigte the ehvior of novel suspension compose of ro-like f virus n thermosensitive polymer Poly(N-isopropylcrylmie) whose phse igrm is temperture n concentrtion epenent. The system exhiits rich vriety of stle n metstle phses, n, more importntly, provies unique opportunity to irectly oserve melting of lmellr phses n single lmelle. Typiclly lmellr phses swell with incresing temperture efore melting into the nemtic phse. The highly swollen lmelle cn even e superhete s result of topologicl nucletion rriers tht slow formtion of nemtic phses. Using opticl forces we prepre metstle nemtic n multilyer structures strting from single colloil memrne, n then oserve the system evolve ck into single memrne. PACS numers: 64.70.M 61.30.-v 64.60.My Melting of three-imensionl (3D) crystls is mong the most uiquitous phse trnsition in nture [1]. In contrst to freezing, melting of 3D crystls hs no ssocite energy rrier. Bulk melting is initite t pre-melte surfce, which then cts s heterogeneous nucletion site n elimintes the nucletion rrier for the phse trnsition [2 4]. In this pper we investigte pthwys for melting of lmellr phses. In contrst to 3D crystls, lmellr phses hve one-imensionl (1D) qusi-long-rnge orer [5 7]. Aitionlly while 3D crystls hve only one surfce etween coexisting mei the lmellr phse is microphse seprte stte in which the entire ulk is spnne y interfces etween immiscile phses. These phses re frequently uilt from mphiphilic molecules such s surfctnts n lock copolymers in solution. Although lmellr phses re very common, melting trnsitions of lmellr into nemtic phses re rrely oserve. Here we crete novel system tht exhiits lmellr-nemtic melting, n we show melting of lmellr phse is funmentlly ifferent from melting of 3D crystlline solis. We hve evelope temperture sensitive queous colloil suspension in orer to crry out these stuies. The suspension is compose of lyotropic monoisperse ros of f virus [8, 9], n thermosensitive Poly(N-isopropylcrylmie) (NIPA) polymer. The thermotropic chrcter of the f/nipa mixture stems from the temperture epenent soluility of NIPA polymer in wter [10]. Below its Θ-temperture of 31 C, wter is goo solvent n NIPA polymer ssumes swollen coil form. Aove the Θ-temperture wter is poor solvent n NIPA hs collpse gloule form [11]. The resultnt mixture forms isotropic, lmellr n nemtic phses epening on temperture n constituent concentrtion. The lmellr phses resemle those of f with hr spheres, in which lyers of spheres re interclte etween lyers of ros [12]. Our temperture sensitive queous suspension, however, enles us to irectly stuy trnsitions from one phse to nother with opticl microscopy. These f/nipa solutions re unusul new mterils whose lmellr phses iffer from those of mphiphilic molecules. The lmelle cn swell consierly n melt into nemtic phse, trnsition lmost never oserve in lock-copolymer lmellr systems. The ifferent phse ehviors rise primrily ecuse of the ro-like chrcter of the f component, n lso ecuse of the sence of covlent cross-links etween f n NIPA polymer. The kinetic pthwys exhiite y these f/nipa mixtures re lso more complex then those foun in temperture-epenent (thermotropic) moleculr liqui crystls n concentrtion-epenent (lyotropic) mphiphilic systems [13, 14]. For exmple the lmellr phses of f/nipa mixtures cn e swollen to the point where isolte lyers (lmelle) of ros re oserve. These isolte memrne-like structures of non-mphiphilic mesogens re stle ginst mechnicl perturtions, melt into nemtic roplets, n rise questions out the simplest intercting mesogens cple of ssemling into two-imensionl (2D) memrnes. Most of the ssemly into 2D memrnes is riven y hyrophoic-hyrophilic interctions [15 17], while our work inictes tht much simpler system of ros with uniform ttrctions is cple of ssemling into memrnes uner specific conitions. Finlly, our temperture sensitive f/nipa solutions offer powerful new moel system for stuies of melting. Lyotropic suspensions such s collois hve proven to e iel for rel-time stuies of freezing [18 20]. Melting trnsitions, however, re much more ifficult to investigte with lyotropic collois. Once stle orere phse is otine, microscopic oservtion n concurrent concentrtion vrition re require to monitor melting, chllenging experimentl tsk. The vntge of our wekly thermotropic f/nipa polymer se suspension is tht simple chnges in temperture enle us to prepre the lyotropic colloil system in metstle orere stte, n then stuy its melting trnsition in situ. Generlly smll increse of temperture increses monomer-monomer ttrctions n thus ecreses the osmotic pressure of the semi-ilute NIPA polymer solution. As result, wter from polymer-rich
Lyer spcing µm 2 c 4 3 ntensit 2 1 5 o C 7 o C 10 o C e c 1.40 1.38 30 le (egr f 13 o C 16 o C g h 17 o C 17 o o C 1.36 1.34 1.32 1.30 1.28 heting cooling 1.26 4 6 8 10 12 14 16 Temperture o C FIG. 1: Melting ehvior of smple S1 (50 mg/ml f n 7.5 mg/ml NIPA). () Lmellr phse t low temperture exhiits visile isloction efects () At 7 C the isloction efects ct s site for nucletion of the nemtic phse. (c) Nemtic omins grow, expelling NIPA polymer into lmellr phse, which les to swelling of lmellr lyers. () Swollen lmellr phse (e) Coexistence etween nemtic phse n highly swollen lmellr phse. Lmellr phse hs the shpe of n elongte stripe while the nemtic phse fills the rest of the spce. (f-g) Isolte monolyer-eforme isotropic tctoi. (h) Isotropic-nemtic coexistence oserve t high temperture. (i-l) Illustrtion of the propose melting processes of the lmellr ro/polymer mixture. Scle rs re 5 µm. lyers flows into ro-rich lyers until osmotic equilirium is re-estlishe. This phenomen ilutes the ro concentrtion loclly, which in turn cn le to melting of the lmellr phse. NIPA polymer ws synthesize t 22 C y polymeriztion of of N-isopropylcrylmie (7 gm) issolve in 100 ml of 20mM (ph=8.15) tris uffer. Ammonium persulfte (61 mg) n tetrmethyleneimine (TEMED) (280 µl) were e s inititor n ccelertor [10]. Bcteriophge f is negtively chrge semi-flexile ro with imeter of 6.6 nm, contour length of 880 nm n persistence length etween 1000 nm n 2200 nm [9, 21]. It forms isotropic, cholesteric(nmetic) n smectic phses with incresing concentrtion [8]. A stnr protocol ws use to grow f virus [9]. The f/nipa mixtures were suspene in uffer (ph=8.15, 20 mm tris) n were plce etween the coverglss n coverslip n then sele. All oservtions were me with n inverte FIG. 2: Melting of the lmellr smple S1 stuie y smll ngle light scttering. The rel spce imges of the melting ehvior re shown in Fig. 1.() Smll ngle light scttering pttern of n orienttionlly isorere lmellr phse t 5 C () Angulrly verge intensity profile t 5 C n 13. (c) Lmellr perioicity for smple uring heting (fille circles) n cooling (open circles). There is lrge hysteresis of lyer spcing etween heting n cooling cycles. microscope (Leic DM IRB) equippe with DIC optics using n oil immersion lens (100X, N.A. 1.4). The temperture of the smple n the ojective ws controlle with wter circultor. Imges were cpture y CCD cmer n igitize using NIH-imge softwre. The opticl trp for the melting experiments ws crete with IR light (λ = 1054 nm) s escrie elsewhere [22]. The melting ehvior of f/nipa mixture with high ro concentrtion is illustrte in Fig. 1. Herefter we refer to this smple s S1. At low temperture (5 C), the mixture forms lmellr phse where lyers of ros re interclte with lyers of polymers; the polymers re in the semiilute regime (Fig. 1). The mesure lyer perioicity (1.2 µm) is much lrger then the perioicity of the smectic phse (0.9 µm) in isolte f suspensions. Disloctions n grin ounries re oserve t low temperture (Fig. 1 n i); with incresing temperture they ct s nucletion sites for formtion of nemtic tctois (Fig.1). The shpe of the nemtic tctoi in the smectic ckgroun is very similr to the shpe of the nemtic tctoi in the isotropic ckgroun [23]. Although polymer is mcroscopiclly miscile with ros in the lmellr phse, it is highly immiscile with ros in nemtic phses [24]. Therefore s the nemtic tctois grow, they expel polymer into coexisting lmellr phse which in turn les to swelling of the lmellr
3 lyers (Fig. 1c n j). In swollen lmellr phse there re regions where iniviul lyers re tightly oun n regions where they re well seprte (Fig. 1). Aove 15 C most of the smple is melte into nemtic phse which coexists with highly swollen lmellr phse. The lmellr phse ssumes form of elongte stripes (Fig. 1e n k). Upon incresing the temperture further, the lmellr stripes trnsform into polymer rich isotropic tctois which re frequently eforme y few isolte lmellr lyers (Fig. 1f-h). Finlly, ove 17 C isolte lyers melt, n the isotropic tctois in the nemtic ckgroun ssume their chrcteristic shpe (Fig. 1h n l) [23]. Lmellr perioicity is lso otine from the iffrction pttern of lmellr phses (Fig. 2). With incresing temperture the pek simultneously roens n shifts to lower ngles (Fig. 2 n c) inicting swelling of the lmellr phse. Aove 15 C most of the smple hs melte into the nemtic: in this cse only lrge forwr scttering is oserve. At this temperture we still oserve highly swollen lmellr stripes with n opticl microscope (Fig. 1e). When the smple is coole from the high temperture nemtic phse, the lmellr phse forms t 13 C. Tken together, the oservtion of coexistence n hysteresis in the heting/cooling cycle suggest lmellr melting into the nemtic phse is first orer phse trnsition with significnt nucletion rriers. Moreover the structure of the lmellr phse epens on smple history. The lmellr spcing of smples prepre t high temperture n susequently coole is smller thn the lmellr spcing of smples prepre t low tempertures n susequently hete. In orer for the polymer to form n isotropic roplet the lmellr lyers must melt. In the next prt of the pper we show there is kinetic rrier for melting of isolte lmelle into nemtic. Therefore with incresing temperture, the smple forms metstle highly swollen lmellr phse until the iniviul lmelle overcome nucletion rrier for melting. In the experiments with smple S1, we oserve swelling of the lmellr phse. The concentrtion of lmellr lyers in these smples ws high, n thus swelling ws limite y steric interctions. In orer to etermine the mximum swelling of the lmellr system, we repete the melting experiments t much lower f concentrtion (Fig. 3). Herefter we refer to this smple s S2. At low tempertures most of the smple is n isotropic polymer solution. At tempertures etween 21-25 C we oserve coexistence etween lmellr roplets (Fig. 3) n isolte lmelle (colloil memrnes) (Fig. 3c). The spcing of the lyere roplets is 1.2 µm, inicting lmellr phse similr to S1. The colloil memrnes we oserve re compose of single liquilike lyer of highly ligne ros; similr memrnes hve een oserve in therml ro/polymer mixtures n perhps in queous solutions of β-feooh ros [9, 19, 25]. Over time isolte lmelle grow from the interfce of the isotropic-lmellr roplet into the isotropic phse; this 21 o C 26 o C c e f 27 o C 30 o C 30 o C 30 o C FIG. 3: Melting ehvior of smple S2 (7.5 mg/ml f n 37 mg/ml NIPA). () At 24 C lmellr roplets form in ckgroun isotropic phse. Simultneously, the formtion of isolte lyers (shown in imge c) is oserve. () At higher temperture, lmellr roplets melt into nemtic phse y the mechnism escrie in Fig. 1. (c) An isolte memrne is stle for tempertures from 22-27 C. (-f) Sequence of imges showing nucletion n growth of 3D nemtic nemtic tctoi from n isolte superhete 2D memrne. (gj) Illustrtion showing the ove escrie melting processes. At tempertures elow 10 C, the smple forms miscile isotropic phse. The reson for this in not unerstoo. Scle rs re 5 µm. inictes tht single memrnes re more stle thn lmellr roplets. The coexistence of isolte memrnes n lmellr roplets with well efine spcing implies nother first orer trnsition, this time from oun to unoun lyers. Relte ining-unining trnsitions hve een stuie theoreticlly in the context of lipi ilyers; the orer of these trnsitions ws foun to e sensitive to the rnge of the intermoleculr interctions [26, 27]. It is lso notle tht the suspension of f ros usully contin low volume frction of imers. The imers cn ffect the swelling kinetics of smectic lyers n proly increse the stility of the multilyer structure since they issolve more esily in ilyer thn in monolyer. We hve investigte melting n lyer reucing ynmics ssocite with isolte memrnes (smple S3). At tempertures ove 25 C lmellr roplets melt into nemtic phse. The mechnism for this melting is similr to melting of the ulk lmellr phse (Fig. 3). The isolte colloil memrnes, however, remin stle t tempertures well ove the melting temperture of the multi-lyer lmellr roplets. Only upon ris-
4 c e g FIG. 4: Lyer reucing trnsition of colloil memrne oserve in smple S3 (12 mg/ml f, 31 mg/ml NIPA). The smple temperture is 28 C, elow the melting of the ulk lmellr phse (29 C). () A memrne is repetely puncture y 2.1 µm silic sphere trppe with lser tweezers. () A metstle nemtic roplet forms ue to sher melting. (c) Upon removl of the lser em the nemtic roplet freezes into multilyer smectic structure. (-f) The structure unergoes sequence of lyer reucing trnsitions. (gh) In seprte experiment formtion of isloction in the min memrnes in visile. The isloction susequently isppers s lyers slie over ech other which reucing the numer of lyers y one. ing the temperture to lmost 30 C is nucletion of three-imensionl nemtic tctoi oserve in the twoimensionl memrne (Fig. 3-f). Once nemtic tctoi is nuclete it quickly grows until the whole memrne melts. If the smple is susequently coole, the formtion of single lyer smectic will e oserve only t tempertures elow 27 C. These oservtions suggest colloil memrnes re metstle superhete structures etween 27-30 C. Nucletion of 3D nemtic tctoi from 2D memrne requires lrge collective fluctution in which mny ros protrue into the thir imension. Our experiments inicte the topologicl rrier for nucletion of nemtic tctoi is very lrge. In this sense the melting ehvior of lmelle re very ifferent from melting of 3D solis. Finlly we explore the stility of isolte colloil memrnes ginst externl mechnicl perturtions (Fig. 4). These experiments were one t tempertures elow the ulk lmellr melting trnsition so tht the f h memrnes were stle. A silic e ws trppe with n opticl tweezer n the colloil memrne repetely puncture (Fig. 4). This mechnicl perturtion prouce locl sher melting of the memrne into nemtic roplet (Fig. 4). We were only le to sher melt memrnes t tempertures close to ulk lmellr-nemtic phse trnsition. At lower tempertures the puncture memrne rpily returne to its equilirium shpe. After the memrne melte, the lser em is kept in close vicinity of the nemtic roplet, loclly heting the smple. When the lser em is on, the nemtic roplet coexists with the memrne. The temperture is reuce when the lser em is turne off, n the nemtic roplet quickly freezes into multilyer smectic structure (Fig. 4c). This multilyer smectic structure is unstle n unergoes sequence of lyer reucing trnsitions until the initil equilirium memrne configurtion is otine. This experiment provies itionl evience tht colloil memrnes re equilirium structures in this temperture rnge. By irectly visulizing the smectic lyers we cn iscern the mechnism y which lyer reucing trnsition tkes plce. The numer of lyers cn e reuce when iniviul lyers slie over ech other (Fig 4e-f) or y spontneous formtion of isloctions in the min memrne n their susequent nnihiltion (Fig. 4g-h). These trnsitions re relevnt to lyer reucing trnsitions in superhete smectic films of thermotropic liqui crystls [28]. To conclue, we hve crete novel f/nipa lmellr phse which cn simultneously melt into nemtic phse n swell into iniviul lmelle. The interply etween these two processes cretes multiple melting pthwys of unexpecte complexity. The lmellr phse iffers from conventionl 3D crystl in tht interfces etween micophse seprte lyers of ligne ros n polymers in isotropic phse spn the entire smple. As result, melting of these interfces correspons to melting of the entire ulk smple n it is possile to superhet the lmellr structure. Another unique feture of our experiment is the ility to control the phse ehvior of lyotropic suspension y temperture, which in turn controls the solvent qulity of the polymer phse. This strtegy cn e use in the future to stuy phse trnsitions of other systems in soft conense mtter. We thnk Tom Luensky n Dennis Discher for useful iscussions. This work ws supporte y the NSF through grnt DMR-0203378 n the PENN MRSEC, DMR-079909. We lso cknowlege support from NASA (NAG8-2172). [1] H. Lowen, Phys. Rep. 237, 249 (1994). [2] J. G. Dsh, Rev. Mo. Phys. 71, 1737 (1999). [3] R. W. Chn, Nture 323, 668 (1986). [4] J. F. vn er Veen, Surf. Sci. 433-435, 1 (1999). [5] F. S. Btes, Science 251, 898 (1991). [6] J. T. Chen, E. L. Thoms, C. K. Oer., n G. P. Mo, Science 273, 343 (1996). [7] M. Muthukumr, C. K. Oer, n E. L. Thoms, Science 277, 1225 (1997). [8] Z. Dogic n S. Fren, Phys. Rev. Lett. 78, 2417 (1997). [9] Z. Dogic n S. Fren, Phil. Trns. R. Soc. Lon. A. 359, 997 (2001).
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