ABSORPTION AND DESORPTION CURRENTS IN POLYSTYRENE DOPED WITH DR1. 1. Introduction

Transcription

1 ABSORPTION AND DESORPTION CURRENTS IN POLYSTYRENE DOPED WITH DR1 1 S. N. Fedsv 1, T. A. Revenyuk 1, J. A. Giacmetti 2 1 Odessa Natinal Academy f Fd Technlgies, Odessa, Ukraine 2 Sa Paul State University, Presidente Prudente, Brazil Abstract Isthermal plarizatin and deplarizatin currents in plystyrene dped with DR1 chrmphre mlecules have been measured and analyzed in rder t study the electrical relaxatin f the pled rder in this guest-hst system. It has been shwn that the superpsitin principle is applicable t the plymer system under study at temperatures lwer than the glass transitin temperature indicating that the dped plystyrene can be cnsidered electrically as a linear system with a negligible electric cnductivity. It has been fund that the respnse functin is nt the expnential ne and characterized by a brad distributin f relaxatin times. By cmparing isthermal absrptin and desrptin currents, the practical infrmatin was btained n the best pling cnditins fr getting high and stable residual plarizatin. 1. Intrductin Accrding t scientific predictins, rganic plymers will gradually replace their inrganic cunterparts in ptelectrnic devices with nn-linear ptical (NLO) plymers being ne f the mst prmising new materials [1-4]. The system must nt pssess a center f symmetry, in rder t have the NLO prperties. Therefre, in guest-hst materials, the NLO chrmphres disslved in a glassy plymer matrix must be permanently aligned fr breaking the symmetry and prducing the residual plarizatin. This is mst easily accmplished by applying a DC electric field t the plymer at the elevated temperature at which the chrmphre diples can be readily riented. The thin plymer film are usually placed between tw parallel electrdes with the plar axis perpendicular t the film planes (sandwich pling) [5]. Perfrmance f the pled material is limited by the structural relaxatin inherent t all glassy plymers leading t eventual randmizatin f the preferential rientatin f the chrmphre mlecules [6-8]. The cnstraints n the degree t which the pling-induced rientatin f chrmphres can be maintained ver lng perids f time abve rm temperature are nt well understd. It is als nt clear what theretical descriptin is apprpriate fr the plymer relaxatin belw the glass transitin temperature (T g ). One des nt understand what pling and physical aging prtcls and methds are mst apprpriate fr maximum micrstrustural alignment and increased tempral stability, because the phenmena f frmatin and relaxatin f the pled rder in NLO plymers are far frm being cmprehensively studied and understd. The diple rientatinal relaxatin is ften prbed by the secnd-harmnic generatin (SHG) [2,8]. Results btained by this methd are nrmally d nt fit t a simple expnential Debye mdel crrespnded t ne relaxatin time, therefre either the Khlrausch-Williams-Watts (KWW) stretched expnential functin, r the bi-expnential functin is used, bth characterizing a brad distributin f relaxatin times. At the same time, we believe that the relaxatin prcesses in NLO plymers can be studied by purely electrical methds adpted frm the well develped field f the electret research [5]. As an example, in this paper we reprt n applicatin f isthermal plarizatin and deplarizatin current measurements t investigate frmatin and relaxatin f the pled rder in amrphus plystyrene (PS) dped with the Disperse Red (DR1) chrmphre mlecules. We cnsider this system as a mdel ne n which sme features f the relaxatin cmmn fr all guest-hst NLO plymers can be btained. Electrical relaxatin and related phenmena have been previusly studied nly in pure PS, but even these data are nt cmprehensive [9-10].

2 2 2. Experimental prcedures Atactic amrphus PS (M w ) was btained by purificatin f a cmmercial resin. Samples were prepared frm a mixture f PS and the Disperse Red 1 (DR1) disslved in chlrfrm. The slutin was spread ver a glass plate and after drying the film was treated at 100 C fr 24 h under vacuum. Then the film was remved by immersing the plate int water. The cntent f the dye was cntrlled frm 0.5% t 2.5% in the films whse thickness was abut 20 µm. The films were characterized using X-ray diffractin and differential scanning calrimetry (DSC). It has been fund that the plymer films were entirely amrphus, with a glass transitin temperature T g arund 90 C. Aluminum electrdes f 2 cm 2 were depsited n bth surfaces f the sample by thermal evapratin in vacuum. Prir t perfrming plarizatin and deplarizatin measurements all samples were shrt circuited and slwly heated t 120 C (abve T g ) in rder t remve stray plarizatin and space charge. During the isthermal experiments, samples were kept at the desired plarizatin temperature T p fr at least 15 min and then the pling vltage V p was applied fr a definite time t p while the pling current I p was cntinuusly recrded. Immediately after cmpletin f pling, the sample was shrt circuited and the deplarizatin current I d was measured and recrded accrdingly. Althugh I p and I d flw in ppsite directins, they are bth presented as psitive currents hereafter in rder t facilitate cmparisn f the currents. 3. Results and discussin 3.1. Isthermal plarizatin and deplarizatin currents In the absence f cnductivity, the isthermal plarizatin current flwing thrugh a dielectric after applicatin f the cnstant vltage, called als the absrptin current, is a pure displacement ne depended n the applied pling field E is assumed t be cnstant and the s called respnse functin ϕ(t) characterizing the definite dielectric at a cnstant temperature. Thus, the plarizatin current density can be presented as j p = E ϕ( t) (1) 10 3 The deplarizatin current density in the case f zer cnductivity is A1 j d [ ϕ( t + t ) ϕ( )] = E t (2) where t is the pling time. If the pling time is lng enugh (t >>t), then ϕ(t+t ) 0, s, as fllws frm Eqs. (1) and (2), B1 A2 j p = j, (3) d B Fig. 1 Isthermal plarizatin (A) and deplarizatin (B) currents: A1-50 C, 3000 s; B1-50 C, 3000 s; A2-25 C, 1000 s; B2-25 C, 2000 s i.e. j p (t) and j d (t) shuld be mirrr images f each ther. Therefre, the difference between j p (t) and j d (t), if bserved experimentally, can be caused by either a finite cnductivity (g 0), r by incmplete pling due t a relatively shrt pling time t. In bth cases ne wuld bserve j p (t) j d (t).

3 T distinguish between influence f the tw factrs, ne shuld analyze the shape f the plarizatin current at lng times t. In the case f absence f the cnductivity and lng pling times, j p (t) and j d (t) cincide with bth currents ging t zer. If the sample has a definite cnductivity, the thrugh cnstant current will be bserved at the j p (t) curve, while the j d (t) curve will g t zer. It fllws frm the abve mentined that by cmparing isthermal plarizatin and deplarizatin currents ne can get infrmatin n the pling time required t btain the sufficient plarizatin and n the value f the cnductivity. Experimentally measured plarizatin and deplarizatin currents are presented in Fig. 1 and 2. At rm temperature and lng pling times (Fig. 1, curves A1 and B1), plarizatin and deplarizatin currents cincide indicating that the cnductivity is very lw and can be neglected at these cnditins. If the pling time is nt lng enugh, a deviatin f I d frm I p is bserved, as in the case shwn in Fig. 1, curves A2 and B2, being in accrdance with Eqs. (1) and (2). It has been fund that the cnductivity can be neglected prvided the pling temperature T p is lwer than the glass 10 3 Fig.2. Isthermal plarizatin (P) at 90 C fr 600 s and deplarizatin (D) at 90 C. Sample PS + 2% DR1. P D transitin temperature T g =90 C. Even at 80 C there was n tendency fr the plarizatin current t reach the saturatin. Hwever, if T p >T g, as ne can see frm Fig. 2, the plarizatin current is much higher than the deplarizatin ne and it tends t becme cnstant at lng pling times (purely cnductive current). It is knwn that in the Debye case f all diples having the same relaxatin time t, the decay f plarizatin and deplarizatin currents shuld crrespnd t the expnential functin I(t)=I exp(-t/t). The curves shwn in Fig. 1 and 2 cnsist f at least tw parts and match rather the pwer law I(t) t -n with n 2 at t frm 0 t 10 s fllwed by a prtracted regin where n This feature is usually cnsidered as an indicatin f a brad distributin f relaxatin times. Straightness f the I d (t) curve in the whle range f time with n > 1 can indicate incmplete pling. Ischrnal values f plarizatin and deplarizatin currents depend n temperature. This dependence is mre sund at small times (I p (80 C) = 60 pa, I p (25 C) = 8 pa at t = 10 s), but then the effect f temperature becmes negligible (I p (80 C) I p (25 C) = 0.1 pa at t = 1000 s). External injectin f charge carriers is nt likely t effect the cnductivity in ur case, since the current - vltage characteristic (nt shwn here) is linear at T>T g and isthermal plarizatin and deplarizatin currents are symmetrical belw T g as shwn in Fig. 1. Therefre, the space charge is frmed lcally due t spatial separatin f intrinsic carriers. Separated charges are mst prbably trapped between plymer chains frming virtual induced diples. Near T g, the trapped charges are shaken ut due t increased mbility f the fragments f the plymer chains but they d nt mve far under actin f the pling field, but rather recmbine, as if the lcal field is higher than the external ne. It is clear that the respnse functin f dped plystyrene is nt the expnent indicating that prcesses f plarizatin build-up and its relaxatin even in the isthermal case are mre cmplicated than thse fr which a simplified mdel, such as the Debye ne, can be cnstructed. It is interesting t find ut whether the material under study behaves electrically as a linear system, r it shws nt nly nn-linear ptical prperties, but als the electrical nes. 3

4 3.2. Superpsitin principle 4 In the case f the electric field E applied t a dielectric, the superpsitin principle establishes that if the respnse functin ϕ(t) fr a unit step variatin f the electric field, as well as the time dependence f the electric field are knwn, then it is pssible t calculate the current I(t) at any mment accrding t the fllwing equatin de I = A ϕ( t θ) dθ (4) dθ where A is the surface area f the sample. Suppse a step vltage is applied prducing a cnstant electric field E during a perid f time t frm t = -t t t = 0 and after that the sample is shrt circuited. In that case ne can write de dt = E δ ( t + t ) E δ (5) where δ(t) is Dirac s delta functin. Therefre, the Eq. (4) fr the transient current can be rewritten fr t > 0 as I = AE [ ϕ( t θ ) δ ( θ + t ) dθ ϕ( t θ ) δ ( θ ) dθ ] (6) After integratin and cnsidering the deplarizatin current as a psitive ne we btain [ ( t t )] I = AE ϕ ϕ + (7) It is clear frm Eq. (7) that if the pling time t is very lng (t ), then ϕ(t + t ) 0 and I (t)=ae ϕ(t). Therefre, if the superpsitin principle is applicable, it is pssible t calculate the deplarizatin current crrespnded t any pling time t frm nly ne experimental curve I (t). Thus, the ppsite is als accurate, i.e. if the experimentally measured and calculated isthermal deplarizatin currents match with each ther, then the superpsitin principle is valid and the system is linear. Crrespnding calculatins were perfrmed fr samples f PS+1.5%DR1 at different temperatures belw T g and results f the calculatins were cmpared with experimental I d (t) curves. As an example, the curves btained at 75 C and 90 C are shwn in Fig. 3 and 4. T btain the experimental curves we applied the pling vltage f 100 V fr a definite time t in the range frm 4 s t abut s and then measured the deplarizatin current. The current crrespnded t the lngest pling time was taken as I (t)=e ϕ(t) fr further calculatins accrding t Eq. (7).It is clear frm Fig. 3 and 4 that even fr a very shrt pling time f 4 s, gd agreement is bserved between experimental and calculated data, prving validity f the superpsitin principle fr a given system. This result is neither bvius nr trivial, because the plymer under study pssesses nnlinear ptical prperties. At the same time, as ur results have prven, the dped PS can be cnsidered electrically as a linear system. Frm the data shwn in Fig. 3 and 4 ne can cnclude als that the respnse functin ϕ(t) deviates cnsiderably frm the ften pstulated pwer law f ϕ(t) t -n, but at lw and medium pling times the curve I d (t) is very clse t the straight line in lgarithmic crdinates. In general, the deplarizatin current curve cnsists f three parts. The current at the first stage (frm t = 0 till t 10 s) des nt depend much n the pling time indicating that it riginates frm deplarizatin f diples having shrt relaxatin times. In the pwer law apprximatin, n 2 is usually bserved at this stage. At the secnd stage (frm 10 s t abut s) the curves btained at different pling

5 times g apart with n ranging frm 2 at very shrt pling times t 1 fr very lng pling times. Therefre, at this stage the shrter pling time, the smaller the pwer cefficient n. At the third stage, abslute values f the currents crrespnded t different pling times are very different, but the cefficient n is almst the same fr all curves which are nw parallel t each ther in lgarithmic crdinates with n 2, similarly t that at the first stage. It fllws frm Eq.(7) that ischrnal values f the deplarizatin current must be prprtinal t the pling field. This feature f the superpsitin principle was examined by cmparing experimental deplarizatin curves btained after pling fr the same time (t = 20 s in ur experiments), but at different pling vltages frm 30 V t 300 V. The results have shwn that the prprtinality f the ischrnal currents t the pling field is actually bserved additinally prving the validity f the superpsitin principle. dp Isthermal deplarizatin current I d = A reflects dynamics f the isthermal relaxatin dt f the residual plarizatin P. Since the deplarizatin experiments are perfrmed under shrt circuit cnditins (E=0), the cnductivity phenmenn des nt effect the measurements. Cnstant vltage Pling time (s) Pling time (s) Fig.3. Isthermal deplarizatin currents at 75 C in isthermally pled PS+1.5% DR1 samples with different pling times Slid lines - experiment, pints - calculatins Fig.4. Isthermal deplarizatin currents at 90 C in isthermally pled PS+1.5% DR1 with different pling times. Slid lines - experiment, pints - calculatins was applied fr a rather lng time and then the sample was shrt circuited at a definite mment and the deplarizatin current was recrded.. The results are shwn in Fig. 5. It is seen frm Fig. 5 that curves fr 30 C and 50 C cincide, while the curve btained at 70 C is parallel t that f 30 C and 50 C indicating qualitatively that the relaxatin functin remained unchanged, but the level f the residual plarizatin and cnsequently, the rate f its decay (the deplarizatin current) increased with temperature in the range frm 50 C t 70 C. It is pssible that the diples are easily riented at higher temperatures due t decreasing f the relaxatin time. It means als that the saturated value f the plarizatin has nt been reached during pling, because the equilibrium plarizatin, accrding t the Langevin-Debye equatin, shuld decrease inversely prprtinal t temperature. Hwever, this in fact happens at T>T g. Due t intensified mtin f the plymer chains, the thermal disrienting during pling prevails ver the rienting effect f the pling field. As the result, the final value f the residual plarizatin at the end f pling decreases. Mrever, a large number f the diples are disriented during the shrt perid f time after the shrt circuiting, prducing the first part f the deplarizatin curve characterized by high values f the current and by its fast decreasing (curves fr 90 C and 110 C in Fig. 5).

6 PS des nt shw cnsiderable cnductivity at temperatures belw T g. On the ther hand, at T > T g, the current measured under a cnstant applied vltage increases shwing existence f sme cnductivity. The rigin f this cnductivity is nt definitely established, but since it is thermally activated, ne can suggest that the cnductivity 90 C 50 C 30 C 110 C C Fig.5. Isthermal deplarizatin f PS+1.5% DR1 after pling at 150 V at different temperatures. current almst certainly riginates frm internal emissin f intrinsic charge carriers, rather than frm the external injectin. This suggestin is supprted by linearity f the current - vltage characteristic at T > T g. It has been fund that the temperature dependence f cnductivity in dped PS is dissimilar fr different cntent f DR1, s as the higher cncentratin f the dpant crrespnds t the higher cnductivity. It is pssible that tgether with the DR1 mlecules sme ther species are added t PS matrix including thse prducing charge carriers at high temperature. As fr the activatin energy f cnductivity, it has the same rder f ev. Cnductivity f the samples at T<T g can be neglected because the saturated steady current was nt bserved even after applicatin f the pling vltage fr a lng time. Mrever, plarizatin and deplarizatin currents in well pled samples are cincide as seen in Fig Cnclusin It has been shwn that the electrical relaxatin f the pled rder in PS dped with DR1 chrmphre mlecules can be successfully studied by measuring isthermal plarizatin and deplarizatin currents. It has been prved that the superpsitin principle is applicable t the plymer system under study, s as the dped PS can be cnsidered electrically as a linear system with a negligible electric cnductivity at temperatures lwer than the glass transitin temperature. It has been fund that the deplarizatin current curve f dped plystyrene cnsists f three parts with the respnse functin being essentially nn-expnential ne, but rather characterized by a brad distributin f the relaxatin times. It means that the behavir f even such a simple system as PS dped with DR1 is much mre cmplicated than that predicted by knwn theretical mdels. By cmparing isthermal plarizatin and deplarizatin currents ne can get practical infrmatin n the preferable pling cnditins, such as the pling time and temperature required fr btaining the adequate and stable residual plarizatin. References 1. C. C. Chang,, C. P. Chen, C.C. Chu J. Macrml. Sci.-Pl. R. 2005, C35, F. Kajzar, K. S. Lee, A. K. Y. Jen Adv. Plym. Sci. 2003, 161, L. Daltn Adv. Plym. Sci. 2002, 158, N. Blembergen J. Nnlinear Opt. Phys. 1996, 5, G. M. Sessler (ed.) Electrets, Vl. 1, Third Editin, Laplacian Press, Mrgan Hill, E. Cecchett, D. Mrni, B. Jerme J. Phys.-Cndens. Mat. 2005, 17, R. D. Dureik, D. E. Schuele, K. D. Singer J. Opt. Sc. Am. B. 1998, 15, A. Dhinjwala, G. K. Wng, J. M. Trkelsn J. Chem. Phys. 1994, 100, V. Sangawar, C. S. Adgankar J Plym. Mater. 1996, 13, V. Sangawar, C. S. Adgankar Indian J. Pure Appl. Phys. 1996, 34,