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A fluorescence study on critical exponents during sol-gel phase transition in complex monomeric systems

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A fluorescence study on critical exponents during sol-gel phase transition in complex monomeric systems
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  This article was downloaded by:[ANKOS 2007 ORDER Consortium]On:14 November 2007Access Details:[subscription number 772815469]Publisher:Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Phase Transitions A Multinational Journal Publication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713647403 Ffast Transient Fluorescence Technique to StudyCritical Exponents at the Glass Transition Demet Kaya a ;Önder Pekcan a ; Yaşar Yılmaz aa Department of Physics Faculty of Science and Letters Istanbul TechnicalUniversity Maslak 80626 Istanbul Turkey.Online Publication Date:01 June 2003To cite this Article:Kaya, Demet,Pekcan, Önder and Yılmaz, Yaşar (2003) 'FfastTransient Fluorescence Technique to Study Critical Exponents at the GlassTransition', Phase Transitions, 76:6, 543 - 556To link to this article: DOI:10.1080/0141159031000085714URL:http://dx.doi.org/10.1080/0141159031000085714PLEASE SCROLL DOWN FOR ARTICLEFull terms and conditions of use:http://www.informaworld.com/terms-and-conditions-of-access.pdf Thisarticlemaybeusedforresearch,teachingandprivatestudypurposes.Anysubstantialorsystematicreproduction,re-distribution,re-selling,loanorsub-licensing,systematicsupplyordistributioninanyformtoanyoneisexpresslyforbidden.Thepublisherdoesnotgiveanywarrantyexpressorimpliedormakeanyrepresentationthatthecontentswillbecompleteoraccurateoruptodate.Theaccuracyofanyinstructions,formulaeanddrugdosesshouldbeindependentlyverifiedwithprimarysources.Thepublishershallnotbeliableforanyloss,actions,claims,proceedings,demandorcostsordamageswhatsoeverorhowsoevercausedarisingdirectlyorindirectlyinconnectionwithor arising out of the use of this material.     D  o  w  n   l  o  a   d  e   d   B  y  :   [   A   N   K   O   S   2   0   0   7   O   R   D   E   R   C  o  n  s  o  r   t   i  u  m   ]   A   t  :   1   3  :   4   4   1   4   N  o  v  e  m   b  e  r   2   0   0   7 Phase Transitions Vol. 76, No. 6, June 2003, pp. 543–556 FFAST TRANSIENT FLUORESCENCE TECHNIQUETO STUDY CRITICAL EXPONENTS AT THEGLASS TRANSITION DEMET KAYA, O ¨ NDER PEKCAN and YA S AR YILMAZ* Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak,80626 Istanbul, Turkey (Received 7 January 2002; Revised 13 February 2002; In final form 13 February 2002) The fast transient fluorescence (FTRF) technique was used to study critical exponents at the glass transition infree-radical crosslinking copolymerization (FCC) for two different monomeric systems, methyl methacrylate(MMA) and styrene (S). Pyrene ( P  y ) was used as a fluorescence probe. The fluorescence lifetimes of   P  y  fromits decay traces were measured and used to monitor the gelation process. Changes in the viscosity of the pregelsolutions due to glass formation dramatically enhance the fluorescent yield of aromatic molecules. This effectis used to study the glass transition upon gelation of MMA and S monomeric systems as a function of time, atvarious temperatures and crosslinker concentrations. The results are interpreted in the view of percolationtheory. The gel fraction and weight average degree of polymerization exponents    and     are found to be0.37  0.02 and 1.66  0.07 in agreement with percolation results. Keywords:  Fast transient fluorescence technique; Gelation; Percolation; Critical exponents 1. INTRODUCTION In chemical gelation, the molecules crosslink into larger clusters by forming covalentbonds in various ways. Free-radical crosslinking coplymerization (FCC) has beenwidely used to synthesize polymer gels. Several theories (Flory, 1941, Stockmayer,1943, Stauffer  et al  ., 1982; Stauffer, 1985; Herrmann, 1986; De Gennes, 1988;Stauffer and Aharony, 1992) have been developed in the past half century to describegel formation in FCC, among which Flory–Stockmayer theory and percolation theoryprovide bases for modeling the sol–gel phase transition. Statistical theories based ontree approximation, which are called mean field or classical theories, srcinate fromFlory (1941) and Stockmayer (1943), and assume equal reactivities of functionalgroups and the absence of cyclization reactions. Most statistical theories derived inthe following decades are fully equivalent, differing only in mathematical languagesuch as Macosko and Miller (1976); Pearson and Graessley (1978); Durand andBruneau (1979). Percolation offers a particulary simple and yet detailed picture in *Corresponding author. Tel.: 0 (212) 285 66 03. Fax: 0 (212) 285 63 86. E-mail: yyilmaz@itu.edu.trISSN 0141-1594 print: ISSN 1029-0338 online    2003 Taylor & Francis LtdDOI: 10.1080/0141159031000085714     D  o  w  n   l  o  a   d  e   d   B  y  :   [   A   N   K   O   S   2   0   0   7   O   R   D   E   R   C  o  n  s  o  r   t   i  u  m   ]   A   t  :   1   3  :   4   4   1   4   N  o  v  e  m   b  e  r   2   0   0   7 terms of which one may seek to understand gelation (Stauffer  et al  ., 1982; Stauffer,1985; De Gennes, 1988; Stauffer and Aharony, 1992). In the language of percolation,one may think of monomers as occupying the vertices of a periodic lattice, and thechemical bonds as corresponding to the edges joining these vertices at any givenmoment, with some probability  p . Then, the gel point can be identified with the perco-lation threshold  p c , where, in the thermodynamic limit, the incipient infinite clusterstarts to form. ldentifying the weight average degree of polymerization  DP w  with theaverage cluster size  S  av  and the gel fraction  G  with the probability  P 1  of an occupiedsite to belong to the incipient infinite cluster, one can predict the scaling behaviour of these and related quantities near the gel point, as a function of   j  p    p c j , S  av  ¼  A ð  p c    p Þ    ,  p  !  p  ð 1 Þ  1  ¼  B ð  p    p c Þ  ,  p  !  p þ ð 2 Þ Here,    and     are the critical exponents and  A  and  B  are the proportionally factors.The critical exponents in percolation theory,   ¼ 0.41 and     ¼ 1.80, differ from thosefound in Flory–Stockmayer,   ¼ 1 and     ¼ 1.Various methods based on solution properties are used to determine the averagemolecular weight of a polymer sample. These include methods based on colligativeproperties, light scattering, and viscosity (Collins  et al  ., 1973; Morawetz, 1975; Slade,1975; Billingham, 1977; Bohdanecky and Kovar, 1982). One would like to measurethe values of the critical exponents with sufficient accuracy to determine their univers-ality class and to verify that they indeed have the non classical values for percolationcomputed from series expansions and Monte Carlo studies as well as renormalizationtheory. The double logarithmic plot of the measured quantity against  j  p   p c j  givesa critical exponent as slope of the straight line fitting the data. A main obstacle liesin the precise determination of the gel point and the critical region. In particular, asmall shift in  p c  results in large shift in the critical exponent. Such a log–log plot revealsthat data should be particularly accurate near the gel point. Preferably, one shouldhave more than one quantity measured in the gelation experiments. Then, one canfix  p c  from the best fit of data and use the same  p c  for the other properties (Stauffer et al  ., 1982).The way to find the critical point in real experiments is to measure and to performthe scaling analysis for, at least, more than one quantity. The critical point can thenbe determined by varying  p c  in such a way as to obtain good scaling behaviourfor both quantities over the greatest range in  j  p   p c j , or  j t  t c j  if the experiments areperformed against time. The first time in 1998, it has been reached the experimentalstate of art to determine    and     exponents (and fractal dimension) for the poly(methylmethacrylate) (PMMA) by means of the steady state fluorescence (SSF) technique(Yılmaz  et al  ., 1998). The dynamic fluorescence anisotropy technique has also beenwidely used for studying segmental mobility and conformational changes in polystyrene(PS) (Viovy,  et al  ., 1985), linear poly(acrylamide) (PAAm) (Ricka  et al  ., 1987), Poly( N  -isopropylacrylamide) (PNIPA) (Binkert  et al  ., 1991) and polyelectrolyte (Bednar  et al  .,1991).It is expected that universal behaviour near the critical point should hold for any kindof monomers not just methylmethacrylate (MMA). But, because of the special features 544 D. KAYA  et al  .     D  o  w  n   l  o  a   d  e   d   B  y  :   [   A   N   K   O   S   2   0   0   7   O   R   D   E   R   C  o  n  s  o  r   t   i  u  m   ]   A   t  :   1   3  :   4   4   1   4   N  o  v  e  m   b  e  r   2   0   0   7 of the chemical constituents used for polymerization, the same technique may not beused for all kind of monomers. PS is an example: Even though the SSF techniqueallows one to get accurate and enough data (up to ten per second) it is not applicablefor studying the critical behaviour of the PS because of the spikes in pyrene (used as afluorescence probe) intensity curves, which is attributed to the existence of criticalopalescence, during the gel formation process of styrene (Okay  et al  ., 1998; Pekcan et al  ., 1999). In this article we studied the FCC polymerization of both styrene (S)and MMA by using the lifetime measurements of aromatic molecules as function of gelation time. In spite of the low frequency of data compared to the SSF technique,we would like to report on a fast transient fluorescence (FTRF) technique whichenables us to surmount this difficulty and makes it possible to directly measure thecritical exponents    and     for both PMMA and PS. 2. FAST TRANSIENT FLUORESCENCE TECHNIQUE When an organic dye absorbs light, it becomes electronically excited, then fluorescenceoccurs from the lowest excited singlet state and decays over a time scale typically of nanoseconds (Birks, 1971; Lakowicz, 1983). In addition unimolecular decay pathwaysfor deexcitation of excited state, there are a variety of bimolecular interactions whichcan lead to deactivation. These are referred to collectively as quenching processes,which enhance the rate of decay of an excited state intensity. For dilute solutionsof dye molecules in isotropic media, exponential decays are common. Because of these features fluorescence dyes can be used to study local environments.Fluorescence and phosphorescence intensities of aromatic molecules are affectedby both radiative and non-radiative processes (Kropp and Dawson, 1969). If thepossibility of perturbation due to oxygen is excluded, the radiative probabilities arefound to be relatively independent of environment and even of molecular species.Environmental effects on non-radiative transitions that are primarily intramolecularin nature are believed to arise from a breakdown of the Born–Oppenheimer approxima-tion (Bixon and Jortner, 1968). The role of the solvent in such a picture is to addthe quasi-continuum of states needed to satisfy energy resonance conditions.The solvent acts as an energy sink for rapid vibrational relaxation that occurs after therate limiting transition from the initial state. Kamioka  et al  . (1988) reported the solventdependence of energy trapping in phenanthrene block polymers and explained thedecrease in fluorescence yield with the static quenching, caused by the solvent inducedtrapping states. A matrix that changes little with temperature will enable one to studymolecular properties themselves without changing environmental influence. PMMAhas been used as such a matrix in many studies (Jones and Siegel, 1964). In situ  fluorescence decay experiments from which  P  y  lifetimes can be determinedwere performed using Photon Technology International’s (PTI) Strobe MasterSystem (SMS). In the strobe, or pulse sampling technique (Ware  et al  ., 1992) thesample is excited with a pulse fight source. The name comes about because the photomultiplier tube (PMT) is gated or strobed by a voltage pulse that is synchronizedwith the pulsed light source. The intensity of fluorescence emission is measured in avery narrow time window on each pulse and saved in a computer. The time windowis moved after each pulse. The strobe has the effect of turning on the PMT and meas-uring the emission intensity over a very short time window. When the data has been FTRF TECHNIQUE TO STUDY CRITICAL EXPONENTS 545     D  o  w  n   l  o  a   d  e   d   B  y  :   [   A   N   K   O   S   2   0   0   7   O   R   D   E   R   C  o  n  s  o  r   t   i  u  m   ]   A   t  :   1   3  :   4   4   1   4   N  o  v  e  m   b  e  r   2   0   0   7 sampled over the appropriate range of time, a decay curve of fluorescence intensityversus time can be constructed. The gating technique has relatively high signal-to-noise advantage upon both Single Photon Counting (SPC) and phase instrument,thereby increasing the dynamic range of the data acquisition. The strobe instrumentis much simpler to use than SPC and data is easier to interpret than with a phasesystem. Because of these advantages SMS is used to monitor gelation processeswhich take around few hours.The rate equation for an excited chromophore with pulse excitation can be written as d  ½ F    dt  ¼     1 ½ F    þ  L ð t    t 0 Þ½ F   ð 3 Þ where [ F  *] and [ F  ] represents the concentration of excited and ground state moleculesand  L ( t  t 0 ) is the light pulse of SMS system The lifetime     of fluorescence moleculeis given by the following relation, which is called Stern–Volmer equation    1 ¼     10  þ  k q ½ Q  :  ð 4 Þ Here  k q  is the quenching rate constant and given by Smoluchowski relation (Birks,1971) and [ Q ] is the quencher concentration. Solution of Eq. (3) produces the decayingfluorescence intensity as I  ð t Þ ¼  C   exp    t      ð 5 Þ where  C   is the preexponential factor. In order to quantify the above observation thearea under the fluorescence decay curves are calculated using Eq. (5) according tofollowing integration I  h i ¼ Z   t 2 t 1 I dt  ¼    c  ð 6 Þ where the integral is taken from the peak ( t 1 ) to the end point ( t 2 ) of the decay curve.The observed fluorescence decay of a sample,   ( t ) is related to the actual fluorescencedecay,  I  ( t ) and the SMS light pulse by the convolution integral  ð t Þ ¼ Z   t 0 L ð t    t 0 Þ I  ð t 0 Þ dt 0 :  ð 7 Þ Because of its long excited singlet lifetime.  P  y  as a chromophore (Birks, 1971) is anattractive choice for studying dynamics in polymers.  P  y  has been successfully employedas the fluorescence probe in the study of micellar (Dorrance and Hunter, 1972)and phospholipid dispersion (Cheng and Thomas, 1974). These studies focus onthe use of dynamics of quenching of   P  y  monomer fluorescence and excimer formationprocesses. The other application of the use of   P  y  as a fluorescence probe is the studyof the vibronic fine structure of its monomer fluorescence. The intensities of thevarious vibronic bands show a strong dependent on the solvent environment 546 D. KAYA  et al  .
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