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Floresans tekniği kullanılarak ince polimer filmlerin çözünürlüğünün incelenmesi

In-situ fluorescence experiments for thin polymer fi̇lm dissolution

  1. Tez No: 66558
  2. Yazar: ŞAZİYE UĞUR
  3. Danışmanlar: PROF. DR. ÖNDER PEKCAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Fizik Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 94

Özet

ÖZET Bu çalışmada kararlı durum floresans tekniği kullanarak PMMA ve Latex filmlerin çözünürlüğü incelendi. Floresans madde olarak Pyrene kullanıldı. Çözücü kalitesinin, karıştırmanın (agitation) ve sıcaklığın çözünme işlemi üzerindeki etkileri araştırıldı. Çözünme işlemi deney boyunca uyrarılmış Pyrene moleküllerinin zamanla değişimi kaydedilerek izlendi. Çözünme paremetreleri Dd (polimer salınma katsayısı) ve ko (relaksasyon sabiti) paremetrelerini hesaplamak için Case I ve Case II difüzyonunu içeren bir model kullanıldı. PMMA filmler için Dd katsayıları («10~6 cm2/s) mertebesinde bulundu. Ayrıca floresans sönme için kq=1.22xl010 M'V1, D=2.05xl0“5 cm2/s ve xo=18.6 ns olarak hesaplandı. Latex filmler için Dd«10”n cm2/s ve ko«10"2 mg/cm2dak mertebesinde bulundu. Sıcaklığa göre yapılan çalışmada Case I ve Case II difüzyonu için aktivasyon enerjileri yaklaşık olarak AEd=24.4 kcal/mol ve AEko=11.3 kcal/mol bulundu.

Özet (Çeviri)

SUMMARY Dissolution of glassy polymer films can be divided into three steps. The first is the diffusion of solvent molecules into the polymer matrix. In the second step, the solvent molecules initiate the relaxation of polymer chains and a solvent swollen gel is formed. The third and the final step consists of diffusion of polymer chains from the gel into the solvent reservoir. The penetration of solvent molecules into glassy polymers often does not proceed according to the Fickian diffusion model. Penetration not described by the Fickian model is called anomalous diffusion, where the rate of transport is entirely controlled by polymer relaxations. This transport mechanism is termed case II in contrast to Fickian diffusion which is called case I. In the case II diffusion model, the second step is the rate limiting step which predicts a linear dependence of the change in film thickness on time. The first and third steps, however follow a case I diffusion model, where the first one is the sorption of solvent molecules by the glassy film and the third is the desorption of polymer chains from the gel layer into solvent (Figure 1). Various mechanism and various mathematical models have been considered for the polymer dissolution. In this study, we employed a simpler model, developed by Enscore etal. to interpret the result of polymer swelling and dissolution experiments. This model includes case I and case II diffusion kinetics. Fickian (case I ) diffusion kinetics includes the solution of a unidirectional diffusion equation for a set of boundary conditions. For a constant diffusion coefficent, D and fixed boundary conditions, the sorption and desorption transport in and out of a thin slab is given by the following relation M,, 8 - 1 ( (2n + \)2D7V2t^ - = 1 r- S r-eXp V (1) Here Mt represents the amount of materials absorbed or desorbed at the time t, M» is the equilibrium amount of material, and d is the thickness of the slab. VIc dJ >“o 1/1 I c_ dJ e >-, o Q_ aj ID to c ai > ”o CO to ru 13 c Ol > o o -. C ^ O nj aj o 5 U_ C/l c > O (-0 10 to 13 ro vııCase II transport mechanism is characterized by the following steps: As the solvent molecules enter into the polymer film, a sharp advancing boundary forms and separates the glassy part from the swollen gel (Figure lb). This boundary moves into the film at a constant velocity. The swollen gel behind the advancing front is always at a uniform state of swelling. If we consider a cross section of a film with thickness d undergoing case II diffusion as in Figure 1, where L is the position of the advancing sorption front, Co is the equilibrium penetrant concentration and ko (mg/cm2min) is defined as the case II relaxation constant. It can be seen that the relaxation front, positioned at L, moves toward the origin with a constant velocity, ko/Co. The algebric relation for L as a function of time t, is described by equation : L = d-^-t. (2) since Mt=koAt and Moo=CoAd, the following relation is obtained: - L = - -t (3) M“ C0d Polymer dissolution processes can be affected by various parameters, including solvent quality, polymer molecular weight, solvent termodynamic compatibility, agitation and temperature. In this work the dissolution mechanism of polymethyl methacrylate (PMMA) thin films in selected organic solvents was investigated. The dissolution was monitored using a Perkin Elmer LS-50 spectrofluorimeter. Pyrene (Py) was used as a fluorescence probe and pyren fluorescence intensity, Ip, was monitored during the dissolution process at 395 nm using the ”time drive“ mode of spectrofluorimeter. Three different sets of experiments were carried out; in the first and second sets AIBN (%0.26 gr) and py (4x1 0”4 M) were dissolved in MMA and this solution was transferred into round glass tube of 15mm internal diameter. Before polymerization, each solution was deoxygenated by bubling nitrojen for 10 minutes. Radical polymerization of the MMA was performed at 65 ±3 °C. After polymerization was completed the tube was broken. Disc shaped, thin films (around 0.2 cm.) were cut for the dissolution experiments.Dissolution of disc shape polymethyl methacrylate (PMMA) films were monitored in real-time by the Py fluorescence intensity change. In the first set,the effect of agitation(stirring) on dissolution process was studied. In order to Vlll2000 8000 Dissolution Time (sec) Fig.2 pyrene intensity, IP, at 395 run versus dissolution time for the three different stirring speed. a,b and c indicate the curves for high,low and no stirring speed. IXunderstand this, dissolution experiments were performed at three different stirring speeds (low, high and no stirring speeds) by using solely chloroform as dissolution agent. Figure 2 presents the Py intensity, Ip versus dissolution time where a and b represent the curves at high and low speeds of magnetic stirrer respectively. Dissolution curve with no magnetic stirrer is shown with c in Figure 2. For high and low stirring speeds, the asymptotic values of the dissolution curves are reached by following the“case I diffusion model. On the other hand, in the same time interval, dissolution curve for no stirrer presents linear behaviour after a short delay which may obey case II model. These results indicate that dissolution process are strongly affected by agitation. In the second set, the effect of solvent quality on film dissolution were studied. Dissolution of disc shape polymethyl methacrylate (PMMA) films in various chloroform-heptane mixtures were performed by the Py fluorescence intensity change. Dissolution curves for various chloroform-heptane mixtures at high stirring speed are shown in Figure 3. It is seen that as the heptane content increases, curves reach a pletau at later times. In other words, PMMA films dissolve slower in higher heptane content solution. In order to understand the smaller intensity values at higher heptane content samples, SSF spectra of Py were taken in various chloroform-heptane mixtures. No shift is observed at maximum Py intensity. Smaller intensity values in Py spectra for high heptane content mixture can be explained by the low viscosity (0.38 cp) effect of heptane, in which excited Py can be quenched much easier than is possible with a high chloroform (0.58cp) content mixture. In processing dissolution data it is assumed that Ip is proportional to the number of Py molecules desorbing from the PMMA film. For a Fickian dissolution behaviour the logarithmic form of Eq (1) is written for n=0, with Ad=Dd7t2/d2 and Bd=Ln(8/7t2) as follow Ln 1 p- = Bd- Adt (4) Here, Ipoo presents the number of Py molecules at equilibrium condition, Dd is the desorption coefficient and d is the thickness of the PMMA disc. Under the assumption above, For case II dissolution behaviour Eq (3) is written as: / k A* (5) -.poo ^0^00 2000 4000 6000 Dissolution Time (sec) 8000 Fig.3. Pyrene intensity, Ip versus dissolution time for film samples dissolved in various chlroform-heptane mixtures at high stirring speed. Numbers above the curves indicates the percents of the heptane content. XIThe curves in Figure 3 seem to follow a case I (Fickian) diffusion model.The logarithmic form of all dissolution curves in Figure 3 according to Eq (4) present linear dependences on time conforming our assumption of relationship with diffusion model. In order to obtain Da values, linear parts of this curves were compared to computations using Eq (4) and desorption coefficients of Py molecules were found to be (»10”6 cm2/sec). İn the third set of experiment, pyrene (Py) labeled PMMA-PIB latex particles were dispersed in heptane in a test tube. Film samples were prepared from this dispersion, by placing numbers of drops on 3x0.8 cm2 glass plates and allowing the heptane to evaporate. Samples were weighted before and after the film casting to determine the film thickness. The average film thickness were around 10 urn. The films were annealed in an oven for 1 hr above the Tg at 180 °C maintained within ±2 °C during annealing. Dissolution experiments were performed as in the first two set, only equipped with a temperature controller. Firstly, the film samples were dissolved in pure toluene at 15, 20, 25, 30, 35 and 40 °C temperatures. Then films samples were dissolved in toluene (80 %)+heptane (20 %) mixture and film samples were kept 25, 30, 35, 40, 45, 50, 55, 60 and 65 °C temperature during dissolution processes. Since toluene is a good solvent for PMMA, heptane is introduced into the mixtures to slow down the dissolution process. Figure 4 shows the dissolution curves for film samples dissolved in toluene at elevated temperatures. One can observed that as the dissolution temperature increases, film samples start to dissolve at early times. The curves reach a plateau almost in the same fashion at long times. Dissolution curves of film samples dissolved in (80+20) % toluene-heptane mixtures show a similar time dependent behaviours which are presented in Figure 5 for various temperatures. Here one has to be noticed that the dissolution curves in Figure 5 are smoother than the curves in Figure 4 at high temperatures, which can be explained by the slowing down effect of heptane during dissolution. In order to quantify the dissolution curves, we aimed to fit the data to Eq (4). All dissolution curves were digitized for numerical treatment. One can observe the deviation from the linearity at early times for film samples dissolved in toluene. Deviations from the linearity at long times present equilibration of dissolution process. The results suggest that there are at least two different mechanisms included during dissolution of annealed high-T latex films. Linear regions of the curves at intermediate times follow the Fickian diffusion model where the starting point shifts to shorter times for the samples dissolved at higher temperatures. When the linear portions of the curves are compared to computations Eq (4) chain desorption coefficients, Dd are obtained. Dj values, for both the films dissolved in toluene and toluene-heptane mixtures are found tobe(«10“ucm2/sec). Short time, non- Fickian regions of the curves were fitted to Eq (5) to obtain ko parameters, ko values varied between (0.63 and 3.38)xl0”2 mg/cm2min. Da and ko Xllvalues of samples dissolved in toluene were found to be much larger at high temperature then they were at low temperature. 70- 50- 30- 10- 40 *uH' w MP“V » *.' V..< ”* ? . efvi 30 sW 3 ! i s !'f !i 'J W»“i! ;; 5|\jS ;..>”-v»- Av'“*'-' '-. it!' r S ! '? ı ”25 s: v''-',^^-W i if!! i ! il : 5 Ün»}! a s 3 5 5,5.':i 'î ' U ;?;"J II,'t i! i 5;!: ! *:İ. ;, ? rf5.?* ^«t* J*' **>- 35

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