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Sert latekslerden film oluşumunun faton soğurma yöntemi ile çalışılması

Başlık çevirisi mevcut değil.

  1. Tez No: 56001
  2. Yazar: FİGEN KENEROĞLU
  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: 1996
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 30

Özet

Bu çalışmada, sert lateks parçacıklardan film oluşumu sırasında optiksel netliğin değişimini araştırmak için foton geçirme metodu kullanıldı. Poli(metil methakrilat)-poli(izobütilen) (PMMA-PIB) parçacıklarından lateks filmler hazırlandı. Genel olarak iki deney yapıldı. Birinci deneyde, bu lateks filmler camsı geçiş sıcaklığı (Tg) nın üzerindeki sıcaklıklarda 10 dak.lık eşit zaman aralıklarında tavlandı. İkinci deneyde ise, yine (Tg) nm üzerindeki 110-220 °C arasındaki sıcaklıklarda değişik zaman aralıklarında tavlandı. Her iki deneyin ölçümleri UV Visible spektrofotometre ile alınarak filmlerin optiksel geçirgenliği araştırıldı. Farklı sıcaklıklarda eşit zaman aralıklarında yapılan ilk deneyde Prager-Tirrell modeli kullanılarak zincir aktivasyon enerjisi 39.15 kcal/mol ve zincir sürünme frekansı 0.008 sn“1 ile 10.83 sn'1 değerleri arasında bulundu. Sabit sıcaklıklarda farklı zaman aralıklarında yapılan ikinci deneyde ise zincir aktivasyon enerjisi 33.42 kcal/mol olarak bulundu. Lateks film oluşumunu simüle ederken de, dikdörtgensel bir latis içinden foton geçirmek için Monte Carlo simülasyonları geliştirildi. Filmin altından çıkan foton sayısı (Ntr), filmin içinde yutulan foton sayısı (Nabs) ve filmin üst yüzeyinden saçılan foton sayısı (Nsc) fotonun film içindeki oratalama yolunun ve parçacık- parçacık arayüzeylerinin kaynaşmasının bir fonksiyonu olarak hesaplandı. Filmin alt yüzeyinden çıkan foton şiddeti I* deki artışın sebebinin kavşak yüzeydeki ”crossing density" artışı olduğu gösterildi. Daha sonra tavlanmış filmlerin fiziksel yapısındaki değişikliği anlayabilmek için saçılma elektron mikroskobu (SEM) kullanıldı. Deneylerde bulunan sonuçlar bu çalışmadan önceki bulunan sonuçlara benzerlik göstermiştir.

Özet (Çeviri)

Latex film formation is a complicated, multistage phenomenon and depends strongly on the characteristic of colloidal particles. In general aqueous or non aqueous dispersions of colloidal particles with glass transition temperature (Tg) above the drying temperature are named hard latex dispersions, however aqueous dispersion of colloidal particles with Tg below the drying temperature is called soft latex dispersion. The term“latex film”normally refers to a film formed from soft particles where the forces accompanying the evaporation of water are sufficient to compress and deform the particles into a transparent, void-free film. However, hard latex particles remain essentially discrete and undeformed during drying process. Film formation from these dispersion can occur in several stages. In both cases first stage corresponds to the wet initial state. Evaporation of solvent leads to second stage in which the particles form a close packed array, here if the particles are soft they are deformed to polyhedra. Hard latex however stay undeformed at this stage. Annealing of soft particles cause diffusion across particle-particle boundaries which leads the film to form a homogeneous continuous material. Annealing of hard latex system however, first leads to void closure upon deformation of particles and then after the voids disappear diffusion across the particle-particle boundaries starts, i.e. the mechanical properties of hard latex films can be evolved by annealing after all solvent has evaporated and all voids have disappeared. This process called coalescence, is an important aspect of latex coatings and represents an important feature that one would like to understand. After void closure process is completed, the mechanism of film formation, by annealing of hard latex films is known as interdiffusion of polymer chains followed by healing at polymer-polymer interface. In general when two identical polymeric materials are brought into intimate contact and heated at a temperature above the glass transition the polymer chains become mobile, interdiffusion of polymer chains across the interface can occur. After this process the junction surface becomes indistinguishable in all respects from any other surface that might be located in the polymeric material. This process is called healing of the junction at which the joint achieves the same cohesive strength as the bulk polymeric material. The word interdiffusion in polymer science is used for the process of mixing, intermingling and homogenization at the molecular level, which implies diffusion among polymer chains. viIn the bulk state, polymer chains have a Gaussian distribution of segments. Chains confined to the half space adjacent to the junction have distorted conformations. Diffusion across the junction leads to configurational relaxation and recovery of Gaussian chain behaviour. Polymers much larger than a certain length are often pictured as confined to a tube, and diffusion occurs by a reptile like motion. In this model each polymer chain is considered to be confined to a tube along the length of which it executes a random back and forth motion. This reptile like motion will cause the chain to slip out of a section of tube at one end or the other. The repetition time tr describes the time necessary for a polymer to diffuse a sufficient distance for all memory of the initial tube to be lost. This is the time it takes for initial configuration to be forgotten and the first relaxation to be completed. In this work, evolution in transparency of films formed from hard latex particles, was studied by measuring the transmitted photon intensity, Itt from latex films. It was shown that the variation in transparency is related to the variation in It,, intensity. Firstly, isothermal experiments were performed by annealing latex films in equal time intervals and Itt was monitored to study crossing density during film formation. Secondly, latex films were annealed in equal time intervals at elevated temperatures above the glass transition (Tg) of Poly (methyl methacrylate) (PMMA) and 1^ intensities were measured by UV-visible (UW) spectrophotometer. Increase in 1^ intensity by increasing annealing time and temperature was attributed to increase in the“crossing density”at the junction surface. The method developed by Prager and Tirrell (PT) was employed to investigate the healing processes at the junction surface. Variation in 1^. intensities with respect to annealing temperature were used to measure the activation energy and repetition frequencies for the randomised polymer chain segment across the polymer-polymer interface. In UW experiments, hard latex particles having two components were used ; the major part, PMMA, comprises 96 mol % of the material and the minor component, polyisobutylene (PIB) (4 mol %), forms an interpenetrating network through the particle interior very soluble in certain hydrocarbon media. A thin layer of PEB covers the particle surface and provides colloidal stability by steric stabilisation. Monte Carlo simulations were performed to calculate the transmitted (N^), absorbed (Nabs) and scattered (N^ photon intensities from a rectangular shape film. The rectangular lattice is partitioned into small square compartments which represent the deformed latex particles in annealed film. An increase in N“. is observed by decreasing the number of disappeared boundaries between compartments. Evolution in transparency of film is modelled by the disappearance of particle-particle interfaces and the increase in I”. is explained by the decrease in number of interfaces in the annealed film samples due to the increasing crossing density. vuPMMA-PIB polymer particles were prepared separately in a two-step process in which MMA in the first step was polymerised to low conversion in cyclohexane in the presence of PIB containing 2 % isoprene units to promote grafting. The graft copolymer so produced served as a dispersant in the second stage of polymerisation, in which MMA was polymerised in a cyclohexane solution of the polymer. A stable spherical dispersion of the polymer particles was produced, ranging in radius from 1 to 3 /zm. A combination of H-NMR and UV analysis indicated that these particles contain 4 mol % PIB. (These particles were prepared by Mr. B. Williamson in Prof. M. A. Winnik Laboratory in Toronto). Latex film preparation were carried out by dispersing PMMA-PIB particles in heptane in a test tube with the solid content taken to be as 1 %. Films were prepared from the dispersion of particles by placing different number of drops on glass plates with the size of 2x0.8 cm2 and allowing the heptane to evaporate. Here we were careful that the liquid drops to cover the whole surface area of the plate and remain there until the heptane has evaporated. Samples were weighed before and after the film casting to determine the film thicknesses. Average size for the particles was taken to be 2 /xm to calculate the number of layers on the films. In this work UW experiments were carried out with the annealed latex film samples. Annealing process of the latex films were performed above Tg of PMMA after evaporation of heptane, at 150, 160, 170, 180, 190 and 200 °C in 10 min. time intervals and in 180, 60, 40, 30, 20, 10 and 5 min. time intervals at elevated temperatures between 110 and 220 °C. The temperature was maintained within ± 2 °C during annealing. After annealing, each sample was placed in the model 160 A UV- Visible spectrophotometer of Shimadzu and absorbance of films were dedected between 300-400 nm. At first experiment, the chain activation energy and repetition frequencies were found respectively 39.15 kcal/mol and among 0.008 sec“1 - 10.83 sec”1. At second experiment, the segmental activation energy to be 33.42 kcal/mol for a partially relaxing polymer chain across the junction surface. Another glass plate was used as a standard for all UW experiments. All measurements were carried out at room temperature after annealing process are completed. When film samples were annealed (above Tg of PMMA) at 150, 160, 170, 180, 190 and 200 °C in 10 min. time intervals and in 180, 60, 40, 30, 20, 10 and 5 min. time intervals at elevated temperatures in various time intervals, a continuous increase in Lj. intensities were observed. The increase in 1^ was already explained in the previous section, by the increase in transparency of latex film due to disappearingof particle-particle interfaces. As the annealing temperature is increased some part of polymer chains may cross the junction surface and particle boundaries start to disappear, as a result the transmitted photon intensity 1^ increase. In order to quantify these results, the Prager-Tirrell (PT) model for the chain crossing density was employed. These authors used de Gennes' s“reptation”model to explain configurational relaxation at the polymer-polymer junction where each polymer chain is considered to be confined to a tube in which it executes a viiirandom back and forth motion. A homopolymer chain with N freely jointed segments of length L was considered by PT, which moves back and forth by one segment with a frequency v. In time the chain displaces down the tube by a number of segments, m. A Gaussian probability density for the net displacement of the polymer chain was obtained for small times and large N. The total“crossing density”a(t) (chains per unit area) at junction surface then was calculated from the contributions a^t) due to chains still retaining some portion of their initial tubes, plus a remainder, a2(t). Here the a2(t) contribution comes from chains which have relaxed at least once. g(t) / cr(oo) = 2;T^[r^ +2]>](-l)fl[r^ exp(-fc2 / r)-^/2erfc(k 1 1%) ]} (1) For small x values the summation term of above equation is very small and can be neglected, which then results in o(t)/o(oo) = 27if1/V/2 (2) In terms of reduced time x = 2vt / N2 the total crossing density can be written as c(t) / 0(00) = 2nm (2vt / N2)1/2 (3) The increase in ^ is already related to the disappearance of particle-particle interfaces i.e. as annealing time interval is increased, more chains relaxed across the junction surface and as a result the crossing density increase. Now, it can be assumed that 1^ is proportional to the crossing density a(t) and then phenomenological equation can be written as Itr(t) / W») = 2nm (2v / N2)1/2 t1/2 (4) In order to compare our results with the crossing density of the PT model, the temperature dependence of a(x) / a(oo) can be modelled by taking into account the following Arrhenius relation for the linear diffusion coefficient v = v0 exp(-AE / kT) (5) IXHere AE is defined as the activation energy for the back and forth motion. Combining Eq 3 and 5 a useful relation is obtained as a(T) / 0(00) = R exp(-AE / 2kT) (6) 1 Ml where R = (8v0t / 7tN ) is a temperature independent coefficient. Here t, is annealing time. The increase in 1^ is already related to the disappearance of particle-particle interfaces i.e. as annealing temperature is increased, more chains relaxed across the junction surface and as a result the crossing density increases. Now, it can be assumed that Itt is proportional to the crossing density cr(T) and then phenomenological equation can be written as Itt(T) / Uoo) = R exp(-AE / 2kT) (7) In order to interpret the temperature behaviour of 1^ intensity, a simple rectangular lattice model is used to simulate the latex film formation process. Rectangular lattice is divided into squares with side length, a and the centre of squares are taken as a scattering centres for photons travel in the lattice. The distance of a photon between each consecutive collision is defined as the mean free path, of a photon during its journey in the lattice. Boundaries between squares are randomly removed to simulate the disappearance of particle-particle interfaces during the annealing process of the film. The thickness of the rectangle, d is taken as 15. The direction of incident photon is taken perpendicular to the film' s front surface and the periodic boundary conditions are applied to the motion of a photon (i.e. photons are not allowed to escape from the sides of the lattice). When a photon travels in the rectangular lattice it always controls the boundary against its moving direction. If there is a boundary then photon is scattered to one of the four directions. During scattering, according to optical scattering rules probability of forward scattering was taken in excess of the other directions. In this picture the early stage of annealing can be simulated by a rectangular lattice where a photon has short . As more boundaries are removed between the square compartments in the lattice, values increase which simulates the latter stage of annealing. As the boundaries are continued to be removed, values increase and more photons can be transmitted through the lattice indicating that annealing causes high transparency. The number of incident photons are taken as 103 and the number of photons transmitted, absorbed and scattered from the back and front surfaces are named as N"., Nabs and Nsc respectively. In order to determine errors, averages calculated upon running the code 200 times for each removed boundary density.In order to support these findings, scanning electron micrographs (SEM) of latex films before and after annealing at 160 °C for 10 and 60 min. time intervals are presented. For SEM images Hummer VII sputtering system was used for gold coating and film of particles were then examined at 5-10 kV in JEOL JSM-T330 microscope. XI

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