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Fotobaşlatıcılar ile vinil/Divinil monomer kopolimerizasyonunda jel oluşumunun ve jel özelliklerinin incelenmesi

Başlık çevirisi mevcut değil.

  1. Tez No: 55538
  2. Yazar: HAMİD JAVAHERİAN NAGHAH
  3. Danışmanlar: PROF.DR. YUSUF YAĞCI
  4. Tez Türü: Yüksek Lisans
  5. Konular: Kimya, Chemistry
  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: 122

Özet

ÖZET Metil metakrilat (MMA) ve etilen glikol dimetakrilat (EGDM) monomerlerinin serbest radikal fotopolimerizasyonu ile çapraz bağlanma reaksiyonu toluen içinde ve % 35 w/v sabit konsantrasyonda incelenmiştir. Foto başlatıcılar olarak benzoin (B), benzoinmetil eter (BME), benzoin etil eter (BEE) 2,2- dimetoksi -2- fenil asetofenon, (DMPA), 2,4,6- trimetilbenzoin difenilfosfin oksit (DPTPO) ve 4-4'-azo-bis-(4- siyanopentanoil) -bis- benzoin (ACPB) kullanılmıştır. Monomer ve asılı vinil gruplannm dönüşümü ile birlikte jelleşme öncesi oluşan polimerin büyüklüğü de makrojelleşmenin başlangıç zamanının fonksiyonu olarak ölçüldü. Asılı vinil gruplarının (B) ve Türevöleri için yaklaşık olarak % 30'u, ve (ACPB) için % 95'i halka oluşum reaksiyonlarına harcandığı bulundu. Halka yapısındaki birimlerin kesri EGDM konsantrasyonundan bağımsızdır. Halka oluşum reaksiyonu nedeniyle radikal merkezlerin ve asılı vinil gruplarının diğer polimer moleküllerine yakınlaşması önemli ölçüde azalmıştır. Sonuç olarak, hem sonlanma hem de çapraz bağlanma reaksiyonları polimer radikallerinin segmental difuzyonu tarafından kontrol edilmektedir. Reaksiyon ortamında bulunan asılı vinil gruplarının reaktivitesi monomerik vinil gruplara nazaran 1-2 kat daha zayıf olduğu hesaplamalardan anlaşılmıştır. Asılı vinil grup reaktivitesindeki bu dikkate değer azalma esas olarak jelleşme noktasında bir gecikmenin nedenidir. Jelleşme teorisinin aksine, polimerizasyonun ilerlemesiyle birlikte jelleşme öncesine kadarki polimer moleküllerinin büyüklükçe dağılımı tekli dağılımdan (mono model) ikili dağılıma (bimodel) değişmiştir. Bu deneysel bulgular yoğun primer taneciklerin kümeleşmesi tipi jelleşme mekanizmasını doğrulamakta ve jelleşmeden önceki polimerlerin polidispersite şeklini göstermektedir.

Özet (Çeviri)

SUMMARY Gel Formation By Chain Crosslinking Photopolymerization of Methyl methacrylate and Ethylene glycol dimethacrylate. Photoinitiated free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate has been investigated in toluene at a monomer concentration of 35 w/v %. The photoinitiators used were benzoin, benzoin methyl ether, benzoin ethyl ether, 2,2'-dimethoxy-2phenyl- acetophenone, diphenyl-2,4,6- trimethylbenzoylphosphineoxide and 4,4'-azo-bis-(4-cyanopentanoyl)-bis-benzoin conversion of monomer and pendant vinyl groups as well as (ACPB) the size of the pregel polymers were measured as a function of the reaction time up to the onset of macrogelation. Approximately 30 % for benzoin and derivatives and 95 % for 4,4'- azo-bis- (4-cyanopentanoyl) -bis- benzoin of the pendant vinyl groups were found to be consumed by cyclization reactions. The fraction of units in cycles is independent on the EGDM concentration. Due to the cyclization reactions, the accessibility of the radical centers and the pendant vinyl groups for other polymer molecules are strongly reduced. As a result, both termination and crosslinking reactions are controlled by the segmental diffusion of the polymer radicals. Calculations indicate a 1-2 orders of magnitude decrease in the average reactivity of pendant vinyls for intermolecular reactions compared to the monomeric vinyls. This drastic decrease in the pendant reactivity is mainly responsible for the delay in the gel point. Contrary to the gelation theories, the size distribution curves of the pregel polymers change from monomodal to bimodal distributions as polymerization proceeds. This finding confirms the coagulation type gelation mechanism of compact primary particles and indicates the shape polydispersity of the pregel polymers. Bulk photopolymerization of multifunctional monomers is one of the most efficient methods to synthesize strongly crosslinked polymers with a high mechanical strength and excellent resistance to solvent penetration. Such materials have found applications as dental materials, aspherical lenses, nonlinear optical materials and coatings. To predict the final properties of these materials, their structural characteristics are extremely important, which in turn depend on the history of the network formation process. Previous works indicate the existence of spatial inhomogeneities in such materials. For instance, the appearance of multiple glass transitions, the existence of trapped radicals and residual unsaturation in the final networks are indications for the presence of regions more crosslinked than the surrounding environment in the material. Such inhomogeneities in the final materials are undesirable for applications because structural inhomogeneity results in a drastic reduction in the strength of the crosslinked polymers. The reason for the inhomogeneous nature of highly crosslinked networks is the nonidealities in their formation process such as the unequal vinyl group reactivity, xivcyclization, and multiple crosslinking. For example, in chain crosslinking copolymerization, the growing chains in the pre-gel stage are rich in divinyl monomer (DVM) units due to the higher reactivity of the DVM. Furthermore these chains are highly diluted by the monomer at low conversions so that cyclization predominates in the early stages of the reaction. As a consequence, the growing chains become internally crosslinked and their structure approaches the microgels. Thus, the appearance of the microgels during to network formation processes can be compared with the formation of primary molecules as intermediates in the classical gelation theories. The interparticular reactions namely crosslinking and multiple crosslinking reactions are responsible for the agglomeration of the microgels leading to the macrogelation. Previous experimental works from our group showed that a large number of multiple crosslinks from between two microgels after they link together in the reaction mixture by a single crosslink. Thus, one may expect that, in the final networks, the microgels or their agglomerates formed by extensive multiple crosslinkages are highly crosslinked than the later formed portions of the network. While the properties of the final networks or partially formed networks have been thoroughly investigated in the last decade, there have not been many reports for the pre-gel period of the photopolymerization, i.e., for the period in which the highly crosslinked regions start to form. This is mainly due to the rapid polymerization of multifunctional monomers in bulk and almost immediate onset of a macrogelation. In the present work, conversion of the monomers and pendant vinyl groups as well as the molecular weight and the size distribution of the soluble polymers were determined experimentally in the pre-gel regime of the photocopolymerization of methyl methacrylate (MMA) / ethylene glycol dimethacrylate (EGDM) monomers. In order to focus on the pre-gel period of the reactions, we magnified this period by adding an inert diluent into the copolymerization system. U.V. photopolymerizations were carried out a 35 w/v % monomer concentration in toluene and using 0.3 wt % of a photoinitiator. The photoinitiators used were benzoin (B), benzoin methyl ether (BME), benzoin ethyl ether (BEE), and 2,2-dimethoxy-2-phenyl-acetophenone (DMPA). These molecules are known to undergo a-cleavage to produce benzoyl and substituted benzyl radicals, as shown in the following scheme: It is the purpose of this thesis to present new measurements which will enable us to estimate the magnitude of cyclization and reduced pendant reactivity depending on the reaction conditions. In this way, we hope to develop a deeper insight into the xvformation mechanism of globular structures in chain crosslinking copolymerization and their influence on the polymerization kinetics. KINETIC MODELLING There are several theories of network formation to describe the relations among the molecular weight of polymers and the conversion or reaction time during crosslinking. These theories include statistical and kinetic methods, and simulation in n-dimensional space, such as the percolation method. In the past half century, statistical and kinetic approaches have been extensively used to describe polymeric gelation. These approaches consider the average properties of the reaction system and therefore, they cannot deal exactly and directly with long-range correlations such as cyclization and with the resulting heterogeneities. On the other hand, the percolation theory that belongs to a non-mean-field theory can take into account the heterogeneities but at present the result of this theory is unrealistic due to the difficulty of introduction of realistic mobilities. The critical region seems to be the main domain of application of percolation techniques to polymer networks. The experimental data obtained in the present study were organized and studied within the framework of a kinetic model which includes constants for cyclization and pendant reactivity. A kinetic scheme for chain crosslinking photopolymerization of vinyl/divinyl monomers can be written as follows. The polymerization system involves three types of vinyl groups; those on vinyl and divinyl monomers and those on polymer chains, i.e., pendant vinyls. Accordingly, the polymerization system can be considered as a special case of terpolymerization in which one of the vinyl groups (pendant vinyls) is created during the course of the reaction when the vinyl on divinyl monomer raacts. The pendant vinyl groups thus formed can then react by cyclization, crosslinking, or multiple crosslinking reactions, or remain pendant. With cyclization the cycle is formed when the macroradical attacks the pendant vinyl groups in the same kinetic chain, while with multiple crosslinking it is formed if the radical attacks double bonds pendant on other chains already chemically connected with the growing radical. It should be noted that cyclization and multiple crosslinking were recently re-defined as primary and secondary cyclizations, respectievely. In the following text the classical definitions will be used. The divinly monomer can be found in the polymer as units bearing pendant vinyl groups, cycles, crosslinks or multiple crosslinks. Since according to the classical theory of gelation only one crosslink per weight-average primary molecule is necessary for the onset of a macrogelation, the content of pendant vinyls is a highly sensitive indicator for the formation of cycles and multiple crosslinks infinite species. Here, we define the pendant conversion, x3, as the fraction of divinyl monomer units with both vinyl groups reacted, i.e., the fraction of fully reacted divinyl monomer units in the polymer. Theories neglecting cyclization predict that every divinyl monomer unit in the polymer should initially bear a pendant vinyl group, i.e., lim X3 =0 where x >0 x is the monomer conversion. Since crosslinking is a second-order reaction, deviation xvifrom zero indicates the existence of cyclization, on the other hand, the rate of change of pendant conversion X3 with the monomer conversion x or with the reaction time t is a measure of the extent of multiple crosslinkages. The greater the slope of X3 versus x or t curve, the higher the number of multiple crosslinks formed per crosslink. Thus, the occurrence of multiple crosslinking will be reflected in a greater decrease in the polymer unsaturation as conversion proceeds than would otherwise occur. In this study photoinitiated copolymerization of MMA and EGDM Was carride out in toluene at 27 ± 0.1°C. The initiators used were B, BME, BEE, DMPA, ACPB and DPTPO. The conversion of the monomers up to the onset of macrogelation was followed by dilatometry. The dilatometers constructed in this laboratory consisted of a blown glass bulb approximately 25 ml in volume connected to a 30 cm length of 1.5 mm precision bore capillary tubing with a ground-glass joint. The thickness of the dilatometers was less than 6 mm to ensure the applicability of the thin film approximation and uniform Light intensity across the sample. The reaction mixture was flushed with nitrogen 20 min prior to the polymerization. Polymerization was initiated with U.V. light of intensity 3.8xl0"9 einsteins generated using a medium mercury lamp. The meniscus of the polymerizing solution was measured throughout the experiment with a millimetric paper to 0.2 mm. The Polymerization technique used was described in experimental section. The reproducibility of the kinetic data was checked by repeating the experiments. The deviation in the initial slopes of time versus conversion data between two runs was always less than 3 %. Different series of experiments were carried out using the photoinitiators listed above. In each series, the initial concentrations of the monomers and the initiator were held constant at 3.5 M (35 w/v %) and 0.3 wt % (with respect to the monomers), respectively, while the crosslinker concentration was varide in a wide range. The polymer Samples for pendant vinyl group measurements were obtained by a gravimetric technique. For this purpose a 15-ml three-neck flask of 0.7 mm thickness containing a magnetic stirrer bar fitted with a nitrogen inlet and pipette outlet was filled with the liquid mixture. After degassing of the mixture, polymerization was initiated with the U.V. light of the mercury lamp at 27 ± 0.1°C. After poly predetermined polymerization times, the content of the flask was poured into 100 ml of methanol and allowed to settle. The precipitated polymer was filtered and dried in vacuo to constant weight. Measurement of the number of pendant vinyls on MMA/EGDM copolymers was carried out using bromination method with pyridine sulfate dibromide (PSDB) reagent in the presence of mercury acetate as catalyst. The method is based on the reaction of pendant vinyl groups with bromine produced in situ from the PSDB reagent and back-titration of the excess bromine. For the calculation of pendant conversion, the reactivities of monomelic vinyls were assumed to be equal. Gel Point measurements were carried out using two different methods. First, the gravimetrik technique was used to follow the polymerization reactions; the gel point was determined as the midpoint between the last time at hich a soluble polymer was obtained and that at which the polymer was not soluble in toluene. For ascertaining the insoluble gel component of samples, the latter were treated with an approximately 50-fold excess of toluene at room temperature. The formation of insoluble polymer xviiwas detected visually from the appearance of gel particles in toluene. Second, dilatometers containing a steel sphere of 4.8 mm diameter was used for the gel point measurements. The midpoint between the last time at which the sphere moves magnetically and that at which it stops moving is taken as the gel point, we found remarkable consistency in the gel points determined using both methods. Each gel point reported in this thesis is an average of at least 4 measurements. Standard derivations were 7%. Apparent molecular weights and the size distribution of the pregel polymers were obtained by size exclusion chromotography (waters, Model M-6000A), equipped with refractive index detector, using two polystryrene gel columns (500, 10.000 A) at a flow rate of 1.0 ml/min in THF at 40°C, and using polystyrene standars. XVlll

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