CHAPTER 2 SYNTHESIS OF DYE DOPED POLYMER RODS AND FILMS

Transcription

1 39 CHAPTER 2 SYNTHESIS OF DYE DOPED POLYMER RODS AND FILMS 2.1 INTRODUCTION Polymers are long chain organic materials or macro molecules having carbon as a common element in their structure. The repeating group is known as monomer or single unit. Natural and synthetic polymers (e.g., polyethylene, phenol formaldehyde, polystyrene, polyamides, Polymethylmethacrylate, etc.) play an ever increasing role in modern technology. Organic dye doped polymer films are attracting a great deal of attention due to their potentially high nonlinearities and rapid response in the electro optic effect compared to inorganic nonlinear optical material. Polymers can easily be fabricated into various forms such as films and rods. Zhang et al (1996) reported nonlinear refractive indices of doped polymer Films. From the application point of view the development of dye doped thin polymer films is more promising because of the benefits of low cost, high compactness and compatibility with other micro optic elements or integrated optical circuits. Dyes incorporated in thin polymer films have been found to be more stable. The thermal characterization of the dye doped polymers were studied by thermo gravimetric analysis. This chapter describes, various polymerization techniques and synthesis of dye doped polymer rods and films.

2 POLYMERIZATION The formation of long, repeating organic polymer chains from the monomer or monomers is called polymerization. Polymerization is the union of two or more smaller and simpler molecules of similar or different types, with or without the elimination of water etc, leading to the formations of new C-C bonds or linkages. There should be at least two reactive or bonding sites in a substance which has to act on a monomer in the formation of the polymer. This number of bonding sites in a monomer is generally called as functionality. There are different forms of polymerization, and different systems exist to categorize them. Categorizations include the addition-condensation system and the chain growth-step growth systems. Another form of polymerization is ring-opening polymerization, which is similar to chain polymerization Condensation Polymerization Condensation polymerization occurs when monomers bond together through condensation reaction of low molecular weight compounds, condensation takes place between two or more poly functional groups to form large poly functional molecules which in turn could condense with each other or in the early stages of the reaction with some of the original reactants. The name indicates that in the process of forming the polymer, another product also occurs, usually water. These condensation reaction can be achieved through reacting molecules incorporating alcohol, amine or carboxylic acid (or other carboxyl derivative) functional groups. And also the monomers must contain more than one functional group so as to enable intermolecular reaction to take place, e.g. Bakelite.

3 41 As addition reactions are called chain-reactions, condensation reactions can be called step-reactions. Unlike in addition reactions, in condensation reactions the different monomers do need to alternate regularly, unless they have identical functional group Chain Growth-step Growth Polymerization Chain growth step growth systems categorizes polymers based on their mechanism. While most polymers fall into their similar category from the addition-condensation method of categorization, there are few exceptions. Chain growth polymers are defined as polymers formed by the reaction of monomers with a reactive center. These polymers grow to higher molecular weight at a very fast rate. Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers. Most step growth polymers are also classified as condensation polymers, but not all step growth-polymers release condensates. Step growth polymers increase in molecular weight at a very slow rate at lower conversions and only each moderately high molecular weight at very high conversion (i.e., greater than 95%) Addition Polymerization Addition polymerization is the construction of a polymer based upon simple monomer units. This monomer joined together is an extremely long chain under the correct conditions of temperature and pressure to become a single polymer chain. This polymer can have cross-linked monomers in its structure, which gives these plastics great strength and resistance to heat.

4 42 This polymerization involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers have extra, internal, bonds which are able to break and link with other monomers to form the repeating chain. The polymerization process undergoes successive stages of initiation, propagation and termination common to chain reaction. The chain carriers are usually free radicals, cations and anions. In this polymerization, the primary molecule reacts with another usually in the presence of a catalyst to form a larger molecules molecule, called dimer. The latter in turn reacts with another monomer to give a trimer and this process continues until some termination occurs or the stock of monomer is finished. One of the common addition polymers is Polymethylmethacrylate (PMMA). It is obtained from the monomer. below: Methylmethacrylate (MMA) whose chemical structure is given (2.1) Mechanism involved in addition or chain polymerization: In addition polymerization, monomer units simply add to one another and the growing polymer chain is formed by consecutive addition of monomer units. Moreover, the monomer is strongly catalyzed by free radicals. In addition polymerization, one of the most important methods used for synthesis of polymers, consists of three important steps:

5 43 (a) Process initiation or the formation of active centers. (b) Chain growth or chain propagation or formation of a polymer having the active center. (c) Chain termination or the removal of the active center. 2.3 FREE RADICAL ADDITION POLYMERIZATION Free radical polymerization requires a polymerizable compound (monomers) and a source of free radicals, which can supply initial radicals. Monomers are stable compounds at room temperature, but react readily with the free radicals, thereby initiating a free radical chain reaction. This process is termed as initiation. The monomer-converted to radical is an active species that adds other monomers, while the growing chain preserves its radical character. The only way to remove the radical from the system is by reacting it with another radical. The process is called termination. The three processes, initiation, propagation, and termination are the necessary features of free radical polymerization Yield of Radicals, from Initiators Those organic compounds that carry an unpaired electron are termed as radical. Radicals are very energetic species; they are therefore rather reactive and usually short-lived. The most common way of generating radicals is by hemolytic decomposition of covalent bonds. This is achieved by increasing the temperature or by radiation. Indeed, any covalent bond will break when the temperature is raised enough. But, in polymerization techniques, we need to produce radicals in a controlled way under possible conditions. The solution of the problem is in selecting compounds with bonds, which can be broken by imparting moderate amounts of energy and by raising the temperature moderately. Labile bonds are single bonds between two like

6 44 electro-negative atoms such as nitrogen-nitrogen, oxygen-oxygen, fluorinefluorine, iodine-iodine, and bromine-bromine bonds. For simple molecules, the disassociation energy is equal to the activation energy for hemolytic decomposition, the lower the energy, the lower the temperature sufficient for noticeable decomposition. For complex molecules, the dissociation of a bond may be accompanied by a simultaneous reorganization of the steric and electronic structure of the radical fragments. This reorganization is visible even when alkyl radicals are compared with radicals made from single atoms (hydrogen and halogens). Substances that decompose to radicals at a convenient temperature are useful as initiators of radical polymerization. The two important classes of such initiators are peroxides and azo compounds. Peroxides decompose based on the low bond energy of the oxygenoxygen bond. They are useful for initiation purposes, and can be used at different temperatures based on different dissociation energies of peroxides. Aromatic peroxides mainly benzoyl peroxides are useful at low temperatures (< 80 o C). The differences are dissociation energy is caused by the interaction of the radical free electron with the aromatic nucleus. Electrons move rather freely throughout the conjugated system, and the unpaired electron may change its residency among several atoms, and thereby lowering the energy by resonance effects. Oxygen molecules easily form peroxides with several types of organic compounds notable ether and hydrogen s on tertiary carbon. Benzoyl peroxide is the most frequently used initiator and it decomposes as per the following scheme: (2.2)

7 Initiation, Propagation and Termination Processes The thermal decomposition of initiators is the first order reaction, and such initiators are most popular for the initiation of radical polymerization. The concentration of the initiator and consequently the production of radicals decrease exponentially with time. Thus, when more or less constant radical production is required, the initiator and/or the room temperature should be selected properly to make the half-time for decomposition of the initiator comparable to the desired overall reaction time. The polymerization is initiated by the addition of free radical R onto the double bond of a molecule of monomer. R + C = C R- C - C (2.3) The resulting radical can then play the role of R in the next addition onto the next molecule of the monomer and thereby leading to chain propagation. All the compounds with carbon-carbon double bonds (C = C) cannot participate in radical chain propagation. For this to happen, the interplay of favorable steric, polymerization and radical stabilization effects are required. Absence of initiators and transfer agents will propagate the chain reaction to continue until it is interrupted by chain termination. Chain termination involves a reaction between two growing leading to non-radical products. Two reactions are possible between two radicals. First is recombination favoured at low temperatures and second is disproportionation at high temperatures. Kinetically, recombination does not go through an activated state (activation energy is low) while dispropagation, which is

8 46 essentially a hydrogen abstraction has appreciable activation energy. In disproportionation, the rate of termination increases by increasing the temperature much faster than for recombination. Growing chains with crowded radical ends would recombine to rather strained structures. For example, methacrylate chain reacts as (2.4) Less crowded chains consequently have a higher probability of recombining than more crowded ones (Petr Munk 1989) Solvent Effects on Free Radical Polymerization A study on the influence of solvent on free radical polymerization, proposes the following probable solvent effects: (a) Viscosity effect. (b) Reactivity of the transferred radical. (c) Modification in initiation rate. (d) Formation of a radical complex between radical and solvent or monomer and (e) Copolymerization with solvent Important Features of Free Radical Polymerization Monomers have double bonds- Polymerization takes place by an attack of free radical on the -electrons of the double bonds.

9 47 Chain growth takes place by a rapid addition of monomer units very quickly, one after the other, and hence free radical polymerization is very fast. Elimination of small molecules such as H 2 O, CO 2, HCl etc., during the formation of polymer is not encouraged. As a result, the elemental composition of the polymer is the same on that of the monomer or monomers involved. As the polymerization continues, the concentration of monomer decreases steadily and concentration of polymer increases simultaneously. However, the concentration of growing chains remains almost constant throughout the reaction. The molecular weight of the polymer depends on monomer concentration of initiator and temperature of the reaction. Since solvent medium effects the rate of decomposition of initiator, the molecular weight also depends on solvent medium. This type of polymerization is suitable for bulk, suspension and emulsion techniques. 2.4 DIFFERENT METHODS OF POLYMERIZATION Polymerization methods may be classified as per technical criteria (1) Bulk polymerization (2) Gas phase polymerization (3) Solution polymerization (4) Emulsion polymerization (5) Solid Phase polymerization

10 Bulk Polymerization Bulk polymerization is carried out in the liquid monomer at a definite pressure and temperature. If the resulting polymer is soluble in a monomer, the viscosity of the medium increases gradually in the course of polymerization, and the result is a monolithic block of polymer. If the polymer is insoluble in a monomer, the polymer is obtained as a powder or a porous body. Bulk polymerization is used to produce polybutadicine, polystyrene, poly chloroprene, polymethylmethacrylate, and other polymers Gas-Phase Polymerization Gas-phase polymerization is a process carried out with the monomer in the gaseous state. Polymer formation begins on the walls of the reaction vessel and occurs on the surface or in bulk of the particles of already formed polymer. This polymerization is used, in particular, for the production of butadiene-sodium rubber Solution Polymerization In solution polymerization, the reaction medium is a suitable solvent (usually organic) besides initiators and modifiers. The two different possibilities are: (i) (ii) Both monomers and polymers are soluble in the solvent polymerization results in a solution of polymer called Lacquer. Lacquer may be used directly for coating or the polymer may be precipitated out of the solution. This method is known as the Lacquer method. A monomer is soluble in liquid, but a polymer is insoluble. In this case, the polymer precipitates out as it is formed, and is then separated from the liquid.

11 Emulsion Polymerization In emulsion polymerization, the liquid monomer is dispersed in a liquid with which it does not mix to form emulsion. Water is the usual dispersion medium. Emulsions are thermo-dynamically not stable. Emulsifiers are surface active agents that are absorbed at the water-monomer interfaces. Emulsifiers form a mechanically stable absorption layers which prevents merging (coalescence) of monomers or polymer droplets. Hence, the substances used as emulsifies usually contains a polar group and a comparatively large hydrocarbon radicals Solid-phase Polymerization Monomers capable of polymerizing are not only in liquid phase, but also in crystalline state, below their melting points. Such polymerization is initiated by irradiating the monomers crystal with X-rays or -rays, fast electrons, or other high energy particles. Here, the rate of chain propagation is affected by the monomer crystal lattice. 2.5 FORMATION OF POLYMETHYLMETHACRYLATE (PMMA) mechanism. Free radical polymerization of MMA is described by the following Chain initiation (2.5)

12 50 Chain Propagation (2.6) Chain termination in two ways. Chain termination of methylmethacrylate polymerization can occur (i) By recombination or coupling. (ii) By disproportionation. (i) By recombination or coupling (2.7)

13 51 ii) By disproportionation (2.8) Azobisisiobutyronitrile (AIBN) and benzoyl peroxide are common radical initiators for MMA (Klaus Kircher 1987). The rate constant of the starting reaction depends on the temperature and the type of radical generator Advantages of PMMA PMMA has high optical transparency. PMMA has high resistance to sunlight PMMA is a hard, fairly rigid material with a high softening point of about 130 o C -140 o C and becomes rubber like at a temperature of above 65 o C. This wide range of temperature from its rigid state to viscous consistency accounts for its outstanding shape forming properties. PMMA has the capacity of transmitting light accurately. PMMA is colourless. It is a close alternative to glass as it does not shatter and can be moulded into almost any shape.

14 52 Scratching on this polymer can be readily removed by rubbing it with a cloth moistened with acetone or other suitable solvent. PMMA is widely used in making of lenses, paints, adhesive, wide screen, TV screens etc. 2.6 MATERIALS FOR THE SYNTHESIS OF DYE DOPED POLYMER RODS AND FILMS The materials involved for the synthesis of dye doped polymer rods in this work, are discussed below Monomer The monomer that was used in this work for the synthesis of dye doped polymer films is Methylmethacrylate (MMA) obtained from Merck India. Methylmethacrylate was chosen as the monomer due to its good transparency at the pump wavelength. The organic dyes exhibited good solubility and compatibility with MMA Additive Adding an additive for modifying the polymer (PMMA) will enhance the solubility of the dye and also increase the laser damage resistance (Dyumaev et al 1992). Low molecular additives such as ethanol, benzene and methanol were used to modify the polymer PMMA. The solvents used as the additives are of HPLC grade in purity Initiator Initiators are thermally unstable compounds and decompose into products called free radicals. The decomposition of the initiator into free radicals (electrically neutral particles with unpaired electrons) can be carried

15 53 out by heat, light or catalysts. Radicals are the active centers in the free radical chain polymerization. Radicals can be formed because of the exciting effect of an elevated temperature. Benzoyl peroxide was chosen as the initiator for polymerization. The concentration of benzoyl peroxide was chosen by trial and error and it was found that 0.5g of benzoyl peroxide per 100 ml was suitable for polymerization Dyes The dyes chosen for the study are given below. Acid Red 27 - Azo family Sudan IV - Azo family Natural Red 25 - Natural family Ethyl Violet - Triarylmethane family Safranin O - Safranin family LD Coumarin family The Chemical structures and their corresponding molecular formulae and molecular weight of dyes used for study are given in Table 2.1. Thin layer chromatography (TLC) tests were carried out to confirm the absence of any impurities on the dyes. The Azo dyes are very distinct and well clearly defined class, characterized by the presence of one or more Azo ( N=N ) groups. Azo dyes give bright hues with high intensity. Their common structural characteristic is the azo chromophore ( N=N ), usually associated with auxochromic hydroxyl(-öh) or amino groups( N H 2 ). The dyes exhibit benzenoidquinonoid tautomerism with the corresponding quinone hydrazones. Acid Red 27 and Sudan IV from azo family are chosen for the study.

16 54 Natural Red 25 dye belong to natural family. It is made up of anthaquinone skeleton and it contains OH and COOH substituents in one of the terminal aromatic ring and 7 [5 [2 (Acetylamino)ethyl] 2 hydroxyphenyl group is covalently attached. It is otherwise called as laccaic acid A. Ethyl Violet belongs to triarylmethane family. Triphenylmethane has the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are ph indicators. The chromophore of this dye is the p-quinonoid group, which may appear as C=Ar=N(C 2 H 5 ) 2, (Ar=aromatic which is associated with auxochromic C 2 H 5 2 N (diethyl amino) groups. Safranin O belongs to safranin dye family. The name is derived French safran. Safranin O are azonium compounds of symmetrical 2,8 dimethyl 3,7 diamino phenazine. It is made up of quinone imine skeleton and their structural characteristic is the p quinonoid ( ) chromophore, which is associated with auxochromic primary amino group ( NH 2 ). Coumarin dyes are a diverse family of dyes containing the coumarin functional group (, benzopyranone) and a primary, secondary or tertiary amine. In some coumarin dyes the basic chromophore is replaced with its hetrocyclic analogues like azo-coumarin, quinoline or azo quinoline in order to enhance the dye properties. The coumarin dyes have the tendency for low photostability. Coumarins degrade due to laser light. The side products formed after degradation also absorb in the laser region which may give undesired effects.ld490 dye belongs to coumarin family dyes. It is widely used laser dye emitting in the blue green region of the spectrum. It is made up of benzopyranone skeleton and their structural characteristics is the quinoliz ( ) chromophore, which is associated with auxochromic tertiary amine group ( ). LD490 dye exhibit intense fluorescence because of the

17 55 above groups present in the structure. These groups are electron donating substituents which tend to enhance emission intensity. Table 2.1 Chemical structures, and their corresponding molecular formulae and molecular weight of dyes used for study Name of the dyes Chemical structure Molecular Formulae Molecular weight Acid Red 27 C 20 H 11 N 2 Na 3 O 10 S Sudan IV C 24 H 20 N 4 O Natural Red 25 C 26 H 19 NO Ethyl Violet C 31 H 42 N 3 Cl Safranin O C 20 H 19 N 4 Cl LD 490 C 15 H 15 NO

18 SYNTHESIS OF DYE DOPED POLYMERS Synthesis of Dye Doped Polymer Rods A calculated quantity of dye for a known concentration was dissolved in the monomer (MMA) completely till a clear solution was obtained. This solution was mixed with the additive in the ratio 4:1 v/v. This ratio was experimentally found by Dyumaev et al in 1992 and he reported that this 4:1 v/v ratio of MMA: additive increased the laser damage resistance of the polymer. To this was added 5 gms / litre of the initiator (benzoyl peroxide) and mixed well till the benzoyl peroxide dissolved completely. This solution was poured into a rectangular polymerizing tube of dimension cm. Since the atmospheric oxygen itself may act as an inhibitor, the tubes were kept under nitrogen atmosphere (Sharma 1998). Thermal bulk free radical polymerization was carried out in a thermostat controlled oil bath. The polymerization was carried out at 35 ºC for the first two days, then at 40 ºC for the next two days and finally at 45 ºC for two more days. The polymer rods, thus obtained after polymerization, were removed from the glass tubes by breaking the latter. Since various characterization and spectral studies had to be carried out with these rods, they were polished at the ends. For this purpose, the polymer rods were cut and their ends polished to the optical quality required. The polishing was done with polishing paper (rough polishing with polishing papers of grade 220, 320, 400, 600, 800 and fine polishing with polishing papers of grade 1/0, 2/0, 3/0, 4/0), while the final polishing was done using 0.3 µm size alumina powder using rotatory wheel to obtain mirror finishing. Hence, dye doped polymer rods were obtained in the form of rectangular rods Synthesis of Dye Doped Polymer Films In order to study the non-linearity of dyes it is essential to synthesize dye doped polymer thin films. As for a non-linear refraction, the

19 57 thickness of the film must be less than the Rayleigh range (Sheik-Bahae 1990) samples of thickness less than 1 mm were synthesized. The mixture of dye-monomer solution with initiator (benzoyl peroxide) is prepared in the same way as for synthesizing dye doped polymer rods. This solution is taken in the cylindrical polymerization tube and kept in the nitrogen gas atmosphere. Bulk free radical polymerization was carried out in thermostat controlled water bath at 40 o C till the solution becomes viscous. By pouring the viscous solution on to a glass slide, dye doped polymer films of desired concentrations are synthesized. The photographs of obtained rods and films are shown in Figure 2.1 (a) (b) (c) (d) (e) (f) Figure 2.1 Photograph of synthesized dye doped polymer rods and films (a) Acid Red 27 (b) Sudan IV (c) Natural Red 25 (d) Ethyl Violet (e) Safranin O (f) LD 490

20 MATERIAL CHARACTERIZATION Material characterization of the dye doped polymer films is essential to know the quality of films, the bond formed between dye molecules and the PMMA, and the dye association with PMMA Optical Quality Optical quality of the synthesized dye doped films are checked by passing a He-Ne laser ( = nm, 5 mw) beam through the samples. The samples which showed no dispersion or distortion were used for further studies Thermo Gravimetric Analysis Thermogravimetric analysis (TGA) is a thermal analysis technique which measures the weight change in a material as a function of temperature and time, in a controlled environment. This can be very useful to investigate the thermal stability of a material, or to investigate its behavior in different atmospheres (e.g. inert or oxidizing). It is suitable for use with all types of solid materials, including organic or inorganic materials. TGA provides quantitative measurement of mass change in materials associated with transition and thermal degradation. TGA records change in mass from dehydration, decomposition, and oxidation of a sample with time and temperature. A differential thermal analysis (DTA) curve can be used to obtain the point at which weight loss is most apparent. Thermo gravimetric analysis (TGA) of the dye doped polymethymethacrylate (PMMA) was carried out using NETZSCH STA 409 C/CD equipment. The thermal analysis of the samples is made from 25 0 C to 1400 o C at a heating rate of 20 o C /min in nitrogen atmosphere.

21 59 The thermograms (TGA and DTA) curves obtained for Ethyl Violet dye doped PMMA and Natural Red 25 dye doped PMMA are shown in Figures 2.2 to Thermo Gravimetric Analysis of Natural Red 25 Dye The TGA trace of Natural Red 25 dye doped PMMA (solventethanol) shown in Figure 2.2 illustrates the absence of any weight loss up to 110 o C. But a weight loss of about 12% occurs at 110 o C. It is due to loss of entrapped solvent in the polymer matrix. It is followed by major weight loss close to 250 o C, which occurs in two stages. The total weight loss of about 99%, occurs at 450 o C, indicating the complete, degradation and dissociation of the polymer. DTA trace of the dye is shown in Figure 2.3, consists of two exothermic peaks and two endothermic peaks. The exothermic peak at 93 o C supports the crystallization of dye doped polymer. The endothermic peaks between 260 o C and 340 o C supports the weight loss due to the entrapped solvent in TGA trace. The major endotherm that follow 340 o C is due to the degradation of the polymer. Figure 2.2 Thermo gravimetric analysis of Natural Red 25 doped PMMA with ethanol

22 60 Figure 2.3 DTA trace of Natural Red 25 doped PMMA with ethanol Thermo Gravimetric Analysis of Ethyl Violet Dye TGA trace of Ethyl Violet dye doped PMMA (solvent ethanol) shown in Figure 2.4 illustrates the absence of any weight loss up to 125 o C. But a minute weight loss occurs between 125 o C and 420 o C. The total weight loss of about 99%, occurs at about 425 o C indicating the complete, degradation and dessociation of the polymer. Since there is absence of newer weight loss, the weight due to the dopand dyes, might be below the detectability limit of TGA. DTA trace of the dye doped PMMA shown in Figure 2.5 illustrates the exothermic peak at 93.8 o C supports the crystallization of dye doped polymer. The endothermic peaks between 225 o C and 350 o C, supports the weight loss due to the entrapped solvent in TGA trace. The major endotherm that follow 350 o C is due to the degradation of the polymer.

23 61 Figure 2.4 Thermo gravimetric analysis of Ethyl Violet doped PMMA with ethanol Figure 2.5 DTA trace of Ethyl Violet doped PMMA with ethanol

24 62 Based on the thermograms, the dopand dyes do not make any covalent interaction with the polymer. They simply remained as non-covalent entities in the polymer matrix. From this analysis it may be concluded that the material is stable up to 250 o C in the N 2 atmosphere. The polymerization is carried out between the temperatures 35 o C to 60 o C. The thermograms reveal that at these temperatures no decomposition of either the dye or PMMA takes place as this might affect the spectral and nonlinear studies undertaken.