The first attempts to develop emulsion polymerization of main diene monomers such as butadiene were undertaken to reduce dependence on natural resources, i.e. natural rubber. The initial publications on emulsion polymerization of butadiene back to shortly after World War II. Much of the fundamental knowledge about the subject, for the first time, was briefly published by the Synthetic Rubber Program . A few years later, a series of articles were published by Morton et al. [2-5] about the cross-linking behavior of polybutadiene and the use of several initiator systems in the emulsion polymerization of butadiene.
Persulfate-mercaptan initiator system was extensively investigated by Kolthoff et al. [6-8] during 1991-1995. They studied persulfate decomposition reactions and the effect of its concentration on the polymerization trend by using various analytical methods for determination of the persulfate  and mercaptan  concentration. Bhankuni  proved that in the persulfate-mercaptan emulsion polymerization of butadiene, the kinetics of reaction is affected by the nature of the emulsifier. Moreover, network formation within the latex particles has no influence on the saturation monomer solubility in the particles. The radiation-induced emulsion polymerization of butadiene was reported in 1974 . Polymerization found to be much faster in the presence of n-dodecylmercaptan. Consistent results about the effect of almost all relevant chemical and physical factors on emulsion polymerization of butadiene were obtained by Weerts and coworkers [11-17]. These investigations led to the following conclusions: 1) Average polymerization rate per particle isn't influenced by the nature and the concentration of emulsifier and/or initiator but is significantly influenced by particle size. 2) Bimolecular termination within the particles is not rate-determining reaction, and radical exit from the particles is significant. Low initiator efficiency limits radical entry into particles. Therefore, it was concluded that emulsion polymerization of butadiene is a typical of Smith-Ewart case I system (n
Chemical structure of polybutadiene consists of three major microstructure: 1,4-cis, 1,4-trans and 1,2 vinyl. High cis-polybutadiene has a high elasticity, whereas the so-called high vinyl is a plastic crystal. Emulsion polymers of butadiene contain all three types of these microstructures although 1,4-trans is the predominant microstructure (~ 60%). Amount of each microstructure in the produced polymer depends on the various parameters such as initiator type, surfactant type and content and polymerization temperature. Temperature dependence of microstructure in emulsion polymerization of polybutadiene has been studied . It was proved that the content of unreacted vinyl groups is not considerably influenced by the polymerization temperature. However, 1,4-trans decreases while 1,4-cis content increases with increasing polymerization temperature. Indeed, the cis-trans ratio is lowerd by lowering the polymerization temperature. The polybutadiene produced at low temperatures contains high amounts of 1,4-trans configuration and this structurally more ordered polymer has improved physical properties. In On the other hand, polybutadiene, which is produced at higher temperatures contains random sequence of 1,2-vinyl, 1,4- trans and 1,4-cis configurations. Such random sequence is the main reason for weak physical properties of produced polybutadiene. Therefore, polybutadiene elastomers are generally synthesized at lower temperatures. Emulsion polymerization is the most usual method for production of acrylonitrile-butadiene-styrene (ABS) engineering terpolymer. For this purpose, polybutadiene latex (PBL) is synthesized via emolsion polymerization method, then styrene and acrylonitrile monomers are added to the latex and make it swelled. The swelling amount of polybutadiene nanoparticles and terpolymer morphology is affected by the content of gel formed during polymerization which itself is controlled by different parameters such as polymerization temperature.
Batch emulsion polymerization of butadiene is long in time. Therefore, finding appropriate strategies to improve the initiator efficiency in the particle nucleation and to increase and consequently increase the polymerization rate has found high industrial importance. In the previous work, we studied the effect of initiator and emulsifier concentations on the reaction kinetics and latex's paricle size . In the continuum of the previous study, effect of temperature on the kinetics of batch emulsion polymerization of butadiene has been studied in the present work. To our knowledge, there is no report on the effect of the reaction temperature on the particle nucleation and growth processes, i.e. on the average number of the particles per unit volume of the continuous phase (Np) and growing chains per particle (n), respectively. For this purpose, the PBL with solid content of 30% was produced through batch emulsion polymerization. Polymerization reaction was performed at different temperatures (60, 70 and 80 oC) in a Buchi reactor equipped with mechanical stirrer (300 rpm) in order to determine temperature dependence of the polybutadiene particle size and its distribution (particle nucleation and growth processes). Moreover, effect of polymerization temperature on the polybutadiene microstructure and the amount of gel formed during the polymerization was evaluated.
2. Research Method
2.1. Materials and instrumentations
Butadiene gas as monomer and disproportionate potassium rosinate (DPR) as anionic emulsifier were supplied from Tabriz Petrochemical Company (TPC). The emulsifier is mixture of abietic acid derivatives including dehydroabietic acid, dihydroabietic acid and tetrahydroabietic acid. Abietic acid content of the emulsifier is less than 2wt%. Potassium persulfate initiator, potassium carbonate and potassium hydroxide electrolytes, laboratory grade, were used without further purification. Tert-dodecyl mercaptan was supplied from TPC and used as chain transfer agent. Hydroquinone (Merck) was used as inhibitor in the conversion measurement. Deionized water was used in all of the polymerizations.
Fourier transform infrared (FTIR) (Model IF505, Bruker) analysis was performed to characterize the final product. Dynamic light scattering (DLS) (Model Zetasizer Nano Series, Malvern) technique was used to measure average particle size and particle size distribution (PSD) of the produced latex. Scanning electron microscopy (SEM) (Model Stereoscan 360, Cambridge Instrument Co.) was applied to determine particle size.
2.2. Synthesis of PBL
Batch emulsion polymerizations were carried out in a stainless steel/glass Buchi reactor (model bmd 300, volume 1dm3) equipped with an anchor shape mechanical stirrer, which was set at 300 rpm. The maximum operating pressure was 20 bar [23, 24]. The required amounts of initiator and emulsifier were dissolved in a part of the needed water for each experiment and the resulting mixture was added to the reactor. The reactor was charged with all ingredients, except for Bu (Table 1). In order to exit oxygen from reactor,