مجله ایرانی پلیمر
سال:1390 | دوره: | شماره: |تعداد مقالات:6

Although reversible addition-fragmentation chain transfer (RAFT) polymerization has attracted great attention of many researchers over recent years, outstanding questions on the mechanism and kinetics of dithioester-mediated RAFT polymerization (especially dithiobenzoates) still have remained unsolved. In this work, based on experimental observations and exact theoretical predictions, the kinetic schemes of RAFT polymerization are extended to a wider range of reactions such as irreversible intermediate radical terminations and reversible transfer reactions.The reactions which have been labeled as kinetic schemes are theoretically the most probable existing reactions and are used for mathematical modelling. The detailed kinetic scheme is applied to three kinds of RAFT polymerization systems by utilizing the method of moments. Unknown kinetic rate constants are obtained by curve fitting of the modelling results and theoretical data, and applying the least square method; or estimation by considering the theoretical facts and experimental findings. The origin of the rate retardation and induction periods has been understood by studying the main and pre-equilibrium stages of dithiobenzoate-mediated RAFT homopolymerization. A copolymerization system in the presence of RAFT agent has also been examined to confirm the capability of introduced kinetic scheme in different monomer/RAFT agent systems. Although unknown parameters were obtained theoretically, their consistency with other researchers' works shows the accuracy of the modelling procedure. The modelling results are in excellent agreement with experimental data which proves the validity and applicability of the detailed kinetic scheme. The results have shown that some reactions may not occur in many RAFT polymerization systems and can be eliminated and therefore more kinetic and mechanistic studies are required.

Biopolymer/layered silicate composites have shown unique advantages and potentials for the passive targeting of drugs because they can overcome the burst drawbacks of organic carrier, increase the drug utilization and enhance the controllable capability of drug release. Furthermore, the biocompatibility and nontoxicity of biopolymers is retained, and therefore it is promising and applicable in pharmaceutical fields. The main research aim of this work was to develop a series of biopolymer/layered silicate composite beads based on chitosan (CTS) and Laponite (LA) by a simple ionic cross-linking reaction using sodium tripolyphosphate as the cross-linker. The resultant beads were characterized by Fourier transform infrared spectroscopy, scanning electronic microscope and X-ray diffraction analysis. The swelling behaviour in physiological pH solutions (7.4 and 1.2), drug encapsulation efficiency and controlled release behaviour were also investigated by using Ofloxacin as the model drug to reveal the effects of introduced LA. The results indicate that the incorporation of LA remarkably improved the swelling behaviour, enhanced the drug entrapment efficiency, and slowed down the drug release behaviour in contrast to the pure organic CTS beads. The exfoliated LA clay could act as a physical cross-linker to facilitate the formation of network structure between the CTS and LA. It is suggested that LA may be developed as an effective additive for fabricating a sustained drug delivery system.

Novel types of polyimides with specific properties were prepared by two ways, namely the conventional two-step and ionic liquid methods (IL) to compare the final polymer characteristics. In this way, nucleophilic substitution reaction of 4, 4' -oxydianiline with 6-chloronicotinoyl chloride yielded a dichlorodiamide compound (DCDA) which by reaction with 5-amino-1-naphthol afforded a new diamine with specific structural features. Polycondensation reaction of the diamine with different aromatic dianhydrides using a two-step method and in an ionic liquid of one step method resulted in formation of novel polyimides. All the precursors and polyimides were fully characterized using routine analytical methods and their physical and thermal properties including solution viscosity, thermal stability and behaviour, crystallinity and solubility were investigated. According to the obtained results polyimides showed high thermal stability and improved solubility. Using IL as a media in one step method for the preparation of polyimides led to improved properties in comparison to conventional two-step method of imidization. The inherent viscosity of the polymers was in the range of 0.40-0.88 dL/g for polyimides prepared via two-step method, and 0.52-0.93 dL/g for polyimides prepared via IL one-step method. The weight loss of 10% as an important criterion for evaluation of thermal stability occurred in the range 430-445°C for polyimides prepared by two-step method, while the same weight loss took place in the range 457-474oC for polyimides prepared by IL one-step method.

To investigate the influence of thermal treatment and interfacial crystalline structure on interfacial shear strength of glass fibre reinforced polypropylene composites (GF/PP), the transcrystalline samples were prepared by isothermal crystallization at 135ºC, while the quenched samples without transcrystallization were obtained by quick cooling at the rate of 20oC/min. In the next step, the transcrystalline and the quenched specimens were each treated at various temperatures of 70oC, 100oC and 130oC, and the interfacial shear strength (IFSS) of these specimens was measured by the microbond test. The results demonstrated that, before thermal treatment, the interfacial shear strength of the transcrystalline specimen is much smaller than the quenched composite specimen. The shear debonding of the trans-crystalline specimen occurred at the rim where the transcrystalline region met the spherulites, because the shear debonding of the quenched specimen happened at the PP and glass fibres interfaces. Whereas, after thermal treatment the shear debonding of the transcrystalline specimens, as same as quenched specimens, occurred at the interfaces of PP and glass fibres. The interfacial shear strength (IFSS) of the trans-crystalline specimen also increased the same as that of the quenched cases.Therefore, we conclude that transcrystallinity alters the failure location at interfacial region and thus it affects the interfacial shear strength. Both the interfacial shear strength of the transcrystalline and that of the quenched samples can be improved by thermal treatment.

The molecular weight and molecular weight distribution were adjusted by controlling polymerization process to meet bimodal poly (ethylene-co -1-butene) using Ziegler-Natta catalyst. The process was carried out through two-step polymerization in a semi-batch reactor where in the first step, a low molecular weight homopolymer of ethylene was produced in the presence of hydrogen and in the next step, the high molecular weight copolymer of ethylene with 1-butene was produced after the hydrogen was removed from the reactor. The effects of hydrogen and comonomer concentrations and the polymerization time for the first and second steps were investigated. Although, the second step was run in the absence of hydrogen, the results indicate that shifting the reaction from the first step to the second step leaves potentially some effects on the active centres. The higher hydrogen concentration decreased the rate of polymerization and molecular weight in the first step while, it indirectly raised the rate of polymerization and consequently the yield of reaction at the second step considerably. Increased comonomer concentration in the second step reduced the average molecular weight and increased the melt flow rate while slightly increased the rate of polymerization. It is found that the effect of comonomer concentration on the rate of copolymerization in the second step is lowered by the increase of hydrogen concentration at the first step.

The effect of AlCl3 as a catalyst in direct condensation of homopolymerization and copolymerization of (rac and l) -lactic acid was studied. The morphological effect of dimer formation via racemization during polymerization with respect to time and catalyst concentration was investigated at 180oC under vacuum conditions (176 mmHg). The sequence distribution of triads and hexads was analyzed using 13C NMR spectroscopy. A combination of Avrami and Ozawa equations for mathematical modelling of the non-isothermal crystallization kinetics of the synthesized poly (d, l -lactic acid) produced consistent results with the experimental data under these conditions. It is shown that AlCl3 promotes racemization which leads to formation of ill-defined polymers, as demonstrated by determinations of molecular weight distribution (Mn<5279 g/mol, PDI=1.4-3), optical purity (91.67% - 99.36%) and melting temperature (143-155oC). The activation energy for crystal growth during the cooling of molten samples of poly (d, l -lactic acid), as determined by Kissinger method, is found to be 45 kcal per mol. In this respect, the microstructural effect of copolymers on their thermal behaviour is discussed.