Iranian polymer journal
Volume:20 Issue: 6, 2011

Although reversible addition-fragmentation chain transfer (RAFT) polymerizationhas attracted great attention of many researchers over recent years,outstanding questions on the mechanism and kinetics of dithioester-mediatedRAFT polymerization (especially dithiobenzoates) still have remained unsolved. In thiswork, based on experimental observations and exact theoretical predictions, thekinetic schemes of RAFT polymerization are extended to a wider range of reactionssuch as irreversible intermediate radical terminations and reversible transfer reactions.The reactions which have been labeled as kinetic schemes are theoretically the mostprobable existing reactions and are used for mathematical modelling. The detailedkinetic scheme is applied to three kinds of RAFT polymerization systems by utilizing themethod of moments. Unknown kinetic rate constants are obtained by curve fitting of themodelling results and theoretical data, and applying the least square method; orestimation by considering the theoretical facts and experimental findings. The origin ofthe rate retardation and induction periods has been understood by studying the mainand pre-equilibrium stages of dithiobenzoate-mediated RAFT homopolymerization. Acopolymerization system in the presence of RAFT agent has also been examined toconfirm the capability of introduced kinetic scheme in different monomer/RAFT agentsystems. Although unknown parameters were obtained theoretically, their consistencywith other researcher's works shows the accuracy of the modelling procedure. Themodelling results are in excellent agreement with experimental data which proves thevalidity and applicability of the detailed kinetic scheme. The results have shown thatsome reactions may not occur in many RAFT polymerization systems and can beeliminated and therefore more kinetic and mechanistic studies are required.

Biopolymer/layered silicate composites have shown unique advantages andpotentials for the passive targeting of drugs because they can overcome theburst drawbacks of organic carrier, increase the drug utilization and enhance thecontrollable capability of drug release. Furthermore, the biocompatibility and nontoxicityof biopolymers is retained, and therefore it is promising and applicable inpharmaceutical fields. The main research aim of this work was to develop a series ofbiopolymer/layered silicate composite beads based on chitosan (CTS) and Laponite(LA) by a simple ionic cross-linking reaction using sodium tripolyphosphate as thecross-linker. The resultant beads were characterized by Fourier transform infraredspectroscopy, scanning electronic microscope and X-ray diffraction analysis. Theswelling behaviour in physiological pH solutions (7.4 and 1.2), drug encapsulation efficiencyand controlled release behaviour were also investigated by using Ofloxacin asthe model drug to reveal the effects of introduced LA. The results indicate that theincorporation of LA remarkably improved the swelling behaviour, enhanced the drugentrapment efficiency, and slowed down the drug release behaviour in contrast to thepure organic CTS beads. The exfoliated LA clay could act as a physical cross-linker tofacilitate the formation of network structure between the CTS and LA. It is suggestedthat LA may be developed as an effective additive for fabricating a sustained drugdelivery 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 thefinal polymer characteristics. In this way, nucleophilic substitution reaction of4,4'-oxydianiline with 6-chloronicotinoyl chloride yielded a dichlorodiamide compound(DCDA) which by reaction with 5-amino-1-naphthol afforded a new diamine withspecific structural features. Polycondensation reaction of the diamine with different aromaticdianhydrides using a two-step method and in an ionic liquid of one step methodresulted in formation of novel polyimides. All the precursors and polyimides were fullycharacterized using routine analytical methods and their physical and thermalproperties including solution viscosity, thermal stability and behaviour, crystallinity andsolubility were investigated. According to the obtained results polyimides showed highthermal stability and improved solubility. Using IL as a media in one step method for thepreparation of polyimides led to improved properties in comparison to conventionaltwo-step method of imidization. The inherent viscosity of the polymers was in the rangeof 0.40-0.88 dL/g for polyimides prepared via two-step method, and 0.52-0.93 dL/g forpolyimides prepared via IL one-step method. The weight loss of 10% as an importantcriterion for evaluation of thermal stability occurred in the range 430-445°C forpolyimides prepared by two-step method, while the same weight loss took place in therange 457-474°C for polyimides prepared by IL one-step method.

To investigate the influence of thermal treatment and interfacial crystallinestructure on interfacial shear strength of glass fibre reinforced polypropylenecomposites (GF/PP), the transcrystalline samples were prepared by isothermalcrystallization at 135ºC, while the quenched samples without transcrystallization wereobtained by quick cooling at the rate of 20ºC/min. In the next step, the transcrystallineand the quenched specimens were each treated at various temperatures of 70ºC,100ºC and 130ºC, and the interfacial shear strength (IFSS) of these specimens wasmeasured by the microbond test. The results demonstrated that, before thermaltreatment, the interfacial shear strength of the transcrystalline specimen is muchsmaller than the quenched composite specimen. The shear debonding of thetrans-crystalline specimen occurred at the rim where the transcrystalline region met thespherulites, because the shear debonding of the quenched specimen happened at thePP and glass fibres interfaces. Whereas, after thermal treatment the shear debondingof the transcrystalline specimens, as same as quenched specimens, occurred at theinterfaces of PP and glass fibres. The interfacial shear strength (IFSS) of thetrans-crystalline specimen also increased the same as that of the quenched cases.Therefore, we conclude that transcrystallinity alters the failure location at interfacialregion and thus it affects the interfacial shear strength. Both the interfacial shearstrength of the transcrystalline and that of the quenched samples can be improved bythermal treatment.

The molecular weight and molecular weight distribution were adjusted bycontrolling polymerization process to meet bimodal poly(ethylene-co-1-butene)using Ziegler-Natta catalyst. The process was carried out through two-step polymerizationin a semi-batch reactor where in the first step, a low molecular weighthomopolymer of ethylene was produced in the presence of hydrogen and in the nextstep, the high molecular weight copolymer of ethylene with 1-butene was producedafter the hydrogen was removed from the reactor. The effects of hydrogen andcomonomer concentrations and the polymerization time for the first and second stepswere investigated. Although, the second step was run in the absence of hydrogen, theresults indicate that shifting the reaction from the first step to the second step leavespotentially some effects on the active centres. The higher hydrogen concentrationdecreased the rate of polymerization and molecular weight in the first step while, itindirectly raised the rate of polymerization and consequently the yield of reactionat the second step considerably. Increased comonomer concentration in the secondstep reduced the average molecular weight and increased the melt flow rate whileslightly increased the rate of polymerization. It is found that the effect of comonomerconcentration on the rate of copolymerization in the second step is lowered by theincrease of hydrogen concentration at the first step.

The effect of AlCl3 as a catalyst in direct condensation of homopolymerization andcopolymerization of (rac and l)-lactic acid was studied. The morphological effectof dimer formation via racemization during polymerization with respect to timeand catalyst concentration was investigated at 180°C under vacuum conditions(176 mmHg). The sequence distribution of triads and hexads was analyzed using13C NMR spectroscopy. A combination of Avrami and Ozawa equations formathematical modelling of the non-isothermal crystallization kinetics of the synthesizedpoly(d,l-lactic acid) produced consistent results with the experimental data under theseconditions. It is shown that AlCl3 promotes racemization which leads to formation ofill-defined polymers, as demonstrated by determinations of molecular weightdistribution (Mn < 5279 g/mol, PDI = 1.4-3), optical purity (91.67% - 99.36%) andmelting temperature (143-155°C). The activation energy for crystal growth during thecooling 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 copolymerson their thermal behaviour is discussed.