Iranian Polymer Journal
Year:2009 | Volume:18 | Issue:2 (104)|Papers Count:7

Cross-linking of polyethylene is a subject of interest, having been emerged as a result of the need to meet application requirements which were not satisfied by the neat polyethylene itself. Although this review is aimed at silane method of cross-linking, the other cross-linking methods, namely: radiation, peroxide and azo methods have been presented briefly for a better understanding of the merits of perspectives of the more recent methods, especially the two-step silane method. Freeradical grafting of unsaturated hydrolysable alkoxy-silanes onto polyethylene chains by a peroxide initiator followed by moisture cross-linking is the most versatile crosslinking method and may be successfully used for other thermoplastics as well. Different techniques of silane cross-linking, i.e., the "one step" Monosil process and the "two-step" Sioplas process have been discussed with more emphases on Sioplas process, as it is less expensive and readily achievable. The grafting step which is performed by reactive processing is the major and key process in Sioplas technique. The state-of-the-art of a two-step silane grafting and cross-linking has been presented. In this regard, the effects of various parameters, such as the type and quantity of silane, peroxide, stabilizing agent, catalyst, micro and macromolecular structures and physical form of polyethylene, additives, and reaction temperature and time have been described in relation to efficiency of grafting and cross-linking. These data were evaluated in turn by torque, MFI, FTIR, and gel content studies.

There has been concern regarding the long-term safety of the current drug-eluting stents (DESs) using "durable-polymer" technology. On this background, a number of second generation DESs using bioabsorbable-polymer are put under tests. In our study, we assessed the in vitro degradation and biocompatibility of poly (D,L-lactic- co-glycolic acid) (PLGA) 90L/10G, as a biodegradable material. In vitro degradation characteristics were evaluated by measuring decrease in the mass loss of PLGA films. In order to evaluate the cytotoxicity of PLGA, calf vascular smooth muscle cells (VSMCs) were incubated with PLGA films. Cells that were incubated in culture medium alone were used as controls. We determined the cell viability by a 3-(4,5-dimethylthiazol- 2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay and the distribution of cell cycle phases by flow cytometry. The morphology of VSMCs seeded on PLGA films was observed by scanning electron microscopy technique. PLGA films were inserted into the anterior wall of the left ventriculators and circumflex arteries of 6 dogs that served as test animals. The animals were sacrificed after 2 weeks, 1 month, and 2 months. Pathohistological and ultra structural changes in the myocytes and vasculature were examined by light and electron microscopy. High-molecular-weight PLGA degraded slowly in the first 4 months. There was 10% mass loss at the 120th day and rapid mass loss thereafter. There were no differences in the cell viability and distribution of cell cycle phases between the control and PLGA groups. The cells attached favourably and grew well on the films. A slight inflammatory change occurred with regard to the pathohistology and ultra structure of the myocardium and vessels. Thus, the low cytotoxicity, good histocompatibilty, and biodegradable nature of PLGA 90/10 make it a promising material as a useful vehicle for locally administered drugs.

The effects of different particle sizes of recycled NBR (NBRr) on curing characteristics, mechanical, and morphological properties of styrene butadiene rubber/ recycled acrylonitrile butadiene rubber (SBR/NBRr) blends were examined. Three different size ranges of NBRr particles, i.e., 117 -mm, 0.85 -15.0 mm, and 10 -19 cm were used in this study. The SBR/NBRr blends with blend ratios of 95/5, 85/15, 75/25, 65/35, and 50/50 were prepared using a two roll-mill at room temperature. The characterization results of the blends show that scorch time, t2, and cure time t90 of the SBR/NBRr blends decreased with increased NBRr content as well as decreasing sizes of NBRr particles due to the existence of cross-linked precursors and unreacted curative in the recycled rubber. Among all blend ratios, the SBR/NBRr blends with smallest size of NBRr particles exhibits lowest minimum torque (ML) compared with the bigger particle sizes of it in SBR/NBRr blends which resulted in more efficient processing. The maximum torque (MH) of all SBR/NBRr blends show the declining trend with increased NBRr content probably due to the poor interactions in SBR/NBRr blends. The SBR/NBRr blends with smallest size of NBRr particles show better mechanical properties (tensile, elongation-at-break, M100, and fatigue) compared with all other blend ratios of bigger sizes of NBRr particles. For physical properties, SBR/NBRr blends with smallest size of NBRr particles exhibited the highest hardness and cross-linking density at all blend ratios whereas resilience decreased, accordingly. As the particle size decreased, the contact surface area increased which provided more efficient interfacial bonds, leading to better properties. The scanning electron microscopy studies showed that the smallest size of NBRr particles in SBR/NBRr blends illustrated a better NBRr-SBR matrix interaction compared with the other NBRr particles in the related SBR/NBRr blends.

Room temperature vulcanizing (RTV) silicone rubber is widely used to coat glass and porcelain insulators to prevent surface flashover on the insulators that are heavily contaminated with industrial pollution and exposure to humidity in the coastal areas which may lead to outages of the power system. Fillers such as alumina trihydrate (ATH) and silica are also used in these coatings to provide tracking and erosion resistance. Therefore, a formulation of RTV coating system should be developed to optimize its performance and in particular to extend its lifetime. In this work, RTV coatings with different quantities of ATH and silica fillers were applied to porcelain suspension insulator. They were then artificially contaminated and exposed to dry and wet conditions. Moreover, contact angle measurements were performed to determine the state of the surface. To carry out surface characteristic studies of the coatings filled with ATH and silica, the samples were gold coated for SEM observation. In addition, TGA was utilized to investigate the role of ATH in RTV samples. The test results are reported on the basis of the role of ATH and silica fillers. The results showed that when ATH is used as filler the highest performance belongs to the sample with 70 pph ATH. Addition of 10 pph silica also improves the performance of coatings at ATH quantity between 35 to 70 pph, but replacement of 10 pph ATH with the same amount of silica was not productively efficient.

Micro emulsion samples of epoxy modified polysiloxane were prepared by hydrolytic polycondensation of active centre’s instead of free radical polymerization. The reactions and mechanisms of the preparation of micro emulsions were elucidated and the micro emulsions were characterized by Fourier transform infrared spectroscopy (FTIR), photon correlation spectroscopy (PCS), transmission electron microscopy (TEM) and UV-visible spectrophotometry techniques. The effects of the amount of catalyzer and epoxy modifier were systematically studied. The results of FTIR spectroscopy indicated that the epoxy group successfully grafted onto the polysiloxane molecules. The micro emulsions showed stability when the amounts of catalyzer and epoxy modifier were low. The particle size decreased and the size distribution widened with the increase of the catalyzer concentration. A slight variation of the particle size occurred when the amount of modifier varied under 1.0 wt%, although the particle size rose rapidly from about 40 nm to 180 nm when the content of modifier was beyond 1.0 wt%. The increasing amount of epoxy modifier gave narrower molecular size distribution and the appearance of micro emulsions changed from a transparent or a semi-transparent state to an opaque state.

The kinetics of non-isothermal crystallizations of polypropylene (PP) and PP containing 0.5 wt% single-walled carbon nanotube (PP/SWNT) were compared by using differential scanning calorimetry technique. The non-isothermal melt crystallization data were analyzed according to Avrami and Mo models. The values of half life and corrected composite rate constant of Avrami equation as well as the rate parameter of Mo equation indicated that the crystallization rate increased with the increasing of PP and PP/SWNT cooling rates. However, the crystallization rate of the nanocomposite was found to be higher than that of PP at a certain cooling rate. Nucleation efficiency (NE) defined by Fillon was obtained for PP/SWNT. It was found that NE varied from 16.4% to 19.4% in the range of cooling rate studied. This implied that SWNT could play as a nucleating agent for the crystallization of PP. In addition, the effective energy barrier (DE) for the non-isothermal melt crystallization process was calculated for the two materials by using the Kissinger method. The DE values were determined to be 226 and 262 kJmol-1 for PP and PP/SWNT, respectively. Based on the theory proposed by Lauritzen and Hoffman, the end surface free energy (se) for the two materials was calculated. The results showed that the value of se for the nanocomposite was lower than that of PP itself, in agreement with the fact that the rate of crystallization is higher in the nanocomposite compared to the pristine polymer.

Vinyl ester polymeric systems linked to ibuprofen were synthesized and evaluated as materials for drug delivery. The carboxyl group of ibuprofen was converted into vinyl ester group by reacting ibuprofen with vinyl acetate in the presence of mercuric acetate as a catalyst. The resultant ibuprofen derivative of vinyl ester was then copolymerized with 2-hydroxyethyl methacrylate or methyl methacrylate (in 1:3 mole ratios) by utilizing azoisobutyronitrile as an initiator at the temperature range of 65-70oC. The structure of all compounds was characterized and confirmed by FTIR, 1H NMR, and 13C NMR spectroscopy techniques and elemental analysis. Gel permeation chromatography was used for determination of the average molecular weights and polydispersity indices of the ibuprofen containing polymers. The hydrolysis of drugpolymer conjugates was carried out in cellophane membrane dialysis bags containing aqueous buffer solutions (pH 1, 7.4, and 10) at 37oC for 48 h. Detection of hydrolysis solutions by UV-vis spectroscopy at selected intervals showed that the drug could be released by hydrolysis of the ester bonds which were formed between the drug and polymer backbone. The release profiles indicated that the hydrolytic behaviour of polymeric prodrugs is strongly based on polymer hydrophilicity and pH of the hydrolysis solution. The results suggest that these prepared polymeric prodrugs could be useful for release of ibuprofen in controlled release systems.