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علوم و تکنولوژی پلیمر (فارسی) | سال:1389 | دوره:23 | شماره:5 (پیاپی 109)

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مرکز اطلاعات علمی SID1
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
Writer: 

ZIAEE F.

Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    359
  • End Page: 

    370
Measures: 
  • Citations: 

    0
  • Views: 

    296
  • Downloads: 

    140
Abstract: 

In this research, the microstructural of low molecular weight 1, 2-polybutadiene (1, 2-PBD) was conducted by 1H and 13C nuclear magnetic resonance spectroscopy (NMR) to determine the isomeric contents of 1, 4-cis, 1, 4-trans and 1, 2-vinyl in 1, 2-PBD polymer structures. Number average molecular weight for low molecular weight 1, 2-PBD was measured by NMR techniques and the results were compared with gel permeation chromatography. Due to the presence of methyl end group and its comparison with repeating units in 1, 2-PBD microstructure, the number average molecular weight was calculated by NMR techniques. For calculation of surface areas, carbon and protons of methyl groups were characterized using distortion enhancement by polarization transfer (DEPT) methods. For proton assignment of methyl end groups in 1H NMR spectral analysis the heteronuclear multiple quantum coherence (HMQC) method was employed. Finally, stereoregularity and tacticity of 1, 2-PBD were investigated through pentad and heptad sequences splitting of olefinic methylene and methine carbons pendant groups with various NMR acquisition temperatures from 20 to 50oC.13C NMR spectra showed that with increasing of NMR acquisition temperature, the number of split peaks of two olefinic carbons increased.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    371
  • End Page: 

    378
Measures: 
  • Citations: 

    0
  • Views: 

    339
  • Downloads: 

    115
Abstract: 

Collagen is the major protein component of cartilage, bone, skin and connective tissue and constitutes the major part of the extracellular matrix. Collagen type I has complex structural hierarchy, which consists of tree polypeptide a-chains wound together in a rod-like helical structure. Collagen is an important biomaterial, finding many applications in the field of tissue engineering. It has been processed into various shapes, such as, gel, film, sponge and fiber. It is commonly used as the scaffolding material for tissue engineering due to its many superior properties including low antigenicity and high growth promotion. Unfortunately, poor mechanical properties and rapid degradation rates of collagen scaffolds can cause instability and difficulty in handling. By crosslinking, the structural stability of the collagen and its rate of resorption can be adapted with respect to its demanding requirements. The strength, resorption rate, and biocompatibility of collagenous biomaterials are profoundly influenced by the method and extent of crosslinking. In this study, the effect of UV irradiation on collagen scaffolds has been carried out. Collagen scaffolds were fabricated using freeze drying method with freezing temperature of -80oC, then exposed to UV irradiation. Mean pore size of the scaffolds was obtained as 98.52±14.51mm using scanning electron microscopy. Collagen scaffolds exposed to UV Irradiation (254 nm) for 15 min showed the highest tensile strain (17.37±0.98%), modulus (1.67±0.15 MPa) and maximum load (24.47±2.38 cN) values.As partial loss of the native collagen structure may influence attachment, migration, and proliferation of cells on collagen scaffolds, we detected no intact a-chains after SDS-Page chromatography. We demonstrate that UV irradiation is a rapid and easily controlled means of increasing the mechanical strength of collagen scaffolds without any molecular fracture.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    379
  • End Page: 

    386
Measures: 
  • Citations: 

    0
  • Views: 

    83
  • Downloads: 

    75
Abstract: 

Bis (indenyl) zirconium dichloride as a catalyst was synthesized using a modified method at the room temperature. Polymerization of ethylene and copolymerization of ethylene and propylene were carried out by this metallocene catalyst. Ethylene propylene (P/E) feed ratios of 0.25, 0.5, 1, and 2 were employed into the copolymerization systems. Copolymerization conditions like cocatalyst/catalyst ratio, copolymerization temperature, feed ratio and the viscosityaverage molecular weight (Mv) changing with copolymerization temperature were investigated. Higher ratio of [Al] / [Zr]=750: 1 increased the activity of the catalyst.However, higher concentration of the cocatalyst slightly reduced the activity. Viscosity-average molecular weight of polymer decreased with high ratio of [Al] / [Zr]. The highest activity was obtained at 60oC. Increases in copolymerization temperature decreased the viscosity-average molecular weight and ethylene content of the polymeric products. Increases in propylene content of the feed ratio produced amorphous polymers with products of increased glass transition temperatures. Kinetic reaction displayed a decay type of reaction.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    387
  • End Page: 

    395
Measures: 
  • Citations: 

    0
  • Views: 

    247
  • Downloads: 

    91
Abstract: 

Styrene polymerization through FRP and ATRP methods was carried out at 110oC, while in-depth studies were performed by Monte Carlo simulation. The changes in monomer conversion, initiator concentration, average molecular weight, and polydispersity index were computed over the course of the polymerization.As the results indicate, compared to ATRP, the FRP reaches higher conversion in a similar reaction time. In addition, the concentration of initiator suddenly drops at the early stages of the ATRP and eventually amounts to zero; chain-length dependent termination rate constant also decreases as the polymerization progresses. However, in case of FRP, the concentration of initiator exponentially falls and termination rate constant rises during the reaction. Furthermore, the average molecular weight increases linearly in the course of ATRP, which testifies the living dynamism of the reaction. Finally, the molecular weight distribution of chains obtained by ATRP process is much narrower.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    397
  • End Page: 

    404
Measures: 
  • Citations: 

    1
  • Views: 

    529
  • Downloads: 

    104
Abstract: 

Poly-b-hydroxybutyrate (PHB), produced by several species of bacteria, has attracted great attention as a biodegradable and biocompatible compound with almost similar properties of polypropylene. Unfortunately, its use is currently limited due to high production costs. One of the most common methods for overcoming this constraint is the use of inexpensive substrates, like methanol.Another strategy which involves the increase in carbon yield which can directly affect the production costs. One method to increase carbon yield is the use of mixed substrates. Therefore, for PHB production the effects of three sources of carbon, ethanol, sodium acetate, and glucose were studied as substrates in different concentrations mixed with methanol. It was found that PHB concentration and PHB content increased by addition of ethanol and acetate, but glucose addition did not have any significant effect on PHB production. Calculation of carbon yield under the best condition indicated that it increased about 3 times by addition of acetate.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    405
  • End Page: 

    413
Measures: 
  • Citations: 

    0
  • Views: 

    80
  • Downloads: 

    79
Abstract: 

The effects of magnesium hydroxide and l-lactide dimer as additives on thermal properties and morphology of poly (l-lactide) (PLLA) films were studied. Hence, neat PLLA films and films containing additives (10% w/w) were prepared in dichloromethane at room temperature via solution casting. To evaluate thermal history on polymer properties, PLLA films were annealed by different processes. In one process, designated as A, PLLA films were heated from 20 up 140oC, then held for 1 h and cooled to room temperature. In another process, designated as B, melted PLLA samples which had been maintained at 200oC for 5 min were cooled with a rate of - 20oC/min to 140oC and annealed for 1 h before being cooled to room temperature. In the next process designated as C, the melted PLLA samples after 5 min of staying at 200oC were being quenched to 0oC. Then the samples were heated to 140oC followed by annealing for 1 h before being cooled to room temperature. The crystallization and morphology properties of PLLA films were studied using polarized optical microscopy and differential scanning calorimetry. It is found that the percentage of crystal and spherulitic formation inside PLLA films were influenced by thermal history and presence of the additives. The type of additives did not affect melting point (Tm) of the films annealed through different processes of A to C, while they had influence on Tm of the films, significantly. It is to be noted that some spherulites were formed in films during B process. During C process, however, the nucleation rate increased due to quenching which enhanced the spherulites formation.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    415
  • End Page: 

    422
Measures: 
  • Citations: 

    0
  • Views: 

    295
  • Downloads: 

    109
Abstract: 

One of the common methods to cure unsaturated polyester resins is to use dual initiators. In order to produce an appropriate composite, it is necessary to know the effect of dual initiators on curing behavior. In this paper the effect of benzoyl peroxide, BPO, and tertiary butyl perbenzoate, TBPB, and their mixtures on thermal curing of an unsaturated polyester resin have been investigated. BPO and TBPB are medium and high temperature initiators, respectively. SPI and DSC tests have been used for this evaluation. SPI test results show that in curing system containing TBPB/BPO, higher gel time, lower curing peak temperature and lower release heat have been achieved by using higher amount of TBPB. DSC results show that a suitable curing system can be achieved by using a dual initiator system.

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Issue Info: 
  • Year: 

    2011
  • Volume: 

    23
  • Issue: 

    5 (ISSUE NO.109)
  • Start Page: 

    423
  • End Page: 

    433
Measures: 
  • Citations: 

    0
  • Views: 

    126
  • Downloads: 

    142
Abstract: 

The incorporation of boehmite nano-particles and its effects on the mechanical properties and bond strength of an experimental dental adhesive were studied. An adhesive solution containing ethanol, Bis-GMA, TMPTMA, HEMA and photo-initiator system (camphorquinone and DMAEMA) was prepared. Silanized boehmite nano-particles were incorporated into the adhesive in different concentrations of 0, 0.2, 0.5, 1, 2, and 5 weight percentages. The suspensions were ultrasonocated to facilitate the nano-particle dispersion. Degree of conversion of the adhesive under visible light irradiation was determined using FT-IR spectroscopy. Depth of cure, diametral tensile strength, flexural strength, and micro-shear bond strength of the adhesives were measured. Scanning electron electron microscopy was utilized to observe the fracture surface topography. The results were compared using one-way ANOVAand Tukey posthoc test at the significance level of 0.05. The results indicated that the diametral tensile strength, flexural strength, and micro-shear bond strength increased at nano-particle contents of 0.2-0.5 wt.% and beyond this range there was a declining trend (p<0.05) observed in all these properties because of the nano-particle agglomeration. No significant difference was observed in flexural moduli of the adhesives with different filler contents. The study revealed that the incorporation of boehmite nanoparticles may improve the properties of dental adhesives.

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مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID
مرکز اطلاعات علمی SID