دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و سوم شماره 5 (پیاپی 109، آذر و دی 1389)

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 and1,2-vinyl in 1,2-PBD polymer structures. Number average molecular weight for lowmolecular weight 1,2-PBD was measured by NMR techniques and the results werecompared with gel permeation chromatography. Due to the presence of methyl endgroup and its comparison with repeating units in 1,2-PBD microstructure, the numberaverage molecular weight was calculated by NMR techniques. For calculation ofsurface areas, carbon and protons of methyl groups were characterized using distortionenhancement by polarization transfer (DEPT) methods. For proton assignmentof methyl end groups in 1H NMR spectral analysis the heteronuclear multiple quantumcoherence (HMQC) method was employed. Finally, stereoregularity and tacticityof 1,2-PBD were investigated through pentad and heptad sequences splitting ofolefinic methylene and methine carbons pendant groups with various NMR acquisitiontemperatures from 20 to 50oC. 13C NMR spectra showed that with increasing ofNMR acquisition temperature, the number of split peaks of two olefinic carbonsincreased.

Collagen is the major protein component of cartilage, bone, skin and connectivetissue and constitutes the major part of the extracellular matrix.Collagen type I has complex structural hierarchy, which consists of treepolypeptide α-chains wound together in a rod-like helical structure. Collagen is animportant biomaterial, finding many applications in the field of tissue engineering. Ithas been processed into various shapes, such as, gel, film, sponge and fiber. It is commonlyused as the scaffolding material for tissue engineering due to its many superiorproperties including low antigenicity and high growth promotion. Unfortunately,poor mechanical properties and rapid degradation rates of collagen scaffolds cancause instability and difficulty in handling. By crosslinking, the structural stability ofthe collagen and its rate of resorption can be adapted with respect to its demandingrequirements. The strength, resorption rate, and biocompatibility of collagenous biomaterialsare profoundly influenced by the method and extent of crosslinking. In thisstudy, the effect of UV irradiation on collagen scaffolds has been carried out.Collagen scaffolds were fabricated using freeze drying method with freezing temperatureof -80oC, then exposed to UV irradiation. Mean pore size of the scaffoldswas obtained as 98.52±14.51 μm using scanning electron microscopy. Collagen scaffoldsexposed 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 α-chainsafter SDS-Page chromatography. We demonstrate that UV irradiation is a rapid andeasily controlled means of increasing the mechanical strength of collagen scaffoldswithout any molecular fracture.

Bis(indenyl)zirconium dichloride as a catalyst was synthesized using a modifiedmethod at the room temperature. Polymerization of ethylene andcopolymerization of ethylene and propylene were carried out by this metallocenecatalyst. Ethylene propylene (P/E) feed ratios of 0.25, 0.5, 1, and 2 wereemployed into the copolymerization systems. Copolymerization conditions likecocatalyst/catalyst ratio, copolymerization temperature, feed ratio and the viscosityaveragemolecular weight (Mv) changing with copolymerization temperature wereinvestigated. 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 copolymerizationtemperature decreased the viscosity-average molecular weight and ethylene contentof the polymeric products. Increases in propylene content of the feed ratio producedamorphous polymers with products of increased glass transition temperatures.Kinetic reaction displayed a decay type of reaction

Styrene polymerization through FRP and ATRP methods was carried out at110oC, while in-depth studies were performed by Monte Carlo simulation.The changes in monomer conversion, initiator concentration, average molecularweight, and polydispersity index were computed over the course of the polymerization.As the results indicate, compared to ATRP, the FRP reaches higher conversionin a similar reaction time. In addition, the concentration of initiator suddenlydrops at the early stages of the ATRP and eventually amounts to zero; chain-lengthdependent termination rate constant also decreases as the polymerization progresses.However, in case of FRP, the concentration of initiator exponentially falls and terminationrate constant rises during the reaction. Furthermore, the average molecularweight increases linearly in the course of ATRP, which testifies the living dynamismof the reaction. Finally, the molecular weight distribution of chains obtained byATRP process is much narrower.

Poly-β-hydroxybutyrate (PHB), produced by several species of bacteria, hasattracted great attention as a biodegradable and biocompatible compoundwith almost similar properties of polypropylene. Unfortunately, its use iscurrently limited due to high production costs. One of the most common methods forovercoming this constraint is the use of inexpensive substrates, like methanol.Another strategy which involves the increase in carbon yield which can directlyaffect the production costs. One method to increase carbon yield is the use of mixedsubstrates. 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

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, PLLAfilms 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 toroom temperature. The crystallization and morphology properties of PLLAfilms were studied using polarized optical microscopy and differential scanningcalorimetry. It is found that the percentage of crystal and spherulitic formationinside PLLA films were influenced by thermal history and presenceof the additives. The type of additives did not affect melting point (Tm) of thefilms annealed through different processes of A to C, while they had influenceon Tm of the films, significantly. It is to be noted that some spheruliteswere formed in films during B process. During C process, however, thenucleation rate increased due to quenching which enhanced the spherulitesformation.

One of the common methods to cure unsaturated polyester resins is to use dualinitiators. In order to produce an appropriate composite, it is necessary toknow the effect of dual initiators on curing behavior. In this paper the effectof benzoyl peroxide, BPO, and tertiary butyl perbenzoate, TBPB, and their mixtures onthermal curing of an unsaturated polyester resin have been investigated. BPO and TBPBare medium and high temperature initiators, respectively. SPI and DSC tests have beenused for this evaluation.

The incorporation of boehmite nano-particles and its effects on the mechanicalproperties and bond strength of an experimental dental adhesive were studied.An adhesive solution containing ethanol, Bis-GMA, TMPTMA, HEMA andphoto-initiator system (camphorquinone and DMAEMA) was prepared. Silanizedboehmite nano-particles were incorporated into the adhesive in different concentrationsof 0, 0.2, 0.5, 1, 2, and 5 weight percentages. The suspensions were ultrasonocated tofacilitate the nano-particle dispersion. Degree of conversion of the adhesive under visiblelight irradiation was determined using FT-IR spectroscopy. Depth of cure, diametraltensile strength, flexural strength, and micro-shear bond strength of the adhesives weremeasured. Scanning electron electron microscopy was utilized to observe the fracturesurface topography.