Iranian Journal of Polymer Science and Technology
Year:2012 | Volume:25 | Issue:1 (ISSUE NO. 117)|Papers Count:8

The final crystalline phase in poly (vinylidene flouride), PVDF, films mainly reflects the polymer structural arrangement or chain conformation during film formation. In this study, the intrinsic viscosity of PVDF solution or even the zero-shear-rate viscosity in polymer melt were used as indications of polymer hydrodynamic chain volume to predict the possible formation of all trans plannar zigzag conformation (b phase crystal). The studies on intrinsic viscosity of each diluted solution were carried out in Ubbelohde viscometer at temperature range of 25-100oC using two individual solvents with different polarities namely, dimethyl acetamide and cyclohexanone. In contrast to solvent polarity, the solution temperature showed negative effect in the degree of hydrodynamic chain volume. This behaviour maybe attributed to higher thermodynamic stability of gauche state rather than trans state in PVDF chains at higher crystallization temperatures. In this regard, through a temperature rise from 25oC to 100oC, the PVDF hydrodynamic chain volume experienced a reduction of about 38% and 25% in dimethyl acetamide and cyclohexanone, respectively. The further reduction of PVDF hydrodynamic chain volume in dimethyl acetamide solution may be referred to higher polymer chain mobility and reduced solvent polarity at higher temperatures. Using zero-shear rate viscosity, the increase in melt temperature showed reduction of about 87% in hydrodynamic chain volume. The temperature dependence of PVDF chain dimension in solution and melt state show a similar trend. These findings are in agreement with the previous results about the unhelpful effect of crystalization temperature in favor of b phase crystal formation in PVDF.

Based on specific applications of electrospinning technology in tissue engineering there is a need for more crucial advances to be made in electrospinning of biocompatible polymer systems. In this study we investigated the effect of surface tension of different solvent systems on the morphology of electrospun nanofibers of gelatin and PLGA. At first the best solvent system was established to optimize nanofiber formation of the polymers. The overall approach of this research was to prepare several solutions of identical concentrations with different solvent systems for electospinning into nanofibes. Finally, the changes in electrospun fiber morphology were studied by scanning electron microscopy (SEM) and the best solvent system was established in order to pave the way for further research on tissue engineering. It is observed that electrospinning of PLGA with lower surface energy solvent system leads to more homogeneous and smooth fibers. This result is similarly obtained for a system of natural gelatin. For ultimate confirmation of these results, it was observed that by changing the solvent system from high surface energy of DMF to lower surface energy of HFIP, the micro-structure of the prepared mat was changed from microspheres to smooth fibers.

Layer-by-layer self-assembly technique is a novel method in nanotechnology for modification of the surface properties of solid materials. Among these materials cellulosic fibers can undergo surface changes by formation of polyelectrolyte multi-layers (PEM) using layer-by-layer method. In this study, the effect of degree of substitution of cationic starch on multi-layer build up of cationic and anionic starches and on the bonding ability of the waste corrugated container (OCC) recycled fiber with layer-by-layer technique was investigated. Experiments were conducted at pH≈7.5 and 750 rpm stirring rate in a dynamic drainage jar (DDJ) for 10 min deposition time to construct 1 to 8 consecutive layers. The handsheets at 60 g/m2 on weight basis were made with untreated and layer-by-layer treated fibers and their strengths have been evaluated. The results showed that the interbonding ability indices of fibers such as tensile index and internal bonding improved significantly with multi-layering of cationic starch. However, there was no meaningful change in quality of the specimens formed. Compared to the untreated fibers, the tensile index of cationic starch increased by 129% at 7th layer with DS≈0.045 and internal bonding increased by 400% at the 5th layer. Moreover, with regard to different degrees of substitution of starch, the increasing trend in properties was observed; in which DS≈0.018 for lower number of layers and DS≈0.045 for higher number of layers. In the first case, more starch of lower DS was needed to neutralize the charge of the fiber surface but in the second case greater charge balance of two polymers resulted in further adsorption and greater stability of cationic starch layer, and consequently higher strength has been achieved for the obtained paper.

Although there are many works published on shape memory polyurethanes, but no report is found on PCL-based polyurethane composite using PCL with molecular weight of 2000 g/mol and nanoclay particles via in-situ polymerization. In this work, we try to approach some problems concerning the chemical structure, dispersion of nanoparticles in the matrix and interaction between nanoparticles and polymer in shape memory of PCL (Mn=2000 g/mol) -based polyurethane/nanoclay composites. While PCL (Mn=2000 g/mol) polyurethane constituted the soft segments, MDI/BD formed the hard segments. The weight ratio of soft to hard segments was found to be 65/35. Composites were synthesized in presence 1 and 3 wt% of nanoclay (Cloisite®30B) via in-situ polymerization by a twostep method. Effect of nano particles on their interactions with polyurethane chains, crystallinity, mechanical properties and shape memory have been investigated using FTIR, DSC, tensile test and shape memory analysis, respectively. Phase mixing and hydrogen bonding intensified with 1 wt% nanoclay particles, whereas 3 wt% nanoclay led to phase separation. XRD, MTDSC, AFM analyses showed exfoliation of samples with 1 wt% nanoclay in the matrix and better tensile strength in comparison with pure sample and higher shape recovery compared to all other samples. Crystallinity of soft segments resulted in higher modulus and improvement of shape memory. TGA results showed a three-step degradation mechanism for PCL, MDI and BDO-based polyurethane.

The effect of nanocaly on crystalline structure of poly (vinylidene fluoride), PVDF, and the morphology of the resulting nano-composite were investigated using different characterization techniques. The presence of 3wt% Cloisite 30B in PVDF matrix results in 11 fold increase in the percentage of beta crystalline content of the polymer. This was found to be attributed to the epitaxial effect of the clay surface. The beta crystalline content of the pure polymer (6%) was raised to 68% in the composite. Addition of 5wt% polyamide 6 (PA6) improved dispersion of nanoclay which led to augmentation of the viscosity and displacement of the crossover frequency of the compatibilized composite towards lower frequencies. However, due to stronger affinity of the PA6 towards organically modified clay the epitaxial effect of the clay on crystalline structure of PVDF was totally eliminated. The reduction of viscosity in incompatibilized nanocomposite was attributed to reduced number of PVDF chain entanglements in the presence of nanoclay. Meanwhile, increase in viscosity and displacement of crossover frequency towards lower frequencies were attributed to formation of clay-PA nanoparticles and PVDF-polyamide 6 interactions. It is expected that the presence of polyamide 6 promotes the formation of oriented-beta crystals in PVDF, which in turn improves the piezoelectric properties of the polymer.

The surface modulus and hardness of photo-crosslinkable cyanoacrylate bioadhesives containing TMPTMA and POSS as crosslinking agents were studied using nanoindentation technique. Adhesives containing 2-octyl cyanoacrylate (2-OCA) and different percentages of POSS nanostructures and TMPTMA as crosslinking agents were prepared.1-Phenyl-1, 2-propanedione (PPD) was incorporated as photo-initiator into the adhesive in 4 wt%. Mechanical properties of the surface of the adhesives were measured using nanoindentation technique applying a Berkovich tip. The results (three replicates) were analyzed and compared using one-way ANOVA and Tukey test at a significance level of 0.05. The results showed that by incorporation of crosslinking agents, surface modulus and hardness increased and force loss during holding decreased. The incorporation of crosslinking agents into the adhesives enhanced their elastic properties. The surface modulus and hardness increased by increasing crosslinking concentration. The results also indicated that the elastic work increased, and plastic work dropped due to increasing crosslinking density. The samples which had been stored in water for 24 h before the tests, showed reduced elastic properties due to the plasticization effect of water.

During the past two decades the use of bioplastics, as a suitable alternative to petroleum-based plastics, has attracted researchers' attention to a great extent. In this study, the whole wheat flour and straw cellulose at different proportions were mixed with glycerol and bioplastics sheets were obtained by a press type molding machine. The mechanical properties of samples were examined on compositions prepared by whole wheat weight in three proportions of 70, 60 and 50% and the cellulose in three proportions 75, 70 and 65%. The tensile tests on the samples indicated that with lowering proportions of both flour and cellulose, the modulus of elasticity and tensile strength of the bioplastics dropped as well. The maximum modulus of elasticity achieved for the flour and cellulose compositions were 12.5, and 8.6 MPa, and the maximum tensile strengths were 878 and 202 kPa, respectively. The TGA tests indicated that the bioplastics prepared from whole wheat flour showed higher temperatures of thermal degradation. The activation energies calculated for the flour and cellulose bioplastics, as estimated by Arrhenius type equation, were 133.0 and 63.8 kJ/mol, respectively.