دو ماهنامه علوم و تکنولوژی پلیمر
سال سی‌ام شماره 6 (پیاپی 152، بهمن و اسفند 1396)

Hypothesis: Core-shell nano-fibrous structures obtained from biodegradable and biocompatible polymers such as silk fibroin (SF) have potential applications in drug delivery and tissue engineering. In this work, coaxial electrospinning of silk fibroin as shell and salicylic acid (SA)/polyvinyl alcohol (PVA) blends as core was studied to fabricate core-shell nano-fibrous structures.
Silk fibroin was extracted from cocoon and dissolved in formic acid. An assembled coaxial nozzle was used to fabricate core-shell nanofibers of SF as shell and PVA/salicylic acid as core components, respectively.
Findings: Effects of variation in viscosity and electrical conductivity of the electrospinning solutions on the final nanofibers morphology, diameters and SA release behaviors were studied using Fourier transforms infrared spectroscopy (FTIR), viscometry, electrical conductometry, ultraviolet spectrophotometry, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Finally, a suitable model for release behaviors of the fabricated core-shell nanofibers was suggested. It was found that the final diameter of fabricated core-shell nanofibers varied from 110 (±22) to 250 (±71) nm and diameter of the core section varied from 40 to 80 nm. The experimental results showed that the shell solution concentration had a significant effect on the final core-shell nanofiber diameter, but increase in the core solution concentration had an insignificant effect on the final nanofibers diameter. It was concluded that the core solution concentration was not the only effective parameter in determining the final diameters of electrospun nanofibers, and the viscosities of shell and core solutions and their electrical conductivities were just as important parameters. According to the results of release profiles, the ratio of the components in core and shell was the effective parameter affecting the profiles of SA released from these structures. The higher PVA concentration in the core increased both the amount of SA and its release rate from fabricated core-shell nanofibers.

Hypothesis: Substitution of carbon black by surface-modified silica in tire tread compound formulation often brings a lower friction coefficient and inadequate vehicle safety. Through modifying polymer-filler interactions, silane chain length is capable of altering viscoelastic properties. The connection between tribological properties and viscoelastic dissipation can be regarded as an important factor to control the frictional behavior of tire tread compounds. It has always been speculated that silane chain length dictates the properties of silane-treated silica filled rubbers through two possible reinforcing mechanisms namely: entropic interaction and/or mechanical engagement. In this contribution, the existence and severity of each mechanism is realized by excluding mechanical contributions of reinforcement from that of entropic interactions by surface-energy theories.
Two aliphatic silanes of short (trimethoxy(propyl)silane) and long chain (hexadecyl trimethoxysilane) with spacer length of 3 and 16 carbons are grafted onto the silica surface. The surface energy of the resulting powders is controlled by controlling the density of silane grafting. The rubber matrix constitutes a solution styrene butadiene rubber (S-SBR) and the compounds have been prepared by means of an internal mixer and a two-roll mill. The surface characteristics of silica as well as the morphological, mechanical and tribological properties of the resulting rubber composites are characterized and compared with a conventional bi-functional silane commonly used for this tread compound.
Findings: For systems in which the surface energy and thus energetic contributions of polymer-filler interaction are controlled to be equivalent for both short- and long-chain silane treated silica, no variations either in dynamic or tribological properties are detected, indicating that no mechanical engagement associated with interlocking of long chain silanes is available. At the same time, entropic interactions played a significant role in final dynamic and tribological properties of the composites. It is also observed that friction coefficient is correlated with loss modulus of the compound better than the loss factor.

Hypothesis: After cellulose, lignin is the most abundant natural polymer in the world. The chemical modification of lignin is the best way to improve its performance in the synthesis of the chemicals and polymeric materials. Among monomers employed in modification of the lignin, acrylamide (AAm) increases the highest yield in graft copolymerization reaction.
A native lignin was extracted from its black liquor derived from paper-making processes by precipitation with HCl, and then purified by dissolving in tetrahydrofuran (THF). Chemical modification of the lignin with acrylamide monomer was investigated in the presence of different initiators including hydrogen peroxide (H2O2)/calcium chloride (CaCl2), H2O2/Fe(II) chloride (FeCl2), ammonium persulfate (APS), potassium persulfate (KPS), KPS/ammonium iron(II) sulfate hexahydrate (AFS.6H2O) and ceric ammonium nitrate (CAN)/HNO3. The graft copolymers were then characterized using Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (1HNMR) spectroscopy methods.
Findings: The highest and lowest conversions of AAm monomer in the presence of different initiators used in synthesizing the lignin graft copolymers were observed for the APS and KPS systems to be 94.32% and 77.83%, respectively. Among all initiators used in the present study, only H2O2/CaCl2 redox system led to a 100% grafting percentage at 30°C without any free homopolymer chain, while in the presence of other systems, free homopolymer chains were also formed in addition to the grafted chains. It was found from the results that, the graft polymerization did not proceed in the presence of CAN/HNO3 system.

Hypothesis: Thermal insulating materials are essential in optimization of energy consumption and to reduce heat and energy loss. Lightweight thermal insulator materials can reduce weight/density and improve the performance of final product. One of the ways to approach lightweight thermal insulator is to develop porous polymeric nanocomposites with low density and good thermal insulation properties. Cork is a cellulosic material with microcells that are widely used in lightweight thermal insulating applications. Porous structure in cellulosic cork allows it to be chosen as an adequate thermal insulator, especially in aerospace applications. Since cork has low density and very low thermal conductivity, many research works are conducted to reduce its thermal conductivity and improving its thermal stability.
In order to improve the thermal insulation performance and ablation of silicone/cork composite, a novolac aerogel nanostructure was used. Novolac aerogel had a nanoporous structure with very low density, thermal conductivity and thermal diffusivity. The presence of novolac aerogel in the microcell structure of cock and filling its porous spaces led to higher density of the cork, eliminated the air thermal convection process in its microcells, and it therefore decreased the thermal conductivity and thermal diffusivity of the composites, significantly.
Finding: The mechanism of heat transfer elimination of novolac aerogel by convection could decrease the thermal conductivity and thermal diffusivity of silicone/cork composites by 39% and 45%, respectively, due to pore size reduction. Also, the aerogel could increase thermal stability and thermal resistance and the residual char with adequate thermal stability. Moreover, a resole/graphene oxide coating layer on the composites surface could significantly improve the composites thermal ablation. Under these conditions, the back surface temperature of composite in the presence of aerogel nanostructure decreased by 55%.

Hypothesis: The influence of hydrophobic nanocellulose on phase separation behavior of off-critical PS/PVME (polystyrene/polyvinyl methyl ether) blends was studied. While the effect of spherical nanoparticles (NPs) on the phase behavior of polymer blends has been previously explored, the impact of rod-like NPs on the phase behavior has not been well studied. Compared to nanospheres, nanorods are associated with much lower critical percolation concentration, due to the high aspect ratio of nanorods.
For this purpose, neat PS/PVME blends with compositions of 40/60 and 10/90 and in the presence of 2% nanocellulose were investigated. The temperature sweep of storage modulus, from the one-phase region temperature to those higher than the two-phase region temperature, was used to investigate the effect of nanoparticles on phase separation temperature. Phase-contrast optical microscopy (OM) was employed to investigate the morphological evolution of PS/PVME blends during the phase separation. TEM images indicated the localization of hydrophobic nanocellulose in the PS-rich phase which was consistent with the prediction of thermodynamic wetting parameter.
Findings: Viscoelastic phase separation (VPS) controlled the phase behavior of PS/PVME 10/90 blend which in the presence of nanoparticles increased the stability of the PS-rich network structure even at high temperatures. The PS/PVME 40/60 blend was phase separated under the nucleation and growth mechanism (NG), and there was a wide distribution of droplets size in the late stage of phase separation. With increasing the quench depth, the dynamic asymmetry increased, leading to transition of the phase separation mechanism from NG to VPS. The addition of nanoparticles enhanced the dynamic asymmetry which induced the VPS at lower temperatures.

Hypothesis: A two-component methylmethacrylate (MMA) traffic paint, used in cold plastic road marking, is considered as a vital part of the safety infrastructure of the road networks and a tool for delineation and traffic control. This system must be well-maintained during its service life. The aim of this study was to evaluate the effect of benzoyl peroxide (BPO) curing agent content on the physical and mechanical properties of MMA traffic paints.
In order to optimize the amount of BPO, samples of two-component traffic paint were prepared and cured with 0.4, 0.8, 1.0, 1.2, 1.4, 1.6 and 2.0 wt% BPO. Different physical and mechanical properties of the samples, such as traffic no pick-up time, hardness, resistance against accelerated weathering conditions were evaluated and their mechanical properties were assessed using stress-strain analysis (SSA) and dynamic mechanical thermal analysis (DMTA).
Findings: The results showed that the amount of BPO was a critical parameter in physical mechanical performance of the paints, in such a way that the low and high levels of curing agent (0.4 wt% and 2.0 wt%, respectively) led to loss of paint properties. This could be related to insufficient curing agent (0.4 wt%) and its overdose (2.0 wt%) in the polymerization of methylmethacrylate. Increasing the curing agent to its optimal amount did not increase the polymer network density, and in some cases impaired the mechanical properties of the samples. The results also showed that the increase in peroxide content reduced the drying time (no pick-up time). Within the optimal range of BPO content level at 1.2 and 1.6 wt%, we achieved traffic paints of two-component formulations with optimum physical and mechanical properties.

Hypothesis: In order to achieve safe and high-quality food products, the use of suitable packaging materials with excellent physical and chemical properties is a key requirement. Pollution resulting from packaging materials made of oil-based plastics and the problems associated with burning, disposal and recycling of these plastic products have attracted the attention of researchers to find appropriate solutions in recent years. Carboxymethyl cellulose (CMC) is one of the important polysaccharide polymers with capability of producing transparent films with relatively good mechanical and inhibition properties that have been used broadly in studies concerning the food stuff packaging.
Nanocomposite films have been prepared by solution casting method in the presence of clay nanoparticles. Pure and modified montmorillonite noanoparticles and Cloisite 30B along with silver and copper were used for improving the functional properties of carboxymethyl cellulose nanobiocomposite films.
Findings: It was observed that the clay nanoparticles incorporated into the nanocomposite films increased the UV absorption and mechanical properties and reduced the vapor permeability of the films. The XRD results showed that the silver was successfully inserted into the gallery space of the nanoclay, because the basal spacing of Ag-modified Cloisite 30B increased from 1.841 nm to 1.855 nm. Also, the compatibility of the nanoparticles with carboxymethyl cellulose was examined by SEM images. The SEM micrographs showed that the Cloisite 30B nanoparticles displayed better interface compatibility with CMC films than Na-montmorillonite. The results of antimicrobial tests showed that the nanobiocomposite film containing
4 wt% of Ag-modified Cloisite 30B exhibited maximum antimicrobial property against Staphylococcus aureus and Escherichia coli bacteria.