دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و یکم شماره 3 (پیاپی 155، امرداد و شهریور 1397)

During the last decades, an increasing attention has been paid to pharmaceutical and biomedical applications of queous polymeric solutions which respond in accordance to the changes in their environmental conditions i.e., stimuli by turning into in situ forming hydrogels. Of all the stimuli-responsive hydrogels, temperature-responsive solutions have been widely investigated due to their simplicity, applicability and relatively high frequency of temperature-responsiveness, in situ gelling polymeric (natural or synthetic) systems. In contrast with the conventional hydrogels, in situ forming temperature-responsive hydrogels can form under physiological conditions and preserve their morphological integrity for a definite time course. Using the materials makes it easier to formulate pharmaceutical formulations by mixing polymer and drug, and also improve the dissolution of hydrophobic drugs with low molecular weight. Due to the simplicity of the pharmaceutical formulation by simple solution mixing, biocompatibility and convenient usage these materials can be used in biomedical and pharmaceutical fields for tissue engineering, solubilizing of sparingly soluble drug molecules, controlled delivery of drugs and biomacromolecules, such as proteins and genes. In this review, temperature-responsive hydrogels are studied regarding their classification, applications and thermodynamics. Moreover, temperature-responsiveness mechanisms, polymeric gels, recent advances in surface, hydrogel and molecular design and biomedical application are investigated. Also, this review focuses on recent investigation based on the designs of temperature-responsive micelles and intelligent bioconjugates. Finally, limitations and potentials of applications of the temperature-responsive in situ forming hydrogels have been reported. The reported information in this paper are necessary to design and develop a desirable temperature-responsive hydrogels with different characteristics and applications.

Hypothesis: Dispersion and stabilization of silica nanoparticles are novel approach in the synthesis of hybrid materials in interacting with cement particles. Branched polymers are able to enhance the dispersion and stability of these nanoparticles. Dispersed nanosilica and branched polymers have direct effects on the microstructure of cement. These materials have wide applications in concrete technology. Methods: Various branched polymers were synthesized through in situ radical polymerization of acrylic acid, maleic anhydride and polyethylene glycol methyl ether methacrylate (Mn=950 g/mol) in presence of nanosilica with different contents.
Findings: The molecular structures of the synthesized branched polymers were characterized by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC) methods. Thermogravimetric analysis was used to identify the nature of interactions between the branched polymers and nanoparticles. The dispersion state of nanosilica particles within branched polymer solution was studied by dynamic light scattering (DLS) and scanning electron microscopy (SEM) analysis. The results showed a low dispersion in acidic conditions due to agglomeration of nanosilica in the cement. Furthermore, after 30 min sonication, in a neutralized condition, the nanoparticles were dispersed well. In addition, the dispersibility of nanosilica dropped with increases in nanosilica loading. The SEM images showed good dispersion of nanoparticles in the cement medium. It was also demonstrated that the branched polymer had a great influence on the morphology of crystal structure formation in the hydrated cement. The SEM images revealed the best distribution of nanoparticles with 4.7 weight percent nanosilica in the presence of neutralized branching polymers under sonication. From the EDAX results it was also found that the nanosilica particles and branched polymers were dispersed well in the cement medium.

Hypothesis: Among available bioplastics, polylactide exhibits superior properties, including high modulus, good processability, and compostability. However, some disadvantages, such as brittleness and low thermal resistance limit its application in some fields. In order to overcome the limitations of polylactide, it is usually modified by addition of nanoparticles or by blending with other thermoplastics. Polylactide/polyethylene (PLA/PE)-based nanocomposites were prepared using 4 phr commercially modified montmorillonite (Cloisite 30B or Cloisite 20A) and polyethylene-g-maleic anhydride compatibilizer (PE-g-MA) by melt blending technique. The structure and morphology development and also the cold crystallization kinetics of the samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), differential scanning calorimetry (DSC) techniques and melt linear and non-linear viscoelastic measurements. Findings: The XRD results showed that polymer intercalation into the clay galleries in PLA/PE/Cloisite 30B was higher than that in PLA/PE/Cloisite 20A. The rheological results along with the calculated data of wetting parameter showed that Cloisite 20A in blend nanocomposite could reduce the droplet size through three different mechanisms a: localization of the organoclay in the interface b: increasing the viscosity of PLA matrix and c: decreasing the extent of coalescence process. The FE-SEM micrographs showed that the nodular morphology of PLA/PE/Cloisite 30B changed to a non-nodular morphology in PLA/PE/Cloisite20A. The melt linear and non-linear viscoelastic measurements showed that a stronger 3D-network structure was formed in PLA/PE/Cloisite 20A compared to that in PLA/PE/Cloisite 30B. It was implied that the crystallization rate followed the Avrami equation with the exponent n of around 2. The results also showed that the addition of compatibilizer (PE-g-MA) into PLA/PE/Cloisite 20A or PLA/PE/Cloisite30B decreased modified crystallization rate constant (Zc), because the cold crystallization rates of compatibilized blend nanocomposites were lower than those of uncompatibilized blend nanocomposites and the role of compatibilizer in the transfer of partial organoclays from PLA matrix into the droplets.

Hypothesis: Worm-like micelles triggered by carbon dioxide (CO2), as an abundant, inert, and green stimulus have recently attracted much interest. These materials have many potential applications, including heat transfer, rheological control, personal protection and enhanced oil recovery (EOR). An ideal CO2-responsive worm-like micelle reveals a reversible transition state (from sol to gel state and vice versa) in response to environmental changes. The most important feature of these systems during these transitions is that CO2 does not accumulate in the system upon repeated cycles. Herein, we prepared two types of materials based on 3-(dimethylamino)-1-propylamine sodium dodecyl sulfate (DMAPA-SDS) as a small molecule, and poly(2-(dimethylamino)ethyl methacrylate-b-polymethyl mthacrylate)-SDS [(PDMAEMA-b-PMMA)-SDS] as a macromolecule to examine possible formation of CO2-responsive worm-like micelles. Amine groups in the structure of DMAPA and PDMAEMA-b-PMMA can be protonated and ionized to quaternary ammonium salts by CO2 bubbling and interact with SDS to possibly form a worm-like micelle through non-covalent electrostatic attraction. The viscosity and structural features of aqueous solutions were evaluated before and after being exposed to CO2 by rheometry and 1H NMR, respectively. The rheometry results showed shear thinning and gel-like behaviors at high shear rates and frequencies, respectively. Findings: The results showed that for a DMAPA-SDS small molecule an ideal reversible CO2-responsive worm-like micelle was formed and a sol-to-gel transition was observed, whereas in using a macromolecule an irreversible agglomeration occurred. The absence of reversible sol-gel transitions and the presence of heavy agglomeration for the (PDMAEMA-b-PMMA)-SDS macromolecule was attributed to entanglements of its long polymer chains. Therefore, DMAPA-SDS as small molecule with its ideal CO2-responsive worm-like micelle has potential in different useful applications, particularly in EOR.

Hypothesis: Electrospinning of nanoparticles/polymer solutions offers the potential to achieve novel composite nanofibers in a variety of high performance applications. In this respect, the aspect ratio and purity of multi-walled carbon nanotubes (MWCNTs) were examined to study the morphological and electrical properties of the electrospun composite nanofibers. In the first step, MWCNTs samples were characterized using scanning electron microscopy (SEM) and Raman spectroscopy. Next, nanocomposite solutions containing MWCNTs were prepared by physical dispersion method in presence of a non-ionic surfactant. Finally, the prepared composite solutions were loaded to a glass syringe and connected to a DC power supply device. The electrospinning was performed by applying a high voltage of 15kV between the needle and the rotating collector. Findings: It is generally accepted that a combination of G-band intensity and G/D ratio is useful to evaluate the purity and crystallinity of MWCNTs. The morphological properties of the electrospun composite nanofibers showed their high dependency on carbon nanotubes dimensions, so that at higher aspect ratios the morphological properties generally improved. The morphological analysis of the composite nanofibers revealed that the deformation of the nanofibers increased with increasing MWCNTs diameter. Very smooth surfaces of the composite electrospun nanofibers even with 1 wt% MWCNT concentration were successfully achieved because of the high stability of MWCNT dispersions. The composite nanofibers with higher purity and aspect ratio presented better electrical conductivity. The electrical conductivity of electrospun composite nanofibers could be adjusted from ~10–8 S/cm (insulator) to ~10–3 S/cm (conductor) by different aspect ratios and purities of MWCNTs. MWCNTs with high aspect ratio and high purity offer a promising way to develop electrospun composite nanofibers with improved morphological and electrical properties.

Hypothesis: Various properties of NBR depend extremely on acrylonitrile (ACN) content of rubber formulation. One of the regular blends of NBR is NBR/PVC. Moreover, as it is widely known nanographene, due to its high specific surface area, is a performance material for producing polymer nanocomposites and for improving thermal and mechanical properties. To study the effect of nanographene on curing, tensile and stress relaxation properties of nanocomposites is interesting and important in practical terms and quality. The hypothesis of this research is to demonstrate the effect of temperature on the physico-mechanical of nanocomposites filled with various amounts of nanographene. NBR/PVC/nanographene nanocomposites were prepared using NBR/PVC blends with a ratio of 70/30 and acrylonitrile butadiene rubber at acrylonitrile contents of 33% and 45%, incorporated with 0.5, 1.0 and 1.5 phr of nanographene by melt intercalation method using a two roll mill. PVC of 30 phr was mixed with the above compounding formulations in an internal mixer. The TEM images, tensile properties and RPA results (curing and stress relaxation behavior) were obtained and compared. The effect of temperature (25, 50, 75°C) on tensile properties was also studied. Findings: Nanographene increased the maximum curing torque, scorch time and curing time. Samples containing higher ACN percentage, showed higher maximum curing torque, scorch time and optimum cure time. In the stress relaxation test, it was observed that the increase in the amount of nanographene increased the initial modulus and reduced the final modulus of the nanocomposites in the uncured and cured states. In formulations with high CAN content, the initial and final moduli showed higher values. By increasing the nanographene content and formulations with higher CAN content, higher elastic and viscous slopes were observed. The tensile properties of nanocomposites, including tensile strength, elongation and modulus were increased by higher amount of nanographene. In addition, the tensile properties of nanocomposites at three different temperatures of 25, 50 and 75°C were investigated and compared. With increasing nanographene concentration and rising temperature, samples showed a much lower reduction in tensile strength and Young's modulus than those in unfilled sample.