دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و سوم شماره 3 (پیاپی 167، امرداد و شهریور 1399)

With serious energy crisis in recent years, phase change materials (PCMs) are among the best choice for thermal energy storage. These materials have attracted special attention in energy saving. PCMs absorb energy during the heating process accompanied by their phase transitions. This energy can be transferred into the surrounding environment during the cooling process. In other words, these compounds have the ability to store and release high amount of heat through phase transitions with a slight temperature variation. PCMs are classified into four groups of organic, inorganic, eutectic and polymeric materials. However, the major limitation of PCMs is their mixing problem with other materials. Encapsulation might be the best way for effective use of PCMs to increase heat transfer rate, preventing their leakage, reducing their interaction with the external media and limit volume changes during phase change. Encapsulation is a coating process of PCMs with a suitable material to protect solids, liquids or gases in a solid shell. Encapsulated materials are categorized into macrocapsules, microcapsules, and nanocapsules, based on the capsule size. Among these, microencapsulation is a successful method to develop PCMs application in construction, pharmaceutics, agriculture, and textile industries. Different types of microcapsules can be made with a wide range of shell materials by various physical, physico-chemical and chemical methods. Depending on the chemical nature, three types of shell materials are used for microcapsulation, including organic, inorganic, and organic/inorganic hybrids. The conventional polymeric compounds in the shell include polypropylene, polyethylene, polyurethane, polystyrene, polyamide, urea-formaldehyde, melamine-formaldehyde and acrylic resins. Due to the importance of encapsulation methods and versatile applications of encapsulated PCMs in various industries, the most important methods for encapsulation of PCMs are reviewed in this paper in preparation of stable PCMs by consideration of the role of polymers.

 Hydrophobic surface modifying macromolecules (SMM) have a main chain of polyurethane with two ends having a fluorine-based hydrophobic polymer. These macromolecules tend to migrate to the membrane surface during membrane fabrication process and alter the physical and chemical properties of the membrane surface by formation of a thin layer on the surface of the membrane. Therefore, they can increase the hydrophobicity of the membrane surface which is an important parameter in gas-liquid membrane contactor system.

Poly(vinylidene fluoride-co-chlorotrifluoroethlene), PVDF-CTFE, hollow fiber membranes were fabricated using surface modifying macromolecules through a dry-wet phase inversion method. Structure and characteristics of fabricated membranes were evaluated and they were used in carbon dioxide absorption and stripping processes in a gas-liquid membrane contactor system.

Water contact angle of outer surface of the fabricated membrane without SMM was measured as 90.51°. Addition of SMM into the membrane increased contact angle to 114.20°. Critical entry pressure of water of the membrane fabricated without/with SMM was measured as 7 and 10.50 bar, respectively. The CO2 absorption flux of 1.76×10-3 mol/m2.s and 9.70×10-4 mol/m2.s, at the liquid phase flow rate of 300 mL/min, was achieved for the fabricated membrane without/with SMM, respectively. The CO2 stripping flux, at the liquid flow rate of 200 mL/min, for the fabricated membrane was achieved at 1.30×10-3 mol/m2.s without SMM and 6.40×10-4 mol/m2.s with SMM. The maximum CO2 stripping efficiency, at the liquid flow rate of 200 mL/min, was achieved for the fabricated membrane at 72.10% without SMM and 42.60% with SMM.

 Nowadays, the growing concern over environmental pollution has attracted attentions towards development of biodegradable micro-particles to replace non-degradable micro-particles used in cosmetic products. In this regard, poly(lactic-co-glycolic acid) (PLGA) micro-particles produced by a simple technique, electrospraying, is used as biodegradable micro-particles. To obtain the maximum efficiency, especially as a drug delivery system, these particles need to be fiber-free with spherical shape and have a uniform size distribution. This goal can be achieved by controlling variables such as polymer molecular weight, solvent type, polymer solution concentration, voltage, needle size, polymer solution feed rate and distance from needle tip to collector.

PLGA particles were electrosprayed considering four variables, including polymer solution concentration (at three levels), needle size (at two levels), voltage (at three levels), and polymer solution feed rate (at three levels). Then, electron scanning microscopic images were obtained from each sample to observe the morphology of particles and to study the effect of process variables on it. To understand the morphological behavior of the particles, the influence of each variable was investigated according to mechanisms involved in electrospraying process.

The four variables affecting droplet size, and consequently, Peclet number and droplet charge play important roles in mechanisms involved in electrospraying process and determining the morphology of the resultant particles. Thanks to the effects of these phenomena, spherical and fiber-free micro-particles with uniform size were obtained by electrospraying PLGA solutions at concentrations of 3% and 5%, using a 27 G needle at a feed rate of 0.3 mL/h and applying a voltage of 10 kV.

Hypothesis: In this study, modification of polyvinyl chloride ultrafiltration membrane was done and the modified membrane was used in the ultrafiltration process for oil-in-water separation. at first, PVC was dehydrochlorinated by sodium hydroxide. Then, the resulting dehydrochlorinated polyvinyl chloride (DHPVC) was grafted by styrene monomer and in the final step the grafted styrene was sulfonated by sulfuric acid. Ultrafiltration membranes were fabricated from PVC, DHPVC, styrene-grafted dehydrochlorinated polyvinyl chloride and sulfonated polymer by phase separation method and were evaluated, using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and pure water permeation flux, determination of mean pore size, porosity and water contact angle tests. The presence of grafted functional groups was confirmed by FTIR spectra. SEM images showed that all the membranes had fingerlike pores and the difference in shape of the pores was due to the thermodynamic instability and viscosity of the polymer solutions. Due to the higher hydrophilicity of the polymer, the sulfonated membrane showed significant increase in pore size and pure water flux, compared to other membranes. The DHPVC membrane showed the lowest contact angle because of the detachment of chlorine as a hydrophobic agent, but for the other membranes there was little difference in their contact angles which could be related to the surface roughness. Subsequently, the oil/water separation experiments were performed by the membranes. The modified membranes showed good performance in the separation of oil and water and had less fouling than the PVC membrane. The rejection of oil particles was 100% for all membranes except the sulfonated membrane. The rejection of oil particles by the sulfonated membrane decreased, because of the higher pore size and significant increase in the flux.

 Graphene oxide (GO) modified with a double bond-containing chemical was used in the preparation of polystyrene composites by combination of “grafting through” and reversible addition-fragmentation chain-transfer (RAFT) polymerization methods. The main objectives were (1) application of two reaction sites for in situ grafting of polystyrene chains from the GO surface, (2) increasing of grafting density in the polymerization process by grafting through two different sites and (3) evaluation of styrene RAFT polymerization in two different grafting densities and various GO contents.

After modification of GO with ethylenediamine (EDA) using nucleophilic ring opening reaction that is resulted in amine-functionalized GO (GOA), coupling reaction was conducted between the GOA and (3-methacryloxypropyl) dimethylchlorosilane (MCS) to obtain GOOHD layers with different modifier contents. Then, grafting through RAFT polymerization method was utilized for preparation of polystyrene composites.

X-ray photoelectron spectroscopy shows successful modification of the GO layers with EDA and MCS. Molecular bonding characterization was performed by Fourier transform infrared spectroscopy for the neat and modified GO. Variations in electron conjugation were studied by Raman spectroscopy. Molecular weight and polydispersity index of the free and anchored polystyrene chains were calculated from the size exclusion chromatography. By using thermogravimetric analysis, thermal properties and grafting ratios of the modifiers and polystyrene chains were studied. Grafting ratio of the modifiers was 12.9% and 7.5% for the modified GO layers with high and low grafting densities, respectively. Morphology of the GO layers was investigated through transmission and scanning electron microscopy. Oxidation changed the flat and smooth graphite to wrinkled layers, and grafting polystyrene chains resulted in turbid layers.

The change in the wetting of rock from hydrophobic to hydrophilic is named "wettability alteration". This is an important factor for enhanced oil recovery (EOR). Because of their unique properties, nanoparticles have attracted much attention for enhanced oil recovery. Despite promising results, the main challenges of using nanoparticles are related to colloidal stability and poor absorption of nanofluids under harsh conditions. In recent years, polymer-grafted nanoparticles have been considered as emerging materials for enhanced oil recovery.

In this study, wettability and absorption of polymer-grafted nanoparticles including silica nanoparticles modified by polyethylene glycol methyl ether (mean molecular weight 2000), silica nanoparticles modified by two polymers: polyethylene glycol methyl ether (mean molecular weights 2000 and 5000) and propyl chains are investigated. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to investigate the chemical bonding and polymer content on the silica surface. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), water contact angle measurement, and UV-Vis spectroscopy were also used to study morphology, material composition, wettability, and absorption of the substrate, respectively.

Best performance for silica nanoparticles modified by polyethylene glycol methyl ether (average molecular weight 5000) and propyl chains at 1000 ppm concentration and salinity range 20000-40000 ppm was obtained. This study shows that silica nanoparticles bonded to different polymers can be considered as an effective and novel approach for enhanced oil recovery.