Benzene physical and chemical organogels: Effect of network scaffolding on the thermodynamic behavior of entrapped solvent molecules

This paper presents a thermodynamic investigation of the benzene physical and chemical organogels, using differential scanning calorimetry (DSC) and intends to draw an appropriate relationship between the gel network structure and the properties. Physical gels, formed by an aluminium soap of fatty acid, and chemical gels, created by in situ cross‐linking of a siloxane copolymer are investigated. The effects of the type and quantity of the gelators and their corresponding network mesh size distribution in the gels on crystallization, melting, and their kinetics are examined. It appears that the kinetics of crystallization of the entrapped solvent is significantly affected by the quality of the gel network scaffolding and can be treated successfully by the Avrami equation of crystallization. From the melting behavior of the entrapped solvent crystallites, quantitative information about the number of solvent molecules bound per molecule of the gelator has been extracted. DSC proves to be a reliable technique to evaluate the population distribution of solvent molecules trapped in the physical and chemical organogel network scaffolding. The state of the solvent may be treated as a probe to understand the structure of the gels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1253–1264, 2004     

Sol-Gel transition and crystallization kinetics of ultra-high molecular weight polyethylene/decalin solution

The gel melting temperature and crystallization kinetics of ultra-high molecular weight polyethylene (UHMW-PE)/decalin systems were investigated. Two methods were used to determine gel melting temperature; thermomechanical analysis (TMA) with a penetration probe and differential scanning calorimetry (DSC). The melting points determined from TMA and DSC were in good agreement, indicating that the crystallization of UHME-PE is an essential step for gelation. The gel melting temperature increases with UHMW-PE concentration. The change in gel melting temperature with composition results from interaction of the components in the amorphous phase. The gelation and crystallization rates increase with decreasing UHMW-PE concentration.     

Crystallization kinetics of poly(1,4‐butylene‐co‐ethylene terephthalate)

The crystallization kinetics of poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and their copolymers poly(1,4‐butylene‐co‐ethylene terephthalate) (PBET) containing 70/30, 65/35 and 60/40 molar ratios of 1,4‐butanediol/ethylene glycol were investigated using differential scanning calorimetry (DSC) at crystallization temperatures (T_c) which were 35–90 °C below equilibrium melting temperature . Although these copolymers contain both monomers in high proportion, DSC data revealed for copolymer crystallization behaviour. The reason for such copolymers being able to crystallize could be due to the similar chemical structures of 1,4‐butanediol and ethylene glycol. DSC results for isothermal crystallization revealed that random copolymers had a lower degree of crystallinity and lower crystallite growth rate than those of homopolymers. DSC heating scans, after completion of isothermal crystallization, showed triple melting endotherms for all these polyesters, similar to those of other polymers as reported in the literature. The crystallization isotherms followed the Avrami equation with an exponent n of 2–2.5 for PET and 2.5–3.0 for PBT and PBETs. Analyses of the Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT and PET had higher growth rate constant G_o, and nucleation constant K_g than those of PBET copolymers. © 2001 Society of Chemical Industry     

Functional Characteristics of Oleogel Prepared from Sunflower Oil with β-Sitosterol and Stearic Acid

β‐Sitosterol (Sit) and stearic acid (SA) were combined at varying ratios (w/w) and added to sunflower oil (SFO) at the concentration of 20 g/100 g oil for preparing edible fat‐like oleogel. The oleogel was characterized using an optical microscope, Fourier transform infrared spectroscopy (FTIR), X‐ray diffractometer (XRD), differential scanning calorimeter, and texture analyzer. The oil‐binding capacity, melting temperature, and firmness of the oleogel increased with the increase in the amount of SA in the gelator combination (Sit:SA, w/w). The microscopic analysis showed that the gel network formed based on the crystallization and self‐organization of gelator molecules, and both gelators showed an independent crystalline behavior in the oleogel. In addition, the FTIR spectra showed that the gel network formed via physical entanglements and was stabilized by non‐covalent interactions such as hydrogen bonding. Furthermore, XRD diffraction patterns indicated high lateral packing of molecular layers in oleogel prepared with the Sit and SA combination compared with oleogel prepared with a single gelator. On the other hand, for studying the effect of varying concentrations of gelator combinations, the Sit3:SA2 (w/w) combination was added to SFO at concentrations of 10, 15, 20, 25, and 30 g/100 g oil. Specific characteristics such as the oil‐binding capacity and firmness of the oleogel improved as the concentration of the gelator combination (Sit3:SA2) increased from 10 up to 30 g/100 g oil. Therefore, it can be concluded that the saturated fat alternative oleogel can be prepared from SFO with a specific Sit and SA combination ratio and concentration.     

Poly(ethyene terephthalate)–solvent interaction: Gelation and melting point depression

The interactions between poly(ethylene terephthalate) (PET) and six high temperature solvents are discussed in terms of gelation and melting temperature depression. The six solvents are 1′-acetonaphthone (AN), phenyl ether (PE), biphenyl (BP), 1-methyl naphthalene (MN), nitrobenzene (NB), and a eutectic mixture of phenyl ether and biphenyl (EU). Although the six solvents have very similar solubility parameter values, the dissolution, gelation, and gel melting temperatures of the PET-solvent systems are vastly different. The characteristic transition temperatures (dissolution, gelation, and gel melting temperatures) of the six solvents decrease in the following order: PE > EU > BP > MN > AN > NB, which is the reverse order of the solvent power. While the transition temperatures of the gel vary with the solvent system, the melting temperature of the dry gel formed from quiescent solution is independent of solvent system. That is, PET-solvent interactions are only discernible in solvated state (wet gel). All the experimental results suggest crystallization is the primary cause of gelation of high temperature PET solutions, with crystals acting as junction points in the network. Based on the dissolution and gel melting temperatures, interaction parameters for the six PET-solvent systems have been calculated.     

Crystallization kinetics and melting behaviour of microbial poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate)

Isothermal and non‐isothermal crystallization kinetics of microbial poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) [P(3HB‐3HHx)] was investigated by differential scanning calorimetry (DSC) and ¹³C solid‐state nuclear magnetic resonance (NMR). Avrami analysis was performed to obtain the kinetic parameters of primary crystallization. The results showed that the Avrami equation was suitable for describing the isothermal and non‐isothermal crystallization processes of P(3HB‐3HHx). The equilibrium melting temperature of P(3HB‐3HHx) and its nucleation constant of crystal growth kinetics, which were obtained by using the Hoffman–Weeks equation and the Lauritzen–Hoffmann model, were, respectively, 121.8 °C and 2.87 × 10⁵ K² when using the empirical ‘universal’ values of U* = 1500 cal mol^?1. During the heating process, the melting behaviour of P(3HB‐3HHx) for both isothermal and non‐isothermal crystallization showed multiple melting peaks, which was the result of melting recrystallization. The lower melting peak resulted from the melting of crystals formed during the corresponding crystallization process, while the higher melting peak resulted from the recrystallization that took place during the heating process. Copyright © 2005 Society of Chemical Industry     

Gelation of methylcellulose hydrogels under isothermal conditions

The current work deals with the gelation of methylcellulose (MC) in aqueous solutions under isothermal conditions. The isothermal gelation was monitored during two consecutive heating and cooling cycles. The gelation was observed to occur early during the second cycle and progress at a higher rate. Micelle‐like structures were found to form in MC solutions during the first gelation cycle. These were largely responsible for the accelerated gelation during the second heating cycle. The influence from the state of water was examined by studying the gelation phenomenon for MC using either cold or hot DI water as solvent. The possible mechanism involved is discussed. A gel indexing method was established to provide a quantitative measure for the state of gelation achievable using different MC concentrations with either cold or hot water solvent. Stabilization kinetics for the gel under isothermal conditions was described using the Malkin and Kulichikhin model. The kinetics parameters were determined. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Polybenzimidazole gel membrane for the use in fuel cell

Thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA) is investigated by studying the gel morphology, thermodynamics of the gelation, and gelation kinetics utilizing test tube tilting and UV-Vis spectroscopy techniques. Gelation kinetics studies reveal that both the gelation rate and critical gelation concentration (C^∗_t=∞) are function of gelation temperature (T_gel) and the molecular weight of PBI. Highly dense fibrillar network morphology with large number of longer and thinner fibrils is obtained for higher gel concentration and higher molecular weight PBI. Both the gel melting (T_gm) and gelation (T_gel) temperature depend upon the gelation concentration and molecular weight of PBI. The presence of self-assembled chains of PA molecules, which help to produce the PBI crystallites, is observed from the thermodynamical study. I.R. and Raman studies prove the presence of strong hydrogen bonding interaction between the PBI and the PA molecules, and the free PA molecules in the gel network. The gelation occurs in two-step processes which include a slow rate determining conformational transition from coil to rod and followed by aggregation of rod via crystallization. The PA loading of PBI membrane obtained from the PBI-PA gel is significantly high compared to the conventional imbibing process membrane. The PBI gel membrane displays very high thermal and mechanical stabilities. The high acid loading and superb thermo-mechanical stability are due to the gel network structure of the membrane. The proton conductivity of the membrane at 160 °C and 0% relative humidity (RH) is ∼0.1 S cm⁻¹, which is higher than the reported values in the literature for the PBI. The activation energy of the proton conduction is 14-15 kJ/mol indicating faster proton transfer by hopping process inside the gel network.     

Crystallization kinetics and affecting parameters on polycaprolactone composites with inorganic and organic additives

The isothermal crystallization and mechanical behavior of biodegradable polycaprolactone (PCL) composites with organic (oleic acid and glycerol monooleate) and inorganic (zinc oxide, organoclay, and hydroxy apatite) additives used alone or simultaneously were investigated. The effect of all additives on the degree of crystallinity percentage (DOC%), isothermal crystallization kinetics parameters, and mechanical test results of PCL composites was studied. The PCL composite films were prepared by solvent casting by using dichloromethane as the solvent. The films were characterized by X‐ray diffraction, differential scanning calorimetry (DSC), and tensile tests. DSC of the first melting and X‐ray diffraction DOC% results (for composites by solvent casting) are compatible. The values by DSC of the second melting (for composites by extrusion method) are lower. Organoclay gives the highest crystallinity among the other inorganic additives used. Small amounts of inorganic additives act as a nucleating agent and increase the crystallinity; the higher amounts decrease. The organic additives act as the plasticizer. When used alone, it lowers the crystallinity, but when used with inorganic additives, it improves the dispersion of inorganic particles in the polymer matrix. The isothermal crystallization kinetics parameters by Avrami analysis showed that crystallization was controlled by nucleation and the crystals had spherical structure. The nucleation type changed between thermal and athermal nucleation. The Pukanzky model interaction parameter B indicated that the organic additives improved the dispersion of inorganic particles in the polymer matrix. Statistically significant, eight correlations (F > 6) were obtained for the crystallinity, crystallization parameters, Young's modulus, and tensile strength as a function of concentration of additives. J. VINYL ADDIT. TECHNOL., 21:174–182, 2015. © 2014 Society of Plastics Engineers     

Erythritol‐Based Medium‐Chain Sugar Amphiphile: Synthesis and Gelling Capability in Edible Oils

The production of organogels from vegetable oils requires low‐molecular‐weight gelators. Herein, a novel small‐molecule sugar ester gelator is synthesized from erythritol and octanoic acid. The purified reaction product is identified as erythritol 1,6‐dioctanoate by high performance liquid chromatography (HPLC) with evaporative light‐scattering detection and nuclear magnetic resonance (NMR). This low‐molecular‐weight erythritol ester exhibits self‐assembly behavior in vegetable oils with a minimum gelation concentration of 4 wt%. Microscopy reveals needle‐like crystals that became slender as the gelator concentration increases. X‐Ray Diffraction (XRD) analyses indicate that the organogels are semicrystalline with both crystalline and amorphous domains. Fourier Transform Infrared (FTIR) spectroscopy reveals that the gel structure is maintained by non‐hydrogen‐bonding interactions. Furthermore, rheological tests show that the mechanical strength of the gel is poor, but as the gelator concentration is increased, the mechanical strength and hardness of the gel also increases. Thus, this erythritol ester effectively promotes the gelation of edible vegetable oils and increases the variety of available organogelators. Practical Applications: The synthesized erythritol ester is successfully applied as a gelator to produce organogels from a variety of vegetable oils, and are expected to advance the development of effective organogelators for edible oils.     

Isothermal crystallization kinetics and melting behavior of modified high‐density polyethylene/barium sulfate nanocomposites

The melting/crystallization behavior and isothermal crystallization kinetics of high‐density polyethylene (HDPE)/barium sulfate (BaSO₄) nanocomposites were studied with differential scanning calorimetry (DSC). The isothermal crystallization kinetics of the neat HDPE and nanocomposites was described with the Avrami equation. For neat HDPE and HDPE/BaSO₄ nanocomposites, the values of n ranges from 1.7 to 2.0. Values of the Avrami exponent indicated that crystallization nucleation of the nanocomposites is two‐dimensional diffusion‐controlled crystal growth. The multiple melting behaviors were found on DSC scan after isothermal crystallization process. The multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae caused during different crystallization processes. Adding the BaSO₄ nanoparticles increased the equilibrium melting temperature of HDPE in the nanocomposites. Surface free energy of HDPE chain folding for crystallization of HDPE/BaSO₄ nanocomposites was lower than that of neat HDPE, confirming the heterogeneous nucleation effect of BaSO₄. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Preparation and Characterization of JP‐10 Gel Propellants with Tris‐Urea Low‐Molecular Mass Gelators

The preparation and characterization of high‐energy density hydrocarbon fuel JP‐10 supramolecular gel propellants are reported. The low‐molecular mass gelator 1,1′,1′′‐((2,4,6‐trioxo‐1,3,5‐triazinane‐1,3,5‐triyl)tris(hexane‐6,1‐diyl))tris(3‐octadecylurea) (HDIT‐18) shows powerful gelation ability for JP‐10, the critical gelator concentration for JP‐10 is as low as 0.0638 wt‐%. The JP‐10 supramolecular gel propellants exhibit high thermal stability. Scanning electron microscopy (SEM) studies reveal that the gelator molecules self‐assemble into 1–3 μm fibers in JP‐10. Rheological studies show the JP‐10 supramolecular gels are thixotropic and shear‐thinning (for a shear rate range from 0.3 to 30 s⁻¹), the dynamic strain sweep test shows that the critical strain percentage of the gel materials is 1 %. The studies reported herein provide a potential fuel material for gel propellants.     

Crystallization kinetics and melting behavior of poly[(trimethylene terephthalate)‐co‐(29 mol % ethylene terephthalate)] copolyester

The copolyester was characterized as having 71 mol % trimethylene terephthalate units and 29 mol % ethylene terephthalate units in a random sequence according to the NMR spectra. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 130 to 170°C. The melting behavior after isothermal crystallization was studied using DSC and temperature‐modulated DSC by varying the T_c, the crystallization time, and the heating rate. The DSC thermograms and wide‐angle X‐ray diffraction patterns reveal that the complex melting behavior involves melting‐recrystallization‐remelting and different lamellar crystals. As the T_c increases, the contribution of recrystallization gradually falls and finally disappears. A Hoffman‐Weeks linear plot yields an equilibrium melting temperature of 198.7°C. The kinetic analysis of the growth rates of spherulites and the change in the morphology from regular to banded spherulites indicate that a regime II→III transition occurs at 148°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Crystallization of 2‐methyl‐1,3‐propanediol substituted poly(ethylene terephthalate). I. Thermal behavior and isothermal crystallization

Differential scanning calorimetry (DSC) was used to evaluate the thermal behavior and isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers containing 2‐methyl‐1,3‐propanediol as a comonomer unit. The addition of comonomer reduces the melting temperature and decreases the range between the glass transition and melting point. The rate of crystallization is also decreased with the addition of this comonomer. In this case it appears that the more flexible glycol group does not significantly increase crystallization rates by promoting chain folding during crystallization, as has been suggested for some other glycol‐modified PET copolyesters. The melting behavior following isothermal crystallization was examined using a Hoffman–Weeks approach, showing very good linearity for all copolymers tested, and predicted an equilibrium melting temperature (T_m⁰) of 280.0°C for PET homopolymer, in agreement with literature values. The remaining copolymers showed a marked decrease in T_m⁰ with increasing copolymer composition. The results of this study support the claim that these comonomers are excluded from the polymer crystal during growth. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2592–2603, 2006     

Comparison of crystallization behaviors of poly(ε‐caprolactone) in confined environment with that in bulk

The spatial confinement of poly(ε‐caprolactone) (PCL) in the matrix of PMMA was synthesized by insitu polymerization and characterized by WAXD and SEM. The nonisothermal crystallization behavior and the kinetics of PCL in PMMA/PCL (85/15) blend and pure PCL were investigated by means of DSC. Jeziorny and Ozawa's theoretical prediction methods were used to analyze the crystallization kinetics. The melting behavior after cooling was also studied. There was an additional interesting phenomenon of double‐melting peak for pure PCL. Peaks at lower temperature shifted to lower temperature, and peaks at higher temperature did not shift with the increasing cooling rate. This behavior can be due to recrystallization. For the high‐crystallization activity energy and low‐crystallization rate, PCL in bulk would recrystallize during the melting process, and displayed a double‐melting behavior. Under spatial confinement of the rigid PMMA, PCL had much lower crystallization activity energy and had only one melting peak. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

[Poly(ethylene terephthalate) ionomer]/silicate hybrid materials via polymer–in situ sol‐gel reactions

A scheme was developed for producing poly(ethylene terephthalate (PET) ionomer)/silicate hybrid materials via polymer–in situ sol‐gel reactions for tetraethylorthosilicate (TEOS) using different solvents. Scanning electron microscopy/EDAX studies revealed that silicate structures existed deep within PET ionomer films that were melt pressed from silicate‐incorporated resin pellets. ²⁹Si solid‐state NMR spectroscopy revealed considerable Si—O—Si bond formation, but also a significant fraction of SiOH groups. ²³Na solid‐state NMR spectra suggested the presence of ionic aggregates within the unfilled PET ionomer, and that these aggregates do not suffer major structural rearrangements by silicate incorporation. For an ionomer treated with TEOS using MeCl₂, Na⁺ ions are less associated with each other than in the unfilled control, suggesting silicate intrusion between PET–SO Na⁺ ion pair associations. The ionomer treated with TEOS + tetrachloroethane had more poorly formed ionic aggregates, which illustrates the influence of solvent type on ionic aggregation. First‐scan DSC thermograms for the ionomers demonstrate an increase in crystallinity after the incorporation of silicates, but solvent‐induced crystallization also appears to be operative. Second‐scan DSC thermograms also suggest that the addition of silicate particles is not the only factor implicated in recrystallization, and that solvent type is important even in second‐scan behavior. Silicate incorporation does not profoundly affect the second scan T_g vs. solvent type, i.e., chain mobility in the amorphous regions is not severely restricted by silicate incorporation. Recrystallization and melting in these hybrids appears to be due to an interplay between a solvent‐induced crystallization that strongly depends on solvent type and interactions between PET chains and in situ‐grown, sol‐gel‐derived silicate particles. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1749–1761, 2002; DOI 10.1002/app.10586     

Span-60-based organogels as probable matrices for transdermal/topical delivery systems

The current study describes the preparation and characterization of thermoreversible span-60 and sunflower oil (SO)-based organogels as a matrix for drug delivery. Effect of gelator concentration on the properties of the organogels was studied by physical property evaluation, stability, light microscopy, FTIR spectroscopy, XRD, thermal analysis, pH, and hemocompatibility studies. The drug release kinetics and antimicrobial efficacy of the salicylic acid loaded organogels were studied. The rate of gelation of the gels was found to be quicker in organogels with higher gelator proportions. The gels were inherently stable when stored below 25°C. The micrographs indicated the presence of needle-shaped crystals which formed aggregates resulting in the formation of three-dimensional networked structures. FTIR indicated intermolecular hydrogen bonding amongst SO and span-60 molecules responsible for the gelation. There was an increase in the crystallinity and the melting point of the organogels as the proportion of the organogelator was increased. The pH of the organogels indicated nonirritant nature of the gels, which were also found to be hemocompatible. The release of SA from organogels followed Higuchian kinetics and showed prolonged antimicrobial activity. The preliminary results indicated that the organogels may be tried as a matrix for controlled drug delivery. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Block copolymer of trans‐polyisoprene and urethane segment: Crystallization behavior and morphology

A new type of urethane segmented copolymer was prepared from hydroxyl‐terminated trans‐polyisoprene (HTTPI) and toluene diisocyanate (TDI). The structures of the copolymer were characterized by FTIR and GPC. Crystalline properties of trans‐polyisoprene (TPI) segments were investigated using WAXD and DSC techniques. The crystals of TPI segments are inclined to exist in low‐melting form (LM). The melting temperature of TPI shifts to a lower temperature as the urethane segment was introduced. DMA studies show that, when TPI crystals were at the melting state, the storage modulus of the copolymer depended on the content of urethane segment. The hard segment here serves as physical crosslinkage. Nonisothermal crystallization kinetics of TPI segment was studied on the basis of the Ozawa equation. It was found that the hard segment suppresses the crystallization of the TPI segment. Morphology of two‐phase separation was observed in the copolymer by SEM. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2286–2294, 2004     

Study on crystal process and isothermal crystallization kinetics of UHMWPE/CA‐MMT composites

Changes in the crystal morphology, crystallinity, and the melting process of thermoplastics resulted in significant changes in the processability and mechanical behavior of composites. In our former study, we prepared a novel antibacterial UHMWPE/CA‐MMT composite. In this study, the crystal process and crystallization kinetics of pure UHMWPE, UHMWPE/MMT, UHMWPE/CA, and UHMWPE/CA‐MMT were characterized by differential scanning calorimeter (DSC). The results showed that the chlorhexidine acetate (CA) and montmorillonite (MMT) could cause strong heterogeneous nucleation. The results of crystallization kinetics indicated that the addition of CA could decrease the crystallization rate constant K value and widen the range of the crystal growth temperature. The reological behaviors of four samples were carried out by a physical MCR301 rheometer. The results showed that the CA could bring down the complex viscosity of composites, thus affecting the crystal process or crystallization kinetics. POLYM. COMPOS., 33:1987–1992, 2012. © 2012 Society of Plastics Engineers     

A heat transfer model and supporting experiments to guide the uniform gelation of molecular oleogels during scale-up

The quest for novel vegetable oil structuring strategies has been progressing since the discovery of the deleterious impacts of trans fats. Although oleogelation using bioderived molecular gelators has been proven to be successful as an alternative to traditional hydrogenation methods, efforts are needed to meet the industrial requirements. A major constraint during the fabrication of oleogels is to achieve consistency in physical properties during scale-up. Experiments showed that gelation fails to occur when larger volumes were prepared based on the minimum gelation concentration (MGC) of gelators, determined using the smallest oil volume (1 ml), a general laboratory practice. This observation was consistent with all the molecular gelators used in this study; sorbitol dioctanoate, mannitol dioctanoate, and 12-hydroxystearic acid. To understand this behavior, a mathematical model was developed since gelator network propagation is governed by the cooling rate. The model indicates that maintenance of a minimal thermal gradient via uniform heat dissipation and gelation time is necessary to achieve homogeneous gel propagation across the vial. With these predictions, we hypothesized and confirmed that oleogels with constant surface area-to-volume ratio could result in identical gelation times and consistent physical properties (MGC, melting temperature, melting enthalpy, yield stress, solid phase content, and oil binding capacity) during scale-up.