Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4′-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M _w = 181 000 g mol ^-1 the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.     

Radiation-induced reactions in poly(1,2,2,2-tetrachloroethyl methacrylate)/monomer mixtures

The work has been aimed at characterizing final products of radiation-induced reactions in polymer/monomer mixtures particularly by analytical and preparative HPLC. Poly(1,2,2,2-tetrachloroethyl methacrylate) (PTCEMA) was used as polymer while p-cumylphenythacrylate (CUPMA) was mainly added as monomer. The substances were irradiated with γ-doses up to 160 kGy. The electron beam doses at an acceleration voltage of 30 kV corresponded to this value.With the HPLC method a CUPMA-grafted PTCEMA and CUPMA oligomers were detected. UV and ¹H-NMR spectra of these separated substances obtained by preparative HPLC confirmed the formation of a graft polymer. IR and Cl-elementary analyses revealed a Cl-substitution in CUPMA oligomers. Evidently, the conversion of unsaturated monomers is decisively determined by the radiation-chemical behaviour of the polymer.     

Miscibility and thermal stability of poly(vinyl alcohol)/chitosan mixtures

The miscibility and the thermal behaviour of chitosan acetate (ChA) with poly(vinyl alcohol) (PVA) have been investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Chitosan is blended with poly(vinyl alcohol) in acetic acid solution and this solution is cast to prepare the blend film. From thermal curves the thermal transitions: T_g, T_m and characteristic temperatures of decomposition: T_di, T_max have been determined and compared. The influence of the degree of PVA hydrolysis on the thermal properties of blend systems has been discussed.Based upon the observation on the DSC analysis, the melting point of PVA is decreased when the amount of ChA in the blend film is increased. Though some broadening of the transition curves could be noticed (DSC, TGA and DMA), the obtained results suggest that in the solid ChA/PVA blends the components are poorly miscible. Only PVA sample with relatively low DH = 88% and hence low degree of crystallinity shows partial miscibility with ChA of relatively low molecular weight.     

Phase diagrams of poly(dimethylsiloxane) and 5CB blends

The phase diagrams of poly(dimethylsiloxane) (PDMS) and 4‐cyano‐4′‐n‐pentyl‐biphenyl (5CB) mixtures are studied for two systems of different molecular weights of the polymer. The experimental diagrams are established by polarized optical microscopy (POM), and analyzed using a combination of the Flory–Huggins theory of isotropic mixing and the Maier–Saupe theory of nematic order. The results are compared with those of polystyrene (PS) and 4‐cyano‐4′‐n‐octyl‐biphenyl (8CB) with analogous molecular weight of the polymer. This investigation could be useful for the choice of systems in electro‐optical devices. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 581–588, 2001     

Thermal degradation studies in poly(vinyl chloride)/poly(methyl methacrylate) blends

The miscibility, morphology, and thermal properties of poly(vinyl chloride) (PVC) blends with different concentrations of poly(methyl methacylate) (PMMA) have been studied. The interaction between the phases was studied by FTIR and by measuring the glass transition temperature (T_g) of the blends using differential scanning calorimetry. Distribution of the phases at different compositions was studied through scanning electron microscopy. The FTIR and SEM results show little interaction and gross phase separation. The thermogravimetric studies on these blends were carried out under inert atmosphere from ambient to 800 °C at different heating rates varying from 2.5 to 20 °C/min. The thermal decomposition temperatures of the first and second stage of degradation in PVC in the presence of PMMA were higher than the pure. The stabilization effect on PVC was found most significant with 10 wt% PMMA content in the PVC matrix. These results agree with the isothermal degradation studies using dehydrochlorination and UV-vis spectroscopic results carried out on these blends. Using multiple heating rate kinetics the activation energies of the degradation process in PVC and its blends have been reported.     

Interpolymer complexation and thermal behaviour of poly(styrene-co-maleic acid)/poly(vinyl pyrrolidone) mixtures

Polymer complexation between poly(styrene-co-maleic acid), (SMA28) and (SMA50) containing 28 and 50 mol% of maleic acid and poly(vinyl pyrrolidone) (PVP), has been investigated by differential scanning calorimeter (DSC), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). All results showed that the ideal complex composition of SMA28/PVP and SMA50/PVP leads, respectively, to 2:1 and 1:1 mole ratio of interacting components.For the investigated systems, the T_g versus composition curve does not follow any of the usual proposed models for polymer blends. Withal, a new model proposed by Cowie et al. is used to fit the T_g data and it is found to reproduce the experimental results more closely. According to n and q obtained values, it seems reasonable to conclude that the inter-associated hydrogen bonds dominate in SMA28/PVP (2:1) complexes. This effect is corroborated by the FTIR study as evidenced by the high displacement of the specific bands and ionic interactions have been clearly identified. Finally, a thermogravimetric study shows that ionic interactions increase the thermal stability of these complexes.     

Calorimetric studies of poly(ethylene terephthalate) stretching over a wide temperature range

Heat effects and structural transformations in amorphous crystallizable poly(ethylene terephthalate) (PET) during uniaxial stretching accompanied by neck formation, have been investigated by calorimetric and x-ray methods over a wide range of temperatures and deformation rates. At small deformation (not exceeding 1–2%) and at temperatures below the glass transition temperature of the polymer, PET behaves as an elastic body. Upon stretching at a constant rate, constant heat power is absorbed, heat effects during loading and unloading coincide completely, and no hysteresis is observed. At large deformations (of the order of 50%), cold drawing develops in this temperature range. The internal energy change in cold drawing is zero within experimental error. A periodic heat release during the self-oscillation regime of drawing PET corresponds to periodic changes in stress, in the rate of the neck formation, and in the appearance of the sample. The temperature limits of the region where crystallization resulting from an uniaxial drawing of the polymer is possible, have been determined, and the heat effect of this phase transition has been measured. Orientation crystallization develops only from 70 to 94°C. These limits are insensitive to changes in deformation rate within one decimal order. The structure of PET in this temperature range has been investigated. The heat of phase transition of orientation crystallization of PET has been determined from the relationship between the measured values of the internal energy change during this process and the limiting degree of crystallinity for the stretched samples. This heat proves to be 5.5 ± 0.1 cal/g.     

Associative and segregative phase separations of poly(N-tert-butylacrylamide)/poly(acrylic acid) mixtures. Effect of solvent

Poly(N-tert-butylacrylamide) (PNtBAm) and poly(acrylic acid) (PAA) form interpolymer complexes in 1- and 2-propanol, blend in ethanol, whereas a segregative phase separation is observed when using methanol as solvent as shown by Fourier transform infrared (FTIR) spectrometry and elemental analysis studies. The composition of PNtBAm/PAA complexes has been determined. Thermal studies demonstrated that all complexes show unique glass transition temperatures, higher than those of the polymer components. Complexation of PAA with PNtBAm results in an improvement of its thermal stability. Solvent effects and specific interactions in the system PNtBAm/PAA have been studied by FTIR, revealing that differences in the polymer–solvent interactions are a decisive factor governing complex formation in solution.     

Thermodynamic of polymer mixtures: The LCT applied to the poly(styrene)/poly(vinyl methylether) system

In this article, the Lattice Cluster Theory developed by Freed and coworkers is applied to the poly(styrene)/poly(vinyl methylether) system by measuring the enthalpy and the volume variation associated with the mixing of the two homopolymers. It comes out that the chains architectures previously chosen for the two macromolecules based on the phase diagram and of the partition function developed by the theory correctly predicts the sign of both ΔH^M and ΔV^M. The foreseen trends as function of mixture composition fit the experimental data only for ΔV^M; ΔH^M is higher than predicted, asymmetrical and independent on the chain lengths of the components. The conclusion is that the theory needs some final touches in order to obtain a better agreement with experiments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 791–797, 2008     

Depolarization thermocurrent studies in poly(hydroxyethyl acrylate)/water hydrogels

Detailed investigations on the dielectric relaxation mechanisms in poly(hydroxyethyl acrylate) (PHEA), by means of the thermally stimulated depolarization currents (TSDC) method in the temperature range 77-300 K are reported. There is particular interest in the dependence of the dielectric relaxation mechanisms on the water content h, h = 0 ? 0.5 w/w, in an attempt to contribute to a better understanding of the physical structure of water in the PHEA hydrogels. We employ thermal sampling (TS) and partial heating (PH) techniques to experimentally analyze the observed complex relaxation processes, due to the secondary (β_sw) and the main (α) relaxation, into approximately single responses and to determine the spectra of activation energies E(T) at different h values. Measurements with different electrode configurations reveal different aspects of the dynamics of the relaxation mechanisms and allow the distinction between dipolar and conductivity relaxation contributions. It is shown that by means of these techniques we can determine certain temperature characteristics for the α relaxation and investigate their dependence on water content. We discuss the relation of these characteristic temperatures to the calorimetric glass transition temperature T_g. © 1994 John Wiley & Sons, Inc.     

Miscibility and enzymatic degradation studies of poly(ε-caprolactone)/poly(propylene succinate) blends

In the present study the miscibility behaviour and the biodegradability of poly(ε-caprolactone)/poly(propylene succinate) (PCL/PPSu) blends were investigated. Both of these aliphatic polyesters were laboratory synthesized. For the polymer characterization DSC, ¹H NMR, WAXD and molecular weight measurements were performed. Blends of the polymers with compositions 90/10, 80/20, 70/30 and 60/40 w/w were prepared by solution-casting. DSC analysis of the prepared blends indicated only a very limited miscibility in the melt phase since the polymer-polymer interaction parameter χ₁₂ was −0.11. In the case of crystallized specimens two distinct phases existed in all studied compositions as it was found by SEM micrographs and the particle size distribution of PPSu dispersed phase increased with increasing PPSu content. Enzymatic hydrolysis for several days of the prepared blends was performed using Rhizopus delemar lipase at pH 7.2 and 30 °C. SEM micrographs of thin film surfaces revealed that hydrolysis affected mainly the PPSu polymer as well as the amorphous phase of PCL. For all polymer blends an increase of the melting temperatures and the heat of fusions was recorded after the hydrolysis. The biodegradation rates as expressed in terms of weight loss were faster for the blends with higher PPSu content. Finally, a simple theoretical kinetic model was developed to describe the enzymatic hydrolysis of the blends and the Michaelis-Menten parameters were estimated.     

Novel tricomponent membranes containing poly(ethylene glycol)/poly(pentamethylcyclopentasiloxane)/poly(dimethylsiloxane) domains

The synthesis and characterization of novel tricomponent networks consisting of well‐defined poly(ethylene glycol) (PEG) and poly(dimethylsiloxane) (PDMS) strands crosslinked and reinforced by poly(pentamethylcyclopentasiloxane) (PD₅) domains are described. Network synthesis occurred by dissolving α,ω‐diallyl PEG and α,ω‐divinyl PDMS prepolymers in a common solvent (toluene), introducing a stoichiometric excess of pentamethylcyclopentasiloxane (D₅H) to the charge, inducing the cohydrosilation of the prepolymers by Karstedt's catalyst and completing network formation by the addition of water. Water in the presence of the Pt‐based catalyst oxidizes the SiH groups of D₅H to SiOH functions that immediately polycondense and bring about crosslinking. The progress of cohydrosilation and polycondensation was followed by monitoring the disappearance of the SiH and SiOH functions by Fourier transform infrared spectroscopy. Because cohydrosilation and polycondensation are essentially quantitative, overall network composition can be controlled by calculating the stoichiometry of the three network constituents. The very low quantities of extractable (sol) fractions corroborate efficient crosslinking. The networks swell in both water and hexanes. Differential scanning calorimetry showed three thermal transitions assigned, respectively, to PEG (melting temperature: 46–60 °C depending on composition), PDMS [glass‐transition temperature (T_g) = ～?121 °C], and PD₅ (T_g = ～?159 °C) and indicated a phase‐separated tricomponent nanoarchitecture. The low T_g of the PD₅ phase is unprecedented. The strength and elongation of PEG/PD₅/PDMS networks can be controlled by overall network composition. The synthesis of networks exhibiting sufficient mechanical properties (tensile stress: 2–5 MPa, elongation: 100–800%) for various possible applications has been demonstrated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3093–3102, 2002     

Wetting behaviour of 5CB and 8CB and their binary mixtures above the isotropic transition

High resolution optical microscopy techniques have been employed to study the wetting properties of (5CB)_x/(8CB)_100-x (x=0, 10, 30, 50, 70, 90, and 100 wt %) binary mixture liquid crystal thin films above the isotropic transitions. Dewetting is found to occurr at T_w=65 and 42.5°C for the 5CB and 8CB samples, respectively, and to depend strongly on the 5CB content in the mixtures. First-order wetting transitions were seen for pure 5CB and 8CB samples; a higher order wetting transitions were obtained for the mixtures. For thin film 5CB, a large hysteresis (ΔT=30°C) between wetting and dewetting during the heating and cooling is obtained. This hysteresis is compared with the corresponding values obtained for pure 8CB and mixture samples. Evidence of the formation of nematic layers on the surface of isotropic droplets was found. Attempts to extract values of the contact line tension for these materials are made. In this respect the applicability of the modified Young's equation is questionable.     

Poly(vinylalcohol)/poly(ethyleneglycol)/poly(ethyleneimine) blend membranes - structure and CO2 facilitated transport

Poly(vinylalcohol) (PVA)/poly(ethyleneimine) (PEI)/poly(ethyleneglycol) (PEG) blend membranes were prepared by solution casting followed by solvent evaporation. The effects of the blend polymer composition on the membrane structure and CO₂/N₂ permeation characteristics were investigated. IR spectroscopy evidenced strong hydrogen bonding interactions between amorphous PVA and PEI, and weaker interactions between PVA and PEG. DSC studies showed that PVA crystallization was partially inhibited by the interactions between amorphous PVA and PEI blend, in which PEG separated into nodules. The CO₂ permeability decreased with an increase in CO₂ partial pressure in feed gas, while the N₂ permeability remained constant. This result indicated that only CO₂ was transported by the facilitated transport mechanism. The CO₂ and N₂ permeabilities increased monotonically with the PEI content in the blend membranes, whereas the ideal selectivity of CO₂ to N₂ transport showed a maximum. When CO₂ is humidified, its permeability through the blend membranes is much higher than that of dry CO₂, but the change in permeability due to the presence of humidity is reversible.     

Intramolecular interactions in poly(styrene-co-acrylonitrile)/poly(ε-caprolactone) blends

Specific interactions in blends of poly(ε-caprolactone) (PCL) and poly(styrene-co-acry-lonitrile) (SAN) were studied as a function of copolymer composition and blend ratio by using Fourier-transform infrared spectroscopy (FTIR). It was shown that miscibility occurred within a certain range of copolymer compositions because the presence of PCL reduced the thermodynamically unfavorable repulsion between styrene and acrylonitrile segments in the random copolymer. This effect was observed in terms of a shift to higher frequencies in the 700 cm^-1 γ-CH out-of-plane deformation vibration absorption of styrene and in the approximately 2236 cm^?1 C?N stretching frequency band in acrylonitrile segments. Specific intermolecular interactions between SAN and PCL were not observed in this study. © 1993 John Wiley & Sons, Inc.     

Low-frequency dilational elasticity of the nematic 4'-pentyl-4-biphenylcarbonitrile (5CB)/water interface

Axisymmetric oscillating pendant drop shape analysis has been used to study the interfacial rheology of the liquid crystal 4'-pentyl-4-biphenylcarbonitrile (5CB) in water with homeotropic anchoring. Nearly spherical 5CB droplets were subjected to low frequency (1-5 mHz) volume oscillations, and the increase in tension with surface dilation was used to calculate the complex modulus. The droplet interface response is completely elastic, with no relaxations occurring on the experimental time scale. This surprising result is attributed to droplet storage of elastic energy in the form of distorted orientational distributions within the bulk (Frank elasticity) and on the surface (anchoring elasticity).