Polarization temperature effect on liquid–liquid transition and space charge detrapping behavior in atactic polystyrene by thermally stimulated depolarization current

Thermally stimulated depolarization current (TSDC) experiments were carried out to investigate the effect of polarization temperature (T _p) on liquid–liquid transition and space charge detrapping behavior in atactic polystyrene. Differential scanning calorimetry (DSC) measurement was applied as a complementary method. When T _p is 130 °C, there are four distinct peaks (α, ρ₁, LL, and ρ₂) showed in TSDC spectrum in the range of 30–200 °C. Compared with the result of DSC, TSDC spectrum show that peak α corresponds to glass transition, peak LL is related to liquid–liquid transition, and peak ρ₁ and peak ρ₂ belong to the space charge current peak. As T _p increases from 100 to 150 °C, the intensity of peak LL maximum increases and that of space charge peaks maxima decreases, which is attributed to different mechanism between liquid–liquid transition and space charge detrapping behavior. In addition, their corresponding characteristic parameters versus T _p are also analyzed in detail.     

Effects of aging and moisture on the dynamic viscoelastic properties of oriental lacquer (urushi) film

The effects of aging and moisture on the dynamic viscoelastic properties of three oriental lacquer films were investigated. With aging over 1000 days at room temperature, the glass‐transition temperature of the lacquer films (T_α) shifted to higher temperatures, the maximum loss tangent (tanδ_α) decreased, and the storage modulus at 20°C (E) increased. These changes were analogous irrespective of lacquers. With increasing moisture content, E decreased and tanδ increased at room temperature. Although the equilibrium moisture content of the virgin lacquer (sap) film was higher than that of the clear lacquer film, its E and tanδ were more stable with an increase of moisture content. It was speculated that the polysaccharides aggregated in the sap film did not effectively contribute to the mechanical properties of the film, while their hygroscopicity resulted in higher moisture content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2288–2294, 2002     

Quantitative study of the annealing of poly(vinyl chloride) near the glass transition

Poly(vinyl chloride) displays a normal DSC of DTA curve for the glass transition when quenched from above its T_g. However if cooled slowly or annealed near the glass transition temperature, a peak appears on the DSC or DTA curve at the T_g. In this paper quantitative studies of the time and temperature effects on the production of this endothermal peak during the annealing of PVC homopolymer and an acetate copolymer are presented. The phenomenon conforms to the Williams, Landell, and Ferry equation for the relaxation of polymer chains, the rate of the peak formation becoming negligible at more than 50°C below T_g. The energy difference between the quenched and annealed forms is small. For a PVC homopolymer annealed 2 hr at 68°C, which is T_g ?10°C, the difference is 0.25 cal/g. For a 13% acetate copolymer of PVC similarly annealed, the difference is 0.36 cal/g. The measured rates of the process give a calculated activation energy of 13–14 kcal/mole for PVC homopolymer and copolymer. This appearance of a peak on the T_g curve for a polymer when annealed near the glass temperature appears to be a general phenomenon.     

Comparison of glass transition measurements between carrimed rheometer and differential scanning calorimetry using methylacrylate-vinylidene chloride copolymer

The potential of the Carrimed CSL 500 rheometer for the thermal analysis of methylacrylate-vinylidene chloride (MA-VDC) random copolymer was evaluated by comparing the glass transition temperature (T_g) of the copolymers with the value obtained from the differential scanning calorimeter. The major relaxation phenomenon in amorphous polymers, namely the glass transition temperature, could be identified clearly using the Carrimed rheometer. Samples of varius contents of methylacrylate in methylacrylate-vinylidene chloride copolymer were prepared as solvent cast films. Small amplitude oscillatory measurements showed as a function of temperature, that methylacrylate-vinylidene chloride copolymer between 12% and 70% methylacrylate (MA) content showed glass transition temperatures between 48°C and 63°C. For the methylacrylate content ranging from 4% to 70%, the saran copolymers had glass transition temperatures between 12°C and 56°C. Both techniques showed the T_g of the methylacrylate-vinylidene chloride copolymer first increased and then decreased with increasing methylacrylate content. Both methods show a peak in the T_g vs. percent methylacrylate content of the copolymer at around 50% methylacrylate content. Small angle amplitude measurements also showed that it is very sensitive to the frequency. The T_g obtained using the Carrimed CSL 500 rheometer is very reproducible and is comparable with that obtained using the standard DSC.     

Significant thermal property and hydrogen bonding strength increase in poly(vinylphenol-co-vinylpyrrolidone) copolymer

A series of poly(vinylphenol-co-vinylpyrrolidone) (PVPh-co-PVP) copolymers were prepared by free radical copolymerization of acetoxystyrene with vinylpyrrolidone (PAS-co-PVP), followed by selective removal of the acetyl protective group. These copolymers were investigated by solid state nuclear magnetic resonance (NMR) and thermal gravimetric analyzer (TGA) to compare with previous results on differential scanning calorimetry (DSC) and Fourier-Transform infrared spectroscopy (FTIR) analyses. The spin-lattice relaxation time in the rotating frame (T_1ρ(H)) of the PVPh-co-PVP is greater than the corresponding PVPh/PVP blend, indicating that the polymer mobility is more restricted and high rigid character of the former. At the same time, the thermal decomposition temperature of homopolymer, copolymer and polymer blend is the order of PVPh-co-PVP copolymer>PVPh/PVP blend>pure PVP homopolymer>PAS-co-PVP copolymer and this order is consistent with previous studies on DSC, FTIR and NMR analyses. In order to understand the mechanism of significant glass transition temperature increase of the PVPh-co-PVP copolymer, the degree of hydrolysis was controlled by varying time of reaction of the PAS-co-PVP copolymer.     

Role of a small amount of comonomer on the physical aging of poly(methyl methacrylate) copolymer investigated by dynamic viscoelasticity

The physical aging of a poly(methyl methacrylate) (PMMA) copolymer having a small amount of ethyl methacrylate was investigated below its glass transition temperature (T_g) using dynamic viscoelastic measurements. The results were compared with those from homo-PMMAs to investigate how the comonomer in the copolymer affects the physical aging of the glass. Annealing temperatures were classified into two categories by the glass transition temperature of a poly(ethyl methacrylate) (T_gPEMA; ca. 65°C). The relaxation rate of the copolymer was faster than those of the homo-PMMAs when the samples were aged below T_gPEMA, but the rates were equal when aged at above T_gPEMA. The experimental results are discussed in terms of the segmental motion.     

Effects of ultrasound on glass transition temperature of freeze-dried pear (Pyrus pyrifolia) using DMA thermal analysis

The effect of ultrasound pretreatment at various power (360 W, 600 W and 960 W, frequency 20 kHz) on the glass transition temperature of freeze dried pear (Pyrus pyrifolia) has been studied. DMA temperature plots were divided into four sections (A – glassy region, B – transition region, C – Rubbery plateau region and D – terminal region) with the aim to analyze their properties changed with sonication. Under the same freeze drying condition, with the increase in ultrasonic power, dried pear showed higher glass transition in terms of storage modulus, loss modulus and loss tangent peak. Also a decrease in a_w (0.31–0.23) and in moisture content (0.12–0.08 g/g d.b.) has been observed. Samples pretreated with ultrasound showed a better texture profile and much porous structure compared to control one. The results from the study indicated that, ultrasound pretreatment prior to freeze drying can improve the stability during storage of freeze dried pear.     

Dielectric studies of poly(vinyl chloride)

Measurements of the dielectric constant and loss factor of polyvinyl chloride (PVC) have been made over a wide range of temperature and frequency. Maxima in dielectric loss were observed at a temperature corresponding to the α or glass transition temperature and secondary β transition temperature. The sensitivity of these maxima to both frequency and thermal pretreatment was investigated. It was found that the position of the β peak on the temperature axis changed markedly with frequency, while the glass transition or α peak showed only small variations. Neither the α nor the β peak were very sensitive to thermal pretreatment, but interesting changes in the minimum between these peaks were observed. In quenched samples a distinct shoulder peak at T_β < T_s < T_α was observed; it was possible to eliminate it by a slow cooling of the samples. The effect of plasticizer content was also studied.     

Hydrogels from 2-(dimethylamino)ethylacrylate with 2-acrylamido-2-methyl-1-propanesulfonic acid: synthesis,characterization, and water-sorption properties

A novel series of copolymer hydrogels of 2-(dimethylamino)ethylacrylate (DMAEA)/2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) were prepared by solution free radical polymerization at different feed monomer mol ratios. The monomer reactivity ratios were determined by Kelen–Tüdös method. According to that, the monomer reactivity ratios for poly(DMAEA-co-AMPS) were r₁ = 0.125 and r₂ = 2.85, (r₁ × r₂ = 0.356). The effect of reaction parameters, including the concentration of cross-linking reagent N,N′-methylene-bis-acrylamide (MBA) and initiator ammonium persulfate (APS), the monomer concentration, pH, temperature, salt solutions, and solvent polarity on the water absorption have been also studied. The hydrogels achieved water-absorption values of 430 g of water/g of xerogel for the copolymer 1:2 richest in AMPS moiety. This copolymer is also very stable to the temperature effect. The optimum pH for the copolymers is 7. Aqueous solutions of the copolymers showed lower critical solution temperature behaviour (LCST). The phase transition temperatures of aqueous solutions of these copolymer increased with increasing of hydrophilic AMPS unit content in the copolymers. The glass transition temperature (Tg) of hydrogels showed a decrease by increasing of comonomer DMAEA content.     

Relaxation time and conductivity of comb‐like gel polymer electrolytes

Using the copolymer of acrylonitrile (AN), methyl methacrylate (MMA), and poly(ethylene glycol) methyl ether methacrylate as a backbone and poly(ethylene glycol) methyl ether (PEGME) with 1100 molecular weight as side chains, comb‐like gel polymers and their Li salt complexes were synthesized. The dynamic mechanical properties and conductivities were investigated. Results showed that the gel copolymer electrolytes possess two glass transitions: α‐transition and β‐transition. Based on the time–temperature equivalence principle, a master curve was constructed by selecting T_α as reference temperature. By reference to T₀ = 50°C, the relation between log τ_c and c was found to be linear. The master curves are displaced progressively to higher frequencies as the content of plasticizer is increased. The relation between log τ_p and the content of plasticizer is also linear. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 576–584, 2007     

Properties of Poly(vinylchloride) Blended with an Emulsion Copolymer of N-p-Tolylmaleimide,Methyl Methacrylate,and Acrylonitrile

Abstract

Copolymers of N-p-tolylmaleimide, methyl methacrylate, and acrylonitrile were synthesized by the semibatch emulsion polymerization method and used for blending with poly(vinylchloride) (PVC) to improve its heat resistance. The thermal properties of the blends with different terpolymer content were investigated by differential scanning calorimeter (DSC), thermogravimatric analysis (TGA), and Vicat softening point tester (T _Vicat). The results showed that the glass transition temperature (T _g ), Vicat softening point, and thermal decomposition temperature of the blends increased with increasing terpolymer content. The tensile strength increased with an increase in the copolymer content. The results of the rheological behavior showed that the apparent viscosity ? _α decreased with increasing shear rate and increased with the copolymer content.     

Synthesis, thermal properties, and specific interactions of high Tg increase in poly(2,6-dimethyl-1,4-phenylene oxide)-block-polystyrene copolymers

We have synthesized a series of block copolymers of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene (PPO-b-PS copolymer) by atom transfer radical polymerization. The PS content in these copolymer systems was determined by using infrared spectroscopy, thermal gravimetric analysis, and solution and solid-state NMR spectroscopy; good correlations exist between these characterization methods. DSC analyses indicated that the PPO-b-PS copolymers have higher glass transition temperatures than do their corresponding PPO/PS blends. Our FTIR and solid-state NMR spectroscopic analyses suggest that the PPO-b-PS copolymers possess stronger specific interactions that are responsible for the observed relatively higher values of T_g. We found one single dynamic relaxation from the dynamic mechanical analysis, which implies dynamic homogeneity exists in the PPO-b-PS copolymer; this result is consistent with the one single proton spin-lattice relaxation time observed in the rotating frame [T_1ρ(H)] during solid state NMR spectroscopic analysis. In addition, the 2D FTIR spectroscopy reveals evidence for the stronger interactions between segments of PPO and PS through the formation of π-cation complexes.     

Calorimetric study of block-copolymers of poly(n-butyl acrylate) and gradient poly(n-butyl acrylate-co-methyl methacrylate)

The nanophase separation in diblock and triblock copolymers consisting of immiscible poly(n-butyl acrylate) (block A) and gradient copolymers of methyl methacrylate (MMA) and n-butyl acrylate (nBA) (block M/A) were investigated by means of their heat capacity, C_p, as a function of the composition of the blocks M/A and temperature. In all copolymers studied, both blocks are represented by their C_p and glass transition temperature, T_g, as well as the broadening of the transition-temperature range. The low-temperature transition of the blocks A is always close to that of the pure poly(n-butyl acrylate) and is independent of the analyzed compositions of the block copolymer, but broadened asymmetrically relative to the homopolymer due to the small phase size. The higher transition is related to the glass transition of the copolymer block of composition M/A. Besides the asymmetric broadening of the transition due to the phase separation, it decreases in T_g and broadens, in addition, symmetrically with increasing acrylate content. The concentration gradient is not able to introduce a further phase separation with a third glass transition inside the M/A block.     

Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermal properties of poly(tetramethyl-p-silphenylene siloxane) and (tetramethyl-p-silphenylene siloxane-dimethyl siloxane) copolymers

Some physical properties of poly(tetramethyl-p-silphenylene siloxane) homopolymer and random block copolymers of tetramethyl-p-silphenylene siloxane-dimethyl siloxane have been determined and correlated with polymer structure. Differential scanning calorimetry (d.s.c.), differential thermal analysis (d.t.a.), density gradient column measurements and optical hot stage melting point determination and diluent techniques were used. The thermodynamic melting temperature of the homopolymer was estimated to be 160°C and its heat of fusion, ΔH_u, found to be 54.4 J/g (13 cal/g or 2710 cal/mol of monomer repeat units). Its limiting glass transition temperature, T_g, was ?20°C. T_g of the copolymer was found to vary almost monotonically with increasing dimethyl siloxane (DMS) content ranging from ?20° (0% DMS) to just above ?123°C, for pure DMS polymer. The copolymer melting temperature was found to increase as the fraction of the crystalline (hard) TMPS constituent was increased. Based upon copolymer theory and extrapolated melting point data, it was estimated that the block size of soft DMS component in the copolymer most probably consists of twelve monomer units distributed amongst TMPS sequences of varying length.     

Thermal analysis of drying process of durum wheat dough under the programmed temperature-rising conditions

The effects of temperature and moisture content on the drying rate of durum wheat pasta were examined using thermogravimetry and differential scanning calorimetry (DSC) at temperature-rising rates of 0.2–1.0 °C/min. The activation energy for the mass transfer coefficient of drying was estimated to be ca. 32 kJ/mol at a moisture contents of 0.14 kg-H₂O/kg-d.m. or higher, but increased rapidly as the moisture content dropped below this level. The conclusion temperature of the endothermic peak in the DSC and the temperature of the inflection point of the drying characteristics curve were located near the glass transition curve of the durum semolina flour.     

The peculiarities of relaxation processes at heating of glasses in glass transition region according to the data of mechanical relaxation spectra (Review)

The results of study of mechanical losses by dynamic methods for silicate, borate, and chalcogenide glasses, metallic glass and glycerol at heating in the glass transition region were analyzed. Essential differences between the dynamic viscosity values η* calculated by means of the Maxwell equation based on the relaxation time at the maximum of losses (ωτ = 1) for labile states of glass, and the experimental values of η for the metastable liquid at the same temperature were revealed. The ratio η*/η was systematized in the framework of kinetic theory of glass transition and thermodynamics. An interpretation of the regularities was proposed based on the theory of dynamic properties of liquids. It was shown that different widths of spectra of relaxation times were the most probable reason of the difference between η* and η. The width of spectrum is determined by the degree of ordering of states of compared metastable liquid and glass at the same temperature; it depends on the thermal prehistory of each state. A wider spectrum of relaxation times corresponds to a more ordered state. For the considered glasses, the ratio of the temperature corresponding to viscosity value η* of the metastable liquid to the temperature of α-relaxation maximum (T _α) is 1.03 ± 0.01 at T _α variation from 190 to 1550 K. It is the evidence that all the relaxation frequencies, constituting both “narrow” and “broad” spectrum are associated with one and the same molecular mechanism. Mechanical losses in the metastable supercooled glycerol are described by the Maxwell equation with high precision for η values from 10¹³ to 10⁵ Pa s.     

Thermal and gas permeation properties of copolymers derived from 3‐methacryloxypropyltris(trimethylsiloxy)silane and adamantyl group‐containing methacrylate derivatives

Novel copolymer membranes derived from three types of adamantyl group‐containing methacrylate derivatives and 3‐methacryloxypropyltris(trimethylsiloxy)silane (SiMA) were synthesized via free radical polymerization. The thermal and permeation properties of these copolymer membranes were investigated. Copolymer membranes with less than 11.9 mol % adamantane content exhibited good membrane forming abilities that are suitable for permeation measurement. The decomposition temperature of all copolymers increased up to approximately 40–80°C with increasing adamantane content compared with poly(SiMA). Moreover, the glass transition temperature (T_g) of all copolymers increased up to approximately 46–60°C with increasing adamantane content compared with the theoretical value, which was estimated from Fox equation. 1‐Adamantyl methacrylate copolymer had the highest fractional free volume among the three types of adamantly group‐containing methacrylate derivatives. The gas permeability coefficient of this copolymer increased by 22–45% with increasing adamantane content compared with that of poly(SiMA). © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43129.     

WATER SORPTION ISOTHERMS AND GLASS TRANSITION PROPERTIES OF GELATIN

ABSTRACT

The water sorption isotherms of gelatin of different molecular weights (317,700, 228,900, and 197,400) were determined at 50°C using an isopiestic method. The sorption isotherms were modeled using the Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–deBoer (GAB) equations. The BET and GAB equations were able to predict the equilibrium moisture content (EMC) with a mean relative error of 5.2 and 5.0%, respectively. The BET monolayer moisture content varied from 4.81 to 5.70% (d.b.) while modeling with the GAB equation predicted monolayer moisture content of 6.14–7.58% (d.b.) depending upon molecular weight. The monolayer moisture content increased with increasing molecular weight. Studies on the effect of moisture content on the “rheological glass transition temperature” (T _g ) showed a smooth increase in the value of T _g as a function of increasing concentration of gelatin solids. This varied from 7 to 35°C at 75% and 97% solids, respectively for the protein sample with MW = 317,700. Pinpointing of the T _g was implemented with the technique of small deformation dynamic oscillation. It was proposed that the “rheological” T _g is the point between the glass transition region and the glassy state. It acquires physical significance by identifying the transition from free volume phenomena of the polymeric backbone in the glass transition region to an energetic barrier to motions in the glassy state involving stretching and bending of chemical bonds.