Thermomechanical Curves Research Articles

Structure and molecular conformation of tussah silk fibroin films treated with water-methanol solutions: Dynamic mechanical and thermomechanical behavior

The thermal response of tussah (Antheraea pernyi) silk fibroin films treated with different water–methanol solutions at 20°C was studied by means of dynamic mechanical (DMA) and thermomechanical (TMA) analyses as a function of methanol concentration and treatment time. The DMA curves of α-helix films (treated with ≥80% v/v methanol for 2 min and 100% methanol for 30 min) showed the sharp fall of storage modulus at about 190°C, and the loss peak in the range 207–213°C. The TMA curves were characterized by a thermal shrinkage at 209–211°C, immediately followed by an abrupt extension leading to film failure. Both storage and loss modulus curves significantly shifted upwards for β-sheet films, obtained by treatment with ≤60% methanol for 30 min. The loss peak exhibited a maximum at 236°C. Accordingly, the TMA shrinkage at above 200°C disappeared. The films broke beyond 330°C, failure being preceded by a broad contraction step. Intermediate DMA and TMA patterns were observed for the other solvent-treated films. The loss peak shifted to higher temperature (219–220°C), and a minor loss modulus component appeared at about 230°C. This coincided with the onset of a plateau region in the storage modulus curve. The TMA extension–contraction events in the range 200–300°C weakened, and the samples displayed a final broad contraction (peak temperature 326–338°C) before breaking. The DMA and TMA response of these films was attributed to partial annealing by solvent treatment, which resulted in the formation of nuclei of β-sheet crystallization within the film matrix. The increased thermal stability was probably due to the small β-sheet crystals formed, which acted as high-strength junctions between adjacent random coil and α-helix domains. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2717–2724, 1998

Morphology of Polyethylene Foamed Crosslinked Specimens made by Hot Mold Injection Molding

Foamed and crosslinked specimens of polyethylene made by hot mold injection molding were investigated. A combination of PELD, dicumilperoxide and azodicarbonamide was used. The three components were mixed and granulated in an extruder-granulator at temperature 100—120°C. The degree of crosslinking was calculated by equilibrium strain ϵr determined at temperature 140°C and pressure stress σp=0.045 MPa. The same parameter was evaluated by thermomechanical curves. The degree of crystal-linity was determined by DSC-kinetics of a nonisothermal melting and crystallization. The morphology was investigated by a polarized microscope MIN 8. It is established that a part of polymer crystallizes as spherulites. The spherulites can observed with crossed polarizers. The samples on the whole are not oriented. There exists only a local orientation around the edge of the bubbles which is proved by the observed bright regions. The number of the spherulites decreases with the increase of crosslinking degree. This also conforms with the degree of crystallinity which decreases with the increase of the crosslinking degree.

Thermomechanical behavior of liquid-crystal polyesters

We have used the thermomechanical method to investigate the processes of self-elongation and shrinkage in uniaxially oriented liquid-crystal (LC) polyesters (CPET — the copolyester of terephthalic acid, phenylhydroquinone, and p-hydroxybenzoic acid; Ultrax — the copolyester of terephthalic acid, isophthalic acid, and bisphenol) and the injection-molded copolyester of hydroxybenzoic and hydroxynaphthoic acids Vectra A900. A universal tensile tester and a thermomechanical tester were used for the investigations. We studied the deformation behavior for the first and second heating under negligible applied stress. We prepared CPET samples in the form of as-spun and annealed fibers, Vectra samples in the form of cubes cut from the nonoriented part of the molded piece (VI) and from the longitudinally oriented central part (VII), Ultrax samples in the form of strands (noncrystalline UI and crystalline UII). We observed a permanent length increase for the as-spun CPET (Fig. 1) and shrinkage in the glass-transition region for the annealed CPET (Fig. 2) for the first and second heating. Note that the as-spun CPET is a smectic polymer, and the annealed CPET contains smectic and crystalline phases. The thermomechanical curves of VI seem to be typical of any molded polymer (Fig. 3). In contrast, for the oriented VII we see spontaneous elongation upon heating in the direction of orientation, with pronounced transverse shrinkage (Fig. 4). The length increase for the noncrystalline UI is quite considerable for the first heating. We observed both spontaneous elongation below the glass-transition temperature and subsequent shrinkage for crystalline UII for the first heating. All the Ultrax curves are much smoother for the second heating (Fig. 5). Spontaneous elongation upon heating appears to be a characteristic feature of oriented LC polymers, the polymer being either fully oriented or oriented within some part of the sample. Shrinkage of LC polyesters upon heating is also possible, as a result of both processing factors and crystallinity. There are two types of restructurization processes: orientation and crystallization (possible for LC polymers capable of forming a crystalline phase).

Influence of chemical modification of a bituminous coal on its behaviour during thermomechanical analysis

The rheological behaviour of a Donbass bituminous coal was characterized by thermomechanical analysis under high pressure (6 MPa) from ambient temperature up to 600°C. Under these conditions the onset of the transfer of the coal from a viscous to a viscofluid state could be determined from the thermomechanical curves in a range of temperature not easily accessible with classical plastometers. The softening temperature and the activation energy of the viscous flow could also be determined. As intramolecular interactions are principally involved in the first stage of the thermoplasticity of coals, chemical modifications were applied to the coal by reductive alkylation and oxidation under several conditions (Sternberg and Lazarov reactions) to modify the intermolecular association. The influence of these treatments is clearly shown by the thermomechanical analysis.

Physical properties and phase separation structure of Antheraea pernyi/Bombyx mori silk fibroin blend films

AbstractThe physical structure and compatibility of solution‐cast Antheraea pernyi/Bombyx mori silk fibroin blend films were stuided by differential scanning calorimetry (DSC), thermomechanical (TMA) and thermogravimetric (TGA) analysis, dynamic viscoelastic measurement, infrared spectroscopy, and x‐ray diffractometry. The DSC curves of the blend films showed independent endotherms at 280 and 358°C, corresponding to the thermal decomposition of B. mori and A. pernyi silk fibroins with random coil conformation. The intensity was roughly proportionate to the amount of each component in the blend. The thermal behavior corresponding to the conformational transitions induced by heating on A. pernyi and B. mori silk fibroins overlapped in the temperature range 190–230°C. Thermal expansion and contraction properties, as well as weight retention behavior of the blend films were intermediate between the pure components, as shown by the TMA and TGA curves. The onset temperature of the storage modulus curve decreased markedly, approaching that of B. mori silk fibroin film when the amount of this component in the blend increased. The loss modulus curve of the blend films showed two peaks at ca. 190 and 210°C, the former corresponding to B. mori, and the latter to A. pernyi silk fibroin. Infrared spectra of the blends exhibited absorption bands characteristic of the pure components overlapping in the spectral region 2000–400 cm−1. The x‐ray diffraction peaks at 23 and 21.5°, attributed to the crystalline spacings of A. pernyi and B. mori fibroins, respectively, overlapped in the diffraction curves of the blends, while the peak at 11.4°, of A. pernyi, increased as the content of this fibroin in the blend increased. The degree of crystallinity, calculated from the x‐ray diffraction curves, diminished as the amount of B. mori silk fibroin decreased. A low degree of compatibility exists between the two fibroins when they are cast from aqueous solution in the experimental conditions adopted in this work. © 1994 John Wiley & Sons, Inc.

Thermal characteristics of organic high-strength, high-modulus fibers containing finely disperse fillers

The behavior of high-strength, high-modulus filled fibers based on aromatic polyamide with heterocyclic fragments was investigated at high temperatures. It was shown that a slight shift in the critical points on the thermomechanical curves to the region of higher temperatures is characteristic of fibers filled with rare-earth element oxides. The glass transition temperature, heat resistance, and thermal stability increase in the filled fibers. These changes are characteristic of fibers containing more than 10 wt. % filler.

Blends of normal high density and ultra-high molecular weight polyethylene, ? irradiated at a low dose

The kinetics of a nonisothermal crystallization and melting ofγ irradiated with dose of 6 Mrad blends of an ultra-high molecular-weight polyethylene (UHMWPE) and a high-density polyethylene with normal molecular weight (NMWPE) is investigated by means of DSC. The blends have been prepared at temperature below the flow temperature of UHMWPE: The enthalpies of melting of the polyethylenes increase, while those of their blends decrease after irradiation. The enthalpies of crystallization of the pure polyethylenes are higher, while those of their blends almost do not change or are a bit higher after irradiation. The rates of a nonisothermal crystallization and melting of the polyethylenes increase, while those of the polyethylenes in the blends decrease after irradiation. Thermomechanical measurements under constant load in wide-temperature interval of irradiated polyethylenes and their blends have been made. A high-elastic plateau in viscous-liquid state is established on the thermomechanical curves of UHMWPE, and the blends with high content of UHMWPE. On the basis of results obtained assumptions have been made about the processes taking place in the blends under the action of irradiation, as well as about the character of the mutual influence between the components in the process of irradiation.

Special features of the structure and properties of segmented oligodiene polyurethane elastomers with rigid segments of different structure

A study of the structure and viscoelastic properties of oligodiene segmented polyurethanes shows that the introduction of urea groups into a rigid segment containing urethane groups significantly increases the degree of segregation of the flexible and rigid segments, and that rigid domains of inhomogeneous structure are formed. The network of physical bonds with rigid domains as network points is capable of rearrangement and fracture when subjected to different mechanical loads and different temperatures, because of the difference in the energy of physical interaction in the rigid heterogeneous domains. The relaxational transitions appear on the thermomechanical curves and the plots of the tangent of the dielectric loss angle against the temperature, their positions depending on the concentration of the urea groups and the magnitude of the applied mechanical load.

Thermomechanical and some mechanical properties of irradiated films prepared from polymer blends

Non-oriented films prepared from EVA-iPP and LDPE-iPP blends, irradiated with fast electrons, have been examined by thermomechanical analysis (TMA) and some mechanical properties (tensile strength and elongation at break) have been determined. The irradiation was carried out at low doses (5–12 Mrad) which brings about the formation of 50–60% gel content in the material. The thermomechanical curves of the irradiated films exhibit a plateau of a high elastic material after the segregation melting of the components, whose elastic modulus E depends on the composition and the irradiation dose. The density of cross-linking, evaluated qualitatively from E, increases with increase of D and the increase of the EVA and LDPE contents in the blends. The tensile strength does not depend on the blend composition and the dose, while the elongation at break decreases with increase of D. The peculiarities in the dependence of the thermomechanical and mechanical properties on the dose and the composition have been explained by the formation of a statistical network with a broad distribution of the lengths between cross-links. The properties of the samples depend on the density of cross-linking, which is determined in turn by the balance between cross-linking and scission in the separate components.

Some thermomechanical properties of polyvinyl alcohol cryogels

Some thermomechanical properties of hydrogels formed as a result of freezing-thawing of concentrated aqueous PVA solutions have been studied. The character of the thermomechanical curves depends on the conditions in which the cryogels were obtained and is largely determined by the regimes of unfreezing of the samples. From the measurements of the melting points of the cryogels of different concentration the authors evaluate the melting enthalpies of the polymeric gel nodes on passing from the gel to solution.

Pecularities of the thermomechanical behaviour of ultra-high molecular weight linear polyethylene and its blends with linear polyethylene of normal molecular weight

Ultra-high molecular weight polyethylene UHMWPE (Mw=4 · 106,Is=O g/ 10 min), high density polyethylene of normal molecular weight NMWPE (Is= 4.8 g/10 min) and their blends have been investigated by means of thermomechanical loading in constant and impulse regime. It has been established that after melting, NMWPE passes to a viscous-liquid state. After melting at 138 °C UHMWPE passes to a high-elastic state. The transition of UHMWPE to a viscous-liquid state takes place at temperatures higher than 180 °C and is accompanied by a high-elastic reversible deformation. The blends of UHMWPE with 10 and 20 mass % of NMWPE show a plateau on the thermomechanical curves, corresponding to a high-elastic state, in a shorter temperature range where the deformation is greater. The blends containing the higher percent of NMWPE show thermomechanical curves lacking such a plateau. All blends are characterized by a singular thermomechanically defined temperature of melting, which increases with increase of UHMWPE content. The existence of the high-elastic state in the curves of UHMWPE and its blends containing NMWPE less than 30 mass % above their melting temperatures is explained by the high degree of physical crosslinking of UHMWPE.

Study of crosslinking of unsaturated polyester resins by relaxation methods

AbstractRadiation and thermal curing of unsaturated polyester resins with styrene were investigated by combining differential thermal analysis with electric and mechanical relaxation techniques. A microprocessor controlled combined relaxation equipment of special construction was used. By radiation initiated curing the reaction was interrupted at different stages and the products were analyzed. Relaxation and simultaneous differential thermal measurements were also made during the course of gamma radiation and peroxide initiated thermal curing. By this way the shift of the characteristic transitions of the resin as a function of conversion could be studied. Also the change of the phase‐structure of the resin caused by the reaction was monitored. By deconvolution of the dielectric spectrum band the physical structure was found to become heterogeneous by crosslinking.Besides the shift of the transition temperatures the oscillator strength of the dielectric transition was found to decrease with conversion. Electrical polarization and depolarization studies were also performed. A special intermittent load thermomechanical technique was used for separating elastic from viscous response of the sample subjected to external mechanical force. The transitions exhibited by the thermomechanical curves were found to shift to higher temperatures by crosslinking and the compliance plateaus decreased.

An investigation of an oligocarbonate-methacrylate polymer synthesized under triaxial compression

The properties of three-dimensional polymers based on bis-(methacryloyloxyethylenoxycarbonyloxyethylene) oxide, synthesized under conditions of triaxial compression at pressures up to 512 MPa, have been investigated. It has been found that the polymer obtained under such conditions has a set of physico-mechanicl properties that differs from that of specimens synthesized at atmospheric pressure: in particular, the polymer synthesized under high pressures has a higher density and modulus of elasticity but a lower glass temperature, as determined from thermomechanical curves.

Synthesis of graft copolymers of the polystyrene-polysiloxane series

The graft copolymerization of octamethylcyclotetrasiloxane with oligostyrenecarbocyclosiloxanes containing cyclosiloxane links attached as side groups to the main has been studied. Graft copolymers of the PS-polysiloxane series have been obtained, in which polydimethylsiloxane branches with various lengths are grafted to the carbon chain of the styrene oligomer. The composition of the copolymers obtained has been analysed by various methods. The kinetic dependence of the yield and viscosity of the systems that have been studied are S-shaped in nature, the slower initial period being longer in duration, the greater is the concentration of the oligostyrenecarbocyclosiloxane in the reagent mixture. The thermomechanical curves of the copolymers obtained point to the presence of transition temperatures corresponding to the glass temperatures of both the polydimethylsiloxane and also the PS blocks in the macromolecules of the graft copolymer.

Thermomechanical, X‐ray and IR investigations of polyamide‐12 films

AbstractThe thermomechanical curves of monoaxially drawn PA‐12 films are recorded. The films were previously subjected to annealing at different temperatures from 40°C to 170°C. The values of Tg and Tm derived from the thermomechanical curves show non‐linear dependence on the applied tension stress. The intensity of the IR absorption bands assigned to the folded conformation of the macromolecules increases with the rise of annealing temperature. In the IR spectrum of the film annealed at 170°C and subjected to additional drawing the intensity of these bands decreased as the degree of drawing increased. These results are interpreted as an indication for crystallization with folding of the macromolecules upon annealing and defolding under the conditions of recording of the thermomechanical curves. WAXS patterns suggest the presence of γ crystalline modification in PA‐12 under the conditions applied.

Relaxation processes in elastomers at low strains from the data of relaxation spectrometry, stress-strain and thermomechanical curves

Relaxation processes in a butadiene-methylstyrene elastomer have been investigated at small strains by the methods of relaxation spectrometry and the direct determination of the stress-strain and thermomechanical curves. In the transition region from the glassy to the rubbery condition, all the methods used revealed the λ′ and λ relaxation processes that are related to molecular mobility in various elements of the supermolecular structures. The results obtained are in agreement with Boyer's data.

Comparative thermal and thermomechanical properties of hot melt adhesives

Abstract Thermal properties of hot melt adhesives as well as their bonds can reasonably be studied by thermogravimetric and thermomechanical methods. In addition to thermogravimetry, determination of thermo-oxidative stability is also suitable for assessing the thermal properties of hot melt adhesives. Effects of their components on the glass transition and melting temperatures ( T g and T m respectively) and the viscoelastic behaviour of hot melt adhesives can be investigated by recording and evaluating of thermomechanical curves. Thermomechanical creep measurements used in the plastics industry are very useful for determining the thermal stability, that is the shear fail temperatures of adhesive bindings. In addition, these methods can be used for qualitative and quantitative investigations of the major ingredients of adhesive compounds.

Microphase separation and properties of urethane elastomers

AbstractSeveral polyurethanes were prepared from poly(ethylene adipate)glycol, 4,4′‐diphenylmethane diisocyanate, 1,4‐butanediol, or 3,3′‐dichloro‐4,4′‐diaminodiphenylmethane. Thermomechanical curves were determined for all investigated systems, and the glass transitions found thereof formed the basis of subsequent analysis. The analysis showed that the investigated systems belonged to those polyurethanes that can undergo microphase separation. It was also shown that the separation in the diamine‐extended elastomers was less sensitive to changes in chemical composition than in the diol‐extended systems. The diamine‐based polyurethanes were also closer to the ideal separation than the diol‐extended elastomers. Examinations of elastomers of varying amount of chemical crosslinking revealed a joint action of the crosslinking and microphase separation in determining mechanical properties of the systems. Together with swelling measurements, the examinations showed that optimum mechanical properties were obtained while keeping a very careful balance between these two factors.

Effect of the structure of an epoxide binder on creep, during cyclic compression at a low frequency

1. The considerable effect of the structure of an epoxide binder on its creep during cyclic loading is revealed and discussed. 2. The causes of the correlation observed between the shape of the deformation diagrams, the creep curves, and the thermomechanical curves are discussed and it is proved that this correlation is based on kinetic effects whose role increases with increasing mobility of the network. 3. The deformational durability of a binder at a high load level is described by a well-known equation for the durability. The special features shown by the effect of the structure of a cross-linked polymer on the parameters of the equation are discussed. An estimate of the intermolecular interactions on the magnitude of the activation energy for the creep process shows that the molecular interaction upon hardening of an epoxide binder increases up to the highest degree of cross-linking.

Regularities of the formation of polystyrene hollow microspheres

The effect of different factors on the formation of hollow microspheres in the process of foaming of suspended polystyrene granules has been investigated at temperatures close to the boiling point of the foaming agent. It is shown that the non-isothermal kinetics of foaming are identical in character to the thermomechanical curves, and critical sizes of initial particles, at which a polycellular structure is formed, have been determined. It was found that these critical particle sizes depend on the diffusion coefficient of the foaming agent, and that as a rule a higher diffusion rate leads to an increase in the critical size of granules, and to increased probability of the formation of monospheres. The data obtained are analysed mathematically, and a set of equations is derived where by kinetic regularities of the foaming of polystyrene granules by liquid foaming agents are described.