Molecular motions in poly(vinyl acetate) revisited. A thermally stimulated current study

The dipolar relaxation mechanisms in poly(vinyl acetate) have been studied in detail using the technique of thermally stimulated currents. The papers published in the literature about this subject are very contradictory, particularly with respect to the assignment of the observed discharges to the corresponding motions at the molecular level. This work aims at clarifying these problems. We detected and characterised three different relaxation mechanisms: (1) a low temperature one (around ?140°C) which was attributed to local internal rotations in the acetate side-groups; (2) a relaxation whose maximum occurs at 42°C, which corresponds to the glass transition relaxation, and shows a compensation behaviour; (3) an upper glass transition relaxation whose maximum appears at 87°C and was attributed to a liquid-liquid transition. These assignments have been made on the basis of the analysis of the behaviour of the samples when submitted to different thermal and electrical treatments.     

Mechanism of physical aging in crystalline polymers

The mechanism of physical aging in isotactic polypropylene was studied in the α-region (15–80°C) by thermally stimulated creep and by creep superposition. Aging for an elapsed time t_e was observed to have no effect on retardation elements at τ for τ ? t_e. Thus the pronounced effects observed in aging cannot be due to the movement of the retardation spectrum as a whole to longer times. The method of time-elapsed time (t?t_e) superposition fails because the retardation spectrum changes shape. This change in shape leads to systematic deficiencies in the superposed curves, an effect which shows up most clearly in superposition of the differentiated creep curves. The possible changes that take place during aging are a change in magnitude of the retardation elements and a shift to longer times. These changes will only occur in retardation elements at τ when τ ? t_e.     

Viscoelastic properties of Japanese lacquer film

The storage modulus (E′) and loss modulus (E″) of Japanese lacquer films were measured over a temperature range of −150 to 400°C. Three relaxation processes labeled α, β, and γ were detected at 80, −60, and −140°C, and their apparent activation energies (ΔE) were 63–91, 13, and 9 kcal/mol, respectively. These were attributed to the micro-Brownian motions of polymerized urushiol, the molecular motion related to the absorbed water, and the motions of methylene groups in the side chains, respectively. With aging at room temperature, the location of the α peak shifted to higher temperature and its ΔE value decreased. This result was ascribed to the autoxidative polymerization of urushiol. The E′ of lacquer films increased with heat treatments at 100°C or above. When treated at temperatures below 200°C, the location of the α peak shifted to higher temperature, with a reduction in the ΔE value. Heat treatments at 200°C or above resulted in remarkable shrinkage and weight loss of films owing to the pyrolysis of lacquer constituents. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1727–1732, 1999     

Antiplasticization and transition to marked nonlinear viscoelasticity in poly(vinyl chloride) (PVC)/poly-ε-caprolactone (PCL) blends

Measurements of mechanical damping (tan δ) in the temperature range of ?120° to +120°C at 110 Hz, of uniaxial tensile creep at 25.0° ± 0.5°C covering creep times from 10 to 1000 sec, and of impact strength at 21°C have been carried out for a series of physical PVC/PCL blends in the composition range of 0%–12% by weight of PCL in the blend. With increasing PCL content in the blend, the α-peak of PVC was shifted to lower temperatures and became broadened. The β-peak of PVC was also shifted to lower temperatures and was markedly suppressed. The tensile creep compliance of approximately linear viscoelasticity showed a maximum decrease of 10%, and the impact resistance was reduced 3.5 times when 5% and 12% by weight of PCL, respectively, was blended with PVC. There was also a considerable increase (25%) in stress level at which the transition from approximately linear to markedly nonlinear viscoelasticity occurred when up to 5% by weight of PCL was added to the PVC. These results are attributed to the antiplasticizing effect of PCL on PVC. They support the importance of β-mechanism in the stress-activated processes proposed to be responsible for the appearance of nonlinear viscoelasticity in glassy polymers, and they are in agreement with the pseudocrosslinking concept of antiplasticization. By comparing the antiplasticization behavior of PVC/PCL blends with that of PVC/DOA and PVC/DOS from reported data, it was possible to obtaing an idea of the level of compatibility in the PVC/PCL blends. The results suggest that PCL is partially compatible with PVC.     

Dynamic viscoelasticity of PUF under small strain and prediction of dynamic viscoelasticity in a wide frequency range

The dynamic viscoelasticity of polyurethane fiber (PUF) under small strain was studied with dynamic mechanical analysis and differential scanning calorimetry, and its viscoelasticity was predicted in a wide frequency range. The results revealed that the loss factor (tan δ) was about 0.05 and the phase angle (δ) was 2.86°, very close to the δ of a perfect elastomer in the range from −100°C to −80°C. The master curves superimposed by time-temperature superposition (TTS), the frequency range of the storage modulus master curve spanned 17 orders of magnitude from 10⁻⁷ Hz to 10¹⁰ Hz, and the loss modulus master curve spanned 13 orders of magnitude from 10⁻³ Hz to 10¹⁰ Hz. The apparent activation energy of the soft segment relaxation decreased with increasing temperature in the range from −70 to −26°C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47070.     

TSD-Untersuchungen zum Einfluß von Wasser auf die molekularen Beweglichkeiten in Polyamid-6

Molecular motions in nylon 6 are studied by thermally stimulated discharge (TSD) techniques. The effect of absorbed water on the relaxation behaviour of nylon 6 is discussed. The γ relaxation process at ?145°C is attributed to local segmental motions in the amorphous phase. The intensity of the γ relaxation is reduced and broadened by absorbed water. The molecular origin of the β relaxation process in the temperature range between ?80 and ?60°C is associated with localized reorientational motions of chain segments involving nonchain-bonded or weakly bonded amide groups. It is quite likely that also rotational rate processes of water molecules bonded to CO-groups contribute to the β process. The intensity increases are due to added water, since associated water molecules are involved with those molecular motions. The α relaxation process at 70°C is associated with large-scale segmental motions at the glass transition. The chain mobility is increased by added water and gives rise to an α peak near 50°C. The α′ relaxation process in the temperature range between 100 and 110°C may either be associated with reorientational motion of stretched chain segments or with the beginning of a crystalline transformation of the γ phase into the monoclinic α form. The temperatures of TSD-peaks correspond to alternating current loss data at a frequency of 5·10^?2 Hz.     

Viscoelastic behavior of poly(cyanate epoxy)

The viscoelastic behavior of poly(cyanate epoxy) has been investigated by dynamic mechanical analysis and thermostimulated creep. First, the coherence of both sets of data has been shown. The analysis of the fine structure of the low temperature retardation mode has revealed the existence of four cooperative sub modes. The high temperature retardation mode corresponds to the anelastic manifestation of the glass transition. The elementary processes constituting this mode are characterized by retardation times following a compensation law with a compensation temperature T_c ～ T_g + 60°. The broad range of activation enthalpies reveals that cooperative movements are highly delocalized. The compensation parameters together with the range of activation enthalpy define the microstructure of poly(cyanate epoxy) and allow us to follow its relationship with the viscoelastic behavior.     

Free radical crosslinking of unsaturated bacterial polyesters obtained from soybean oily acids

Summary Poly(-3-hydroxy alkanoate) containing unsaturated side chains, PHA-soybean, were produced by feeding Pseudomonas oleovorans with soybean oily acids obtained from soybean oil. Unsaturation of PHA-soybean were found to be 10 mol-% of unsaturated side chains. Main saturated part of the biopolymer was Poly(3-hydroxy octanoate) with minor hexanoate and decanoate units. PHA films were crosslinked via free radical mechanism by means of thermally or under UV irradiation in the presence of benzoyl peroxide, benzophenon, and /or ethylene glycol dimethacrylate (EGDM). Crosslinking yield of the PHA films were found to be from 81 to 93 wt.-% from the sol-gel analysis. Swelling properties of the crosslinked PHA films in chloroform and toluene were also studied. Mc values of crosslinked PHAs were also calculated using Flory-Rehner equation. The crosslinked biopolyester obtained by thermally at 60 °C with benzoyl peroxide indicated the highest crosslinking density. Glass transition temperatures (Tg) of crosslinked biopolyester samples were changed from −33 to −45 °C while that of PHA-soybean was −60 °C. Received: 16 June 2000/Revised version: 22 January 2001/Accepted: 20 May 2001     

High temperature compressive creep behavior of BaCe0.65Zr0.2Y0.15O3−δ in air and 4% H2/Ar

The proton conductive material BaCe_0.65Zr_0.2Y_0.15O_3−δ has great potential for the separation and purification of hydrogen. However, due to the demanding application conditions regarding both temperature and atmosphere, the elevated temperature structural stability needs to be characterized and warranted. Hence, in this research work, the elevated temperature compressive creep behavior of BaCe_0.65Zr_0.2Y_0.15O_3−δ in the temperature regime of 850°C to 1200°C was studied in both air and 4% H₂/Ar as a function of the applied stress. The results indicate different creep mechanisms depending on atmosphere and temperature range. While dislocation creep was observed in 4% H₂/Ar over the full range, a dislocation creep mechanism was observed in air at temperatures ≤1050°C and a diffusional creep mechanism at temperature ≥1100°C. A detailed microstructural analysis of the post-creep test specimens revealed that the exposure to oxygen leads to localized stoichiometric changes and a decomposition at the surface.     

TSC study of secondary dielectric relaxations in a polyepoxy matrix composite

Two thermal analysis techniques—Differential Scanning Calorimetry and Thermostimulated Current—have been used to characterize a mica/glass fiber-reinforced composite; the matrix is a novolac epoxy resin. The glass transition temperature of the composite as determined by DSC is 80°C. Below the glass transition temperature, two complex relaxation modes γ and β, respectively situated at −90 and −40°C, have been observed. The analysis of the fine structure shows that they are constituted of elementary processes characterized by relaxation times following compensation laws. Because the compensation temperature corresponding to the γ mode is located in the vicinity of the glass transition, it has been assigned to cooperative movements precursors of the glass transition. The mobile units might be the phenyl rings in the bisphenol A. Considering its dependence upon water content, the β mode has been related to the molecular mobility of bound water. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 135–139, 1997     

Fabrication and characterization of bone china using synthetic bone powder as raw materials

The synthetic bone powder was studied as a raw materials for bone china, completely replacing natural bone ash raw materials. The physical and thermal properties of samples obtained by the two bone powders were tested and comparatively studied. Performance tests included pyroplastic deformation, flexural strength, bulk density, sintering shrinkage, water absorption, transmittance, thermal expansion coefficient and the thermal shock resistance. The phase composition and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicated that using synthetic bone powder could shorten the preparation time, reduce the sintering temperature and result in high-quality bone china. The pyroplastic deformation decreased from 9.14% to 7.92%, the three-point flexural strength increased from 123 MPa to 191 MPa, the light transmittance (at a 2-mm thickness) increased from 6.7% to 11.2%, the thermal expansion coefficient decreased from 8.24×10⁻⁶ °C⁻¹ to 7.69×10⁻⁶ °C⁻¹, and the thermal shock resistance increased from 140 °C to 180 °C. A continuous interface layer without cracks was produced by using the synthetic bone powder.     

Order-Disorder Conformation Change of Succinoglycan in Aqueous Sodium Chloride as Studied by Static and Dynamic Light Scattering

Summary Static and dynamic light scattering measurements have been made on a sodium salt sample of succinoglycan in 0.01 M aqueous NaCl at different temperatures between 25 and 75°C where the polysaccharide undergoes a thermally induced change from an ordered (helical) to disordered conformation with raising temperature T. The weight-average molecular weight M _w, the z-average radius of gyration, and the hydrodynamic radius sharply decrease in a relatively narrow T range (around 55°C) in which the specific rotation was previously found to change sigmoidally with T. In particular, the value of M _w (4.55 × 10⁵) in the ordered state at 25°C is twice as large as that (2.27 × 10⁵) in the disordered state at 75°C, giving decisive evidence that the helical structure of the polysaccharide in aqueous NaCl is composed of paired chains. It is concluded that this structure is a double-stranded helix and breaks directly into two disordered chains with increasing T. Received: 6 July 2001 / Accepted: 24 July 2001     

Time-dependent properties of epoxy resin with imidazolium ionic liquid

This study investigates creep and viscoelastic behavior of the diglycidyl ether of bisphenol A (DGEBA) epoxy resin and triethylenetetramine (TETA) system containing an imidazolium ionic liquid (IL), the 1-n-butyl-3-methylimidazolium chloride ( C ₄ MImCl ). Different time-dependent analysis methods are studied using data from tensile creep, tensile creep/recovery, and three-point and four-point flexural creep tests of epoxy with 1.0 or 4.0 phr of IL. From the results, the composition containing 1.0 phr of C ₄ MImCl , cured at 60°C, presented greater viscoelasticity and crosslink density compared to compositions cured at 30 and 40°C, which was attributed to higher free volume and higher molecular mobility induced by the presence of the IL. In tensile creep tests using the stepped isostress method (SSM), no important degrading effects were found after the addition of 1.0 phr of IL over long time periods. This composition also showed the best overall performance in flexural SSM creep tests.     

Synthesis and thermally stimulated current study of molecular relaxations of the grafted polymethylphenylsiloxane segment in thiodiphenol-modified epoxy resin

A series of grafted polymethylphenylsiloxane (PMPS) segments in thiodiphenyl-containing epoxy resin (ESTP) was prepared. The structure was evaluated by IR, ¹H-NMR, and ¹³C-NMR measurements. DSC measurements on the grafted ESTP epoxy resins showed a decreasing trend for T_g with increasing content of PMPS siloxane. The TSC measurements further confirmed this trend. This result suggests that the matrix of ESTP copolymer was less rigid with grafting of PMPS oligomer, due to the Si—O—C linkage at the opened glycidyl ether chain located approximately between the BPA and 4,4′-thiodiphenoxyl chains in the epoxy backbone. A new and broad sub-T_g transition appeared at −60°C in the TSC spectrum was observed for the cooperative motion of this siloxane moiety. All copolymers showed sub-T_g relaxations of γ- and δ-modes, observed at −100 and −130°C, respectively. These two relaxations may be attributed to the motions of BPA and the terminal groups in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1523–1530, 1998     

The limits of linear viscoelasticity in poly(methyl methacrylate) and poly(ethyl methacrylate)

The limit of linear viscoelasticity is determined for poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) in uniaxial tension creep over the temperature range of 20° to T_g ?10°C. The time span covered is from 10 to 1000 sec. The linear limit is defined as the point at which the creep compliance deviates by more than 1% from its mean value in the linear viscoelastic range. For both materials, the stress limit of linear viscoelasticity falls to a minimum or plateau level at a temperature below T_g. It is suggested that the β-mechanism plays an important role in the existence of this minimum.     

Thermally stimulated current in glass-laminated unsaturated polyester resins

Experimental results are reported on the electrical properties of glass-laminated unsaturated polyester (UP) resins. Measurements of the poling current as a function of temperature reveal two different activated processes. Above 50°C the activation energy is 160 kJ/mol and has been associated with ionic condition, possibly by protons, while at lower temperatures (below −70°C) the activation energy is very low (less than 40 kJ/mol) and is associated with dipole relaxation. It is concluded that the laminated UP resin undergoes a phase transformation between low and high temperatures, which gives rise to a completely different electrical response. This phase transformation can be associated with the glass transition of UP resin phase. In view of the low activation energy, giving rise to a polarization peak, it is concluded that the low temperature range (glassy state) favors strictly localized motions of charges—most likely electrons. At higher temperatures, the behavior corresponds to d.c. conductivity and is associated with extended motions of charge carriers with limitations at contacts and at internal barriers. According to the air gap measurements, the pronounced dip found in the temperature dependence of the poling current (in the range from 20 to −30°C) may be due to separation of conduction and dipole relaxation current. Thermally stimulated depolarization (TSD) current measurements reveal the presence of high internal fields due to space charges accumulated at the phase boundaries of glass fibers.     

Synthesis and Characterization of First Generation Dendritic Azidoesters

A new class of first generation azido‐terminated dendritic esters have been synthesized by four step processes. The final products and the intermediates are characterized by IR, ¹H NMR, and elemental analysis. Thermal studies revealed that these dendritic esters were thermally stable up to ∼190 °C and the glass transition temperature was found to be −54 and −32 °C, respectively.     

Study on porous silk fibroin materials. I. Fine structure of freeze dried silk fibroin

Silk fibroin solution was prepared by dissolving the silk fibroin in triad solvent CaCl₂ · CH₃CH₂OH · H₂O. In this article we tested and analyzed the state of frozen silk fibroin solution and fine structure of freeze dried porous silk fibroin materials. The results indicated that the glass transition temperature of frozen silk fibroin solution ranges from −34 to −20°C, and the initial melting temperature of ice in frozen solution is about −8.5°C. When porous silk fibroin materials are prepared by means of freeze drying, if freezing temperature is below −20°C, the structure of silk fibroin is mainly amorphous with a little silk II crystal structure, and if freezing temperature is above −20°C, quite a lot of silk I crystal structure forms. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2185–2191, 2001     

Effect of viscoelasticity on the spherical and flat adhesion characteristics of photopolymerizable acrylate polymer networks

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1–62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/−20 °C away from transition region , opening the possibility of thermally switchable adhesives.     

A comparative study of hydroxyapatite nanostructures produced under different milling conditions and thermal treatment of bovine bone

In the present investigation the effects of milling parameters (time, atmosphere, and media) and chemical composition of raw materials on the mechanochemical synthesis of nanocrystalline hydroxyapatite (n-HAp) were studied. For a comparative study of the mechanically activated samples versus thermally treated specimen (natural origin), n-HAp was also produced via annealing of bovine bone at 800 °C for 2 h. The gained powders exhibited average sizes about 32 and 27 nm under air atmosphere, and about 32 and 34 nm under argon atmosphere. TEM images confirmed the formation of n-HAp with various morphologies under different experimental conditions.