Thermoplastic Elastomer Phase Changing Materials for Stiffness Modulation

Abstract

Four natural rubber-based phase changing materials (PCMs) were synthesized and evaluated with the goal of achieving controlled stiffness modulation at elevated temperatures (～100-120?°C). The phase changing was achieved through either the glass transition or melt transition of one of four phase changing fillers: polystyrene, poly(methyl methacrylate), low density polyethylene, and indium tin alloy. The PCM stiffness was analyzed using dynamic mechanical analysis and the results showed that all four PCMs exhibited two thermal transitions: a low temperature transition at ?58?°C due to the glass transition of the natural rubber matrix and a high temperature transition at ～100-120?°C due to the thermal transitions in the four filler materials. The degree of stiffness change of all PCMs at 100-120?°C was found to be strongly influenced by the type of the phase changing filler as well as the type of thermal transition (glass transition versus melt transition). The compatibilities of the phase changing materials were analyzed using the Flory-Huggins interaction parameter and it was shown that natural rubber and low density polyethylene had the best compatibility. All systems exhibited effective stiffness modulation with the change in temperature, enabling them to be used for applications requiring variable stiffness control.     

Phase transitions in ferroelectric polymer films

Phase transitions in ferroelectric films of vinylidene fluoride-trifluoroethylene (70/30) copolymer are studied by differential scanning calorimetry (DSC). Films 40 nm thick are prepared from solution by centrifugation. DSC thermograms reveal peaks typical of the first-order phase transition that are associated with heat absorption and heat release, and the enthalpies of the transitions are determined. Upon cooling, a kink in the slope of the DSC curve that determines the temperature of the glass transition is observed at T = ?30°C.     

Observation and analysis of a glass transition in amorphous frozen solutions of iron-II chloride

This paper deals with Mössbauer investigations, X-ray diffraction studies and differential calorimetric measurements of the amorphous state of frozen solutions of FeCl₂ in water. This glassy state persists from at least ?180 °C until ?90 °C. All three experimental methods reveal the existence of a glass transition at ?110 °C from an amorphous state to a supercooled liquid. It is shown that for such transitions important conclusions can be drawn from a comparison between the Mössbauer and X-ray diffraction Debye-Waller factor respectively by determining the transmitted Mössbauer intensity far off resonance. Out of the analysis we conclude that the glassy state of quenched ice is due to the hexaquo complexes as implanted impurities which prevail their surrounding from a regular crystallisation. These impurities are also responsible for the glass transition into a supercooled liquid state by releasing new degrees of freedom as e.g. hindered rotational modes.     

High Thermally Stable and Organosoluble New Poly(amide-imide)s Derived from Bis(4-trimellitimido phenoxy)phenyl Triptycene

A triptycene-containing dicarboxylic acid monomer, bis(4-trimellitimido phenoxy)phenyl triptycene, was successfully synthesized by refluxing the diamine, bis(4-aminophenoxy)phenyl triptycene with trimellitic anhydride in glacial acetic anhydride. A series of aromatic poly(amide-imide)s were prepared by condensation polymerization of the dicarboxylic acid monomer and seven different commercially available diamines. The resulting poly(amide-imide)s were soluble in various solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and pyridine. The polymers were amorphous and had inherent viscosities in the range 0.40–0.67?dL/g. These polymers had low dielectric constants ranging from 2.09 to 2.18. The glass transition temperatures of these polymers were observed between 229°C and 277°C. These polymers showed good thermal stability without significant weight loss up to 500°C. The temperatures at 10% weight loss ranged from 512°C to 570°C in nitrogen. The UV-Vis absorption spectra revealed that most of the polymers had absorption maxima around 312–348 nm, indicating the conjugation between the aromatic rings and nitrogen atoms. Solvent cast films had tensile strengths from 75 to 108?Mpa, elongates at break of 7%–9% and tensile moduli of 1.7–2.0?Gpa, indicating they were strong materials.     

Mechanical behavior of poly(ethyleoe terephthalate) at cryogenic temperatures

The mechanical behavior of poly(ethylene terephthalate) (PET) was studied at temperatures from 4 to 300°K with a free oscillating torsion pendulum and an Instron tester.

Torsion pendulum data show the presence of relaxation maxima below 80°K. Noncrystalline specimens show a small peak at 48°K (δ) and a shoulder at 8-12° K (?). A peak at 20° K is observed in crystalline samples, both oriented and unoriented. In addition drawn specimens show a pronounced peak at 48° K and the assymmetric maximum in the 100-240° K region is split into two peaks. The logarithmic decrement in the β-loss region decreases with increasing crystallinity and orientation, while the cryogenic loss peaks increase in intensity.

Tensile tests with biaxially oriented and heat-set films show a twofold increase in elastic modulus and a tenfold increase in toughness at cryogenic temperatures when compared with “amorphous” PET.     

Thin-layer chromatography of copolyesters from blends of poly(ethylene terephthalate)/polycarbonate

Copolymers made by ester exchange reaction have been obtained from poly(ethylene terephthalate) (PET)/poly(bis-phenol-A carbonate) (PC) blends during melt mixing. The copolyesters were isolated by thin-layer chromatography (TLC) and identified by infrared spectroscopy. It was found that the quantity of copolymer formed was increased by the temperature and duration of melt mixing. The PET/PC blend was found to react at 270°C within 10 min, as detected by TLC. After 60 min, the pure PC had disappeared. The miscibility of PET/PC blends was found to be markedly aided by the addition of as little as 2% of the copolymer isolated by TLC.     

Relaxation phenomena of poly-γ-benzyl-L-glutamate,poly-γ-methyl-L-glutamate,and copoly(γ-methyl-L-glutamate, γ-benzyl-L-glutamate)

Relaxation phenomena of poly-α-amino acids in the solid state have been investigated using poly-γ-benzyl-L-glutamate (PBLG), poly-γ-methyl-L-glutamate (PMLG), and copoly (γ-methyl-L-glutamate, γ-benzyl-L-glutamate) (PMBG) by means of dielectric, dynamic mechanical, NMR, dilatometric, and X-ray diffraction measurements at temperatures between ?196 and 180°C.

Each of the samples exhibits two relaxation regions, one at room temperature (β-relaxation) and the other in the range from ?150 to ?100°C (γ-relaxation). The γ-relaxation is attributed to motion of the side chains with small amplitude. The β-relaxation is due to large-scale motion of the side chain. It has been found that the β-relaxation is well described by the WLF-equation.

The intensity of the X-ray diffraction peak at 2θ = 7° for PBLG increases with increasing temperature, which is similar to results obtained in small-angle X-ray scattering for polymer crystals consisting of two phases, amorphous and crystalline. A break point is observed at 18°C where the specific volumetemperature curve also shows a break point.

It is concluded that the side chains of these polymers are almost amorphous, and that they undergo a glass-like transition while the backbones keep an α-helical conformation.     

Concerning free volume quantities and the glass temperature

Free volume quantities proposed earlier by Boyer and Simha in connection with the glass transition are reformulated by taking into account the temperature dependence of the thermal expansivities α _l and α_g for the liquid and the glass, respectively. This necessitates an extrapolation of the liquid to temperatures below Tg which is performed by means of the reduced volume-temperature function established and given a theoretical foundation previously. For the glass, low temperature experimental data, encompassing all relaxations occuring below T_g, are required.

Two polymer series are examined in detail, namely, poly(methacrylates) and poly(vinyl) alkyl ethers, where α_g has been measured between at least 30°K and T_g. Results for poly(methylacrylate) and poly(styrene) are also given. The systematic decrease in the product (αl - αg) · T|_T=Tg with increasing length of the side chain noted previously is considerably reduced but not eliminated when the appropriately corrected expression is substituted instead. However, the free volume fraction related to the quantity αlT|_T=Tg remains more nearly invariant in the polymers analyzed.

An alternative treatment is discussed which considers an occupied volume expanding below T_g by a mechanism of thermal vibrations solely. Experimental and theoretical means of obtaining this quantity arc suggested.     

The Slow Molecular Mobility in Semicrystalline Poly(Ethylene Oxide)

The thermally stimulated depolarization current (TSDC) technique has been used to study the slow molecular mobility in the amorphous part of the semicrystalline polymer, poly(ethylene oxide) (PEO). Experiments were carried out in the temperature range that includes the glassy state, the glass transformation region and the rubber state. The dipole moments in the polymeric main chain originated a broad and low intensity secondary relaxation in the temperature region from ?130°C up to the glass transition region; the activation energy of the motional modes of this secondary relaxation was in the range between 35 and 100 kJ mol^?1. The glass transition temperature of the PEO, provided by the TSDC technique, was T_g = ?53°C, and the fragility index was found to be m = 43. A strong relaxation above T_g was observed, whose molecular origin was discussed. The thermal behavior of the PEO was also characterized by differential scanning calorimetry.     

Effects of thermal contraction on structure and properties of PET fibers

A series of poly(ethylene terephthalate) fibers of various molecular weights was first drawn to a practical maximum draw ratio and then allowed to contract thermally under tension for 10, 20, and 38%. These contracted fibers exhibit a high degree of plasticity even when tested at—100°C and extension rates of 1300%/sec. An attempt is made to explain this behavior by means of a systematic study of morphological changes which occur during thermal contraction. The interpretation of the results of small-angle and wide-angle x-ray diffractions, infrared spectroscopy, and birefringence suggest the existence of two types of amorphous domains; those separating the adjacent crystallites in the microfibril and those separating the microfibrils. It is speculated that the molecules in these two domains respond differently to thermal effects and stress, and that the interfibrillar amorphous domain consists of highly extended molecules.

It is shown that the thermal contraction, which does not involve major changes in the degree of crystallinity, proceeds by several mechanisms. At low degrees of contraction, the most important mechanism is the contraction of the microfibrils. At high levels of contraction, the shrinkage proceeds to a large degree via relative displacement of the microfibrils and the contraction of extended interfibrillar tie molecules.

The conclusions regarding the structure of these fibers are corroborated by means of transmission electron microscopy of thin cross-sections.     

Measurements of the complex shear modulus of polymers under hydrostatic pressure usina the quartz resonator method

Abstract

The quartz resonator method measures the complex shear modulus or compliance of viscoelastic materials in the frequency range from 50 kHz to 140 MHz at temperatures between ?150°C and 300°C and pressures up to 1 GPa. This method can be applied to viscous fluids or polymer melts -even in their glassy or seminystalline regime.

The phase diagram of poly(diethylsiloxane) PDES (a mesophase polymer) was determined for two samples with different molecular weight at pressures up to 400 MPa and temperatures between 20°C and 100°C. Phase transitions are indicated by a sharp bend in the shear compliance although the volume effect of the mesophase-isotropic transition vanishes around 80 MPa.

The pressure dependence of the glass relaxation process (in PVAc), was studied by measuring the change of the complex shear modulus with pressure at constant temperatures between 95°C and 145°C and pressures up to 600 MPa. Additionally to the relaxation process, also the pressure dependence of the real part of the shear modulus in the glassy region can be determined for testing the dislocation concept in the meandermodell by W. Pechhold.     

The glass transition temperature of amorphous poly(ethylene terephthalate) by thermally stimulated currents

The relation between the temperature T_α of the dipolar relaxation, obtained by the technique of thermally stimulated currents (TSC) and the glass transition temperature T_g has been studied in amorphous poly(ethylene terephthalate) samples. The temperature T_α depends fundamentally on the polarization temperature T_p, the polarization time t_p, and the heating rate v. For each heating rate a maximum T_α, T_M, was obtained for an optimum polarization temperature T_po. The value of T_po is 70°C, independent of the heating rate, and very close to the glass transition temperature obtained by differential scanning calorimetry (69°C). The resulting value for T_M coincides with T_po in the limits of null heating rate and null isothermal polarization time, and, consequently, T_M gives the value of the glass transition temperature for each heating rate as a function of the isothermal dipolar contribution on polarizing at the temperature T_po.     

Mechanical Behavior of Bulk Poly(Ethylene Terephthalate) Subjected to Simple Shear

Abstract

Mechanical behavior of bulk semicrystalline poly(ethylene terephthalate) (PET) processed through simple shear is investigated. The equal channel angular extrusion (ECAE) process was used to achieve the simple shear condition. The PET samples were processed in one and two ECAE passes in the same direction, with the sample rotated 180° about the extrusion axis for the second pass. Microstructural features at the nanometer and micrometer scales were studied by small‐angle x‐ray scattering (SAXS) and scanning electron microscopy (SEM). SAXS results showed that at the nanometer scale, two types of lamellar orientations are induced in both samples, but with different extents of orientation. In the ECAE‐oriented PET structures on the micrometer scale, as revealed by SEM, are well‐defined macrofibrils. However, the fibrillar structures in the sample extruded once are more oriented than those in the sample extruded twice. Fractography investigations suggest that the ECAE‐induced fibrillar structure and stretched amorphous chains are responsible for the change in mechanical properties.     

Dynamic mechanical characterization of relaxations in poly(oxymethylene), miscible blends,and oriented filaments

Multifrequency dynamic mechanical analysis (DMA) data were obtained for molded poly(oxymethylene) (POM) and its blends from-150°C to 150°C. Because of the high crystallinity, the assignment of the glass transition in POM has been controversial in the literature. Low and high glass transition temperature (T _g) phenolated compounds, including poly(vinyl phenol), were found to be miscible with POM. The shift of the β transition in the POM blends favors an assignment of the β transition detected at ?3°C(1 Hz), not the ?80°C γ transition, as the T _g in semicrystalline POM because the latter is invariant with diluent. The peak at the β transition in pure POM is weak and can only be seen clearly by DMA measurements on samples that have not “aged” at ambient temperature. This is further evidence that the β transition arises from a cooperative glass-transition-like motion. The γ transition is not influenced by aging because it is due to a concerted localized main chain motion. The β transition of an oriented POM filament can be seen in the DMA flexural loss spectrum at-18°C (1 Hz), but not in a tensile loss spectrum. The broad a relaxation was detected at about 110°C (1 Hz) in molded POM and its blends, while it was shifted to about 135°C in the higher crystallinity, oriented system. The α peak is also independent of diluent, consistent with a crystalline origin for this transition, as was proposed earlier.     

Glass transition of poly(ethylene terephthalate)

It has been found that commercial poly(ethylene terephthalate) film exhibits current glow curves which have maxima at 73.5 ± 3.4°C and at 105.3 ± 3.4°C. These current glow curves were obtained by measuring the current flowing under zero bias as the temperature was raised 1°C/min. A typical curve, for untreated 1-mil du Pont Mylar A, is seen in Fig. 1. Although a paper on this study will shortly be submitted for publication, some conclusions of that paper may be stated here.     

Effect of thermal history on Tg and corresponding Cp changes in PVC of different stereoregularities

Samples of poly(vinyl chloride) polymerized at temperatures from ?60°C to +90°C, and therefore of different degrees of syndiotacticity and crystallinity, have been thermally treated in an attempt to reduce their content of ordered structures. Changes in order have been monitored by measuring Tg and the corresponding specific heat increment Δc_p. Partially syndiotactic samples have higher T_g and lower Δc_p than samples that are nearly atactic. After thermal treatment the differences decrease but do not vanish, even for samples heat-treated to about 250°C. This seems to indicate that some ordered structures are very stable even at relatively high temperatures. A quantitative estimate of the ordered structure content, based on Δc_p data, is proposed.     

Phase transition of quaternary alkyl halide salts of diazabicyclo [2.2.2] octane

A series of quaternary alkyl halide salts of diazabicyclo [2.2.2] octane were found to exhibit first-order phase transitions at 64–103°C. Drastic changes of halide anion conductivities by three to four orders of magnitude were found to occur at the transition temperatures. X-ray diffractions of powdered sample revealed that the transition was induced by a structural change of the crystal. Differential scanning calorimetries were also measured.     

The compatibility and transesterification for blends of polyethylene terephthalate)/poly(bisphenol-A carbonate)

A study has been made of the influence of transesterification on the miscibility in binary blends of poly(ethylene terephthalate) and poly(bisphenol-A carbonate). The blends were melt mixed in the range 260–300°C and studied by differential scanning calorimetry, dynamic mechanic analysis, and by Fourier transform infrared spectrometry. It was found that copolymer produced by a transesterification reaction can enhance the miscibility of this system. The new compositions were uniquely identified by FTIR. Gel permeation chromatography showed that molecular weight decreases were not the origin of miscibility. The ester exchange reaction itself was found to be initiated by the residual catalyst in the commercial polymers selected for study. This has been shown by the absence of reaction after polymer purification by solution and reprecipitation prior to melt mixing.     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below T_g carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.