Shift In Tg Research Articles

Degradation in Thermal Properties and Morphology of Polyetheretherketone-Alumina Composites Exposed to Gamma Radiation

Sheets of polyetheretherketone (PEEK) and PEEK-alumina composites with micron-sized alumina powder with 5, 10, 15, 20, and 25% by weight were fabricated, irradiated with gamma rays up to 10 MGy and the degradation in their thermal properties and morphology were evaluated. The radicals generated during irradiation get stabilized by chain scission and crosslinking. Chain scission is predominant on the surface and crosslinking is predominant in the bulk of the samples. Owing to radiation damage, the glass transition temperature, Tg increased for pure PEEK from 136 to 140.5 °C, whereas the shift in Tg for the composites decreased with increase in alumina content and for PEEK-25% alumina, the change in Tg was insignificant, as alumina acts as an excitation energy sink and reduces the crosslinking density, which in turn decreased the shift in Tg towards higher temperature. Similarly, the melting temperature, Tm and enthalpy of melting, ΔHm of PEEK and PEEK-alumina composites decreased on account of radiation owing to the restriction of chain mobility and disordering of structures caused by crosslinks. The decrease in Tm and ΔHm was more pronounced in pure PEEK and the extent of decrease in Tm and ΔHm was less for composites. SEM images revealed the formation of micro-cracks and micro-pores in PEEK due to radiation. The SEM image of irradiated PEEK-alumina (25%) composite showed negligible micro-cracks and micro-pores, because of the reinforcing effect of high alumina content in the PEEK matrix which helps in reducing the degradation in the properties of the polymer. Though alumina reduces the degradation of the polymer matrix during irradiation, an optimum level of ceramic fillers only have to be loaded to the polymer to avoid the reduction in toughness.

Influence of the sorption of polar and non‐polar solvents on the glass transition temperature of polyamide 6,6 amorphous phase

AbstractThe effect of a series of polar, amphiphilic, and non‐polar solvents of various molecular weights on the glass transition (Tg) of polyamide 6,6 (PA66) amorphous phase was studied. The sorption at equilibrium was measured by immersing the polymer in each solvent. Differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) were used to characterize the Tg of solvent‐saturated samples. It was observed that the solvent sorption provokes a drop on the polymer Tg, depending on the interactions between the polymer and the solvents. Polar molecules are absorbed in large amounts and decrease the Tg by more than 80°C, whereas non‐polar molecules are absorbed in very small quantities but have a large effect on Tg as well. The sorption equilibrium and Tg shift in the presence of the E10 mixture (10%vol ethanol, 45%vol toluene, and 45%vol isooctane) were also analyzed. A large E10 intake and a Tg drop close to that with pure ethanol are observed. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Synthesis and phase behaviour of a poly{2,5-bis[(p-ethoxyphenoxy) carbonyl] benzyl acrylate}-based mesogen-jacketed liquid crystalline copolymer

Mesogen-jacketed liquid crystal polymers have attracted considerable interest in recent years. In this paper, a series of poly (PEPCS)-based mesogen-jacketed liquid crystal copolymers (MJLCPs) with a different 4-hydroxybutyl acrylate (HEA) content has been synthesised and characterised using Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The hydroxyl groups are attached to the backbone in the PEPCS-co-HEA copolymers to facilitate further chemical crosslinking. These PEPCS-based MJLCPs show a reversible phase transition when the HEA content is lower than 30 wt%. Tg and Ti shift to lower temperatures as the HEA content increases. POM studies reveal that the PEPCS and PEPCS-co-HEA copolymers all show a transition from the nematic to isotropic phase on heating and from the isotropic to nematic phase on cooling. Below Tg , the long range orientational ordering of the mesogenic units is fixed.

Characterization of poly(vinyl acetate)/sugar cane bagasse lignin blends and their photochemical degradation

The acetone-soluble lignin fraction (ASLF) of sugar cane bagasse, from a sugar and alcohol factory residue, was obtained after extraction with formic acid and used to prepare blends with poly(vinyl acetate) (PVAc) by casting. PVAc and ASLF/PVAc blends were irradiated with ultraviolet light (Hg lamp). Blend formation and the irradiation effects were examined through thermal analysis (TG and DSC), scanning electron microscopy, and atomic force microscopy. The DSC results show PVAc glass transition temperature (T g) shifts because of both, irradiation and ASLF incorporation. Non-irradiated pure PVAc presented a smooth surface, while after UV irradiation, light surface spots are observed. ASLF/PVAc 10/90 and 5/95 blends did not exhibit differences before and after UV irradiation, suggesting that lignin protects PVAc from photochemical degradation.

Polyvinylchloride (PVC) and natural rubber films plasticized with a natural polymeric plasticizer obtained through polyesterification of rice fatty acid

The use of natural plasticizers with low toxicity and good compatibility in several polymer applications has become more attractive. In this study, a natural plasticizer was synthesized in the laboratory by esterification of rice fatty acids and polyols. The resulting product was added to PVC and natural rubber latex films (5% w/w). Mechanical properties of the films, as well as tests of migration and exudation of the plasticizer, were performed in order to evaluate the efficiency of plasticization. Optical microscopy (OM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to characterize the pristine and plasticized polymer. Mechanical tests indicated that the addition of the natural plasticizer resulted in an increase in the elongation at break and a consequent decrease in the tensile strength of the films. The plasticizing effect induced by the natural plasticizer was also confirmed by a glass transition temperature (Tg) shift towards lower temperatures in the plasticized films.

Properties of polystyrene/poly(dimethyl siloxane) blends partially compatibilized with star polymers containing a γ-cyclodextrin core and polystyrene arms

A star polymer with a γ-CD core and PS arms (CD-star) is used to partially compatibilize blends of the immiscible polymers polystyrene (PS) and poly(dimethylsiloxane) (PDMS). The mechanism of compatibilization is threading of the CD core by PDMS and subsequent solubilization in the PS matrix facilitated by the star arms. Films cast from clear solutions in chloroform exhibit large wispy PDMS domains, indicating that some dethreading of CD-star and agglomeration of PDMS takes place during the slow process of solvent evaporation. However, DSC and DMA data show that partial compatibilization takes place, as evidenced by a shift in the PS and PDMS Tgs toward each other. The shift in PS Tg is greater when CD-star is present compared to samples without CD-star. PDMS also tends to leach out of the solution-cast films during solvent evaporation and post-processing of the films. The amount of retained PDMS is significantly increased when CD-star is present. The DMA data also show that PDMS has a lower molecular mobility when CD-star is present.

Effects of composition and transesterification catalysts on the physico-chemical and dynamic properties of PC/PET blends rich in PC

Melt blending of polycarbonate (PC)/poly(ethylene terephthalate) (PET) rich in PC at absence/present of different type of tranesterification catalysts was carried out by using reactive extrusion method. The thermal, dynamic, and morphological properties were studied. It was found that all blends are formed by a PC matrix and a semicrystalline (12–20% of crystallinity) of PET dispersed phase. The addition of a catalyst in the mixing process promotes a refined and homogeneous dispersion of PET, as well as it enhances the dynamicmechanical behavior of PC/PET blends compared with PC. These effects are attributed to the emulsifying effect of the PC–PET copolymer generated by transesterification. Additionally, this copolymer contributes to the miscibility between phases as demonstrated by the glass transition (Tg) shift of PC phase and PET phase.

Effect of impurities on the quantum plasticity of β-tin single crystals

The effect of small additions (0.01at.%) of indium, cadmium, and zinc impurities on the temperature dependences of the plasticity characteristics of β-tin single crystals in the interval 0.5K<T<4.2K is studied. Plastic deformation of the samples occurred by means of dislocation slip in the system (100)⟨010⟩. It has been shown in previous work that Peierls barriers are the main obstacles to the motion of dislocations of this system in pure β-tin. There exists a threshold temperature Tg≈1.2K above which dislocations overcome barriers by means of thermal fluctuations (thermally activated plasticity) and below which by means of quantum-mechanical tunneling (quantum plasticity). The present investigation shows that for weak doping of β-tin by various types of impurities the influence of Peierls relief on dislocation mobility remains dominant at low temperatures but Tg shifts appreciably in the direction of high temperatures (δTg≈0.2–1K), i.e. the temperature interval of quantum plasticity becomes wider. The experimentally observed shift of the threshold temperature depends in a complicated manner on the size of the impurities atoms, which create local barriers for dislocation slip. Current theory cannot give an unequivocal interpretation of such a dependence.

Fast in situ copolymerization of PET/PEN blends by ultrasonically-aided extrusion

An ultrasonically-aided extrusion process was developed for fast in situ compatibilization of immiscible polymer blends. PET, PEN, and their 50/50 blend were ultrasonically extruded at various amplitudes. PET underwent homopolymerization and degradation, respectively, at ultrasonic amplitudes of 7.5 and 10 μm, while PEN underwent degradation at amplitudes of 5, 7.5, and 10 μm. MALDI-TOF mass spectrometry revealed greater amounts of hydroxyl and carboxyl terminated oligomers in ultrasonically treated PET and PEN, indicating their greater reactivity. Ultrasonic treatment at short residence time led to the enhancement of transesterification reaction in the PEN/PET blend, as shown by 1H NMR and MALDI-TOF, indicating greater randomization with ultrasonic treatment. The latter was also observed through a shift in Tg that closely follows Gibbs–DiMarzio relation and an increase in viscosity of blend with treatment at an amplitude of 10 μm. No crystallinity was observed in the blend due to the already high level of transesterification introduced by extrusion without treatment. Accordingly, crystallinity, mechanical properties, oxygen permeability, and optical clarity of the blend were not influenced by ultrasonic treatment.

Wplyw syntetycznego polihydroksymaoelanu na wybrane wlasciwosci nowych, otrzymanych z jego udzialem poliuretanow do zastosowan medycznych. Cz. I. POLIURETANY Z AROMATYCZNYM DIIZOCYJANIANEM W SEGMENCIE SZTYWNYM

Polyurethanes (PUR) containing 37 % of soft segments, obtained by prepolymer chain extension in the reaction of diphenylmethane 4,4'-diisocyanate (MDI) with 1,4-butanediol (1,4-BD) were synthesized. Soft segments of PUR were built either of poly([R,S]-3-hydroxybutyrate) (a-PHB) and polycaprolactonediol (PCL) or a-PHB and polyoxytetramethylenediol (PTMG) (Table 1). The effects of a presence of synthetic a-PHB in the PUR chain on the thermal and mechanical properties, water and vegetable oil sorption, and sterilization efficiency of the materials prepared, designed to medical applications, were investigated. It was shown by DSC method that the introduction of a-PHB with methyl side group to the soft segment disturbed phase segmentation leading to glass transition temperature (Tg) shift to higher values (Tables 2, 3). The presence of a-PHB in the chain increases the hydrophilicity of the products, causing better water absorption and lower absorption of vegetable oil (Table 4, Fig. 1). The results of microbiological tests and the mechanical properties of a-PHB containing PUR samples show that such materials can be subjected to plasma sterilization without surface flaw (Table 5) and PUR obtained can be used for medical applications if not high mechanical strength is required.

Preparation and Properties of PP/PLA /Multiwall Carbon Nanotube Composites Filaments Obtained by Melt Compounding

The present paper studied the thermal and mechanical properties of atmospheric pressure plasma jet (APPJ) treated multiwall carbon nanotubes/polypropylene/polylactic acid nanocomposite filaments. The experiments included tensile tests, differential scanning calorimeter (DSC), Scanning electron microscopy (SEM) experiments. DSC studies showed that there were a distinct shift in Tg and a relatively moderate change in Tm for different systems. The activation volumes of CNTs/PP/PLA nanocomposite filaments have been calculated to describe strain rate sensitive behavior of CNTs/PP/PLA nanocomposite filaments by following Eyring’s equation based on the tensile test results.

Adaptation of polystyrene/multi‐wall carbon nanotube composite properties in respect of its thermal stability

AbstractPolystyrene (PS)/multi‐wall carbon nanotube (MWCNT) nanocomposites were prepared by melt mixing and by coagulation of PS/MWCNT sonicated dispersion. Before mixing and coagulation particles of MWCNT were covered with PS nanospheres by microemulsion polymerization to increase their dispersibility and compatibility with the matrix. Microemulsion polymerization was carried out under sonication in the presence of surfactants and a cross‐linking agent. The structure of modified MWCNT filler was revealed by SEM and TEM microscopy analyses. They show that PS is well grafted onto surface of MWCNT in form of ca 30 nm nanospheres or as ca 3–8 nm PS film layer. Significant changes in properties were recorded mainly for the coagulated sample. Although no shift of Tg was recorded for the melt mixed composite, the coagulated sample shows an increase by 9°C. Also the reinforcing effect of MWCNT was more pronounced for the same method of preparation (further supported by the filler modification). The creep response of this material shifts to longer times not only in the Tg area, but also as low as 70°C below it, which is perceived as an improvement of temperature stability (the same properties kept for a longer time). This was also proved by the increase of creep modulus, and consequently by the prolonged time needed to reach the same tensile creep compliance after aging. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Magneto-resistance analysis of nanometer A12O3 added Bi-2223 polycrystalline superconductors

We report magneto-resistivity analyses on (Bi,Pb)2Sr2Ca2Cu3Ox (denoted as (Bi.Pb)-2223) granular samples added with nano-sized A12O3 particles. A12O3 is added to the precursor powders during the final sintering cycle of a multi-step preparation process. Resistivity measurements of 0.0 and 0.2 wt. % A12O3 added (Bi.Pb)-2223 samples in applied magnetic field ranging from 0 to 250 mT have been compared and analyzed by the thermally assisted flux creep model. The effective activation energie in the A12O3 added sample is higher than in free one. The vortex phase diagrams in the H-T space show that characteristic temperatures Tg and Tp shift to higher values with 0.2 wt.% A12O3 addition.

Phase Structure and Mechanical Property of Blends of Organosolv Lignin Alkyl Esters with Poly(ε-caprolactone)

Organosolv lignin (OSL) alkyl esters (carbon number n, 2–5) were synthesized by homogeneous reaction with different acyl anhydrides/4-dimethylamino pyridine in pyridine solution. Miscibility of the esterified OSL with poly(ε-caprolactone) (PCL) was investigated through DSC, morphological observation, and spectroscopy. DSC revealed a composition-dependent shift in Tg for blends of OSL esters (n = 3–5) with PCL; thus these three systems can be regarded as miscible on a Tg-detection scale. AFM indicated that no remarkable phase separation occurred in these blends. In observations of crystallization behavior of the PCL component, the OSL derivatives of n = 4 and 5, the alkyl ester substituents of which make a higher similarity in chemical structure with a repeating unit of PCL, exhibited a higher level of miscibility with the aliphatic polyester, compared to the other OSL derivative of n = 3. Thermal-molded blend sheets of OSL esters of n = 3–5 with PCL attained ≥500% of elongation at rupture at 20 °C even when the blends contained 50 wt% of the respective esterified lignin components, reflecting the good miscibility of these OSL esters with PCL.

Preparation and Characterization of Organoclay-Filled, Vinyl Ester-Modified Unsaturated Polyester Nanocomposites

Organophilic montmorillonite clay — unsaturated polyester (UP) and vinyl ester oligomer (VEO) — unsaturated polyester nanocomposites were prepared and the formation of nanocomposites was evaluated by X-ray diffraction (XRD), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM). The VEO was prepared by reacting commercially available epoxy resin LY556 and acrylic acid and was used as a toughening agent for unsaturated polyester resin. Organoclay-filled hybrid VEO—UP matrices, developed in the form of castings, were characterized for their thermal and mechanical properties. Dynamic mechanical measurements indicated the presence of higher crosslink density for the clay-filled systems than unfilled systems. X-ray diffraction analysis infers the intercalation of the polymer molecules between the clay layers which in turn restricts the mobility of polymer in the vicinity of clay layers. A shift in Tg towards lower temperature was observed for nanocomposites. Significant improvement in the mechanical properties was also observed in the case of nanocomposites when compared with neat resin matrix.

Glass transition and its characteristic length for thin crosslinked polystyrene shells of rodlike capsules

AbstractRodlike capsules consisting of a calcium carbonate core and a crosslinked polystyrene shell were synthesized, and the glass transition temperature (Tg) and characteristic length of the glass transition ξ(Tg) for the thin outer shells were investigated by temperature‐modulated differential scanning calorimetry. The shell thickness ranged from 20 to 129 nm. The ratio of the Tg for the outer shell to the bulk Tg increases with decreasing shell thickness d. The d‐dependence of Tg is interpreted in terms of a simple two‐layer model which assumes that an immobile layer exists near the core‐shell interface. Shells of hollow capsules unexpectedly exhibit a similar d‐dependence of Tg to that for the filled capsules. This is characteristic of the crosslinked polymeric shells, and is attributed to certain spatial heterogeneity of crosslink distribution, and/or to the unstable configuration in the ultrathin shell that does not undergo relaxation due to the crosslink. The latter idea is based on the assumption that unstable configurational state is responsible for the Tg shift from the bulk value observed for nanosized polymeric materials. The ratio of the characteristic length for the shell of the filled capsule to that of the bulk ξf(Tg)/ξb(Tg) decreases with decreasing d. The results are interpreted in terms of the configurational entropy, and it is also suggested that the configurational state of network polymer chains in the shell affects the characteristic length. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2116–2125, 2008

Heat Aging Behavior of Novel Poly (Phenylene-Ether) Based Thermoplastic Elastomers

Abstract The heat aging performance of a series of novel poly (phenylene ether) (PPE) based thermoplastic elastomers (TPEs) from styrene-ethylene-butylene-styrene (SEBS), ethylene vinyl acetate (EVA) and PPE-polystyrene (PS), was studied. This quaternary blend showed superior heat aging performance due to the high Tg thermoplastic component (PPE). At 80 °C, different compositions of the quaternary blends were exposed for 500 hours. Effects of compositions, vinyl acetate (VA) content of EVA and different molecular weights (MW) of SEBS, on the mechanical properties upon heat aging were analyzed in detail. A representative composition (based on the mechanical properties) of the quaternary blend (SEBS/EVA/PPE-PS: 45/30/25) was exposed at different temperatures, i.e. 80 °C, 120 °C, 140 °C and 170 °C, for 2000 hours. Thermal degradation profiles of change in tensile strength and percent elongation at break due to thermal degradation of the blends were monitored and “half-life” temperature was estimated. Using the Arrhenius equation, the “lifetime” of the quaternary blend was predicted (100,000 hours at ∼131 °C). Change in functionalities due to chemical degradation was also monitored using Fourier Transform Infrared Spectroscopy (FTIR). As a consequence of degradation, the shift in Tg was observed by temperature modulated DSC (Differential Scanning Calorimeter). Detailed microstructural studies were done to establish the structure-property correlation, for degraded as well as pristine materials. The degradation mechanism was elucidated on the basis of morphology and structure studies of the blends.

Interfacial Interaction and Morphology of EVOH and Ionomer Blends by Scanning Thermal Microscopy and Its Correlation with Barrier Characteristics

In a blend, the interfacial interaction between the component phases can be effectively utilized to bring about homogeneous mixing and unique performances. While in conventional blends, preserving the morphology of the melt mixed state is unfeasible because of the strong thermodynamic tendency of the components to phase separate, herein, we report the intermolecular interaction of two hydrogen bonded polymers such as a barrier polymer poly(ethylene-co-vinyl alcohol) (EVOH) with an ionic polymer in their blends, which work symbiotically to achieve the desirable characteristics. We demonstrate the creation of a unique ellipsoid microfibrilliar morphology and melt exfoliation of one polymer in the blends through intermolecular interaction and achieve high oxygen barrier characteristics. Scanning thermal microscopy and scanning electron microscopy investigations confirm the presence of such unique morphology. The interfacial interaction and formation of interphase was evident from the local thermal analysis results combined with photoacoustic Fourier transform infrared spectroscopy (PA-FTIR). PA-FTIR confirms the chemical nature of the interaction, while the differential scanning calorimetry results indicate modification of the EVOH phase by the ionomer. The shift of Tg and broadening of the tan delta curve is evident from dynamic mechanical analysis confirming the interaction of the blend components. The blend B(60) with microfibrillar morphology shows fourfold drop in oxygen permeability indicating the role of interfacial interaction and desired morphology.

Role of Diblock Copolymers toward Controlling the Glass Transition of Thin Polymer Films

The physical properties of thin polymer films are often thickness, h, dependent, influenced by confinement and by interfacial interactions between the chains and the external interfaces. We show that the magnitude and film thickness dependence of the average glass transition temperature, Tg, of the polystyrene−silicon oxide (PS/SiOx/Si) system are influenced appreciably with the addition of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. The Tg can be “tailored” to increase, or decrease, with decreasing h or to remain independent of h. Tg shifts of as much of 35 °C are obtained for films of h ≈ 20 nm. Additionally, we report that the critical micelle concentration, φcmc, of the copolymer in thin films is considerably larger than for the bulk; specifically, micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains in the system. The h dependence of Tg is not influenced by the ϕcmc or by the number of micelles in this system.

Mechanical Spectroscopy of PVCN with Increasing Cross-Linking Degree

The effect of cross-linking on the dynamics of amorphous poly(vinyl cinnamate) (PVCN) below and above the glass transition temperature (Tg) was studied by means of mechanical spectroscopy. The polymer is cross-linked by irradiating it with ultraviolet light, and it is controlled by the irradiation time, where the kinetics is observed by means of Fourier transform infrared spectroscopy. The evolving links between molecules hinder molecular mobility and thus influence the mechanical loss in the material, leading to (i) a shift of Tg to higher temperatures, (ii) a substantial decrease of the strength of the corresponding α-relaxation, (iii) an increase of Young's modulus, and (iv) an increased damping of PVCN below the glass transition.