The heterogeneity of the network structure of epoxy polymers studied by dynamic and DSC tests

The final network structure of epoxy polymers cured with a crosslinking agent is strongly connected with the type and amount of the curing agent and conditions. Concerning the morphology of this network, it has been reported that cured epoxies are two-phase systems, containing roughly spherical floccules arranged in a matrix resembling the constituent phases. In this paper and by using an epoxy-polymer cured with different amounts (i.e., different than the stoichiometric one) of a curing agent, strong indications of their network-structure heterogeneity have been disclosed. For this study, two different experimental techniques, such as dynamic mechanical measurements and DSC tests have been utilised. It was found that cured epoxy-polymers consist of two extreme regions, with low and high crosslink densities, whose volume fractions have been estimated by using a mechanical model applicable for two-phase systems.     

Microstructure and mechanical properties of polymers studied by positron annihilation

The recent progress of positron studies on polymers are briefly reviewed. The correlation between free-volume holes and mechanical properties are discussed. The results indicate that the positron spectroscopy is a potential tool to characterize the microstructure and mechanical properties of polymeric materials.     

Physico-chemical changes taking place in gamma irradiated bovine globulins studied by thermal analysis

Structural transformations induced in gamma and alpha globulins under influence of gamma irradiation using doses of 2.5 and 24 kGy were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG, DTG). Thermal decomposition of the globulins irradiated in water suspensions occurs at higher temperatures, in comparison to the reference non-irradiated samples. This was related to formation of covalent linkages in the irradiated proteins, apart to chemical changes induced in amino-acids. Essential modification of thermal decomposition was detected already after irradiation with a dose of 2.5 kGy performed for water suspensions. Irradiation of solid native proteins induces decrease in decomposition temperature and gives evidence of proteins degradation.     

Online monitoring of thermoset post-curing by dynamic mechanical thermal analysis DMTA

Thermosetting moulding compounds are synthetic materials which can be easily formed in the molten state and achieve high temperature stability due to a cross-linking process which takes place during manufacture. To ensure thermal and mechanical properties, post-curing of moulded phenolic resin components is necessary for high quality applications. In the industrial practice, post-curing time–temperature-programs are heuristically acquired. In this paper, dynamical mechanical thermal analysis is employed to determine optimal post-curing conditions for injection moulded parts from phenolic resin.     

Molecular motions in a polycarbonate composite as studied by thermally stimulated recovery and dynamic mechanical analysis

The viscoelastic properties of a short carbon-fibre reinforced polycarbonate are studied in the glassy state and in the glass transition region by means of Thermally Stimulated Recovery (TSR) and Dynamic Mechanical Analysis (DMA) techniques in the flexural mode. The temperature calibration of the apparatus is discussed. Each thermal sampling experiment is analysed according to a simple Voigt-Kelvin model in which the temperature dependence of the retardation time follows the Arrhenius equation with two adjustable thermokinetic parameters. Non-linear fittings of the experimental data are performed with the corresponding constitutive equation. Both the activation energy and the pre-exponential factor follow the usual trends with temperature and present the well-known compensation phenomena. A divergence between the TSR results of the two materials is found above ± 145°C and may be a result of the effect of the interface. The observed movements in the glassy state which are still found to have a cooperative character are analysed according to the Adam-Gibbs theory. Dynamic-mechanical relaxation properties of the composite are reported and analysed in terms of Takayanagi's block model.     

Superposition of dynamic mechanical properties in the glassy state

Superposition of the loss tangent curves could be achieved for the β-transition of a series of homologous epoxy resins. It was found that both a vertical and horizontal shift were necessary to achieve superposition when the curves were plotted as the logarithm of the loss tangent versus reciprocal absolute temperature. Resins from the diglycidyl ether of bisphenol A (DGEBA) were prepared with five different curing agents and their loss tangent curves measured on a free-oscillation torsion pendulum (ca. 1 cps). The β transition is caused by DGEBA, which was found via molecular models to contain a mobile group. The intensity of the loss for three of the resins was found to be proportional to the concentration of DGEBA, molecular models revealing that no additional mobile groups were introduced by these curatives. The remaining two curing agents introduced mobile groups into their systems and for these two, no separate transitions were identified but the intensity of the DGEBA β transition was increased. This may be caused by a coupling of the DGEBA mobile groups through the flexibility of the curative-introduced mobile groups.     

Glass transition of semi-crystalline PLLA with different morphologies as studied by dynamic mechanical analysis

Poly(l-lactic acid) was crystallized from the glassy state at different temperatures to produce fully transformed semi-crystalline specimens exhibiting different lamellar morphologies. The materials were tested by dynamic mechanical analysis, where a T _g decrease was found with an increasing crystallization temperature. Considering a three-phase model, this tendency was related to the corresponding increase in the thickness of the rigid amorphous phase. It is suggested that this phase could, in some extent, accommodate through local translational/rotational motions the cooperative motions taking place within the mobile amorphous phase. This could be due to the non-compact structure of the cooperatively rearranging regions, which can present a string-like or fractal structure in their edges. The width of the loss factor peak associated to the glass transition increases with increasing crystallization temperature, suggesting an increase in the broadness of the distribution of relaxation times. The drop in the storage modulus across T _g varies systematically with the crystallization temperature in the different materials and could be correlated with the crystalline content. Above T _g, the loss factor exhibits a plateau-like behaviour at significantly high values, which seems to be a rather general behaviour in semi-crystalline systems that could be related to the contribution of pure irreversible flow in the overall viscoelastic behaviour.     

Viscoelastic properties of chitosan with different hydration degrees as studied by dynamic mechanical analysis

Dynamic mechanical analysis, DMA, is an adequate technique for characterizing the mechanical features of biomaterials, as one can use test conditions that can more closely simulate the physiological environments in which they are going to be applied. In this work it was possible to perform different tests on chitosan membranes using low/moderate hydration levels, as well in completely wet conditions. In the first case the data obtained at different relative humidity environments were rationalized under a time-humidity superposition principle, where a master curve for the storage modulus could be obtained along a wide range of frequencies. The temperature dependence of the shift factors exhibited a curvature opposite to that expected by the WLF equation, and is consistent with relaxation dynamics behavior below the glass transition. Temperature scans above room temperature in both dry and wet conditions did not reveal strong variations in the viscoelastic properties. It was possible to follow in real time the water uptake in an initially-dry membrane. During the initial strong and fast decrease of the storage modulus the loss factor exhibited a peak that should correspond to the occurrence of the glass transition resulting from the plasticization effect of water. Upon equilibration the loss factor reached similar values as for the dry material (tandelta approximately equal to 0.5). The viscoelastic characterization reported in this work for chitosan may be useful in the use of such material for a variety of biomedical applications.     

Use of dynamic mechanical thermal analysis (DMTA)

A Mark III DMTA (Polymer Laboratories, Loughborough, U.K.) was used to measure the glass transition temperatures (T _g) of a commercial cracker and its dough, each equilibrated to various water activities covering a range of 0.11–0.75 for the cracker and 0.11–0.90 for the cracker dough. DMTA measures the change in the elastic modulus (E′) and loss modulus (E″), as well as that in tanδ (E″/E′), with temperature. The change in the elastic modulus with temperature for the two systems followed a pattern similar to that found for complex food polymers (gluten, amylopectin), withT _g decreasing as moisture content increased. Baking did not change the location of the glass transition curve (T _g vs. moisture content); i.e. the curves for raw dough and baked finished product were somewhat superimposable, and similar to the published gluten curve, indicating that for this type of cracker containing ～5% sugars, the protein fraction is most responsible for theT _g curve.     

Quantitative analysis of gaseous diffusion in glassy polymers

The current theoretical treatment of diffusion of gases in glassy polymers is based on the “dual sorption” model, but also includes certain other important assumptions, at least some of which cannot be fully justified a priori. They require experimental validation, which, however, is not possible by the procedures used or proposed so far. Methods suitable for this purpose are discussed here.     

On some mechanical effects in glassy polymers attributed to chain entanglements

The important role of the entanglements in the deformation of high-molecular-weight glassy polymers is demonstrated by two phenomena: the build-up of material resistance in polymethylmethacrylate after chain interpenetration and the intrinsic crazing of polycarbonate which is observed when the entanglement network reaches its limits of extension.Dedicated to Prof. Dr. F. H. Müller.     

Extensional flow of bulk polymers studied by dynamic Monte Carlo simulations

Dynamic Monte Carlo simulations of bulk lattice polymers driven through planar geometries with sequentially converging,parallel and diverging spaces between two neutrally repulsive solid plates are reported.The spatial profiles of polymer velocity and deformation along the course of such a laminar extensional flow have been carefully analyzed.The results appear consistent with experimental observations in literature.In the entrance and exit regions,a linear dependence of chain extension upon the excess velocity has been observed.Moreover,an annexed shear flow and a molecular-dispersion effect are found.The results demonstrate a useful strategy of this approach to study polymer flows and bring new insights into the non-Newtonian-fluid behaviors of bulk polymers in capillary rheometers and micro-fluidic devices.     

Thermal and dynamic mechanical thermal analysis of lignocellulosic material-filled polyethylene bio-composites

Thermal and rheological properties of plant-based natural filler-reinforced polyethylene bio-composites applying various filler loadings as well as the impacts of the different compatibilizers were investigated by means of differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA). As lignocellulosic materials, such as rice-husk flour and wood flour, are eco-friendly biomaterials and a thermoplastic polymer, for example, high-density polyethylene, has good physico-mechanical and thermal properties, therefore their bio-composites can combine and utilize these two advantages at the same time. The temperature of the α-relaxation (T _α) slightly increased and melting temperatures (T _m) of the matrix polymer in the case of the studied bio-composites did not shift significantly as the filler loading changed, because the rigid interphase hinders the motion of polymer segments resulting in the increase in T _α and only weak interactions developed at the interface between the matrix polymer and the reinforcement in the case of non-compatibilized composites. However, compatibility between the reinforcement and the matrix polymer was enhanced by incorporating compatibilizers, which further improved stiffness. From the DMTA experiment, the reinforcements result in composite samples having higher storage modulus (E′) than the neat polymer sample, indicating that incorporating lignocellulosic filler increased their stiffness.     

Structural changes in Fe-doped SrSnO3 perovskites during thermal analysis

Perovskite-type strontium stannate, SrSnO₃, is of technological interest because of its potential applications. In the present work, Fe(III)-doped SrSnO₃ perovskites were synthesized using Sn metal and the polymeric precursor method, derived from the Pechini method. Thermogravimetric evaluations in N₂ and O₂ atmospheres were performed to evaluate the possible formation of oxygen vacancies resulting from Fe(III) doping. The X-ray diffraction patterns showed the formation of an orthorhombic structure with high crystallinity after calcination at 1,073 K. The first thermal analysis, in a N₂ atmosphere, led to the partial elimination of strontium carbonate and hydroxyl groups, while the thermal analysis in an O₂ atmosphere led to an increase in the mass, indicating O₂ adsorption on the lattice, which could be related to the presence of Fe(III) and Fe(IV) in the perovskite structure. Moreover, the thermal analysis changed the symmetry of the iron-doped perovskites.     

Structure and dynamics of MG rubber studied by dynamic mechanical analysis and solid‐state NMR

Methyl‐methacrylate‐grafted natural rubber was prepared by free radical polymerization of methyl methacrylate in natural rubber latex, and their structure and dynamics were investigated by dynamic mechanical analysis and solid‐state nuclear magnetic resonance (NMR). Samples were prepared by chemical initiation and high‐energy radiation. The changes of glass transition temperature and tan δ max with different total poly(methyl methacrylate (PMMA) content are reported. The effect of the change in composition in copolymers on tan δ peak width, tan δ max, and area under the tan δ curve are used to understand the miscibility and damping properties. Solid‐state ¹³C‐NMR measurements were carried out to determine several relaxation time parameters, such as rotating frame and laboratory frame proton and carbon relaxation times. Cross polarization times and carbon relaxation times were interpreted based on the changes in the molecular motion. Proton relaxation times were interpreted based on the heterogeneity of the matrix. Results confirmed phase separation and a presence of an interfacial region. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1141–1153, 1999     

Amorphous-amorphous transition in glassy polymers subjected to cold rolling studied by means of positron annihilation lifetime spectroscopy

In this study, polycarbonate (PC) and polystyrene (PS) are subjected to plastic deformation by means of cold rolling and the resulting variation of the free volume and its subsequent time evolution after rolling is investigated by means of positron annihilation lifetime spectroscopy (PALS). The value of the long lifetime component that is attributed to the decay of ortho-positronium (tau(o-Ps)) and its intensity (I(o-Ps)) are used to characterize, respectively, the size and the concentration of the free-volume holes. In addition to the PALS experiments, the effect of plastic deformation on the dynamic tensile modulus is investigated. The PALS results show that both for well-aged PC and PS an increase of tau(o-Ps) and a decrease of I(o-Ps) occur upon plastic deformation. During the subsequent aging, tau(o-Ps) tends to return to the value assumed before plastic deformation, while I(o-Ps) remains constant with time. These results corroborate the idea of an amorphous-amorphous transition, rather than that of a "mechanical rejuvenation" as proposed in the past to explain the ability of plastic deformation to reinitiate physical aging. Finally, a linear relation between the size of the free-volume holes and the dynamic tensile modulus is found, which suggests that the stiffness of amorphous glassy polymers is fully determined by their nanoscopic structure.     

Diffusion in glassy polymers

Two versions of the free‐volume theory of diffusion are compared by considering differences in the predictions for the activation energy for the diffusion process. A number of data‐theory comparisons for free‐volume theory are discussed and evaluated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 785–788, 2003     

Kinetic phenomena in non-crystalline materials studied by thermal analysis

The structural relaxation and viscosity behavior of Ge₃₈S₆₂ glass has been studied by thermomechanical analysis. The relaxation response to any thermal history is well described by the Tool-Naraynaswamy-Moynihan model. The apparent activation energy of structural relaxation is very close to the activation energy of viscous flow (Eη=478±12 kJ mol^-1). However, the activation energy of crystal growth obtained by optical microscopy is about one half of this value. Similar result has been obtained from isothermal DSC measurement (Ea=220±20 kJ mol^-1). The kinetic analysis of these data reveals interface controlled crystal growth with zero nucleation rate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural morphology of poly(styrene)-poly(butyl acrylate) polymer-polymer composites studied by dynamic mechanical measurements

The emulsion polymerization process allows production of polymer particles with different structural morphologies. Films obtained after coalescence keep some memory of this morphology, but large modifications can occur during coalescence. In the present case, one of the polymers, polystyrene (PS), exhibits a glass temperature (Tg) much higher than the filmification temperature (close to room temperature), while the other one, poly(butyl acrylate) (PBA), has a much lowerTg. Furthermore, it is well known that dynamic mechanical measurements can be very helpful in providing information on the morphology of polymer materials, i.e., on geometrical and topological arrangement of homopolymer domains. At first, this method was used for comparison of two types of films: i) the first one obtained from structured-core (PS)-shell (PBA) particles, ii) the second one obtained from a blend of homopolymer particles (PS and PBA). It appears that the expected core-shell particles lost their geometric structure in the second film. Second, comparison of the predicted dynamic modulus and experimental data shows that i) strong interactions exist between PS nodules unless their coalescence has occured, leading to an abnormally high modulus at room temperature, ii) after achieving their coalescence, PS forms a more or less continuous phase. Both phenomena strongly depend on the particle size and their respective volume fractions.