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Polymer glasses are rheologically complex materials whose viscoelastic response is a function of the kinetics of structural relaxation. To circumvent the complexity, several experimental loading protocols have been established over the years, but the results have not led to a unifying concept. From a theoretical point of view, we tackled the problem by coupling the structural relaxation kinetics with the constitutive equation for linear viscoelasticity written in the reduced time domain. In this article, we substantiate the reliability of our approach. Geometrical descriptions and thermodynamic rules motivated a constitutive coupling of viscoelastic and structural relaxation phenomena, and that represents the novelty in our theory. The axial creep response is simulated following the thermal/mechanical loading sequence known as Struik's Protocol. Simulation is extended to a vanishing stress, and we showed that the time-aging time superposition procedure holds true above a given stress threshold. Furthermore, we extend our study to a single-step stress relaxation experiments to predict the glass densification that occurs in uniaxial tension (known as “volume implosion”) and compression. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 724–739, 2009

The principal features of the volumetric as well as the viscoelastic response of mechanically stimulated glasses can be summarized as follows: i) the time-aging time shift factors contract upon increasing the probe stress (i.e. the stress apparently modifies the volume recovery kinetics), ii) the volume recovery baseline remains unaltered (i.e. the underlying structure of the stimulated glass remains unchanged) iii) yielding scales linearly with the logarithmic of the strain rate. Here we present a review of the above features with aid of a series of numerically simulated results concerning the responses of glassy polycarbonate. Simulations are obtained coupling a modified KAHR equation with the constitutive law for linear viscoelasticity within the domain of the reduced time. It will be shown that by using a minimum of experimental (PVT and linear viscoelastic) data inputs even the subtle intricacies can be predicted. Furthermore a new class of results concerning the stress-strain behaviour of glassy polymers is presented that never appeared before.

The origin of internal stresses in a polymeric component, subjected to an arbitrary cooling, will be described. The differential volume relaxation arising as a result of the different thermal history suffered by each body point was considered as the primary source of stresses build up. A numerical routine was developed accounting for the simultaneous stress relaxation processes and implemented within an Ansys® environment. The volume relaxation kinetics was modelled by using the four-parameter TNM (Tool-Narayanaswamy-Moynihan) phenomenological theory using the PVT (Pressure-Volume-Temperature) data obtained on a polystyrene sample. The numerical algorithm translates the specific volume theoretical predictions at each body point as applied non-mechanical loads acting on the component. Thermorheological complexity emerged due to coupling of volume and stress relaxation phenomena. A linear dependence of Poisson's ratio on the tensile relaxation modulus was postulated. A quantitative predictions of time dependent stress distribution was realized for two specific test cases.

A procedure able to describe the curing process of a particulate composite material used in a dental restoration is developed in the ANSYS environment. The material under concern is a multifunctional methacrylate-based composite for dental restoration, activated by visible light. The model accounts for the dependence of the viscoelastic functions on temperature and degree of cure. Three geometries have been considered in the analysis that are representative of three different classes of dental restoration and mainly differ by the C (constrained)-factor, (i.e. the bounded to unbounded surface ratio). It was found that the temperature could give a necrosis in the vicinity of the tooth nerve and that the average stress at the interface between the composite and the tooth scales exponentially with the C-factor. The residual stress at the dental restoration interface is also compared with the uniaxial tensile strength of twelve commercially available composite materials: it clearly appears that the level of residual stress may overcome the strength of the composite, especially at high C-factors.

Amorphous polymers show a time dependent response to an external solicitation. The time dependent response is usually measured in terms of shear response, whereas the bulk response is a more difficult to measure. In the framework of linear viscoelasticity at least two viscoelastic functions are needed in order to evaluate the mechanical behavior of viscoelastic amorphous polymers. Often the one of viscoelastic function is available in terms of creep compliance and the other one is available in terms of relaxation modulus: a reliable procedure for the inter-conversion is needed. Here the inter-conversions between relaxation moduli and creep compliances are described and a numerical method for the inter-conversion is presented. It is showed that the numerical solution lays on the analytical one and it is also less time consuming.

A finite element methodology for residual stresses calculation in dental restorative materials is proposed. The material under concern is a multifunctional methacrylate-based composite for dental restorations, activated by visible light. Reaction kinetics, curing shrinkage, and viscoelastic relaxation functions were required as input data on a structural finite element solver. Post cure effects were considered in order to quantify the residual stresses coming out from natural contraction with respect to those debited to the chemical shrinkage. The analysis showed for a given test case that residual stresses frozen in the dental restoration at uniform temperature of 37°C are of the same order of magnitude of the strength of the dental composite material per se.

The adhesive performances of a PSA are usually defined by three properties: tack, shear resistance, and peel strength. All these properties can qualitatively be correlated to the linear viscoelastic spectrum of PSA. In many cases, selected frequency intervals can be isolated within the linear viscoelastic spectrum, representing “windows” associated to the in-service performances of the adhesive Indeed, the “fine tuning” of linear viscoelastic properties was considered by PSA manufacturers of being a tool to tailor the individual products in order to meet the user’s requirements in different applications, a case that in not reflected in our work.

Structural relaxation is accompanied by a simultaneous change in all structure-sensitive physical mechanical properties. Naturally, a fundamental correlation between structural relaxation and the variation of the properties of the glass would be of great interest to scientists and engineers. Changes in mechanical and dielectric properties during the course of structural relaxation are of particular interest, but one must remember that their measurements involve the use of external stress (or strain), which, regardless of its magnitude, will affect the ongoing structural relaxation. The application of even the smallest stress or strain could shorten the relaxation time by one or more orders of magnitude and hence caution must be exerted in seeking correlations between volume and enthalpy relaxation on the one hand, and viscoelastic (mechanical or dielectric) measurements on the other. The two approaches can be viewed as manifestations of the same structural relaxation although one must bear in mind that they differ fundamentally in terms of the cause and the effect, and while the theoretical background of correlation between structural and viscoelastic relaxation has not been explored, in this paper an empirical correlation between the two will be reported.

Poisson’s ratio is defined as the ratio of the lateral contraction to the elongation in the infinitesimal uniaxial extension of a homogeneous isotropic body. In a viscoelastic material, Poisson’s ratio is a function of time (or frequency). In this paper, the time-dependence of the Poisson’s ratio is analytically evaluated from the bulk and shear responses using the relations between the viscoelastic functions in the Laplace domain. It has been found that, in the region of α-relaxation, Poisson’s ratio may be a nonmonotonic function of time, with a weak minimum at short times, when the shear response is broader than bulk response such that the ratio τG/τK is much larger than 1, or a monotonically increasing function of time if the shear and bulk responses share similar timescales and relaxation time distributions. The latter case is verified using experimental data from the literature for a cross-linked polymer, whereas the former case is verified for two linear polymers.

Glassy polymers confined at nano‐scale do exhibit anomalous thermo‐viscoelastic behavior in respect to their bulk response. For example, in ultra‐thin polymer films both negative coefficient of thermal expansion (NCTE) and glass transition temperature (Tg) depression are observed, experimentally. Theoretical conjectures have been drawn that qualitatively explain some features of highly confined glassy polymers [1,2,3] yet quantitative prediction of NCTE and Tg depression would allow a reliable design of devices operating at nano‐scale. In this paper the thin films anomalies are shortly reviewed and their explanation is tempted with classical existing theories.

An attempt to calculate the internal stresses in a cylindrically shaped polycarbonate (LEXAN‐GE) component, subjected to an arbitrary cooling rate, will be described. The differential volume relaxation arising as a result of the different thermal history suffered by each body point was considered as the primary source of stresses build up [1–3]. A numerical routine was developed accounting for the simultaneous stress and structural relaxation processes and implemented within an Ansys® environment. The volume relaxation kinetics was modeled by coupling the KAHR (Kovacs, Aklonis, Hutchinson, Ramos) phenomenological theory [4] with the linear viscoelastic theory [5–7]. The numerical algorithm translates the specific volume theoretical predictions at each body point as applied non‐mechanical loads acting on the component. The viscoelastic functions were obtained from two simple experimental data, namely the linear viscoelastic response in shear and the PVT (pressure volume temperature) behavior. The dimensionless bulk compliance was extracted from PVT data since it coincides with the memory function appearing in the KAHR phenomenological theory [7]. It is showed that the residual stress scales linearly with the logarithm of the Biot’s number.

A procedure is described and analyzed for the determination of the relative placement of linear viscoelastic functions and their relative relaxationretardation spectra for a commercial polycarbonate (LEXAN GE). The complete set of viscoelastic functions in creep and relaxation was obtained from two simple experimental data, namely the linear viscoelastic response in shear and the pressure volume temperature (PVT) behavior. The dimensionless bulk compliance was extracted from PVT data showing that it coincides with the memory function appearing in the Kovacs, Aklonis, Hutchinson, Ramos phenomenological theory [Kovacs, A. J., J. J. Aklonis, J. M. Hutchinson, and A. R. Ramos, J. Polym. Sci., Polym. Phys. Ed. 17, 1097 (1979)]. Our results are compared with the relevant literature data obtained on different polymers and show that polycarbonate fulfills simultaneously the responses features concerning the relative placement of the bulk and shear moduli [Kono, R. J., Phys. Soc. Jpn. 15, 4 (1960)], the shapes of the bulk and the shear retardation spectra [Bero, C. A., and D. J. Plazek, J. Polym. Sci., Part B: Polym. Phys. 29, 39-47 (1991)], the relative placement of the bulk and shear compliance [Deng, T. H., and W. G. Knauss, Mech. Time-Depend. Mater. 1, 33-49 (1997); Sane, S. B., and W. G. Knauss, Mech. Time-Depend. Mater. 5, 293-324 (2001)], the relative placement of bulk modulus and shear compliance [Meng, Y., and S. L. Simon, J. Polym. Sci., Part B: Polym. Phys. 45, 3375-3385 (2007)], the relative placement of the bulk compliance and the axial relaxation modulus [Knauss, W. G., and I. Emri, Polym. Eng. Sci. 27, 86-100 (1987)], and the shape of the Poisson's ratio [Lakes, R. S., Cell. Polym. 11, 466-469 (1992); Von Koppelmann, J., Rheol. Acta 1, 20-28 (1958); Ferry, J. D., Viscoelastic Properties of Polymers, 3rd ed. (Wiley, New York, 1980)], and its scaling with the shear modulus [O'Brien, D. J., N. R. Sottos, and S. R. White, Exp. Mech. 47, 237 (2007)]. © 2009 The Society of Rheology.

The viscoelastic properties of the amorphous and semicrystalline PEEK and of a PEEK-based composite (APC2) have been analyzed as a function of the aging time and related free volume relaxation, at temperatures close to the glass transition temperature. The effect of thermal treatments (physical aging) on the polymer amorphous phase of the three systems is presented. Creep tests approaching the “zero stress” were performed on samples subjected to different thermal histories. The presence of a second phase, namely crystallites and fibers, determines a loss of the mobility of the polymeric amorphous chains and consequently a decrease of the rate of free volume relaxation.

The principal features of the volumetric as well as the viscoelastic response of mechanically stimulated glasses can be summarized as follow: (i) the time-aging time shift factors contract upon increasing the probe stress (i.e., the stress apparently modifies the volume recovery kinetics), (ii) the volume recovery baseline remains unaltered (i.e., the underlying structure of the stimulated glass remains unchanged). Here we present a series of numerically simulated results concerning the responses of glassy polycarbonate that simultaneously fulfill these apparent contradictions. The problem was tackled coupling a modified Kalroush, Aklonis, Hutchinson, Ramos equation with the constitutive law for linear viscoelasticity within the domain of the reduced time. It was argued that the relaxation times under isobaric conditions depend on the temperature, the dimen- sionless volume, and the isotropic components of the stress tensor. Simulations are obtained with a minimum of experimental (PVT and linear viscoelastic) data inputs. Different loading protocols consisting of complex combinations and/or sequences of large and small mechanical stimuli were tested. Volumetric as well as viscoelastic behavior are systematically reported. A tentative explanation of the origin of the time-aging time contraction was finally proposed while some additional features concerning the volumetric response emerged.

Water-based pressure sensitive adhesives (PSA) obtained by emulsion polymerization have peculiar characteristics compared with their solvent counterparts. In particular they contain significant amount of microgels, permeated by uncrosslinked molecules, and generate micronetworks after film formation. Viscoelastic characterisation of three PSAs, both in the linear limit and in uniaxial elongation is reported in this paper. Modelling of tensile stress-strain curves is proposed, based on a network theory accounting for the presence of entanglements (Ball et al. Polymer, 1981, 22, 1010–1018). A rate-dependence of some of the physical parameters of the model is introduced in order to describe experimental data.

The use of Wilhelmy plate techniques for studying the interactions of fibers with fluids of variable viscosity is discussed. The influence of hydrodynamic effects of force (F) VS. DEPTH (z) plots is interpreted on the basis of surface tension-independent added terms of the constitutive equation of the Wilhelmy experiment. Wetting experiments using pseudoplastic fluids (polyethyleneglycol/water solution) show that viscoelasticity is indeed important in the interpretation of Wilhelmy plate experiments. The comparison of wetting experiments performed on untreated and plasma-treated carbon fibers showed that at low stage speed and fluid viscosity, surface chemistry and morphology determines contact angles. At higher stage speeds and viscosities and advancing and receding angles are affected by surface chemistry-independent and hydrodynamics-dependent added contributions.

It has been recognized for some time that, immediately upon cooling to temperatures below their glass transition, polymers are characterized by departure from the equilibrium chain conformation. The viscoelastic properties of the amorphous and semicrystalline PEEK and of a PEEK-based composite (APC2) have been analyzed as a function of the aging time and related free volume relaxation at temperatures close to the glass transition region. The effect of thermal treatments on the three systems is presented. The presence of a second phase, namely crystallite and fibers, determines a loss of the mobility of the polymeric amorphous chains and consequently a decrease of the rate of the free volume relaxation. Creep tests approaching "zero stress" were performed on samples subjected to different thermal histories.

It has been known since the work of Batchelor in 1972 that concentrated suspensions of rigid spherical particles may be anisotropic, the orientation of the particles being induced by the external flow field. This in particular implies that the ratio of the elongational to the shear viscosity may be different from the Trouton value of three even in the absence of viscoelastic effects. No direct experimental evidence of this conclusion is, as far as we have been able to ascertain, available, and some such evidence is presented in this paper.

Analyse transitoire (fluage sous flexion dans diverses conditions) et dynamique rigidite au voisinage de la temperature de transition vitreuse

Thermal treatment of amorphous poly(aryl-ether-ether ketone) below the glass transition temperature has been studied. The extent of aging was measured by differential scanning calorimetry. X-ray and Fourier transform infrared spectra were used to study both melt-cast and annealed samples. The effect of the thermal treatment of viscoelastic response was evaluated using creep tests. Aging has been shown to be accompanied by a marked change in the transport properties of the material.

The mechanical characterization of rabbit Achilles' tendon and anterior cruciate ligament is presented. Both static and dynamic mechanical tests have been performed on fresh explanted tissues. Experimental results are presented and discussed in terms of a relationship between structural architecture and mechanical properties. The differences in collagen fibre configuration and composition between tendon and ligament influence the tensile and viscoelastic properties. In particular, tendons and ligaments showed a gradual transition from a rubber-like to a glassy-like behaviour during the loading process due to the straightening of collagen fibres.