G. Marin

Rheological models based on the double reptation mixing rule: The effects of a polydisperse environment

We present a comparison of various rheological models based on the double reptation concept, which relates linear viscoelastic data to molecular-weight distribution in order to determine the most efficient way to describe polydispersity effects. We have used for this study a two-step process. First, we have performed a systematic comparison of the predictions of various models with experimental data already published in the literature for binary blends of polystyrene (PS) and polymethylmethacrylate (PMMA). Then, we have compared the values of the complex shear modulus derived from these molecular models with rheological data obtained on “commercial” polydisperse polymers (PS, PMMA, and high-density polyethylene). It is shown that only the two models which explicitly take into account the effects of the polydisperse surrounding of a macromolecular chain through “tube renewal” effects are able to describe correctly the polydispersity effects on zero-shear viscosity and steady-state compliance.

Rheology of polydisperse polymers: relationship between intermolecular interactions and molecular weight distribution

An expression of the relaxation function of linear polydisperse polymers is proposed in terms of intermolecular couplings of reptative chains. The relaxation times of each molecular weight are assumed to be shifted according to a tube renewal mechanism accounting for the diffusion of the surrounding chains. The subsequent shift is applied to the relaxation function of each molecular weight obtained from an analytical expression of the complex compliance J*(ω). Therefore the complex shear modulus G*(ω) is derived from the overall relaxation function using the probability density accounting for the molecular weight distribution and four species-dependent parameters: a front factor A for zero-shear viscosity, plateau modulus GN0, activation energy E and characteristic temperature T∞. All the main features of the theology of polydisperse polymers are described by the proposed model.

Viscoelastic behaviour of non-compatible polymer blends: Polystyrene-polycarbonate

Abstract The linear and non-linear viscoelastic properties of a series of non-compatible polymer blends (polystyrene-bisphenol A polycarbonate) have been studied in the temperature range 180–250°. Glass transition temperature measurements show a small degree of compatibility at low PS contents. Variations of zero-shear viscosities as a function of blend composition have been related to the glass transitions and to the values of the thermodynamic interaction parameter λ 23 reported by Lipatov. An attempt was made to derive the linear viscoelastic properties of the blends as a function of the composition of the dispersed phase, given the viscoelastic behaviour of the pure components, using phenomenological models. Morphology of the dispersed phase has also been studied using scanning electron microscopy.

Blending law for binary blends of fractions of linear polystyrene

Abstract The dynamic behaviour of binary blends of linear polystyrene fractions in the terminal zone of the relaxation spectrum presents interesting peculiarities which have been discussed in a previous paper by Montfort. The viscoelastic properties are characterized by a representation in the complex plane of viscosities. In this paper, we propose an empirical blending law which represents such behaviour over a large range of frequencies. The conclusions of this law on the limiting values of η 0 and J 0 e are compared with those deduced from the blending laws of Graessley and BMEO.

Rheological properties of hot melt pressure-sensitive adhesives based on styrene--isoprene copolymers. Part 1: A rheological model for [sis-si] formulations

The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, y . In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.

Molecular weight distribution from viscoelastic data: The importance of tube renewal and Rouse modes

Rheological models based on molecular dynamics (as opposite to empirical relationships) are now preferred to link the molecular weight distribution (MWD) of linear polymers to their rheological properties. These models incorporate the double reptation concept, which represents the relaxation modulus as an integral over the molecular weight distribution. We propose a method that incorporates a detailed modeling of all the relevant relaxation processes, including Rouse fast and longitudinal modes and glassy relaxation. In addition, we take into account the effect of polydispersity on the relaxation times for reptation, i.e., “tube renewal.” In order to demonstrate the importance of these features of our technique, we compare it with one involving the direct inversion of the double reptation integral without accounting for tube renewal and additional relaxation processes. To invert the relaxation modulus in terms of the molecular weight distribution, one must either solve the ill-posed problem using an effic...

Rheology and Adherence of Pressure-Sensitive Adhesives

We have studied the relationship between rheological and peeling properties for hot-melt pressure-sensitive adhesives based on homopolymers or copolymers blended with tackifying resins. In this article, we particularly try to demonstrate that it is possible to define a quantitative link between rheology and adherence when the model formulations are deposited on substrates with strong (thermodynamic) adhesion. We describe the experimental results obtained on these model formulations and discuss the quantitative relationships obtained. In the case of “adhesion modulation” (derived from different treatments of the substrates), we show that the relationships become much more complicated, even with the same model adhesives. At the end, we discuss on the competition between adhesion and dissipation in the case of poor adhesion.

Dynamic viscoelasticity of entangled polymers

Abstract The study of dynamic viscoelastic properties of polystyrene samples shows the influence of molecular parameters on characteristic parameters of linear viscoelastic behaviour. An analysis of complex viscosity in a complex plane shows the influence of polydispersity on the broadness of the relaxation spectrum and on the shape of steady-flow curve η a ( γ ) .

Viscoelastic and thermal behaviour of partially compatible polymer blends polycarbonate/tetramethylpolycarbonate

Abstract Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) measurements show that, in the case of polycarbonate (PC) with tetramethylpolycarbonate (MPC), the homopolymers are miscible up to 70% PC weight fraction; at higher PC content, an additional PC phase appears. The partial miscibility of PC and MPC has been confirmed by ultrasonic attenuation measurements and scanning electron microscopy. The viscoelastic behaviour of these blends has been correlated with the blend composition and compatibility. The free volume contraction, found to explain the variations of glass transition temperature and viscosity with concentration, suggests strong intermolecular interaction in the compatible range.

Rheological Properties of Hot Melt Pressure Sensitive Adhesives (HMPSAs) Based on Styrene–Isoprene Copolymers, Part 3: Rheological Behavior of Different Block Copolymers With High Diblock Content

ABSTRACT The processing and application properties of hot-melt pressure-sensitive adhesives (HMPSA) are governed, to a large extent, by their rheological properties. Coating of the HMPSA is performed at high temperatures in the molten state. At room temperature, the adhesive satisfies the Dahlquist criterion and, consequently, has permanent tack. We have particularly studied the full formulations based on triblock and diblock copolymers, and also those based on newly designed molecules, such as tetrablock or radial copolymers. We have demonstrated in the previous articles of this series that, for these systems, the volume fraction of the free polyisoprene is the most important parameter that drives the tack performances by controlling the level of the secondary elastic plateau modulus observed in the low-frequency range. To improve the end-user properties, we have increased the diblock content in the blends. We describe here the dynamic mechanical properties, at room temperature, of the pure copolymer blends (i.e., without addition of a tackifying resin) and the full HMPSA formulations. We focus particularly in this article on blends that contain a high diblock content. The effect of the morphology of the diblock copolymer on the rheological behavior of the adhesive is discussed in detail. Finally, we propose a model, based on molecular dynamics concepts, which describes the rheological behavior in a very wide range of frequencies for all copolymers and full formulations of the study.