Interphase viscoelastic properties

Lipatov et al. [116,124-127] who simulated the polymeric composite behavior with a view to estimate the effect of the interphase characteristics on composite properties preferred to break the problem up into two parts. First they considered a polymer-polymer composition. The viscoelastic properties of different polymers are different. One of the polymers was represented by a cube with side a, the second polymer (the binder) coated the cube as a homogeneous film of thickness d. The concentration of d-thick layers is proportional to the specific surface area of cubes with side a, that is, the thickness d remains constant while the length of the side may vary. The calculation is based on the Takayanagi model [128]. From geometric considerations the parameters of the Takayanagi model are related with the cube side and film thickness by the formulas ... 

In the case of PO blends, compatibilization most frequently aims for improved ductility and/or transparency. The Z-N-LLDPE obtained using multi-sited catalyst constitutes a specific case - the homopolymer may have phase-separated morphology that requires compatibilization. It has been known that addition of 5-20 wt% LDPE needs to be used for improved performance. However, explanation for this is rather recent (Robledo et al. 2009). The relaxation spectrum of the blend may be decomposed into three components (1) Z-N-LLDPE matrix, (2) LDPE dispersed drops, and (3) a thick interphase with its own viscoelastic properties, obtained by interaction between the high-MW linear fraction of the LLDPE and the low-MW linear LDPE macromolecules. 

A linear viscoelastic constitutive model of dilute emulsion viscoelastic properties was proposed by Oldroyd [111, 112]. The model considered low deformation of monodispersed drops of one Newtonian liquid in another, with an interphase. Choi and Schowalter [113] extended their cell model to dilute emulsions with Newtonian matrix and viscoelastic drops under infinitesimally small oscillatory deformation. Oldroyd s model was modified by Palierne [126, 127] for dilute viscoelastic hquids emulsions with polydispersed spherical drops (thus, subject to small deformations) with constant interfacial tension coefficient, Vu, at concentrations below that where the drop-drop interactions start complicating the flow field, that is, < 0.1 ... 

One of the reasons for deviation of theoretical equations, connecting elasticity modulus with filler amount from the experimental data, is the formation of surface layers at the poljmier-filler interface (interphase layers). The properties of these layers are different than in bulk. It is very important to estimate the contribution of the interphase to the viscoelastic properties of composites. 

Let us now consider some experimental data on the influence of the interphase on the viscoelastic properties. One of the most convenient ways to study the problem is to use some model composites, consisting of the reinforcement covered by polymer, modelhng the interphase layer, and polymer matrix. The system epoxy resin-glass beads covered with poly(butyl methacrylate) was... 

The presence of an interphase layer requires us to consider the polymer matrix as consisting of two parts interphase and free matrix, i.e., the model for a two-component system must be used. The viscoelastic properties of such a sys-... 

The principles of modelling of viscoelastic roperties which account for the interphase participation were developed. Also, the dependence of viscoelastic properties on thickness of the interphase was theoretically studied using a mechanical model. ... 

The conclusion can be drawn that if the strong intermolecular bonds are present, between filler and matrix (strong adhesion), interphase has a higher glass transition temperatrme and Tg of composite wiU also be higher. For a weak adhesion, the opposite is true. From this consideration, a great contribution of the interphase and its volmne fraction to the viscoelastic properties is evident. Theocaris states that the presence of interphase leads to well-separated regions... 

Dynamic mechanical thermal analysis (DMTA) has also been used by Brinson, et al.,t l to determine the suitability of the technique for evaluating damage in the adhesive bond from the viscoelastic properties of bonded beams and for evaluating the effects of various environmental conditions and various surface treatments. The authors considered that if the bond becomes damaged (either adhesive and/or interphase) due to excessive load, fatigue, moisture, or corrosion, it would seem likely that dissipation mechanisms or loss modulus and tan 5 would change. Therefore, they used DMTA to measure the viscoelastic properties of beams with simulated flaws and beams taken from lap specimens, which had been exposed to humidity and/or corrosion for extended periods. 

An important problem in describing viscoelastic properties of IPNs consists of taking into account the properties of an interphase. There is great difficulty... 

This effect is of importance, as it was shown that the reaction rates determine the time of the onset of phase separation, the degree of phase separation, and the fraction of an interphase. Correspondingly, the viscoelastic properties of compatibihzed IPNs depend not only on the presence of the compatibilizing agent, but also on its effect on the reaction kinetics. 

The results disoissed above indicate that in heterogeneous polymeric materials, in accordance with general physico-chemical principles stated in section 1, the viscoelastic properties of a polymeric matrix are strongly dependent on the type of heterogeneity and on the nature of the interphase interactions. In all cases the changes in molecular mobility and structure of the polymer in the boundary layer close to... 

Wei P J and Huang Z P (2004) Dynamic effective properties of the particle-reinforced composites with the viscoelastic interphase, Int J Solids Struct 41 6993-7007. 

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