Interface toughness

By combining Eqs. 4-6 one obtains the basic relation between interface toughness and the force required to break or pull-out a chain... 

The toughness of interfaces between immiscible amorphous polymers without any coupling agent has been the subject of a number of recent studies [15-18]. The width of a polymer/polymer interface is known to be controlled by the Flory-Huggins interaction parameter x between the two polymers. The value of x between a random copolymer and a homopolymer can be adjusted by changing the copolymer composition, so the main experimental protocol has been to measure the interface toughness between a copolymer and a homopolymer as a function of copolymer composition. In addition, the interface width has been measured by neutron reflection. Four different experimental systems have been used, all containing styrene. Schnell et al. studied PS joined to random copolymers of styrene with bromostyrene and styrene with paramethyl styrene [17,18]. Benkoski et al. joined polystyrene to a random copolymer of styrene with vinyl pyridine (PS/PS-r-PVP) [16], whilst Brown joined PMMA to a random copolymer of styrene with methacrylate (PMMA/PS-r-PMMA) [15]. The results of the latter study are shown in Fig. 9. 

Bulge Testing to Determine Modulus and Interface Toughness. .. 

Interfacial stability is predicted using the ERR, which is defined as the mechanical energy that is released during an incremental increase in crack length. Crack advance occurs when the ERR reaches a critical value, often referred to as the interface toughness, denoted here as Fj. Thus, the crack growth criterion is G > Fj. The ERR is defined as [33]... 

The simplification that = 0 implies that crack advance can be predicted in the context of a mixed-mode interface toughness, that is. 

Since interface toughness depends strongly on the amount of mode-mixity, the ERR alone is not sufficient to predict delamination one must also compute the phase angle from the relevant elasticity solution and have measurements that describe Fj = /( ). 

Despite the fact that the pressure seems to act purely to open the crack, it should be noted that there is a mode II component arising from bending of the film at the edge of the delamination the phase angle is = —45 . This highlights the need for mixed-mode interface toughness measurements for material systems relevant to microfluidic systems. 

In ideal world, one might modulate the adhesion energy of the valve seat to achieve the desired performance for a specific set of dimensions. This would likely be difficult as the chemistry of the fluidic environment, which obviously depends on the application, often affects interface toughness. However, one can measure the interface toughness for a given application and then choose the dimensions of the chambers to satisfy Equation 39.34 the result is a check valve that actuates to a specific clearance for a given interface condition. 

This section outlines several approaches to characterizing the elastic modulus of thin films and the interface toughness when bonded to substrates. Naturally, there is an elastic enormous variety of test configurations that can be used attention here is limited to those with specimen fabrication that is similar to that used for typical microfluidic devices. The text by Freund and Suresh [32] describes additional test approaches, particularly those pertaining to quantifying the effects of mode-mixity on interface toughness. 

Perhaps the most straightforward test to determine interface toughness is the peel test, wherein one measures the applied force required to peel a film from the substrate. The test is illustrated in Figure 39.15 and has the advantage of measuring at least part of the dependence of the interface toughness on mode-mixity. The ERR for this configuration is [32,49]... 

The energy release rate, or crack driving force, which is used to predict crack stability. The critical value of the energy release rate at which crack extension occurs, often referred to as the fracture toughness, or adhesion energy the subscript indicates that the quantity refers to the interface toughness (as opposed to a bulk material). 

LOCA (Interfacing) Tough structure of S G tubes reduce initiator frequency... 

Chicot, D., Demarecaux, P., Lesage, J., 1996. Apparent interface toughness of substrate and coating couples from indentation tests. Thin Solid Films 283, 151—157. 

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