Interfacial width between polymers

According to Dee and Sauer (5,6), the interfacial width between a bulk polymer melt and pure air (assuming the vapor pressure of the polymer is negligible) is of the order of 10 to 15 A. There are two aspects to be considered. First, the depth of the surface of interest depends on the nature of the quantity being studied. Second, the measurable thickness depends on the instrument being used. Both of these aspects will be discussed in the following sections. 

Experimental values of interfacial widths between incompatible polymers are larger than expected and there has been considerable discussion as to the origin of this discrepancy. Capillary wave fluctuations are considered to be responsible for the diSerence between calculated and experimental results [112]. Detailed discussion of these effects can be found in Reference [107]. 

Schnell, R., Stamm, M. and Creton, C., Direct correlation between interfacial width and adhesion in glassy polymers. Macromolecules, 31, 2284-2292 (1998). 

Polymer-polymer interfaces are an important area of study since the interfacial behaviour is fundamental to the bulk properties of the system. This is particularly true when two or more polymers are mixed to form a blend, but the interface also plays a dominant role in areas such as adhesion, welding, surface wetting and mechanical strength. To understand fully polymer behaviour in such applications, the interface must be characterised at a microscopic level. Through deuterium labelling the interface between otherwise indistinguishable polymers can be studied, and neutron reflectivity provides unprecedented detail on interfacial width and shape. In addition to the inherent interdiffusion between polymers at a polymer-polymer interface, the interface is further broadened by thermally driven capillary waves. Capillary waves... 

Fig. 54. Fracture mechanisms map for interfaces between glassy polymers as a function of normalized degree of polymerization N/Ne and normalized interfacial width a(/dc...

A different behavior is observed [76] for bilayers composed of partially miscible polymers below their critical temperature Tc. In this case two pure blend components interdiffuse until the equilibrium of two coexisting phases is established. The above equilibrium state is characterized by the coexistence compositions ( q and (]>2 and the interfacial width w. The relaxation of the initial interface between pure constituents involves two processes (see Fig. 3) ... 

The reinforcement of polypropylene and other thermoplastics with inorganic particles such as talc and glass is a common method of material property enhancement. Polymer clay nanocomposites extend this strategy to the nanoscale. The anisometric shape and approximately 1 nm width of the clay platelets dramatically increase the amount of interfacial contact between the clay and the polymer matrix. Thus the clay surface can mediate changes in matrix polymer conformation, crystal structure, and crystal morphology through interfacial mechanisms that are absent in classical polymer composite materials. For these reasons, it is believed that nanocomposite materials with the clay platelets dispersed as isolated, exfoliated platelets are optimal for end-use properties. 

Neutron reflection has been used to stu die interfaces between the melt phases of ciystallisable polymers as well as real time interdiffusion of polymers and oligomers. Both systems are experimentally demanding and have required the use of specialised cells and data collection procedures. The interfacial widths for a number of polymer systems have been determined and the Flory Huggins interaction parameters obtained. In addition, the interdiffiision process has been followed for a polystyrene-polystyrene system above its Tg and also for a polystyrene-oligostyrene in-situ in real time using very rapid reflectivity scans. 

The interfacial behaviour between the component polymers in blends to a large extent controls the bulk polymer blend characteristics. The understanding of these interfaces is therefore of vital importance. The neutron reflection (NR) technique is ideally suited to study polymer interfaces since it provides a composition profile perpendicular to the interface with a resolution on a sub-nanometer length scale. From self consistent mean-field theory for immiscible homopolymers of infmite molecular weight the interfacial width (w) is is related to the Floiy Huggins interaction parameter ( f) by iv=( )f-j6x a is the segment length) [1]. Therefore, simply by... 

One solution to overcome these problems is to observe the polymers in the molten state when sandwiched between a polished silicon substrate and a heated trough which contains a bulk layer of one of the polymers [13,19-21]. This produces both molecularly smooth and macroscopically flat samples allowing accurate determination of the interfacial width and profile. A simple heating cell, shown in Figure 2, has been developed which allows such experiments to be carried out This cell consists of a brass trough into which a plug of hydrogenous polymer is moulded. 

Due to the restricted Q range in the data from die real time measurements, interpretation of the NR data is limited to specific models and functional form fits based on the fits to the as made sample. The interfacial widths were determined from the reflectivity data by fitting the profiles widi a standard two layer model and assuming a simple Gaussian into-fiicial profile between die dPS and the hPS. The interfacial profile is seen to be symmetric as may be expected for a system whae the polymers are of approximately equal molecular weight... 

Polymer interfaces are usually much broader than inorganic interfaces because, even in the case of immiscible polymers, some segment interpenetration occurs at the interface between the individual domains, being the interfacial width (iv) proportional to where x is the Flory-Huggins chi-parameter [13]. Therefore,... 

The phyllosilicate particles present a very high aspect ratio of width/thickness, in the order of 10-1000 and the complete exfoliation of the layered silicate in the polymer matrix is the main goal for the successful development of clay-based nanocomposites. For very low concentrations of particles, the total interface between polymer and layered silicates is much greater than that in conventional composites. Depending on the strength of the interfacial interaction, four types of morphology are possible in nanocomposites (Fig. 2) [24] ... 

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