Blend Morphology Characterisation By Microscopy

When two immiscible polymers are brought together by melt blending, a distinct two-phase blend is formed. The size, shape and distribution of one phase into the other depends on material parameters (i.e., blend composition, viscosity ratio, elasticity ratio, and surface tension) as well as processing conditions (i.e., temperature, time, and intensity of mixing, and the nature of the flow). It is well known that the properties of such a blend (e.g., mechanical, rheological, optical, barrier, and dielectrical properties) are closely related to the state of dispersion or blend morphology which describes the size, shape, and spatial distribution of the component phases with respect to each other. Therefore, characterisation of the morphology of phase separated blends is the key to understanding structure-property relationships.

Figure 2.30 [165] shows schematically various useful polymer blend morphologies for different end properties such as high strength and toughness, toughness coupled with stiffness, good barrier and high flow properties that can be produced by polymer-polymer melt blending processes.

Figure 2.30 Schematic of useful morphologies of polymer blends [165]

In polymer blends the main application of microscopy is not so much to

Notes: No. 1 total range of available magnification within each category;

No. 2 finest detail the microscope can resolve;

No. 3 nearly planar vision (2 dimensions) in TEM and at high resolution OM;

No. 4 ability to discern details perpendicular to the field direction;

No. 5 the diagonal size of field under observation;

No. 6 only OM allows observation of liquid/liquid phase changes.

2.4.3.1 Optical Microscopy (OM)

Optical microscopy is readily available, low cost and generally offers the starting point for morphological characterization of polymer blends [38].

However, the classical light microcopy is limited by diffraction to domains larger than 500nm. In OM, the necessary contrast for detecting different phases might arise from a number of different sources such as colour, opacity, refractive index, orientation, absorption, or dichroic differences [166].

OM is typically used for the characterisation of heterophases, such as fillers [167], or pigments and additives [168], which are commonly added to blends.

These features are normally visible in reflected or transmitted light using polished or microtomed samples.

For polymer blends a minimum domain size of 500 nm can be examined in the optical microscope using one or more of the imaging methods like phase contrast, polarised light, bright-dark field, and interference microscopy, [169, 170].

Polarised light optical microscopy constitutes a very useful technique to obtain qualitative and quantitative features of the microstructure and crystallisation kinetics of polymer blends [171]. Qualitative studies, such as those of morphology of the phases, miscibility, phase separation, and compatibilisation, can easily be performed; quantitative studies regarding crystallisation kinetics of the components, size and size distribution of the dispersed phase, and orientation can also be done if accessories like a hot stage and a video camera coupled to a computer with digital image processing are used. Optical microscopy applied to polymer blends has been reviewed in [170-175]. Examples of the application of polarising microscopy for the investigation of spherulitic structures have been reported by Lovinger and Williams [176] on polyethylene/polypropylene blends, Martuscelli and co-workers [177] on poly(ethylene oxide)/poly(methyl methacrylate) blends, and Bulakh et al [178] on poly(phenylene sulphide)/amorphous polyamide blends.

The use of OM in the analysis of polymer blends is often limited by the small size of typical dispersions. As a result, many researchers routinely defer to higher magnification techniques like scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

2.4.3.2 Electron Microscopy

Compared with optical microscopes, electron microscopes like the SEM and TEM are capable of much higher resolution. SEM is normally employed for the analysis of surfaces and is of lower resolution than TEM. This method focuses an electron beam onto a surface, and the emission of electrons from the specimen is detected and amplified to obtain an image. The accelerating voltage is typically in the range of 1-40 kV; much lower than TEM. As SEM

exhibits a relatively large depth of field, it can show topological features better than other microscopy methods (except perhaps atomic force microscopy).

Samples require surface conductivity, thus a thin layer (~10 nm) of a conductive metal (gold or platinum) is normally sputtered onto the surface of samples before analysis. Staining and etching process are commonly employed to provide improved contrast. To obtain information of morphology in the bulk of the material, it is necessary to remove the surface layer. Only when adhesion between the phases is poor, a new surface that reflects the bulk morphology can be created by fracturing the sample. Cryogenic fracture is usually employed to prevent plastic deformation and provide surfaces with better defined topological features than possible with higher temperature fracture. Particles, such as fillers or impact modifiers, will often be exposed and debonded during cryogenic fracture revealing the desired contrast.

On the other hand, TEM is analogous to transmission optical microscopy except that an electron beam instead of a light beam is employed. The electron beam is formed with high accelerating voltage (100-400 kV) and viewed on a fluorescent screen. The wavelength of electrons allows for higher resolution and thus much smaller dimensional resolution than optical methods. TEM samples have to be sufficiently thin (usually microtomed into thin slices of less than 100 nm) to allow electron beam penetration [170].

Since polymers are mainly composed of C, H, N and O atoms, the electron density difference between polymers is not large enough to achieve sufficient contrast in heterogeneous materials. Thus the key to success employing TEM involves developing phase contrast of the components. Osmium tetraoxide, OsO4, is the most common staining material employed and is particularly useful for polymers with unsaturation, because it reacts with the double bond to yield an osmate ester providing excellent contrast.

The application of SEM and TEM in the characterisation of polymer blends has been reviewed in references [170, 173-175, 179]. Microscopic methods are frequently used in parallel, the SEM/TEM pair being the most frequent.

Lee and Han investigated the evolution of polymer blend morphology of

various blend systems during compounding in an internal mixer [180] and a twin screw extruder [181] using SEM and TEM.