Polystyrene fracture

The O/C ratios observed on the corresponding polystyrene fracture surfaces were 0.01 or lower, indicating that crack growth at the polystyrene/ poly(methyl methacrylate) interface contains both cohesive and adhesive components, with cohesive fracture only occurring on the polystyrene side. 

Figure 12.6 SEM micrograph of polystyrene fracture surfaces welded to poly(methyl methacrylate), T sA 140°C.

It has been demonstrated that with SBR polystyrene blends the rubber should exist in discrete droplets, less than 50 p.m in diameter where a good finish is required, within the polystyrene matrix. It is believed that in such a form the rubber can reduce crack propagation and hence fracture in various ways. The most favoured current explanations of this were discussed in Chapter 3. Suffice it to say here that the following features appear necessary for a suitable blend ... 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

The main results of this miero-mechanical model in the quasi-static regime have been compared with experimental results obtained by placing polystyrene (PS)-polyvinyl pyridine (PVP) diblock copolymers with a short PVP block between PS and PVP homopolymers. The fracture toughness was found to increase linearly with E from that of the bare PS/PVP interface, while the slope of the line increased with the degree of polymerization of the block being pulled out. If the data for the different copolymers were plotted as AG vs. (where... 

The latter equation contains constants with well-known values and can therefore be used to predict the fracture stress of most polymers. For example, the bond dissociation energy Do, is about 80 kcal/mol for a C-C bond. For polystyrene, the modulus E 2 GPa, A. 4, p = 1.2 g/cm, = 18,000, and we obtain the fracture stress, o A1 MPa, which compares well with reported values. Polycarbonate, with similar modulus but a lower M. = 2,400 is expected to have a fracture stress of about 100 MPa. In general, letting E 1 GPa, p = 1.0 g/cm, and Do — 335 kJ/mol, the tensile strength is well approximated by... 

This diagram also helps to illustrate why the inherent fracture toughness of a material is not the whole story in relation to brittle fracture. For example. Table 2.2 shows that polystyrene, which is known to be a brittle material, has a K value of about 1 MN However, LDPE which has a very high... 

There are other conditions that result from the frozen-in stresses. In materials such as crystal polystyrene, which have low elongation to fracture and are in the glassy state at room temperature, a frequent result is crazing it is the appearance of many fine microcracks across the material in a direction perpendicular to the stress direction. This result may not appear immediately and may occur by exposure to either a mildly solvent liquid or vapor. Styrene products dipped in kerosene will craze quickly in stressed areas. 

Anh, T.H. and Vu-Khanh, T. Fracture and Yielding Behaviors of Polystyrene/Ethylene-Propylene Rubber Blends Effects of Interfacial Agents, Polym. Eng. Set 41(12), 2073-2081, December 2001. 

The use of ionomers such as lightly sulfonated polystyrene as an oil-based fracturing fluid viscosifier has been studied (70). 

The morphology of these two systems is shown in Figures 1 and 2, respectively. Figure 1 shows electron photomicrographs of fracture replicas of SBR vulcanizates containing polystyrene fillers of two different particles sizes, and the existence of the individual polystyrene particles is easily confirmed. Figure 2 shows a schematic of the morphology of a styrene-diene-styrene block copolymers, in which the formation of a... 

Looking at the melt fracture of specific polymers, we see many similarities and a few differences. Polystyrene extrudates begin to spiral from smooth at t 105 N/m2, and at higher shear stresses, they are grossly distorted. Visual observations show a wine glass entrance pattern with vortices that are stable at low stress values and spiral into the capillary and subsequently break down, as t is increased. Clearly, melt fracture is an entrance instability phenomenon for this polymer. 

Because the components must initially form miscible solutions or swollen networks a degree of affinity between the reacting components is needed. Therefore, most of the investigations into epoxy IPNs have involved the use of partially miscible components such as thermoplastic urethanes (TPU) with polystyrenes [57], acrylates [58-61] or esters which form loose hydrogen-bound mixtures during fabrication [62-71 ]. Epoxy has also been modified with polyetherketones [72],polyether sulfones [5] and even polyetherimides [66] to help improve fracture behavior. These systems, due to immiscibility, tend to be polymer blends with distinct macromolecular phase morphologies and not molecularly mixed compounds. 

FIG. 2 Two-dimensional colloidal crystal formed by a single layer of polystyrene spheres of diameter 3 /mi. The particles are initially suspended in water. The crystal is formed by confining the suspension between two glass plates and reducing the separation until it equals the diameter of particles. Here, one can see some typical features of crystals such as defects, fractures and vacancies. 

Fracture Surface Morphology and Phase Relationships of Polystyrene/Poly(methyl Methacrylate) Systems... 

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