Fracture behavior at partially miscible polymer interfaces

Polymer Engineering and Science, May, 2004 by Russell E. Gorga, Balaji Narasimhan

RESULTS AND DISCUSSION

Thermal Analysis

To ensure that sample annealing is well above the glass transition temperature ([T.sub.g]) of PBS, differential scanning calorimetry (DSC) was used to measure [T.sub.g] as a function of f (0.04 < f < 0.55) at a heating/cooling rate of 10[degrees]C/min for all three degrees of polymerization (N = 424, 1370, and 7144). Within the range studied. N has no effect on [T.sub.g]. The [T.sub.g] increased linearly with f from 105 to 130[degrees]C. Since the highest [T.sub.g] reported is 130[degrees]C, the lowest temperature for annealing was chosen to be 150[degrees]C.

Unstable Crack Propagation

From the MDCB test, crack length, a. as a function of time or wedge displacement is determined. The crack length should be constant throughout the experiment. In some samples, significant fluctuation in a was observed which is believed to be a result of inhomogeneous bonding at the interface due to possible contamination induced by impurities or air bubbles. Samples that exhibited such behavior were not used in the fracture energy measurement.

Fracture Behavior: N-Symmetric PS/PBS

Figure 3 shows the equilibrium fracture energy as a function of f for N = 1370 at 150 and 175[degrees]C and N = 7144 at 175 and 200[degrees]C. The data indicate that [G.sub.c] decreases as f increases for each N at a given temperature. Using an expression for [chi], which was experimentally determined for this system in an earlier publication (39) as shown in Eq 3, the critical volume fraction of brominated repeat units in PBS, [f.sub.c], at which the system crosses the phase boundary, is determined as a function of N and T.

[FIGURE 3 OMITTED]

[chi] = [f.sup.2](-0.0833 [73.75/T]) (3)

[[chi].sub.s] = 1/2([1/[[N.sub.PS][phi]]] [1/[[N.sub.PBS](1-[phi])]]) (4)

[f.sub.c] = [square root of ([[chi].sub.s]/[-0.0833 [73.75/T]])] (5)

Here, T is the temperature, [[chi].sub.s] is [chi] at the spinodal, and [phi] is the volume fraction of PS. The critical temperature, [T.sub.c], obtained using Eqs 3 and 4 for each PS/PBS pair studied is shown in Table 2. For [N.sub.PS] = [N.sub.PBS] = 1370, [f.sub.c] is 0.13 at 150[degrees]C (see Table 3). When f < [f.sub.c], the system is miscible and [G.sub.c] is high (as shown when f = 0.09). When f > [f.sub.c], the system has a miscibility gap, interdiffusion is thermodynamically limited (preventing the development of entanglements that can support stresses), and [G.sub.c] is low (as shown when f = 0.25 and 0.55). Therefore, two broad interdiffusion/fracture regimes can be postulated. Regime I: when f < [f.sub.c], there is no thermodynamic barrier to interdiffusion and the entanglement density is high, leading to high [G.sub.c] values. Regime II: when f > [f.sub.c], the system has a miscibility gap and interdiffusion is thermodynamically limited, which results in a low entanglement density and decreased [G.sub.c], which continues to systematically decrease with increasing f.

When the annealing temperature is increased from 150 to 175[degrees]C, the fracture energy increases, as expected, and the trend is similar to that at 150[degrees]C (see Fig. 3). For [N.sub.PS] = [N.sub.PBS] = 1370, [f.sub.c] is also 0.13 at 175[degrees]C since the 25[degrees]C change in temperature does not affect [f.sub.c] within 2 significant figures. The difference in [G.sub.c] between the two temperatures is less pronounced at higher f, owing to immiscibility effects dominating the interfacial transport. Therefore, increased annealing temperature has little effect on interdiffusion and the fracture energy when the system is within the 2-phase (or phase separated) region due to the inability to develop entanglements across the interface.