Fracture behavior at partially miscible polymer interfaces

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

EXPERIMENTAL

Materials

Monodisperse PS was obtained from Polymer Source, Inc (Dorval, Quebec). The degree of polymerization, N, ranged between 1281 and 7670 for both polymers (see Table 1 for molecular properties). PBS was synthesized by the procedure described by Kambour and coworkers (44) and explained in detail in a previous publication (39). [.sup.13]C-NMR, elemental analysis, DSC, TGA, GPC, and UV-Vis spectroscopy are used to characterize the PBS copolymer to ensure substitution of bromine in the para-position of the benzene ring and an unchanged degree of polymerization. By controlling the amount of bromine added and the reaction time, PBS with varying volume fractions of bromine (0.04 < f < 0.66) was synthesized (see Table 1).

Fracture Sample Preparation

PS Beams

The PS beams are 1 X 5 c[m.sup.2] with a thickness of 2 mm. The beams are molded by melt pressing polymer powder using a Carver Press (Wabash, Ind.). The two male ends of the mold are lined with DuPont Teflon[R] sheeting to create an airtight seal in the mold. The beams are pressed at 160[degrees]C for 30 minutes at 200 psi.

PBS Films

PBS films are cast from a 0.1 g/ml solution of PBS in toluene onto a pretreated silicon wafer (45) to obtain a film thickness of ~200 [micro]m. The samples are dried in a flow hood for 24 h and then in vacuum at 50[degrees]C for 6 h. Originally a 100 [micro]m film thickness was used, but transfer of the crack across the film from one interface to the other (during the fracture experiment) was observed with the unaided eye and confirmed by measuring the elemental composition of the fracture surface via energy-dispersive X-ray (EDX) spectroscopy. As a result, the PBS film thickness was increased to 200 [micro]m (which was verified by visual inspection and SEM), which eliminated the transfer of the crack.

PS-PBS-PS Trilayers

Trilayers are formed by joining one PBS film between two PS beams (see Fig. 1). The trilayers are placed in a mold, similar to the one used to form the PS beams, and annealed (150 < T < 200[degrees]C) under slight pressure in the Carver press for times up to 12 h. Our previous interdiffusion studies (40) clearly show that these times are sufficient for equilibrium. Two different types of interfaces are fabricated: N-symmetric (where [N.sub.PS] = [N.sub.PBS]) and N-asymmetric (where [N.sub.PS] [not equal to] [N.sub.PBS]).

Experimental Techniques

Modified Double Cantilever Beam

In general, many different fracture mechanics geometries can be used to measure fracture energy, such as the double cantilever beam, compact tension, double torsion, single edged notch, peel adhesion, and blister (1). Most of these have been adapted from bulk material tests to study interfaces. The fracture energy of polymer-polymer interfaces is measured using an adaptation of the double cantilever beam test, called the wedge cleavage, or modified double cantilever beam (MDCB) test. For polymers with different moduli, one way to eliminate mode mixity is to use trilayers instead of bilayers (46). The use of the trilayer will eliminate mode mixity as long as: 1) the outside beams are of the same thickness and modulus; and 2) the center layer is thin relative to the outside beam thickness. In the case for PS and partially brominated (f < 1) PBS, the moduli of the materials are close (within 10% when f = 0.11) so that mode mixity is further minimized (18). Using the MDCB test and a trilayer geometry, the fracture energy is calculated using Eq 1, based on an elastic foundation model originally proposed by Kanninen (47) and modified by Kalb et al. (27).