Polystyrene ripple experiment

Fig. 6. The ripple experiment at the interface between a bilayer of HDH- and DHD-labeled polystyrene, showing the interdifussion behavior of matching chains. The protonated sections of the chain are marked by filled circles. The D concentration profiles are shown on the right. Top the initial interface at / = 0. The D concentration profile is flat, since there is 50% deuteration on each side of the interface. Middle the interface after the chain ends have diffused across (x < / g). The deuterated chains from Que side enrich the deuterated centers on the other side, vice ver.sa for the protonated sections, and the ripple in the depth profile of D results. A ripple of opposite sign occurs for the H profile. Bottom the interface when the molecules have fully diffused across. The D profile becomes flat [20,56].

The Ripple Experiment Agrawaletal. (37) carried out what they called the ripple experiment, using neutron reflectometry on selectively deuterated polystyrene block copolymer chains in two layers. In one of the layers, the central 50% of the mers were deuterated, the two end 25% portions were normal, that is, bearing hydrogen. This material was denoted as HDH (1/4H-1/2D-1/4H). In the second layer, the chain labeling was reversed to make DHD (1/4D-1/2H-1/4D). The molecular weights of the two polymers were nearly identical, 2.25 x 10 g/mol, and 2.50 x 10 g/mol, respectively, synthesized by anionic polymerization to have narrow polydispersity indices. 

Welp et al. (43) continued the ripple experiment, going to 400,000 g/mol polystyrene to obtain a better definition of the ripple. They concluded that the reptation model proposed by de Gennes (44) with parallel development by Doi and Edwards (45) was the best model to describe the dynamics of polymer interdiffusion. There were six dominant ripple characteristics that were examined ... 

In order to give some perspective, let us consider one of the block copolymers studied by us, namely polystyrene-poly(methyl methacrylate) symmetric diblock of molecular weight 37,000 at 200°C. Using appropriate numbers for this material, and our theoretical prediction for the lamellar moduli B and K [54], the critical field is estimated to be 260 kV/cm for a l-/i,m gap and 8.2 kV/cm for a 1-mm gap. The incipient ripple wavelengths would be 0.14 and 4.3 /xm, respectively. The 18 kV/cm field used in several of our experiments can induce a ripple instability only if the gap size exceeds —200 /xm. 

---