The order in very thin films (12 nm to 92 nm) of the symmetric diblock copolymer poly(styrene-b-paramethylstyrene) near the order-to-disorder transition has been investigated by "N nuclear-reaction analysis. Lamellar ordering parallel to the surface is observed, qualitatively very similar to that of thick films. The order parameter L is found to depend linearly on film thickness d, when d is smaller than the natural order parameter Lo observed in the absence of constraints. The thickness dependence of L arises from the competition between the two characteristic dimensions of the system, Lo and d. The results are compared with recent theoretical predictions.

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