The thermodynamic and dynamic properties of several partially deuterated symmetric (equal block volume fraction) polyethylene-poly(ethylethylene) (PE-PEE), poly(ethylene-propylene)-poly-(ethylethylene) (PEP-PEE), and polyethylene-poly(ethylene-propylene) (PE-PEP) diblock copolymers were characterized above and below the order-disorder transition (ODT). A lamellar morphology was established for all temperatures below the ODT using small-angle neutron scattering (SANS) measurements with shear-aligned specimens. As TODT is approached, the lamellar order weakens as evidenced by the loss of higher order SANS reflections and azimuthal smearing of the principal equitorial scattering peaks. SANS and rheology experiments also provided unambiguous evidence of composition fluctuations above the order-disorder transition temperature in each system. However, the temperature range of the fluctuations in the disordered state depends on the degree of polymerization, in accordance with fluctuation theory. A crossover in the disordered state between slightly and moderately stretched coils was also documented by SANS, coincident with the temperature where fluctuations become apparent Theologically.