A novel experimental set up consisting of dielectric spectroscopy and small angle light scattering (or potentially any other optical technique used in combination) is introduced. The sample is enclosed in a Dielectric-Optical-Shear (DOS) cell capable of precisely controlling temperature and thickness. This method allows for simultaneous in situ, real-time monitoring of the changes in the molecular dynamics and corresponding development of supramolecular structure of a given polymer system during various time-sensitive processes. The DOS cell is utilized to investigate relaxation phenomena related to the glass transition, T-g, of poly(L(-)-lactide) during isothermal crystallization under quiescent, partially constrained conditions. A decrease in T, was observed in situ via a shift in dipolar relaxation to higher frequency (confirmed calorimetrically, ex. situ). The shift was most rapid in the spherulitic impingement stage of crystallization, at approximately 30% overall crystallinity. (C) 2003 Elsevier Science Ltd. All rights reserved.