(464g) Gradient Copolymers with Continuous Distributions of Nanoenvironments Yield Glass Transition Temperatures over 50-80 K in Breadth
Gradient copolymers have molecular structures between those of random copolymers and block copolymers, with a continuous change in comonomer composition along the chain backbone. Predictions were made several years ago (Buzin et al., Polymer 2002, 43, 5563) that gradient copolymers may possess unique thermal properties in comparison to other copolymer types. Here we demonstrate experimentally for the first time the very distinct glass transition behavior of a number of gradient copolymer systems involving continuous transitions of many tens of degrees in breadth.
Studies have been performed on four gradient copolymer systems with comparisons being made to random and block copolymers from the same comonomers used in the gradient copolymers. These include styrene-acetoxystyrene (S-AS) copolymers, S-hydroxystyrene (HS) copolymers (made by hydrolysis of the S-AS copolymers, S-tert-butyl acrylate (TBA) copolymers, and S-acrylic acid (AA) copolymers (made by hydrolysis of the S-TBA copolymers). By employing the derivatives of the heat curves obtained in differential scanning calorimetry, it is possible to demonstrate that all random copolymers exhibit one relatively narrow glass transition temperature (Tg) while block copolymers of sufficient size exhibit two distinct Tgs with an inactive temperature range (no glass transition response) between the two Tgs. In contrast, gradient copolymers exhibit continuous Tgs of very significant breadth, as much as 80 K in the S-HS system, due to the range of nanoscale compositions present in the gradient copolymer samples. These results are consistent with recent predictions by Lefebvre, Olvera de la Cruz, and Shull (Macromolecules 2004, 37, 1118) who employed numerical self-consistent field methods to compare the ordered lamellar states in block and gradient copolymers. Lefebvre et al. found that the unit cell composition tends toward a sinusoidal shape with gradient copolymers, meaning that a broad, continuous distribution of nanoscale compositions are present in gradient copolymers. Such a distribution is expected to yield a continuous, broad distribution of Tgs, in agreement with the Tg behavior obtained in the current experimental study.
The potential applications of such materials and other unique behavior expected for gradient copolymers with continous, broad Tgs will also be discussed.