(667f) Glass Transition Temperature of Isolated Polymer Chains Dispersed within a Bulk Phase: Novel Characterization by Fluorescence
Since the publication of "Self-concentration and effective glass transitions in (miscible) polymer blends" (Macromolecules 2000, 33, 5278) by Lodge and McLeish in the year 2000, many dozens of experimental studies have focused on characterizing the alpha-relaxation dynamics and glass transition temperatures (Tgs) of particular components in miscible polymer blends. In numerous cases, these studies have demonstrated that the two polymer components in miscible polymer-A/polymer-B blends can exhibit substantially different cooperative segmental dynamics if the Tgs of the two homopolymers are sufficiently different in magnitude. However, the lack of instrumental sensitivity via conventional techniques to a minor component in highly asymmetric blends (≤ ~1 wt% minor component) has hindered the direct characterization of Tg associated with trace levels of one polymer dispersed in a sea of a second polymer. Such measurements are useful to test the validity of the Lodge-McLeish model in the limits of infinite dilution, both in blends that are miscible at all compositions and in blends that are normally immiscible except near the limit of infinite dilution of one component. Such measurements are also useful to provide information regarding the length scales over which the cooperative segmental mobility of a single polymer-A chain may be impacted by perturbations to its environment consisting of bulk polymer-B, e.g., by the presence some hundreds of nanometers away of other polymer-A chains as the concentration of polymer-A is slowly increased from infinite dilution.
In response to this challenge, we have developed a novel fluorescence technique that utilizes the intrinsic fluorescence of styrene repeat units to determine the Tg of a styrene-containing minor component dispersed homogeneously near trace levels in a polymer blend (e.g., at ≤ 2 wt % where the blend is miscible and the minor component is dissolved in the bulk phase). In particular, we have applied this technique to determine the Tg of polystyrene (PS) chains near infinite dilution in other polymers via the intersection of the rubber-state and glassy-state temperature dependences of fluorescence intensity. This technique also allows us to confirm that the PS chains are individually dispersed in the second polymer and not segregated or in significant contact with other PS chains because single PS chains isolated in a second polymer exhibit only monomer fluorescence of the phenyl rings while segregated PS chains exhibit both excimer and monomer fluorescence. When dispersed at 0.1 wt% within poly(methyl methacrylate) (PMMA) (neat, bulk Tg = 125 °C), polystyrene (PS) (neat, bulk Tg = 100 °C) exhibits a Tg of 118 °C. However, when PS chains are dispersed at 0.1 wt% within poly(t-butyl acrylate) (PtBA) (neat bulk Tg = 45 °C), the PS Tg is reduced to 64 °C. This demonstrates that the bulk environment can have dramatic effects on the chain dynamics of the individually dispersed PS chains. This technique has also been extended to block copolymer/homopolymer blends. This study is the first to allow direct Tg measurements of isolated polymer chains dispersed within a bulk phase and also allows for a critical test of the Lodge-McLeish model at highly asymmetric blend ratios.