Block Copolymer/Homopolymer Mesoblends:  Preparation and Characterization

Abstract

Miscible block copolymer/homopolymer blends are typically prepared from homogeneous solutions in a nonselective solvent. During solvent removal and subsequent annealing, the molecular species comprising such blends organize in such fashion as to lower the system free energy and ideally attain thermodynamic equilibrium. In this work, we investigate nonequilibrium triblock copolymer/homopolymer (ABA/hB) blends generated by diffusing hB molecules from a hB-selective solvent into a lamellar ABA copolymer. Since the copolymer is already microphase-ordered during homopolymer incorporation, we refer to such blends as mesoblends. The mass uptake of hB is found to be strongly dependent on homopolymer molecular weight (M hB), with the maximum solubility scaling as M hB -1. An induction period that scales as M hB 1/2 is also observed. Transmission electron microscopy reveals that the morphology of these mesoblends appears to be perforated lamellar, which, in some cases, transforms to cylindrical upon annealing. Dynamic mechanical analysis and differential scanning calorimetry confirm that the A-rich microdomains in the mesoblends are plasticized. The mechanical properties of these mesoblends can be improved upon annealing, but nonetheless differ from those of composition-matched conventional blends.

Solvent-Facilitated Homopolymer Sorption in Swollen Block Copolymer Matrices

“…This trend likewise applies to the S hI (C) data for the SIS73 series with the exception that appreciable hI solubility is not achieved until sufficient driving force exists for the hI molecules to diffuse into the swollen copolymer matrix (C > 1% w/v). Such delayed homopolymer transport has been previously observed 21 in systems wherein the molecular weight of hI (M hI ) was systematically varied at constant M I . At higher values of C, S hI increases abruptly and eventually exceeds the values of S hI attained by the other two copolymers.…”

“…Within experimental uncertainty, these results suggest the existence of a monotonically increasing linear relationship between (∂S hI /∂C) T and M I , which confirms that hI solubility depends on C (a measure of the number of hI molecules available per unit volume) and M I (a measure of the volume that, upon swelling in H, becomes available for hI uptake) at constant M S . While it is not known whether such linearity is achieved with homopolymer/ copolymer combinations differing in M hI or chemical constitution, we have previously reported 21 that S hI varies as M hI -1/2 at constant M I for related systems. To ascertain the value of the hI diffusion coefficient from the sorption data presented in Figure 1, we first reformat the data in a form amenable to analysis.…”

“…Although the SIS40, SIS51, and SIS73 copolymers initially possess lamellar morphologies, as discerned 20 from transmission electron microscopy (TEM), their morphologies evolve in the presence of H. This solvent, while selective for the I midblocks of the copolymers, is sufficiently soluble in the glassy S end blocks to plasticize the S lamellae. Swelling of the I midblocks, coupled with internal stress buildup in the S lamellae, 22 promotes chain rearrangement, void formation, and, ultimately, a morphological transition to a more complex, nonlamellar morphology 21 (see the inset provided in Figure 1). This morphology retains sufficient mechanical integrity to permit handling of the swollen films after exposure.…”

“…After a given immersion time, the squares were removed, washed with a detergent-containing aqueous solution, and rinsed with distilled water prior to drying under vacuum (for 2 days) and reweighing. Longer drying times under vacuum did not result in any further weight change.Residual solvent remaining in the PS microdomains after solvent removal was accounted for by performing complementary solubility tests in pure H, as described earlier 21.…”

Ordered Structures and Phase Transitions in Mixtures of a Polystyrene/Polyisoprene Block Copolymer with the Corresponding Homopolymers in Thin Films and in Bulk

“…Many studies on triblock copolymers have been concerned with the bulk behavior of almost symmetric copolymers having a lamellar morphology blended with midblock associating homopolymer. [9][10][11] Lamellar swelling induced by homopolymer has been characterized. Temperature-dependent bulk phase transitions have been investigated in binary blends consisting of the highly asymmetric triblock copolymer (Vector 4111) and the end block associating homopolymer polystyrene.…”