Evolution of Phase Morphology of Mixed Poly(tert-butyl acrylate)/Polystyrene Brushes Grafted on SilicaParticles with the Change of Chain Length Disparity.

We report in this article the synthesis of a series of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene(PS) brushes with PtBA number-average molecular weight(Mn) being fixed at 24.5 kDa and PS Mnranging from 14.8 to 30.4 kDa on 160 nm silicaparticles and the study of their microphase separation behaviors usingdifferential scanning calorimetry (DSC) and transmission electronmicroscopy (TEM). The samples were synthesized by a two-step “graftingfrom” process from asymmetric difunctional initiator (Y-initiator)-functionalizedsilica particles using two different “living”/controlledradical polymerization techniques. PtBA brushes weregrown first from Y-initiator-functionalized particles by surface-initiatedatom transfer radical polymerization of tert-butylacrylate at 75 °C in the presence of a free initiator, followedby nitroxide-mediated radical polymerization (NMRP) of styrene at120 °C. The “living” nature of NMRP allowed thesynthesis of mixed PtBA/PS brushes with differentPS molecular weights (14.8, 18.7, 24.9, and 30.4 kDa) in a one-potpolymerization. DSC studies showed that all thermally annealed mixedbrush samples exhibited two glass transitions with the middle pointslocated at ∼47 and ∼90 °C, suggesting that thetwo tethered polymers microphase separated in the brush layer. ForTEM studies, the samples were dispersed in CHCl3, a goodsolvent for both PtBA and PS, drop-cast onto carbonfilms, thermally annealed in vacuum at 120 °C for 3 h, and thenstained with RuO4vapor. All samples showed clear microphaseseparation, consistent with the DSC results. With increasing PS Mnfrom 14.8, to 18.7, and 24.9 kDa, the morphologyof mixed brushes evolved from isolated, nearly spherical PS microdomainsburied inside the PtBA matrix, to short PS cylindricalmicrodomains in the PtBA matrix, and to nearly bicontinuousnanostructures. Further increasing the molecular weight of PS to 30.4kDa resulted in the formation of isolated PtBA microdomainswhich were covered by PS chains. [ABSTRACT FROM AUTHOR]