Phase Behavior of Poly(2-vinylpyridine)-block-Poly(4-vinylpyridine) Copolymers Containing Gold Nanoparticles

We studied the phase behavior of the poly­(2-vinylpyridine)-block-poly­(4-vinylpyridine) copolymer (P24VP) containing gold nanoparticles, by rheometry, small-angle X-ray scattering, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). Although both blocks of P24VP exhibited attractive interaction to gold precursors, unusual phase behavior was observed depending on the amount of gold nanoparticles. As the amount of gold nanoparticles increased, the order-to-disorder transition temperature (T_ODT) of P24VP with gold nanoparticles decreased first, then increased, and finally decreased again. To explain this phenomenon, we prepared two block copolymers: polystyrene-block-poly­(2-vinylpyridine) copolymer (PS2VP) and polystyrene-block-poly­(4-vinylpyridine) copolymer (PS4VP) containing gold nanoparticles. With increasing the amount of gold particles, the T_ODT of PS2VP increased continuously, whereas that of PS4VP gradually decreased. For PS4VP containing gold nanoparticles, because P4VP chains can interact with the gold nanoparticle surface, density fluctuations exist near the gold nanoparticle surfaces, which causes the T_ODT to decrease. On the other hand, although the pyridine ring in P2VP could be associated with the gold surface, P2VP chains become stretched due to steric hindrance arising from the ortho position of nitrogen in P2VP. The chain stretching increases the T_ODT. Thus, the decrease of T_ODT for P24VP originates from P4VP microdomains containing gold nanoparticles, while the increase of T_ODT is attributed to the P2VP microdomains containing gold nanoparticles. With increasing the amounts of gold nanoparticles, the contribution of P4VP microdomains containing gold nanoparticles on the T_ODT becomes dominant, causing the T_ODT to redecrease. To verify the gold nanoparticle position in both P2VP and P4VP microdomains, we performed TEM and scanning transmission electron microscopy (STEM) experiments. At lower amounts of gold nanoparticles, they are mainly located inside P4VP microdomains. With increasing the amount of the gold nanoparticles, they are distributed in both P2VP and P4VP microdomains, though the amount of gold nanoparticles in P4VP microdomains is larger than that in P2VP microdomains.