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  納米材料表面的分子狀態(tài)是決定材料表面現(xiàn)象（潤(rùn)濕、摩擦、粘結(jié)等）和物理性能（力學(xué)、流變、結(jié)晶等）的關(guān)鍵因素。研究高分子表面分子的弛豫行為不僅有助于尋找調(diào)控納米材料性能的新方法，還可為揭示玻璃化的本質(zhì)這一凝聚態(tài)物理長(zhǎng)期懸而未決的問(wèn)題提供新的視角和證據(jù)。研究發(fā)現(xiàn)，玻璃態(tài)高分子表面存在一層運(yùn)動(dòng)能力增強(qiáng)的“活性層”；隨著遠(yuǎn)離表面，分子運(yùn)動(dòng)能力逐漸減弱直至回到本體狀態(tài)。然而，對(duì)于表面增強(qiáng)分子運(yùn)動(dòng)能力的傳遞距離和梯度分布規(guī)律的爭(zhēng)論，以及實(shí)驗(yàn)上高空間分辨率測(cè)量表面附近局部分子弛豫時(shí)間（或玻璃化轉(zhuǎn)變溫度，T_g）的困難，阻礙了該領(lǐng)域的進(jìn)一步發(fā)展。

Fig. 1. Experimental depth profiles T_g^ne(z) taken at the surface of various c-PMMA polymers. Dashed lines are the best fits to a linear gradient.

Fig. 2. Simulation results revealing intrinsic nonequilibrium effects on near-surface τ(z) of long-chain films. (A) Segmental relaxation time τ vs. inverse temperature for various segmental layers at different distances z/σ from the surface (σ is the length scale of the bead-spring potential). The dotted lines represent the low-temperature cutoff for inclusion of equilibrium relaxation time data and τ = t_ANN,max = 10⁶. The curved lines show fits of the data to an established model for equilibrium dynamics. (B) Normalized alteration gradients of T_g^eq and T_g^ne relative to bulk for systems with t_ANN,max = 10⁵ and 10⁶. The solid line shows a fit of T_g^eq (z) to an exponential gradient form to the T_g^eq data, while the dotted lines represent linear fits to the surface region.

  論文鏈接：www.pnas.org/doi/10.1073/pnas.2406262121

  課題組網(wǎng)頁(yè)：https://www.chem.zstu.edu.cn/gfzclbjmsys.htm