环形高分子单链塌缩的熔球中间态及其热力学解释

Intermediate Molten Globule State and Its Thermodynamic Interpretation in the Collapse Transition of Single Ring Polymer

采用动态蒙特卡洛分子模拟研究了环形高分子单链在不良溶剂中发生塌缩转变时可逆地出现具有核-壳结构特征的熔球中间态,发现该结构特征与相同链长的线形单链基本相同,表明其只与链的长短有关,而与链端基的特殊效应无关.本工作将这一现象与单链单晶在其平衡熔点附近出现的类似现象相互关联,采用表面预溶模型来解释单链塌缩出现熔球中间态的热力学机理.分子量越低,熔球越小,表面预溶现象就越显著,塌缩转变随热力学条件变化就越缓慢.实际的高分子体系由于链内拓扑缠结,在表面未必能充分释放片段链,达不到理论预期的平衡态.表面预溶使得相分离临界点或晶体熔点附近在界面厚度方向上存在链单元能量状态不连续分布,这在微观分子水平上与临界界面连续浓度梯度的传统理论处理不一致,为我们深入理解高分子流体界面的微观结构带来帮助.

An isolated single polymer suspending in the dilute solution exhibits a collapse transition when switched from good solvent to poor solvent. The transition passes through a thermally reversible intermediate molten globule state holding a characteristic core-shell structure with a concentrated core and a dilute shell, which appears more obvious for shorter chains. Therefore, the collapse transitions of short-chain polymers appear more gradual than those of long-chain polymers. This intermediate molten globule state is interesting because it allows a better understanding of the protein folding and its bio-functions as well as of polymer interface properties. We performed dynamic Monte Carlo simulations to compare the intermediate molten globule state of a single ring polymer to that of a single linear polymer. Their molten globule structures appeared very similar. This observation thus ruled out the specific effect of chain ends, and allowed us to focus on the surface effect of the globules in the intermediate states, which was also obvious for the limited chain length. We observed that, right below the theta point as the critical demixing point for solution of the polymer with infinite chain lengths, the intermediate molten globule states showed two energy states of monomers separately in the core and in the shell, appearing as a coexistence of two concentrations separately deviating from the critical 0 point. We, therefore, considered the molten globule state as a result of pre-dissolution, similar to the pre-melting phenomenon of single-chain single crystals around their equilibrium melting points. The surface dissolution model was then applied to describe the characteristic core-shell structures near the critical point during collapse transition of single-chain polymers. According to the surface dissolution model, near the critical theta point, the globule-solvent interface energy of the collapsed globules became high, which was then optimized by forming a thin layer of coexisting dilute phase,i.e. replaced by the globule-coil plus coil-solvent interface energy. The situation is something similar to the complete wetting of the vapour on a solid substrate, except that the partial surface dissolution achieves a minimum free energy with an equilibrium thickness of the surface layer. In fact, on the phase diagram, right below the theta point, any concentration deviation from the theta state will enter the metastable region for first-order phase transition. The equilibrium layer thickness defines the soluble length of chain segments on the surface. For a real single-chain polymer, however, the extent of surface dissolution may not be able to reach the equilibrium end, because the topological constraint inside the globule, for example, a knotting, will not allow the equilibrium lengths of chain segments to release at the globule surface. This kind of pre-dissolution behaviours could be unique for polymers at interfaces near the critical point. Since shorter chains make smaller globules, their surface dissolution phenomena become more obvious and their collapse transitions appear more gradual. Owing to the surface dissolution near the critical point for phase separation, the distribution of monomer energy states becomes discontinuous along the direction normal to interface. This behaviour reveals a discrepancy at the molecular level if compared to the traditional theoretical treatment of a continuous concentration gradient along this normal direction for the critical interfaces. Thus, the pre-dissolution model may provide a profound approach to better understand the microscopic nature of polymers at the near-critical fluid-fluid interfaces.