以β-環狀糊精與三嵌段共聚物自組裝形成準聚輪烷之結構與流變性質

Abstract

先前研究中指出Pluronic (PL)及β-cyclodextrin (β-CD)可以藉由主客交互作用形成準聚輪烷(polypseudorotaxane, PPR)。然而，β-CD的在水相中之低溶解度卻限制了此系統之應用性。利用24%檸檬酸(CA)作為溶劑，β-CD的溶解度可以提高至比水相中高約10倍，因此可以製備出以PL及β-CD自組裝形成之PPR膠體。所形成的PPR膠體亦被證實具有搖變減黏之特性。然而，檸檬酸對於PL自身自組裝結構之影響尚未被研究。此外，在檸檬酸溶液中，PL及β-CD之自組裝結構及其結構對於流變性質之關係亦尚未被研究。在低於 PL F108、F127、F88及P84之臨界微胞濃度時，可以利用動態光散射、小角度X光/中子散射觀察到一異常聚集之行為。分子動力學模擬結果指出，在小於PL之臨界微胞濃度時，PPO之疏水作用力為造成此異常聚集之原因。而在此溶液中添加檸檬酸、醋酸、鹽酸及氫氧化鈉可以有效抑制聚集生成長達20天。在酸性溶液中，由於H3O+分子與PL分子上之EO單體結合進而使分子間產生斥力，以及在鹼性溶液中，因鹽析現象而降低其臨界微胞濃度則為其抑制聚集產生之原因。了解檸檬酸對於F108自身聚集之影響後，則繼續探討添加不同β-CD對於此系統之自組裝結構影響。在此可以發現，尚未添加β-CD時，5% F108將自組裝形成一半徑為1 nm而長度為8 nm之柱狀微胞。當添加β-CD後，則可觀察到柱狀之PPR。在1%至9% β-CD添加量下，散布在F108分子鏈上之β-CD數目從1-2個增加到3-4個。當β-CD濃度大於10%時，將藉由β-CD之分子間氫鍵形成以兩個組成單元構成之相關性結構(correlated structure)。而在10% β-CD添加量時，可以觀察到兩個大小分別為50 (α相) 及46 nm (β相)之domain。然而，此兩個domain之內部結構中卻無法觀察到週期性結構。當β-CD添加量大於12%後，週期性結構則可以在domain內部被觀察到。在12-19% β-CD系統中，此α相 及β相之domain大小縮小至41 & 32及 30 & 10 nm。根據分析19% β-CD系統散射圖譜之繞射峰，更可以知道單一F108分子鏈上有48個β-CD與化學計量之計算結果(49個)相互吻合。之後，對於PL/β-CD系統之結構特徵和流變性進行研究。使用流變學小角度中子散射(Rheo-SANS)可以知道，在流場中由PPR組成之層板結構可以沿著速度梯度之方向排列。移除剪切力後，高度排列的層板結構回復至其勻相性排列狀態之鬆弛時間，可能大於2小時，亦甚至不會發生(不可逆)。此層板結構間則以F108分子鏈串接成一更大尺度之結構。在此，我們認為晶粒(grain)中即包含了此網絡結構。晶粒大小的動態變化則源自於F108分子鏈所形成網絡的破壞及重新建立，進而導致了搖變減黏及降伏應力之行為。層板結構在系統中之位向(orientation)導致了預剪切系統中黏度的不可逆變化(在實驗時間尺度上)。上述這些知識為PL/β-CD系統提供了基本的理解。

Polypseudorotaxane (PPR) prepared by the host-guest interaction between Pluronic (PL) and β-cyclodextrin (β-CD) had been studied in the past. However, the low solubility of β-CD in water limited its application. In this study, by utilizing 24% citric acid (CA), the solubility of β-CD can be increased to ~10 times higher than that in aqueous phase and, hence, makes the gelation of β-CD based PPR system possible. The formed PPR gel bear thixotropic behavior which makes it a potential candidate for the injectable gel. However, the fundemental understanding about the effect of CA to PL is not clear. The self-assembly structure of PPR formed by such high concentration of β-CD as well as the the relation between the structure characteristics to the rheological properties also need to be revealed. Anomalous large PL aggregates below the critical micelle concentration (CMC) were found throughout four PL types (F108, F127, F88 and P84). We characterized their structures using dynamic light scattering and small-angle X-ray/neutron scattering. Molecular dynamic simulations suggest that the PPO segments, through weakly hydrophobic (insufficient to form micelles), promotes the formation of large aggregates. Addition of acid or base (e.g. citric acid, acetic acid, hydrochloric acid and sodium hydroxide) in F108 solution significantly suppresses the aggregate formation up to 20 days due to the repulsion force from the attached H3O+ molecules on EO segment in both PEO and PL and the reduction of CMC though the salting-out effect. The self-assembly structure of F108 in various amount of β-CD solution was investigated. 5% F108 formed cylindrical micelles of 1 nm in radius and 8 nm in length in the presence of 24% citric acid through the dehydration of citric acid and citrate. PPR was formed once β-CD was added. In dilute β-CD system (1%), the single chains of PPR with separated β-CD stacks on F108 were formed. The numbers of β-CD in each stack increased from 1 to 4 with increasing β-CD concentration to 9%. In dense β-CD system, PPR condensed to correlated structures majorly composed of two unit blocks through the hydrogen bonds between PPRs. Two distinguishable correlated domains with correlation lengths of 50 nm (marked α-phase) and 46 nm (marked β-phase) along the chains, but without fine periodic structure within each individual domain, were identified in the 10% β-CD solution. Periodic stacking of β-CD in the domains developed in the 12% solution. As β-CD concentration increased from 12 to 19%, the correlated heights of α and β phases shrank from 41 and 32 nm into 30 and 10 nm, respectively. There were 48 β-CDs that stabilized on each PL F108 chain in the 19% β-CD system, which is in a good agreement with the stoichiometry.Then, the structural characteristic and the rheological properties were studied. The PPR based lamellar structure could be aligned with the velocity gradient direction in shear flow evidenced by small angle neutron scattering for rheology (Rheo-SANS). The relaxation of highly aligned lamellae to the isotropic state is either slower than 2 h or irreversible after ceasing the shear flow. We propose that the lamellar stacks which are connected by the protruded PL F108 chains. The secondary structure is “grain” within which several lamellar stacks network with each other through F108. The dynamic variation of grain size through dissociation and reestablishment of the networks of the PL F108 chain is responsible for the thixotropic behavior and the observed yielding behavior. The orientation of the lamellar stacks results in the irreversible change (on the experimental time scale) of the viscosity in the pre-sheared system. This knowledge provides fundamental understanding of the PL/β-CD system.