Inverse molecular weight dependence of cloud points for aqueous poly(N-isopropylacrylamide) solutions
Inverse molecular weight dependence of cloud points for aqueous poly(N-isopropylacrylamide) solutions
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DOI：
10.1021/ma990062d
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发表时间：
1999-06-29
期刊：
影响因子：
5.5
通讯作者：
Sato, T
中科院分区：
文献类型：
--
作者：
Tong, Z;Zeng, F;Sato, T
导论.聚（N-异丙基丙烯酰胺）（PNIPAm）由于其水溶液和交联水凝胶的热敏性而成为一种极具吸引力的高分子材料。1，2众所周知，PNIPAm/水溶液在升高温度超过约32 ℃时经历由疏水相互作用引起的相分离。3 PNIPAm同时含有亲水性酰胺基团和疏水性异丙基基团，可以作为模型化合物用于揭示具有疏水键的溶液的相行为。对疏水键诱导相分离的充分认识将有助于基于这种相变的智能材料的设计以及对蛋白质热致变性过程的认识。该体系的热敏性行为通常被认为是基于其低临界溶解温度（LCST），这是所有聚合物溶液的共同现象。4-9然而，关于PNIPAm水溶液的相图的信息仍然令人困惑。通过仅对一个样品加热时的相分离的可见观察，Heskins和Guillet 10确定了作为聚合物浓度的函数的浊点温度。Fujishige等人11从光学透射率测量结果表明，分子量在50 000至840万之间的PNIPAm的浊点曲线是可重叠的。最高分子量的曲线几乎是平的，即平行于浓度轴。相反，Schild和Tirrell 12认为PNIPA在水中的浊点取决于聚合物分子量和浓度。更严重的是，该系统的下临界点浓度和双结点至今还没有得到。这整个情况是由于缺乏单分散PNIPAm样品，这是相当困难的，通过通常的沉淀分级，因为它是两亲性的。最近，我们成功地分馏PNIPAm，并获得了一系列不同分子量的样品，Mw/Mn小于1.3。13使用这些窄分布的PNIPAm样品，我们确定了其水溶液的浊点温度，覆盖浓度从0.58到70重量%，并发现了一种新的分子量依赖性，与在溶剂升高的温度下具有LCST的普通聚合物溶液相反。N-异丙基丙烯酰胺
Introduction. Poly (N-isopropylacrylamide)(PNIPAm) becomes an attractive polymeric material both academically and industrially due to the thermosensitivity found in its aqueous solutions and cross-linked hydrogels. 1, 2 It is well-known that PNIPAm/water solution undergoes phase separation upon raising temperature beyond about 32 C induced by the hydrophobic interaction. 3 PNIPAm can also act as a model compound for revealing the phase behavior of solutions with hydrophobic bonding since it contains both hydrophilic amide groups and hydrophobic isopropyl groups. Fully understanding the hydrophobic bonding induced phase separation would help the design for intelligent materials based on this phase change and the recognition to the heating-induced denaturation process of proteins as well. The thermosensitivity behavior of this system is commonly believed on the base of its lower critical solution temperature (LCST) which is a common phenomenon for all polymer solutions. 4-9 However, the information about the phase diagram for the aqueous solution of PNIPAm is still confusing. By visible observation of the phase separation upon heating on only one sample, Heskins and Guillet10 determined the cloud point temperature as a function of the polymer concentration. From optical transmittance measurements Fujishige et al. 11 showed that the cloud point curves for PNIPAm ranging in molecular weight from 50 000 to 8400000 were superimposable. The curve for the highest molecular weight was almost flat, ie, parallel to the concentration axis. In contrast, Schild and Tirrell12 argued that the cloud point for PNIPA in water depends on polymer molecular weight and concentration. To be more serious, neither the concentration of the lower critical point nor binodals for this system are available as yet. This entire situation is due to the lack of monodisperse PNIPAm samples, which are considerably difficult to be acquired by usual precipitation fractionation because it is amphiphilic. Recently, we were successful in fractionation of PNIPAm and obtained a series of samples of different molecular weights with Mw/Mn less than 1.3. 13 Using these narrowly distributed PNIPAm samples, we determined the cloud point temperature of its aqueous solutions covering the concentration from 0.58 up to 70 wt% and found a novel molecular weight dependence, in contrast to common polymer solutions with the LCST at solvent elevated temperatures.Experimental Procedures. N-Isopropylacrylamide