Deane Oliver J, Jennings James, Armes Steven P
Dainton Building, Department of Chemistry, University of Sheffield Brook Hill Sheffield South Yorkshire S3 7HF UK
Chem Sci. 2021 Oct 7;12(41):13719-13729. doi: 10.1039/d1sc05022b. eCollection 2021 Oct 27.
2-Hydroxypropyl methacrylate (HPMA) is a useful model monomer for understanding aqueous dispersion polymerization. 4-Hydroxybutyl acrylate (HBA) is an isomer of HPMA: it has appreciably higher aqueous solubility so its homopolymer is more weakly hydrophobic. Moreover, PHBA possesses a significantly lower glass transition temperature than PHPMA, which ensures greater chain mobility. The reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of HBA using a poly(ethylene glycol) (PEG) precursor at 30 °C produces PEG-PHBA diblock copolymer nano-objects. Using glutaraldehyde to crosslink the PHBA chains allows TEM studies, which reveal the formation of spheres, worms or vesicles under appropriate conditions. Interestingly, the partially hydrated highly mobile PHBA block enabled linear PEG-PHBA spheres, worms or vesicles to be reconstituted from freeze-dried powders on addition of water at 20 °C. Moreover, variable temperature H NMR studies indicated that the apparent degree of hydration of the PHBA block increases from 5% to 80% on heating from 0 °C to 60 °C indicating uniform plasticization. In contrast, the PHPMA chains within PEG-PHPMA nano-objects become dehydrated on raising the temperature: this qualitative difference is highly counter-intuitive given that PHBA and PHPMA are isomers. The greater (partial) hydration of the PHBA block at higher temperature drives the morphological evolution of PEG-PHBA spheres to form worms or vesicles, as judged by oscillatory rheology, dynamic light scattering, small-angle X-ray scattering and TEM studies. Finally, a variable temperature phase diagram is constructed for 15% w/w aqueous dispersions of eight PEG-PHBA diblock copolymers. Notably, PEG-PHBA can switch reversibly from spheres to worms to vesicles to lamellae during a thermal cycle.
甲基丙烯酸2-羟丙酯（HPMA）是用于理解水分散聚合的一种有用的模型单体。丙烯酸4-羟丁酯（HBA）是HPMA的异构体：它在水中的溶解度明显更高，因此其均聚物的疏水性更弱。此外，聚（丙烯酸4-羟丁酯）（PHBA）的玻璃化转变温度明显低于聚（甲基丙烯酸2-羟丙酯）（PHPMA），这确保了更高的链流动性。在30℃下使用聚乙二醇（PEG）前体对HBA进行可逆加成-断裂链转移（RAFT）水分散聚合，可制备聚乙二醇-聚（丙烯酸4-羟丁酯）（PEG-PHBA）二嵌段共聚物纳米粒子。使用戊二醛交联PHBA链可进行透射电子显微镜（TEM）研究，结果表明在适当条件下会形成球形、蠕虫状或囊泡状结构。有趣的是，部分水合的高流动性PHBA嵌段使得线性PEG-PHBA球形、蠕虫状或囊泡状结构在20℃加水时能够从冻干粉末中重构出来。此外，变温核磁共振氢谱（¹H NMR）研究表明，从0℃加热到60℃时，PHBA嵌段的表观水合度从5%增加到80%，表明其均匀增塑。相比之下，聚乙二醇-聚（甲基丙烯酸2-羟丙酯）（PEG-PHPMA）纳米粒子中的PHPMA链在温度升高时会脱水：鉴于PHBA和PHPMA是异构体，这种定性差异非常违反直觉。通过振荡流变学、动态光散射、小角X射线散射和TEM研究判断，较高温度下PHBA嵌段更大程度的（部分）水合驱动了PEG-PHBA球形结构向蠕虫状或囊泡状结构的形态演变。最后，构建了八种PEG-PHBA二嵌段共聚物15% w/w水分散体的变温相图。值得注意的是，PEG-PHBA在热循环过程中可以可逆地从球形转变为蠕虫状，再转变为囊泡状，最后转变为片层状。
