Many biopolymer hydrogels are environmentally responsive because they are held together by physical associations that depend on pH and temperature. Here we investigate how the pH and temperature response of the rheology of hyaluronan hydrogels is connected to the underlying molecular interactions. Hyaluronan is an essential structural biopolymer in the human body with many applications in biomedicine. Using two-dimensional infrared (2DIR) spectroscopy, we show that hyaluronan chains become connected by hydrogen bonds when the pH is changed from 7.0 to 2.5, and that the bond density at pH 2.5 is independent of temperature. Temperature-dependent rheology measurements show that due to this hydrogen bonding the stress relaxation at pH 2.5 is strongly slowed down in comparison to pH 7.0, consistent with the sticky reptation model of associative polymers. From the flow activation energy we conclude that each polymer is crosslinked by multiple (5-15) hydrogen bonds to others, causing slow macroscopic stress relaxation, despite the short time scale of breaking and reformation of each individual hydrogen bond. Our findings can aid the design of stimuli-responsive hydrogels with tailored viscoelastic properties for biomedical applications.

中文翻译：

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将生物聚合物水凝胶的刺激响应流变学与潜在的氢键相互作用联系起来

许多生物聚合物水凝胶对环境敏感，因为它们通过依赖于pH和温度的物理缔合而结合在一起。在这里，我们研究了透明质酸水凝胶流变学的pH和温度响应如何与潜在的分子相互作用有关。透明质酸是人体中必不可少的结构生物聚合物，在生物医学中有许多应用。使用二维红外（2DIR）光谱，我们显示了透明质酸链在pH从7.0更改为2.5时通过氢键连接，并且pH 2.5时的键密度与温度无关。与温度相关的流变学测量表明，由于氢键的存在，与pH 7.0相比，pH 2.5下的应力松弛大大减慢，与缔合聚合物的粘性复制模型一致。根据流动活化能，我们得出结论，尽管每个单独的氢键断裂和重整的时间尺度都很短，但每个聚合物都通过多个（5-15）氢键交联到其他聚合物上，从而导致宏观应力松弛缓慢。我们的发现可以帮助设计具有针对生物医学应用的量身定制的粘弹性的刺激响应水凝胶。