Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel.,Biomacromolecules

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Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel.
Biomacromolecules ( IF 5.5 ) Pub Date : 2018-05-18 , DOI: 10.1021/acs.biomac.8b00392
Ville Hynninen ₁ , Sami Hietala ₂ , Jason R McKee ₃ , Lasse Murtomäki ₄ , Orlando J Rojas _{1,

5} , Olli Ikkala _{1,

5} , Nonappa _{1,

5}

Affiliation

We show that composite hydrogels comprising methyl cellulose (MC) and cellulose nanocrystal (CNC) colloidal rods display a reversible and enhanced rheological storage modulus and optical birefringence upon heating, i.e., inverse thermoreversibility. Dynamic rheology, quantitative polarized optical microscopy, isothermal titration calorimetry (ITC), circular dichroism (CD), and scanning and transmission electron microscopy (SEM and TEM) were used for characterization. The concentration of CNCs in aqueous media was varied up to 3.5 wt % (i.e, keeping the concentration below the critical aq concentration) while maintaining the MC aq concentration at 1.0 wt %. At 20 °C, MC/CNC underwent gelation upon passing the CNC concentration of 1.5 wt %. At this point, the storage modulus ( G') reached a plateau, and the birefringence underwent a stepwise increase, thus suggesting a percolative phenomenon. The storage modulus ( G') of the composite gels was an order of magnitude higher at 60 °C compared to that at 20 °C. ITC results suggested that, at 60 °C, the CNC rods were entropically driven to interact with MC chains, which according to recent studies collapse at this temperature into ring-like, colloidal-scale persistent fibrils with hollow cross-sections. Consequently, the tendency of the MC to form more persistent aggregates promotes the interactions between the CNC chiral aggregates towards enhanced storage modulus and birefringence. At room temperature, ITC shows enthalpic binding between CNCs and MC with the latter comprising aqueous, molecularly dispersed polymer chains that lead to looser and less birefringent material. TEM, SEM, and CD indicate CNC chiral fragments within a MC/CNC composite gel. Thus, MC/CNC hybrid networks offer materials with tunable rheological properties and access to liquid crystalline properties at low CNC concentrations.

中文翻译：

甲基纤维素/纤维素纳米晶体水凝胶中的逆热可逆机械硬化和双折射。

我们表明，包含甲基纤维素（MC）和纤维素纳米晶体（CNC）胶体棒的复合水凝胶在加热时表现出可逆且增强的流变储能模量和光学双折射，即逆热可逆性。使用动态流变学、定量偏光光学显微镜、等温滴定量热法 (ITC)、圆二色性 (CD) 以及扫描和透射电子显微镜（SEM 和 TEM）进行表征。水介质中CNC 的浓度变化最大为3.5 wt%（即，保持浓度低于临界水浓度），同时保持MC 水浓度为1.0 wt%。在 20°C 下，当 CNC 浓度达到 1.5 wt% 时，MC/CNC 发生凝胶化。此时，储能模量（G'）达到稳定水平，双折射率逐步增加，从而表明存在渗透现象。复合凝胶的储能模量 (G') 在 60 °C 时比 20 °C 时高一个数量级。ITC 结果表明，在 60°C 时，CNC 棒在熵驱动下与 MC 链相互作用，根据最近的研究，MC 链在该温度下塌陷成具有中空横截面的环状胶体尺度持久原纤维。因此，MC 形成更持久聚集体的趋势促进了 CNC 手性聚集体之间的相互作用，从而提高了储能模量和双折射。在室温下，ITC 显示 CNC 和 MC 之间存在焓结合，后者包含水性、分子分散的聚合物链，从而产生更松散且双折射较小的材料。TEM、SEM 和 CD 显示 MC/CNC 复合凝胶内的 CNC 手性片段。因此，MC/CNC 混合网络提供了具有可调流变特性的材料，并在低 CNC 浓度下获得液晶特性。

更新日期：2018-05-07

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