玻璃化转变温度可区域数字化调控的形状记忆聚合物

Shape Memory Polymers with Regionally Defined Glass Transition Temperature via Digital Photocuring

用热聚合合成了一种带有未反应不饱和双键的形状记忆环氧树脂.通过第二步光固化过程,可以将未反应的双键聚合,形成第二重交联点以增加材料的交联密度,同时提高其玻璃化温度.利用数字化光掩膜技术对热聚合后的材料进行区域化地曝光,并控制各局部区域的曝光时间,可引发网络中的双键进行第二步光固化,并使材料区域的玻璃化温度在40～70°C区间内可调.二次固化前后的材料均表现出良好的形状记忆性能,形状固定率与回复率均接近100%.区域化玻璃化温度可控的材料可进行程序化的多形状记忆回复过程,并有望用做具有应变隔离功能的新型柔性电子基底材料.

Thermal transition temperatures (e.g. glass transition temperatures) play an important role in various applications of shape memory polymers that can recover from temporary shapes to original (permanent) shapes upon heating. This study reports a shape memory epoxy with unsaturated double bonds, enabling a secondary photocuring process. Dual-functional epoxy monomer, E44, and monofunctional epoxy monomer, glycidyl methacrylate, are first thermally cured via a polyether amine crosslinker. Properties of the cured epoxy can be finely tuned by controlling the feed compositions. The increase of feed ratio of glycidyl methacrylate will lead to lower glass transition temperature and elastic modulus, whereas larger strain at break. In the secondary photocuring process, the unsaturated bonds provided by glycidyl methacrylate are polymerized after light exposure, forming additional crosslinking points. As a result, the glass transition temperature of the material increases. Herein, digital photo-masking technique is applied for the photocuring process via a commercial projector. The regional exposure time of the material can be conducted, and thus, the local glass transition temperature can be controlled in a range of approximately 40-70℃. Storage modulus at the rubbery state of the material increases from approximately 0.5 MPa to 8 MPa after photo exposure for 240 s, implying that there is a remarkable increase of the crosslinking density via the secondary photocuring. Both the materials before and after the secondary photocuring provide ideal shape memory performance. The shape fixity ratios and shape recovery ratios retain nearly 100% during six shape memory cycles, indicating good thermo-mechanical stability of the material. The material with regional controllable glass transition temperature enables a programmable multi-shape recovery process. Complex original shapes can be fabricated if the secondary photocuring is conducted towards a thermally cured epoxy. In addition, the material can be potentially applied as a novel substrate for stretchable electronics due to its strain isolation functionality.