Shear Alignment Mechanisms of Close-Packed Spheres in a Bulk ABA Triblock Copolymer
Shear Alignment Mechanisms of Close-Packed Spheres in a Bulk ABA Triblock Copolymer
复制标题
DOI：
10.1021/acs.macromol.2c01245
复制
发表时间：
2022-10
期刊：
影响因子：
5.5
通讯作者：
Wenyue Ding;Josiah Hanson;W. Burghardt;C. López‐Barrón;Megan L. Robertson
中科院分区：
文献类型：
--
作者：
Wenyue Ding;Josiah Hanson;W. Burghardt;C. López‐Barrón;Megan L. Robertson
采用原位小角X射线散射(SAXS)和傅立叶变换(FT)流变学相结合的方法，对聚苯乙烯-丙烯酸十二酯-b-苯乙烯三嵌段共聚物中封闭堆积微球的取向和剪切诱导的微观结构变化进行了定量研究。通过应用非线性振荡剪切，将模压成型产生的无序球体转化为紧密堆积的球体。FT流变学确定了在固定温度和频率下小幅度、中等幅度和大幅度振荡剪切行为(分别为SAOS、MAOS和LAOS)的应变幅度区域。振荡剪切的存在引起了六角封闭堆积(HCP)层的取向变化。增加应变幅度会产生更快的有序化和更高的取向度，通过对现场SAXS和FT流变学数据拟合拉伸指数函数来量化。在MAOS区，观察到剪切变薄行为，由于形成的HCP层表现出较小的流动阻力，储能系数(G‘)降低。三次谐波(i3/1)随应变幅值的平方而增大，假设来源于尖锐的晶界(由于缺陷)，切比雪夫系数表明循环内剪切增厚和应变硬化。在老挝地区，HCP层采用平行取向的曲折路径滑动机制，且g‘和损耗模数(G“)均随应变幅值的增大而减小。有趣的是，在较高的应变幅值下，在振荡剪切作用下，紧密堆积的球体中首次观察到剪切无序，这从原位SAXS和FT流变学数据的特征特征中得到了证明。结构重排与微区溶解和重整机制一致。
The alignment and shear-induced microstructural changes of closed-packed spheres observed in a poly(styrene-b-lauryl acrylate-b-styrene) triblock copolymer were quantified with a combination of in situ small-angle X-ray scattering (SAXS) and Fourier-transform (FT) rheology. Disordered spheres produced through compression molding were transformed to close-packed spheres through the application of nonlinear oscillatory shear. FT rheology identified strain amplitude regimes for small-, medium-, and large-amplitude oscillatory shear behaviors (SAOS, MAOS, and LAOS, respectively) at fixed temperature and frequency. The presence of oscillatory shear induced orientational changes in the hexagonally-closed packed (HCP) layers. Increasing the strain amplitude produced faster ordering and higher degree of orientation, quantified through fitting a stretched exponential function to in situ SAXS and FT rheology data. In the MAOS regime, shear thinning behavior was observed, with decrease in the storage modulus (G′) due to the formation of HCP layers that exhibited less resistance to flow. The third-order harmonic (I3/1) increased with the square of the strain amplitude, hypothesized to originate from the presence of sharp grain interphase boundaries (due to defects), and Chebyshev coefficients indicated intracycle shear thickening and strain stiffening. In the LAOS regime, HCP layers adopted a parallel orientation with a zigzag path sliding mechanism, and bothG′and the loss modulus (G″) decreased with increasing strain amplitude. Interestingly, at higher strain amplitude, shear deordering was observed for the first time in close-packed spheres under oscillatory shear, as evidenced by characteristic signatures in both in situ SAXS and FT rheology data. Structural rearrangements were consistent with the domain dissolution and reformation mechanism.