How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals

<b>Abstract</b><br/><p>Emulsion polymerized latex-based pressure sensitive adhesives (PSAs) are more environmentally benign because they are synthesized in water but often underperform compared to their solution polymerized counterparts. Studies have shown a simultaneous improvement in the tack, and peel and shear strength of various acrylic PSAs upon the addition of cellulose nanocrystals (CNCs). This work uses atomic force microscopy (AFM) to examine the role of CNCs in (i) the coalescence of hydrophobic 2‐ethyl hexyl acrylate/n‐butyl acrylate/methyl methacrylate (EHA/BA/MMA) latex films, and (ii) as adhesion modifiers over multiple length scales. EHA/BA/MMA. Thin films with varying solids content and CNC loading were prepared by spin coating. AFM revealed that CNCs lowered the solids content threshold for latex particle coalescence during film formation. This improved the cohesive strength of the films, which was directly reflected in the increased shear strength of the EHA/BA/MMA PSAs with increasing CNC loading. Colloidal probe AFM indicated that the nano-adhesion of thicker continuous latex films increased with CNC loading when measured over small contact areas where the effect of surface roughness was negligible. Conversely, the beneficial effects of the CNCs on macroscopic PSA tack and peel strength were outweighed by the effects of increased surface roughness with increasing CNC loading over larger surface areas. This highlights that CNCs can improve both cohesive and adhesive PSA properties, however, the effects are most pronounced when the CNCs interact favorably with the latex polymer and are uniformly dispersed throughout the adhesive film.</p><b>摘要</b><br/><p>乳液聚合乳胶基压敏胶（pressure sensitive adhesives, PSAs）因以水为合成介质而更具环境友好性，但其性能通常逊于溶液聚合制备的同类胶黏剂。研究表明，向各类丙烯酸酯压敏胶中添加纤维素纳米晶（cellulose nanocrystals, CNCs）可同时提升其初粘性、剥离强度与剪切强度。本研究采用原子力显微镜（atomic force microscopy, AFM），探究了纤维素纳米晶在两方面的作用：一是疏水型丙烯酸-2-乙基己酯/丙烯酸正丁酯/甲基丙烯酸甲酯（2‐ethyl hexyl acrylate/n‐butyl acrylate/methyl methacrylate, EHA/BA/MMA）乳胶膜的粒子聚结过程，二是作为多尺度黏附改性剂的应用效果。通过旋涂法制备了固含量与纤维素纳米晶添加量各不相同的EHA/BA/MMA乳胶薄膜。原子力显微镜观测结果显示，纤维素纳米晶降低了成膜过程中乳胶粒子聚结所需的固含量阈值。这提升了薄膜的内聚强度，具体体现为随着纤维素纳米晶添加量增加，EHA/BA/MMA压敏胶的剪切强度同步提升。胶体探针原子力显微镜测试表明，当在接触面积较小、表面粗糙度影响可忽略的条件下测量时，较厚的连续乳胶膜的纳米级黏附力随纤维素纳米晶添加量的增加而提升。与之相反，在更大的接触面积下，表面粗糙度随纤维素纳米晶添加量增加所带来的负面影响，抵消了纤维素纳米晶对宏观压敏胶初粘性与剥离强度的有益作用。该研究结果表明，纤维素纳米晶可同时改善压敏胶的内聚与黏附性能，但仅当纤维素纳米晶与乳胶聚合物相互作用良好且在胶黏剂薄膜中均匀分散时，其改性效果才能得到充分发挥。</p>