Materials Science Centre and the Northwest Composites Centre, School of Materials, University of Manchester, Grosvenor Street, Manchester M1 7HS, United Kingdom.
Biomacromolecules. 2010 Mar 8;11(3):762-8. doi: 10.1021/bm1001203.
Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stress-transfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of mum. Polarized Raman spectroscopy of the 1095 cm(-1) Raman band, associated with C-O ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers.
通过拉曼光谱技术深入了解了决定被囊动物纤维素晶须/聚醋酸乙烯酯纳米复合材料力学性能的应力传递机制。应力传递的程度受晶须局部取向（或各向异性）的影响，而这又受到用于制备纳米复合材料的加工条件的控制。溶液浇铸材料没有微观各向异性，而经过浇铸和随后压缩成型的样品则包含各向同性区域和局部取向晶须的区域。偏光显微镜显示这些区域的尺寸在数百微米。用 1095 cm(-1)拉曼带的偏振拉曼光谱（与纤维素主链的 C-O 环伸缩有关）来量化纤维素晶须的局部取向。纳米复合材料薄膜单轴变形时，该拉曼带明显可辨的偏移进一步用于确定纤维素晶须所经历的应力水平，这最终反映了主要在聚醋酸乙烯酯基质和被囊动物晶须之间以及网络内晶须之间的应力传递水平。在各向同性区域，晶须形成了一个渗透网络，与应变相关的观察到的拉曼位移速率比晶须单轴取向的区域小。拉曼位移受水的存在强烈影响，当样品完全水合时，会导致缺乏应力传递，这一点可以通过拉曼技术清楚地检测到。将纳米复合材料加热到聚醋酸乙烯酯基质的玻璃化转变温度以上也会降低单个晶须所经历的应力。
