Characterization of nanoscale property variations in polymer composite systems: 1. Experimental results

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Abstract

A technique utilizing the indenting capabilities of the atomic force microscope is used to evaluate local changes in the response of polymer composite systems near the fiber–matrix interface. Room temperature and elevated temperature indentation response is measured for several model composite systems. Results of indentation studies are compared to finite element model predictions to understand the influence of interphase properties on the measured responses. For sized fiber systems, unexpected property variations are observed, leading to the discovery of a possible interphase formation mechanism in these systems.

Introduction

The use of polymers in composite materials and adhesives requires sensitive nanoscale property evaluation of polymer systems. In these systems, nanoscale properties that control various aspects of material performance can be different from bulk properties. For example, the behavior of polymer composites is highly dependent on the interfacial strength between the fiber and matrix. However, interfacial strength is controlled by the matrix material adjacent to the fiber, i.e. the fiber–matrix interphase region. In some epoxy matrix systems, fiber–matrix interphase regions form because of preferential adsorption of reacting species to the fiber [1], [2], [3]. Models have predicted that this process leads to compositional gradients near the fiber surface [4], [5], as shown in Fig. 1. Recent experimental results using neutron reflectivity have verified these modeling predictions for several epoxy–amine systems reinforced with unsized carbon fibers [1]. From these results, compositional gradients were observed to extend only a few nanometers away from the fiber for each of the systems studied. However, the local microstructure can be altered significantly, causing property differences between the interphase region and the bulk matrix [6], e.g. a lower interphase glass transition temperature, Tg.

Experimental evidence of the property differences between the interphase region and the bulk matrix has been obtained using several different methods [7], [8], [9], [10], [11], [12], [13], [14], [15], each of which infer interphase properties indirectly from the test data. The atomic force microscope (AFM) has the ability to measure differences in response directly. Traditionally, the AFM has been used as a nanoscale profilometer, measuring the topography of surfaces through direct contact between the sample surface and a probe tip mounted on the end of a cantilever microbeam. Development of the AFM's imaging capabilities has focused on the tip–surface interaction forces, leading to the utilization of the AFM as a surface force apparatus. This mode of operation, termed force mode, is generally used to minimize tip–sample forces during imaging. Information regarding the tip–sample forces is given by a force curve, which is a plot of tip deflection as a function of the motion of the piezoelectric actuator in the z-direction. Recent developments in software and hardware have been made that allow for nanoscale indentation studies to be performed using the AFM in force mode [6], [16], [17].

In this paper, the indentation response near fibers in three composite systems is measured using the AFM. These measurements are made at room temperature and at several elevated temperatures using a heating stage, the design of which is described in the following section. In the companion paper, a three-dimensional finite element model of indentation near a fiber is presented. This model is then used to predict interphase effects on indentation response, and the results are compared to experimental measurements.

Section snippets

Indentation measurements

Indentation of several polymer composite systems was performed with the AFM using a technique described in detail elsewhere [6], [16], [17]. In this study, AFM force curves were produced from the interactions between single-fiber composite samples and ultrastiff silicon cantilever probes, for which the estimated spring constants, kc, ranged from 200 to 500 N m−1. For each study, indentations were made across a sample surface moving toward the fiber edge with increments in lateral distance ranging

Unsized carbon fiber–epoxy composite system

Indentation responses were measured on unsized carbon fiber–epoxy samples at 20, 65, 80, 100, and 120°C. In each case, several rows of indents were made from the bulk epoxy matrix toward and onto the fiber at various angles of approach. In general, the indentation response throughout each row was similar to the response measured on the bulk epoxy (i.e. at large distances from the fiber), as shown in Fig. 3, Fig. 4, Fig. 5. In each figure, all responses have been normalized by the average bulk

Conclusions

The AFM indentation technique was used to investigate the local indentation responses of several polymer composite systems. In each case, the calculated relative stiffnesses were significantly higher for indents that were less than 200–300 nm from the fiber. This stiffening effect is caused by the restriction imposed by the fiber on the adjacent matrix material and might be related to a change in indentation mechanics due to the presence of the fiber. Although the fiber–matrix interphase for

Acknowledgments

The authors are grateful for the financial support of the U.S. Army Research Laboratory (ARL) under the Composite Materials Research Collaborative Program (CMR), ARL agreement number DAAL01-96-2-0048. Also, special thanks goes to Rod Don of the Center for Composite Materials for his help in the design and debugging of the heating stage.

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