Thanks to a grant from the National Science Foundation (Divisions of Materials Research, and Civil, Mechanical, and Manufacturing Innovation Award No. 11000806), researchers from Michigan Technological University and Oregon State University, US, have teamed up to help fill in some of the missing details.

In the study, the group used the nanoindentation module in an atomic force microscope to analyse the nanomechanical characteristics of cellulose nanocrystal (CNC)-based composites. The scientists then compared these properties with the results of macroscale testing.

Nanocomposite selection

Poly(vinyl alcohol) (PVOH) is a multipurpose polymer with potential applications as artificial human tissue and controlled drug-release hydrogels. Because of its hydrophilicity, PVOH is not stable in water unless it is cross-linked. In this study, cross-linking of PVOH was accomplished by poly(acrylic acid) (PAA).

Key results

The study showed that if the dispersion of CNCs in the polymer matrix is homogenous then the nanoscale elastic properties follow the same trend as macroscale data. When CNCs were distributed inhomogeneously, the team found a discrepancy between macroscale tensile and nanoscale tests. Interestingly, in comparison to the macroscale elastic properties, the measured nanoscale elastic moduli were higher.

In addition, the work highlights an interesting link to the chemistry of polymer matrix. In particular, the effective reinforcement of CNCs depends on the presence of PAA. This means that when PAA was added to PVOH, the addition of CNCs up to 20 wt.% improved the elastic modulus of PVOH. But when no PAA was added, increasing the CNC content above 10 wt.% resulted in the agglomeration of CNCs and a degradation in the mechanical properties of PVOH.

The collaboration has made a good start in better understanding the link between chemistry, nanoscale and macroscale properties. Next, the researchers plan to conduct real-time nanoscale characterization of CNCs and related nanocomposites to generate a roadmap for this paradigm.

More details can be found in the journal Nanotechnology.