Resistance to time-dependent deformation of polymer-based nanocomposites

The broad engineering applications of polymers and composites have become the state of the art due to their numerous advantages over metals and alloys, such as lightweight, easy processing and manufacturing, as well as acceptable mechanical properties. However, a general deficiency of thermoplastics is their relatively poor creep resistance, impairing service durability and safety, which is a significant barrier to further their potential applications. In recent years, polymer nanocomposites have been increasingly focused as a novel field in materials science. There are still many scientific questions concerning these materials leading to the optimal property combinations. The major task of the our work is to study the improved creep resistance of thermoplastics filled with various nanoparticles and multi-walled carbon nanotubes.

A systematic study of three different nanocomposite systems by means of experimental observation and modeling and prediction was carried out. In the first part, a nanoparticle/PA system was prepared to undergo creep tests under different stress levels (20, 30, 40 MPa) at various temperatures (23, 50, 80 °C). The aim was to understand the effect of different nanoparticles on creep performance. 1 vol. % of 300 nm and 21 nm TiO2 nanoparticles and nanoclay was considered. Surface modified 21 nm TiO2 particles were also investigated. Static tensile tests were conducted at those temperatures accordingly. It was found that creep resistance was significantly enhanced to different degrees by the nanoparticles, without sacrificing static tensile properties. Creep was characterized by isochronous stress-strain curves, creep rate, and creep compliance under different temperatures and stress levels. Orientational hardening, as well as thermally and stress activated processes were briefly introduced to further understanding of the creep mechanisms of these nanocomposites.

The second material system was PP filled with 1 vol. % 300 nm and 21 nm TiO2 nanoparticles, which was used to obtain more information about the effect of particle size on creep behavior based on another matrix material with much lower Tg. It was found especially that small nanoparticles could significantly improve creep resistance. Additionally, creep lifetime under high stress levels was noticeably extended by smaller nanoparticles. The improvement in creep resistance was attributed to a very dense network formed by the small particles that effectively restricted the mobility of polymer chains. Changes in the spherulite morphology and crystallinity in specimens before and after creep tests confirmed this explanation.

In the third material system, the objective was to explore the creep behavior of PP reinforced with multi-walled carbon nanotubes. Short and long aspect ratio nanotubes with 1 vol. % were used. It was found that nanotubes markedly improved the creep resistance of the matrix, with reduced creep deformation and rate. In addition, the creep lifetime of the composites was dramatically extended by 1,000 % at elevated temperatures. This enhancement contributed to efficient load transfer between carbon nanotubes and surrounding polymer chains.

Finally, a modeling analysis and prediction of long-term creep behaviors presented a comprehensive understanding of creep in the materials studied here. Both the Burgers model and Findley power law were applied to satisfactorily simulate the experimental data. The parameter analysis based on Burgers model provided an explanation of structure-to-property relationships. Due to their intrinsic difference, the power law was more capable of predicting long-term behaviors than Burgers model. The time-temperature-stress superposition principle was adopted to predict long-term creep performance based on the short-term experimental data, to make it possible to forecast the future performance of materials.