The Viscoelastic Behavior Of Semi-Crystalline Polymers At Micro-Scale

The viscoelastic behaviour of polymers is very important for engineering design and application. With the extensive use of polymers in the micro-nano structure, developing the measurement method with high resolution to characterize their viscoelastic behavior at micro-nano scales has an important realistic significance. In this paper, the viscoelastic behavior of semi-crystalline polymer-Polyamide 12(PA12) was investigated by micro-/nano-indentation. Based on the test results from loading, holding and unloding section in indentation process, the viscoelastic behavior of PA12 was characterized from the mesoscopic view, meanwhile the measurement results of micro-indentation creep and macro-creep were contrastively analysed. The specific research contents and conclusions are as follows:1. The effects of loading rates and strain rates on the responses of the load versus indentation depth in indentation process of PA12 were investigated using load controlled method and strain-rate controlled method(combined with Continuous Stiffness Measurement technique, CSM). Results show thatthe exponent m during loading is a material parameter related to hardness, m<2indicating strain softening while m>2 indicating strain hardening. For load controlled method, the strain rate is independent of loading rate but inversely proportional to m. Due to the coexisting deformation mechanism of crystalline dislocation and thermal activation in amorphous domains, PA12 shows the softening for load controlled method but hardening for strain-rate controlled method with CSM(the target strain rate is less than 0.2 s-1). The calculation method of unloading stiffness at the initial unloading stage based on elasticity contact theory invalidates, and the backward analysis results of apparent unloading stiffness through contact hardness show that the unloading stiffness and elastic modulus decrease with the increasing unloading rate.2. In order to eliminate the overestimate of elastic modulus caused by the“nose” at the initial unloading stage, a simplified calculation method is proposed on the basis of elasticity contact theory. Results from the definition of creep correction factor show the elastic modulus no longer depend on unloading rate and holding time, meanwhile the correction procedure is significantly simplified. The creep correction factor decreases and approaches1 with the increasing unloading rate and holding time. The corrected elastic modulus is a stable value.3. The viscoelastic response of indentation creep was investigated using a phenomenological rheology model for the creep behavior under three different preloading modes(step loading, ramp loading, strain-rate loading).Results show that the creep compliances and retardation spectrums are basically unchanged under different peak load, and PA12 manifest as linear viscoelasticity for the step preloading. For ramp loading, with the decreasing pre-loading rate, the creep compliance increase due to the structure relaxation in pre-loading stage, the peak intensity of retardation spectrum reduce and the peaks representing small molecular motion disappear. For strain-rate loading,the peak intensity of retardation spectrum gardually decrease and disappear eventually with the decrease of strain-rate.4. In order to characterize the viscoelastic creep response of viscoelastic material under different holding loads, a phenomenological viscoelasticity model used to describe the creep deformation was built based on quadratic element mechanical model. Results indicate that the model can effectively predict the whole process of indentation creep response with geometrically self-similar sharp indenter. The creep displacement and displacement rate are proportional to the square root of holding load, but creep strain rate is independent of the holding load.5. For the multi-scale correlation of macro-/micro- creep test results, the conventional compression creep tests and indentation creep tests with cylindrical flat punch were carried out and the viscoelasticity parameters obtained from micro-indentation tests were contrastively analyzed with that of macro-creep. Results show that PA12 exhibit nonlinear viscoelasticity for the macro-creep tests, the creep compliance and peak intensity of retardationspectrum for micro-indentation tests are lower than that for macro-creep tests.The peaks denoting activated motion of large molecules disappear at the retardation time spectral line for micro-indentation tests, while the peaks representing activated motion of small molecules are disappeared for the macro-creep tests.