Viscoelastic and nanomechanical behaviors of polymeric materials

The mechanical and viscoelastic properties of polymers at different length scales (from nanometers to bulk) have attracted great attention. One method recently widely used to characterize the nanomechanical properties of materials is nanoindentation technique. For nanoindentation of polymers, in order to obtain precise results, the sample surface needs to be accurately determined. In this work, we are able to show that the reported abnormal extreme surface stiffening of soft rubber, PDMS, is associated with surface detection error, i.e., the instrument determined sample surface is far below the actual sample surface. Moreover, we propose a geometrical model for the Berkovich probe to quantitatively estimate the surface detection error from the reported depth dependent universal hardness behaviors for PDMS. In addition, the effect of surface detection error on the determination of mechanical properties for polymers is further investigated by varying polymer modulus (3 GPa to 2 MPa), probe geometry (Spherical and Berkovich), and surface detection methods. The results suggest that surface detection error can lead to apparent large stiffening, especially for soft materials. Furthermore, the high pressure beneath the probe is also found to be a contribution to the apparent stiffening when surface detection error is absent. Advanced rheometry has been widely used to characterize the viscoelastic properties of polymers in the macroscale. In this work, the strain-controlled mechanical spectral hole burning (Strain_MSHB) technique is successfully expanded to stress-controlled (Stress_MSHB) fashion. The dynamic heterogeneity of a polymer solution probed with both MSHB techniques are compared. In addition, the local temperature changes, which are thought to be the origin of dynamic heterogeneity observed in the dielectric spectral hole burning of small molecule glass formers, are not significant enough to explain the heterogeneity in MSHB of polymers. The van Gurp-Palmen (vGP) plot, which can be easily obtained from the dynamic rheological measurements, was originally proposed to verify the validity of Time-Temperature Superposition. It was found to be sensitive to polymer molecular weight, polydispersity, and structure. In the current work, a novel use of the vGP plot is proposed after the re-examination of the vGP plot by compiling literature data for polymer melts with different topological structures: linear (pure and blend), ring (pure and mixed with linear chain contaminants), comb, and bottlebrush. A new parameter: the reciprocal of complex modulus at the first minimum moving from the terminal regime (1⁄(G_(δ_(min,1))^* )) is defined and compared with the steady-state recoverable compliance J_s. Results show that 1⁄(G_(δ_(min,1))^* ) and J_s while related to each other, as they follow similar dependences on the molecular weight, weight fraction of high molecular weight content and backbone concentration, there are also differences. Furthermore, this new parameter shows exciting potential in quantifying the linear chain contaminations in the ring polymers. The structure-property relationship in polymer rheology is of special interest. In this work, this relationship is established by investigating the linear rheological properties of a series of second-generation dendronized wedge polymers. Due to the densely grafted bulky side group, up to degree of polymerization of 400, entanglement is absent. An extremely low glassy modulus of approximately 100 MPa is observed and is related to its high degree of freedom in the glassy state introduced by the side group, which is further confirmed by the absolute heat capacity measurements.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Chemical Engineering

Grantor

Texas Tech University

Year dc:date.issued

2018

Author dc:creator

• Qian, Zhiyuan

Chair dc:contributor.committeechair

• McKenna, Gregory B.

Committee members dc:contributor.committeemember

• Simon, Sindee L.
• Khare, Rajesh
• Christopher, Gordon

• Polymeric materials
• Nanoindentation
• Surface detection
• Linear rheology
• Dynamic heterogeneity
• Mechanical spectral hole burning
• Van Gurp-Palmen plot
• Wedge polymer

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/2346/74437

OAI identifier oai:identifier

oai:ttu-ir.tdl.org:2346/74437