Study of the temperature-dependent stress-cracking behavior of amorphous polymer-based nanocomposites

• Applicant: Professor Dr.-Ing. Alois K. Schlarb
• Subject Area: Plastics Engineering ; Polymer Materials
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 270466437

• Final Report Year: 2019

Final Report Abstract

The objectives of this project are focused on the systematic characterization of the ESC resistance complemented by standard mechanical, thermal, optical, and morphological properties investigation. The influences of intrinsic properties of polymer (molecular weight and structure) and nanofiller (size and content), as well as the external factors such as environmental conditions (temperature and stress cracking agent), were thoroughly investigated. Amorphous based polymers, i.e., PMMA, PS and two different molecular weights of PC were used as polymer matrices. As nanofiller, two different sizes of hydrophobic fumed silica (nano-SiO2) were used. Incorporation of nano-SiO2 into polymers causes a reduction in optical properties, but effective in improving the tensile, notched impact and fracture toughness properties of the nanocomposites. The distinctive enhancement of the mechanical properties is attributed to the adequate level of dispersion and the interfacial interaction of the SiO2 nanoparticles in the polymer matrix. This is particularly in the case with the smaller sized nano-SiO2, which provides an outstanding enhancement in both mechanical properties and transparency compared to the composites filled with a lager particle. The order of the properties improvement by adding nanofiller is more effective in PC-H>PC-L>PS>PMMA, respectively. Although, the nano-SiO2 content and size do not significantly affect the glass transition temperature of the polymer matrix. The best compromise of stiffness, strength, toughness and transparency is achieved at a filler content of 1 vol.%. The investigation of ESC behavior of polymer-based nanocomposites was performed using two different ESC-test levels, namely, fracture mechanic tests and a bent strip test, with optical crack detection. The online detection and quantification of material changes (e.g. micro-cracks, crazes, plastic deformations or cracks) leads to better characterization, interpretation and ultimately understanding the ESC phenomenon of polymer-based nanocomposites. Interestingly, the nano-SiO2 enhances the ESC resistance of polymer in different fluids and extend the endurance of the product, particularly in the case with the smaller sized nano-SiO2. This result confirms that the enormous nanofiller-polymer interface was a dominant influence of the structural arrangements of the macromolecules, and thus greatly increased ESC resistance of the polymer in all investigated fluids. However, the improvement degree of ESC resistance is a function of intrinsic polymer properties such as chemical structure and molecular weight. Therefore, the effective ESC resistance of different materials is in the following order: PC-H/SiO2>PC- L/SiO2>PS/SiO2> PMMA/SiO2, which is also related to their mechanical performance. The correlation of the Mode I critical stress intensity factor with the Hansen solubility parameter and molar volume of agent shows a very good agreement for different fluids, thus allowing the determination of the stress cracking behavior of nanocomposites using only simple equations. An accelerated testing method has been developed for predicting slow crack growth behavior by applying a modified superposition approach. The good reproducibility and agreement of the construction of master curves suggested that the superposition approach could be used as a comparative method for predicting the crack growth behavior, especially for long-term ESC testing, as in mild agents. Both approaches can be useful tools for a quick prediction of ESC behavior, which allows minimizing the test time and the expense of stress-cracking tests for various applications. Additionally, the results of stress cracking tendency from two different test method is a similar and good agreement for different environmental conditions.

Publications

• Improving environmental stress cracking resistance of glassy polymers by the incorporation of nanofillers. Young Researchers Symposium 2016, Kaiserslautern, Germany, 14-15 April 2016
  Nomai, J.; Schlarb, A.K.
  
• Spannungsrissbeständigkeit effizient beurteilen. Jahresmagazin Ingenieurwissenschaften/Kunststofftechnik 10 (2016), pp. 110-113
  Nomai, J.; Schlarb, A.K.
  
• Stress cracking of medical devices: securing care for patients due to the incorporation of nano-scaled fillers into glassy polymers. Medical plastics conference 2016, Bangkok, Thailand, 25-26 April 2016
  Schlarb, A.K.
  
• Enhancement of environmental stress cracking resistance of luer-connectors. Beutel, Schläuche, Folien in der Medizintechnik, Heidelberg, Germany, 26-27 April 2017
  Nomai, J.; Schlarb, A.K.
  
• Environmental stress cracking (ESC) resistance of polycarbonate/SiO2 nanocomposites in different media. Journal of Applied Polymer Science 134 (2017)
  Nomai, J.; Schlarb, A.K.
  
• Efficient analysis and determination of environmental stress cracking of polymers in different environment. Polymer Testing & Analysis, Pittsburgh, USA, 11-12 September 2018
  Nomai, J.; Schlarb, A.K.
  
• Effects of nanoparticle size and concentration on optical, toughness, and thermal properties of polycarbonate. Journal of Applied Polymer Science (2019)
  Nomai, J.; Schlarb, A.K.