Computational fluid dynamics (CFD) simulation is an important tool as it enables engineers to study different design options without a time-consuming experimental workload. However, the prediction accuracy of any CFD simulation depends upon the set boundary conditions and upon the applied rheological constitutive equation. In the present study the viscoelastic nature of an unfilled gum acrylonitrile butadiene rubber (NBR) is considered by applying the integral and time-dependent Kaye–Bernstein–Kearsley–Zapas (K-BKZ) rheological model. First, exhaustive testing is carried out in the linear viscoelastic (LVE) and non-LVE deformation range including small amplitude oscillatory shear (SAOS) as well as high pressure capillary rheometer (HPCR) tests. Next, three abrupt capillary dies and one tapered orifice die are modeled in Ansys POLYFLOW. The pressure prediction accuracy of the K-BKZ/Wagner model was found to be excellent and insensitive to the applied normal force in SAOS testing as well as to the relation of first and second normal stress differences, provided that damping parameters are fitted to steady-state rheological data. Moreover, the crucial importance of viscoelastic modeling is proven for rubber materials, as two generalized Newtonian fluid (GNF) flow models severely underestimate measured pressure data, especially in contraction flow-dominated geometries.

The rubber manufacturing processes and the parameters set, e.g., crosslinking time and temperature, have a major influence on the crosslinking reaction as well as on the state of cure and thus significantly affect the physical properties and the quality of the end product. In this work, the quality of rubber parts produced by compression molding (CM) with different processing parameters was studied. Besides, specimens with preexisting reversion effect were prepared to analyze the influence of differently-formed sulfur networks, due to increasing vulcanization time, on the component’s morphology and, subsequently, on its final properties. All CM parts were quality controlled by compression set (CS) and dynamic mechanical analysis (DMA) in tension mode over a wide temperature range applying a MCR 702e MultiDrive (Anton Paar GmbH, Austria), providing information on changes in thermal transitions in the material, e.g., the glass transition temperature T_G. Such change in T_G can be directly correlated with the network structure of the rubber. The investigations yielded to four main observations. (i) The reproducibility of the DMA is very high, e.g., ΔT_G=±0.24 K, and thus it is an excellent method to detect changes in polymer morphology. (ii) The crosslinking state of the samples, i.e., whether low, high, or over-crosslinked, is detectable both at T_G and in the application temperature range. However, the influence of the crosslinking reaction or the state of cure is more pronounced in the application temperature range, where a significant difference between the extensional loss factors exists. (iii) A comparison of the measured T_G as well as a correlation between CS and DMA indicated that over-crosslinked samples or an already accompanying reversion of the material can be detected excellently. Finally, (iv) the DMA in tension mode allows a distinction between different manufacturing and crosslinking methods.

Dynamic covalent polymer networks exhibit a cross-linked structure like conventional thermosets and elastomers, although their topology can be reorganized by thermoactivated bond exchange. This characteristic enables a unique combination of repairability, recyclability and dimensional stability, crucial for a sustainable industrial economy. We herein report the application of a photoswitchable nitrogen superbase for the spatially resolved and reversible control over dynamic bond exchange within a thiol-ene photopolymer. By the exposure to UV or visible light, we successfully gain control over the associative exchange between thioester links and thiol groups, and thereby the macroscopic mechanical material properties, in a locally controlled manner. Consequently, the resulting reorganization of the global network topology enables us to utilize our material for previously unrealizable advanced applications such as spatially resolved, reversible reshaping as well as micro-imprinting over multiple steps. Finally, the presented concept contributes fundamentally to the evolution of dynamic polymers and provides universal applicability in covalent adaptable networks relying on a base-catalyzed exchange mechanism.