Tom A. P. Engels

Bio: Tom A. P. Engels is an academic researcher from Eindhoven University of Technology. The author has contributed to research in topics: Creep & Materials science. The author has an hindex of 13, co-authored 39 publications receiving 648 citations. Previous affiliations of Tom A. P. Engels include Delft University of Technology & DSM.

Rate- and Temperature-Dependent Strain Softening in Solid Polymers

Abstract: It is demonstrated that a large number of solid poly- mers (PMMA, PLLA, iPP, PS) display a pronounced change in kinetics (strain-rate and temperature dependence) after yield. The phenomenon finds its origin in the fact that, in specific ranges of temperature and strain rate, two different molecular processes may contribute to the yield stress. Because of strain softening, the post-yield response is only controlled by one of the two, resulting in a strain-rate dependence of the yield drop. The universality of the phenomenon is discussed in connection to the alleged influ- ence of secondary transitions on the impact response of polymer glasses. A modification of the finite-strain elasto-viscoplastic EGP-model is proposed to enable an accurate description of the mechanical response of solid polymers in the transition range. The versatility of the model is demonstrated on the temperature and strain-rate dependence of the intrinsic mechanical behav- ior of PMMA, iPP, PS, and PLLA. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1757-1771, 2012 INTRODUCTION Structural failure in static or dynamic (impact) loading conditions is a major concern in the applica- tion of polymers in load-bearing components. 1 Hence, under- standing of the fundamental causes leading to that process, and, ideally, identification of the role of details in the molec- ular architecture, is of critical importance. Likewise, the need for quantitative predictive modeling tools for failure phenom- ena is of extreme relevance and importance in the design and optimization of reliable load-bearing polymer components. The short-term failure of polymers is known to originate in usually rapid development of localized irreversible (plastic) strain, 2, 3 as manifested in moderate localization in shear bands and/or necks, 4, 5 or in extreme localizations in crazes that lead to cracks. 6, 7 A loss of structural integrity of the product results, and product failure can therefore be either ductile, involving the development of large localized plastic deformation zones accompanied by (more stable) tearing phenomena, or brittle, which gives fragmentation of the part. Changing the tempera- ture or loading rate can cause the failure mode to switch: the so-called brittle-to-ductile (B-D) or ductile-to-brittle (D-B) transitions. 8, 9

Thermoplastic elastomers based on strong and well-defined hydrogen-bonding interactions

Abstract: In order to investigate the effects of strong and well-defined hydrogen bonding on the properties of thermoplastic elastomers (TPEs), we have applied two distinct and strongly dimerizing 2-ureido-4-[1H]-pyrimidinone (UPy) quadruple hydrogen-bonding units as physical cross-linker. While the UPy groups consequently serve as the “hard phase” or “hard block” in these TPEs, an amorphous polyester has been used as the “soft phase” or “soft block”. The UPy unit has been flanked with either a sterically demanding isophorone spacer or a linear hexamethylene, where these spacers have been derived from isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI), respectively. This difference on a molecular level leads to a homogeneous amorphous material in the IPDI case (polymer 1) and to a nanophase-separated material in the HDI case (polymer 2) as revealed by AFM and DSC experiments. Apart from this distinctive difference in morphology and nanoscopic organization, the macroscopic properties of both material...

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo-Responsive Actuators

Abstract: Arbitrary shape (re)programming is appealing for fabricating untethered shape-morphing photo-actuators with intricate configurations and features. We present re-programmable light-responsive thermoplastic actuators with arbitrary initial shapes through spray-coating of polyethylene terephthalate (PET) with an azobenzene-doped light-responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami-like folds and left- and right-handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area-specific actuation such as winding, unwinding and unfolding. By shape re-programming, the same sample can be re-designed and light-actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo-responsive actuators with highly tuneable geometries and actuation modes.

An untethered magnetic- and light-responsive rotary gripper: shedding light on photoresponsive liquid crystal actuators

Abstract: Here, a remotely controlled dual magneto- and photoresponsive soft robotic gripper is reported, capable of loading, transport, rotation, and release of cargo. The untethered soft actuator consists of a magnetically responsive polydimethylsiloxane layer containing magnetic iron powder coated onto the central region of a light-responsive liquid crystal polymer film hosting photochromic azobenzene dyes. Light is used to trigger the actuator to autonomously grab and pick up cargo with a high degree of control. Magnetic response is employed to conduct the locomotion as magnetic guidance, allowing the gripper to have both translational freedom and rotational freedom in its locomotion, differentiating the device from other soft robotic grippers. Control can be attained even in enclosed and/or confined spaces, through solely remote actuation. Through combined video, mechanical, and thermal analyses, the actuation mechanism of the light-responsive liquid crystal network is investigated, shedding light on the decisive role of the temperature evolution in governing both rate of motion and deformation amplitude of the light-responsive soft actuator.

Does the Strain Hardening Modulus of Glassy Polymers Scale with the Flow Stress

Abstract: Using a generic coarse-grained bead-spring model, Hoy and Robbins reproduced important experimental observations on strain hardening, specifically the generally observed Gaussian strain hardening response and its dependence on net- work density and temperature. Moreover, their simulation results showed that the strain hardening response at different strain rates collapses to a single curve when scaled to the value of the flow stress, a phenomenon that has not yet been verified experimentally. In the present study, the proposed scaling law is experimentally investigated on a variety of polymer glasses: poly(methyl methacrylate), poly(pheny- lene ether), polycarbonate, polystyrene, and poly(ethylene terephthalate)-glycol. For these polymers, true stress-strain curves in uniaxial compression were collected over a range of strain rates and temperatures and scaled to the flow stress. It was found that, generally, the curves do not collapse on a mastercurve. In all cases, the strain hardening modulus is observed to increase linearly, but not proportionally to the flow stress. The experimental data, therefore, unambiguously demonstrate that the pro- posed scaling law does not apply within the range of temperature and strain rate covered in this study. V C 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2475-2481, 2008

Tough Stimuli-Responsive Supramolecular Hydrogels with Hydrogen-Bonding Network Junctions

Abstract: Hydrogels were prepared with physical cross-links comprising 2-ureido-4[1H]-pyrimidinone (UPy) hydrogen-bonding units within the backbone of segmented amphiphilic macromolecules having hydrophilic poly(ethylene glycol) (PEG). The bulk materials adopt nanoscopic physical cross-links composed of UPy–UPy dimers embedded in segregated hydrophobic domains dispersed within the PEG matrix as comfirmed by cryo-electron microscopy. The amphiphilic network was swollen with high weight fractions of water (wH2O ≈ 0.8) owing to the high PEG weight fraction within the pristine polymers (wPEG ≈ 0.9). Two different PEG chain lengths were investigated and illustrate the corresponding consequences of cross-link density on mechanical properties. The resulting hydrogels exhibited high strength and resilience upon deformation, consistent with a microphase separated network, in which the UPy–UPy interactions were adequately shielded within hydrophobic nanoscale pockets that maintain the network despite extensive water content....

Physical chemistry of supramolecular polymer networks

Abstract: Supramolecular polymer networks are three-dimensional structures of crosslinked macromolecules connected by transient, non-covalent bonds; they are a fascinating class of soft materials, exhibiting properties such as stimuli-responsiveness, self-healing, and shape-memory. This critical review summarizes the current state of the art in the physical–chemical characterization of supramolecular networks and relates this knowledge to that about classical, covalently jointed and crosslinked networks. We present a separate focus on the formation, the structure, the dynamics, and the mechanics of both permanent chemical and transient supramolecular networks. Particular emphasis is placed on features such as the formation and the effect of network inhomogeneities, the manifestation of the crosslink relaxation dynamics in the macroscopic sample behavior, and the applicability of concepts developed for classical polymer melts, solutions, and networks such as the reptation model and the principle of time–temperature superposition (263 references).

Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging.

Abstract: The effects of confinement on glass transition temperature (Tg) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The Tgs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, Tg values can be enhanced or depressed relative to neat, bulk Tg (Tg,bulk) or invariant with nanofiller content. For alumina nanocomposites, Tg increases relative to Tg,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit Tg enhancements as large as 5 K or Tg reductions as large as 17 K relative to Tg,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006

From supramolecular polymers to multi-component biomaterials.

Abstract: The most striking and general property of the biological fibrous architectures in the extracellular matrix (ECM) is the strong and directional interaction between biologically active protein subunits. These fibers display rich dynamic behavior without losing their architectural integrity. The complexity of the ECM taking care of many essential properties has inspired synthetic chemists to mimic these properties in artificial one-dimensional fibrous structures with the aim to arrive at multi-component biomaterials. Due to the dynamic character required for interaction with natural tissue, supramolecular biomaterials are promising candidates for regenerative medicine. Depending on the application area, and thereby the design criteria of these multi-component fibrous biomaterials, they are used as elastomeric materials or hydrogel systems. Elastomeric materials are designed to have load bearing properties whereas hydrogels are proposed to support in vitro cell culture. Although the chemical structures and systems designed and studied today are rather simple compared to the complexity of the ECM, the first examples of these functional supramolecular biomaterials reaching the clinic have been reported. The basic concept of many of these supramolecular biomaterials is based on their ability to adapt to cell behavior as a result of dynamic non-covalent interactions. In this review, we show the translation of one-dimensional supramolecular polymers into multi-component functional biomaterials for regenerative medicine applications.