Interlayer defects in a three-dimensionally (3D) printed structure are expected from the residual stress and commonly observed warpage, but their detection has been challenging and elusive. They are to be distinguished from intralayer defects (e.g., discernible voids), which can be quite easily detected. Interlayer defects in the form of local inadequate interlayer bonding in a 3D-printed polymer structure have been unprecedentedly shown in this work to be detectible by capacitance measurement. The interlayer defects are artificially obtained and judiciously positioned. The printing involves conventional stereolithography, using an acrylate resin. Two coplanar electrical contacts are positioned on the exterior surface of the structure. The capacitance decreases monotonically with increasing number of defects at a given layer, and with decreasing N , where N is the number of layers above the defect, thus providing an effective indicator of the presence and location defects. However, the capacitance can be the same for different numbers of defects if N is different. This issue may be circumvented in future work by having additional electrical contacts at various layers. The maximum value of N that allows capacitance-based defect detection exceeds 210. The fractional decrease in capacitance due to the defects (relative to the case of no defect) increases monotonically with increasing number of defects at a given layer, but it essentially does not vary with N . Thus, the fractional decrease in capacitance indicates the presence of defects, but is not an effective indicator of the defect location. The capacitance increases with N , due to the larger depth of penetration of the current when N is higher. The greater is the number of defects at a layer, the more is the reduction in the depth of current penetration by these defects. The technique of this work may be extended to the detection of the 3D positions of defects by capacitance tomography.

An accurate molecular identification of plastic waste is important in increasing the efficacy of automatic plastic sorting in recycling. However, identification of real-world plastic waste, according to their resin identification code, remains challenging due to the lack of techniques that can provide high molecular selectivity. In this study, a standoff photothermal spectroscopy technique, utilizing a microcantilever, was used for acquiring mid-infrared spectra of real-world plastic waste, including those with additives, surface contaminants, and mixed plastics. Analysis of the standoff spectral data, using Convolutional Neural Network (CNN), showed 100% accuracy in selectively identifying real-world plastic waste according to their respective resin identification codes. Standoff photothermal spectroscopy, together with CNN analysis, offers a promising approach for the selective characterization of waste plastics in Material Recovery Facilities (MRFs).

A three-dimensionally (3D) printed acrylate polymer without filler, printed by stereolithography, has previously been shown to be inherently piezoelectric in the out-of-plane and in-plane directions in the absence of electrical poling. This work has shown that the longitudinal piezoelectric coupling coefficient d is increased by poling at an electric field of 2400 V m ⁻¹ by up to about 200%. The poling takes 60 min for either direction. The effect of in-plane poling on the in-plane piezoelectric effect is greater than the effect of out-of-plane poling on the out-of-plane piezoelectric effect. This is due to the lower piezoelectric coefficient without poling for the in-plane direction, consistent with the fact that the molecular alignment is 2D in the plane of the printed layers and the in-plane poling is conducted in a particular in-plane direction. Without poling, d is smaller for the in-plane direction than the out-of-plane direction. With poling, d is essentially equal for the two directions. This suggests that the in-plane poling possibly causes the in-plane molecular alignment to develop a slight degree (if any) of 1D molecular alignment. The natural depoling that occurs immediately after the completion of the poling takes as little as 60 min, with the depoling being faster in the out-of-plane direction than the in-plane direction. Due to the depoling, the inherent piezoelectric behavior without poling remains practically attractive.