TPU-assisted adhesive PDMS film for dry or underwater environments

The study addresses the challenge of creating reusable adhesive films that function effectively in both dry and underwater environments, a need in applications such as epidermal adhesives, underwater sensors, and leak repair. Conventional multilayer tape adhesives can suffer from poor mechanical properties and interfacial adhesion. PDMS is attractive for stretchable, waterproof uses due to its transparency, hydrophobicity, and flexibility, but fully cross-linked PDMS has poor surface adhesion while lightly cross-linked PDMS has reduced mechanical strength. The research proposes an approach to engineer an adhesive PDMS film (APF) with anisotropic cross-linking—lightly cross-linked on one side and highly cross-linked on the other—by curing PDMS on TPU to achieve both good mechanics and strong, reusable adhesion in dry and wet conditions.

Background examples of bioinspired adhesion include gecko setae, mussel adhesion, and octopus suction, which motivate reusable adhesives. Tape adhesives typically offer better reusability than glue adhesives but often have mechanical and interfacial limitations. Prior methods to enhance PDMS adhesion involve manipulating cross-link density via curing agent content, thermal conditions, additives, or inhibitors that interrupt hydrosilylation when mixed with oligomers, surfactants, or amine-based materials. However, these approaches trade off stiffness and adhesion, motivating a new strategy that yields strong adhesion without compromising mechanical robustness.

Materials: Sylgard 184 PDMS (base and curing agent), TPU film (polymeric glycol and polyisocyanate), and a Pt catalyst solution (Pt(0)-1,3-diethyl-1,1,3,3-tetramethyldisiloxane complex). Preparation of APF: TPU substrate cleaned with IPA, DI water, dried. PDMS precursor mixed at 10:1 base:curing agent, degassed 30 min, poured on TPU, and cured sequentially 6 h at 40 °C, 4 h at 60 °C, 2 h at 80 °C. Samples designated 40APF, 60APF, 80APF. Acrylic plate molds controlled thickness and shape. Preparation of NPF: Same process but PDMS cured on acrylic substrate (40NPF, 60NPF, 80NPF). Mechanical testing: Dogbone-like strips, thickness 4.5 mm, width 10 mm, gauge length 40 mm, tensile speed 100 mm/min; Young’s modulus from slope at 20% strain; ≥3 replicates. Adhesion tests: Tack tests with 2 cm × 2 cm, 5 mm thick samples on a custom probe of same area; nonadhesive side plasma-treated and glued to probe. Test speeds 20–40 mm/min, preloads 5–20 N. Substrates: Al, glass, Si, PLA, PET, TPU; roughness effects evaluated on 3D-printed ABS surfaces with varied patterns. Underwater adhesion measured on Al, Si, PLA. Cyclic tests: 20 cycles, 5 N preload at 20 mm/min, in dry and underwater conditions. Preloading tests: 1–50 N at 20 mm/min. Creep tests: 200 g hanging weight after 20 N, 5 s preload, in dry and underwater; ≥3 replicates. Material characterization: Raman spectroscopy (532 nm, 10 mW, 0.02 s, 1000 scans) and Raman mapping (10 mW, 0.083 s, 2 µm pixel, 5 scans, 100 µm × 500 µm). ICP-MS quantified Pt content in TPU before/after PDMS casting (200× dilution). Contact angle measurements on PET, Si, glass, Al, 3D-printed PLA, TPU, and PDMS surface (droplet 7.32 µl). Surface roughness via profilometry; SEM used to observe bonding in double-PDMS samples. Double-PDMS experiments: Additional PDMS precursor cast on previously cured APF or NPF to probe interfacial curing/bonding; additional Pt catalyst dosing experiments assessed adhesion dependence on Pt content.

• Mechanical properties: Increasing curing temperature increased PDMS modulus. For comparable curing conditions, 60APF and 80APF had approximately 16% lower Young’s modulus than their NPF counterparts, attributable to a soft adhesive gel layer on one side. - Adhesion performance (dry): APF showed higher adhesion than NPF across all tested substrates. 60APF achieved a maximum adhesion strength of 47 kPa, about 13.6× greater than 60NPF. 80APF also outperformed 80NPF (38 kPa for 80APF). 40APF had relatively low adhesion due to low bulk modulus and weaker properties. On various substrates, APF adhered strongest to PET at 34.8 kPa, followed by Si wafer, glass, Al, PLA, and TPU. On rough 3D-printed PLA, APF’s adhesion was approximately 7.71× higher than NPF; smaller surface feature sizes (from 400 µm to 200 µm) increased adhesion due to better conformal contact. - Mechanism: Raman spectra indicated incomplete consumption of vinyl C=C groups at the APF surface (stronger ~1600 cm⁻¹ band), signifying a lightly cross-linked layer rich in free and dangling chains. Raman mapping showed this adhesive layer extends to roughly 300 µm from the surface. TPU adsorbed Pt catalyst atoms from PDMS (N–H groups in TPU served as binding sites; 1539 cm⁻¹ band), confirmed by ICP-MS detection of Pt in TPU after PDMS casting, while pristine TPU had Pt below detection. Double-PDMS tests showed partial bonding when new PDMS was added to APF (Pt-depleted interface), but complete bonding when added to NPF. Adding Pt catalyst to the condensed gel enabled full curing; adding extra Pt to PDMS precursors reduced adhesion, linking catalyst content and cross-linking to adhesive performance. - Application context: APF adhered strongly in both dry and underwater environments and maintained adhesion after repeated use; effective as a waterproof patch adhering to submerged surfaces (as demonstrated in applications).

Curing PDMS on TPU induces anisotropic cross-linking through catalyst (Pt) depletion at the interface: TPU adsorbs Pt, locally inhibiting hydrosilylation and producing a lightly cross-linked, compliant surface layer rich in free and dangling chains. This adhesive layer enhances conformability and interfacial interactions, increasing real contact area and adhesion, especially on rough substrates, while the opposite, more fully cross-linked side preserves mechanical robustness. The mechanism is supported by Raman spectroscopy/mapping, ICP-MS, and interfacial bonding experiments. Consequently, APF achieves substantially higher dry adhesion than conventional PDMS films and demonstrates utility in underwater scenarios and repeated use. The simple process (casting on TPU) enables tunable adhesion via curing conditions and can be scaled for practical applications such as waterproof patches, wearable sensors, and underwater grippers.

The work introduces a simple, scalable method to fabricate adhesive PDMS films by curing PDMS on TPU, creating an anisotropic structure with a lightly cross-linked adhesive surface layer and a robust backing. Mechanistic studies reveal TPU-induced Pt catalyst adsorption, yielding a ~300 µm-thick adhesive layer responsible for strong adhesion. APF exhibits significantly enhanced adhesion compared to normal PDMS across diverse substrates, superior performance on rough surfaces, and effective use in dry and underwater environments with reusability. Future work could optimize curing profiles and TPU chemistry to tailor adhesive layer thickness and properties, systematically map underwater performance across salinity/temperature conditions, and integrate APF into device-scale waterproofing and sensing systems.