Researchers tuned a photocrosslinked polymer network to test whether a single flexible film can combine softness, degradability, repeated stretching, and light-controlled heating.
Paper: Graphene-integrated flexible films with a three-dimensional elastic network and photothermal responsiveness. AI-generated abstract conceptual image created using ChatGPT/OpenAI
A recent study in the Polymer Journal reports a highly stretchable, polyester-based composite film with a hydrolyzable polymer backbone. The film elongated by 163.00% before breaking and, under near-infrared light, its surface temperature rose from 24.2 °C to 50.6 °C in 60 seconds.
Researchers made the material by adding graphene to a three-dimensional (3D) elastic polymer network formed by UV photocuring. The combination gives the film high stretchability and rapid light-induced heating, properties that could suit flexible and wearable devices.
Why flexible, degradable materials are needed
Demand for soft, flexible polymeric materials has grown with technologies such as wearable electronics and deformable sensors. These technologies need substrates that can bend and stretch repeatedly without fracturing.
Many conventional elastomers, including common silicone and polyurethane materials, are mechanically compliant but non-degradable and can remain in the environment after disposal. Biodegradable polymers such as poly(L-lactic acid) (PLLA) and poly(ε-caprolactone) (PCL) offer another route, but their mechanical behavior can limit their use in soft devices.
Some biodegradable polymers are relatively stiff or brittle, while PCL is flexible but usually has weak elastic recovery. Materials for soft devices still need to combine softness, elasticity, structural integrity, and a response to external stimuli.
Making the hydrolytically degradable elastomer
Researchers synthesized a triblock copolymer, poly(l-lactide-co-ε-caprolactone)-b-poly(ethylene glycol)-b-poly(l-lactide-co-ε-caprolactone) diacrylate (PLCL-PEG-PLCL DA). The structure pairs biodegradable PLCL segments with flexible PEG. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) was used in thiol-ene photocuring to form a crosslinked polymer network.
The researchers dissolved the polymer precursors in chloroform with the photoinitiator Irgacure 2959. They varied the molar ratio of thiol groups to carbon-carbon double bonds to change the network structure. They injected the precursor solutions into custom glass molds, exposed them to 365 nm ultraviolet light for 30 minutes, and vacuum-dried the films at 40 °C.
To add a photothermal response, the researchers mixed graphene nanoplatelets 6-8 nm thick into the polymer matrix before photocuring. Graphene content was fixed at 2 wt%.
Proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared (FTIR) spectroscopy were used to examine the polymer precursor, and FTIR also confirmed network formation after UV curing. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to examine morphology and crystalline ordering, while differential scanning calorimetry (DSC) measured thermal properties.
Mechanical performance, degradation, and photothermal response
Tests showed that UV photocrosslinking suppressed crystallization and produced a fully amorphous elastic network. Adding PETMP raised elongation at break from 65.55 ± 6.23% in the no-PETMP formulation, DA-0T, to 109.13 ± 7.18% in the highest-PETMP formulation, DA-1T. DA-0T was still UV-cured. Adding 2 wt% graphene raised elongation at break to 163.00 ± 8.11%. In this material, graphene did not behave as a rigid filler or make the film brittle.
In cyclic tensile testing, the graphene composite retained its elasticity through 20 loading and unloading cycles at 70% strain, with an elastic loss of about 3.70%. SEM images suggested that graphene sheets could slide and rearrange at interfaces inside the polymer during stretching. The authors propose that this process dissipated mechanical energy and delayed crack growth, but the study did not directly observe the sliding.
The films were also tested under accelerated alkaline hydrolysis in a pH 9.0 buffer at 37 °C. They first absorbed water and swelled, then fragmented as ester bonds in the polyester backbone were cleaved. More PETMP changed the structural failure pattern.
The more-connected DA-1T network underwent relatively abrupt bulk disintegration after hydrolysis progressed, while graphene delayed visible fragmentation and apparent mass loss. The test does not establish how long the material would last in the body, soil, compost, or normal disposal conditions.
Under 700-1000 nm irradiation, the graphene-free DA-1T film showed little temperature increase. The graphene-containing DA-1T/G film rose from 24.2 °C to 35.3 °C in 10 seconds and reached 50.6 °C after 60 seconds. The comparison ties the heating response mainly to graphene.
Prospects in wearable technologies
The film combines stretchability, hydrolytic degradability, and a fast photothermal response, which could help it conform to moving or irregular surfaces. The paper discusses wearable devices and soft interfaces, but the researchers did not build or test a wearable sensor or biointerface.
The graphene-containing films can generate localized heat under near-infrared light, enabling light-controlled heating. The authors mention wearable thermal interfaces, soft photothermal patches, flexible heating elements, and light-triggered actuation. The study did not test thermotherapy, biological safety, soft robotics, or actuator operation. Those medical, biological, robotic, and device-level questions remain open.
Next steps for composite films
The study produced a graphene-integrated elastomer with high stretchability, cyclic elastic recovery, hydrolytic degradation under the accelerated test conditions, and rapid photothermal heating. The measurements establish both high deformability and a near-infrared heating response in the same formulation.
The crosslinked polymer architecture altered the material's mechanical and degradation behavior while preserving its photothermal response. No other nanofillers were tested, so applying this fabrication route to other responsive components requires further study.
Future work could test the film in flexible-device prototypes, measure performance during simultaneous stretching and near-infrared heating, and examine degradation under conditions closer to its intended use.
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