Effect of Water on Crystallization and Melting of Telechelic Oligo(ε‐caprolactone)s in Ultrathin Films

Abstract

The thermal behavior of ultrathin, semi‐crystalline films of oligo(ε‐caprolactone)s (OCLs) with hydroxy or methacrylate end groups, is studied by the Langmuir technique in dependence on mean molecular areas and crystallization temperatures. The films on solid substrate as obtained by Langmuir–Schaefer transfer exhibit different lamellar thicknesses, crystal number densities, and lateral sizes. The melting temperature of OCL single crystals at the water and solid surface is proportional to the inverse crystal thickness and generally lower than in bulk PCL. An influence of OCL end groups on the melting behavior is observed mainly at the air–solid interface, where methacrylate end capped OCL melts at lower temperatures than hydroxy end capped OCL. Comparing the underlying substrate, melting/recrystallization of OCL ultrathin films is achievable at lower temperatures at the air–water interface than at the air–solid interface, where recrystallization is not identifiable. Recrystallization at the air–water interface generally occurs at higher temperature than the initial crystallization temperature. The surface pressure, as an additional thermodynamic variable, seems to further affect the crystallization behavior, with crystal thickness and lateral growth rate increasing with surface pressure. The results presented here are important when designing temperature‐sensitive or active nanostructured materials or interfaces based on OCL.

Rapid depolymerization of poly(ethylene terephthalate) thin films by a dual-enzyme system and its impact on material properties

“…The plasticizing effect of water has been evidenced for PET in a range of temperatures and humidity values, 20 , 28 , 32 , 39 and its effect in lowering melting temperatures of semicrystalline polyesters on Langmuir films has been reported by our group. 40 It has been also observed by several authors that T g of amorphous PET films prepared on solid substrates (e.g., Si wafers) is lower than its bulk (between 65°C and 71°C) and decreases with the reduction of the film thickness, leading to a T gs as low as 48°C for a surface thickness of 13.6 nm, as described by Shinotsuka et al. 28

Figure 3 Determination of the T g of PET ultrathin films on an aqueous subphase (A) G′ versus temperature plot at different surface pressures, indicating the first and second tangent lines for calculation of T g .

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Degradation kinetics of oligo(ε-caprolactone) ultrathin films: Influence of crystallinity

“…S1. A detailed explanation on differences in the compression isotherms, crystallization, and melting behavior of OCDOL and OCDME films at the air-water interface can be found elsewhere [8][9][10].…”

“…2b). This effect is attributed to differences in chain-packing and larger free volume in OCDME crystals, which is evidenced by the prevalence of growth sectors with a lower melting temperature [8]. Nevertheless, for both OCDOL and OCDME, the highly crystalline films degrade slowly and almost linearly at 37 °C.…”

“…Langmuir technique setup and details on the cleaning of the Teflon trough used for investigation of OCL films at air-water interface were described previously [8]. Briefly, to prepare OCL ultrathin films differing in their crystallinity, chloroform solution (0.2-0.3 mg mL −1 ) of OCDOL or OCDME was spread dropwise on the surface of water at 21 ± 0.5 °C in the Langmuir trough using a micro syringe (Hamilton Co., Reno, NV, USA).…”

“…By means of the Langmuir technique, we are able to generate ultrathin OCL films at the air-liquid interface with precisely adjusted semi-crystalline morphology, including crystal size, number density, thickness, and melting temperature. The polymer films with adjustable semi-crystalline morphology are transferable to solid substrates for further characterization [8]. It is also possible to subject these films to thermal treatments that reflect the activation of a shapeshifting effect while measuring the influence on degradation in real time.…”