Characterization of optical properties of acrylate based adhesives exposed to different temperature conditions

Highly Sensitive Temperature Sensor Based on Cascaded Polymer-Infiltrated Fiber Mach–Zehnder Interferometers Operating near the Dispersion Turning Point

High-accuracy temperature measurement plays a vital role in biomedical, oceanographic, and photovoltaic industries. Here, a highly sensitive temperature sensor is proposed and demonstrated based on cascaded polymer-infiltrated Mach–Zehnder interferometers (MZIs), operating near the dispersion turning point. The MZI was constructed by splicing a half-pitch graded index fiber (GIF) and two sections of single-mode fiber and creating an inner air cavity based on femtosecond laser micromachining. The UV-curable polymer-infiltrated air cavity functioned as one of the interference arms of MZI, and the residual GIF core functioned as the other. Two MZIs with different cavity lengths and infiltrated with the UV-curable polymers, having the refractive indexes on the different sides of the turning point, were created. Moreover, the effects of the length and the bending way of transmission SMF between the first and the second MZI were studied. As a result, the cascaded MZI temperature sensor exhibits a greatly enhanced temperature sensitivity of −24.86 nm/°C based on wavelength differential detection. The aforementioned result makes it promising for high-accuracy temperature measurements in biomedical, oceanographic, and photovoltaic applications.

Mechanical Behavior and Constitutive Model Characterization of Optically Clear Adhesive in Flexible Devices

Optically clear adhesive (OCA) has been widely used in flexible devices, where wavy stripes that cause troublesome long-term reliability problems often occur. The complex mechanical behavior of OCA should be studied, as it is related to the aforementioned problems. Therefore, it is necessary to establish reasonable mechanical constitutive models for deformation and stress control. In this work, hyperelastic and viscoelastic mechanical tests were carried out systematically and relative constitutive models of OCA material were established. We found that temperature has a great influence on OCA’s mechanical properties. The stress and modulus both decreased rapidly as the temperature increased. In the static viscoelasticity test, the initial stress at 85 °C was only 12.6 kPa, 57.4% lower than the initial stress at 30 °C. However, in the dynamic test, the storage modulus monotonically decreased from 1666.3 MPa to 0.6628 MPa as the temperature rose, and the decline rate reached the maximum near the glass transition temperature (Tg = 0 °C). The test data and constitutive models can be used as design references in the manufacturing process, as well as for product reliability evaluation.

Using Mie Scattering to Determine the Wavelength-Dependent Refractive Index of Polystyrene Beads with Changing Temperature

Polystyrene beads are often used as test particles in aerosol science. Here, a contact-less technique is reported for determining the refractive index of a solid aerosol particle as a function of wavelength and temperature (20-234 °C) simultaneously. Polystyrene beads with a diameter of 2 μm were optically trapped in air in the central orifice of a ceramic heating element, and Mie spectroscopy was used to determine the radius and refractive index (to precisions of 0.8 nm and 0.0014) of eight beads as a function of heating and cooling. Refractive index, n, as a function of wavelength, λ (0.480-0.650 μm), and temperature, T, in centigrade, was found to be n = 1.5753 - (1.7336 × 10^-4)T + (9.733 × 10^-3)λ^-2 in the temperature range 20 < T < 100 °C and n = 1.5877 - (2.9739 × 10^-4)T + (9.733 × 10^-3)λ^-2 in the temperature range 100 < T < 234 °C. The technique represents a step change in measuring the refractive index of materials across an extended range of temperature and wavelength in an absolute manner and with high precision.

Temperature Sensor Based on Side-Polished Fiber SPR Device Coated with Polymer

A highly sensitive temperature sensor based on surface plasmon resonance (SPR) of a side-polished single mode fiber is demonstrated. The sensor consists of a gold film coated side-polished fiber covered by a layer of UV-curable adhesive. Before introducing the UV-curable adhesive, the gold-coated fiber exhibits refractive index (RI) sensitivity of 1691.6 nm/RIU to 8800 nm/RIU in the range of 1.32 to 1.43. The resonant wavelength of the SPR sensor shifts to 650 nm when the adhesive is coated on the gold film, and is fixed at about 725 nm when the adhesive is cured. Due to the high thermo-optic and thermal expansion coefficient of the adhesive, the sensor structure achieves a temperature sensitivity of −0.978 nm/°C between 25 °C and 100 °C. The proposed optical fiber SPR sensor is simple, highly sensitive and cost effective, which may find potential applications for temperature measurements in the biomedical and environmental industries.

Highly sensitive temperature sensor based on a polymer-infiltrated Mach-Zehnder interferometer created in graded index fiber

A highly sensitive temperature sensor is proposed and demonstrated based on a UV-curable polymer-infiltrated Mach-Zehnder interferometer (MZI) created in a graded index fiber (GIF). The device was constructed by splicing a half-pitch GIF between two single-mode fibers and creating an inner air cavity in one lateral side of the GIF core by means of femtosecond laser micromachining. The air cavity and the residual GIF core functioned as two interference arms of the MZI. Moreover, the GIF was used as a miniature in-fiber collimator to reduce insertion loss of the air cavity. Experimental results show such an MZI device has a high refractive index (RI) sensitivity of 24611.54 nm/RIU (RI=1.545-1.565). Subsequently, thermo-sensitive polymer liquid was infiltrated into the air cavity, then cured with UV illumination, and annealed at 50°C for 12 h. The infiltrated MZI exhibits a high temperature sensitivity of -13.27  nm/°C. In addition, this MZI also has excellent thermal stability and repeatability, compact structure, low insertion loss, and high fringe visibility. As such, the proposed MZI could be developed for high-accuracy temperature measurements in many areas such as biomedical or oceanographic applications.

Simultaneous Determination of Glass Transition Temperatures of Several Polymers

AIMS

A simple and easy optical method is proposed for the determination of glass transition temperature (Tg) of polymers.

METHODS & RESULTS

Tg was determined using the technique of microsphere imaging to monitor the variation of the refractive index of polymer microsphere as a function of temperature. It was demonstrated that the method can eliminate most thermal lag and has sensitivity about six fold higher than the conventional method in Tg determination. So the determined Tg is more accurate and varies less with cooling/heating rate than that obtained by conventional methods. The most attractive character of the method is that it can simultaneously determine the Tg of several polymers in a single experiment, so it can greatly save experimental time and heating energy.

CONCLUSION

The method is not only applicable for polymer microspheres, but also for the materials with arbitrary shapes. Therefore, it is expected to be broadly applied to different fundamental researches and practical applications of polymers.

Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers

We demonstrate the suitability of terahertz time-domain spectroscopy as a non-destructive, contact-free tool to monitor the glass transition in polymers--a core feature of the amorphous phase. Below the glass transition temperature T(g), segmental motions along the polymer chain are frozen due to the lack of free volume between neighboring macromolecules. We show that this transition also reflects in the temperature dependence of the refractive index at terahertz frequencies. Two domains can be identified, which differ in their sensitivity to temperature changes. To verify the proposed approach, we determine the glass transition temperature T(g) of semi-crystalline poly(oxymethylene) (POM) with terahertz time-domain spectroscopy and validate the results by destructive differential scanning calorimetry (DSC) measurements.