Engraving the Polymer Spherulites with the Pump-Probe Setup Based on Quantum Cascade Laser

A pump-probe setup consisting of two mid-infrared quantum cascade lasers (QCLs) is designed to engrave polymer spherulites via selective melting induced by infrared resonance absorption. When the wavelength of QCL is tuned to the infrared absorbance peak of specific molecular group and the vibration direction of the group is the same as the direction of light polarization, infrared resonance absorption results in strong thermal effect. Combing with the high polarization property of QCL, selective melting can be realized. The molecular chains in polymer spherulites are aligned in all directions and the corresponding orientation direction is fixed, suitable for verifying the feasibility of the spherulites engraving by pumping different parts of the spherulites. The wavelength of the pump beam is set to a strong absorption peak in the infrared spectrum, inducing crystal melting by resonant heating, while the wavelength of the probe beam is turned to the conformation absorption peak of specific groups to trace the structure change. There are mainly three highlights of this method: (1) the melting process and the melting degree are studied in situ based on the change of the transmission intensity; (2) both pump and probe beams can reflect the structure information; (3) selectively melting of designated position in spherulites can be realized. Results of isotactic polybutene-1 (iPB-1) show that, when the energy of the pump stays constant, the melting effect is different at distinct positions of the spherulites. Melting degree of the sample decreases with the increase of the angle between the vibration direction of the groups and polarization direction of the pump beam, which proves feasible to use QCL to perform engraving on spherulite scale. Selective melting induced by infrared resonance absorption provides a new way for in situ study of rapid melting process and a guideline for controlling crystal form transition and aggregation morphology at specific position.