8CB, 9CB VE 8OCB MEZOGENLERİNİN SICAKLIK VE DALGA BOYUNA BAĞLI OPTİK ÖZELLİKLERİ VE DSC ANALİZİ

Öz

Bu çalışma, 8CB, 9CB ve 8OCB sıvı kristal mezojenlerinin sıcaklık ve dalga boyuna bağlı kırılma indisleri (n, no, ne) ile çift kırılma (Δn) davranışlarını kapsamlı biçimde incelemektedir. Ölçümler λ = 0.465 μm, 0.525 μm, 0.625 μm ve beyaz ışık altında hem ısıtma hem de soğutma döngülerinde gerçekleştirilmiş, böylece doğrudan ve ters yönlü SmA–N–I faz geçişlerinin optik özellikler üzerindeki etkileri değerlendirilmiştir. Ayrıca, Diferansiyel Taramalı Kalorimetri (DSC) analizleri yapılarak faz geçiş sıcaklıkları sistematik olarak belirlenmiş ve tarama hızına bağlı termal davranışlar ayrıntılı olarak incelenmiştir. Bulgular, izotropik kırılma indisi n’de sıcaklığa bağlı doğrusal bir azalma, olağanüstü kırılma indisi ne’de faz geçişlerine duyarlı güçlü değişimler ve olağan kırılma indisi no’da daha sınırlı bir sıcaklık bağımlılığı ortaya koymuştur. Δn değeri sıcaklıkla birlikte azalmış ve izotropik fazda sıfırlanarak moleküler düzensizliğe işaret etmiştir. DSC sonuçları ile birlikte değerlendirilen veriler, ters geçişlerde belirgin termal histerezis gözlendiğini ve bu faz geçişlerinin birinci mertebeden karakter taşıdığını doğrulamaktadır. Sonuçlar, siyanobifenil türevlerinin sıcaklığa duyarlı optoelektronik uygulamalardaki potansiyelini vurgulamakta ve sıvı kristallerin faz geçiş davranışına ilişkin literatüre önemli katkılar sunmaktadır.

Anahtar Kelimeler

• Sıvı kristaller
• Siyanobifenil
• Çift kırılma
• Kırılma indisi
• DSC
• Faz geçişleri
• Termal histerezis

TEMPERATURE AND WAVELENGTH DEPENDENT OPTICAL PROPERTIES AND DSC ANALYSIS OF 8CB, 9CB, AND 8OCB MESOGENS

Öz

This study presents a comprehensive investigation of the refractive indices (n, no, ne) and birefringence (Δn) of three cyanobiphenyl-based liquid crystal mesogens—8CB, 9CB, and 8OCB—under varying temperatures and wavelengths. Measurements were conducted at λ = 0.465 μm, 0.525 μm, 0.625 μm, and white light during both heating and cooling cycles, thereby enabling the evaluation of optical responses across direct and reverse SmA–N–I phase transitions. In addition, Differential Scanning Calorimetry (DSC) analyses were performed to systematically determine the phase transition temperatures and to examine the thermal behavior under different scanning rates. The results reveal a linear decrease in the isotropic refractive index n with temperature, pronounced temperature- and transition-dependent variations in the extraordinary refractive index ne, and moderate behavior in the ordinary refractive index no. Birefringence Δn decreased steadily with increasing temperature and vanished at the isotropic phase transition, indicating molecular disordering. When combined with the DSC findings, the data confirm the presence of significant thermal hysteresis during reverse transitions, demonstrating the first-order nature of these phase transformations. Overall, the results emphasize the potential of cyanobiphenyl derivatives for temperature-sensitive optoelectronic applications and contribute valuable insights into the phase transition behavior of liquid crystals.

Anahtar Kelimeler

• Liquid crystals
• Cyanobiphenyl
• Birefringence
• Refractive index
• DSC
• Phase transitions
• Thermal hysteresis

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