Nabila Shamim*,Yung P. Koh, Sindee L. Simon, and Gregory B. McKenna Glass Transition Temperature of Thin Polycarbonate Films and trinitrotoluene (TNT): Measured by Flash Differential Scanning Calorimetry Nabila Shamim*,Yung P. Koh, Sindee L. Simon, and Gregory B. McKenna *Department of Chemical Engineering, Prairie View A & M University Department of Chemical Engineering, Texas Tech University Office of Naval Research under Project No. N00014-11-1-0424 and the John R. Bradford endowment at Texas Tech University

Tg of Nanoconfined Glasses and Polymer Films Flash DSC has added a new dimension in rapid-scanning calorimetry . Gao et. al (2013) have measured the glass transition temperature of single polystyrene films using Flash DSC as a function of cooling rate. They observed Tg depression with decreasing cooling rate. Nanocalorimetry and ac-calorimetry have been used to measure Tg and Cp of thin films. No noticeable difference from bulk was reported. Nanocalorimetry at 1000 K/s cooling (Allen et al. 2003, 2004) AC-chip calorimeter at 1 to 1000 Hz (Schick et al. 2006) Siyang Gao, Yung P. Koh, and Sindee L. Simon, Macromolecules, 46, 562-570, 2013. Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z.; Allen, L. H. Phys. Rev. E 2003, 91 (8), 085703. Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z.; Allen, L. H.Macromolecules 2004, 37, 4607−4616. 6. H. Huth, A. A. Minakov and C. Schick, J. Polym. Sci., Part B: Polym.Phys., 2006, 44(20), 2996–3005.

Rate of cooling on the observed Tg depression Most recently, Yin et. al has reported calorimetric glass transition temperature of polycarbonate (2012 ) and poly(vinyl methyl ether) (PVME) (2013) thin films using AC calorimetry at a frequency 1- 1000Hz. The quantities that arise from the calorimetric measurements are the impact of : Rate of cooling on the observed Tg depression The influence of types of surface e.g. rigid substrate or liquid substrate H. Yin, and A. Schönhals, Soft Matter, 2012, 8, 9132-9139. H. Yin, and A. Schönhals, Polymer, 2013, 54, 2067-2070.

Gao et. al.(2013) has shown for PS that calorimetric investigation of thin film floating on liquid layer exhibit reduced glass transition relative to the bulk value for low cooling rates. The results from the liquid substrate exhibit effects more like those observed for rigidly supported films as reported first by Wang and McKenna in liquid dewetting experiments. Gao. et. al (2013), Macromolecules Wang and Mckenna (2013) Macromolecules.

Motivation Thin films floating on a liquid substrate behave more like films supported on rigid substrates than like freely standing films. The techniques developed in the TTU thermal analysis labs with the flash DSC permit further investigation of this situation. Most prior studies have mainly focused on polystyrene. Therefore, it is of interest to investigate polycarbonate using direct calorimetric measurements.

Objectives In the present investigation we study polycarbonate thin films floating on Apiezon grease using Flash differential scanning calorimetry. Film thickness ranged from 350 nm to 22nm Cooling rates varied from 1000 K/s to 0.1 K/s, Thickness and cooling rate dependence of the Tg Activation energy and dynamic fragility at the glass transition temperature for the different film thicknesses.

Methodology Materials Polycarbonate from General Electric Lexan LS-2 Mw = 47 kg/mol PDI = 2.55 Thin film preparation Spin-coat from chloroform Anneal under vacuum, 50 oC, 24 hrs Measure film thickness by AFM Use Apiezon grease as liquid layer 0.5 x0.5 mm2 sample area

Tg Measurements The fictive temperature (Tf') on heating as a function of cooling rate. For thin films, Mettler Toledo Flash DSC was used with a heating rate of 600K/s and 1000K/s for cooling rate from 0.1 to 1000 K/s. For bulk samples, conventional DSC was used with a heating rate of 10K/min for cooling rate from 0.002 to 0.5 K/s

Calculation of the Fictive Temperature *Polystyrene The Tf' value obtained using Moynihan’s integration method. Tf' ≈ Tg for a given cooling rate. Moynihan, C. T.; Easteal, A. J.; DeBolt, M. A.; Tucker, J. J. Am.Ceram. Soc. 1976, 59, 12−16. *Siyang Gao, Yung P. Koh, and Sindee L. Simon, Macromolecules, 46, 562-570, 2013.

Flash DSC Response for PC Thin Film 125nm PC thin film 125nm film Heating rate 600K/s

Flash DSC Response for PC Thin Film 22nm PC thin film 22 nm film Heating rate 1000K/s

VFT Fit

Fragility and Activation Energy Table 1: VFT parameters for polycarbonate thin films obtained from DSC measurements. Film Thickness (nm) Tg,ref (K) at 0.1 K/s T∞(K) m Eg (kJ/mol)  Bulk 412.0 396.4±5 98  774.9 350 412.02 360.5±10 80 634.6 125 411.4 335.5±13 75.6 595.3 75 403.5 328.5±17 61.2 472.3 40 398.3 288.8±10 58.2 444.2 22 393.8 277.9±16 52.3 394.5

Tg of PC as a function of film thickness

Comparisom with Literature Results for PC

Calorimetery chip sensor showing the silver paste with TNT sample The glass transition of trinitrotoluene (TNT) by flash DSC; Nabila Shamim, Yung P. Koh, Sindee L. Simon, Gregory B. McKenna∗ Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA

Flash DSC Response for TNT The glass transition of trinitrotoluene (TNT) by flash DSC; Nabila Shamim, Yung P. Koh, Sindee L. Simon, Gregory B. McKenna∗ Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA

The glass transition of trinitrotoluene (TNT) by flash DSC; Nabila Shamim, Yung P. Koh, Sindee L. Simon, Gregory B. McKenna∗ Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA

Summary • The calorimetric glass transition temperature of polycarbonate thin films having thicknesses from 22 nm to 350 nm has been measured using flash differential scanning calorimetry over a cooling rate range’ from 0.1 to 1000 K/s. The Tg values for 350 nm and 125 nm thick films are consistent with the bulk Tg of polycarbonate obtained by conventional DSC. The results show a strong reduction in Tg (approximately 18 oC) from the macroscopic Tg for 22 nm thick film at a cooling rate of 0.1 K/s.

Summary • It was also found that the reduction in Tg for a given film thickness increases with decreasing cooling rate, and that the dynamic fragility m and activation energy at the Tg decreases as film thickness decreases.

Thank You nashamim@pvamu.edu