Nanoscale Heat Conduction in CNT-POLYMER Nanocomposites at Fast Thermal Perturbations.

Nanometer scale heat conduction in a polymer/carbon nanotube (CNT) composite under fast thermal perturbations is described by linear integrodifferential equations with dynamic heat capacity. The heat transfer problem for local fast thermal perturbations around CNT is considered. An analytical solution for the nonequilibrium thermal response of the polymer matrix around CNT under local pulse heating is obtained. The dynamics of the temperature distribution around CNT depends significantly on the CNT parameters and the thermal contact conductance of the polymer/CNT interface. The effect of dynamic heat capacity on the local overheating of the polymer matrix around CNT is considered. This local overheating can be enhanced by very fast (about 1 ns) components of the dynamic heat capacity of the polymer matrix. The results can be used to analyze the heat transfer process at the early stages of "shish-kebab" crystal structure formation in CNT/polymer composites.

Nanometer scale thermal response of polymers to fast thermal perturbations.

Nanometer scale thermal response of polymers to fast thermal perturbations is described by linear integro-differential equations with dynamic heat capacity. The exact analytical solution for the non-equilibrium thermal response of polymers in plane and spherical geometry is obtained in the absence of numerical (finite element) calculations. The solution is different from the iterative method presented in a previous publication. The solution provides analytical relationships for fast thermal response of polymers even at the limit t → 0, when the application of the iterative process is very problematic. However, both methods give the same result. It was found that even fast (ca. 1 ns) components of dynamic heat capacity greatly enhance the thermal response to local thermal perturbations. Non-equilibrium and non-linear thermal response of typical polymers under pulse heating with relaxation parameters corresponding to polystyrene and poly(methyl methacrylate) is determined. The obtained results can be used to analyze the heat transfer process at the early stages of crystallization with fast formation of nanometer scale crystals.

Ultrafast thermal processing and nanocalorimetry at heating and cooling rates up to 1 MK/s.

To generate artificial materials with advanced physical and chemical properties and to study phase transition kinetics on submillisecond time scale, an ultrafast nonadiabatic membrane nanocalorimeter was constructed. A set of commercially available membrane gauges for ultrafast nanocalorimetry has been developed. The gauges placed in a thermostat with controlled helium gas pressure and temperature can be utilized as devices for thermal processing and calorimetry with resolution of 1 nJ/K. Controlled ultrafast cooling, as well as heating, up to 10(6) K/s can be attained for nanogram samples. The maximum cooling rate is inversely proportional to the radius of the heated region of the gauge, which was in the range of 10-100 microm depending on the gauge. The minimum addenda heat capacity was 3 nJ/K. The dynamic heat-transfer problem for the temperature distribution in the membrane-gas system at ultrafast processing has been solved. The characteristic rate R(0) corresponding to quasistatic limit of the temperature change in the membrane-gas system has been found to be equal to 10(5) K/s for a 1 microm thick silicon nitride membrane in helium gas. Calorimeter performance at ultrafast rates has been verified by a set of test experiments. The method was applied for thermal processing and calorimetric measurements in a set of linear polymers. It has been established that nearly amorphous polyethylene can be obtained at a cooling rate of 10(6) K/s.

Isothermal reorganization of poly(ethylene terephthalate) revealed by fast calorimetry (1000 K s(-1); 5 ms).

Reorganization of semicrystalline polymers on heating is a fast process. For poly(ethylene terephthalate) (PET) heating rates of several thousand Kelvin per second are needed to prevent reorganization of unstable crystals. Utilizing a thin film vacuum gauge as a fast calorimeter we are able to extend the scanning rate range of commercial DSC's (microK s(-1) to 10 K s(-1)) to rates as high as 10000 K s(-1) on heating and cooling. Because of the fast equilibration time isothermal experiments can be performed after scanning at several thousand Kelvin per second. The dead time after such a quench is in the order of 10 ms and the time resolution is in the order of milliseconds. These ultra fast calorimeters allow us to study the kinetics of extremely fast processes in semicrystalline polymers like reorganization. For PET crystallized at 130 degrees C reorganization needs less than 40 ms between 150 degrees C and 200 degrees C. Isothermal reorganization at 223 degrees C is about two orders of magnitude faster than isothermal crystallization from the isotropic melt at the same temperature. The melt memory for the remaining structures needed for reorganization is removed 25 K above the equilibrium melting temperature of PET.