Crystallization dilatometry

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

Using dilatometry in parallel with cyclic voltammetry (CV) measurements in lmolL 1 LiC104 EC-l,2-dimethoxy-ethane (DME), Besenhard et al. [87] found that over the voltage range of about 0.8-0.3 V (vs. Li/Li+), the HOPG crystal expands by up to 150 percent. Some of this expansion seems to be reversible, as up to 50 percent contraction due to partial deintercalation of solvated lithium cations was observed on the return step of the CV. It was concluded [87] that film formation occurs via chemical reduction of a solvated graphite intercalation compound (GIC) and that the permselective film (SEI) in fact penetrates into the bulk of the HOPG. It is important to repeat the tests conducted by Besenhard et al. [87] in other EC-based electrolytes in order to determine the severity of this phenomenon. 

The copolymers consist of strictly alternating sequences of diene and olefin. C-NMR measurements Showed the microstructure of the butadiene units in BPR to be exclusively of the trans-1,4 configuration (Figure 8). The isoprene units in isoprene-ethylene copolymer (IER) contain 84 % trans-1,4, 15 % cis-1,4, and 1 % 3,4 structures (Figure 9). Spontaneous crystallization in unstretched BPR samples was detected by dilatometry and confirmed by X-ray diffraction and DSC measurements. The extrapolated equilibrium melting point is about -10 °C. 

Crystallization in a range of PDMS-PEO-PDMS triblocks was investigated using dilatometry, DSC and SAXS by Galin and Mathis (1981). They found that depending on the composition of the copolymer the PEO crystallized within... 

List, G. R., Steidley, K. R., Palmquist, D., and Adlof, R. O., Solid fat index vs. solid fat content A comparison of dilatometry and pulsed nuclear magnetic resonance for solids in hydrogenated soybean oil, in Crystallization and Solidification Properties of Lipids, N. Widlak, R. Hartel, and S. Narine (Eds.), pp. 146-152, AOCS, Champaign, IL, 2001. 

Various techniques are available for the investigation of the solid state. These include microscopy (including hot-stage microscopy, HSM), infrared spectrophotometry (IRS), single-crystal X-ray and X-ray powder diffraction (XRPD), thermal analysis, and dilatometry. 

Since the sensitivity of pulse NMR is very high and H Ti values for usual polymers are less than 1 s due to the spin diffusion, rapid measurements with short repetition times are possible. This gives us the real time measurement of nonequilibrium phenomena such as crystallization in the polymer. The crystallization process of polymers has been studied by an optical microscope, dilatometry and X-ray diffraction. These methods only gives static information about the crystallization process. The pulse NMR measurements provide both the fraction and the molecular mobility of each phase. Figures 7.19 and 7.20 show the temperature change of the fractions and T2 values of crystalline, interfacial and amorphous components for poly(e-caprolactone)... 

The crystal phases in the glass-ceramics were determined by XRD analysis. All instruments were precisely and identically set to ensure a high precision to obtain the integral peak area. The microstructure of the fresh fractured cross section of the glass-ceramics was observed by SEM. The thermal expansion coefficient (TEC) was calculated from room temperature to 500 °C at a heating rate of 5°C/min in the dilatometry analyser (NETZSCH, DIL402PC). The flexural strength was determined in a 3-point bend test at a constant strain ratio of 0.5mm/min. 

Dilatometric analysis is a less widely used technique for characterising polymorphs, but has, for example, been applied to the study of polymorphic transformations occurring in theobroma oil, methyl stearate and chloramphenicol [25]. Substances which contract as they transform from a metastable (less dense) polymorph to a stable (more dense) polymorph can be studied by measuring their specific volume as a function of temperature. A recent study involved the combined use of dilatometry and neutron scattering to characterise the orthorhombic and monoclinic polymorphs of ra-nitrophenol [ 130]. Crystals of both polymorphs showed significant anisotropy of expansion and it was possible to reconcile the direction of lowest expansion with that of the hydrogen bonding interactions. Attempts to correlate these results with those obtained from IR and Raman spectra were subsequently reported [131]. 

Calorimetry and dilatometry were used to estimate super-high molar masses for some crystallizable polymers. For such high molar masses, crystallization is increasingly impeded and the fraction of the molecules able to crystallize has been calibrated with respect to molar mass for polytetrafluoroethylene [25,26]. 

The quantitative influence of self-nucleation is shown in Fig. 3.66 for the case of crystallization of poly-l-butene with crystal form 11 when followed by dilatometry. 

At sufficient supercooling, the sphemhtic superstmcture may be described with only one growth rate, v, as shown in Fig. 3.83. The increase in crystal volume, V, is given by the second equation and allows the computation of the enthalpy evolved by multiplication with the product of density and specific heat of fusion (Ahf in 1 g ). With nucleation data, the crystallization rate of the whole sample can be computed and linked to growth rates measured by dilatometry or calorimetry. Results for the LiPOj crystallization, which is discussed in Sect. 3.1.6, are also listed in Fig. 5.83 [15]. The changes of the growth rale with temperature are given in the table. Note that the crystallization of the polymer is in this case coupled with the polymerization reaction [1], and increases with temperature. 

In this section experimental results are discussed, concerned with analyses of melting and crystallization kinetics, as well as reversibility of the phase transition. The frame of the discussion is set by Fig. 3.76, which will be supported by experimental data on poly(oxyethylene). The thermal analysis tools involved are TMDSC, optical and atomic-force microscopy, DSC, adiabatic calorimetry, and dilatometry. Most of these techniques are described in more detail in Chap. 4. Results from isothermal crystallization, and reorganization are attempted to be fitted to the Avrami equation. This is followed by a short remark on crystallization regimes and finally some data are presented on the polymerization and crystallization of trioxane crystals. 

A quantitative, isothermal measurement of the crystallization kinetics, usable for analysis by the Avrami method, is illustrated in Fig. 3.98 by the upper left curve. Similar curves can be generated by dilatometry or adiabatic calorimetry as described in Sects. 4.1 and 4.2. At time zero, one assumes that the isothermal condition has been reached. The dotted segments of the heat-flow response are then proportional to the heat of crystallization evolved in the given time intervals and can be converted directly into the changes of the mass fraction of crystallinity after calibration or normalization to the total heat evolved. An independent crystallinity determination... 

---