Parameters affecting Tg

In polymers of the type —CH2—CH(X)— restriction to rotation is imposed by increasing the size of X, thereby raising Tg. In the acrylate example it is possible to copolymerize a mixture and the Tg of the random copolymer increases with increasing amounts of the methacrylate. 

The above examples illustrate dipole-dipole interchain attraction. Thus, Tg in chlorinated polyethylene (obtained using CI2) increases with increasing percentage chlorine. 

Flexible side chains reduce interchain forces (this is sometimes called internal plasticization). 

Other factors, such as packing density of the polymer chains, may contribute. 

2 Controllable parameters. The above comparisons are uncontrollable parameters in that they cannot be altered without changing the chemical nature of the polymer. However, the following question often arises—given a specific polymer, what parameters can be applied to alter its Tg The following are examples. 

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Sample mass, volume and form are important for recording accurate and reproducible TG curves. Reliable TG curves rely on minimization of deviation between sample temperature and programmed temperature. Deviation typically results from either endothermic or exothermic reactions in the sample and subsequent heat transfer between heat source and sample. Sample mass is the most important parameter affecting TG curves. In general, small sample mass is better than large mass for minimizing temperature deviation. TG sample mass is usually about several milligrams. The low limit of sample mass depends on the resolution limit of the microbalance. 

Samples can be in block, flake, fiber or powder form. Sample form is the second most important parameter affecting TG curves. The sample form effect on a TG curve is demonstrated in Figure 10.23, which shows the TG curves of different forms of polymethyl methacrylate... 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

Thermal as well as oxidative dimers have also been isolated and characterized from pure fatty acid and TGs oxidized under simulated deep fat frying conditions. These model systems have been employed in order to simplify and control the various parameters affecting the thermal-oxidative reactions and to facilitate the structure elucidation of the decomposition products. 

Define the fractional expansion volume as [V(T)-V(0)1/V(0), so that it denotes the fraction by which the molar volume increases as a result of thermal expansion in going from T=0K up to some finite temperature T. It will be seen that, for amorphous polymers, to within the limits of the accuracy of the equations listed below, Tg is the only material parameter affecting the fractional expansion volume, which increases linearly with T. 

The details of the molecular structure of polymers profoundly influence the observed Tg s, as illustrated by Table 5-2, where we may contrast the Tg of polydimethyl siloxane, -123 °C, with that of poly(calcium phosphate), + 525 °C. At least approximately, we may separate the observed effects into intermolecular and intramolecular parts. The latter refer to structural parameters affecting the stiffness of the chain backbone we shall examine these first. 

External factors affecting Tg, such as the cooling rate and the frequency of the mechanical stress, depend on the service conditions of the material considered. With respect to the structural or compositional parameters, they can be modified at will by the chemist or the formulator in order to adjust Tg and thus meet the requirements of a given application. 

Though the factors that govern Tg have been known for some years, there is still a wide variation in values for particular polymers. Polymer Tg s are sensitive to parameters which may or may not have been evaluated by the authors. Published values should be reviewed considering all the factors which affect T,. The main factors affecting Tg values are polymer structure, sample crystallinity, diluent types and concentrations, molecular weight distributions, previous thermal history of the sample, and system pressure. More detailed treatments are given in reviews (6,48,49,1241-1249). 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

The effect of structural parameters on KIc and GIc can be summarized as follows any change in the chemical structure (use of monomers with different molar masses, use of nonstoichiometric formulations, etc.), will produce a variation in Tg this will directly affect the value of fracture resistance. An increase in Tg will lead to an increase in constant temperature, and to a decrease in the fracture resistance. This is why high-Tg epoxy networks exhibit very low values of fracture resistance. 

The secondary structure, such as a conformation, is studied mainly by solid-state NMR.2 In the solid state, NMR chemical shift is characteristic of specific conformations because the internal rotation around the chemical bonds is restricted. This shows that the NMR chemical shift can be used for elucidating the conformation of polymers in the solid state. In the amorphous phase, the conformation of the polymer chain is not fixed above Tg. Even in such a case, NMR chemical shift and the relaxation parameters can give us useful information such as the averaged conformation or the dynamics of the exchange. Solid-state NMR can also provide information about the crystalline structures, which are classified under the higher order structures through NMR chemical shift, since for most polymers, different crystalline structures accompany conformational changes which affect their NMR chemical shift. 

In this study, we control the film growth solely by substrate surface processes, by varying Ug and/or Tg, without affecting the bulk plasma parameters. This is possible when a third electrode, used as the substrate holder, is placed in the plasma system as shown in Fig. 1. A small amount of RF power delivered to this electrode results in a bias potential Vg which controls bombardment of the growing films by low energy ions. If the area of this third electrode is substantially smaller than that of the main RF electrode, its presence does not appreciably influence the plasma characteristics this has recently been confirmed by actinometric optical emission spectroscopy (8). 

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