Crystallinity onset

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are similar techniques. They measure change in the heat capacity of a sample. These techniques can be used to determine various transition temperatures (T , Tg, T , Tp, etc.), specific heat, heat of fusion, percent crystallinity, onset of degradation temperature, induction time, reaction rate, crystallization rate, etc. A DSC instrument operates by compensating electrically for a change in sample heat. The power for heating is controlled in such a way that the temperature of the sample and the reference is the same. The vertical axis of a DSC temperature scan shows the heat flow in cal/s. 

Degree of crystallinity, onset- and peak temperatures for melting inf and 2" heating-cooling cycle for two parallel polypropylene samples containing PIB grafted silicas were measured. The true mass fraction of polypropylene in each composite was used in the calculation of crystallinity from the heat of fusion for 100% crystalline PP (190 J/g). 

Addition of talc into isotactic polypropylene (PP) induces two types of crystalline forms, i.e., a and forms [89,92,93 ]. Some reported that PP can form 4 different crystalline forms, i.e., a,jS,y,and(5 (smectic) forms [93]).Ferrageetal. [94] showed crystalline onset temperature shift by addition of talc and an a nucleating agent (sodium 2,2 -methyl-bis-(4,6-di-fert-butylphenyl phosphate) in polypropylene. Addition of triphenodithiazine, pimelic acid with calcium stearate, or quinacridone dye permanent red can increase thephase [94 to 96]. 

Figure 8.2 shows the melting endotherms from differential scaiming calorimetry (DSC) for 0,1 and 3 wt% drawn samples (to a 12 1 draw ratio), respectively. The crystallinity was calculated using a AH value of 207.1 J/g for 100% crystalline PP and Equation 8.1, shown below (where AH is the enthalpy measured from the experiment). The percent crystallinity, onset temperature and melting peak temperature are tabulated in Table 8.1. 

Crystallization. Acidified aluminum sulfate solutions can be supercooled 10 °C or more below the saturation point. However, once nucleation begins, the crystallization rate is rapid and the supersaturated solution sets up. The onset of nucleation in a gentiy stirred supersaturated solution is marked by the appearance of silky, curling streamers of microscopic nuclei resulting from orientation effects of hydraulic currents on the thin, platelike crystals. Without agitation, nucleation in an acidified solution, in glass tubes, can yield extended crystalline membranes of such thinness to exhibit colors resulting from optical interference. 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

The second phase of polymer degradation is characterized by a decrease in the rate of chain scission (Fig. 19) and the onset of weight loss. Weight loss has been attributed to (1) the increased probability that chain scission of a low molecular weight polymer will produce a fragment small enough to diffuse out of the polymer bulk and (2) the breakup of the polymer mass to produce smaller particles with an increased probability of phagocytosis. The decrease in the rate of chain scission, as well as the increased brittleness of the polymer, is the result of an increase in the crystallinity of PCL,... 

Regular insulin is unmodified crystalline insulin commonly referred to as natural insulin. It is a clear solution that has a relatively rapid onset and short duration of action. On subcutaneous injection, regular insulin forms small aggregates called hexamers that undergo conversion to dimers followed by monomers before systemic absorption can occur. Therefore, patients should be counseled to inject regular insulin subcutaneously 30 minutes prior to consuming a meal. Regular insulin is the only insulin that can be administered intravenously. 

In any state preceding the onset of crystallization at T < To we assume that bundle stability is favored by localized attractive interactions between contacting (short) stems, some enthalpy advantage being balanced by a corresponding entropy loss (see Fig. 3). Depending upon the core structure of the crystalline stems, various bundle models were examined [8,9]. In the present... 

Little is known about the variation of the critical stress ", with structure and temperature. For the polyethylene discussed abovedecreased from 620 psi at 22X to 39general trend with all polymers. Turner (84) found that the value of (r(. for polyethylenes increased by a factor of about 5 in going from a polymer with a density of 0.920 to a highly crystalline one with a density of 0.980. Reid (80,81) has suggested that for rigid amorphous polymers. ", should be proportional t° (Tt - T) For brittle polymers, the value of ", may be related to the onset of crazing. 

As such, nuclear contributions to the heat capacity due to the interaction between germanium crystalline electric field gradients and the quadrupole moments of boron nuclei could account for the observed onset of the Schottky anomaly. 

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