Poly induction time

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. 

Increasing temperature shortens the induction time and increases the maximum chemiluminescence intensity in the case of chemiluminescence of PP powder (type (a), see Figure 15), whereas it increases the initial chemiluminescence intensity in the case of poly(2,6-dimethyl-l,4-phenylene oxide) (type (b), see Figure 5). This is perhaps not surprising as the rate of oxidation reaction increases with temperature as well. 

The nonisothermal crystallization process of isotactic poly(butene-l) as a function of cooling rate was investigated by Silvestre et al. [77]. The samples were melted at 160°C for ten minutes and cooled to room temperature at different rates 0.5, 1, 2 and 4°C/min. The induction time and the time required for the completion of the phase transition were found to decrease with cooling rate. The calculated CRC parameter was 18 h. ... 

Linear low density poly(ethylene)-based nanocomposites exhibit a faster photo oxidation in comparison to the unfilled matrix. The acceleration is not due to a faster rate of the photo oxidation but due to the reduction of the induction time of the oxidation reaction. It is suspected that the presence of trace amoimts of metal ions in the organoclays promotes a catalytic photo oxidation. Thus, metal deactivators have been introduced into the formulations. Combinations of metal deactivators with UV absorbers show synergistic effects (12). 

Using isothermal differential thermal analysis for the characterization of the long-term oxidation of poly(butene) and crosslinked poly(ethylene), it has been pointed out that straight line extrapolations from short-term experiments at elevated temperatures to low temperatures and long times are not possible (3). Also, oxidation induction times introduce another difficulty for interpretation of the experiments. In contrast, for isotactic poly(propylene) the application of the relationship of Arrhenius does not make problems (4). 

More recently, Poli and Smith have presented evidence of VAc-controlled polymerization initiated by V-70 in the presence of complexes CpCr(nacanac ) (Ar = Xyl, XVIII Dipp, XIX Figure 16). ° As shown in the initial report, the experiment carried out in the presence of XVIII at 50 °C led to a <15% conversion before polymerization stopped, while that carried out in the presence of XIX at 30 °C proceeded to 70% conversion in 40 h. Although the level of control was poor (Ain =67 000 vs. targeted 30 000 at 70% conversion, Al /Aln= 1.8), the molecular weight increased with conversion suggesting the intervention of a reversible termination process (Figure 5). The substoichiometric amount of V-70 and the absence of an induction time rule out an OMRP-DT mechanism. 

It was found from a study of isotactic poly(propylene), up to shear rates of 2 s", that when the induction time was subtracted from the real time more conventional values of n were obtained.(72) In this work n = 2 was found for quiescent crystallization. 

Preparation and Properties of High-Molecular-Weight Poly (propylene oxide). Figure 4 shows a typical conversion-time plot for polymerization of propylene oxide by a hexacyanometalate salt complex catalyst. This reaction is characterized by an initial period during which almost no conversion occurs, followed by a period of rapid polymerization. The initial period, termed the induction period, is highly... 

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