Model polymers, polypropylene

A. Marcilla, A. Gomez, J. A. Reyes-Labarta and A. Giner, Catalytic pyrolysis of polypropylene using MCM-41 kinetic model. Polymer Degradation and Stability, 80, 233-240 (2003). 

Spectra of model compounds are also utilized to assign stereochemical sequences in polymer.182 However, model compounds with defined stereochemical configurations are rather difficult to obtain. Zambelli et a/.183 reported that heptad models for polypropylene, 3(5), 5(R), 1 (RS), 9(RS),... 

Fully aware of this conflicting situation, we decided, nevertheless, to perform mechanistic studies on low molecular models of polypropylene, in the hope of achieving on this basis some new information and ideas with respect to the basic processes taking place in this polymer. Isooctane, as a model for the structural unit of polypropylene, seemed to us to be an appropriate substance for this kind of investigation. 

The isolated polyether matrix was modeled using polypropylene glycol (2000 MW) and isotactic polypropylene oxide. The polypropylene glycol was degassed and placed over molecular sieves to remove residual water present in the polyol. The isotactic polypropylene oxide was isolated by repeated crystallization from acetone (9). Inherent viscosity was 1.85 in benzene (0.5% concentration) at 25°C. Films of the isotactic polypropylene oxide were cast onto glass plates (cleaned as described previously) from a 6% solution of the polymer in N,N-dimethylformamide, dried in a forced air draft oven for 1 h at 75°C, and then placed in a vacuum desiccator (0.1 mm mercury) for 24 h to insure complete removal of residual solvent. 

FIGURE 3 Illustration of the stereochemistry in a vinyl polymer. Below each structural formula is an illustration of the stereochemistry with a ball and stick model for polypropylene. 

L. Cauvin, D. Kondo, M. Brieu, N. Bhatnagar, Mechanical properties of polypropylene layered silicate nanocomposites characterization and micro-macro modelling. Polymer Testing 29 (2) (2010) 245-250. 

As noted, the field of molecular simulation is relatively new, and a detailed review of it is beyond the scope of this text and we introduce here a few of the more relevant references. One of the first applications of molecular mechanics to polymers was by Theodorou and Suter (93,94), who modeled atactic polypropylene as an amorphous cell subjected to a range of stress conditions (hydrostatic pressure, pure strain, and uniaxial strain). Such modeling generally gives reasonable estimates of the elastic constants of a material [within 15% (79)], providing the density of the glass is correctly modeled. 

Other possible simultaneous techniques in combination with CL are /zFTIR, FTIES and oxygen uptake. Simultaneous FTIES-CL analysis (both emission spectroscopies) has been used to evaluate oxidation models for polypropylene [598]. In a further instrumental advancement, imaging chemiluminescence (ICL) has become available cfr. Chp. 5.6.4.1). Early stages of polymer oxidation can also be studied using 02 exposure and ToF-SIMS analysis [599,600]. 

O Brien, A. and Cooper, I. Polymer additive migration to foods-a direct comparison of experimental data and values calculated from migration models for polypropylene. Food Additives and Contaminants, 18(4), 343-355 (2001). 

Zaki, O., Abbes, B., and Safa, L. Non-Fickian diffusion of amyl acetate in polypropylene packaging Experiments and modelling. Polymer Testing, 28, 315-323 (2009). 

Nitta K, Asuka K, Liu B and Terauo M (2006) The effect of the addition of silica particles on linear spherulite growth rate of isotactic polypropylene and its explanation by lamellar cluster model, Polymer 47 6457-6463. 

Kalaitzidou K, fVikushima H, Miyagawa H and Drzal L T (2007) Flexural and tenale moduli of polypropylene naiiocomposites and comparison of experimental data to Halpin-Tsai and Tandon-Wang models, Polym Eng Sci 47 1796-1803. 

Foster, R.J., Mine, P.J., and Ward, l.M. (2009) Characterisation and modelling of polypropylene/carbon nanofibre nanocomposites. Polymer, 50, 4018-4027. 

Pabedinskas, A., Cluett, W. R., Balke, S. T. Modeling of polypropylene degradation during reactive extrusion with implications for process control. Polym. Eng. Set. (1994) 34, pp. 598-612... 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

The model has also been found to work well in describing the mechanics of the interface between the semicrystalline polymers polyamide 6 and polypropylene coupled by the in-situ formation of a diblock copolymer at the interface. The toughness in this system was found to vary as E- where E was measured after the sample was fractured (see Fig. 8). The model probably applied to this system because the failure occurred by the formation and breakdown of a primary craze in the polypropylene [14],... 

More recently, two-state E/B models have been proposed by Chujo and Doi (9.10) for the analysis of polypropylene. Similar E/B models were proposed by Cheng(11) and Asakura, et al(12) for polybutylene. For copolymers, two-state B/B models have been proposed for ethylene-propylene copolymers,(11,13-15) and propylene-butylene copolymers.(11,13) Recently, Cheng(11) generalized these multi-state models and developed computer methodology for the general analysis of such systems. A number of polymer systems were treated. 

Very low molecular weight polypropylenes are now known to be useful industrial additives (8). Such polymers can be made by using very high concentrations of peroxide in the degradation. Extension of the model to high initiator concentrations is therefore of practical interest, o The model was found to be sensitive to step size in reaction time used. 

Simulation programs for the ESR line shapes of peroxy radicals for specific models of dynamics have been developed for the study of oxidative degradation of polymers due to ionizing radiation [66]. The motional mechanism of the peroxy radicals, ROO, was deduced by simulation of the temperature dependence of the spectra, and a correlation between dynamics and reactivity has been established. In general, peroxy radicals at the chain ends are less stable and more reactive. This approach has been extended to protiated polymers, for instance polyethylene and polypropylene (PP) [67],... 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

The case of isotactic polypropylene is somewhat different issues related to the crystallization of the chiral /1-form [42,43] of this polymer have been discussed by various authors and probably the model presented by Lotz is the most articulated and detailed [14]. It is known that iPP, unlike sPP, adopts in all its crystalline modifications its stable conformation, namely the 3i helix with a 6.50 A periodicity [36], which may be right-handed or... 

The wall-PRISM theory has been extended to multiple site models [95], A simple example of a multiple-site model is a vinyl polymer (e.g., polypropylene) where there are three types of united-atom sites corresponding to CH2, CH, and CH3 groups. Ignoring end effects as before the PRISM equations take the form... 

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