Quenched polypropylene

Figure 4 Light permeability of polypropylene as a function of the sample thickness 1-3-quenched polypropylene 4-6-normal polypropylene 1 and 4-100 /ac thick 2 and 5-150 jLtc thick 3 and 6-500 /jlc thick.

Rapidly quenched polypropylene, PP, is an example of metastable condis crystals. The fast crystallization prohibits the development of proper handedness of the helices of Fig. 5.26. The X-ray diffraction-scan in Fig. 5.146 shows the disorder caused by random assembly of left- and right-handed helices. The two broad diffraction maxima at room temperature correspond to the spacings along and between the chains. The state is a glass, since no conformational mobihty is detectable. As soon as the sample is heated to about 360 K, conformational mobility permits perfection of the helices. 

For the quenched polypropylene, the stress-strain behavior becomes linear only in the region of strain below 1%. Observe that at a strain of 7.6% the isochrones are flattening to a zero slope, and the modulus has been reduced by a factor of at least 3.2 from what it would have been if the behavior were linear. Also, at strains above about 2% the nonlinearity becomes essentially independent of time, i.e. the isochrones form a set of nearly parallel curves. 

The use of quenching agents in polymers is a recent development. Of particular interest are the nickel(II) chelates in polypropylene film and fibre and the even newer hindered amines which appear to combine the roles of antioxidant cmd quenching agent. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

We find an additional crystalline structure in rapidly quenched isotactic polypropylene products. In these materials, the polymer chains do not have the necessary time to orient,... 

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. 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

Vogtle F, Gestermann S, Kauffmann C et al (1999) Polypropylene amine) dendrimers with peripheral dansyl units protonation, absorption spectra, photophysical properties, intraden-drimer quenching, and sensitization processes. J Am Chem Soc 121 12161-12166... 

For polypropylene, by using the spectrum of an annealed sample and subtracting it from a quenched sample it is possible to obtain a difference spectrum characteristic of the amorphous regions of polypropylene 210). In Fig. 13, the difference spectrum characteristics of the amorphous phase of the quenched sample (a) is compared with the difference spectrum characteristic of the ordered phase of an annealed... 

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

Fig. 21. X-ray diffraction pattern of the annealing effect on polypropylene quenched to the CD-glass (control trace). The changes may suggest a glass transition at 340 to 360 K. Curve courtesy of Dr. W. W. Cox of the Research Laboratory of Hercules Inc.

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