Structure of PVC

Hierarchical Structure of PVC. PVC has stmcture that is built upon stmcture which is, in turn, built upon even more stmcture. These many layers of stmcture are all important to performance and are interrelated. A summary of these stmctures is Hsted in Table 2 Figure 5 examines a model of these hierarchies on three scales. 

It is useful to compare the structures of PVC and polyethylene Figure 12.8) since this enables predictions of the properties of the former to be made. Both materials are linear polymers and substantially thermoplastic. The presence of the... 

Thermogravimetric analysis and other studies made on low-molecular weight model compounds such as 1,3, 5,-trichlorohexane [7,8] corresponding to the idealized head-to-tail structure of PVC show these structures to be considerably more stable than the polymer. This abnormal instability of the polymer is attributed to structural irregularities or defects in the polymer chain, which serve as initiation sites for degradation. 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

One of the first commercial copolymers, introduced in 1928, was made of VC (87%) and VAc (13%) (vinylite). As the presence of the VAc mers disrupted the regular structure of PVC, the copolymer was more flexible and more soluble than PVC itself. 

Above the melting point, disappearance of microcrystallites destroys the network structure of PVC consequently it behaves like a typical high molecular weight linear amorphous polymer. 

Like other polymers, the structure of PVC is identified by IR spectroscopy [4]. Figure 5.8 shows the spectra of PVC, indicating the presence of saturated C-H groups with CH2 deformation band at 1430 cm"1, which has moved about 30 cm 1, suggesting the presence of polar substituents. The C-Cl stretching bands are observed in the range of 600-700 cm"1. 

The PVC crystallites are small, average 0.7 nm (3 monomer units), in the PVC chain direction, and are packed laterally to a somewhat greater extent (4.1 nm) (21,33). A model of the crystallite is shown in Figure 6. The crystalline structure of PVC is found to be an orthorhombic system, made of syndiotactic structures, having two monomer units per unit cell and 1.44—1.53 specific gravity (34—37). 

Dechlorination of head-to-head, tail-to-tail structure can be expected to go to 100% completion. If dechlorination of head-to-tail structure starts at random positions, then 13.5% of the chlorine should remain at the end of reaction. Dilute solutions of PVC treated with zinc removes 87% of the chlorine, proving the head-to-tail structure of PVC (71). 

A review is given of the fundamental chemistry and physical structure of PVC to provide an insight into property and performance versatility. Mention is made of how specific mechanical and thermal properties can be tailored via compounding and fabrication processes to serve specific purposes in medical applications. 12 refs. USA... 

The results given in the preceding section 9.4 on model compounds would seem to suggest that the normal structure of PVC is very stable. Anomalous weak links are thus probably responsible for the degradation of the polymer. The potential weak links in PVC are... 

There are also cases where the surface structure of PVC is altered using UV irradiation resulting in a significant decrease in the migration of phthalate plasticiser [8]. 

Temperature and orientation are the most influential factors which impact secondary crystallization (the number of crystallites, their stmcture and size, and their spatial distribution and interaction). Therefore thermal processes (heating and cooling) during polymerization, storage, and PVC processing affect the resultant crystalline structure of PVC product. 

The variety of products reflects the value of basic PVC properties. The widely recognised versatility of PVC stems from the many forms in which the polymer can be made and from the fact that each form can be compounded further to alter its properties. In PVC compounding, the interaction of additives with the polymer is a complex matter depending upon the structure of PVC and the structural changes during the fusion process. When heat and shearing forces are applied to PVC, molecular deformation flow and particle flow take place. Such a flow is called particle slippage. 

Low-MW acrylics, styrene/acrylics and poly(a-methylstyrene) blend easily with PVC and improve processing, thermoformability, gloss and ductility [33]. Commercial PMMA systems have been found to be miscible with PVC [34]. Miscibility allows improved processability by breaking the structures of PVC particles. 

Table 2. Transport of inert gases in 30 A micro-structures of PVC at 318 K. Henry s constants S are in cm (STP)/cm -atm, diffusion coefficients D in cm /s, and permeability coefficients P in cm (STP)/cm-s-atm ...

The mathematical aspects for a general case of polymers with defects in tacticity were first worked out, formulae are available and can be applied to any case. We have applied these mathematical techniques to the case of polyvinylcloride (PVC) which is known to be strongly configurationally disordered. Our work was done when the problem of the structure of PVC was highly controversial. 

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