Melting of crystallites in polymers with low degree of crystallinity under shear flow conditions I: Poly(vinyl chloride)

Abstract

The aggregates found in dilute solutions of nearly atactic PVC have been shown to consist of 10 to 15 single molecules held together by a crystalline nucleus. Based on the postulate that the last crystallites in a PVC melt have the same structure as found in dilute solution, a relation between the melting temperature in shear (Tdyn), the static melting temperature (Tm) and the shear stress (τ) is found: where Q is a constant. The model predictions are in accordance with data from a Brabender rheometer and a Rheometrics mechanical spectrometer. Q is found to be a constant for samples with weight average molecular weights in the interval between 70,000 and 160,000 corresponding to polymerization temperatures between 40 and 70°C.

Rheological Images of Poly(vinyl chloride) Gels. 6. Effect of Temperature

“…This assumption is of meaning because the gel junction can be considered to be a dense crystallite domain in which the solvent (DOP in this case) is excluded. According to the literature, [28][29][30][31][32][33] the heat of fusion for PVC was reported to be the values between 2.76 and 11.3 kJ/mol. Here, we use 3.28 kJ/ mol 28 as ∆h, because this value is thought to be most probable.…”

Dynamic‐mechanical behavior of poly(vinyl chloride), PVC, and chlorinated PVC compounds in the range of processing temperatures

“…N o general agreement seems to exist as to the exact nature of secondary crystallinity in PVC; a nematic structure of lower heat of fusion than the orthorhombic primary crystallites has been proposed by Juijn,et a2. (24). Such structures might be particularly sensitive to disrupting shear forces (25) and responsible for various anomalies in the rheological behavior as e.g.. the effect of therinomechanical history. Melting and recrystallization take place rather rapidly in the lower temperature region (T < 170°C) where complex dynamic viscosity becomes nearly independent of temperaturr ( Fig.…”

Rheological behavior of polyvinyl chloride‐g‐diallyl phthalate resin

“…The melt rheology shifts from particulate flow to viscoelastic flow. [ 39–41 ] Accompanying the gelation, there is the change of the primary crystallites: those with melt temperature lower than processing temperature will melt to facilitate the fusion of the primary particles; the more perfect crystallites with melt point higher than the processing temperature remains in the melt to cause the extra yield‐like complex viscosity, the extra elasticity, and the VGP typical with long relaxation modes. To further verify the rheology is indeed associated with the gelation, the tensile fracture surfaces of the grafted PVC are examined with SEM in comparison to the un‐grafted sample.…”

“…The physical entities for such "crosslinks" likely are the entanglements and residual crystallites. [39][40][41] Due to the particulate nature and presence of primary crystallites in the nascent PVC resins, [42] it has to experience the process of gelation, that is, the melting of at least some of the primary crystallites that free the chains to allow primary particles to fuse to a F I G U R E 4 Loss modulus versus frequency for PVC and PVC-g-DAP samples continued network. The residual primary crystallites, that is, those with melting temperature higher than the processing temperature, serve as such physical "crosslinking".…”