Effect of chain architecture on the shear induced crystallisation of branched and linear poly(ethylene terephthalate) (PET)

Tracey Hanley, Robert Knott, David Sutton (ANSTO) and Ellen Heeley (The University of Sheffield, UK)

 The majority of commercial polymer processing involves the application of shear stresses to molten polymer. To analyse the effects of polymer architecture combined with shear stress, a study was undertaken on the crystallization kinetics of a series different grades of poly(ethylene terephthalate) (PET) prepared using a novel reactive extrusion process [1].

Reactive extrusion of bottle grade PET with the branching and chain extension agents pyromellitic dianhydride (PMDA) and pentaerythritol (penta), resulted in enhanced rheological properties, such as higher melt strength and higher viscosity [1,2]. A key finding of a previous isothermal crystallisation study of these novel PETs under quiescent conditions [3] was reactive extrusion significantly modified the crystallisation kinetics and changed the final polymer morphology.

In this study both linear and branched PET materials were investigated, six different samples were chosen to cover a wide range of the possible sample preparation variables and thus the end products (Table below). Sample V is a commercially available PET, Eastman PET 9663 E0002, Sample A was a control sample prepared by processing the E9963 in the extruder under the same extrusion conditions as used for the reactive extrusion but without any chemical additives.

This process gives a direct measure of the degree of degradation and change that occurred as a result of the heating and shearing processes. The other four samples (B, C, D and E) were prepared by reactive extrusion of the E9663 with chemical additives; a tetrafunctional acid anhydride pyromellitic dianhydride (PMDA) (supplied by Indspec, USA) and a polyhydric alcohol pentaerythritol, added in different amounts for the different samples. The reactive extrusion was performed using a JSW TEX-30 twin screw extruder in a co-rotating mode at 280 °C.

SAXS data were obtained on the DUBBLE beamline (BM26) at the European Synchrotron Radiation Facility, Grenoble, France. Samples were dried in a vacuum oven at 125 °C for 8 hours prior to SAXS measurements. The Linkam shear cell was modified to be purged with dry nitrogen gas for all experiments to minimise hydrolysis of PET at elevated temperatures.

Samples were placed in the shear cell between stainless steel plates fitted with mica windows and a distance of 500 ± 10 µm between plates. All experiments were conducted according to following protocol: samples were first melted at 300 °C for 5 minutes to remove previous thermal history, and then cooled at 30 °C/min to 280 °C where a step shear was applied for 2 seconds at a shear rate of 53 s-1.

Finally samples were immediately cooled at 30 °C/min to the crystallisation temperature of 245 °C where isothermal crystallisation proceeded. SAXS data was recorded for (i) the molten polymers at the melt temperature, and (ii) time-resolved data following the application of the step shear.

The crystallisation of the various PET samples was strongly dependent on the type of PET used. The highly branched, high Mw PET-D and E have a very large amount of orientation induced by the shear event whereas the linear PET-A and V had the crystallisation induced by the shear event (i.e. crystallise more rapidly than if allowed to crystallise isothermal without shear). While the two intermediate PET-B and C had orientation induced initially followed by isotropic crystallisation.

Overall it was found that the introduction of branching acts as nucleation sites with initiation of crystallisation faster in all the branched materials. However with low levels of branching, the kinetics of the crystallisation proceeded slower, in contrast to long chain hyperbranched PET samples where both initiated and completed crystallisation very rapidly.

This can be explained by the branch points acting as nucleating sites but then at low levels of branching the crystallisation proceeds at a rate that allows relaxation of the chains i.e. the observation that the crystallisation then becomes isotropic and the chain folding required for crystallisation is inhibited by the chain entanglements that have occurred as a result of the initial shear induced crystallisation.

Conversely, the hyperbranch molecules with long chain branching crystallise so rapidly upon the application of shear that there is very little time for relaxation as evidenced by the amount of shear introduced orientation.

Reactive extrusion increased melt viscosity and had a large influence on the shear induced crystallisation. The presence of long chain hyperbranched structures in the PET samples had a substantial impact on zero-shear isothermal crystallisation (Hanley et al. 2006), but this effect was accelerated under shear. The degree of orientation induced in the shear field was directly related to increased intrinsic viscosity.

The kinetics observed in this study and the degree of orientation achieved was strongly dependent on the type of PET used. Linear PET-A and V exhibited shear induced isotropic crystallisation that was slower with increasing molecular weight, while the reactive extruded long chain branched PET-D and E showed the opposite trend where increasing the molecular weight increased both the rate of crystallisation and the degree of induced orientation of the final product.

The intermediate PET-B and C which have increased melt viscosity in comparison to linear PET-A and V, have increased kinetics and orientation initially but then proceed at a slower rate than the linear PET. In summary, the findings from this study establish that crystallisation behaviours are critically dependent on the type of PET molecular architecture.

Consequently, the commercial processing conditions for branched PET should be significantly different to those required for conventional linear PET. Conversely, branched PET would be capable of different processing regimes to those accessible by standard linear PET grades.

References

1. G. Van Diepen, M. O'Shea and G. Moad, Modified polyesters. WO 98/33837 (1998).
2. J. S. Forsythe, K. Cheah, D. R. Nisbet, R. K. Gupta, A. Lau, R. Donovan, M. O'Shea and G. Moad. J Appl. Polym. Sci. 100 3646-3652 (2006).
3. T. L. Hanley, J. S. Forsythe, D. Sutton, G. Moad, R. P. Burford and R. B. Knott. Polymer Int. (2006); in press.