A small-angle X-ray scattering study of the effect of chain architecture on the shear-induced crystallization of branched and linear poly ( The synchrotron-based small-angle X-ray scattering (SAXS) technique was used to investigate the shear-induced crystallization kinetics of branched/unbranched poly(ethylene terephthalate) (PET). Reactive extrusion of bottle-grade PET with the branching and chain-extension agents pyromellitic dianhydride and pentaerythritol results in enhanced rheological properties, such as higher melt strength and higher viscosity. In this study, six samples of PET were investigated: linear PET [intrinsic viscosity (IV) ' 0.76 dm 3 g À1 ]; four branched PETs produced from linear PET by a reactive extrusion technique (IV ' 0.86-1.06 dm 3 g À1 ); and a control PET (IV ' 0.73 dm 3 g À1 ) extruded under the same conditions without reactive agents. SAXS data were recorded for the PET at the melt temperature and time-resolved SAXS data were recorded following the application of a step shear (53 s À1 for 2 s). As the PET IV was increased, the extent of shear-induced orientation increased, whilst the time taken for the polymer to initiate and complete crystallization decreased.
IntroductionThe majority of commercial polymer processing involves the application of shear stresses to molten polymers. To analyse the effects of polymer architecture combined with shear stress, a study was undertaken on the crystallization kinetics of a series of different grades of poly(ethylene terephthalate) (PET) prepared using a novel reactive extrusion process (Van Diepen et al., 1998). 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 (Van Diepen et al., 1998;Forsythe et al., 2006).A key finding of a previous isothermal crystallization study of these novel PETs under quiescent conditions (zero shear) (Hanley et al., 2006) was that the reactive extrusion process significantly modified the crystallization kinetics and changed the final polymer morphology. In summary, the rate of melt crystallization of the branched PET was reduced by up to~20% and the lamella spacing reduced by~10%. In particular it was found that varying the relative amounts of PMDA and penta gave significantly higher melt strengths and viscosities compared to the addition of PMDA alone. The improvements as a result of co-addition included lower extruder torque values, die pressures and melt temperatures, and consequently lower levels of degradation. The additives worked in two ways: (i) the alcoholysis of the PET by the added penta produced a branched PET; and (ii) the PMDA caused chain extension and repair of the chain scissions that occur during alcoholysis. The resulting PET was investigated by a combination of rheology, end-group analyses and gel permeation chromatography (GPC) and was found to be a hyperbranched polyester with long-chain branching . The exten...