(50e) Integrated Real Time Birefringence and Light Depolarization Studies On Structural Evolution During Complete Processing Cycle: Heating, Stretching, Holding and Cooling of Polymer Films

A hybrid real-time spectral birefringence technique depolarized light intensity method^1,2 was developed to capture the mechanistic changes that take place during heating, stretching, holding and cooling cycles of polypropylene/maleic anhydride modified polypropylene (PP/PPGMA) films in partially molten state. For this purpose, we investigated the PP/PPgMA blend and with the help of using complementary off-line techniques: Wide Angle X-ray Diffraction (WAXD), Small Angle X-ray Scattering (SAXS), Atomic Force Microscopy (AFM), Small Angle Light Scattering (SALS) and Differential Scanning Calorimetry (DSC), the structural evolution details were elucidated.

Polypropylene can be processed in a partially molten state to produce very highly oriented films. The mechanical and optical properties of the film is directly depended on the processing conditions that the material was submitted. An understanding of the effects of processing parameters on the stress-optical relationship during deformation can provide a way of targeting material properties through control of structural development (molecular orientation and crystallization).

Many studies have already been carried out to correlate the final structure of polypropylene with the processing conditions^3,4,5. Vast majority of the studies do not follow the mechano-structural evolution during the course of full processing stages. As a result, the mechanistic details that occur later in the process tend to mask the details that occur in the early stages. For this reason, with the on-line measurement system used in this research, we are able to track the true stress, strain and birefringence as the material undergoes melting, stretching, holding and cooling stages (Figure 1).

During the heating stage, the birefringence and the degree of melting of the as-cast films were determined by real-time depolarizing light intensity technique. The results indicate the initial fraction of crystallites, which govern the deformation behavior of the PP films, remains in dynamic equilibrium with molten phase prior to the deformation after the initial transient stage. Throughout the uniaxial deformation of PP/PPgMA at this mushy state (partially molten state), the true stress-birefringence results showed three regime stress optical behaviors. Regime I is characterized by the slight decrease of birefringence with stress. At this small strain level, the original spherulites deform and some of these superstructures are destroyed, resulting in broken and deformed lamellar remnants. Regime II shows a rapid rise of birefringence accompanied by orientation and stress induced crystallization. During this stretching microfibrillar morphology develops. Polymer chains approach their limit of extensibility in Regime III, where birefringence approaches a plateau.

After the deformation, the film is held in a stretched state at the processing temperature to simulate the holding stage. During this period, birefringence and strain increase, while stress decreases. This stage was found to involve the growth of lamellae, primarily in the transverse direction on the previously developed fibrillar morphology.

During cooling, the stress initially decreases, and birefringence and strain show significant development. Following this stage, the thermal and stress-induced crystallization under the influence of substantial contraction forces develops.

References

1.   Valladares, D; Toki, S.; Sen, T.Z.; Yalcin, B.; Cakmak, M., Macromol. Symp., 2002, 185, 149-166. 

2.   Ding, Z.; Spruiell, J. E., J. Polym. Sci. Part B: Polym. Phys., 1996, 34, 16, 2783-2804

3.   Shimomura, Y., Spruiell, J. E., White, J. L., J. Appl. Polym. Sci. 1982, 27, 2663.

4.   Shimomura, Y., Spruiell, J. E., White, J. L., Polym. Eng. Rev. 1983, 2, 417.

5.   Choi, D., White, J. L., Polym. Eng. Sci, 2001, 41, 1743.

Figure 1 - Birefringence, true stress, temperature and Hencky strain measurements during complete processing cycle a PP/PPgMA film undergoes:  heating, stretching, holding and cooling stages