Effect of Extensional Viscosity on Cocontinuity of Immiscible Polymer Blends

Transcription

1 Effect of Extensional Viscosity on Cocontinuity of Immiscible Polymer Blends Aaron T. Hedegaard a, Liangliang Gu and Christopher W. Macosko* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota a present address: 3M Co. St. Paul MN SUPPORTING INFORMATION Additional Phase Inversion Predictions In addition to the Utracki s model shown in Eq. (4) of the article [Utracki (1991)], many other predictive models for determining the phase inversion composition have observed that it is strongly dependent on the viscosity ratio, p, of the immiscible blended polymers: p PE PLA (S.1) where is the shear rate dependent viscosity of the polyethylene (PE) phase and is the PE shear rate dependent viscosity of the polylactic acid (PLA) phase. It should be noted that all the models shown here are purported to be material system independent. The specific materials (PE and PLA) used in this study are listed in the equations here for clarity and ease of application to the current results. Paul and Barlow empirically observed that phase inversion in polymer-polymer blends occurred at a volume fraction ratio equal to the viscosity ratio p [Paul and Barlow (1980)]: PLA p PE, PLA, (S.2) 1

2 where and are the phase inversion volume fractions of the PE and PLA phases, PE, PLA, respectively. Work by Metelkin and Blecht (1984) predicted the phase inversion of polymer-polymer blends by applying work by Tomotika (1935) concerning the breakup of threads into droplets. The theory predicts phase inversion to occur when the deformation rate of stretching droplets into fibers is matched by the rate of fiber breakup into droplets. 1 PLA, 1 F p * p (S.3) log 1.8 log 2 F p p p (S.4) This equation was derived using a LDPE/rubber blend. However, the original thread breaking work from Tomotika suggests that interfacial tension and thread size, which are not variables in the work presented by Metelkin and Blecht, play a key role in the rate of thread stretching and breakup, so there is dispute as to whether this model is generally applicable to other material systems [Pötschke and Paul (2003)]. Despite this extensive work on predicting phase inversion by viscosity ratio, subsequent studies have found these models lacking in their general applicability to all blended polymer systems. Other models have been proposed that go beyond just including shear viscosity to incorporate shear elasticity into the phase inversion prediction, particularly in work by Bourry and Favis (1998). These are based on an observation that a more elastic phase has a tendency to encapsulate a less elastic phase during mixing, due to a decrease in effective interfacial tension under dynamic conditions when the more elastic phase serves as the matrix [Favis and Chalifoux (1988); Vanoene (1972)]. This elastic dependence of the phase inversion can be described in terms of a ratio of G : [Bourry and Favis (1998)] PE, PLA, G ' G ' PLA PE (S.5) Similarly, the elastic dependence can also be described by a ratio of tan(δ), with the materials 2

3 inverted compared to the G ratio so as to retain the same dependency on G : [Bourry and Favis (1998)] PE, PLA, tan( ) tan( ) PE PLA (S.6) FIG. S.1. Phase inversion predictions from different literature-reported models, relative to the ranges and centers of cocontinuity for each blend system. The Utracki model (Eq. (4)) and the ratio of tan(δ) (Eq. (S.6)) predicted the behavior most successfully, but inconsistently, showing large deviations for blends with dissimilar branching architecture between the phases. While previous studies have reported better predictions of phase inversion from Eq. (S.5) and Eq. (S.6) than from models that only incorporate shear viscosity [Steinmann (2001); Bourry and Favis (1998); Favis and Chalifoux (1988)], no such improvement was observed here, as shown Fig. S.1. This figure shows graphically the experimental range and center of cocontinuity for each blend system compared to the all the phase inversion predictions. The accuracy of the predictions and was evaluated by plotting the model-predicted phase inversion points against the experimentally observed results, as shown graphically in Fig. S.2. Between the predictions incorporating elasticity, the one based on the ratio of tan(δ) 3

4 performed nearer to the observed phase inversion results. However, it was inconsistent for blends containing HDPE, and was in general not as accurate as the Utracki model. Fig. S.2 particularly shows that predictions based on tan(δ) also predicted that the PLA phase inversion composition decreased with increased PLA branching, which was opposite of the experimentally observed trend, confirming a need for incorporating extensional rheological effects into predictions of cocontinuity. FIG. S.2. Evaluation of the accuracy of phase inversion predictions. The diagonal line represents agreement between the experimental center of cocontinuity ranges and the model predictions. Additional reconstructed 3D structures of PE/PLA blends at 45/55 vol.% Fig. S.3 shows the representative reconstructed 3D structures of PE/PLA blends at 45/55 vol.%, following extraction of the PLA phase. The characteristic size of each image is shown in Fig. 8. 4

5 FIG. S.3. Reconstructed 3D structures of PE/PLA blends at 45/55 vol.%, following PLA extraction, of a) HDPE/L-PLA, b) HDPE/M-PLA, c) HDPE/B-PLA, d) LDPE/L-PLA, e) LDPE/M-PLA, and f) LDPE/B-PLA. Additional reconstructed 3D structures of 45/55 vol.% LDPE/mixed PLA blends during annealing Fig. S.4 shows the reconstructed 3D structures of LDPE/mixed PLA blends, 45/55 vol.%, during annealing. It illustrates self-similar coarsening and provides pore size used to make Fig 9. 5

6 FIG. S.4. Reconstructed 3D structures of LDPE/mixed PLA blends after PLA extraction: a) 0 min annealing b) 1 min annealing c) 3 min annealing d) 10 min annealing. The annealing temperature is 180 C. Additional SEM images for discussion of elongated features in branched matrix To aid the discussion of elongated features in a branched matrix, SEM images for blends of HDPE/L-PLA, HDPE/B-PLA, LDPE/L-PLA, and LDPE/B-PLA at various compositions can be found in Fig. S.5 through Fig. S.8, respectively. All of the blends shown in these figures are cocontinuous according to gravimetric solvent extraction experiments. 6

7 FIG. S.5. Cocontinuous blends of HDPE/L-PLA across the range of cocontinuity. Red arrows point out elongated domains. FIG. S.6. Cocontinuous blends of HDPE/B-PLA across the range of cocontinuity. Red arrows point out elongated domains. 7

8 FIG. S.7. Cocontinuous blends of LDPE/L-PLA across the range of cocontinuity. Red arrows point out elongated domains. FIG. S.8. Cocontinuous blends of LDPE/B-PLA across the range of cocontinuity. Red arrows point out elongated domains. Evidence of stabilized elongated features of a linear minor phase with a branched major phase are seen in the LDPE/L-PLA blends of Fig. S.7, where the onset of stable elongated features of the linear 8

9 PLA (represented by the holes in the blend) can already be seen at only 33 vol% PLA (Fig. S.7b), with a greater probability of elongated features as L-PLA content increases. Meanwhile, HDPE/B-PLA blends shown in Fig. S.6 exhibit droplet-like behavior of the PLA phase for higher amounts of PLA, with extended droplets not seen until 55 vol% B-PLA (Fig. S.6b). Elongated features in Fig. S.5 Fig. S.8 are highlighted by the red arrows. Additional confocal images of blends at cocontinuity limits Fig. S.9 - Fig. S.14 show the morphology of PE/PLA blends with compositions at the cocontinuity limits. All these blends are cocontinuous according to gravimetric solvent extraction method. We observed that at the when PE is major phase, the minor phase (PLA) more like interconnected droplets. However, when PLA is the major phase, the minor phase (PE) seems to be interconnected fibers. FIG. S.9. HDPE/L-PLA (a) 61/39 vol.%, (b) 41/59 vol.%. FIG. S.10. HDPE/M-PLA (a) 65/35 vol.%, (b) 29/71 vol.%. 9

10 FIG. S.11. HDPE/B-PLA (a) 55/45 vol.%, (b) 24/76 vol.%. FIG. S.12. LDPE/L-PLA (a) 76/24 vol.%, (b) 42/58 vol.%. FIG. S.13. LDPE/M-PLA (a) 75/25 vol.%, (b) 24/76 vol.%. 10

11 FIG. S.14. LDPE/B-PLA (a) 66/34 vol.%, (b) 24/76 vol.%. Morphology of LDPE/B-PLA and HDPE/B-PLA during coarsening Fig. S.15 shows the 2D LSCM image of LDPE/B-PLA during coarsening. The initial morphology is more like interconnected droplets. After 30 minutes of annealing, the droplets became rounded. This may indicate a breakup into a relatively more dispersed morphology, which could explain the slower coarsening at long times. Fig. S.16 shows 2D LSCM images of HDPE/B-PLA during coarsening. For this blend, the morphology is still cocontinuous at longer annealing time. All blends other than LDPE/B- PLA showed self-similar cocontinuous coarsening similar to HDPE/B-PLA. FIG. S.15. Morphology of LDPE/B-PLA during coarsening: a) 0 min, b) 3 min, c) 30 min. FIG. S.16. Morphology of HDPE/B-PLA during coarsening: a) 0 min, b) 3 min, c) 30 min. References for Supporting Information Bourry, D. and B. D. Favis, "Cocontinuity and phase inversion in HDPE/PS blends: Influence of interfacial modification and elasticity," J. Polym. Sci., Part B: Polym. Phys. 36, (1998). 11

12 Favis, B. D. and J. P. Chalifoux, "Influence of composition on the morphology of polypropylene/polycarbonate blends," Polymer 29, (1988). Metelkin, V. I. and V. P. Blekht, Colloid Journal of the USSR 46, 425 (1984). Paul, D. R. and J.W. Barlow, "Polymer Blends," Poly. Rev. 18, (1980). Pötschke, P. and D. R. Paul, "Formation of co-continuous structures in melt-mixed immiscible polymer blends," J Macromol. Sci. C, Polym. Rev. 43, (2003). Steinmann, S., W. Gronski, and C. Friedrich, "Cocontinuous polymer blends: influence of viscosity and elasticity ratios of the constituent polymers on phase inversion," Polymer 42, (2001). Tomotika, S. "On the instability of a cylindrical thread of a viscous liquid surrounded by another viscous fluid," P Roy Soc Lond A Mat 150, (1935). Utracki, L. A. "On the viscosity-concentration dependence of immiscible polymer blends," J. Rheol. 35, (1991). Vanoene, H. "Modes of dispersion of viscoelastic fluids in flow," J. Colloid Interface Sci. 40, (1972). 12