Rheology⁻Microstructure Relationships in Melt-Processed Polylactide/Poly(vinylidene Fluoride) Blends

Abstract

In this study, small amplitude oscillatory shear tests are applied to investigate the rheological responses of polylactide/poly(vinylidene fluoride) (PLA/PVDF) blends and to correlate their viscoelastic properties with the morphological evolutions during processing. Although the analysis of the elastic moduli reveals some changes as a function of blend composition and processing time, the weighted relaxation spectra are shown to be more useful in detecting changes. The analysis demonstrates that when PVDF, i.e., the more viscous phase, is the matrix, the blend relaxes cooperatively and only a single relaxation peak is observed. By contrast, blends with highly concentrated morphologies do not fully relax, showing instead an upward increasing trend at longer times. This outcome is attributed to the broad distribution of highly concentrated droplets with a high probability of droplet⁻droplet contacts. Dynamic mechanical analysis (DMA) reveals that crystalline segmental motions attributed to the α-relaxation of PVDF at around 100 °C are restricted by the highly concentrated morphology of the 50/50 PLA/PVDF blend processed for 10 min. Relaxation analyses of the blends via dynamic oscillatory shear tests and DMA are shown to be powerful tools for investigating small microstructural changes in immiscible polymer blends.

Keywords: PLA/PVDF blend; processing-driven morphology; rheology.

Figure 1

Storage moduli G′(ω) of (a) neat polymers and (b) 70/30, (c) 50/50, and (d) 30/70 PLA/PVDF blends processed for different times as a function of frequency from 0.01 to 100 rad/s at strain amplitude of 0.5% and a temperature of 200 °C under a nitrogen atmosphere; (e) storage moduli of the blends normalized relative to the storage moduli of the blends processed for 5 min at a fixed frequency of 0.1 rad/s.

Figure 1

Storage moduli G′(ω) of (a) neat polymers and (b) 70/30, (c) 50/50, and (d) 30/70 PLA/PVDF blends processed for different times as a function of frequency from 0.01 to 100 rad/s at strain amplitude of 0.5% and a temperature of 200 °C under a nitrogen atmosphere; (e) storage moduli of the blends normalized relative to the storage moduli of the blends processed for 5 min at a fixed frequency of 0.1 rad/s.

Figure 2

SEM images of (a–c) 70/30, (d–f) 50/50, and (g–i) 30/70 PLA/PVDF blends processed for (a,d,g) 5 min, (b,e,h) 7.5 min, and (c,f,i) 10 min. Scale bars are always equal to 1 µm, with the exception of (d,f), where the scale bars indicate 2 µm. This figure is reprinted with permission from [21].

Figure 3

(a) Number average (R_n) and (b) volume average (R_v) radii sizes of droplets in blends of various compositions calculated based on Equations (5) and (6) from SEM images. The 30/70 PLA/PVDF blend processed for 5 min did not show a distinct droplet morphology, and, hence, the calculations for this composition were done after processing for 7.5 and 10 min only.

Figure 4

Van Gurp–Palmen plots of (a) neat polymers and (b–d) 70/30, 50/50, and 30/70 PLA/PVDF blends processed for (b) 5, (c) 7.5, and (d) 10 min at 200 °C.

Figure 5

Cole–Cole plots of PLA/PVDF blends of different compositions processed for (a) 5 min, (b) 7.5 min, and (c) 10 min. The plots are normalized relative to the values of η′ and η″ of the respective initial viscosities η_i.

Figure 6

Weighted relaxation spectra of (a) neat polymers and (b–d) 70/30, 50/50, and 30/70 PLA/PVDF blends processed for (b) 5 min, (c) 7.5 min, and (d) 10 min at 200 °C.

Figure 6

Weighted relaxation spectra of (a) neat polymers and (b–d) 70/30, 50/50, and 30/70 PLA/PVDF blends processed for (b) 5 min, (c) 7.5 min, and (d) 10 min at 200 °C.

Figure 7

Schematic presentation of a relaxation spectra of a blend with a condensed morphology where the round-shaped domain phases are located in very close vicinity to one another as in the case of the 10 min processed (70/30) and 5 min and 10 min processed (50/50) PLA/PVDF blends.

Figure 8

Palierne fittings of the 10 min processed (70/30), (50/50), and (30/70) PLA/PVDF blends measured at 200 °C. The dashed lines represent the fitted results.

Figure 9

(a) Storage moduli E′ and (b) tan δ values of blends processed for 10 min as a function of temperature (−60 to 140 °C) and measured at a frequency of 1 Hz.