The Distribution of Nanoclay Particles at the Interface and Their Influence on the Microstructure Development and Rheological Properties of Reactively Processed Biodegradable Polylactide/Poly(butylene succinate) Blend Nanocomposites

Abstract

The present work investigates the distribution of nanoclay particles at the interface and their influence on the microstructure development and non-linear rheological properties of reactively processed biodegradable polylactide/poly(butylene succinate) blend nanocomposites. Two types of organoclays, one is more hydrophilic (Cloisite^®30B (C30B)) and another one is more hydrophobic (Betsopa^TM (BET)), were used at different concentrations. Surface and transmission electron microscopies were respectively used to study the blend morphology evolution and for probing the dispersion and distribution of nanoclay platelets within the blend matrix and at the interface. The results suggested that both organoclays tended to localize at the interface between the blend's two phases and encapsulate the dispersed poly(butylene succinate) phase, thereby suppressing coalescence. Using small angle X-ray scattering the probability of finding neighboring nanoclay particles in the blend matrix was calculated using the Generalized Indirect Fourier Transformation technique. Fourier Transform-rheology was utilized for quantifying nonlinear rheological responses and for correlating the extent of dispersion as well as the blend morphological evolution, for different organoclay loadings. The rheological responses were in good agreement with the X-ray scattering and electron microscopic results. It was revealed that C30B nanoparticles were more efficient in stabilizing the morphologies by evenly distributing at the interface. Nonlinear coefficient from FT-rheology was found to be more pronounced in case of blends filled with C30B, indicating better dispersion of C30B compare with BET which was in agreement with the SAXS results.

Keywords: morphology development; non-linear rheological properties; reactively compatibilized clay-containing PLA/PBS blends.

Figure 1

Chemical structure of (a) Joncryl^® ADR 4368, where x, y and z are between 1 and 20 R₁, R₂, R₃, R₄ and R₅ are H, CH₃, a higher alkyl group, or a combination of them; R₆ is an alkyl group [5], (b) dimethyl dihydrogenated-tallow quaternary ammonium and (c) methyl tallow bis-2-hydroxyethyl quaternary ammonium, used in the modifications of South African bentonite and modified montmorillonite (MMT), respectively. T is tallow and it is a mixture of homologs C18, C16 and C14. HT is hydrogenated tallow.

Figure 2

SEM images of (a) PLA/PBS, (b) PLA/PBS/J, (c–e) PLA/PBS/J/BET, and (f–h) PLA/PBS/J/C30B blends. The scale bars are 1 µm in all images. The scale bar in the inset image in (a) is 10 µm. The inset images in (c,f) are magnified showing two individual droplets in 1.5 wt % BET and C30B filled blends. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 3

(a) Number average and (b) volume average droplet radii of the PLA/PBS/J blends filled with C30B and BET. The dashed and solid line plots represent the blends with BET and C30B, respectively. The morphology of the 5 wt % C30B-filled blend was not quite distinct for calculating droplet sizes. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 4

Schematic presentation of possible chemical reaction and formation of linear chain branched at the interface.

Figure 5

TEM images of the (a,a’) PLA/PBS/J/1.5%C30B and (b,b’) PLA/PBS/J/5%C30B blends. (a’) and (b’) are the high-magnification localized images of 1.5 wt % C30B (scale bar = 200 nm) and 5 wt % C30B (scale bar = 500 nm) blends. C30B is Cloisite^®30B and J is Joncryl.

Figure 6

TEM images of the (a,a’) PLA/PBS/J/1.5% BET and (b,b’) PLA/PBS/J/5% BET blends. (a’) and (b’) are the high magnification localized images of 1.5 wt % BET (scale bar = 1 μm) and 5 wt % BET (scale bar = 1 μm) blends. BET is Betsopa™ and J is Joncryl.

Figure 7

Background (scattering pattern of the PLA/PBS/J blend) subtracted scattering profiles of (a) PLA/PBS/J/C30B and (b) PLA/PBS/J/BET for different clay loadings. “ift” stands for the experimental scattering curve after background subtraction and “app” stands for the approximate scattering curves determined on the basis of GIFT analysis. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 8

The pair-distance-distribution functions [p(r)] for (a) PLA/PBS/J/C30B and (b) PLA/PBS/J/BET for different clay loadings. “POR” denotes p(r) determined using the GIFT technique. The deconvolution of the approximate electron density distribution function provides a p(r) function, as denoted by “PDC”. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 9

Electron density profiles for (a) PLA/PBS/J/C30B and (b) PLA/PBS/J/BET, for different clay loadings. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 10

Storage (elastic) moduli of the PLA/PBS and PLA/PBS/J blends at various concentration of (a) C30B and (b) BET, at 190 °C. Strain amplitudes were small (0.5−1%) to ensure linear response. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 11

Storage (elastic), G’(ω) and loss (viscous), G’’(ω) moduli of the PLA/PBS/J blends, for various concentrations of (a) C30B and (b) BET, at 190 °C. Moduli are arbitrarily shifted by 10° (I = 0 wt % clay), 10¹ (II = 1.5 wt % clay), 10² (III = 3 wt % clay) and 10³ (IV = 5 wt % clay), respectively, for the sake of clarity. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 12

Storage (elastic) moduli G’(${\gamma }_0$) of the PLA/PBS/J blends, for various loadings of (a) C30B and (b) BET, at 190 °C, under the LAOS flow and a fixed frequency of 6.28 rad/s. (c,d) represent the normalized moduli of the their counterpart plots in (a,b). BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 13

Normalized third relative ($I_{3/1}$) intensities of (a) PLA/PBS/J/C30B, (b) PLA/PBS/J/BET blends and $Q\equiv I_{3/1}/{\gamma }_0^2$ values of (c) PLA/PBS/J/C30B and (d) PLA/PBS/J/BET blends as a function of strain amplitude at 190 °C and fixed frequency of 6.28 rad/s. $Q$ is fitted using a model in analogy with Carreau-Yasuda model $[Q=Q_0(1+{\left(C_1{\gamma }_0\right)}^{C_2}{)}^{\frac{C_3-1}{C_2}}]$ where $Q_0$ is the zero-strain non-linear coefficient and $C_1,C_2$and $C_3$ are fitting parameters. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.

Figure 14

(a) $Q_0$ and (b) NLR values for the (60/40) PLA/PBS/0.6J blends filled with C30B and BET organoclays, for different loadings. Inset images in (b) are TEM images of 5 wt % blends of their corresponding blend. BET is Betsopa™, C30B is Cloisite^®30B, and J is Joncryl.