Cationic Cyclopentadienyliron Complex as a Novel and Successful Nucleating Agent on the Crystallization Behavior of the Biodegradable PHB Polymer

Abstract

Cationic cyclopentadienyliron (CpFe⁺) is one of the most fruitful organometallic moieties that has been utilized to mediate the facile synthesis of a massive number of macromolecules. However, the ability of this compound to function as a nucleating agent to improve other macromolecule properties has not been explored. This report scrutinizes the influence of the cationic complex as a novel nucleating agent on the spherulitic morphology, crystal structure, and isothermal and non-isothermal crystallization behavior of the Poly(3-hydroxybutyrate) (PHB) bacterial origin. The incorporation of the CpFe⁺ into the PHB materials caused a significant increase in its spherulitic numbers with a remarkable reduction in the spherulitic sizes. Unlike other nucleating agents, the SEM imageries exhibited a good dispersion without forming agglomerates of the CpFe⁺ moieties in the PHB matrix. Moreover, according to the FTIR analysis, the cationic organoiron complex has a strong interaction with the PHB polymeric chains via the coordination with its ester carbonyl. Yet, the XRD results revealed that this incorporation had no significant effect on the PHB crystalline structure. Though the CpFe⁺ had no effect on the polymer's crystal structure, it accelerated outstandingly the melt crystallization of the PHB. Meanwhile, the crystallization half-times (t_0.5) of the PHB decreased dramatically with the addition of the CpFe⁺. The isothermal and non-isothermal crystallization processes were successfully described using the Avrami model and a modified Avrami model, as well as a combination of the Avrami and Ozawa methods. Finally, the effective activation energy of the PHB/CpFe⁺ nanocomposites was much lower than those of their pure counterparts, which supported the heterogeneous nucleation mechanism with the organometallic moieties, indicating that the CpFe⁺ is a superior nucleating agent for this class of polymer.

Keywords: FTIR analysis; bacterial Poly(3-hydroxybutyrate) (PHB); cationic organoiron complex; crystal structure; isothermal and non-isothermal crystallization; nanocomposites; spherulitic morphology; thermal stability.

Figure 1

Polarized light optical micrographs of Poly(3-hydroxybutyrate) (PHB) spherulites of the pure PHB and PHB/CpFe⁺ nanocomposites with ratios of (a) 100:0, (b) 99.5:0.5, (c) 99:1, and (d) 97:3, respectively, after the isothermal crystallization at 100 °C scale bar 200 µm.

Figure 2

SEM micrographs of the pure PHB and PHB/ CpFe⁺ nanocomposites with 0.5%, 1%, and 3% of the CpFe⁺ moieties (right); zoomed-in image of the 97:3% of PHB/CpFe⁺ (left) that shows the helix structure with x 2200, scale bar 10 µm.

Figure 3

FTIR spectra of the CpFe⁺ complex, pure PHB, and PHB/ CpFe⁺ nanocomposites.

Scheme 1

Proposed mechanism of the PHB/CpFe⁺ interaction.

Figure 4

XRD pattern of pure PHB and PHB/ CpFe⁺ nanocomposites with ratios of 100:0, 99.5:0.5, 99:1, 97:3 respectively (top). XRD pattern of a higher ratio of CpFe⁺ 1%, 3%, and 10% (bottom).

Figure 5

DSC curves of the pure PHB and PHB/CpFe⁺ nanocomposites; (a) cooling curves from the melt at a cooling rate 20 °C/min, (b) second heating curves with a heating rate 20 °C/min.

Figure 6

Relative crystallinity ($\mathrm{X}\left(\mathrm{t}\right))$ (right) and its corresponding typical Avrami plot (left) in the isothermal crystallization process for the pure PHB and PHB/ CpFe⁺ nanocomposites.

Figure 7

A maximum crystallization temperature as a function of a cooling rate for the non-isothermally crystallized pure PHB and PHB/ CpFe⁺ nanocomposites.

Figure 8

Relative crystallinity as a function of crystallization temperatures and time in the non-isothermal crystallization process for the pure PHB and PHB/CpFe⁺ nanocomposites at various cooling rates.

Figure 9

Typical Avrami plots of $\log (1-\ln \left(1-\mathrm{X}\left(\mathrm{t}\right)\right)$ versus $\log \left(\mathrm{t}\right)$ of the pure PHB and PHB/CpFe⁺ nanocomposites that non-isothermally crystallized at various cooling rates.

Figure 10

Plots of $\log \left(\mathsf{\phi }\right)$ versus $\log \left(\mathrm{t}\right)$ for the pure PHB and PHB/CpFe⁺ nanocomposites that non-isothermally crystallized at a given X(t).

Figure 11

Dependence of the effective activation energy of the pure PHB and PHB/CpFe⁺ nanocomposites on (a) $\mathrm{X}\left(\mathrm{t}\right)$ and (b) average temperature.

Figure 12

Thermogravimetry and differential thermogravimetry curves of the pure PHB and PHB/CpFe⁺ nanocomposites.