Effects of ZnO crystals synthesized in presence of CMI biopolymer on PHBV properties

Synthesis and characterization of ZnO crystals

ZnO crystals were synthesized by chemical precipitation method [12] from aqueous solution of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, Fluka Analytical, 99%) and hexamethylenetetramine [(CH₂)₆N₄, HMT, Merck, 99%]. CMI (Dequest, PB11625) was added into the solution in varying concentrations. Crystallization reaction was carried out in a triple-neck jacketed vessel with 2 L capacity, connected to a thermostatic oil bath at 95°C. Inlet concentrations of zinc nitrate hexahydrate and HMT were 0.06 M and the concentrations of CMI added were 0, 50, and 450 ppm. Reaction time was 90 min and the particles of CMI added were 0, 50, and 450 ppm. Reaction time was 90 min and the particles were separated from the suspension by filtration at the end of the reaction, followed by drying.

Synthesized ZnO particles were subjected to ultrasonic dispersion due to particle agglomeration using an ultrasonic horn (Sonics, Vibracell) of 13-mm diameter at 20 kHz frequency and 40% amplitude. Ultrasonication time was 60 min and was carried out under nitrogen cooling.

Scanning electron microscopy (SEM) images of the particles were taken using Zeiss EVO-LS at an accelerating voltage of 10.0 kV. Samples were coated by Au–Pd prior to the examination.

Fourier transform infrared spectroscopy (FTIR) analysis was performed using Bruker Optics Alpha-P, in the 400–4000 cm⁻¹ region and at a resolution of 4 cm⁻¹. Spectra were obtained by collecting the average of at least 20 scans.

Incorporation of ZnO crystals into PHBV matrix

ZnO crystals synthesized with different concentrations of CMI (0, 50, and 450 ppm) were incorporated into PHBV (AdMajoris Company, Maj’Eco FN000HA) matrix by melt-extrusion method. A twin-screw extruder (Rondol, 10 mm Microlab, L/D=20) was used with a temperature profile of 90-135-160-160-150°C from feed to die and a screw speed of 80 rpm. PHBV/ZnO composite pellets obtained after extrusion were compression molded into sheets of 0.8 mm thickness using a mechanical press operated at 165°C temperature and 30, 70, and 90 bar consecutive pressures.

Characterization of PHBV/ZnO composites

SEM images of PHBV/ZnO composites were taken by Philips XL30 ESEM-FEG at an accelerating voltage of 5.0 kV. Samples were coated with Au prior to the examination.

Melting and crystallization behavior of PHBV samples was analyzed by differential scanning calorimetry (DSC). Samples of 10 mg mass were analyzed using Perkin Elmer, Pyris 1 under a nitrogen atmosphere. Samples were heated from 0°C to 190°C at a heating rate of 10°C/min, and kept at this temperature for 2 min to erase thermal history. Then the samples were cooled to 0°C at a cooling rate of 10°C/min. Melting and crystallization temperatures and enthalpies were calculated from the heating and cooling curves. The degree of crystallinity (X_c) was calculated from the equation; X_c, %=ΔH_m/ΔH⁰*100/w, where ΔH⁰ is the melting enthalpy of 100% crystalline PHBV (146 J/g [20]) and w is the weight fraction of PHBV.

Thermogravimetry analysis (TGA) was performed to evaluate the thermal stability of PHBV samples. Perkin Elmer, Pyris Diamond thermal analysis instrument was used. Samples of 10 mg mass were heated from room temperature to 550°C with a heating rate of 10° min and under nitrogen flow. Peak degradation temperature (T_max) values were determined from the derivative TG curves, and initial degradation temperature (T_i) was calculated as the temperature corresponding to 2% of weight loss.

Mechanical properties of the samples were analyzed by uniaxial tensile testing according to ASTM D882-12 standard. Instron 5982 universal testing machine was used with an initial load of 1 N and the cross-head speed was 5 mm/min.

Formation of ZnO crystals and the effect of CMI

In absence of CMI, formation reaction of ZnO precipitate from the aqueous solution of zinc nitrate hexahydrate and HMT took place according to the following reaction steps [12]:

Rod-shaped, hexagonal prismatic ZnO crystals were obtained at the end of the crystallization reaction (Fig. 1a). When CMI was added to the reaction, the growth of crystals in the axis of length was limited, whereas crystal growth in the lateral face was not suppressed. The addition of CMI resulted in the production of shorter and thicker crystals, and this effect was more pronounced as the concentration of CMI was increased (Fig. 1b and c). Formation of ZnO rod arrays connecting to each other at the center was observed for the particles synthesized in absence of CMI and with 50 ppm of CMI addition. This behavior was previously explained by a self-aggregation process occurring at high reaction temperatures [12]. When CMI concentration was increased to 450 ppm, the aggregated asterisk-like structures were no longer observed, and singular rod particles were obtained. It is well known that high surface energy of small particles lead to particle agglomeration and this effect might have been hindered for the higher concentration of CMI due to the increased diameter of particles.

Fig. 1:

SEM images of ZnO particles synthesized (a) without CMI addition, and (b) with 50 ppm and (c) with 450 ppm CMI addition.

Particle agglomeration is an undesirable phenomenon in the fabrication of polymer composites since it affects polymer properties negatively. Ultrasonic dispersion was applied to the particles prior to inclusion into the polymer matrix since severe agglomeration was observed for the particles synthesized with no and 50 ppm CMI addition. Particles synthesized in presence of 450 ppm CMI were also subjected to ultrasound for comparison purpose. SEM images of particles after 60 min of ultrasound application are given in Fig. 2. Ultrasound application was effective in breakage of aggregated array structures and singular, separated rod particles were obtained. For the particles synthesized with 450 ppm CMI addition, small crystal imperfections observed on the surface of the crystals prior to ultrasonication were ruptured and segregated by ultrasonic dispersion.

Fig. 2:

SEM images of ZnO particles after ultrasound application (a) without CMI addition, and (b) with 50 ppm and (c) with 450 ppm CMI addition.

Mean length and diameter values of the particles were calculated after deagglomeration by ultrasound application. Statistical analysis was performed on SEM images taking at least 100 particles into consideration, and the mean values of crystal length, diameter and aspect ratio (L/D) are given in Table 1. D is defined as the highest distance between opposite corners across the hexagonal face, measured at the thickest part of the crystals; and L is the extension normal to the hexagonal face. Standard deviation values calculated for the mean length and diameter were relatively high due to the breakage of ZnO rods by ultrasound application, although more monodisperse particles were synthesized by crystallization reaction before the application of ultrasound. Increasing CMI concentration clearly produced shorter and thicker crystals.

FTIR analysis was applied to evaluate surface chemistry of the particles. The peak observed around 400 cm⁻¹ is the characteristic peak related to Zn-O stretching (Fig. 3). The peaks observed at 1325–1590 cm⁻¹ range were attributed to the carbonyl group in CMI molecule [21]. These peaks vaguely appeared on the spectrum of ZnO(50) sample, whereas were clearly observed on the spectrum of ZnO(450). The appearance of these peaks suggested CMI molecules were either chemically bonded or physically adsorbed on ZnO crystals.

Fig. 3:

FTIR spectra of ZnO particles synthesized with varying CMI concentration.

Fabrication and characterization of PHBV/ZnO composites

Ultrasound dispersed ZnO, ZnO(50), and ZnO(450) crystals were incorporated into PHBV matrix by melt-extrusion. Particle content of the PHBV composites was selected as 1% (w/w) since homogeneous particle dispersion within the matrix is easier for lower particle loadings. SEM images of the composites fabricated are given in Fig. 4, indicating considerably good particle dispersion for all samples.

Fig. 4:

SEM images of the composites (a) PHBV/ZnO, (b) PHBV/ZnO(50), and (c) PHBV/ZnO(450).

PHBV samples displayed bimodal melting behavior in DSC heating scan (Fig. 5a). Two-peak melting curves are common for PHBV samples [22] and are explained by a melting-crystallization-remelting behavior [23]. The appearance of the first melting peak (T_m1) was around 174°C and the second melting peak (T_m2) was around 181°C for all samples (Table 2), suggesting main crystal structure and lamellar size was not affected by ZnO addition. However, total enthalpy of fusion (ΔH_m) and thus X_c were decreased in composite samples compared to neat PHBV. ZnO particles had a retarding effect on nucleation, decreasing the degree of crystallinity. The intensity of second melting peaks was increased compared to neat PHBV, probably due to the increased mobility of polymer chains and recrystallization of more crystals into more stable crystals. This effect was more pronounced for the PHBV/ZnO and PHBV/ZnO(50) composites. PHBV and its composites exhibited similar cooling scan curves (Fig. 5b), with a crystallization temperature (T_c) around 121°C. The decrease in crystallization enthalpy (ΔH_c) with ZnO addition also suggested a decrease in the degree of crystallinity of PHBV.

Fig. 5:

DSC thermograms of PHBV and its composites: (a) heating and (b) cooling scans.

Fig. 6:

Tensile properties of PHBV samples: (a) tensile strength and modulus, and (b) toughness and elongation at break.

ZnO particles were previously reported to accelerate crystallization of PHB (polyhydroxybutyrate) [24] and PHBV [25] due to heterogeneous nucleation effect. On the other hand, another study [26] reported the retarding effect of ZnO particles on crystallization of electrospun PHBV nanofibers, decreasing crystallinity while showing no effect on the value of T_m. This behavior was explained by the formation of hydrogen bonds between the ZnO particles and PHBV, disturbing the mobility of PHBV chains and decreasing crystallinity.

TGA analysis revealed that addition of ZnO particles slightly decreased the degradation temperatures T_i and T_max compared to neat PHBV sample (Table 2). Fortunately, the T_max values obtained were considerably above the T_m values, only slightly narrowing the processing window of the polymer. considerably above the T_m values, only slightly narrowing the processing window of the polymer. Decrease in thermal stability was previously reported for incorporation of cellulose nanowhiskers [27] and graphene [28] into PHBV matrix. The increase of thermal conductivity of the matrix due to the addition of particles was reported to have a negative effect on thermal degradation.

Tensile strength, Young’s modulus, and elongation at break values were calculated for PHBV and its composites from the stress-strain curves obtained from tensile testing (Figure 6). ZnO particles might have contributed to effective stress transfer across the polymer matrix, since tensile strength was observed to increase with ZnO addition into PHBV. Young’s modulus values were not affected significantly. Toughness values were calculated as the area underneath the stress-strain curves. The addition of ZnO particles into PHBV matrix significantly increased elongation at break and toughness of the polymer. These results were in correspondence with DSC findings, since lower crystallinity is known to result in higher ductility. The increase in ductility is important since one of the major drawbacks of PHBV is its brittle nature. Increase in elongation and toughness were higher for PHBV/ZnO and PHBV/ZnO(50) samples, compared to PHBV/ZnO(450) sample. Aspect ratio (L/D) of particles is known to be an effective parameter in ductility, as well as crystallinity, affecting the shear stress at the filler-matrix interface. This could be the reason for higher toughness values obtained for composites fabricated with ZnO particles with higher L/D values.