XRD Studies of the Morphological Changes Generated by Interface Agents for Obtaining New Scalable Sustainable Blends Based on Starch and PCL ^†

Abstract

The starch–PCL blends are important from a practical point of view. Due to the hydrophobic nature of PCL, its blends with starch are immiscible. Therefore, these blends must be compatibilized to control the interface’s properties and the thermodynamic stability over time. A physical compatibilization solution, easier to scale up in industrial conditions, was attempted. The results showed better compatibility between the two tested polymers because of the grafted olefin polymer that eliminates the net difference between the in-contact phases, by creating an interface area, strengthening the multicomponent system and reducing the morphological defects as voids.

1. Introduction

The limitation of oil, natural gas and coal resources as well as the climate issues that society is currently facing have caused new insurance strategies with polymer materials for the perspective of 2050 [1,2,3,4] to consider, on the one hand, materials based on renewable origin polymers including their blends with conventional polymer for sustainable applications [5,6,7,8,9], as a possibility of reducing environmental pollution and saving conventional resources [10,11,12] and, on the other hand, as general strategic direction of the mechanical recycling of all multiphase known polymeric compounds.

Among the shortcomings related to the use of renewable origin polymers are those related to their low mechanical properties and durability and the rapid loss of these properties as a result of characteristic high hydrolytic instability. That is why, nowadays, the scientific community is concerned with the development of new durable materials based on renewable origin polymers, especially considering the multicomponent compounds [13,14].

Abundant in the environment, natural polymers, such as starch or cellulose, represent an interesting category of raw materials useful for various applications that require biodegradable properties (such as flexible packaging [15,16], mulching films for agriculture [17], etc.). However, these polymers also present mentioned disadvantages, mainly incompatibility with many other polymers, poor functional properties and processability through melt-processing techniques into finished product due to the high melt viscosity in the melt.

In order to develop new starch-based polymeric materials of practical interest, improving its functional properties can be achieved through physical modification, a research possibility already with important application results, some of them even industrially scaled, such as the modification with polymers as: synthetic hydrophobic, non-degradable or hydrophilic, degradable [18,19,20], polyesters or renewable grades (cellulose, natural rubber, etc.) [21,22].

Poly(3-caprolactone) (PCL) is a linear, semi-crystalline, biodegradable, hydrophobic, aliphatic polyester with a glass transition temperature of ~−60 °C, low melting temperature (58–60 °C), easy to be melt-processed [23,24], used also to modify polymers, including renewable ones, such as starch [25,26,27,28,29].

Since PCL is a hydrophobic polymer and starch is a hydrophilic one, their blends are not miscible and therefore present undesirable phase separation [30,31]. In the starch–PCL blends, the mechanical properties become weak with the increase of the starch content [32]. To increase the compatibility between the two polymers, an interface agent can be efficient so that the process can be easily applied at a scaled level. For these blends, many compatibilization solutions are known, but all of them are difficult to apply in scalable condition [33,34,35,36,37,38].

The aim of the paper was to study the compatibilization efficiency of a grafted polyolefin for the starch–PCL compounds by studying the morphological structure.

2. Materials and Methods

2.1. Materials

The new compounds were obtained using the polymers listed below:

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Corn starch (A): powder, amylase/amylopectin ratio of 30/70, T_g of 67 °C, 0.6 g/cm³ density (at 25 °C) (M.K. Group D.O.O.) (Richest Group, Shanghai, China);

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Poly epsilon caprolactone (PCL): Molecular weight (Mw) of 85,000–105,000 g * mol, 3–6 melt flow index (MFI) g/10 min (160 °C, 2.16 kg) (Richest Group, Shanghai);

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Maleinized polyethylene (PE-g-MA): 0.95 g/cm³ density, 1.8% maleinization degree (Lyondell Basell, Rotterdam, The Netherlands).

2.2. Blend Preparation Procedure

The new blends were prepared considering a classic Brabender (Bbd)-roller procedure for melt compounding and usual conditions (T Bbd: 170–190 °C, t = 3 min after gelation, T roller = 40–80 °C, friction ratio of 1:2). The S/PCL/C compounding ratio was between from the following, 100/0/0–40/40/20–0/0/100, and the new obtained materials were characterized on roller sheets.

2.3. Characterization

The characterization was performed using methods that highlight the morpho-structural and functional properties of the new materials, considering a binary blend with a composition of 1/1 starch–PCL and a ternary blend with 10 [p] compatibilizer at 100 [p] binary blend with 1/1 starch–PCL. The morphological structure characteristics of the new blends comparatively analyzed with those of the used polymers obtained were as follows:

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X-ray Diffraction (XRD): Rigaku-Smart Lab diffractometer (Rigaku, Tokyo, Japan), acceleration voltage 45 kV and current intensity 200 mA, incident radiation CuKα1 at wavelength 1.54059 Å, in the beam configuration parallel, continuously for 2θ values between 5 and 60°, with a resolution of 0.02° and a scanning speed of 4°/min, PDXL 2.7.2.0 software.

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Morphological fracture analysis: breaking in liquid nitrogen; 60 s time, on a high-resolution scanning electron microscope, SEM-QUANTA Inspect F50 (FEI, Thermo Fisher Scientific, Hillsboro, OR, USA).

3. Results and Discussions

3.1. XRD Morphology of the Individual Polymers and Those of the New Blends

The diffractograms of the individual polymers and of the new mixtures can be found in Figure 1a, Figure 2a, Figure 3a, Figure 4a and Figure 5a, and those of the data obtained by processing these diffractograms are displayed in Figure 1b, Figure 2b, Figure 3b, Figure 4b and Figure 5b and

Table 1. The intensity of the specific diffractions of the basic polymers.

Starch		PCL		PE-g_MA
Diffraction Angle 2-Theta, deg	Intensity, cps	Diffraction Angle, 2-Theta, deg	Intensity, cps	Diffraction Angle, 2-Theta, deg	Intensity, cps
15.134	3172	15.57	1293	19.254	6798
17.051	3430	19.188	3905	20.164	20,125
18.09	3270	20.68	13,721	21.465	79,673
20.06	628	21.37	70,882	23.748	16,521
22.98	31.098	22.006	10,108	39.73	1080
30.64	555	23.69	22,846	46.87	862
32.96	468	29.76	1041
34.44	271
47.64	256

,

Table 2. The intensity of the specific diffractions of the binary blend.

Diffraction Angle, 2-Theta, deg	Intensity, cps
19.254	6798
20.164	20,125
21.465	79,673
23.748	16,521
39.73	1080
46.87	862

and

Table 3. The intensity of the specific diffractions of the ternary blend.

Diffraction Angle, 2-Theta deg	Intensity, cps
9.43	1159
15.13	1490
16.989	1572.53
17.956	1169.08
19.199	413.9
21.202	7049
21.384	29,472
23.627	10,640

. From these figures, it can be noticed that the primary polymers have the following diffraction peaks: starch 9 diffraction peaks 2θ (15.13 deg; 17.05 deg; 18.09 deg; 20.06 deg; 22.98 deg; 30.64 deg; 32.96 deg; 34.44 deg; 47.64 deg) (Figure 1a,b); PCL-7 diffraction peaks 2θ (15.57 deg; 19.18 deg; 20.62 deg; 21.37 deg; 22.006 deg; 23.69 deg; 29.76 deg) (Figure 2a,b) and Pe-g-MA 7 diffraction peaks 2θ (19.25 deg; 20.16 deg; 21.46 deg; 23.74 deg; 39.73 deg; 46.87 deg; 52.89 deg) (Figure 3a,b). For the studied binary compound, the diffraction peaks were registered at the following 2θ angles (17.030; 18.010; 21.760; 22.020; 23.700; 29.860) (Figure 4a,b). Additionally, for the ternary compound, these 2θ angles are (9.430 deg; 15.130 deg; 16.9890 deg; 17.950 deg; 19.190 deg; 1.220 deg; 21.230 deg; 23.630 deg).

If the diffraction characteristics of the binary blend are comparatively analyzed with those typical to each individual polymer, it can be noticed that the binary blend has only four peaks coming from the nine of those of starch (17.030 deg and 18.010 deg) and one of the seven peaks of PCL (23.700 deg) (Figure 1a,b, Figure 2a,b and Figure 4a,b).

If the intensities of the two peaks from the primary compound that comes from starch and those from PCL are compared with the intensity for the corresponding intensities describing the two individual polymers, it can be observed that an important decrease in intensity in all three cases was registered, namely, by 88% for the starch diffraction intensity from 17.051 deg, with 23% for starch intensity of the diffraction from 18.01 deg and with 27% for the PCL diffraction intensity from 23.70.

The other four diffraction peaks in the diffractogram of the binary blend represent shifted diffractions from PCL peaks, three of them with lower and one with higher intensity. Thus, the diffraction peak from 21.76 deg is the shifted PCL peak from 21.37 deg but lower with 7%. The 22.02 deg diffraction resulted from shifting the PCL diffraction from 22.006 deg but now has a lower intensity with 34%. The diffraction from 29.86 deg represents the shifted peak from starch registered at 30.64 deg, now more intense with 61%. Diffraction from the binary compound from 36.2 deg occurs at an angle that does not exist in the two polymers from this blend.

Summarizing the above, it can be stated that in the binary compound, a new diffraction peak appears, another three are peaks shifted from the two polymers used for obtaining this blend and three are peaks describing these polymers alone. The intensity only for two blend diffractions from the six coming, shifted or unshifted, from the two base polymers are higher than the similar peaks from these polymers. In the binary mixture, the following peaks from the base polymers no longer appear:

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Starch: 15.134 deg, 20.06 deg, 22.98 deg, 32.96 deg, 34.44 deg (five drops);

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PCL: 15.57 deg, 19.188 deg, 20.68 deg, 21.37 deg, 23.69 deg (five drops).

In the ternary S-PCL-PE-g–MA blend, three new peaks appear at 9.43 deg, 16.989 deg and 21.22 deg. The diffraction from 17.95 deg results from shifting the one from 19.051 deg of starch, but now it is 56% less intense. The diffraction from 15.13 deg is identical to the starch, similar to those from the same position, but has a lower intensity with 56%. In the studied ternary blend, there are also three diffraction peaks similar with PCL ones placed in the same angle, but in all cases, with lower intensity than in the case of an individual polymer. The diffraction from 19.15 deg is lower by 11% than the similar ones from PCL from 19,188, that from 21.38 deg is lower by 42% than the similar ones from PCL at 21.37 deg, and the one from 23.62 deg is lower by 47% compared to similar ones from this PCL registered at 23.69 deg. So, in the ternary blend, there are three new diffractions, one shifted and four coming from the basic polymers, one from starch and three from PCL, but in all cases with 10–46% lower intensity than for the individual polymers. Six of the eight diffractions describing starch no longer appear in the ternary mixture (namely, those at 15.134 deg, 18.09 deg, 20.06 deg, 30.64 deg, 32.96 deg, 34.44 deg) From the seven diffractions of PCL, four of them no longer appear in the ternary blend (namely, those from 15.57 deg, 20.68 deg, 22,006 deg, 29.76 deg). Any PE-g-MA diffraction appears in the spectrum of the studied ternary mixture.

From the comparative analysis of the diffractograms of the mixtures with and without the interface agent, it is found that its presence determines:

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The appearance of three new diffractions compared to one in the case of the good mixture.

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The existence of a single diffraction shifted compared to three for the binary mixture.

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The appearance of only one unmodified starch-describing diffraction compared to two for the binary mixture.

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Greater decreases in the intensity of diffractions with values in the interval 10–56% compared to 7–36% for the binary mixture.

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Disappearance of six of the eight diffractions of starch compared to five in the case of the mixture without interface agent.

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None of the diffractions of the interface agent can be identified in the diffractogram of the ternary mixture.

3.2. SEM

SEM micrographs are shown in Figure 6, Figure 7 and Figure 8. Their analysis prove that the presence of the interface agent creates a transition zone between different phases from the system instead of clear boundaries. These zones often present discontinuities generated by the branched amylopectin chains that cannot be oriented into the flow direction due to its branched chemical structure. In addition, no defects of morphological structure such as voids can be noticed in the ternary blends. These results prove that between the three components of the ternary blends, secondary valences bonds were established which led to the formation of the mentioned interface area. These results can be the basis for the development of a scalable procedure to manufacture new biodegradable compounds based on starch.

4. Conclusions

The starch–PCL blends are interesting from a practical point of view due to their biodegradability, which generates the reduction in the use of petroleum resources and environmental protection through the development of new polymeric materials for various applications.

Due to the hydrophobic nature of PCL, its blends with starch are immiscible and therefore they must be compatibilized to control the interface’s properties and thus the thermodynamic stability over time. A physical compatibilization solution was attempted, for easier applicability into industrial conditions.

Better compatibility between the two polymers was obtained. The grafted olefinic polymer used eliminates the net difference between the in-contact phases by creating an interface area with morphological and structural defects.

Better compatibility efficiency was demonstrated for the maleinized olefinic polymers. The improved compatibility generated the lack of morphological defects (i.e., voids) and the disappearance of some of the diffraction peaks characterizing the individual components.