Biodegradable Compounds of Poly (Ɛ-Caprolactone)/Montmorillonite Clays

Biodegradable Poly (ɛ-caprolactone) (PCL) was compounded with Brasgel PA (MMT) and Cloisite Na (CLNa), and compatibilized with maleic anhydride grafted poly(ɛ-caprolactone) (PCL-g-MA). No evidence of degradation was verified during processing, as well as particle aggregates were absent as displayed through scanning electron microscopy (SEM) images. Compatibilized compounds upon clay addition achieved higher thermal stability as visualized by thermogravimetry (TG). Montmorillonite clays acted as nucleant agent during PCL melt crystallization as evidenced by differential scanning calorimetry (DSC). Mechanical behavior of PCL was preserved, and all compounds presented Elongation higher than 350% meanwhile the heat distortion temperature (HDT) was increased by 20% in PCL compounds, which may have wider range of application and better performance.


Introduction
Nowadays, society is concerned in how to preserve the environment wellbeing, scientists and researchers have focused their attention on finding solutions to the large amounts of waste that are daily produced, mainly, due to the use of quickly discarded products. In this sense, sustainable methods have been developed to stimulate "green technologies", that is, development of materials that favor the closed life cycle, producing biodegradable waste without damaging to ecosystems. 1,2 Biodegradable polymers can be decomposed in bioactive environments through microorganisms' action. There are several advantages in using these polymers, such as increased soil fertility, lower accumulation of plastic products in landfills and lower costs in waste management, for instance. 3,4 In this work poly (ɛ-caprolactone) (PCL) compounds were investigated; PCL is a linear, biodegradable and biocompatible aliphatic polyester commonly used in pharmaceuticals and dressings for wounds. 5,6 PCL is synthesized through ε-caprolactone ring-opening polymerization or by free-radical polymerization of 2-methylene-1,3-dioxepane. [7][8][9] Biodegradable polymers can be used in blends, composites and nanocomposites. However, for an effective compatibilization between components, the best strategy used has been the incorporation of graft copolymers as compatibilizers in original incompatible systems. Methods used to obtain these compatibilizers include chemical modification of non-polar polymers with monomers by reactive processing, such as maleic anhydride (MA). 10,11 Grafting with maleic anhydride is intended to achieve compatibility between polymers in blends or between filler and polymer in composites or nanocomposites. Reactions are usually performed in presence of organic peroxides and can take place in solid, melt or solution states. The reaction in the melt, by reactive processing, has been the most used method due to lower cost and easy operation. 12,13 Continuous or discontinuous mixing equipment can be used as a reactive chemical reactor, maleic anhydride grafting reactions with peroxides were performed on both torque rheometers and single and double screw extruders. During reactive extrusion, polymer modification reactions and processing take place simultaneously. 9,12 Adding clays in polymer compounds has been a common practice due to the improvement of several properties, such as permeability and thermal resistance. Single characteristics presented by mineral clays as: lamellar or fibrous structures, ion exchange capacity, wide range of chemical composition as well as electric charge of lamellae; provide special physicochemical behaviors that determine their surface properties and interactions in organic substances, such as polymers. 14,15 This work aims at the development of biodegradable compounds based on PCL produced in an internal mixer, maleic anhydride grafted poly(ɛ-caprolactone) (PCL-g-MA) is added as compatibilizer, and montmorillonite clays Brasgel PA (MMT) and Cloisite Na (CL Na) as fillers.

Grafting MA groups onto PCL
This work was based on the studies of Siqueira et al. 16 , where the selected parameters were those that presented the best conditions for processing and functionalization of poly (ɛ-caprolactone) with maleic anhydride (MA). Functionalization was performed in a Thermocientific Polylab QC torque rheometer, operating at 160°C, 60 rpm for 10 min. The functionalized copolymer was synthesized at 1.0% content of initiator dycumil peroxide (DCP) and 5.0% of maleic anhydride.

Purification of Functionalized Polymer
Modified poly (ε-caprolactone) was purified for taking out residual monomers and other possible by-products. Part of grafted polymer was dissolved in xylene at 120°C under stirring and refluxed for 3-4 h. The solution was filtered and precipitated. After this procedure, the material was dried under vacuum for 24 h at 40°C. This procedure also was used by Siqueira et al. 16 .
The anhydride content in the grafted polymer was determined by titration of the acid groups derived from the anhydride functions using the procedure described by John et al. 17 and used by Siqueira et al. 16 , where the extracted sample was refluxed for 1 h in xylene saturated with water. The solution was titrated with 0.05 N ethanoic KOH using 1% drops of thymol blue in dimethylformamide as an indicator. An excess of KOH was added and the dark blue color was titrated through a yellow end point by addition of 0.05N isopropanic hydrochloric acid. PCL-g-MA was completely soluble in water-saturated xylene and did not precipitate during titration.

Compounding in the Internal Mixer
PCL/MMT, PCL/PCL-g-MA/MMT, PCL/CLNa and PCL/PCL-g-MA/CLNa systems were compounded in a Thermocientific Polylab QC torque rheometer in contents as described in Table 1. Previously, compounds were cold mixed and then fed into the mixer chamber. Compounds were processed at 160°C, 60 rpm for 10 min.

Extrusion
Initially, a concentrate of PCL/MMT and PCL/CLNa was processed using a high-speed homogenizer MH-50H from MH. Compounds with 1:1 PCL/Clay were homogenized. Afterwards the concentrates were crushed in a knife mill.
PCL and concentrates were processed in a Coperion (Werner-Pfleiderer ZSK 18) double screw extruder (Wesseling, Rhein-Erft-Kreis, Germany) under processing parameters presented in Table 2. The polymer/clay concentrates were incorporated into PCL, in nominal content of 3% by weight of clay, under the same processing parameters applied for neat PCL.
With the extrudate, injected specimens were produced according to ASTM D638 (Type I), D256 and D648, for tensile, impact and HDT tests, respectively. Specimens were made using an Arburg Allrounder 270C Golden Edition, operating at 110°C, with mold at 20°C. An average of 10 specimens was used for each investigated composition.

Characterizations
For torque rheometry analyses a Rheocord 600 coupled to a Haake System 90 was used, with roller type rotors operating at 60 rpm and 160 °C, under an air for 10 minutes. Analyses were done in duplicates.
The X ray diffraction (XRD) experiments carried out in a Bruker D2Phaser, using copper Kα radiation, voltage of 40 kV, current of 30 mA, scanning from 2 to 30 o and scanning speed of 0.2 o /min.
Fourier-transform infrared spectroscopy (FTIR) was performed in a Bruker -Vertex 70 Spectrometer, with scans from 4000 to 450 cm -1 . Experiments were executed using impact specimens.
Thermogravimetric (TG) analyzes were performed in a TGA 51H Shimadzu, using 5 ± 0.5 mg of sample. The heating rate used was 10°C/min, from room temperature (23°C) to 500°C, under inert nitrogen atmosphere, at gas flow rate of 50 mL/min. Differential scanning calorimetry (DSC) analyzes were performed in a Shimadzu DSC-50, samples were heated from room temperature (23 o C) to 150 o C, and then cooled to room temperature at a heating/cooling rate of 10 o C / min, under nitrogen atmosphere with gas flow rate of 50 mL/min.
The tensile tests carried out according to ASTM D 638, Elastic Modulus, Tensile Strength and Elongation were measured. The tests were performed in a universal EMIC model DL10000, using a load cell of 200 kgf, with deformation of 50 mm/min, operating at room temperature. Presented results are an average of 10 tests.
The IZOD impact strength tests were performed on notched specimens, using a Resil 5.5 from Ceast and a pendulum of 2.75 J, according to ASTM D 256, at room temperature. Presented results are an average of 10 tests.
Heat deflection temperature (HDT) was obtained according to ASTM D 648, in a Ceast HDT 6 VICAT/N 6921.000,  Torque rheometry was used to investigate the thermal stability of PCL compounds during processing. As torque is directly proportional to viscosity, under constant processing parameters, i.e., temperature and rotor speed, torque results can be understood as an indirect measure of molecular weight. At constant temperature torque dropping means decreasing in molecular weight, suggesting that compounds degradation occurred during processing. A constant torque plateau indicates absence of degradation as observed in Figure 1a. 18 It can be observed in Table 3 that neat PCL has an average torque of 2.90 N.m. In general, upon clay addition, compounds viscosity increases, as expected, due to the interaction among clay lamellae and entangled macromolecular chains. Thus, indicating there was an increase in friction or shear rate during compounding. 19   Brito et al. 20 investigated the rheological behavior of PE/ PE-g-MA/Bentonite clay by torque rheometry, as PE-g-MA has a polar structure, interaction evidences between clay and PE were verified, resulting in higher torque values. In the present work, as PCL already presents a polar structure Gorrassi et al. 21 , Wu and Liao, 22 suggest upon clay addition, its most like interactions take place even in the absence of PCL-g-MA, providing increased torque of both compositions, i.e., compounds with MMT and CLNa.

Torque Rheometry
Subtle differences between viscosity compounds with MMT and CLNa are most due from exchangeable cation linkages. In this case, CLNa would have higher bond energy than MMT. [23][24][25] Meanwhile, compatibilizer addition to PCL/clay compounds slightly reduced the torque, probably due to the compatibilizer acting as a solvent or plasticizer, thus decreasing the viscosity. Figure 2 shows X-ray diffractograms of MMT and CLNa, and Figure 3 presents X-ray diffractograms of PCL compounds.

X-ray diffraction (XRD)
addition, can be observed PCL crystalline character did not change after addition of MMT and CLNa.
which refer to the segments of chains that bend in an orderly manner generating crystalline palms at (110) Diffractograms in Figure 3 illustrate that all compounds presented two characteristic peaks associated to PCL, at 2θ = 21.6° and 2θ = 23.7°, which are due to chain segments ordering bending providing crystalline plans at (110) e (200). The peak at 2θ = 22.0° is believed to be characteristic of the compatibilizer maleic anhydride (MA), as also reported by Vertuccio et al. 31

Spectroscopy in the Infrared Region with
Fourier Transform (FTIR) Figure 4 presents FTIR spectra of MMT and CLNa clays, and PCL compounds. As expected, bands attributed to montmorillonite (the main mineral clay in bentonites) are observed in MMT and CLNa spectra in the region between 1004 -1045cm -1 characteristic of Si-O bonds, around 916 and 514cm -1 corresponding to the octahedral layers of aluminosilicate Si-O-Al, and around 3600cm -1 due to the structural stretching vibrations of OH (hydroxyl) group. 34  XRD is a powerful technique to observe the degree of clay dispersion as well as the disorder degree of clay's crystalline structure in polymeric compounds, in this present work in PCL compounds. Generally intense reflections in the range of 2θ = 3-10° indicate an intercalated ordered system with alternating layers of polymer/silicate. On the other hand, when exfoliation is achieved, i.e. when individual silicate layers (1 nm thick) are homogeneously dispersed in the matrix, the XRD diffractograms do not show peaks due to loss of clay's structural identity. 24 In diffractograms of Figure 3 can be verified that no composition presented peaks due to MMT and CLNa clays. This can be an indicative of an exfoliated structure, that is, disordering is suggested in the clay's stacking layers, increasing their basal spacing between the lamellae, thus making it difficult to evaluate the diffraction angle 2θ. 30 In Concerning PCL spectra, in general, there is no significant change in the compounds relative to neat PCL. It is also believed PCL peaks overlap the clays ones once they were not observed in the spectra.
In Figure 4 are observed bands in the range between 2943 and 2865 cm -1 , which are attributed to asymmetric and symmetrical stretch of CH 2 ; in 1720 cm -1 peaks are referred to carbonyl stretch (C=O) of PCL's ester group; bands in 1470 and 1366 cm -1 represent the stretching of CH groups in CH 2 ; bands in 1241, 1170 and 1294 cm -1 refer to the elongation in the crystalline phase of C-O and C-C linkages; in 1240 cm -1 asymmetric stretching of C-O-C; in 1170 cm -1 symmetrical stretching of C-O-C and in 1156 cm -1 elongation of C-O and C-C linkages of PCL's amorphous phase. 35

Thermogravimetry (TG)
Thermal stability of PCL compounds was investigated using thermogravimetry, Figure 5 displays TG plots. It is observed weight loss takes place in a single step for all PCL compounds, for neat PCL it starts ~ 280°C whereas compounds with clay and PCL-g-MA have an onset temperature ~ 350°C. Upon addition of MMT and CLNa thermal stability of PCL was significantly increased, which may be provided to a likely protective barrier effect of the clay on the polymer, improving greatly its thermal resistance 36 . Regarding the compatibilizer PCL-g-MA effect, it subtly decreased the weight loss onset temperature; this was most due its action as a solvent/plasticizer between PCL/Clays. 37

Mechanical properties
Mechanical properties of PCL compounds are presented in Table 4, regarding the Elastic Modulus an increase of 34.19% and 22.57% was observed for PCL/MMT and PCL/ CLNa, respectively, upon compatibilizer PCL-g-MA addition it increased by 18.31% and 20.69% in PCL/PCL-g-MA/ MMT and PCL/PCL-g-MA/CLNa. No significant effect was verified in Tensile Strength, whereas subtle decreases were observed in Maximum Elongation.
In general, polymer/clay systems with low nano-clay content (<5%) often have higher mechanical properties when compared to the properties of neat polymers. The main reason for this higher performance is the strongest interfacial interaction between the matrix and the silicate layers compared to the reinforced systems with conventional loading. 20,38 In principle, these results presented in this work are encouraging, since the use of nano-clays MMT and CLNa as lower cost filler was added in the PCL, providing an increase in thermal stability (TG) and Elastic Modulus without damaging the Tensile Strength.
Related to HDT an increase of ~ 25% was observed in PCL compounds, which can be resulted from chemical interactions and well dispersed fillers in PCL matrix (see SEM images) due to successful processing. In general, adding nano-fillers in polymeric matrices increases their brittle character; an interesting result obtained in this work was the increase in HDT values, with no significant change in the Maximum Elongation whereas all compounds presented values higher than 350%.
Regarding Impact Strength all the compounds had lower values than neat PCL, with higher decrease in MMT systems,

Differential Scanning Calorimetry (DSC)
DSC scans acquired during cooling, i.e., during melt crystallization, of PCL compounds are presented in Figure 6a, bell shaped and single exotherms are observable. Upon clay addition the exotherms were displayed in higher temperatures, most likely to a nucleant action of clays, meanwhile the melting endotherms presented in the right side of Figure  6b were not significantly affected being displayed in similar temperature ranges.   39 reported that impact strength is generally reduced upon increasing of maleic anhydride content in PP/montmorillonite clay. PCL, as shown above, is a ductile polymer with higher impact strength, which is a measure of energy dissipation during short-term deformation in solid polymers. It is believed that in PCL/Clay compounds the added particles can act as stress concentrators preventing the proper energy dissipation mechanisms thus reducing the impact strength. [40][41][42] Figure 7 shows SEM images of neat PCL with evidence of extensive roughness characterizing ductile fracture with elastic and plastic deformation, thus providing fracture with high energy absorption, results that agree with those presented for maximum elongation in Table 2. Figures 8 and 9 present SEM images for PCL/CLNa and PCL/MMT, respectively. Clearly, clay particles are observable in PCL; after fracturing, some particles remained adhered to PCL matrix meanwhile others pulled out from it. In general clay particles are well dispersed -as result of successful processing-in PCL promoting higher performance as above mentioned.

Scanning Electron microscopy (SEM)
SEM images of compatibilized compounds are shown in Figures 10 and 11. A particulate structure embedded in PCL matrix is verified. Fibrils are also observed which act as anchor avoiding "pulling out" of clay particles, and this is an indication that there was interaction between matrix and reinforcement, i.e., PCL-g-MA/Clay, they are strong enough to increase the thermal stability as shown in TG thermograms and HDT while keeping good mechanical performance.

Conclusions
PCL compounds upon addition of MMT and CLNa clays, and compatibilized with PCL-g-MA were successful melt extruded in this study, without evidences of degradation and particles aggregates during processing. Thermal resistance of PCL compounds was 20% higher as verified by thermogravimetry and heat distortion temperature, additionally clays acted as nucleant agent accelerating the melt crystallization. Tensile Strength was lightly changed meanwhile Elastic Modulus increased and Elongation was higher than 350% for all investigated compounds. Summing up, better performance of PCL was achieved that can be translated in wider applications and faster processing cycles, i.e. lower cost of processing/products.

Acknowledgement
To CNPq, to MCTI/CNPq, to Bentonit União Nordeste for the supply of clay, to the Polymer Materials Processing Laboratory/CCT/UFCG, to CAPES and CAPES/PNPD.