Abstract
The main purpose of this study was to investigate influences of three parameters on the mechanical and thermal properties of the polylactide (PLA) matrix nanocomposites filled with polyhedral oligomeric silsesquioxane (POSS) particles. For the first parameter of “Filler Content”, nanocomposites with 1, 3, 5, 7 wt% basic POSS structure were compared. For the second parameter of “Functional Group,” basic POSS structure having only nonpolar isobutyl groups were compared with three other functionalized POSS structures; i.e. aminopropylisobutyl-POSS (ap-POSS), propanediolisobutyl-POSS (pd-POSS) and octasilane-POSS (os-POSS). Finally, for the third parameter of “Copolymer Compatibilization,” all specimens were compared before and after their maleic anhydride (MA) grafted copolymer compatibilization. Specimens were produced with twin-screw extruder melt mixing and shaped under compression molding. Various tests and analyses indicated that the optimum filler content for the improved mechanical properties was 1 wt%; while the optimum structure for strength and modulus was pd-POSS structure, in terms of fracture toughness it was basic POSS structure. Additional use of MA compatibilization was especially effective for the basic POSS and os-POSS particles.
Keywords
Introduction
Polylactide (PLA) is today a well-known biopolymer having aliphatic thermoplastic polyester structure polymerized from renewable sources especially from corn starch. In the last decade it has been particularly used for biomedical, agricultural and food packaging applications due to its inherent biodegradable and compostable nature. Today, PLA has been considered for many other engineering applications; in which its mechanical and thermal properties should be improved. In this respect, there is tremendous number of works especially using micro or nanocomposite approach; for example reinforcing PLA structure with montmorillonite1–3 and halloysite4–6 nanoclays, carbon nanotubes7–9 and graphene,7,8,10–12 nanosilica3,13–15 and nanotitania16–19 particles.
From this point of view, Polyhedral Oligomeric Silsesquioxane (POSS) particles could be also considered as an important candidate with their inorganic-organic nanosized cage structures. When incorporated into polymer matrices, the inorganic core of the cage may provide molecular reinforcement while the organic groups may increase compatibility with the polymer matrices. Depending on the application area, the chemistry of POSS can be changed by altering the organic groups attached to the cage corners. In order to prepare polymer nanocomposites, POSS particles can be mixed with the matrix directly, or attached to the macromolecules during polymerization.
Literature survey revealed that PLA/POSS nanocomposites have been produced by different techniques for certain purposes. For instance, some studies,20–23 especially investigated effects of POSS content on the mechanical and thermal properties including crystallinity amount of PLA matrix nanocomposites. They basically indicated that addition of POSS particles in the range of 1–3 wt% could increase certain mechanical properties and crystallinity degree.
Another group of studies in the literature24–29 investigated effects of having different functional groups at the corners of the POSS cage on certain properties of neat or blended PLA matrix nanocomposites. Examples of the functionalized POSS structures used were: aminopropylheptaisobutyl-POSS, aminopropylheptaisooctyl-POSS, aminopropylisobutyl-POSS, trisilanolisobutyl-POSS, glycidylisobutyl-POSS, octavinyl-POSS, epoxycyclohexyl-POSS and trans-cyclohexanediolisobutyl-POSS. In one of these studies, 29 influences of PLA-g-MA copolymer was also investigated. Due to the different chemical interactions between the functional groups of POSS structures and PLA-based matrices, they24–29 generally revealed that there were certain differences in the mechanical properties (elastic modulus, yield strength, tensile strength, toughness, ductility, etc.); thermal behavior (transition temperatures, enthalpies, crystallinity amounts, etc.) and morphology of the nanocomposites.
Literature survey summarized above indicated that there were very limited number of publications on the effects of POSS content, POSS functional groups and maleic anhydride compatibilization on the performance of PLA matrix nanocomposites. Therefore, the main purpose of this study is, as the first time, to contribute PLA/POSS nanocomposites literature by investigating the effects of these three parameters (filler content, functional groups, MA compatibilization) all on the same manuscript. For this purpose, mechanical and thermal properties of the PLA matrix nanocomposites were compared first by reinforcing with 1, 3, 5, 7 wt% basic POSS structure having only isobutyl groups, and then POSS structure having three different functional groups (aminopropyl, propanediol, dimethylsilane), and finally using PLA-g-MA copolymer for each specimen groups.
Experimental
Materials used
PLA matrix material used was L-lactic acid type polylactide (NaturePlast PLE 001, France). According to its data sheet, it melts between 145°C and 155°C and degrades in the range of 240–250°C; its melt flow index range at 190°C under 2.16 kg is 2–8 g/10 min, with a density of 1.25 g/cm 3 . Gel permeation chromatography analysis conducted in this study revealed that its weight average molecular weight was 105 800 g/mol.
Polyhedral oligomeric silsesquioxane (POSS) hybrid nanoparticles having four different functional groups were purchased from Hybrid Plastics Inc. (USA). Details of these functional groups will be given in the second part of the results and discussion section.
During copolymer compatibilization studies maleic anhydride (MA) (Sigma Aldrich, purity 99%) used for grafting of PLA has a molecular weight of 98.06 g/mol, a melting temperature range of 51–56°C, and a boiling temperature of 200°C. The initiator used for MA grafting reaction was dicumyl peroxide (DCP) (Sigma Aldrich, purity 99%) with melting temperature of 39°C.
Production of PLA/POSS nanocomposites
PLA granules were first pre-dried overnight in a vacuum oven at 60°C and then pre-mixed with certain amounts of POSS particles manually. This mixture was melt compounded via Rondol Microlab 300 laboratory size (D = 10 and L/D = 20) twin-screw extruder (Rondol Technology Ltd., UK). Typical temperature profile from feeder to die used was 115°–170°–180°–175°–150°C while the typical screw speed used was 75 rpm.
Before shaping of the test specimens by compression molding; continuous strands coming out from the twin-screw extruder die were cut into 2–3 mm granules by using a four-blade cutter. Then, pellets were again allowed to re-dry for 15 h in a vacuum oven at 60°C. Standard size specimens required for testing and analyses were melt shaped via laboratory scale compression molding (MSE LP-M2SH05, Turkey) at 160°C under 25 kN with 5 minutes of melting and then pressing time.
In the third part of the study, effects of copolymer compatibilization was investigated via MA grafted PLA (PLA-g-MA) copolymer which was produced by using reactive extrusion technique via twin-screw melt mixing of PLA and 2 wt% MA including 0.5 wt% dicumyl peroxide (DCP) as the free radical initiator. By using titration method, the amount of grafted MA on PLA was found as 2.67. Details of these copolymer formation procedures are explained in our former study. 30
Structural and morphological characterization
Fourier transform-infrared (FTIR) spectroscopy was used in order to reveal possible interfacial interactions between PLA, MA and POSS nanoparticles. At least 32 scans were signal-averaged by the attenuated total reflectance (ATR) unit of the IR spectrometer (Bruker ALPHA, Germany) in the wavenumber range of 400 to 4000 cm−1 with a resolution of 4 cm−1. Fracture surface morphology of the PLA/POSS nanocomposite specimens were analyzed under scanning electron microscope (SEM) (FEI Nova Nano 430, USA).
Mechanical tests and thermal analysis
In order to determine mechanical properties of the PLA/POSS nanocomposites; tension tests were applied according to ISO 527-2 standard while three-point-bending flexural tests were carried out according to ISO 178 standard. These tests were performed under 5 kN universal testing system (Instron 5565A, USA). Fracture toughness tests were also carried out to determine the KIC and GIC values of the nanocomposites by using single-edge-notched-bending specimens according to ISO 13586 standard again under Instron 5565A system. Notching device (Ceast Notchvis Instron, USA) was used to form the notches and pre-cracks on these specimen edges as described in the standard. All these mechanical tests were repeated five times for each specimen group, and the average values including their standard deviations were determined.
In order to investigate thermal behavior of all PLA/POSS nanocomposite specimens; first differential scanning calorimetry analyses (DSC) (SII X-DSC 700 Exstar, Japan) were used to determine the important transition temperatures and enthalpies of melting and crystallization of the samples during a heating profile from −80° to 220°C at a rate of 10°C/min under nitrogen flow. Then, thermogravimetric analyses (TGA) (SII TG/DTA 7300 Exstar, Japan) were conducted to determine the thermal degradation temperatures of the specimens under a heating rate of 10°C/min from 30° to 550°C under nitrogen flow.
Results and discussion
As will be discussed in detail below, behavior of PLA/POSS nanocomposites in terms of morphological, mechanical and thermal properties were investigated by revealing the influences of three different parameters. The first one was the effects of using different POSS contents, the second one was effects of having different functional groups on the POSS structure, and the third one was effects of using maleic anhydride grafted copolymer compatibilization.
Effects of POSS content
In this first part, the POSS structure used had only isobutyl (a rather non-polar organic group) attached to each eight corner of the inorganic cage. Effects of the filler content was studied by reinforcing the PLA matrix with 1, 3, 5 and 7 wt% POSS nanoparticles. These specimens were designated by using the format of PLA/POSS x, where x denotes the amount of the nanoparticles used.
Since distribution and agglomeration level of the nanoparticles in the matrix has significant influences on the mechanical and other properties of the nanocomposites, SEM studies were conducted on the fracture surface of the fracture toughness test specimens of all compositions. SEM images taken at a magnification of 40000× given in Figure 1 show that lower POSS contents, i.e. 1 and 3 wt%, resulted in rather uniform distribution with lower degree of agglomeration in PLA matrix. For instance, for the 1 wt% POSS content, the average size range of the agglomerates were not more than 100 nm. On the other hand, the level of agglomeration for the higher POSS contents i.e. 5 and 7 wt%, were much larger.
SEM fractographs showing effects of POSS content on the distribution and agglomeration level of particles in PLA matrix.
Effects of POSS content on the mechanical properties were first evaluated by conducting tension tests and three-point-bending tests to determine the influences on the strength and elastic modulus values of PLA. While “tensile stress-strain” curves and “flexural stress-strain” curves of specimens are given separately in Figure 2, the values of “Tensile Strength (σTS)” and “Tensile Modulus (E)” determined by tension tests; and the values of “Flexural Strength (σFlex)” and “Flexural Modulus (EFlex)” determined by bending tests are all tabulated in Table 1. Moreover, influences of increasing POSS content on the strength, elastic modulus and fracture toughness values of PLA/POSS nanocomposites were compared in Figure 3.
Stress–strain curves of the neat PLA and its nanocomposites with different POSS contents obtained during tensile and three-point-bending flexural tests.
Tensile strength (σTS), flexural strength (σFlex), tensile modulus (E), flexural modulus (EFlex) and fracture toughness (KIC and GIC) values of the all specimen groups.
Effects of POSS content on the strength (σTS and σFlex), modulus (E and EFlex) and fracture toughness (KIC and GIC) values of the specimens.
Due to the basic strengthening and stiffening mechanisms of “load transfer from the matrix to the reinforcement” and “decreased mobility of the macromolecular chains of matrix,” Table 1, Figures 2 and 3 simply show that use of 1 wt% POSS content could improve strength and modulus values of PLA matrix slightly; which was for instance around 3% in Flexural Strength (σFlex) and 6% in Flexural Modulus (EFlex). However, beyond 1 wt% POSS content, due to the higher degree of agglomeration, the effectiveness of the basic strengthening and stiffening mechanisms started to decrease gradually.
Effects of POSS content on the fracture toughness, i.e. ability of the materials to withstand crack initiation and propagation, values of the specimens were evaluated in terms of both “Critical Stress Intensity Factor (KIC)” and “Critical Strain Energy Release Rate (GIC)” values. Due to the effectiveness of the POSS nanoparticles on the basic toughening mechanisms of “crack deflection,” “shear band formation,” “debonding and pull out,” etc.; Table 1 and Figure 3 show that the increases in the fracture toughness values of the PLA matrix continue beyond 1 wt%, reaching maxima at 3 wt% POSS content. At this composition, increases in KIC and GIC fracture toughness values were as much as 11% and 77%, respectively. Beyond 3 wt% content, fracture toughness values started to decrease gradually, again due to the same reason mentioned before.
Effects of POSS content on the thermal behavior of the specimens were studied by DSC and TGA analyses. First heating DSC thermograms of the specimens were given in Figure 4, while the important transition temperatures such as glass transition (Tg), cold crystallization (Tc) and melting (Tm) temperatures were tabulated in Table 2 together with the enthalpies of melting (ΔHm) and crystallization (ΔHc) including the percent crystallinity (Xc) of the PLA matrix. The relation used in calculation of percent crystallinity is given below;
where wPLA is the weight fraction of the PLA matrix and ΔHmº is the melting enthalpy of 100% crystalline PLA determined as 93 J/g in literature. 31
First heating DSC thermograms and TG curves of the specimens with different POSS contents.
Transition temperatures (Tg, Tc, Tm), enthalpies (ΔHm, ΔHc) and crystallinity percent (%Xc) of the all specimen groups during DSC first heating profile.
It was observed that use of POSS nanoparticles resulted in no significant influences on the glass transition temperature (Tg) and melting temperature (Tm) of the PLA matrix. On the other hand, due to the nucleation agent effect of the nanoparticles, increasing POSS content resulted in substantial decreases in the Cold Crystallization Temperature (Tc) of the PLA matrix leading to significant increases in the crystallinity amount. For instance, the increase in crystallinity amount (Xc) was more than two times with only 1 wt% POSS, while this increase was more than three times with 5 wt% POSS.
Thermogravimetric (TGA) curves indicating the thermal degradation temperatures and %residue of each specimen were also given in Figure 4, while the data determined were tabulated in Table 3 as T5%, T10% and T25% representing the degradation temperatures at 5%, 10% and 25% mass losses; and Tmax representing the temperature at maximum mass loss. It was generally seen that use of POSS nanoparticles resulted in slight decreases in the T5%, T10% and T25% thermal degradation temperatures of the PLA matrix. There were a few degrees of increase only in the Tmax degradation temperature. Table 3 also indicates that inorganic residue% increases parallel to the POSS content in the matrix.
Thermal degradation temperatures (T5%, T10%, T25%) of the all specimen groups at 5, 10 and 25 wt% mass losses, the maximum mass loss temperature (Tmax) and %residue at 550°C.
Effects of the functional groups on the POSS structure
Depending on the application, corners of the inorganic cage structure of POSS could be functionalized by attaching different organic groups. In the first part of the study, the POSS structure used had only “isobutyl” (a rather non-polar group) attachment at each eight corners. That structure simply designated as
In this second part of the investigation, influences of having different organic functional groups on the corners of the POSS structure were explored by comparing the performances of the three more POSS structures with each other. In the second POSS structure, one of the corner was functionalized by “aminopropyl” group; it is named as
Functional groups of the four different POSS structures compared in the study.
Note that in the previous part, since use of 1 wt% POSS resulted in lowest degree of agglomeration in PLA matrix, performance comparison of the four different POSS structures were evaluated by using this optimum POSS content for each. Therefore, in this part, in the designation of each nanocomposite specimen group, 1 wt% filler contents were not indicated.
In this study, ATR-FTIR analyses were first conducted in order to reveal different functionalities of each POSS structure as given in Figure 6(a). In the literature32–34 distinctive IR bands for the basic POSS structure were reported as; stretching vibration peaks of siloxane (Si–O–Si) between 1050 and 1150 cm−1, Si–C vibration peak at 1250 cm−1, C–H stretching vibrations between 2800 and 3000 cm−1, and C–H bending vibration in the band of 1295–1365 cm−1. In this study, Figure 6(a) indicated that Si–O–Si and Si–C peaks were observed at 1085 cm−1 and 1229 cm−1, respectively; while C–H stretching vibrations were at around 2870 and 2952 cm−1, including the C–H bending vibration in the band of 1332–1366 cm−1.
ATR-FTIR spectra of the (a) POSS structures; (b) neat PLA and PLA/POSS nanocomposites; (c) PLA-g-MA (i.e. gMA) copolymer and PLA/POSS nanocomposites with MA compatibilization.
For the ap-POSS structure the expected additional IR bands should be due to the N–H stretching and bending present in the aminopropyl group. For the pd-POSS structure the additional IR bands could be due to the -OH stretching present in the propanediol group. However, since these functional groups were attached to only one corner of the basic POSS structure, it was difficult to recognize these additional bands from the IR spectra given in Figure 6(a).
For the os-POSS structure, the additional IR bands reported in the literature35,36 were Si–H stretching vibrations at 900 and 2140 cm−1 due to the silane groups present. Since eight corners of the structure was functionalized with silane groups, it was very easy to recognize these additional Si–H peaks at 890 and 2140 cm−1 on the last IR spectrum given in Figure 6(a).
In Figure 6(b), IR spectra for neat PLA and its 1 wt% POSS nanocomposites are given. The first spectrum in this figure indicates typical six distinctive bands observed for the neat PLA matrix, which is consistent with the literature 37 ; i.e. C–C stretching peak at 868 cm−1, C–O stretching peaks at 1089 cm−1 and 1185 cm−1, C–H deformation peak at 1325 cm−1, CH3 bending absorption peak at 1480 cm−1; ester carbonyl C=O stretching peak at 1753 cm−1, and C–H stretching at 2995 cm−1.
In the rest of the Figure 6(b), IR spectra for each PLA matrix nanocomposites were given. However, due to the very low amount (only 1 wt%) POSS particles and due to the overlapping with typical PLA peaks, it was not easy to recognize differences in the IR spectrum of the nanocomposite specimens.
On the other hand, certain slight differences observed from Figure 6(b) might be mentioned. For instance; when POSS particles were incorporated, there was broadening of the typical C–O stretching peaks (1080 cm−1) of PLA matrix. Another difference observed was the increased intensities of the C–H stretching peaks (2950 cm−1) of PLA matrix. These changes could be speculated that certain level of interfacial interactions between the PLA matrix and functionalized POSS structures were achieved.
SEM examinations (Figure 7(a)) conducted on the fracture surfaces of each specimen group indicated that replacement of the at least one of the rather nonpolar (isobutyl) group on the corners of the POSS structure by functional groups (such as aminopropyl, propanediol, dimethylsilane) might improve the interfacial interactions between the PLA matrix. Thus, Figure 7(a) shows that, compared to basic POSS structure; ap-POSS, pd-POSS and os-POSS particles had rather more homogeneous distribution with lower degrees of agglomeration.
SEM fractographs showing (a) effects of POSS functional groups on the distribution and agglomeration level of the particles in PLA matrix; (b) effects of MA compatibilization on the interfacial morphology between PLA matrix and POSS structures.
In order to reveal effects of POSS functional groups on the mechanical properties of the PLA matrix nanocomposite specimens; tension, bending and fracture toughness tests were conducted. Tensile and flexural stress-strain curves are given in Figure 8, while all the mechanical properties; Tensile Strength (σTS), Flexural Strength (σFlex), Young’s Modulus (E), Flexural Modulus (EFlex), Fracture Toughness (KIC and GIC) values were compared in Figure 9. The data with standard deviations were tabulated again in Table 1.
Stress-strain curves of the 1 wt% PLA/POSS nanocomposites before and after their MA compatibilization obtained during tensile and three-point bending flexural tests.
Effects of POSS functional groups and MA compatibilization on the (a) strength (σTS and σFlex); (b) modulus (E and EFlex); (c) fracture toughness (KIC and GIC) values of the specimens.
It was generally observed that due to the certain interfacial interactions between the PLA matrix and the organic functional groups of POSS structure, there were different levels of improvement in the mechanical properties of the PLA/POSS nanocomposites.
In terms of strength (σTS and σFlex) and elastic modulus (E and EFlex) values, Figures 9(a) and (b) and Table 1 show that “propanediol” functional group on one of the corner of POSS structure resulted in slightly higher improvements compared to other POSS functional groups. This could be due to the higher efficiency of the pd-POSS structure on the strengthening and stiffening mechanisms of “load transfer” and “decreased chain mobility.” For this nanocomposite (i.e. PLA/pd-POSS) the increases in σTS and σFlex were 11% and 6%; while in E and EFlex the increases were 5% and 16%, respectively.
On the other hand, in terms of fracture toughness (KIC and GIC) values, Figure 9(c) and Table 1 indicate that the basic POSS structure having rather nonpolar “isobutyl” on its eight corner resulted in slightly higher improvements compared to other POSS structures having functional groups. This behavior could be due to the lower degree of interfacial interactions between the PLA matrix and the basic POSS structure having only “isobutyl” groups on its corners. Having lower degree of interfacial interactions lead to higher efficiency in the well-known toughening mechanisms of “debonding” and “pull-out.” For this nanocomposite (i.e. PLA/POSS) the increases in the values of KIC and GIC were 5% and 42%, respectively.
Effects of POSS functional groups on the thermal behavior of the specimens were evaluated from the first heating DSC curves and TG curves as given in Figure 10. Data obtained from these curves are tabulated again in Tables 2 and 3. DSC analyses revealed that the most significant influence of using 1 wt% POSS having functional groups was the substantial decreases in the cold crystallization temperature (Tc) leading to higher amounts of crystallinity (% Xc). For instance, crystallinity amount of neat PLA increases as much as three times when filled with 1 wt% pd-POSS or os-POSS; this increase was two times when filled with basic POSS structure. Because, efficiency of the “nucleation agent” action of POSS particles would be higher when their at least one corner was functionalized leading to more homogeneous distribution in the PLA matrix. TG analyses also revealed that, compared to basic POSS structure use of functionalized POSS structures (ap-POSS, pd-POSS, os-POSS) resulted in similar influences on the thermal degradation temperatures of the specimens.
First heating DSC thermograms and TG curves of the 1 wt% PLA/POSS nanocomposites before and after their MA compatibilization.
Effects of MA compatibilization
It is known that interfacial interactions between polymer matrix chains and inorganic or organic fillers could be improved by using a suitable graft copolymer structure; in which their graft groups are attracted to the filler surfaces. This technique, which can be named as “graft copolymer compatibilization” is especially applied by using “Maleic Anhydride” (MA) grafts for many polymer matrices and for many fillers.
Thus, in this third part of the study, use of MA grafted PLA (PLA-g-MA) copolymer compatibilization technique for each specimen group was also conducted in order to reveal whether there would be further interfacial interaction improvement between the PLA matrix and basic and functionalized POSS structures.
For simplicity, “PLA-g-MA” copolymer is designated as “gMA.” Details of the production method of this gMA copolymer was explained in our former study. 30 In this section, PLA matrix nanocomposites were produced again using only 1 wt% of each POSS structures and 2 wt% of gMA copolymer. In the designation of these gMA compatibilized specimen groups since the wt%’s are the same, they are not indicated, only the “gMA” designation is inserted between the PLA matrix and all POSS structures.
In order to reveal possible interfacial interactions between MA and other groups, ATR-FTIR analyses were conducted first for the PLA-g-MA copolymer (i.e. gMA) and then for all nanocomposite groups having gMA compatibilization, as given in Figure 6(c).
It is known that 38 the distinctive IR bands for MA structure are cyclic C=C stretching peak at 1590 cm−1, asymmetric C=O stretching vibration at 1774 cm−1 and symmetric C=O stretching vibration at around 1850 cm−1. As discussed in detail in our former study 30 and indicated in the first spectrum of Figure 6(c), when PLA was grafted with MA, the most significant evidence is the absence of the cyclic C=C stretching peak of MA structure at 1590 cm−1 due to chemical interaction between PLA and MA structure.
Other spectra in Figure 6(c) belong to PLA matrix nanocomposites filled with 1 wt% four different POSS particles and 2 wt% gMA copolymer. Unfortunately, due to the very low amounts of the ingredients and overlapping of the common IR bands in these nanocomposites, it was very difficult to speculate those possible interfacial interactions between MA and functional groups of the POSS structures.
Since it was difficult via ATR-FTIR analyses, the second analyses conducted to reveal the effects of MA compatibilization on the interfacial interactions was SEM examination of the fracture surfaces of all specimen groups. Compared to the SEM fractographs of the previous section (i.e. Figure 7(a)); it is seen in Figure 7(b) that use of MA compatibilization had no detrimental effects on the interfacial morphology between PLA matrix and basic POSS and os-POSS particles.
On the other hand, it was very obvious in Figure 7(b) that, there were certain levels of “debonding” and “pull-out” when PLA matrix was filled with ap-POSS and pd-POSS particles. As will be seen in the following paragraphs, there was also substantial reductions in the mechanical properties of these specimens. Therefore, it could be speculated that use of MA compatibilization resulted in no additional interfacial interactions between PLA matrix and ap-POSS and pd-POSS particles.
Then, the same mechanical tests mentioned before were applied for all nanocomposite specimen groups after their MA compatibilization. Stress-strain curves of these specimens were compared in Figure 8, while values of all mechanical properties (strength, modulus, fracture toughness) of the specimens before and after MA compatibilization were compared again in Figure 9 and tabulated in Table 1 with standard deviations.
Generally, it was seen in these figures and table that, due to the improved interfacial interactions, all mechanical properties of the nanocomposites filled with 1 wt% basic POSS and functionalized os-POSS particles were increased after their MA compatibilization.
For instance, because of the higher efficiency in strengthening and stiffening mechanisms, Figures 9(a) and (b) and Table 1 indicate that, compared to neat PLA the increase in flexural strength (σFlex) was 3% when filled with 1 wt% basic POSS particles; in which after MA compatibilization this increase was risen to 8%. Similarly, the increase in flexural modulus (EFlex) was 13% when filled with 1 wt% os-POSS particles; in which after MA compatibilization this increase was risen to 19%.
Again, due to the further improvements in the toughening mechanisms, Figure 9(c) and Table 1 show that the increase in KIC fracture toughness for the PLA/POSS was 5%, while that increase was risen to 7.5% for the PLA/gMA/POSS specimen. Similarly, the increase in GIC fracture toughness for PLA/os-POSS was 39%, while that increase was risen to 58% for the PLA/g-MA/os-POSS specimen.
On the other hand, Figure 9 and Table 1 indicate that, due to the no further improvements in the strengthening, stiffening and toughening mechanisms, all mechanical properties of the PLA matrix composites filled with ap-POSS and pd-POSS particles decreased substantially after their MA compatibilization.
Changes in the thermal behavior of the PLA matrix nanocomposites before and after MA compatibilization were evaluated by DSC and TG analyses as shown in Figure 10 while the data obtained were tabulated again in Tables 2 and 3. Figure 10 and Table 2 reveal that the most significant change in the DSC analyses was the reductions in the crystallinity amounts (%XC) of the PLA matrix nanocomposites after their MA compatibilization. It can be speculated that the reason for these reductions might be due to the reduced conformational mobility of the matrix chains required for crystallization. TGA results in Figure 10 and Table 3 show that all thermal degradation temperatures (T5%, T10%, T25%, Tmax) including %Residue values of the nanocomposite specimens increased substantially after their MA compatibilization.
Conclusions
Effects of POSS content
Compared to the higher POSS contents of 5 and 7 wt%, lower POSS contents (i.e. 1 and 3 wt%) resulted in rather uniform distribution with lower degree of agglomeration in PLA matrix.
Due to the higher efficiency of uniformly distributed lower POSS contents in the strengthening, stiffening and toughening mechanisms; use of 1 wt% POSS resulted in highest improvements in strength and modulus, while 3 wt% POSS resulted in highest improvements in fracture toughness values of the PLA matrix.
Due to the nucleation agent effect of the nanoparticles, increasing POSS content increased crystallinity amount of the PLA matrix; while that they had no significant influences on the thermal degradation temperatures.
Effects of POSS functional groups
Compared to the basic POSS structure having only nonpolar isobutyl groups, functionalized POSS structures having aminopropyl groups (ap-POSS), propanediol groups (pd-POSS) and dimethylsilane groups (os-POSS) resulted in more homogeneous distribution with lower degrees of agglomeration in PLA matrix.
Due to the certain interfacial interactions between the PLA matrix and the organic functional groups of POSS structure, different levels of improvement were observed in the mechanical properties of the nanocomposites. Generally, compared to others, use of pd-POSS structure resulted in slightly higher improvements in strength and modulus values, while use of basic POSS structure was better in terms of fracture toughness values.
Since efficiency of the nucleation agent action of the more homogeneously distributed POSS particles were higher, use of ap-POSS, pd-POSS and os-POSS led to further increases in the crystallinity amount of PLA matrix, while no significant difference was observed in thermal degradation temperatures.
Effects of MA compatibilization
Additional use of maleic anhydrate (MA) grafted copolymer compatibilization had no detrimental effects on the interfacial morphology between PLA matrix and basic POSS and os-POSS particles; while there were certain levels of debonding with ap-POSS and pd-POSS particles.
Consequently, due to the improved interfacial interactions, all mechanical properties of the nanocomposites with basic POSS and os-POSS particles were increased after their MA compatibilization.
After MA compatibilization of all nanocomposites, although there were reductions in the crystallinity amounts of their PLA matrices, all thermal degradation temperatures increased substantially.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
