Aleksandra Ostafinska,
Ivan Fortelny,
Martina Nevoralova,
Jiri Hodan,
Jana Kredatusova and
Miroslav Slouf*
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic. E-mail: slouf@imc.cas.cz
First published on 12th November 2015
Poly(lactic acid) (PLA) is a promising material for biomedical applications due to its biodegradability and high stiffness, but suffers from low toughness. We report that blending of PLA with another biodegradable polymer, poly(ε-caprolactone) (PCL), can increase the impact strength above the values of the individual components, while the other important macro- and micromechanical properties remain at well-acceptable level (above the theoretical predictions based on equivalent box model). Although some previous studies indicated incompatibility of PLA and PCL polymers, we demonstrate that the melt-mixing of the polymers with optimized viscosities (PLA/PCL viscosity ratio ∼ 1), the optimized composition (PLA/PCL = 80/20 by weight), and the optimal processing (compression molding with fast cooling) leads to optimal morphology (∼0.6 μm particles of PCL in PLA matrix) and synergistic effect in the mechanical performance of the systems. In an additional set of experiments, we show that the addition of TiO2 nanoparticles slightly improves stiffness, but significantly reduces the toughness of the resulting nanocomposites. The investigated systems were characterized by electron microscopy (SEM and TEM), notched impact strength, dynamic mechanical analysis, and microindentation hardness testing.
However, application of neat PLA is strongly limited by its brittleness. Very efficient method of the improvement of toughness of brittle polymers is their blending with elastomers.4 Good candidate for blending with PLA is biocompatible polyester PCL, which exhibits low glass transition temperature and high toughness. Indeed, PLA/PCL blends have been suggested and/or studied as a promising material for the controlled drug release,5 the tissue engineering,6 bone fixation devices,7 and food packaging.8 Nevertheless, the studies focused on the mechanical properties of the PLA/PCL blends have been rather contradictory. López-Rodríguez et al.9 found strong decrease in strength at yield and at break and no increase in strain at break for PLA/PCL (80/20) blends in comparison with neat PLA. Such results are typical of incompatible polymers. Vilay et al.10 found moderate decrease in strength at yield and remarkable increase in strain at break with increasing content of PCL. Mittal et al.11 concluded that PCL hinders crystallization of PLA due to some extent of intermixing of PCL and PLA phases. Fine but non-uniform morphology was observed in PLA/PCL (50/50) blends.12 Bai et al.13 studied dependence of the notched Izod impact strength on the size of PCL particles and crystallinity of PLA matrix in PLA/PCL (80/20) blends. They found that blends with crystalline PLA matrix achieved maximum impact strength for substantially smaller PCL particles than for blends with amorphous PLA matrix. Achieved maximum value of the strength for blends with crystalline PLA was about twice of the maximum value for blends with amorphous PLA. Recently, Urquijo et al.14 found fine morphology and remarkable improvement in the elongation at break and impact strength of PLA/PCL blends; in this case the polymers seemed to have good interfacial adhesion.
Prevailing meaning in scientific community is that PLA and PCL are incompatible polymers and their blends should be compatibilized in order to achieve good mechanical properties.15 A number of papers have been focused on compatibilization of PLA/PCL blends.16–21 It was found that the addition of PCL-b-PLA diblock copolymer16 and PLA-PCL-PLA triblock copolymers16,18 led to finer morphology of PLA/PCL blends and enhanced their yield stress.16 Maleic-anhydride-grafted PLA (PLA-g-PCL) was used as a reactive compatibilizer.19 Addition of PLA-g-PCL decreased the size of PCL particles and enhanced elongation at break of PLA/PCL blends. Compatibilization of PLA/PCL blends with polyhedral oligomeric silsesquioxane (POSS) led to a decrease in PCL particle size in dependence on POSS functionalization or grafting with PCL-b-PLA. On the other hand, substantial positive effect of the compatibilization on maximum strength and elongation at break for PLA/PCL (70/30) was not detected.17 Takayama et al.21 found that the addition of lysine triisocyanate (LTI) to PLA/PCL blends reduced the immiscibility and thereby decreased the particle size of PCL. Furthermore, annealing of PLA/PCL/LTI blends increased the crystallinity, improved the bending modulus and strength as well.21
Recently, we have studied phase structure evolution in PLA/PCL blends with very similar viscosities of the components.22 The blends exhibited particulate structure with quite small PCL particles up to 20 and 30 wt% for compression molded and quenched samples, respectively. In well-defined, compression molded samples, the particles were slightly larger than in the quenched ones, but even the volume average of their radius was smaller than 1 μm till 20 wt% of PCL. Moreover, PCL particles have been poorly visible in cryogenically fractured samples of PLA/PCL blends without etching, which indicated good interfacial adhesion. The combination of stiff PLA matrix with small, well-dispersed, toughening PCL particles was promising from the point of view of mechanical performance.
In this study, we optimized composition and processing of PLA/PCL blends so that we could obtain PLA/PCL blends fine particulate morphology and well-balanced combination of mechanical properties, namely stiffness and toughness, required for bone tissue engineering applications. Moreover, we prepared PLA/PCL/TiO2 composites and investigated the effect of TiO2 nanoparticles on morphology and mechanical performance. It is well known that TiO2 nanoparticles have positive effect on tissue regeneration.23 TiO2 nanoparticles were also reported to influence crystallization kinetics, crystallinity, melting point and glass transition of polymers.24,25 Nakayama et al.26 showed that addition of TiO2 nanoparticles into PLA matrix lead to the photodegraded product in comparison with pure PLA. Therefore, the structure, thermal and mechanical properties of PLA/PCL/TiO2 nanocomposites besides neat PLA/PCL blends were the second object of this study. Last but not the least, we characterized the blends and composites also by microindentation hardness testing and investigated the relationship among morphology, micromechanical properties, macromechanical properties and predictive theory based on equivalent box model.27
![]() | ||
Fig. 1 The absolute value of complex shear viscosity at 180 °C as a function of angular frequency of PLA and PCL biopolymers. |
Sample | PLA (%) | PCL (%) | TiO2 (%) | Preparation |
---|---|---|---|---|
76/19/5/a | 76 | 19 | 5 | All components mixed together |
72/18/10/a | 72 | 18 | 10 | All components mixed together |
72/18/10/b | 72 | 18 | 10 | From masterbatch of PLA with TiO2 |
72/18/10/c | 72 | 18 | 10 | From masterbatch of PCL with TiO2 |
Xc(PLA) = [|ΔHm(PLA)| − |ΔHcc(PLA)|]/ΔH0m(PLA) × 100%/w(PLA) | (1) |
Eb = E1v1 + E2v2 | (2) |
Eb = E1v1p + E2v2p + vs2/[(v1s/E1) + (v2s/E2)] | (3) |
E ∝ |G*| ∝ EIT | (4) |
The predictive models (eqn (2) and (3)) can be applied also on moduli from DMA and microindentation experiments. Analogously to eqn (2) and (3), also the upper limit of yield stress of a polymer blend (σYb) can be estimated from RoM (eqn (5)) and more realistic estimate of σYb can be obtained from EBM model (eqn (6)):
σYb = σ1v1 + σ2v2 | (5) |
σYb = σY1v1p + σY2v2p + AσY1vs | (6) |
HIT ≈ 3σY | (7) |
The sign ≈ in eqn (7) means that the Tabor relation was developed and justified for plastic materials, while for viscoelastic materials, such as polymers, it is accepted as the first approximation.32
PLA/PCL | μ(N) (μm) | σ(N) (μm) | μ(V) (μm) | σ(V) (μm) |
---|---|---|---|---|
a μ(N) and σ(N) = arithmetic mean and width of number distribution of particle size. μ(V) and σ(V) = arithmetic mean and width of volume distribution of particle size. The values of μ(N), σ(N), μ(V) and σ(V) were calculated with program MDISTR.28 | ||||
90/10 | 0.30 | 0.17 | 0.58 | 0.24 |
85/15 | 0.56 | 0.29 | 0.97 | 0.33 |
80/20 | 0.58 | 0.38 | 1.34 | 0.54 |
75/25 | 0.72 | 0.74 | 3.29 | 1.88 |
70/30 | 0.83 | 0.96 | 6.96 | 4.00 |
As both PCL and PLA are semicrystalline polymers, their crystallinity (wc) and lamellar thickness (lc) could have an impact on their mechanical properties.33,35,38 These morphological parameters were assessed from DSC in the form of DSC crystallinities (Xc ∼ wc) and melting points (Tm ∼ lc). As for the minority phase, PCL, the melting points (varying around 62 °C) and crystallinities (varying around 55%) did not show any apparent trend, which suggested that given preparation procedure did not influence PCL supermolecular structure. For PLA matrix (Table 3) we observed cold crystallization around 98 °C and melting of the crystalline phase around 168 °C. The cold crystallization evidenced that the cooling process was quite fast and, as a result, the final crystallinity values of PLA matrix (calculated according to eqn (1)) were quite low.
Composition of PLA/PCL/TiO2 | Tcc,PLA (°C) | ΔHcc,PLA (J g−1) | Tm,PLA (°C) | ΔHm,PLA (J g−1) | Xc,PLA (%) |
---|---|---|---|---|---|
a Tcc,PLA = temperature of cold crystallization, ΔHcc,PLA = enthalpy of cold crystallization, Tm,PLA = melting temperature, ΔHcc,PLA = melting enthalpy, Xc,PLA = degree of crystallinity. | |||||
100/0 | 106.0 | −22.2 | 168.2 | 31.2 | 9.7 |
90/10 | 92.2 | −19.3 | 167.4 | 27.2 | 9.5 |
80/20 | 94.4 | −19.0 | 170.1 | 24.4 | 7.2 |
70/30 | 97.2 | −18.0 | 170.2 | 22.1 | 6.4 |
76/19/5/a | 96.7 | −19.3 | 168.3 | 24.2 | 7.0 |
72/18/10/a | 97.6 | −19.4 | 166.9 | 24.0 | 6.9 |
72/18/10/b | 94.5 | −17.6 | 167.7 | 24.3 | 10.0 |
72/18/10/c | 99.2 | −18.2 | 167.6 | 22.9 | 7.1 |
It is worth reminding that the fine phase structure of the PLA/PCL blends with particulate structure (up to composition 75/25 wt%) was achieved intentionally and in accord with theoretical predictions. In order to obtain small PCL particles in PLA matrix, we optimized the composition and preparation as follows: firstly, we selected the components with similar viscosities (Section 2.2; Fig. 1), which led to the fine phase morphology.22 Secondly, we optimized the compression molding procedure with the aim to avoid frozen stresses and simultaneously to minimize particle coalescence and subsequent coarsening of phase structure. The fine particulate morphology of PCL was expected to result in the highest toughening of PLA matrix.4,39 As the increase of PLA stiffness with crystallinity is quite moderate (less than 15% if the crystallinity is increased from 9 to 70%),40 we sacrificed the crystallinity and slightly higher stiffness in favor of fine phase morphology and strongly improved toughness.
![]() | ||
Fig. 3 Charpy notched impact strength of PLA/PCL blends; the error bars represent standard deviations. |
The toughness of 80/20 blend exceeded even the toughness of pure PCL impact modifier, which meant that there was a synergistic effect. Moreover, the impact energy of 80/20 blend was more than 16 times higher in comparison with pure PLA. This was well above the increase reported in analogous studies. For example, Odent et al.41 used 10% of random aliphatic copolyesters as impact modifiers for PLA (impact energy of pure PLA ∼2.5 kJ m−2 like in this work) and increased the impact strength ca. 3 times (the best impact energy achieved ∼7.1 kJ m−2). In another recent study of Urquijo et al.14 the improvement of the impact strength of the PLA/PCL blend (80/20) was ca. twofold, slightly increasing for higher concentrations of PCL. Therefore, it seems that our strong, synergistic improvement of PLA toughness could be attributed partially to the optimized composition, preparation and morphology of the blends, and partially to serendipity when combining all favorable effects together.
Recent analysis42 pointed out that the notched impact strength of polymer blends is a complex function of multiple factors, the most important of which are particle diameter, D, volume fraction of the dispersed phase, φ, and particle–matrix adhesion. Both theoretical analyses and experimental studies of the fracture process in rubber-toughened polymers4,42–44 showed that the brittle–ductile (BD) and ductile–brittle (DB) transitions could be observed for impact strength as a function of D. BD transition occurs when the blend contains particles above a critical minimum size that initiate cavitation, while DB transition is associated with crazes initiated at bigger particles. Therefore, high impact strength for a combination of polymers with given φ can be achieved only for the droplet diameters between BD and DB transitions. An increase in the impact strength with D near BD transition is frequently very steep (cf. Fig. 25.16 in ref. 44), whereas the related decrease near DB transition is usually slower. Experimental studies rubber toughened thermoplastics showed that the optimal D is higher for brittle amorphous thermoplastics (such as PS and PMMA; optimal D ∼ 1 μm) than for semicrystalline thermoplastics (such as PP and PA; optimal D ∼ 0.2–0.3 μm). As the above mentioned semicrystalline thermoplastics exhibit substantially higher ductility in comparison with the amorphous ones, it not quite clear if matrix crystallinity or its toughness has more decisive effect on optimal value of D. The optimal D for PLA/PCL blends was studied by Bai et al.13 who found that it is larger for blends with the amorphous PLA matrix than with the semicrystalline one. They obtained twice larger notched Izod impact strength for optimum PCL particle size in PLA/PCL blend with semicrystalline matrix in comparison to the blend with the amorphous matrix. The optimum weight-average diameter for PLA/PCL (80/20) blends with a low crystallinity of PLA obtained by Bai et al.13 (0.86 μm) is in between number-average and volume-average D for our system with the same composition (see Table 2). This is in agreement with the fact that our combination of PLA and PCL grades with the method of the blend preparation led to optimum morphology and toughness of our PLA/PCL blend with composition 80/20. For higher concentrations of PCL (Fig. 3, compositions 75/25 and 70/30) the average D was strongly increased (Table 2) and evidently exceeded the optimal value for our system. For lower concentrations of PCL (Fig. 3, compositions 85/10 and 90/10) the decrease in D below the optimal value (Table 2) combined with decreasing amount of rubber phase with the toughening effect44 resulted in observed steep decrease in the impact strength.
The stiffness of PLA/PCL blends was assessed in the form of absolute values of complex moduli, |G*|, at angular frequency 1 rad s−1 from DMA measurements (Fig. 4, black squares). As expected, the addition of soft PCL component into PLA matrix decreased the final modulus of the PLA/PCL blends. This represented an obvious penalty for toughening of brittle polymers with elastomers.4 However, the decrease in |G*|was not dramatic: it was lower than predicted by EBM model (eqn (3)) with both default (Fig. 4, dashed line) and morphology-adjusted parameters (Fig. 4, dash-and-dot line). The adjustment of the EBM parameters consisted in changing the value of volume fraction, at which the blend morphology changes from particulate to co-continuous (eqn (3); ref. 27 and 35): the default value based on the percolation theory (0.16) was changed to more realistic value based on SEM micrographs (0.35; Fig. 2). More detailed explanation about usage/adjusting EBM model parameters can be found in Section 2.10 and references therein. In any case, the experimental moduli were above both EBM predictions. Therefore, we conclude that PLA/PCL (80/20) blend offers a combination of an excellent toughness and well acceptable stiffness.
Macromechanical properties (Fig. 3 and 4) demonstrated that PLA/PCL blends under study did not behave as incompatible systems. This was in agreement with conclusions of Urquijo et al.14 The results documented that the interfacial adhesion between PLA and PCL is sufficient for achievement of the high impact strength if blends with optimum size of the PCL particles are prepared. Insufficient enhancement of the impact strength of PLA with the addition of PCL found in some studies9,14,41 was apparently caused by an improper size of the PCL particles, as explained above, in the second paragraph in this section. Moreover, the recent study of Bai et al.13 suggested that even higher increase in the impact strength could have been achieved for the system PLA/PCL systems with higher crystallinity of the matrix, on condition that the morphology would be optimized.
It has been shown that crystallization of PLA leads to a change in the toughening mechanism in PLA blends from crazing to shear-yielding.41,45,46 It is generally supposed that shear-yielding causes dissipation of more energy prior to fracture with respect to crazing. We confirmed that the interrelations among the composition of PLA/PCL blends, the size of PCL particles and the crystallinity of PLA matrix are quite complex and that they should be an object of further studies.
E = 2G(1 + ν) | (8) |
![]() | ||
Fig. 5 Correlation between macroscopic modulus, |G*|, from DMA measurements and microscopic indentation modulus, EIT, from microindentation hardness testing. |
The experimental values of micromechanical properties (EIT and HIT) were also compared with theoretical predictions based on EBM (eqn (3) and (6); Fig. 6 and 7). Although the EBM model has been developed for macroscopic properties, it can be applied to microscopic properties as well (Section 2.9; ref. 35). Fig. 6 and 7 show experimental values of EIT and HIT as functions of the blend composition, compared with various models like in Fig. 4: (i) simple linear model (dotted line), (ii) EBM model with default parameters (dashed line; the model is based on the assumption that partial continuity of minority phase occurs at volume fraction ≈ 0.16 – this assumption derives from percolation theory),27 and (iii) EBM model with parameters adjusted according to real morphology of our blends (dash-and-dot lines; the model calculated with PCL continuity starting at volume fraction ≈0.35 – this value was estimated from SEM micrographs in Fig. 2; the other parameters of the EBM model were left at their default values).27,35,36
The indentation moduli EIT (Fig. 6) were below the ideal case represented by linear model (eqn (2)), but above the more realistic prediction based on EBM models (eqn (3)). In other words, the decrease in micromechanical properties caused by PCL was not critical, having been even better than theoretical predictions. This was in excellent agreement with the results for the shear moduli from DMA measurement |G*| (compare Fig. 4 and 6), confirming not only the reliability of our results, but also the better-than-expected stiffness of the resulting PLA/PCL blends.
The experimental HIT values were even above the linear model for the low PCL concentrations (eqn (5)), and safely above EBM predictions (eqn (6)) for all compositions (Fig. 7), even when the EBM curves were calculated for maximal interfacial adhesion (EBM debonding parameter at its maximum value A = 1, see Section 2.9). This behavior, i.e. positive deviations of elastic modulus (which is proportional to EIT according to eqn (4)) and yield strength (which is proportional to HIT according to eqn (7)) is typical of compatible and partially miscible blends. Such blends form strong interfacial layer with improved properties and frequently exhibit synergistic effects.35,49 We conclude that microindentation experiments confirmed good interfacial adhesion and synergistic effects that were observed at macroscopic scale (Fig. 3 and 4).
Morphology of the prepared blends is summarized in Fig. 8. SEM micrographs of smoothed and etched surfaces (Fig. 8a–d) evidenced that the addition of TiO2 particles had negligible effect on the size distribution of PCL particles. SEM micrographs of fracture surfaces prepared under liquid nitrogen (Fig. 8e–h) indicated that interfacial adhesion between PLA and PCL is quite strong (fracture frequently propagated through the particles instead along the interface) and that the TiO2 nanoparticles tend to form agglomerates regardless of preparation procedure (the preparation procedures are listed in Table 1). TEM micrographs of ultrathin sections (Fig. 8i–l) showed the dispersion of TiO2 nanoparticles between PLA and PCL components in higher detail: the nanoparticles were localized partly in PLA matrix, partly on the PLA/PCL interface and just occasionally in PCL inclusions. This suggested why the addition of TiO2 nanoparticles had little effect on the size distribution of PCL inclusions: nanofillers can affect the morphology of polymer blends through the change of the viscosity ratio of the dispersed phase and matrix or by direct suppression of droplet coalescence due to their localization at the interface. In our case, the particles localized at the interface did not cover the surface of PCL particles completely and, as a result, they were inefficient at the coalescence suppression. A small amount of TiO2 dispersed in PLA and PCL probably did not change viscosity ratio of the blend components remarkably and so their effect on the blend morphology was almost negligible. The TiO2 nanoparticles also tended to form small agglomerates, but their size did not exceed 1 μm. This PLA/PCL/TiO2 morphology was similar to that observed by Mofokeng et al.,50 where the most of TiO2 nanoparticles was dispersed in PLA matrix, but the size of nanoparticle agglomerates was in micrometer range. Somewhat better dispersion observed in our work might be attributed to slightly different melt-mixing conditions and smaller nanoparticle size in the above mentioned study.50 From the microscopic point of view, it is interesting that the PLA matrix (light-gray background) was quite easily distinguished from PCL particles (dark-gray spheres and ellipses), although the microscopic specimens were not stained and the accelerating voltage was as high as 120 kV. The observed contrast could be attributed to somewhat different densities of the two polymers (PCL = 0.98 g cm−3 and PLA = 1.12 g cm−3) and to the high sensitivity of modern digital cameras for TEM microscopes (in our case 11MPix CCD camera Morada; Olympus).
![]() | ||
Fig. 8 Morphology of PLA/PCL/TiO2 composites; description of the composites is given in Table 1. (a–d) SEM/SE micrographs of smoothed and etched surfaces, (e–h) SEM/BSE micrographs of fracture surfaces and (i–l) TEM micrographs of ultrathin sections. |
The crystallinity of PLA in all composites was measured as well. The results are collected in Table 3. The addition of TiO2 particles did not change the crystallinity of PLA matrix, which was similar as for pure PLA (∼10%). The crystallinity of PCL inclusions (∼55%) was not changed either.
The notched impact strength of all PLA/PCL/TiO2 composites with 5 and 10 wt% of TiO2 (Table 1) was measured and compared with that of PLA/PCL (80/20) blend (Fig. 9). All PLA/PCL/TiO2 composites exhibited lower toughness than the PLA/PCL (80/20) blend. The decrease was correlated with TiO2 concentration. The way of sample preparation had a significant impact on the composite with 10 wt% of TiO2; the highest impact strength was noticed for composite prepared by simultaneous mixing of all components (sample 72/18/10/a), but still the toughness was reduced to 52% in comparison with the original blend PLA/PCL (80/20). For composite with 5 wt% of TiO2 (sample 76/19/5/a) we got 66% of PLA/PCL (80/20) blend value.
![]() | ||
Fig. 9 Charpy notched impact strength of PLA/PCL/TiO2 composites; description of the composites is given in Table 1. |
The stiffness of all composites increased slightly with respect to the neat blend (Fig. 10). The sample preparation influence on the composite stiffness was negligible. The stiffness increased about 6% and 11% with 5 and 10 wt% of TiO2 content, respectively. Generally, the addition of TiO2 could be used to fine-tune the toughness and stiffness of PLA/PCL blends, as illustrated in Fig. 3, 9 and 10. The effect of the TiO2 particles on the stiffness was somewhat smaller than predicted by the Einstein equation for non-interacting particles well adhering to the matrix,51 which indicated that the adhesion between PLA and TiO2 was not perfect. The weak effect of TiO2 on the blend stiffness also confirmed, in agreement with the morphology observed in Fig. 8, that the TiO2 nanoparticles tended to form small distinct aggregates rather than a stiffening physical network. In any case, the expected increase in stiffness due to TiO2 nanoparticles showed to be quite modest and the impact on toughness was negative. We conclude that the best combination of mechanical properties can be achieved by optimizing PLA/PCL ratio without the addition of TiO2.
![]() | ||
Fig. 10 Absolute values of complex modulus of PLA/PCL/TiO2 composites; the values were taken from DMA measurements at ω = 1 rad s−1; description of the composites is given in Table 1. |
σY ≈ E/30 | (9) |
![]() | ||
Fig. 11 Indentation modulus (EIT) and indentation hardness (HIT) of PLA/PCL/TiO2 composites obtained from microindentation hardness testing; the composites are described in Table 1. |
The model represented by eqn (9) was successfully tested for various amorphous and semicrystalline polymers.38,53 If we consider eqn (4), the combination of eqn (7) and (9) gives the approximate relation between microhardness (HIT) and tensile modulus (EIT), valid for semicrystalline polymers:
HIT ≈ 3σY ≈ EIT/10 | (10) |
According to eqn (10), our systems containing ca. 80% of semicrystalline PLA matrix should give approximate ratio EIT/HIT ≈ 10. The actual average EIT/HIT ratio of all composites in Fig. 11 ranged from 15 to 17. The EIT/HIT ratio for all PLA/PCL blends (Section 3.1) was in the same range. This was in reasonable agreement with theory and confirmed the consistency of our measurements. In conclusion, the microindentation hardness measurements re-confirmed the very good agreement between micro- and macromechanical properties (Section 3.1.3) and the fact that the addition of TiO2 did not bring any important benefit (Section 3.2.2) for mechanical properties.
We have demonstrated that proper choice of the components (PLA/PCL viscosity ratio ∼ 1/1; Fig. 1) combined with specific processing conditions (melt-mixing followed by compression molding combined with fast cooling), resulted in optimal morphology (PLA matrix containing small PCL particles) and synergistic improvement of toughness at the expense of moderate decrease in stiffness. We have also tested addition of biocompatible filler – TiO2 nanoparticles – to the blend with optimized composition. Morphology of the blends and composites was characterized by electron microscopy and DSC. Toughness was assessed by means of Charpy notched impact testing and stiffness was obtained from DMA experiments. Microindentation hardness testing yielded the values of indentation modulus and hardness. The results of macro- and micromechanical measurements were compared with each other and with the predictive theory based on equivalent box model (EBM). The main results are summarized below:
(1) Toughness of the prepared PLA/PCL blends (Fig. 2) achieved local maximum at the composition 80/20, where it was 16 times higher in comparison with pure PLA, exceeding even the impact strength of pure impact modifier, PCL (Fig. 3). This was a clear synergistic effect, achieved partially due to the careful optimization of the blend preparation and partially due to serendipity in combining all favorable factors such as phase morphology of the blend and crystallinities of both components.
(2) Stiffness of the PLA/PCL blends inevitably decreased with increasing concentration of soft PCL component. However, the decrease was mild, lower than predicted by EBM theory (Fig. 4). In other words, the high stiffness of the PLA matrix was not influenced adversely by the impact modifier particles.
(3) The moduli from microindentation hardness testing were in excellent agreement with the macroscopic moduli from DMA experiments (Fig. 5). The results from micromechanical testing confirmed that the decrease in stiffness, observed at macroscopic scale, was better than predicted from the properties of the individual components. Both indentation modulus and indentation hardness showed positive deviations from EBM model, which indicated formation of very strong interfacial layer and synergistic improvements of mechanical properties (Fig. 6 and 7).
(4) The addition of TiO2 nanoparticles (5 and 10 wt%) did not affect the morphology of PLA/PCL (80/20) blend significantly (Fig. 8). It caused substantial decrease in the impact strength (Fig. 9), but only slight improvement of stiffness (Fig. 10), as confirmed by micromechanical testing (Fig. 11). Although the impact strength of PLA/PCL/TiO2 composites remained several times higher in comparison with pure PLA, we concluded that the addition of TiO2 did not bring any apparent benefit to mechanical properties.
This journal is © The Royal Society of Chemistry 2015 |