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The bifunctionality of poly[(R)-3-hydroxybutyrate] in self-reinforced composite materials
Polymer Testing 63 (2017) 614e620
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Behaviour
The bifunctionality of poly[(R)-3-hydroxybutyrate] in self-reinforced composite materials Sebastian Jurczyk a, Jakub Włodarczyk b, Michał Kawalec b, Henryk Janeczek b, Michał Michalak b, Michał Sobota b, * a b
Institute for Engineering of Polymer Materials and Dyes, Paint and Plastics Department, 50A Chorzowska St., 44-100 Gliwice, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34, M. Curie-Skłodowska St., 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2016 Received in revised form 31 May 2017 Accepted 19 September 2017 Available online 21 September 2017
The poly[(R)-3-hydroxybutyrate] (PHB) is a highly crystalline, biosourced polymer. The advantages of the PHB are its biodegradability and biocompatibility; however, the brittleness caused by its high crystallinity decreases the application ability of the PHB in comparison with the polyolefins. Excellent results were observed for the reactive extrusion of PHB in the presence of peroxides in many investigations of the modifications of PHB mechanical properties. The disadvantage must be considered to be the thermal degradation of PHB during extended extrusion and its limitation in natural composite preparation. The peroxides are highly reactive with natural fillers, and this causes a decrease of the filler's mechanical properties. Consequently, the reactive extrusion may be a useful tool for the production of additives only. The results we present of this investigation is based on a different material preparation strategy. The two-stage method incorporated additives preparation via reactive extrusion of PHB and the blending of the obtained product with neat PHB. Theself-reinforced composite material obtained in this way revealed significantly higher values of stress and strain compared to neat PHB. The thermal degradation of the PHB matrix was retarded and total crystallinity of the composite was decreased. The materials were characterized using DSC, SEM and SEC techniques. The samples were also investigated employing tensile and impact strength tests. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Mould injection Biopolyester Reactive extrusion PHB
1. Introduction Biosourced polymer, high crystalline poly[(R)-3hydroxybutyrate] (PHB) is produced by various bacteria as an intracellular carbon and energy storage material [1]. PHB has received considerable attention as an environmentally benign packaging plastic, due to its biodegradability and biocompatibility, while its synthetic analogue poly[(R,S)-3-hydroxybutyrate] has been considered as a material for medical use [2e6]. Although natural PHB has many ecological advantages over polyolefines, it also has several drawbacks, such as the brittleness caused by high crystallinity, and its relatively poor mechanical properties. In addition, the crystallization process is rapid at room temperature and its total crystallinity degree is independent on rate of heating [7]. There are a few means of modification which may improve PHB mechanical properties. The method of combined modification and
* Corresponding author. E-mail address: [email protected] (M. Sobota). https://doi.org/10.1016/j.polymertesting.2017.09.028 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
processing is probably the most interesting from point of view of application. The example of one method mentioned is be reactive processing. The direct modification of PHBV into the intended material during processing has been described by Asrar et al. Their investigation revealed the effect of PHBV (poly([R]-3hydroxybutyrate-co-[R]-3-hydroxyvalerate)) extrusion with peroxide. The product analyses indicated partially cross-linked material with improved mechanical properties. The peroxides generated free-radicals at methine groups of the polymer backbone and converted linear structure of the polymer into branched one [8e12]. The dicumyl peroxide (DCP) is a popular peroxide, commonly used in industry, and it decomposes within 5 min at 180 C [13]. In the case of using related conditions for the processing of poly[(R,S)-3-hydroxybutyrate], the PHB sample may significantly degraded [14e17]. The poly[(R,S)-3-hydroxybutyrate] reveals poor stability at 180 C. We shall now describe, the results of PHB modification via the new method by which the effect of thermal degradation was reduced. The modification presented here was based on a two-step
S. Jurczyk et al. / Polymer Testing 63 (2017) 614e620
process, one which could be an alternative to direct reactive processing. The first step was the preparation of PHB modifier (filler) via the reactive processing of PHB (radical cross-linking using DCP). Further, the clPHB (cross-linked PHB) was used as a crystallization retarder additive for PHB. The investigations presented include the thermo-mechanical characterization of the material obtained in function of clPHB additive loading.
2. Experimental 2.1. Materials Poly([R]-3-hydroxybutyrate) (PHB) (Biomer, Mn ¼ 140 000, ƉM ¼ 2.4), Poly([R]-3-hydroxybutyrate) (oligoPHB)((Mn ¼ 1600, ƉM ¼ 2.1), dicumyl peroxide (DCP) 98% (Sigma-Aldrich) and chloroform (anhydrous, 99%) (Sigma-Aldrich) were used as received. 2.2. Fabrication of materials employing micro-compounding technique The clPHB additive was prepared by the reactive blending of PHB mixed with 0.2% (w/w) DCP carried out for 5 min at 180 C in a micro-extruder Minilab (Thermo-Haake) equipped with corotating two-cone shape screws. Further material was pulverized using a cryogenic mill. Next, a part of the clPHB powder was fractionated in Soxhlet apparatus into a chloroform-soluble fraction (clPHB1) and a chloroform-insoluble fraction (clPHB2). Low-molar mass linear PHB filler (oligPHB) (Mn ¼ 1600, ƉM ¼ 2.1) was also prepared similarly to [18]. Next, the four types of PHB blends were prepared in the same processing condition. The clPHB/PHB in various weight ratios were obtained and additionally conditioned for 1 h at 100 C in the oven before the mechanical test. The residual three blends, such as clPHB1/PHB, clPHB2/PHB and oligPHB/PHB in 50:50 wt ratio, were prepared(obtained) as reference samples. All the samples were processed and formed into bone-shape bars ISO 527e2 (1BA) using a micro-extruder Minilab and MiniJet piston injection molding system (Thermo-Haake).
615
3.3. Scanning electron microscope - morphological analysis The characterization of the obtained material was performed with a Nanolab 7 (Semco) scanning electron microscope (SEM) at an accelerating voltage of 15 kV on samples sputter-coated with gold. 3.4. Polarized light microscopy (PLM) examination of crystallization process The crystallization process clPHB/PHB blends was studied with a polarized light microscopy (PLM) (Zeiss, Opton-Axioplan) equipped with a Nikon Coolpix 4500 color digital camera and a Mettler FP82 hot plate with Mettler FP80 temperature controller. Microscopic observations were carried out at a magnification of 160. The polymer blend samples were heated to 200 C on the hot plate (amorphous state), and then the samples were rapidly cooled down in an ice-water bath. 3.5. Mechanical properties The mechanical properties of the prepared materials were determined using the tensile testing machine Instron 4204 (Instron, Norwood, USA); the crosshead section rate was 20 mm/min. The impact strength test was carried out with an A-notch Charpy Zwick Impact Testing Machine, 5102 model (Zwick GmbH, Ulm, DE) with pendulum of impact energy 1.0 J. The impact strength tests were conducted according to ISO 179/1eA. All the composites were tested at ambient temperature (þ23 C). 3.6. Thermal properties and morphology of the material obtained
3. Testing and characterization
The thermal properties were determined using a TA DSC 2010 differential scanning calorimeter (DSC) (TA Instruments, New Castle, USA). The instrument was calibrated using high purity indium and gallium. The specimens were heated from 100 C to 200 C under a nitrogen atmosphere (flow ¼ 50 mL/min) at heating rate of 20 deg/min. The melting temperature (Tm) of the composites was determined from the first heating run as the peak maximum of melting endotherm. The glass transition temperature (Tg) was determined as the midpoint of heat capacity change of amorphous sample obtained by quenching from melt to liquid nitrogen.
3.1. Granulometric analysis
3.7. SEC analysis
The granulometric (grain size) analysis was performed by dry sieve analysis using sieves sifter (model No. LPzE-2e “Labindex”). The metal sieves possess the following dimensions: (mesh size): 0.500 mm, 0.250 mm, 0.120 mm, 0.063 mm, 0.045 mm were applied to carry out the test. The powder was sifted during a 5 min period under the following conditions: amplitude e 3.0 mm, frequency e 50.0 Hz.
The determination of number-average and weight-average molecular weights and molecular weight distributions (ÐM) values of polymers were conducted using size exclusion chromatography. The analyses were carried out in chloroform at 35 C and at a flow rate of 1 mL/min. The equipment included a pump (VE 1122, Viscotek) and a set of two PLgel 5 m MIXED-C ultra-high efficiency columns and a Shodex SE 61 differential refractive index detector. A volume of 100 mL of sample solution in chloroform (concentration 0.3% w/v) was injected. Polystyrene standards (Polymer Laboratories) with narrow molecular weight distributions were used to generate calibration curve.
3.2. The determination of PHB cross-linking efficency The amount of cross-linked PHB was obtained by extraction of 10.0 g clPHB in 200 ml of chloroform using a Soxhlet extractor for 72 h. The percentage of cross-linked fraction was calculated using the following equation: Weight percent of cross-linked fraction ¼ mx/my x 100% Where my is the starting weight of the samples and mx is dry residues obtained after extraction.
4. Results and discussion 4.1. The properties of cross-linked PHB(clPHB) Partially cross-linked PHB was obtained in a free-radical process initiated with DCP and carried out directly in the extruder. The literature data refer full decomposition of the radical reaction initiator at 180 C within 5 min [9]. Therefore, the first experiment
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Table 1 Parameter of composite samples preparation. Sample
Time [min] Temperature of plasticizing zone [ C] Rate of screw rotating [rpm] Mould temperature [ C]
Additive clPHB PHB clPHB/PHB clPHB1/PHB clPHB2/PHB PHB/ oligoPHB
5.0
180
20
1.5 1.5
180 180
100 100
assessed degradation of the PHB during processing. Apparently, the molar mass comparison of PHB processed 5 min at 180 C with the initial material revealed 20% decrease of its Mn after the processing. Thus, it was decided to prepare the final material in a two-step process comprising: (i) reactive extrusion of PHB to obtain crosslinked PHB and (ii) blending of neat PHB with the desired amount of the cross-linked additive (See Table 1). 4.1.1. Granulometric analysis of clPHB The clPHB material obtained via reactive extrusion was grinded. Further, the investigations of the powder dimensions of grinded clPHB revealed three fractions: 0.063mm-0.124 mm, 125mm0.249 mm and 0.250mm-0.499 mm, the content of each being over 20% w/w (Fig. 1). A mixture of all fractions was used to prepare bone-shape samples for further mechanical testing. 4.1.2. Estimation of clPHB cross-linked structure and SEM morphological analysis The weight percent of cross-linked polymer fraction in clPHB was estimated on the basis of the limited solubility of the crosslinked material in chloroform. In fact, 58% w/w of insolube fraction was found after Soxhlet extraction of clPHB with chloroform, and it is considered as a real part equal with the cross-linked PHB. The solube extract(clPHB1) from clPHB were determined using SEC (Mn ¼ 4300, Mw ¼ 52000, Mw10% low fraction ¼ 900, Mw10% high fraction ¼ 304000). The clPHB additive was blended with neat PHB in desired ratios. Apparently, a fraction of clPHB insolube in chloroform is also insoluble in PHB, therefore the blend of clPHB/PHB forms composite in which PHB-soluble fraction of clPHB penetrate the structure of neat PHB and may modify the morphology of the material obtained in comparison to extruded PHB. The SEM analysis confirmed the composite structure of the materials obtained.
Fig. 1. Result of granulometric analysis of grinded clPHB.
Injection Temperature [ C]
Injection pressure [bar]
e
e
e
60 60
180 180
350 700
The presence of particles partially soluble clPHB in PHB matrix is shown in Fig. 2).
4.2. Thermal properties of clPHB/PHB composties In fact, the addition of clPHB does influence the properties of molds made of PHB In order to reveal the doubt, several composites with varying content of clPHB (mixture of all powder fractions) were prepared by mould injection and their thermal properties were examined (Table 2.). The DSC data (first run) revealed a decrease of melting enthalpy DHm with increasing content of the clPHB additive. The observed changes of melting enthalpy DHm suggest that, during processing, the matrix interacted with the filler. The introduction of equation 1 allows us to understand the influence of the filler on the matrix. The theoretical 1DHm takes into account weight shares of energy (DH) of plain substances (excluding the interactions). Although empirical DHm and theoretical 1DHm (calculated according to equation 1) revealed similarities, differences between experimental DHm (first run) and the theoretically predicted 1DHm are rather significant. The phenomenon may be connected to the presence of the branched structure of clPHB which retarded the crystallization of the PHB component. At the same time, the thermograms of the second DSC run showed that predicted 2DHms were comparable to the experimental ones. These results confirmed the retardation of crystallization only in the fast cooling processing method, such as mould injection. Nonetheless, the addition of clPHB decreased the overall crystallinity in the clPHB/PHB composites. The DSC curves of the second DSC run (Fig. 3) also depict decrease of both: cold crystallization temperature and DHcc; with increasing content of cross-linked additive. The difference between value of DHcc and DHm (2nd
Fig. 2. The SEM photograph of composite structure clPHB/PHB 25/75.
S. Jurczyk et al. / Polymer Testing 63 (2017) 614e620
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Table 2 The DSC results for matrix and modified matrix. Sample
PHB clPHB/PHB 25/75 w/w clPHB/PHB 50/50 w/w clPHB1/ PHB 50/50 w/w clPHB2/ PHB 50/50 w/w oligPHB/ PHB 50/50 w/w clPHB
Tcc DHm (1st run) melting enthalpy [J/ 1DHm g] melting enthalpy [J/ [ C] g]
DHcc (2nd run)enthalpy [J/ DHms (2nd run)melting enthalpy [J/ 2DHms melting enthalpy [J/ g]
g]
g]
76.8 60.2
76.8 69.1
52.1 56.9
80.5 77.2
99.3 83.6
99.3 87.0
57.5
61.4
60.5
71.5
73.0
74.6
58.4
e
56.3
70.9
70.9
e
54.0
e
58.5
65.1
65.4
e
73.2
e
44.4
80.5
81.3
e
46.0
46.0
58.9
38.53
49.9
49.9
run) is interesting. This difference for neat PHB was the greatest, while in the case of the composites, the difference decreased with increasing content of the cross-linked additive. The presence of the clPHB additive retarded the crystallization rate in the materials obtained. The clPHB/PHB 50/50 which contained the highest content of clPHB was totally amorphous after quenching before the second DSC run. The lowest value of DHm was observed for clPHB. It was almost two times lower than the value of DHm of PHB. Such a difference of DHm can be explained as a consequence of the existence of the insoluble or partially soluble branched/cross-linked part of the clPHB structure which, due to steric hindrance, probably does not crystallize. Another explanation of the observed phenomenon might be the plasticization of the PHB matrix with oligomeric products. For these reasons, reference samples with clPHB1, clPHB2 and oligPHB were prepared. It was observed (Fig. 4) that the addition of chloroform-soluble clPHB1 or insoluble clPHB2
fractions to PHB also decreased the crystallinity of PHB. Values of DHm for both materials are lower than in PHB and also the Tccs are in range of Tcc of clPHB/PHB composities. The results revealed that plasticizing effect was observed only in reference sample with oligoPHB. The DSC result for this sample showed the lowest value of Tg(glass transition) and Tcc and DHm comparable with PHB. However, the lower content of fraction branched oligomers clPHB in blend is not visible for DSC technique but still may generate as a plasticizer the changes in the mechanical properties of the material.
4.3. Crystallization process observation in polarized light microscopy (PLM) The result of the DSC analysis was proven by PLM examinations of the samples. The microscopic observation revealed that the high rate of crystallization of PHB sample was decreased by the addition
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Fig. 3. Thermograms of (2ndDSC run): 1) PHB, 2) clPHB/PHB 25/75 w/w, 3) clPHB/PHB 50/50 w/w, 4) clPHB.
Fig. 4. Thermograms of (2ndDSC run) of references samples: 1)clPHB1/PHB 50/50 w/w, 2)clPHB2/PHB 50/50 w/w, 3)oligoPHB/PHB 50/50 w/w.
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Fig. 5. The micrographs (160 magnification) of crystallization progress of melted samples after being quenched in water. Time of crystallization: a) 3 min at 23 C; b) 7 min(3 min at 23 C and 4 min at 30 C); c) 8min (3 min at 23 C and 4 min at 30 C and 1 min at 35 C); d) 9min (3 min at 23 C and 4 min at 30 C, and 2 min at 35 C).
of clPHB. Fig. 5 shows the crystallization process of PHB and its composites with clPHB after quenching. The PHB crystallized into small crystals population in whole volume at room temperature in less than 1 min. The high rate of crystallization of neat PHB was observed in the DSC result. The second DSC run of the sample revealed melting enthalpy DH ¼ 20 J/g after rapid cooling. In contrast, the composites demonstrated retarded crystallization at 35 C into crystal phase with many amorphous areas (Fig. 5d). The amorphous defects observed in the crystalline phase crystillized when the sample temperature was increased to 50 C. Another difference was that the crystals which appeared were bigger(Fig. 6). 4.4. Tensile properties and impact strength of clPHB/PHB composites The DSC experiment of mould injected samples clPHB/PHB revealed frozen crystallinity which resulted from the processing method. The results of the mechanical tensile test of composites are shown in Table 3. The composite's highest value of stress at the break was 42 MPa for conditioned material clPHB/PHB 25/75 w/w. In addition, Young modulus of the material was almost 20% lower than in the case of moulded PHB, while elongation at the break was 2% higher. The modulus of conditioned composite having 50% w/w clPHB content was 1008.6 MPa which was the lowest value, and
Fig. 6. The micrograph (160 magnification) during crystallization progress of melted sample after being quenched in water. The crystallization time: 12min (3 min at 23 C and 4 min at 30 C, and 2 min at 35 C, and 3 min 50 C).
about 50% lower than neat PHB modulus. This composite was also less brittle than neat PHB and the value of elongation at the break
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S. Jurczyk et al. / Polymer Testing 63 (2017) 614e620 Table 3 The tensile test data for PHB and clPHB/PHB composites. Sample
PHB PHB conditioned clPHB/PHB 25/75 w/w clPHB/PHB 25/75 w/w conditioned clPHB/PHB 50/50 w/w clPHB/PHB 50/50 w/w conditioned
s stress at maximum load [MPa] (S.D)
Е Young modulus [Mpa] (S.D)
Strain at Auto Break[%]
33.8(0.7) 43.1(1.2)
1884.2(61.0) 2010.1(120.0)
2.0(0.3) 1.6(0.5)
38.3(1.1)
1629.2(67.1)
3.5(0.3)
42.3(1.3)
1661.6(14.6)
5.7(0.8)
39.5 (2.1)
1389.6(60.2)
5.1(0.1)
36.7(0.2)
1008.6(87.2)
9.1(0.9)
was 9%. The increase of clPHB content in composite decreased their value of stress and modulus. In contrast, its elongation increased with the increase of clPHB content. The differences observed were connected to changing morphology, as was shown in the DSC results. The crystallinity of the prepared composites decreases and Tcc increases with the increase of clPHB content. Moreover, the filler interacted with the PHB matrix and acted as a crystallization retardant and blocker. The following experiment, in which the sample was conditioned, also showed the plasticizing properties of clPHB. The comparison of unconditioned and conditioned samples showed differences. The expected effect was observed in the conditioned blend clPHB/PHB 50/50%(w/w). This sample exhibits a 4% higher value of elongation than in the case of the unconditioned. The significant influence of the content of low-molecular crosslinked (which not co-crystallized with PHB) additive might well be the explanation for this phenomenon. The additive clPHB shows, in temperature of conditioning, melting endotherm (DSC thermogram Fig. 3) and probably diffuses from amorphous region into the bulk of the continuous PHB phase and efficiently plasticizes the material. In fact, the observed properties of clPHB were also proven by an additional mechanical test (impact resistance). The composite clPHB/PHB 25/75 demonstrated 1.2 kJ/m2(S.D ¼ 0.1) which is a higher impact strength than prepared analogously PHB 0.9 kJ/m2 (S.D ¼ 0.1). According to the literature data, the amorphous phase of material absorbs the shocks [19]. 5. Conclusion The influence of the addition of cross-linked PHB additive on the mechanical properties of PHB self-composite and its thermal properties were investigated. The experimental results pointed out that the addition of cross-linked additive decreased the crystallinity of mould-injected PHB and improved its mechanical properties. During PHB processing via mould injection, the method applied allowed us to obtain lower crystalline materials. The method of modification presented is not laborious and easy to apply in thermo-processing line, allowing for the elimination of the side effects of peroxide interaction with low stable fillers (e.g. natural fiber) which undergo undesired reactions in the presence of a strong oxidation agent. Acknowledgements This work was supported by the National Science Centre (NCN SONATA 3 project no. 2012/05/D/ST5/03384, “Synthetic analogues of aliphatic biopolyesters generating controlled response in the form of a mechanical effect on temperature stimulus”).
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Behaviour
The bifunctionality of poly[(R)-3-hydroxybutyrate] in self-reinforced composite materials Sebastian Jurczyk a, Jakub Włodarczyk b, Michał Kawalec b, Henryk Janeczek b, Michał Michalak b, Michał Sobota b, * a b
Institute for Engineering of Polymer Materials and Dyes, Paint and Plastics Department, 50A Chorzowska St., 44-100 Gliwice, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34, M. Curie-Skłodowska St., 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2016 Received in revised form 31 May 2017 Accepted 19 September 2017 Available online 21 September 2017
The poly[(R)-3-hydroxybutyrate] (PHB) is a highly crystalline, biosourced polymer. The advantages of the PHB are its biodegradability and biocompatibility; however, the brittleness caused by its high crystallinity decreases the application ability of the PHB in comparison with the polyolefins. Excellent results were observed for the reactive extrusion of PHB in the presence of peroxides in many investigations of the modifications of PHB mechanical properties. The disadvantage must be considered to be the thermal degradation of PHB during extended extrusion and its limitation in natural composite preparation. The peroxides are highly reactive with natural fillers, and this causes a decrease of the filler's mechanical properties. Consequently, the reactive extrusion may be a useful tool for the production of additives only. The results we present of this investigation is based on a different material preparation strategy. The two-stage method incorporated additives preparation via reactive extrusion of PHB and the blending of the obtained product with neat PHB. Theself-reinforced composite material obtained in this way revealed significantly higher values of stress and strain compared to neat PHB. The thermal degradation of the PHB matrix was retarded and total crystallinity of the composite was decreased. The materials were characterized using DSC, SEM and SEC techniques. The samples were also investigated employing tensile and impact strength tests. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Mould injection Biopolyester Reactive extrusion PHB
1. Introduction Biosourced polymer, high crystalline poly[(R)-3hydroxybutyrate] (PHB) is produced by various bacteria as an intracellular carbon and energy storage material [1]. PHB has received considerable attention as an environmentally benign packaging plastic, due to its biodegradability and biocompatibility, while its synthetic analogue poly[(R,S)-3-hydroxybutyrate] has been considered as a material for medical use [2e6]. Although natural PHB has many ecological advantages over polyolefines, it also has several drawbacks, such as the brittleness caused by high crystallinity, and its relatively poor mechanical properties. In addition, the crystallization process is rapid at room temperature and its total crystallinity degree is independent on rate of heating [7]. There are a few means of modification which may improve PHB mechanical properties. The method of combined modification and
* Corresponding author. E-mail address: [email protected] (M. Sobota). https://doi.org/10.1016/j.polymertesting.2017.09.028 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
processing is probably the most interesting from point of view of application. The example of one method mentioned is be reactive processing. The direct modification of PHBV into the intended material during processing has been described by Asrar et al. Their investigation revealed the effect of PHBV (poly([R]-3hydroxybutyrate-co-[R]-3-hydroxyvalerate)) extrusion with peroxide. The product analyses indicated partially cross-linked material with improved mechanical properties. The peroxides generated free-radicals at methine groups of the polymer backbone and converted linear structure of the polymer into branched one [8e12]. The dicumyl peroxide (DCP) is a popular peroxide, commonly used in industry, and it decomposes within 5 min at 180 C [13]. In the case of using related conditions for the processing of poly[(R,S)-3-hydroxybutyrate], the PHB sample may significantly degraded [14e17]. The poly[(R,S)-3-hydroxybutyrate] reveals poor stability at 180 C. We shall now describe, the results of PHB modification via the new method by which the effect of thermal degradation was reduced. The modification presented here was based on a two-step
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process, one which could be an alternative to direct reactive processing. The first step was the preparation of PHB modifier (filler) via the reactive processing of PHB (radical cross-linking using DCP). Further, the clPHB (cross-linked PHB) was used as a crystallization retarder additive for PHB. The investigations presented include the thermo-mechanical characterization of the material obtained in function of clPHB additive loading.
2. Experimental 2.1. Materials Poly([R]-3-hydroxybutyrate) (PHB) (Biomer, Mn ¼ 140 000, ƉM ¼ 2.4), Poly([R]-3-hydroxybutyrate) (oligoPHB)((Mn ¼ 1600, ƉM ¼ 2.1), dicumyl peroxide (DCP) 98% (Sigma-Aldrich) and chloroform (anhydrous, 99%) (Sigma-Aldrich) were used as received. 2.2. Fabrication of materials employing micro-compounding technique The clPHB additive was prepared by the reactive blending of PHB mixed with 0.2% (w/w) DCP carried out for 5 min at 180 C in a micro-extruder Minilab (Thermo-Haake) equipped with corotating two-cone shape screws. Further material was pulverized using a cryogenic mill. Next, a part of the clPHB powder was fractionated in Soxhlet apparatus into a chloroform-soluble fraction (clPHB1) and a chloroform-insoluble fraction (clPHB2). Low-molar mass linear PHB filler (oligPHB) (Mn ¼ 1600, ƉM ¼ 2.1) was also prepared similarly to [18]. Next, the four types of PHB blends were prepared in the same processing condition. The clPHB/PHB in various weight ratios were obtained and additionally conditioned for 1 h at 100 C in the oven before the mechanical test. The residual three blends, such as clPHB1/PHB, clPHB2/PHB and oligPHB/PHB in 50:50 wt ratio, were prepared(obtained) as reference samples. All the samples were processed and formed into bone-shape bars ISO 527e2 (1BA) using a micro-extruder Minilab and MiniJet piston injection molding system (Thermo-Haake).
615
3.3. Scanning electron microscope - morphological analysis The characterization of the obtained material was performed with a Nanolab 7 (Semco) scanning electron microscope (SEM) at an accelerating voltage of 15 kV on samples sputter-coated with gold. 3.4. Polarized light microscopy (PLM) examination of crystallization process The crystallization process clPHB/PHB blends was studied with a polarized light microscopy (PLM) (Zeiss, Opton-Axioplan) equipped with a Nikon Coolpix 4500 color digital camera and a Mettler FP82 hot plate with Mettler FP80 temperature controller. Microscopic observations were carried out at a magnification of 160. The polymer blend samples were heated to 200 C on the hot plate (amorphous state), and then the samples were rapidly cooled down in an ice-water bath. 3.5. Mechanical properties The mechanical properties of the prepared materials were determined using the tensile testing machine Instron 4204 (Instron, Norwood, USA); the crosshead section rate was 20 mm/min. The impact strength test was carried out with an A-notch Charpy Zwick Impact Testing Machine, 5102 model (Zwick GmbH, Ulm, DE) with pendulum of impact energy 1.0 J. The impact strength tests were conducted according to ISO 179/1eA. All the composites were tested at ambient temperature (þ23 C). 3.6. Thermal properties and morphology of the material obtained
3. Testing and characterization
The thermal properties were determined using a TA DSC 2010 differential scanning calorimeter (DSC) (TA Instruments, New Castle, USA). The instrument was calibrated using high purity indium and gallium. The specimens were heated from 100 C to 200 C under a nitrogen atmosphere (flow ¼ 50 mL/min) at heating rate of 20 deg/min. The melting temperature (Tm) of the composites was determined from the first heating run as the peak maximum of melting endotherm. The glass transition temperature (Tg) was determined as the midpoint of heat capacity change of amorphous sample obtained by quenching from melt to liquid nitrogen.
3.1. Granulometric analysis
3.7. SEC analysis
The granulometric (grain size) analysis was performed by dry sieve analysis using sieves sifter (model No. LPzE-2e “Labindex”). The metal sieves possess the following dimensions: (mesh size): 0.500 mm, 0.250 mm, 0.120 mm, 0.063 mm, 0.045 mm were applied to carry out the test. The powder was sifted during a 5 min period under the following conditions: amplitude e 3.0 mm, frequency e 50.0 Hz.
The determination of number-average and weight-average molecular weights and molecular weight distributions (ÐM) values of polymers were conducted using size exclusion chromatography. The analyses were carried out in chloroform at 35 C and at a flow rate of 1 mL/min. The equipment included a pump (VE 1122, Viscotek) and a set of two PLgel 5 m MIXED-C ultra-high efficiency columns and a Shodex SE 61 differential refractive index detector. A volume of 100 mL of sample solution in chloroform (concentration 0.3% w/v) was injected. Polystyrene standards (Polymer Laboratories) with narrow molecular weight distributions were used to generate calibration curve.
3.2. The determination of PHB cross-linking efficency The amount of cross-linked PHB was obtained by extraction of 10.0 g clPHB in 200 ml of chloroform using a Soxhlet extractor for 72 h. The percentage of cross-linked fraction was calculated using the following equation: Weight percent of cross-linked fraction ¼ mx/my x 100% Where my is the starting weight of the samples and mx is dry residues obtained after extraction.
4. Results and discussion 4.1. The properties of cross-linked PHB(clPHB) Partially cross-linked PHB was obtained in a free-radical process initiated with DCP and carried out directly in the extruder. The literature data refer full decomposition of the radical reaction initiator at 180 C within 5 min [9]. Therefore, the first experiment
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Table 1 Parameter of composite samples preparation. Sample
Time [min] Temperature of plasticizing zone [ C] Rate of screw rotating [rpm] Mould temperature [ C]
Additive clPHB PHB clPHB/PHB clPHB1/PHB clPHB2/PHB PHB/ oligoPHB
5.0
180
20
1.5 1.5
180 180
100 100
assessed degradation of the PHB during processing. Apparently, the molar mass comparison of PHB processed 5 min at 180 C with the initial material revealed 20% decrease of its Mn after the processing. Thus, it was decided to prepare the final material in a two-step process comprising: (i) reactive extrusion of PHB to obtain crosslinked PHB and (ii) blending of neat PHB with the desired amount of the cross-linked additive (See Table 1). 4.1.1. Granulometric analysis of clPHB The clPHB material obtained via reactive extrusion was grinded. Further, the investigations of the powder dimensions of grinded clPHB revealed three fractions: 0.063mm-0.124 mm, 125mm0.249 mm and 0.250mm-0.499 mm, the content of each being over 20% w/w (Fig. 1). A mixture of all fractions was used to prepare bone-shape samples for further mechanical testing. 4.1.2. Estimation of clPHB cross-linked structure and SEM morphological analysis The weight percent of cross-linked polymer fraction in clPHB was estimated on the basis of the limited solubility of the crosslinked material in chloroform. In fact, 58% w/w of insolube fraction was found after Soxhlet extraction of clPHB with chloroform, and it is considered as a real part equal with the cross-linked PHB. The solube extract(clPHB1) from clPHB were determined using SEC (Mn ¼ 4300, Mw ¼ 52000, Mw10% low fraction ¼ 900, Mw10% high fraction ¼ 304000). The clPHB additive was blended with neat PHB in desired ratios. Apparently, a fraction of clPHB insolube in chloroform is also insoluble in PHB, therefore the blend of clPHB/PHB forms composite in which PHB-soluble fraction of clPHB penetrate the structure of neat PHB and may modify the morphology of the material obtained in comparison to extruded PHB. The SEM analysis confirmed the composite structure of the materials obtained.
Fig. 1. Result of granulometric analysis of grinded clPHB.
Injection Temperature [ C]
Injection pressure [bar]
e
e
e
60 60
180 180
350 700
The presence of particles partially soluble clPHB in PHB matrix is shown in Fig. 2).
4.2. Thermal properties of clPHB/PHB composties In fact, the addition of clPHB does influence the properties of molds made of PHB In order to reveal the doubt, several composites with varying content of clPHB (mixture of all powder fractions) were prepared by mould injection and their thermal properties were examined (Table 2.). The DSC data (first run) revealed a decrease of melting enthalpy DHm with increasing content of the clPHB additive. The observed changes of melting enthalpy DHm suggest that, during processing, the matrix interacted with the filler. The introduction of equation 1 allows us to understand the influence of the filler on the matrix. The theoretical 1DHm takes into account weight shares of energy (DH) of plain substances (excluding the interactions). Although empirical DHm and theoretical 1DHm (calculated according to equation 1) revealed similarities, differences between experimental DHm (first run) and the theoretically predicted 1DHm are rather significant. The phenomenon may be connected to the presence of the branched structure of clPHB which retarded the crystallization of the PHB component. At the same time, the thermograms of the second DSC run showed that predicted 2DHms were comparable to the experimental ones. These results confirmed the retardation of crystallization only in the fast cooling processing method, such as mould injection. Nonetheless, the addition of clPHB decreased the overall crystallinity in the clPHB/PHB composites. The DSC curves of the second DSC run (Fig. 3) also depict decrease of both: cold crystallization temperature and DHcc; with increasing content of cross-linked additive. The difference between value of DHcc and DHm (2nd
Fig. 2. The SEM photograph of composite structure clPHB/PHB 25/75.
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Table 2 The DSC results for matrix and modified matrix. Sample
PHB clPHB/PHB 25/75 w/w clPHB/PHB 50/50 w/w clPHB1/ PHB 50/50 w/w clPHB2/ PHB 50/50 w/w oligPHB/ PHB 50/50 w/w clPHB
Tcc DHm (1st run) melting enthalpy [J/ 1DHm g] melting enthalpy [J/ [ C] g]
DHcc (2nd run)enthalpy [J/ DHms (2nd run)melting enthalpy [J/ 2DHms melting enthalpy [J/ g]
g]
g]
76.8 60.2
76.8 69.1
52.1 56.9
80.5 77.2
99.3 83.6
99.3 87.0
57.5
61.4
60.5
71.5
73.0
74.6
58.4
e
56.3
70.9
70.9
e
54.0
e
58.5
65.1
65.4
e
73.2
e
44.4
80.5
81.3
e
46.0
46.0
58.9
38.53
49.9
49.9
run) is interesting. This difference for neat PHB was the greatest, while in the case of the composites, the difference decreased with increasing content of the cross-linked additive. The presence of the clPHB additive retarded the crystallization rate in the materials obtained. The clPHB/PHB 50/50 which contained the highest content of clPHB was totally amorphous after quenching before the second DSC run. The lowest value of DHm was observed for clPHB. It was almost two times lower than the value of DHm of PHB. Such a difference of DHm can be explained as a consequence of the existence of the insoluble or partially soluble branched/cross-linked part of the clPHB structure which, due to steric hindrance, probably does not crystallize. Another explanation of the observed phenomenon might be the plasticization of the PHB matrix with oligomeric products. For these reasons, reference samples with clPHB1, clPHB2 and oligPHB were prepared. It was observed (Fig. 4) that the addition of chloroform-soluble clPHB1 or insoluble clPHB2
fractions to PHB also decreased the crystallinity of PHB. Values of DHm for both materials are lower than in PHB and also the Tccs are in range of Tcc of clPHB/PHB composities. The results revealed that plasticizing effect was observed only in reference sample with oligoPHB. The DSC result for this sample showed the lowest value of Tg(glass transition) and Tcc and DHm comparable with PHB. However, the lower content of fraction branched oligomers clPHB in blend is not visible for DSC technique but still may generate as a plasticizer the changes in the mechanical properties of the material.
4.3. Crystallization process observation in polarized light microscopy (PLM) The result of the DSC analysis was proven by PLM examinations of the samples. The microscopic observation revealed that the high rate of crystallization of PHB sample was decreased by the addition
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Fig. 3. Thermograms of (2ndDSC run): 1) PHB, 2) clPHB/PHB 25/75 w/w, 3) clPHB/PHB 50/50 w/w, 4) clPHB.
Fig. 4. Thermograms of (2ndDSC run) of references samples: 1)clPHB1/PHB 50/50 w/w, 2)clPHB2/PHB 50/50 w/w, 3)oligoPHB/PHB 50/50 w/w.
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Fig. 5. The micrographs (160 magnification) of crystallization progress of melted samples after being quenched in water. Time of crystallization: a) 3 min at 23 C; b) 7 min(3 min at 23 C and 4 min at 30 C); c) 8min (3 min at 23 C and 4 min at 30 C and 1 min at 35 C); d) 9min (3 min at 23 C and 4 min at 30 C, and 2 min at 35 C).
of clPHB. Fig. 5 shows the crystallization process of PHB and its composites with clPHB after quenching. The PHB crystallized into small crystals population in whole volume at room temperature in less than 1 min. The high rate of crystallization of neat PHB was observed in the DSC result. The second DSC run of the sample revealed melting enthalpy DH ¼ 20 J/g after rapid cooling. In contrast, the composites demonstrated retarded crystallization at 35 C into crystal phase with many amorphous areas (Fig. 5d). The amorphous defects observed in the crystalline phase crystillized when the sample temperature was increased to 50 C. Another difference was that the crystals which appeared were bigger(Fig. 6). 4.4. Tensile properties and impact strength of clPHB/PHB composites The DSC experiment of mould injected samples clPHB/PHB revealed frozen crystallinity which resulted from the processing method. The results of the mechanical tensile test of composites are shown in Table 3. The composite's highest value of stress at the break was 42 MPa for conditioned material clPHB/PHB 25/75 w/w. In addition, Young modulus of the material was almost 20% lower than in the case of moulded PHB, while elongation at the break was 2% higher. The modulus of conditioned composite having 50% w/w clPHB content was 1008.6 MPa which was the lowest value, and
Fig. 6. The micrograph (160 magnification) during crystallization progress of melted sample after being quenched in water. The crystallization time: 12min (3 min at 23 C and 4 min at 30 C, and 2 min at 35 C, and 3 min 50 C).
about 50% lower than neat PHB modulus. This composite was also less brittle than neat PHB and the value of elongation at the break
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S. Jurczyk et al. / Polymer Testing 63 (2017) 614e620 Table 3 The tensile test data for PHB and clPHB/PHB composites. Sample
PHB PHB conditioned clPHB/PHB 25/75 w/w clPHB/PHB 25/75 w/w conditioned clPHB/PHB 50/50 w/w clPHB/PHB 50/50 w/w conditioned
s stress at maximum load [MPa] (S.D)
Е Young modulus [Mpa] (S.D)
Strain at Auto Break[%]
33.8(0.7) 43.1(1.2)
1884.2(61.0) 2010.1(120.0)
2.0(0.3) 1.6(0.5)
38.3(1.1)
1629.2(67.1)
3.5(0.3)
42.3(1.3)
1661.6(14.6)
5.7(0.8)
39.5 (2.1)
1389.6(60.2)
5.1(0.1)
36.7(0.2)
1008.6(87.2)
9.1(0.9)
was 9%. The increase of clPHB content in composite decreased their value of stress and modulus. In contrast, its elongation increased with the increase of clPHB content. The differences observed were connected to changing morphology, as was shown in the DSC results. The crystallinity of the prepared composites decreases and Tcc increases with the increase of clPHB content. Moreover, the filler interacted with the PHB matrix and acted as a crystallization retardant and blocker. The following experiment, in which the sample was conditioned, also showed the plasticizing properties of clPHB. The comparison of unconditioned and conditioned samples showed differences. The expected effect was observed in the conditioned blend clPHB/PHB 50/50%(w/w). This sample exhibits a 4% higher value of elongation than in the case of the unconditioned. The significant influence of the content of low-molecular crosslinked (which not co-crystallized with PHB) additive might well be the explanation for this phenomenon. The additive clPHB shows, in temperature of conditioning, melting endotherm (DSC thermogram Fig. 3) and probably diffuses from amorphous region into the bulk of the continuous PHB phase and efficiently plasticizes the material. In fact, the observed properties of clPHB were also proven by an additional mechanical test (impact resistance). The composite clPHB/PHB 25/75 demonstrated 1.2 kJ/m2(S.D ¼ 0.1) which is a higher impact strength than prepared analogously PHB 0.9 kJ/m2 (S.D ¼ 0.1). According to the literature data, the amorphous phase of material absorbs the shocks [19]. 5. Conclusion The influence of the addition of cross-linked PHB additive on the mechanical properties of PHB self-composite and its thermal properties were investigated. The experimental results pointed out that the addition of cross-linked additive decreased the crystallinity of mould-injected PHB and improved its mechanical properties. During PHB processing via mould injection, the method applied allowed us to obtain lower crystalline materials. The method of modification presented is not laborious and easy to apply in thermo-processing line, allowing for the elimination of the side effects of peroxide interaction with low stable fillers (e.g. natural fiber) which undergo undesired reactions in the presence of a strong oxidation agent. Acknowledgements This work was supported by the National Science Centre (NCN SONATA 3 project no. 2012/05/D/ST5/03384, “Synthetic analogues of aliphatic biopolyesters generating controlled response in the form of a mechanical effect on temperature stimulus”).
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