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Thermal and Rheological Properties of Modified Polyhydroxybutyrate (PHB).

INTRODUCTION

Bioplastics have already found their way in industry, replacing conventional plastics to some extent which are fossil-based and non-biodegradable. One main representative of bioplastics is polylactide (PLA) which is meanwhile used for many technical applications [1]. A further important representative is poly-[beta]-hydroxybutyrate (PHB), a polyester belonging to the family of polyhydroxyalkanoates. A great advantage of PHB is its production via biosynthesis yielding in a bio-based, non-toxic and catalyst-free polymer. At the biosynthesis, bacteria store the polymer in form of granules in their cells, having an energy source when they grow under stress conditions. The granules have diameters between 0.2 and 0.5 [micro]m. They are amorphous within the cells, whereas their stage is changing to semi-crystalline when they were isolated by extraction of the bacteria [2]. The structure of PHB becomes extremely regular since it is almost completely isotactic with the carbon atoms in the R(-)-configuration. This plays an important role in biodegradation processes. Only the R(-)-constitution can be recognized and degraded by microorganisms [3]. On the contrary, synthetically generated PHB cannot be degraded to a high extend, which is a further advantage for bio-based synthesized PHB.

Degradation takes place in natural environments like soil, water, compost, sewage sludge and marine sediment. There, some other factors like microbial activity, moisture level, temperature and pH of the environment as well as the surface area and the molecular weight of the polymer influence the degradation rate [4]. The PHB then shows completely biodegradation to C[O.sub.2] and [H.sub.2]O, without forming any toxic products [5].

PHB resembles in its properties like crystallinity, melting point and tensile strength the non-degradable and fossil-based isotactic polypropylene (iPP). A replacement of iPP by PHB and its potential application in industrial processes is currently only possible to a very limited extent because of its brittleness, deficiency in thermal stability and difficult processability [6].

The deficiency in thermal stability has been discussed in many reports [7, 8], in which a non-radical random chain scission reaction, the so-called cis-elimination, is considered as the main reason. Degradation products are crotonic acid, linear oligomers having a crotonate end group and dimers and trimers of crotonic acid [9].

In Ref. [10], further studies on the degradation of PHB were done and described to clarify the complex mechanism. It was found that the thermal degradation behavior is dependent on the reaction time and/or temperature. At the beginning also a random degradation--a nonauto-catalytic degradation--is suggested, whereas an auto-catalytic reaction based on the changes in molecular weight [11] is assumed to proceed subsequently in the middle stage of the degradation process. Here, especially the crotonic acid which is produced in an unzipping reaction at the end of the polymer chains, strongly accelerates the degradation and therefore an immense weight loss takes place. Furthermore, if the PHB is additionally exposed to high shear deformations for example in melt processing techniques like extrusion or injection molding it may also lead to a scission on the polymeric chain, causing a further decrease in molecular weight. As a consequence the crystallization temperature decreases and the crystallization time increases resulting in an increased stickiness on metallic surfaces, for example, in molds and dies. Additionally, due to the reduced molecular mass the formation of crystallization nuclei at cooling is reduced. Only less but big spherulites were formed resulting in a progressive decrease of the mechanical properties, such as tensile strength, in later products [12].

To overcome these problems some efforts have been made by the preparation of PHB-copolymers like poly(3-hydroxybutyrateco-hydroxyvalerate) or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate. Due to the longer side chains a reduction of intermolecular interactions is achieved, yielding in a reduced melting temperature and crystallization rate. For some compositions an improved thermal stability during processing could be reached as well as improved mechanical properties [12-14].

Despite this, the thermal and mechanical properties of PHB can be improved by adding plasticizers. So far the use of many plasticizers has been studied. Examples are oxypropylated glycerin, glycerol, glycerol triacetate, acetyl tributyl citrate, salicylic ester, soybean oil--to name only a few [4]. A major disadvantage of many plasticizers is their migration to the surface along with a reduced plasticizing effect over time if they are not linked chemically to the polymer matrix.

Another possibility to retard thermal degradation of PHB is the grafting of various unsaturated monomers, like acrylics as methyl methacrylate, 2-hydroxyethyl methacrylate, and acrylic acid or maleic anhydride and styrene, respectively [14, 15]. Also the partial branching and crosslinking by an organic peroxide like dicumyl peroxide and a multifunctional coagent, such as triallyl trimesate [16] as well as by the epoxy-functionalized chain extender Joncryl are routes to improve thermal stability by an increase in activation energy of thermal decomposition [13]. But most of these mentioned routes were performed in solution, in discontinuous batch processes with the help of a laboratory kneader (e.g., from Thermo Scientific Inc. or Brabender GmbH & Co KG) or in a mini-extruder where in a separate step the PHB have to be coated with the adequate reactive component before feeding into the extruder. The received amounts of each modified PHB were small--mostly only in laboratory-scale.

In this work, the aim was to retard degradation processes and thus to increase the thermal melt stability of PHB by branching and crosslinking. Chemical modification was performed by means of reactive extrusion on a twin screw extruder. The new feature is that the chemical modification of the PHB takes place continuously in a single stage process. Due to the direct feeding of PHB and the reactive components into the extruder this one-step process would be a solvent-free and thus environmentally friendly method.

One way for branching and crosslinking is the hydrogen abstraction from the tertiary carbon of PHB backbone which can be initiated by organic peroxides. In this work TBPB seemed to be a suitable peroxide because complete decomposition of the peroxide as well as sufficient reaction time during extrusion could be ensured. But as reported [16] at polymers like polypropylene and PHB processing in the presence of free-radicals cause often severe chain scission. The reaction has then to be assisted by a bi- or multi-functional coagent/modifier with unsaturated sites which can react with the induced free radical on the polymer backbone. As suitable modifiers triallyl cyanurate (TAC) with three allylic groups and divinylbenzene (DVB) with two vinyl groups were used to induce long-chain branching or crosslinking on PHB.

In a next step, plasticizing of the PHB materials was carried out by two different plasticizers. As a low molecular weight plasticizer rapeseed oil was added and as high molecular weight plasticizers two PEGs, namely PEG 1500 and PEG 4000, were chosen. In case of rapeseed oil, its unsaturated groups could react with the free-radicals on the PHB-backbone induced by the peroxide TPBP. In case of PEG, no peroxide was used to induce free radicals on the PHB-backbone. Here, a chain extension reaction takes place with the bi-functional 4,4'-methylenebis(phenyl isocyanate) (MDI) as coupling agent between the PHB and PEG, reacting with their end-groups. By both methods the plasticizer should be chemically linked to the PHB. The thermal and rheological properties of all materials were subsequently characterized.

EXPERIMENTAL

Materials

The PHB used in this study was a fine white powder purchased from Biomer, Krailling, Germany. As organic peroxide the TBPB with a half-live time of ~1 min at 165[degrees]C [17] was chosen because the mean residence time of the polymer and the reactants in the extruder (from feed section to the die) was around 4 min and thus, a complete decomposition of the peroxide as well as a sufficient reaction time was ensured. For modification various coagents/modifiers with different functionalities were used. An overview of them as well as the concentration and the material name is given in Tables 1 and 2. The selected modifier concentrations are based on literature studies.

Reactive Extrusion

PHB and PEGs were dried in a vacuum oven at 40[degrees] C for 20 h before use, while the other modifiers were used as received. The modifications were carried out on a twin screw extruder type ZSK26MC from Coperion GmbH, Stuttgart, Germany, equipped with a double-strand die. During extrusion, the temperature profile was of 60[degrees]C (zone 1), 150[degrees]C (zone 2), and 185[degrees]C [+ or -] 3[degrees]C (zone 4-10). The die was also heated to 185[degrees]C. For analyzing the influence of the screw speed on the degradation of PHB experiments were carried out at screw speeds of 100 and 150 [min.sup.-1]. Modification of PHB was done at a constant screw speed of 150 [min.sup.-1]. The fine PHB powder and the modifiers in solid powder form were added gravimetrically in zone 1. The throughput of PHB varied from 2 to 4 kg/h. Liquid modifiers were added volumetrically by means of a medical infusion pump in zone 1, too. If the viscosity of the modifier was too high for pumping, it was diluted by anhydrous toluene. The polymer strands were air-cooled on a conveyor belt before granulated. All materials were compounded in a one-step process.

Thermal Characterization

The thermal behavior of the samples was analyzed by differential scanning calorimetry (DSC) on a DSC 2/400 from Mettler-Toledo. Samples of about 10 mg were weighed and sealed in aluminum DSC crucibles and placed in the DSC cell. They were first heated from -20[degrees]C to 200[degrees]C at a rate of 10 K [min.sup.-1] under nitrogen atmosphere. Then the samples were cooled from 200[degrees]C to -20[degrees]C at 10 K [min.sup.-1]. They were then heated again to 200[degrees]C at 10 K [min.sup.-1]. The crystallinity of the materials was calculated by the use of a heat of fusion of 146.6 J [g.sup.-1] for 100% crystalline PHB homopolymers [15].

To study the thermal stability of PHB and the modified materials thermogravimetric analysis (TGA) were done on a Mettler TGA 850. The samples were first heated from 30[degrees]C to 450[degrees]C under nitrogen atmosphere and subsequently further heated to 850[degrees]C under normal atmospheric conditions in order to pyrolyze the samples.

Rheological Properties

The rheological characterization in shear flow was performed with a plate-plate rheometer from Rheometrics Inc. at 180[degrees]C with a diameter of 25 mm and a gap of 1 mm under nitrogen atmosphere. Dynamic mechanical experiments were carried out in a frequency range from 100 to 0.1 rad [s.sup.-1]. The strain was of 5% for all measurements and thus inside the linear viscoelastic region.

RESULTS AND DISCUSSION

Influence on Thermal Properties

An overview of the transition temperatures and enthalpies as well as the crystallinity of the unmodified and modified PHBs, which resulted from the second heating scan, are listed in Table 3. The values of the glass transition [T.sub.g] are not listed, because no significant changes were observed. The transition from the glassy state to the rubbery state takes place between -5[degrees]C and 10[degrees]C in all materials. The unmodified extruded PHB_ex exhibit a cold crystallization of only small amounts of the polymer ([DELTA][H.sub.cc] = 5.7 J/g) over a broad range from around 65[degrees]C to 130[degrees]C. In this range imperfect metastable crystals of different types were formed [18]. Subsequently, a broad melting area with a peak maximum at 178[degrees]C followed. In case of the peroxide modified PHBs (PHB_1A-C) as well as of the PHBs treated with peroxide and coagent (PHB_2A-D) no cold crystallization occurs, but also a broad melting peak can be observed, which maximum is shifted to slightly lower values. A similar behavior displays the material being treated with peroxide and DVB (PHB_3A) and the one additionally plasticized with rapeseed oil (PHB_3B). The PHB plasticized with PEG (PHB_4A-C), on the other hand, resemble in their thermal behavior the PHB_ex. They show also a cold crystallization over a broad range where the crystals melt and recrystallize continuously during the heating until crystalline phase finally melt. Thereby, the higher the molecular mobility, the faster the process occurs. The materials with PEG 4000 exhibit a farther small peak ([DELTA][H.sub.m0] < 5 J/g) with a maximum at ~45[degrees]C. Since this temperature is between the melting point of PEG 4000 ([T.sub.pm] = 53[degrees]C-58[degrees]C) and 4,4'-MDI ([T.sub.pm] = 40[degrees]C) it is assumed that this peak is attributed to small amounts of a mixture of PEG and 4,4'-MDI or maybe to PEG reacted with 4,4'-MDI. To get more information about these components dissolution of them out of the polymer matrix and subsequent characterization should be done in further studies.

In terms of crystallinity, all materials which were branched/partially crosslinked by the organic peroxide TBPB and by a coagent (PHB_1A-3B), respectively, exhibit a little bit higher values than the PHB_ex. In contrast, the PEG-plasticized materials (PHB_4A-C) display a slightly decrease of crystallinity, suggesting that the crystallization rate may be reduced due to the interference of PEG-chains being located in the interlamellar regions of the PHB.

During cooling of PHB materials a significant increase of the crystallization temperature is shown for all modified PHBs with the exception of plasticized materials (Table 4). For the peroxide modified PHB (PHB_1B-C) the crystallization temperature rises by around 16 K.

From literature [14, 16] it is known that the spherulite size of PHB decreases enormous by branching/partial crosslinking resulting in a pronounced nucleation and thus in an enhanced crystallization temperature. An explanation might be that the mobility of the PHB chain segments which are near the chemical linkage of the modifier is limited. Packing of the polymer chains starts in these areas, which then can act as nucleating points upon further cooling [15]. Thereby, the size of the spherulites reduces with increasing content of the modifier. Furthermore, the range where crystallization occurs could be reduced due to a faster crystallization rate. In case of PEG-modified PHB no branching points arise on the PHB backbone since modification was only done by chain extension reaction at the end-groups of PHB. Therefore, the mobility of the chain segments almost remains the same compared to the unmodified PHB, and no nucleating points are formed within the PHB backbone. The only restriction is the mobility of the complete chain due to its higher chain length accompanying with an increase of entanglements and the formation of a physical network. But this does not lead to any significant improvement of nucleation as it can be seen from the crystallization temperatures. Besides this, the crystallization process extends over a wide range, due to the low crystallization rate which may be ascribed to an interference of the lamellar structure by PEG-chains.

The effect of modification on the thermal behavior of PHB materials was additionally investigated by TGA (Fig. 1). It is noted that decomposition of the PHB_ex and the branched/partial cross-linked PHBs (PHB_1A - 3B) occurs in a single stage process whereas all PEG-modified materials (PHB_4A-C) show a second decomposition step. With the exception of PHB_3A which exhibit an onset degradation temperature already at 240[degrees] C, all other modified materials as well as the unmodified PHB start to decompose at 260[degrees]C, having its major weight loss of around 50% in a similar range. It is proposed that this major step can be ascribed to the random chain scission of PHB by intramolecular cis-elimination, where degradation products like crotonic acid, linear oligomers with a crotonate end group and dimers and trimers of crotonic acid were formed. The incorporation of branches/crosslinks does not alter the thermal decomposition process. In case of PEG-modified materials (PHB_4A-C) the second decomposition step appears in a range of 310[degrees]C-430[degrees]C. The height of this step at its beginning is approximately proportional to the amount of PEG and MDI added at the reactive extrusion to the PHB polymer. Therefore, this stage is attributed to the decomposition of PEG-extended PHB.

Influence on Rheological Properties

It is known [19] that the screw speed of the extruder and thus the shear deformation affects the degradation behavior of PHB in addition to thermal stress. In order to find out how large this influence is, extrusion of PHB powder was done at screw speeds of 100 and 150 [min.sup.-1], respectively. At smaller screw speeds no sufficient feeding of the fine powder was possible whereas at higher speeds the residence time of the polymer gets too short with regard to the reaction time of the modifiers. The degree of degradation was then determined by measuring the complex shear viscosity I[eta]*I as a function of the angular frequency [omega] (Fig. 2). At higher angular frequencies, nearly down to 10 rad/s, almost no differences in the complex shear viscosity of both samples can be seen. The largest difference of almost 500 Pa s appears at an angular frequency of 1 rad/s. This shows that the influence of the applied screw speeds is not relevant on degradation process. At further experiments, the material PHB ex 150 [min.sup.-1] was used as reference and is hereinafter only referred as PHB_ex.

Much more conspicuous is the enormous decrease of the complex viscosity at small frequencies. The reason is that the measurements were carried out from high to small frequencies and due to deficiency in thermal stability the PHB starts to degrade by random chain scission already during the measurement causing a reduction of molecular weight and thus of viscosity.

In order to increase thermal melt stability of PHB chemical modification was done by reactive extrusion. As mentioned before, it was supposed that degradation process can be retarded by branching and crosslinking of PHB due to the hindrance of chain scission mechanisms. In a first step, the organic peroxide TBPB was added in three different concentrations to the PHB in order to identify the dependence of the peroxide content on the extent of chain scission and thus loss of viscosity by free-radical mediated reaction. As depicted in Fig. 2, at a small content of 0.2 wt% of TBPB (PHB_1A) the course of the curve is nearly the same as that of the unmodified PHB_ex, showing that no remarkable reaction occurred. Adding 0.3 and 0.4 wt% TBPB (PHB_1B + 1C), respectively, resulted in much more pronounced shear thinning properties as well as in a significantly enhanced complex viscosity at lower frequencies. This behavior is typical for polymers with long chain branches [20]. At 0.4 rad/s the melt viscosity of PHB_1C was nearly three times higher than that of the PHB_ex. These results are in contrast to that described in Ref. [16] where processing in the presence of free-radicals cause severe chain scission on the PHB. By the use of TBPB branching/crosslinking of PHB occurred. Besides this, it is noteworthy that the degradation process could be retarded through these free-radical mediated reactions. Whereas the unmodified PHB shows a reduction of complex viscosity and thus of molecular mass already at a frequency of around 1 rad/s the PHB modified with 0.4 wt% TBPB exhibits degradation still at 0.3 rad/s.

Regarding the storage modulus G' (Fig. 3) the relaxation behavior of the unmodified PHB and the low modified PHB (PHB_1A) differ from that of the high modified PHBs (PHB_1B + 1C). In the low frequency range higher values for G' and a lower frequency dependency of G' was found for PHB_1B + 1C. It is assumed that the increased storage modulus is due to a higher elasticity of the materials caused by the higher molecular weight and the entanglements of chain branches and/or the formation of a network through partial crosslinking. The PHB_ex and the PHB_1 A show a typical behavior of linear polymers with a slope of 2 in the low frequency range.

In order to evaluate the influence of branches, possibly long chain branches and/or broad molecular weight distributions on the shear rheological behavior, phase angle [delta] was plotted against the complex modulus IG*I (van Gurp Palmen Plot). As shown in Fig. 4 the curve of PHB_1A is again nearly the same as that of the unmodified PHB, which shape is characteristic for polymers with linear structure. At low values of IG*I the phase angle [delta] is nearly 90[degrees], meaning that for both materials a viscous deformation behavior dominates. The curve of PHB_1B + 1C, respectively, shifts to lower phase angle values. This can be attributed to a structural change like branching and higher polydispersity through the free-radical mediated reaction, which leads to higher relaxation times compared to the linear PHB_ex. If branching has occurred, it is likely that a star-like structure was formed because in case of polyethylene the phase angle decreases monotonously in the presence of star-like structures (a special long chain branched structure),whereas for comb-like polyethylenes, a wavy change of the 5 appears with the increase of [absolute value of G*] [20,21].

In a second step, a coagent with unsaturated sites was used to assist the free-radical mediated reaction. In case of TAC three allylic groups are available that can react with the peroxide-induced radicals on the polymer backbone. As depicted in Fig. 5 the course of the curves of the unmodified (PHB_ex) and low modified PHB (PHB_2A + 2B) are similar. At PHB_2A degradation reactions still dominate branching since complex viscosity is lower than that of PHB_ex over the whole range. By enhancing the amount of coagent only a little bit degradation and branching reactions are in equilibrium and therefore no higher viscosities could be achieved. A strong rise of the viscosity at the low frequency range can be observed when the amount of TAC and TBPB grows up (PHB_2C + 2D). At a frequency of 1 rad/s the complex viscosity of the material with 0.4 wt% of TAC (PHB_2D) is more than three times higher than that of the unmodified PHB. At 0.2 rad/s the viscosity is even about a factor of 10 higher and no degradation is still observed whereas the PHB_ex already degrades. These results show that by the modification with a peroxide in combination with a coagent thermal melt stability could be substantially enhanced.

The behavior of the storage modulus G' (Fig. 6) is quiet similar to that observed for the peroxide-modified PHB materials (PHB_1A-C) depicted in Fig. 3. The values of the low modified material (PHB_2A) also resemble them of the PHB_ex demonstrating viscous properties. With increasing content of TAC G' shifts to ever higher values in the low frequency range. This can be ascribed to the presence of long branches or crosslinks induced by the tri-functional coagent which leads to an enhanced elasticity of the materials.

The Van Gurp Palmen plot (Fig. 7) presents a pronounced reduction of the phase angle upon increasing the content of TAC, indicating longer relaxation times. In case of PHB_2A the course of the curve corresponds to that of PHB_ex. No considerable change neither in molecular structure, nor in molecular weight has occurred. For PHB_2B the phase angles are smaller than 90[degrees] at lower IG*I and decrease then monotonously with the increase of [absolute value of G*], suggesting a star-like structure [21]. With enhanced amount of TAC [delta] becomes more and more independent of IG*I. The phase angle of the material with 0.4 wt% of TAC is of a constant value of ~30[degrees] and thus completely independent of IG*I.

A similar behavior of coagent-modified PHB is described in Ref. [16]. It is proposed that this can be ascribed to a highly long chain branched structure with partially crosslinking and pronounced elastic properties. But since the PHB_2D exhibit still a melting transition with an enthalpy value higher than that of the PHB_ex the network can be only of less crosslinking points. The mobility of the chain segments between these crosslinking points as well as the mobility of the long chain branches will then be still high enough for melting at higher temperatures.

As mentioned before, plasticization of PHB materials was carried out by two different types of plasticizer. As a low molecular weight plasticizer, rapeseed oil was added and as high molecular weight plasticizers PEG 1500 and PEG 4000 were chosen. By means of both plasticizers, the extensibility of the melt strands after reactive extrusion was significantly increased. Detailed characterization of the melt strength by Rheotens measurements and determination of the mechanical properties will be done in further studies and will be subject of a subsequent paper. Regarding the complex viscosities in dependence of the angular frequency (Fig. 8), the curves of the PHB_ex and the PHB modified with TBPB and DVB (PHB_3A) nearly overlap. No enhanced branching and thus, no change in melt viscosity could be achieved by means of the bifunctional DVB and the applied concentration ratio of TBPB and DVB, respectively. But by adding 7 wt% rapeseed oil the viscosity could be increased by more than one and a half times (0.5 rad/s). Unfortunately, degradation reactions could only be less retarded as demonstrated by the loss of viscosity at low frequencies. In case of plasticizing with PEG, the course of the curve resembles that of the PHB_ex by adding 6 wt% of PEG 4000 (PHB_4B). The complex viscosity could only be increased slightly and the random chain scission reaction also could not be stopped. On the contrary, a completely different behavior could be observed by increasing the amount of PEG 4000 to 10 wt% (PHB_4C). Degradation reactions could almost be prevented and the complex viscosity could be increased by a factor of nearly 4.5 at a frequency of 0.1 rad/s. It is assumed that this can be ascribed to the higher number of hydroxyl-groups. Thus, random chain scission of PHB is more hindered, since more PEG-end-capped PHB-chains exist. The same behavior could be observed by the use of 6 wt% of PEG 1500 (PHB_4A). Although the concentration of PEG 1500 is equal to the concentration of PEG 4000 in PHB_4B degradation could be greatly reduced and the complex viscosity could be increased by a factor of about 6 (0.1 rad/s). An explanation might be that due to the lower molecular weight of PEG 1500 its number of hydroxyl-groups is of 2.7 times higher than that of PEG 4000 at equal concentrations. Thus, also more PEG-end-capped PHB-chains could be formed which prevent degradation processes.

Regarding the storage modulus (Fig. 9), at the low-frequent range a shift to higher values of G' can be observed for all modified materials. For PHB_3B a less dependency of G' is achieved at frequencies above 1 rad/s, being an indication for branched structures. The curves of the PEG-modified PHB (PHB_4A-C) exhibit a similar course than that of the PHB_ex indicating a linear structure due to chain extension reaction. In case of PHB_4A and PHB_4C the course of the curve is flatter at low frequencies, which can be attributed to the formation of a physical network due to a high number of entanglements. The highest elasticity could be achieved for the PHB_4A since here the number of chain extended polymer chains and thus the polydispersity is most severe.

Figure 10 provides more information about the molecular structures. The course of the curves of PHB_3A, PHB_4B, and PHB_4C are very similar to that of the linear PHB_ex. They also exhibit a phase angle [delta] near 90[degrees] at low values of IG*I, which confirm the presence of linear structures. The curve of PHB_3B and PHB_4A, respectively, shifts to lower phase angle values indicating a structural change. For PHB_3B branching and the formation of a star-like structure will be likely. For the PHB_4A longer relaxation times were obtained although a linear structure should have been resulted due to chain extension. To get more information about the molecular structure further studies would be necessary.

CONCLUSIONS

Chemical modification of PHB with different modifiers was successfully performed in a solvent-free and environmental friendly one-step process by reactive extrusion. The resulting branched and partially crosslinked materials affect the thermal properties of PHB substantially. A significant increase of the crystallization temperature and rate lead to an improved crystallization behavior. The rheological characterization of the shear flow behavior revealed that the PHB modified with a combination of TBPB and TAC (PHB_2D) exhibit a melt viscosity being about one order of magnitude higher than that of the unmodified PHB. Additionally, a distinct retardation of degradation processes and thus an enhanced increase of thermal melt stability could be achieved. Plasticization of PHB by different PEG leads to a higher extensibility of the melt strands as well as to a reduction of crystallinity and a change in thermal decomposition. By the use of PEG 1500 (PHB_4A) also a remarkable improvement in the melt viscosity and the melt stability could be achieved. These results show that it is possible to affect and improve the properties of PHB substantially by chemical modification. The approach to combine the good properties of PHB_2D and PHB_4A is currently in progress with the aim to open the way for its potential application in industrial processes.

Sandra Weinmann (iD), Christian Bonten

Institut fur Kunststofftechnik, University of Stuttgart, Stuttgart, 70569, Germany

Correspondence to: S. Weinmann; e-mail: sandra.weinmann@ikt.uni-stuttgart.de

Contract grant sponsor: Institut fur Kunststofftechnik.

DOI 10.1002/pen.25075

Published online in Wiley Online Library (wileyonlinelibrary.com).

ACKNOWLEDGMENTS

The research work described in this paper has been financed exclusively by the Institut fur Kunststofftechnik. Funding by any organization has not taken place.

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Caption: FIG. 1. TGA curves of the thermal decomposition of unmodified and modified PHB.

Caption: FIG. 2. Complex shear viscosity as a function of angular frequency of unmodified PHB ex 100 [min.sup.-1] and PHB ex 150 [min.sup.-1] (PHB_ex) as well as peroxide-modified PHBs (PHB_1A-C).

Caption: FIG. 3. Storage modulus G' as a function of angular frequency of unmodified PHB (PHB_ex) and peroxide-modified PHBs (PHB_1A-C).

Caption: FIG. 4. Phase angle as a function of complex modulus of unmodified PHB (PHB_ex) and peroxide-modified PHBs (PHB_1A-C).

Caption: FIG. 5. Complex shear viscosity as a function of angular frequency of unmodified PHB (PHB_ex) and PHB modified with a mixture of TBPB and TAC (PHB_2A-D).

Caption: FIG. 6. Storage modulus G' as a function of angular frequency of unmodified PHB (PHB_ex) and PHB modified with a mixture of TBPB and TAC (PHB_2A-D).

Caption: FIG. 7. Phase angle as a function of complex modulus of unmodified PHB (PHB_ex) and PHB modified with a mixture of TBPB and TAC (PHB_2A-D).

Caption: FIG. 8. Complex shear viscosity as a function of angular frequency of unmodified PHB (PHB_ex), PHB treated with a mixture of TBPB and DVB (PHB_3A) and additionally plasticized with rapeseed oil (PHBJB) and PHB plasticized with PEG (PHB_4A-C).

Caption: FIG. 9. Storage modulus G' as a function of angular frequency of unmodified PHB (PHB_ex), PHB treated with a mixture of TBPB and DVB (PHB_3A) and additionally plasticized with rapeseed oil (PHBJB) and PHB plasticized with PEG (PHB_4A-C).

Caption: FIG. 10. Phase angle as a function of complex modulus of unmodified PHB (PHB_ex), PHB treated with a mixture of TBPB and DVB (PHB_3A) and additionally plasticized with rapeseed oil (PHBJB) and PHB plasticized with PEG (PHB_4A-C).
TABLE 1. Overview of used coagents/modifiers.

Coagent/modifier       Abbreviation           Manufacturer

Tert.-                     TBPB       Merck KGaA, Darmstadt, Germany
Butylperoxybenzoate

Triallyl cyanurate          TAC       Merck KGaA, Darmstadt, Germany

Divinylbenzene              DVB       Merck KGaA, Darmstadt, Germany

4,4'-methylenebis           MDI       Merck KGaA, Darmstadt, Germany
  (phenyl
  isocyanate)

Polyethylene glycol       PEG 1500    Merck KGaA, Darmstadt, Germany
1500

Polyethylene glycol       PEG 4000    Merck KGaA, Darmstadt, Germany
4000

Rapeseed oil                --          Brokelmann+co--Oelmuhle
                                        GmbH+co, Hamm, Germany

TABLE 2. Overview of the modified PHBs and the extruded
unmodified PHB (PHB_ex) as reference.

Sample name      Coagent/modifier      Concentration
                                          in wt%

PHB_ex                  --                  --
PHB_1A                 TBPB                 0.2
PHB_1B                 TBPB                 0.3
PHB_1C                 TBPB                 0.4
PHB_2A               TBPB/TAC           0.02/0.125
PHB_2B               TBPB/TAC            0.02/0.2
PHB_2C               TBPB/TAC            0.03/0.3
PHB_2D               TBPB/TAC            0.04/0.4
PHB_3A               TBPB/DVB             0.2/0.5
PHB_1B         TBPB/DVB/rapeseed oil     0.2/0.5/6
PHB_4A             PEG 1500/MD1             6/2
PHB_4B             PEG 4000/MDI             6/2
PHB_4C             PEG 4000/MDI            10/4

TABLE 3. Summarized DSC results of the second
heating scan of PHB_ex and modified PHBs.

Sample    [T.sub.cc] in   [DELTA][H.sub.cc]
           [degrees]C      in J [g.sup.-1]

PHB_ex         95                5.7
PHB_1A         93                5.9
PHB_1B         --                --
PHB_1C         --                --
PHB_2A
PHB_2B         --                --
PHB_2C         --                --
PHB_2D         --                --
PHB_3A
PHB_3B         --                --
PHB_4A         93                9.7
PHB_4B         95                7.0
PHB_4C         94                6.2

Sample    [T.sub.p,m] in   [DELTA][H.sub.m]   [alpha]
            [degrees]C     in J [g.sup.-1]     in  %

PHB_ex         177               90.8           61
PHB_1A         176               92.1
PHB_1B         172               94.8           65
PHB_1C         169               92.2           63
PHB_2A         175                94            64
PHB_2B         175               92.2           63
PHB_2C         174               94.3           64
PHB_2D         176               94.2           64
PHB_3A         174               94.1           64
PHB_3B         174               95.0           65
PHB_4A         176               84.3           58
PHB_4B         174               83.5           57
PHB_4C         176               80.1           55

[T.sub.cc], cold crystallization peak temperature;
[T.sub.p,m], melting peak temperature; [DELTA][H.sub.cc],
enthalpy of cold crystallization; [DELTA][H.sub.m],
enthalpy of melting; [alpha], crystallinity.

TABLE 4. Summarized DSC results of the 1st cooling
scan of unmodified and modified PHBs.

Sample   [T.sub.c] in   [DELTA][H.sub.c]
          [degrees]     in J [g.sup.-1]

PLA_ex        90              76.3
PHB_1A        92              76.7
PHB_1B       116              84.6
PHB_1C       117              82.3
PHB_2A       100              78,9
PHB_2B       106              79.0
PHB_2C       110              82.2
PHB_2D       111              83.9
PHB_3A       107              82.6
PHB_3B       108              81.9
PHB_4A        83              67.2
PHB_4B        88              68.2
PHB_4C        90              66.1

[T.sub.c] crystallization peak temperature,
[DELTA][H.sub.c] enthalpy of crystallization.
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Author:Weinmann, Sandra; Bonten, Christian
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EUGE
Date:May 1, 2019
Words:6184
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