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Understanding the Morphology Formation and Properties of Polyamide 6 and Bio-based Poly(Trimethylene Terephthalate) Blends.

INTRODUCTION

Responding to a worldwide demand of environmental improvement, the circular economy is promoting the concept of regeneration and restoration of the materials while creating a guide line for the modern research area [1]. In this context, the initiatives to reduce the negative impacts of chemicals by synthesizing new eco-responsible building blocks and the redefining of postconsumed products are in constant increase. Environmental concerns caused by petro-based polymer materials, on both health and environment, have enabled the emergence of an exponential number of researches and legislation on ways to propel the use of vegetal fibers and biomaterials in the current plastic applications. The use of green carbon allows reducing the C[O.sub.2] footprint, increasing the safety of the building blocks extraction processes on the environment while limiting our dependence to fossil feedstock supplies.

Through all the polymer classes, polyamides, commercially known as nylons, present a particular interest due to their high engineering benefits (high range temperature and easy processability, good mechanical and thermal properties, etc.) and high applications benefits (low density, durability, etc.). To satisfy the increasing demand for bio-materials, bio-nylons are beginning to emerge in either fully or partially bio-based forms. For a couple of years, partially bio-based nylons such as nylon 11 [2, 3] and more recently: nylon 4.6, [2, 3] nylon 4.10, [2, 3] nylon 6.10, [2-4] and nylon 10.10, [2, 3] have been developed. The feedstock issued from biomass allows, in certain cases, the development of new polymers, as for the novel bio-nylons, PA5.4 and PA5.10. The relatively high costs to produce biobased nylons compared to their petrochemical homologues, makes their use difficult for wide industrial applications. For this reason, nylon 6 (issued from the ring opening polymerization of a lactam) and nylon 6.6 (synthesized via the polycondensation of a diamine with a diacid), although mainly petro-based, are still the most commonly consumed. They mainly share the market in textiles, packaging and automotive applications, with about 7.4 million tons of nylon 6 and nylon 6.6 being produced in 2016 [5]. Every year about 4-6 million tons of fibers worldwide used for carpet industries are disposed whose 60% are nylon 6/6.6 [6]. For fibers applications, while PA6.6 has a greater resistance than PA6, the latter has a better resistance to light, dyeability, elastic recovery, wear resistance and thermal stability [7].

Poly(ethylene terephthalate) PET, poly(trimethylene terephthalate) PTT and poly(butylene terephthalate) PBT are all polyesters of which partially bio-based varieties exist commercially, by the replacement of the diol. One of the most successful partially bio-based polyesters is PTT, which is synthesized from 1,3-propanediol (PDO) and terephthalic acid or dimethylterephthalate [8]. This linear aromatic thermoplastic is, nowadays, commercialized by Dupont from renewable sources containing between 20 and 37% of renewable material (35% in this study). The bio-content arises from the derivatization of glucose into PDO, through a fermentation stage, making the monomer bio-based [8]. As with nylon 6, PTT is particularly suitable for carpet fibers application due to its low modulus, better bending and tensile elastic recovery in comparison to PET and PBT; providing soft and stretch fabrics with appreciable touching [9, 10]. Moreover, in comparison to its homologous PET and PBT, PTT is less sensitive to moisture [11].

Studying PTT/PA6 blends is particularly relevant from a carpet recycling perspective. Carpet recycling activities are expanding due to the increasing amount of fibers and potential resources represented by post-consumed carpets. Many organizations, such as: The Carpet America Recovery Effort (CARE) in USA and the Carpet Recycling Europe (CRE) in Europe, have started to collect and recycle post-consumer carpets. The carpet market is predominantly using PA6/6.6 (70%), PET/PTT (20%) and PP (10%). However, it is expected that the PP application will be reduced and partially replaced by PET/PTT which should reach the same consumption amount to PA6 from now to 2020 [12, 13]. The recovery of individual polymers is a long and costly process, for example, the density difference between PTT (1.32 g [cm.sup.-3]) and PA6 (1.15 g [cm.sup.-3]) makes difficult to mechanically separate these two polymers by floatation. For this reason, most of the time recycled fibers are extruded and reused for car applications where the mechanical properties of the blends can be improved by the addition of chemicals or fillers [14].

The works relating to the effects of blending a polyamide with a polyester show that they are immiscible and have poor interaction. For example, PET/PA6 blends without the addition of compatibilizer, present worse properties than the neat polymers, PET and PA6 [15]. The process of polyamide with PTT has been the topic of several researches already in the literature: PTT/polyamide 12 [16, 17] and PTT/polyamide 6.10 [18] focusing mainly on the viscosity, morphology and crystallinity aspects, while no mechanical properties were provided. The blend ratio 50/50 wt% was showing unexpected droplet morphology for all the formulations prepared. However, four main parameters are influencing the blend morphology evolution: the blend composition, the Theological properties of the starting materials, the processing temperature and screw speed. According to the processing parameters used: the size, shape and distribution of the phase can change influencing the blend morphology. The blend is also highly dependent of the interfacial tension between the two (or more) polymers. Indeed, most hybrids result in an immiscible blend composed of coarse morphology when interfacial modifications with compatibilizers are not applied. In this study however, the goal is to better understand the intrinsic interaction of PTT/PA6 during blending in several processing conditions, in absence of compatibilizers. A special focus on the 50/50 wt% blend of PTT/PA6 was selected since any type of morphology is possible at this ratio and following the accepted model trends of polymer blending, such as the Kerner and Nielsen models, this ratio is where the morphology influences the properties the most. Then, in addition of being the most challenging ratio, the 50/50 wt% blend additionally presents a very relevant case for the carpet recycling of PTT/PA6 blends as presented before.

By varying the processing conditions, a preliminary investigation on the morphological development of the 50/50 wt% PTT/PA6 blends was carried on atomic force microscopy (AFM) and scanning electron microscopy (SEM) highlighting the unexpected behavior of the 50/50 wt% blends processed. Therefore, rheology measurements and surface tension calculations were conducted to explain the morphological change observed as a consequence of the processing conditions applied. Finally, the mechanical performances (tensile, flexural and tensile) of the different extruded samples were measured and related to the developed morphologies.

EXPERIMENTAL METHODS

Materials

Commercially available grades of polyamide 6 (PA6) and partially bio-based poly(trimethylene terephthalate) (PTT) were used for processing. Polyamide 6, product name Ultramid B27E with a density of 1.12-1.15 g [cm.sup.-3] and a melting point of 220[degrees]C, was supplied by BASF. Weight average molecular weight ([M.sub.w]) of B27E is 65,200 g [mol.sup.-1] [19], Poly(trimethylene terephthalate), PTT product name SORONA 3301 BK001 with a density of 1.32 g [cm.sup.-3] and a melting point of 228[degrees]C, was supplied by Dupont. Weight average molecular weight ([M.sub.w]) of Sorona is 56,300 g [mol.sup.-1] [8]. For contact angle measurements, diiodomethane with a purity >99% was ordered from Acros organics and ethylene glycol with a purity [greater than or equal to] 99% was purchased from Sigma-Aldrich. To solvent etch PA6 phase before SEM investigation, formic acid, 98+% pure was used and supplied from Acros organics.

Blends Preparation

Prior to blending, PA6 and PTT were predried for 16 h under vacuum at 120[degrees]C in order to remove moisture which can lead to hydrolysis. The moisture content of PTT (max <0.02%) and PA6 (max <0.10%) after drying was ~0.015% and ~0.10% respectively. Then, PTT/PA6 blends at the weight ratio 50/50 wt. % were melt-processed in a twin screw extruder (LeistritzMicro-27 in co-rotation mode) at three different temperatures: 240[degrees]C, 250[degrees]C and 265[degrees]C. The pure polymers, PTT and PA6, were processed at 265[degrees]C. The extruder screw speed was set at 70 rpm for all the samples and one test at 265[degrees]C and 200 rpm was done to investigate the influence of shearing rate on the properties. At the nozzle, the extrudates were quenched in a water bath before being repelletized. Then the pellets were dried at 120[degrees]C for another 16 h under vacuum before injection molding. The pellets were injection molded (Arburg AllRounder 77 Ton Co-Injection Moulding Machine) into tensile, flexural and impact test specimens at the temperature as used for processing. The pressure was kept at 5 bars and held for 20 sec. at a mold temperature 70[degrees]C prior to demolding. Finally, the samples were conditioned for 2 days at 25[degrees]C under 50% humidity before testing (ASTM D618-08, procedure A).

Measurements

Atomic force microscopy, AFM, measurements were performed using an AFM instrument Multimode 8 (Bruker) and Nanoscope software for scans analysis. The peak force quantitative nanomechanical (PFQNM) mode was applied to map the morphology surface of the samples. Before any testing, injection molded samples were prepared using a Leica ultramicrotome for the purpose of obtaining polished surfaces which were analyzed in their transverse direction.

The scanning electron microscope, SEM, was a Phenom ProX set to an accelerating voltage of 10 kV. All specimens tested in the SEM were impact fracture surfaces from the notched Izod impact tests of the blends. The specimens were etched in formic acid for 5 min at room temperature to remove the PA6 phase.

The rheology measurements were performed using a MCR302 rheometer (Anton Paar GmbH) using a parallel plate configuration working in oscillatory mode. The plates diameter used was 25 mm, with a measurement gap distance set at 1 mm. A strain of 10% was applied which was determined to be within the linear viscoelastic region. After drying, the rheological behavior of neat pellets of PTT, neat pellets of PA6 and the 50/ 50 wt% extruded blend pellets were investigated at the processing temperatures that is, 240[degrees]C, 250[degrees]C, and 265[degrees]C, under oxygen atmosphere. Sweeps in the range of 1-1,000 rad [s.sup.-1] were performed.

Contact angle measurements of the extruded resins were done at room temperature on a goniometer Model 260 (RameHart Instruments Co) employing water, ethylene glycol and diiodomethane as liquids. The static sessile drop method was applied at 50% humidity on injection molded samples to assess the interfacial tension of binary blends, the contact angles were measured using the tangential method, and were calculated based on the Young's Equation. The contact angle was measured six times on two different regions of three surfaces, per sample. The variation of contact angle values was around 2[degrees].

The tensile and flexural tests were performed on a Universal Testing Machine (Instron). The tensile properties were measured following the ASTM D638 procedure. The tensile properties of the blends were measured at 5 mm [min.sup.-1]. The flexural measurements were carried out following the ASTM D790 specification, procedure B. The crosshead speed used for flexural tests was 14 mm [min.sup.-1]. The notched izod impact strength properties were measured on impact mode, a notching cutter was used to notch all the specimens before testing. The measurements were performed on a Monitor Impact Tester (Testing Machines Inc.) according to the ASTM D256 procedure. For all the tensile and flexural samples investigated an average of five measurements was performed whereas six tests were tested for impact, the standard deviation are reported.

RESULTS AND DISCUSSION

Blends Morphology Observation

Surface Analysis via Atomic Force Microscopy. Atomic force microscopy (AFM) was used to analyze the overall morphology surface of the PTT/PA6 blends and to measure the size of thendomains. The observation of the 50 [micro]m topographic image of the processed blends at 240[degrees]C Fig. 1a and 265[degrees]C Fig. 1b highlights the immiscibility of PTT/PA6 blends for both processing temperature investigated. While most of the blends processed with a ratio 50/50 show a co-continuous morphology, in this case a droplet like morphology is achieved after processing in both conditions. A similar result was observed by AFM for PTT/PA12 blends [17] though the phases were not identified. By using FESEM on PTT/PA6, 10 [20] a columnar structure was suggested and Jordhamo relationship allowed predicting that PTT would act as the continuous phase while the dispersed cylinders would correspond to PA6,10. In this work, by revealing the material contrast in polymer blends surface, AFM scanning mode allowed distinguishing two phases: a region of dark contrast corresponding to the polymer of lower modulus that is, PTT (tensile modulus ~2.1 GPa) and a region of brighter contrast relating to the PA6 phase of higher modulus (tensile modulus ~3.0 GPa). According to this result, PTT would act as the matrix while PA6 would play the role of the dispersed phase.

On the other hand, the domain size is strongly influenced by the processing temperature applied. Accordingly, a process at 240[degrees]C results in droplets of ~4 [micro]m (Fig. 1c) whereas the dispersed phase reached ~l-2 [micro]m when processing at 265[degrees]C. Moreover, very small occlusions (typically below 0.5 [micro]m) of the continuous in the dispersed phase are distinctly observed after a process at 265[degrees]C, see Fig. 1d. This partial encapsulation was also obtained in other polyester/amide blends by varying the polyamide/polyester ratio [21].

Three Dimensional Structure via Scanning Electron Microscopy. To observe the real morphology of the bulk of the blend, scanning electron microscopy was applied on processed samples after solvent etching the PA6 phase in formic acid for 5 min at room temperature, see Fig. 2. By dissolving the PA6 phase, the morphology in the three dimensions of the remnant phase, that is PTT, can be observed.

The remaining PTT phase shows a sea-island morphology on the surface, while closer to the center of the sample a columnar structure is observed, see Fig. 2a. This columnar morphology, observable by three dimensions analysis, highlights a better ability of the PA6 phase to flow, covering the PTT dispersed phase. This observation is in opposition with the result obtained by AFM analysis where the dispersed phase, of higher modulus, was attributed to PA6. While the difference observed could be imparted to the limitation of the AFM analysis: as this latest is only an analysis of surface measuring small area of the sample and consequently is not reflecting the homogeneity of the bulk, another reason can also explain the result distorts observed. As shown elsewhere on PTT/PA12 blends, in presence of PA 12, PTT shows a degree of crystallinity increase [17]. A concurrent increase of the degree of crystallinity would result in an increase of the PTT modulus justifying the higher modulus observed for the dispersed phase. Indeed, Zhang has shown an increase of tensile modulus from 2.35 to 2.82 GPa after an increase of the degree of crystallinity of PTT from 7.5 to ~30% [22].

Understanding the Morphology Formation

AFM and SEM revealed that the selection of an adapted processing temperature is essential as directly influencing the blend morphology. Based on the preliminary blend morphology investigation, a process at higher temperatures appeared to be favorable to smaller dispersed phase droplets. The interface between the polymer components must be considered to understand the effect of temperature on droplet dispersion. Therefore, the rheological behavior and the interfacial tension of the polymers were studied in an attempt to establish the relationship between the blend theory and the resulting droplet like morphology.

Melt Viscosity Measurements. The viscosity of immiscible blends is complex and influenced by parameters such as individual rheological properties of the phases, miscibility of the system, droplet interface, interfacial adhesion, etc. The rheological behavior of the neat polymers was investigated as function of the frequency at different temperatures, in order to give the input of the flow behavior on the morphological change observed. In Fig. 3 are displayed the complex viscosity ([[eta].sup.*]) of the neat polymers, tested at different temperatures, against angular frequency ([omega]).

In Fig. 3, PTT (squares) shows a complex viscosity which is independent of the frequency in the range 1-100 rad [s.sup.-1], presenting a Newtonian-like behavior of the melt. In the same range of frequencies PA6 (circles) shows a slight increase of complex viscosity. For all the temperatures investigated, PA6 shows a lower complex viscosity than PTT. The easier ability of PA6 to flow explains the morphology observed earlier by SEM, namely PA6 acts as the matrix and PTT the dispersed phase. In this work, neat polymers of equivalent molecular weight (~60 kg [mol.sup.-1]) were used discriminating the influence of chain length on the complex viscosity values. Moreover, based on the viscosity values measured, was calculated the viscosity ratio by dividing the viscosity of the dispersed phase (PTT) by the viscosity of the matrix (PA6). As intended for the three processing temperatures selected, we obtained 1.96 at 240[degrees]C, 1.44 at 250[degrees]C and 1.14 at 265[degrees]C. The temperature increase leads to a decrease of the viscosity ratio decreasing from ~2 to ~1. This result is in accordance with Wu, [23] who highlighted a significant reduction of the average droplet diameter with the decrease of viscosity ratio. Wu observed that the viscosity ratio influences the immiscible blends morphology and that a droplets size optimum can be reached. A viscosity ratio around unity would be the optimum condition for the polymer elongation, droplet breakup and consequently to get the smallest droplets. This observation is consistent with the droplet breakup and small polymer encapsulation into the dispersed phase observed in Figs. 1 and 2 at higher temperature.

Interfacial Tension and Polarity. The estimation of the surface tension and associated components is of relevance importance to understand the processes of adhesion, wettability, and so forth of multiphase materials as the surface/interfacial tensions are connected to intermolecular forces. Regarding the blend applications, the interfacial tension is the main calculation used to get an estimation of the level of compatibility of the phases, giving a prediction of the final morphology. Because there is no related interfacial tension data for the pair PTT/PA6 available in the literature, the interfacial tension was calculated by using the contact angle method based on two models: the Owens-Wendt theory (or geometric mean method) [24] and the Oss-Chaudhury-Good approach (acid-base theory) [25]. Derived from Fowkes Equation [26] and combined with the Young's Equation, the well-known Owens-Wendt theory is often applied for polymer surfaces and its uses in the literature is widespread. Although powerful this approach is sometimes controversial due to the questionable hydrogen bonding contribution in the polar component. Thereby, another approach, the Oss-Chaudhury-Good was applied in complement since the polar component accounts for hydrogen bonding and other Lewis acid/base interactions [27]. In their acid-base theory they divided in two main components, the electrodynamic interactions (dispersives forces) [[gamma].sup.LW], and the acid-base contribution (including hydrogen bonding) [[gamma].sup.AB]. This latest approach enables to tightly correlate surface tension components and chemical nature of the materials.

For the Owens-Wendt theory, a minimum of two liquids is required to solve the Equation but in order to compare the data with the Oss-Chaudhury-Good approach, in this work three solvents were used for both methods.

Contact angle. The contact angles were measured employing water as the polar solvent, ethylene glycol as the semipolar solvent and diiodomethane as the apolar solvent, as presented in Table 1. It is well admitted that the contact angle data as well as the pair of liquids chosen are greatly influencing the component composing the surface tension value.

The contact angles values measured for both neat polymers are lower than 90[degrees] indicating a partial wetting behavior (amphiphilic character) between the polymers surface and the liquids. Higher is the affinity between the surface and the solvent and lower is the contact angle. Like this, the presence of amide functions on the surface of PA6 favors the hydrogen bonds with the polar solvents, making the wetting easier. In the opposite fashion, the apolar solvent (diiodomethane) shows a higher affinity with the less polar surface (PTT). Focusing more specifically on the values measured, [[theta].sub.PTT/water] was found by Lin et al. [28] at 87.0[degrees] while 83.6[degrees] was obtained in this work, stressing a good agreement between the data. Regarding [[theta].sub.PA6/water], a wide range of values can be found in the literature 55[degrees] < [[theta].sub.water] < 70[degrees]. Indeed, the contact angle measurements are influenced by three main parameters, the roughness of the sample surface, the environmental humidity and the functional groups contained on the polymer surface. Despite the strong influence of operatory conditions, we measured 69.7[degrees] which is in the abovementioned range found in literature. The contact angle of PA6, measured with other solvents, for example [[theta].sub.ethylene glycol] [29] and [[theta].sub.diiodomethane] [30], was found in the literature at 50.7[degrees] and 46.0[degrees], respectively, meaning that we are in an acceptable range as we measured 47.0[degrees] and 42.7[degrees].

Owens and Wendt approach: Geometric mean method. First, the surface tensions were calculated for PTT and PA6 using the geometric mean method. The assessment of the surface tension ([[gamma].sub.SV]) value of the neat polymers has been made possible by the preliminary calculation of both the polar ([[gamma].sub.LV.sup.p]) and the dispersive ([[gamma].sub.LV.sup.d]) contributions. The liquid surface energy parameters, the surface tension measured, work of adhesion and interfacial tension calculated for all the solvents, polymers and blends are summarized in Table 2. Because the melt blending was carried out at high temperatures, the interfacial tension value requested to be extrapolated to the processing temperature; using the Eq. 1:

[mathematical expression not reproducible] (1)

With [[gamma].sub.SV] the surface tension of the solid (PA6 or PTT) measured at 20[degrees]C, [alpha] the temperature coefficient of surface tension (d[gamma]/dT) found in the literature for PTT = -0.067 mN [m.sup.-1] [degrees][C.sup.-1] [31] and PA6 = -0.065 mN [m.sup.-1] [degrees][C.sup.-1] [32].

The surface tensions values measured for PA6 (42.4 mN [m.sup.-1]) and PTT (44.8 mN [m.sup.-1]) are almost equal meaning that the sum of the intermolecular forces involved on the sample surface is equivalent. Albeit the similar surface tension of PTT and PA6, PA6 shows a higher polar component over PTT resulting in a higher surface polarity value (Table 2). This result can be explained by the presence of amide linkage groups representing a double donor ability through the amine N-H and the ester C=O, increasing the polar component of PA6 over PTT. Moreover, as expected an increase of temperature results in a decrease of the surface tension, for both PTT and PA6.

Once the surface tension of neat polymers was calculated, the interfacial tension between PTT and PA6 was determined as shown in Table 2. At room temperature the interfacial tension between PTT and PA6 is still relatively high (~2.7 mN [m.sup.-1]). However, the extrapolation at high temperatures highlight a high interaction between PTT and PA6 yielding to a low interfacial tension ~0.7 mN [m.sup.-1]. Furthermore, the work of adhesion [W.sub.A] corresponding to the surface free energy required to separate two materials [33] was calculated. At room temperature the work of adhesion (84.5 mN [m.sup.-1]) is almost twice the value of the one extrapolated at the processing temperatures (~45 mN [m.sup.-1]). At the temperature of processing, [W.sub.A] is progressively decreasing with the increase of temperature meaning that a higher temperature of process is beneficial for the dispersion and break-up of the droplet dispersed phase. The lower the surface tension of the polymers, the lower the strength of contact between PA6 and PTT leading to better phases dispersion during the process emphasizing the morphology previously observed in Fig. 1.

In addition, the calculation of the capillary (or Taylor) number (Ca) [34, 35] which determines the limit of droplet deformation, was calculated based on the Eq. 2:

[C.sub.a] = [[eta].sub.m] [gamma] [R.sub.0]/2[sigma] (2)

With [[eta].sub.m] the viscosity of the matrix phase, [gamma] the shear rate, [R.sub.0] the diameter of the dispersed component, and a the interfacial tension.

According to this theory, two main regimes exist, either Ca [much greater than] 1 and particles are stable enough to elongate into a cocontinuous morphology, or Ca ~ 1 and the important interfacial stress results into the break-up of thread into smaller droplets. The application of this formula on the PTT/PA6 blends prepared at 240[degrees]C and 265[degrees]C gives the values ~1.1 and ~0.2, respectively. At such low Ca values the PTT/PA6 blends were not stable enough to form a cocontinuous morphology, which is consistent with the blend morphology observed earlier.

Oss-Chaudhury-Good method: Acid-base theory. The benefit of the acid-base method is the dissociation of the polar component into its acceptor and donor characters. The surface free energy parameters of the solvents used are listed in Table 3 and issued from literature [36]. Solvents with strong characteristic differences were selected in order to get consistent polymer surface free energies.

As we can see in Table 3, PA6 shows a higher electron donor component ([[gamma].sub.SV.sup.-]) resulting in an increase of its polar component ([[gamma].sub.SV.sup.AB]) which can be explained by the amide group of PA6 acting as a slight Lewis base (higher ability to give electron). The phenyl contained in PTT is showing a low effect despite the donor character of its [pi] electrons. The high hydrogen network surrounding the PA6 chains provide an increase of the acid-base character of PA6 over PTT. Although PA6 and PTT are considered as polar polymers a significant apolar (Lifshitzvan der Waals or dispersive) component is observed, as in many other materials found in the literature [37]. According to the components obtained for PTT, it almost appears as a monopolar material while PA6 would be considered as a weakly bipolar surface.

The comparison of the surface tension calculated by the geometric mean method and acid-base theory highlights an excellent correlation for both neat polymers. Indeed, by the geometric method the values 42.4 and 44.8 mN [m.sup.-1] were calculated while by using the acid-base theory the values 41.1 and 44.8 mN [m.sup.-1] were obtained for PA6 and PTT, respectively. In the literature the PA6 is measured by the geometric equation at [[gamma].sub.SV] ~42-44 mN [m.sup.-1] and by the acid-base theory at [[gamma].sub.SV] ~38-40 mN [m.sup.-1] [31]. Considering PTT, the main references are giving the surface tension at the processing temperature directly. One value obtained by the harmonic equation was found for PTT in the literature at [[gamma].sub.SV] ~49 [38] whereas no measurement was found by these authors via the acid-base method. While PA6 is in adequacy with the literature, our value disagrees somewhat with the sole literature value found by these authors for PTT. The interfacial tension between PTT and PA6 is measured around 0.5 mN [m.sup.-1] by the acid-base approach, highlighting a thermodynamically favorable interaction between the components. However, this value is a bit lower that the one calculated by the geometric mean method (2.7 mN [m.sup.-1]). The low interfacial tension measured is consistent with the good dispersion observed previously by AFM and SEM. On the other hand, according to the equation proposed by Oss et al. [38] the free energy of adhesion can be calculated more accurately than with the geometric mean method as both the dispersive and acid/base components are considered. At ambient temperature, the work of adhesion equals the value measured previously by the Owens and Wendt method, and is obtained at ~84 mN [m.sup.-1] highlighting the high consistency of our results over the two methods used.

Mechanical Properties

The influence of the blending ratio on the mechanical properties of a blend is a subject widely investigated in the literature, showing the benefice of having smaller dispersed phase droplets size, for example, PBT/PA6 blends [39]. According to the rule of mixture it is expected that favor the polymer with the higher mechanical properties would increase the mechanical performance of the final blend. The highly sensitive 50/50 wt% blending ratio shows a significant dependency of the morphology to the processing conditions applied. Accordingly, the influence of the morphological dependence over temperature highlighted previously was investigated by means of tensile, flexural and impact tests after injection molding at 240[degrees]C, 250[degrees]C, and 265[degrees]C (Fig. 4). The tensile strength (a), indicates the capacity of a sample to support a load before tending to elongate. The impact strength (c) refers to the ability of a material to absorb the energy applied by dissipating the load through the matrix. The elongation at break (d) expresses the ability of a material to continue its deformation before critical failure occurs.

The neat PA6 (1) is a tough polymer showing a high stiffness and ductility. In comparison, the neat PTT (2) is less tough and stiff than PA6 but as ductile. The comparison of the different blends (Fig. 4: 3, 4, 5, and 6) with neat nylon 6 and neat PTT shows that the blends are not leading to improved mechanical performances. All strengths, modulus and impact strengths values obtained for the composites appeared between those of the neat polymers or in the same range. A special focus on the tensile modulus values of the blends show a slight increase in comparison to the stiffer polymer that is, PA6. This stiffness increase supports the idea that the modulus of PTT is increased when blended with PA6 in the proportion 50/50 wt%. In opposition, the elongation at break suffered significant reduction with the PTT/PA6 blending due to the low adhesion or high interfacial tension between the polymers as already observed after extrusion of immiscible blends [6]. While the immiscibility of the blends generates moderated mechanical performances, the increase of processing temperature slightly raises the overall performance. A smaller calculated WA value after a process at 265[degrees]C indicated a better compatibility of the phases, which clearly resulted in small PTT droplet in the PA6 dispersed phase. The higher compatibility would allow for, smaller dispersed phase droplets and a better stress transfer between the PTT droplets and the PA6 phase improving the toughness, ductility and stiffness of the extruded blends. In addition, while a slow shearing (70 rpm) is beneficial for the properties increase, a faster shearing rate (200 rpm) results in the properties decreasing.

CONCLUSIONS

The influence of processing temperature on bio-based poly(trimethylene terephthalate)--PTT and polyamide 6--PA6 blends via twin-screw extrusion was successfully investigated in this work. After the process of 50/50 wt% blends at varied temperatures and shearings, the morphological investigation by AFM, and SEM after solvent etching the surface of the blends, stressed two main observations: a droplet like morphology was observed for all processing temperatures applied and a significant reduction of the dispersed phase size, with the temperature increase, was highlighted. While AFM and SEM experiments drew a different phases attribution, the result provided by SEM that is, PA6 forms the continuous phase and PTT the dispersed phase, was retained as more representative of the bulk of the blend. This behavior was emphasized by the lower viscosity of PA6 over PTT for all the temperatures investigated. Moreover, the droplet size reduction observed was explained by both a reduction of the viscosity ratio with the temperature increase measured by rheology, and a decrease of the interfacial tension calculated by both Owens-Wendt approach and Oss-Chaudhury-Good method. Furthermore, the smaller phases dispersion resulted in a significant improvement of the mechanical properties stressing the importance of selecting adapted processing parameters.

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Amandine Codou, (1) Manjusri Misra, (1, 2) Amar K. Mohanty (iD) (1, 2)

(1) Byproducts Discovery and Development Centre, Department of Plant Agriculture, Crop Science Building, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

(2) School of Engineering, University of Guelph, Thornbrough Building, Guelph, Ontario, N1G 2W1, Canada

Correspondence to: A. K. Mohanty; e-mail: mohanty@uoguelph.ca Contract grant sponsor: Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), Canada/University of Guelph-Bioeconomy for Industrial Uses Research Program Theme; contract grant numbers: 030176; 200283; 050155; contract grant sponsor: Natural Sciences and Engineering Research Council (NSERC); Canada-Discovery Grants; contract grant number: 401111.

DOI 10.1002/pen.24837

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

Caption: FIG. 1. AFM modulus mapping of PTT/PA6 50/50 wt% blends after processing at 240[degrees]C (a) 50 [micro]m (b) 50 [micro]m, and 265[degrees]C (c) 50 [micro]m, and (d) 5 [micro]m.

Caption: FIG. 2. SEM micrographs at x 3,000 magnification of PTT/PA6 50/50 wt% blends, after solvent etching of the PA6 phase, processed at (a) 240[degrees]C and (b) 265[degrees]C.

Caption: FIG. 3. Complex viscosity vs. frequency for neat PA6 (circles) and neat PTT (squares) at (a) 240[degrees]C, (b) 250[degrees]C, and (c) 265[degrees]C.

Caption: FIG. 4. (a) Tensile, (b) flexural, (c) impact tests, and (d) elongation at break of neat polymers processed at 265[degrees]C and the blends processed at 240, 250, and 265[degrees]C, the latter at two different speed screw. (1) neat PA6 (2) neat PTT (3) PTT/PA6-240[degrees]C, (4) PTT/PA6-250[degrees]C, (5) PTT/PA6-265[degrees]C, (6) PTT/PA6-265[degrees]C-200 rpm
TABLE 1. Contact angle measurements of pure PA6 and PTT processed at
265[degrees]C, measured at room temperature in water, ethylene glycol
and diiodomethane.

                Contact                      Contact
                 angle                        angle
Samples   [[theta].sub.Water]    [[theta].sub.Ethylene glycol]
              ([degrees])                  ([degrees])

PA6        69.7 [+ or -] 1.2            47.0 [+ or -] 1.9
PTT        83.6 [+ or -] 0.6            48.4 [+ or -] 2.4

                     Contact
                      angle
Samples   [[theta].sub.Diiodomethane]
                   ([degrees])

PA6             42.7 [+ or -] 2.2
PTT             29.7 [+ or -] 2.0

TABLE 2. Liquid surface energy parameters, surface tension measured,
work of adhesion and interfacial tension calculated by geometric mean
method and extrapolated to the processing temperatures.

Surface tension     20[degrees]C (mN [m.sup.-1])

                    [[gamma].sub.LV]   [[gamma].sub.LV.sup.P]

Water/air                 72.8                  51.0
Ethylene                  48.0                  19.0
  glycol/air
Diiodomethane/air         50.8                   0

                    [[gamma].sub.SV]   [[gamma].sub.SV.sup.P]

PA6/air                   42.4                  7.8
PTT/air                   44.8                  1.7
Interfacial               2.7
  tension PA6/PTT
Work of adhesion          84.5

Surface tension     20[degrees]C             Polarity
                    (mN [m.sup.-1])

                    [[gamma].sub.LV.sup.d]

Water/air                    21.8             0.7005
Ethylene                     29.0             0.3958
  glycol/air
Diiodomethane/air            50.8               0

                    [[gamma].sub.SV.sup.d]

PA6/air                      34.6             0.2921
PTT/air                      43.1             0.1486
Interfacial
  tension PA6/PTT
Work of adhesion

Surface tension      240[degrees]C      250[degrees]C
                    (mN [m.sup.-1])    (mN [m.sup.-1])

                    [[gamma].sub.SV]   [[gamma].sub.SV]

Water/air
Ethylene
  glycol/air
Diiodomethane/air

PA6/air                   20.3               19.7
PTT/air                   28.4               27.7
Interfacial               0.7                0.7
  tension PA6/PTT
Work of adhesion          48.0               46.7

Surface tension      265[degrees]C
                    (mN [m.sup.-1])

                    [[gamma].sub.SV]

Water/air
Ethylene
  glycol/air
Diiodomethane/air

PA6/air                   18.7
PTT/air                   26.7
Interfacial               0.7
  tension PA6/PTT
Work of adhesion          44.7

With L standing for liquid, V for vapor, S for
solid, p for polar and d for dispersive.

TABLE 3. Surface free energy parameters of water, ethylene glycol and
diiodomethane (top), surface tension of PTT, PA6 and surface
interaction obtained for PTT/PA6 blend (bottom).

Model liquids      Total surface           Dispersive
                      tension              component
                  (mN [m.sup.-1])       (mN [m.sup.-1])
                  [[gamma].sub.LV]   [[gamma].sub.LV.sup.LW]

Distilled water         72.8                  21.8
Ethylene glycol         48.0                  29.0
Diiodomethane           50.8                  50.8

Solids            [[gamma].sub.SV]   [[gamma].sub.SV.sup.LW]

PA6                     41.1                  39.1
PTT                     44.8                  44.2
PA6-PTT                 0.5                    0.2

Model liquids        Polar component           Elec. acceptor
                     (mN [m.sup.-1])              component
                  [[gamma].sub.LV.sup.AB]      (mN [m.sup.-1])
                                            [[gamma].sub.LV.sup.+]

Distilled water            51.0                      25.5
Ethylene glycol            19.0                      1.9
Diiodomethane               0.0                      0.0

Solids            [[gamma].sub.SV.sup.AB]   [[gamma].sub.LV.sup.+]

PA6                         2.1                      0.1
PTT                         0.7                      0.0
PA6-PTT                     0.3

Model liquids          Elec. donor              Work of
                        component          adhesion [W.sub.A]
                     (mN [m.sup.-1])        (mN [m.sup.-1])
                  [[gamma].sub.LV.sup.-]

Distilled water            25.5
Ethylene glycol            47.0
Diiodomethane              0.0

Solids            [[gamma].sub.LV.sup.-]

PA6                        13.8
PTT                        2.9
PA6-PTT                                           84.2
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Author:Codou, Amandine; Misra, Manjusri; Mohanty, Amar K.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Dec 1, 2018
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