Effect of Glass Fibers with Different Surface Properties on the

Article pubs.acs.org/IECR

Effect of Glass Fibers with Different Surface Properties on the Morphology and Properties of Polyamide 6/Poly(butylene terephthalate) Blends Li Wang,†,‡ Zhao-Xia Guo,*,§ and Jian Yu*,§ †

The State Key Laboratory of Chemical Resource Engineering, Beijing 100029, China College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China § Key Laboratory of Advanced Materials, Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: The morphology and properties of polyamide 6 (PA6)/poly(butylene terephthalate) (PBT) blends filled by three types of glass fibers (GF) with different surface properties were investigated. The GF were unmodified or surface-modified for PA6 or PBT, denoted as GF(Pris), GF(PA6), and GF(PBT), respectively. The incorporation of 15 wt % of GF with different surface properties all led to a transition from a cocontinuous (at least a part of each phase penetrates the whole volume in a coherent and continuous manner) to a sea-island (separated domains dispersed in a continuous matrix) morphology with PA6 being the matrix phase when PA6/PBT equaled 45/55. GF(Pris) was always encapsulated by PA6, while the encapsulating layers on the surfaces of GF(PA6) and GF(PBT) changed from PBT to PA6 with increasing PA6 contents. The morphological changes induced by GF caused more PBT to crystallize at a lower temperature and enhanced the alkali tolerance of the blend significantly.

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INTRODUCTION Glass fibers (GF) are a class of microsized and one-dimensional fillers that are frequently used to enhance the mechanical strength and rigidity of polymeric materials. Up to now, there has been considerable literature on single polymer/GF composites documenting that their mechanical properties are closely related to the length and orientation degree of GF and the interfacial adhesion between GF and polymer matrix.1−6 Polymer multiphase and multicomponent systems containing GF have been developed and reported by many researchers to pursue better overall properties.7−16 In some cases, a second polymer was added into a first polymer/GF composite to improve the interfacial adhesion between the first polymer and GF or to compensate for the loss in toughness and processing properties caused by GF. In some other cases, GF were added into polymer blends to endow them with higher mechanical strength and rigidity. As we all know, the final properties of immiscible polymer blends are closely related to their phase morphology. Therefore, tailoring the morphology of polymer blends has become an important route to reach optimized overall properties. The potential of inorganic fillers (organoclay, carbon nanotubes, silica, calcium carbonate, etc.) in tailoring the morphology of polymer blends has been exhibited many times during the past decade.17−25 A rich diversity of morphological changes can be obtained by incorporating inorganic fillers, which seems to have opened a door to the next generation of morphology modifier. However, these findings were focused on the effect of the burgeoning nanofillers. An interesting question arises after a reversed thinking on whether microfillers such as GF can also influence the morphology of polymer blends. Despite the great attention © 2013 American Chemical Society

paid to the encapsulation of GF by polymer and their interfacial adhesion in GF-filled polymer blends, the answer to this question is rare in the literature. Zheng et al.26 found that 30 wt % of GF could cause a transition of a liquid crystalline polymer (LCP) from spherical to fibrillar shape in polyamide 6 (PA6)/ LCP (65/5) blend. This was attributed to the enhanced deformation and fibrillation of LCP phase by the intensified elongation flow field in the entrance of the “microcapillary” formed by stacked GF. Laura et al.27 observed a reduction in the average size of rubber particles in PA6/ethylene−propylene rubber (EPR) (80/20) and PA6/styrene−ethylene/butylene− styrene (SEBS) (80/20) blends by adding 15 wt % of GF, which was believed to be caused by the increase in apparent viscosity induced by GF. Despite these reports, whether and how GF can influence the morphology of polymer blends remains an interesting subject that needs to be explored. In the present study, PA6/poly(butylene terephthalate) (PBT) blend was chosen as a model system, as it represents a class of immiscible polymer blends of polar polycondensates. Furthermore, PA6/PBT blends are important from the point of view of industry, because blending PA6 with PBT can increase mechanical strength and rigidity and reduce materials cost and sensitivity to moisture, as well as avoid difficulties in separation during recycling. We focused on the effect of GF with different surface properties on the morphology of PA6/PBT blends at different blending compositions. The crystallization behavior Received: Revised: Accepted: Published: 206

August 9, 2013 November 15, 2013 November 29, 2013 November 29, 2013 dx.doi.org/10.1021/ie4026133 | Ind. Eng. Chem. Res. 2014, 53, 206−213

Industrial & Engineering Chemistry Research

Article

condenser and refluxed for 18 h. The hydrolytic degradation degree of the PBT phase is defined as m − m2 Φ= 1 mPBT

and alkali resistance were also characterized and correlated with morphology.

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EXPERIMENTAL SECTION Materials. PA6 (1013B; density 1.14 g/cm3, melting temperature 220 °C) was provided by UBE Co. PBT (density 1.31 g/cm3, melting temperature 224 °C) was supplied by Huahui Plastic Additives Factory (Zhangjiagang, China). PA66 (Zytel 101L; density 1.14 g/cm3, melting temperature 260 °C) was purchased from Dupont. Poly(ethylene terephthalate) (PET; density 1.37 g/cm3, melting temperature 247 °C) was provided by Ninghe Plastic Corp. (Tianjin, China). GF (T435E and T436) were provided by Taishan Fiberglass Inc. (Tai’an, China), with their filament diameter and chopped length being 10 μm and 4.5 mm, respectively. GF (T435E) showed good compatibility with PA matrix, while GF (T436) showed good compatibility with PBT, PET, and PC matrices according to the products’ description. In addition, pristine GF were obtained by placing GF (T435E) into a muffle furnace at 600 °C for 2 h to eliminate the organic modifier on the surface of the GF. The GF with different surface properties are denoted as GF(PA6), GF(PBT), and GF(Pris). Sample Preparation. Samples were prepared using a torque rheometer (HAAKE PolyDrive, HAAKE Co.) at a temperature of 250 °C and a screw speed of 60 rpm. Thus, the desired amounts of PA6, PBT, and the GF with different surface properties were premixed manually and fed into the chamber of the torque rheometer. Samples for further characterizations were obtained after melt mixing for 6 min. PA6 and PBT were blended at weight ratios of 30/70, 45/55, and 70/30 in the absence and presence of 15 wt % of the GF with different surface properties. The designations are in the form of PBTx for the samples without GF and PBTxGF(y) for the samples with 15 wt % of GF, with x being the weight percentage of PBT relative to all polymers and y being the type of GF. Characterization. Field Emission Scanning Electron Microscopy (FESEM). Samples subjected to FESEM were prepared with two methods: cryo-fracturing directly as well as cryo-fracturing followed by selectively etching the PA6 phase using formic acid to obtain a clear reveal of the phase morphology. FESEM was performed with a JEOL apparatus (JSM-7401F) operating at an accelerating voltage of 1 kV. Smileview software of the microscope was used in the determination of average particle size. Melt Rheology. An Anton Paar rotational rheometer (Physica MCR 301) was used for the characterization of melt rheological properties. The tests were conducted with parallel plate geometry (ϕ 25 × 1 mm) in a frequency sweep mode in N2 atmosphere. The temperature was 250 °C and the strain amplitude was kept at 1%. Differential Scanning Calorimetry (DSC). A Q100 calorimeter (TA Instruments) was utilized to study the nonisothermal crystallization behavior of different samples. Samples were heated to 280 °C at a heating rate of 10 °C/min and then annealed at 280 °C for 5 min to eliminate thermal history. After that, they were cooled to 25 °C at a cooling rate of 10 °C/min. The data of the cooling scan were recorded. Alkali Tolerance Tests. About 1 g of different samples prepared by melt compounding was cut into cubes with side length being approximately 5 mm. Then, they were placed into 100 mL of NaOH aqueous solution (10 wt % in concentration) in a 250 mL one-port flask with a magnetic stirrer and a

where m1 and m2 are the weights of a sample before and after the alkali tolerance test, respectively, and mPBT is the weight of PBT phase in the sample before the alkali tolerance test.

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RESULTS AND DISCUSSION Morphology. Figure 1 shows the FESEM micrographs of the cryo-fractured (left) and formic acid (a selective solvent for

Figure 1. FESEM micrographs of the samples with PA6/PBT = 30/70 (1, cryo-fractured; 2, cryo-fractured and etched by formic acid): (a) PBT70, (b) PBT70GF(PA6), (c) PBT70GF(PBT), and (d) PBT70GF(Pris).

PA6)-etched (right) surfaces of the samples with PA6/PBT = 30/70. A typical sea-island morphology28 with discrete spherical PA6 domains dispersed in the PBT matrix was obtained in sample PBT70. The type of morphology did not change with the addition of the GF with different surface properties. A statistical particle size analysis was carried out on the FESEM micrographs of the recovered PA6 particles after degrading the PBT phase using boiling NaOH aqueous solution (10 wt %), and the results are given in Table 1. It 207

dx.doi.org/10.1021/ie4026133 | Ind. Eng. Chem. Res. 2014, 53, 206−213


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Table 1. Number-Average Particle Diameters (Dn) of Different Samples sample name PBT70 PBT70GF(PA6) PBT70GF(PBT) PBT70 GF(Pris)

Dn (μm)

sample name

Dn (μm)

sample name

± ± ± ±

PBT55 PBT55GF(PA6) PBT55GF(PBT) PBT55GF(Pris)

/ 1.78 ± 0.63 1.63 ± 0.60 1.82 ± 0.64

PBT30 PBT30GF(PA6) PBT30GF(PBT) PBT30GF(Pris)

1.05 0.95 0.94 1.00

0.37 0.34 0.24 0.31

Dn (μm) 0.98 0.95 0.91 0.95

± ± ± ±

0.17 0.17 0.16 0.19

sense, while requiring only at least a part of each phase to penetrate the whole volume coherently and continuously in a broad sense. This latter definition permits the existence of discrete domains apart from the continuous network structures28 and is utilized in the present study. It can be seen from Figure 2a2 that many cylindrical holes were left after etching using formic acid for sample PBT55, instead of the hemispherical holes as are shown in Figure 1a2 for sample PBT70. Besides, the weight loss ratio of sample PBT55 after etching using formic acid was close to 45%, which was impossible if no part of the PA6 phase penetrated the whole sample.29 Therefore, a cocontinuous morphology was formed in sample PBT55. It is surprising that spherical particles could be observed for samples incorporated with the GF with different surface properties after etching using formic acid in FESEM micrographs. In other words, the addition of 15 wt % of GF turned the cocontinuous morphology into a sea-island morphology with PA6 being the matrix phase for the samples with PA6/PBT = 45/55. In this case, the particle size analysis was performed on the FESEM micrographs of the recovered PBT particles after etching using formic acid. The Dn values of the samples with PA6/PBT = 45/55 are also listed in Table 1, which show no statistically significant differences. It can be seen from Figure 3a that a typical sea-island morphology with PA6 being the matrix phase was formed for sample PBT30 when PA6/PBT reached 70/30. No obvious change in morphology could be distinguished in the presence of the GF with different surface properties, judging from Figure 3 and the statistical data listed in Table 1. Encapsulation of the GF with Different Surface Properties by Polymers. The encapsulation of GF by polymers in GF-filled polymer blends is a crucial factor closely associated with materials properties; thus, it should not be neglected. Fortunately, such information can be detected without difficulty by FESEM. It can be seen from Figure 1 that the GF with different surface properties were encapsulated by different polymers. The polymer that encapsulated GF(PA6) or GF(PBT) had no clear interface with the PBT matrix and was maintained after etching using formic acid for PBT70GF(PA6) and PBT70GF(PBT). This indicates that the PBT matrix adhered on the surfaces of GF(PA6) and GF(PBT) in PBT70GF(PA6) and PBT70GF(PBT). A sharp interface existed between the polymer encapsulating GF(Pris) and the PBT matrix for PBT70GF(Pris). This encapsulating layer disappeared and smooth GF surface was exposed after etching using formic acid. Thus, GF(Pris) were encapsulated by the PA6 phase in PBT70GF(Pris). The situation was quite similar for the samples with PA6/PBT = 45/55 (Figure 2), keeping in mind the formation of the sea-island morphology with PA6 being the matrix phase. GF(PA6) and GF(PBT) were encapsulated by thick layers of PBT in PBT55GF(PA6) and PBT55GF(PBT), while the PA6 matrix adhered on the surface of GF(Pris) in PBT55GF(Pris). It was still the PA6 phase that adhered on the surface of GF(Pris) when PA6/PBT reached 70/30. Nevertheless, the situation changed dramatically for

can be seen that the differences in number-average particle diameter (Dn) are smaller than the standard deviations. Therefore, the incorporation of the GF with different surface properties did not cause any obvious change in the size of the dispersed PA6 domains. Figure 2 gives the FESEM micrographs of the samples with PA6/PBT = 45/55. A weight ratio of 45/55 corresponds to a

Figure 2. FESEM micrographs of the samples with PA6/PBT = 45/55 (1, cryo-fractured; 2, etched by formic acid): (a) PBT55, (b) PBT55GF(PA6), (c) PBT55GF(PBT), and (d) PBT55GF(Pris).

volume ratio of approximately 50/50 (concerning the discrepancy between PA6 and PBT in density), at which a cocontinuous morphology is likely to evolve when the two blended components have similar viscosities.28 Generally speaking, there are two different definitions of cocontinuous morphology. It refers to the ideal case where at least two continuous structures coexist in the same volume in a narrow 208



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Figure 4. FESEM micrographs of the samples with PA6/PET = 45/55 and filled by 15 wt % of GF (1, cryo-fractured; 2, etched by formic acid): (a) GF(PA6), (b) GF(PBT), and (c) GF(Pris).

Figure 3. FESEM micrographs of the samples with PA6/PBT = 70/30 (1, cryo-fractured; 2, etched by formic acid): (a) PBT30, (b) PBT30GF(PA6), (c) PBT30GF(PBT), and (d) PBT30GF(Pris).

GF(PA6) and GF(PBT). They were mainly adhered with the PA6 matrix, with only a few small areas on GF surfaces encapsulated by the PBT phase in PBT30GF(PA6) and PBT30GF(PBT). It seems that GF(PA6) and GF(PBT) exhibit affinity for both PA6 and PBT to some extent (no specific selectivity), which differs from the case of GF(Pris). Furthermore, the encapsulation of GF(PA6) and GF(PBT) by either PA6 or PBT was influenced by their blending ratio. This phenomenon can be well-understood if competition exists between PA6 and PBT in encapsulating GF(PA6) and GF(PBT), due to the similarities in their physicochemical properties. FESEM observations were conducted to detect the encapsulation of the GF with different surface properties in PA6/PET (45/55) and PA66/PBT (55/45) blends to confirm this presumption, considering that the molecular structure and chemical properties of PA6 and PA66 (PBT and PET) are quite alike. The results are given in Figures 4 and 5. PA6 and PBT have similar melting temperature, while the melting temperature of PET is about 30 °C higher. As a result, the polyester phase melted after the melting of the polyamide phase in the GF-filled PA6/PET (45/55) blends, endowing the polyamide melt with the opportunity to contact GF in priority. In such a case, the polyester phase encapsulating the surfaces GF(PA6)

Figure 5. FESEM micrographs of the samples with PA66/PBT = 55/ 45 and filled by 15 wt % of GF (1, cryo-fractured; 2, etched by formic acid): (a) GF(PA6), (b) GF(PBT), and (c) GF(Pris).

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and GF(PBT) was thin and incomplete (Figure 4), which was in stark contrast with the case of PBT (Figure 2). The melting temperature of PA66 is about 40 °C higher than that of PBT; thus, the polyester phase melted before the melting of the polyamide phase in PA66/PBT blend. A blending ratio of 55/ 45 was chosen for PA66/PBT to ensure the formation of a seaisland morphology with polyamide being the matrix phase. It can be seen from Figure 5a,b that the encapsulation of the polyester phase on the surfaces of GF(PA6) and GF(PBT) resembled the case in Figure 2, although the volume fraction of PBT was actually lower. Therefore, the encapsulation of the polyester phase on the surfaces of GF(PA6) and GF(PBT) could be tuned by changing the melting sequence of the polyester and polyamide phases. Our presumption that competition exists between PA6 and PBT in encapsulating GF(PA6) and GF(PBT) can be verified. Besides, whether the polyamide phase melted before or after the polyester phase (Figures 4c and 5c), the polyamide phase adhered to the surface of GF(Pris). This indicates that GF(Pris) have much higher selectivity for the polyamide phase under the conditions in the present study. Interpretation of the Morphological Changes Induced by GF. The GF with different surface properties did not exert any obvious change in the morphology of the PA6/PBT blends at asymmetric compositions (30/70 or 70/30) but caused a transition from the cocontinuous morphology to a sea-island morphology with PA6 being the matrix phase at a symmetric composition (45/55). This is rather interesting because the morphological transition may lead to a combination of the properties of PA6 and PBT in a quite different manner. It is well-known that the morphology of immiscible polymer blends is mainly influenced by blending composition, viscoelasticities, and interfacial properties of the blended components, as well as processing conditions. Three possible explanations can be proposed to interpret the morphological changes induced by GF by considering the following influencing factors. (1) PA6/PBT system has a narrow composition range for a cocontinuous morphology; thus, the cocontinuous morphology formed in the present study may be unstable. Figure 6 presents the FESEM micrographs of the samples with PA6/PBT = 40/

60 and 50/50. The cocontinuous morphology vanished completely and a typical sea-island morphology with PBT (PA6/PBT = 40/60) or PA6 (PA6/PBT = 50/50) being the matrix phase was formed with a slight shift in composition from PA6/PBT = 45/55. Therefore, the cocontinuous morphology at PA6/PBT = 45/55 may be quite unstable and sensitive to the incorporation of GF. A similar situation has been reported by Fu et al. for carbon nanotube (CNT) filled poly(p-phenylene sulfide) (PPS)/PA66 (60/40) blends.30 A morphological transition from sea-island to cocontinuous and then to coarse sea-island took place when the amount of CNTs increased from 0 to only 0.5 parts per hundred rubber, demonstrating that the morphology of PPS/PA66 (60/40) blends was quite sensitive to the dispersion state of CNTs. (2) GF(PA6) and GF(PBT) were encapsulated by thick layers of PBT in the PA6/PBT (45/55) blends, which equaled a reduction in the volume fraction of PBT phase. This may shift the effective volume ratio into the range for a sea-island morphology concerning the narrow composition range for a cocontinuous morphology, like in the case of the PA6/PBT (50/50) blend. Unfortunately, the amount of polymers adhering to the surfaces of GF(PA6) and GF(PBT) is difficult to determine because an efficient method to separate the discrete PBT domains and the GF encapsulated by PBT is lacking. Despite this, it is reasonable to believe that the encapsulation of PBT on GF(PA6) and GF(PBT) causes the morphological changes. However, this does not interpret the case of GF(Pris) because the adherence of PA6 on the surface of GF(Pris) did not lead to a sea-island morphology with PBT being the matrix phase. Thereby, there must be other factors that are dominating in the case of GF(Pris). (3) GF can affect the rheological properties of PA6 or PBT, resulting in a shift in the composition range for a cocontinuous morphology. Many researchers have observed filler-induced sea-island to cocontinuous morphological transition and attributed it to the filler-induced changes in rheological properties, especially the viscosity ratio between different phases.31−33 Thus, the influence of GF with different surface properties on the rheological properties of PA6 and PBT needs to be evaluated. GF(PA6 & PBT) and GF(Pris) were mixed with PBT and PA6, respectively, at a weight ratio equal to that in the GF-filled PA6/PBT (45/55) blends, according to the encapsulation of the GF with different surface properties in the samples with PA6/PBT = 45/55. Pristine PA6 and PBT as well as these composites were subjected to dynamic rheological tests, and the results are plotted in Figure 7. The complex viscosity (η*) of pristine PBT melt was slightly higher than that of PA6, and their η* exhibited weak dependence on angular frequency, due to the rigidity of PA6 and PBT macromolecules. The storage modulus (G′) of pristine PBT melt was close to that of PA6. With the addition of GF(PA6) and GF(PBT), the η* and G′ of PBT melt were enhanced obviously, and the enhancement induced by GF(PA6) was larger than that by GF(PBT). Besides, GF(PA6) and GF(PBT) caused a plateau in the low-frequency range of the G′ curve, together with enhanced shear thinning behavior in the low-frequency range of the η* curve. These are clear signs that GF achieved rheological percolation in PBT, thus causing a transition from liquid to quasisolid behavior of the PBT melt, according to the literature reports.34,35 The effect of GF(Pris) on the rheological properties of PA6 melt was similar to the case of GF(PA6 & PBT) on PBT. An increase in viscosity ratio between polymers 1 and 2 demands more polymer 1 for phase inversion to take

Figure 6. FESEM micrographs of the samples with PA6/PBT = 40/60 (a, b) and 50/50 (c, d): (a, c) cryo-fractured, (b) etched by 10 wt % of NaOH aqueous solution, and (d) etched by formic acid. 210



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Figure 7. Plots of melt complex viscosity (a) and storage modulus (b) versus angular frequency for PA6 and PBT without and with different GF.

one in the middle corresponding to the crystallization of PA6. It can be seen that the intensity of P3 became much stronger compared with the crystallization curve of sample PBT55 and it increased as that of P1 decreased. This can not be attributed to the direct contribution of different GF, because DSC showed that the crystallization of PA6 and PBT remained almost unchanged with the addition of different GF (see Supporting Information, Figure S1). DSC was conducted on the samples with PA6/PBT = 30/70 and 70/30 (see Supporting Information, Figures S2 and S3) to understand this phenomenon. PA6 crystallized at a temperature lower than that of pristine PA6 in the samples with PA6/PBT = 30/70 (PA6 domains dispersed in PBT), whereas the crystallization temperature of PBT was similar to that of pristine PBT. The situation was reversed for the samples with PA6/PBT = 70/30 (PBT domains dispersed in PA6). It can be inferred that the confined crystallization of PA6 (or PBT) occurred in the samples with PA6/PBT = 30/70 (or 70/30) due to the formation of fine PA6 (or PBT) droplets dispersed in PBT (or PA6) matrix, as has been reported by many researchers.40,41 Then the crystallization behavior of the GF-filled PA6/PBT (45/55) blends is understandable. Different GF brought about a transition from the cocontinuous morphology to a sea-island morphology with PA6 being the matrix phase for the PA6/PBT (45/55) blend, which led to a sharp increase in the proportion of the dispersed PBT domains that underwent confined crystallization. Consequently, the intensity of P3 increased substantially, accompanied by a decrease in the intensity of P1. Besides, the intensity of P3 for sample PBT55GF(PA6) was smaller than that for sample PBT55GF(PBT), hinting that more PBT was encapsulated on the surface of GF for sample PBT55GF(PA6). Alkali Tolerance. Polyester is prone to hydrolytic degradation, especially under alkaline conditions because of the inherently labile ester bond. This is a problem that should not be neglected during the application process of polyestercontaining materials. It can be deduced that the alkali tolerance ought to be improved by adding GF, taking into account the morphological changes induced by GF for the PA6/PBT (45/ 55) blend. Alkali tolerance tests were carried out by refluxing in 10 wt % of NaOH aqueous solution for 18 h, and the hydrolytic degradation degree (Φ) of the PBT phase was determined. The results are listed in Table 2. About 70.0% of the PBT phase in sample PBT55 degraded in the alkali tolerance test. Actually, the cubic shape of sample PBT55 had been destructed after the alkali tolerance test. All the hydrolytic degradation degrees of the PBT phase did not exceed 30%, and the samples preserved

place, whereas an increase in storage modulus ratio between polymers 1 and 2 has an opposite effect, according to the prediction equations of the phase inversion point.36−39 Thus, on the one hand, the changes in viscosity ratio by GF support the sea-island morphology induced by GF(PA6 & PBT) for the PA6/PBT (45/55) blend, but they do not support the case of GF(Pris). On the other hand, the sea-island morphology induced by GF(Pris) for the PA6/PBT (45/55) blend is easy to understand, but the cases of GF(PA6) and GF(PBT) are not, from the point of view of the changes in storage modulus ratio by GF. It is possible that the contribution of storage modulus was shielded by the decrease in effective volume ratio of PBT and the contribution of viscosity ratio in the cases of GF(PA6) and GF(PBT), while it dominated in the case of GF(Pris). Nevertheless, this is just a deduction without any convincing evidence. Crystallization Behavior. An interesting question is whether the morphological changes induced by GF for the PA6/PBT (45/55) blends will be reflected in crystallization behavior. DSC was carried out and the results are shown in Figure 8. The crystallization curve of sample PBT55 exhibited

Figure 8. DSC cooling curves of PA6, PBT, and the samples with PA6/PBT = 45/55.

two intense peaks at 199.6 (P1) and 188.5 °C (P2), corresponding to the crystallization temperatures of pristine PBT and PA6, respectively. Besides, a weak peak appeared at 182.4 °C (P3) partially overlapped with P2. It should be noted here that the pristine PBT and PA6 for DSC underwent the same melt compounding process as that of all other samples. Three obvious peaks showed up on crystallization curves in the presence of the GF with different surface properties, with the 211



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Table 2. Hydrolytic Degradation Degree (Φ) of PBT Phase after the Alkali Tolerance Test for the Samples with PA6/ PBT = 45/55 sample name

PBT55

PBT55GF(PA6)

PBT55GF(PBT)

PBT55GF(Pris)

Φ (%)

70.0

29.8

18.9

15.0

their original cubic shape well in the presence of the GF with different surface properties. Therefore, the morphological transition induced by GF for the PA6/PBT (45/55) blend improved its alkali tolerance significantly. This is fairly valuable from an industrial point of view, because it allows the use of a high volume fraction of the less expensive PBT in PA6/PBT blends, without harming the service performance and lifetime under alkaline conditions obviously. Besides, sample PBT55GF(PA6) had a higher Φ than sample PBT55GF(PBT), which agrees with the deduction from DSC results that more PBT was encapsulated on the surface of GF for sample PBT55GF(PA6).

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CONCLUSIONS The GF with different surface properties all led to a transition from a cocontinuous to a sea-island morphology with PA6 being the matrix phase when PA6/PBT equaled 45/55. GF(Pris) had selectivity for PA6, while GF(PA6 & PBT) experienced the competitive encapsulation by PA6 and PBT, causing the encapsulating layers on their surfaces to change with the blending ratios between PA6 and PBT. The morphological changes induced by GF caused more PBT phase to crystallize at a lower temperature and enhanced the alkali tolerance of the blend significantly. Therefore, the present research provides a successful case of tailoring the morphology of polymer blends using GF, which may be quite useful in industry and engineering fields.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. cn. Fax: 0086-010-62782345. Notes

The authors declare no competing financial interest.

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REFERENCES

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Effect of Glass Fibers with Different Surface Properties on the

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