A Microrheological Study of Poly(Methyl Methacrylate) Elastomer/Poly(Ethylene Terephthalate) (PMMA_elast/PET) Blends

Abstract

This study involved an evaluation of the influence of phase inversion in poly(methyl methacrylate) elastomer/poly(ethylene terephthalate) binary blends (PMMA_elast/PET) and the effect of the addition of poly(methyl methacrylate-glycidyl methacrylate-ethyl acrylate) (MGE) interfacial compatibilizer on the microrheological properties of this polymer blend. Thermal, dynamic mechanical thermal, rheological and morphological analyses were performed using sensitive techniques such as differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), parallel plates rheometry in the linear viscoelastic region, and atomic force microscopy (AFM), respectively. In this study, it was found that variations in the percentage of the PET phase influenced the correlation between the rheological properties at low shear rates and the morphology of the PMMA_elast/PET binary blend and of the PMMA_elast/PET/MGE compatibilized blend.

Keywords:
PMMA/PET blend; parallel plates rheometry; AFM

1. Introduction

PMMA/PET blends are used in electrical and automotive applications which require materials with good dimensional stability, mainly for electronic circuits. However, these polymer blends are immiscible and incompatible^1-4 so they require the addition of an interfacial compatibilizer to improve the dispersion of the dispersed phase, thereby reducing the interfacial tension, stabilizing the morphology by reducing coalescence, and improving the interfacial adhesion between the phases^5,6.

The purpose of using a compatibilizer in immiscible polymer blends is to improve their final properties, and several analytical methods should therefore be employed to verify the efficiency of interfacial compatibilization. Microscopic characterizations by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) can provide evidence of the efficiency of compatibilization in polymer blends. Other characterization techniques such as rheological studies using low shear rates in the linear viscoelastic region of melt polymers can be used to complement the results obtained in the evaluation of possible changes in the rheological behavior of molten compatibilized polymer blends⁷.

Several authors, cited below, report the effect of compatibilization on the morphology and rheology of immiscible polymer blends compatibilized with MGE terpolymer, composed of methyl methacrylate (MMA), glycidyl methacrylate (GMA) and ethyl acrylate (EA). In studies using MGE terpolymer as a compatibilizer for PBT/ABS, PBT/AES and PBT/SAN blends ^8-12, it was found that the GMA epoxy group can react with the PBT terminal groups and may lead to the formation of a copolymer (PBT-g-MGE) at the interface of these polymer blends during the melt processing, which will alter their final properties.

Larocca et al.¹⁰, who examined the efficiency of the reactive compatibilization of MGE on the properties of PBT/AES blends, found that the formation of PBT-g-MGE copolymer at the interface of these blends altered their morphological and rheological properties. These effects became more intense in response to an increase in the overall concentration of GMA (present in the composition of the MGE) in the blend and to a reduction in the molecular weight of MGE. Costa et al.¹¹ carried out a study on SAN/PBT blends compatibilized with MGE, and reported that an analysis by TEM revealed a significant reduction in the dispersed phase of PBT in response to the addition of MGE, thus demonstrating the reaction efficiency of MGE with the final groups of PBT chains due to the in situ reactions that occurred during processing.

MGE terpolymer has also proved to be an excellent compatibilizer in PMMA/PET blends, since PET chain ends have the same carboxyl and hydroxyl groups, which are similar to those found in PBT. In this studies¹², TEM and AFM analyses revealed that increasing the percentage of molecular weight of PET in the PMMA/PET binary blend led to a significant increase in the mean diameters of the dispersed phase. Conversely, the use of MGE in these blends reduced the mean diameters of the dispersed phase in compositions containing the same percentage of PET of the binary blend.

Poly (methyl methacrylate) with elastomeric particles (PMMA_elast)^13,14 and PET have complementary properties¹⁵; hence, variations in the composition of their mixtures alter their structural and morphological characteristics. Earlier studies of PMMA_elast/PET immiscible polymer blends conducted by Reinaldo et al.¹⁶ revealed synergistic properties that can be used in specific applications. As for the efficiency of compatibilization, the authors found evidence of dual reactivity between MGE and PET, and also with PMMA_elast, due to the presence of the elastomer phase of PMMA.

In studies of the morphology of immiscible polymer blends, rheological characterization is extremely important because it sheds light on the final properties of these blends^17-19, especially in polymer blends with complex morphologies constituted by copolymers with elastomeric particles, such as the PMMA_elast contained in PMMA_elast/PET blends. Therefore, our goal was to perform a microrheological study of PMMA_elast/PET blends with and without the use of an interfacial compatibilizer, MGE, up to phase inversion, using parallel plates rheometry and atomic force microscopy.

2. Materials and Methods

2.1 Materials

The materials used in this work were PMMA copolymerized with elastomeric particles (PMMA_elast) (ECP800) (MFI = 3. 8 g/10 min, ASTM D 1238²¹, 230ºC/2.16 kg, according to the manufacturer) produced by Unigel S.A. and PET from soft drink bottles (MFI = 15 g/10 min, ASTM D 1238²¹, 250ºC/2.16 kg, according to the authors). The interfacial compatibilizer used here was poly(methyl methacrylate-glycidyl methacrylate-ethyl acrylate) (MGE) terpolymer with a composition of: 88 wt% methyl methacrylate, 10 wt% glycidyl methacrylate and 2 wt% ethyl acrylate, which was synthesized in the laboratory as described in the literature^9,20.

2.2 Extrusion mixing and injection molding

The PMMA_elast/PET binary blend up to phase inversion and the compatibilized PMMA_elast/PET/MGE blend were processed in a co-rotating twin screw extruder (D = 16 mm and L/D=40) under the following processing conditions: temperature profile of 102/160/190/210/220/230/220/210ºC; feed speed of 80 rpm; and screw speed of 220 rpm. After extrusion, the pure polymers and the polymer blends were granulated and dried at 60ºC in a vacuum oven for 12 hours. These materials were then molded (ASTM standard D638²²) in an Arburg 270V injection molding machine using a temperature profile of 230/240/240/250/250ºC, mold temperature of 50ºC and a cooling time of 50 s. The injection molded test specimens were used for the thermal, dynamic mechanical thermal, rheological and morphological analyses.

2.3 Thermal characterization by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA)

The DSC analyses were performed in a Netzsch DSC 200 F3 Maia differential scanning calorimeter using about 6 mg of sample, which was placed in an aluminum crucible and subjected to the following conditions: nitrogen atmosphere at a flow rate of 50 mL∙min^-1 and a heating rate of 10ºC∙min^-1 from 20 to 290ºC. The DMTA analyses were carried out in an Anton Paar parallel plates rheometer equipped with a SRF 12 accessory in torsion at a frequency of 1 Hz and the temperature ranged from 20 to 130ºC at a scanning rate of 2ºC∙min^-1.

2.4 Rheological characterization by parallel plates rheometry

The rheological analyses of the pure polymers and the PMMA_elast/PET and PMMA_elast/PET/MGE blends were carried out in the linear viscoelastic region, using an Anton Paar MCR 302 parallel plates rheometer, at a temperature of 270ºC in the frequency range of 1000 to 0.01 rad∙s^-1.

2.5 Morphological characterization by Atomic Force Microscopy (AFM)

Samples taken from the injection molded test specimens were first trimmed and then cut into ultra-thin sections in a RMC Power Tome-X Cryo-Ultramicrotome, using a Diatome Diamond Cryo Histo 45º knife, with sample temperature at -80ºC, knife temperature at -60ºC, liquid nitrogen cooling, cutting speed of 0.2 mm∙s^-1, and final section thickness of 25 nm. The morphological analysis was performed via non-contact atomic force microscopy, using a Shimadzu SPM-9700 microscope at a frequency of 1 Hz.

3. Results and Discussion

3.1 Thermal Characterization by Differential Scanning Calorimetry (DSC) and by Dynamic Mechanical Thermal Analysis (DMTA)

Thermal characterizations were performed by DSC and DMTA to evaluate the parameters that affect processability and the effects of the MGE compatibilizer on the PMMA_elast/PET blends. The DSC curves shown in Figure 1, and their values listed in Table 1, indicate that the behavior of PMMA_elast is typical of an amorphous polymer, showing only the PMMA glass transition temperature (T_g = 94.4ºC), because the T_g of the elastomeric phase was not measured by this technique (the start of test temperature was above ambient temperature). While PET, a semicrystalline polymer, presented a glass transition temperature (T_g = 70.4ºC), a crystallization temperature (T_c = 121.2ºC) and a crystalline melting temperature (T_m = 254ºC). The PMMA_elast/PET blends exhibited clear thermal transition temperatures (Figure 1), except for the blend containing 15 wt% of PET phase (see Table 1) that could not be quantified using this technique.

Figure 1
DSC curves of the pure polymers and the PMMA_elast/PET and PMMA_elast/PET/MGE blends.

Table 1
Thermal transitions and percent crystallinity of the pure polymers and the PMMA_elast/PET and PMMA_elast/PET/MGE blends

Compositions (wt%)	TgPMMA (ºC)	TgPET (ºC)	Tcc (ºC)	ΔHcc (J/g)	Tm (ºC)	ΔHm (J/g)	xc (%)
PMMAelast (100)	94.4	-	-	-	-	-	-
PMMAelast/PET (85/15)	87.2	64.0	-	-	252.0	0.92	4.4
PMMAelast/PET (70/30)	103.4	72.3	123.7	5.83	251.4	11.14	26.5
PMMAelast/PET/MGE (65/30/5)	119.1	69.8	119.1	4.10	246.3	10.36	24.7
PMMAelast/PET (50/50)	103.1	71.5	122.4	10.05	251.7	18.92	27.0
PMMAelast/PET (30/70)	97.3	71.5	123.6	13.44	252.0	30.13	30.7
PET (100)	-	70.4	121.2	25.32	254.0	44.83	32.0

• T_g^PMMA, T_g^PET, T_cc, T_m and ΔH_cc: Values related to temperatures of glass transition, crystallization, and crystallization and melting point enthalpy in the first heating cycle. Calculation of _c according to the following equation: xc=ΔHfΔH°fx100%purePET23xc=ΔHfWPETxΔH°fx100% (polymer blends)²⁴. The ΔH_fº of PET is 140 (J/g)²³

The DSC results also indicated the reactive compatibilization obtained with MGE was effective, based on signs of a decrease in crystalline phase in the blend containing 30 wt% of PET in response to this interfacial compatibilizer. The binary blend PMMA_elast/PET contained 26.5 wt% of crystalline phase (PET) and the compatibilized blend PMMA_elast/PET/MGE had 24.7 wt% of crystalline phase, as can be seen in Table 1. This decrease in the crystallinity of polymer blends in the same composition using a reactive compatibilizer has been reported by other researchers¹⁰, who attributed it to phase anchoring and increased molecular weight due to the reaction between the interfacial compatibilizer and the semicrystalline polymer.

Although the crystalline melting temperature of the PMMA_elast/PET binary blend is lower than the pure PET, it did not vary as a function of increased of molecular weight percentage up to the phase inversion of the binary blend. Given the reactivity of the epoxy group of MGE with the chain ends of PET and also with the elastomeric phase of PMMA¹⁴ significantly reduces the crystalline melting temperature of PET in the PMMA_elast/PET/MGE blend.

The DMTA results shown in Figure 2 pertain to the glass transition temperature (T_g) of pure PET, pure PMMA and the PMMA in the PMMA_elast/PET blend. The T_g of the elastomeric phase was not measured by this technique (the start of test temperature was above ambient temperature) and the T_g of the PET in the PMMA_elast/PET blends was not measured due to overlapping with the T_g peak of the PMMA.

Figure 2
Tan δ curves of the pure polymers and the PMMA_elast/PET and PMMA_elast/PET/MGE blends.

The α-relaxations pertaining to the glass transition temperatures of the polymers can be observed by means of the tan δ peak, i.e., the highest characteristic value of the tan δ peak corresponds to the material's glass transition temperature²³. Figure 2 shows that the differences in the tan δ peak values of PMMA_elast and PET are related to the different values of the glass transition temperatures (T_g) of these polymers. Because PMMA is an amorphous polymer, its peak was higher than that of PET, which is a semicrystalline polymer. As for the PMMA_elast/PET blends, the curves of the compositions showed a shift in the T_g of PMMA in response to the increase in the proportion of PET.

3.2 Microrheology of the PMMA_elast/PET and PMMA_elast/PET/MGE blends

Figure 3 correlates the morphological results obtained by AFM with the rheological results of the storage modulus (G') versus the loss modulus (G'') of the pure polymers and the PMMA_elast/PET binary blend as a function of the variation in the dispersed phase of PET. A significant difference was found in the values of the G' vs. G'' curves of PMMA_elast and PET, because PMMA_elast is a copolymer that contains an elastomeric phase and PET is a homopolymer. Changes in the PET content of the polymer blends caused variations in both the morphological characteristics and rheological behavior shown on the G' vs. G'' curves, indicating that the addition of PET phase affects the microrheological behavior of the PMMA_elast/PET blend.

Figure 3
Correlation between the morphology and rheology of the PMMA_elast/PET binary blend at 270ºC.

Figure 4 shows how the addition of MGE compatibilizer influences the rheological and morphological behavior when compared to the binary blends. The G'G'' vs. frequency (ω) curves of the pure polymers reveal the differences in the rheological behavior of these two polymers, which are directly related to the morphology of PET (homopolymer) and of PMMA_elast (copolymer). The presence of the elastomeric phase in the PMMA_elast copolymer showed higher G'G'' values at low frequency than the PET.

Figure 4
Correlation between rheology and morphology determined by low shear rate rheology and AFM: a) PMMA_elast (100 wt%); b) PMMA_elast/PET blend (70/30 wt%); c) PMMA_elast/PET/MGE blend (65/30/5 wt%); d) PET (100 wt%), at 270ºC.

Two G' = G'' crossover points were also observed, where the dissipative and elastic properties of the PMMA_elast at 270ºC were the same at the same frequency, with G' = G'' values at high frequency (ω = 160 rad/s) and low frequency (ω = 0.65 rad/s), while only one G' = G'' crossover point was observed at low frequency (ω = 1 rad/s) in PET at 270ºC. The behavior of PMMA_elast observed here is attributed to the crosslinks in rubber, which influence the rheological behavior of rubber-toughened acrylic copolymers, as has been reported by other authors¹⁹.

The compatibilized blend with MGE, i.e., the PMMA_elast/PET/MGE blend, did not show the G' = G'' crossover point on the G' G'' versus ω curve, and in the low frequency region, the storage modulus (G') is much higher than the loss modulus (G''). This behavior is attributed to the chemical reactions that occur between PMMA_elast and PET with MGE. The presence of these bonds at low shear rates causes the material to behave like a solid; hence, the value of G' is higher than the G''. The reactive compatibilization of MGE with the carboxyl and hydroxyl groups of PET, and also with the elastomeric particles in PMMA_elast copolymer, has been reported previously by Reinaldo et al.³. A smaller size distribution of the dispersed phase of PET was found in the PMMA_elast matrix of the compatibilized blend than in the binary blend with the same PET content.

Figure 5 illustrates the 3D morphology of the PMMA/PET interfacial region up to phase inversion, as well as that resulting from the use of MGE as interfacial compatibilizer in the PMMA_elast/PET/MGE blend. Sectioning the material using a diamond knife below the glass transition temperature (T_g) of the polymers was successful in cutting without structural deformations of the interface. In addition, the expansion of the phases as a function of the specific thermal expansion coefficient of each polymer allowed evaluations to be made of the differences at the interfaces of the blends as a function of phase inversion (30 to 70 wt% of PET) and the use of 5 wt% of MGE in the PMMA_elast/PET blends.

Figure 5
Interfacial analysis of the 3D morphology of the binary blend, the use of interfacial compatibilizer, and the phase inversion: a) PMMA_elast/PET blend with 30 wt% of PET; b) PMMA_elast/PET/MGE blend with 30 wt% of PET and 5 wt% of MGE; c) PMMA_elast/PET blend with 50 wt% of PET; d) PMMA_elast/PET blend with 70 wt% of PET.

Figure 6 shows the mean values of the height of the interface between PMMA and PET in the PMMA_elast/PET blends. The interface values for the compatibilized blends with MGE were lower than those of the binary blends containing 30, 50 and 70 wt% of PET phase. Therefore, the decrease in interfacial height, which has to do with the thermal expansion coefficient, provided evidence of better phase dispersion in the PMMA_elast/PET blends in response to the use of MGE.

Figure 6
Measures of the difference in the interface of PMMA_elast/PET and PMMA_elast/PET/MGE blends.

Figure 7 depicts the results of complex viscosity (η*) as a function of frequency (ω) in the linear viscoelastic region of the pure polymers, the PMMA_elast/PET binary blend and the PMMA_elast/PET/MGE blend compatibilized at 270ºC. As can be seen, neither PMMA_elast nor PET presented Newtonian behavior at low frequencies at the temperature of 270ºC.

Figure 7
Complex viscosity (η*) vs. frequency (ω) of the pure polymers and the PMMA_elast/PET and PMMA_elast/PET/MGE blends at 270ºC.

As for the behavior of the PMMA_elast/PET blends, the findings indicate that the complex viscosity (η*) of the PMMA_elast matrix did not change significantly in response to the addition of 15 and 30 wt% of the PET phase. Only the binary blends containing 50 and 70 wt% of PET showed such changes, and an increasing PET content should tend to bring the values close to the pure PET.

The compatibilized blend showed much higher complex viscosities (η*) at low shear rates than the pure polymers and the binary blend with the same content of PET phase (30 wt% of PET). This corroborates with the efficiency results of the dual reactive interfacial compatibilization of PMMA_elast/PET blend with MGE¹⁶. According to other researchers²⁵, the compatibilization mechanism involves the induction of interfacial interactions between the components of the polymer blend.

4. Conclusions

This microrheological study of PMMA_elast/PET binary blend and PMMA_elast/PET/MGE compatibilized blend was possible through the use of parallel plates rheometry in dynamic-oscillatory mode and atomic force microscopy (AFM). Based on low shear rate rheology, a study was made of the storage modulus as a function of the loss modulus and the complex viscosity of the pure polymers, the binary blend up to phase inversion and compatibilized blend. The use of sample preparation by cryo-ultramicrotomy enabled the study of the PMMA/PET interface by atomic force microscopy.

5. Acknowledgments

The authors gratefully acknowledge the financial support of the Brazilian research funding agencies CAPES (Federal Agency for the Support and Improvement of Higher Education), CNPq (National Council for Scientific and Technological Development) and FINEP (Research and Projects Financing Agency). We also thank the company UNIGEL S.A. for donating the PMMA used in this study.

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Publication Dates

• Publication in this collection
  30 Nov 2017
• Date of issue
  2017

History

• Received
  06 Dec 2016
• Reviewed
  04 Sept 2017
• Accepted
  27 Oct 2017