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Keywords:

  • reactive plasticizers;
  • processing;
  • rheology;
  • DMTA;
  • crystallization

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The effect of two different bisphenol-A-based diepoxides—nearly pure DGEBA340 and a DGEBA381 oligomer—and an aromatic diamine curative (MCDEA) on the solubility and processability of poly(phenylene oxide) (PPO) was studied. The solubility parameters of the diepoxies and the curative calculated from Fedors's method suggest miscibility of PPO with the components, and this was observed at the processing temperature; however, some of the blends were not transparent at room temperature, indicating phase immiscibility and/or partial PPO crystallization. The steady shear and dynamic viscosities of the systems agreed well with the Cox–Merz relationship and the logarithmic viscosities decreased approximately linearly with increasing amounts of DGEBA381, DGEBA340 or MCDEA, thus causing a processability enhancement of the PPO. The dynamic rheology of intermediate PPO:DGEBA compositions at 200 °C showed gel-like behaviour. Dynamic mechanical analysis of blends with varying PPO:DGEBA ratios showed that the main glass transition temperature (Tg) of the blends decreased continuously with increasing epoxy content, with a slightly higher plasticizing efficiency being exhibited by DGEBA340 compared to DGEBA381. However, blends with 50 and 60 wt% PPO had almost identical Tg due to the phase separation of the former blends. The blends of MCDEA and PPO were miscible over the concentration range investigated and Tg of the blends decreased with increasing MCDEA concentration. © 2013 Society of Chemical Industry


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Thermoplastics such as poly(phenylene oxide) (PPO), bisphenol-A polycarbonate (PC) and polyetherimide possess many desirable properties such as stiffness, strength and toughness, but their high melt viscosities require processing at elevated temperatures where they are exposed to severe degradation and loss of desirable properties, thus limiting their processing and final applications. In contrast, thermosetting monomers, such as epoxy resins, can be processed at low temperatures but they tend to be brittle. To avoid these problems, a number of workers have blended thermoplastics with thermosetting monomers, with the primary aim either to improve the processability of a high glass transition temperature (Tg) thermoplastic with a crosslinkable monomer as a reactive plasticizer, or to toughen a brittle thermoset by the addition of a ductile thermoplastic, without sacrificing the excellent properties of the host polymer.[1]

PPO possesses many desirable properties such as low water absorption, high dimensional stability, good thermal, electrical and chemical resistance, low flammability and excellent mechanical properties, and as a result this polymer is widely used in the electrical and electronics industries.[2] A few researchers[3-7] have reported the use of small quantities of PPO in blends with epoxies in order to modify the thermoset's brittleness and observed an increased fracture toughness and only a slight reduction in tensile modulus and tensile yield strength. On the other hand, several researchers[8-15] have reported the use of epoxy resin as a reactive plasticizer to improve the processing of PPO. In this case, the crosslinkable epoxy resin can act as a plasticizer for the PPO in the early stage of processing which decreases the viscosity and thus allows processing at a lower temperature. At the end of processing, the reactive plasticizer is polymerised in situ to form a morphology with PPO either as a co-continuous phase or as the matrix containing crosslinked epoxy particles.

PPO, like PC, is normally considered to be an amorphous polymer since PPO cannot easily crystallize in bulk due to a small difference between Tg (ca 225 °C[16]) and melting temperature (ca 267 °C16) which results in a high-viscosity melt and retarded molecular rearrangement into the crystalline state. However, PPO can crystallize in the presence of a solvent,[17] and in our previous studies of blends of PPO with diallyl o-phthalate (DAOP)[18] and of PC with either DAOP[19] or diglycidyl ether of bisphenol-A (DGEBA),[20] crystallization of the thermoplastics was observed thus making the systems more complex.

Several workers have previously investigated different aspects of the uncured[10, 11, 21] or curing rheology,[10, 12, 14] the morphology of cured blends[11, 21] or the mechanical properties of PPO:epoxy blends with PPOs having differing molecular weights,[3, 10, 21, 22] differing DGEBA molecular weight and thus structure,[3, 9] different blend ratios, different epoxy curing agents[8, 9] and different curing temperatures,[11, 12, 14] which has made comparisons difficult. Therefore, in this paper (and in a companion paper), we combine all of these techniques to investigate one common system, and we extend this work by investigating whether PPO crystallizes in the epoxy plasticizer during processing of the blend.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Materials

Poly(2,5-dimethylphenylene oxide) powder, PPO (Scheme 1), was supplied by GE Plastics as C2000. The molecular weight distribution was determined in chloroform using a Tosoh Ecosec HLC8320 gel permeation chromatograph and refractive index detector and four Tosoh TSKGEL polystyrene columns (two SuperHZ2000, one SuperHZ3000×1 and one SuperHZM-M) at a flow rate of 0.35 mL min−1. The number- and weight-average molecular weights were calculated to be 19 200 and 44 300 g mol−1, respectively, and are quoted in polystyrene equivalents. Before processing, the PPO was dried in an oven at 110 °C for 12 h.

image

Scheme 1. Structures of monomers and poly(phenylene oxide) (PPO).

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Two epoxies with different molecular weights were studied as potential plasticizers for PPO: (1) a pure form of DGEBA, coded DGEBA340, with a molecular weight of 340 g mol−1 (n = 0, see Scheme 1) from Aldrich and (2) an oligomer of DGEBA, coded DGEBA381, with a molecular weight of 381 g mol−1 (n = 0.15, see Scheme 1) and containing 15 mol% of hydroxyl groups obtained as Araldite Algy 9708–1 from Ciba-Geigy. In a companion paper[23] we investigate several curing agents for these monomers and show that 4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA; supplied by Aldrich), shown in Scheme 1, is the favoured material. Thus in the present paper PPO:MCDEA blends are also studied.

The composition of the samples is described here by the mass ratio of PPO to epoxy, or of PPO to MCDEA, so that, for example, the code 80PPO:20DGEBA refers to a blend of 80 wt% PPO and 20 wt% of either of the DGEBA monomers. The specific epoxy monomer used, either DGEBA340 or DGEBA381, is specified in the text.

Sample preparation

For melt blending of PPO:epoxy blends, PPO powder was pre-mixed with epoxy using a spatula at room temperature and then allowed to interdiffuse for 18 h. The blend was then added to the hopper of a Minilab microextruder (Thermo Electron, Germany) and was mixed for 5 min in the extruder by a pair of tapered screws (L/D linearly varied from 10 to 20, with a screw length 100 mm) and a recycling channel, before finally being extruded through the extruder's die. The screw speed was fixed at 100 rpm and the temperatures of the barrel, recycling channel and die were equal and pre-set at the lowest usable temperature which was 240 °C for 80PPO:20DGEBA, 220 °C for 70PPO:30DGEBA, 200 °C for 60PPO:40DGEBA and 180 °C for 50PPO:50DGEBA blends. Similarly, for PPO:MCDEA blends, melt blending of 86.67PPO:13.33MCDEA was performed at 250 °C, 80PPO:20MCDEA at 240 °C and 73.33PPO: 26.67MCDEA at 230 °C—these ratios were selected to facilitate mixing of the epoxy and MCDEA-based blends reported in a companion paper.[23] The resulting molten blends were then immediately injected into a cold steel mould (1.5 mm × 5.0 mm × 50 mm) using a 3.5 mL Micro Injection Moulder (DSM Research, The Netherlands) and rapidly cooled to room temperature.

Blends containing less than 50 wt% of PPO in epoxy formed a slurry at room temperature and so the blends were prepared by directly dissolving PPO in epoxy at 180 °C in an oven for up to 30 min (depending on PPO content) in a closed container until PPO completely dissolved in the monomer.

Isothermal rheological behaviour

Isothermal dynamic and steady shear rheology measurements of the uncured blends of different ratios of PPO:DGEBA or PPO:MCDEA were conducted at 200 °C using an Anton Paar Physica MCR501 rheometer. This temperature was selected because, as shown in a companion paper,[23] the cure of DGEBA with MCDEA at 200 °C proceeds at a rate which is compatible with the processing of the blends. For blends with PPO concentrations of 50 wt% or more, the test samples were prepared by injection moulding into squat cylindrical moulds of 1.5 mm. For blends with PPO concentrations less than 50 wt%, the test samples were prepared by direct dissolution of the components at 180 °C before casting into a silicon mould well to produce disc samples about 1–2 mm thick. The disc samples were placed between two parallel plates of the rheometer with various diameters (15–50 mm, depending on the viscosity of the blends) preset at 200 °C and a fixed gap of 0.5 mm imposed. For steady shear studies, the viscosity was measured at various shear rates increasing from 0.1 to 100 s−1; for dynamic oscillation measurements, the maximum strain was 5% and the imposed frequency was varied from 0.1 to 100 rad s−1 at 200 °C. The dynamic rheology of either DGEBA- or MCDEA-based blends showed no hysteresis when the frequency was ramped up and then down, indicating stability of the blends.

Dynamic mechanical thermal analysis (DMTA)

A Rheometric Scientific Mark IV model dynamic mechanical thermal analyser was used at 1 Hz in dual cantilever bending mode over a temperature range of 25 to 250 °C at 2 °C min−1 to measure the glass transition behaviour of various PPO:DGEBA or PPO:MCDEA blends. Bar-type specimens had dimensions of 1.5 mm × 5.0 mm × 20 mm and were cut from the injection moulded sample. Tg was defined as the temperature corresponding to the maximum in tan δ.[24]

Differential scanning calorimetry

Dynamic DSC studies were conducted using uncured samples of PPO:DGEBA340 in hermetically sealed pans, to monitor crystallization and the glass transition region in the blends. The heating and cooling experiments were performed at 10 °C min−1 with a PerkinElmer Pyris 1 DSC calibrated using high-purity indium and zinc and fitted with a Flexicool (ETS system) cooling system.

Results And Discussion

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Solubility parameter

To assess the likely solubility of the monomers and the epoxy resins and thus their ability to act as a reactive plasticizer, the Hildebrand solubility parameters of the epoxy resins and curatives were calculated using Fedors's[25] group additivity approach (since only Fedors had all of the group contribution data for the functional groups in the materials) and these are listed in Table 1. As can be seen, the differences in solubility parameters of the DGEBA-based epoxies and MCDEA from PPO are much less than 2 (J cm−3)1/2 or 2 MPa1/2 and this limit is often considered to be a useful criterion for miscibility.[26] The solubility parameter of a DGEBA oligomer should vary with its molecular weight and hydroxyl content and Table 1 suggests that DGEBA381 should be a marginally better solvent for PPO than DGEBA340. However, in fact PPO has been reported[21] to have lower miscibility with a higher molecular weight DGEBA-based epoxy due to its lower entropic contribution to the free energy of mixing, thus reducing miscibility.

Table 1. Predicted solubility parameters of PPO and monomers
MaterialSolubility parameter, δ ((J cm−3)1/2)
PPO22.9
DGEBA38122.6
DGEBA34022.3
MCDEA23.1

Rheological properties

The PPO melt was processed at 280 °C. As discussed above, it is observed that the minimum operating temperature required for efficient mixing and extrusion of the blends decreases with increasing epoxy or MCDEA content, as expected. For example, for 80PPO:20DGEBA340, the extruder temperature used is 240 °C while the temperature is reduced 180 °C for 50PPO:50DGEBA. All blends are transparent at the processing temperature, but the blended samples of the epoxies with less than 60 wt% PPO are opaque at room temperature suggesting a two-phase structure caused by partial immiscibility of PPO- and DGEBA-rich phases or by partial crystallization of PPO from the melt. PPO crystallization occurs in solvents[17] and is observed in DSC studies of some of the PPO:DGEBA samples as discussed below. For the PPO:MCDEA blends, the minimum operating melt blending temperature is reduced as the content of PPO is reduced as noted above. All PPO:MCDEA blends are transparent at room temperature, suggesting high miscibility, which is consistent with the close correspondence of their solubility parameters (Table 1).

The isothermal rheological properties of blends of PPO with either DGEBA381 or DGEBA340 were investigated in order to examine the improvement in processability of PPO on addition of epoxy resin at high PPO levels, as well as the impairment in flow behaviour of the epoxy caused by PPO at low PPO levels. Figure 1 shows the steady shear viscosities of PPO:DGEBA340 blends at 200 °C at various shear rates. Very similar behaviour was also found for PPO:DGEBA381 (not shown). The blends possess typical polymer melt characteristics with plateau viscosities at low shear rates (Newtonian region) and a reduction of viscosities at high shear rates (pseudoplastic or shear rate thinning behaviour). For blends with 50 wt% PPO or more, the transition from Newtonian to pseudoplastic region occurs at a lower shear rate with increasing PPO content, as is normally found in other polymer solutions.[27, 28] For blends with 40 wt% PPO or less, the viscosity is close to Newtonian over the entire range of shear rates studied.

image

Figure 1. Steady shear viscosity of PPO:DGEBA340 at 200 °C.

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Figure 2 shows that the logarithm of viscosity in the Newtonian region drops rapidly as the concentration of DGEBA340 increases in the melt blend; this type of behaviour is usually interpreted as being due to an increase of free volume with the presence of solvent.[29, 30] The logarithm of the viscosity decreases in an approximately linear fashion as the PPO content is increased, as commonly observed in polymer blends,[19, 31, 32] but deviation occurs at lower levels of PPO. Thus at high concentrations of PPO, the viscosity of blends decreases approximately tenfold per 10 wt% increase in epoxy content and this behaviour gives a quantitative basis for the previously noted improved processability of PPO after blending with epoxy. However at low PPO concentrations, the change is approximately twofold per 10 wt% increase in PPO which is more desirable for its use in toughening the crosslinked epoxy.

image

Figure 2. Steady shear viscosity of PPO:DGEBA340 and PPO:MCDEA at 0.1 s−1 and 200 °C and complex viscosity of PPO:DGEBA340, PPO:DGEBA381 and PPO:MCDEA at 0.1 rad s−1 and 200 °C as a function of PPO content.

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Figure 3 shows the frequency dependence of the complex viscosity of PPO:DGEBA340 at 200 °C in the range 0.1 to 100 rad s−1. As found for the steady shear data, the viscosity progressively decreases with increasing epoxy content suggesting a significant improvement of processability. Nearly identical behaviour is observed for PPO:DGEBA381, but as demonstrated in Figure 2, the PPO:DGEBA381 blends have slightly higher complex viscosities than the PPO:DGEBA340 blends. This may be due to the higher molecular weight of the epoxy in the former system, or may be caused by the formation of hydrogen bonds in the presence of hydroxyl groups in DGEBA381. Figure 3 also shows that the dynamic viscosity becomes frequency dependent at higher frequencies, particularly for higher levels of PPO in the blend, and similar behaviour has been reported by Venderbosch et al.[10]

image

Figure 3. Dynamic viscosity of PPO:DGEBA340 at 200 °C.

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A comparison of the steady shear viscosity (Fig. 1) and complex viscosity (Fig. 3) shows very similar behaviour, indicating that the system complies with the empirical Cox–Merz relationship:[33]

  • display math

where η is the steady shear viscosity in Pa.s, η* is the complex viscosity in Pa.s, ω is the frequency in rad s−1 and inline image is the shear rate in s−1. The steady shear viscosity at 0.1 s−1 and the complex viscosity at 0.1 rad s−1 are very similar as shown in Figure 2, but at shear rates greater than 10 s−1, the steady shear viscosity tends to be lower than the dynamic viscosity.

The variations of storage modulus (G′) and loss modulus (G″) and tan δ with angular frequency of the PPO:DGEBA340 blends at 200 °C are plotted in Figures 4 and 5. Similar behaviour was also observed for the PPO:DGEBA381 blends (data omitted for brevity). Most of the blends show power-law dependencies on frequency over the majority of the frequency sweep. Blends with 50 wt% PPO and more show a higher power-law dependence of G′ on frequency than G″, and tan δ decreases with increasing frequency suggesting liquid-like behaviour.[34, 35] However, tan δ is virtually independent of frequency for blends with 20–40 wt% PPO, which is characteristic of a gel-like state. This behaviour may be caused by small amounts of crystallization of PPO[17] in the DGEBA solution or by an entanglement structure of the PPO chains. Physical gels such as hydrogen-bonded proteins, polysaccharides, block copolymer solutions and crystallizing polymer solutions[36-39] have also been found to obey the power-law dependencies of G′ and G″ on frequency which is similar to those for chemically crosslinked incipient gels.

image

Figure 4. Storage modulus (filled symbols) and loss modulus (open symbols) versus frequency for PPO:DGEBA340 at 200 °C.

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image

Figure 5. Tan δ versus frequency for PPO:DGEBA340 at 200 °C.

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The dynamic and steady-state rheology of 57PPO:43MCDEA was also studied at 200 °C—other blends with higher PPO levels were too viscous at the chosen processing temperature (200 °C) for the fully formulated blends to be measured. Figure 6 illustrates the dependence of moduli (G′ and G″) and tan δ on frequency. Tan δ decreases with increasing frequency and the scaling exponents for G′ and G″ are close to 2 and 1, respectively, which are typically found for simple liquids and polymer melts at high temperature.[35] The steady shear viscosity (not shown) of 57PPO:43MCDEA has a zero shear rate viscosity of 500 Pa.s and starts showing shear-rate thinning above 1 to 10 s−1 and this is comparable to the complex viscosity at 1 rad s−1 of 466 Pa.s. As observed for the PPO:DGEBA data, the shear rate and frequency dependence of the steady shear viscosity and dynamic viscosity are similar indicating that the system complies approximately with the Cox–Merz relationship.[33]

image

Figure 6. Moduli (storage modulus and loss modulus) and tan δ versus frequency of 57PPO:43MCDEA blend at 200 °C.

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Dynamic mechanical properties

DMTA was performed on the PPO:DGEBA340 blends to determine their Tg. Blends with less than 50 wt% PPO could not be studied because the samples were very fragile and so attempts to carry out DMTA of these samples were unsuccessful. Figure 7 shows the storage modulus (E′) and loss factor (tan δ) of uncured PPO with different contents of DGEBA340 fron which it can be seen that the glass transition region shifts to lower temperature with increasing levels of DGEBA due to the plasticization effect which reduces Tg by approximately 40 °C per 10 wt% DGEBA340. Almost identical behaviour (not shown here) was observed for PPO:DGEBA381 blends. The PPO:DGEBA blends show similarly shaped tan δ curves—a main symmetric transition between 70 and 120 °C followed by a rise in tan δ at high temperatures which is commonly seen in uncrosslinked polymers with few entanglements.[40] Similarly the elastic modulus passes through a sigmoidal transition before entering a brief plateau (entanglement) region and then the flow regime. However, the tan δ curves also exhibit a small shoulder below the main glass transition peak which increases in strength as the concentration of DGEBA is increased, suggesting the presence of an uncured epoxy-rich phase. Concomitantly, the modulus curves have small inflections between 20 and 150 °C indicating the glass transition of a second phase. These observations are consistent with the observed cloudiness at room temperature of the blends with more than 40 wt% DGEBA. It should be noted that optical transparency of polymer blends does not prove miscibility because the differences in refractive indices or the size of the domains may be too small to significantly scatter visible radiation. Further evidence of phase separation is apparent in the dependence of the peak temperature in tan δ on composition (Fig. 8)—Tg of the blends continuously decreases with increasing DGEBA340 content from 0 to 50 wt% DGEBA340. However, the blend with 60 wt% DGEBA has almost the same Tg as the 50PPO:50DGEBA340 blend (Fig. 8). This is further demonstrated when the Tg vales are compared with those predicted from the Fox equation[41]—for 60 to 80 wt% PPO, the experimental values are slightly lower than those calculated but a large deviation can be observed for the 50 wt% DGEBA381 blend. This behaviour is consistent with large-scale phase separation for this blend at room temperature.

image

Figure 7. E′ and tan δ of various uncured PPO:DGEBA340 blends at 1 Hz as a function of temperature.

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image

Figure 8. Tg calculated (Fox equation) and observed (DMTA) of PPO:DGEBA340 and PPO:DGEBA381 blends with various concentrations of PPO. Tg of DGEBA340 and DGEBA381 was measured using DSC—note that Tg determined using DMTA at 1 Hz is usually about 20 °C above the value determined using DSC.[42]

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PPO:DGEBA-based epoxy blends have been reported to exhibit upper critical solution temperature phase behaviour with cloud point temperatures ranging from 60 to 190 °C (and thus immiscibility below these temperatures) for 10–30 wt% PPO, depending on the structure[9] or molecular weight of the DGEBA epoxy resin[3, 31] and on the molecular weight of the PPO.[10, 21] Venderbosch et al.[9, 10] observed that PPO:DGEBA blends with high concentrations of PPO (≥70 wt% PPO) are visually homogeneous at room temperature and have Tg values that appear to obey the Fox equation. In addition, using dynamic rheology during cooling the blends from the molten miscible state, Meijer et al.[43] reported that blends with lower concentrations of PPO (10–60 wt% PPO) exhibit one constant Tg irrespective to PPO content. This behaviour can be explained by a thermoreversible phenomenon[8-10, 15] in which phase separation into polymer-rich and plasticizer-rich phases of the equilibrium composition is hindered by the vitrification of the polymer-rich phase, thus producing a single and similar Tg for differing polymer concentrations, known as the Berghmans point.[44, 45] Thus for blends with low PPO concentrations, the observed Tg values are equal to the temperatures at the intersection of the cloud point curve (for up to 60 wt % PPO) and the ideal Tg–composition line of a homogeneous system.[9, 10, 43]

For the PPO:DGEBA381 blends, the main Tg values are approximately 20 °C higher than those for blends with DGEBA340. This difference can only be partly ascribed to the higher Tg of DGEBA381 (−14 °C) compared to DGEBA340 (−17 °C) which would result in higher Tg of the PPO:DGEBA381 blends. However, the difference is more likely to be associated with a lower solubility of DGEBA381 in PPO, in agreement with the miscibility results of Merfeld et al.[21] causing less of a plasticizing effect in terms of reduction of Tg of the blends. As found with PPO:DGEBA340, the Tg–composition plot (Fig. 8) shows that Tg of the main relaxation is approximately independent of composition at less than 60 wt% PPO. These results suggest that the Berghmans points for PPO:DGEBA340 and PPO:DGEBA381 are approximately 90 and 112 °C at 50–60 wt% PPO, respectively. In agreement, Ishi and Ryan[11] observed a Berghmans point (using DSC) of 100 °C at 63 wt% PPO in a blend with an oligomer similar to DGEBA381.

Pure PPO crystals have a melting temperature of approximately 260 °C and a heat of fusion of approximately 40 J g−1.[46] To determine whether PPO is able to crystallize from the PPO:epoxy melt, dynamic DSC studies (Fig. 9) were conducted on some of the samples with low and high concentrations of PPO. For the blend with 20 wt% PPO, one endothermic melting transition is observed at 34 °C. This transition could be due to either the melting of DGEBA crystals (DGEBA has a melting point near 42 °C[47]) or impure PPO crystals. The former assignment is unlikely because the presence of PPO in DGEBA should decrease the melting point of DGEBA whereas melting point depression[48] of PPO in the blends could cause this low melting point. The heat of fusion measured for the peak at 34 °C is 2.72 J g−1 (based on the total mass of the blend) which equates to 33.7% crystallinity of PPO. After cooling and reheating, this transition is replaced by a broad maximum at 157 °C with an endotherm of 7.47 J g−1 which might be due to a devitrification process. For the blend with 50 wt% PPO, an endotherm is observed at 70 °C in the first heating stage, which is most likely due to the fusion of PPO crystals because the endotherm is too high for DGEBA. The heat of fusion for this peak (13.3 J g−1) equates to 66.3% crystallinity for PPO. On cooling and reheating, this transition is replaced by two DSC glass transitions at 22 and 130 °C with heat capacity step (ΔCp) of 0.23 and 0.16 J g−1 °C−1, respectively. At first sight this result appears to contradict the DMTA data which give a DMTA Tg of 90 °C for the PPO-rich phase and another Tg due to the DGEBA-rich phase near 25 °C. Aside from the small difference in Tg determined using DSC or DMTA, the reason for the differences in these data is probably associated with the differing times allowed for phase separation to occur in the two specimens. In contrast, the 80PPO:20DGEBA340 blend shows only one DSC glass transition during heating and reheating at temperatures near 120 °C (ΔCp of 0.21 J g−1 °C−1). Since this system is believed to be a miscible blend, this value is in accord with the DMTA Tg of ca 155 °C, since Tg values measured using DMTA are invariably higher.[42] Thus it appears that PPO can crystallize slowly from solutions in DGEBA when the blend composition is of an intermediate value, such as 50 wt% PPO, but not for high PPO contents, as with the 80 wt% PPO blend. The values measured here are also lower than the temperature (200 °C) used in the dynamic rheology experiments (Fig. 5) which indicate pseudo-gelation behaviour for blends with 20–40 wt% PPO, and so crystallization of PPO is unlikely to be the cause.

image

Figure 9. Dynamic DSC of heating and reheating of various PPO:DGEBA340 uncured blends (10 °C min−1).

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DMTA was also performed on the PPO:MCDEA blends. They only exhibit one glass transition (Fig. 10) which is consistent with the observation that all samples are transparent at room temperature. The variation Tg of the blend with composition is also shown in Figure 10 with Tg being reduced by approximately 38 °C per 10 wt% MCDEA. These results are in agreement with the qualitative information discussed above which indicates that addition of MCDEA to PPO makes it more readily processable.

image

Figure 10. E′ and tan δ of various uncured PPO:MCDEA blends at 1 Hz as a function of temperature and plot of Tg versus PPO content. Tg of pure MCDEA is estimated from its melting point based on the two-thirds rule.[49]

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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The solubility parameters of the materials calculated using Fedors's method suggest that DGEBA340, DGEBA381 and MCDEA should be miscible with PPO. While the PPO:MCDEA blends with 13.3, 20 and 26.3 wt% MCDEA were transparent at room temperature and all blends were transparent at the processing temperature, PPO:DGEBA blends with more than 40 wt% DGEBA were cloudy at room temperature, suggesting either partial immiscibility of PPO and DGEBA or crystallization of PPO. In agreement with this suggestion, DSC demonstrated that PPO could crystallize from a solution in DGEBA.

The rheological properties of PPO with various concentrations of DGEBA381 or DGEBA340 at 200 °C showed that increasing amounts of either epoxy in the blends caused a continuous reduction of viscosity of the blends thus improving the processability of PPO. MCDEA similarly reduced the viscosity of the PPO blends. The dynamic rheological characteristics of the DGEBA:PPO blends with intermediate compositions indicated gel-like behaviour.

Dynamic mechanical studies of PPO:DGEBA340 and PPO:DGEBA381 blends indicated a slightly lower Tg for the former and thus a slightly higher plasticizing efficiency of DGEBA340. DMTA of the PPO:DGEBA blends showed a tan δ shoulder on the main glass transition peak which indicated partial phase separation at room temperature. At high concentrations of PPO (more than 60 wt% PPO), the main Tg values of the blends were only slightly different from those predicted by the Fox equation. Thus even if these blends are partially immiscible at room temperature, it appears that the phases interdiffuse during the DMTA experiment resulting in only one phase when the temperature reaches that of the main tan δ peak (primary Tg). However, at lower PPO contents, the primary Tg deviated from the predicted line suggesting immiscibility in the blend at the temperature of the main glass transition. In contrast, DMTA of MCDEA blends showed no evidence of phase separation and, for the data available, Tg of PPO:MCDEA varied smoothly with composition.

Overall, these results indicate the applicability of epoxy monomers as reactive plasticizers to improve the processability of PPO, and provide information on the increase in viscosity when PPO is added to epoxies for the purpose of toughening the thermoset matrix.

ACKNOWLEDGEMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The authors thank the Australian Research Council for partial support through grant DP0557737.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. Results And Discussion
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES