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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

New epoxy thermosets have been prepared via cationic UV-photopolymerization introducing two different multiarm star-like polymers. Both stars have a poly(glycidol) core but one has poly(methylmetacrylate) arms and the other poly(ε−caprolactone) ones. The characterization of the curing process has been performed by Real-Time FTIR and photo-DSC, observing a slight reduction in the curing rate on increasing the proportion of star. The thermosets prepared were characterized by gel content determination, DMTA and TGA, and finally the morphology observed by FE-SEM, demonstrating the formation of nanophases in the case of the star with poly(ε−caprolactone) arms. POLYM. ENG. SCI., 54:17–23, 2014. © 2013 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Epoxy resins are one of the most used thermosets in the field of coatings. They are versatile materials that present high chemical resistance and good mechanical and thermal properties [1-3]. However, they present some drawbacks being the most noticeable the fact that they are materials inherently fragile, which limits their range of application. During the past decades, considerable efforts have been made to improve toughness of epoxy thermosets [4-7] In most cases, the strategies followed lead to losing some of the good properties of the resin, especially in terms of rigidity and glass transition.

Recently, a lot of attention has been paid to the formation of nanostructured epoxy coatings. The final goal of this strategy is the improvement of the toughness of the epoxy thermoset without compromising other properties. In the literature, it is possible to find many different approaches. One strategy is the use of linear block copolymers. Lipic et al. [8] described the phase separation of a polyethylene oxide/polyethylene-propylene copolymer in an epoxy matrix. Ryan et al. [9] used different block copolymers to induce nanophase separation in epoxy resins. In some cases one block was miscible with the resin while the other not and the separation appears on preparing the reacting mixture. In other copolymers both blocks where miscible but one of them was reactive towards the resin, and then the phase separation occurred during the curing reaction by a reaction induced phase separation mechanism (RIPS). Also Fan et al. [10] obtained a nanostructured epoxy resin using a triblock copolymer containing poly(dimethyl siloxane), poly(ε-caprolactone) and poly(styrene) to modify DGEBA.

Hyperbranched polymers have also been used as a convenient alternative to obtain phase separation in epoxy systems. Their structure gives them excellent flow and processing properties, since they are characterized by lower viscosity than that of linear polymers of comparable molecular weight. The highly branched structures gives further access to a large number of reactive end groups and thus, HBPs have been successfully used in various coating and resin applications including epoxy systems in order to improve some of their properties such as processability or toughness [11, 12]. One aspect is e.g. their use as toughening agents that phase-separate during curing [13].

More recently, Meng et al. [14] have introduced the use of a star-shaped block copolymer and they obtained phase separation in diglycidyl ether of bis-phenol-A (DGEBA) matrix, and even an increase in the Tg of the resulting thermosets.

Regarding the use of star polymers in UV-cured systems, our group has recently studied the cationic photopolymerization of a commercially available cycloaliphatic epoxy resin in the presence of a star polymer made of a hyperbranched aromatic-aliphatic core and poly(ethylene glycol) arms [15]. In that case, the presence of phase separation was demonstrated.

However, not only the additive used but also the polymerization procedure seems to be important in the induction of micro/nano phase separation. For instance, Ratna et al. [16] reported the preparation of phase separated thermosets when working with DGEBA/Boltorn H30 mixtures and curing with diamines. On the contrary, phase separation was not observed in DGEBA/Boltorn H30 mixtures using Yb(OTf)3 as cationic thermal initiator [17] or curing with anhydrides [18]. Moreover, we have already demonstrated that the particle size that one can obtain using a new hyperbranched-linear-hyperbranched polymer as modifier strongly depends on whether the epoxy thermoset is prepared by cationic initiation via UV or thermally [19].

Here we present the use of two different star polymers in the preparation of photo-cured epoxy resins. The influence of the structure of the arms of the star polymer on the curing process and in the final properties and morphologies obtained will be discussed. Moreover, a comparison between the morphology of the thermosets herein presented and thermosets prepared using the same star polymers but prepared by cationic thermal curing will be also shown.

EXPERIMENTAL PART

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Materials

The multiarm star-like polymers used in the present work polyglycidol-polycaprolactone (PGOH-PCL) and polyglycidol-polymethylmetacrylate (PGOH-PMMA) (Scheme 1) were synthesized following two procedures previously described by us [20, 21]. The 1H- and 13C-NMR signals matched with the previously reported. PGOH-PCL has a poly(glycidol) core (PGOH) and poly(ε-caprolactone) (PCL) arms which were grown by means of ring-opening polymerization of ε-caprolactone from the hydroxyl groups of PGOH core. Its Mn was 225,700 g/mol, molecular weight dispersity of 2.05 (determined by SEC-MALLS) and presented no Tg but a melting at 60°C, as determined by DSC. The number of arms was calculated to be between 100 and 110 by 1H-NMR. The average degree of polymerization of PCL arms was 30. PGOH-PMMA was synthesized by atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) from an activated PGOH core modified with terminal bromine groups. It possesses an average of 45 arms of poly(methyl methacrylate) (PMMA) determined by 1H-NMR spectroscopy. The degree of polymerization of the PMMA arms was calculated after hydrolysis and analysis of the arms by SEC-RI and it resulted to be 55. PGOH-PMMA presented a Mn of 324,500 g/mol (determined by SEC-MALLS) and a molecular weight dispersity of 2.2 and it presents two Tg values, which are 97 and 113°C (determined by DSC).

image

Scheme 1. Schematic representation of all the reactants used.

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The bis-cycloaliphatic diepoxy resin 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexyl carboxylate, (CE) and triphenylsulfonium hexafluoroantimonate (PI, Ph3S+SbF6) (Scheme 1) were purchased from Aldrich and used as received.

Film Preparation

Photocurable formulations were prepared by adding the appropriate amount of either PGOH-PCL or PGOH-PMMA (0, 5, 10, or 15 g per hundred g of resin, phr) to epoxy resin (CE). To achieve a homogeneous mixture, formulations were heated at 80°C and after cooling down no precipitation by visual inspection was observed. Once the mixtures were at room temperature, 2 phr of photoinitiator (PI) were added. The liquid formulations were coated onto a glass substrate or in a polypropylene template (for free-standing films) using a wirewound applicator. Then, the films were exposed to UV radiation with a fusion lamp (H-bulb) in air at a conveyor speed of 5 m min−1, with radiation intensity on the surface of the sample of 280 mW cm−2.

Characterization

1H-NMR and 13C-NMR measurements were carried out at 400 MHz and 100.6 MHz, respectively, in a Varian Gemini 400 spectrometer. CDCl3 and DMSO-d6 were used as solvents for NMR measurements. For internal calibration the solvent signals were used: δ (13C) = 77.16 ppm, δ (1H) = 7.26 ppm for CDCl3 and δ (13C) = 39.52 ppm, δ (1H) = 2.50 ppm for DMSO-d6. Quantitative 13C-NMR experiments were recorded using a delay time between sampling pulses equal to 8 s and the sequence inverse gated decoupling.

The determination of molecular weights and molecular weight distributions were carried out on a modular build SEC-system using an Agilent 1200 series pump coupled with a multi-angle laser light scattering (MALLS) detector Tristar MiniDawn (Wyatt Technology) and Knauer RI detector in combination with a PolarGel-M-column (Polymer Laboratories) in DMAc mixed with 3 g L−1 LiCl. The evaluations of the molecular weights were made using software ASTRA 4.9 (Wyatt Technology).

The kinetics of photopolymerization was determined by real-time (RT) FTIR spectroscopy, employing a Thermo-Nicolet 5700 FTIR device. Epoxy group conversion was followed in real-time upon UV exposure, by monitoring the decrease in the absorbance due to epoxy groups in the region 760–780 cm−1. A medium pressure mercury lamp equipped with an optical guide was used to induce the photopolymerization (light intensity on the surface of the sample of about 5 mW cm−2). Variation in the experimental conditions (light intensity, humidity, and temperature) caused slight differences in the kinetic curves. For this reason all the conversion curves in the figures were performed on the same day and under the same conditions and thus, good reproducibility was obtained. All the polymerization reactions were performed at room temperature at constant humidity (25–30%). The samples were stored for at least 24 h before properties were evaluated.

The conversion (XUV,FTIR) was calculated by monitoring the disappearance of the epoxy band (760–780 cm−1) with the time and using Eq. (1).

  • display math(1)

where At is the normalized absorbance of the epoxy band at a given time and A0 is the initial normalized absorbance.

Photocalorimetric experiments were performed in order to study the effect of the temperature on the photocuring kinetics. The various samples were photocured at different temperatures using a Mettler DSC-821e calorimeter appropriately modified to permit irradiation with a Hamamatsu Lightningcure LC5 (Hg-Xe lamp) with two beams, one for the sample pan and the other for the reference pan. Samples weighing ca. 4 mg were cured in open aluminium pans in a nitrogen atmosphere. Two scans were performed on each sample in order to substract the thermal effect of the UV irradiation from the photocuring experiment, each one consisting of 2 min of temperature conditioning, 4 min of irradiation and finally 2 more minutes without UV light. A light intensity of 50 mW cm−2 (calculated by irradiating graphite-filled pans on only the sample side) was employed. Dynamic postcuring experiments were carried out on a Mettler DSC-821e, from 30 to 200°C at 15°C min−1 in nitrogen atmosphere. The degree of conversion XUV during the photocuring stage was calculated based on the residual heat evolved during the postcuring scan as follows (Eq. (2)):

  • display math(2)

where Δhpost is the heat released during the dynamic post-curing process and Δhtheor corresponds to the total heat evolved during complete cure of the formulation.

The gel content was determined on the cured films by measuring the weight loss after 24 h extraction with chloroform at room temperature according to the standard test method ASTM D2765-84.

Dynamic-mechanical thermal analyses (DMTA) were performed on a Triton Technology DMA from Mettler-Toledo at 1 Hz frequency in the tensile configuration. The storage modulus, E′, and the loss factor, tan δ, were measured from 30°C to the temperature at which the plateau of rubbery state was observed. The Tg value was assumed as the maximum of the loss factor curve (tan δ). The samples were prepared by placing the corresponding formulation in a silanized glass template, cured in the UV lamp and post-cured in an oven for 2 h at 100°C to obtain specimens with dimensions ca. 2 × 0.5 × 0.1 cm3.

Thermogravimetric analysis (TGA) was performed with a METTLER TGA/SDTA 851 instrument between 30 and 800°C at a heating rate of 10°C min−1 under a 60 ml min−1 air flow.

SEM analyses were performed on fracture surfaces of the hybrid systems by using a ZEISS SUPRA™ 40 Field Emission Scanning Electron Microscope (FE-SEM) with an acceleration voltage of 10 Kv and WD= 2 mm (Nominal resolution: 1.5 nm). Samples were prepared by breaking specimens of each thermoset under liquid nitrogen and the fracture area was observed after coating the surface with carbon.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Study of the Curing Kinetics

The curing reaction of the studied formulations was followed by means of RT-FTIR. Monitoring the disappearance of the epoxide band (760–780 cm−1) and using Eq. (1) it was possible to calculate the conversion at different curing times. As an example, Fig. 1 shows the conversion against time plots for the neat formulation and for formulations containing 15 phr of both multiarm star polymers. Moreover, Table 1 shows the final conversion (after 3 min of UV irradiation) for all the studied formulations. As one can see, the conversion reached decreases on increasing the proportion of modifier, regardless of the nature of the arms. Additionally, paying attention to the inset of Fig. 1, which represents the first stage of the photopolymerization (5 s), it is possible to conclude that both additives produce a decelerative effect, since the slope of the curve diminished. The increase in the viscosity of the curing mixture caused by the presence of the polymeric additive can explain the reduction in the curing rate. In previous works using those star-like polymers in DGEBA-based thermosets, we already observed an increase in the viscosity, and that this increase was higher for the case of PGOH-PMMA star [20, 21]. This is in agreement with the fact that in this study, the reduction in curing rate is more pronounced for this star modifier.

image

Figure 1. Conversion against time plots for the neat material and those obtained from 15 phr PGOH-PCL and 15 phr PGOH-PMMA modified formulations by RT FTIR. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Table 1. Heat released during the UV curing, heat released in a post-curing thermal experiment and conversions obtained by photo-DSC and FTIR measurements
SampleΔhUVa (kJ/ee)Δhthermalb (kJ/ee)Δhtotal (kJ/ee)XDSC (%)XFTIRc (%)
  1. a

    Obtained by photo-DSC mesurments.

  2. b

    Obtained as the residual heat in a second thermal scan.

  3. c

    Conversion calculated by integrating the epoxy band in FTIR measurements.

CE3132634961
5 phr PGOH-PCL2435594159
10 phr PGOH-PCL2138593654
15 phr PGOH-PCL1537622952
5 phr PGOH-PMMA2433574258
10 phr PGOH-PMMA2237593754
15 phr PGOH-PMMA1435592453

The photocuring reaction was also investigated by means of photo-DSC. This technique puts more clearly into evidence, the reduction on the conversion achieved in the final material on increasing the proportion of star (see Fig. 2 and Table 1). Moreover, performing a thermal scan to the UV-cured samples we could see that all the materials could resume its curing and continue to react up to the same extent since in all cases the total heat evolved (Δhtotal) was of the same order. This indicates that at room temperature vitrification during curing takes place, and thus, a thermal post-curing is required in order to achieve fully cured materials.

image

Figure 2. Heat evolved during the photopolymerization reaction obtained by photoDSC for the formulations neat CE, and the formulations containing 15 phr PGOH-PCL and 15 phr PGOH-PMMA. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Characterization of the Cured Materials

After studying the influence of both polymeric additives to the curing system, the following step consisted in the characterization of the cured materials. In order to verify the covalent incormporation of the multiarm star polymers to the system we determined gel content of all prepared thermosets. As we can see in Table 2, all the materials prepared presented quantitative gel content. This is, as mentioned before, an indirect evidence of the covalent incorporation of the star polymers to the epoxy matrix since if they were only dispersed within the matrix they should be extracted out. In the case of PGOH-PCL, the presence of hydroxyl groups at the end of the arms in the structure of the polymer is the responsible of the chemical incorporation: during the ring opening polymerization of epoxides, the occurrence of the so called activated monomer propagation mechanism (AM) [22] allows the chain transfer to occur between the activated monomer and the alcohol groups present in the multiarm star structure. PGOH-PMMA was synthesized by a first partial modification of poly(glycidol) core with 2-bromoisobutyryl bromide, which allows the selection of the number of initiating sites to grow the PMMA arms. Thus, in the multiarm star synthesized, there are 45 PMMA arms and subsequently an average number of 60 unmodified OH groups per molecule are present and can undergo the chemical incorporation of the structure to the epoxy matrix by AM mechanism. In the thermal cationic curing of DGEBA with PGOH-PCL multiarm stars a reduction of the reactivity and the covalent linkage of the dendritic additive to the epoxy matrix were also observed [23].

Table 2. Gel content, thermomechanical and thermal stability data for all the studied thermosets
SampleGel content (%)tan δa (°C)E250°Cb (MPa)T5%c (°C)Tmaxd (°C)
  1. a

    Maximum of the tan δ against temperature curve.

  2. b

    Storage modulus at the rubbery state plateau.

  3. c

    Temperature of the onset of degradation, where 5% in weight is lost.

  4. d

    Temperature where the maximum degradation rate is achieved.

CE1002052.9287407
5 prh PGOH-PCL991982.5289409
10 phr PGOH-PCL991935.15285408
15 phr PGOH-PCL1001993.03285410
5 phr PGOH-PMMA1001912.77290407
10 phr PGOH-PMMA991809.98290407
15 phr PGOH-PMMA1001693.35286407

Specimens of all the studied formulations were prepared in order to perform dynamic mechanical thermal analysis (DMTA). By means of this technique, the Tg of the prepared thermosets was obtained as the temperature at the maximum of the tan δ against temperature plot and it is reported in Table 2. As one can see, the addition of PGOH-PMMA causes a reduction on the Tg proportional to the amount of star polymer used. This is typical for homogeneous systems where the resulting glass transition temperature is a combination of the Tgs of the different components, as predicted by the Fox equation [24]. On the contrary, the addition of PGOH-PCL yields no significant changes in the Tg. This could stand for a phase separated morphology and additionally, as seen in Fig. 3, two relative maxima of the tan δ are observed. However, the presence of zones with different crosslinking density cannot be discarded. The bigger one corresponds to the glass transition temperature of the thermosetting matrix, and fits with the Tg of the neat material, while the second one at around 120°C may be attributed to the relaxation of the PGOH-PCL structure or to less crosslinked regions. It should be commented that the intensity of this broad peak increases with the proportion of modifier, which seems to support the phase separation of the multiarm star. It should be commented that the use of PGOH-PCL modifiers in thermal cationic curing of DGEBA led to a significant reduction of the tan δ maximum temperature of the thermoset, without any sign of phase separation [23], causing a plasticization of the matrix.

image

Figure 3. Tan δ against temperature curves for the cured neat material and the modified thermosets containing 15 phr PGOH-PCL and 15 phr PGOH-PMMA. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Additionally to the determination of the glass transition temperature, DMTA provides data about the modulus of the resulting thermosets. As observed in the values collected in Table 2, the relation between E′ and the amount of star is not linear but reaches a maximum value at a concentration of 10 phr. Moreover, the thermosets containing PGOH-PMMA present higher E′ values. This can be rationalized on the characteristics of the modifier with less OH groups, which produce chain transfer reactions leading to a higher crosslinked structure.

The thermal stability of the prepared materials was evaluated by means of thermogravimetric analysis (TGA). In systems that potentially can be used at high temperatures this factor is of the greatest importance. As seen in Fig. 4 and in the values of T5% and Tmax of Table 2, the thermal stability of all the prepared thermosets is of the same order. Since the structure of the neat resin already contains thermally labile ester groups, the introduction of poly(ε-caprolactone) or poly(methyl methacrylate) chains in the matrix does not produce any further degradation process. Additionally, since the TGA experiments were carried out in air, a second degradation process at higher temperatures is appreciated, typical for the combustion of carbon residues.

image

Figure 4. TGA thermograms for the cured neat material and the modified thermosets containing 15 phr PGOH-PCL and 15 phr PGOH-PMMA. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Finally, the morphology of the prepared materials was observed by means of FE-SEM. Figure 5 shows the micrographs obtained for (a) the neat system, (b) the material modified with 15 phr of PGOH-PMMA and (c,d) the thermoset with 15 phr of PGOH-PCL. The neat system, as seen in the picture, is completely homogeneous, and shows a fracture profile typical of brittle materials: no cracks are appreciated in the fracture and the surface has a smooth appearance. On the contrary, in the sample containing 15 phr of PGOH-PCL particles can be clearly appreciated. The fact that phase separation is observed is in agreement with the DMTA results discussed previously. In a previous work using this star polymer as a modifier of a thermal curing system no phase separation was observed [24]. Thus, the fact that the curing process plays an important role in the morphology of the materials is put into evidence. In the thermal case, the higher temperatures and the slower curing rate of the curing process favors that the polymer dissolves within the matrix. On the contrary, in the UV curing process the morphology of the formulation (where the star polymer is dispersed in the nanoscale within the epoxy monomer) is blocked since the process is very fast, and as the result the materials maintain the nanophase separation. The fact that the particles obtained are not perfectly spherical and of different size (100–500 nm) is also a consequence of the quickness of the UV curing process. In RIPS phase separated materials a more defined structure is usually observed [25]. We would like to underline the fact that although we have not been able to test it, the presence of nanophase separation is usually related to an improvement in toughness [26].

image

Figure 5. FESEM micrographs for the (a) neat cured material, (b) the modified thermosets containing 15 phr PGOH-PMMA and (c,d) 15 phr PGOH-PCL.

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Interestingly, PGOH-PMMA did not yield phase separation. Therefore, also the chemical structure of the star plays an important role. One could say that the compatibility between the poly(methyl methacrylate) arms and the epoxy matrix is better than the one of poly(ε-caprolactone) and thus the star polymer is homogeneously incorporated to the matrix. Although no phase separation is observed, it should be commented that the morphology of the fracture changes dramatically, and becomes that of a tough material, where cracks and specially a rough surface, is appreciated.

So, as a summary of this morphological study, we would like to point out the fact that choosing the appropriate structure as well as the curing systems is of key importance in order to achieve nanophase separation in these epoxy/multiarm star-like polymer modified systems.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

New thermosets obtained by the cationic UV photopolymerization of a cycloaliphatic epoxy resin, containing 0–15 phr of two different multiarm star polymers have been successfully prepared. Because of the increase in the viscosity of the formulation the addition of star polymers generates a decrease in the curing rate.

Thermosets containing PGOH-PMMA where homogeneous and a reduction in the Tg was observed on increasing its proportion. Moreover, the fracture area observed by electron microscopy of these materials seems tougher in respect to the neat CE thermoset.

Materials containing PGOH-PCL presented nanometric phase separation as demonstrated by FE-SEM miscroscopy. Another evidence of this characteristic was obtained by DMTA, since two peaks in the tan δ plot were observed.

Up to 15 phr, the addition of both stars did not cause a decrease in the thermal stability of the resulting thermosets.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL PART
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES