Miscibility, Phase Morphology, Thermomechanical, Viscoelastic and

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Miscibility, phase morphology, thermo-mechanical, viscoelastic and surface properties of PCL modified epoxy systems: Effect of curing agents Jyotishkumar Parameswaranpillai, Sisanth Krishnan Sidhardhan, Seno Jose, Nishar Hameed, Nisa V. Salim, Suchart Siengchin, Jürgen Pionteck, Anthony Magueresse, and Yves Grohens Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01713 • Publication Date (Web): 05 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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Industrial & Engineering Chemistry Research

Miscibility, phase morphology, thermo-mechanical, viscoelastic and surface properties of PCL modified epoxy systems: Effect of curing agents Jyotishkumar Parameswaranpillai1*, Sisanth Krishnan Sidhardhan1, Seno Jose2, Nishar Hameed3, Nisa V. Salim4, Suchart Siengchin5, Jürgen Pionteck6, Anthony Magueresse7, Yves Grohens7

1. Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682022, Kerala, India. 2. Department of Chemistry, Government College Kottayam, 686013, Kerala, India. 3. Factory of the Future, Swinburne University of Technology, Hawthorn, VIC, Australia. 4. Carbon Nexus, Institute for Frontier Materials, Deakin University, Waurn Ponds Campus, Geelong, VIC, 3220, Australia. 5. Department of Materials and Production Engineering, King Mongkut's University of Technology North Bangkok 1518 Pracharaj 1, Wongsawang Road, Bangsue, Bangkok 10800, Thailand. 6. Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany. 7. FRE CNRS 3744, IRDL, Univ. Bretagne Sud, F-56100 Lorient, France.

*Corresponding author: Jyotishkumar Parameswaranpillai Email: [email protected]

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Abstract In this paper, we report on the effect of curing agents on the miscibility, morphology, thermomechanical properties and surface hydrophobicity of diglycidyl ether of bisphenol-A (DGEBA)/poly(ε-caprolactone) (PCL) blends. Two curing agents, viz., 4, 4'-diamino diphenyl sulfone (DDS) and 4, 4'-diamino diphenyl methane (DDM) were used. Studies revealed that epoxy/PCL/DDM system was completely miscible due to the intermolecular hydrogen bonding interactions between carbonyl groups of PCL and hydroxyl groups of epoxy resin. On the other hand, epoxy/PCL/DDS system exhibited phase separated matrix/droplet type morphology, primarily due to the intramolecular hydrogen bonding interactions within the epoxy phase between sulfonyl groups of DDS and hydroxyl groups generated during epoxy-DDS reaction. The storage modulus of epoxy/PCL/DDM system was greater than that of epoxy/PCL/DDS system, and the dependence of modulus on PCL content was more pronounced in the former. Moreover, epoxy/PCL/DDM system exhibited better tensile properties and thermal stability.


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1. Introduction Epoxy resin is an important thermosetting polymer widely used for making high performance composites, electrical laminates, adhesives and coatings because of its excellent mechanical properties, good thermal stability, chemical resistance and long pot life period.1-3 However, the cured epoxy resins are highly brittle, which, in fact limits their end applications. One way to improve the toughness is to incorporate functionalized liquid rubbers like hydroxyl terminated liquid natural rubber (HTLNR), epoxidized natural rubber (ENR), amine-terminatedbutadiene-acrylonitrile (ATBN) and carboxyl terminated butadiene-acrylonitrile (CTBN).4-8 Alternatively, the incorporation of high performance engineering thermoplastics such as poly (ethersulfone) (PES), poly(etherimide) (PEI) and poly-(acrylonitrile-butadiene-styrene) (ABS) enhances toughness.9-13 Furthermore, epoxy based thermosets can be made tougher by introducing block copolymers (BCPs), which phase separate to nanostructures.14-16 Although good improvement in toughness can be achieved by these methods, addition of liquid rubbers and BCPs significantly decrease the modulus and glass transition temperature (Tg) of the cured epoxy system due to the presence of partially dissolved rubber or BCP in the epoxy system.17-19 Semi-crystalline polymers such as polyoxymethylene, poly(ε-caprolactone), poly(butylene terephthalate), poly(ethylene oxide) and syndiotactic polystyrene have been used to modify the epoxy resins.20-24 In such cases, crystallization has a strong effect on the miscibility and phase morphology. Poly(ε-caprolactone) (PCL) is a widely accepted polyester for it low melting point (shape memory applications) and biodegradable properties. PCL is miscible with epoxy and may retain miscibility or phase separate, depending on the type of curing agent used. Typically, PCL modified epoxy blends cured with 4,4'-diamino diphenyl methane (DDM) are miscible. But curing with anhydride leads to miscible or immiscible system, depending on the molecular weight of PCL.23, 25-26 On the other hand, the blends cured with 4,4'-diamino diphenyl sulfone (DDS) always exhibit phase separated morphology.21 The miscibility of PCL with epoxy resin is due the hydrogen bonding interactions between CO groups of PCL and OH groups generated during the epoxy-DDM crosslinking reaction.27 Miscibility and phase separation of PCL modified epoxy resin have been extensively studied. For example, Groeninckx and coworkers investigated the effect of the cure temperature, the concentration and the molecular mass of PCL on the morphology and the mechanism and kinetics of phase separation of epoxy/DDS/PCL system.21 Guo et al studied the miscibility, crystallization and spherulitic morphology of uncured and cured epoxy/PCL blends.25 Although 3 ACS Paragon Plus Environment


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the miscibility and phase separation behavior of epoxy/PCL blends were well established, little attention has been paid to investigate the thermo-mechanical properties of these blends and correlation of these properties with phase morphologies. It is important to mention that PCL is miscible with epoxy/DDM system, but undergoes phase separation in epoxy/DDS system. Hence, the curing agents have profound effect on the physical properties of the blends. The present paper presents the effect of curing agents on the miscibility, morphology, cure kinetics, crystallinity, viscoelastic, thermo-mechanical and surface properties of epoxy/PCL system. 2. Experimental 2.1. Materials The epoxy matrix material (Lapox L-12), diglycidyl ether of bisphenol-A (DGEBA), and the curing agent (Lapox K-10), 4,4'-diaminodiphenylsulfone (DDS), were supplied by Atul Ltd., Gujarat, India. Poly(ε-caprolactone) and the curing agent, 4,4'-diaminodiphenyl methane (DDM), were procured from Sigma-Aldrich. The chemical structures of epoxy resin, DDM, DDS and PCL are given in Figure 1. The molecular weight, melting temperature (Tm) and glass transition temperatures (Tg) of the materials used for blending are given in Table 1.

CH3

O CH2

CH

CH2

O

C

CH3

OH O

CH2

CH

CH2

CH3

O 0.15

C

O O

CH2

CH3

Diglycidyl ether of bisphenol-A (DGEBA)

H2N

CH2

4,4'–diamino diphenyl methane (DDM)


NH2

CH

CH2

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H 2N

SO2

NH2

4,4'–diamino diphenyl sulfone (DDS)

Poly(ε-caprolactone) (PCL)

Figure 1. Chemical structures of epoxy resin, DDM, DDS and PCL

Table 1. Molecular weight, melting temperature (Tm) and glass transition temperature (Tg) of the materials used for blending Materials

Molecular weight

Tm (°C)

Tg (°C)

(gmol-1) Epoxy

340

-

-17

DDM

198

90

-

DDS

248

178

-

PCL

10,000 (Mn)

60

-65

2.2. Preparation of epoxy blend Blends of epoxy resin containing 5, 10, 15, 20, and 30 phr of PCL were prepared. For the preparation of epoxy/PCL/DDM blends, required amount of PCL was added to epoxy resin at 100 °C with constant stirring, till the thermoplastic was completely dissolved in the epoxy resin. After complete dissolution of thermoplastic in epoxy resin, the hardener DDM (26 wt%), was added and stirring was continued until a homogeneous mixture was obtained. The resulting mixture was poured into a preheated open mould and cured at 100 °C for 2 h. Post curing was done at 100 °C for 4 h. For the preparation of epoxy/PCL/DDS system, PCL was added to the epoxy resin at 180 °C with constant stirring. Stirring was continued till the thermoplastic was 5 ACS Paragon Plus Environment


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completely dissolved in the epoxy resin. After complete dissolution of thermoplastic in epoxy resin, the hardener DDS (33 wt%) was added, stirring was continued until a homogeneous mixture was obtained. The resulting mixture was poured into a preheated open mould and cured at 180 °C for 2 h. Post curing was done at 200 °C for 4 h. 2.3. Characterization techniques 2.3.1. Fourier transform infrared spectroscopy Infrared studies were conducted to investigate (1) the completion of curing reaction and (2) the hydrogen bonding interaction between the blend components. Completely cured samples powdered with KBr and made in the form of pellets were scanned from 4000 to 400 cm-1 using a Thermo Nicolet, Avatar 370 spectrometer of Thermo scientific. 2.3.2. Differential Scanning Calorimetry (DSC) A DSC Q1000 modulated differential scanning calorimeter of TA-Instruments was used to analyze the thermal behavior of non-cured samples in N2 atmosphere. Samples of mass approximately 6 mg were used. Modulated DSC measurements were performed at a heating rate of 2 K/min, modulated with 0.31 K/40 s, from – 80 °C to 300 °C. The cooling was done without modulation at a rate of 40 K/min. 2.3.3. Dynamic Mechanical Analysis (DMA) Variations in storage modulus, loss modulus and tan δ of the blends with respect to temperature were analyzed using a DMA Q-800 dynamic thermal analyzer of TA instruments. Samples of 60 x 10 x 3 mm3 in size were used for analysis. Measurements were taken using a dual cantilever clamp at a frequency of 1 Hz from 30 to 250 °C, with a ramp of 3 K/min. 2.3.4. Mechanical properties Tensile measurements were taken using a Tinius Olsen machine, Model H 50 KT, at a cross head speed of 50 mm/min, according to ASTM D 638. Samples of 80 × 10 × 3 mm3 in size were used for determining the tensile properties. 2.3.5. Scanning Electron Microscopy (SEM) A Jeol JSM-6460LV scanning electron microscope was used to investigate the fractured surface of crosslinked epoxy blends. Each specimens were sputter-coated with a thin layer of gold before analysis.


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2.3.6. Thermogravimetric analysis (TGA) A Perkin Elmer, Diamond TG/DTA thermogravimetric analyzer was used to investigate the thermal stability of epoxy blends. The measurements were done at a heating rate of 20 K/min from 25 to 800 °C in N2 atmosphere, using samples of mass 6-8 mg. 2.3.7. Contact angle Measurements Contact angles were measured by means of a Digidrop from GBX, Model MSE contact angle analyzer (sessile drop method). The drop image was processed by Windrop image-analysis system with an accuracy of ± 0.1. All measurements were carried out at 23 °C using distilled water as the probe. Ten measurements were taken on different specimens of the same sample and the average was taken as the final value. 3. Results and discussion 3.1. Miscibility of PCL in epoxy/DDM and epoxy/DDS systems: Interchain specific interactions All the cured epoxy/PCL/DDM blends are transparent indicating that the blends are homogeneous. The FTIR spectra of epoxy/PCL/DDM blends are shown in Figure 2. The region ranging from 3100 to 3700 cm-1 in the spectrum, indicates the stretching region of hydroxyl groups of epoxy resin. A careful examination of this region reveals a shoulder at around 3570 cm-1, which is assigned to non-associated free hydroxyl groups generated during the epoxyamine reaction. These generated hydroxyl groups can form hydrogen bond with the carbonyl group of PCL.25 Note that PCL exhibits miscibility with many polymers including epoxy resin due to its great tendency to form hydrogen bonds with these polymers. The band at 3400 cm-1 is ascribed to the hydrogen bonded hydroxyl groups. When PCL is added to epoxy, the associated hydroxyl band is shifted to higher frequencies, suggesting intermolecular hydrogen bonding interactions. It can be seen from the figure that addition of PCL decreases the intensity of the shoulder, indicating that as the amount of PCL increases the intermolecular hydrogen bonding interactions between the carbonyls groups of PCL and the free hydroxyl groups generated during the epoxy-DDM reaction also increase. It is important to mention that the epoxy peak at 916 cm-1 is absent in FTIR spectra, which shows that the blends are cured.



80

% Transmittance

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3400 60

3427 40

20

916

epoxy + DDM 5 phr PCL 10 phr PCL 15 phr PCL 20 phr PCL 30 phr PCL 3500

1000

500 -1

Wave number (cm )

Figure 2. FTIR spectra of cured epoxy/DDM system modified with PCL. The FTIR spectra of epoxy/PCL/DDS blends are shown in Figure 3. In all systems, the epoxy peak at 916 cm-1 is not visible suggesting that the blends are cured. Note that the free hydroxyl groups in epoxy/DDS system are absent as there is no shoulder at 3570 cm-1. Moreover, peak of hydrogen bonded hydroxyl groups at 3400 cm-1 remains at the same position regardless of the blend composition, which signifies the absence of intermolecular interactions (hydrogen bonding) between the crosslinked epoxy and PCL. This may be attributed to the formation of intramolecular hydrogen bonding interactions within the epoxy phase, between the sulfonyl groups of DDS and the generated secondary hydroxyl groups, in the course of epoxy/DDS reaction. It is important to mention that the intramolecular interactions suppress intermolecular hydrogen bonding interactions between the epoxy phase and PCL.28-30 Thus, from the FTIR spectra, it can be concluded that epoxy/PCL/DDM is completely miscible owing to the strong intermolecular hydrogen bonding between the carbonyl groups of PCL and the free hydroxyl groups of amine-crosslinked epoxy system. In contrast, epoxy/PCL/DDS system is phase separated because intramolecular interactions within the epoxy phase between sulfonyl groups and the secondary hydroxyl groups inhibit intermolecular interactions between the epoxy phase and PCL.


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80

% Transmittance

60

40

916 cm

-1

epoxy + DDS 5 phr PCL 10 phr PCL 15 phr PCL 20 phr PCL 30 phr PCL

20

0 3600

3200

700 -1

Wave number (cm )

Figure 3. FTIR spectra of cured epoxy/DDS system modified with PCL 3.2. Thermal properties: Differential scanning calorimetry Figure 4 shows the DSC thermograms obtained on curing of PCL modified epoxy/DDM blends during first heating in modulated DSC scan. Only one exothermic peak is observed for all samples. The exothermic peak is caused by oxirane ring and the reaction with the amine of the hardener.31 Since the reactivity of the primary and in the first step formed secondary amines to oxirane rings is similar, no steps in the heat release are observable. The total enthalpy of reaction (∆H) was calculated from the peak area under the baseline extrapolated to end of the reaction. Table 2 summarizes the DSC data obtained from the first and second DSC scans as a function of PCL content.

0.5 st

epoxy + DDM 15 phr PCL 30 phr PCL

1 heating 0.4

exo endo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 W/g

0

50

100

150

200

250

Temperature [°C]

Figure 5. DSC second heating scans of pure epoxy and PCL modified epoxy/DDM systems

Figure 6 shows the DSC thermograms obtained on curing of PCL modified epoxy/DDS blends, during first heating in modulated DSC scan. Interestingly, in addition to the main exothermic peak, a shoulder appeared after the exothermic peak for the epoxy blend system. This phenomenon is due to the phase separation of PCL from the crosslinked epoxy matrix.35 The parameters derived from the thermograms are given in Table 3. Tp slightly increases with increasing the PCL content, specifying the retardation of curing due to the dilution of miscible PCL on epoxy rich phase. The Tg of the epoxy resin (first heating scan) decreases from 4 to -7 °C with the addition of 20 phr of PCL, signifying the miscibility of PCL with epoxy resin. On the other hand, the Tg of the cured blend (second heating, Figure 7) is comparable with the neat epoxy, suggesting phase separation of PCL from the epoxy resin. In short, unlike in epoxy/PCL/DDM system, miscibility of PCL with epoxy/DDS resin is suppressed when the sample is completely cured. Further, the melting of PCL at 60 °C (seconding heating) is more pronounced in PCL modified epoxy/DDS system. The ∆Hm(corr) value for the PCL modified epoxy/DDS blend is shown in Table 3. The high ∆Hm(corr) value indicates that most of the PCL gets segregated as microdomains. The stronger phase separation of the cured blends could be responsible for higher crystallization of PCL in the epoxy/DDS based blends compared to the



epoxy/DDM blend system. In other words, it is easier for PCL to crystallize from the PCL rich phase than miscible epoxy/PCL blend.36 0.4 epoxy + DDS 20 phr PCL

st

1 heating

exo endo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 W/g

0

50

100

150

200

250

Temperature [°C]

Figure 7. DSC second heating scans of pure epoxy and PCL modified epoxy/DDS system 3.3. Viscoelastic properties: Dynamic mechanical analysis Figures 8a and 8b show the influence of added PCL on the storage moduli (E') of epoxy resins cured with DDM or DDS, respectively. The E' represents the stiffness of the sample, which decreases with increasing temperature. For DDM cured systems, a rapid drop in modulus is observed at around 110 °C, near the Tg of neat epoxy. As the concentration of PCL in the blend increases, E' and Tg decrease. The decrease in E' and Tg is due to the miscibility of flexible PCL with the epoxy phase.37 Blend containing 30 phr of PCL exhibits lowest E' values, at all temperatures due to dilution of the epoxy network by dissolved PCL. Further, it is interesting to note that in this blend, reduction in E' occurs within the broad temperature range (ca. from 65 to 155 °C) probably due to the very broad glass transition (ca. from 70 to 135 °C, from DSC). For DDS cured systems, the E' decreases with the addition of PCL and the E' is lowest for epoxy system modified with 30 phr of PCL. It is obvious from the figure that the modulus profile involves two steps: the first step at 60 °C (melting of PCL) and the second step at 200 °C (the Tg of the epoxy matrix phase).31 It is important to recognize that the first step is more prominent at higher concentration of PCL. Interestingly, the Tg of epoxy phase remains almost unaffected with the addition of PCL due to phase separation of PCL from the epoxy matrix. On the other hand, the Tg of the 30 phr modified epoxy system is considerably reduced, probably due to the presence of partially dissolved PCL.



(b)

(a) 1000

Storage modulus (MPa)

1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

epoxy + DDM 5 phr 10 phr 15 phr 20 phr 30 phr

10

100

epoxy + DDS 5 phr PCL 10 phr PCL 15 phr PCL 20 phr PCL 30 phr PCL 10

40

60

80

100

120

140

160

50

180

100

150

200

o

o

Temperature ( C)

Temperature ( C)

Figure 8. Storage modulus of (a) PCL modified epoxy/DDM blends (b) PCL modified epoxy/DDS blends It is worth noting that although E' and thereby stiffness of epoxy/PCL/DDS system is less compared to epoxy/PCL/DDM system at relatively low temperatures, the latter is found to be more susceptible for high temperature applications. Moreover, the decrease in E' with PCL content is more pronounced in epoxy/PCL/DDM system than in epoxy/PCL/DDS system. The E' of epoxy/PCL/DDS system registers no appreciable deterioration with increase of temperature up to 200 °C (except a marginal decrease in modulus beyond 60 °C due to the melting of PCL) while the epoxy/PCL/DDM system undergoes a sharp decrease in E' with temperature, especially at high concentrations of PCL. This can be explained in terms of the difference in the miscibility behavior of the two systems. Epoxy/PCL/DDS system is phase separated and therefore the E' values are mainly contributed by the major component (epoxy matrix) and the minor component (PCL) gets dispersed as domains, as shown in the SEM micrographs (Figure 13). But, in epoxy/PCL/DDM system, due to its miscible nature, PCL phase also contributes to the overall modulus of the sample. Figures 9a and 9b show the influence of added PCL on the loss modulus (E'') of epoxy resin cured with DDM and DDS, respectively. The Tg of the epoxy phase (α-transition) in the DDM cured neat epoxy system is found to be 120 °C. For PCL modified epoxy/DDM blends, the Tg considerably decreased with the addition of thermoplastic. This decrease in Tg is ascribed to the plasticization effect of miscible PCL chains in the epoxy matrix.38 As mentioned earlier, the driving force for the miscibility between PCL and epoxy resin is the intermolecular hydrogen bonding interactions between them. For the DDS cured epoxy system, the main peak at around 200 °C represents the Tg of the epoxy phase. It should be noted that the Tg of the epoxy phase


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remains invariant regardless of the PCL content. However, the Tg of 30 phr modified epoxy system is slightly decreased probably because of the partial miscibility of PCL chains with cured epoxy resin at higher PCL content. Careful examination of the E'' curves for higher concentrated epoxy blends reveals a minor peak at around 60 °C. This peak is related with the melting of the PCL minor phase.

epoxy + DDM 5 phr PCL 10 phr PCL 15 phr PCL 20 phr PCL 30 phr PCL

200

180

(a)

140

150

100

50

(b)


160

Loss modulus (MPa)

250

Loss modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80 60 40 20

0

0

50

100

150

200

40

60

80

o

100

120

140

160

180

200

220

240

o

Temperature ( C)

Temperature ( C)

Figure 9. Loss modulus of (a) PCL modified epoxy/DDM blends (b) PCL modified epoxy/DDS blends Figure 10a represents the tan δ curves of PCL modified epoxy/DDM blends. The Tg values obtained from the tan δ curves are given in Table 4. The cured epoxy system shows the Tg (αtransition of epoxy phase) at 141 °C. The tan δ peak height decreases and the peak point shifts to lower temperature at higher PCL content. As mentioned earlier, the depression in Tg is one of the criteria of miscibility. Further, the decrease in peak height is also an indication of miscibility. This means that PCL chains get effectively interpenetrated into the crosslinked epoxy matrix at the segmental level. Moreover, the epoxy matrix gets effectively plasticised by PCL chains and exhibits a lower Tg than the control epoxy.39 The plot of tan δ for PCL modified epoxy/DDS system as a function of temperature is shown in Figure 10b and the Tgs obtained from the tan δ curves are given in Table 4. It is important to note that unlike epoxy/PCL/DDM system, the Tg of epoxy phase remains unaffected by the addition of PCL. However, interestingly, the blend with 30 phr of PCL exhibits a pronounced shoulder at the lower temperature side indicating the presence of second epoxy phase with lower Tg besides the main transition of crosslinked epoxy resin. The shoulder at lower temperature is due to the presence of partially dissolved PCL in the crosslinked epoxy phase.40 15 ACS Paragon Plus Environment


0.8


1.0

0.8

(a)

0.7 0.6 0.5


(b)

Tan δ

0.6

Tan δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.4 0.3 0.2

0.2

0.1 0.0

0.0 50

100

150

150

200

200

250

o

o

Temperature ( C)

Temperature ( C)

Figure 10. Tan δ profile of (a) PCL modified epoxy/DDM blends (b) PCL modified epoxy/DDS blends The molecular weight between crosslinks (Mc) and effective cross-link density ( υ e ) were calculated from the tan δ profile, using equations 1 and 2.41 Mc =

3.9 × 104 Tg − Tg 0

------------------- (1)

Where Tg is the glass transition temperature of the cross-linked epoxy thermoset obtained from the tan δ profile and Tg0 is that of the uncross-linked epoxy having the same chemical composition as the cross-linked epoxy thermoset. The value of Tg0 is taken as 76 and 91 °C for DGEBA/DDM and DGEBA/DDS systems, respectively.42

υe =

ρ NA Mc

------------ (2)

where ρ is the density and N A is Avogadro’s number

Table 4 gives the values of Mc and υe. Epoxy/PCL/DDM system has greater Mc but lower υe values compared to epoxy/PCL/DDS system, irrespective of blend compositions. For the blends with epoxy/DDM system, the Tg and υe decrease while Mc increases. This implies that the epoxy matrix gets effectively plasticised by the miscible PCL chains. On the other hand, for PCL modified epoxy/DDS system Tg, Mc, and υe are almost unaffected, suggesting that there is some degree of phase separation of PCL from the crosslinked epoxy matrix.

Table 4. Network parameters of DGEBA/PCL blends calculated from glass transition


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PCL content

Epoxy/PCL/DDM blends

Epoxy/PCL/DDS blends

(phr) Tg

Mc

υe×1027

Tg

Mc

υe×1027

(°C)

(g/mol)

chains /m3

(°C)

(g/mol)

chains /m3

0

140.6

603.7

1.2

213.3

318.9

2.27

5

135.2

658.8

1.1

212.3

321.5

2.25

10

132.7

687.8

1.05

208.2

332.8

2.17

15

127.8

752.9

0.96

209.4

329.4

2.19

20

125.4

789.5

0.92

210.0

327.7

2.21

30

117.8

933.0

0.77

211.7

323.1

2.24

Cole -Cole Plots The Cole-Cole plots of PCL modified epoxy resin cured with DDM and DDS are shown in

Figures 11a and 11b. Cole−Cole plots are extensively used to examine the homogeneity of polymer blends. Cole−Cole plots give smooth semicircular curves for miscible blends. From

Figure 11a, it is seen that all the blends show smooth semicircular curves, strongly supporting the homogeneity of the system. Note that the height of the semicircular curve decreases and shifts to lower modulus at higher PCL content due to the plasticizing effect of miscible PCL chains. PCL modified epoxy/DDS blends also show semicircular curves with some irregularities, especially at higher modulus (Figure 11 b). In fact, 30 phr PCL modified epoxy blend shows maximum disturbance in the curve, indicating greater heterogeneity. Note that the height of the semicircular curve is not much affected, which in turn supports the phase separated nature of the system. 300


250

200

180


(b)

160 140

Loss Modulus (MPa)

(a)

Loss Modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

100

120 100 80 60 40

50 20 0

0 0

500

1000

1500

2000

0

500


1000



1500


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Page 18 of 27

Figure 11. Cole-Cole plots of (a) PCL modified epoxy/DDM blends and (b) PCL modified epoxy/DDS blends

3.4. Mechanical properties: Tensile testing Tensile properties of PCL modified epoxy/DDM and epoxy/DDS blends are given in Table 5. For PCL modified epoxy/DDM blends, the elongation at break (EB) and tensile toughness (TT) increase by the addition of PCL content up to 15 phr. On the other hand, the tensile strength (TS) and tensile modulus (TM) decrease with the increasing concentration of PCL. However up to 15 phr of PCL, the decrease in tensile strength and tensile modulus is modest. Epoxy/DDM/PCL (15 phr) blend exhibits nicely balanced properties in terms of mechanics. Compared to neat epoxy/DDM system this blend possesses improved tensile toughness (13 % increase) and elongation at break (17 % increase), combined with acceptable tensile strength (3 % decrease) and modulus (< 5 % decrease).

Table 5. Tensile properties of PCL modified epoxy blends

PCL



content

TS

TM

EB

TT

(Phr)

(MPa)

(MPa)

(%)

(Jm-3

TS

TM

EB

TT

(MPa)

(MPa)

(%)

(Jm-3

x 104)

x104)

neat

74.3 ± 4

2342 ± 200

8.7 ± 0.3

424

57.4 ± 4.2

2320 ± 170

6.7 ± 0.4

247

5

72.5 ± 3

2283 ± 180

9.5 ± 0.5

438

59.3 ± 4.6

2155 ± 210

8.7 ± 0.5

325

10

71.6 ± 6

2228 ± 230

9.6 ± 0.4

440

52.4 ± 5.0

1994 ± 135

8.0 ± 0.2

271


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15

71.8 ± 5

2233 ± 240

10.2 ± 0.3

479

49.3 ± 3.9

1861± 167

7.9 ± 0.4

250

20

61.3 ± 5

2031 ±160

7.7 ± 0.4

284

51.0 ± 3.9

1835 ± 122

8.1± 0.4

206

30

63.5 ± 6

2050 ± 270

8.8 ± 0.3

351

38.9 ± 3.3

1561 ± 198

6.4 ± 0.5

165

For PCL modified epoxy/DDS blends, maximum tensile properties are obtained for blends containing 5 phr of PCL. The TS marginally increases (ca. 3%), EB and TT register ca. 30 % increase and modulus slightly decreases (< 10 %). The tensile properties gradually decrease with increasing amount of PCL. Thus, between epoxy/PCL/DDM and epoxy/PCL/DDS systems, the former exhibits better mechanical properties. Addition of PCL has greater reinforcing effect in the former and optimum concentration of PCL is found to be 15 phr. On the other hand, for epoxy/DDS blends, incorporation of PCL has no appreciable effect, in terms of tensile properties. Thus the addition of only small amounts of PCL is favorable in regard of balanced mechanical properties. The difference in performance between both epoxy systems can be correlated with the difference in miscibility and phase structure of these blends.

3.5. Phase morphology: Scanning electron microscopy (SEM) The SEM micrographs of PCL modified epoxy/DDM and epoxy/DDS blends are shown in

Figures 12 and 13. It is worth emphasizing that the two systems exhibit entirely different phase morphology. PCL modified epoxy/DDM blends are homogeneous and transparent with single phase as evidenced by FTIR and DSC studies. All the fracture surfaces of the blends show generated uninterrupted linear primary cracks. In addition to linear cracks, couples of parabolic markings are observed for 5 phr modified blends. The limited improvement in tensile properties is due the generation of primary cracks by the application of high stress which restricts the energy dissipation.43 On the other hand, SEM micrographs of epoxy/PCL/DDS blends revealed phase separated matrix/droplet morphology in which PCL droplets are dispersed in epoxy matrix. These observations are in agreement with FTIR, DSC and DMA results. It is important to mention that before crosslinking, the blends had a homogeneous morphology. During crosslinking, reaction induced phase separation (RIPS) occurs. The fracture surface of 5 phr PCL modified epoxy blend reveals small PCL domains of 1 µm size uniformly dispersed in epoxy matrix, which enhances the tensile properties. With increasing PCL content, the particle size of dispersed PCL domains increases (at 30 phr of PCL up to 4 µm) and size distribution becomes non-uniform, generating more and more as catastrophic failure initiators. It is interesting to note that for 30 phr PCL modified epoxy/DDS system, a few epoxy substructures are visible in the 19 ACS Paragon Plus Environment


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Page 20 of 27

PCL domains. This may be attributed to the mixing of a fraction of epoxy matrix nearer to the PCL dispersed microdomains with miscible PCL block. Due to the greater chain mobility, during curing, this fraction of epoxy may diffuse from the matrix through the matrix/droplet interface and be trapped inside the PCL domains.

Figure 12. SEM micrographs of PCL modified epoxy/DDM blends. (a) 5 phr PCL, (b) 15 phr PCL, (c) 30 phr PCL.

Figure 13. SEM micrographs of PCL modified epoxy/DDS blends. (a) 5 phr PCL, (b) 15 phr PCL, (c) 30 phr PCL.

3.6. Thermal stability: Thermogravimetric analysis Thermal stability of PCL modified epoxy/DDM and epoxy/DDS blends was analyzed using TGA, in nitrogen atmosphere. It is obvious from the Figure 14 that all the blends follow single step degradation and are stable up to 350 °C, irrespective of the curing agent and curing condition. Note that when compared with the neat epoxy, the blends show no deterioration in thermal stability. However, the residual weight fraction for the blends is less than that of neat epoxy system. To conclude, all the prepared blends are thermally stable up to 350 °C and can be used for high temperature applications. It is also observed that epoxy/PCL/DDM system is thermally more stable (Tmax = 410 °C) than epoxy/PCL/DDS system (Tmax = 380 °C).


Page 21 of 27

100

100

(a)

(b)

80

80

60

60

40

% Mass

% Mass

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



20

40


20

0

0 100

200

300

400

500

600

700

100

200

o

300

400

500

600

700

o

Temperature ( C)

Temperature ( C)

Figure 14. TGA curves of (a ) PCL modified epoxy/DDM blends, (b) PCL modified epoxy/DDS blends

3.7. Surface hydrophobicity: Contact angle measurements Contact angle is an angle used to measure hydrophilicity/hydrophobicity of samples. The variation of contact angles against blend composition for both PCL modified epoxy/DDM and epoxy/DDS blends is shown in Table 6. It can be observed that irrespective of the curing agent, PCL modified epoxy systems possess higher contact angles compared to neat epoxy resin. This may due to the surface roughness of the samples which in turn depends on PCL content. The influence of hydrophobic nature of PCL cannot be negeleted.44 Epoxy/DDS system and their blends are by tendency more hydrophobic than the blends based on epoxy/PCL/DDM. The reason why the contact angle at higher PCL loading in the epoxy/PCL/DDM blends is reduced to that of the pure resin is not explainable by us in the moment. We like just to mention that differences in surfaces roughness, surface heterogeneities or surface enrichment processes also influence the contact angle measurements.

Table 6. Contact angle values for PCL modified epoxy/DDM and epoxy/DDS blends Samples



Contact angle (°)

Contact angle (°)

Neat epoxy

76 ± 6.1

82.5 ± 2.6

5 phr PCL

87.3 ± 4.5

81.4 ± 3.4



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Page 22 of 27

10 phr PCL

88.9 ± 2.4

84.5 ± 2.5

15 phr PCL

81.4 ± 2.3

91.5 ± 3.6

20 phr PCL

75.1 ± 4.1

87.7 ± 3.6

30 phr PCL

75.8 ± 3.7

87.7 ± 3.5

4. Conclusions Blends of diglycidyl ether of bisphenol-A (DGEBA) and polycaprolactone (PCL) cured with 4,4'-diaminodiphenylsulfone (DDS) and 4,4'-diaminodiphenylmethane (DDM) were prepared. The miscibility, thermal properties, phase morphology, viscoelasticity, mechanical and surface properties were studied. Epoxy/PCL/DDM system was completely miscible due to the intermolecular hydrogen bonding interactions between carbonyl groups of PCL and hydroxyl groups of epoxy resin, while epoxy/PCL/DDS system exhibited phase separated matrix/droplet type morphology due to the intramolecular hydrogen bonding interactions between sulfonyl groups of DDS and hydroxyl groups. The type of curing agent has profound effect on the viscoelastic properties of the blends. Addition of PCL resulted in improvement in tensile properties for epoxy/PCL/DDM than epoxy/PCL/DDS system and the optimum concentration of PCL was found to be 15 phr. Furthermore, epoxy/PCL/DDM system was thermally more stable than epoxy/PCL/DDS system. Contact angle measurements revealed that regardless of the curing agent, PCL modified epoxy systems possessed lower hydrophilicity than neat epoxy resin.

Supporting Information DSC heating scan of PCL, lower and higher magnification AFM images of 30 phr PCL modified epoxy/DDM blends, DTA curves of epoxy/PCL/DDM and epoxy/PCL/DDS system.

Acknowledgement J P acknowledges the Department of Science and Technology, Government of India, for financial support under an Innovation in Science Pursuit for Inspired Research (INSPIRE) Faculty Award (contract grant number IFA-CH-16).

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For Table of Contents Only

Homogeneous morphology of epoxy/PCL/DDM system

Phase separated morphology of epoxy/PCL/DDS system


Miscibility, Phase Morphology, Thermomechanical, Viscoelastic and

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