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PEG-ran-PPG modified epoxy thermosets: A simple approach to develop tough shape memory polymers Jyotishkumar Parameswaranpillai, Sreekanth Panachikunnel Ramanan, Jinu Jacob George, Seno Jose, Ajesh K Zachariah, Suchart Siengchin, Krittirash Yorseng, Andreas Janke, and Jürgen Pionteck Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04872 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Industrial & Engineering Chemistry Research

PEG-ran-PPG modified epoxy thermosets: A simple approach to develop tough shape memory polymers

Jyotishkumar Parameswaranpillai12*, Sreekanth Panachikunnel Ramanan 2, Jinu Jacob George2, Seno Jose3, Ajesh K. Zachariah4, Suchart Siengchin5, Krittirash Yorseng5, Andreas Janke6, Jürgen Pionteck6

1. Center of Innovation in Design and Engineering for Manufacturing, King Mongkut’s University of Technology North Bangkok, 1518 Pracharaj 1, Wongsawang Road, Bangsue, Bangkok 10800, Thailand. 2. Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682022, Kerala, India. 3. Department of Chemistry, Government College Kottayam, Kottayam 686013, Kerala, India 4. Department of Chemistry, Mar Thoma College, Tiruvalla 689103, Kerala, India 5. Department of Mechanical and Process 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 Str. 6, 01069 Dresden, Germany. Corresponding authors Dr. Jyotishkumar Parameswaranpillai Email: [email protected] Dr. Suchart Siengchin Email: [email protected]

1 ACS Paragon Plus Environment

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Abstract In this work, we prepared thermo-responsive shape memory epoxy thermosets by blending epoxy resin with poly(ethylene glycol-ran-propylene glycol) random copolymer (PEG-ran-PPG or RCP). Incorporation of RCP precisely tuned the temperature showing the shape memory effect of epoxy thermoset, which was established by dynamic mechanical analysis (DMA) and fold-deploy test. FTIR spectroscopy confirmed that the compatibility of the system is caused by intermolecular hydrogen bonding and only at high RCP concentration some phase separation starts. DMA and thermomechanical analysis (TMA) provided evidence for the interactions of RCP chains with epoxy thermoset. The impact strength considerably increased especially for 30 and 40 wt% RCP modified blends. Furthermore, the blends exhibited good thermal stability in conjunction with excellent UV resistance.


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Introduction Shape memory polymers (SMPs) are next generation smart materials that have the capacity to change their shape upon exposure to external stimuli (1-4). The external stimulus such as heat (5), light (6), pH (7), magnetic field (8), electricity (9), or water (10) can stimulate a shape memory polymer to recover its original shape. Heat is the most common stimulus since shape memory effect in polymers is mostly connected with thermal phase transition (11). Since polymers are viscoelastic materials, the glass transition temperature (Tg) allows the reversibility of a polymer material between the elastic and viscous states. This is considered as the principal reason for the shape memory effect in crosslinked epoxy polymers. Therefore, Tg is known as the shape recovery or shape memory transition temperature and can be tuned according to the practical applications at a desired temperature (12). Several types of SMPs based on different polymers such as perfluorosulphonic acid ionomer (PFSA) (5), polynorbornene (13), natural rubber (14), poly(methyl methacrylate) (15), poly(vinyl alcohol) (PVA) (16), thermoplastic polyurethane (TPU)/poly(ε-caprolactone) (PCL) blends (17), polyurethane (18), crosslinked polypropylene/polyethylene blend (19), epoxy (20), etc. are reported in the literature. They are used for many applications such as deployable structures for aircraft and spacecraft (21-22), temperature sensors (2), biomedical devices (23-24), actuators (25), etc. However, SMPs based on thermoplastics possess low stiffness and shape fixity that limits their range of end-use applications. Epoxy resin is widely used for making advanced composites, for coating in electric and electronic applications, as adhesive material, etc., because of the versatility in curing, easy processing, high modulus, good thermal stability, good chemical resistance, low shrinkage, long pot life period, low cost, and highly tunable Tg (26). Therefore, lots of interests are going on to make shape memory epoxy thermosets (27-30). The shape memory properties of epoxy 3 ACS Paragon Plus Environment


thermoset can be tuned by the addition of thermoplastics such as PCL (31), poly(ethyleneimine) (32), or by linear epoxy monomer (33). Epoxy shape memory thermosets have many advantages over shape memory alloys, such as low cost, less weight, corrosion resistance, easy processability, and tailorable Tg. Moreover, they have good fixity and recoverability when compared with other SMPs, making them important shape memory materials suitable for advanced applications (34). However, they quite often possess inferior mechanical properties, in particular poor impact strength, that limits the possibility for using them in advanced composite industry. Therefore, it is necessary to tune the thermo-mechanical properties of SMPs in order to take full advantage of them. For the last two decades, copolymers were used to develop nanostructured epoxy blends with excellent thermo-mechanical properties (35 - 40). In fact, block copolymer is supposed to be one of the best materials to improve the toughness of epoxy blends. To the best of our knowledge, no work has been reported on random copolymer modified epoxy shape memory polymers. It is well know that most homopolymers and random copolymers phase separate from the epoxy matrix by polymerization-induced phase separation, resulting in macrophase separated epoxy blends (41-42). However, PEG is miscible with epoxy resin and in this work, poly(ethylene glycol-ran-propylene glycol) (PEG-ran-PPG) random copolymer (RCP) with 75% PEG was used to get a completely miscible homogeneous thermoset, which decreases the Tg of the epoxy matrix with increasing amount of RCP. In this work, we precisely tuned the temperature showing shape memory properties of epoxy thermosets by changing the concentration of PEG-ran-PPG random copolymer. The roles of intermolecular hydrogen bonding interaction and blend composition on the shape memory effect, phase morphology, thermo-mechanical properties, and UV resistance of the RCP modified epoxy/DDM system were investigated.


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Experimental Materials and preparation of blends Epoxy resin used was diglycidyl ether of bisphenol-A with weight average molecular weight of 340 gmol-1 was obtained from Atul Ltd, India. The modifier, poly(ethylene glycol-ran-propylene glycol) (PEG-ran-PPG) containing approximately 75 wt %. ethylene glycol and having a number average molecular weight of 12000 gmol-1 and the curing agent, 4,4'-diaminodiphenylmethane (DDM) with weight average molecular weight of 198.26 gmol-1 was obtained from SigmaAldrich. Epoxy blends containing 0, 5, 10, 15, 20, 30, and 40 wt% of RCP were prepared. The procedure for the preparation of sample is given in our previous publication (43). In brief, the RCP was mixed with epoxy resin at 100 °C, until a homogeneous mixture was obtained. The stoichiometric amount of DDM was dissolved in this mixture and cured for 2 h at 100 °C and then postcured for 4 h at 120 °C to specimen. Structural formula of epoxy resin is given in Figure 1.

Epoxy resin Figure 1. Structural formula of epoxy resin. Characterisation techniques Fourier transform infrared spectroscopy (FTIR) The hydrogen bonding interactions between the blend components were evaluated by Bruker Vertex 80v FTIR from 4000 to 600 cm-1.



Dynamic mechanical analysis (DMA) The viscoelastic properties of the PEG-ran-PPG modified epoxy thermosets were analysed by a DMA Q-800 (TA Instruments) at a heating rate of 3 K/min at 1 Hz. The sample dimensions of 30 × 10 × 3 mm3 were used for the analysis. Thermomechanical analysis (TMA) The dimensional stability of the PEG-ran-PPG modified epoxy thermosets with respect to temperature was analyzed using a TMA Q-400 (TA Instruments) at a heating rate of 3 K/min. Atomic force microscopy (AFM) The morphology of PEG-ran-PPG modified epoxy thermosets were characterized on microtomed cut surfaces using a Dimension 3100 Nanoscope IIIa (Veeco, USA) AFM in tapping mode. Shape memory test (fold-deploy test) The shape memory properties of the PEG-ran-PPG modified epoxy thermosets were studied by the widely accepted fold-deploy test. The shape fixity and shape recovery ratios were calculated from the fold-deploy test. The sample dimensions of 10 × 1 × 0.1 mm3 were used for the analysis. The samples were heated in an oven at Tg + 20 °C and deformed into a “U” shape with a folding angle θmax by an external stress. The sample was cooled to room temperature keeping the shape due to constant stress, and after releasing the stress some relaxation occurred, changing the folding angle from θmax to θfix. For shape recovery the deformed samples were reheated at Tg + 20 °C. The recovery is not complete and some deformation with an angle θfinal remains. The maximum folding angle (θmax), the folding angle after fixing (θfix), and the residual folding angle after shape recovery (θfinal) were measured to determine the shape retention ratios (= [θfix/θmax] × 100) and shape recovery ratios (= [θfix − θfinal/θfix] × 100) for each composition.


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Mechanical properties The tensile properties of the PEG-ran-PPG modified epoxy thermosets were examined by a Tinius Olsen machine, model H 50 KT, according to ASTM D 638. The dimensions of the samples used was 80 × 10 × 3 mm3. The impact strength was examined on a Resil impactor junior tester, according to ISO 180. The dimensions of the samples used was 80 × 10 × 3 mm3. Average of 5 measurements were taken for getting the final value. Thermogravimetric analysis (TGA) The thermal stability of the PEG-ran-PPG modified epoxy thermosets in N2 atmosphere was examined using a TGA-Q-500 thermogravimetric analyzer (TA instruments) at a heating rate of 20 K/min. UV/VIS absorbance Cary 6000i spectrometer (Varian) was used to study the stability of the PEG-ran-PPG modified epoxy thermosets against ultra violet/visible light from 200 to 1000 nm. Results and discussion Dynamic mechanical analysis (DMA) DMA measurements were done using a single cantilever mode from -100 to 250 °C to evaluate the shape memory properties of the modified epoxy blends. The storage modulus (E'), loss modulus (E´´), and tan δ curves of the epoxy blends are shown in Figure 2a-c. From the storage modulus curves the glass transition temperature (Tg) decreases with increasing RCP content. The ratio of elastic (at 30 °C) to rubbery moduli (Tg + 30 ° C) (Ee/Er) is an important factor that influences the shape memory effect of polymeric materials. A large value of Ee/Er is essential for good shape memory effect (11, 33, 43). Table 1 presents the calculated values of Ee/Er of neat epoxy and its blends. It is obvious from the table that the ratio increases with



increase in RCP concentration. However, for 30 and 40 wt% RCP modified epoxy blends the modulus starts dropping at 30 °C. Therefore Ee/Er dips below the value of the neat epoxy system. The level of interaction between the RCP and epoxy matrix could be established from the variation of crosslink density of the blends (44). Crosslink density of the rubbery region of E' vs. temperature profile was formally calculated according to equation 1, (45) and the values obtained are given in Table 1. ------------------

ܰ = ‫ݎܧ‬/(ɸRT)

(1)

(N is crosslink density; ɸ is front factor and is equal to one (46-47); R is the gas constant; Er is rubbery modulus at Tg + 30°C; T is absolute temperature (in K)).

(a) 1000

E' (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

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100

10 -100

neat epoxy 5 wt% RCP 10 wt% RCP 15 wt% RCP 20 wt% RCP 30 wt% RCP 40 wt% RCP -50

0

50

100

150

200

250

o

Tem perature ( C)

Figure 2a. Storage modulus vs. temperature curves of epoxy/DDM and its blends with PEG-ranPPG


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300

200

E'' (MPa)

(b)

neat epoxy 5 wt% RCP 10 wt% RCP 15 wt% RCP 20 wt% RCP 30 wt% RCP 40 wt% RCP

250

150

100

50

0 -100

-50

0

50

100

150

o

Temperature ( C)

Figure 2b. Loss modulus vs. temperature curves of epoxy/DDM and its blends with PEG-ranPPG

1.0

0.8

0.6

(c)


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


0.4

0.2

0.0 -100

-50

0

50

100

150

200

o

Temperature ( C)

Figure 2c. tan δ vs. temperature curves of epoxy/DDM and its blends with PEG-ran-PPG



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Table 1. Ee/Er, N and the parameters obtained from the tan δ of epoxy/DDM and its blends with PEG-ran-PPG. RCP content (wt%)

0

5

10

15

20

30

40

Ee/Er

75.7

77.6

78.3

87

95

69.8

64

N (mol/dm3)

7.5

6.2

6.8

5.7

6.4

5.3

3.7

Tg (°C)

141

137

133

129

112

88

61

FWHM (K)

30

24

22

26

35

39

42

Peak height

0.99

0.78

0.65

0.54

0.47

0.46

0.48

The crosslink density of the blends decreases with increase in RCP content implying the presence of intermolecular interaction between the epoxy matrix and RCP phase. Besides that, the glass transition peaks observed in the loss modulus vs. temperature profile (Figure 2b) and tan δ vs. temperature profile (Figure 2c) provide additional support for this interaction (44). The Tg of epoxy phase in the modified epoxy blends obtained from the tan δ vs. temperature curves is shown in Table 1. The neat epoxy shows a Tg at 141 °C and with the incorporation of RCP, Tg of epoxy phase decreases. A remarkable shift in glass transition peak is observed in the blends when the RCP content exceeds 15 wt%. There is gradual decrease in Tg of epoxy phase with the addition of up to 15 wt% of RCP. The shift in Tg for 5, 10, and 15 wt% RCP modified blends is 4, 8, and 12 °C, respectively. But beyond that level of RCP, there is drastic reduction in Tg of epoxy phase. For example, addition of 20 wt% of RCP resulted in a decrease of 29 °C in the Tg of epoxy phase. The reduction in Tg of epoxy phase in blends containing 30 and 40 wt% of RCP, with respect to the Tg of neat epoxy, are 53 and 80 °C respectively. This means favourable interactions between both the components are more especially at higher concentrations of RCP.


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It is also important to mention that the tan δ peak height decreases, while the full width at half maximum (FWHM) increases with the addition of PEG-ran-PPG especially at higher RCP content, (Table 1) that also signifies the miscibility and interaction between PEG-ran-PPG and epoxy matrix (48). Besides that, 30 and 40 wt% RCP modified systems show an additional glass transition peak corresponding to the RCP phase at around -50 °C, signifying some separation of pure RCP in the blends. The Fox equation (equation 2) was used to evaluate the miscibility of PEG-ran-PPG modified epoxy blends 1/Tg = W1/Tg1 + W2/Tg2

-----------

2

Where W1 and W2 are weight fractions of epoxy thermoset and RCP, respectively. For complete miscibility, the experimental Tg must be in agreement with calculated Tg by the Fox equation. From Figure 3 it is obvious that the experimental Tg shows slightly higher values when compared with the theoretical values calculated by the Fox equation. The difference in Tg values may be due to the intermolecular interaction between the epoxy thermoset and RCP because the Fox equation disregards intermolecular forces. These results reveals that the major fraction of the RCP is miscible with epoxy matrix.



Experimental data Fox equation

400

350

Tg (K)

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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300

250

0.0

0.2

0.4

0.6

0.8

1.0

Wt fraction of RCP

Figure 3. Comparison of experimental Tg with that calculated using the Fox equation The FTIR spectra (Figure S1) and the analysis of the shift in wave number with the addition of RCP into epoxy (Table 2) are helpful in assessing the extent of interactions in these systems. The red shift in the band at 3396 cm-1 confirms the interaction between PEG blocks and epoxy through hydrogen bonding. The red shift further implies that the intermolecular hydrogen bonding interactions are stronger than intramolecular hydrogen bonding with in the epoxy matrix (49). From the table it seems that at higher concentration of the RCP, the extent of intermolecular forces is greater. This will, indeed, decrease the crosslink density and Tg, as we observed earlier. Table 2. The shift in wave number with respect to PEG-ran-PPG content PEG-ran-PPG content (wt % )

0

10

20

30

40

Wave number (cm-1)

3396

3382

3365

3359

3355

Shift (cm-1)

Nil

-14

-31

-37

-41

A further support in this direction comes from the thermomechanical analysis (TMA) where the thermal expansion behavior of the blends has been investigated. Note that the changes 12 ACS Paragon Plus Environment

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in dimensional stability with respect to temperature are related to molecular vibrations of amorphous polymer chains. Figure 4 shows that neat epoxy, because of its highly crosslinked phase structure, exhibits maximum dimensional stability. Addition of RCP decreases the dimensional stability. However, it is interesting to note that the variation in dimensional behavior of blends containing up to 15 wt% RCP is similar to that of neat epoxy. A stress relaxation is observed around the glass transition temperature and the slope of the dimension change dependence on temperature is increased above Tg. Beyond that level of RCP content, a different behavior has been observed. The dimensional stability decreases at higher RCP concentration primarily due to the better interaction of PEG chains with epoxy matrix in these blends. It is important to mention that DMA, FTIR, and TMA results complement each other.

5.0

neat epoxy 5 wt% RCP 10 wt% RCP 15 wt% RCP 30 wt% RCP 40 wt% RCP

4.5 4.0

Dimension Change (%)

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


3.5 3.0 2.5 2.0

124

66

1.5 1.0 0.5 0.0 -50

0

50

100

150

200

o

Temperature ( C)

Figure 4. Dimension change vs. temperature curves of neat epoxy and its blends with PEG-ranPPG AFM images of the 20 and 40 wt% RCP modified epoxy blends are given in Figure 5. There is no typical phase separated structure detectable. The soft spots appearing dark brown in



the phase contrast mode are due to debris or other impurities or artefacts coming from preparation and correlate with elevated structures in the height profile. Thus AFM supports that major portion of RCP is miscible with epoxy matrix, due to the favourable interaction of between both blend components.

(a) 20 wt% RCP modified epoxy blends

(b) 40 wt% RCP modified epoxy blends Figure 5. The AFM images of (a) 20 wt% RCP modified epoxy blends, (b) 40 wt% RCP modified epoxy blends


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Shape memory test (fold-deploy test) Fold-deploy test was used to study the shape memory properties of the epoxy blends. The shape memory experiments were repeated 5 times, and the average values are given in Table 3. The RCP modified epoxy blends show excellent shape retention and shape recovery ratios (>96 %). Interestingly, there is no change in shape retention and shape recovery in dependence on RCP content. Note that the same values are observed even for the neat epoxy resin. This implies that the prepared blends can be effectively employed for making high performance shape memory polymers that can be used at relatively low temperature. Table 3. The parameters obtained from fold-deploy test PEG-ran-PPG

Folding/recovery

Retention ratio

Recovery ratio

content (wt%)

temperature

(%)

(%)

(approximately) 0

161

97

97

10

153

96.4

96.4

20

132

96.5

96.5

30

108

96.5

96.5

40

81

97

97



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Mechanism of shape memory effect

Heat, Deform, Cool

Heating

Permanent shape

Temporary shape

Figure 6. Shape memory behaviour of 40 wt% RCP modified epoxy blends. The size of the sample is 80×10×3 mm3. Shape memory behaviour of 40 wt% RCP modified epoxy blends are given in Figure 6. In epoxy thermosets, thermal shape memory effect may be attributed to the co-existence of rigid and brittle glassy state and soft and flexible rubbery state. Above the Tg, the material enters into rubbery state and becomes soft and flexible so that it can easily be deformed into any independent shape (temporary shape). Up on cooling and when it reaches below the Tg, the material loses its flexible nature and becomes rigid in such a way that its temporary shape is fixed. The original shape (permanent shape) can be regained by heating the polymer again to temperatures above its Tg. It is believed that shape memory effect is an entropic phenomenon. When the polymer is in its permanent shape, the conformation of the polymer chains possesses highest entropy. Up on heating, the segmental motion of polymer chains gets activated and when an external load is applied, the confirmation of the polymer chains changes to a stressed state with lower entropy. This state is trapped by freezing the polymer chains and results in temporary 16 ACS Paragon Plus Environment

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shape fixation. Again, up on heating above the Tg of the system, translational segmental motion of polymer chains gets reactivated and the polymer regains its original stable conformation with the highest entropy (43). This means that for crosslinked polymeric systems, it is not essential to have different phases to exhibit shape memory effect as the Tg acts as the reversible phase (switch) and the crosslinked networks act as the net points. At this point, it is also important to mention the role of RCP in tuning the shape memory effect in epoxy system. As stated earlier the intermolecular forces between both components resulting in molecular dissolution of RCP in the cured epoxy. This is reason for the reduction in shape memory transition temperature, increased miscibility and decreased crosslink density of the prepared blends. A schematic of polymer network of original permanent shape, deformed temporary shape, and recovered shape is shown in Figure 7.

Figure 7. Schematic of polymer network of original permanent shape, temporary shape, and recovered shape. (red dots represent epoxy crosslinks, black line represent epoxy chains and blue lines represent RCP chains homogeneously mixed with epoxy phase) Mechanical properties The mechanical properties of neat epoxy and its blends are given in Table 4. It is important to mention that up to 20 wt% of RCP concentration in the blend, there is no significant change in the mechanical properties, except that impact strength decreases at lower RCP concentration. However beyond 20 wt% of RCP concentration, tensile strength and modulus drastically 17 ACS Paragon Plus Environment


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decrease, whereas elongation and impact strength significantly enhance. Note that the interactions between RCP and epoxy matrix are more intense for higher concentrated blends, which reduces the crosslink density of the epoxy matrix. Therefore, the addition of PEG-ran-PPG will result in the reduction of tensile strength and modulus, especially for higher concentration RCP concentrations. The significant increase in elongation at break is due to the interpenetration of RCP chains with epoxy network, which imparts a certain level of plasticization of epoxy network by reducing the crosslinking density (50). It is worth emphasizing that with 20, 30 and 40 wt% of RCP content, the impact strength increases by ca. 20, 60 and 90 %, respectively. The increase in impact strength is due to the strong hydrogen bonding between the epoxy matrix and the miscible RCP chains, which result in excellent adhesion between both the phases. Table 4. Mechanical properties of neat epoxy and its blends PEG-ran-PPG

Tensile

Tensile

Tensile

Impact

content (wt%)

strength

elongation

modulus

strength

(MPa)

(%)

(MPa)

(kJm-2)

0

59.3 ± 6.6

6.7 ± 1.2

2037 ± 15

28.5 ± 6.8

5

54.8 ± 5.1

6.8 ± 1.1

2072 ± 151

20.5 ± 6.5

10

59.6 ± 6.3

8.3 ± 1.5

2068 ± 80

16.2 ± 4.5

15

55.2 ± 9.9

7.4 ± 1.6

2020 ± 179

22.7 ± 5.2

20

55.1 ± 3.4

8.5 ± 1.5

1966 ± 106

34.4 ± 10

30

37.8 ± 2.8

8.9 ± 1.3

1434 ± 115

45.9 ± 11.2

40

12.9 ± 0.8

19.6 ± 3.4

500 ± 22

53.4 ± 15.5


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Thermal stability The TGA curves given in Figure 8 show the thermal degradation behavior of neat epoxy and its blends. Important thermal degradation characteristics such as initial degradation temperature (IDT), maximum degradation temperature (MDT), final degradation temperature (FDT) and % char content calculated from the thermograms are shown in Table S1. Although one would expect a significant deterioration in thermal stability of epoxy blends with the addition of RCP, thermal stability is retained in all the blends. This is because the interpenetrated nature of RCP chains into the epoxy crosslinks may restrict the thermal degradation of RCP phase. Further, the UV absorption spectra shown in Figure 9 illustrate that neat epoxy and its blends absorb UV light and thereby effectively screen the UV radiation. The blends up to 20 wt% RCP shows a strong absorption below 350 nm. However, for 20, 30 and 40 wt% of RCP modified epoxy blends the UV absorption range was shifted to visible range. Thus the higher concentrated blends are very effective in shielding the UV light (200-380 nm) that can cause skin cancer, sun burn etc., and blue light (380-500 nm) that is very harmful to human eyes.

100

80

Weight (%)

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


60

40

20


0 100

200

300

400

500

o

Temperature ( C)

Figure 8. TGA thermograms of neat epoxy and its blends 19 ACS Paragon Plus Environment

600


5

neat epoxy 5 wt% RCP 10 wt% RCP 20 wt% RCP 30 wt% RCP 40 wt% RCP

4

3

Absorbance

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

2

1

0

300

400

500

600

700

Wavelength (nm)

Figure 9. UV absorption spectra of neat epoxy and its blends Conclusion Shape memory and thermo-mechanical properties of the poly(ethylene glycol-ran-propylene glycol) (RCP) modified epoxy blends were systemically studied. The intermolecular interaction between epoxy matrix and RCP phase was confirmed by FTIR spectroscopy. The modification of epoxy networks by RCP drastically decreased the crosslink density and thereby the glass transition temperature of epoxy phase. The glass transition temperature was decreased from 141 to 61 °C by the addition of 40 wt% of the RCP. Shape memory properties were measured by using fold-deploy test. No change in shape retention and shape recovery was observed irrespective of the blend composition. These blends exhibited excellent thermo-mechanical properties and UV resistance that make them suitable for developing potential materials of tremendous technological significance. Acknowledgment JP 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 20 ACS Paragon Plus Environment

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(contract grant number IFA-CH-16). S.S. acknowledges King Mongkut’s University of Technology North Bangkok (grant no. KMUTNB-62-GOV-001) Supporting Information Figure S1 shows the FTIR spectra of PEG-ran-PPG modified epoxy shape memory polymers. Table S1 shows the thermal degradation characteristics of PEG-ran-PPG modified epoxy shape memory polymers. References (1) Lendlein, A.; Kelch, S. Shape-Memory Polymers, Angew. Chem. Int. Ed. 2002, 41, 20342057. (2) Hu, J.; Zhu, Y.; Huang, H.; Lu, J. Recent advances in shape–memory polymers: Structure, mechanism, functionality, modeling and applications. Prog. Polym. Sci. 2012, 37, 1720-1763. (3) Zhao, Q.; Zou, W.; Luo, Y.; Xie, T. Shape memory polymer network with thermally distinct elasticity and plasticity. Sci. Adv. 2016, 2, e1501297 (4) Behl, M.; Razzaq, M. Y.; Lendlein, A. Multifunctional Shape-Memory Polymers. Adv. Mater. 2010, 22, 3388-3410. (5) Xie, T. Tunable polymer multi-shape memory effect. Nature 2010, 464, 267-270. (6) Lendlein, A.; Jiang, H.; Junger, O.; Langer, R. Light-induced shape-memory polymers. Nature 2005, 434, 879-882. (7) Ying, L.; Chen, H.; Liu, D.; Wang, W.; Liu, Y.; Zhou, S. pH-Responsive Shape Memory Poly(ethylene

glycol)−Poly(ε-caprolactone)-based

Polyurethane/Cellulose

Nanocrystals

Nanocomposite. ACS Appl. Mater. Interfaces 2015, 7, 12988-12999. (8) Zheng, X. T.; Zhou, S. B.; Xiao, Y.; Yu, X. J.; Li, X. H.; Wu, P. Z. Shape Memory Effect of Poly(D,L-lactide)/Fe3O4 Nanocomposites by Inductive Heating of Magnetite Particles. Colloids Surf B Biointerfaces 2009, 71, 67-72. 21 ACS Paragon Plus Environment


(9) Xiao, Y.; Zhou, S. B.; Wang, L.; Gong, T. Electro-active Shape Memory Properties of Poly(ε-caprolactone)/Functionalized Multiwalled Carbon Nanotube Nanocomposite. ACS Appl. Mater. Interfaces 2010, 2, 3506-3514. (10) Wang, W.; Lu, H.; Liu, Y.; Leng, J. Sodium dodecyl sulfate/epoxy composite: water induced shape memory effect and its mechanism. J. Mater. Chem. A 2014, 2, 5441-5449. (11) Xiao, X.; Kong, D.; Qiu, X.; Zhang, W.; Liu, Y.; Zhang, S.; Zhang, F.; Hu, Y.; Leng, J. Shape memory polymers with high and low temperature resistant properties. Sci. Rep. 2015, 5, 14137. (12) Hager, M. D.; Bode, S.; Weber, C.; Schubert, U. S.; Shape memory polymers: Past, present and future developments. Prog. Polym. Sci. 2015, 49–50, 3–33 (13) Nagai, H.; Ueda, A.; Isomura, S. Shape memory norbornene polymer molded products, Jpn. Pat. 06080768, 1994. (14) Heuwers, B.; Beckel, A.; Krieger, A.; Katzenberg, F.; Tiller, J. C. Shape-memory natural rubber: an exceptional material for strain and energy storage. Macromol. Chem. Phys. 2013, 214, 912–923. (15) Li, X.; Liu, T.; Wang, Y.; Pan, Y.; Zheng, Z.; Ding, X.; Peng, Y. Shape memory behavior and mechanism of poly(methyl methacrylate) polymer networks in the presence of star poly(ethylene glycol). RSC Adv. 2014, 4, 19273–19282. (16) Chen, H.; Li, Y.; Tao, G.; Wang, L.; Zhou, S. Thermo- and water-induced shape memory poly(vinyl alcohol) supramolecular networks crosslinked by self-complementary quadruple hydrogen bonding. Polym. Chem. 2016, 7, 6637-6644. (17) Zheng, Y.; Dong, R.; Shen, J.; Guo. S. Tunable Shape Memory Performances via Multilayer Assembly of Thermoplastic Polyurethane and Polycaprolactone. ACS Appl. Mater. Interfaces 2016, 8, 1371−1380. 22 ACS Paragon Plus Environment

Page 22 of 27

Page 23 of 27 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


(18)

Tobushi, H.; Hayashi, S.; Hoshio, K.; Makino, Y.; Miwa, N.

Bending Actuation

Characteristics of Shape Memory Composite with SMA and SMP. J Intell Mater Syst Struct. 2006, 17, 1075-1081. (19) Zhao, J.; Chen, M.; Wang, X.; Zhao, X.; Wang, Z.; Dang, Z.; Ma, L.; Hu, G.; Chen, F. Triple Shape Memory Effects of CrossLinked Polyethylene/Polypropylene Blends with Cocontinuous Architecture. ACS Appl. Mater. Interfaces 2013, 5, 5550–5556. (20) Liu, Y.; Zhao, J.; Zhao, L.; Li, W.; Zhang, H.; Yu, X.; Zhang, Z. High Performance Shape Memory Epoxy/Carbon Nanotube Nanocomposites. ACS Appl. Mater. Interfaces 2016, 8, 311−320. (21) Liu, C.; Qin, H.; Mather, P. T. Review of progress in shape-memory polymers. J.Mater.Chem. 2007, 17, 1543-1558. (22) Liu, Y.; Du, H.; Liu, L.; Leng, J. Shape memory polymers and their composites in aerospace applications: a review. Smart Mater. Struct. 2014, 23, 023001 (22pp) (23) Meng, H.; Li, G. A review of stimuli-responsive shape memory polymer composites. Polymer 2013, 54, 2199-2221. (24) Dolynchuk, O.; Kolesov, I.; Jehnichen, D.; Reuter, U.; Radusch, H.; Sommer, J. Reversible Shape-Memory Effect in Cross-Linked Linear Poly(ε-caprolactone) under Stress and Stress-Free Conditions. Macromolecules 2017, 50, 3841−3854. (25) Dye, D.; Shape memory alloys: Towards practical actuators. Nat. Mater., 2015, 14, 760-761 (26) Parameswaranpillai, J.; Hameed, N.; Pionteck, J.; Woo, E.M. eds. Handbook of Epoxy Blends. Springer New York; ISBN: 978-3-319-18158-5 (27) Kumar, K.S.S.; Khatwa, A.K.; Nair, C.P. R.; High transition temperature shape memory polymers (SMPs) by telechelic oligomer approach. React. Funct. Polym. 2014, 78, 7-13.



(28) Yao, Y.; Wang, J.; Lu, H.; Xu, B.; Fu, Y.; Liu, Y.; Leng, J. Thermosetting epoxy resin/thermoplastic system with combined shape memory and self-healing properties. Smart Mater. Struct. 2016, 25, 015021. (29) Hu, J.; Chen, W.; Fan, P.; Gao, J.; Fang, G.; Cao, Z.; Peng, F. Epoxy shape memory polymer (SMP): Material preparation, uniaxial tensile tests and dynamic mechanical analysis. Polym. Test. 2017, 62, 335-341. (30) Biju, R., Reghunadhan Nair, C. P. R.; Synthesis and characterization of shape memory epoxy-anhydride system. J. Polym. Res. 2013, 20, 82. (31) Lützen, H.; Gesing, T. M.; Kim, B. K.; Hartwig, A. Novel cationically polymerized epoxy/poly(ɛ-caprolactone) polymers showing a shape memory effect. Polymer 2012, 53, 60896095. (32) Santiago, D.; Fernández-Francos, X.; Ferrando, F.; De la Flor, S. Shape-memory effect in hyperbranched poly(ethyleneimine)-modified epoxy thermosets. J. Polym. Sci. Part B Polym. Phys. 2015, 53, 924-933. (33) Leng, J.; Wu, X.; Liu. Y. Effect of a linear monomer on the thermomechanical properties of epoxy shape-memory polymer. Smart Mater. Struct. 2009, 18, 095031. (34) Xie, F.; Huang, L.; Leng, J.; Liu, Y. Thermoset shape memory polymer and their composites. J. Intell. Mater. Syst. Struct. 2016, 27, 2433-2455. (35) Hillmyer, M. A.; Lipic, P. M.; Hadjuk, D. A.; Almdal, K.; Bates, F. S. Self-assembly and polymerization of epoxy resin-amphiphilic block copolymer nanocomposites. J. Am. Chem. Soc. 1997, 119, 2749-2750. (36) Martin-Gallego, M.; Verdejo, R.; Gestos, A.; Lopez-Manchado, M. A.; Guo, Q. Morphology and mechanical properties of nanostructured thermoset/block copolymer blends with carbon nanoparticles. Compos. Part A. Appl. Sci. Manuf. 2015, 71, 136-143. 24 ACS Paragon Plus Environment

Page 24 of 27

Page 25 of 27 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


(37) George, S. M.; Puglia, D.; Kenny, J. M.; Parameswaranpillai, J.; Pionteck, J.; Thomas, S. Volume shrinkage and rheological studies of epoxidised and unepoxidised poly (styrene-blockbutadiene-block-styrene) triblock copolymer modified epoxy resin–diamino diphenyl methane nanostructured blend systems. Phys. Chem. Chem. Phys. 2015, 17, 12760-12770. (38) Yu, R.; Zheng, S.;

Li, X.; Wang, J. Reaction-Induced Microphase Separation in Epoxy

Thermosets Containing Block Copolymers Composed of Polystyrene and Poly(ε-caprolactone): Influence

of copolymer

Architectures

on

Formation

of

Nanophases.

Macromolecules 2012, 45, 9155–9168. (39) Li, T.; He, S.; Stein, A.; Francis, L. F.; Bates, F. S. Synergistic Toughening of Epoxy Modified by Graphene and Block Copolymer Micelles. Macromolecules 2016, 49, 9507–9520. (40) Garate, H.; Goyanes, S.; D’Accorso, N. B. Controlling Nanodomain Morphology of Epoxy Thermosets Modified with Reactive Amine-Containing Epoxidized Poly(styrene-bisoprene-b-styrene) Block Copolymer. Macromolecules 2014, 47, 7416–7423. (41) Kortaberria, G. Morphology of Epoxy/Block Copolymer Blends. In: Parameswaranpillai, J.; Hameed, N.; Pionteck, J.; Woo, E. (eds) Handbook of Epoxy Blends. Springer, Cham, 2015. (42) Garate, H.; Morales, N.J.; Goyanes, S.; D’Accorso, N. B. Miscibility, Phase Separation, and Mechanism of Phase Separation of Epoxy/Block-Copolymer Blends. In: Parameswaranpillai, J.; Hameed, N.; Pionteck, J.; Woo, E. (eds) Handbook of Epoxy Blends. Springer, Cham, 2015. (43) Parameswaranpillai, J.; Ramanan, S. P.; Jose, S.; Siengchin, S.; Magueresse, A.; Janke, A.; Pionteck, J. Shape memory properties of epoxy/PPO-PEO-PPO triblock copolymer blends with tunable thermal transitions and mechanical characteristics. Ind. Eng. Chem. Res., 2017, 56 , 14069–14077.



(44) Liu, J.; Sue, H.; Thompson, Z. J.; Bates, F. S.; Dettloff, M.; Jacob, G.; Verghese, N.; Pham, H. Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles. Polymer 2009, 50, 4683–4689. (45) Iijima, T.; Yoshioka, N.; Tomoi, M. Effect of cross-link density on modification of epoxy resins with reactive acrylic elastomers. Eur. Polym J. 1992, 28, 573-581. (46) Nouailhas, H.; Aouf, C.; Guerneve, C. L.; Caillol, S.; Boutevin, B.; Fulcrand, H. Synthesis and Properties of Biobased Epoxy Resins. Part 1. Glycidylation of Flavonoids by Epichlorohydrin. J. Polym. Sci. A Polym. Chem. 2011, 49, 2261-2270. (47) Zhao, S.; Abu-Omar, M. M. Renewable Epoxy Networks Derived from Lignin-Based Monomers: Effect of Cross-Linking Density. ACS Sustainable Chem. Eng. 2016, 4, 6082-6089. (48) Reghunadhan, A.; Datta, J.; Kalarikkal, N.; Thomas, S. Development of nanoscale morphology and role of viscoelastic phase separation on the properties of epoxy/recycled polyurethane blends. Polymer 2017, 117, 96-106. (49) Hameed, N.; Salim, N. V.; Walsh, T. R.; Wiggins, J. S.; Ajayan, P. M.; Fox, B. L. Ductile thermoset polymers via controlling network flexibility. Chem. Commun. 2015, 51, 9903-9906. (50) Misaki, T.; Hirohata, T.; Yoshii, M.; Hamasaki. T. Properties of networks obtained by internal plasticization of epoxy resin with aromatic and aliphatic glycidyl compounds. J. Appl. Polym. Sci. 1989, 37, 2617- 2625.


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Schematic of polymer network of original permanent shape, temporary shape, and recovered shape. (red dots represent epoxy crosslinks, black line represent epoxy chains and blue lines represent RCP chains homogeneously mixed with epoxy phase) 559x231mm (300 x 300 DPI)

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PEG-ran-PPG modified epoxy thermosets: A ... - ACS Publications

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