PPO-PEO-PPO triblock copolymer

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Shape memory properties of epoxy/PPO-PEO-PPO triblock copolymer blends with tunable thermal transitions and mechanical characteristics Jyotishkumar Parameswaranpillai, Sreekanth Panachikunnel Ramanan, Seno Jose, Suchart Siengchin, Anthony Magueresse, Andreas Janke, and Jürgen Pionteck Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03676 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Industrial & Engineering Chemistry Research

Shape memory properties of epoxy/PPO-PEO-PPO triblock copolymer blends with tunable thermal transitions and mechanical characteristics

Jyotishkumar Parameswaranpillai12*, Sreekanth Panachikunnel Ramanan1, Seno Jose3, Suchart Siengchin4*, Anthony Magueresse5, Andreas Janke6, Jürgen Pionteck6

1. Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682022, Kerala, India. 2. 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. 3. Department of Chemistry, Government College Kottayam 686013, Kerala, India. 4. Department of Materials and Production Engineering, King Mongkut's University of Technology North Bangkok 1518 Pracharaj 1, Wongsawang Road, Bangsue, Bangkok 10800, Thailand. 5. FRE CNRS 3744, IRDL, Univ. Bretagne Sud, F-56100 Lorient, France. 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]

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Abstract In this paper, we report a simple method to prepare novel, transparent, hard-tough and hard-flexible shape memory and soft-flexible epoxy polymers based on poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) (PPO-PEO-PPO) triblock copolymer (TBCP) and diglycidyl ether of bisphenol-A (DGEBA)/4,4'-diaminodiphenylmethane (DDM) system. The PPO-PEO-PPO triblock copolymer was used to tailor crosslink density and flexibility in epoxy thermosets and thereby their glass transition temperature (Tg). The formed blends exhibit a phase separated morphology. The phase separation was initiated by immiscible PPO blocks via self-assembly. Three types of shape memory polymers, viz. stiff, intermediate and soft-flexible epoxy systems, with entirely different physical and mechanical properties were prepared only by adjusting the blend composition. All the blends were UV resistant, thermally and dimensionally stable and could be used for various outdoor applications. To the best of our knowledge, no work has been reported on the shape memory properties of epoxy modified block copolymers.

Key words: shape memory properties; epoxy polymers; triblock copolymer; miscibility; thermo-mechanical properties


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Introduction Shape memory polymers (SMPs) are stimuli-responsive and self-adjusting active smart polymers. They can switch from original permanent (primary) shape to a temporary (secondary) shape and later recover the primary shape on demand. This on-demand movement is called shape memory effect1-3. Shape memory technology offers ample opportunities for researchers to design the desired polymer architecture and tune the thermo-mechanical programming conditions to fabricate novel, high performance polymeric materials with increasingly sophisticated properties suitable for several advanced applications4. In this technology, by conventional processing, the polymer takes the permanent shape and afterwards sample is deformed into temporary shape by programming5. Upon application of suitable stimuli such as heat6-9, light10-11, moisture12 pH13, electrical field14-15, and magnetic field16, the polymer recovers its original shape. Epoxy resins are important thermosetting resins widely used for several advanced applications due their ease of processing, good thermo-mechanical properties, dimensional stability, chemical resistance, good adhesion and environmental resistance.17 Because of the superior thermo-mechanical properties, great interest is there among researchers to develop shape memory epoxy polymers (SMEPs).18 It is reported that the thermal flexibility and shape memory effect in epoxy thermoset can be induced by varying the content of curing agent or the addition of perfectly ductile thermoplastics, linear epoxy monomers, elastomers or elastomer like materials, etc.18-25 The present research is devoted to develop thermo-responsive shape memory polymers based on epoxy resin and triblock copolymer (TBCP). TBCP is a class of materials that can be used as modifiers for epoxy thermoset to generate nanostructured thermoset with superior thermo-mechanical properties.26-28 To the best of our knowledge, no studies on TBCP modified



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SMEPs have been reported, till date. A low molecular weight poly(propylene glycol)-blockpoly(ethylene glycol)-block-poly(propylene glycol) TBCP was employed to adjust the crosslink density of epoxy thermoset and thereby induce shape memory effect in epoxy thermosets at low temperatures. When heated above the Tg, the resulting polymer blends become soft and flexible. They can be deformed into different temporary shapes through twisting and bending, and upon cooling the temporary shapes are fixed. These temporary shapes, which cannot be achieved by molding, can be transformed into the original permanent shape when heated again above the Tg. The present paper examines the shape memory effect, thermo-mechanical properties, intermolecular hydrogen bonding interactions between the TBCP and epoxy thermoset, formation of nanostructures, UV resistance and thermal stability of TBCP modified epoxy/DDM system. Miscibility and hydrogen bonding interactions were evaluated by FTIR. AFM and SEM were used to examine the phase morphology and fracture mechanics, respectively. DMA provided storage modulus, loss modulus and tan δ behavior of the blends. UV spectrophotometry was used to investigate the stability of the blends against ultra violet light. TMA and TGA were used to investigate the dimensional and thermal stabilities of the blends, respectively. Experimental Materials Diglycidyl ether of bisphenol-A (DGEBA) (Lapox L-12, Mw-340 g mol-1) from Atul Ltd, India, was used as the matrix material for preparing epoxy blends. The curing agent, 4,4'diaminodiphenylmethane (DDM, Mw-198 g mol-1) was procured from Sigma-Aldrich India. The triblock copolymer, poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) (PPO-PEO-PPO, TBCP) PPG-PEG-PPG, Pluronic® 10R5 with 50 wt% PEO, also was


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procured from Sigma-Aldrich India. The TBCP has an Mw of 2000 g mol-1, melting point of 15 °C and Tg of -65 °C. Preparation of blends The required amount of TBCP was mixed with epoxy resin at 100 °C, until the mixture became homogeneous. The stoichiometric amount of curing agent (DDM) was added to the homogenous mixture with continuous stirring until the curing agent was completely dissolved. The mixture was poured into preheated mold and cured in an air oven for 2 h at 100 °C and then post-cured for 4 h at 120 °C. Blends with 0, 5, 10, 15, 20, 30, 40, and 50 wt% of TBCP were prepared. Techniques Fourier transform infrared spectroscopy (FTIR) FTIR experiments were carried out using a Bruker Vertex 80v system. The cured samples were scanned from 4000 to 600 cm-1, at a resolution of 4 cm-1, with 100 scans per measurements. Atomic force microscopy (AFM) The AFM measurements were done in the tapping mode by a Dimension 3100 Nanoscope IIIa (Veeco, USA). A microtome machine was used for cutting and smoothing the samples for AFM study. Dynamic mechanical analysis (DMA) A DMA Q-800 from TA Instruments was used to obtain the storage modulus, loss modulus and tan δ of the cured blends between -100 and 250 °C at a heating rate of 3 K/min. Analysis was performed at a frequency of 1 Hz, with sample dimensions of 30 x 10 x 3 mm3. Mechanical properties Tensile testing was carried out in accordance with ASTM D 638 with sample dimensions of 80 × 10 × 3 mm3, using a Tinius Olsen machine, model H 50 KT, at a cross head speed of 50



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mm/min. The average of 5 test was taken as the final value. The impact test was done according to ISO 180, (Izod, without notch) with sample dimensions 80 x 10 x 3 mm3, using a Resil impactor junior. Scanning electron microscopy (SEM) The fracture surface of the blends after impact testing was examined using a Jeol JSM-6460LV scanning electron microscope, operating at an accelerating voltage of 20 kV. The fracture surface were sputter-coated with gold before SEM analysis. Thermomechanical Analysis (TMA) TMA Q-400 from TA Instruments was used to analyze the dimensional stability of the cured blends between- 50 and 250 °C, at a heating rate of 3 K/min. The sample thickness was 3 mm. UV/VIS Absorbance The UV absorbance of the blends was studied using a Cary 6000i spectrometer (Varian). The samples were scanned from 200 to 1000 nm. Thermogravimetric analysis (TGA) Thermal degradation behavior of the blends was investigated using a TGA-Q-500 thermogravimetric analyzer (TA instruments). In order to measure the thermal stability, samples of about 6 mg were heated from 25 to 700 °C at a heating rate of 20 K/min in N2 atmosphere. Shape Memory Test (Fold-deploy Test) Fold-deploy test was carried out to investigate the shape memory properties of the epoxy blends. Rectangular samples of 10 × 1 × 0.1 mm3 were used for the analysis. The investigation was done as follows29; firstly the sample was kept in an oven at Tg + 20 °C and deformed into the U shape (temporary shape) with an deformation angle θmax by the application of the stress. In the next step, sample was brought to room temperature under a constant stress for fixing this shape. Due


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to some relaxation the deformation angle change slightly to θfix. Finally, shape recovery was carried out by reheating the sample at Tg + 20 °C, with a remaining deformation angle (θfinal). The maximum bending angle (θmax), the bending angle after fixing (θfix), and the residual bending angle after shape recovery (θfinal) were measured. The shape retention and shape recovery ratios were determined by the equations 1 and 2. Shape retention ratio = [θfix/θ max] × 100 ----Shape recovery ratio = [θfix–θfinal/ θfix] × 100

(1) -----

(2)

Results and discussion The storage modulus (E') of TBCP modified epoxy system is shown in Figure 1a. Three types of behavior can be deduced from the Figure. Blends containing up to 20 wt% of TBCP exhibit an E' dependency on temperature very similar to that of a typical epoxy thermoset (Type 1). The E' profile of blend containing 50 wt% of TBCP is very similar to that of an elastomer (Type 3). The other two compositions manifest intermediate behavior (Type 2). All in common is that with increasing TBCP the glassy-rubbery transition shift to lower temperature. Type 1 and type 2 blends have higher E' values than that of neat epoxy up to temperature very close to 0 °C. Type 3 blend possesses very low E' value compared to other blends in the entire temperature range. The increase in E' at lower temperatures may be attributed to the strong intermolecular hydrogen bonding interactions between the TBCP and epoxy matrix. This interaction, in the present case, is temperature sensitive. This means that as temperature increases, due to the greater extent of translational segmental motion of polymer chains, a fraction of hydrogen bonds may be disrupted causing a decrease in E'. For Type 2 blends, E' drastically decreases near 0 °C, because of the melting of the crystalline fraction of phase separated PEO blocks of the TBCP within the cross-linked epoxy resin. The interesting behavior of 50 wt% TBCP modified blend is



derived from its co-continuous type of morphology. A large difference in elastic and rubbery modulus is essential for good shape memory effect. Here the difference in elastic and rubbery modulus increases with the addition of TBCP; this is detectable from the DMA profiles.30 It is important to mention that expect the 50 wt% TBCP containing epoxy blends, all the prepared blends (including neat epoxy/DDM system) shows shape memory effect above room temperature. The ratio of elastic modulus (Ee at 30°C) to rubbery modulus (Er at Tg + 30°C) of the blends (Table 1) increases with increasing concentration of TBCP, which signifies the enhanced shape memory effect in TBCP modified epoxy thermoset.

1000

E' (MPa)

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100

10

-100

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

0

50

100

150

200

o

Temperature ( C)

Figure 1 (a). Storage modulus vs. temperature profiles of epoxy/DDM and its blends with PPOPEO-PPO From the storage modulus curves it is obvious that the Tg drastically decreases with the addition of TBCP. This may be attributed to an apparent decrease in crosslink density by the addition of 8 ACS Paragon Plus Environment

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TBCP because of the interaction of TBCP chains with epoxy matrix.31 The storage modulus in epoxy blends depends on the composition, moduli of the individual components and phase morphology. Therefore changes in crosslink density with the addition of TBCP were calculated from the rubbery plateau region in storage modulus profile according to the rubber elasticity theory, using the following equation.32-33 ܰ = ‫ܧ‬௥ /(ɸRT) ----------

(3)

where N is the crosslink density, ɸ is the front factor and is equal to unity in Flory theory33-34, R is the gas constant, Er is the rubbery modulus at Tg + 30°C, and T is the temperature in Kelvin at Tg + 30°C. The calculated crosslink density of the modified blends is given in Table 1. It is worth noting that, the decrease in crosslink density with increasing TBCP content is due to the dilution of network by molecular mixed TBCP.35 Table 1. The calculated Ee at 30°C/Er at Tg + 30°C ratio and N from storage modulus profile of the prepared blends. TBCP content

Ee at 30°C

Er at Tg +

Ee at 30°C/Er at

N

(wt%)

(MPa)

30°C (MPa)

Tg + 30°C

(mol/dm3)

0

2099

27.7

75.7

7.5

5

2143

24.1

89.1

6.6

10

2224

23.6

94.3

6.5

15

2358

22.5

104.8

6.3

20

2152

21.5

100.1

6.2

30

1658

12.4

133.7

3.9

40

760.8

11.27

67.5

3.7

50

43.5

14.1

-

-



300

neat epoxy 15 wt% TBCP 40 wt% TBCP

250

5 wt% TBCP 20 wt% TBCP 50 wt% TBCP

10 wt% TBCP 30 wt% TBCP

E'' (MPa)

200

150

100

50

0 -50

0

50

100

150

200

o

Temperature ( C)

Figure 1 (b). Loss modulus vs. temperature curves of epoxy/DDM and its blends with PPOPEO-PPO 1.0

neat epoxy 5 wt% TBCP 10 wt% TBCP 15 wt% TBCP 20 wt% TBCP 30 wt% TBCP 40 wt% TBCP 50 wt% TBCP

0.8

0.6

Tan δ

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0.4

0.2

0.0 -50

0

50

100

150

200

o

Temperature ( C)

Figure 1 (c). tan δ vs. temperature curves of epoxy/DDM and its blends with PPO-PEO-PPO The loss modulus (E'') curves of the TBCP modified epoxy blends (Figure 1 b) also show a behavior similar to storage modulus curves. Note that the loss peak at the higher temperature side represents the Tg of the epoxy thermoset. Type 1 blends show distinct peaks at higher temperatures, type 2 blends show a very broad peak at intermediate temperatures and type 3 blend exhibits a distinct peak at negative temperature. The tan δ curves given in Figure 1c 10 ACS Paragon Plus Environment

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shows a clear picture. The data obtained from the tan δ curve are shown in Table 2. It is important to note that the Tg of epoxy resin steadily decreases by the addition of TBCP. The Tg is considered as the most critical parameter for shape memory polymers because the Tg acts as the shape memory transition temperature. Thus, it is important to mention that one can tune the Tg of the epoxy thermoset precisely by varying the concentration of TBCP. For neat epoxy system, the Tg was observed at 141 °C and it reduces to 114 °C by the addition of 20 wt% of TBCP. Thus Type 1 blends with TBCP contents up to 20 wt% exhibit the shape memory effect at rather high temperatures. Our manual studies by using an air oven revealed that neat epoxy shows excellent shape memory effect at around 200 °C, while Type 1 blends can show nice shape memory effect at around 150 °C. However, further increase in TBCP content decreases the Tg to 80 °C for 30 wt% and 65 °C for 40 wt% addition of TBCP. These blends (Type 2) exhibit the shape memory effect below 100 °C. Interestingly, shape memory effect is lost above room temperature for higher TBCP contents of 50 wt%, although the Tg decreases just to 32 °C, but the glass transition region broadens dramatically with increasing TBCP content and the softening starts below room temperature in Type 3 blends. To understand extent of miscibility of the polymer blends, Fox equation (eq 4) was used. 1/Tg = W1/Tg1 + W2/Tg2

----- (4)

Where W1 is weight fractions of crosslinked epoxy with Tg1 and W2 is weight fractions of TBCP with Tg2. For a perfect blend, the experimental Tg of the polymer blends must follow the Tg calculated by Fox equation. The experimental Tg (tan δ peak point) shows a near perfection to the Tg calculated by Fox equation (Figure 2). The variations in experimental and theoretical Tg may be due to the hydrogen bonding interaction between the blend components. Note that Fox



equation is based on weight fractions of the individual polymers. It does not account for intermolecular forces between the blend components.

Experimental data Fox equation

400

350

Tg (K)

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300

250

200 0.0

0.2

0.4

0.6

0.8

1.0

Wt fraction of TBCP

Figure 2. Results of epoxy/TBCP blends as a function of composition Table 2. Glass transition parameters obtained from the tan δ curve. TBCP content

0

5

10

15

20

30

40

50

Tg (°C)

141

138

133

124

114

80

65

32

FWHM

30

24

27

35

40

42

52

73

Peak height

0.985

0.677

0.549

0.46

0.417

0.479

0.395

0.309

(wt%)

Further, broadening of the tan δ peak is caused by the interaction between the TBCP and epoxy matrix and can be quantified from the values of full width at half maximum (FWHM) of the tan

δ curves of the blends, given in Table 2. FWHM gradually increases with the addition of TBCP showing increased miscibility, interaction and interpenetration of TBCP with epoxy matrix at higher concentrations of TBCP.36 Similarly, the tan δ peak height decreases with increasing 12 ACS Paragon Plus Environment

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amount of TBCP due to the enhanced miscibility between the epoxy and TBCP.35, 37 Thus, it can be concluded that the TBCP chains get effectively interpenetrated into the crosslinked epoxy matrix, at the molecular level. Shape Memory Test (Fold-deploy Test) The shape memory properties were quantified by applying stress on the material by a widely accepted fold-deploy test. The shape retention and shape recovery ratios were calculated and are given in Table 3. It is seen from the table that the shape retention and shape recovery ratios of the blends are more than 93%, and are retained after repeated cycles. These results show the reliability of the prepared blends for the advanced applications. The shape retention and shape recovery ratio slightly decreases especially for the higher concentrated blends may be due to the decreased crosslink density of these blends. Table 3. The parameters obtained from Fold-deploy test Cycle

Retention

Recovery

Cycle

Retention

Recovery

No

Ratio (%)

Ratio (%)

No

Ratio (%)

Ratio (%)

1

97

97

1

97

97

0 wt%

2

97

97

5 wt%

2

97

97

TBCP

3

97

97

TBCP

3

97

97

4

97

97

4

97

97

5

97

97

5

97

97

Cycle

Retention

Recovery

Cycle

Retention

Recovery

No

Ratio (%)

Ratio (%)

No

Ratio (%)

Ratio (%)

1

97

97

1

97

97

10 wt%

2

97

97

15 wt%

2

97

97

TBCP

3

97

97

TBCP

3

97

97

4

97

97

4

97

97

5

97

97

5

97

97



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Cycle

Retention

Recovery

Cycle

Retention

Recovery

No

Ratio (%)

Ratio (%)

No

Ratio (%)

Ratio (%)

1

97

97

1

97

97

20 wt%

2

97

97

30 wt%

2

97

97

TBCP

3

97

97

TBCP

3

97

97

4

97

97

4

96

96

5

97

97

5

96

96

Cycle

Retention

Recovery

Cycle

Retention

Recovery

No

Ratio (%)

Ratio (%)

No

Ratio (%)

Ratio (%)

1

95

95

1

94

94

40 wt%

2

95

95

50 wt%

2

94

94

TBCP

3

95

95

TBCP

3

94

94

4

95

95

4

94

94

5

95

95

5

94

94

Heat, Deform, Cool

Heating

Permanent shape

Temporary shape 30 wt% TBCP modified epoxy

Figure 3. Photographs showing the shape memory effect in 30 wt% TBCP modified epoxy The shape memory property of the representative samples (30 wt% TBCP modified epoxy) is shown in Figure 3. The samples should be heated above the Tg (by dipping it in hot water at 100 °C), so that they become soft and flexible. The heated samples thus can be 14 ACS Paragon Plus Environment

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deformed into any temporary shapes through twisting and bending and upon cooling, the temporary shapes are fixed. The original shape can be restored by heating again above the Tg (by dipping the sample in hot water at 100 °C). It is worth noting that the recovered shape is indistinguishable from the permanent shape, which indicates that the modified epoxy thermosets exhibit excellent shape fixity and recovery. Furthermore, it is important to add that the modified epoxy thermosets are stable after repeated manual shape memory test. For all SMPs, there should be two basic requirements: firstly, net points or physical/chemical crosslinks. The physical/chemical crosslinks determine the permanent shape which could ensure the storage and release of the entropic energy that is responsible for shape fixing and recovery of SMP. Secondly, molecular switches responsible for shape memory cycle. Molecular switches correspond to the Tg or Tm and are known as shape memory transition temperature. In the present case, strong intermolecular hydrogen bonding interaction between TBCP and epoxy resin modifies the epoxy network. The reciprocal hydrogen bonding established between the polyether based triblock copolymer and epoxy based NH and OH groups renters the TBCP part into the epoxy network. This is also the reason for their miscibility and diminishes the apparent crosslink density. The schematic representation of the structure of epoxy/DDM and PPO-PEO-PPO modified epoxy/DDM matrix is shown in Figure 4. As mentioned earlier, the temporary shape is obtained by deforming the material by heating it above its Tg, and then cooling to below Tg. During this process, the hydrogen bond breaks and the chemical cross-links adapt to the external load via conformational rearrangements and are frozen when suddenly cooled to room temperature. When the sample is reheated to temperatures above the Tg, the polymer chains between the crosslinking points become more mobile and the strain energy stored during the



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deformation is released. Thus, the shape memory polymer regains its original (permanent) shape by maximizing the chain entropy.

Figure 4. Schematic representation of the structure of epoxy/DDM and PPO-PEO-PPO modified epoxy/DDM matrix. The extent of hydrogen bonding interaction between the TBCP and epoxy network can be understood from the FTIR spectra shown in Figure 5. Hydrogen bonds are formed by the interactions of OH and NH groups of epoxy resins with ether groups of miscible PEO blocks of TBCP. Hydrogen bonding leads to miscibility/compatibility between TBCP and cross-linked epoxy resin. The region ranging from 3100 to 3700 cm-1 is highlighted in the spectrum; this region represents the stretching vibrations of OH and NH groups of epoxy resin. Further, this region is characterized by a main peak at 3403 and a shoulder at 3550 cm-1. The main peak is due to the hydrogen bonded (self-associated) hydroxyl groups and the shoulder is due to free hydroxyl groups generated during the epoxy-amine reaction.35,

37

The shoulder gradually

disappears with the addition of TBCP. This is because free hydroxyl groups generated during the 16 ACS Paragon Plus Environment

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epoxy-amine reaction may form hydrogen bonds with the ether group of PEO blocks. The intermolecular hydrogen bonds between the epoxy resin and ether group of PEO blocks imparts compatibility between TBCP and epoxy matrix. On the other hand, peaks due to the hydrogen bonded hydroxyl groups are shifted to lower frequencies with better intensity indicating that hydroxyl groups of epoxy network are stretched due to hydrogen bonding with ether group of PEO blocks of TBCP (Table 4). This red shift confirms that the hydrogen bonding between the hydroxyl groups of epoxy and ether group of miscible PEO blocks of TBCP is stronger than the self-associated hydrogen bonding in cured epoxy resin. Hence, TBCP modifies the epoxy network by becoming a part of the epoxy network and induces the shape memory effect at low temperatures.38

Neat epoxy 10 wt% TBCP 20 wt% TBCP 30 wt% TBCP 40 wt% TBCP 50 wt% TBCP

1.0

0.8

ATR Units

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


0.6

0.4

0.2

3369 3550 3403

0.0 3500

3250

1500

1250

1000

-1

Wavenumber (cm )

Figure 5. FTIR spectra of TBCP modified epoxy blends



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Table 4. The shift in wave number corresponding to hydrogen bonded hydroxyl groups of epoxy as a function of TBCP content in the blends Wt % of TBCP

0

10

20

30

40

50

Wavenumber (cm-1)

3403

3369

3377

3367

3369

3392

Shift (cm-1)

Nil

-26

-36

-34

-11

-34

Impact and tensile properties of TBCP modified epoxy blends are shown in Table 5. Among type 1 blends, 5 wt% TBCP modified epoxy system increases the impact strength of epoxy thermoset by ca.36 %, without appreciably affecting other mechanical properties. Among type 2 blends, the one with 30 wt% of TBCP possesses good mechanical properties. Type 3 blend, although exhibits considerably higher tensile elongation compared to other blends, shows inferior mechanical performance, typical of an uncross-linked elastomer. Table 5. Impact and tensile properties of TBCP modified epoxy blends. TBCP

Impact

Tensile

Tensile

Tensile

Tensile

Content

strength

strength

elongation

modulus

toughness

(wt %)

(Jm-3)

(MPa)

(%)

(MPa)

(Jm-3 x104)

0 wt%

25.3 ± 14

66.7 ± 5

10.2 ± 1.7

2080 ± 67

391 ± 30

5 wt%

34.3 ± 9.8

64.8 ± 6.3

10.3 ± 1.8

1845 ± 126.7

341.5 ± 113

10 wt%

19.6 ± 3.9

54.3 ± 3.9

7.8 ± 1.2

1961 ± 133.6

233.6 ± 65.5

15 wt%

18.8 ± 9.6

53.9 ± 4.6

8.4 ± 1

1881 ± 222

241.7 ± 54.6

20 wt%

20.0 ± 6.5

56.3 ± 5.9

9.1 ± 0.9

1807 ± 331.7

257.7 ± 46.6

30 wt%

27.9 ± 9

30 ± 0.3

11.2 ± 1.6

1085 ± 85.7

242.3 ± 47.4

40 wt%

17.4 ± 8.7

11.7 ± 1.1

16.9 ± 4.4

402 ± 86.9

154.4 ± 58.8

50 wt%

15 ± 4.2

1.5 ± 0.4

23.8 ± 3.3

9.4 ± 0.3

21.7 ± 3.6


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Composition dependence of properties of epoxy/TBCP systems can be related to their morphologies as observed by AFM (Figure 6) and SEM (Figure 7). Figure 6 shows the representative phase morphologies of each type of blend. Figure 6a shows the matrix/droplet type phase morphology of type 1 blend, in which spherical TBCP domains are dispersed in epoxy matrix. It is important to mention that PPO-PEO-PPO is an amphiphilic block copolymer, in which PEO blocks are miscible with epoxy resin, while PPO blocks are immiscible. Once epoxy resin starts reacting with the amine curing agent, the molecular weight of the epoxy resin increases. This decreases the combinatorial entropy contribution towards free energy, which in turn triggers the partial phase separation of miscible PEO blocks. On the other hand, the immiscible PPO blocks self-assemble into nanodomains in the uncured resin, the nanodomains will be fixed during the curing process while the phase separated PEO blocks may form an interphase between the cured epoxy resin and PPO nanodomains. One of the peculiar properties of this type of morphology is the improved interfacial adhesion, a better stress transfer may occur between the matrix and dispersed phase, which leads to enhanced impact strength. But there should be an optimum dispersed phase concentration for maximum toughness, which is in the present case about 5 wt%. The SEM micrograph of fracture surface of the 5 wt% TBCP modified blend is given in Figure 7a. No major cracks are observed for 5 wt% modified samples. AFM images given in Figure 6b shows that type 2 blends with 40 wt% of TBCP still retains the matrix/droplet type morphology, but with much larger dispersed domains. The greater extent of hydrogen bonding interaction between the polymer components and larger TBCP dispersed domains make the system soft and flexible enough to achieve adequate level of segmental mobility to decrease the Tg to around 65°C to ensure shape memory effect below 100°C. The difference in morphology



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can be supported by the SEM micrograph of the fracture surface given in Figure 7b, where parabolic marking are observed all over the fracture surface. It is interesting to note that 50 wt% TBCP modified epoxy system exhibits entirely different co-continuous phase morphology, very similar to a thermoplastic elastomer, as seen in Figures 6c and 7c. Thus, it can be concluded that the change in blend composition leads to blends with wide range of properties such as hardtough, hard-flexible and soft-flexible epoxy systems, which depend on the type of phase morphology.

(a) 5 wt% TBCP modified epoxy thermoset

(b) 40 wt% TBCP modified epoxy thermoset


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(c) 50 wt% TBCP modified epoxy thermoset Figure 6. AFM images of TBCP modified epoxy thermosets

(a) 5 wt% TBCP modified epoxy thermoset

(b) 40 wt% TBCP modified epoxy thermoset



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(c) 50 wt% TBCP modified epoxy thermoset Figure 7. SEM micrographs showing fracture surfaces of TBCP modified epoxy thermosets Further, the UV absorption spectra of TBCP modified epoxy blends given in Figure 8 revealed that all the blends can effectively filter ultraviolet (UV) light, which is very harmful to human health, and can act as UV shield. Epoxy blends have higher shielding efficiency than the neat epoxy, such that the blends can absorb UV light below 400 nm. On the other hand, addition of 50 wt% of TBCP improves the absorption further to visible range up to 500 nm and may be used to develop materials to screen blue-violet light, between 380 and 440 nm, that can cause damage to the retina. It is important to add that the optical transparency is retained in all the blends.


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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

0 200

400

600

800

Wavelength (nm)

Figure 8. UV absorption spectra of TBCP modified epoxy thermosets Thermal expansion is an important parameter for technical applications of any type of material. The thermal expansion behavior of TBCP modified epoxy thermosets was studied by TMA. Figure 9 shows the change in dimension of TBCP modified epoxy systems as a function of temperature. For all the blends, the change in dimension increases with temperature. This increase in dimensional change is related with molecular vibrations. Near to Tg some relaxation occurs and the slope of the dimensional changes is higher in the flexible rubbery state than in the stiff glassy state. The dimensional stability is highest for neat epoxy system; the dimensional stability slowly decreases by the addition of TBCP and is lowest for 50 wt% TBCP modified epoxy system, due to its co-continuous phase structure with a continuous TBCP phase. Overall, the thermal expansion coefficient of epoxy can be controlled by mixing with TBCP to desired values according to the requirements of the intended application.




3.5

3.0

Dimension Change (%)

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2.5

2.0 1.5

1.0

0.5

0.0 -50

0

50

100

150

o

Temperature ( C)

Figure 9. The plot of dimensional change vs. temperature for TBCP modified epoxy blends Finally, the TGA thermograms of the TBCP modified epoxy systems given in Figures 10 a-b show that the thermal stability decreases by the addition of TBCP. Note that the modified blends follow single step degradation and are stable up to 250 °C. The important parameters obtained from thermograms are given in Table 6. The initial decomposition temperature (IDT) is unaffected by the addition of up to 10 wt% of TBCP, but it decreases by ca. 100 °C by the addition of 50 wt% of the TBCP. The maximum decomposition temperature (MDT) remains almost same in all the blends, while the decrease in the final decomposition temperature (FDT) is not significant.


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Page 25 of 33

2.5


100

2.0

o

Deriv.Weight (%/ C)

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

1.5

1.0

0.5

0.0

0 100

200

300

400

500

600

100

700

200

300

400

500

600

700

o

Temperature ( C)

o

Temperature ( C)

Figure 10. TGA thermograms of TBCP modified epoxy blends Table 6. Parameters obtained from the TGA thermograms Sample

IDT(°C)

MDT(°C)

FDT(°C)

Neat epoxy

359

379

423

5 wt% TBCP

359

379

418

10 wt% TBCP

358

379

417

15 wt% TBCP

350

380

414

20 wt% TBCP

315

380

411

30wt% TBCP

2 75

380

409

40wt% TBCP

246

378

407

50wt% TBCP

256

385

407

Conclusion Epoxy/DDM/TBCP blends having different blend compositions were prepared and characterized. All the blends exhibited shape memory properties. It is found that blend composition has a crucial role on the final properties. Three types of shape memory polymers 25 ACS Paragon Plus Environment


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viz. stiff-tough, intermediate and soft-flexible epoxy systems were obtained. The blend with 5 to 20 wt% of TBCP is stiff exhibiting shape memory properties at around 150 °C, while that with 50 wt% of TBCP was elastomeric. However, the blends with 30 and 40 wt % of TBCP showed shape memory effect below 100 °C. Addition of TBCP decreased the Tg of epoxy thermosets and hence shape memory properties could be effectively tuned. These shape memory epoxy polymers (SMEPs) have high and low temperature cycle effect. In fact all the prepared blends shows very high shape retention and shape recovery ratios (more than 93 %) irrespective of the repeated cyclic experiments. Further, they were thermally stable, stable to ultra violet radiations and hence are suitable for developing materials for outdoor applications. Since TBCP was used as the modifier, the SMEPs were nanostructured with good impact strength. They can withstand high number of recovery cycles and can be used under extreme conditions (high temperature and stress). Moreover, we can easily tune the Tg and stiffness of the epoxy thermoset by changing the concentration of the TBCP. 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 (contract grant number IFA-CH-16). SS acknowledges the King Mongkut’s University of Technology North Bangkok (KMUTNB-61-KNOW-003.). JP thanks Prof. C. P. Reghunadhan Nair for fruitful discussions.


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(10) Zhang, H. J.; Han, D. H.; Yan, Q.; Fortin, D.; Xia, H. S.; Zhao, Y. Light-Healable Hard Hydrogels through Photothermally Induced Melting−Crystallization Phase Transition. J. Mater. Chem. A 2014, 2, 13373-13379. (11) Lendlein, A.; Jiang, H.; Junger, O.; Langer, R. Light-induced shape-memory polymers. Nature 2005, 434, 879-882. (12) Wang, W.; Lu, H.; Liu, Y.; Leng, J. Sodium dodecyl sulfate/epoxy composite: waterinduced shape memory effect and its mechanism. J. Mater. Chem. A 2014, 2, 5441-5449. (13) Li, Y.; 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. (14) Xiao, Y.; Zhou, S.; 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. (15) Wang, W.; Liu, D.; Liu, Y.; Leng, J.; Bhattacharyya, D. Electrical Actuation Properties of Reduced Graphene Oxide Paper/Epoxy-Based Shape Memory Composites. Compos. Sci. Technol. 2015, 106, 20-24. (16) Razzaq, M. Y.; Behl, M.; Lendlein, A. Magnetic Memory Effect of Nanocomposites. Adv. Funct. Mater. 2012, 22, 184-191. (17) Parameswaranpillai, J.; Hameed, N.; Pionteck, J.; Woo E.M. (eds.), Handbook of Epoxy Blends, Springer International Publishing AG, DOI: 10.1007/978-3-319-18158-5 (18) Kumar, K.S.S.; Biju, R.; Nair, C.P. R. Progress in shape memory epoxy resins. React. Funct. Polym. 2013, 73, 421- 430


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Supporting Information The blends of epoxy resin with 50 wt% TBCP are soft and flexible like an elastomer, this is because the softening of these samples starts below the room temperature. The 50 wt% TBCP modified epoxy system exhibits co-continuous phase morphology. These blends can show shape memory effect at low temperatures with good fixity and shape recovery ratio more than 93 %.


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Photographs showing the shape memory effect in 40 wt% TBCP modified epoxy system 271x120mm (300 x 300 DPI)

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PPO-PEO-PPO triblock copolymer

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