Morphology development and mechanical properties

Research Article Received: 27 November 2016

Revised: 28 December 2016

Accepted article published: 9 February 2017

Published online in Wiley Online Library: 13 March 2017

(wileyonlinelibrary.com) DOI 10.1002/pi.5343

Morphology development and mechanical properties of unsaturated polyester resin containing nanodiamonds Mohammadjafar Hashemi and Akbar Shojaei* Abstract Unsaturated polyester/styrene (UP) resin was filled with nanodiamonds (NDs) containing carboxyl and methacrylate functionalities using mechanical mixing. Field emission SEM exhibited a uniform dispersion of tightly bound aggregates of nanosized spherical NDs with good interfacial interaction. Rheological measurements exhibited a step increment in the shear viscosity of a UP/ND suspension at 0.6 wt% ND resembling a percolation state at this loading. Shear viscosity data supported by dynamic mechanical analysis results suggested the development of effective ND particles in which ND aggregates were covered by only polyester macromolecules. Accordingly, the morphology of UP/ND composites approached a quasi-percolation state at 0.6 wt% in which effective ND particles were connected thoroughly, instead of direct ND−ND contact, forming a co-continuous polyester phase covering the ND particles. Based on such morphology, DSC and Fourier transform infrared analysis suggested the development of heterogeneous microgels in cured UP resin containing NDs which in turn governed the overall mechanical properties of the composites. © 2017 Society of Chemical Industry Keywords: curing; morphology; nanodiamond; unsaturated polyester resin

INTRODUCTION

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Unsaturated polyester (UP) resin is a class of thermosetting polymers which is widely used in various industrial applications such as marine, automotive, coatings, storage tanks, piping and construction.1 UP resins are also commercially attractive matrix materials for producing fiber reinforced composites, particularly with glass fiber.2 Such widespread applications of UP resins are due to the flexible processing characteristics, good dimensional and thermal stability, high chemical resistance and comparatively low price. These characteristics have made UP resin the most popular among thermosetting resins with almost 80 wt% of the global market of all thermosetting resins.3 – 5 Depending on the monomers used during the polymerization, UP resin can be either orthophthalic (general purpose) or isophthalic.6 In general, UP resins typically contain 30–40 wt% vinyl monomers like styrene, known as the reactive diluent, to control the viscosity of the resin while taking part in the final crosslinked structure during the curing process.7 Despite many desirable characteristics of UP resins, their limited stiffness and strength compared to other thermosetting resins, in particular epoxy resins,8 have restricted their applications for high performance services. To overcome such deficiencies, the mechanical properties of UP resins are traditionally improved by incorporating suitable fibrous or particulate reinforcements with various geometries. However, reinforcement by nanoparticles has become promising in the field of polymer composite materials. This is due to the fact that nanoparticles offer a large specific surface area and exceptional mechanical and thermal properties with multifunctional features compared to conventional particulate or fibrous reinforcements. Polym Int 2017; 66: 950–959

Silicate layers or clays (either natural or organically modified ones) have been extensively investigated in various polymeric matrices (thermosetting, thermoplastics and rubbers) to improve the mechanical, thermal and barrier properties. To get greater improvement in the properties and obtain nanocomposites with multifunctional features, carbon based nanoparticles such as carbon nanotubes (CNTs) and graphene oxide (GO) have attracted the attention of researchers in both academia and industry. However, to attain successful nanocomposites with the full potential benefit of nanoparticles, they should be dispersed in a nanoscale state which is often a challenging task in the field of polymeric nanocomposites because of the strong interparticle tendency between the nanoparticles. The role of nanoparticles with various shapes and properties like CNTs,9 – 13 GO,14 – 16 clay,17 – 19 carbon black,20 nanoalumina,21 nano calcium carbonate22 and nano titanium23 on the performance of UP resins has been investigated. In all cases the optimum improvement in mechanical properties can be achieved by a fine dispersion of nanoparticles providing the maximum interfacial area. Depending on the nanoparticles used, such goals can be achieved by using a suitable mixing process and/or surface modification of the nanoparticles.

∗

Correspondence to: A Shojaei, Department of Chemical and Petroleum Engineering, Sharif University of Technology, PO Box 11155–9465, Tehran, Iran. E-mail: [email protected] Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

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© 2017 Society of Chemical Industry

Unsaturated polyester resin containing nanodiamonds

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Scheme 1. Sequence of reactions used for the synthesis of ND-HEMA.

Recent studies have shown that nanodiamonds (NDs) produced by a detonation process can be a suitable alternative for expensive carbon based materials in improving the mechanical and physical properties of polymers.24,25 This can be due to many interesting features of NDs such as a rich surface chemistry with various oxygen containing groups, high mechanical properties, thermal conductivity and good biocompatibility.26,27 However, like many nanoparticles, the achievement of a uniform dispersion of NDs and development of a strong UP − ND interfacial interaction can be of prime concern in ND filled UP resin. In the present work, we describe the preparation and properties of UP resin filled with NDs at low concentrations (less than 1 wt%). To achieve uniform dispersion and develop improved interfacial interaction, NDs with two different surface functional groups, i.e. the carboxylic group (COOH) and 2-hydroxyethyl methacrylate (HEMA), were examined in the present investigation.

EXPERIMENTAL Materials Orthophthalic UP resin containing 33 wt% styrene (UP2500) was obtained from ARAD SHIMI Co., Tehran, Iran. Hereafter, UP will be used as an abbreviation for polyester plus styrene monomer mixture in this study. Methyl ethyl ketone peroxide as initiator and cobalt naphthenate containing 6 wt% cobalt as promoter were purchased from Aldrich (Steinheim, Germany) and used as-received. NDs were supplied from NaBond Technologies Co., China. According to the manufacturer, NDs were prepared by a detonation process with an average particle size of 4–6 nm, a purity of 98% − 99%, a density of 3.05 − 3.3 g cm−3 and a specific surface area of 282 m2 g−1 . 2-hydroxy ethyl methacrylate (2-HEMA) was used for functionalization of the NDs. Thionyl chloride and tetrahydrofuran with a purity of 99.99% were obtained from Merck and used without purification. Functionalization of ND As-received NDs were first oxidized in atmospheric conditions at 425 ∘ C for 1.5 h, as in the literature.28 Based on this thermal oxidation process, the carboxylic content of as-received NDs increased from 0.3 mmol g−1 to 1.7 mmol g−1 .29 Therefore, thermally oxidized NDs were considered as carboxylic group functionalized NDs, referred to as ND-COOH in this investigation. To obtain HEMA functionalized NDs (ND-HEMA), ND-COOH was first activated by thionyl chloride to obtain intermediate acyl chloride grafted ND (ND-COCl), and then ND-COCl was grafted by HEMA using the esterification reaction. Further details for the synthesis of ND-HEMA can be found elsewhere.25 Scheme 1 summarizes the steps used to modify the exterior surface of NDs with HEMA.

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Characterization The morphology of UP resin and UP/ND composites was investigated by field emission SEM (FE-SEM) using a model Mira III, TeScan, Czech Republic. In order to obtain a smooth surface, the samples were fractured in cryogenic conditions (liquid nitrogen flask). Also the surfaces of the samples were coated with platinum before FE-SEM analysis. The tensile tests were carried out at room temperature with a HIWA 2126 universal testing machine from Hiwa Eng. Co., Iran, according to ASTM D 638–10. The crosshead speed was 5 mm min−1 and the machine was equipped with an extensiometer. Five samples of each category were examined for tensile testing and the average data were reported. Dynamic mechanical analysis (DMA) was performed using a Tritec 2000 dynamic mechanical analyzer in single point bending on samples with dimensions of 2 × 6 × 18 mm3 at a fixed frequency of 1 Hz with an amplitude of 0.01 mm over the temperature range −50 to 250 ∘ C with a temperature ramp of 5 ∘ C min−1 . DSC analysis was performed on the uncured samples to explore the role of the NDs on the curing behavior of the UP resin, using a differential scanning calorimeter (Q-100, TA Instruments, USA) under a nitrogen atmosphere at a heating rate of 10 ∘ C min−1 . Steady shear experiments were performed using a Physica MCR 301, Anton Paar Inc., with parallel plates to analyze the rheological behavior and obtain further insight into the morphology of the UP/ND suspension. The rheological characterization was performed on neat UP and UP/ND suspension samples without curing agents at ambient temperature. Fourier transform infrared (FTIR) spectroscopy analysis with a resolution of 2 cm−1 in the transmission mode was carried out using a Bruker (Tensor 27 IR) spectrophotometer on a KBr pellet.

RESULTS AND DISCUSSION Morphology Figure 1 displays FE-SEM microphotographs of the fracture surface of the neat cured UP and UP/ND composites at low and high magnifications. Neat UP resin shows a smooth fracture surface suggesting brittle failure for this resin. As shown in Fig. 1(B), UP/ND-COOH(0.1) exhibits a smooth surface containing a few ND aggregates (see the white circles in Fig. 1(B)) with an average


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Preparation of UP/ND composites Composites containing ND-COOH and ND-HEMA at various concentrations of 0.1, 0.3 and 0.6 wt% were prepared by mechanical mixing coupled with ultrasonication. To do this, the desired

amount of NDs was first incorporated into the UP resin using mechanical mixing at 700 rpm for 20 min. Subsequently, the mixture was sonicated for 15 min at room temperature followed by further mechanical mixing at 1000 rpm for 90 min at ambient temperature. Finally, methyl ethyl ketone peroxide initiator (1.5 wt% by weight of resin) and cobalt naphthanate accelerator (0.1 wt%) were added to the mixture to activate the curing process. Afterwards, the mixture was poured into silicone molds that were made according to standard ASTM D638-10, degassed under vacuum and allowed to cure at room temperature. Then, the samples were post cured at 80 ∘ C for 3 h. The post-cured samples were analyzed after resting at room temperature for at least 2 days.

www.soci.org Low magnification

High magnification

M Hashemi, A Shojaei interaction with the particles, because no detached particles and no cavity formation can be observed inside and around the clusters. In the case of ND-COOH, the clusters are converted to bigger agglomerates by increasing the ND-COOH up to 0.6 wt% with a maximum diameter of almost 10 μm. Meanwhile, the fractured surface becomes rougher at 0.6 wt% concentration suggesting the relative toughness of UP/ND-COOH(0.6) which can be associated with a decrease of the crosslinking density of the resin, as explained in the literature as well.31 – 33 The role of ND loading on the crosslinking density of UP will be discussed later based on DMA data. Comparing the FE-SEM microphotographs of ND-HEMA and ND-COOH at the same loading (0.3 wt%), it is inferred that smaller clusters and a more uniform distribution is achieved by employing HEMA functionalization on the NDs. In summary, it can be concluded that good interfacial interaction is achieved between the UP resin and both ND-COOH and ND-HEMA; however, in the case of ND-HEMA, a finer and more uniform distribution is obtained. Steady shear rheology Rheological measurements were carried out to obtain a further understanding of the structure of the NDs and the dispersion morphology of the UP/ND suspensions. The shear viscosity results for UP and UP/ND suspensions are depicted in Fig. 2. As can be seen, the viscosity of neat UP resin shows a very slight reduction with shear rate; however, it can be considered as a Newtonian fluid over the frequencies studied. By adding NDs in the form of both ND-COOH and ND-HEMA, the viscosity − shear rate behavior is almost retained; however, an increase in viscosity in particular at low shear rates is observed. The enhancement in the viscosity at low shear rate with ND loading and the independence of the viscosity with shear rate for UP/ND suspensions can be associated with the strong interfacial interaction between UP and NDs and the development of a stable microstructure that does not undergo alteration at high shear rates. This behavior is in accordance with the literature for amine functionalized multiwalled CNTs incorporated in UP at low concentrations.9 According to the viscosity increment inferred from the rheological data, it can be concluded that both carboxyl and HEMA functional groups can interact appropriately with UP resin, as was corroborated by FE-SEM as well. The stability of the microstructure at high shear rates can be explained by the fact that direct particle − particle interaction has not been developed at the compositions studied in this work and that NDs are spherical particles which are inherently not able to deform at high shear rates. To obtain an insight into the morphology of the UP/ND suspension based on the rheological data, the Krieger − Dougherty model,34 which is one of the most popular viscosity models for suspensions, was used. The model is as follows:

Figure 1. FE-SEM microphotographs of (A) neat UP resin, (B) UP/ ND-COOH(0.1), (C) UP/ND-COOH(0.3), (D) UP/ ND-COOH(0.6) and (E) UP/ND-HEMA(0.3) at low (left) and high (right) magnifications.

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diameter of less than 1 μm. These aggregates contain a number of nanosized spherical particles of ND-COOH with an average diameter of ca 10 nm (see the relevant micrograph with high magnification) contacted tightly together, which is in agreement with our data examined by TEM25 and rheological measurements.30 It appears that UP resin infiltrates into the clusters very well, wets ND-COOH particles appropriately and establishes good interfacial


𝜂s = 𝜂m

( 1−

𝜑eff 𝜑m

)−2.5𝜑m (1)

where 𝜂 s and 𝜂 m are the viscosity of suspension and matrix, respectively, 𝜑m is the maximum packing fraction of particles which is considered to be 0.63 in this study35 and 𝜑eff is the effective volume fraction of NDs. The effective volume fraction is the actual volume of aggregates/agglomerates of NDs plus the volume of tightly adhered matrix molecules covering the NDs per whole volume of the suspension. The effective volume of particles can be approximated by an equivalent ellipsoid which contains ND particles and matrix molecules.36 Accordingly, the


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(a)

(b)

10

6

UP/ND-COOH 0.1 UP/ND-COOH 0.6

Viscosity [Pa·s]

UP UP/ND-COOH 0.3 UP/ND-HEMA 0.3

ND-COOH ND-HEMA φa=0.65 φa=0.75 φa=0.95

ηs/ηm

4

2

0

1 0.1

1 10 Shear Rate [1/s]

100

0

0.1 0.2 Particle Content (Vol.%)

0.3

Figure 2. (a) Shear viscosity of neat UP resin and UP suspensions with ND-COOH and ND-HEMA at room temperature as a function of shear rate and (b) relative shear viscosity of different composites versus actual volume percentage of NDs. The solid lines are the fitting curves by the Krieger − Dougherty model for various 𝜑a .

(a)

(b) 1.00E+10

0.8 UP UP/ND-COOH 0.1 UP/ND-COOH 0.3 UP/ND-COOH 0.6 UP/ND-HEMA 0.3

1.00E+09 E'(pa)

Tan δ

0.6

UP UP/ND-COOH 0.1 UP/ND-COOH 0.3 UP/ND-COOH 0.6 UP/ND-HEMA 0.3

0.4

1.00E+08 0.2

0

1.00E+07 0.0

50.0 100.0 150.0 Temperature (°C)

200.0

0.0

50.0 100.0 150.0 Temperature (°C)

200.0

E'(pa)

(c)

3E+09 0.0

20.0 40.0 Temperature (°C)

Figure 3. DMA data for neat UP, UP/ND-COOH and UP/ND-HEMA: (a) dissipation factor (tan 𝛿); (b) storage modulus; (c) magnified section of the storage modulus at low temperatures.

local volume fraction of ND particles in hypothetical ellipsoidal aggregates/agglomerates (𝜑a ) can be correlated with the actual volume fraction of particles, i.e. 𝜑 and 𝜑eff , as follows:36 𝜑a =

𝜑 𝜑eff

(2)

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Dynamic mechanical analysis DMA is a suitable technique to get a deep insight into the microstructure of polymers and polymer composites. The tan 𝛿 versus temperature curve of the samples obtained from DMA experiments is shown in Fig. 3. For neat cured UP resin, a prominent peak is observed at 90.8 ∘ C (see Table 1 and Fig. 3(a)) followed by



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Indeed, 𝜑a reflects the number of ND primary particles in the equivalent ellipsoid and can be an indication of the aggregate/agglomerate structure. For instance, 𝜑a = 1 shows no matrix molecules adhering tightly on the surface of NDs and consequently the effective volume fraction of NDs (𝜑eff ) is equal to the actual volume fraction (𝜑). Contrarily, a considerably lower value of𝜑a (less than 1) shows that the nanoparticles are able to adsorb many matrix molecules and therefore the effective nanoparticles contain more tightly adhered molecules. Figure 2(b) displays the dependence of the normalized viscosity of the suspension (ratio of the suspension viscosity to the neat resin viscosity) on the actual volume percentage of NDs with

various 𝜑a values calculated on the basis of Eqns (1) and (2). It is obvious that 𝜑a at low ND concentrations, i.e. 𝜑 ≤ 0.3 wt%, is completely different from the value at 0.6 wt%. At low concentrations, 𝜑a is close to 0.95 demonstrating highly compacted ND particles with minor (adhered) resin in the hypothetical effective agglomerates. At 0.6 wt% loading, 𝜑a is considerably lower (almost 0.65) suggesting the development of a higher amount of adhered polymers in this case which seems to be somewhat strange. However, such a low 𝜑a value and significantly higher viscosity of UP/ND-COOH(0.6) will be discussed in the next sections based on the distinctive morphology developed at this concentration.

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Table 1. Physical properties of the cured UP and UP/ND composites obtained by DMA Samples UP UP/ND-COOH(0.1) UP/ND-COOH(0.3) UP/ND-HEMA(0.3) UP/ND-COOH(0.6)

T g (∘ C)

𝜐e (mol m−3 )

90.8 95.2 87.4 86.8 63.6

Mc (g mol−1 )

2433 2558 2025 1462 1332

460 438 553 766 841

a small shoulder at 36.4 ∘ C. As presented by Tanaka37 and further proved by Cook and Delatychi,38 the prominent peak, representing the primary relaxation of cured UP resin called 𝛼 relaxation, reflects the glass transition temperature of the whole crosslinked structure which indeed contains polystyrene bridges.39 Moreover, the minor peak which is an indication of the secondary relaxation referred to as 𝛽 relaxation is associated with the segmental motion of phthalate groups of the polyester backbone which are far from the crosslink points.38 It should be noted that the 𝛼 relaxation temperature is greatly dependent on the degree of crosslinking of the UP which is governed by the styrene content. In other words, on increasing the crosslinking density caused by the increasing styrene content, the 𝛼 relaxation temperature shifts to higher temperatures.7,38 Contrarily, the 𝛽 relaxation temperature stays unchanged on varying the crosslinking density; however, such secondary relaxation appears at certain styrene contents ranging between 20 and 70 wt%.38 Below 20 wt% concentration of styrene where the crosslinking density is sufficiently low, the segmental motion of the polyester backbone and that of the whole crosslinked structure are cooperative leading to a single peak in tan 𝛿 curve at low temperatures. It is clear that the relaxation peaks of the UP/ND-COOH composite are greatly dominated by the concentration of ND-COOH. For UP/ND-COOH(0.1), both 𝛼 and 𝛽 relaxation temperatures are still observed, similar to the cured UP resin. However, in the UP composite containing 0.3 wt% ND-COOH, the peak relevant to 𝛼 relaxation becomes broader with a slight increment in its height. The 𝛽 relaxation is still distinguishable in this composite; however, it is superimposed by an 𝛼 relaxation peak forming a

(a)

M Hashemi, A Shojaei skewed shape. The tan 𝛿 curve for the UP composite containing 0.6 wt% ND-COOH exhibits totally different behavior to the composites with lower concentrations of ND-COOH. It demonstrates a single relaxation behavior at 63.6 ∘ C which almost corresponds to the 𝛽 relaxation peak of neat cured UP with a significantly broadened peak with much increased height. This behavior for UP/ND-COOH(0.6) is similar to the relaxation behavior of cured UP resin having very low crosslinking density obtained at quite low styrene concentrations, as reported in the literature.38 According to the tan 𝛿 versus temperature data, it is deduced that incorporation of ND-COOH at concentrations ≥ 0.3 wt% disturbs the network structure of the UP resin during curing resulting in lower crosslinking density. However, this effect is much more pronounced at ND-COOH concentrations of 0.6 wt%. The role of ND-COOH in the crosslink structure of the UP resin can be associated with the polarity of the ND surface and in particular its carboxyl group which could cause further immiscibility of the styrene and polyester molecules. This explanation is consistent with the literature that the hydroxyl and carboxyl termination groups of the polyester chain have an unfavorable effect on the miscibility of the polyester and styrene solution.7,40 In fact, the ND-COOH particles are able to interact only with polyester molecules through the hydrogen bonding between the ester group and carboxyl group of ND-COOH. Accordingly, it can be assumed that microdomains consisting of ND particles wetted and covered by only polyester molecules (interphase) which are surrounded by a mixture of polyester/styrene (see Fig. 4(a)) are formed in the UP/ND suspensions. Therefore, the crosslinking is developed inside and outside the microdomains in the course of the curing process. Accordingly, the network structure is assumed to consist of polyester − polyester crosslinks in microdomains and polyester molecules crosslinked with polystyrene bridges out of the microdomains. Such a morphological model can be representative for UP/ND-COOH(0.3), because the broader tan 𝛿 peak of this sample suggests a diversity of crosslink structures. Even though microdomains are also available for UP/ND-COOH(0.1), such morphological features cannot dominate the general performance of the composite due to minor concentrations of microdomains in this case.

(b)

Styrene molecule

Neat polyester molecule

Nanodiamond

Unsaturated polyester shell covering the ND

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Figure 4. (a) Morphological model for a UP/ND suspension containing microdomains. Microdomains consist of an ND core surrounded by only polyester molecules (shell). (b) Interconnected microdomains at high ND concentrations.



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By increasing the ND concentration (upward of 0.3 wt%), the presented morphology approaches interconnected microdomains (see Fig. 4(b)). Therefore, an apparent percolation is achieved in which the microdomains are connected together instead of the real particles. Indeed, in this morphological model (Fig. 4(b)), there exist co-continuous polymer phases including neat polyester molecules surrounding the ND-COOH and polyester/styrene mixture. A significant enhancement of the low shear viscosity of the UP/ND-COOH(0.6) suspension and its very low 𝜑a value obtained by rheological data support such a morphological feature. The step change of the rheological behavior at the percolation state in polymer nanocomposites has already been reported in the literature.41 Figure 3 shows that the tan 𝛿 curve of UP/ND-HEMA(0.3) is very similar to that of UP/ND-COOH(0.3) suggesting that these two composites have similar morphological features. However, the intensity of the 𝛽 relaxation peak of UP/ND-HEMA(0.3) is larger than that of UP/ND-COOH(0.3). As 𝛽 relaxation is associated with the motion of segments of the polyester backbone far from crosslink points, the higher intensity of the 𝛽 relaxation peak of UP/ND-HEMA(0.3) can be associated with its lower overall crosslinking density. Figures 3(b) and 3(c) display the storage modulus curves for all the samples. The effect of ND loading on the storage modulus of UP resin is different below and above the glass transition temperature (T g , corresponding to the 𝛼 relaxation temperature). Below T g where the macromolecule stays in the glassy state, the storage modulus increases steadily on incorporating ND-COOH and increasing its content up to 0.6 wt%. In this situation, the storage modulus of neat cured UP resin increases from 3.9 GPa at 10 ∘ C to 5.2 GPa proportionally with ND loadings up to 0.6 wt% at the same temperature. It is also inferred that ND-HEMA has better reinforcing efficiency with respect to ND-COOH at 0.3 wt% loading. These observations lead us to conclude that in the glassy state the reinforcing efficiency depends on the nanoparticle content, the state of dispersion and the degree of interfacial interaction. In the rubbery state (above T g ), UP/ND-COOH(0.6) shows a minimum storage modulus compared with other samples, despite its maximum storage modulus in the glassy state. It appears that the storage modulus of the samples in the rubbery state is mainly dominated by the crosslinking density. To evaluate this idea, the crosslinking density of the samples was estimated based on rubber elasticity theory using the relation42 𝜐e =

Er 3RTr

(3)

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Cure behavior analysis The influence of NDs on the cure behavior of the UP resin was investigated using dynamic DSC runs performed on uncured samples from room temperature to 250 ∘ C. Figure 5 shows the DSC thermograms and the cure rate (d𝛼/dt, s−1 ) versus the degree of conversion (𝛼) obtained from the DSC thermograms. The degree of conversion and the cure rate can be calculated as follows: 45 t

𝛼=

1 dH dt ΔH0 ∫0 dt

1 dH d𝛼 = dt ΔH0 dt

(4)

(5)

where ΔH0 and H stand for the total heat of reaction and the heat of reaction at a given time (temperature), respectively. It should be noted that the temperature scanning rate 𝛽 = dT/dt used in this study was 10 ∘ C min−1 . The DSC thermograms show a single main exothermic peak for all the samples. However, the peak position as well as the total heat of reaction and the reaction rate are significantly dependent on the ND content. Consistent with the DMA data, this observation suggests that NDs alter the cure reaction mechanism which more probably originates from the alteration of the microstructure of the UP/ND suspension. By incorporating 0.1 wt% ND-COOH, the general feature of the DSC peak of neat UP (without NDs) is almost retained, with a small shift of the reaction peak (temperature corresponding to maximum peak temperature) to lower temperatures and no effect on the reaction rate but a significant increment in the heat of reaction (see Table 2 and Fig. 5). This cure characteristic is similar to clay filled UP resin explored by Xu and Lee3 who attributed such behavior to the role of nanoparticles in facilitating the initiation of radicals at an early stage of the curing process. Such behavior in our case can be explained by the adsorption of initiator on the surface of ND-COOH due to the polarity of the initiator. The presence of carboxyl groups of NDs more probably facilitates initiator decomposition in the system. On increasing the ND-COOH content to 0.3 wt%, the cure initiation temperature (T i ) and peak temperature (T p ) move to considerably higher temperatures, the cure rate decreases and the



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where 𝜐e is the crosslinking density, E r is the storage modulus at the reference temperature T r and R is the universal gas constant. Normally, the reference temperature is considered as T r = T g + 40 (∘ C).42 Equation (3) has been successfully used to estimate the crosslinking density of thermosetting polymers42,43 and thermosetting nanocomposites.44 Table 1 presents 𝜐e and Mc (average molecular weight between two adjacent crosslink points in the polyester backbone which is determined by Mc = 𝜌/𝜐c ). It can be inferred that UP/ND-COOH(0.6) shows significantly lower crosslinking density as explained based on the tan 𝛿 data and corroborated by the fracture surface roughness using FE-SEM (Fig. 1(D)). As can be seen, Mc ranges between 450 and 850 g mol−1 which is plausible based on the average molecular weight of the polyester molecules used in this study (1580 g mol−1 according to the manufacturer).

Interestingly, as shown in Fig. 3(b), the storage modulus of UP/ND-COOH(0.6) increases steadily on increasing the temperature above 120 ∘ C in the rubbery state which is not the case for the other samples studied in this investigation. This behavior can be explained by the evolution of the crosslinking density of UP/ND-COOH(0.6) at higher temperatures in the course of the DMA experiment.44 As all the samples experienced a similar post-curing process at 80 ∘ C before DMA testing, it can be concluded that the apparent percolation morphology postulated in Fig. 4(b) retards the curing reaction of UP/ND-COOH(0.6) while it can be accelerated at higher temperatures. This observation led us to conclude that, to obtain fully crosslinked UP/ND composites with higher ND concentrations, post curing should be performed in multi-stage heating up to temperatures much above T g . Consistent with the 𝛽 relaxation behavior, it is deduced (Table 1) that the crosslinking density of UP/ND-HEMA obtained based on Eqn (3) is lower than that of UP/ND-COOH(0.3). However, like UP/ND-COOH(0.3) it remains unchanged on increasing the temperature to 200 ∘ C, as the corresponding storage modulus in the rubbery state is almost retained (Fig. 3(b)). A possible mechanism for such difference will be discussed in a later section.

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(a)

M Hashemi, A Shojaei

(b)

1000

0.006 UP UP/ND-COOH 0.3 UP/ND-COOH 0.6 UP/ND-HEMA 0.3

200

-200

3

0.005 Reaction Rate (s-1)

Heat Flow (mW.g-1)

UP/ND-COOH UP/NDP COOH 0.1

600

0.004 0.003 UP UP/ND-COOH 0.1 UP/ND-COOH 0.3 UP/ND-COOH 0.6 UP/ND-HEMA 0.3

0.002 0.001

-600

0 0 Temperature (°C)

0.5 Degree of Cure

1

Figure 5. (a) DSC thermograms at 10 ∘ C min−1 dynamic scan; (b) reaction rate versus conversion.

Table 2. Cure behavior of UP and UP/ND composites characterized by DSC thermograms Samples UP UP/ND-COOH(0.1) UP/ND-COOH(0.3) UP/ND-COOH(0.6) UP/ND-HEMA(0.3)

T i (∘ C)

T p (∘ C)

T f (∘ C)

61.54 58.74 65.09 87.9 67.99

92.59 90.96 97.43 115.48 101.52

142 140 152.4 159.56 158.23

ΔH (J g−1 ) 221.3 268.7 222.8 207.6 229.1

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total heat of reaction exhibits a relative decrease with respect to UP/ND-COOH(0.1). These characteristics indicate that at this concentration of ND-COOH, i.e. 0.3 wt%, the cure reaction has been retarded, the cure rate decreased and the overall degree of conversion lowered in the presence of ND-COOH. Such behavior can be explained based on the morphological model presented in Fig. 4 and the fact that curing of the polyester/styrene mixture consists in the formation and development of microgels.46,47 In fact, the microgels are started in the early stage of the curing reaction through the cyclization and intramolecular crosslinking between vinylene groups of polyester molecules. However, the final crosslinked structure of the UP resin is obtained via inter-microgel crosslinking mainly by styrene monomers. According to the morphological model presented in Fig. 4, the interphase region contains polyester molecules without the styrene monomer due to the polarity of the ND surfaces. This causes a relative increment in the styrene concentration and consequently a decrease in the polyester/styrene ratio beyond the interphase region. As the microgel morphology and the kinetics of the polyester/styrene copolymerization are essentially dependent on the polyester/styrene ratio,47,48 the observed kinetics behavior for UP/ND-COOH(0.3) is expected. Basically, for the interphase region without styrene monomers, microgels are formed by intramolecular crosslinking of UP molecules as usual. However, inter-microgel crosslinking in the interphase region, forming the final network structure, is more probably performed by polymerization of vinylene groups of the polyester at the surface of the microgels. Consequently, a highly overlapped and severely entangled microgel particle morphology, flake-like, is achieved47 in the interphase region. Therefore, it is anticipated that the reactivity of the polyester vinylene groups decreases significantly due to the reduction in segmental mobility of polyester molecules originating from


interaction with the ND surface and because of steric hindrance of large polyester molecules.46 These characteristics of the interphase region result in retardation of the curing reaction, starting at higher temperatures, and many unreacted vinylene groups. Therefore, the interphase region contains concentrated microgel particles with low crosslinking density. Beyond the interphase region where the styrene/polyester ratio has increased due to the expelling of styrene monomer from the interphase region, a dumbbell shaped microgel morphology in which microgel particles are connected by long polystyrene chains can be anticipated.46 Indeed, homopolymerization of styrene is more probable due to a higher styrene monomer concentration and the microgel concentration is low outside the interphase region. Therefore, the network structure in this region contains long polystyrene crosslinks but with low crosslinking density due to the lower concentration of microgel particles. Basically, the activation energy of the styrene − styrene vinylene reaction (homopolymerization) (70 kJ mol−1 )49 is higher than that of the styrene − polyester vinylene reaction (25–35 kJ mol−1 ),50,51 showing that the styrene − polyester vinylene reaction is more favorable.47,51,52 Therefore, it is plausible that the curing reaction governed by homopolymerization of styrene shifts to higher temperature and the reaction rate decreases with respect to UP resin without NDs. Based on the cure mechanism of ND-COOH filled UP resin governed by the microstructure of UP/ND-COOH, as mentioned above, a shift of the reaction temperature to higher values and a reduction in the overall crosslinking density of UP/ND-COOH(0.3) can be anticipated. For UP/ND-COOH(0.6) in which a quasi-percolation structure was assumed (see Fig. 4), the cure kinetics mechanisms presented for UP/ND-COOH(0.3) based on the heterogeneous microgel formation model are intensified, leading to significantly higher T i and T p (retardation effect), as given in Table 2. The distinctive cure characteristics of UP/ND-COOH(0.6) are that the heat of reaction decreases significantly and the reaction rate increases sensibly compared with UP/ND-COOH(0.3) (see Fig. 5). The lower heat of reaction is associated with a lower degree of reaction in this composite which is in line with the lower crosslinking density estimated by DMA. In fact, the continuous interphase region containing severely entangled microgels provides an environment with many unreacted carbon double bonds. Additionally, in such a highly packed morphology, the termination reaction is significantly retarded which in turn causes the concentration of radicals to increase leading to a higher reaction rate.


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Figure 6. FTIR spectra of UP and UP/ND-COOH(0.6).

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Figure 7. Stress–strain curves of (a) UP/ND-COOH and (b) UP/ND-HEMA composites.

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cure temperature to higher values, increases the reaction rate but provides a slightly higher heat of reaction. As the HEMA functional group contains carbon double bonds, it can contribute to the crosslinking reaction in the interphase region which can lead to a slightly higher total heat of reaction. Accordingly, it is plausible to assume that the interphase region in UP/ND-HEMA is stronger than that of UP/ND-COOH. The relatively higher T p of UP/ND-HEMA(0.3) compared with UP/ND-COOH(0.3) can be attributed to its higher styrene/polyester ratio promoting longer homopolystyrene with a relatively lower overall crosslinking density, as discussed above. This explanation of the lower crosslinking density is consistent with the DMA data mentioned above. The higher styrene/polyester ratio for UP/ND-HEMA(0.3) can be associated with the finer dispersion of ND-HEMA which provides a greater overall interphase region for this composite.

Tensile properties Figure 7 displays the stress–strain curves for UP resin and its composites with ND-COOH and ND-HEMA at all concentrations of 0.1, 0.3 and 0.6 wt%. As can be observed, the tensile behavior is improved with ND loading up to 0.3 wt%. However, at 0.6 wt% concentration for both ND-HEMA and ND-COOH the tensile behavior has decreased dramatically with a slight increment in



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The FTIR spectra of the cured UP and UP/ND-COOH(0.6) are compared in Fig. 6 to investigate the relative content of unreacted carbon double bonds in each sample. The absorption peak at 980 cm−1 is relevant to the C − H out-of-plane bending in polyester C = C bonds whose intensity reflects the unreacted carbon double bonds in the samples.48 Thus, the normalized absorption peak at 980 cm−1 (A980 ), which is the ratio of the absorption peak area at 980 cm−1 (A980 ) to the peak area at 1730 cm−1 (A1730 , which is relevant to the carbonyl group C = O remaining unchanged during curing),48 was calculated to be 0.014 and 0.034 for UP and UP/ND-COOH(0.6), respectively. This indicates that the unreacted C = C bonds in UP/COOH(0.6) are much greater than in UP resin. It should be noted that microgel formation governed by the morphological characteristics of the UP/ND-COOH mixture is also valid for UP/ND-COOH(0.1). However, the concentration of ND-COOH is more probably too low to alter the cure kinetics governed by the heterogeneous microgel formation in UP composites mentioned above. Consequently, the ND-COOH(0.1) effect on the cure behavior is limited to its role in the initiation stage of the cure reaction as mentioned above. The cure behavior of UP/ND-HEMA(0.3) is compared with that of UP/ND-COOH(0.3) in Fig. 5. Overall, the general feature of the DSC thermogram and the cure rate curve for both composites is the same, suggesting that the cure mechanism for NDs with both functional groups is similar. However, ND-HEMA shifts the

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Figure 8. (a) Tensile stress and (b) Young’s modulus of UP/ND-COOH and UP/ND-HEMA samples.

elongation at break. This behavior is interestingly consistent with the DMA and DSC data. The tensile strength and Young’s modulus extracted from the stress–strain curves are shown in Fig. 8. For the UP/ND-COOH composite series, the maximum improvement in tensile modulus (16% increment) and tensile strength (13%) is observed at 0.1 wt%. However, the maximum improvement in tensile strength and tensile modulus for UP/ND-HEMA is obtained at a higher concentration, i.e. 0.3 wt%. Moreover, while the extent of the improvement in tensile strength for ND-HEMA is similar to that of ND-COOH, the tensile modulus for this nanoparticle (ND-HEMA) is considerably greater (55% increase). It appears that such a difference in the elastic modulus of ND-HEMA filled UP can be attributed to the finer state of dispersion and the formation of a stronger interphase, as shown by FE-SEM, DMA and DSC data. The extent of improvement obtained in this study is consistent with the UP/nanofillers reported in the literature. Thanh et al.18 reported an enhancement in tensile strength (12%) and elastic modulus (56%) in the presence of nanoclay at concentrations above 1 wt%. The improvement in tensile properties of UP with CNTs was reported to be marginal.11 However, amine functionalization of CNTs9 and using pre-dispersed CNTs10 improved the tensile properties of UP resin up to ca 20 at 0.5 wt% loading. Liu et al.53 prepared in situ polymerized unsaturated polyester resin in the presence of ethylene glycol functionalized GO and observed an ca 50% improvement in tensile properties compared to neat resin at 0.08 wt% loading. Almost the same improvement was reported for UP filled with graphene nanosheets.14 It is found that the tensile strength and elastic modulus decreased considerably at 0.6 wt% concentration for both ND-COOH and ND-HEMA, while the elongation at break shows a relative increase. This behavior can be explained by the morphological model presented in Fig. 4 in which a continuous interphase region with low crosslinking density is developed. Lower crosslinking density actually results in higher segmental mobility of molecules leading to higher elongation at break.

CONCLUSIONS

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The microstructure of UP/ND composites was investigated and discussed in depth based on FE-SEM, steady shear viscosity, DMA and DSC analyses. It was observed that both ND-COOH and ND-HEMA were dispersed in the UP matrix as tightly bound aggregates and interacted appropriately with the UP matrix. Incorporation of NDs in the UP resin enhanced the shear viscosity of the suspension proportionally with the filler loading, with a sharp step increment


at 0.6 wt% resembling a percolation state in this composition of the UP/ND suspension. The tan 𝛿 curve and storage modulus obtained from DMA analysis exhibited distinctive features at various ND concentrations of 0.1 wt%, 0.3 wt% and 0.6 wt%. At 0.6 wt% ND loading, the tan 𝛿 peak height increased and the degree of crosslinking decreased considerably. Such behavior was explained using a morphological model consisting of effective ND aggregates in which the ND aggregates were covered by only polyester macromolecules. At 0.6 wt% ND loading, the effective ND aggregates were connected together, instead of direct contact of the ND aggregates, forming a quasi-percolation state. In such a case, a co-continuous styrene-free polyester phase covering the ND aggregates was formed within the continuous polyester/styrene matrix. The co-continuous styrene-free polyester phase was a low segmental mobility region and contained many unreacted carbon double bonds after curing and post curing. It was deduced that such unreacted carbon double bonds could be reactivated by post curing at temperatures much higher than the T g of the UP resin. DSC analysis revealed that the cure behavior of UP resin was altered significantly on incorporation of NDs and their concentration. At 0.1 wt% loading, the effect of the NDs on the promotion of initiation of radicals dominated the overall cure characteristics of the UP resin, while at higher concentrations, i.e. 0.3 wt% and 0.6 wt%, the heterogeneous morphology mentioned above controlled the cure behavior. It was also speculated that the microgels formed in the UP/ND composites were heterogeneous as well. In the co-continuous styrene-free polyester phase severely entangled microgels were formed while a dumbbell shaped microgel morphology was obtained in the polyester/styrene matrix in which microgel particles were connected by long polystyrene chains. The tensile properties of the UP/ND composites were closely correlated with the morphological model described in this paper.

ACKNOWLEDGEMENTS The financial support provided by the Sharif University of Technology and Iran Nanotechnology Initiative Council for this research is gratefully acknowledged.

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