Synthesis of Polyurethane Acrylate Oligomers Using Microwave Irradiation Energy as Aqueous Binder for Textile Printing

Abstract

Polyurethane acrylate oligomer has been synthetized using microwave irradiation as a green chemistry and synthetized using thermal heating for comparison. Using microwave irradiation, it was possible to either synthesize polyurethane acrylate oligomers without catalyst and/or solvent or achieved at a record time representing 1/12 of the reaction time needed for normal thermal heating. Polyurethane acrylate oligomers synthesized using microwave irradiation possess enhanced thermal stability than the thermal heating synthesized one. The crystallinity percentages of microwave-synthesized polyurethanes are higher than the thermal heating-synthesized polymer. Several experimental measurements applied to the samples like X-ray diffraction (XRD), IR spectra, and transmission electron Microscopy (TEM) etc. The overall morphology of the synthesis of polyurethane acrylate oligomers using microwave irradiation was investigated by TEM, which indicated regular, ordered, and homogeneous polymers within nanosized particle distribution. The disappearance of isocyanate bands on IR charts are strong evidence for the success of the preparation processes for polyurethane acrylate oligomers by all used methods.

1. Introduction

The use of microwave irradiation as a source of energy in chemical reactions with its magnificent properties, such as enhancing the rate of reaction, decreasing the reaction time, and reducing or eliminating the reaction auxiliary, shows the most important features of microwave-assisted chemistry. On the other hand, the application of microwave irradiation in polymerization reactions is a good alternative to thermal heating with improved environmental scope [1, 2, 3, 4, 5]. Under controlled reaction conditions and catalyst concentration, one or two hydroxyl groups in the polypropylene glycol (PPG) and/or sorbitol react with the primary isocyanate group of isophorone diisocyanate (IPDI) to form an isocyanate – terminated polyurethane prepolymer leaving the second isocyanate group unreacted for subsequent reaction with hydroxyethyl acrylate (HEA) to introduce unsaturation sites at the ends of polyurethane prepolymers to form polyurethane acrylate. Isophorone diisocyanate (IPDI) is an unsymmetrical molecule and therefore has isocyanate groups with different reactivities. The primary isocyanate group is more reactive than the secondary one [6, 7].

Improvement of the properties of acrylated urethane may be achieved by using sorbitol (bio-based compound) as a chain extender to increase the ability of cross-linking and hydrogen bond formation producing higher molecule weight and more flexible isocyanate functional prepolymer, which is subsequently caped, by hydroxyl ethyl acrylate monomer. Polyurethane acrylate, based on aliphatic isocyanates (e.g., isophorone diisocyanate), is more flexible than aromatic urethane acrylates with the same functionality [8]. The main advantage of aliphatic polyurethane acrylates is the fact that they are virtually non-yellowing and therefore can be used for long-lasting applications, on white- or light-colored substrates [9]. The reaction rate enhancement occurs through increasing the rotation, friction, and collision of the molecules of the monomer having specific groups (NCO and OH) [10, 11]. Pigment fixation on textiles relies on a binding agent that requires a curing process to hold the pigments on a textile fabric. The binding agents are polymers or preferably copolymers of unsaturated monomers such as ethyl acrylate, butyl acrylate, styrene, acrylonitrile, vinyl acetate, and butadiene. However, pigment coloration has some industrial and ecological problems such as relatively high-temperature cure, stiff hand, poor crock fastness, formaldehyde emissions, and clogging nozzles and screens in both textile inkjet and screen printing processes. These disadvantages are related to the binding agent. Thus, to improve the quality of the textile pigmented colored goods, the overall properties of the binding agents should be improved [12, 13, 14, 15].

The present work was carried out with the following objectives synthesis of polyurethane acrylate oligomers as aqueous binder, by both of microwave’s irradiation and conventional thermal heating under comparable conditions, and it is utilized these binders for printed cotton fabrics using pigment color.

2. Materials and methods

2.1 Chemicals

Polypropylene glycol (PPG) (2000 g/mol) is supplied by Fluka chemical [Co. Switzerland)], Germany. Hydroxyethyl acrylate (HEA) is supplied by Degussa, Germany. N, N-dimethylacetamide (DMA) is supplied by ACROS Chemical Co. All chemicals are dried before being used. Dibutyltin dilaurate (DBTDL), isophorone diisocyanate (IPDI), and sorbitol were supplied by across Chemical Co, used as received.

2.2 Methods

2.2.1 Synthesis of polyurethanes acrylate (PUAmcs, PUAms, and PUAm) via MW irradiation

The reaction was carried out under microwave irradiation in a pulses multimode Milestone microwave reactor with a frequency of 2.45 GHz and maximum microwave power of 1200 W. Where PUAmcs is polyurethane acrylate synthesized with the aid of microwave irradiation using catalyst and solvent, PUAms is polyurethane acrylate synthesized with the aid of microwave irradiation without assistance of the catalyst, and PUAm is polyurethane acrylate synthesized with the aid of microwave irradiation in the absence of solvent and catalyst.

System setup: a 500-ml round-bottom flask with a magnetic stirrer sited on an electromagnetic plate located below the bottom of the microwave cavity, and a stirring bar was used to promote a high-speed magnetic stirring during the reaction. There were two joints for gas inlet and outlet to achieve continuous flow of nitrogen (nitrogen gas was pumped) into the vessel to provide inert reaction conditions and chemical injection (by mean of a syringe). The column was connected from the upper side to a refluxed condenser and a drying tube. Moreover, the system was supplied by infrared sensor directed toward the reaction flask for automatic measuring of the temperature change as simplified in Figure 1.

Figure 1.

Modified microwave reactor system.

Polyurethane acrylate is synthesized with the aid of microwave irradiation where the catalyst and solvent are involved during synthesis of PUAmcs, while polyurethane acrylate prepared without the assistance of the catalyst via microwave irradiation is symbolized as PUAms, and synthesis of polyurethane acrylate PUAm in the absence of solvent and catalyst has been done via microwave irradiation. The reactions were performed at a temperature range between 40 and 60°C by and microwave power between 100 and 250 W. A calculated amount of polypropylene glycol (PPG) (2000 g/mole) and sorbitol were added in N, N dimethylacetamide as solvent in a modified one-necked flask located inside Milestone microwave reactor as shown in Figure 1. The reaction mixture was left about 10 min at 60°C and 200 W to ensure complete mixing of the reaction mixture. The reaction temperature and microwave power were reduced again to 40°C and 100 W, respectively. A calculated amount of IPDI with and without 0.05 (w/w) DBTDL was slowly dropped into the reaction medium for 5 min. The reaction mixture was stirred for an additional 10 min at 60°C and 200 W to achieve an acceptable reaction rate without gelation [8]. The reaction temperature and microwave power were reduced to 40–50°C and 150 W, and a calculated amount of hydroxy ethyl acrylate (HEA) was gradually added to the reaction mixture. The reaction was complete without any further change in temperature with constant stirring for approx 10 min. This action allows the polypropylene glycol (PPG) (microwave inert) to absorb the microwave energy, and this step was continued for 5 min at 200 W (depending on the amount of PPG), i.e. the synthesis of PUAmcs. A calculated amount of IPDI without either DBTDL (i.e. without catalyst for synthesis of polyurethane acrylate (PUAms) or without solvent and catalyst for the synthesis of polyurethane acrylate (PUAm) was slowly dropped into the reactor at 40°C during a period of 5 min. The reaction mixture was stirred, and the reaction temperature was raised again to 60°C and 200 W for an additional 10 min. The mixture was allowed to react to generate a microwave reactive intermediate (urethane prepolymer), and then the silicon carbide bar was removed from the reaction medium. The end point was determined as the theoretical NCO value reached. After the end point has been reached, the reaction temperature and power are reduced to 50–40°C and 150 W, respectively. A calculated amount of HEA was gradually added to the reaction mixture, and the reaction mixture was left without any change in conditions with constant stirring for 4 min continuously to cap the terminal NCO groups.

2.2.2 Synthesis of polyurethane acrylate (PUAt) via conventional thermal heating

The reaction of polypropylene glycol with isophorone diisocyanate was conducted according to the modification of procedure described elsewhere [16, 17, 18, 19] as follows: A calculated amount of polypropylene glycol (PPG) (2000 g/mole) was added in DMA (solvent) into a three-necked flask equipped with a stirrer, thermometer, and reflux condenser under nitrogen atmosphere and heating oil bath. The reactant was left for about an hour at 40°C to ensure complete mixing of the reaction mixture. A calculated amount of IPDI containing 0.05 (w/w) DBTDL as catalyst was slowly dropped into the reaction medium at 40°C for over an hour. The reaction mixture was stirred for additional 2 h at 60°C to obtain an acceptable reaction rate without gelation. The mixture was allowed to react until the theoretical NCO content is reached. The end point of this step has been detected based on the NCO concentration which is determined by using a standard dibutylamine back-titration method [8]. The reaction temperature was reduced again to 45–50°C. A calculated amount of hydroxyethyl acrylate (HEA) was gradually added to the reaction mixture during an hour. After the addition of HEA, the reaction was complete without any further change in temperature with constant stirring for 2 h. Reduced reaction temperature was needed to avoid any possibility of thermal cross-linking on unsaturated sites in the system. There was a noticeable change in the viscosity of the reaction mixture during this part of the reaction. The PU prepolymer is viscous transparent liquid, with just the addition of HEA, its viscosity decreased sharply. Again, the viscosity of reactants starts to increase after a few minutes of addition. Therefore, at the end of reaction, a thick, viscous, and transparent liquid is obtained. Black bottles were used for saving PUA to avoid any possible photoreaction. Moreover, after the evaporation of the solvent, a white collected powder of polyurethane acrylate has been formed.

2.3 Measurements and analysis

2.3.1 X-ray diffraction analysis

Phase identification, purity, relative crystallinity, and crystallite size of the products were performed at room temperature by using a Philips diffract meter (PW 3710). The patterns were run with Ni-filtered copper radiation (λ = 1.5404 Å) at 30 kV and 10 mA with a scanning speed of 2θ = 2.5°/min0.

2.3.2 Infrared analysis

The infrared of the synthetized polyurethanes was measured using infrared spectrometer, Perkin Elmer system 2000 FT-IR (Fourier transform IR spectrometer). Single-beam spectrometer has a resolution of 2 cm⁻¹. The samples were ground with KBr (1:100 ratio) as a tablet and mounted on the sample holder in the cavity of the spectrometer.

2.3.3 Transmission electron microscopy (TEM)

Synthesized polyurethanes are mounted on aluminum stubs and sputter-coated with gold in a 150-Å sputter (Coated Edwards) and examined by Jeol (JXA-840A) Electron Probe Microanalysis (Japan), magnification range of 35–10,000, and accelerating voltage of 19 kV in order to confirm the presence of fragrance particles.

3. Results and discussion

3.1 Synthesized polyurethane acrylate (PUAmcs, PUAms, and PUAm) and (PUAt)

The completion of the reaction without the use of the catalyst is considered an example of green chemical reaction aimed to minimize or to eliminate the chemical auxiliary during the reaction addition to the nonuse of substances with a toxic nature such as DBTDL that has a bad impact on both public health and on the environment [20]. The properties of prepared polyurethane PUAm has been compared with the rest of all synthesized polyurethanes prepared either in the microwave’s reactors or by the conventional thermal heating (PUAt) as well explained in former characterization. The most important observation during the preparation of polyurethane acrylate PUAm is the greater increase in viscosity compared to all solvent involved reactions, especially in acrylation step. This may be due to that the presence of a solvent during the reaction is carried out quieting the monomers interaction which leads to reduce the viscosity of yields. However, it is clear that the operating of the reaction in microwave reactor without the presence of solvent accelerates the rates of reaction beside increases the viscosity, but the reaction for general is hard to control. This technique is useful for the production of dry polymers because of the difficulty of complete solvent removal in polyaddition polymerization [5]. On the contrary, of that, in conventional heating synthesis, the heat introduced in the reaction medium from the outside and the walls of the reaction vessel are generally the hottest parts of the reaction, especially during the initial ramp to the desired temperature. This causes nonhomogeneous temperature distribution though reactor wall, which results in dropping in reaction rate. Consequently, an increase in time is needed to achieve the desired reaction. Pulsed microwave heating is used to control temperature and eliminate the exothermic temperature peak from the fast exothermic reaction that will lead to maintain the same temperature distribution at all of the reaction stages [10]. The reaction rate acceleration using microwave’s irradiation technique, compared with common thermal heating under similar reaction conditions [21], can be determined by using relationship:

$$\mathit{\text{rate enhancement}}=\frac{\mathit{\text{conventional reaction time}}}{\mathit{\text{microwave reaction time}}}$$E1

By the application of this relationship to the reactions studied here, it was found that the synthesis of polyurethane acrylate inside microwave reactor accelerates the rate of reaction by 12 times compared to the conventional heating methods. In addition, the most important observation during the microwave’s synthesis is that the gelation effect is minimized to a higher degree compared to normal thermal heating. So, microwave’s irradiation can be used successfully in the synthesis of polyurethane acrylate instead of thermal heating. The suggested reactions for the prepared polyurethane acrylate can be represented in Figures 2 and 3.

Figure 2.

The synthesis of urethane prepolymer.

Figure 3.

The synthesis of polyurethane acrylate oligomer, where D is isophorone.

3.2 FT-IR spectral of synthesized polyurethane acrylate oligomers

Infrared spectra of the synthesized polyurethane acrylate oligomers are shown in Figure 4. During polyurethane synthesis, there are two major peaks used as a monitor for checking the reaction pathway. The reaction development is tested by the disappearance of NCO peak at around 2260 to 2270 cm⁻¹and appearing of ѵ-NH peak at around 3000 to 3400 cm⁻¹ on FT-IR spectra [8]. From the infrared spectrum of IPDI, there is a characteristic peak for ѵ–NCO group around 2100–2270 cm⁻¹ due to the presence of diisocyanate groups.

Figure 4.

FT-IR chart of synthesized polyurethane acrylate oligomers: (a) PUAt, (b) PUAmcs, (c) PUAms, (d) PUAm, and (e) isophorone diisocyanate.

It is clear from the IR charts of all synthesized polyurethane acrylates that there are no bands observed at approximately 2100–2270 cm⁻¹. This indicates that all entire amount of IPDI is completely consumed during the reaction, and the final products are free from free isocyanate groups [22, 23, 24]. By taking a general look on IR charts of the prepared polymers, we can conclude that all prepared polymers are characterized by strong absorption bands at 3300 cm⁻¹ attributed to ѵ-N▬H, bands around 1720 cm⁻¹ corresponding to ѵ-C〓O, absorption bands in range of 1640 cm⁻¹ for ѵ-C〓C, and absorption bands at approximately 1100 cm⁻¹ on fingerprint range of infrared. Spectral scale is corresponding to ѵ-C▬O▬C [25]. The appearance of all these peaks and the disappearance of the isocyanates bands on IR charts are strong evidence for the success of the preparation processes for those polymers by all used methods [25]. Therefore, the shift of FT-IR peak position to a lower frequency is an evidence of the existence of hydrogen bonding. The amount of the shift is a sign for the strength of the bonding [26]. Examination of the FT-IR spectra of prepared polyurethanes revealed that the position of ѵ-N▬H are to be at 3299 cm⁻¹ for PUAms, 3322 cm⁻¹ for PUAm, 3334 cm⁻¹ for PUAmcs, and 3361 cm⁻¹ for PUAt. Moreover, ѵ-C〓O are found to be at 1725 cm⁻¹ for PUAt, 1717 cm⁻¹ for PUAmcs, 1717 cm⁻¹for PUAms, and 1716 cm⁻¹ for PUAm. Principally, the appearance of ѵ-N-H peaks at lower wavenumber refers to the existence of hydrogen bonds and consequently to the existence of polymers phase separation [27].

The highest value of ѵ-N▬H shifting is for PUAms which refers to the highest phase separation degree occurred with PUAms polymer next value for, PUAmcs and finally PUAm between microwave polyurethane groups. In addition, the appearance of broad ѵ-N▬H bands introduces another proof about the exciting of N▬H groups in the bonded state. Moreover, ѵ-N▬H for PUAt is at 3361 cm⁻¹ which may refer to less degree of hydrogen bonding and consequently to a lower-phase separation manner. These results were supported by the shifting values in ѵ-C〓O; all microwave-synthesized polyurethanes show more shifting in values for ѵ-C〓O bands (1717 cm⁻¹) indicating the presence of C〓O in the bonding state which supported the presence of segment phase separation. While ѵ-C〓O for PUAt appeared at 1725 cm⁻¹ referring to a relatively less bonded one. From the aforementioned results, we can conclude that the presence of a solvent during the synthesis of polyurethanes under microwave irradiation develops the phase separation between polymer units. In addition, microwave irradiation polyurethane acrylates synthesis developed the hydrogen bond formation between the polymer segments which forces unites toward a more phase separation manner than thermal heating synthesis method.

3.3 Gel permeation chromatography (GPC) of polyurethane acrylate oligomers

Determination of polyurethane acrylate oligomers molecular weight is considered as an essential factor because it determines many physical properties. The data obtained from gel permeation chromatography (GPC) analysis of the prepared polyurethane acrylate oligomers are listed in Table 1.

Sample	Mn g/mol	Mw g/mol	PDI
1.1432e0	3.5093 e3	3.0696 e3	PUAt
1.1547e0	1.8894e4	1.6362 e4	PUAmcs
1.11907e0	1.7586e4	1.4769 e4	PUAms
1.3454 e0	1.3028 e4	9.6836 e3	PUAm

Table 1.

Gel permeation chromatography data of prepared polyurethane acrylates oligomers.

Where PUAt polyurethane acrylate was synthesized via conventional thermal heating;

PUAmcs polyurethane acrylate was synthesized with the aid of microwave irradiation using catalyst and solvent;

PUAms polyurethane acrylate was synthesized with the aid of microwave irradiation without assistance of the catalyst;

PUAm polyurethane acrylate was synthesized with the aid of microwave irradiation in the absence of solvent and catalyst.

From gel permeation chromatography data, it can be noted that, generally, Mw values for synthetized polyurethanes (PUAmcs, PUAms, and PUAm) using microwave are higher than that of the conventional thermally synthesized one (PUAt). These results agree very well with the previous reports. Microwave synthesis of polyurethane acrylate oligomers is producing higher molecular weight compared with thermal heating [5]. This may be due to that microwave irradiation is not depending on the thermal conductivity of the vessel materials. Thus, leading to a rapid raising in matter temperature directly related to the rotational motion of the molecules.

This action increases molecule mobility and therefore increases the reactivity of molecule functional groups, that is, OH and NCO groups. Consequently, this leads to enhance the reaction between functional groups [10]. Then, the interaction rate will increase, which results in higher molecular weight than that obtained from the thermal heating synthesis method. That increase in molecular weight of polyurethane acrylate oligomers is required, because it is well known that to obtain good mechanical properties as a molecular weight higher than 10,000 g/mole is needed [28].

From gel permeation chromatography data, it is clear that PUAmcs and PUAms have the highest Mw values in microwave PU group. While, lower value is corresponding to PUAm. This means that the presence of a solvent during microwave synthesis of PUA favored the formation of higher-molecular-weight polymer chains. This may be attributed to the action of a solvent during the polymerization process. The solvent enhances the chain growth of polymers instead of chain transfer resulting in increasing in the molecular weight of polymer as whole [29]. This is also may be due to the dependence of the microwave irradiation character on the properties of the solvent used [5].

In addition to that, by comparing gel permeation chromatography data of PUAmcs and PUAms in detail, it is evident that PUAmcs has slightly higher Mw value than PUAms, while PDI value of PUAms is better than PUAmcs. This can be explained as follows: the basic function of the catalyst during polyurethane synthesis is to catalyze chain extension in addition to developing several side reactions [8]. Based on this fact, the catalyst-free reaction powered by microwave irradiation may guide the reaction to develop in the desired direction (the formation of the main product), at the same time, reduce the side reactions to a minimum degree, and to improve the polymerization mechanism pathway (PDI value) as whole. PDI or polydispersity index is a value, which expresses the scale of nonuniformity during the polymerization reactions. PDI is used as a measure for the broadness of a molecular weight distribution of a polymer. The larger the PDI is, the broader the molecular weight will be. A monodispersed polymer, where all chain lengths are equal (such as a protein), has PDI = 1. The best-controlled synthetic polymers have PDI of 1.02 to 1.10. Chain reactions yield PDI values between 1.5 and 20. From GPC data, the PDI values for both microwave and thermal prepared polyurethanes have values between 1.11907 and 1.3454 which indicate that all reactions are carried out under uniform and homogenous mechanism. The best PDI value is 1.11907 for PUAms. This result proves that microwave irradiation synthesis of PUA is more ordered, homogenous, and uniform process than a thermal heating process [10].

3.4 Thermogravimetric analysis (TGA) of polyurethane acrylate oligomers

This analysis is important to be performed in finding out the suitable environment for the processing of materials. In addition, any undesired byproduct can be distinguished from TGA data. Figure 5 shows the thermogravimetric analysis (TGA) of synthesized polyurethane acrylate oligomers. It is seen that all synthesized PUA via microwave irradiation (PUAmcs, PUAms, and PUAm) show enhanced thermal stability than conventional thermal ones (PUAt).

Figure 5.

Thermogravimetric analysis (TGA) of synthesized polyurethane acrylates.

By comparing the values of the initial decomposition temperature of the synthesized samples T_i (the temperature corresponding to the decomposition process is started) and T_max (the temperature corresponding to major decomposition process), it is found that T_i and T_max values of all PUAm are higher than that of PUAt. Determination of kinetic parameters related to thermal decomposition is considered a key factor for understanding the thermal character of polyurethane under dynamic conditions [30]. Activation energy (E*) is considered as a semiquantitative factor describing the thermal stability of material, the higher E* value is, the greater the thermal stability of polymer. The calculated activation energy values for the synthesized PUA agreed well with the previous results.

The temperature variations corresponding to different percentages of weight loss during the process of thermal decomposition of the tested synthesized polyurethane acrylate oligomers are listed in Table 2.

Temperature (°C) Sample	Ti	Tmax	T20%	T30%	T40%	T50%
PUAt	344	397	352	370	379	386
PUAmcs	349	401	355	367	384	392
PUAms	361	406	371	386	394	400
PUAm	354	403	367	382	390	396

Table 2.

Thermogravimetric analytical data of synthesized polyurethane acrylate oligomers.

Where PUAt polyurethane acrylate was synthesized via conventional thermal heating;

PUAmcs polyurethane acrylate was synthesized with the aid of microwave irradiation using catalyst and solvent;

PUAms polyurethane acrylate was synthesized with the aid of microwave irradiation without assistance of the catalyst;

PUAm polyurethane acrylate was synthesized with the aid of microwave irradiation in the absence of solvent and catalyst.

The results show that the microwave PUA record improved thermal stability than PUAt. This is proved by recording higher decomposition temperatures corresponding to microwave PUA during degradation processes than that of PUAt.

From the results stated above, PUA synthesized using microwave irradiation recorded improved results than PUAt in all of T_i, T_max, activation energy, and the rest of the other kinetic parameters [30]. From these results, we can conclude that microwave irradiation synthetic method of polyurethane acrylates improves their thermal stability under all reaction conditions. Moreover, PUAms shows enhanced kinetics parameter results. So, PUAms can be considered as the highest thermally stable synthesized polymer.

3.5 Differential scanning calorimetry (DSC)

Thermal history of the synthesized polyurethane acrylate samples is studied by means of differential scanning calorimetry (DSC) analysis. In the temperature range below 100°C, DSC curves for the synthesized PUAS show two transitions. The first one is between 5.6 and 8.5°C. The second transition is around 57°C. These temperatures are corresponding to the glass transition temperature (Tg) and melting point temperature (Tm) of the soft segment, respectively. The appearance of Tg corresponding to soft segments in a separated manner enhances the presence of a suggested phase separation performance [31]. DSC curves are shown in Figure 6. The lowest Tg value is corresponding to PUAt. While, all microwave-synthesized PUAS show higher values.

Figure 6.

Differential scanning calorimetry of synthesized polyurethane acrylate oligomers.

This may be explained as follows: the value of Tg is mainly affected by segment mobility [32]. Therefore, as described above, all microwave-synthesized PUAS have higher ability for hydrogen bond formation between segments than PUAt. Thus, they exhibit higher phase separation manner than PUAt. In addition, it is known that phase separation restricts the chain mobility and hinders their movement [33]. This causes an increase in Tg values for all microwave-synthesized PUA compared to PUAt. From the above data, we can conclude that MW irradiation synthesis of polyurethane acrylate enhances Tg values of soft segments than thermal heating method.

3.6 X-ray diffraction

The crystallinity is a term that is referring to a high degree of atom arrangement (tightly packed, repeating, and regular structure). In the field of polymers, there is no crystalline polymer even with a highly ordered one. There must be some crystals disorder [7]. On the other hand, the brilliant physical properties of polyurethane polymer are due to its ability for the formation of what is called segments as reported before [7]. So, the polymer matrices containing both amorphous and crystalline domains are deployed in each other. This action plays a major role in the crystallization of polyurethane polymers. These bonds effectively link neighboring chains to build crystals which aid in reinforcing polyurethane unite and increase its stability [34]. Moreover, PU morphology and macroscopic properties are governed by the crystallinity of the phases, size of domains, and molecular composition. Both of the hydrogen bonding and the interaction among the units in the hard domains (Van der Waal’s force) control polyurethane crystallinity [35]. By calculating the crystallinity % of the synthesized PUA, the degree of phase separation can be estimated. All PUA polymers are scanned over the range of 2theta (θ) angle from 5 to 70 degrees. From Figure 7, which represents XRD spectra of the synthesized PUAS, we can note that all samples show a crystalline structure represented by the emergence of strong crystallinity peaks on XRD charts. Highly characteristic crystallinity peaks appear at around 2θ = 19.1, 23.2, 26.1, respectively [36].

Figure 7.

XRD chart of synthesized polyurethane acrylate oligomers.

In addition to that, a broad amorphous peak is observed on XRD charts. Crystallinity% has been calculated from the following equation [37]:

$$\%\text{Crystallinity}=\frac{\mathit{\text{Area under crystalline peaks}}}{\mathit{\text{Total Area under}}\mathit{all}\mathit{\text{peaks}}!}\times 100$$E2

The calculated crystallinity % shows that all samples possess good crystallinity degree. This gives a good evidence about the existence of hydrogen bonds between polymers units, and therefore, the presence of phase separation pattern. The crystallinity % values of microwave-synthesized polyurethane group are 45.25, 42.7, and 38.6 for PUAms, PUAmcs, and PUAm respectively. The highest crystallinity % is found for PUAms. This may be due to that it has the highest hydrogen bond formation ability among all synthesized PUAs as shown from FT-IR data. The next value is attributed to PUAmcs, and at the end of microwave PUA list PUAm is located. These results can be explained by the following: the presence of a solvent can develop the polyurethanes crystallinity during microwave irradiation synthesis through the enhancement of hydrogen bond formation between segments leading to a higher phase separation degree. Generally, the crystallinity % values for PUAm are higher than PUAt (32.5%). These results show a good agreement with FT-IR spectral data previously explained. From the above results, we can conclude that microwave irradiation synthesis of polyurethane acrylates develops hydrogen bond formation between polymer units leading to enriching the phase separation action, resulting in increasing the ability of the polymer molecules to be ordered in well-arranged manner. Therefore, the crystallinity percentage is increased than the thermal heating.

3.7 Transmission electron microscopy

By comparing TEM images of microwave-synthesized polyurethane acrylate oligomers (Figures 8b–8d) and that of the thermal synthesized one (PUAt) (Figure 8a), it is observed that all microwave-synthesized polyurethanes show smaller particle size, regular, and uniform particle structure with a narrower distribution compared to PUAt particles. In addition to this feature, all PUAm appear to be in a nanoscaled size with particles size in range between 37 and 17 nanometer.

Figure 8.

Transmission electron microscopy (TEM) images of (a) PUAt, (b) PUAmcs, (c) PUAms, and (d) PUAm.

From the above results, it is clear that the ability of microwave irradiation synthesis of PUA to yield nanoscale-uniform polymers particles; whether the reaction in MW reactor is carried out in all systematic reaction condition (PUAmcs) or not (PUAm, PUAm). It is evident also that the absence of the catalyst or even the solvent from the reaction conditions do not affect the brilliant character of synthesized PUA via microwave irradiation.

On the other hand, TEM image of PUAt shows larger range of particles size distribution, as the particles of PUAt appear to be out of nanoscale, where TEM image shows the spreading of all particles in the range between 118 and 184 nanometer. But all microwave-synthesized polyurethanes show a nanosized cubic crystal in the range between 35 and 39 nanometers. So, the synthesized polyurethane acrylate oligomers can be classified as nanostructured oligomers [11].

Due to the severe conditions in which the polymer was synthesized (under microwave irradiation without using catalyst and solvent), it shows an additional different trends in its TEM images. Tree-like shape and broad networks in addition to subsequent aggregation are observed [38]. This may be explained as follows: the absence of a solvent during the synthesis process results in a direct harsh interaction between monomer and molecules in the presence of microwave irradiation resulting in a concentrated branched polymer network.

Finally, from the above results, it could be concluded that the microwave irradiation may guide the reaction mechanism toward the producing of nanoscaled particles with uniform and homogeneous particle distribution during PUA synthesis.

4. Conclusion

From the results obtained from different analyses of the polyurethane acrylate oligomers synthesized by either microwave irradiation or thermal heating, it is possible to conclude the following: successful synthesis of polyurethane acrylate has been verified by FT-IR analytical tool under both thermal heating and microwave irradiation. Due to the ultra-properties of microwave irradiation, it was possible to synthesize polyurethane acrylate without catalyst and/or solvent. This represents an excellent environmental additive in polyurethane synthesis chemistry. The synthesis of polyurethanes using microwave irradiation was achieved in a record time representing 1/12 of the reaction time needed for the normal thermal heating. Phase separation manner develops when microwave irradiation was used as the energy source instead of the normal thermal heating method. Both the Mw and Mn of microwave-synthesized polyurethane oligomers recorded higher values compared to those obtained by the thermal heating process. In addition, the PDI values indicate that microwave irradiation syntheses of polyurethanes resulted in a more uniform and homogenous reaction mechanisms. Polyurethane acrylates synthesized by microwave irradiation possess enhanced thermal stability than the thermal heating synthesized one. The crystallinity percentages of microwave-synthesized polyurethanes are higher than the thermal heating-synthesized polymer. Regular, ordered, and homogeneous polymers within nanosize particle distribution are obtained because of the application of microwave irradiation in the synthesis of polyurethane acrylate.