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Effect of soft chain length and generation number on properties of flexible hyperbranched polyurethane acrylate and its UV-cured film
Progress in Organic Coatings 114 (2018) 216–222
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Effect of soft chain length and generation number on properties of flexible hyperbranched polyurethane acrylate and its UV-cured film
MARK
Hongping Xiang, Xiaowei Wang, Lu Xi, Haihui Dong, Peng Hong, Jiahui Su, Yanyan Cui, ⁎ Xiaoxuan Liu Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, Guangdong, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hyperbranched polymers Polyurethane acrylate Soft chain length Flexibility UV-curable film
A series of UV-curable flexible hyperbranched polyurethane acrylate (F-HBPUA) with various generation numbers and soft chains are successfully developed by modifying different generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH) with diverse soft chains contained semiadduct urethane monoacrylate. The influence of generation number and soft chain on the performances of F-HBPUA and its UV-cured freestanding film is studied. The degree of branching of F-HBPUA from the second to the fifth generation is proved to be 0.74, 0.79, 0.82 and 0.94, respectively. The glass transition temperature of F-HBPUA increases with increasing generation number but it decreases with the increase of soft chain length. The thermal stability of FHBPUA improveswith higher generations while the longer soft chain is only beneficial to the heat resistance at below 300 °C. As the increase of generation number and chain length, the UV-curing time of F-HBPUA film is significantly shortened from 12 s to 3 s, the flexibility of the UV-cured film is greatly increased from 4 mm to 1 mm, adhesion is improved to 3 grades. This novel flexible hyperbranched polyurethane acrylates can be applied to coatings, membranes and 3D print parts with excellent flexibility.
1. Introduction Hyperbranched polyurethane acrylates (HBPUA) not only exhibit the low viscosity, high solubility and abundant terminal functional groups of hyperbranched polymers (HBP) [1–7], but also possess the excellent photoreactivity, abrasive resistance and adhesion of polyurethane acrylates (PUA) [8]. They are mainly synthesized by modifying different hydroxyl-terminated hyperbranched polymers (HBPOH) with various semiadducts of urethane monoacrylate [9–15]. Additionally, as the hydroxyls in HBP are partially modified with anhydrides and successively neutralized with triethylamine, the HBPUA obtained become waterborne [16–20]. HBPUA have been widely applied in coating, adhesive, photoresist, printing ink and nanocomposite as modifier, crosslinker and other additives to reduce viscosity, accelerate reaction, improve mechanical properties and enhance toughness of matrix [9–24]. However, due to the nature of compactly packed periphery, high crosslinking density and lack of chain entanglements, HBPUA are usually too brittle to form freestanding films, and the mechanical performances of films are quite weak [25]. Several strategies, like grafting reactive polymer chains, modifying functional end-groups with short
⁎
flexible chain and copolymerizing with linear analogues [25–28], are taken to address the high brittleness and weak mechanical properties, but the flexibility of UV-cured freestanding films remains pretty poor (5–8 mm) yet [9,12,29]. In order to greatly reduce the mechanical brittleness and produce UV-curable flexible HBPUA freestanding films, different soft chains like polyethylene glycol (PEG) and isopentydiol (IPD) are firstly introduced into semiadduct urethane monoacrylate of IPDI (isophorone diisocyanate)-HEA (2-hydroxyethyl acrylate) to form flexible IPDI/PEG/HEA and IPDI/IPD/HEA arms, in this study. The flexible arms are then reacted with hydroxyl terminated hyperbranched polyurethane (HBPUOH, deemed as core) to produce UV-curable flexible hyperbranched polyurethane acrylates (F-HBPUA). The influence of different flexible chains on the properties of F-HBPUA and their UV-cured freestanding films are studied. Because of the significant impact of degree of branching (DB) on properties of HBP [3–5], F-HBPUA with different DB are also synthesized to investigate its influence on performances of FHBPUA and their UV-cured freestanding films.
Corresponding author. E-mail address: [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.porgcoat.2017.10.019 Received 24 July 2017; Received in revised form 19 October 2017; Accepted 21 October 2017 0300-9440/ © 2017 Published by Elsevier B.V.
Progress in Organic Coatings 114 (2018) 216–222
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was reacted at 70 °C for about 140 min until the peak at 2270 cm−1 for the NCO groups completely disappeared. Precipitated with ether, washed with acetone and vacuum dried, different generations of F-HBPUA (abbreviated as F-HBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5) obtained. To study the effect of flexible chain on properties of F-HBPUA, the semiadduct of IPDI/PEG600/HEA, IPDI/IPD/HEA and IPDI/HEA were reacted with the fourth generation of hydroxyl terminated hyperbranched polyurethane (HBPU-OH-4) to produce F-HBPUA-PEG600, FHBPUA-IPD and HBPUA, respectively.
2. Experimental section 2.1. Materials Polyethylene glycol (PEG, Mn = 200, 600), isophorone diisocyanate (IPDI), 2-hydroxyethyl acrylate (HEA), dimethylacetamide (DMA), dibutyltin dilaurate (DBTDL), diethanol amine (DEA), isopentyldiol (IPD), diethyl ether, hydroquinone monomethyl ether (MEHQ), dibutylamine and pyridine were purchased from Aladdin Reagent Co. (Shanghai, China) and used without further purification. 2.2. Synthesis of different generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH)
2.5. Preparation of UV-cured F-HBPUA films F-HBPUA were dissolved in anhydrous ethanol to prepare 85 wt.% resin solutions, and then mixed with 4 wt.% of Irgacure1173 used as the photoinitiator. The films were prepared by a doctor blade technique on cleaned tinplate with a typical thickness of ∼30 μm. After drying for 5 min at 60 °C under vacuum, UV-cured F-HBPUA films obtained by exposing to 365 nm UV light with a light intensity of 30 mW/cm−2.
IPDI (33.33 g, 0.15 mol) and DMA (33.33 g, 0.38 mol) were added to a 250 mL four – neck flask, equipped with a mechanical stirrer, nitrogen inlet, thermometer, and dropping funnel. As the mixtures were cool to 0 °C, DEA (15.77 g, 0.15 mol) and DMA (15.77 g, 0.18 mol) were added dropwise and reacted for 2 h. Then, the reaction temperature was gradually increased to 50 °C. The value of NCO was determined every 20 min. Excessive dibutylamine was added to terminate the reaction as soon as the value of NCO was equal to its theoretical value which was calculated based on its composition and structure and shown in Table 1. Different generations of HBPU-OH were synthesized by directly controlling reaction time, or indirectly controlling the value of NCO [29]. After precipitating with ether and vacuum drying, different generations of HBPU-OH (from the second to the fifth, abbreviated as HBPU-OH-2, HBPU-OH-3, HBPU-OH-4 and HBPU-OH-5) obtained. The synthesis process of HBPU-OH was shown in Scheme 1a.
2.6. Characterization FTIR spectrum was recorded on a Nicolet Magna 360 spectrometer by KBr disk method, scanned from 4000 cm−1 to 400 cm−1 with a resolution of 4 cm−1. 1H NMR and 13C NMR were performed on a AVANCE III HD 400 MHz spectroscopy, DMSO-d6 was used as the solvent. Pencil hardness of the UV-cured film was measured according to ASTM D3363-05 standard. Adhesion was tested in accordance with ASTM D3359-09 standard. The flexibility of films was determined according to ASTM D4338-97 standard. Intrinsic viscosity ([η]) tests were carried out with an Ubbelohde viscometer at 30 ± 0.1 °C, where the samples were dissolved to form 5 g/L DMF solution. Differential scanning calorimetry (DSC) analysis was measured by TA Instruments Q20 under N2 atmosphere with a heating rate of 10 °C/min from 25 °C to 150 °C. Thermal gravimetric analysis (TGA) was carried out on NETZSCH STA 499C in N2 atmosphere from 40 °C to 800 °C by 10 °C/ min. The gel content of the UV-cured film was estimated by measuring the weight variation after extracting with acetone in Soxhlet extractor for 36 h and dried at 60 °C under vacuum. Gel content could be calculated by Eq. (1).
2.3. Synthesis of different semiadduct urethane monoacrylate Different flexible semiadduct urethane monoacrylate (IPDI/ PEG200/HEA) was synthesized as Scheme 1b. IPDI (22.22 g, 0.10 mol) and DMA (22.22 g, 0.26 mol) were well-mixed, and then PEG200 (10 g, 0.05 mol) and DMA (10 g, 0.11 mol) was added dropwise. After that, the mixtures were reacted at 60 °C. As the value of NCO was close to its theoretical value, the temperature was decreased to 35 °C. HEA (7.55 g, 0.065 mol), DBTAL (0.037 wt.%), MEHQ (0.1 wt.%) and DMA (7.55 g, 0.09 mol) were dropped, and reacted at 35 °C for 2 h. When the value of NCO was close to its theoretical value, the reaction was terminated and IPDI/PEG200/HEA obtained. In addition, to investigate the influence of different flexible chains, PEG600 and isopentyldiol (IPD) contained semiadduct urethane monoacrylate, IPDI/PEG600/HEA and IPDI/IPD/HEA were also prepared following synthetic process of IPDI/PEG200/HEA. The semiadduct urethane monoacrylate without flexible chain (IPDI/HEA) was also synthesized, as control.
Gel content = Wv/We × 100
(1)
where Wv was the weight of the UV-cured film before extracting and We was the weight of the UV-cured film after extracting. 3. Result and discussion 3.1. Characterization of F-HBPUA
2.4. Synthesis of different flexible hyperbranched polyurethane acrylate
The molecular structures of different generations and soft chains contained F-HBPUA are firstly characterized by FTIR, and shown in 1 and S1 . As for HBPU-OH-4 in Fig. S1a, the broad peak at 3030–3740 cm−1 is mainly assigned to eOH, while the intense peaks at 3150–3700 cm−1 and 1540 cm−1 for all F-HBPUA and HBPUA should attribute to stretching vibration and bending vibration of eNH [30,31]. Obviously, there is no the characteristic peak of NCO group at 2270 cm−1 in all curves, which means NCO groups have been completely reacted. The peaks at 1720 cm−1 are ascribed to C]O of eNHCOO- resulted from the reaction of NCO and OH, while the peaks at 1630 cm−1 are assigned to C]O of −NHCON- caused by reaction between NCO and NH [14,32]. Compared with HBPU-OH-4, all F-HBPUA and HBPUA in Fig. 1 appear new peaks at 810 cm−1 attributed to eCH]CH2 of HEA [13,31]. The peaks at 1130 cm−1 attributed to CeO are greatly enhanced with the introduction of flexible chains, especially with PEG200 and PEG600. Due to lacking flexible chain, the CeO peak
As shown in Scheme 1c, different generations of HBPU-OH were severally mixed with IPDI/PEG200/HEA, where the molar ratio of OH in HBPU-OH and NCO in semiadduct urethane monoacrylate was 1:1. After adding DBTAL (0.05 wt.%) and MEHQ (0.1 wt.%), the mixture Table 1 The values of NCO and hydroxyl for the different generations of HBPU-OH [29]. Generations
Theoretical NCO value/%
Tested NCO value/%
Theoretical OH value/%
Tested OH value/%
Yield/%
Second Third Fourth Fifth
6.41–4.28 4.28–1.83 1.83–0.86 0.86–0.41
4.52 2.15 1.06 0.68
257.00–284.50 228.45–195.81 195.81–182.76 182.76–176.86
240.33 215.34 192.35 179.14
85 80 86 89
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Scheme 1. Schematic representation for the synthesis of F-HBPUA.
HBPUA and HBPUA-IPD without PEG segment, both curves at 3.51 ppm do not show the peak belonged to eCH2eCH2e of PEG. This also indicates the different semiadducts urethane monoacrylate have successfully reacted with various generations of HBPU-OH. 13 C NMR spectra of different generations and flexible chains contained F-HBPUA are presented in Fig. 3 and S3 . The resonance at 155.73 ∼ 157.17 ppm, 132.46 ppm, 69.36 ∼ 70.23 ppm and 63.33 ppm should be ascribed to eC]O of eNHCOOeand eNHCONe, eCH]CH2 of HEA, eCH2eCH2e of PEG and eCH2eCH2e of HEA, respectively. Clearly, HBPUA and F-HBPUA-IPD in Fig. 3b do not reveal the resonance at 69.36–70.23 ppm, due to the lack of PEG segments. HBPs synthesized from AB2 monomer mainly consist of three different types of repeating units, i.e. a dendritic unit (D), a linear unit (L) and a terminal unit (T) [4]. The degree of branching (DB) can be indirectly
of HBPUA does not be enlarged. Hence, it represents different semiadducts urethane monoacrylate have successfully reacted with various generations of HBPU-OH to generate different F-HBPUA. The 1H NMR spectra of different generations and flexible chains contained F-HBPUA are displayed in Fig. 2 and S2 . Overall, the peaks at 0.5 ∼ 1 ppm and 1.5 ∼ 4 ppm in Fig. S2 are mainly ascribed to eCH3 and eCH2-. In Fig. 2, the peak at 7.06 ∼ 7.23 ppm is attributed to NeH of −NHCOO- and −NHCON-. Compared with HBPU-OH-4, all FHBPUA and HBPUA appear new peaks at 5.93 ∼ 6.37 ppm, 4.17 ∼ 4.26 ppm and 3.51 ppm, which are distributed to eCH]CH2 in HEA, eCH2eCH2e in HEA and eCH2eCH2e in PEG [13], respectively. All F-HBPUA exhibit similar curves because their chemical structures are analogical only with different degree of branching, i.e. different generation numbers. As for different flexible chains in Fig. 2b, due to
Fig. 1. FTIR spectra of (a) different generations of FHBPUA and HBPU-OH-4, and (b) different flexible chains contained F-HBPUA.
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Fig. 2. 1H NMR spectra of (a) various generations of F-HBPUA and HBPU-OH-4, and (b) diverse flexible chains contained F-HBPUA.
estimated from the ratio of these units in culated according to Fréchet Eq. (2) [31].
DB =
D+T D+T+L
13
C NMR spectra, and cal-
(2)
The peaks at 62.23 ppm, 60.91 ppm and 60.67 ppm in Fig. 3 can be severally assigned to D, L and T unit of different F-HBPUA [32,33]. The DB of F-HBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5 in Fig. 3a are proved to be 0.74, 0.79, 0.82 and 0.94, respectively, which are increased with the increase of generation number. These results are similar with the reported DB of different generations of HBPU [29]. The DB of HBPUA, F-HBPUA-IPD, F-HBPUA-4 and F-HBPUA-PEG600 in Fig. 3b are 0.81, 0.80, 0.82 and 0.80, respectively, which are not significantly distinct from one another. This is because these different FHBPUA derive from the same generation of HBPU-OH-4 regarded as the core, only with different flexible arms on the surface.
Fig. 4. The [η] of all synthesized hyperbranched polyurethane acrylates.
3.2. Intrinsic viscosity of F-HBPUA
weights are also increased with the increase of flexible chain length, the [η] increases correspondingly.
The intrinsic viscosity ([η]) of polymers is deemed to one of the most important parameters as it is an estimation of hydrodynamic volume, intermolecular chain entanglement and inter/intramolecular interaction, which principally depends on the molecular weight of polymers. It can affect the flow ability, photopolymerization rate and properties of cured product [14,20]. As shown in Fig. 4, the [η] of FHBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5 are 13.72 mL g−1, 16.14 mL g−1, 19.92 mL g−1 and 24.19 mL g−1, respectively. With the increase of generation numbers, the molecular weight of F-HBPUA is increased, and the amounts of polar groups also are increased which enhance the intermolecular interaction [20], so the [η] of F-HBPUA is increased with increasing generation number. The [η] of HBPUA, FHBPUA-IPD, F-HBPUA-4 and F-HBPUA-PEG600 are 12.61 mL g−1, 15.67 mL g−1, 19.92 mL g−1 and 25.85 mL g−1, respectively. These FHBPUA are developed based on the same generation of HBPU-OH only grafting with different soft chains on the surface, so their molecular
3.3. Thermal properties of different F-HBPUA The glass-transition temperature (Tg) of different F-HBPUA are determined by DSC and represented in Fig. 5. Tg of F-HBPUA-2, F-HBPUA3, F-HBPUA-4 and F-HBPUA-5 in Fig. 5a are 83.7 °C, 87.5 °C, 89.3 °C and 93.3 °C, respectively. Apparently, Tg increases with the increase of generation numbers from the second to the fifth, because the molecular structures become more compact and rigid for higher generations, and thus the molecular mobility becomes more difficult [29,34]. Moreover, with the increase of generation numbers, the amounts of polar groups are increased, the intermolecular interaction is enhanced. Therefore, Tg of F-HBPUA increases with increasing generations or DB. As for FHBPUA with different flexible chains in Fig. 5b, Tg greatly varies with the type of flexible segments. HBPUA due to lacking flexible chains has Fig. 3. 13C NMR spectra of (a) different generations of F-HBPUA and (b) different flexible chains contained F-HBPUA.
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Fig. 5. DSC curves of F-HBPUA (a) effect of generation numbers and (b) effect of flexible chains.
Not only generation numbers but also soft chain lengths have a significant influence on flexibility of UV-cured F-HBPUA films. FHBPUA-4 and F-HBPUA-5 films reveal better flexibility than that of FHBPUA-2 and F-HBPUA-3 films, as the generation number increases, the dendritic arms become more flexible [34,36], the cured films are more flexible, accordingly. The flexibility is also greatly enhanced by more flexible chains [37]. Owing to lack of flexible chains, HBPUA films exhibit the lowest flexibility (4 mm) than others. F-HBPUA-IPD films containing soft chain have relatively better flexibility (3 mm) than HBPUA films. As the soft chain length further increases, the flexibility of F-HBPUA-4 and F-HBPUA-PEG600 films are distinctly improved to 1 mm, which is much higher than that of HBPUs (5–8 mm) [9,12,29]. Overall, pencil hardness of these films is obviously decreased with the increase of generation number and soft chain length. As the generation number or DB increases, the dendritic arms become more flexible and F-HBPUA contains more soft chains [12,34], so F-HBPUA-2 and F-HBPUA-3 films are harder than F-HBPUA-4 and F-HBPUA-5 films. As the increase of soft chain length, films become softer, thus hardness reduces [37]. Therefore, HBPUA films without soft chain shows the highest hardness (2H)while F-HBPUA-4 and F-HBPUA-PEG600 films have the lowest hardness (HB). These films also show better adhesion on tin plates with increasing DB and soft chain length. The content of polar groups (oxygen and nitrogen) are increased with the increase of DB and soft chain length, which reinforces the intermolecular interaction and interfacial adhesion between resin molecules and metal plates [35,38]. The gel content of F-HBPUA films is decreased from the second to the fifth generations. Within the rapid polymerization process, the diffusion and mobility of both radicals and pendant double bonds are severely constrained in the crosslinked network. Consequently, it is difficult for the unreacted double bonds trapped in the polymeric networks to continue polymerizing and crosslinking [19]. On the other hand, the mobility of acrylate is improved with the increase of soft chain length to some extent, which facilitate the photopolymerization [37], so the gel content is improved with longer soft chain.
the highest Tg (113.8 °C), which is close to that of its analogues [29]. Tg decreases with the increase of flexible segment length which improves the molecular motion, and F-HBPUA-PEG600 possesses the lowest Tg (78.2 °C). The thermal stability of polymers is vital for its long-term use and can be determined through TG analysis. TG curves of all F-HBPUA are shown in Fig. 6. The initial weight loss below 150 °C is attributed to the volatilization of solvent. After that, all F-HBPUA samples exhibit a similar thermal decomposition and include two main weight loss regions. The first decomposition below 260 °C is mainly caused by the urethane groups in hard segments, and the second step between 260 °C and 450 °C results from the decomposition of soft segments [13]. Therefore, the higher the generation of F-HBPUA, the more urethane linkages, the stronger intermolecular interaction, and thus F-HBPUA-2 in Fig. 6a exhibits weaker thermal stability at 160–260 °C than other samples [17]. After the decomposition of urethane linkages, all F-HBPUA samples reveal very homologous thermal degradation behavior at higher temperature above 300 °C. As for thermal stability of different flexible chain contained F-HBPUA in Fig. 6b, it is quite clear that length of soft segments has different influence at different degradation stage. FHBPUA with a long soft chain show an excellent thermal stability at below 300 °C resulted from the decomposition of hard segment while those with a short soft chain exhibit a better thermal stability at 300–450 °C rooted in the depolycondensation and polyol degradation [34]. 3.4. Performances of UV-cured F-HBPUA films The performances of UV-cured F-HBPUA films are listed in Table 2. The UV-curing time is significantly decreased from 12 s to 3 s from the second to fifth generation, because the functionality of terminal acrylic double bonds located at the outer layer of F-HBPUA greatly increases in higher generations, which can significantly accelerate the photopolymerization [19,24,34]. However, as the functionality is enough, the curing rate does not further increase [35], thus F-HBPUA-4 and FHBPUA-5 exhibit equal curing time. F-HBPUA with different flexible segment has similar curing time due to their parallel DB.
Fig. 6. TGA curves of (a) different generations of FHBPUA and (b) various flexible chains contained FHBPUA.
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Table 2 Properties of the UV-cured F-HBPUA films. Samples
Curing time/s
Flexibility/mm
Pencil hardness
Adhesion/grades
Gel content/%
F-HBPUA −2 F-HBPUA −3 F-HBPUA −4 F-HBPUA −5 HBPUA F-HBPUA-IPD F-HBPUA-PEG600
12 9 3 3 3 3 3
2 2 1 1 4 3 1
F F HB HB 2H H HB
4 4 3 3 4 4 3
96.5 95.2 94.4 94.1 91.4 93.2 95.2
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4. Conclusion Different generations and various lengths of soft chain contained UV-curable flexible hyperbranched polyurethane acrylates (F-HBPUA) are synthesized by reacting various generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH) with different soft chains contained semiadduct urethane monoacrylate. The degree of branching of F-HBPUA from the second to the fifth generation turns out to be 0.74, 0.79, 0.82 and 0.94, and for HBPUA, F-HBPUA-IPD and F-HBPUAPEG600, it is 0.81, 0.80 and 0.80, respectively. As the increase of generation number, Tg of F-HBPUA is increased but it is greatly decreased with increasing soft chain length. The higher generation number F-HBPUA has, the better thermal stability it exhibits. However, the longer soft chain will deteriorate the heat resistance as the temperature is higher than 300 °C. With increasing generation numbers and chain lengths, the UV-curing time of F-HBPUA films decreases to 3 s, flexibility of F-HBPUA films improves to 1 mm, adhesion also enhances to 3 grades, only pencil hardness slightly decreases to HB. The gel content of these films decreases with higher generations while it increases with longer soft chain. This novel F-HBPUA not only can be used as functional additives, but also can produce UV-cured freestanding films with excellent flexibility. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant numbers: 21604014 and 51641302). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.porgcoat.2017.10.019. References [1] D. Wang, T. Zhao, X. Zhu, D. Yan, W. Wang, Bioapplications of hyperbranched polymers, Chem. Soc. Rev. 44 (2015) 4023–4071. [2] D. Wang, Y. Jin, X. Zhu, D. Yan, Synthesis and applications of stimuli-responsive hyperbranched polymer, Prog. Polym. Sci. 64 (2017) 114–153. [3] W. Wu, R. Tang, Q. Li, Z. Li, Functional hyperbranched polymers with advanced optical, electrical and magnetic properties, Chem. Soc. Rev. 44 (2015) 3997–4022. [4] Y. Zheng, S. Li, Z. Weng, C. Gao, Hyperbranched polymers: advances from synthesis to applications, Chem. Soc. Rev. 44 (2015) 4091–4130. [5] Y. Huang, D. Wang, X. Zhu, D. Yan, R. Chen, Synthesis and therapeutic applications of biocompatible or biodegradable hyperbranched polymers, Polym. Chem. 6 (2015) 2794–2812. [6] F. Sun, X. Luo, L. Kang, X. Peng, C. Lu, Synthesis of hyperbranched polymers and their applications in analytical chemistry, Polym. Chem. 6 (2015) 1214–1225. [7] Y. Segawa, T. Higashihara, M. Ueda, Synthesis of hyperbranched polymers with controlled structure, Polym. Chem. 4 (2013) 1746–1759. [8] A. Prabhakar, D.K. Chattopadhyay, B. Jagadeesh, K.V.S.N. Raju, Structural investigations of polypropylene glycol (PPG) and isophorone diisocyanate (IPDI)based polyurethane prepolymer by 1d and 2d NMR spectroscopy, J. Polym. Sci. Polym. Chem. 43 (2005) 1196–1209.
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[37] Y. Zhang, A. Asif, W. Shi, Highly branched polyurethane acrylates and their waterborne UV curing coating, Prog. Org. Coat. 71 (2011) 295–301. [38] A.K. Mishra, R. Narayana, K.V.S.N. Rajua, T.M. Aminabhavi, Hyperbranched polyurethane (HBPU)-urea and HBPU-imide coatings: effect of chain extender and NCO/OH ratio on their properties, Prog. Org. Coat. 74 (2012) 134–141.
[35] G. Xu, Y. Zhao, W. Shi, Properties and morphologies of UV-cured epoxy acrylate blend films containing hyperbranched polyurethane acrylate/hyperbranched polyester, J. Polym. Sci. Part B: Polym. Phys. 43 (2010) 3159–3170. [36] A. Asif, W. Shi, X. Shen, K. Nie, Physical and thermal properties of UV curable waterborne polyurethane dispersions incorporating hyperbranched aliphatic polyester of varying generation number, Polymer 46 (2005) 11066–11078.
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Effect of soft chain length and generation number on properties of flexible hyperbranched polyurethane acrylate and its UV-cured film
MARK
Hongping Xiang, Xiaowei Wang, Lu Xi, Haihui Dong, Peng Hong, Jiahui Su, Yanyan Cui, ⁎ Xiaoxuan Liu Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, Guangdong, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hyperbranched polymers Polyurethane acrylate Soft chain length Flexibility UV-curable film
A series of UV-curable flexible hyperbranched polyurethane acrylate (F-HBPUA) with various generation numbers and soft chains are successfully developed by modifying different generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH) with diverse soft chains contained semiadduct urethane monoacrylate. The influence of generation number and soft chain on the performances of F-HBPUA and its UV-cured freestanding film is studied. The degree of branching of F-HBPUA from the second to the fifth generation is proved to be 0.74, 0.79, 0.82 and 0.94, respectively. The glass transition temperature of F-HBPUA increases with increasing generation number but it decreases with the increase of soft chain length. The thermal stability of FHBPUA improveswith higher generations while the longer soft chain is only beneficial to the heat resistance at below 300 °C. As the increase of generation number and chain length, the UV-curing time of F-HBPUA film is significantly shortened from 12 s to 3 s, the flexibility of the UV-cured film is greatly increased from 4 mm to 1 mm, adhesion is improved to 3 grades. This novel flexible hyperbranched polyurethane acrylates can be applied to coatings, membranes and 3D print parts with excellent flexibility.
1. Introduction Hyperbranched polyurethane acrylates (HBPUA) not only exhibit the low viscosity, high solubility and abundant terminal functional groups of hyperbranched polymers (HBP) [1–7], but also possess the excellent photoreactivity, abrasive resistance and adhesion of polyurethane acrylates (PUA) [8]. They are mainly synthesized by modifying different hydroxyl-terminated hyperbranched polymers (HBPOH) with various semiadducts of urethane monoacrylate [9–15]. Additionally, as the hydroxyls in HBP are partially modified with anhydrides and successively neutralized with triethylamine, the HBPUA obtained become waterborne [16–20]. HBPUA have been widely applied in coating, adhesive, photoresist, printing ink and nanocomposite as modifier, crosslinker and other additives to reduce viscosity, accelerate reaction, improve mechanical properties and enhance toughness of matrix [9–24]. However, due to the nature of compactly packed periphery, high crosslinking density and lack of chain entanglements, HBPUA are usually too brittle to form freestanding films, and the mechanical performances of films are quite weak [25]. Several strategies, like grafting reactive polymer chains, modifying functional end-groups with short
⁎
flexible chain and copolymerizing with linear analogues [25–28], are taken to address the high brittleness and weak mechanical properties, but the flexibility of UV-cured freestanding films remains pretty poor (5–8 mm) yet [9,12,29]. In order to greatly reduce the mechanical brittleness and produce UV-curable flexible HBPUA freestanding films, different soft chains like polyethylene glycol (PEG) and isopentydiol (IPD) are firstly introduced into semiadduct urethane monoacrylate of IPDI (isophorone diisocyanate)-HEA (2-hydroxyethyl acrylate) to form flexible IPDI/PEG/HEA and IPDI/IPD/HEA arms, in this study. The flexible arms are then reacted with hydroxyl terminated hyperbranched polyurethane (HBPUOH, deemed as core) to produce UV-curable flexible hyperbranched polyurethane acrylates (F-HBPUA). The influence of different flexible chains on the properties of F-HBPUA and their UV-cured freestanding films are studied. Because of the significant impact of degree of branching (DB) on properties of HBP [3–5], F-HBPUA with different DB are also synthesized to investigate its influence on performances of FHBPUA and their UV-cured freestanding films.
Corresponding author. E-mail address: [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.porgcoat.2017.10.019 Received 24 July 2017; Received in revised form 19 October 2017; Accepted 21 October 2017 0300-9440/ © 2017 Published by Elsevier B.V.
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was reacted at 70 °C for about 140 min until the peak at 2270 cm−1 for the NCO groups completely disappeared. Precipitated with ether, washed with acetone and vacuum dried, different generations of F-HBPUA (abbreviated as F-HBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5) obtained. To study the effect of flexible chain on properties of F-HBPUA, the semiadduct of IPDI/PEG600/HEA, IPDI/IPD/HEA and IPDI/HEA were reacted with the fourth generation of hydroxyl terminated hyperbranched polyurethane (HBPU-OH-4) to produce F-HBPUA-PEG600, FHBPUA-IPD and HBPUA, respectively.
2. Experimental section 2.1. Materials Polyethylene glycol (PEG, Mn = 200, 600), isophorone diisocyanate (IPDI), 2-hydroxyethyl acrylate (HEA), dimethylacetamide (DMA), dibutyltin dilaurate (DBTDL), diethanol amine (DEA), isopentyldiol (IPD), diethyl ether, hydroquinone monomethyl ether (MEHQ), dibutylamine and pyridine were purchased from Aladdin Reagent Co. (Shanghai, China) and used without further purification. 2.2. Synthesis of different generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH)
2.5. Preparation of UV-cured F-HBPUA films F-HBPUA were dissolved in anhydrous ethanol to prepare 85 wt.% resin solutions, and then mixed with 4 wt.% of Irgacure1173 used as the photoinitiator. The films were prepared by a doctor blade technique on cleaned tinplate with a typical thickness of ∼30 μm. After drying for 5 min at 60 °C under vacuum, UV-cured F-HBPUA films obtained by exposing to 365 nm UV light with a light intensity of 30 mW/cm−2.
IPDI (33.33 g, 0.15 mol) and DMA (33.33 g, 0.38 mol) were added to a 250 mL four – neck flask, equipped with a mechanical stirrer, nitrogen inlet, thermometer, and dropping funnel. As the mixtures were cool to 0 °C, DEA (15.77 g, 0.15 mol) and DMA (15.77 g, 0.18 mol) were added dropwise and reacted for 2 h. Then, the reaction temperature was gradually increased to 50 °C. The value of NCO was determined every 20 min. Excessive dibutylamine was added to terminate the reaction as soon as the value of NCO was equal to its theoretical value which was calculated based on its composition and structure and shown in Table 1. Different generations of HBPU-OH were synthesized by directly controlling reaction time, or indirectly controlling the value of NCO [29]. After precipitating with ether and vacuum drying, different generations of HBPU-OH (from the second to the fifth, abbreviated as HBPU-OH-2, HBPU-OH-3, HBPU-OH-4 and HBPU-OH-5) obtained. The synthesis process of HBPU-OH was shown in Scheme 1a.
2.6. Characterization FTIR spectrum was recorded on a Nicolet Magna 360 spectrometer by KBr disk method, scanned from 4000 cm−1 to 400 cm−1 with a resolution of 4 cm−1. 1H NMR and 13C NMR were performed on a AVANCE III HD 400 MHz spectroscopy, DMSO-d6 was used as the solvent. Pencil hardness of the UV-cured film was measured according to ASTM D3363-05 standard. Adhesion was tested in accordance with ASTM D3359-09 standard. The flexibility of films was determined according to ASTM D4338-97 standard. Intrinsic viscosity ([η]) tests were carried out with an Ubbelohde viscometer at 30 ± 0.1 °C, where the samples were dissolved to form 5 g/L DMF solution. Differential scanning calorimetry (DSC) analysis was measured by TA Instruments Q20 under N2 atmosphere with a heating rate of 10 °C/min from 25 °C to 150 °C. Thermal gravimetric analysis (TGA) was carried out on NETZSCH STA 499C in N2 atmosphere from 40 °C to 800 °C by 10 °C/ min. The gel content of the UV-cured film was estimated by measuring the weight variation after extracting with acetone in Soxhlet extractor for 36 h and dried at 60 °C under vacuum. Gel content could be calculated by Eq. (1).
2.3. Synthesis of different semiadduct urethane monoacrylate Different flexible semiadduct urethane monoacrylate (IPDI/ PEG200/HEA) was synthesized as Scheme 1b. IPDI (22.22 g, 0.10 mol) and DMA (22.22 g, 0.26 mol) were well-mixed, and then PEG200 (10 g, 0.05 mol) and DMA (10 g, 0.11 mol) was added dropwise. After that, the mixtures were reacted at 60 °C. As the value of NCO was close to its theoretical value, the temperature was decreased to 35 °C. HEA (7.55 g, 0.065 mol), DBTAL (0.037 wt.%), MEHQ (0.1 wt.%) and DMA (7.55 g, 0.09 mol) were dropped, and reacted at 35 °C for 2 h. When the value of NCO was close to its theoretical value, the reaction was terminated and IPDI/PEG200/HEA obtained. In addition, to investigate the influence of different flexible chains, PEG600 and isopentyldiol (IPD) contained semiadduct urethane monoacrylate, IPDI/PEG600/HEA and IPDI/IPD/HEA were also prepared following synthetic process of IPDI/PEG200/HEA. The semiadduct urethane monoacrylate without flexible chain (IPDI/HEA) was also synthesized, as control.
Gel content = Wv/We × 100
(1)
where Wv was the weight of the UV-cured film before extracting and We was the weight of the UV-cured film after extracting. 3. Result and discussion 3.1. Characterization of F-HBPUA
2.4. Synthesis of different flexible hyperbranched polyurethane acrylate
The molecular structures of different generations and soft chains contained F-HBPUA are firstly characterized by FTIR, and shown in 1 and S1 . As for HBPU-OH-4 in Fig. S1a, the broad peak at 3030–3740 cm−1 is mainly assigned to eOH, while the intense peaks at 3150–3700 cm−1 and 1540 cm−1 for all F-HBPUA and HBPUA should attribute to stretching vibration and bending vibration of eNH [30,31]. Obviously, there is no the characteristic peak of NCO group at 2270 cm−1 in all curves, which means NCO groups have been completely reacted. The peaks at 1720 cm−1 are ascribed to C]O of eNHCOO- resulted from the reaction of NCO and OH, while the peaks at 1630 cm−1 are assigned to C]O of −NHCON- caused by reaction between NCO and NH [14,32]. Compared with HBPU-OH-4, all F-HBPUA and HBPUA in Fig. 1 appear new peaks at 810 cm−1 attributed to eCH]CH2 of HEA [13,31]. The peaks at 1130 cm−1 attributed to CeO are greatly enhanced with the introduction of flexible chains, especially with PEG200 and PEG600. Due to lacking flexible chain, the CeO peak
As shown in Scheme 1c, different generations of HBPU-OH were severally mixed with IPDI/PEG200/HEA, where the molar ratio of OH in HBPU-OH and NCO in semiadduct urethane monoacrylate was 1:1. After adding DBTAL (0.05 wt.%) and MEHQ (0.1 wt.%), the mixture Table 1 The values of NCO and hydroxyl for the different generations of HBPU-OH [29]. Generations
Theoretical NCO value/%
Tested NCO value/%
Theoretical OH value/%
Tested OH value/%
Yield/%
Second Third Fourth Fifth
6.41–4.28 4.28–1.83 1.83–0.86 0.86–0.41
4.52 2.15 1.06 0.68
257.00–284.50 228.45–195.81 195.81–182.76 182.76–176.86
240.33 215.34 192.35 179.14
85 80 86 89
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Scheme 1. Schematic representation for the synthesis of F-HBPUA.
HBPUA and HBPUA-IPD without PEG segment, both curves at 3.51 ppm do not show the peak belonged to eCH2eCH2e of PEG. This also indicates the different semiadducts urethane monoacrylate have successfully reacted with various generations of HBPU-OH. 13 C NMR spectra of different generations and flexible chains contained F-HBPUA are presented in Fig. 3 and S3 . The resonance at 155.73 ∼ 157.17 ppm, 132.46 ppm, 69.36 ∼ 70.23 ppm and 63.33 ppm should be ascribed to eC]O of eNHCOOeand eNHCONe, eCH]CH2 of HEA, eCH2eCH2e of PEG and eCH2eCH2e of HEA, respectively. Clearly, HBPUA and F-HBPUA-IPD in Fig. 3b do not reveal the resonance at 69.36–70.23 ppm, due to the lack of PEG segments. HBPs synthesized from AB2 monomer mainly consist of three different types of repeating units, i.e. a dendritic unit (D), a linear unit (L) and a terminal unit (T) [4]. The degree of branching (DB) can be indirectly
of HBPUA does not be enlarged. Hence, it represents different semiadducts urethane monoacrylate have successfully reacted with various generations of HBPU-OH to generate different F-HBPUA. The 1H NMR spectra of different generations and flexible chains contained F-HBPUA are displayed in Fig. 2 and S2 . Overall, the peaks at 0.5 ∼ 1 ppm and 1.5 ∼ 4 ppm in Fig. S2 are mainly ascribed to eCH3 and eCH2-. In Fig. 2, the peak at 7.06 ∼ 7.23 ppm is attributed to NeH of −NHCOO- and −NHCON-. Compared with HBPU-OH-4, all FHBPUA and HBPUA appear new peaks at 5.93 ∼ 6.37 ppm, 4.17 ∼ 4.26 ppm and 3.51 ppm, which are distributed to eCH]CH2 in HEA, eCH2eCH2e in HEA and eCH2eCH2e in PEG [13], respectively. All F-HBPUA exhibit similar curves because their chemical structures are analogical only with different degree of branching, i.e. different generation numbers. As for different flexible chains in Fig. 2b, due to
Fig. 1. FTIR spectra of (a) different generations of FHBPUA and HBPU-OH-4, and (b) different flexible chains contained F-HBPUA.
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Fig. 2. 1H NMR spectra of (a) various generations of F-HBPUA and HBPU-OH-4, and (b) diverse flexible chains contained F-HBPUA.
estimated from the ratio of these units in culated according to Fréchet Eq. (2) [31].
DB =
D+T D+T+L
13
C NMR spectra, and cal-
(2)
The peaks at 62.23 ppm, 60.91 ppm and 60.67 ppm in Fig. 3 can be severally assigned to D, L and T unit of different F-HBPUA [32,33]. The DB of F-HBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5 in Fig. 3a are proved to be 0.74, 0.79, 0.82 and 0.94, respectively, which are increased with the increase of generation number. These results are similar with the reported DB of different generations of HBPU [29]. The DB of HBPUA, F-HBPUA-IPD, F-HBPUA-4 and F-HBPUA-PEG600 in Fig. 3b are 0.81, 0.80, 0.82 and 0.80, respectively, which are not significantly distinct from one another. This is because these different FHBPUA derive from the same generation of HBPU-OH-4 regarded as the core, only with different flexible arms on the surface.
Fig. 4. The [η] of all synthesized hyperbranched polyurethane acrylates.
3.2. Intrinsic viscosity of F-HBPUA
weights are also increased with the increase of flexible chain length, the [η] increases correspondingly.
The intrinsic viscosity ([η]) of polymers is deemed to one of the most important parameters as it is an estimation of hydrodynamic volume, intermolecular chain entanglement and inter/intramolecular interaction, which principally depends on the molecular weight of polymers. It can affect the flow ability, photopolymerization rate and properties of cured product [14,20]. As shown in Fig. 4, the [η] of FHBPUA-2, F-HBPUA-3, F-HBPUA-4 and F-HBPUA-5 are 13.72 mL g−1, 16.14 mL g−1, 19.92 mL g−1 and 24.19 mL g−1, respectively. With the increase of generation numbers, the molecular weight of F-HBPUA is increased, and the amounts of polar groups also are increased which enhance the intermolecular interaction [20], so the [η] of F-HBPUA is increased with increasing generation number. The [η] of HBPUA, FHBPUA-IPD, F-HBPUA-4 and F-HBPUA-PEG600 are 12.61 mL g−1, 15.67 mL g−1, 19.92 mL g−1 and 25.85 mL g−1, respectively. These FHBPUA are developed based on the same generation of HBPU-OH only grafting with different soft chains on the surface, so their molecular
3.3. Thermal properties of different F-HBPUA The glass-transition temperature (Tg) of different F-HBPUA are determined by DSC and represented in Fig. 5. Tg of F-HBPUA-2, F-HBPUA3, F-HBPUA-4 and F-HBPUA-5 in Fig. 5a are 83.7 °C, 87.5 °C, 89.3 °C and 93.3 °C, respectively. Apparently, Tg increases with the increase of generation numbers from the second to the fifth, because the molecular structures become more compact and rigid for higher generations, and thus the molecular mobility becomes more difficult [29,34]. Moreover, with the increase of generation numbers, the amounts of polar groups are increased, the intermolecular interaction is enhanced. Therefore, Tg of F-HBPUA increases with increasing generations or DB. As for FHBPUA with different flexible chains in Fig. 5b, Tg greatly varies with the type of flexible segments. HBPUA due to lacking flexible chains has Fig. 3. 13C NMR spectra of (a) different generations of F-HBPUA and (b) different flexible chains contained F-HBPUA.
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Fig. 5. DSC curves of F-HBPUA (a) effect of generation numbers and (b) effect of flexible chains.
Not only generation numbers but also soft chain lengths have a significant influence on flexibility of UV-cured F-HBPUA films. FHBPUA-4 and F-HBPUA-5 films reveal better flexibility than that of FHBPUA-2 and F-HBPUA-3 films, as the generation number increases, the dendritic arms become more flexible [34,36], the cured films are more flexible, accordingly. The flexibility is also greatly enhanced by more flexible chains [37]. Owing to lack of flexible chains, HBPUA films exhibit the lowest flexibility (4 mm) than others. F-HBPUA-IPD films containing soft chain have relatively better flexibility (3 mm) than HBPUA films. As the soft chain length further increases, the flexibility of F-HBPUA-4 and F-HBPUA-PEG600 films are distinctly improved to 1 mm, which is much higher than that of HBPUs (5–8 mm) [9,12,29]. Overall, pencil hardness of these films is obviously decreased with the increase of generation number and soft chain length. As the generation number or DB increases, the dendritic arms become more flexible and F-HBPUA contains more soft chains [12,34], so F-HBPUA-2 and F-HBPUA-3 films are harder than F-HBPUA-4 and F-HBPUA-5 films. As the increase of soft chain length, films become softer, thus hardness reduces [37]. Therefore, HBPUA films without soft chain shows the highest hardness (2H)while F-HBPUA-4 and F-HBPUA-PEG600 films have the lowest hardness (HB). These films also show better adhesion on tin plates with increasing DB and soft chain length. The content of polar groups (oxygen and nitrogen) are increased with the increase of DB and soft chain length, which reinforces the intermolecular interaction and interfacial adhesion between resin molecules and metal plates [35,38]. The gel content of F-HBPUA films is decreased from the second to the fifth generations. Within the rapid polymerization process, the diffusion and mobility of both radicals and pendant double bonds are severely constrained in the crosslinked network. Consequently, it is difficult for the unreacted double bonds trapped in the polymeric networks to continue polymerizing and crosslinking [19]. On the other hand, the mobility of acrylate is improved with the increase of soft chain length to some extent, which facilitate the photopolymerization [37], so the gel content is improved with longer soft chain.
the highest Tg (113.8 °C), which is close to that of its analogues [29]. Tg decreases with the increase of flexible segment length which improves the molecular motion, and F-HBPUA-PEG600 possesses the lowest Tg (78.2 °C). The thermal stability of polymers is vital for its long-term use and can be determined through TG analysis. TG curves of all F-HBPUA are shown in Fig. 6. The initial weight loss below 150 °C is attributed to the volatilization of solvent. After that, all F-HBPUA samples exhibit a similar thermal decomposition and include two main weight loss regions. The first decomposition below 260 °C is mainly caused by the urethane groups in hard segments, and the second step between 260 °C and 450 °C results from the decomposition of soft segments [13]. Therefore, the higher the generation of F-HBPUA, the more urethane linkages, the stronger intermolecular interaction, and thus F-HBPUA-2 in Fig. 6a exhibits weaker thermal stability at 160–260 °C than other samples [17]. After the decomposition of urethane linkages, all F-HBPUA samples reveal very homologous thermal degradation behavior at higher temperature above 300 °C. As for thermal stability of different flexible chain contained F-HBPUA in Fig. 6b, it is quite clear that length of soft segments has different influence at different degradation stage. FHBPUA with a long soft chain show an excellent thermal stability at below 300 °C resulted from the decomposition of hard segment while those with a short soft chain exhibit a better thermal stability at 300–450 °C rooted in the depolycondensation and polyol degradation [34]. 3.4. Performances of UV-cured F-HBPUA films The performances of UV-cured F-HBPUA films are listed in Table 2. The UV-curing time is significantly decreased from 12 s to 3 s from the second to fifth generation, because the functionality of terminal acrylic double bonds located at the outer layer of F-HBPUA greatly increases in higher generations, which can significantly accelerate the photopolymerization [19,24,34]. However, as the functionality is enough, the curing rate does not further increase [35], thus F-HBPUA-4 and FHBPUA-5 exhibit equal curing time. F-HBPUA with different flexible segment has similar curing time due to their parallel DB.
Fig. 6. TGA curves of (a) different generations of FHBPUA and (b) various flexible chains contained FHBPUA.
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Table 2 Properties of the UV-cured F-HBPUA films. Samples
Curing time/s
Flexibility/mm
Pencil hardness
Adhesion/grades
Gel content/%
F-HBPUA −2 F-HBPUA −3 F-HBPUA −4 F-HBPUA −5 HBPUA F-HBPUA-IPD F-HBPUA-PEG600
12 9 3 3 3 3 3
2 2 1 1 4 3 1
F F HB HB 2H H HB
4 4 3 3 4 4 3
96.5 95.2 94.4 94.1 91.4 93.2 95.2
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4. Conclusion Different generations and various lengths of soft chain contained UV-curable flexible hyperbranched polyurethane acrylates (F-HBPUA) are synthesized by reacting various generations of hydroxyl terminated hyperbranched polyurethane (HBPU-OH) with different soft chains contained semiadduct urethane monoacrylate. The degree of branching of F-HBPUA from the second to the fifth generation turns out to be 0.74, 0.79, 0.82 and 0.94, and for HBPUA, F-HBPUA-IPD and F-HBPUAPEG600, it is 0.81, 0.80 and 0.80, respectively. As the increase of generation number, Tg of F-HBPUA is increased but it is greatly decreased with increasing soft chain length. The higher generation number F-HBPUA has, the better thermal stability it exhibits. However, the longer soft chain will deteriorate the heat resistance as the temperature is higher than 300 °C. With increasing generation numbers and chain lengths, the UV-curing time of F-HBPUA films decreases to 3 s, flexibility of F-HBPUA films improves to 1 mm, adhesion also enhances to 3 grades, only pencil hardness slightly decreases to HB. The gel content of these films decreases with higher generations while it increases with longer soft chain. This novel F-HBPUA not only can be used as functional additives, but also can produce UV-cured freestanding films with excellent flexibility. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant numbers: 21604014 and 51641302). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.porgcoat.2017.10.019. References [1] D. Wang, T. Zhao, X. Zhu, D. Yan, W. Wang, Bioapplications of hyperbranched polymers, Chem. Soc. Rev. 44 (2015) 4023–4071. [2] D. Wang, Y. Jin, X. Zhu, D. Yan, Synthesis and applications of stimuli-responsive hyperbranched polymer, Prog. Polym. Sci. 64 (2017) 114–153. [3] W. Wu, R. Tang, Q. Li, Z. Li, Functional hyperbranched polymers with advanced optical, electrical and magnetic properties, Chem. Soc. Rev. 44 (2015) 3997–4022. [4] Y. Zheng, S. Li, Z. Weng, C. Gao, Hyperbranched polymers: advances from synthesis to applications, Chem. Soc. Rev. 44 (2015) 4091–4130. [5] Y. Huang, D. Wang, X. Zhu, D. Yan, R. Chen, Synthesis and therapeutic applications of biocompatible or biodegradable hyperbranched polymers, Polym. Chem. 6 (2015) 2794–2812. [6] F. Sun, X. Luo, L. Kang, X. Peng, C. Lu, Synthesis of hyperbranched polymers and their applications in analytical chemistry, Polym. Chem. 6 (2015) 1214–1225. [7] Y. Segawa, T. Higashihara, M. Ueda, Synthesis of hyperbranched polymers with controlled structure, Polym. Chem. 4 (2013) 1746–1759. [8] A. Prabhakar, D.K. Chattopadhyay, B. Jagadeesh, K.V.S.N. Raju, Structural investigations of polypropylene glycol (PPG) and isophorone diisocyanate (IPDI)based polyurethane prepolymer by 1d and 2d NMR spectroscopy, J. Polym. Sci. Polym. Chem. 43 (2005) 1196–1209.
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