structurally enhanced self‐plasticization of poly(vinyl chloride) via...

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A highly self-plasticized poly(vinyl chloride) (PVC) is demonstrated for the first time via click grafting of hyperbranched polyglycerol (HPG). The plasticizing effect of the grafted HPG on PVC is systematically investigated by various analytical methods. The amorphous and bulky dendritic structure of HPG efficiently increases the free volume of the grafted PVC, which leads to a remarkably lower glass transition temperature comparable to that of the conven-tional plasticized PVC. Viscoelastic analysis reveals that HPG considerably improves the softness of the grafted PVC at room temperature and promotes the segmental motion in the system. The HPG-grafted PVC films exhibit an exceptional stretchability unlike the mixture of PVC and HPG because the covalent attachment of HPG to PVC allows it to maintain its homogeneous and well-organized architecture under ten-sile stretching. The work provides valuable insights into the design of highly flexible and stretchable polymeric materials by means of introducing hyperbranched side chains.

Structurally Enhanced Self-Plasticization of Poly(vinyl chloride) via Click Grafting of Hyperbranched Polyglycerol

Kyu Won Lee, Jae Woo Chung,* Seung-Yeop Kwak*

K. W. Lee, Prof. S.-Y. KwakDepartment of Materials Science and Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, South KoreaE-mail: [email protected]. J. W. ChungDepartment of Organic Materials and Fiber Engineering Soongsil University 369 Sangdo-ro, Dongjak-gu, Seoul 156-743, South KoreaE-mail: [email protected]

devices.[2] Among the several types of plasticizers avail-able, phthalates are the most commonly used due to their high miscibility with PVC and excellent plasticization effi-ciency.[3] However, phthalates have a tendency to migrate from the articles during use, which raises concerns about serious human risks such as endocrine disruption and car-cinogenic effects in the human body.[4] The migration of plasticizer also leads to severe deterioration in flexibility of the products.

Several approaches have been investigated to resolve these urgent phthalate migration problems: development of alternative plasticizers,[5] PVC block copolymers,[6] modification of the PVC surface,[7] and incorporation of nanomaterials.[8] Although these approaches suppress the migration of the plasticizer to a certain degree, they decrease the physical properties of the plasticized PVC, and most of all, the migration is still not completely pre-vented. Recently, the covalent attachment of plasticizer onto the PVC backbone has been presented as the most

1. Introduction

Poly(vinyl chloride) (PVC) is one of the most widely used polymers due to its high versatility, excellent physical properties, and low cost.[1] Because pristine PVC exhibits rigidity and poor processability, a considerable amount of plasticizer is added to facilitate its use in a wide range of applications, such as toys, pipes, wallpaper, flooring, cables, curtains, packaging material, and medical

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effective approach to prevent the migration of plasticizer. Navarro et al. reported phthalates covalently bound PVC via the displacement of chlorine with thiol-substituted phthalates.[9] This method rendered “self-plasticized” PVC with zero migration, but the material showed a quite low plasticization efficiency compared with that of a mix-ture of PVC and phthalate, and the reaction mixture still contained hazardous phthalates. The covalent grafting of cardanol[10] and poly(ε-caprolactone)[11] onto PVC were achieved by “click” chemistry. However, these PVCs were not flexible enough for actual use due to the small molec-ular structure of cardanol and high crystallinity of poly(ε-caprolactone). Moreover, the mechanical properties of the grafted PVCs were not investigated at all.

Hyperbranched polyglycerol (HPG) is an aliphatic poly-ether with 3D globular architecture and multiple hydroxyl terminal groups.[12] HPG has been intensively studied in various fields such as an antifouling membrane,[13] a drug delivery agent,[14] and a template for nanomaterials[15] owing to its advantages of low chain entanglement, large population of functional groups, excellent biocompat-ibility, and facile one-pot synthesis.[16] In particular, HPG exhibits high molecular mobility originated from its ran-domly branched amorphous structure and a number of flexible ether lingkages.[17] From this point of view, we expected that combining linear PVC with HPG would pro-duce a novel linear-hyperbranched architecture exhib-iting extraordinary self-plasticization performance.

Herein, we report a highly self-plasticized and migra-tion-free PVC via click grafting of HPG. Introducing highly branched polyether side chains provided the PVC with a structurally enhanced self-plasticization ability, which led to a remarkably lower glass transition temperature of −28.9 °C and significantly enhanced elongation at break of 912%. Furthermore, HPG grafting considerably improved the thermal stability and migration resistance of the PVC, indicating that HPG grafted PVC is suitable for use in harsh environmental conditions.

2. Results and Discussion

Recently, there have been several reports about the grafting of hyperbranched polyglycerol (HPG) onto linear polymers using “grafting to” strategy.[18] In this study, we prepared hexyl-terminated HPG (HPG-C6) grafted PVC (PVC-g-HPG-C6) via alkyne-azide click reaction with HPG component containing exactly one focal alkyne group and azide functional PVC (see the Supporting Informa-tion for experimental details). A schematic illustration of the synthesis of PVC-g-HPG-C6 is presented in Scheme 1. HPG was synthesized by the anionic ring-opening poly-merization of glycidol using propargyl alcohol as an alkyne source.[19] In the 1H nuclear magnetic resonance (NMR)

spectra (Figure S1, Supporting Information), the specific peaks corresponding to the glycidol disappeared and broad peaks attributable to methylidyne and methylene protons adjacent to the hydroxyl and ether groups of HPG appeared in the range of 3.37–3.94 ppm. Additionally, the peak cor-responding to the proton adjacent to alkyne group shifted from 4.15 to 4.17 ppm, which clearly indicated that prop-argyl alcohol initiated the polymerization of glycidol. To improve its miscibility with PVC, the hydroxyl groups of the HPG were esterified with hexanoic anhydride.[17] In the Fourier transform infrared (FT-IR) spectrum of HPG-C6, a CO stretch at 1740 cm−1 was observed due to the ester groups in HPG-C6 (Figure S2, Supporting Information). The 1H NMR spectrum exhibited a peak shift for the methylene proton of the main chain adjacent to the ester groups from 3.37–3.94 to 3.97–4.33 and 5.10–5.29 ppm, and new peaks corresponding to aliphatic chains in HPG-C6 at 0.90, 1.32, 1.63, and 2.31 ppm (Figure S3, Supporting Information). These results indicated that the hydroxyl group of HPG was successfully modified to an aliphatic ester. The degree of branching of HPG-C6 was determined by 13C NMR, and the value was 0.436 (Figure S4, Supporting Information). The size exclusion chromatography (SEC) trace of HPG-C6 showed that its number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were 1606 g mol−1 and 1.74, respectively (Figure S5, Supporting Information). The Mw/Mn of HPG-C6 was slightly larger than that of Frey and co-workers work[16b] due to the monofunctional ini-tiator[20] and sodium counterion of the initiator[21] used in the polymerization of HPG-C6. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was carried out to obtain an absolute molecular weight range for HPG-C6. The spectrum showed a regular interval of 172.1 m/z and the most intense peak appeared at m/z 865.7 (corresponding to 4-mer of HPG-C6), indicating that HPG-C6 possessed multiple side chains in its molecular structure (Figure S6, Supporting Information).

Azide functional PVC (PVC-N3) was prepared by nucleo-philic substitution of chlorine atoms into the PVC main chain.[22] FT-IR spectra of the PVC-N3 showed that the rela-tive intensity of the N3

− stretch at 2110 cm−1 increased proportionally with the reaction time, implying that the degree of azidation was well-controlled by varying the reaction time (Figure S7, Supporting Information). The degree of azidation values obtained from elemental analysis (EA) results were 1.8%, 3.6%, 5.8%, and 9.0% for 0.5, 1.0, 1.5, and 2.0 h reaction time, respectively (Table S1, Supporting Information).

Finally, PVC covalently bound with HPG-C6, i.e., PVC-g-HPG-C6 was synthesized by click reaction between HPG-C6 and PVC-N3. The formation of PVC-g-HPG-C6 was monitored by FT-IR. The specific band corresponding to the N3

− stretch (2110 cm−1) disappeared after the click reaction, indicating that the azide groups were fully


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converted to triazoles (Figure S8, Supporting Informa-tion). As shown in Figure 1a, each characteristic proton peaks corresponding to PVC and HPG-C6 was observed in the 1H NMR spectrum of PVC-g-HPG-C6, and a new peak developed at 7.73 ppm (assigned as 6) owing to the tria-zole group. These results confirmed that HPG-C6 was suc-cessfully grafted onto the PVC backbone via the click reac-tion. A series of PVC-g-HPG-C6 was prepared with varying the HPG-C6 content (Figures S9 and S10, Supporting Infor-mation). As can be seen in Figure 1b, the SEC traces of the different PVC-g-HPG-C6s showed typical unimodal peaks with Mn/Mw generally below 2.7, and Mn values that increased with HPG-C6 content. Solid-state 1H NMR was used to determine the homogeneity of the PVC-g-HPG-C6s. As presented in Figure 1c, the decay plots of all the PVC-g-HPG-C6s exhibited a single slope for the whole

τ range, indicating that the PVC-g-HPG-C6s each had con-stant T1. This means that the PVC-g-HPG-C6s were homo-geneous as a result of the high compatibility of PVC and HPG-C6. T1 decreased gradually with increasing HPG-C6 concentration because spin relaxation was promoted by the introduction of flexible HPG-C6 in the molecular structure.

Differential scanning calorimetry (DSC) was used to investigate the thermal properties of the PVC-g-HPG-C6s with different HPG-C6 content. As shown in Figure 2a, the DSC curves of neat PVC and PVC-g-HPG-C6s did not exhibit any melting peak, indicating their amorphous character-istics. Additionally, a single glass transition temperature (Tg) was observed in all cases, implying that the samples were homogeneous on the scale of 20–30 nm. This result correlated well with the solid-state 1H NMR results. The


Scheme 1. The synthesis of hyperbranched polyglycerol grafted poly(vinyl chloride) by alkyne-azide click chemistry.


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Figure 2. a) DSC curves for PVC-g-HPG-C6s. b) Tg depression of PVC-g-HPG-C6s according to HPG-C6 content. Temperature dependence of c) storage modulus and d) tanδ for PVC-g-HPG-C6s.

Figure 1. a) 1H NMR spectra of neat PVC, HPG-C6, and PVC-g-HPG-C6. b) SEC curves for neat PVC and PVC-g-HPG-C6s. c) Logarithmic plots of solid-state 1H NMR decay for neat PVC and PVC-g-HPG-C6s.


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Tg values of neat PVC, PVC1.8-g-HPG-C6, PVC3.6-g-HPG-C6, PVC5.8-g-HPG-C6, and PVC9.0-g-HPG-C6 were 81.3, 38.6, 16.4, −4.5, and −28.9 °C, respectively. The sharp decline in Tg with HPG-C6 content was mainly caused by the plas-ticizing effect of HPG-C6. Especially, PVC5.8-g-HPG-C6 and PVC9.0-g-HPG-C6 had a Tg below 0 °C, which is comparable with that of the conventional PVC/phthalate mixture (Figure S11, Supporting Information). As displayed in Figure 2b, the PVC-g-HPG-C6s exhibited remarkably lower Tg than those of PVCs modified with cardanol[10] or poly(ε-caprolactone)[11] at an equivalent degree of grafting. This high plasticization efficiency can be explained by the structural effect of HPG-C6. The amorphous, unentangled, and bulky dendritic structure as well as the numerous mobile polyether segments of HPG-C6 efficiently increase the free volume of the PVC-g-HPG-C6. These results con-firm that grafting a hyperbranched polyglycerol onto PVC is an effective method for providing PVC with intrinsic flexibility.

The storage modulus (E′) and loss tangent (tanδ) of the PVC-g-HPG-C6s as a function of temperature are shown in Figure 2c,d, respectively. The storage modulus dropped as the temperature was increased, indicating the occur-rence of energy dissipation. With increasing HPG-C6 con-tent, the onset temperature of the E′ drop (TE′onset) and the E′ value at 25 °C (E′25 °C) of the PVC-g-HPG-C6s noticeably decreased from 46.1 to −15.0 °C and from 235 to 0.12 MPa, respectively (Table S2, Supporting Information). These observations revealed that HPG-C6 promoted segmental motion in the system and improved the softness of PVC-g-HPG-C6 at room temperature. Additionally, we found that the addition of HPG-C6 shifted the α relaxation tem-perature (Tα relaxation) to lower temperature and increased the height of corresponding tanδ peak (Figure 2d; Table S2, Supporting Information). These results clearly demonstrated that HPG-C6 increased the ductility of the PVC-g-HPG-C6s. It was observed that TE′onset and Tα relaxa-

tion of the mixture of PVC and di(2-ethylhexyl) phthalate (DEHP) (PVC/DEHP) were between those of PVC5.8-g-HPG-C6 and PVC9.0-g-HPG-C6 (Figure S12a, Supporting Infor-mation). These results were in accordance with the Tg values estimated by DSC. However, E′25 °C of PVC/DEHP was between that of PVC3.6-g-HPG-C6 and PVC5.8-g-HPG-C6 (Figure S12b, Supporting Information). This result indi-cated that PVC/DEHP is less soft and flexible than PVC-g-HPG-C6 with equivalent values of Tg.

Next, we investigated the mechanical properties of the PVC-g-HPG-C6s. Figure 3a shows representative stress–strain curves for neat PVC and the PVC-g-HPG-C6s. It is evident that a high concentration of HPG-C6 dramatically changed the type of the stress–stain curve from brittle to ductile deformation. To confirm the effect of the cova-lent bonding between PVC and HPG-C6, we prepared a series of mixtures of PVC and HPG-C6 (PVC/HPG-C6s)

as a counter part of PVC-g-HPG-C6s, and tested them in the same manner (Table S3, Supporting Information; Figure 3b). As displayed in Figure 3c, the tensile strength (σts) tended to decrease in both series with increasing addition of HPG-C6. It seems that HPG-C6 acted as a lubri-cant for the PVC, which weakened the intermolecular fric-tion force between PVC chains.[23] The σts values of all the PVC-g-HPG-C6s were higher than those of PVC/HPG-C6s at about the same HPG-C6 content because the covalent bonding between PVC and HPG-C6 was stronger than the noncovalent intermolecular interactions in the PVC/HPG-C6. The most interesting result is that the elongation at break (εb) of the PVC-g-HPG-C6s increased proportion-ally with the HPG-C6 content; the εb reached 912%, 380 times greater than that of neat PVC (Figure 3d). In contrast, PVC/HPG-C6 exhibited a maximum εb value of 153% when the HPG-C6 content was 1.7 mol% and then decreased con-sistently with increasing HPG-C6 content. It is suggested that the covalent attachment of HPG-C6 to PVC allowed it to maintain its homogeneous and well-organized archi-tecture under tensile stretching, which would impede the propagation of cracks and enable the chains to extend maximally. Meanwhile, HPG-C6 mole cules in the mixture aggregated together at high concentration, which would act as a structural defect in the system, thus resulting in rapid crack propagation during deformation (Figure 3f).[24] PVC-g-HPG-C6 exhibited enhanced toughness due to the concurrent improvement of tensile strength and elonga-tion at break (Figure 3e). The tensile strength and elonga-tion at break of PVC/DEHP were very similar with those of PVC3.6-g-HPG-C6 despite a large difference between Tg values of PVC/DEHP (−23.8 °C) and PVC3.6-g-HPG-C6 (16.4 °C) (Figure S13, Supporting Information). In other words, PVC-g-HPG-C6 was stretchable even better than expected considering its Tg depression ability. It can be explained by the free-volume theory, in which case the covalent bonding between PVC and plasticizer molecules hindered the long-range segmental motions at low tem-perature. On the other hand, the improved stretchability of the PVC-g-HPG-C6 may be accounted by the lubricity theory. Since the flexible alkyl chain of HPG-C6 would act as a lubricant, chain concentration in the system would be a dominant factor under tensile stretching. From the above results, we conclude that the use of a hyperbranched plasticizer and its covalent attachment to PVC play crucial roles in realizing highly flexible and stretchable PVC.

Plasticized PVCs generally require high thermal stability for their practical application. Figure S14 (Sup-porting Information) shows thermogravimetric analysis (TGA) curves obtained for neat PVC and the PVC-g-HPG-C6s. The onset temperature (Tonset) of the PVC-g-HPG-C6s was between 279 and 287 °C, whereas that of neat PVC is 268 °C, indicating that the PVC-g-HPG-C6s were more stable than neat PVC at elevated temperature



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(Table S5, Supporting Information). Derivative thermo-gravimetry (DTG) analysis was conducted to investigate the thermal degradation behavior of PVC-g-HPG-C6 in detail (Figure S15, Supporting Information). There are two major degradation steps for neat PVC; the first step corre-sponds to the loss of HCl (Td,PVC1) and the second step cor-responds to the break of polyene segments (Td,PVC2). In the case of the PVC-g-HPG-C6s, Td,PVC1 and Td,PVC2 were shifted to higher temperature and an additional degradation peak appeared due to the decomposition of HPG-C6 (Td,HPG-C6) (Table S5, Supporting Information). Commonly, the acid strength of the catalyst determines the rate of thermal degradation of PVC.[25] Because hexanoic acid (pKa = 4.88)

released by the hydrolysis of HPG-C6 is weaker than HCl (pKa = −5.9), this would retard the thermal degradation of PVC-g-HPG-C6. PVC1.7/HPG-C6 showed similar TGA and DTG curves to those of PVC1.8-g-HPG-C6, implying that the covalent attachment of HPG-C6 had a negligible effect on the thermal stability of PVC-g-HPG-C6 (Figures S16 and S17, Supporting Information).

Finally, we estimated the migration resistance of PVC-g-HPG-C6s using a leaching test based on American Society for Testing and Materials (ASTM) procedures. The degree of migration was determined gravimetrically (Figure S18, Supporting Information). As expected, no migration was observed for any of the PVC-g-HPG-C6s

Figure 3. Representative stress–strain curves of a) PVC-g-HPG-C6s and b) PVC/HPG-C6s. Variation in c) tensile strength, d) elongation at break, and e) toughness of PVC-g-HPG-C6s and PVC/HPG-C6s with the HPG-C6 content. f) Schematic diagram showing different architec-ture and fracture behavior of PVC-g-HPG-C6 and PVC/HPG-C6.


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during the test, whereas 14.1% of the HPG-C6 migrated out from PVC1.7/HPG-C6. This confirmed that the PVC-g-HPG-C6s were significantly stable in harsh conditions owing to the covalent bonding between PVC and HPG-C6.

3. Conclusion

In conclusion, we have presented highly self-plasticized PVC achieved through click grafting with hyperbranched polyglycerol. The obtained PVC-g-HPG-C6 exhibited out-standing self-plasticization performance owing to the amorphous, unentangled, and bulky dendritic structure as well as the numerous mobile polyether segments of HPG-C6. HPG-C6 considerably improved the soft and ductile characteristics of PVC-g-HPG-C6 and promoted segmental motion in the system. The synergistic effect of the pres-ence of hyperbranched plasticizer and its covalent attach-ment provided PVC-g-HPG-C6 with significantly enhanced flexibility and stretchability. The obtained materials also exhibited improved thermal stability and perfect migra-tion resistance. Therefore, we believe that PVC-g-HPG-C6 is a promising candidate for the development of advanced flexible PVC for a wide range of applications.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No. NRF-2015R1A2A2A01005651).

Received: August 30, 2016; Revised: September 29, 2016; Published online: October 14, 2016; DOI: 10.1002/marc.201600533

Keywords: click chemistry; hyperbranched polyglycerols; poly(vinyl chloride); self-plasticization; stretchability

[1] Y. Saeki, T. Emura, Prog. Polym. Sci. 2002, 27, 2055.[2] D. Braun, J. Polym. Sci. Part A: Polym. Chem. 2004, 42, 578.[3] B. Yin, M. Hakkarainen, J. Appl. Polym. Sci. 2011, 119, 2400.[4] S. Oishi, Arch. Toxicol. 1990, 64, 143.[5] P. Jia, M. Zhang, C. Liu, L. Hu, G. Feng, C. Boa, Y. Zhou, RSC

Adv. 2015, 5, 41169.[6] a) J. F. J. Coelho, M. Carreira, P. M. O. F. Gonçalves,

A. V. Popov, M. H. Gil, J. Vinyl Addit. Technol. 2006, 12, 156; b) J. F. J. Coelho, M. Carreira, P. M. O. F. Gonçalves, A. V. Popov, M. H. Gil, Eur. Polym. J. 2006, 42, 2313.

[7] X. Zhang, C. Zhang, J. M. Hankett, Z. Chen, Langmuir 2013, 29, 4008.

[8] B. Y. Yu, J. W. Chung, S.-Y. Kwak, Environ. Sci. Technol. 2008, 42, 7522.

[9] R. Navarro, M. P. Perrino, M. G. Tardajos, H. Reinecke, Macro-molecules 2010, 43, 2377.

[10] P. Yang, J. Yan, H. Sun, H. Fan, Y. Chen, F. Wang, B. Shi, RSC Adv. 2015, 5, 16980.

[11] G. Demirci, M. A. Tasdelen, Eur. Polym. J. 2015, 66, 282.[12] D. Wilms, S.-E. Stiriba, H. Frey, Acc. Chem. Res. 2010, 43, 129.[13] X. Li, T. Cai, C. Chen, T.-S. Chung, Water Res. 2016, 89, 50.[14] S. Son, E. Shin, B.-S. Kim, Biomacromolecules 2014, 15, 628.[15] J. Iocozzia, Z. Lin, Langmuir 2016, 32, 7180.[16] a) R. K. Kainthan, J. Janzen, E. Levin, D. V. Devine,

D. E. Brooks, Biomacromolecules 2006, 7, 703; b) A. Sunder, R. Hanselmann, H. Frey, R. Mülhaupt, Macromolecules 1999, 32, 4240.

[17] K. W. Lee, J. W. Chung, S.-Y. Kwak, Green Chem. 2016, 18, 999.

[18] a) C. Schüll, H. Frey, ACS Macro Lett. 2012, 1, 461; b) C. Schüll, L. Nuhn, C. Mangold, E. Christ, R. Zentel, H. Frey, Macromol-ecules 2012, 45, 5901.

[19] a) A. Zill, A. L. Rutz, R. E. Kohman, A. M. Alkilany, C. F. Murphy, H. Kong, S. C. Zimmerman, Chem. Commun. 2011, 47, 1279; b) T. Cai, M. Li, K.-G. Neoh, E.-T. Kang, J. Mater. Chem. B 2013, 1, 1304.

[20] R. Hanselmann, D. Hölter, H. Frey, Macromolecules 1998, 31, 3790.

[21] F. Wurm, J. Nieberle, H. Frey, Macromolecules 2008, 41, 1184.[22] a) A. Earla, R. Braslau, Macromol. Rapid Commun. 2014, 35,

666; b) J. Sacristán, H. Reinecke, C. Mijangos, Polymer 2000, 41, 5577.

[23] L. M. Espinosa, A. Gevers, B. Woldt, M. Graß, M. A. R. Meier, Green Chem. 2014, 16, 1883.

[24] A. M. Bishai, F. A. Gamil, F. A. Awni, B. H. F. Al-Khayat, J. Appl. Polym. Sci. 1985, 30, 2009.

[25] P. Tiemble, G. Martínez, J. L. Millán, J. Polym. Sci. Part A: Polym. Chem. 1995, 33, 1243.

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