Preparation of poly(vinyl alcohol) and hydroxypropyl-[beta]-cyclodextrin inclusion complex through polymer processing.
Polyfvinyl alcohol) (PVA) as a polar and biodegradable polymer, which could be obtained from non-petroleum resources, has attracted concerns for developing ecofriendly materials. Because of relatively good water solubility, film formation, adhesive properties, excellent biocompatibility, biodegradability and non-toxicity, PVA has been widely used in fibers, coatings, paper adhesives, emulsifiers, oil chemicals and dug slow-release systems [1, 2],
Cyclodextrins (CDs) are cyclic starch oligomers consisting of six ([alpha]), seven ([beta]), or eight ([gamma]) glucose units, which can form host-guest type of inclusion complexes (ICs) with polymers , One of their important applications is their ability to act as antibacterial agents used in packaging materials. Recently, PVA/ CDs ICs have attracted appreciable interests due to the potential commercial application in packaging materials.
Solution methods are the most common way to prepare polymer-CD, such as co-precipitation and slurry-complexation [4-6], In co-precipitation, the guest polymer solutions are mixed with aqueous solutions of the host CDs, followed by precipitation of the polymer-CD ICs. In slurry-complexation, guest molecules are introduced into a supersaturated CD solution. Because the solution is saturated, the complexes crystallize and then precipitate from the liquid. However, to our best knowledge, most PVA/CDs ICs are limited in the solution processing, which could only yield low dimension PVA products because of the large consumption of energy and time at the dissolving and drying process. Thus, its application areas are significantly constrained. Generally, the melt-processing of PVA becomes possible by using water as plasticizer which can form intermolecular complexes with PVA through hydrogen bond. The polymer processing technology of PVA/CDs ICs is much more effective, simple and cost-saving compared with the traditional solution-based processing technology.
In fact, there are two major factors that will influence the formation of ICs in melt-processing. The first one is high temperature , When CD thread on the guest polymer chain, the main driving force of formation of complex is the large number of heat released from water molecules evaporation. Temperature has a significant influence on the formation of pseudo polyrotaxanes by affecting the threading-dethreading progress and the rate of threading process. The second one is high viscosity of melt . The viscosity of polymer melt is 10 to hundred times of polymer solution, and will notably reduce the diffusion of the two reactants and retard the yield of complex. It should be estimated by experiments to determine whether the final inclusion compound can be formed.
To our best knowledge, few publications have been reported concerning on PVA/CDs ICs by polymer processing. Hydroxypropyl-[beta]-cyclodextrin (HP-[beta]-CD) was used as host because of its good dissolvability in water. The aim of this paper was to prepare melt-processed PVA/CDs ICs by rotor mixing, single-screw and twin-screw extrusion using water as plasticizer and solvent. The illustration of fabrication ICs in a twin-screw extruder was shown in Fig. 1. As can be seen in Fig. 1, ICs between host HP-[beta]-CD and guest PVA were prepared through the force of mixing and shearing in twin-screw processing. The structure of products was confirmed by [sup.1]H-NMR and 2D-NOESY NMR. The influence of three different melt-processing on the yields, rheological, mechanical and thermal properties of the PVA/HP-[beta]-CD ICs was investigated and discussed in detail.
HP-[beta]-CD with substitution degree of 8.38 measured by [sup.1]HNMR was purchased from Zibo Qianhui Fine Chemical (Shandong, China). PVA 1799, with Mn of 74,800, degree of hydrolysis of 99%, was provided from Sinopec Sichuan Vinylon Works (Chongqing, China). Water was purified by distillation.
Preparation of PVAIHP-[beta]-CD Through Melt-Processing PVA (100 g) and HP-[beta]-CD (7, 14, 21, 28 g) dissolved in distilled water (70 g) were mixed at room temperature. The content of additional HP-[beta]-CD was expressed as phr (7, 14, 21, 28), respectively. Then, the mixtures were sealed for 24 h to enable the mixture to achieve sufficient diffusion of water into PVA.
The swelled PVA blends were melt mixed using a torque rheometer (Model RC-90, Hakke, USA) equipped with a mixer (Model 600) with rotor speed of 30 rpm at 90[degrees]C and load of 5 kg for 7 min. Torque profiles as function of temperature were recorded for subsequent analysis.
Twin-screw extrusion of swelled PVA blends was conducted using a Hakke PolyLab QC Rheometer and a co-rotating twin-screw extruder (model Rheomex OS PTW16, screw diameter (D) =16 mm, length to diameter ratio (L/D) = 40) with screw speed of 30 rpm and retention time of 8 min. The extrusion barrel was consisting of 10 heating zones at different temperature (90[degrees]C, 95[degrees]C, 100[degrees]C, 100[degrees]C, 105[degrees]C, 105[degrees]C, 105[degrees]C, 105[degrees]C, 100[degrees]C, 100[degrees]C), respectively.
Single-screw extrusion of swelled PVA blends was conducted by a Hakke Rheocord 90 and a single-screw extruder (model Hakke Rheomex 254, D = 19 mm, L/D = 25) with screw speed of 30 rpm. The temperature of the each heating zones was 90[degrees]C, 90[degrees]C, 100[degrees]C, and 95[degrees]C, respectively.
Fourier transform infrared (FTIR) spectroscopy of PVA and PVA/HP-[beta]-CD was conducted on a Nicolet 560 spectrophotometer (USA), ranging from 4000 to 400 [cm.sup.-1]. The samples were dried at 50[degrees]C for 12 h in a vacuum oven before test. The film tested was prepared by pressing a mixture. The structures of PVA/HP-[beta]-CD were determined by 400 MHz and 600 MHz [sup.1]H NMR (Bruker AV II) at 25[degrees]C using D20 and tetramethylsilane as the solvent and internal standard, respectively. Samples were extracted with methanol for 24 h using a soxhlet to remove residential HP-[beta]-CD. X-ray diffraction (XRD) patterns were collected on A Phillips diffractometer (X'Pert Pro MPD) using CuKa radiation at an accelerating voltage of 40 kV and scanning rate of 3%nin. The data were collected from 29 = 5-60[degrees]. The mixtures were molded into sheets with the thickness of about 1.0 mm at 120[degrees]C for 5 min. Tensile strength and elongation at break were evaluated using an Instron 5567 universal testing machine at a tensile speed of 50 mm/min. all the five dumbbell specimens were kept at a temperature of 25[degrees] C and a relative humidity of 50% for 72 h. The thermal stability of the samples (about 7 mg) was investigated using a thermal degradation analysis (TGA) (TA-Q600) under nitrogen from 50[degrees]C to 800[degrees]C at a heating rate of 20[degrees]C/min. Differential scanning calorimetry (DSC) measurements of samples were taken on a NETZCH 204 Phoenix differential scanning calorimeter with sample weight of 7-8 mg using nitrogen as the purge gas. The thermal properties of all samples were measured by heating from 25[degrees]C to 250[degrees]C at a heating rate of 10[degrees]C/min.
RESULTS AND DISCUSSION
Figure 2 shows the FTIR spectra of (a) PVA, (b) HP-[beta]-CD, and (c) PVA/HP-[beta]-CD. The typical bands of HP-[beta]-CD observed at 3401, 2928 and 1035 [cm.sup.-1] are assigned to stretching vibration of --OH, CH/CH2 and the coupled C--C/C--O, respectively . For pure PVA, the peaks at the wave numbers of 3309 [cm.sup.-1] (the stretching vibration peak of its side hydroxyl groups), 2922 [cm.sup.-1] (the -C[H.sub.2] group stretching vibration), 1139 [cm.sup.-1] (C--O stretching vibration of crystalline) and 1090 [cm.sup.-1] (C--O stretching vibration of amorphous) are characteristic peaks . The absorption bands of PVA/HP-[beta]-CD are similar to those of PVA. However, compared to pure PVA, the peak at about 1035 [cm.sup.-1] is shown in the FTIR spectra of PVA/HP-[beta]-CD corresponded to the coupled C-C/C-O stretching vibrations of HP-[beta]-CD, which proved that HP-[beta]-CD molecules existed in PVA/HP-[beta]-CD. It is also found that the absorption peak at about 3309 [cm.sup.-1] assigned to -OH stretching vibrations becomes stronger and blue shifts to 3340 [cm.sup.-1] after the addition of HP [beta]-CD. This result might be ascribed to the inter-molecular force formed between HP-[beta]-CD and PVA, which will be proved by the analysis shown later.
Figure 3 shows the *H NMR spectra of PVA (a), HP-[beta]-CD (b), and PVA/HP-[beta]-CD inclusion complex (c), respectively. The peaks corresponding to -CH and -OOCCH3 of PVA are observed at 3.92 and 1.8 ppm. However, those for the PVA/HP-[beta]-CD inclusion complex appeared at 3.91 and 1.7 ppm, which suggest that the protons of the PVA chain are included in HP-[beta]-CD and shielded by it. Compared with the chemical shifts of HP-[beta]-CD as shown in Fig. 3c, those for PVA/HP-[beta]-CD inclusion complex are not changed. The peaks corresponding to protons of PVA are labeled as 2, 2', 3, 3', 4 and those of HP-[beta]-CD are labeled as a-g [11-13].
[sup.1]H NOESY spectra for the host-guest complexes prepared by rotor mixing was recorded in Fig. 4. The spectrum of PVA/HP-[beta]-CD complex displays clear and strong NOE cross peaks between the protons of PVA methylene groups ([H.sub.2]) and the protons of HP-[beta]-CD cavity (Hc). Another proton located in HP-[beta]-ICD cavity ([H.sub.e]) and protons located outside cavity ([H.sub.a], [H.sub.h], and [H.sub.d]) have no cross peaks with PVA. These findings are in accordance with the results obtained from [sup.1]H-NMR analysis that the hydrophobic part of the guest PVA is embedded into the cavity of the CD.
In addition, using the Ha in the 4.96 and 5.14 ppm as the internal standard, the integration area of the protons of PVA/HP-[beta]-CD was calculated. As shown in Table 1, yields of the ICs prepared by melt-processing are 62.5%, 51.4%, and 46.8% in twin-screw, rotor and single-screw processing, respectively. It is conceivable that the yield of PVA/HP-[beta]-CD ICs is influenced by intensity of mixing and shearing in twin-screw, rotor, and single-screw processing. The sample experienced the most sufficient mixing in twin-screw processing because of the strongest shearing comparing to the other two processing methods, so the yield is highest in three different processing. As a result, the product of PVA/HP-[beta]-CD ICs is prepared successfully by melt-processing with yield of about 50%.
XRD is a powerful instrument for characterization of polymer crystalline structure . Figure 5 presents the XRD patterns of HP-[beta]-CD, neat PVA and PVA/HP-[beta]-CD complexes prepared by three different melt-processing. Neat PVA exhibits a significant crystalline peak at about 19.4[degrees], which is due to the strong inter- and intra-molecular hydrogen bonding. On the other hand, the pattern of the HP-[beta]-CD reveals two broad peaks in the range of 10-20[degrees], confirming its amorphous character. Obviously, with the addition of HP-[beta]-CD into PVA, the diffraction pattern of the IC exhibits a characteristic peak fairly similar to that of the crystalline PVA. This is probably because the cylindrical cavities of HP-[beta]-CD doesn't completely inclusive the PVA chain side by side, that is to say, PVA/HP-[beta]-CD ICs formed is sparse [9, 15]. Therefore, the difference in crystalline structure of complexes prepared by three different melt-processing might be ascribed to the degree of inclusion or the yield of PVA/HP-[beta]-CD complexes.
The torque curve of melting polymer is considered to be related with the viscosity and processability of the blends . The torque of steady-state is a measurement of melt viscosity for a stabilized morphology. It is desirable that the torque value of steady-state and stabilization time were bright to a minimum. Figure 6 compares the torque curves recorded for the PVA complexes with different content of HP-[beta]-CD in rotor mixing. Due to resistance of solid granules to the free rotation, the torque of PVA without HP-[beta]-CD increases rapidly and reaches a high value at the loading stage , As the polymer is melted, the torque decreases and reached a steady-state after about 3 minutes of mixing. Obviously, with the increase of HP-[beta]-CD content, the torque peaks are significant weaken. Meanwhile, the stabilization time of PVA/HP-[beta]-CD blends was shorten, which is benefit for the processability of the blends. Furthermore, the torque of PVA with the incorporation of 28 per HP-[beta]-CD increased slowly to a constant without peak. It is probably because the presence of hydrophilic HP-[beta]-CD can interfere with swelled PVA by forming hydrogen bonds with PVA matrix at the first stage, indicating that HP-[beta]-CD may act as plasticiser. However, the stable torques of PVA exhibit slight increase from 18.5 N m to 21.5 Nm with the increasing HP-[beta]-CD content. This also can be ascribed that the HP-[beta]-CD thread on the guest polymer chain protects PVA from melting by forming ICs with PVA. The result is in accordance with the analysis of DSC which will be shown later.
The incorporation of HP-[beta]-CD is also expected to improve mechanical properties. PVA/HP-[beta]-CD ICs prepared by Rotor mixing were stayed at 25[degrees]C and RH of 50% for 72 h before testing. Table 2 shows the tensile strength, elongation at break and strength at break of ICs.
The tensile strength and elongation at break and strength at break of PVA/HP-[beta]-CD complexes increases slightly at first with increasing 11P-[beta]-CD content. This property may be attributed to the formation of a complex structure by interactions of PVA and HP-[beta]-CD cavity, resulting in the combination of chain and increase of the molecular weight , The tensile strength, elongation at break and strength at break of PVA/HP-[beta]-CD complexes decrease gradually when the content of HP-[beta]-CD is above 7, 14, 21 phr, respectively, which may be ascribed that extra HP-[beta]-CD without forming IC with PVA weaken the interaction between PVA molecules. Thus, the content of HP [beta]-CD has an optimum value according on mechanical properties.
Thermal Properties Analysis
The thermal properties of various samples are investigated by DSC, as shown in Figure 7. The neat PVA shows a characteristic endothermic peak with onset temperature of 190.5[degrees]C, peak temperature of 221.2[degrees]C and melting enthalpy of 73.4 J/g. However, HP-[beta]-CD exhibits a broad endotherm in temperature of around 150[degrees]C and a sharp endotherm in temperature of around 230[degrees]C. It can be seen that the melting peaks of PVA/HP-[beta]-CD ICs shift to lower onset temperature of 162[degrees]C, 155[degrees]C, and 150[degrees]C for single-screw, rotor mixing and twin-screw, respectively. But the peak melting temperature has nearly no change range from 220.0[degrees]C to 222.8[degrees]C. Besides, there's a significant decrease in the melting enthalpy of 72.8 J/g, 66.2 J/ g, and 56.7 J/g for single-screw, rotor mixing and twin-screw, respectively, which is in reverse to their IC yield. This phenomenon can be ascribed to the fact that PVA chains partly included into the cylindrical cavities of HP-[beta]-CD are prevented from moving and crystallizing. Therefore, we can demonstrate that the thermal properties of PVA/HP-[beta]-CD complexes are also affected by melt-processing because of the degree of inclusion or the yield of PVA/HP-[beta]-CD complexes. This result confirms that the PVA is entrapped in the HP-[beta]-CD cavity by forming IC.
Figure 8 shows the TGA curves of PVA, HP-[beta]-CD, and PVA/HP-[beta]-CD complexes prepared by three melt processing. As shown in Fig. 6, the degradation of HP-[beta]-CD with excellent thermal stability starts from about 325[degrees]C and shows maximum decomposition at around 360[degrees]C. There is a weight loss below 10 wt% for samples, which attributes to water evaporation. Obviously, there are two major weight losses for neat PVA and PVA/HP-[beta]-CD complexes in the range of 200-500[degrees]C, which could be ascribed to the structural decomposition of PVA. Compared with the major degradation peak temperature of neat PVA (300[degrees]C), that of PVA/HP-[beta]-CD complexes notably shift toward higher temperature (about 355[degrees]C), which demonstrates that the HP-[beta]-CD improves the thermal stability of the PVA/HP-[beta]-CD complexes. The enhancement of thermal stability of PVA with the incorporation of HP-[beta]-CD could be explained by the protection of HP-[beta]-CD rings. Under protection of CD rings, it takes more time for PVA chains to dehydrate to form conjugated double bond structure , and finally to form carbon residue. The residues of complexes are 9.9 wt%, 6.7 wt%, and 5.1 wt% for twin-screw extrusion, rotor mixing and single-screw extrusion samples, respectively, which are in accordance with the HP-[beta]-CD inclusion yields measured by [sup.1]H-NMR.
In this article, polymer processing methods including rotormixing, single-screw and twin-screw extrusion have been proved to be rapid methods to prepare ICs in quantities. FTIR, [sup.1]HNMR, 2D NOESY 'H-NMR, and XRD characterization results showed that ICs between host HP-[beta]-CD and guest PVA were prepared through polymer processing, and cylindrical cavities of HP-[beta]-CD didn't completely inclusive the PVA chain side by side, that is to say, PVA/HP-[beta]-CD ICs formed was sparse. The yields of PVA/HP-[beta]-CD ICs were 62.5%, 51.4%, and 46.8% in twin-screw, rotor, and single-screw processing, respectively. By introducing cylindrical cavities of HP-[beta]-CD into the PVA chains, the thermal and mechanical properties of PVA were improved, however, the melting temperature and melting enthalpy were decreased.
NOMENCLATURE CD Cyclodextrin DSC Differential scanning calorimetry FTIR Fourier transform infrared HP-[beta]-CD Hydroxypropyl-[beta]-cyclodextrin IC Inclusion complex PVA Poly(vinyl alcohol) TGA Thermal degradation analysis XRD X-ray diffraction
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Hongqin Xiao, Xiangkun Kong, Jianjun Bao, Sheng Zhang
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
Correspondence to: J. Bao; e-mail: firstname.lastname@example.org and S. Zhang; email: email@example.com
TABLE 1. Yield of inclusion complexes (PVA/Water/HP-P-CD: 100/70/28) calculated from 'H-NMR spectra. Melt-processing Average CD numbers PVA/HP-[beta]-CD methods on a PVA molecule weight ratio Twin-screw extrusion 8.10 100/17.5 Rotor mixing 6.69 100/14.4 Single-screw extrusion 6.06 100/13.1 Melt-processing Yield of methods HP-[beta]-CD (%) Twin-screw extrusion 62.5 Rotor mixing 51.4 Single-screw extrusion 46.8 TABLE 2. Tensile strength and elongation at break of PVA/HP-[beta]-CD complex prepared by rotor mixing. PVA/Water/HP-[beta]-CD Tensile Elongation at (phr) strength (MPa) break (%) 100/70/0 24.8 335 100/70/7 26.6 398 100/70/14 28.0 360 100/70/21 25.5 303 100/70/28 23.7 301
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|Author:||Xiao, Hongqin; Kong, Xiangkun; Bao, Jianjun; Zhang, Sheng|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 2015|
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