Thermal, Crystallographic, and Mechanical Properties of Poly

Mar 16, 2017 - Poly(butylene succinate)/magnesium hydroxide sulfate hydrate whisker (MHSH) composites were prepared using the prepolymer from 1,4-buta...
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Thermal, Crystallographic, and Mechanical Properties of Poly(butylene succinate)/Magnesium Hydroxide Sulfate Hydrate Whisker Composites Modified by in Situ Polymerization Chuanhui Gao,* Zetian Li, Yuetao Liu, Xinhua Zhang, Jing Wang, and Yumin Wu College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ABSTRACT: Poly(butylene succinate)/magnesium hydroxide sulfate hydrate whisker (MHSH) composites were prepared using the prepolymer from 1,4-butanediol, succinic acid, and modified MHSH, which were characterized by FTIR and 1H NMR. The thermal, crystallographic, and mechanical properties of the PBS composites were investigated. Differential scanning calorimetry (DSC) demonstrated that glass transition temperature (Tg) and crystallization temperature (Tc) were increased with increasing of filler content. The thermal degradation process of PBS composites was analyzed by TG-FTIR, and a possible thermal degradation mechanism of PBS was proposed. The results indicated that the prepolymer was better than UMHSH in interfacial adhesion with PBS. The tensile strength, flexural strength, and modulus were significantly increased with the addition of filler mass fraction, especially when the composites were blended with the prepolymer.

compatibility and miscibility. Someya et al.14 modified montmorillonites using five different amine compounds to prepare PBS/clay composites. It was also reported that the modified montmorillonites were fine dispersion in PBS matrix. Among the much of the literature, the inorganic fillers (whether modified or not) and matrix were mostly simple mechanical mixing. Although the mechanical properties of the PBS composite were improved to some degree, the increment of tensile strength reported by some literatures was small when the inorganic fillers were used as reinforcing agent. Hence, enhancing the effect of mechanical properties, in particular the tensile strength, was not prominent when the composites were prepared by simple mixing. In this sense, it is urgent to develop the new filler or exploit the novel synthesis processes for PBS composites. Magnesium hydroxide sulfate hydrate whisker (MHSH), as the inorganic materials with high strength, modulus, and toughness, is a fibrous monocrystal. Which is a new kind of reinforcing filler for the composites.15,16 The molecular formula of MHSH is determined as xMg(OH)2·yMgSO4·zH2O and the family of MHSH comprises no more than 20 members.17 In most cases, MHSH was used as the flame retardant because the degradation of MHSH was an endothermic process and accompanied by the release of water vapor. At present, reports about the using of MHSH as filling material to enhance the polymer, especially for the PBS, are quite few. With regard to the preparation of PBS composites, the coprecipitation and in situ polymerization have been reported recently,18 and it is

1. INTRODUCTION As an emerging biodegradable aliphatic polyester, poly(butylene succinate) (PBS) has attracted tremendous attention because it is nontoxic and able to substitute petroleum-based materials in some areas.1−3 PBS is synthesized from succinic acid and 1,4-butanediol by the esterification reaction and condensation polymerization.4 Recently, due to the excellent biocompatibility,5 biodegradability,6−8 thermal stability, and processability,9 PBS has been widely used in high tonnage application such as disposable tableware, food and cosmetics packaging, and mulch films.10,11 Nevertheless, there are also some drawbacks limiting the extensive application of PBS in industries because of its unsatisfactory mechanical behavior, low melting point, high cost, and so on. Many studies about PBS composites are focused on the application of various fillers, such as inorganic whisker materials like silica, silicon carbide, potassium titanate, calcium carbonate, calcium sulfate, as well as cellulous materials like bamboo powder, wood flour, plant fiber, starch, and others (graphene oxide), which have been reported to reinforce PBS to improve the mechanical ability, enhance the thermal stability, reduce the cost, or accelerate degradation.12 Guo et al.13 prepared poly(butylene succinate)/hydroxyapatite (HA) composites by melt-mixing. The addition of HA significantly improved the mechanical properties and impacted on the crystallization of PBS. Compared to the neat PBS, the tensile modulus and flexural modulus were increased 66.4% and 69%, respectively. The crystallization rate was increased when HA content was up to 20 wt %. However, the poor interfacial adhesion between the fillers and polymer becomes the crucial factor to limit the development of inorganic nanocomposites. Recently, organic modification, maleic anhydride grafting, and addition of compatibilizers are the main method to heighten the © 2017 American Chemical Society

Received: Revised: Accepted: Published: 3516

September 29, 2016 March 9, 2017 March 16, 2017 March 16, 2017 DOI: 10.1021/acs.iecr.6b03784 Ind. Eng. Chem. Res. 2017, 56, 3516−3526

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Industrial & Engineering Chemistry Research proved that the fillers could disperse evenly in the matrix and keep the effective interfacial adhesion. Therefore, the comprehensive research on the MHSH reinforced the PBS combined with in situ polymerization is also lacking in the literature. In this paper, the functional fillers were prepared by organomodified reaction with the monomers and subsequent polymerization. The prepolymer was used as compatibilizer for the PBS. The MHSH was first modified chemically by silane coupling agent (KH560) to incorporate the epoxy groups which could react with −OH or −COOH. Second, the prepolymer was prepared by in situ polymerization through addition of modified MHSH to 1,4-butanediol and succinic acid instead of mixing simply. After purification, the obtained prepolymer contained fillers were further blended with the commercial PBS. For comparison, the PBS/UMHSH composites were also prepared. After that, morphology, mechanical abilities, nonisothermal crystallization behavior, and thermal degradation process were investigated. It is interesting that MHSHs show an unexpected enhancement effect on the mechanical properties. That is, the tensile strength is increased by 33.7% for PBS/UMHSH and 42.1% for PBS/MMHSH, respectively. The increment is far better than most of the other inorganic whiskers in previous reports. This is not only attributed to the characteristics of fillers but also the preparation method.

water. Then, the prepolymer, which was prepared by the chemical reaction of MMHSH and PBS oligomer, was obtained by condensation and polycondensation at 200 °C under vacuum (0.1 MPa) for about 3 h. The MMHSH mentioned in Results and Discussion was all via in situ polymerization. 2.2.3. Preparation of PBS Composites. The PBS/MMHSH composites were prepared by a Mini corotating twin screw extruder using commercial PBS and the PBS prepolymer obtained by in situ polymerization. The extrudate was directly injected into standard specimen with the mold temperature of 35 °C and the injection pressure of 0.6 atm. During the mixing process, the commercial PBS was 100 g and the PBS prepolymer (including PBS oligomer 11−12 g, MMHSH 5, 10, 15, 20, 25 g, respectively) was 16.53, 22.08, 27.26, 31.78, 36.36 g, respectively. Before the preparation of PBS/MMHSH composites, the PBS prepolymer was purified three times by Soxhlet extractor and then dried at 80 °C in drying oven. In comparison with the influence of different preparation methods, PBS/UMHSH composites were also prepared by blending with the commercial PBS (100 g) and unmodified whisker (5, 10, 15, 20, 25 g, respectively). 2.3. Characterization. 2.3.1. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra of MHSH, MMHSH, and KH-560 were recorded on a Thermo-Nicolet Avatar-360 apparatus in the wavelength range of 4000−400 cm−1 with resolution of 2 cm. 2.3.2. Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy. The chemical structure of oligomer was recorded on a Bruker AV500 NMR spectroscopy at ambient temperature with dissolving in deuterated chloroform (CDCl3). 2.3.3. Differential Scanning Calorimetry (DSC). The Tg, Tc, and Tm of the PBS/MHSH composites with various MHSH mass fraction were analyzed by DSC (204-F1, NETZSCH, Germany). A specimen of about 10 mg was held in aluminum seal before the measurement process. During the measurement process, the samples were first held at 150 °C for 2 min to eliminate thermal history and then quenched to −80 °C under a nitrogen atmosphere and again heated to 150 °C at a heating rate of 10 °C/min to evaluate the thermal properties. Subsequently, the samples were cooled to room temperature by 10 °C/min to evaluate the crystallization behavior. 2.3.4. Thermogravimetric Analysis Coupled with Fourier Transform Infrared Spectroscopy (TG-FTIR). Thermal stability of all the samples was characterized by thermogravimetric analysis (209-F3, NETZSCH, Germany) at the heating rate of 20 °C/min under nitrogen conditions. The gases produced during the decomposition process were qualitatively analyzed by FTIR (Avatar-360, Thermo-Nicolet, America). The two kinds of instruments were linked through interface connection. All the samples were heated from room temperature to 700 °C. 2.3.5. X-ray Diffraction Analysis. X-ray diffraction (XRD) profiles of pure PBS, MHSH, and various PBS/MHSH composite samples were measured under air at room temperature by a Rigaku D/max-rA X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.542 Å). The patterns were recorded for 2θ range from 5° to 60° with 8°· min−1 scan speed. The operation voltage and current were maintained at 40 kV and 150 mA, respectively. All the samples for XRD measurements were powder. 2.3.6. Scanning Electron Microscopy. Morphology of the composites was observed on SEM (JSM-6700F, JEOL, Japan) at an accelerating voltage of 12 kV at high vacuum. All the samples were cryofractured using liquid nitrogen and sputter-

2. EXPERIMENTAL SECTION 2.1. Material. Poly(butylene succinate) (PBS) (Mn = 5.3 × 104, Mw/Mn = 2.6) was produced by Mitsubishi Chemical Corporation, Japan. Magnesium hydroxide sulfate hydrate whiskers (molecular formula was MgSO4·5Mg(OH)2·3H2O, specific gravity = 2.3 g/cm3, length = 10−60 μm, diameter < 0.1 μm, aspect ratio = 30−40) were provided by NP Whisker Co.Ltd., Shanghai, China. Silane coupling agent (KH560) was purchased from Qingdao Haihua Flame Retardant Materials Co., Ltd., China. 1,4-Butanediol, succinic acid, anhydrous ethanol along with the catalyst tetrabutyl titanate were analytical grade and used as received. 2.2. Preparation of Composites. 2.2.1. Modification of MHSH. The MHSH needed to be modified by the silane coupling agent first. The process was as follows: the ethanol and distilled water based on the volume ratio 9:1 were added in a conical flask, and the pH was regulated to about 4−5 by acetic acid. The silane coupling agent about 4% (mass fraction) of the MHSH content was added, and the mixture was hydrolyzed at room temperature for 120 min. Then, the MHSH was dispersed in ethanol aqueous solution solution and accompanied by continuous stirring for 90 min at the temperature of 80 °C. After that, the MHSH was filtered and oven-dried at 80 °C for 3 h to obtain the modified MHSH. For convenience, the modified and unmodified MHSHs were noted as MMHSH and UMHSH, respectively. 2.2.2. Preparation of Prepolymer. To obtained the prepolymer by in situ polymerization, the 1,4-butanediol (54 g, 0.6 mol), 1,4-succinic acid (59 g, 0.5 mol), tetrabutyl titanate (0.2 g, 0.0006 mol), and MMHSH (5, 10, 15, 20, 25 g, respectively) were added into a four-necked round-bottom flask (250 mL capacity). The flask was equipped with a mechanical stirrer. A torque meter was immersed in a thermostatic oil bath and connected to a water-cooled condenser. First, the esterification reaction was kept for 5 h at 160 °C with mechanical agitation and purged by nitrogen to distill off the 3517

DOI: 10.1021/acs.iecr.6b03784 Ind. Eng. Chem. Res. 2017, 56, 3516−3526

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Industrial & Engineering Chemistry Research coated with gold to eliminate electron charging prior to examination. 2.3.7. Mechanical Properties. Tensile tests for the dumbbell-shaped specimen were carried out by the Universal Testing Machine (AI-7000S, China) equipped with a 10 kN electric load cell. The crosshead speed was 50 mm/min. The flexural measurements were carried out at a crosshead speed of 2 mm/ min. The sample dimensions for tensile and flexural tests were 75 × 10(4) × 2 mm3 and 75 × 10 × 4 mm3, respectively. The hardness of composite was characterized by Rockwell apparatus with 4 mm thickness specimens. The indenter adopts the ordinary steel ball. The hardness value was dimensionless and calculated according to the indentation. Five samples were measured, and final results were reported as averaged values. All of the tests were done at around room temperature (21 °C).

3. RESULTS AND DISCUSSION 3.1. Characterization of Organo-Modified and Chemical Structures. The modification of MHSH, as well as the preparation of PBS prepolymer, was described in the Experimental Section. The FTIR spectra of MHSH, modified MHSH, and KH-560 are shown in Figure 1. The adsorption peaks from 3400 to 3650 cm−1 are ascribed to the stretching vibration of hydroxyl groups. The peak observed at 1630.79 cm−1 is attributed to the stretching vibration of the crystal water. The strong signals which are visible at 1116.91 and 641.21 cm−1 are the absorption peak of SO42−.19 By comparison to Figure 1a, it can be seen from Figure 1b that the absorption peak detected at 1630.79 cm−1 shifted to 1616.18 cm−1. This is because the formation of intramolecular hydrogen bonding makes the peak shift to lower wavenumber, which further manifests that the KH-560 is successful to link with the MHSH by the hydrogen bonding. There are also many intensive absorption peaks observed in the figure as indicated by the arrows, which is the characteristic absorption band of water. After modification, the crystal water may be changed to free form partly. The FTIR spectra of silane coupling agent are also shown in Figure 1c, and the characteristic absorption band of Si−O−C stretching vibration is detected at 1087.20 cm−1. During the modified process, −OCH3 is replaced by −OH due to the hydrolysis. The absorption peak of Si−O−C disappears, and the absorption peak of Si−OH is overlapping with the original hydroxyl groups. The chemical structure of prepolymer was confirmed by 1H NMR. As shown in Figure 2, the resonance shift observed at 2.63 (δHa) ppm belongs to the methylene proton of succinic acid unit in the prepolymer. The chemical shifts observed at 1.71 (δHb) and 4.12 (δHc) ppm are caused by the two different methylene protons of 1,4butanediol unit. The weak peaks of 3.67 (δHd) and 1.63 (δHe) are attributed to the protons on terminal methylene groups.20 It is indicated that the excessive 1,4-butanediol successfully links with terminal carboxyl of succinic acid. The schematic diagram of modified reaction mechanism is exhibited in Figure 3. The silane coupling agent after the hydrolysis first attaches to the surface of the whisker by hydrogen bonding. The epoxy group is at the other end. In the in situ polymerization, a new kind of ester bond is synthesized by ring-opening reaction of epoxy group with the repeating unit with end carboxyl of PBS (shown in “a”). Simultaneously, the new formation of hydroxyl could further react with the carboxyl of the repeating units (shown in “b”). It is believed that the combination of chemical bonds contributes to better miscibility with the matrix and improves the mechanical properties. That is

Figure 1. FTIR spectra of (a) UMHSH, (b) MMHSH, and (c) KH560.

Figure 2. 1H NMR spectrum of the PBS oligomer. 3518

DOI: 10.1021/acs.iecr.6b03784 Ind. Eng. Chem. Res. 2017, 56, 3516−3526

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Industrial & Engineering Chemistry Research

PBS/MMHSH composites are both adopted. The DSC curves of PBS/MHSH composites have been performed in order to evaluate the effect of MHSH on the thermal properties and crystallization properties of PBS matrix. Figure 4 shows the second DSC heating and subsequent cooling scans of PBS/ UMHSH and PBS/MMHSH composites. It is worth noting that the melting temperature of PBS/MHSH composites is nearly unacted on MHSH whisker. However, another mild melting peak is detected at about 44 °C after the PBS prepolymer is added, resulting from the small molecular weight of the PBS oligomer. Nevertheless, the crystallization temperature (Tc) is increased with the increasing amount of UMHSH whisker. The Tc value obtained by cooling is increased from 68.3 to 79.4 °C, when the UMHSH content is 25 wt %. For the PBS/MMHSH composites, the Tc is slightly reduced rather than increased comparing with the pure PBS. All results obtained from the DSC analysis, including glass transition temperature (Tg), crystallization temperature (Tc), crystallization enthalpy (ΔHc), melting temperature (Tm), melting enthalpy (ΔHm), as well as the calculated degree of crystallinity (Xc, %), are summarized in Table 1. The degree of crystallinity (Xc, %) is calculated by ΔHm/ΔH0m(1 − Wf),21 where ΔH0m is the theoretical value of 100% crystallized crystal and Wf is the weight fraction of whisker in the composites. The ΔH0m of PBS is taken as 110.5 J/g.22 As can be seen from Table 1, the Tg value is increased slightly with the increasing addition of MHSH whisker, suggesting that the fillers can hinder the migration of polymer amorphous chain segments, which is reasonable since the inorganic materials have high stiffness and strength. Similarly, the incorporation of the MHSH whisker not only reduced the crystallization enthalpy (ΔHc) but also decreased the melting enthalpy (ΔHm). Compared to the pure PBS, when the UMHSH content is 25 wt %, the values of ΔHc and ΔHm are reduced by 23.3% and 41.7%, respectively. As a result, the

Figure 3. Modified reaction mechanism between whisker, silane coupling agent, and the repeating unit with end carboxyl of PBS: (1) modified reaction; (2) in situ polymerization.

confirmed by the gradually increasing tensile strength, flexural strength, flexural modulus, very different hardness as well as the morphology results. The good miscibility with the PBS is also confirmed by SEM images shown in Figure 9. 3.2. Differential Scanning Calorimetry (DSC). To study the influence of whisker and prepolymer for the thermal properties of PBS, in the present paper, the PBS/UMHSH and

Figure 4. DSC heating (a, c) and cooling (b, d) curves of PBS, PBS/UMHSH, and PBS/MMHSH composites. 3519

DOI: 10.1021/acs.iecr.6b03784 Ind. Eng. Chem. Res. 2017, 56, 3516−3526

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Industrial & Engineering Chemistry Research Table 1. Thermal Properties and Degree of Crystallinity of Neat PBS and Its Composites ΔHm (J/g)

Tm (°C) sample

Tg (°C)

Tc (°C)

ΔHc (J/g)

PBS PBS−5%UMHSH PBS−15%UMHSH PBS−25%UMHSH PBS−5%MMHSH PBS−15%MMHSH PBS−25%MMHSH

−35.3 −34.8 −33.7 −33.3 −35.7 −34.0 −34.4

68.3 76.2 77.2 79.4 66.4 66.8 67.7

65.53 60.39 57.61 50.29 62.09 54.93 52.95

Tm1

Tm2

ΔHm1

111.5 111.7 111.5 111.0 43.4 43.5 44.3

ΔHm2

Xc (%)

63.39 55.9 52.13

73.4 55.3 58.4 57.0 62.9 60.2 60.4

81.04 58.11 54.86 47.28 112.6 111.9 113.2

3.11 2.76 2.34

Figure 5. TGA and DTG profiles of UMHSH, PBS oligomer, and PBS composites. (d) was the corresponding local amplification figure from 250 to 400 °C.

3.3. Thermogravimetric Analysis Coupled with Fourier Transform Infrared Spectroscopy (TG-FTIR). The thermal stability of PBS and PBS/MHSH composites is shown in Figure 5. The weight loss of all the samples shows a similar downward trend (Figure 5a), while the plot (Figure 5b) mainly reflects the degradation process. As reported by Tran et al.,22 the peaks on the derivative curves, corresponding to the highest weight loss rates, were used to determine the degradation temperature of the PBS and composites. The PBS matrix and PBS−5% UMHSH composite have only one peak; however, the composites present two peaks at about 334−338 °C and 391−396 °C when the adding of MHSH reaches 15 and 25 wt %. This is attributed to the dehydration of MHSH at 295 and 401 °C, which is also reported by Gao et al.17 The decomposition reaction equations are as follows:

calculated degree of crystallinity (Xc, %) is also be affected, which is significantly reduced from 73.4% to 55.3% for the PBS−5% UMHSH. By comparison of the PBS/UMHSH and PBS/MMHSH composites, the ΔHc and ΔHm increase slightly with incorporation of PBS prepolymer when the composites contain the same amount of MHSH. In particular, the value of Tc has an obvious difference. All of these are caused by the additional prepolymer. The decreasing of Xc, %, greatly benefits the industrial processing of PBS/UMHSH composites because the high degree of crystallinity can lead to the poor ductility. The decreasing of Xc, %, is conducive to film-forming. The whisker is a kind of acicular crystal, which grows in the form of monocrystal. The incorporation of the UMHSH makes the composites easy to form the crystal nucleus, so the Tc value is increased. On the other hand, the weight percentage of PBS in the composites is reduced with the addition of whisker. The interaction between the whisker and polymer limits the movement of polymer chains, so the molecular chains participating in the crystallization are reduced. It can be concluded that the MHSH serves as nucleating agent, which promotes the nucleation of PBS and accelerates the rate of crystallization at higher temperature, although the PBS oligomer can be weakened by this effect.

MgSO4 ·5Mg(OH)2 · 3H 2O → MgSO4 ·5Mg(OH)2 + 3H 2O (from 260 to 360 °C) MgSO4 ·5Mg(OH)2 → MgSO4 ·5MgO + 5H 2O (from 360 to 800 °C) 3520

DOI: 10.1021/acs.iecr.6b03784 Ind. Eng. Chem. Res. 2017, 56, 3516−3526

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Industrial & Engineering Chemistry Research The TGA heating trace and derivative curve of the PBS oligomer are shown in picture Figure 5c. It can be seen that the maximum weight-loss temperature is 396 °C. The specific data afforded by TGA are listed in Table 2. The T5wt%, T50wt%, and Table 2. TGA Data for the Samples sample

T5wt% (°C)

T50wt% (°C)

Tmax (°C)

residue at 700 °C (wt %)

UMHSH PBS oligomer PBS PBS−5%UMHSH PBS−15%UMHSH PBS−25%UMHSH

298 278 336 334 330 323

385 382 388 394 395

401 396 390 393 396 392

71.2