A study on mediating the crystallization behavior of PBT through intermolecular hydrogen-bonding

Abstract

Intermolecular hydrogen-bonding could be formed between poly(butylene terephthalate) (PBT) and 4,4′-thiodiphenol (TDP) which was verified using advanced quantum-chemistry calculations. Blends of PBT and TDP were prepared through melt blending and the intermolecular hydrogen-bonding was characterized using Fourier transform infrared spectroscopy (FTIR). The results showed that intermolecular hydrogen-bonding formed between the carbonyl group of PBT and hydroxyl group of TDP, the carbonyl and hydroxyl absorption bands shifted to a lower wavenumber and the shape of the hydroxyl peaks became wider asymmetrically with increasing TDP content. The effects of hydrogen bonding on the crystallization and melting behaviors of PBT were investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). The results showed that both the non-isothermal melt-crystallization behavior and isothermal crystallization kinetics of PBT were inhibited by the addition of TDP. The overall isothermal crystallization rates of PBT in the PBT/TDP blends were obviously slower than that of pure PBT at the same crystallization temperature. The crystal structure of PBT did not change through the incorporation of TDP, while the crystallinity and the crystal size of PBT decreased with increasing TDP content.

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1 Introduction

Poly(butylene terephthalate) is one of the most important engineering thermoplastic materials due to its good chemical resistance, thermal stability, excellent processability and has wide applications in many fields such as electrical components, automotive, precise instruments and as a replacement for many metals.^1–5 However, the crystallization rate is very fast which results in flexural deformation of final products. Therefore, it is meaningful to overcome the defects of flexural deformation of final products by adjusting the crystallization behavior. Generally, incorporation other polymers including polycarbonate (PC)⁶ and poly(ethylene octane) (POE)⁷ into PBT to adjust the crystallization rate of PBT have been favored for easy realization. Nevertheless, poor compatibility of the polymer blends has been a big problem. In recent years, the fast development of nanotechnology has offered the possibility to mediate the crystallization and physical properties of polymers such as PBT. Organo-montmorillonite (OMMT) can increase the degree of crystallinity of PBT, which was reported by Chow.⁸ Fang et al. reported that multiwalled carbon nanotubes act as a nucleating agent which greatly increases the crystallization rate of PBT/multiwalled carbon nanotube nanocomposites.⁹ Bian et al. described that microwave exfoliated graphite oxide nanosheets (MEGONSs), used as a hetero-nucleating agent for PBT, and MEGONSs facilitated the crystallization of PBT.¹⁰ The mediation of the crystallization of PBT may also be achieved through blending with some filler. Deshmukh et al.^11,12 found that interfacial interaction between the functional groups of a filler and polymer would affect the overall crystallization process of PBT through comparative analysis on the effects of functional fillers including CaCO₃, nanoCaCO₃, wollastonite and talc on the crystallization of PBT. The results revealed that CaCO₃, wollastonite and a low amount of nanoCaCO₃ accelerate the crystallization rate of PBT, but the crystallization of PBT was inhibited because of the formation of chemical bonding interactions between talc and PBT. Furthermore, a novel technique has been employed, mediating the crystallization of PBT through blending with a reactive solvent (epoxy) and the overall crystallization rate of PBT at all temperatures increased with the incorporation of epoxy resin.¹³ In addition, some kind of low molecular weight additive, especially some low molecular weight dihydric phenols, may obviously change the crystallization-melt behavior and the mechanical and other properties of the polymers for the formation of intermolecular hydrogen-bonding between the phenol group and carbonyl, ether, ester, or other proton-acceptor functional groups of polymers. It has been reported that bisphenol A (BPA) inhibited the overall isothermal crystallization kinetics and the spherulite growth rate of poly(butylene succinate).¹⁴ Li et al. found that the crystallization of poly(ε-caprolactone) (PCL) was greatly hindered by the addition of 4,4′-dihydroxydiphenyl ether (DHDPE), when the content of DHDPE reached 40%, PCL changed to a fully amorphous elastomer.¹⁵ He et al. found that the thermal and dynamic mechanical properties of poly(L-lactic acid) (PLLA) and the physical properties of poly(3-hydroxybutyrate) [P(3HB)] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were greatly modified through blending with 4,4′-thiodiphenol (TDP) because of intermolecular hydrogen-bonding.^16,17 The blends of dihydric phenols and polymers mentioned above were all prepared through a solution blending method. However, the shortcoming of solution blending is that the process is more complicated and time-consuming and the use of a solvent has a certain harm to the environment. A melt blending method is closer to the actual production process. Besides, it is unknown whether or not the intermolecular hydrogen-bonding between dihydric phenols and polymers blends prepared through melt blending would be damaged. As far as we know, such research has not been reported so far. Therefore, in this work, TDP was used as a low molecular weight organic crystal mediator to blend with PBT, and PBT/TDP blends were prepared through a melt blending method. It is expected that intermolecular hydrogen-bonding would form between PBT and TDP, and it might have an obvious influence on the crystallization behavior of PBT. The intermolecular interactions and crystallization behavior of PBT/TDP blends were investigated using Fourier transform infrared spectroscopy (FTIR), quantum-chemical calculations, differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The crystal structure of PBT was identified using wide angle X-ray diffraction (WAXD).

2 Experimental

2.1 Materials and sample preparation

The poly(butylene terephthalate) (PBT) used in this study was DURANEX® 2002 EF2001 purchased from WinTech Polymer Ltd (Japan), with a density of 1.31 g cm⁻³. The viscosity average molecular weight is 1.3 × 10⁵ g mol⁻¹, measured using an Ubbelohde viscometer. 4,4′-Thiodiphenol (TDP) (AR) was purchased from Meryer Chemical Technology Co. Ltd. (Shanghai, China). The chain structure of PBT and the molecular structure of TDP are shown in Fig. 1.

Fig. 1 The chain structure of PBT and the molecular structure of TDP.

PBT/TDP blends were prepared through melt mixing in a co-rotating twin-screw extruder (type TSZ-30 China). The temperature profiles of the barrel were 240–250–250–240 °C from the hopper to the die. Before melt mixing, PBT was dried in a vacuum oven at 120 °C for 12 h. Binary blends of PBT/TDP with weight ratios of 100/0, 95/5, 90/10, 80/20 and 70/30 wt/wt were prepared for testing non-isothermal thermal properties, XRD and FTIR analysis. PBT/TDP ratios of 100/0, 98/2, 96/4 and 94/6 wt/wt were prepared for isothermal crystallization analysis.

2.2 Characterization and measurements

Quantum chemical calculations in this work were conducted under the framework of density functional theory (DFT) combined with the self-consistent field molecular orbital (SCF-MO) method. The DFT exchange–correlation functional was treated with Truhlar’s M06-2X.¹⁸ We have chosen this specific functional because of the accuracy to explore systems with hydrogen-bonding,^19,20 which usually plays an important role in the current work. Double-ζ plus double polarization basis sets 6-31G(d,p) were employed for C, H, O and S. IR spectroscopic properties were obtained based on the calculation of vibration frequencies on the basis of the optimized geometrical structures at the same theory level. All calculations were performed with the aid of Gaussian 09 code throughout the work.²¹

FTIR spectra of the PBT/TDP blends were recorded at room temperature by means of a Spectrum Two spectrometer (America). The scanned wavenumber range was 4000–400 cm⁻¹. All spectra were recorded at a resolution of 4 cm⁻¹ and 32 scans for each sample. Films of the pure PBT and PBT/TDP blends with a thickness suitable for FTIR measurements were prepared in 1% (wt/wt) chloroform/trichloroacetic acid solution. The ratio of chloroform and trichloroacetic acid in the solution was 4 : 1 (v/v). Then the solution was directly dropped onto the surface of a KBr plate and dried in a vacuum oven at 100 °C for 12 h to remove the solvent. The FTIR measurement of pure TDP was conducted using the KBr powder compression method for different sample characteristics and KBr powder was dried in a vacuum oven at 120 °C for 12 h before sample preparation.

Differential scanning calorimetry (DSC, TA Q20) was used to study the melting and crystallization behaviors of the blends. The calorimeter was calibrated in temperature and energy using indium. Dry nitrogen was used as a purge gas at a rate of 50 ml min⁻¹ during all the measurements. The samples for all of the measurements were 4–6 mg.

For recording the non-isothermal crystallization process, samples were first heated from 40 °C to 260 °C at a heating rate of 20 °C min⁻¹ and held at this temperature for 3 min to erase thermal history. They were then cooled to 40 °C at a heating rate of 10 °C min⁻¹ and subsequently subjected to second melting through heating to 260 °C at a heating rate of 10 °C min⁻¹ to determine the melting peak values (T_m).

For the isothermal crystallization process, samples were first heated to 260 °C and held for 3 min to erase thermal history, followed by rapid cooling to the selected temperature (T_c) at a cooling rate of 150 °C min⁻¹. After completing the crystallization, the blends were heated again at 10 °C min⁻¹ until completely melted.

WAXD measurements were performed at room temperature on a Rigaku D/MARX2200 PC diffractometer using Cu Kα radiation (λ = 1.542 Å). The measurements were recorded at 40 kV and 30 mA in 2θ ranges from 5° to 40° at a scanning rate of 2° min⁻¹. The samples used for XRD analysis were injection molded using a plastic injection molding machine (SZS-15 China).

The spherulite morphologies of pure PBT and PBT/TDP blends were observed using a LEICA DM2500P polarizing optical microscope (Germany). Samples were dissolved in chloroform/trichloroacetic acid solution and then cast on a glass substrate. The ratio of chloroform and trichloroacetic acid in the solution was 4 : 1 (v/v). The obtained thin films were melted at 260 °C for 3 min to erase thermal history and then rapidly cooled to 202 °C at a rate of 70 °C min⁻¹ for isothermal crystallization and kept at this temperature for observations.

Tensile tests were carried out at room temperature using a CTM 8050S electrical universal material testing machine (China) at speeds of 10 mm min⁻¹. At least five specimens were tested for each recorded value of elongation-at-break.

3 Results and discussion

3.1 Theoretical investigation of the hydrogen bond structure

The development of quantum-chemical calculations provides greater possibility to investigate the intermolecular interaction between PBT and TDP. Fig. 2(a) shows the obtained optimized structures of PBT and (b) the hydrogen-bonding between PBT and TDP, in which the OH group of the TDP molecule is hydrogen-bonded to the C=O group of PBT. The obtained C=O⋯HO distance (d) is 2.035 Å, which is calculated using:

d = [(x_i − x_j)² + (y_i − y_j)² + (z_i − z_j)²]^1/2 (1)

where (x_i, y_i, z_i) and (x_j, y_j, z_j) are the atomic Cartesian coordinates of the two atoms. It can be noticed that this distance is larger than 0.97 Å for general O–H bonds, while much smaller than 3.75 Å for the van der Waals distance for the interaction of oxygen and hydrogen atoms considering that their van der Waals radii are 1.55 and 1.20 Å for the carbon and hydrogen atoms, respectively.²² Thus the obtained C=O⋯HO distance here is within the scope of hydrogen bonding. Table 1 gives the structure parameters obtained from optimized structures (a) and (b) as well as the carbonyl stretching frequencies of pure PBT and the hydrogen bonded carbonyl group of PBT. The vibrational frequencies are directly obtained with the aid of Gaussian code through determining the second derivatives of the energy with respect to the Cartesian nuclear coordinates based on:where x = R − R₀ is the vector with the displacements of the atomic positions from their equilibria, while the derivatives are calculated at R = R₀. In structure (a), the calculated carbonyl stretching frequency of pure PBT is 1864 cm⁻¹, while the carbonyl stretching frequency of the hydrogen bonded carbonyl group of PBT is lower by as much as 45 cm⁻¹ than pure PBT, meaning that intermolecular hydrogen-bonding formed between PBT and TDP. Correspondingly, the distance d(C=O (ester)) of pure PBT is 1.204 Å, which is smaller than the d(C=O (ester)) of the hydrogen bonded carbonyl group of PBT (1.214 Å). The distance is indeed lengthened, indicating that hydrogen-bonding also formed between the OH of TDP and the ester C=O of PBT which cause lengthening of the carbonyl bond length.

Fig. 2 Optimized structures of (a) pure PBT and (b) hydrogen-bonded PBT and TDP (the blue balls denote the carbon atoms, red balls denote the oxygen atoms and yellow balls denote the sulphur atoms).

Table 1 Calculated C=O stretching frequencies of (a) pure PBT and (b) hydrogen-bonded PBT

	ν (C=O) (cm^−1)	d (C=O) (Å)
(a)	1864	1.204
(b)	1819	1.214

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3.2 FTIR analysis on specific interactions between PBT and TDP

In order to undertake more in-depth studies of the intermolecular hydrogen-bonding of PBT and TDP, experimental techniques were used combined with the theoretical method. FTIR spectroscopy is a particularly suitable technique for investigating a specific intermolecular interaction. The changes of the strength and position of IR absorption peaks resulting from some characteristic functional groups can be attributed to the existence of intramolecular or intermolecular interactions.^{15–17,23,24} Fig. 3(a) shows the FTIR spectra of the PBT/TDP blends in the carbonyl vibration region. The carbonyl group of pure PBT has its stretching mode located at 1718 cm⁻¹, whereas TDP shows no absorption in the carbonyl vibration region ranging from 1650 cm⁻¹ to 1800 cm⁻¹. For the PBT/TDP blends, the carbonyl absorption band slightly shifted to a lower wavenumber, indicating that intermolecular hydrogen-bonding formed between carbonyl groups in PBT and the hydroxyl groups in TDP which leads to the variation of the carbonyl absorption band. The data shows the high consistency between FTIR experiments and theoretical calculations. In addition, a weak absorption peak is observed in pure PBT at 1681 cm⁻¹ and it should be ascribed to intramolecular hydrogen-bonding which usually forms with hydroxyl groups and the carbonyl groups of conjugated systems.²⁵ When adding 5 wt% TDP to the PBT matrix, this absorption peak shifted to 1680 cm⁻¹ and then disappeared gradually with the further increase of TDP content. These observed changes result from switching from the intramolecular hydrogen-bonding of PBT to the intermolecular hydrogen-bonding between PBT and TDP. The FTIR spectra of PBT/TDP blends in the hydroxyl vibration region are shown in Fig. 3(b). According to Fig. 3(b), pure TDP has a broad absorption peak in the region from 3000 cm⁻¹ to 3650 cm⁻¹ and it is attributed to the hydroxyl stretching vibration absorption of hydroxyl group self-association. However, a very weak absorption centered at 3421 cm⁻¹ is observed in PBT and it should be attributed to the vibration of hydroxyl groups at the chain terminal of PBT. In PBT/TDP blends, the hydroxyl stretching vibration absorption band shifts to lower wavenumber and the peak shape becomes wider asymmetrically with increasing TDP content. All of these observed variations confirm the formation of hydrogen-bonding between PBT and TDP and this hydrogen-bonding interaction may further influence the crystallization behavior of the PBT in PBT/TDP blends.

Fig. 3 FTIR spectra in the carbonyl vibration region (a) and hydroxyl vibration region (b) of the PBT/TDP blends.

3.3 DSC analysis

According to the analysis above, it is clear that it is not a simple blending of PBT and TDP because of the hydrogen-bonding interaction. Therefore, studying the effect of the hydrogen bonding interaction on the melt and crystallization behaviors of the PBT in PBT/TDP blends is essential. The non-isothermal crystallization behavior of pure PBT and PBT/TDP blends is shown in Fig. 4(a). The crystallization exothermal peak (T_c) of pure PBT is sharp and the crystallization peak is located at 193.8 °C, indicating the fast crystallization rate of PBT. For the PBT/TDP blends, it is obvious that the crystallization exothermal peaks shift dramatically downward to the low temperature range and become wider with an increased weight fraction of TDP in the blends. When the TDP content reached 30 wt%, the value of T_c dropped sharply to 160.8 °C, showing a 33.0 °C reduction. It has been reported that PC is able to decrease the T_c values of PBT by about 13 °C with a content of up to 50 wt% and thermotropic liquid crystalline polymer (LCP) decreased the T_c values by 0.6 °C with a content of 20 wt% at a cooling rate of 10 °C min⁻¹.^26,27 The sharply decreased T_c of PBT in the PBT/TDP blends suggests that the formation of a hydrogen-bonding interaction between PBT and TDP effectively impedes the movement of the molecular chain and suppresses the crystallization ability of PBT. Various thermal parameters determined from the non-isothermal melt and crystallization behaviors are shown in Table 2. It can be seen from Table 2 that the crystalline enthalpy (ΔH_c) of pure PBT was measured to be 94.92 J g⁻¹. Moreover, the ΔH_c values were determined to be 80.30, 69.64, 58.99 and 49.03 J g⁻¹ for PBT/TDP blends with the TDP content of 5, 10, 20 and 30 wt%, respectively. The crystallinity of the PBT in the blends was also estimated from eqn (3) and the results are listed in Table 2.where ΔH_c is the measured heat of crystallization for the sample, and ΔH⁰_m is the heat of fusion for 100% crystalline PBT, which is 142.0 J g⁻¹.²⁸ w_f is the weight percent of the PBT matrix in the blend. From Table 2, the calculations demonstrate that the crystallinity of the PBT in the blends decreases with the increase of TDP content. Fig. 4(b) shows the heating scans of pure PBT and PBT/TDP blends and double melting endotherm peaks of these samples are displayed. The double melting behavior of these samples can be ascribed to melting-recrystallization phenomena during the heating process.²⁹ The low melting peak (T_m1) is associated with the fusion of imperfect crystals grown through normally primary crystallization and the high melting peak (T_m2) is for the more perfect crystals formed after recrystallization. Besides, it can be observed that the intensity of T_m2 decreased with the increasing content of TDP, indicating that imperfect crystals were relatively prone to exist and the recrystallization of PBT was less easy to occur because of the hydrogen bonding interaction. Moreover, the decrease of the melting temperature and melting enthalpy of the blends with increasing TDP content reconfirms the conclusion that the hydrogen-bonding interaction between PBT and TDP suppresses the crystallization ability of PBT.

Fig. 4 DSC curves for PBT and the PBT/TDP blends, (a) cooling curves (10 °C min⁻¹) and (b) heating curves (10 °C min⁻¹).

Table 2 Parameters of PBT and the PBT/TDP blends obtained from non-isothermal crystallization at a cooling rate of 10 °C min⁻¹

PBT/TDP (wt/wt%)	Tc (°C)	Tc(onset) (°C)	ΔHc (J g^−1)	Tm1 (°C)	ΔHm1 (J g^−1)	Tm2 (°C)	ΔHm2 (J g^−1)	Xc (%)
100/0	193.8	197.1	94.92	213.0	12.34	222.6	61.45	66.85
95/5	190.6	194.7	80.30	209.3	9.10	221.2	66.31	59.53
90/10	187.7	192.2	69.64	202.1	4.03	216.8	50.26	54.49
80/20	174.5	180.1	58.99	183.9	—		58.86	51.93
70/30	160.8	166.6	49.03	158.8	—		52.49	49.33

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3.4 Isothermal crystallization kinetics

The non-isothermal melt and crystallization behavior of PBT and the PBT/TDP blends were investigated in the above section and all of the results showed that the crystallization behavior of PBT was hindered by the intermolecular hydrogen-bonding formed between PBT and TDP. To further confirm the inhibitory effect of TDP on the crystallization rate of PBT, it is necessary to study the isothermal crystallization kinetics of the PBT/TDP blend system. However, we are aware that the temperature may be one of the important factors that affects hydrogen-bonding. The isothermal crystallization temperature of PBT is usually above 200 °C. Under an isothermal crystallization temperature such as this, does the hydrogen-bonding still exist? So molecular dynamics (MD) simulations were conducted by using the atom centered density matrix propagation (ADMP) model^30–32 with the M06-2X/6-31G(d,p) theory level at 202 °C (that is the crystallization experimental temperature in this work) for 2 ps with the time step size as 1 fs. It was found that the hybrid PBT/TDP structure at the end of the MD simulation exhibited not much difference compared with that at room temperature. The obtained distance for the C=O⋯HO interaction was also within the scope of hydrogen-bonding (2.198 Å), though it is a bit longer than 2.035 Å at room temperature for a rising temperature. Thus it is true that hydrogen-bonding still forms between PBT and TDP at the isothermal crystallization temperature. Fig. 5 shows curves of the relation between relative crystallinity (X(t)) and crystallization time for PBT and the PBT/TDP blends at a crystallization temperature of 206 °C. For the sake of brevity, only the results for isothermal crystallization at 206 °C are described, while other similar results have also been observed for the PBT/TDP blends at crystallization temperatures of 198, 200, 202 and 204 °C. The value of X(t) was calculated according to the following equation:where ΔH_t is the total heat evolved at time t and ΔH_∞ is the total heat evolved as time approaches infinity. From Fig. 5, the values of crystallinity up to 100% correspond to a time of about 12 minutes for PBT, while it is 62 minutes for the PBT/TDP blends with 6 wt% TDP, the total crystallization time being extended by about 50 min. The required time for achieving complete crystallization continues to increase with increasing TDP content at a certain crystallization temperature. This indicates that the formed hydrogen-bonding dramatically suppresses the crystallization of PBT.

Fig. 5 Relative crystallinity of pure PBT and PBT/TDP blends as a function of time when isothermally crystallized at 206 °C.

The Avrami equation is widely used to describe the isothermal crystallization behavior of polymer and polymer blends, as follows:

X_(t) = 1 − exp(−ktⁿ) (5)

Eqn (5) can also be converted to:

ln[−ln(1 − X_(t))] = ln k + n ln t (6)

where k is the Avrami rate constant containing the nucleation and the growth parameters, n is the Avrami exponent which is dependent on the mechanism of nucleation and the form of crystal growth. Fig. 6 shows the Avrami plot for pure PBT and the PBT/TDP blends when isothermally crystallized at 206 °C. It can be seen that each curve shows an initial linear portion, but some points deviate from the Avrami equation. The early non-linear stage corresponds to the primary crystallization process, which consists of the outward growth of lamellar stacks.³³ The upward deviation at the end of the plots is mainly attributed to the secondary crystallization, which is caused by impingement and nonlinear growth patterns of spherulites in the later stage of the crystallization process.³⁴ The values of k and n can be calculated from the intercept and slope of the straight lines and are listed in Table 3. According to Table 3, the n values of PBT and the PBT/TDP blends range from 2.24–2.84, indicating an athermal nucleation process followed by a hybrid crystal structure of planar lamellae and three-dimensional spherulitic growth.^33–35 The values of n are non-integer in all cases, which may be a result of the mixed growth or surface nucleation of crystals^36,37 and crystalline branching or the two-stage crystal growth during the crystallization process.³⁸ It is unscientific to compare the overall crystallization rate directly from the k values, because the unit of k is min⁻ⁿ and n is a variable at different T_c in the present work. Another important parameter is the half-time of crystallization (t_1/2), which is defined as the time at which the extent of crystallization is 50%. It can be calculated from eqn (7) which is derived from the Avrami equation and the values of t_1/2 are listed in Table 3. Generally, the reciprocal of t_1/2 is taken as a parameter of the crystallization rate of the polymer,^14,39,40 the relation between 1/t_1/2 and T_c is shown in Fig. 7. The values of 1/t_1/2 decrease with the increasing weight fraction of TDP at the same T_c (198–206 °C). When T_c = 198 °C, pure PBT shows a 1/t_1/2 of 1.51 min⁻¹, while with the addition of 6 wt% TDP the value of 1/t_1/2 decreases to 0.577 min⁻¹. The overall isothermal crystallization rates of PBT in PBT/TDP blends are obviously slower than that of pure PBT at the same crystallization temperature, indicating that crystallization of PBT is suppressed by the hydrogen-bonding interaction between the PBT and TDP. In addition, the values of 1/t_1/2 gradually decrease with increasing T_c for pure and blended PBT, which should arise from the low supercooling at higher T_c.⁴⁰ In short, the blending with TDP inhibits the isothermal melt crystallization process of PBT in the blends.

Fig. 6 Avrami analysis of pure PBT and the PBT/TDP blends isothermally crystallized at 206 °C.

Table 3 Crystallization kinetic parameters of pure PBT and the PBT/TDP blends at different temperatures

PBT/TDP (wt/wt%)	Tc (°C)	n	k (min^−n)	t1/2 (min)	1/t1/2 (min^−1)
100/0	198	2.65	20.6 × 10^−1	0.663	1.51
	200	2.62	5.85 × 10^−1	1.07	0.937
	202	2.47	2.25 × 10^−1	1.58	0.634
	204	2.62	4.49 × 10^−2	2.84	0.352
	206	2.60	8.21 × 10^−3	5.50	0.182
98/2	198	2.40	5.77 × 10^−1	1.08	0.927
	200	2.68	1.23 × 10^−1	1.90	0.526
	202	2.42	4.49 × 10^−2	3.10	0.323
	204	2.57	6.95 × 10^−3	6.00	0.167
	206	2.74	1.01 × 10^−3	10.8	0.0926
96/4	198	2.63	5.10 × 10^−1	1.12	0.890
	200	2.86	7.94 × 10^−2	2.14	0.468
	202	2.61	3.07 × 10^−2	3.29	0.304
	204	2.55	9.15 × 10^−3	5.46	0.183
	206	2.42	1.37 × 10^−3	13.1	0.0763
94/6	198	2.43	1.83 × 10^−1	1.73	0.577
	200	2.48	3.91 × 10^−2	3.18	0.314
	202	2.50	5.78 × 10^−3	6.77	0.148
	204	2.29	3.11 × 10^−3	10.6	0.0943
	206	2.24	7.67 × 10^−4	20.8	0.048

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Fig. 7 Plots of 1/t_1/2 versus T_c for pure PBT and the PBT/TDP blends.

3.5 Wide-angle X-ray measurements

Fig. 8 shows X-ray diffractograms for PBT and the PBT/TDP blends. The main peaks of PBT are located at around 15.9°, 17.2°, 20.4°, 23.2° and 24.8° corresponding to the planes of (011̄), (010), (10̄2), (100) and (111̄), respectively. The characteristic peaks of the PBT/TDP blends are similar to those of neat PBT. There are no new characteristic peaks appearing in the X-ray patterns of the blends, indicating that the inclusion of TDP has little effect on the crystal structure of PBT, and TDP may exist in the amorphous region of PBT. Generally, the XRD spectra of samples with small crystallite sizes have wide, low intensity diffraction peak profiles and samples with large crystallite sizes have sharp, high intensity peak profiles.¹¹ It is clearly seen from Fig. 8 that the intensity of the diffraction peaks weaken and the peak shapes become flat with an increasing TDP content, meaning that the crystallinity and the crystallite size of PBT reduce with the addition of TDP.

Fig. 8 WAXD curves for PBT and the PBT/TDP blends.

3.6 Polarized optical microscopy

The crystallization morphologies of pure PBT and the PBT/TDP blends isothermally crystallized at 202 °C for 10 min are shown in Fig. 9. It can be seen that the sizes of the spherulites in the PBT/TDP blends are smaller than that of pure PBT, which is in accordance with the XRD analysis. The observed result of smaller sized spherulites can be related to the cross-linking structure which formed in the PBT/TDP blends because of the hydrogen-bonding interactions shown in Fig. 10. This possible cross-linking structure actually belongs to a type of physical cross-linking and probably hinders the movement of molecular chains and the growth of crystals, resulting in the lower crystallization rate. In addition, it can be observed that the higher the TDP content, the more blurry the blend crystal is. This may be because of the formation of more defects resulting from more hydrogen-bonding due to the increasing TDP content.

Fig. 9 Polarized optical micrographs of PBT/TDP blends with different TDP content: (a) 0 wt%, (b) 2 wt%, (c) 4 wt%, and (d) 6 wt%, isothermally crystallized at 202 °C for 10 min.

Fig. 10 A schematic representation of the PBT–TDP inter-associated existence in the mixtures.

3.7 Elongation-at-break

The elongation-at-break (%) of the PBT/TDP blends is shown in Fig. 11. An obvious dependence of the elongation-at-break of PBT on the TDP content is observed and this dependence is embodied in the increasing elongation-at-break of blends with the increase of TDP content. Pure PBT shows an elongation-at-break of 449.7%, and with the addition of 30 wt% TDP, the value of elongation-at-break reaches 736.2%, indicating the enhancement of the ductility of the blend system.

Fig. 11 Variation of the elongation-at-break (%) of the PBT/TDP blends.

4 Conclusions

Quantum-chemical calculations and FTIR analysis verified that intermolecular hydrogen-bonding formed between PBT and TDP. TDP could be regarded as an effective crystal mediator and significantly suppressed the crystallization properties of PBT. When the TDP content reached 30 wt%, the crystallization temperature and crystallinity of PBT reduced by 33 °C and 17.52%, respectively. The elongation-at-break of PBT increased from 449.7% to 736.2%. For the overall isothermal crystallization process, the crystallization kinetics and spherulite morphology of PBT were obviously influenced by the TDP content. The PBT/TDP blends had slower crystallization rates than pure PBT. The crystallite size of PBT decreased with increased TDP content while the crystal structure of PBT did not change with the addition of TDP.

Acknowledgements

This work was gratefully supported by the National Natural Science Foundation of China (No. 21264012 and 51403210).

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