Melting and β to α transition behavior of β-PBA and the β-PBA/PVPh blend investigated by synchrotron SAXS and WAXD

Abstract

Synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) are used to monitor the melting and βα transition behavior of β-PBA and the β-PBA/PVPh blend during the heating process. After melt recrystallized at 10 °C, the β crystals are obtained. The lamellar thickness of β crystals in the neat PBA is similar to that in its PBA/PVPh blend and the long period for the former one is shorter than the latter one. At lower heating rate, α crystals start to appear at 39 °C suggesting the βα phase transition occurs. Compared with the neat PBA, the β-PBA in the blend transits to α crystals at a higher rate. At a higher heating rate, β crystals melt directly. Both at the lower and higher heating rate, the crystal structure along the a-axis is much more stable than that along b-axis in the β crystals. Moreover, the heating rate does not play a role in the melting mechanism of the β crystals. The melting of the β crystals in the neat PBA and its PBA/PVPh blend chose different mechanisms. The melting mechanism of the β crystals in the PBA/PVPh blend is attributed to the sequential melting. On the contrary, the melting of the β phase in the neat PBA is complex. Besides the peak of q = 0.54 nm⁻¹, a new peak appears at q = 0.32 nm⁻¹ in the SAXS profiles in the heating process suggesting the appearance of a population with a larger long period. The presence of two peaks is caused by the coexistence of two kinds of lamellar stacks. And the partial melting of thinner β-PBA lamellae occurs firstly in one kind of lamellar stacks.

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1 Introduction

Biodegradable polymeric materials have attracted a lot of interest of researchers in recent years due to their environmental advantages. For this kind of materials, biodegradability is one of the most important properties. However, the biodegradation rate is a key issue for the application of these polymers. To control the biodegradation rate, great efforts have been made. These efforts are generally focused on two approaches. One of them is the chemical structure regulation through synthesizing new materials. The other one is the supermolecular structure control via physical way. Taking the semicrystalline biodegradable polymers as an example, the supermolecular structures including lamellar thickness, lamellar orientation, crystal modification and size are all important factors to govern the biodegradation rate of the materials.

Poly(butylenes adipate) (PBA) is one of the typical biodegradable aliphatic polyesters, which exhibits two types of crystal forms, i.e. the α- and β-forms.^1,2 Its thermodynamically most stable α-form is characterized by a monoclinic unit cell with parameters of a = 0.67 nm, b = 0.8 nm, c (fiber axis) = 1.42 nm, and β = 45.5°. The β-form of PBA is characterized by chains in a planar zigzag conformation packed in an orthorhombic unit cell with lattice parameters of a = 0.505 nm, b = 0.736 nm and c (fiber axis) = 1.467 nm.^1,2 Recent studies demonstrated that the α- and β-PBA crystals exhibit different biodegradability. The α-PBA crystals degrade faster than the β-PBA crystals.³ Therefore, it is of great importance for regulating the crystal modification so as to control the biodegradation rate of PBA. The crystal structure modification of polymers is usually controlled by using nucleating agent,^4,5 controlling the thermal treatment process,^6,7 or crystallizing on a solid surface with epitaxial ability.^8,9 For PBA, isothermal melt-crystallization at high temperature produces α-form crystals, while β-form crystals are predominately produced at low crystallization temperature.^10–12 In the case of epitaxial crystallization, when crystallizing the PBA on highly oriented polyethylene or isotactic polypropylene substrates, β-PBA crystals will be developed regardless of the crystallization temperature.^13,14

It should be pointed out that blending provides an economical and widely used approach to produce new polymeric materials with desired properties. According to the crystallinity of polymers, the blend systems can be divided to crystalline/crystalline, amorphous/amorphous, crystalline/amorphous blends.^15–22 Once the semicrystalline polymer is involved, phase segregation of the crystalline phase with respect to amorphous or the other crystalline phase, such as the interlamellar, interfibrillar, or interspherulitic segregations, will be induced. The induced phase segregation consequently affects the crystallization kinetics, crystalline morphology, and even crystal modification.^23–27 For example, the incorporation of PVDF facilitates the formation of α-PBA crystals. It also accelerates the β to α phase transition of PBA upon annealing at a high temperature.^28,29 For crystalline/amorphous blends, PVPh has attracted tremendous interest in the preparation of novel polymer blends. Due to the existence of the hydroxyl group in poly(vinyl phenol) (PVPh), which readily leads to a specific hydrogen bonding interaction. A number of investigations have proved that PVPh exhibits excellent miscibility benefiting from the hydrogen bonding with aliphatic polyesters. In our previous studies, we have prepared PVPh/PBA blends. The hydrogen bonds between C=O group of PBA and OH group of PVPh have been identified.³⁰ It has been identified that the hydrogen bonds accelerate the phase transition of β to α crystals during the heating process. The melting of β phase before the β to α transition is observed in both neat PBA and blends of PBA and PVPh.

Although it is known that β phase is a thermally unstable phase and it transforms to α phase upon heating, the quantitative analyses have not been performed. The phase transition mechanism of PBA upon heating, either in pure sample or in the blend, remains unclear. In the present studies, we investigated the melting of β phase and its transition to α phase via synchrotron small-angle X-ray scattering and wide-angle X-ray diffraction during the heating process. Moreover, the effect of hydrogen bonds formed in the blend was also explored. The way of β phase melting is only related to the hydrogen bonds, irrespective of heating rate. On the contrary, the heating rate plays a decisive role on the transition of β phase: at slow heating rate (1 °C min⁻¹) β transits to α phase whereas at fast heating rate (10 °C min⁻¹) β phase melts directly.

2 Experimental section

2.1 Material and preparation procedures

Bacterially synthesized PBA (Mw = 12 000 g mol⁻¹) was purchased from Aldrich Co. It was purified by dissolving it in hot chloroform, precipitated in methanol and vacuum-dried at 60 °C. PVPh with glass transition temperature of 105 °C was purchased from Aldrich Cop. and used without further purification. PBA/PVPh blends were prepared by dissolving them together in tetrahydrofuran (THF) with blend ratio of PBA/PVPh = 80/20. After the majority of solvent had been evaporated, the resultant films were placed under vacuum at room temperature for one week to completely remove the residual solvent. The homogeneity of the blend has been checked by infrared spectroscopy. Thereafter, the polymers were melt molded at 60 °C under compression and cooled to room temperature, giving 0.5 mm thick sheets. The molded specimens were kept at 60 °C for 5 min to erase thermal history and then rapidly transferred to another hot stage precooled to 10 °C for isothermal crystallization.

2.2 Synchrotron SAXS and WAXD measurements

The SAXS and WAXD experiments were performed at the BL03XU beamline with wavelength λ = 1.0 Å in SPring-8, Harima, Japan. The distances between sample and detector were 1749.5 mm for SAXS and 76 mm for WAXD. The two-dimensional SAXS and WAXD patterns were simultaneously recorded with a CCD camera (Hamamatsu Photonics, Shizuoka, Japan, V7739P+ORCA R2) and an imaging plate (IP) system (Rigaku, Tokyo, Japan, RAXIS VII), respectively. The SAXS and WAXD profiles were obtained by circularly averaging their two-dimensional patterns as a function of magnitude of the scattering vector, q (0.1–1.7 and 5–30 nm⁻¹, respectively). Here q = (4π/λ)sin θ, λ is the wavelength of the incident X-ray beam, and 2θ is the scattering angle of SAXS or Bragg angle for WAXD, respectively. Each two-dimensional (2D) SAXS data was normalized by an ionization chamber placed in front of the sample and corrected further for the background run. WAXD data was only corrected for the background run.

The samples were placed in the DSC pan with two windows for the X-ray to transmit. The windows were covered by polyimide films. The SAXS, WAXD and DSC measurements were carried out simultaneously during the heating process of samples over 27–60 °C with two different heating rates: 1 °C min⁻¹ and 10 °C min⁻¹.

WAXD data analysis. The WAXD profiles were observed as functions of diffraction angle (2θ) and temperature (T). Degree of crystallinity was determined from WAXD data using a curve-fitting program where the diffraction profile was separated into crystalline PBA reflections and the amorphous halo. The apparent degree of crystallinity was defined as the ratio of the area under the resolved crystalline peaks to the total unresolved area. The length of a-axis and b-axis can be determined from the formulas for calculating crystal spacing.

SAXS data analysis. The background scattering due to the thermal density fluctuation was subtracted from the measured SAXS intensity. The one-dimensional correlation function was calculated from the scattering curves and defined as³¹
where the I(q) is the scattering intensity after subtracting thermal diffuse scattering and z is the correlation distance. Since the observed SAXS profiles can be collected only over the accessible finite q range, it is necessary to extrapolate them to both low and high q values for integration. The extrapolation to high and low q data has been conducted on the basis of the Porod law and Guinier law, respectively.^32–35

The determination of the characteristic parameters (L, l_a, l_c) from the one dimensional correlation function is shown in detail in ESI 1.† Here L is l_a plus l_c. L and l_c are long period of lamellae and lamellar thickness, respectively.

3 Results and discussion

Fig. 1 shows the WAXD profiles of PBA and its blend with PVPh. The Bragg diffractions at 2Θ ≈ 13.7°, 14.3°, 15.7° and 19.4° are respectively attributed to the (110), (111), (020) and (120) lattice planes of the β-PBA crystals. This indicates that the addition of PVPh does not change the crystallization behavior of PBA. With careful inspection, shift of the diffraction peaks toward larger diffraction angles can be identified for the blend, which is more evident in the enlarged profiles (see the inset of Fig. 1). This demonstrates unambiguously that the length of b-axis shortens in the blend.

Fig. 1 The WAXD profiles of neat PBA and the PBA/PVPh blend at room temperature.

Fig. 2a shows representative Lorentz-corrected SAXS profiles of the PBA and its blend with 20 wt% PVPh at room temperature. It is found that neat PBA shows the first- and second-order scattering peaks at q ≈ 0.53 nm⁻¹ and 1.12 nm⁻¹, respectively. This originates from the stacked crystalline lamellae in the sample. For the PBA/PVPh blend, there is also an obvious first-order scattering peak at 0.43 nm⁻¹. The left shift of first-order scattering peak suggests that the long period of PBA crystals increases with the addition of PVPh molecules. On the other hand, the second-order scattering peak can hardly be discerned. This indicates that the order of PBA lamellar stacks in the blend is not good.

Fig. 2 (a) Lorentz-corrected SAXS profiles of neat PBA and blend at room temperature. (b) The plots of Stroble–Schneider’s one dimensional correlation function.

To disclose the different structures of PBA blended with PVPh, the Lorentz-corrected SAXS profiles are analyzed by using the 1-D correlation function, which will provide us the information of lamellar thickness (l_c) and the long period (L) of the lamellar stacks. As shown in Fig. 2b, for the neat PBA sample, the long period is calculated to be 11.7 nm, which is composed of a thin layer of 5.1 nm and relatively thicker layer of 6.6 nm. One of these two layers corresponds to the lamellar thickness (l_c) while the other one is the distance of the interlamellar amorphous region (l_a). The assignment of l_c and l_a is governed by the magnitude of crystallinity within the lamellar stacks, which can be calculated by Φ_cs = l_c/L. The overall crystallinity of the sample can be obtained through Φ_c = Φ_s × Φ_cs, where Φ_s is the volume fraction of lamellar stacks in whole the sample. Considering that the Φ_s cannot exceed 1, Φ_c should less than Φ_cs. For the present case, the overall crystallinity of neat PBA (Φ_c) is estimated to be around 0.55. This demonstrates the crystallinity of PBA within the crystalline lamellar stacks should be higher than 0.55.

Therefore, the lamellar thickness of the PBA formed under present condition should be 6.6 nm, while the interlamellar amorphous layer is about 5.1 nm. For the PBA/PVPh blend, the long period is calculated as ca. 14.6 nm, which is composed of a 5.6 nm and a 9.9 nm layers. The fact that the melting point of PBA crystals in the blend is much lower than that of neat PBA as judged from DSC measurements indicates a thinner lamella of PBA in the blend than that of the neat PBA sample. Taking this into account, we assign the 5.6 nm to the lamellar thickness of the PBA in blend, while the 9.9 nm to the thickness of the interlamellar amorphous region. The notable decrease in overall crystallinity, ca. 0.32, also supports this assignment. It is reasonable by considering that the PVPh is excluded to the interlamellar region of PBA, which expanses the interlamellar amorphous region. From these data, the calculated Φ_s values for PBA in neat sample and blend are 0.98 and 0.8, respectively. This reveals an even distribution of PBA lamellar crystals in neat sample and the formation of mostly interlamellar with some interfibrillar (lamellar bundles) phase segregation of the blend. The interlamellar phase segregation also suggests that PVPh and PBA are miscible in the amorphous phase.

The above results show that blending PVPh with PBA does not change the crystallization behavior of the PBA, which crystallize all in its β-form. It influences, however, the crystalline morphology of the PBA with reduced lamellar thickness and increased long period of the lamellar stacks. To find out whether the PVPh affects the β to α phase transition of PBA or not, the heating processes of both PBA and its blend with 20 wt% PVPh at a rate of 1 °C min⁻¹ were monitored. Fig. 3 shows the WAXD results obtained during the heating processes of the PBA and its PVPh blend. From Fig. 3, several features should be addressed here. First of all, the intensities of the diffraction peaks contributed by the β-PBA crystals decrease gradually and disappear finally with the increase of temperature for both samples. At the same time, the peaks shift to left because of thermal expansion. To show the different thermal expansion coefficient in different direction of the unit cell, the temperature dependence of a and b unit cell parameters of the β-PBA crystals for both the neat and blend samples is presented in Fig. 4. It is clear that, in both cases, the lattice parameter along b-axis increases in a similar way with temperature. On the other hand, both temperature and the addition of PVPh do not affect the lattice parameter along a-axis evidently. This indicates that the packing of β-PBA molecules in a-axis direction is more stable compared with that in the b-axis direction. There are C=O groups which are able to form hydrogen bonds with H of CH₃ groups. The formation of sheets of hydrogen bonds between neighboring chains along the a-axis may favor the stability of a-axis. The exact molecular origin of the different expansion of a-and b-axes is still under study.

Fig. 3 The WAXD measurement results obtained from the (a) PBA and the (b) blend of PBA/PVPh sample at the heating rate of 1 °C min⁻¹.

Fig. 4 The temperature dependence of the a- and b-axis for β crystals in neat PBA and the blend of PBA/PVPh at the heating rate of 1 °C min⁻¹.

Second, when the intensity of the β-PBA diffraction peaks decreases with temperature, new diffraction peaks at 13.98° and 14.39° can be clearly seen at the latter stage of heating process. These two peaks are attributed to the (110) and (020) Bragg peaks of α-PBA crystals, respectively. Actually, these peaks appear already as shoulders of the (110) and (111) diffraction peaks of the β-PBA crystals when the temperature reaches about 39 °C. Henceforth, their intensities increase firstly with temperature and then decreases till disappearance. This reflects the occurrence of β to α phase transition first and then the melting of the transformed α-PBA crystals. The WAXD patterns of the pure PBA and the PBA/PVPh blend exhibit essentially a similar feature except for the different temperature dependence. To display the β to α phase transition process, enlarged WAXD profiles of neat PBA from 12° to 16° are shown in Fig. 5a. The curves are shifted on the intensity scale for clarity. As the (110) peaks of β- and α-PBA crystals coexist and overlap in a wide temperature range, the Gauss method is employed to do curve-fitting analysis for quantitatively estimating the change of these two phases. The WAXD pattern of neat PBA at 43.5 °C is chosen as an example and the fitted curves are shown in Fig. 5b.

Fig. 5 (a) The enlarged representative WAXD profiles for neat PBA from 13° to 16° displaying the β to α transition process. (b) Curve-fitting analysis for the WAXD pattern of neat PBA at 43.5 °C.

After curve-fitting, the (110) peak areas of β- and α-crystals have been obtained. The changes of the (110) peak areas of β- and α-crystals during heating are shown in Fig. 6a and b, respectively. The crystallinity measured simultaneously by DSC is also displayed in Fig. 6c. From Fig. 6, it can be found that the change of PBA crystals with temperature experiences three regions: melting of β-crystals; β to α phase transition; melting of α-crystals. Such result is consistent with our previous studies.³⁰ It is further found that the PBA in the blend has a lower crystallinity and the β-PBA crystals in the blend melt evidently earlier than those in the neat PBA sample. The melting of the β-PBA crystals in the blend proceeds also faster as elucidated by the dramatically decrease of the crystallinity. This is related to the thinner lamellae formed in the blend and reveals the influence of PVPh on the crystallization of PBA. It should be pointed out that the α-PBA crystals appear somewhat earlier in the neat PBA sample than in the blend. However, the increment of the α-PBA crystals in the blend is clearly faster than in the neat PBA sample. To check the β to α phase transition rate of PBA in different samples, a transition rate is defined as

where A_β(T) and A_α(T) are the (110) peak areas of β- and α-crystals at temperature T, respectively. The ϕ_β of neat PBA and the PBA/PVPh blend are displayed in Fig. 7.

Fig. 6 The change of (110) peak areas of (a) β- and (b) α-crystals, (c) crystallinity, (d) DSC for the neat PBA and the blend of PBA/PVPh sample at the heating rate of 1 °C min⁻¹.

Fig. 7 The transition rate of ϕ_β for the neat PBA (—○—) and the blend of PBA/PVPh (—●—) sample at the heating rate of 1 °C min⁻¹.

Fig. 7 clearly shows that the β-PBA crystals in the blend transform to α-phase at a faster rate. Moreover, the melting temperature of transformed α-PBA crystals in the neat PBA sample is much higher than that in the blend. One can notice that the transformed α phase even experiences a recrystallization process in the neat PBA indicated by the arrows in Fig. 6b and c.

From above WAXD results, it is evident that the crystallinity of the β-PBA crystals in both the neat and blend samples decreases continuously with temperature (Fig. 6c). Moreover, α phase shows up at around 38 °C (Fig. 6b). The appearance of α phase occurs after the melting of β phase. To further elucidate the melting process of β-PBA crystals, SAXS experiments were performed during the heating process. The SAXS profiles for neat PBA and its blend with PVPh are very different. Fig. 8 shows the Lorentz-corrected one-dimensional profiles of the sample in the heating process. As can be seen from Fig. 8a, the profile of neat PBA is similar to that of PBA/PVPh blend at room temperature except for the different scattering peak position due to different long period of the β-PBA crystals. During the heating from 27 °C to 38 °C, the two samples behave completely differently. For the neat PBA, the original scattering peak at 0.54 nm⁻¹ (designed as peak I) shifts slightly to a lower scattering angle and its intensity decreases gradually with the increase of temperature. Even a new peak at 0.32 nm⁻¹ (designed as peak II in Fig. 8a) appears at temperature above 32 °C. The position of this peak does not change with temperature but its intensity increase slightly. From the 1-D correlation profile, the corresponding long period of the peak I at room temperature is estimated to be 11.7 nm and the peak II corresponds to long period of 19.6 nm. The coexistence of the two peaks indicates there are two populations of β-PBA crystals with different long periods in the sample. These peaks diminish finally with the complete melting of all PBA crystals. On the other hand, the blend PBA/PVPh shows only one scattering peak locating at 0.43 nm⁻¹ and its position keeps at the same place in the whole melting process (see Fig. 8b). It shows no obvious peak shift but decreased peak intensity with temperature. The long period was estimated as 14.6 nm. These results demonstrate that the β-PBA crystals have different melting mechanisms in the neat and blend samples.

Fig. 8 The Lorentz-corrected one-dimensional SAXS profiles of (a) neat PBA from 28 to 47 °C, (b) blend PBA/PVPh from 28 to 44 °C.

Generally speaking, there are three different mechanisms to predict a morphological evolution of crystalline lamellar stacks under thermal treatment.³⁶ One model relies on sequential melting of individual lamellar crystals. In the crystalline lamellar stacks, there are variations in the lamellar thickness of the crystals (see ESI† S2a). At relative lower melting temperature, thinner crystalline lamellae melt firstly while the thicker ones are unaffected. It should be noted that the crystals of polymer melts in a wide temperature region. In this case, the average long period will increase evidently. The second mode is surface melting in which all lamellar crystals melt simultaneously from the surface. This will lead to crystalline lamellae lose a fraction averagely to each of two neighboring amorphous layers, leading to a decrease in lamellar thickness and crystallinity but unchanged long period (see Fig. S2(b)†). The third mode is stack melting. It means that crystalline lamellar stacks composed of thinner and/or less stable crystals, will melt completely at relatively lower melting temperature while the internal structure of the surviving crystalline lamellar stacks keep unchanged (see Fig. S2(c)†). SAXS accompanying to stack melting has two characteristic features, namely the peak maximum moves to smaller q values and the intensity decreases steadily due to the reduced fraction of sample giving rise to the observed SAXS pattern during proceeding of melt.

According to the SAXS patterns of neat PBA and PBA/PVPh blend taken during the melting process, we prefer to attribute the melting mechanism of the blend PBA/PVPh to the surface melting with the signatures of unshifted peak but gradually reduced intensity. The surface melting mode can also be proved by the temperature dependence of PBA lamellar thickness. As presented in Fig. 9, the lamellar thickness decreases linearly with the increase of temperature. The melting of neat PBA seems do not fit any of the above referred melting mechanism. According to the SAXS evolution of neat PBA, the melting mechanism of neat PBA may be attributed to the combination of two of those models. There may be most likely two types of crystalline lamellar stacks. One type of crystalline stacks consists of alternately arranged thick and thin lamellae (see Fig. 10a). The other one consists of lamellae with uniform thickness (see Fig. 10b). At the initial stage, these lamellar stacks may have a similar average long period at room temperature. During the heating process, the thinner lamellae in the first type of crystalline stacks melt first, which leads to an increase of long period of these crystalline lamellar stacks and results in the appearance of peak II, as can be seen in Fig. 10c. The melting of some lamellae at the temperature above 32 °C is also observed by in situ AFM measurement (see Fig. S3†). The second type of crystalline lamellar stacks with even lamellar thickness remains unchanged in the heating process (see Fig. 10d). The in situ AFM results (see Fig. S3†) also prove that some kinds of lamellae do not change their morphology in a wide temperature range. The formation of lamellae with different thickness may be caused by the secondary crystallization.

Fig. 9 The lamellar thickness of PBA crystals in the blends as a function of temperature heating at 1 °C min⁻¹.

Fig. 10 The sketch for the melting process of lamellar stack in neat PBA.

This results in the coexistence of peak II and peak I with reduced intensity in the heating process. Due to the existence of double peaks, it is difficult to calculate the lamellar thickness of neat PBA. Although the exact melting mechanism is unknown, it is clear that two populations of β-PBA crystals with different long periods coexist in partially melting sample and one of them is caused by the early melting of thinner β-PBA crystalline lamellae. As for the blends, in the amorphous region the molecular chains of PBA bonds with PVPh molecular chains which limit the secondary crystallization and leads to uniform distribution of lamellar thickness.

It was further found that heating rate plays a very important role on the transition of PBA from β to α phase. Fig. 11 shows the WAXD measurement results obtained from the neat PBA and PBA/PVPh blend samples at the heating rate of 10 °C min⁻¹. At room temperature the WAXD profile is similar to that shown in Fig. 1 and the (110), (111), (020), (120) Bragg peaks of β crystals are observed at 13.7°, 14.3°, 15.7°, 19.4°. With the increase of temperature, all the Bragg peaks weaken gradually and disappear finally. No new peak appears in the heating process, indicating that the β to α phase transition does not occur. In this case, the fast heating suppresses the transition. The thermal expansion in a- and b-axis directions of the β-PBA unit cell when heated at 10 °C min⁻¹ (see Fig. 12) behaves differently to that of the sample heating at 1 °C min⁻¹ (see Fig. 4). When the neat PBA is heated at 10 °C min⁻¹, the b-axis expands linearly in the whole melting process. In the case of PBA–PVPh blend, the expansion of b-axis with temperature is not so sensitive to heating rate.

Fig. 11 The change of (110) peak areas of (a) β-crystals, (b) DSC, (c) crystallinity for the neat PBA and the blend of PBA/PVPh sample at the heating rate of 10 °C min⁻¹.

Fig. 12 The temperature dependence of the length of a- and b-axis for β crystals in neat PBA and the blend of PBA/PVPh at the heating rate of 10 °C min⁻¹.

To observe the simultaneous change of β-PBA crystals in the melting process, the variation of (110) peak area of β-PBA crystals, the DSC heating scans and the crystallinity are shown together in Fig. 13. It can be seen that there is only one endothermic peak which corresponds to the melting of the β-PBA crystals. This further confirms the suppression of the β to α phase transition. Moreover, the melting temperature of β-PBA crystals in the blend is much lower than that in the neat PBA sample. The lower melting point can be attributed to the thinner β-PBA lamellae affected by the addition of PVPh. The above experimental results reveal that heating rate and blending with PVPh significantly affect the thermal behavior of β crystals.

Fig. 13 The change of (110) peak areas of (a) β-crystals, (b) DSC, (c) crystallinity for the neat PBA (empty circle) and the blend of PBA/PVPh (solid circle) sample at the heating rate of 10 °C min⁻¹.

The evolution of SAXS patterns for neat PBA and PBA/PVPh blend during the heating process at a rate of 10 °C min⁻¹ is also checked. There is also a new peak appearing at lower q value in the neat PBA samples in the heating process.

To elucidate the effect of heating rate on the two peaks of neat PBA, the peak positions and their intensity ratio (I_peakI/I_peakII) as a function of temperature at the two different heating rate are compared (see Fig. 14a and b). To our surprise, the change of peak position and I_peakI/I_peakII with temperature shows no dependence on the heating rate in the melting process. In addition, the peak position for the blends is also independent of the heating rate (see Fig. 14c). The results clearly suggest that heating rate does not play a role on the melting process of the β crystals, although it significantly alters the phase transformation of β crystals in the later stage.

Fig. 14 (a) The two peaks: peak I and peak II position of neat PBA as a function of temperature. (b) The intensity ratio of I_peakI/I_peakII of neat PBA as a function of temperature. (c) The peak position of the PBA/PVPh blend as a function of temperature. Here the empty and solid symbols demonstrate the heating rate of 1 °C min⁻¹ and 10 °C min⁻¹, respectively.

4 Conclusions

The synchrotron SAXS and WAXD profiles of the neat PBA and PBA/PVPh blend during the heating process are analyzed. The heating rate plays a decisive role on the β to α phase transition: at low heating rate (1 °C min⁻¹), β to α phase transition occurs. For the phase transition, the β crystals in the neat PBA start to transit to α crystals at relatively lower temperature compared with that in the blend PBA/PVPh. The former has, however, a slower transition rate than the latter. The melting of β crystals in the PBA and PBA/PVPh blend chose different mechanisms. The melting mechanism of the β crystals in the PBA/PVPh blend is attributed to the sequential melting. On the contrary, the melting of β phase in the neat PBA is somewhat complex. The appearance of a new peak at q = 0.32 nm⁻¹ in the SAXS profiles during heating suggests the generation of a population of PBA lamellar stacks with a larger long period. It may be caused by the partial melting of thinner β-PBA lamellae in some of the lamellar stacks.

At high heating rate (10 °C min⁻¹), β-PBA crystals melt directly. Both at the lower and higher heating rate, the molecular arrangement along a-axis is more stable than that along b-axis in the β-PBA crystals. Moreover, the heating rate does not play a role on the melting process of the β-PBA crystals, although it significantly alters the phase transformation of β-PBA crystals in the later stage.

Acknowledgements

This study was financially supported by the National Natural Science Foundations of China (no. 21004003, 21274010 & 51221002) and the Fundamental Research Funds for the Central Universities (no. ZY1104). The synchrotron radiation experiments were carried out in the second hutch of BL03XU, SPring-8 as constructed by the Consortium of Advanced Soft Material Beamline (FSBL) with the approval of the JASRI (Proposal no. 2010B7254).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04752d

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