Introduction

The miscible blends and block copolymers comprised of semicrystalline polymers often show the formation of interpenetrating spherulites (IPS), i.e., spherulitic growth of one component that continues to grow inside the spherulites of the other component [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. The constituent with a lower melting point usually has a higher crystallinity and growth rate in the blends reported so far.

The formation process of IPS is classified into two types: the simultaneous and stepwise growth of spherulites. In the simultaneous growth process, both substances, which generally have a difference in the melting point, T m , of approximately 30 K or less, simultaneously develop spherulites, and IPS are formed after the growth fronts of the different components contact each other [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. In the stepwise process, which is generally observed when the T m difference is approximately 50 K, the spherulites with a higher T m grow first and fill the whole volume. Then, the spherulites of the other component nucleate and develop inside the existing spherulites [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].

When IPS are formed, the crystals of the penetrated spherulites may suppress the crystallization of the penetrating spherulites, which develop in restricted regions such as the interlamellar and interfibrillar regions [11, 22,23,24, 33]. Such suppression may result in a reduced degree of crystallinity, which is one of the most fundamental properties of semicrystalline polymers from scientific and engineering viewpoints. However, such effects have not been examined sufficiently thus far. The morphological formations and crystallization kinetics have mainly been investigated in the papers on IPS published so far.

The only work that has focused on the degree of crystallinity in IPS is, to the best of our knowledge, a paper on blends of poly(ethylene succinate) (PES) and poly(ethylene oxide) (PEO) using pulsed nuclear magnetic resonance (pulsed NMR) [34]. Nevertheless, the simultaneous crystallization of PES and PEO made the evaluation of the crystal content and the degree of crystallinity less precise. Moreover, the two types of PEO crystals in the simultaneous process, i.e., those that had grown inside and outside the PES spherulites, also made discussing the influences of the PES crystals difficult. Only the former type of PEO spherulites is influenced by the PES crystals. Furthermore, no discussion on the spin-spin relaxation time, T2, or the mobility of the chain segments was presented.

The difficulties described in the previous paragraph can be resolved by investigating IPS that exhibit stepwise spherulitic growth. The blends of poly(butylene succinate) (PBS) and PEO represent such a system, and PBS is the higher-T m component [19,20,21,22,23,24]. All PEO spherulites grow inside the PBS spherulites under the influence of the PBS crystals, and the amount of the crystals of the two substances can be separately determined. PBS/PEO blends exhibit homogeneous melts above T m . At elevated temperatures, the melts are reported to undergo phase separation [35]. The phase diagram has a lower critical solution temperature that is substantially higher than the T m of PBS.

Pulsed NMR is an effective method to determine the absolute values of the degree of crystallinity based on the number of protons by analyzing the spin-spin relaxation [34, 36,37,38,39,40,41,42]. The degree of crystallinity can be obtained from the magnitude of the Gaussian or Weibull function with a T2 on the order of 10−5 s. The crystallization process can also be analyzed if the crystallization rate is sufficiently slower than the acquisition rate of the data [34, 36, 37]. Slow processes such as secondary crystallization can also be analyzed by pulsed NMR, whereas the small heat flow of the secondary process makes an analysis by differential scanning calorimetry very difficult.

The aim of this paper is to discuss the degree of crystallinity in PBS/PEO blends, which exhibit stepwise spherulitic growth in the IPS formation process. The degree of crystallinity of each component in the blends is derived from the crystal content obtained by pulsed NMR. The mobility of the chain segments and the factors contributing to the degree of crystallinity of the constituents are discussed.

Experimental

PBS (Mn = 43,000, Mw = 84,000, Tm° = 120 °C, T g  = −32 °C) and PEO (Mn = 70,000, Mw = 404,000, Tm° = 65 °C, T g  = −65 °C) were purchased from Sigma-Aldrich Company. Weighed PBS and PEO were dissolved in chloroform. A solution of approximately 1 wt.% was then dried in a fume hood for 3 days and then under a vacuum at 35 °C for 3 days. The weight ratios of the blends were PBS/PEO = 2/8, 3/7, 4/6, and 5/5. Hereafter, the first and second digits indicate the contents of PBS and PEO, respectively, in the blends.

The pulsed NMR measurements were carried out with a JNM-25 MU (Jeol) as follows. The solid samples in NMR tubes with a diameter of 10 mm were first melted in a dry block bath at 120 °C before they were quickly transferred into the NMR probe, whose temperature was controlled at the crystallization temperature, T c . The spin-spin relaxation signals of 1H at a resonant frequency of 25 MHz were repeatedly collected using a solid echo pulse sequence [43]. Four decay signals were averaged for the noise reduction. The details of the pulsed NMR measurements can be found in other papers [34, 36, 37].

The spin-spin relaxation decay, I, of the blends was decomposed into Weibull and exponential functions after the onset of crystallization, as

$$I\left( t \right) = A_1\exp \left[ { - \frac{1}{\mu }\left( {\frac{t}{{T_2^{(1)}}}} \right)^\mu } \right] + A_2\exp \left( { - \frac{t}{{T_2^{(2)}}}} \right)$$
(1)

where A i and \(T_2^{\left( i \right)}\) are the amplitude and spin-spin relaxation time of component i, respectively, t is the relaxation time, and μ is the parameter of the Weibull function. Components 1 and 2 were assigned to the immobile crystalline and mobile amorphous parts with T2 values typically on the order of 10−5 s and 10−3‒10−4 s, respectively. The fractional amount of the crystals, f, based on the number of protons is defined by

$$f = A_1/\left( {A_1{\mathrm{ + }}A_2} \right)$$
(2)

The crystallization process was also monitored using a polarized optical microscope (Olympus, BX51) equipped with a temperature controller (Linkam, 10002 and TMS-600) and a digital camera (Roper Scientific, CoolSNAP5.0). The blend samples, whose film thickness was approximately 10 µm, were placed between two optical glass plates. They were melted at 140 °C for approximately 3 min and then quenched to the desired T c .

Results and discussion

The polarized optical micrographs of the crystallization process at 50 °C in a 3/7 blend are displayed in Fig. 1. The spherulites of PBS, which is the higher-T m component, nucleated first and filled the whole sample, as shown in Fig. 1a. Then, PEO nucleated in the spherulites of PBS and grew with a spherulitic shape, as shown in Fig. 1b, c. The ranges of the T c and the blend composition for the pulsed NMR measurements were determined by optical microscopy to ensure the crystallization rate was suitable for the pulsed NMR measurements.

Fig. 1
figure 1

The polarized optical micrographs of the crystallization process in a PBS/PEO = 3/7 blend at 50 °C. a Completion of the growth of PBS spherulites, which filled the whole volume, and (b, c) nucleation and growth of a PEO spherulite inside the PBS spherulites. The pictures in (b) and (c) were taken 10 s and 31 s after (a), respectively

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Figure 2 shows an example of a normalized spin–spin relaxation function, I. The magnitude of the fast-decay component, which was fitted to the Weibull function, indicates the fraction of the crystals based on the number of protons.

Fig. 2
figure 2

Normalized spin-spin relaxation function, I, of the relaxation time, t, decomposed into Weibull and exponential functions for a PBS/PEO = 3/7 blend crystallized for 1.341 × 103 s at 48 °C

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The changes in f and T2 for the crystalline component with a crystallization time, t, for the two homopolymers are indicated in Fig. 3, which was obtained by curve fitting the relaxation functions repeatedly acquired at different t. The increase in f represents the primary crystallization, namely, the crystal growth. The time range of the primary crystallization for PBS was earlier than that for PEO (Fig. 3a). This result is in accordance with that in Fig. 1. PBS, the higher-T m component, crystallizes before the onset of PEO, the lower-T m component. The slight increase in f for PEO after the primary crystallization is ascribed to the secondary crystallization. In contrast, the secondary process was almost negligible for PBS.

Fig. 3
figure 3

a The fraction of crystals, f, and (b) the spin-spin relaxation time, T2, of the PBS (open) and PEO (filled) homopolymers crystallized at 50 °C as a function of the crystallization time, t

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As displayed in Fig. 3b, the T2 of PBS was nearly constant throughout the crystallization process, while that of PEO exhibited an increase during the primary process and a decrease during the secondary process. A possible reason for the increase in the primary crystallization process of PEO is its fast crystallization rate, which results in the formation of disordered crystalline structures. The segments in the disordered crystals are expected to have a higher mobility and longer T2. The slight decrease in T2 during the secondary process of PEO can be ascribed to the perfection of the PEO crystals. The longer T2 of PEO compared with that of PBS indicates that the segments in the PEO crystals are more mobile than those in the PBS crystals and that the perfection of the crystals in the secondary process is more apparent for PEO than PBS. However, the nearly constant T2 of PBS after the primary process indicates that the secondary process is also negligible from the viewpoint of segmental mobility.

Figure 4 shows the changes in f and T2 for the 3/7 blend crystallized at 48 °C. The results showed a combination of the crystallization behavior of the homopolymers in Fig. 3. The spin-spin relaxation functions of the blends could not be decomposed into the components representing the PBS and PEO crystals because the T2 values of both components were too close. The value of f first increased in the time range t ≤ ca. 102 s and then leveled off, as indicated by line A, before another increase, which is represented by line B. The former and latter increases in f are assigned to the primary crystallization of PBS and PEO, respectively. The secondary crystallization of PBS was also negligible in the blends, whereas the slight increase indicated by line C represents the secondary crystallization of PEO.

Fig. 4
figure 4

a The fraction of crystals, f, and (b) the spin-spin relaxation time, T2, in the PBS/PEO = 3/7 blend crystallized at 48 °C as a function of the crystallization time, t. Line A represents the fraction of the crystals after the primary crystallization of PBS. The intersection point of lines B and C represents the fraction of the crystals after the primary crystallization of PEO

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As displayed in Fig. 4b, when only PBS crystallized, the crystalline component exhibited a nearly constant T2, which was comparable with that of the PBS homopolymer in Fig. 3b. After the onset of the primary crystallization of PEO, T2 varied as seen for the PEO homopolymer in Fig. 3b; it first increased in the primary crystallization process and then slightly decreased in the secondary process of PEO. The increase in T2 can be ascribed to the longer T2 of PEO than that of PBS, as indicated in Fig. 3b. The values of f and T2 as a function of t with the changing T c are displayed in Fig. 5, and the data for f and T2 are shifted by 0.2 and decades, respectively. The time range of the crystallization shifted to the right, namely, toward longer t, in Fig. 5 as the Tc increased. Other aspects were nearly independent of the Tc. Note that the induction period of PEO for the blends was longer than that for the homopolymer, which can be ascribed to the dilution effect.

Fig. 5
figure 5

a The fraction of crystals, f, and (b) the spin-spin relaxation time, T2, in the PBS/PEO = 3/7 blend crystallized at various temperatures as a function of the crystallization time, t. The plots were shifted by 0.2 for (a) and by decades for (b)

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The fractional amount of PBS crystals, fPBS and PEO crystals, fPEO, after the primary crystallization process of each component can be obtained using lines A‒C in Fig. 4a. The value of fPBS is indicated by line A, and fPEO can be obtained by subtracting fPBS from f at the intersection point of lines B and C.

Figure 6 shows fPBS as a function of the mass fraction of PBS, wPBS, and T c . The value of fPBS increased with wPBS. This is simply because the blends with higher PBS contents contain larger amounts of PBS crystals. In contrast, fPBS was nearly independent of T c . Figure 7 displays fPEO as a function of the mass fraction of PEO, wPEO, and T c . As seen for PBS, fPEO increased with wPEO, and it was nearly independent of T c .

Fig. 6
figure 6

The fraction of the PBS crystals, fPBS, as a function of (a) the weight fraction of PBS, wPBS, and (b) the crystallization temperature, T c

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Fig. 7
figure 7

The fraction of the PEO crystals, fPEO, as a function of (a) the weight fraction of PEO, wPEO, and (b) the crystallization temperature, T c

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The degree of crystallinity of component j (1 for PBS and 2 for PEO), ϕ j , can be expressed by

$$\phi_j = f_j/F_j$$
(3)

where f j is fPBS or fPEO obtained via pulsed NMR, and F j is the fractional amount of the protons of component j, expressed by

$$F_j = \frac{{w_jN_j/M_j}}{{\mathop {\sum } w_jN_j/M_j}}$$
(4)

where w j is the weight fraction, N j is the number of protons in the repeating unit, and M j is the molar mass of the repeating unit of component j.

Figure 8 displays the degree of crystallinity of PBS, ϕPBS, as a function of wPBS and T c . Figure 8a also contains the data for the PBS homopolymer obtained from Fig. 3. The dependence of ϕPBS on wPBS was completely different from that of fPBS. As wPBS decreased from unity, ϕPBS first increased and then decreased (Fig. 8a). This result indicates that adding PEO to PBS first enhances and then suppresses the crystallinity of PBS. The value of ϕPBS is comparable with that of the homopolymer even when wPBS decreased to 0.3. The crystallinity of PBS was suppressed when wPBS was further lowered to 0.2. Figure 8b shows the monotonic decrease in ϕPBS with the increasing T c . However, the 2/8 blend did not show a clear dependence on T c .

Fig. 8
figure 8

The degree of crystallinity of PBS, ϕPBS, as a function of (a) the weight fraction of PBS, wPBS, and (b) the crystallization temperature, T c

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Figure 9 indicates the degree of crystallinity of PEO, ϕPEO, as a function of wPEO and T c . Figure 9a contains the PEO homopolymer data obtained from Fig. 3. As seen for PBS, the dependence of ϕPEO on wPEO was completely different from that of fPEO. Although the data points are relatively scattered, they were nearly constant for 0.6 ≤ wPEO ≤ 1 and slightly decreased as wPEO further decreased to 0.5. The T c dependence of ϕPEO exhibited a monotonic decrease, as seen for PBS. The result in Fig. 9a implies that adding PBS to PEO does not enhance the crystallinity of PEO, which occurs when adding PEO to PBS. Note that the crystallization of PBS reduces the PBS content in the amorphous regions of the PBS spherulites. However, the qualitative dependence in Fig. 9a is unchanged because the data points merely shift to the right-hand side of the figure.

Fig. 9
figure 9

The degree of crystallinity of PEO, ϕPEO, as a function of (a) the weight fraction of PEO, wPEO, and (b) the crystallization temperature, T c

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The slight decrease in the degree of crystallinity, ϕ, with the increasing T c for both PBS and PEO (Figs. 8b and 9b, respectively) can be ascribed to the polydispersity of the polymers [34, 37, 44]. The degree of supercooling for the fractions with lower molar masses is further reduced as T c increases. This is because the polymer with a lower molar mass generally has a lower equilibrium melting point, resulting in a reduced ϕ.

As discussed in other works [34, 45], the behavior of ϕ in crystalline/amorphous blends can be classified into several patterns. Either a monotonic decrease or a maximum is often observed when the content of the amorphous component is increased. In this paper, changing the blend composition induced different influences on the crystallinity of PBS and PEO; Fig. 8a displays a maximum, and Fig. 9a shows the intermediate between the monotonic decrease and the maximum. Note that PEO was still in an amorphous state when PBS crystallized, and un-crystallized PBS acted as the amorphous component when PEO crystallized (note ϕPBS < 100%).

To account for the different behaviors of ϕPBS and ϕPEO, at least three factors that affect the crystallinity should be discussed: (i) the dilution effect, (ii) the order of crystallization of the components, and (iii) the difference in the glass transition temperature, T g . The dilution effect, the first factor, should dominate the reduced crystallinity for wPBS = 0.2 and wPEO = 0.5, i.e., when the content of each crystallizing component is lower.

The order of crystallization, the second factor, affects the crystallinity of PBS and PEO differently. The crystals of PBS, which crystallize before the onset of the crystallization of PEO, can influence the crystallization process of PEO. When PBS crystallizes, the PEO in the molten state acts as a diluent for the crystallization of PBS. The PEO crystals, which crystallize after PBS, cannot influence the crystallization of PBS. After the spherulitic growth of PBS, PEO molecules are confined in the interlamellar and interfibrillar regions inside the PBS spherulites. [11, 22,23,24, 33] This leads to suppression of the crystallinity of PEO.

Figure 10 displays ϕPEO as a function of fPBS and shows a monotonic decrease in ϕPEO with the increasing fPBS. This result implies that the amount of PBS crystals before the onset of the crystallization of PEO is actually a factor that influences the crystallinity of PEO. When the blends contain a larger amount of PBS crystals before the onset of the crystallization of PEO, the crystallinity of PEO is further suppressed.

Fig. 10
figure 10

The degree of crystallinity of PEO, ϕPEO, as a function of the fraction of the PBS crystals, fPBS

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The difference in T g (the third factor indicated in the former paragraph) can also affect the crystallinity by changing the mobility of the segments. The nucleation rate, I n , of the polymeric crystals can be expressed by [46]

$$I_n = G_0\exp \left[ { - \frac{U}{{R\left( {T - T_\infty } \right)}}} \right]\exp \left( { - \frac{K}{{k_BT}}} \right)$$
(5)

where G0 is the constant, U is the activation energy of molecular transport, R is the gas constant, T is the absolute temperature, T∞ is the temperature at which the motion of the chains is frozen, K is the activation energy of nucleation, and k B is the Boltzmann constant. The change in T g affects T∞, which is generally approximately 30 K lower than T g , accounting for nucleation [47]. A miscible blend of PBS and PEO has a single T g , which is lower and higher than that of the PBS and PEO homopolymers, respectively [19]. Note that PBS is the component with the higher T g . The change in T g results in enhanced and suppressed mobility for the segments of PBS and PEO, respectively. According to the first exponential factor in Eq. (5), the change in T g upon blending enhances the crystallinity of PBS, although I n and ϕ are different physical quantities.

Conclusions

The degree of crystallinity and the mobility of the PBS and PEO segments in PBS/PEO blends were examined by pulsed NMR. The T2 data indicated that the mobility of the segments in the PBS crystals was nearly constant throughout the crystallization process. However, the mobility in the PEO crystals first increased in the primary crystallization process and decreased in the secondary crystallization process. The fractional amount of the crystals, f, and the degree of crystallinity, ϕ, exhibited different behaviors. The values of f were dominated by the blend composition. Adding PEO to PBS resulted in an enhancement in ϕPBS. In contrast, ϕPEO was nearly constant for wPEO ≥ 0.6. These results were discussed based on the dilution effect, order of crystallization, and difference in the T g of the constituents. The dilution effect was apparent when the content of one of the constituents was reduced. PBS crystals, which develop before the onset of the crystallization of PEO, suppressed the crystallinity of PEO, which crystallizes after the completion of the PBS crystallization in the confined regions of the PBS spherulites. The single T g of the miscible blends can be a factor that contributes to the enhancement of the crystallinity of PBS, i.e., the higher-T g component.