Critical Stress for Crystal Transition in Poly(butylene succinate)-Based

Oct 23, 2014 - Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of ... Institute of Fashion Technology, Beijing 100029, China...
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Critical Stress for Crystal Transition in Poly(butylene succinate)Based Crystalline−Amorphous Multiblock Copolymers Guoming Liu,† Liuchun Zheng,† Xiuqin Zhang,‡ Chuncheng Li,† and Dujin Wang*,† †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Beijing Key Laboratory of Clothing Materials R & D and Assessment, Department of Materials Science & Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China ABSTRACT: Multiblock copolymers consisting of crystalline poly(butylene succinate) and amorphous poly(1,2-propylene succinate) (PBS-co-PPS) are synthesized. The microstructure of the materials is investigated by the combination of thermal analysis and wide-angle/smallangle X-ray scattering (WAXS/SAXS). The noncrystalline PPS blocks are found to locate predominately in the amorphous phase between crystalline lamellae of PBS. By means of in situ WAXS coupled with optical-assisted strain measurement, the deformation process of PBS-co-PPS is studied. The stiffness and strength of PBS-co-PPS decrease with increasing PPS fraction, while the strain recovery behavior of PBS-co-PPS is similar to PBS homopolymer. Transition from α crystal to β crystal is observed for all the PBS-co-PPS samples. The critical stress for α−β transition of PBS-co-PPS is determined, which is found to be independent of PPS blocks. The universal critical stress for crystal transition is interpreted through a single-microfibril-stretching mechanism.

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unloading. The deformation behavior of PBS film was studied by in situ small/wide-angle X-ray scattering (SAXS/WAXS).18 The crystallographic α−β transition took place after the morphological lamellar−fibrillar transition. A “lamellar thickening” phenomenon was observed simultaneously with the reversible crystal transition, arising from the different c values in α and β crystals. Recently, multiblock copolymers consisting of PBS blocks and amorphous poly(1,2-propylene succinate) blocks (PPS) were synthesized to improve the impact toughness of PBS.21−23 Based on the single glass transition temperature (Tg), the PPS blocks were assumed to be miscible with the amorphous phase of PBS.24 These copolymers provide an opportunity to explore the effect of chain composition on the stress-induced crystal transition and to further understand the micromechanical deformation mechanism of semicrystalline polymers. For example, does crystal transition occur in those multiblock copolymers? How does the amorphous block influence the critical stress for crystal transition? In this work, we elucidate extensively the miscibility and microstructure of PBS-co-PPS copolymers. The critical stress for α−β transition is measured on the basis of optical-assisted strain measurement and in situ WAXS. It turns out that the critical stress for α−β transition is independent of the PPS fraction. A single-microfibril-stretching micromechanical serial model is proposed.

INTRODUCTION Crystalline polymers generally exhibit polymorphism; i.e., multiple crystal forms exist in one polymer. As different crystals usually possess different intrinsic properties, it is therefore crucial to understand phase transition mechanisms. Transitions between different crystal phases can be induced by thermal treatments or external fields.1,2 Two types of stressinduced crystal transition have been reported depending on their reversibility. Irreversible transitions were reported in isotactic polypropylene (iPP),3,4 polyamide 6 (PA 6),5 poly(vinylidene fluoride) (PVDF),6 poly(ethylene oxybenzoate) (PEB),7 polybutene-1 (PB-1),8 etc., which usually correspond to transitions from a less stable form to a more stable one. Reversible crystal transitions induced by stress were discovered in several polymers such as poly(butylene terephthalate) (PBT),9,10 poly(ethylene oxide) (PEO),11 poly(butylene succinate) (PBS),12,13 poly(ethylene succinate) (PES),14,15 etc. Treating as first-order phase transition, Tashiro and co-workers established a thermodynamic framework for the stress-induced reversible crystal transition in PBT.16 This mechanism has been widely utilized in understanding the stress-induced crystal transition in other polymers.12,15,17−19 Poly(butylene succinate) (PBS) is a promising biodegradable polymer with balanced mechanical properties. PBS crystallizes into a monoclinic α crystal form under normal conditions,20 in which chains adopt a T7GTG̅ conformation (a = 0.523 nm, b = 0.912 nm, c = 1.090 nm, and β = 123.9°).13 Under uniaxial tensile stress, a new β crystal could be formed,12 with T10 chain conformation and cell parameters of a = 0.584 nm, b = 0.832 nm, c = 1.186 nm, and β = 131.6°.13 The transition between α and β crystal of PBS occurred reversibly upon loading and © XXXX American Chemical Society

Received: September 4, 2014 Revised: October 3, 2014

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EXPERIMENTAL SECTION

the analysis of glass transition temperature, it was proposed that PPS block is miscible with the amorphous phase of PBS.22−24 Here we present a more systematic investigation on the microstructure of PBS-co-PPS. Figure 1 shows the WAXS

Materials and Sample Preparation. Poly(butylene succinate) (PBS) homopolymer and poly(butylene succinate)−poly(1,2-propylene succinate) (PBS-co-PPS) copolymers were synthesized by a twostep method. Low molecular weight dihydroxytelechelic-polyester prepolymers (PBS and PPS oligomer) were first synthesized. The number-average molecular weights (Mn) of the prepolymers were comparable, 4.4 × 103 and 4.9 × 103 g/mol for PBS and PPS oligomer, respectively, as determined by 1H NMR. Then, PBS-co-PPS with different composition was synthesized from PBS and PPS by a chain extension reaction using hexamethylene diisocyanate as the chain extender. The details of the polymer synthesis can be found elsewhere.21−23 The molecular weight of the final polymer was determined by gel permeation chromatography (GPC, equipped with a refractive index detector), using chloroform as the solvent and monodisperse polystyrene as the calibration standard, as listed in Table 1. The chain extension reaction can be hypothesized as a

Table 1. Molecular Weight and Dispersity of PBS Homopolymer and Copolymers sample

Mn (×103), g/mol

Mw (×103), g/mol

PDIa

PBS co-5b co-10 co-15 co-20

189 131 126 111 83.8

408 402 404 361 284

2.07 3.07 3.21 3.24 3.39

Figure 1. 1D WAXS intensity profiles of PBS and its multiblock copolymers with PPS transformed from 2D patterns by averaging 360° azimuthally.

intensity profiles of PBS-co-PPS. All the reflections can be indexed as the α form of PBS. No visible shift of PBS reflections can be observed. These results indicate that PPS is totally amorphous, which is probably attributed to the irregularity of PPS chains imparted by the randomness in “head and tail” incorporation of 1,2-propylene. Figure 2 shows the DSC thermograms of PBS and copolymers. The extracted data are summarized in Table 2.

a

PDI: polydispersity index, the ratio of weight-average molecular weight and number-averaged molecular weight, Mw/Mn. b“co-5” means PBS−PPS copolymer with a PPS mass fraction (wPPS) of 5%.

random inclusion process of PBS and PPS blocks, resulting in a statistical block distribution. The average number of blocks per chain was estimated to be 17−30, according to the molecular weight of polymer and oligomer. Polymer plaques with thickness of 1 mm were compressed at 160 °C and then rapidly quenched in water. Mini tensile bars with 28 mm long and 2 mm wide (parallel length: 4 mm) were cut from those plaques. Characterization Methods. The melting and crystallization behavior of PBS and copolymers was examined with a differential scanning calorimeter (TA, Q2000). The instrument was calibrated with indium before measurements. Temperature scans were performed in the temperature range from −90 to 160 °C under a nitrogen atmosphere. For the measurement of crystallization and melting behavior, the heating/cooling rate was set as 10 °C/min, while for the measurement of glass transition temperature (Tg) the heating rate was chosen as 20 °C/min. In situ WAXS measurements were carried out at the beamline BL16B1 in the Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the radiation source was λ = 1.24 Å. The mini tensile bars were stretched at a crosshead speed of 6.00 mm/min on a Linkam TST350 hotstage. Scattering patterns were collected by a MAR 165 detector with a resolution of 2048 × 2048 pixels (pixel size: 79 × 79 μm2). Image acquisition time was 10 s. The sample-to-detector distance was 194 mm. Ex situ SAXS measurements were carried out at the beamline 1W2A in the Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the radiation source was λ = 1.54 Å. Scattering patterns were collected by a MAR 165 detector. Image acquisition time was 10 s. The sample-to-detector distance was 1535 mm. All the X-ray patterns were corrected for detector noise, air scattering, and sample absorption.

Figure 2. DSC thermograms of PBS and its multiblock copolymers with PPS. The inset shows the glass transition.

Only one Tg was observed in all the samples. The Tg of PBS-coPPS is higher than that of PBS homopolymer, which increases with the PPS content. The T*g listed in Table 2 is estimated by the Fox equation25 * 1 − wPPS w* 1 = + PPS Tg,PBS Tg,PPS Tg*

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where Tg, PBS and Tg, PPS are 240.55 and 267.85 K,24 respectively. The weight fraction of PPS (w*PPS) is calculated taking account of the crystallinity of the system: wPPS * = wPPS 1 − Xc

RESULTS AND DISCUSSION Microstructure of PBS-co-PPS. In the previous reports,22−24 PPS was termed as amorphous block in PBS-coPPS without providing any diffraction information. Based on B

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out to account for the isotropic distribution. As the experimental accessible q range is finite, it was necessary to extend the data to both lower and higher q values. The intensity versus q data was extrapolated from the smallest measured q value to zero based on the Guinier law. Large q values were damped to infinite q by using the Porod law.27 Figure 4 shows a

Table 2. Data Summary of the DSC Results sample

Tc (°C)

Tm (°C)

ΔHc (J/g)

Xca

Xc,PBSb

Tg (°C)

Tg* c (°C)

PBS co-5 co-10 co-15 co-20

67.8 66.0 63.9 62.6 61.4

108.9 108.3 108.3 107.8 107.7

57.8 54.4 53.4 50.0 48.5

0.289 0.272 0.267 0.250 0.243

0.289 0.286 0.296 0.294 0.304

−32.6 −29.9 −26.4 −24.1 −20.7

−30.9 −29.2 −27.6 −25.9

a Crystallinity Xc is calculated by dividing ΔHc by the enthalpy of fusion of 100% crystalline PBS (200 J/g).26 bXc,PBS is the crystallinity of PBS calculated by normalizing Xc by the fraction of PBS. cTg* is the estimated glass transition temperature of PBS-co-PPS by the Fox equation.

It can be observed that the measured Tg deviated from the estimated T*g remarkably as the PPS fraction increasing. A single crystallization peak (Tc) was observed in all the samples, which, together with crystallization enthalpy (ΔHc), decreases gradually with PPS content. On the other hand, the Tm of all the samples are similar. These observations indicate that the PPS is excluded from the crystalline phase of PBS and is miscible or partially miscible to the amorphous phase of PBS. The decreased Tc can be explained by the “dilute” effect of amorphous PPS chains. PBS-co-PPS displayed space-filling spherulites.24 As such, spherulitic inclusion model is favored, where the PPS chains are included in the spherulites of PBS. Small-angle X-ray scattering (SAXS) is frequently applied in discriminating microstructure of block copolymers. Figure 3 shows the SAXS curves of PBS

Figure 4. Example showing the analysis of lamellar structure using electron density correlation analysis.

typical correlation function of PBS, demonstrating how the long period L, the crystalline layer thickness lc, and the amorphous layer thickness la = L − lc can be estimated from the correlation function curve. It is generally impossible to decide whether it is the amorphous or the crystalline thickness that is read out from the correlation function without prior knowledge of crystallinity. Because the crystallinity of the sample used is much lower than 50%, the smaller value is assigned as the average thickness of the crystalline layer, which is in accordance with other reports.18,29 Figure 5 shows the structural parameters extracted from the correlation analysis. The L increases from 7.6 to 8.5 nm with

Figure 3. SAXS curves for PBS and its multiblock copolymers with PPS.

and copolymers. A broad peak can be observed in the q range of 0.6−0.8 nm−1, corresponding to a length scale of 10 nm. The peak maximum moves to lower q with increasing PPS content. In the high-q region, the SAXS intensity decay is proportional to q−4, falling into the Porod region, corresponding to a twophase structure separated by a sharp interface.27 The average thickness of the amorphous and crystalline layers can be evaluated from the one-dimensional electron density correlation function K(z)28

Figure 5. Long period (L), crystalline layer thickness (lc), and amorphous layer thickness (la) of PBS copolymers as a function of PPS content.

∞

K (z ) =

PPS content. Interestingly, the lc keeps nearly constant for all the samples. The la increases with PPS content, indicating that the amorphous layer is “filled” and “stretched” by amorphous PPS chains. This result unambiguously confirms that the PPS chains are included in the amorphous phase of PBS. True Stress−Strain Curves. To determine the critical stress for crystal transition in PBS and its copolymers, we need

∫0 I(q)q2 cos(qz) dq ∞

∫0 I(q)q2 dq

where z is the correlation distance along the direction from which the electron density distribution is measured and I(q) is the 1D intensity profile. Multiplication of q2 to I(q) is carried C

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markers on the surface of the specimen. Figure 7 shows the true stress versus Hencky strain curves of PBS and its copolymers. It can be seen that the copolymers are “softer” than pure PBS, and the stress for co-20 is smaller than co-10. This behavior can be easily explained by the mechanical contribution of the rubbery PPS, which has a modulus much lower than that of semicrystalline PBS. The plastic (εH,p) part of the strain is defined as

to measure the true stress−strain curves. For this purpose, we used an optical-assisted step-cycle measurement: stretching was conducted stepwise to progressively higher tensile strains with a displacement interval of 2 mm. The force was totally relaxed to 0 between two successive steps. Ink points were marked on the surface of the mini tensile bars. The displacements of the ink points at the irradiated region were monitored by a digital camera. Given that the width, thickness, and marked distance of the initial specimen and stretched specimen are (W0, T0, b0) and (W, T, b), respectively, after load removal, the dimension becomes (Wp, Tp, bp). No volume change during deformation is assumed because no whitening is observed, i.e.

⎛ bp ⎞ εH,p = ln⎜ ⎟ ⎝ b0 ⎠

The elastic (εH,e) part of the strain can be calculated according to εH,e = εH − εH,p

W0T0b0 = WTb

the true stress can be calculated according to σ=

As shown in Figure 8, εH,e together with εH,p increases with total strain until εH = 0.3 for all the samples. Then εH,e reaches

Fb W0T0b0

The true strain (or Hencky) is defined according to Strobl and co-workers30−32

⎛b⎞ εH = ln⎜ ⎟ ⎝ b0 ⎠ Figure 6 shows an example of the force−displacement curves of the step-cycle measurements. The inset displays the ink

Figure 8. Elastic (εH,e) and plastic (εH,p) strain as a function of total strain for PBS and copolymers stretched at 30 °C.

a plateau value of around 0.15−0.2, which nearly does not change with further deformation. Meanwhile, εH,p dominates the later deformation region, increasing linearly with the total strain εH. It is interesting that the three samples show similar mechanical recovery behavior. Crystalline Phase Fraction Determination. In order to measure the critical stress for crystal transition, it is essential to determine the phase fractions under tension. Figure 9 shows the 2D WAXS patterns at different strains. It is clear that samples become gradually orientated with increasing strain.

Figure 6. Force−displacement curves of the step-cycle measurement of PBS, inset showing the ink points on the mini tensile bars.

Figure 9. 2D WAXS patterns for PBS and copolymers stretched at different strains at 30 °C. The rightmost panels show the unloaded pattern after the adjacent strain. The stretching direction is vertical.

Figure 7. True stress (σ)−Hencky strain (εH) plot of PBS and copolymers at 30 °C. D

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Figure 10. Example illustrating the process of estimating fractions of different phases in a stretched PBS sample.

New reflection peaks appear during stretching, corresponding to crystal transition. Upon unloading, the WAXS pattern corresponds to α crystal. Detailed indexing of the 2D pattern of α and β crystal can be found in our previous paper.18 All the three samples show similar feature during deformation. As the sample consists of mixed α, β and amorphous phase, it is not possible to estimate the phase fractions precisely by peak deconvolution methods. In our case, the two well-separated reflections on the meridian, indexed as the (1̅03) of β and α crystals, are selected to calculated the fractions of different phases in the material during stretching. For this purpose, we adopt a methodology as illustrated in Figure 10. After corrected for detector noise, air scattering, and absorption as mentioned in the Experimental Section, two-dimensional WAXS patterns are further normalized by sample thickness T, which is estimated according to T = T0

Figure 11. Plot of β (1̅03) intensity (Iβ) versus α (1̅03) intensity (Iα) in PBS and copolymers.

b0 b

Table 3. Summary of the Linear Fitting Results of Figure 11

Afterward, the 2D pattern is separated into two parts, i.e., isotropic and anisotropic part, by the “halo method”, which is first proposed by Hsiao and co-workers.33,34 The details of the method can also be found in our previous work.4 As the crystal transition takes place in a highly oriented fibrillar structure, the isotropic part contains only the diffuse scattering from unoriented amorphous phase. Thus, the 1D intensity profile at the meridian contains only the scattering of the (1̅03) reflections from α and β crystals. By fitting the 1D curve using two Gaussian peaks, the integrated intensities of each reflection can be calculated. The intensity of the (1̅03)β reflection versus the intensity of the (1̅03)α reflection is plotted in Figure 11. The linear fitting results are listed in Table 3. The relationship can be expressed as

sample

slope (k)

intercept (C)

C/Xc

PBS co-10 co-20

0.38 ± 0.03 0.37 ± 0.02 0.37 ± 0.02

210 ± 10 172 ± 6 164 ± 6

726 ± 35 644 ± 22 675 ± 25

It should be pointed out that a fiber pattern with incident Xrays normal to the fiber axis has a “blind region” of scattering vector near the fiber axis in reciprocal space. Therefore, only a part of intensity of the (10̅ 3) reflection was collected and accounted for in our analysis. We calculated the structure factor |F| of the (1̅03) reflection of α and β crystals using the crystal structures presented by Ichikawa and co-workers.13 The calculated value, |F|β2/|F|α2 is 0.36, very close to k, indicating that our method is applicable for PBS-co-PPS. Critical Stress for α−β Transition. The fraction of β crystal as a function of true stress is plotted in Figure 12. It is very interesting to observe that the data points from different samples are highly overlapped, forming a master curve. The transition is rather broad, covering the range of 70−250 MPa. The onset stress for the transition is ∼70 MPa, which is defined as the “critical stress” for α−β transition. Data on PBS fibers from Ichikawa17 are also plotted in Figure 12. Similar with our results, the critical stress for crystal transition is also ∼70 MPa. However, the crystal transition in their study is much sharper. In actuality, the “sharpness” of the transition reflects the local

Iβ + kIα = C This linear relationship confirms that the crystal transition in PBS is a solid−solid transition, which provides a calibration factor k for comparing the two reflections. All the samples show a similar slope of k ∼ 0.37. The intercept C represents the total amount of crystalline phase. If we normalize C by Xc as shown in Table 2, it is clear that C/Xc is comparable for all the samples. E

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layer. Note that the PPS chains are drawn as red dashed coils located in the amorphous layer. The alternating crystalline and amorphous layer within microfibrils can be simplified as a serial model, where the crystalline layer shares identical stress with the amorphous layer. On the crystalline layer basis, the local stress required for crystal transition is identical for PBS and PBS-co-PPS because of the same chemical and physical nature of the crystalline layer. Therefore, the apparent stress is equal to the local stress in crystalline layer, which is independent of PPS fraction. A similar serial model has been applied previously by Tashiro et al.16 in a study of stress-induced crystal transition in PBT. However, our observation provides straightforward structural evidence for the justification of such a micromechanical model; i.e., microfibrillar structure provides the structural basis for serial model. It is worth noting that microfibrils are laterally connected by “tie” chains.36 These tie chains are important for stress transfer between adjacent microfibrils, which definitely play an important role in crystal transition. The importance of tie chains in mechanical properties of polymers has been realized for long, especially on the slow crack growth.42,43 Quantification of the contribution of tie chains is still difficult, which will be the topic for further studies. Nevertheless, it seems that the inter- or intra-microfibril stress transfer of PBSco-PPS is not affected by the existence of PPS blocks. The net result is effectively similar as stretching of individual microfibrils.

Figure 12. Fraction of β crystal as a function of true stress of PBS and PBS-co-PPS stretched at 30 °C.

stress distribution within a specimen. Therefore, the local stress distribution within fibers is more uniform than that of cold drawn specimens. Single-Microfibril-Stretching Mechanism. The universal master curve for α−β transition in PBS-co-PPS reveals several molecular level structure transformation features. The plastic deformation of semicrystalline polymers generally includes several steps. At small strains, inter- and inner-lamellar slips35−38 or block slippage30−32,39 takes place first, followed by the stress-induced fragmentation and recrystallization of polymer chains at larger deformation.30−32,39 The mechanism of stress-induced recrystallization has been a topic of numerous debates because of lack of direct evidence.18,40 Until recently, strong supportive evidence for the existence of recrystallization was reported.41 Nevertheless, as demonstrated in our previous study,18 the α−β transition in PBS takes place after the lamellar−fibrillar transition. Therefore, we confine our discussions on an already existing fibrillar structure, in which crystalline and amorphous layers pack periodically. Polymer chains in the crystalline layer preferentially align along the stretching direction. Figure 13 illustrates the structure of microfibril of PBS and PBS-co-PPS. The straight lines in the gray region represent the PBS chains in the crystalline layer, and the coil-like curves represent the chains in the amorphous

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CONCLUSIONS Multiblock copolymers consisting of crystalline blocks of PBS and amorphous blocks of PPS provide an opportunity to explore the effect of chain composition on the revisible crystal transition. A microstructure study of the PBS-co-PPS samples demonstrated that the amorphous PPS chains locate predominately in the amorphous phase of PBS. As expected, transition from α to β crystal was observed for all the PBS-co-PPS samples. Although the PBS-co-PPS was softer than PBS homopolymer due to the rubbery PPS blocks, the critical stress for α−β transition of PBS-co-PPS was shown to be the same as that of pure PBS. The universal critical stress for crystal transition was illustrated by a serial model on the basis of the highly oriented fibrillar structure with alternating crystalline and amorphous layers. The single-microfibril-stretching mechanism sheds new lights on the understanding of deformation mechanism of semicrystalline polymers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail (D.W.): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (Grant No. 51203170). The SSRF and BSRF are acknowledged for kindly providing the beam time.

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Figure 13. Schematic illustrating the origin of universal critical stress for PBS-co-PPS based on single-microfibril-stretching mechanism. The straight green lines in gray region represent the PBS chains in crystalline layer. The coil-like green curves represent PBS chains in amorphous layer. The PPS chains are drawn as red dashed curves.

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