Processing, Mechanical Properties, and Structure Analysis of Melt

Chapter 5

Downloaded by UNIV OF ROCHESTER on November 5, 2014 | http://pubs.acs.org Publication Date (Web): August 16, 2012 | doi: 10.1021/bk-2012-1105.ch005

Processing, Mechanical Properties, and Structure Analysis of Melt-Spun Fibers of P(3HB)/UHMW-P(3HB) Identical Blend Taizo Kabe,1 Takeharu Tsuge,2 Takaaki Hikima,3 Masaki Takata,4 Akio Takemura,1 and Tadahisa Iwata*,1,4 1Graduate

School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan 2Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan 3Research Infrastructure Group, RIKEN Harima Institute/SPring-8 Center, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan 4Structural Materials Science Laboratory, RIKEN Harima Institute, SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan *E-mail: [email protected]

High tensile strength melt-spun fibers of poly[(R)-3hydroxybutyrate] (P(3HB)) with the addition of a small amount of ultra-high-molecular-weight P(3HB) (UHMW-P(3HB)) were processed by cold-drawing and two-step cold-drawing methods. The mechanical properties and highly ordered structure were investigated by tensile testing, and wide-angle and small-angle X-ray diffraction measurements (WAXD and SAXS) using a synchrotron radiation beam at SPring-8. The molecular weight of P(3HB) decreased drastically within a few minutes during melt-spinning. However, in the case of the blended samples, the degree of molecular weight decrease was slightly reduced by the addition of UHMW-P(3HB). The melt-spun blend fibers stretched at a cold-drawing ratio of 14 showed a tensile strength of 530 MPa. When two-step cold-drawing was applied, the tensile strength increased further to 740 MPa. WAXD and SAXD revealed that the addition of a small amount of UHMW-P(3HB) into P(3HB) induces

© 2012 American Chemical Society In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

the formation of the β-form (planar zigzag conformation) of molecular chain, increasing the mechanical properties. Based on these results, it seems that the addition of a small amount of UHMW-P(3HB) has a great effect on the thermal stability and processability of melt-spun fibers and increases their final mechanical properties.


Introduction Poly[(R)-3-hydroxybutyrate] (P(3HB)) is produced from renewable carbon resources such as sugar, organic acids or plant oils by a number of microorganisms (1, 2). This polymer is expected to become an alternative to petroleum-based plastics because of its biodegradability and thermoplasticity. However, it is well known that the elongation at break of P(3HB) material is very poor (only 5%), and its mechanical properties degrade with time as a result of secondary crystallization because its glass-transition temperature (Tg) of 4 °C is lower than room temperature (3, 4). For this reason, improvement of the mechanical properties of P(3HB) is of both scientific and practical concern. Until now, many researchers have tried to prepare high tensile strength fibers of P(3HB) by hot-drawing and cold-drawing methods (5–8). However, the tensile strengths of the processed fibers were 190-420 MPa, significantly lower than that of petroleum-based plastics such as polypropylene. The major cause of this problem is probably that the melt-spinning temperature of P(3HB) is close to its thermal degradation temperature, and thus thermal degradation occurs during melt-spinning. Iwata et al. reported production of ultra-high-molecular-weight P(3HB) (UHMW-P(3HB)) biosynthesized by using recombinant Escherichia coli under specific fermentation conditions and succeeded in obtaining high tensile strength fibers from this UHMW-P(3HB) (9). They developed a new drawing method (two-step drawing combined with cold-drawing), and the tensile strength of their UHMW-P(3HB) fiber reached 1,320 MPa, which is higher than that of common plastic fibers. Furthermore, it was revealed by using micro-beam X-ray diffraction that, in this high tensile strength fiber, a new molecular conformation (planar zigzag conformation, β-form) was generated at the center part of the fiber, together with the normal molecular conformation (2/1 helix conformation, α-form) of P(3HB) (10). However, the biosynthesis of UHMW-P(3HB) using high-density cultivation is very difficult and costly. Accordingly, to use UHMW-P(3HB) efficiently, we report the addition of a small amount of UHMW-P(3HB) into P(3HB), and its effect on thermal properties, melt-spinning processability, and mechanical properties of the P(3HB)/UHMW-P(3HB) identical blend. The highly ordered structure in the P(3HB)/UHMW-P(3HB) blend fibers, which shows two kinds of molecular conformations (α-form and β-form) was analyzed by wide-angle and small-angle X-ray diffractions using synchrotron radiation at SPring-8. This information will be useful for making P(3HB) fibers with high tensile strength, which is an important step in increasing the industrial use of this polymer. 64 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Experimental Section


Materials P(3HB) with weight average molecular weight (Mw) = 5.2 × 105 and polydispersity (Mw/Mn) = 1.6 was supplied by Monsanto Japan Co. UHMW-P(3HB) with Mw = 3.47 × 106 and Mw/Mn = 2.67 was biosynthesized from glucose by recombinant E. coli JM109 according to the method of Kahari et al (11). Both samples were purified by dissolving and precipitating using chloroform and hexane and subsequently dried in vacuum. An identical blend sample consisting of two different molecular weight P(3HB)s was prepared by mixing them in chloroform, followed by precipitating and drying. The composition ratio of UHMW-P(3HB) to P(3HB) was chosen as 5/95.

Melt-Spun Fibers with Cold-Drawing and Two-Step Drawing The molecular weights of P(3HB)s normally decrease during melt-spinning as a result of the heating process. The change of molecular weight with melt-spinning time and melt-spinning temperature was monitored to determine the optimized melt-spinning conditions. The P(3HB) or blend sample was placed into the spinning apparatus at 180 °C, 190 °C or 200 °C. Die diameter, volume velocity of extruded sample from die, and take-up speed were set at 1 mm, 4 mm3/s, and 900 mm/s, respectively. The melt-spun fiber was taken up for 1 minute, and the molecular weight was measured using gel permission chromatography (GPC). The melt-spun fibers were quenched into an ice water bath at around 0 °C to obtain amorphous fibers. These amorphous fibers were then cold-drawn in a water bath at various drawing temperatures (0–8 °C) to various drawing ratios (up to 14 times). Furthermore, two-step cold-drawing (to 2.8 times) was applied to a six times cold-drawn fiber. All drawn fibers were annealed at 80 °C for 30 min in an oven.

Molecular Weight Measurement Molecular-weight measurements were obtained by GPC at 40 °C, using a Shimadzu 10A GPC system and a 10A refractive index detector with two joint columns of Shodex K-806M and K-802. Chloroform was used as the eluent at a flow rate of 0.8 ml/min, and sample concentrations of 1 mg/ml were used. Polystyrene standards were used to make a molecular-weight calibration curve.

Stress−Strain Test Mechanical properties of fibers were measured with a tensile tester (EZ-test, Shimadzu Co., Japan) at a cross-head speed of 20 mm/min. The gauge length was 10 mm. These results obtained were averaged over five samples for each condition. 65 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Wide-Angle and Small-Angle X-ray Measurements


The highly ordered structures of the cold-drawn and two-step-drawn fibers were investigated by wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) using a synchrotron radiation beam. WAXD and SAXS measurements with a wavelength of 0.09 nm at 13.8 keV of synchrotron radiation were carried out at BL45XU beam line at SPring-8, Harima, Japan. The diffraction patterns were recorded with a CCD camera (C7330-12-NR, Hamamatsu Photonics, Japan) with exposure times of 76–1058 ms. The camera lengths for WAXD and SAXS were 126 mm and 2500 mm, respectively.

Results and Discussion Molecular Weight Changes during Melt-Spinning Figure 1 shows the changes of weight-average and number-average molecular weights (Mw and Mn) during melt-spinning. Both Mw and Mn decreased drastically within a few minutes, because of thermal degradation of P(3HB). However, the thermal degradation of the blend sample was less than that of the P(3HB). If the thermal degradation of the P(3HB) molecular chains follows a random cutting model, the number-average degree of polymerization (Pn,t) at time t is given by a reversible reaction with second-order kinetics (Eq. 1) (12).

In Eq. 1, Pn,t and Pn,0 indicates the number average degree of polymerization at melt-spinning time, 0 and t, respectively. And Kd is a constant of thermal degradation calculated from the slope of 1/ Pn vs melt-spinning time as shown in Figure 1C and F. The Kd values for each sample at various temperatures are summarized in Table I. The Kd value of the blend sample was lower than that of P(3HB) at every melt-spinning temperature. This result indicates that thermal degradation of blend sample was slowed by the addition of a small amount of UHMW-P(3HB). Optimum Conditions for Melt-Spinning at Various Melt-Spinning Temperatures It is well known that the melt viscosity of polymers depends on the melt-spinning temperature and melt-spinning time. Table II summarizes the melt-spinning processability of the blend sample at various melt-spinning temperatures. When the melt-spinning temperature was 180 °C, it was impossible to spin the fibers because the “molten” sample remained hard and brittle, 66 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


indicating that the polymers did not completely melt. When the blend sample was melted at 190 °C, 8 min was required to obtain the melt-spun fibers. At a melt-spinning temperature of 200 °C, 2 min of melting was enough to produce the melt-spun fibers. This melt-spun fiber was easily stretched until a cold-drawing ratio of over 10 times. When the longer melt-spinning time (8 min) was used at 200 °C, continuous spinning was difficult because of the low melt viscosity of the sample. Based on these results, the optimum melt-spinning conditions were selected as 200 °C for 2 min.

Figure 1. Relationships between molecular weight and melt-spinning time. Left and right columns show results for P(3HB) and the blend, respectively. Weight-average (A, D), and number average (B, E) molecular weights and inverse of number-average degree of polymerization (C, F) are displayed. 67 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Table I. Heat decomposition rate constant of each sample at three temperatures (180, 190, 200 °C), as calculated from the slope of melt-spinning time vs. 1/Mn in Figure 1 (C and F) Sample


Melt-spinning temperature (°C)

P(3HB)

blend

180

(0.323±0.016)×10-4

min-1

(0.253±0.009)×10-4

min-1

190

(0.533±0.010)×10-4

min-1

(0.463±0.016)×10-4

min-1

200

(1.021±0.025)×10-4

min-1

(0.976±0.013)×10-4

min-1

Table II. The effects of melt-spinning time and melt-spinning temperature on spinning processability of the blend Melt-spinning time [min]

Melt-spinning temperature [°C]

2

4

8

180

×a

×a

▴

190

×a

▴

○

200

○

○

×b

xa: The melted blend was unable to be spun because that was solid. ▴ : The melted blend was able to be spun but unable to draw. ○ : The melted blend was able to be applied the spinning and drawing. xb: The melted belnd was unable to be spun because the viscosity of that was low.

Effect of Cold-Drawing Temperature on Tensile Strength Figure 2 shows the tensile strengths of 6 or 12 times cold-drawn blend fibers as a function of the cold-drawing temperature from 0 to 8 °C. Maximum tensile strength appeared when cold-drawing was applied at 4 °C. The Tg of P(3HB) is around 4 °C. Accordingly, cold-drawing at temperatures below Tg did not induce molecular chain orientation because the chain mobility was too low. On the other hand, increasing the cold-drawing temperature seems to accelerate the orientation of molecular chains as a result of increasing molecular chain mobility. However, a high cold-drawing temperature of around 8 °C progresses the growth of crystal nuclei prior to molecular chain orientation. For subsequent experiments, a colddrawing temperature of 4 °C was selected. 68 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 2. Tensile strength of cold-drawn blend fibers as a function of drawing temperature. Drawing ratios of 6 times and 12 times were used for each sample.

Mechanical Properties of Cold-Drawn and Two-Step-Drawn Fibers Cold-drawn fibers of both P(3HB) and UHMW-P(3HB)/P(3HB) blend were processed under optimized conditions: melt-spinning temperature (200 °C), meltspinning time (2 min) and cold-drawing temperature (4 °C). Figure 3 shows the tensile strength of cold-drawn fibers as a function of cold-drawing ratio. Tensile strength increased at cold-drawing ratios of over 4 and 6 for the blend and P(3HB), respectively, suggesting that the addition of a small amount of UHMW-P(3HB) increased the molecular orientation. At a cold-drawing ratio of 14, the tensile strength of the blend and P(3HB) reached 530 MPa and 370 MPa, respectively. Elongation at break and Young’s modulus are summarized in Table III. Figure 4 shows the tensile strength of two-step drawn P(3HB) and blend fibers, as a function of total drawing ratio. Melt-spun fibers were stretched until 6 times at 4 °C in an ice water bath and then two-step cold-drawing was applied until 2.8 times at 8 °C. The two-step drawn fibers were annealed in an oven to fix the two-step drawn molecular chains. Mechanical properties are summarized in Table III. P(3HB) fiber was able to be stretched up to total drawing ratio of 17 and its tensile strength increased to 630 MPa. This value is higher than previously reported (5–8). For the two-step drawn blend fibers, the tensile strength further increased to 740 MPa. This value is almost the same as that of common plastic fibers such as polypropylene. This result also indicates that the addition of a small amount of UHMW-P(3HB) contributes to much improved mechanical properties.

69 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 3. Tensile strength of cold-drawn fibers as a function of drawing ratio.

Figure 4. Mechanical properties of P(3HB) and blend fibers prepared by two-step cold-drawing.



X-ray Diffraction and Scattering Measurements Figure 5 shows WAXD and SAXS of cold-drawn and two-step-drawn P(3HB) and blend fibers. The WAXD and SAXS patterns of undrawn P(3HB) and blend fibers in Figure 5A and B indicate that the molecular chains are slightly oriented in the as-spun fibers. In the case of P(3HB), despite a cold-drawing ratio of 6 times, WAXD and SAXS images were almost the same as those of the as-spun fibers (Figure 5A′ and a′). However, in the case of the blend fiber, one can see a well-oriented diffraction diagram in WAXD (Figure 5B′) and two clear strong reflections along the meridian in SAXS (Figure 5b′), suggesting that molecular chains and lamellar crystals with a 2/1 helix conformation (α-form) (13, 14)are both strongly oriented. Furthermore, a new reflection on the equatorial line indicates that the addition of a small amount of UHMW-P(3HB) induces the planar zigzag conformation (β-form) (15–17)in the cold-drawn blend fiber. When 14 times cold-drawing was applied, the WAXD patterns became sharp, indicating highly orientated molecular chains along the cold-drawing direction; the degrees of orientation of P(3HB) and blend fibers are calculated as 0.88 and 0.92, respectively. The intensity of the peaks attributed to the β-form is greater for the blend fiber than for the P(3HB) fiber, suggesting that the addition of a small amount of UHMW-P(3HB) contributes to increased orientation of molecular chains and generation of the planar zigzag conformation. As a result, the mechanical properties are improved. Until now, it was considered the β-form was generated from the amorphous region between lamellar crystals by two-step cold-drawing (16, 17). However, in this particular cold-drawing process, two-step cold-drawing was not applied. Despite this, in the blend, at a relatively low drawing ratio of 6 times, β-form was generated as shown in Figure 5B′. It is well-known that highly oriented fibers have a shish-kebab structure inside. UHMW-P(3HB) chains seems to be the “shish” region of the shish-kebab structure, which typically consists of extended molecular chains (“shish”) and lamellar crystals (“kebab”) in a three-dimensional structure. The β-form might be generated from the “shish” region of UHMW-P(3HB). For the two-step drawn blend fibers at a total drawing ratio of 13, the WAXD pattern (Figure 6 B) shows a strong reflection of the β-form compared with that in the cold-drawn fiber (Fig. 5 B″). It turns out that the two-step cold-drawing method promotes the formation β-form and thus improves the mechanical properties. The SAXS pattern of the two-step-drawn blend fiber (Fig. 6 B′) showed a drop-shaped pattern towards the center, which indicates the lengthening of the long period of stacked lamellar crystals by two-step cold-drawing. From these results, it is found that the molecular chains of the P(3HB) in the blend were also oriented highly as a result of the addition of a small amount of UHMW-P(3HB).


Drawing method

Sample

P(3HB) one-step cold-drawing Blend

72


Table III. Tensile strengths, Young’s modulus, and elongation at break for one- and two-step cold-drawn fibers

P(3HB) two-step cold-drawing Blend

First drawing ratio (λ)

Second drawing ratio (λ)

Total drawing ratio (λ)

Tensile strength (MPa)

Elongation at break (%)

Young’s modulus (GPa)

1

-

1

37 ± 27

9 ± 32

2.49 ± 2.85

6

-

6

78 ± 45

75 ± 37

3.34 ± 1.64

14

-

14

370 ± 84

52 ± 45

4.80 ± 4.56

1

-

1

37 ± 2

16 ±11

1.44 ± 0.78

6

-

6

187 ± 25

168 ± 18

2.58 ± 1.73

14

-

14

530 ± 31

40 ± 12

6.41 ± 0.69

6

1

6

78 ± 45

75 ± 37

3.34 ± 1.64

6

1.7

10

347 ± 114

58 ± 31

4.37 ± 0.86

6

2.2

13

324 ± 59

50 ± 29

5.04 ± 0.71

6

2.8

17

630 ± 209

46 ± 18

9.46 ± 4.53

6

1

6

187 ± 25

168 ± 18

2.58 ± 1.73

6

1.6

10

506 ± 97

60 ± 11

5.67 ± 1.32

6

2.2

13

740 ± 90

50 ± 14

10.6 ± 0.35

In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 5. WAXD (capital letters) and SAXS (lower case letters) patterns of cold-drawn fibers. (A,a and (B,b) are P(3HB) and the blend, respectively. Drawing ratios are 6 (′) and 14 (″).



Figure 6. WAXD and SAXS patterns of two-step cold-drawn fibers. (A) and (A′): drawing ratio of 6 (i.e. first step); (B) and (B′) two-step cold-drawn fibers with total drawing ratio of 13.

Conclusions In this paper, the effect of addition of a small amount of UHMW-P(3HB) on the melt-spinning processability, mechanical properties, and highly ordered structure of P(3HB)/UHMW-P(3HB) blend fibers was investigated. The addition of a small amount of UHMW-P(3HB) inhibited the thermal degradation and improved the melt-spinning processability. The tensile strength of the cold-drawn blend fibers is 530 MPa. On the other hand, the two-step-drawn blend fiber had a tensile strength of 740 MPa and Young’s modulus of 10.6 GPa together with elongation at break of 50 %. This result indicates that P(3HB) becomes a high tensile strength and high modulus fiber, as well as having adequate elongation at break. WAXD and SAXS measurements showed that the addition of UHMW-P(3HB) to P(3HB) leads to formation of β-form crystals at relatively low drawing ratio. This phenomenon seems to be caused by the extended UHMW-P(3HB) molecular chains becoming the “shish” region of the “shish-kebab” structure in the fiber.


Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research of Japan (A) No.22245026 (2010) and the by New Energy and Industrial Technology Development Organization (NEDO) of Japan.


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Processing, Mechanical Properties, and Structure Analysis of Melt

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