Isothermal crystallization of Trans-1, 4-polyisoprene from solutions: Kinetic parameters and crystal modification

Accepted Manuscript Isothermal crystallization of Trans-1, 4-polyisoprene from solutions: Kinetic parameters and crystal modification Xiao Han, Dongbo Zhang, Huarong Nie, Aihua He PII:

S0032-3861(18)30103-4

DOI:

10.1016/j.polymer.2018.01.077

Reference:

JPOL 20337

To appear in:

Polymer

Received Date: 10 November 2017 Revised Date:

15 January 2018

Accepted Date: 28 January 2018

Please cite this article as: Han X, Zhang D, Nie H, He A, Isothermal crystallization of Trans-1, 4polyisoprene from solutions: Kinetic parameters and crystal modification, Polymer (2018), doi: 10.1016/ j.polymer.2018.01.077. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Isothermal Crystallization of Trans-1, 4-polyisoprene from Solutions: Kinetic Parameters and Crystal Modification

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Xiao Han, Dongbo Zhang, Huarong Nie*, Aihua He* Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key aboratory of Rubber-Plastics (Ministry of Education ), School of Polymer Science and Engineering,

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Qingdao University of Science and Technology, Qingdao, Shandong 266042, China. *Corresponding Author, E-mail: [email protected]; [email protected]

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Tel: +86-0532-84022951, Fax: +86-0532-84022951

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Isothermal Crystallization of Trans-1, 4-polyisoprene from Solutions: Kinetic Parameters and Crystal Modification Xiao Han, Dongbo Zhang, Huarong Nie*, Aihua He*

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Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization，Key Laboratory of RubberPlastics (Ministry of Education )，School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China.

*Corresponding Author, E-mail: [email protected]; [email protected]

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Tel: +86-0532-84022951, Fax: +86-0532-84022951

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Abstract: Solution crystallization of TPI is relatively less concerned, though the crystallization kinetics from solution is profoundly related to the advance in fraction purification and the precipitation polymerization of TPI. In this work, isothermal crystallization of TPI from its solutions with varied concentrations and solvents at temperatures ranged from -20 °C to 30 °C

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were studied to attain the kinetic parameters and crystal fcrms. The solubility curves of TPI in the solvents and the conversion temperatures between isothermal and non-isothermal crystallization were supplied to propose the available concentrations and temperatures for TPI

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isothermal crystallization. In most cases, the kinetics of TPI crystallization was subject to the

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Avrami-equation, revealing as the crystallization rate of TPI increase with TPI concentration but decrease with crystallization temperature (Tc) though few deviations arising from the influence of non-isothermal crystallization and transient nucleation were also observed under several special conditions. The better quality solvents of TPI, like THF and toluene exhibited the relatively slower crystallization rate. The α form of TPI crystals was found to appear as samples crystalized from heptane and THF. Keywords: Trans-1, 4-polyisoprene (TPI), kinetics of solution crystallization, crystal forms.

ACCEPTED MANUSCRIPT 1. Introduction Trans-1,4-polyisoprene (TPI), the trans isomer of natural rubber (cis-1,4-polyisoprene) attracts great attention in the elastomer applications, however, one of the typical characters of TPI is the crystallization as temperature falls below 60 °C [1-5]. The crystallization process or

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the respective chain folding during TPI crystallization can be considered as a compromise between a thermodynamically stable structure and the crystallization conditions [6-8].

To date, the Avrami equation is one of the typical analysis modes for understanding of the

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kinetic process during polymer isothermal crystallization from solution. It has been reported that the kinetic parameters of TPI crystallization from isotropic melt are different with the

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molecular weights [9-11]. The Avrami exponent is claimed to be the molecular weight dependence because of the changes of nucleation mechanism probably involving the timedependent heterogeneous nucleation [9]. TPI with medium molecular weight enables the favourable nucleation conditions arising from the activation of nucleation catalysts.

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Unfortunately, the deviation of the Avrami equation is oftentimes found and possibly involves the complicated nucleation and growth mode [12]. As example, the crystallization kinetics of an unfractionated sample of the synthesized TPI from isotropic melt, reported by Chaturvedi,

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proposed that the observed deviation from the Avrami theory towards completion of the process was due to the low molecular weight species of the low melting crystals [11]. In other

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cases, the transient nucleation process is considered to be responsible for the deviation, in particular the Avrami exponent larger than 4 [13]. Progress in research of TPI crystallization also reveals that two major forms, denoted as monoclinic (α) and orthorhombic (β), are dependent of the crystallization temperature (Tc), the types of solvents, the concentration and even the heating process as well [14]. The α form achieved frequently at high crystallization temperature (Tc) is more stable than the β form that normally appears at low Tc with TPI solidification from melt. Differently, Boochathum [14,15] found that the α form presented dominantly in various solutions no matter what the temperature 2

ACCEPTED MANUSCRIPT was, while β form just appeared when Tc was lower than the special temperature named conversion temperature, which could be regarded as the boundary between isothermal crystallization and non-isothermal crystallization. For isothermal crystallization, Tc should be set above conversion temperature, and the quality of solvents exerts considerable influence on

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the crystal modification. The α form instead of β form, was obtained when TPI crystallized from hexane at 10 °C, while both appeared in amyl acetate at most Tc. Although great significant progress has been made in polymorph analysis [14-22], the most studied solvents,

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heptane and amyl acetate are both out of scope of good solvents for TPI, moreover, the detailed kinetics of TPI crystallization from different solutions has not been much concerned to date

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[22].

It is not denied that solution crystallization of polymers could take place during polymerization in the presence of solvents or monomers, and technically be employed for fractionation of polymers. The success in the popular synthesis of TPI via bulk precipitation

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polymerization and purification fractionation in our previous work motivated our present study on TPI crystallization kinetics from solution to deep understanding the polymerization method of TPI and achieve the high quality products [23,24].

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In this work, the dilatometer method, combined with the Avrami-type analyses, are employed to access a series of kinetic courses, reflecting as the influence of temperature,

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solvent and concentration on overall crystallization rate and polymorphic behaviour of TPI. The used solvents include heptane, the poor solvent of TPI, and toluene and tetrahydrofuran (THF), good solvents of TPI. To assure the isothermal crystallization of TPI from different solvents, the conversation temperatures between isothermal and non-isothermal crystallization are proposed, following the achievement of solubility curves of TPI in different solvents. Thereafter, the adopted Tc and concentration are set above the conversion temperatures and saturated level, respectively. 2. Experimental section 3

ACCEPTED MANUSCRIPT 2.1 Materials Trans-1,4-polyisoprene (Mooney viscosity (ML3+4100

°C

is 23 Pas) with 99% trans-1,4

content was purchased from DIPAI chemical co, China. Before using, TPI was purified by dissolving it in THF and then precipitated with the mixture solvents consisting of 99% ethanol

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and 1% hydrochloric acid to remove the gel and catalyst residue. All solvents used in the crystallization experiments, like heptane, toluene and THF were high purity. 2.2 Dilatometric experiments

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TPI solutions were prepared directly in ampoule of dilatometer at about 60 °C, which is near the melting point and defined as the dissolution temperature. Subsequently, the equipped

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capillary altimeter was rapidly put into the ampoules. To eliminate the volatilization of solvents, the top outlet of capillary was sealed with water column. The equipped dilatometers were transferred to an oil bath (40 °C) for 5 minutes for the initial height of the capillary before placed in a thermostat bath set at Tc ± 0.01 °C. The solution heights in capillary heretofore were

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continuously recorded. As a control, another dilatometer with pure solvents in ampoule was used to undergo the same procedure for the deduction of the height variation arising from the cold shrinkage of solvents [25].

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It is noted here solubility curves have been firstly measured by precipitation method for the suitable concentration range of TPI crystallization from solution. The detailed process is

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based on the precipitation method in which the massed supersaturated solutions are exposed directly at the selected temperature. Thereafter, collecting, drying and weighing of the solid surplus to calculate the concentrations of the saturated solutions. 2.3 Characterization TPI crystals after isothermal crystallization were collected by filtrating from TPI solutions, washing with the corresponding solvents and dried in a vacuum oven at room temperature. All differential scanning calorimetry (DSC 8500, PerKin Elmer) measurements were performed under a nitrogen atmosphere with a flow rate of 50 mL/min. Around 5-10 mg of 4

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samples were heated from 20 °C to 80 °C with a scanning rate of 10 °C/min. Temperatures of DSC were calibrated with In. Wide angel X-ray diffraction (WAXRD, Rigaku/XRD-Uitima IV) records were operated at a generator voltage of 40 kV and a current of 200 mA with Cu Ka radiation (λ = 0.154 nm).

from 400 to 4000 cm-1 at a resolution of 4 cm-1 with 32 scans. 3. Results and discussion

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3.1 Documented crystallization condition and data analysis

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FTIR analysis of TPI crystals was performed on a Bruker VERTEX 70 FTIR spectrometer

Solution-crystallization of TPI occurs and reasonably manifests as thermoreversible

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gelation when solution concentration is enough high for substantial chain overlap at a given Tc. It is acceptable that the crystallization rate of TPI is strictly counted on the level of supersaturation attaining, apart from the Tc as well as the quality of solvents [26]. Shown in Fig. 1a is the saturation curve (red one) of TPI/heptane solution, measured by precipitation method,

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revealing the maximum dissolution of TPI in heptane ranged from 0.04 w/v% to 0.1 w/v% as temperatures changing from -10 °C to 25 °C. The saturation level of TPI/THF solutions increases from 0.3 w/v% to 1 w/v% in the ranged temperatures from -20 °C to 10 °C, which is

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about 20 times higher than that of TPI/heptane at the same temperature (Fig. 1b). Consequently, TPI concentrations in the good solvents, like THF and toluene are ranged from 1 w/v% to 5

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w/v% for the observation of TPI crystallization, while TPI concentrations in heptane, the intermediated quality solvent for TPI, are scoped from 0.5 w/v% to 2 w/v% (shown as the black line in Fig. 1). It’s obvious that all the studied solutions are high beyond their respective saturation level.

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ACCEPTED MANUSCRIPT 2.5

5

2.0

4

1.5

3

1.0

2

0.5

1

0.0

-20

-15

-10

-5

0

5

TPI/heptane solubility curve

(b)

10

-15

-10

Temperature/oC

-5

0

5

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TPI/THF solubility curve

(a)

C (g/mL)

C (g/mL)

6

10

15

20

25

30

Temperature/oC

Fig. 1. Solubility curve of (a) TPI/heptane solutions and (b) TPI/THF solutions, both exhibited as the red

crystallization.

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lines. The red points are the designed supersaturation concentration of TPI for the observation of solution

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At a given temperature, TPI in different solvents are recognized to have different saturation levels. Similarly, in a given solvent, solutions reach saturation at different Tc showing different concentrations. For polymer crystallization from solution, isothermal crystallization and non-isothermal crystallization meet different mechanisms. A certain temperature, called conversion temperature is proposed to distinguish both of them. Owing to

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the competition between the crystallization rate and cooling rate related to the temperature difference between the dissolution temperature and crystallization temperature, solvent quality,

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the thermal conductivity of solution, etc., the gelation above conversion temperature brings the isothermal crystallization, otherwise, non-isothermal crystallization takes place. That is to say,

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the low Tc below conversion temperature generally causes the premature crystallization of polymer before the solution temperature in ampoule falls to the set value. As a result, the crystallization of polymer proceeds as solution temperature keeps dropping. Moreover, the conversion temperature is reported to be independent of concentration of polymer solutions, yet to do with the quality of solvent. As shown in Fig. 2, the conversion temperature for TPI/heptane solution was found to be 15 °C, but lower than -10 °C for TPI/toluene and TPI/THF solutions. To make sure the isothermal crystallization of TPI, the Tc for TPI

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crystallization from heptane is set above 15 °C, and higher than -10 °C for TPI from toluene and THF. heptane solvent 2% TPI/heptane solution

(a)

100 80

40 conversion temperature

20 0 -10

-5

0

5

10

3000 30

toluene solvent 5% TPI/toluene solution

3000

20

Time/min

Time/min

Time/min

1200

15 10

-14

-12

-10

-8

30

THF 5% TPI/THF

40

THF solvent 5% TPI/THF solution

2000

20 10 0

1000

-6

Teperature/oC

600

25

30

25

1800

4000

Time/min

2400

20

(c)

toluene solvent 5% TPI/toluene solution

(b)

15

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Temperature/oC

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-15

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Time/min

60

-14

-12

-10

-8

-6

Teperature/oC

-10

-5

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0

0

0

5

10

o

Teperature/ C

-10

-5

0

5

10

o

Temperature/ C

Fig. 2. Plots of cold shrink time of solutions (black line) and initial crystallization time of TPI (red line) in

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different solvents (a) heptane, (b) toluene and (c) THF. The conversion temperature is defined as the crosslinking point of both lines.

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Data on crystallization kinetics of TPI at different concentrations, Tc and solvent types by dilatometric measurement are obtained and analyzed with Avrami-type analyses as follows [27].

( ht − h∞ )

( h0 − h∞ )

= exp(−kt n )

(1)

Where h0 , ht and h∞ , respectively, represent the height of liquid surface in the capillary at the initial time, t time and termination time of crystallization. k is the crystallization rate 7

ACCEPTED MANUSCRIPT constant, and n is the Avrami exponent (=1-4) depending upon the nucleation and growth mechanisms. In general, transient nucleation and heterogeneous nucleation give rise to different Avrami exponent values, n > 4 is claimed to be arising from the transient nucleation while n values between 3 and 4 is the result of homogenously nucleation and three-dimensional growth

Eq. (1) can also be rewritten as

(2)

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  h − h  lg − ln  t ∞  = lg k + n lgt   h0 − h∞ 

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[13].

Thus, a plot of lg(-ln) vs lgt at a constant temperature should be linear with a slope of “n”

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and intercept lgk. The half time (t1/2) can be calculated by the following equation [28].

 ht − h∞   1/2  = 0.5  h0 − h∞ 

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3.2 The influence of concentration (c)

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Fig. 3 shows the detailed transformation-time isotherms for different solutions plotted on an Avrami basis for the studied concentration ranges. Evidently, a linear relation holds over a

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transformation range for the studied concentrations. The deviation from the fitted line at the late stage is observed and attributed to the secondary transition, which also brings the turbulence of

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the liquid level in the capillary [27]. As shown in Table 1, the common feature for TPI crystallization from different solvents is that the higher the concentration the lower the K. At the given Tc, the half time (t1/2) decreases with the concentration in almost solutions. Similar tendency in Avarmi exponent and crystallization rate constant were observed as well. Yet, the deviations of the Avrami equation are also observed as TPI crystallization from good solvents at low concentrations, 1 w/v% of TPI in toluene and THF. The two Avrami exponents are larger than 4. As we have known that the Avrami model assumed the cluster size distribution closely related to the production of nuclei approaching steady-state instantaneously, resulting in

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ACCEPTED MANUSCRIPT a time-independent nucleation rate. However, the 1 wt% of TPI in THF and toluene makes the solutions just approaching the saturation point (Figure 1), therewith the chain rearrangements of polymers could not make chain cluster size distribution attaining steady state regime instantaneously but with some time lag [29]. As a result, the transient nucleation possibly

0.5% TPI/heptane 1% TPI/heptane 2% TPI/heptane

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(a)

0.0

-0.5

-1.0

-1.5 1.0

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log(-ln(ht-h∞)/(h0-h∞))

0.5

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occurs which accounts for the deviation of the Avrami exponent values.

1.2

1.4

1.6

1.8

2.0

log(t)

0.0

-0.5

log(-ln(ht-h∞)/(h0-h∞))

(b)

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log(-ln(h t-h∞)/(h0 -h∞))

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1% TPI/toluene 3% TPI/toluene 5% TPI/toluene

0.5

-1.0

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-1.5

1.5

1.8

2.1

2.4

2.7

1.0 0.5

1% TPI/THF 3% TPI/THF 5% TPI/THF

(c)

0.0 -0.5 -1.0 -1.5 -2.0

3.0

1.5

2.0

2.5

3.0

3.5

4.0

log(t)

log(t)

Fig. 3. Avrami plots of TPI isothermal crystallization from (a) heptane at 15 °C, (b) toluene and (c) THF at 0 °C with different concentrations.

Upon crystallization, the dominant presence of α TPI or β TPI, or even both co-existences greatly depends on the crystallization conditions. Fig. 4 shows the DSC curves of TPI crystals collected from heptane at 0 °C. Two melting peaks appear at 55 °C and 63 °C, traditionally

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ascribed to the melting profiles of β form and α form, respectively [15]. However, as we further characterize the structure of crystals by XRD, only the feature peaks of α form present (Fig. 5a). To identify the crystal forms in accuracy, TPI crystals are further characterized by FTIR. The α and β forms could be distinguished by the characterization peaks presented in the range of

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850 cm-1 - 890 cm-1. β form is identified as the presence of peak at 877 cm-1 while peaks at 862 cm-1 and 882 cm-1 are ascribed to the α form [30,31]. Shown in Fig. 5b illustrates the yielding of α form as TPI crystallizes from heptane whatever the concentration is. Therefore, the initial

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melting peaks in DSC curves at 55 °C possibly arise from the loosen packing of α crystals once

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the crystallization rate is allowed [32]. It is easily found that the relative content of the loosen α form for TPI crystals prepared from 2 w/v% solution is higher than that obtained from 0.5 w/v % solution. This can be explained by the lower concentration leading to the slower crystallization rate (shown in Table 1), therewith TPI chains have enough time to adjust their

2% 1% 0.5%

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packing mode to reduce the deficiency.

40

50

60

70

80

Temperature /oC

Fig. 4. DSC curves of TPI crystals filtered from heptane solutions with different concentration.

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(b) α

Intensity (a.u.)

6000

α

(b)

2% 1% 0.5%

α

α

4500

3000

0 12

16

20

24

28

36 950

32

900

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1500

850

800

750

700

Vive number/cm-1

2theta

Fig. 5. (a) WAXRD photograph and (b) FTIR images of TPI crystals filtered from heptane with different

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concentration. Table 1 Kinetic parameters of TPI crystallization from different solutions

0.5

K

-7

Half time/min

15

3.38

5.90×10

56

15

2.93

1.10×10-7

43.5

15

3.28

2.24×10-5

24

20

1.77

1.66×10-7

119.5

25

1.85

1.00×10-8

323

0

6.62

7.38×10-21

1083

0

2.94

-9

8.45×10

517

-10

1.69

1.33×10-3

39

0

1.82

8.51×10-5

112

5

3.68

1.06×10-12

1462

10

3.74

4.71×10-15

6530

1

0

4.41

7.03×10-17

3745

3

0

3.34

2.67×10-11

1294

-10

2.19

7.27×10-6

154

0

1.81

4.11×10-6

663

5

3.85

7.53×10-14

2147

10

4.34

1.76×10-17

6664

1 Heptane (15.1) 2

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1

3

Toluene (18.0)

n

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Concentration /wt% Tc / °C

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5

THF (18.5)

5

3.3 The influence of crystallization temperature (Tc) The 5 w/v% TPI/toluene and TPI/THF solutions, and 2 w/v% TPI/heptane solutions are used to study the effects of Tc on TPI crystallization kinetics. Fig. 6 shows the products of 11

ACCEPTED MANUSCRIPT equation (1) as the function of crystallization time, and the calculated parameters are also displayed in Table 1. Upon TPI crystallization from good solvents, toluene and THF, the half time (t1/2) as well as the crystallization rate constant (k) increase exponentially with Tc. The values of t1/2 for TPI crystallization from toluene is 39 min at -10 °C, 112 min at 0 °C and 6530

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min at 10 °C, respectively. Also, the increases of the Avrami exponents (n) with Tc in toluene and THF are also found and can be rationalized by the variation of the crystal growth mode [19]. The high Tc brings the long crystallization time, which probably leads to the more perfect

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crystal structure from lamella to spherulite, in turn the increase of the Avrami exponent. However, such trend was not observed as 2 w/v% of TPI precipitated from the relatively poor

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solvent, heptane. The largest Avrami exponent (3.28) at 15 oC brings the abnormal decrease of the Avrami exponent with Tc. However, shown in Figure 2 exhibits that 15 °C is so approaching the conversion temperature of TPI crystallization from heptane that the nonisothermal crystallization of TPI at 15 °C should not be neglected, which possibly results in a

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complicated nucleation and crystal growth mode.

In addition, it is believed that the crystallization rate is controlled by two processesnucleation and crystal growth. Low Tc prompts the nucleation while high Tc contributes to the

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crystal growth rate, therewith the crystallization rate as the function of temperature always show the parabola profile [33]. In this event, the decrease of K with Tc increasing indicates that

log(-ln(ht-h∞)/(h0-h∞))

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TPI crystallization from solutions is clearly a nucleation control process. 1.0 0.5

-10 °C 0 °C 5 °C 10 °C

(a)

0.0 -0.5 -1.0 -1.5 -2.0 1.5

2.0

2.5

log(t)

12

3.0

3.5

4.0

ACCEPTED MANUSCRIPT 15 °C 20 °C 25 °C

(b)

log(-ln(ht-h∞)/(h0-h∞))

0.5

log(-ln(ht-h∞)/(h0-h∞))

1.0

-10 °C 0 °C 5 °C 10 °C

0.0 -0.5 -1.0 -1.5

0.5

(c)

0.0

-0.5

-1.0

-1.5

-2.0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

1

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1.0

2

3

4

5

log(t)

log(t)

Fig. 6. Avrami plots of TPI isothermal crystallization from solutions of (a) 5 w/v% TPI/THF, (b) 5

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w/v% TPI/toluene and (c) 2 w/v% TPI/heptane at different temperatures.

Shown in Fig. 7 and Fig. 8 are the DSC and XRD results of TPI crystals that collected at

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different Tc from heptane. Two endothermic peaks around 55 °C and 63 °C appears as TPI crystallizes at relatively low temperatures, such as below 0 °C from heptane and -20 °C from THF. Although the initial melting peak previously is claimed to be the contribution of imperfect α form, WAXD is further used to identify the crystal forms. Clearly, there is still

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only α form presenting in XRD patterns. It is noted here that Tc (-10 °C, 0 °C and 10 °C) below 15 °C enables the non-isothermal crystallization for TPI/heptane solutions, likewise for those

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samples crystallization at Tc (-20 °C) below -10 °C for TPI/THF solutions. Tc at low temperature allows the high crystallization rate because of the large undercooling. Resultantly,

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the rapid solidification of TPI chains causes their insufficient time to align orderly to the crystal lattice. As shown in DSC, the lower Tc is accompanied by the visible initial melting peak of α forms, which illustrates the high content of amorphous TPI in crystals. The high Tc, although, involves the slow crystallization rate, TPI chains are allowed to diffuse and align orderly attaching to the crystal growth front. Therefore, the higher Tc the higher Tm of α form was found whatever TPI crystallizes from either TPI/heptane or from TPI/THF solutions.

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75

(a)

25 °C

60

20 °C

45

10 °C

40

EXO

EXO

60

(b)

15 °C

5 °C

30

10 °C 20

0 °C

0 °C

15 -20 °C

30

40

50

60

70

80

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-10 °C 0

0 40

50

Temperature/oC

60

70

80

Temperature/oC

Fig. 7. DSC thermograms of TPI crystals collected from 2 w/v% TPI/heptane solutions at

20000

4000

(a)

α

(b)

α

25 °C

15000

Intensity (a.u.)

20 °C

15 °C

2000

10 °C 0 °C

1000

α

α

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3000

Intensity (a.u.)

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different Tc (a), and 3 w/v% TPI/THF solutions at different Tc (b).

10 °C 5 °C

10000

0 °C

5000

-20 °C

-10 °C

0

0 18

24

30

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12

2theta

36

12

16

20

24

28

32

36

2theta

Fig. 8. Wide angel X-ray diffraction patterns of TPI crystals collected from 2 w/v% TPI/heptane solutions at different Tc (a), and 3 w/v% TPI/THF solutions at different Tc (b).

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3.4 The influence of solvents

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In this case, three different solvents, identified with the solubility for TPI from high to low as THF, toluene and heptane, are employed for the kinetic analysis of TPI crystallization. TPI belongs to the non-polar polymers. Surprisingly, THF, a polar solvent shows the highest solubility for TPI. It is acknowledged that the dependence of solubility for polymers is actually referred to the approach of another two parameters, including dispersion component parameter (δd) and hydrogen bonding component parameter (δh) between polymer and solvent, apart from the close polar component parameter (δp) [34]. According to the Hansen solubility parameters, the values for 1, 4-polyisoprene are δd = 16.6, δp = 1.4 and δh = 0.8, respectively. THF (δd = 16.8,

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δp = 5.7, δh = 8) can dissolve more content of TPI than toluene (δd = 18, δp = 1.4, δh = 2) and heptane (δd = 15.3, δp = δh = 0) due to the dominant interaction between non-polar TPI and the solvent is the dispersion force. As the comparison between toluene and heptane is concerned, the similar δp = 1.4 between TPI and toluene accounts for the higher solubility for TPI in

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toluene than that in heptane. The common sense about the quality of solvents for polymers is that the better the quality of the solvent the more extension the polymer chains, thus more polymers normally can

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dissolve in the solutions. For solvents with poor quality, polymers tend to coil and easily precipitate from solution at a relatively low concentration. As shown in Table 1 and Fig. 9, for

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1 w/v% of TPI/THF solutions and TPI/toluene solutions, the half time of TPI crystallization from toluene at 0 °C is about 1083 min, which means that crystallization is slight faster than that from THF, whose half time is about 3745 min. For 1 w/v% of TPI/heptane solutions, Tc (0 °C) is still lower than the conversion temperature. The crystallization is so rapid that the height

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change of dilatometer arising from the cold shrinkage and crystallization could not be distinguished. Therefore, the partial overlap in height changes results in the crystallization being not recorded by dilatometer method, and, which finished within 10 minutes (Fig. 9a). It is

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believed that the solvent quality of TPI is responsible for the crystallization rates. The

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crystallization rate of TPI from high to low trend in different solvents is just converse with the solvent quality changing from good to poor direction. 80

0 °C n-Heptane solvent

（ a）

log(-ln(ht-h∞ ∞ )/(h0-h∞ ))

70

h

60 50 40

1% TPI/toluene 1% TPI/THF

（ b）

0.0

-0.5

-1.0

-1.5

30 -5

0

5

10

15

20

t/min

15

0.5

0 oC 1% TPI/n-Heptane

25

30

35

40

2.6

2.8

3.0

3.2

log(t)

3.4

3.6

3.8

4.0

ACCEPTED MANUSCRIPT Fig. 9. (a) The origin curve of liquid surface for pure heptane solvent (black points) and 1 w/v% of TPI/heptane (red points). (b) Avrami plots of TPI isothermal crystallization from 1 w/v% of TPI/THF and TPI/toluene solutions at 0 °C.

4. Conclusions

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In this study, we addressed the isothermal crystallization of trans-1, 4-polyisoprene (TPI) from different solutions at different temperatures to get the kinetic parameters and the corresponding crystal forms. The solubility curves of TPI in the different solvents and the conversion temperatures between isothermal and non-isothermal crystallization are proposed to

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attain the available experimental conditions. Independent of the solvents, TPI crystallization

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from different solutions is scope of the Avrami equation. The half time (t1/2) and the crystallization rate constant of TPI decreased with the concentrations. Because of the nucleation controlled process for TPI crystallization from solutions, the t1/2 as well as the k increased exponentially with the crystallization temperature. Also, the solvent quality of TPI was responsible for its crystallization rates, and a relatively high crystallization rate was found in

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the poor solvent (heptane) of TPI. However, the deviation cases are also found when TPI solutions approach the critical point between isothermal crystallization and non-isothermal

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crystallization or the transient nucleation occurs. Whatever the concentrations and crystallization temperatures of TPI solutions were, only a-form, however, presented while TPI

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crystallized from the studied solutions.

Acknowledgements

This work was supported by the National Basic Research Program of China (2015CB654700, (2015CB654706)); the National Natural Science Foundation of China (51773105,51473080); the Natural Science Foundation of Shandong Province (ZR2016EMM05); the Significant Basic Research Program of Shandong province (ZR2017ZA0304); Taishan Scholar Program and Yellow River Delta Scholar program. 16

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[34] C.M. Hansen. Hansen solubility parameters. Springer, New York, 2007.

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ACCEPTED MANUSCRIPT 1. TPI isothermal crystallization from different solutions and conditions, involving the factors of concentrations, solvents and temperatures are investigated to achieve the kinetic parameters and the polymorphic behaviors. 2. The solubility curves of TPI in the different solvents and the conversion

attain the available experimental conditions.

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temperatures between isothermal and non-isothermal crystallization are proposed to

3. TPI crystallization in most cases is subject to the Avrami-equation while few deviations resulting from the transient nucleation or the influence of non-isothermal crystallization are observed.

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4. All the collected TPI crystals from different solutions are proved to be α form by multiple techniques.

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5. This work is helpful to deep understand the polymerization method of TPI by the popular bulk precipitation method and achieve the high quality products by

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fractionalization purification due to both associated with solution crystallization.