Crystallization through mesophase in poly(butylene terephthalate): Approach from dependence of growth rate on lamellar thickness

Accepted Manuscript Crystallization through mesophase in poly(butylene terephthalate): Approach from dependence of growth rate on lamellar thickness Takashi Konishi, Waki Sakatsuji, Yoshihisa Miyamoto PII:

S0032-3861(17)30488-3

DOI:

10.1016/j.polymer.2017.05.021

Reference:

JPOL 19681

To appear in:

Polymer

Received Date: 7 March 2017 Revised Date:

2 May 2017

Accepted Date: 9 May 2017

Please cite this article as: Konishi T, Sakatsuji W, Miyamoto Y, Crystallization through mesophase in poly(butylene terephthalate): Approach from dependence of growth rate on lamellar thickness, Polymer (2017), doi: 10.1016/j.polymer.2017.05.021. 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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Crystallization through mesophase in poly(butylene

rate on lamellar thickness

Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan

b

Division of Physics and Astronomy, Graduate School of Science, Kyoto University, Kyoto

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a

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Takashi Konishi a,*, Waki Sakatsuji b and Yoshihisa Miyamoto a

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terephthalate): approach from dependence of growth

606-8502, Japan

(T. Konishi)

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*Corresponding author. E-mail address: [email protected]

ABSTRACT:

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Keywords: poly(butylene terephthalate); mesomorphic phase; crystal growth

The temperature dependence of the spherulitic growth rate of poly(butylene

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terephtharate) (PBT) has been investigated by optical microscopy and compared with the temperature dependence of the lamellar thickness.

The changes in the dependence of the growth

rate on the lamellar thickness are observed at three temperatures, 218, 208 and 157 °C.

The middle

temperature, 208 °C, corresponds to the temperature at which the temperature dependence of the lamellar thickness changes.

The correspondence leads to the interpretation that the crystalline stem

directly forms at the growth front above 208 °C and that the mesomoprhic stem forms below 208 °C.

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1. Introduction.

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Poly(butylene terephthalate) (PBT) is a semi-crystalline polyester and widely used as an PBT has two triclinic crystalline forms, α-form and β-form[1-7] and the

smectic liquid crystal[8,9].

The α-crystalline structure is mainly obtained when PBT is crystallized The β-crystalline structure forms by stretching the α-form and

from the melt or the glass.

transforms reversibly to the α-form on removal of the strain.

The smectic phase is obtained by

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engineering plastic.

stretching the glass below room temperature and transforms into the α-form by heating.

The c-axis

(fiber axis) length of the α-crystalline unit cell, ca. 11.6 Å is similar to that of the smectic periodicity,

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ca. 11.7 Å[8,9].

The morphology[10-12] and the growth rate[11,13-15] of the spherulite in PBT have been PBT has three types of spherulitic morphologies in the isothermal

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investigated for a long period.

crystallization from the melt, usual, mixed and unusual types.[10-12] Di Lorenzo and Righetti have reported that the unusual type spherulite forms at the crystallization temperature Tc = 193 °C, the mixed type at Tc = 204 °C and the usual type at Tc = 214 °C and that the Tc-dependence of the

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spherulitic growth rate changes at 211 °C.[11] Yoshioka and coworkers also have confirmed that the usual type spherulite forms at 210 °C and the unusual type at 163 and 68 °C.[12]

In regard to

the spherulitic growth rate, Di Lorenzo and Righetti have reported that the temperature dependence

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of the spherulitic growth rate changes at 211 °C and pointed that the temperature, 211 °C, is the

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regime II-III transition temperature by using the equilibrium melting temperature as 250 °C.[11] In a previous study[16,17], we found that the equilibrium melting temperature is 270 °C by the Gibbs-Thomson relation and that the Tc-dependence of the lamellar thickness l in PBT changes at a temperature TX = 208 °C.

According to the crystallization model through the mesophase[16-22],

the results indicate that PBT crystallizes directly from the supercooled melt above TX = 208 °C, while through the mesophase below TX.

Although the model also predicts the change in the

Tc-dependence of the growth rate at TX due to the difference of the crystallization processes[18,21,22], the research from the viewpoint of the comparison between the growth rate and lamellar thickness has not been performed in PBT.

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The aims of this report are to obtain experimentally the Tc-dependence of the growth rate of

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spherulite in the wide temperature range in PBT by optical microscopy, to compare the Tc-dependence of the growth rate with that of the lamellar thickness, and to interpret the relation between the growth rate and the lamella thickness using the crystallization model through the

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mesophase.

2. Experimental Section.

The polymer used in this study was PBT with Mv = 38,000 purchased from Sigma-Aldrich co. ltd. The

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The PBT pellet is sandwiched by cover glasses at 280 °C at a thickness of ca. 20 µm.

isothermal crystallization process at the crystallization temperature Tc between 120 and 220 ºC after

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melting at 280 ºC for 2 min was observed by a polarized or a bright-field optical microscope (Nikon ECLIPSE ME600) in order to measure the features and the sizes of the crystalline morphologies, respectively.

For the observation of optical retardation, 530 nm sensitive color plate was inserted

between the sample and the analyzer.

The temperature of the sample were controlled by

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temperature-controller hot stages, Linkam THMS-600 and Linkam LK300 which give the cooling rates as 90 K/min and 300 K/sec, respectively. ºC and Linkam LK300, for Tc ≤ 170 ºC.

Linkam THMS-600 was mainly used for Tc ≥ 170

In order to make immediately the temperature of the

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sample to be constant after cooling to Tc, the temperature-controlled sample stage in Linkam LK300

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was covered with a copper cover plate.

3. Results and Discussion.

3.1. Temperature dependences of spherulitic growth rate and crystalline lamellar thickness. Figures 1(a)-(d) show polarized optical micrographs taken with a sensitive color plate of the spherulites in PBT isothermally crystallized at Tc = 197, 201, 214 and 221 °C, respectively.

The

spherulites for Tc = 197 °C in Figure 1(a) indicate that the Maltese cross is oriented at about 45° to the crossed polars and are categorized as the unusual type[10-12].

The morphology of the

spherulites is of the unusual type at Tc < 200 °C (Figure 1(a)) as reported by the other authors[10-12],

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but cannot be defined at Tc > 200 °C (Figure 1(b)).

The spherulites for Tc = 221 °C in Figure 1 (c)

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Figure 1.

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are axialites.

The polarized micrographs of PBT isothermally crystallized at (a) 197 °C for 16 sec, (b)

201 °C for 26 sec, (c) 214 °C for 11 min and (d) 221 °C for 5 hours.

The polarizing directions are equatorial and meridional ones.

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with a sensitive color plate.

These micrographs are taken The

bright-field micrographs of spherulites in PBT isothermally crystallized at (c) 214 °C for 11 min and Each scale bar indicates 20 µm.

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(d) 221 °C for 5 hours.

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For Tc > 210 °C the boundaries of the spherulites are obscure in the polarized micrograph (Figure 1(c) and (d)), but those in the bright-field micrographs are clearer (Figure 1(e) and (f)).

Figure 2

shows the time-evolutions of diameters of spherulites or the maximum lengths of axialites, d, in PBT isothermally crystallized at several Tc from the melt; both of the spherulites and the axialites linearly grow with time.

Figures 3(a) and (c) show the Tc-dependence of the radial growth rate of spherulite

or axialite, u, for PBT.

The growth rate has a maximum at ca. 140 °C. The Tc-dependence of u

obtained by Di Lorenzo[11] is also plotted in Figure 3(a) and (c) and is similar to the present results.

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Time-evolutions of diameters of spherulites or maximum lengths of axialites, d, in PBT

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isothermally crystallized at Tc.

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Figure 2.

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Temperature dependence of (a) growth rate u of the spherulites or axialites and (b) lc in

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Figure 3.

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isothermally crystallized PBT. in (c).

The temperature region between 180 and 240 °C in (a) is enlarged

The circles and the crosses in (a) and (c) are our data and Di Lorenzo’s ones[11],

respectively.

The bold broken (purple) and solid (red) curves in (a) and (c) are fitting curves with

Eq. (1) using parameters u0,LC(1) and KLC(1), and u0,LC(2) and KLC(2), respectively.

The thin solid

(yellow) and broken (green) curves are fitting curves using parameters u0,LM(1) and KLM(1), and u0,LM(2) and KLM(2), respectively. KLM’.

The chain (blue) curve is fitting curve using parameters u0,LM’ and

The fitting curves in (a) and (c) are the same as those in Figure 5.

The red and blue

broken curves above and below 208 °C in (b) are fitting curves using Eqs. (9) and (10), respectively.

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The fitting curves in (b) are the same as that in Figure 6.

These curves and their parameters will be

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described in detail later.

The error bounds in (a), (b) and (c) are approximately equal to the symbol

sizes.

In a previous study[17], we investigated the melting behavior of the lamellae in the heating

X-ray scattering (SAXS) measurement.

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process of the isothermally crystallized PBT by differential scanning calorimetry and small angle The melting temperature of the crystalline lamella with

thickness lc mainly formed at Tc is determined.

Figure 3(b) shows the Tc-dependence of lc.

The

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broken curve in Figure 3(b) represents the fitting one for the relation between lc and Tc, which will

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be mentioned later in detail.

3.2. Analysis of the Tc–dependences of u compared with the lc–dependence of u. In order to examine the Tc-dependence of u we consider the secondary nucleation process on the growth face of the crystalline lamella[23].

The growth rate of lamella depends on the size of the

nucleus.[24-26]

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secondary critical nucleus and the secondary nucleus thickens by passing through the critical The growth rate of spherulite u is represented by

  

(1)

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 Ki u = u 0,i ⋅ β (Tc ) ⋅ exp −  Tc ∆Ti

where u0,i is a prefactor, β(Tc), the temperature dependent factor related to the diffusion constant, Ki, the nucleation constant related to the activation energy of the secondary nucleus, and ∆Ti, the degree of supercooling expressed by ∆Ti = Ti 0 − Tc using the equilibrium transition temperature between two phases, Ti0.

The subscript index i refers to each phase transition between Liquid(L) and

Crystal(C), and Liquid(L) and Mesophase(M), respectively: i = LC and LM.

The Tc-dependence of

β(Tc) is the Vogel-Fulcher type described by β(Tc) = exp[−d*TV(Tc − TV)−1], where d* is the fragility index and TV is the Vogel-Fulcher temperature.

The values of d* and TV have been reported as 4.9

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and 276 K, respectively, from the results of the dielectric measurement[27].

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When the multiple

secondary nuclei form on the substrate, which is categorized as regime II[28], Ki is defined as

k B ∆H i

(2)

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Ki =

2σ e ,iσ s ,i b0,i Ti 0

where σs,i and σe,i are the lateral and the folding surface energies between two phases, respectively, b0,i, the thickness of the stem added on the substrate, kB, the Boltzmann constant, and ∆Hi, the Since the length of the critical nucleus li* equals to 2σe,i

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enthalpy difference between two phases.

 σ s ,i b0,i l i *   . u = u 0,i ⋅ β (Tc ) ⋅ exp − k T B c  

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Ti 0/∆Ti∆Hi, Eq. (1) is rewritten as

(3)

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The growth rate u of the normal L-C transition is given by Eqs. (1) and (2) with i = LC.

Since

the lamella with lLC* melts at Tc, the crystalline lamellar thickness lc formed at Tc requires the excess

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length δl to thermodynamically stabilize the crystal: lc = lLC* + δlLC.

  σ b l  ⋅ exp − s , LC 0, LC c k B Tc  

  . 

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 σ s b0 , LC δl LC u = u 0 , LC ⋅ β (Tc ) ⋅ exp k B Tc 

Equation 3 can be replaced by

(4)

The Tc-dependences of u and lc in Figure 3 gives the relation between u and lc using Eq. (4). relation between uβ−1 and lcTc−1 in a semi-logarithmic scale is shown in Figure 4. changes at 218 and 208 °C.

The

The relation

The relations between 208 and 218 °C and above 218 °C are linear,

but the plot deviates from the linear relation below 208 °C.

Since it has been predicted that the

folded chain crystal has the constant δl[24,26] and δl << l, the linear relations above 208 °C in Figure 4 indicate that the critical crystalline stems of thickness lLC* nucleate on the substrate.

On

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the other hands, the relation below 208 °C cannot be explained by the growth mechanism same as

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Figure 4.

Plot of uβ−1 against lcTc−1.

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that above 208 °C.

The error bounds are approximately equal to the symbol

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sizes.

In order to clarify the origin of the changes at 218 and 208 °C in Figure 4, the Tc-dependence of u is examined using Eqs. (1) and (2).

TLC0 has been estimated as 270 °C by the Gibbs-Thomson

semi-logarithmic scale.

Figure 5(a) shows uβ−1 against [Tc(TLC0 − Tc)]−1 in a

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relation from the SAXS results[16,17].

The plot above 208 °C also shows two linear relations.

rate for Tc > 208 °C has been fitted using two fitting parameters, u0,LC and KLC.

Thus the growth The non-linear

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regression method gives u0,LC(1) = 5.63 x 1012 µm/s and KLC(1) = 7.44 x 105 K2 for Tc > 218 °C, and The change at

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u0,LC(2) = 2.55 x 107 µm/s and KLC(2) = 4.26 x 105 K2 for 208 °C < Tc < 218 °C.

218 °C can be explained as regime I-II transition when KLC(1) / KLC(2) = 1.75 is regard as 2[23, 29-32]. The spherulitic morphological change at regime I-II [29-32] has been reported, and the morphology of the spherulite also changes into the axialite around at 218 °C in Figure 1.

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Plots of uβ−1 against [Tc∆Ti]−1 for (a) TLC0 = 270 °C, (b) TLM0 = 252 °C and (c) TLM’0 =

233 °C.

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Figure 5.

The bold broken (purple) and solid (red) lines in (a) give u0,LC(1) = 5.63 x 1012 µm/s and

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KLC(1) = 7.44 x 105 K2 and u0,LC(2) = 2.55 x 107 µm/s and KLC(2) = 4.26 x 105 K2, respectively, in Eqs. (1) and (2).

The thin solid (yellow) and broken (green) lines in (b) give u0,LM(1) = 3.99 x 107 µm/s

and KLM(1) = 3.12 x 105 K2 and u0,LM(2) = 1.68 x 1010 µm/s and KLM(2) = 5.59 x 105 K2, respectively. The thin chain (blue) line in (c) gives u0,LM’ = 1.15 x 109 µm/s and KLM’ = 3.59 x 105 K2. fitting was performed using the filled marks in each figure.

Each

The crosses in (a) and (b) are Di

Lorenzo’s results[11].

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Although the lc-dependence of u clearly changes at 208 °C in Figure 4, the plot below 208 °C only

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slightly deviates from the fitted linear relation above 208 °C in Figure 5(a). Righetti have pointed out the regime II-III transition at 211 °C.[11]

Di Lorenzo and

The regime II-III transition

does not occur at the around 208 °C since the ratio of the slope at Tc < 208 °C to that at Tc > 208 °C is 1.34 and much smaller than two in Figure 5(a).

In a previous study[17], we have pointed out that

the results of the temperature dependence of the lamellar thickness.

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the existence of the crystallization process through the mesophase at Tc < TX = 208 °C in PBT from The Tc-dependence of u at Tc <

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208 °C will be analyzed using the crystallization model through the mesophase in the next section.

3.3. Analysis of the Tc-dependence of u using the crystallization model through the mesophase.

forms on the growth front below TX.

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According to the crystallization model through the mesophase[16-18], the mesomorphic stem Thus the growth rate of the spherulite can be expressed by

Eqs. (1)-(3) with i = LM, which represents the L-M transition: the semi-logarithmic relation between uβ−1 and [T(TLM0 − T)]−1 should be linear.

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The fitting parameters in Eqs. (1) and (2) with i = LM are u0,LM, KLM and TLM0.

The non-linear

regression method to fit the experimental result in the Tc range between 165 and 206 °C gives u0,LM(1) = 3.99 x 107 µm/s, KLM(1) = 3.12 x 105 K2 and TLM0 = 252 °C.

The relation of uβ−1 against [T(TLM0

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− T)]−1 is shown in Figure 5(b) in a semi-logarithmic scale.

Figure 5(b) shows that the

experimental results above 157 °C obey the fitting line and follows another line below 157 °C.

The

K2 .

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broken line for Tc < 157 °C in Figure 5(b) gives u0,LM(2) = 1.68 x 1010 µm/s and KLM(2) = 5.59 x 105 The four calculated curves above and below 208 °C are shown in Figure 3 and well fit not only

our experimental result but also Di Lorenzo’s one in Figures 3 and 5. According to the crystallization model through the mesophase, the mesomorphic stem forms, thickens and transforms into the crystalline lamella below TX.

On the basis of the Gibbs-Thomson

relation the relation between mesomorphic or crystalline lamellar thicknesses l and the transition temperatures between L and M TLM, between L and C TLC and between M and C TMC are represented by

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Ti (l ) = Ti 0 −

2σ e ,i Ti

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⋅

∆H i

1 l

(i = LM, MC, LC)

(5)

σe,MC = σe,LC − σe,LM

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where TMC0 is the equilibrium TMC, and it is assumed that

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∆HMC = ∆HLC − ∆HLM.

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and

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(7)

TLM(l), TMC(l) and TLC(l) in Eq. (5) are called as Liquid-Mesophase (L-M), Mesophase-Crystal (M-C), Liquid-Crystal (L-C) transition lines, respectively.

above TX.

TLM(l) and TMC(l) exist below TX, and TLC(l)

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are shown as the T−l-1 phase diagram in Figure 6.

The L-M, M-C and L-C transition lines

Since the L-M, M-C and L-C transition lines cross at a point (TX, lX), Eq. (5) gives the

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relations lX and TX represented by[16,18,33]

(8)

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TLM(lX) = TLC(lX) = TMC(lX) = TX.

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Figure 6.

Phase diagram of l−1 − T for PBT.

The bold solid lines represent the boundaries for two

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of the phases, liquid, crystal and mesophase (Eq. (5)). the boundaries.

The thin solid lines are the extrapolations of

The bold broken curves represents lc given by Eqs. (9) and (10).

The thin broken

curves are the curves extrapolating the bold ones.

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The values of the terms, σe,MC/∆HMC, σe,LC/∆HLC, TMC0, TLC0 and TX have been estimated as 5.58 Å, 3.05 Å, 335 °C, 270 °C and 208 °C, respectively, in a previous paper.[17]

Since lc needs δl

because of the driving force for crystallization, the Tc-dependence of lc has been experimentally

(

0 ∆H LC TLC − Tc

) + δl

0 2σ e , MC TMC

(

0 ∆H MC TMC − Tc

LC

) + δl

MC

(Tc > TX)

(9)

(Tc < TX)

(10)

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lc =

0 2σ e , LC TLC

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lc =

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obtained from the SAXS results and represented by

where δlLC and δlMC are the excess length for Tc > TX and Tc < TX, respectively.

Both of δlLC and

δlMC have the same constant value 9 Å[17]. The Tc-dependences of lc for the L-C and the M-C

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transitions are shown in Figure 6.

The previous SAXS result gives the values of five terms, σe,MC/∆HMC, σe,LC/∆HLC, TMC0 TLC0 and The optical microscopicy results in Figure 4 give the values of three terms, KLC(2) = 4.26 x

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TX.[17]

105 K2, KLM(1) = 3.12 x 105 K2 and TLM0 = 252 °C.

Substituting these values into Eqs. (2) with i =

LM and LC and Eqs. (5) and (8) with i = LM, LC and MC gives b0,LCσs,LC = 1.77 x 10−21 JÅ−1, b0,LMσs,LM = 1.83 x 10−21 JÅ−1 and σe,LM/∆HLM = 2.23 Å.

The values of σe,LC/∆HLC, σe,LM/∆HLM

and σe,MC/∆HMC gives ∆HLM/∆HLC = 0.757 and σe,LM/σe,LC = 0.556 by using Eqs. (6) and (7).

The

L-M transition line in Eq. (5) drawn using the value of TLM0 is shown in Figure 6. We calculate each values of the terms, ∆HLC, ∆HLM, ∆HMC, σe,LC, σe,LM, σe,MC, b0,LC and σs,LC using the literature values[3,4]: the heat of fusion, 32.0 kJ mol-1 [3], and the density of the crystal,

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1.404 g cm-3 [4], at room temperature, gives ∆HLC as 2.04 x 10−22 J Å−3. Using the value of ∆HLC,

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∆HLM = 1.54 x 10

−22

J Å , ∆HMC = 4.95 x 10−23 J Å−3, σe,LC = 6.22 x 10−22 J Å−2, σe,LM = 3.64 x 10−22 −3

J Å−2 and σe,MC = 2.76 x 10−22 J Å−2 are obtained.

The values of the terms estimated above are

listed in Table 1.

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Table 1. The values of the terms determined in this study. TLM0 (°C)

TMC0 (°C)

TX (°C)

∆HLC (JÅ-3)

∆HLM (JÅ-3) ∆HMC (JÅ-3)

270 a

252

335 a

208 a

2.04 x10-22 b

1.54 x10-22

σe,LC (JÅ-2)

σe,LM (JÅ-2)

σe,MC (JÅ-2)

σs,LC (JÅ-2)

b0,LC (Å)

6.22 x10-22

3.64 x10-22

2.76 x10-22

: Ref. (17), b: Ref. (3), c: Ref. (4).

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a

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TLC0 (°C)

3.03 x10-22

5.86 c

4.95 x10-23

b0,LMσs,LM (JÅ-1) 1.83 x 10-21

The value of ∆HLM/∆HLC for PBT indicates that the order of the mesophase is close to that of the The both values of ∆HLM/∆HLC and σe,LM/σe,LC for PBT are

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crystal rather than that of the liquid.

larger than those of the other polymers reported by Strobl, ∆HLM/∆HLC = 0.59 and σe,LM/σe,LC = 0.25

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for poly ε-caprolactone, 0.45, 0.12 for polyethylene, and 0.58, 0.37 for isotactic polystylene.[21] For isotactic polypropylene, the metastable smectic phase forms near its glass transition.

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Regarding the smectic phase as the mesophase for iPP, ∆HLM/∆HLC for iPP is estimated as 0.81 [34] or 0.73 [35] by using the densities for the crystal 0.916 g cm-3 and the smectic phase 0.938 g cm-3 [34]. Since ∆HLM/∆HLC for PBT are similar to that for iPP, the mesophase in PBT might be the smectic phase observed when stretching the PBT glass[8,9].

We have proposed that the formation

of the smectic phase for PBT needs the alignment of the polymer chain by stretching for PBT since the glass transition temperature Tg of the smectic phase, Tg,sm, is located below Tg of the amorphous, Tg,am, and that the smectic phase for iPP does not need because of Tg,sm > Tg,am.[9]

Thus the smectic

phase of the stretched PBT might be similarly treated as that of non-stretched iPP.

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Yoshioka and coworkers have determined the growth direction at Tc = 210 °C as the [010]*

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direction of the α-form.[12]

Thus the result indicates that the length of a0,LC and b0,LC can be

regarded as the lengths of a- and b-axes in the unit-cell projecting along c-axis, respectively, and are estimated as 4.37 Å and 5.86 Å, respectively, using the unit cell parameters of PBT[4].

The values are also listed in Table 1.

The

relation between σs,LC and ∆HLC is given by the well-known Thomas-Staveley relation[36],

σs,LC =

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of b0,LC and b0,LCσs,LC give σs,LC = 3.03 x 10−22 J Å−2.

The values

The

αTS∆HLC(a0,LCb0,LC)0.5, where αTS is a constant depending on material properties and a0,LC is the width of the stem.[31]

The value of αTS is estimated as 0.3-0.4 for ordinary organic material[36], Using a0,LC = 4.37 Å [4], αTS for PBT are estimated as 0.293.

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and 0.1-0.2 for polymer[37].

value of αTS is slightly larger than that obtained by Di Lorenzo and Righetti, 0.23[11], and is larger

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than those of the other polymers and is similar to those of organic materials.

The Tc-dependences of lLC* above 208 °C is given by TLC(l) and that of lLM* below 208 °C, by TLM(l).

The relation between uβ−1 and li*Tc−1 is shown in Figure 7.

Figure 7 shows that the plots

above and below TX show similar linear behaviors because the ratios of the intercept, log(u0,LM/u0,LC)

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= 0.20, and the slope, b0,LMσs,LM/b0,LCσs,LC = 1.03, are close to 1 in Eq. (3).

Note that lLM* is the

lamellar thickness of the mesomorphic stem firstly formed at the growth front and lc at Tc < TX in

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Figure 4 is that of the lamella after the M-C transition expressed by Eq. (10). The enhancement of the Tc-dependence of u in the lower Tc-region has been also observed for

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many semi-crystalline polymers.[38-48] the regime II-III transition.

The origin of the enhancement has been mostly treated as

It is reported, however, that the disordered crystal enhances the growth

rate in the lower Tc-region for poly(L-lactic acid).[47,48] enhancement has also been observed.[49]

For the polymeric liquid crystals the

For some of the other polymers the enhancement is

possibly due to formation of mesophase or disordered crystal before that of the stable crystal.

15

Figure 7.

Plot of uβ−1 against li*Tc−1.

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The circles and squares indicates lLC* for the crystal above

208 °C and lLM* for the mesophase below 208 °C, respectively.

The bold broken, bold solid, thin

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solid and thin broken lines are given by Eq. (3) with the parameters for the fitting lines in Figure 5.

around 157 °C in Figure 5.

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Finally we comment on the three possible origins of the inflection of the Tc-dependence of u The first is that the inflection might indicate that the growth face of the

mesophase changes around 157 °C.

The second is that the inflection temperature can be explained The

third is that another disordered-mesophase (M’) forms on the growth front at Tc < 157 °C.

The

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as the regime II-III transition temperature when KLM(2)/ KLM(1) = 1.79 is regarded as about 2.

change of the T-dependence of the lamellar thickness or the nodular crystal size around 157 °C has not been detected from the SAXS results.[16,17,33]

The SAXS results suggest that the

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disordered-mesophase transforms into the normal mesophase and then into the crystal.

When the

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fitting parameters in Eqs. (1) and (2) with i = LM’ are u0,LM’, KLM’ and TLM’0, the non-linear regression method in the Tc range between 118 and 155 °C gives u0,LM’ = 1.15 x 109 µm/s, KLM’ = 3.59 x 105 K2 and TLM’0 = 233 °C.

The fitting results are shown in Figure 3 and Figure 5(c).

growth rate of the disordered-mesophase intersects that of the mesophase at 157 °C.

The

Substituting

these values into Eqs. (2) and (5) with i = LM and LM’ gives σe,LM’/∆HLM’ = 1.86 Å and b0,LM’σs,LM’ = 2.64 x 10-21 JÅ-1. possible origins.

However there are no definitive evidences for any of the three addressed

More detailed experimental research for the origin of the inflection will be needed

in the future.

16

4. Conclusion.

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In this study, the Tc-dependence of spherulitic growth rate of PBT observed by optical microscopy changes at 157, 208 and 218 °C.

The higher temperature of the three, 218 °C, may be explained as

the regime I-II transition because of the results of the Tc-dependence of u and the morphologies of the spherulites.

The middle temperature, 208 °C, corresponds to that of the Tc-dependences of the The examination of the Tc-dependences of growth rate and the lamellar

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lamellar thickness.

thickness shows that the crystalline stems nucleate and grow at Tc > 208 °C and the mesophase

Acknowledgements.

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stems, at Tc < 208 °C. The analysis gives the parameter values of the mesophase of PBT.

This work was partially supported by KAKENHI(Grant-in-Aid for Young

and Technology, Japan.

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Scientists (B) (No. 21740311, 25800236)) from the Ministry of Education, Culture, Sports, Science The measurements of the lamellar thickness were performed at the

BL-40B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2010A1291, 2010A1200, 2010B1479, 2010B1482, 2011B1398, 2012A1393

[1]

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Highlight

1. The T-dependence of spherulitic growth rate was investigated in poly(butylene terephtharate). 2. The T-dependence of spherulitic growth rate was compared with that of the lamellar thickness.

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3. Both of the T-dependences change at 208 °C.

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4. The crystallization proceeds through the mesophase below 208 °C and directly do above 208 °C.