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Aug 14, 2013 - molecular mass parameters were calculated from the SEC-MALLS data ...... (44) Nofar, M.; Zhu, W.; Park, C. B.; Randall, J. Ind. Eng. Ch...
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Shear-Induced Nucleation and Morphological Evolution for Bimodal Long Chain Branched Polylactide Huagao Fang, Yaqiong Zhang, Jing Bai, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China ABSTRACT: The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). Shear-induced nucleation density enhancements for the long chain branched PLA (LCB PLA) were studied on the basis of the determination of the critical shear rate, for which the stretch of the longest chains of the linear component is expected. The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate compared with that of linear PLA but also forms the shish-kebab structure after sheared for sufficient time. The quantitative evaluation of the shear-induced nucleation densities from rheological measurements is based on the space-filling model by using the Avrami equation, and the obtained nucleation density values are well consistent with that estimated from POM observations. A saturation of nucleation density under shear can be reached for both linear PLA and LCB PLA. The saturated nucleation density values are higher than that under the quiescent condition by a factor of over 3 orders of magnitude, and the saturated nucleation density value for LCB PLA is more than that for linear PLA by a factor of 1 order of magnitude under the same shear condition. The enhancement of nucleation ability and the morphological evolution from the spherulitic to shish-kebab structures induced by shear flow can be ascribed to the broadened and complex relaxation behaviors of LCB PLA.

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INTRODUCTION Processing and shaping operations of semicrystalline polymer materials, such as injection molding, film blowing, and fiber spinning, etc., make polymer chains oriented in intensive shear and/or elongation flow. Such flow fields not only accelerate crystallization kinetics but also affect final crystalline morphologies, thus inference the product properties and performance.1−7 Considerable works have devoted to understand the interplay between flow fields and polymer crystallization. It is widely known that the applied flow field can drastically enhance polymer crystallization, mainly through an acceleration of the nucleation process, which is attributed to the oriented structures developed in the flow field.8−11 Investigation on macromolecular conformation in flow fields is crucial to understand shear-induced crystallization of polymer materials. van Meerveld et al. had proposed a rheological classification of the flow regimes to describe macromolecular conformation, based on two characteristic Weissenberg numbers: Wirep = τrepγ̇ and Wis = τRγ̇, with γ̇ being the shear rate and τrep and τR being the reptation time (disengagement time) and the Rouse time (stretch relaxation time) of the high molecular mass chains.12 The extensive evaluation of experimental results illustrates that the transition of crystalline morphology correlates with the transition degree of chain © 2013 American Chemical Society

conformation from orientation to stretch in the preshear process.10,13−16 Some key fundamental characteristics of the polymers, including polymer molecule architecture such as the chain length and its distribution, rigidity, interchain friction, and chain branching, are crucial to interfere with the rheological properties to demonstrate different orientation and structural evolution state under the given flow field. The importance of the high molecular mass (HMW) tail in polymer melt on the formation of oriented, anisotropic structures was pointed out by several researchers.17−20 An important conclusion of these studies is that a small part of HMW component drastically increases the crystallization rate and enhances the formation of the shish-kebab structure, because these chains with long relaxation time can accumulate stretch at deformation rates that are accessible in the experimental conditions. Long chain branched (LCB) polymers possess advantageous properties, which are important in industrial processing applications, such as high melt strength, extensive shear thinning, and strain hardening.21−25 Understanding the crystallization behavior of LCB polymers in flow field is crucial Received: June 12, 2013 Revised: July 29, 2013 Published: August 14, 2013 6555

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induced crystallization of PLA materials have not been reported so far. Recently, a new class of LCB PLA polymers was prepared from linear PLA precursor by applying γ radiation with addition of the trifunctional monomer, trimethylolpropane triacrylate (TMPTA).49 These LCB PLA polymers exhibit unique bimodal architectures containing a linear chain fraction with low molecular mass and a LCB fraction with high molecular mass, resulting in the improved melt rheological and foaming properties.49 The unique molecular architecture of these LCB PLA polymers provides an opportunity to study the effects of LCB on the shear-induced crystallization kinetics. In the present work, we investigated the effects of long chain branching on the crystallization kinetics of LCB PLA under isothermal crystallization conditions by applying a shearing experimental protocol adopted in the study on the shearinduced crystallization of PLA with linear chain architecture.43 First, we present some of the characteristic properties of LCB PLA and determine the critical shear rate at which the long PLA chains are expected to be stretched. Then, the effects of long chain branching on the nucleation density enhancements and morphological evolution of LCB PLA under the different shear conditions were investigated by in-situ POM and rotational rheometry. Finally, the numbers of nuclei, which were estimated from space filling using the Avrami equation, were obtained to quantitatively evaluate the influences of LCB on the enhancements of shear-induced isothermal crystallization kinetics of PLA.

with respect to the fundamental scientific concerns and industrial application interests. However, to our knowledge, only sparse studies on the effects of long chain branching on shear-induced crystallization of polymers were reported in the literature.26−30 Agarwal et al. studied the shear-induced crystallization of LCB isotactic polypropylene (LCB iPP) produced by a diene monomer copolymerization technique using metallocene catalysts. Their results demonstrated that oriented crystalline fraction was higher in LCB iPP than that in the linear iPP as shown by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements, and the crystallization kinetics of LCB iPP was enhanced by more than 1 order of magnitude when compared to linear iPP under shear. They ascribed these unique properties to the broadened and complex relaxation behaviors of LCB iPP.26 Heeley et al. studied the quiescent and shear-induced crystallization of welldefined linear monodisperse hydrogenated polybutadiene blends with a high molecular mass long chain branched comb-shaped additive using the time-resolved X-ray scattering technique, which showed that the addition of LCB combs to the blends had significantly increased the crystallization rates compared to the quiescent conditions, and the formation of an oriented crystal occurred sharply when the LCB contents were 5 and 10%, which were around c*.27 Bustos et al. investigated the effect of the molecular structure of polyethylene on the crystallization kinetics by following the dynamic modulus, in which the enhancement of crystallization kinetics was conditioned by the molecular architecture via its effect on the relaxation time.28 Vega et al. compared the shear-induced crystallization behaviors of linear iPP and LCB iPP by using rheometry, in which lower values of the induction time for the onset of crystallization process were found at the low and moderate preshear conditions, and the effect was much more pronounced for LCB iPP than for linear iPP.29 Recently, Kitade et al. studied the shear-induced crystallization of LCB iPP at 170 °C, close to the melting temperature, and at a low shear rate of 2 s−1 by SAXS, WAXS, and POM, which demonstrated that shish-like structures predominately formed and the further growth to kebabs was suppressed in LCB iPP.30 However, it is clearly concerned that overall there is a lack of systematic investigation on the relationship between the chosen shear conditions and crystallization kinetics and morphological evolution for the LCB polymer materials, which is definitely vital to understanding the crystallization behaviors of LCB polymer materials for processing, compared with extensive studies on the shear-induced crystallization of their linear counterparts. Polylactide (PLA), as an environment friendly semicrystalline polymer produced from renewable resource, has received great attention in recent years. The processing behavior can be remarkably optimized by the introduction of long chain branching (LCB) structures in linear PLA matrix to improve the melt rheological properties such as melt strength and strain hardening.31−35 Although the crystallization behavior of linear PLA has been extensively investigated in quiescent conditions as well as under shear flow,36−43 the studies on crystallization behavior of PLA materials with LCB structures were only performed in quiescent conditions, aiming to investigate the influences of LCB structures. The results showed that the different crystallization behaviors of LCB PLA compared with that of linear ones were tightly associated with the degree of branching, the type of chain extender, and its functional groups.44−48 The effects of long chain branching on the shear-

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EXPERIMENTAL SECTION

Materials and Preparation of Samples. Commercial available polylactide (PLA) (Natureworks product PLA 2002D with 96 wt % of L-isomeric content) was used in this study as linear precursor. Two model PLA samples with different chain architectures were prepared by γ irradiation. The details about the sample preparation and their rheological properties can be found elsewhere49 and are briefly described here. Long chain branched PLA sample (named as LCB PLA in this study) was prepared by γ irradiation with addition of the trifunctional monomer, trimethylolpropane triacrylate (TMPTA). TMPTA of 0.4 wt % content was first melt mixed with linear PLA precursor and then subjected to γ radiation of 2.2 kGy/h from 60Co source for 5 kGy to produce long chain branched structure. The linear PLA reference was prepared by experiencing the same thermal and irradiation history of the PLA precursor but with no addition of TMPTA. Characterization of Molecular Masses and Distributions. The characterization of the PLA samples with respect to molecular mass distribution and long chain branching level was carried out by using size-exclusion chromatography (SEC) coupled with a DAWN HELEOS II multiangle laser light scattering (MALLS) detector, a refractive index (RI) detector Optilab T-rEX, and a viscometer ViscoStar II. The measurements were performed using the eluent tetrahydrofuran (THF) with a flow rate of 1.0 mL/min at 30 °C. The molecular mass parameters were calculated from the SEC-MALLS data using the commercial software ASTRA 6 (Wyatt Technology, Santa Barbara, CA). Measurements of Melt Rheological Properties. Rheological measurements were performed on a rotational rheometer (TAAR2000EX, TA Instruments, New Castle, DE) with a parallel plate geometry of 25 mm in diameter and a gap of 0.9 mm at constant temperatures in a nitrogen atmosphere. Dynamic frequency sweep measurements were carried out in the linear viscoelastic regime from frequency of 500 to 0.1 rad/s for linear PLA and from 500 to 0.05 rad/ s for LCB PLA. Measurements on Shear-Induced Crystallization Kinetics. The isothermal crystallization processes for linear PLA and LCB PLA under the quiescent conditions and after shear application, 6556

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to avoid the effect of PLA thermal degradation, a fresh PLA film sample was used for each measurement.

respectively, were studied by a rotational rheometry (TA-AR2000EX, TA Instruments) in oscillatory time sweep mode. A plate−plate geometry of diameter of 8 mm was used to avoid transducer instabilities. In general, the crystallization process was monitored through the response of the sample to a low enough oscillatory strain at a fixed frequency. It must be emphasized that the imposed strain must be low enough not to influence the crystallization kinetics and not to damage the sample. Thus, in this study, a much low strain of 0.5% was adapted to avoid the crystallization kinetics to be modified. During the crystallization process the sample usually experienced a contraction, which induced an obvious variation of the sample dimension. Because of the dimensional change, the sample was subjected to a large longitudinal force, which might cause errors on the measurements of storage modulus. In order to keep the force value at zero, the gap between the parallel plates was automatically adjusted throughout the crystallization process according to the normal force transducer of the rheometer. The experimental procedure for the shear-induced crystallization is schematically shown in Figure 1 and is

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RESULTS AND DISCUSSION Molecular Characteristics of Linear PLA and LCB PLA. Linear PLA and LCB PLA samples with different molecular characteristics and chain architectures were prepared by applying γ radiation with and without addition of the trifunctional monomer, trimethylolpropane triacrylate (TMPTA), respectively. The obtained molecular parameters, the weight-average molecular mass, Mw, and the polydispersity index, Mw/Mn, are listed in Table 1. It can be seen that overall the Mw and Mz of LCB PLA and linear PLA differ by a factor of about 2, and the LCB PLA sample possesses the higher polydispersity index. Table 1. Molecular Mass Parameters for Linear PLA and LCB PLA sample code

Mw (kg/mol)

Mz (kg/mol)

Mw/Mn

LCB contenta (%)

linear PLA LCB PLA

87 169

142 299

1.4 2.2

0 6.4

LCB content was obtained by fitting the curve of molar mass distribution.

a

A close look at the SEC curves and the conformation plots of radius of gyration as functions of molecular mass (Figure 2)

Figure 1. Schematic indication of thermal and shear applications for isothermal crystallization process for linear PLA and LCB PLA.

described as follows. At first, a disk-shaped PLA sample was loaded at the temperature of 200 °C (T1) and held for 5 min to erase previous thermal history. Subsequently, the PLA sample was cooled at 15 °C/ min to the desired crystallization temperature of 120 °C (T2), and then a steady shear pulse was applied to the undercooled melt. For the isothermal crystallization process at T2 a time sweep at an angular frequency of 5 rad/s was performed to trace the evolution of storage modulus of the sample with time until the crystallization process was completed. It should be noted that PLA shows the maximum spherulitic growth rate at the isothermal crystallization temperature of 120 °C, which was reported in our previous publication.50 Therefore, in this study, we had chosen the experimental crystallization temperature of 120 °C to investigate the effects of shear on the crystallization behaviors of LCB PLA materials. Shear-Induced Morphological Evolutions Observed by POM. The nucleation and growth of spherulites for PLA samples during isothermal crystallization process under application of shear were observed by using a polarized optical microscopy (POM, Olympus BX51, Japan) equipped with a Linkam optical shearing system (Linkam CSS-450, Linkam Scientific Instruments, UK). The gap between the two quartz windows in Linkam CSS-450 was set at 20 μm. The PLA film samples were melted at 200 °C for 5 min to eliminate thermal history and then cooled to the isothermal crystallization temperature of 120 °C at a cooing rate of 8 °C/min. Once the temperature reached 120 °C, steady shears with different selective shear rates or shear times were immediately applied to PLA melt to examine the changes of nucleation density and growth rate of PLA spherulites with the shear conditions by continuously taking the optical micrographs at appropriate time intervals just after the shear cessation and until impingements of the growing spherulites. In order

Figure 2. Molecular mass distributions for linear PLA and LCB PLA as determined from SEC-MALLS. The solid lines represent the fitted lognormal bimodal distributions in LCB PLA. The inset shows the changes of root-mean-square radius of gyration, ⟨rg2⟩0.5, as functions of molecular mass, MLS, for linear PLA and LCB PLA.

indicates that linear PLA exhibits a unimodal molecular mass distribution, whereas LCB PLA exhibits a bimodal molecular mass distribution that contains the long chain branches in the higher molecular mass fraction, which results in the coil contraction phenomenon in the conformation plot, and the linear chains in the lower molecular mass fraction. When LCB PLA is described with the weighted sum of two log-normal distributions (see the blue solid lines in Figure 2), the best fit on the distribution curve indicates that the Mw of linear fraction is close to that of linear PLA and the mass percentage of LCB fraction is 6.4 wt %. Therefore, the bimodal LCB PLA can be considered as obtained from linear PLA with the addition of the high molecular mass long chain branched fraction. The preparation protocol ensures that despite of the difference in molecular mass the two fractions are actually mixed at the molecular level. Rheological Properties of Linear PLA and LCB PLA. It is well-known that the polymer molecular architecture, 6557

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regression simultaneously adjusts not only Gi and τi parameters but also the number of relaxation modes, N, during the iterative calculations to obtain the best fit of G′ and G″ data. The best sets of modulus, Gi, and relaxation time, τi, are presented in Figure 4, which can be used to nicely fit the measured G′ and

especially the topological structure such as long chain branching, has a strong effect on the melt rheological properties. The LCB polymers demonstrate the extensive shear thinning, high viscoelastic property, strain hardening, and also the thermorheologically complex behavior in the melt rheological properties. The rheological properties of linear PLA and LCB PLA at the temperature of 180 °C are shown in Figure 3. In the terminal region the storage modulus, G′, and

Figure 4. Relaxation behaviors of linear PLA and LCB PLA melts at 180 °C. Figure 3. (a) Changes of storage modulus, G′, and loss modulus, G″, as functions of angular frequency, ω, for linear PLA and LCB PLA at 180 °C. The lines are obtained from the discrete Maxwell relaxation time spectra. (b) Changes of complex viscosity, |η*|, as functions of angular frequency, ω, for linear PLA and LCB PLA at 180 °C.

G″ data (see the lines shown in Figure 3a). Figure 4 indicates that the shortest relaxation modes are similar for both PLA samples. However, the LCB structure in LCB PLA results in broadening of the relaxation spectrum in the longest relaxation time regime, and the longest relaxation time increases from 1.7 s for linear PLA to 12.5 s for LCB PLA. From a rheological viewpoint, the effect of shear flow on the macromolecular conformation can be assessed by defining two characteristic Weissenberg numbers for molecular orientation and stretch: Wirep = τrepγ̇ and Wis = τRγ̇, with γ̇ being the shear rate and τrep and τR being the reptation time (disengagement time) and the Rouse time (stretch relaxation time) of the high molecular mass chains, respectively, which was proposed by van Meerveld et al.12 The transitions between these regimes are characterized by critical values of the Weissenberg numbers. In regime I, for Wirep < 1 and Wis < 1, the polymer chains are in their equilibrium state; thus, shear flow has no effects on crystallization. Generally, polymer chains tend to be oriented for Wirep > 1 and Wis < 1 (regime II), to be stretched for Wis > 1 (regime III), and in the final regime (regime IV), the polymer chains are strongly stretched for a sufficiently long time to fulfill the condition that the molecular stretch ratio λ is larger than a critical value, λ*(T). Regime II is necessary for the enhancement of the number density of activated nuclei, and regime III ensures sufficient stretch of the polymer chains into a conformation ideal for the formation of oriented nuclei. These transitions then suggest that there is a minimum shear rate (1/τR), below which the oriented nuclei cannot be obtained, and therefore the shish-kebab crystalline morphology is unlikely to be observed for such a crystallized polymer. On the basis of the analyses of the shear-induced crystallization studies as reported in the literature, it is concluded that stretch of the longest chains (high molecular mass tails, HMW tails) of polymers dominates the dynamics of the shear-induced crystallization. Thus, the longest relaxation time from the relaxation spectrum is used as a measure of the HMW tails. The Rouse time (the stretch relaxation time), τR, of PLA can be calculated using the relation based on the tube model of Doi and Edwards:54 τrep τR = (3) 3Z

loss modulus, G″, of linear PLA follow the well-known frequency dependences, i.e., G′ ∝ ω2 and G″ ∝ ω, for which only the longest relaxation times contribute to the viscoelastic behavior. The G′ values in the low-frequency region of the experimental window pronouncedly increase for LCB PLA as compared with linear PLA. The slopes at the low-frequency region decrease from 1.88 for linear PLA to 1.37 for LCB PLA. Note that the deviation of the slope from an ideal fluid at the low-frequency region indicates the terminal zone is not reached for the PLA samples in our experimental frequency range. However, LCB PLA presents the more pronounced nonterminal behavior as compared with linear PLA. The storage modulus, G′, and loss modulus, G″, in the lower frequency range could be obtained with the help of creep measurements.51,52 The complex viscosity of LCB PLA at low frequencies is higher than linear PLA by a factor of 5 orders of magnitude and LCB PLA demonstrates a more pronounced shear thinning behavior (see Figure 3b). Therefore, the much more obvious nonterminal behavior for LCB PLA than linear PLA suggests that there exist longer relaxation times for LCB PLA, which can be ascribed to the long chain branched structure. Similar results were observed for long chain branched PLA materials prepared by melt radical reactions and electron beam irradiation.31,45,46,53 The dynamic moduli, G′ and G″, can be expressed in terms of the discrete Maxwell relaxation time spectra (Gi, τi):54 N

G′(ω) =

∑ i=1 N

G″(ω) =

∑ i=1

Giω 2τi 2 1 + ω 2τi 2

(1)

Giωτi 1 + ω 2τi 2

(2)

where Gi is the modulus corresponding to the relaxation time, τi. The parameters Gi and τi can be obtained by calculating from the G′ and G″ data by using the standard nonlinear regression method in the Trios Software (TA Instruments). The nonlinear 6558

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where Z is the average number of entanglements (Z = Mw/Me, Me is the molecular mass between entanglements). The calculated Z value for linear PLA is about 11 by using the Me value of 8000 g/mol for PLA. Table 2 lists the longest

rheometry. This widely used experimental technique has been applied to investigate the shear-induced crystallization for several polyolefins and PLA materials, which possesses several important advantages, which can be described as follows: (1) it is much easier and faster than other methods such as optical microscopy, small-angle light scattering (SALS), SAXS, WAXS, and differential scanning calorimetry (DSC) to track the crystallization processes of polymers, (2) it can combine the applying shear with subsequently measuring the mechanical spectrum to detect even small microstructure changes in the polymer materials, and (3) it is applicable to the systems where some other methods do not work, for examples, the colored system and/or the filled composite system.29,58−62 The rise of storage modulus, G′, has been correlated with the growth of crystals. The growth of G′ is attributed to the filler effects of the crystals, based on modeling the crystallizing polymer as a suspension of growing particles in an amorphous matrix. Figure 5 shows the evolutions of storage modulus, G′, measured during the shear-induced isothermal crystallization for linear PLA and LCB PLA at the temperature of 120 °C after controlled shear at 120 °C with a fixed shear rate (γ̇ =1 s−1) associated with different shear time, ts, respectively. Note that the corresponding Wis values are higher than 1, for which stretch of the high molar mass chains is predicted. The data for the quiescent crystallization process are plotted as well in Figure 5 for the comparison purposes. For the quiescent isothermal crystallization process, with the sporadic nucleation and growth of spherulites of PLA from melt the change of storage modulus, G′, with time shows a sigmoidal shape, exhibiting a progression of storage modulus from about constant values before the starting of crystallization to a rapid increase and then to approaching the plateau values at the ending of the primary crystallization stage. The time at the intersection of the highest slope of the storage modulus−time curve with the line through the initial constant storage modulus values (indicated by the green dashed lines in Figures 5a,b) is often defined as the induction time, t0, for crystallization. At the temperature of 120 °C the induction times needed for the onset of crystallization of linear PLA and LCB PLA are about 7300 and 7700 s, respectively. When a shear is applied, the storage modulus curve shifts to the shorter time region, and the induction time is significantly reduced with increasing shear time in comparison with the quiescent condition for both linear PLA and LCB PLA, indicating an acceleration of the crystallization kinetics. However, the shift becomes less obvious at the sufficiently longer shear time, which implies that the

Table 2. Longest Reptation Time (τHMW rep ) and Rouse Time (τHMW ) Determined from the Relaxation Spectra and the R Corresponding Critical Shear Rates for the OrientationStretch Regime Transition of Linear PLA T (°C)

τHMW (s) rep

τHMW (s) R

γ̇I→IIa (s−1)

γ̇II→IIIa (s−1)

180 120

1.69 38.3

0.052 1.17

0.59 0.026

19.23 0.85

γ̇I→II and γ̇II→III are the critical values of shear rate for the transition of flow regimes for linear PLA between regimes I and II, and regimes II and III, respectively.

a

relaxation times together with the characteristic shear rates for the transitions between the orientation and stretch regimes for linear PLA at 180 and 120 °C, respectively. The relaxation times and corresponding characteristic shear rates at 120 °C were calculated using the Arrhenius type of temperature dependence (with Ea = 77 kJ/mol49), from which the shear conditions for studying the shear-induced crystallization kinetics of linear PLA could be chosen. It must be noted that the determination of the Rouse time for LCB PLA is quite complicated because the stretching of LCB is controlled by the Rouse time of LCB, which is mediated by the linear PLA matrix.27,55 Indeed, the determination of the longest Rouse relaxation time for LCB Polymer is not trivial, and some methods have been provided to solve this problem. Verbeeten et al. have proposed a useful method to estimate the longest Rouse relaxation time by determination of the parameters through fitting the experimental data with the extended pompom model.56,57 In our case, the terminal regime could not be determined in the experimental time scale, and the longest relaxation time of 12.5 s from the spectrum underestimates the true value for the LCB fraction. Therefore, in the followed shear-induced crystallization measurements the shear rate of 1 s−1 was selected, which was sufficiently high to guarantee Wis > 1, at which the stretch of the longest chains (high molecular mass tails or LCB chains) of linear PLA and LCB PLA was expected. Shear-Induced Isothermal Crystallization Kinetics. Dynamic mechanical measurement has been applied to follow the crystallization behaviors of PLA by using rotational

Figure 5. Changes of storage modulus, G′, with time during crystallization at Tc of 120 °C at the shear rate of γ̇ = 1 s−1 with different shear time for (a) linear PLA and (b) LCB PLA. 6559

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Figure 6. Changes of relative crystallinity, x(t), with time during crystallization at Tc of 120 °C at the shear rate of γ̇ = 1 s−1 with different shear time for (a) linear PLA and (b) LCB PLA.

acceleration of crystallization kinetics by shear can become saturated after a certain shear time. The shortening of induction time for crystallization is a signature of formation of extra nuclei activated by shear, which will be discussed more in the later section. It should be pointed out that the plateau values for G′ at the end of the primary crystallization stage show some variations among the curves probably due to slight differences in the sample position between the parallel plates of the rheometer and due to the fluctuations in the gap distance, which influence the sample diameters. Note that the contact state of solidified polymer sample within the geometry in every individual experiment is different from each other because shrinkage of PLA samples in the geometry during the crystallization process could not be precisely identical. Figure 6 shows the changes of relative crystallinity with time during the shear-induced isothermal crystallization of linear PLA and LCB PLA at the temperature of 120 °C after controlled shear at 120 °C with a fixed shear rate (γ̇ =1 s−1) associated with different shear time, ts, respectively. Note that the relative crystallinity is estimated by applying a logarithmic normalization of the G′ data in dynamic mechanical measurements, following that Pogodina et al. have proposed as follows:60 x(t ) =

log G′(t ) − log G′min log G′max − log G′min

Figure 7. Changes of crystallization half-time, t1/2, as functions of shear time for linear PLA and LCB PLA crystallized at Tc of 120 °C with shear rate of 1 s−1. The lines are drawn to guide the eye.

of polymer molecules and the formation of a stable crystallization precursor become easier for the sheared melts of LCB PLA than linear PLA. Morphological Evolution for Linear PLA and LCB PLA after Shear Pulse. The effects of shear flow with the shear rate of 1 s−1 on morphological evolutions of PLA samples during isothermal crystallization were studied by using polarized optical microscopy equipped with an optical shear stage. Figures 8 and 9 illustrate the nucleation and spherulitic growth for linear PLA and LCB PLA, respectively, during isothermal crystallization at Tc of 120 °C when the samples were subjected to shear at the shear rate of 1 s−1 for different shear time, ts. It can be seen from Figure 8 that after the undercooled melt of linear PLA is applied to a steady shear pulse the nucleation density is remarkably enhanced, and the impingement time for the growing spherulites becomes shortened with increasing shear time. Meanwhile, the crystalline morphologies change from the well-defined spherulites with clear Maltase cross pattern to closely stacked fine-grained spherulitic morphology with increasing shear time. It is interesting to find that under shear two kinds of crystallites can be recognized in the morphological evolution process (panels B and C in Figure 8). Most of the growing spherulites have about the same sizes for each shear condition, indicating that these nuclei are produced at about the same time. Some spherulites appear during the late crystallization stage, indicating that their nuclei are generated according to the sporadic nucleation mechanism. When the shear time increases, a large number of point-like small spherulites can be distinguished in the micrographs (see panels D−F in Figure

(4)

where G′min and G′max are the values of the starting storage modulus and ending plateau storage modulus, respectively. Because LCB PLA can be treated as a blend of linear PLA and long chain branched PLA with mass ratio of 93.6/6.4, the result can be used to evaluate the effects of long chain branching on the crystallization kinetics under shear. The influence of LCB on the isothermal crystallization kinetics of LCB PLA under shear can be simply quantified by defining the crystallization half-time, t1/2, at which the relative crystallinity reaches a value of 0.5. The effect of LCB on the melt crystallization kinetics under shear can be clearly seen in Figure 7. LCB PLA crystallizes much faster than linear PLA under the same shear condition, and the saturation of shear effect at the longer shear time (higher than 20 s) on the crystallization kinetics for linear PLA and LCB PLA is also observed, consistent with our previous report.43 The saturated value of crystallization halftime for LCB PLA is 490 s, which is nearly half of that for linear PLA (970 s), indicating that LCB structure accelerates 1-fold of the shear-induced maximum crystallization rate for PLA. This increase in the crystallization kinetics is generally attributed to an increase of crystal nuclei due to the fact that the orientation 6560

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Figure 8. Selected POM micrographs during isothermal crystallization at Tc of 120 °C for linear PLA after sheared at shear rate of 1 s−1 for different shear time of (A) 0, (B) 1, (C) 5, (D) 20, (E) 30, and (F) 40 s. The scale bar in the top micrograph of panel A represents 200 μm and is applied to all the micrographs. The crystallization times are indicated in the micrographs. The red arrow in panel B indicates the spherulite whose nucleus forms according to the sporadic nucleation mechanism. The olive arrows on the right side of the figure indicate the shear flow direction.

Figure 9. Selected POM micrographs during isothermal crystallization at Tc of 120 °C for LCB PLA after sheared at shear rate of 1 s−1 for different shear time of (A) 0, (B) 1, (C) 5, (D) 20, (E) 30, and (F) 40 s. The scale bar in the top micrograph of panel A represents 200 μm and is applied to all the micrographs. The crystallization times are indicated in the micrographs. The red arrow in the panel B indicates the spherulite whose nucleus forms according to the sporadic nucleation mechanism. The blue arrows in panels E and F indicate the shear-induced row-like crystals. The olive arrows on the right side of the figure indicate the shear flow direction.

8), which is in consistence with the result in our previous publication,43 indicating that the increase of crystallization kinetics is mainly achieved through the enhancement of nucleation density by shear flow. LCB PLA demonstrates a different crystalline evolution phenomenon with increasing shear time at the shear rate of 1

s−1 from that of linear PLA, which is shown in Figure 9. It can be seen that under the quiescent condition the nucleation ability of LCB PLA is higher than linear PLA because the branched points can act as nucleating sites for the nucleation process. A similar phenomenon has been observed in other 6561

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Figure 10. Changes of spherulite radius as functions of time for (a) linear PLA and (b) LCB PLA during isothermal crystallization at Tc of 120 °C at shear rate of 1 s−1 for different shear time.

Figure 11. Changes of nucleation density as functions of space filling for (a) linear PLA and (b) LCB PLA crystallized at Tc of 120 °C with shear rate of 1 s−1 for different shear time.

quantitatively evaluation of the shear-induced enhancement of nucleation density will be made based on the data from the rheological measurements. The growth of storage modulus, G′, has been attributed to the filler effects of the crystals, based on modeling the crystallizing polymer as a suspension of growing particles in an amorphous polymer matrix. In general, the Avrami equation (eq 5) was applied to describe the space-filling effect of spherulitic growth in the case of quiescent isothermal crystallization, for which all nuclei, N, appear at the same time, t0, and the growth rate, G(t), is constant with time.

LCB PLAs prepared by other technical routines in the literature.44,46 When the undercooled melt of LCB PLA is applied to the same intensity of steady shear pulse at the shear rate of 1 s−1, the nucleation density of LCB PLA is remarkably enhanced in a more pronounced degree than that of linear PLA. When the shear time increases to 30 s, the shear-induced rowlike crystals (characteristic of shish-kebab structure) can be distinguished in the micrographs in panel E of Figure 9, accompanied by a great number of point-like fine-grained spherulites. It is found that the shear time has a significant effect on the crystalline morphological evolution from the point-like spherulites to row-like crystals. The micrographs in panel F of Figure 9 for the crystalline morphological evolution under the shear time of 40 s show significant increase of the density of row-like crystals as compared with that under the shear time of 30 s, indicating that the content of shish structure increases with increasing shear time. The shish-kebab structure usually cannot be formed instantaneously at the shear rate above the inverse Rouse time, and its formation also needs sufficient shear time for the stretched deformation. The existence of critical shear intensity for the formation of shish-kebab morphology has also been reported in the literature in forms of critical shear time,12,63 critical shear strain,64 and critical mechanical work.14,15 Determination of Shear-Induced Nucleation Density for PLA Melts. It has been shown in the above sections that shear flow can significantly enhance the crystallization kinetics for PLA under isothermal crystallization conditions. The shear effects on crystallization kinetics of PLA are achieved by acceleration of the nucleation rate, which has been morphologically confirmed by POM observations. In this section,

⎤ ⎡ 3 ϕ(t ) = 1 − exp⎢ − πNG(t )3 (t − t0)3 ⎥ ⎦ ⎣ 4

(5)

In our experiments, most of nucleation takes place during shear flow and the spherulitic growth process occurs after cessation of shear flow, and the observations of crystalline morphology for linear PLA and LCB PLA (Figures 8 and 9) confirm that the shear effect has not changed the nature of spherulites (except the case of LCB PLA being sheared for more than 30 s). Note that increasing the shear time for LCB PLA will cause the transition from spherulitic to shish-kebab morphologies, which cannot be captured by this method. In our case, there is no secondary increase of crystallization kinetics with increasing shear time as demonstrated by Housmans et al.10 Therefore, it is safe to determinate the shear-induced nucleation density by this method. Furthermore, the growths of PLA spherulites are distinctively observed for linear PLA and LCB PLA samples subjected to the selected shear conditions in this work. Thus, the shear effects on the growth rates of spherulites in the undercooled PLA melts can be determined. 6562

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Avrami modeling on the rheological data to decide the nucleation density, we compared the number of nuclei derived from the rheological measurements with that obtained from POM observations for linear PLA after sheared for different time at the shear rate of 1 s−1. As shown in Figure 12, the values derived from the rheological measurements are consistent with the values determined by POM observations. Note that the numbers of nuclei from POM observations were determined by the method described as follows. By counting the number of spherulites by using the Image J software (NIH) in a certain area, the area nucleus density, NA, could be determined, from which the volume nucleus density, NV, was calculated using the simple approximation according to eq 7:

Figure 10 shows the changes of spherulite radius as functions of time for PLA during isothermal crystallization at Tc of 120 °C after various applied shear conditions. It is found that the PLA spherulites in the sheared melts all grow linearly with time, and the linear fitted lines demonstrate the same slope in a good precision, indicating that the spherulitic growth rates, G, of 1.0 × 10−8 m/s for linear PLA and 0.9 × 10−8 m/s for LCB PLA in the sheared melts remain unchanged, which are in accordance with the results from shear-induced crystallization process for other polymers and our previous result.9,43,65 The degree of space filling can be calculated by following the same empirical equation to decide the relative crystallinity. With the assumption that all nuclei are present after the cession of shear flow, the nucleation densities for linear PLA and LCB PLA under different shear conditions can be estimated from space filling using the Avrami equation (eq 6),10 and the results are shown in Figure 11. N=

−3 ln[1 − ϕ(t )] 4πG(t )3 t 3

NV (m−3) = NA(m−2)3/2

(7)

In general, it can be observed that shear strongly enhances the formation of nuclei and a saturation plateau of nucleation density is approached for each sample at longer shear time under shear. The saturated nucleation density under shear at the shear rate of 1 s−1 for linear PLA is 2.4 × 1014, which is more than that under quiescent condition by a factor of over 3 orders of magnitude. The saturated nucleation density value for LCB PLA is more than that for linear PLA by a factor of 1 order of magnitude under the same shear condition, indicating that an incorporation of 6.4 wt % LCB structures in the linear PLA matrix can enhance the nucleation ability under shear. This can be understood by the assumption that the highly branched PLA chains take part in the formation of the crystallization precursor for the primary nuclei because the relaxation of the orientated structure in LCB PLA will take longer time than that of linear PLA after shear is ceased, resulting in the increased value of nucleation density for LCB PLA under the same shear condition. Mechanism for Shear-Induced Nucleation Enhancement and Shish-Kebab Formation of LCB PLA. In the current work we have performed a qualitative investigation on the relationship between the shear-induced nucleation enhancement and morphological evolution and shear time at a constant shear rate of 1 s−1 for LCB PLA, especially compared with the case of linear PLA. Both LCB PLA and linear PLA show the similar shear time dependences of enhancement and saturation of the nucleation density. However, the saturated nucleation density value for LCB PLA is more than that for linear PLA by a factor of 1 order of magnitude under the same shear

(6)

It can be seen that the nucleation density, N, is almost constant when the values of space filling, ϕ, are between 0.1 and 0.9. The nucleation density can be determined by taking the value at which ϕ = 0.5 as a reasonable approximation. The changes of nucleation density as functions of shear time for linear PLA and LCB PLA after sheared at 1 s−1 are shown in Figure 12. To investigate the reliability of this method of

Figure 12. Changes of nucleation density, Nv, as functions of shear time for linear PLA and LCB PLA crystallized at Tc of 120 °C with shear rate of 1 s−1. The red data points in the plot were obtained from POM observation.

Figure 13. Schematic illustration of evolutions of crystalline morphologies for linear PLA and LCB PLA after sheared for a sufficient time. 6563

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enhanced compared to the quiescent conditions, and the crystallization kinetics was accelerated with the increase of shear time. LCB PLA crystallized much faster than linear PLA under the same shear conditions. A saturation effect of shear time on crystallization kinetics was observed for both linear PLA and LCB PLA. In-situ POM observations demonstrated the LCB PLA not only possessed higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate as compared with that of linear PLA but also formed the shish-kebab structure after being sheared for the sufficiently longer time. The quantitative evaluation of the shear-induced nucleation density, which was estimated from the space-filling model using the Avrami equation, demonstrated that the saturated nucleation density under shear at the shear rate of 1 s−1 for linear PLA was much more than that under the quiescent condition by a factor of over 3 orders of magnitude and the saturated nucleation density value for LCB PLA was more than that for linear PLA by a factor of 1 order of magnitude under the same shear condition. The improvement of nucleation ability and morphological evolution from the spherulitic to shish-kebab structures in the shear flow field for LCB PLA could be ascribed to the broadened and complex relaxation behaviors of LCB PLA compared with linear PLA.

condition. Moreover, LCB PLA demonstrates the formation of shish-kebab crystalline morphology after sheared for more than a critical duration, whereas the crystalline morphology for linear PLA shows less obvious dependence on shear time, remaining the spherulitic morphology. It is generally believed that when the polymer melts are sheared, the macromolecular chains are stretched and aligned along the flow direction, and the stretched chains then assemble into parallel array and form the fibril bundles, which can behave as the primary nuclei for polymer crystallization. According to Heeley et al., two regimes of stretching can be identified based on the chain configuration.27 In the moderate stretching regime (τRγ̇ > 1), the contour length of the tube is increased by flow. However, the chain configuration remains essentially Gaussian, where the degree of bond orientation is low. Conversely, in the strong stretching regime (τRγ̇ ≫ 1), the amount of bond orientation becomes high. It is generally assumed that large bond orientation is required for the formation of shish-kebab structure. Figure 13 presents a schematic illustration of the mechanism for understanding the effects of LCB structures on the crystallization behavior of PLA materials when the PLA melts are sheared for long duration (more than the saturation shear time). The PLA material is first completely melted and quickly cooled to the crystallization temperature, for which the undercooled melt is disordered and the polymer chains in the undercooled melts are amorphous with Gaussian conformation. With the short shear time, both linear PLA and LCB PLA demonstrate the enhanced nucleation density of point-like nuclei and the nucleation density for LCB PLA is higher than that of linear PLA, because more segments of chains are aligned in LCB PLA to participate in the shear-induced nucleation process. With the long shear time, LCB PLA shows a different scenario from that of linear PLA. When linear PLA melts are sheared at the shear rate of 1 s−1 (τRγ̇ = 1.17), the stretch of the linear chain is weak and the stretched chain is easy to relax after the cession of shear flow, only resulting in the shear-induced point-like nucleation density enhancement. On the other hand, when LCB PLA is sheared at the shear rate of 1 s−1, the Weissenberg number (τLCB R γ̇) for LCB is far greater than 1, suggesting that the LCB chains can be strongly stretched in the shear flow field. Thus, when the melt is sheared for more than a critical shear time, large bond orientation of LCB PLA chains is achieved, and the stretched chain conformation can be reserved to form the fibrillar core (shish) after the cessation of shear, due to the fact that LCB molecules possess the long relaxation time (see Figure 13). In the subsequent isothermal crystallization process, the free coiled PLA chains can be absorbed on the surface of the shish to form the chain-folding lamellae (kebabs).

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AUTHOR INFORMATION

Corresponding Author

*Tel +86 0551-63607703; Fax +86 0551-63607703; e-mail [email protected] (Z.G.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z. G. Wang acknowledges the financial support from the National Science Foundation of China with Grant No. 21174139 and National Basic Research Program of China with Grant No. 2012CB025901.

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REFERENCES

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CONCLUSIONS In this study the effects of long chain branching on the nucleation density enhancements and crystalline morphological evolution for shear-induced crystallization of LCB PLA under isothermal conditions were thoroughly studied by using a rotational rheometer and POM. Prior to the shear-induced crystallization measurements, the molecular characteristics and rheological properties of PLA at the molten state were comprehensively investigated. The critical shear rates for the orientation and stretch of the longest chains (high molecular mass tails) of linear PLA at the temperature of 120 °C were determined to be 0.026 and 0.85 s−1, respectively. Investigation on shear-induced isothermal crystallization kinetics showed that the crystallization process at the shear rate of 1 s−1 was greatly 6564

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